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

Iron based alloys with carbon like steel find a wide range of applications in industries. The applications include, but are not limited to pipes, structural components used in automobiles, bridges and buildings, surgical tools, electrical components and the like. One application which widely uses steel is automobile industry. In automobile industry, out of the other requirements, there is an ever increasing demand for components with mechanical characteristics like light weight, resilient, high toughness or rigidity and high impact resistance. These mechanical characteristics of the components are a pre-requisite for safety of the occupants in the vehicle. Hence, ultra-high strength steels are commonly used in vehicles in view of their superior mechanical properties over other conventional materials. The ultra-high strength steel components are used for chassis, impact beams, bumpers, A and B pillars, roof tails, cross members, tunnels and the like among other components in the vehicles.

Conventionally, steel components used in automobiles are heat treated and subjected to forming process, to obtain required shape for assembly. Generally, hot stamping is the preferred heat treatment process used for forming the steel components used in vehicle, as the process is simple and cost effective. The hot stamping process can be performed in two ways viz. direct hot stamping process and indirect hot stamping process. In the direct hot stamping process, the steel blank is heated up to 950°C in a furnace and the steel blank is held at that temperature for 4 to 5 minutes for austenization of steel. The steel blank is then transferred to the press, where the steel blank is formed to required shape. The formed steel blank is then quenched in the closed tool to obtain the component. On the other hand, in the indirect hot stamping process, the blank is initially cold pre-formed, followed by austenization and subsequent quenching to obtain the required component. However, in either direct or indirect hot stamping process, the steel is usually heated in a furnace maintaining an ambient/oxidised atmosphere. The oxidised atmosphere causes surface oxidation of steel during austenization that degrades the mechanical properties as well as aesthetics of the component. To mitigate the problem of surface oxidation, coating may be performed to protect the surface of the steel from oxidation, and to maintain the surface appearance, mechanical properties of the steel. The steel whose surface is coated for protection from oxidation may be referred as a coated steel. Conventionally, many coatings are available for protecting steels from oxidation.

One of the most widely used coatings for protecting steel from oxidation is zinc based coating. The zinc coating on the steel surface is a sacrificial coating. The zinc coating on the steel surface gets oxidized, instead of the steel surface, thereby protecting the steel surface. However, the zinc based coatings may not be suitable for direct hot stamping process. Since, in direct hot stamping process, the operating temperature is above 900°C and at this temperature, interdiffusion of zinc and iron takes place. This results in the formation of liquid iron (Fe) - Zinc (Zn) phase (s) on the surface of the steel. The liquid phases present at the interface readily gets diffused into the grain boundaries, resulting in Liquid Metal Induced Embrittlement (LMIE). Due to LMIE, the material fails at an early stage, when subjected to tensile forces during forming. To mitigate the aforementioned problems instead of pure zinc coating, galvannealed coatings are tried out which is an alloy coating of iron and zinc having overall 10-12 wt.% iron in the coating and Fe-Zn gamma phase, Fe-Zn delta phase and Fe-Zn zeta phase from the substrate coating interface up to the coating top surface. The layer structure of the developed galvannealed (GA) steel sheet after hot stamping consists of two layers on the a-Fe substrate. The surface layer is zinc oxide, and second is Fe-Zn solid solution. However, if the heat treatment time is small then the high melting solid solution of Fe-Zn does not form instead the low melting Fe-Zn intermetallics (gamma, delta etc.) are formed at the interface and again causes LMIE. Hence, longer heat treatment period is required for galvannealed coating to avoid LMIE which is unrealistic from production point of view. Further, zinc coating by indirect hot stamping process is possible but not feasible due to requirement of high forming force at cold condition.

One of the other conventionally used coatings is aluminium based coatings. The aluminium based coaling or aluminium silicon coating process is carried out by hot dipping the steel in a bath composition containing from 9% to 10% silicon, 2% to 3.5% iron, the remainder being aluminium. The dipping temperature is about 675°C. This coating forms a microstructure having primary Al-Si eutectic matrix. The coating may form 5 μm thick Fe2SiAl ₇ inhibition layer at the coating/steel interface. Additionally, a thin layer of FezAls and FeAh with a thickness of less than 1 μτη may be formed between the Fe ₂ SiAl ₇ layer and substrate steel. The intermetallic phases may be solid at the hot stamping heat treatment temperature, which may eliminate chances of LMIE formation. Hence, the aluminium based coating on the steel may withstand the elongation of 40% at 700-850°C without failure of the material. However, the intermetallics at the interface are brittle in nature and thus during shaping, cracks are formed within the coating and interfacial de-bonding occurs. The cracks in the coating interface, exposes the substrate for oxidation at that position. Also, aluminium cannot offer any cathodic/sacrificial corrosion protection.

To overcome some of the problems associated with pure aluminium based coaling and pure zinc based coating, a dual layer Zn-Al coating, Zn-Al-Mg post-process coating are proposed in conventionally. In this process, steel is first hot-dip coated with Al-10% Si coating, and subsequently hot dipped in Zn having Al (<1 wt%). The final coating is composed of a 5μm Zn layer, a 15μm Al-Si alloy layer and a Fe2SiAl ₇ intermetallic layer at the coating/steel interface. In addition, a phosphating process is conducted prior to hot stamping. The Zn-Al- Mg post process coating is done on the hot stamped parts. However, such coating does not meet the requirement of surface protection during hot stamping process. Moreover, during hot dip coating process of Zn-Al-Mg coating the martensite is tempered and the strength level is significantly reduced.

Conventionally, there is also another coating process employed for press hardenable steels. Such process includes electrodeposition of Ni-Zn phase having composition of llwt-% Ni, 0.6wt-% Fe and balance zinc on steel substrate. This corresponds to Ni-Zn gamma (γ) phase with higher melting point (880°C) than the Fe-Zn phases (782°C). The final microstructure composed of iron based solid solution as well as Ni-Zn gamma phase that do not cause the formation of LMIE. This coating was able to withstand LMIE, however electrodeposition of Ni-Zn phases on bare steel may a cause of hydrogen embrittlement which would eventually lead to premature failure of steel. Further, the iron concentration in the coating becomes largest with very few Ni-Zn islands that will reduce the sacrificial nature of the coating. In the light of the foregoing discussion, there is a need to develop a coated steel substrate and a method for coating a steel substrate to overcome one or more limitations stated above.

SUMMARY OF THE DISCLOSURE

One or more drawbacks of conventional methods of coated steel substrates are overcome, and additional advantages are provided through a product and a method as claimed in the present disclosure. Additional features and advantages are realized through the technicalities of the present disclosure. Other embodiments and aspects of the disclosure are described in detail herein and are considered to be a part of the claimed disclosure.

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 coating on the steel substrate. The nickel-zinc coating forms an iron-nickel solid solution layer on the steel substrate, a nickel layer on the iron-nickel solid solution layer, a nickel-zinc solid solution layer on the nickel 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.

In an embodiment, the nickel-zinc coating 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 thickness of the nickel-zinc coating ranges from 25μm 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-nickel solid solution layer is formed of Body Centered Cubic crystals, phase fraction of iron-nickel solid solution is about 2-4% of total coating.

In an embodiment, the nickel 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, and thickness of the nickel layer ranges from 1μm to 3μm. In an embodiment, the nickel layer is formed of Face Centered Cubic crystals, and phase fraction of nickel layer is about 4% to about 6% of total coaling, and wherein, the nickel layer has hardness of about 6.71 GPa. In an embodiment, 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, and thickness of the nickel-zinc solid solution layer ranges from 0.5μm to 1μm.

In an embodiment, the nickel-zinc solid solution layer is formed of Face Centered Cubic crystals, and phase fraction of the nickel-zinc solid solution layer is about 2% to about 4% of total coating.

In an embodiment, the nickel-zinc gamma layer 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 ranges from 6μm to ΙΟμm.

In an embodiment, the nickel-zinc gamma layer is formed of cubic structure crystals, and phase fraction of the nickel-zinc gamma layer is about 10% to about 12% of total coaling, and wherein the nickel-zinc gamma layer has a mechanical resistance of about 4.83 GPa.

In an embodiment, the nickel-zinc delta layer 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 3μm to 5μm. In an embodiment, the nickel-zinc delta layer is formed of monoclinic crystals, and phase fraction of nickel-zinc delta layer about 20% to about 25% of the total coaling, and wherein the nickel-zinc delta layer has a mechanical resistance of about 2.82 GPa.

In an embodiment, the overlay zinc layer comprises iron of about 0.1% to about 1% by weight and zinc of about 80% to about 100% by weight and remainder of the composition includes oxygen by weight, and thickness of the overlay zinc layer ranges from 5μm to 15μm.

In an embodiment, the overlay zinc layer is formed of hexagonal closed pack crystals, and phase fraction of the overlay zinc layer comprises of zinc and zinc-oxide, and wherein the overlay zinc layer has a mechanical resistance of about 0.61 GPa. In an embodiment, the steel substrate, 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.

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 coating on the steel substrate. The nickel-zinc coating forms an iron-nickel-zinc solid solution layer on the steel substrate, an upper coating layer on the iron-nickel-zinc solid solution layer and an oxide layer on the upper coating layer.

In an embodiment, the nickel-zinc coating comprises iron of about 25% to about 35% by weight, zinc of about 15% to about 30% by weight, and nickel of about 30% to about 40% by weight, and the thickness of the nickel-zinc coating ranges from 30μm to 7()μm. In an embodiment, the iron-nickel-zinc solid solution layer comprises iron of about 25% to about 40% by weight, zinc of about 20% to about 30% by weight and nickel of about 25% to about 40% by weight and thickness of the iron-nickel solid solution layer ranges from 5μm to 15μm.

In an embodiment, the iron-nickel-zinc solid solution layer is formed of Face Centered Cubic crystals, and phase fraction of the iron-nickel solid solution layer is about 15% to about 20% of the total coating. In an embodiment, the upper coating layer comprises iron of about 25% to about 30% by weight, zinc of about 10% to about 25% by weight, nickel of about 40% to about 45% by weight and oxygen of about 2% to about 5% by weight and thickness of the upper coating layer ranges from 24μm to 55μm. In an embodiment, the upper coating layer is formed of Face Centered Cubic crystals, and phase fraction of the upper coating layer is about 75% to about 80% of the total coating.

In an embodiment, the oxide layer comprises iron of about 0.5% to about 1% by weight, zinc of about 75% to about 80% by weight, nickel of about 0.01% to about 0.05% by weight and rest oxygen and thickness of the oxide layer ranges from 1 μm to 3μm.

In an embodiment, the phase fraction of the oxide layer is about 3% to about 5% of the total coating. embodiment, the steel substrate, 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.

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 70°C to about 90°C, heating the steel substrate in an inert atmosphere up to a temperature ranging from about 450°C to about 470°C and applying a coat of zinc onto the steel substrate.

In an embodiment, cleaning the steel substrate comprises acts of washing the steel substrate by a caustic solution at a temperature ranging from about 50°C to about 70°C for a time ranging from 2 minutes to 5 minutes to remove oil remnants and thereafter rinsing the steel substrate in water to clean carry overs of the caustic solution. After rinsing the steel substrate is pickled in an acidic solution at temperature ranging from about 60°C to about 70°C 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 bath includes 150-200 g/1 of

of and the bath is maintained at a pH ranging from 2 to 7; and the electroplating of

the steel substrate is carried out by configuring a plate of nickel 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 -I V respectively, for 1-30 minutes.

In an embodiment, the coat of zinc is applied by dipping the steel substrate in a molten zinc solution at a temperature ranging from about 450°C to about 470°C 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 3 illustrates graphical representation of Glow Discharge Optical Emission Spectroscopy depth profile of the coating on the steel substrates before hot stamping process, in accordance with an embodiment of the present disclosure. Figure 4 illustrates graphical representation of Grazing Incidence X-ray diffraction of the coating on the steel substrate staring form top surface to interface, in accordance with an embodiment of the present disclosure.

Figure 5a illustrates cross-sectional elemental view of the coated boron steel, in accordance with an embodiment of the present disclosure. Figure 5b illustrates cross-sectional elemental view of coated interstitial phase steel, 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 micrograph and analysis plots of final microstructure of the coating on the steel substrate with 5μm of overlay zinc layer, in accordance with an embodiment of the present disclosure. Figure 9 illustrates micrograph and analysis plots of final microstructure of the coating on the steel substrate with 15μm of overlay zinc layer, in accordance with an embodiment of the present disclosure.

Figure 10a 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.

Figure 10b illustrates graphical representation of stress-strain curve of tensile test carried on coated steel till 40% elongation, in accordance with an embodiment of the present disclosure. Figure 10c illustrates graphical representation of potentiodynamic polarization test carried before and after treatment of the coaled steel, 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 a coated steel which offers high mechanical resistance after thermal treatment. The coated steel enables both direct and indirect hot stamping process, without deteriorating mechanical properties like strength or ductility of steel. The coated steel comprises a steel substrate and a nickel-zinc 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. The present disclosure also discloses a method for applying a Ni-Zn coating on the steel substrate. 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 then heated in an inert atmosphere up to a predetermined temperature. Lastly, a coat of zinc is applied by dipping the heated steel substrate into a zinc solution, thereby coating a nickel-zinc layer on the steel substrate.

The nickel-zinc 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 on the steel substrate. In an embodiment, the nickel-zinc coating on the steel substrate comprises of iron, zinc, nickel and oxygen in predetermined quantities and is of predetermined thickness as per requirement. In an embodiment, each of the layers in the nickel-zinc coating comprises of iron, zinc, nickel and oxygen in predetermined quantities and is of predetermined thickness.

The coated steel after heat forming forms hot worked coated steel. The hot worked coated steel comprises steel substrate and a nickel-zinc 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 coating on the steel substrate forms an iron-nickel solid solution, 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 retains the strength as well as ductility of the steel. Simultaneously, 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 (1). In an embodiment, the steel substrate (1) 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.

Table. 1

wherein, the balance being iron optionally along with incidental elements of the alloy.

The presence of boron in the steel enhances hardenability of steel. Boron also enables formation of martensite phase of iron particles in the steel substrate (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 resistance 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 or any other foreign entities deposited on the surface of the steel substrate (1). In the cleaning process, the steel substrate (1) is subjected for washing in a caustic solution maintained at a temperature ranging from about 50°C to about 70°C, and the time ranging from 2 minutes to 5 minutes. Washing the steel substrate (1) in the caustic solution removes oily contaminants on the surface of the steel substrate (1). The steel substrate (1) is then rinsed in water, to clean carry overs of the caustic solution on the surface of the steel substrate (1) during washing. Subsequent to rinsing, the steel substrate (1) is pickled in an acidic solution maintained at a temperature ranging from about 60°C to about 70°C, and the time ranging from about 1 minute to about 5 minutes to remove corrosion from the surface of the steel substrate (1). After pickling the steel substrate (1) may be 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 NiSO ₄ of about 150 g/1 to about 200 g/1, NiCI ₂ of about 30 g/1 to about 40.9 g/1 and H ₃ BO ₃ of about 5 g/1 to about 8 g/1. The nickel bath is maintained at a pH ranging from about 2 to about 7. The electroplating is carried out by configuring a plate of nickel as anode and the steel substrate (1) as cathode, while maintaining a current from about 2 mA/cm2 to about 5mA/cm2 and a voltage of 0.5V to about IV respectively for about 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 deposition on the steel substrate (1) will act as a potential barrier to hydrogen, and therefore prevents diffusion of hydrogen into the steel substrate (1) and hence, prevents hydrogen embrittlement.

In step 103, the electroplated steel substrate is heated in an inert atmosphere up to a temperature of about 450°C to about 470°C in an inert chamber. Healing the steel substrate (1) in an inert atmosphere prohibits the increase of oxygen content on the electroplated surface. This inherently eliminates possibility of oxygen concentration in subsequent steps of the process of coating the steel substrate (1). In an embodiment, the inert chamber is a hot dip process simulator. In step 104, the heated steel substrate is dipped in a molten zinc bath maintained at a temperature of 460°C for about 2 seconds to about 10 seconds, for applying a coat of zinc. The zinc particles adhere to the steel substrate (1), when the steel substrate (1) is immersed in the molten zinc solution. The excess zinc adhered to the surface, may be wiped out using wipers installed at both sides of a control panel. Thus, the process from steps 101-104 provides a steel substrate (1) coated with a nickel-zinc coating of predetermined thickness. In an embodiment, the nickel-zinc 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 thickness of the nickel-zinc coating ranges from 25μm to 50μm. Reference is now made to Figure 3 which illustrates graphical depth profile of the coating on the steel substrate (1) after coating. In an embodiment, the depth profile of the coating on the steel substrate (1) is obtained by Glow Discharge Optical Emission Spectroscopy. As shown in the figure 3 the nickel-zinc coating on the steel substrate (1) carried by the steps 101-104 when galvanised, results in a plurality distinct layers and interfaces between them. In an embodiment, the plurality of layers formed on the steel substrate (1) includes an iron-nickel solid solution layer on a surface of steel substrate (1). Subsequently, a nickel 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. Each of the layers and interfaces between them 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 iron-nickel solid solution layer forms a thickness ranging from 0.5μm to 2μm. Further iron-nickel solid solution layer is formed of body centered cubic crystals, and phase fraction of the iron-nickel solid solution layer is about 2% to about 4% of the total coating.

In an embodiment, the nickel layer (2) shown as Ni rich layer in FIG. 3 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, and thickness of the nickel layer (2) ranges from 1μm to 3μm. The nickel layer (2) has a mechanical resistance of about 6.71 GPa. Further, the nickel layer (2) is formed of face centered cubic crystals, and phase fraction of the nickel layer (2) is 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. 3 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, and thickness of the nickel-zinc solid solution layer ranges from 0.5μm to 1μm. Further, the nickel-zinc solid solution layer is formed of face centered cubic crystals, and phase fraction of the nickel-zinc solid solution layer is of about 2% to about 4% of the total coating. In an embodiment, the nickel-zinc gamma layer (3) shown as gamma layer in Figure 3 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, and thickness of the nickel-zinc gamma layer (3) ranges from 6μm to ΙΟμm. The nickel-zinc gamma layer (3) has a mechanical resistance of about 4.83 GPa. Further, the nickel-zinc gamma layer (3) is formed of cubic structure crystals, and phase fraction of the nickel-zinc gamma layer (3) is about 10% to about 12% of the total coating. In an embodiment, the nickel-zinc delta layer (4) shown as delta layer in figure 3 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 10% to about 12% by weight, and thickness of the nickel-zinc delta layer (4) ranges from 3μm to 5μm. The nickel-zinc delta layer (4) has a mechanical resistance of about 2.82 GPa. Further, the nickel-zinc delta layer (4) is formed of monoclinic crystals, and phase fraction of nickel- zinc delta layer (4) is about 20% to about 25% of the total coating

In an embodiment, the overlay zinc layer (5) shown as overlay zinc in figure 3, 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 remainder of the composition includes oxygen by weight, and thickness of the overlay zinc layer (5) ranges from 5μm to 15μm. The overlay zinc layer (5) has a mechanical resistance of about 0.61GPa. Further, the overlay zinc layer (5) is formed of hexagonal closed pack crystals, and phase fraction of the overlay zinc layer (5) comprises of zinc and zinc-oxide.

In the exemplary embodiment illustrated in figure 3, the overlay zinc layer (5) extends up to 5 μm, the nickel-zinc delta layer (4) and the nickel-zinc gamma layer (3) extends up to 15 μm, the nickel-zinc solid solution layer extends up to 17 μm, the nickel layer (2) extends up to 20 μm and the iron-nickel solid solution layer extends up to 25 μm.

The aforementioned characteristics of each layer of the coating provide the necessary strength and ductility for the resultant coated steel (CS). The coating on the steel also enhances the mechanical resistance of the steel due to the presence of nickel in the coating. Figure 4 in an exemplary embodiment of the present disclosure illustrates phase identification of the coated steel (CS). To ascertain the layers formed on the steel substrate (1), the phase identification of the coated steel (CS) is carried out. In an embodiment, grazing angle X-ray diffraction [XRD] may be used for phase identification.

To determine the phase identification, the overlay zinc layer (5) is polished to obtain signals from underneath the layers. In an embodiment, grazing angle XRD is performed with copper target and with a grazing angle of 3°. The data obtained by the XRD is stored in a memory unit [not shown in figures] of a system used for performing XRD. The memory unit is also configured to store reference values of the coatings on the steel substrate (1). In an embodiment, the reference values include peak positions that are to be attained by each layers and interfaces on the steel substrate (1). The data from XRD is analysed by a computing unit [not shown in figures] by comparing the data obtained from XRD with reference values stored in the memory unit.

Figure 4 shows the coating on the steel substrate (1) from a top surface (a) to an interface (e). The top surface of the coated steel (CS) shows peaks for zinc [as shown in (a) row]. There are also peaks observed that can be assigned to both zinc as well as Ni-Zn delta phase (26=36.5°). Additionally, there are two low angle peaks exclusively for Ni-Zn delta phase with very low intensity (2Θ=35°, 44°). After the first stage of polishing of the top surface of the coating the intensity of the peaks for zinc is greatly reduced [as shown in (b) row]. The intensity of the peaks for delta phase is increased and there are new peaks for the Nickel-Zinc delta and gamma phase. At a further depth within the coating [as shown in (c) row], it is observed that the intensity of the zinc peaks are very small. There is presence of low intensity peaks of delta and gamma phases and there are new peaks for solid solutions of zinc in nickel and nickel in iron. There are peaks at 29=65°, 82° which are broadened. This confirms the presence of solid solutions and micro strain in the coating structure. Further polishing of the coating [as shown in (d) row] results in lowering of the intensity for Ni-Zn delta and gamma phase. Also, there are prominent peaks of solid solutions of iron for the coated steel (CS) [as shown in (e) row].

From the above experimental data illustrated in figure 4, it is evident that there is a nickel rich layer at the steel substrate (1) coating interface along with two nickel-zinc phases in the coating [cross sectional view shown in figure 5 a]. This configuration in the coating will enable hot stamping process, without any coating cracks or embrittlement, as the iron-nickel solid solution layer along with the nickel layer (2) has a melting point of 1400°C and are ductile at higher temperatures having FCC crystal structure. In an embodiment, the nickel-zinc coating applied on the steel substrate (1) can also be applied on the interstitial free steels (6) [as shown in figure 5b] and dual phase steels (7) [as shown in figure 5c]. In an embodiment, the dual phase steels is selected from group consisting of mechanical strength from about 600 MPa to about 1000 MPa.

The coated steel (CS) may be subjected for hot forming to form the steel to a required shapes and dimensions. Now referring to, Figure 6 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 10°C/s up to 950°C, and is maintained at this temperature for about 300 seconds. Then, the coated steel (CS) is cooled at a rate of 30°C/s up to 850°C 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 7]. 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 resistance 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) takes place due to its boiling point of 907°C. Also, during heating of the coated steel (CS) the overlay zinc layer (5) oxidises. Thus, due to evaporation and oxidation of the overlay zinc layer (5), the amount of zinc in the final microstructure varies, based on the thickness of the overlay zinc layer (5). Hence, greater the thickness of the overlay zinc layer (5) on the coated steel (CS), greater will be the amount of zinc in the final microstructure. Similarly, the amount of nickel in the subsequent layer deteriorates based on the thickness of the nickel layer (2) in the coated steel (CS) before hot stamping. Thus, the final coating thickness depends on initial nickel coating thickness, galvanising time and thickness of overall zinc layer.

An example of variation of final microstructure of the coating on the steel substrate (1) with initial 5μm and 15μm of overlay zinc layer (5) is illustrated in figures 8 and 9 respectively. As illustrated, in both the cases a continuous interfacial layer rich in nickel and iron is present in the coaling. However, amount of zinc is higher at some positions where iron concentration is less [as shown in figure 9] whereas the zinc rich areas are very less for the sample shown in figure 8. Thus, the coating interface may arrest any crack when subjected to tensile forces and is also ductile due to formation of FCC structure of iron and nickel. Thus, the mechanical properties of the steel are retained, even after coating of the steel substrate (1). Also, due to the presence of iron-nickel phases in the coating, the possibility of LMIE of zinc is prevented.

Exemplary Experimental results:

Tensile tests were conducted for the coated steel (CS) and the bare steel (B) as per the test specifications in the table 2 below. Consequently, stress-strain curves were plotted based on the values obtained [as shown in figure 10a and 10b for coated test sample 1 and 2 respectively]. The test 1 is shown in figure 10a, and is conducted for coated steel (CS) sample up to fracture point and the results are compared with the bare steel (B). Also, test 2 is shown in figure 10b, and is conducted for coated steel (CS) up to 40% elongation and the results are compared with the bare steel (B). The results of the tensile tests are tabulated in table 2.

Table 2

Referring to Figure 10a, it is evident that the mechanical strength values of the coated steel (CS) sample overlaps with the mechanical strength values of the bare steel (B), till fracture during the tensile test. Thus, the critical strength parameters of the coated steel (CS) which are, elastic strength, yield strength, ultimate strength and rupture strength resembles that of the bare steel (B). Now referring to figure 10b, it is evident that the strength values of the coated steed overlaps with that of the bare steel (B), during tensile test up to 40% elongation of samples. Thus, the critical strength parameters of the coated steel (CS) and coated steel whose tensile test is repeated (CSi) which are, yield strength and ultimate strength up to 40% elongation of the sample resembles that of the bare steel (B). The strength values of the coated steel (CS) specimen still resemble that of the coated steel (CS), when the tensile test up to 40% elongation of the specimen is carried out. Therefore, it is evident from figures 10a and 10b, that there is no deterioration of the mechanical properties of the coated steel (CS) with respect to the bare steel (B), even after hot stamping treatment. The test on the coated test sample 1 and 2 are repeated for deformation temperature up to 900°C and the preferred temperature is 850°C.

Referring to Figure 10c, potentiodynamic polarization tests were carried out in 3.5% sodium chloride solution [NaCl]. The potentiodynamic polarization tests were accomplished in the range of about -250mV to about +250mV than the corrosion potential [Ε _∞ ΙΓ] at a scan rate of 0.5 mV per second in a potentiostat. In an embodiment, the potentiostat is a Gamry potentiostat. The results of the potentiodynamic polarization tests are tabulated in table 3.

Table 3 From the polarization curve, it is evident that the corrosion parameters [w and Econ-] of the coated steel (CS) correlates or overlaps to that of the conventional galvanised coating (GS). The overlapping of the Ε _∞ ΙΓ values may be due to the presence of overlay zinc layer (5) at the top of the surface of the coating. Further, the coating after heat treatment (CS ₂ ) is showing a large potential shift in positive direction than the conventional galvanised coating (GS) and have lesser w values. This shift of potential in positive direction may be due to the compactness of the coating after heat treatment. This shows that, the coating of Ni-Zn on the steel substrate offers resistance to corrosion even after heat treatment. Advantages:

The present disclosure provides a coated steel, which provides superior properties as compared to conventional coated steels. The present disclosure provides a coated steel, which retains the mechanical properties of the bare steel (B), even after heat treatment.

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.

It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as "open" terms (e.g., the term "including" should be interpreted as "including but not limited to," the term "having" should be interpreted as "having at least," the term "includes" should be interpreted as "includes but is not limited to," etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present. For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases "at least one" and "one or more" to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles "a" or "an" limits any particular claim containing such introduced claim recitation to inventions containing only one such recitation, even when the same claim includes the introductory phrases "one or more" or "at least one" and indefinite articles such as "a" or "an" (e.g., "a" and/or "an" should typically be interpreted to mean "at least one" or "one or more"); the same holds true for the use of definite articles used to introduce claim recitations. In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should typically be interpreted to mean at least the recited number (e.g., the bare recitation of "two recitations," without other modifiers, typically means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to "at least one of A, B, and C, etc." is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., "a system having at least one of A, B, and C" would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.)- In those instances where a convention analogous to "at least one of A, B, or C, etc." is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., "a system having at least one of A, B, or C" would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase "A or B" will be understood to include the possibilities of "A" or "B" or "A and B."

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.

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