Claims:We Claim:

1. A coated steel, comprising:
a steel substrate, having a composition comprising, by weight percent of:
Carbon 0.2 % to 0.25 %;
Manganese 1.15 % to 1.4 %;
Sulphur less than 0.01 %;
Phosphorus less than 0.05 %;
Silicon 0.2 % to 0.35 %;
Aluminium less than 0.1 %;
Copper less than 0.05 %;
Chromium 0.15 % to 0.35 %;
Nickel less than 0.1 %;
Molybdenum less than 0.01 %;
Vanadium less than 0.01 %;
Niobium less than 0.01 %;
Titanium 0.02 % to 0.05 %;
Nitrogen less than 50ppm;
Boron 0.002 % to 0.005 %; and the balance being iron optionally along with incidental elements; and
a nickel-copper composite coating on the steel substrate, wherein the nickel-copper composite coating forms a nickel-copper alloy layer on the steel substrate.

2. The coated steel as claimed in claim 1, wherein the steel is a bare steel substrate.

3. The coated steel as claimed in claims 1 and 2, wherein the nickel-copper alloy layer on the bare steel substrate comprises iron of 2 % to 5 % by weight, nickel 66 % to 74 % by weight and copper 19 % to 27 % by weight and, thickness of the nickel-copper alloy layer ranges from 3 µm and 15 µm.

4. The coated steel as claimed in claim 1, wherein the steel substrate is coated with an iron-zinc alloy.
5. The coated steel as claimed in claim 4, wherein the iron-zinc alloy coating on the steel substrate forms an iron-zinc alloy layer comprising iron of 8 % to 15 % by weight and rest being zinc and, thickness of the iron-zinc alloy layer ranges from 6 µm to 15 µm on the steel substrate.

6. The coated steel as claimed in claims 1 and 4, wherein the nickel-copper alloy layer on the iron-zinc alloy coated steel substrate comprises iron of 1 % to 2 % by weight, nickel of 57 % to 64 % by weight, copper of 8 % to 36 % by weight and zinc of 1 % to 2 % by weight and thickness of the nickel-copper alloy layer ranges from 3 µm and 15 µm.

7. The coated steel as claimed in claim 1, wherein the nickel-copper composite coating, after hot working forms:
an iron-nickel-copper solid solution layer on the steel substrate; and
an iron-nickel-copper oxide layer on iron-nickel-copper solid solution layer.

8. The coated steel as claimed in claim 7, wherein the iron-nickel-copper solid solution layer comprises iron of 30 % to 78 % by weight, nickel of 18 % to 53 % by weight, copper of 4 % to 8 % by weight, oxygen of 1 % to 6 % by weight and the thickness of the iron-nickel-copper solid solution layer ranges from 2 µm to 15 µm.

9. The coated steel as claimed in claim 7, wherein the iron-nickel-copper oxide layer comprises iron of 35 % to 56 % by weight, nickel of 20% to 40% by weight, copper of 2 % to 6 % by weight, oxygen of 17 % to 28 % by weight and the thickness of the iron-nickel-copper oxide layer ranges from 1 µm to 5 µm.

10. The coated steel as claimed in claim 7, wherein the steel substrate is coated with the iron-zinc alloy.

11. The coated steel as claimed in claim 10, wherein the nickel-copper composite coating on the iron-zinc alloy coated steel substrate after hot working forms:
an iron rich iron-nickel-copper-zinc solid solution layer; and
a zinc rich iron-nickel-copper-zinc solid solution layer on the iron rich iron-nickel-copper-zinc solid solution layer.

12. The coated steel as claimed in claim 11, wherein the iron rich iron-nickel-copper-zinc solid solution layer comprises iron of 83 % to 98 % by weight, nickel of 5 % to 8 % by weight, copper of 1 % to 2 % by weight, zinc of 1 % to 11 % by weight and the thickness of the iron rich iron-nickel-copper-zinc solid solution layer ranges from 5µm to 30 µm.

13. The coated steel as claimed in claim 11, wherein the zinc rich iron-nickel-copper-zinc solid solution layer comprises iron of 70 % to 76 % by weight, nickel of 7 % to 8 % by weight, copper of 1 % to 2 % by weight, zinc of 16 % to 18 % by weight and the thickness of the zinc rich iron-nickel-copper-zinc solid solution layer ranges from 0.5 µm to 6 µm.

14. A method for manufacturing a coated steel, the method comprising:
providing a steel substrate having a composition comprising, by weight percent of:
Carbon 0.2 % to 0.25 %;
Manganese 1.15 % to 1.4 %;
Sulphur less than 0.01 %;
Phosphorus less than 0.05 %;
Silicon 0.2 % to 0.35 %;
Aluminium less than 0.1 %;
Copper less than 0.05 %;
Chromium 0.15 % to 0.35 %;
Nickel less than 0.1 %;
Molybdenum less than 0.01 %;
Vanadium less than 0.01 %;
Niobium less than 0.01 %;
Titanium 0.02 % to 0.05 %;
Nitrogen less than 50ppm;
Boron 0.002 % to 0.005 %; and the balance being iron optionally along with incidental elements;
coating the steel substrate with a nickel-copper composite coating by a coating method to form the coated steel substrate, wherein the coated steel substrate comprises a nickel-copper alloy layer.

15. The method as claimed in claim 14, wherein the coating method employed is a co-deposition process.

16. The method as claimed in claim 14, wherein the steel substrate is bare steel substrate.

17. The method as claimed in claims 14 and 15, wherein the nickel-copper alloy layer on the bare steel substrate comprises iron of 2 % to 5 % by weight, nickel of 66 % to 74 % by weight and copper of 19 % to 27 % by weight and, thickness of the nickel-copper alloy layer ranges from 3 µm and 15 µm.

18. The method as claimed in claim 14, wherein the steel substrate is coated with an iron-zinc alloy.

19. The method claimed in claim 14 , wherein iron-zinc alloy coating on the steel substrate forms an iron-zinc alloy layer comprising iron in the range of 8 % to 15 % by weight and rest being zinc and, thickness of the iron-zinc alloy layer ranges from 6 µm to 15 µm on the steel substrate.

20. The method as claimed in claim 14 and 18, wherein the nickel-copper alloy layer on the iron-zinc alloy coated steel substrate comprises iron of 1 % to 2 % by weight, nickel of 57 % to 64 % by weight, copper of 8 % to 36 % by weight and zinc of 1 % to 2 % by weight and thickness of the nickel-copper alloy layer ranges from 3 µm and 15 µm.

21. A method for manufacturing a hot worked coated steel, the method comprising:
providing the steel substrate with a nickel-copper composite coating by a method as claimed in claim 14;
heating the coated steel substrate to a temperature Tc to obtain a heated substrate;
hot working the heated substrate, to obtain a hot worked substrate; and
cooling the hot worked substrate to a temperature less than 400 °C to obtain the hot worked coated steel substrate.

22. The method as claimed in claim 21, wherein the hot working is a hot stamping process, and is performed in a die and punch assembly.

23. The method as claimed in claim 21, wherein the nickel-copper composite coating on the steel substrate after hot working forms:
an iron-nickel-copper solid solution layer on the steel substrate; and
an iron-nickel-copper oxide layer on the iron-nickel-copper solid solution layer.

24. The method as claimed in claim 23, wherein the iron-nickel-copper solid solution layer on the iron-zinc alloy coated steel substrate comprises iron of 30 % to 78 % by weight, nickel of 18 % to 53 % by weight, copper of 4 % to 8 % by weight, oxygen of 1 % to 6 % by weight and the thickness of the iron-nickel-copper solid solution layer ranges from 2 µm to 15 µm.

25. The method as claimed in claim 23, wherein the iron-nickel-copper oxide layer on the iron-nickel-copper solid solution layer comprises iron of 35 % to 56 % by weight, nickel of 20% to 40% by weight, copper of 2 % to 6 % by weight, oxygen of 17 % to 28 % by weight and the thickness of the iron-nickel-copper oxide layer ranges from 1 µm to 5 µm.

26. The method as claimed in claim 21, wherein the steel substrate is coated with an iron-zinc alloy.

27. The method as claimed in claim 21 and 26, wherein the nickel-copper composite coating on the iron-zinc alloy coated steel substrate after hot working forms:
an iron rich iron-nickel-copper-zinc solid solution layer on the steel substrate; and
a zinc rich iron-nickel-copper-zinc solid solution layer on the iron rich iron-nickel-copper-zinc solid solution layer.

28. The method as claimed in claim 27, wherein the iron rich iron-nickel-copper-zinc solid solution layer comprises iron of 83 % to 98 % by weight, nickel of 5 % to 8 % by weight, copper of 1 % to 2 % by weight, zinc of 1 % to 11 % by weight and the thickness of the iron rich iron-nickel-copper-zinc solid solution layer ranges from 5 µm to 30 µm.

29. The method as claimed in claim 27, wherein the zinc rich iron-nickel-copper-zinc solid solution layer comprises iron of 70 % to 76 % by weight, nickel of 7 % to 8 % by weight, copper of 1 % to 2 % by weight, zinc of 16 % to 18 % by weight and the thickness of the zinc rich iron-nickel-copper-zinc solid solution layer ranges from 0.5 µm to 5 µm.

30. A coated steel, comprising:
a steel substrate, having a composition comprising, by weight percent of:
Carbon 0.2 % to 0.25 %;
Manganese 1.15 % to 1.4 %;
Sulphur less than 0.01 %;
Phosphorus less than 0.05 %;
Silicon 0.2 % to 0.35 %;
Aluminium less than 0.1 %;
Copper less than 0.05 %;
Chromium 0.15 % to 0.35 %;
Nickel less than 0.1 %;
Molybdenum less than 0.01 %;
Vanadium less than 0.01 %;
Niobium less than 0.01 %;
Titanium 0.02 % to 0.05 %;
Nitrogen less than 50ppm;
Boron 0.002 % to 0.005 %; and the balance being iron optionally along with incidental elements;
an iron-zinc coating on the steel substrate; and
a nickel coating on the iron-zinc alloy coating, wherein the nickel coating forms a nickel layer on the iron-zinc alloy coated steel substrate.

31. The coated steel as claimed in claim 30, wherein the iron-zinc alloy coating forms an iron-zinc alloy layer comprising iron in the range of 8 % to 15 % by weight and rest being zinc and, thickness of the iron-zinc alloy layer ranges from 6 µm to 15 µm on the steel substrate.

32. The method as claimed in claim 30, wherein the nickel layer comprises iron of 0.1 % to 15 % by weight, nickel 80 % to 90 % by weight and zinc of 1 % to 10 % by weight and the thickness of nickel layer on the iron-zinc alloy coated steel substrate ranges between 2 µm and 10 µm.

33. The coated steel as claimed in claim 30, wherein the nickel coating after hot working forms:
an iron rich iron-nickel-zinc layer on steel substrate; and
a zinc rich iron-nickel-zinc layer on the iron rich iron-nickel-zinc layer.

34. The coated steel as claimed in claim 33, wherein the iron rich iron-nickel-zinc layer comprises iron of 10 % to 84 % by weight, nickel of 10 % to 45 % by weight, zinc of 6 % to 44 % by weight and the thickness of the iron rich iron-nickel-zinc layer ranges from 5 µm to 30 µm.

35. The coated steel as claimed in claim 33, wherein the zinc rich iron-nickel-zinc layer comprises iron of 1 % to 1.5 % by weight, nickel of 1 % to 3 % by weight, zinc of 77 % to 84 % by weight and the thickness of the zinc rich iron-nickel-zinc layer ranges from 0.5 µm to 5 µm.

36. The coated steel as claimed in claims 1 and 30, wherein the hot worked steel substrate has a martensite body centered tetragonal crystal structure (BCT) structure.

37. The coated steel as claimed in claims 1 and 30, wherein the steel substrate is a steel sheet.

38. A method for manufacturing a coated steel, the method comprising:
providing a steel substrate having a composition comprising, by weight percent of:
Carbon 0.2 % to 0.25 %;
Manganese 1.15 % to 1.4 %;
Sulphur less than 0.01 %;
Phosphorus less than 0.05 %;
Silicon 0.2 % to 0.35 %;
Aluminium less than 0.1 %;
Copper less than 0.05 %;
Chromium 0.15 % to 0.35 %;
Nickel less than 0.1 %;
Molybdenum less than 0.01 %;
Vanadium less than 0.01 %;
Niobium less than 0.01 %;
Titanium 0.02 % to 0.05 %;
Nitrogen less than 50ppm;
Boron 0.002 % to 0.005 %; and the balance being iron optionally along with incidental elements;
coating an iron-zinc alloy coating on the steel substrate; and
coating a nickel coating by coating method on the iron-zinc coating to form a coated steel substrate, wherein the coated steel substrate comprising a nickel layer on the iron-zinc alloy layer.

39. The method as claimed in claim 38, wherein the coating method employed is an electrodeposition process.

40. The method claimed in claim 38 , wherein the iron-zinc alloy coating on the steel substrate forms an iron-zinc layer comprising iron in the range of 8 % to 15 % by weight and rest being zinc and, thickness of the iron-zinc layer ranges from 6 µm to 15 µm on the steel substrate.

41. The method as claimed in claim 38, wherein the nickel layer on the iron-zinc alloy coated steel substrate comprises iron of 0.1 % to 15 % by weight, nickel 80 % to 90 % by weight and zinc of 1 % to 10 % by weight and the thickness of nickel layer on the iron-zinc alloy coated steel substrate ranges between 2 µm and 10 µm.

42. A method for manufacturing a hot worked coated steel, the method comprising:
providing the steel substrate with a nickel coating by a method as claimed in claim 38;
heating the coated steel substrate to a temperature Tc to obtain a heated steel substrate;
hot working the heated steel substrate, to obtain a hot worked steel substrate, and
cooling the hot worked substrate to a temperature less than 400 °C to obtain the hot worked coated steel substrate.

43. The method as claimed in claim 42, wherein the hot working is a hot stamping process, and is performed in a die and punch assembly.

44. The method as claimed in claim 42, wherein the nickel coating on the iron-zinc alloy coated steel substrate after hot working forms:
an iron rich iron-nickel-zinc layer on steel substrate; and
a zinc rich iron-nickel-zinc layer on the iron rich iron-nickel-zinc layer.

45. The method as claimed in claim 44, wherein the iron rich iron-nickel-zinc layer comprises iron of 10 % to 84 % by weight, nickel of 10 % to 45 % by weight, zinc of 6 % to 44 % by weight and the thickness of the iron rich iron-nickel-zinc layer ranges from 5 µm to 30 µm.

46. The method as claimed in claim 44, wherein the zinc rich iron-nickel-zinc layer comprises iron of 1 % to 1.5 % by weight, nickel of 1 % to 3 % by weight, zinc of 77 % to 84 % by weight and the thickness of the zinc rich iron-nickel-zinc layer ranges from 0.5 µm to 5 µm.
, Description: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 the industries. The applications of the steel may 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, resilience, 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 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 may be 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.

Zinc coating is used to protect steel surface from corrosion due to the sacrificial nature of the 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 the 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 may be 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 coating 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 Fe2SiAl7 inhibition layer at the coating/steel interface. Additionally, a thin layer of Fe2Al5 and FeAl3 with a thickness of less than 1 µm may be formed between the Fe2SiAl7 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 oC 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 coating and pure zinc-based coating, a dual layer Zn–Al coating, Zn–Al–Mg post-process coating are proposed in some of the conventional arts. In such 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 5 µm Zn layer, a 15 µm Al–Si alloy layer and a Fe2SiAl7 intermetallic layer at the coating/steel interface. In addition, a phosphating process may be 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 may be significantly reduced. The brittle nature of the coating is a major negative issue of aluminium silicon coating and it can cause coating fracture during deformation at high temperature as well as low temperature i.e. during the service life of hot stamped parts; thereby the indirect stamping process is not suitable for this coating.

Another major drawback of the aluminium silicon coating is that the coatings cannot offer any cathodic corrosion protection. The uncoated regions of the coated steel, such as cut edges, are therefore not protected against corrosion. Moreover, hydrogen uptake during austenitization process is a major issue with Aluminium silicon coatings.

Other type of coatings being tried out recently as a potential replacement of zinc coatings and Al-Si coatings. One such type of coating is the galvalume coating by dipping the substrate in 55 wt.% Al–Zn bath, with 1.6 wt.% Si, and at a bath temperature of 680 °C. It was reported that 55 wt.% Al–Zn coated substrate was probably less susceptible to LME although the coating contained a large volume fraction of liquid Zn at grain boundaries at the press forming temperature. However, it is required that before hot stamping the coated steel must go through a pre-conditioning stage at 550-730 °C for 9-66 min which will increase the amount of Fe in the coating and decrease the probability of LMIE. However, such conditioning may not be suitable to existing hot stamping line parameters and the product is not yet established successfully.

There are other types of coatings being researched like, dual Layer Zn–Al coating, Zn–Al–Mg post-process coating etc. In the dual Layer Zn–Al coating 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 5 µm Zn layer, a 15 µm Al–Si alloy layer and a Fe2SiAl7 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. Essentially, this 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 11 wt.% Ni, 0.6 wt.% Fe and balance zinc on steel substrate. This corresponds to Ni-Zn gamma (?) phase with higher melting point (880 oC) than the Fe-Zn phases (782 oC). 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 may 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.

The present disclosure is directed to overcome one or more limitations stated above, or any other limitations associated with the conventional coatings on the steel.

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.

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.

In a first non-limiting embodiment of the present disclosure, a coated steel is disclosed. The coated steel, comprises a steel substrate, having a composition comprising, by weight percent of: Carbon 0.2 % to 0.25 %; Manganese 1.15 % to 1.4 %; Sulphur less than 0.01 %; Phosphorus less than 0.05 %; Silicon 0.2 % to 0.35 %; Aluminium less than 0.1 %; Copper less than 0.05 %; Chromium 0.15 % to 0.35 %; Nickel less than 0.1 %; Molybdenum less than 0.01 %; Vanadium less than 0.01 %; Niobium less than 0.01 %; Titanium 0.02 % to 0.05 %; Nitrogen less than 50 ppm; Boron 0.002 % to 0.005 %; and the balance being iron optionally along with incidental elements and a nickel-copper composite coating on the steel substrate, wherein the nickel-copper composite coating forms a nickel-copper alloy layer on the steel substrate.

In an embodiment, the steel is a bare steel substrate. The nickel-copper alloy layer on the bare steel substrate comprises iron of 2 % to 5 % by weight, nickel 66 % to 74 % by weight and copper 19 % to 27 % by weight and, thickness of the nickel-copper alloy layer ranges from 3 µm and 15 µm.

In an embodiment, the steel substrate is coated with an iron-zinc alloy. The nickel-copper alloy layer on the iron-zinc alloy coated steel substrate comprises iron of 1 % to 2 % by weight, nickel of 57 % to 64 % by weight, copper of 8 % to 36 % by weight and zinc of 1 % to 2 % by weight and thickness of the nickel-copper alloy layer ranges from 3 µm and 15 µm.

In an embodiment, the nickel-copper composite coating, after hot working forms an iron-nickel-copper solid solution layer on the steel substrate; and an iron-nickel-copper oxide layer on iron-nickel-copper solid solution layer. The iron-nickel-copper solid solution layer comprises iron of 30 % to 78 % by weight, nickel of 18 % to 53 % by weight, copper of 4 % to 8 % by weight, oxygen of 1 % to 6 % by weight and the thickness of the iron-nickel-copper solid solution layer ranges from 2 µm to 15 µm. Further, the iron-nickel-copper oxide layer comprises iron of 35 % to 56 % by weight, nickel of 20% to 40% by weight, copper of 2 % to 6 % by weight, oxygen of 17 % to 28 % by weight and the thickness of the iron-nickel-copper oxide layer ranges from 1 µm to 5 µm.

In an embodiment, the steel substrate is coated with the iron-zinc alloy.

In an embodiment, the nickel-copper composite coating on the iron-zinc alloy coated steel substrate after hot working forms an iron rich iron-nickel-copper-zinc solid solution layer; and a zinc rich iron-nickel-copper-zinc solid solution layer on the iron rich iron-nickel-copper-zinc solid solution layer. The iron rich iron-nickel-copper-zinc solid solution layer comprises iron of 83 % to 98 % by weight, nickel of 5 % to 8 % by weight, copper of 1 % to 2 % by weight, zinc of 1 % to 11 % by weight and the thickness of the iron rich iron-nickel-copper-zinc solid solution layer ranges from 5µm to 30 µm. Further, the zinc rich iron-nickel-copper-zinc solid solution layer comprises iron of 70 % to 76 % by weight, nickel of 7 % to 8 % by weight, copper of 1 % to 2 % by weight, zinc of 16 % to 18 % by weight and the thickness of the zinc rich iron-nickel-copper-zinc solid solution layer ranges from 0.5 µm to 6 µm.

In another non-limiting embodiment of the present disclosure, a method for manufacturing a coated steel is disclosed. The method for manufacturing a coated steel comprises: providing a steel substrate having a composition comprising, by weight percent of: Carbon 0.2 % to 0.25 %; Manganese 1.15 % to 1.4 %; Sulphur less than 0.01 %; Phosphorus less than 0.05 %; Silicon 0.2 % to 0.35 %; Aluminium less than 0.1 %; Copper less than 0.05 %; Chromium 0.15 % to 0.35 %; Nickel less than 0.1 %; Molybdenum less than 0.01 %; Vanadium less than 0.01 %; Niobium less than 0.01 %; Titanium 0.02 % to 0.05 %; Nitrogen less than 50 ppm; Boron 0.002 % to 0.005 %; and the balance being iron optionally along with incidental elements and coating the steel substrate with a nickel-copper composite coating by a coating method to form the coated steel substrate, wherein the coated steel substrate comprises a nickel-copper alloy layer.

In an embodiment, the coating method employed is a co-deposition process.

In one non-limiting embodiment of the present disclosure, a method for manufacturing a hot worked coated steel is disclosed. The steel substrate is provided with a nickel-copper composite coating. The coated steel substrate is heated to a temperature Tc to obtain a heated substrate. Later, the heated substrate is subjected to hot working to obtain a hot worked substrate; and finally, the hot worked substrate is cooled to a temperature less than 400 °C to obtain the hot worked coated steel substrate.

In an embodiment, the hot working is a hot stamping process, and is performed in a die and punch assembly.

In a second non-limiting embodiment of the present disclosure a coated steel. The coated state comprises a steel substrate, having a composition comprising, by weight percent of: Carbon 0.2 % to 0.25 %; Manganese 1.15 % to 1.4 %; Sulphur less than 0.01 %; Phosphorus less than 0.05 %; Silicon 0.2 % to 0.35 %; Aluminium less than 0.1 %; Copper less than 0.05 %; Chromium 0.15 % to 0.35 %; Nickel less than 0.1 %; Molybdenum less than 0.01 %; Vanadium less than 0.01 %; Niobium less than 0.01 %; Titanium 0.02 % to 0.05 %; Nitrogen less than 50 ppm; Boron 0.002 % to 0.005 %; and the balance being iron optionally along with incidental elements; an iron-zinc coating on the steel substrate; and a nickel coating on the iron-zinc alloy coating, wherein the nickel coating forms a nickel layer on the iron-zinc alloy coated steel substrate.

In an embodiment, the iron-zinc alloy coating forms an iron-zinc alloy layer comprising iron in the range of 8 % to 15 % by weight and rest being zinc and, thickness of the iron-zinc alloy layer ranges from 6 µm to 15 µm on the steel substrate.

In an embodiment, the nickel layer comprises iron of 0.1 % to 15 % by weight, nickel 80 % to 90 % by weight and zinc of 1 % to 10 % by weight and the thickness of nickel layer on the iron-zinc alloy coated steel substrate ranges between 2 µm and 10 µm.

In an embodiment, the nickel coating after hot working forms: an iron rich iron-nickel-zinc layer on steel substrate; and a zinc rich iron-nickel-zinc layer on the iron rich iron-nickel-zinc layer. The iron rich iron-nickel-zinc layer comprises iron of 10 % to 84 % by weight, nickel of 10 % to 45 % by weight, zinc of 6 % to 44 % by weight and the thickness of the iron rich iron-nickel-zinc layer ranges from 5 µm to 30 µm. Further, the zinc rich iron-nickel-zinc layer comprises iron of 1 % to 1.5 % by weight, nickel of 1 % to 3 % by weight, zinc of 77 % to 84 % by weight and the thickness of the zinc rich iron-nickel-zinc layer ranges from 0.5 µm to 5 µm.

In an embodiment, the hot worked steel substrate has a martensite body centered tetragonal crystal structure (BCT) structure.

In one non-limiting embodiment of the present disclosure, a method for manufacturing a coated steel is disclosed. The method for manufacturing a coated steel comprises providing a steel substrate having a composition comprising, by weight percent of: Carbon 0.2 % to 0.25 %; Manganese 1.15 % to 1.4 %; Sulphur less than 0.01 %; Phosphorus less than 0.05 %; Silicon 0.2 % to 0.35 %; Aluminium less than 0.1 %; Copper less than 0.05 %; Chromium 0.15 % to 0.35 %; Nickel less than 0.1 %; Molybdenum less than 0.01 %; Vanadium less than 0.01 %; Niobium less than 0.01 %; Titanium 0.02 % to 0.05 %; Nitrogen less than 50 ppm; Boron 0.002 % to 0.005 %; and the balance being iron optionally along with incidental elements; coating an iron-zinc alloy coating on the steel substrate; and coating a nickel coating by coating method on the iron-zinc coating to form a coated steel substrate, wherein the coated steel substrate comprising a nickel layer on the iron-zinc alloy layer.

In an embodiment, the coating method employed is an electrodeposition process.

In an embodiment, the iron-zinc alloy coating on the steel substrate forms an iron-zinc layer comprising iron in the range of 8 % to 15 % by weight and rest being zinc and, thickness of the iron-zinc layer ranges from 6 µm to 15 µm on the steel substrate.

In an embodiment, the nickel layer on the iron-zinc alloy coated steel substrate comprises iron of 0.1 % to 15 % by weight, nickel 80 % to 90 % by weight and zinc of 1 % to 10 % by weight and the thickness of nickel layer on the iron-zinc alloy coated steel substrate ranges between 2 µm and 10 µm.

In yet another non-limiting embodiment of the present disclosure, a method for manufacturing a hot worked coated steel is disclosed. The steel substrate is provided with a nickel-copper composite coating. The coated steel substrate is heated to a temperature Tc to obtain a heated substrate. Later, the heated substrate is subjected to hot working to obtain a hot worked substrate; and finally, the hot worked substrate is cooled to a temperature less than 400 °C to obtain the hot worked coated steel substrate.

In an embodiment, the hot working is a hot stamping process, and is performed in a die and punch assembly.

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 cross-sectional elemental view of the nickel-copper composite coating on steel substrate, in accordance with an embodiment of the present disclosure.

Figure 2 illustrates cross-sectional elemental view of the iron-zinc coating on the steel substrate, in accordance with an embodiment of the present disclosure.

Figure 3 illustrates cross-sectional elemental view of nickel-copper composite coating on the iron-zinc coated steel, in accordance with an embodiment of the present disclosure.

Figure 4 cross-sectional elemental view of the hot worked coated steel with nickel-copper composite coating in accordance with an embodiment of the present disclosure.

Figure 5 illustrates cross-sectional elemental view of the hot worked iron-zinc coated steel with nickel-copper composite coating in accordance with an embodiment of the present disclosure.

Figure 6 illustrates cross-sectional elemental view of the nickel coating on iron-zinc coated steel coated steel, in accordance with another embodiment of the present disclosure.

Figure 7 illustrates cross-sectional elemental view of the hot worked coated steel with nickel coating on iron-zinc coated 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 embodiments thereof have 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.

From the prior arts, it can be stated that a single solution for coating for hot stamping having right proportion of solid phases with ductility at high temperature that can be usable in both direct and indirect stamping process as well as having sacrificial corrosion protection is still not been developed. So, a single coating solution having all these required properties is of prime importance for ultra-high strength press hardenable steels. Herein, the present invention provides a coated steel with superior qualities over prior art coatings.

The present disclosure provides a coated steel which offers high mechanical resistance even 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 nickel-copper composite 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 nickel-copper composite coating of the bare steel substrate forms a nickel-copper alloy layer on the bare steel substrate. Further, the steel substrate may be coated with an iron-zinc alloy coating prior to nickel-copper composite coating. The iron-zinc alloy coating on the steel substrate forms an iron-zinc alloy layer. The nickel-copper composite coating is provided on the ion-zinc alloy coated steel substrate. The nickel-copper composite coating on the iron-zinc alloy coated steel forms a nickel-copper alloy layer on it.

In an embodiment, the steel substrate is a steel sheet.

The present disclosure also provides another 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 coated with an iron-zinc alloy coating on it. The iron-zinc alloy coating on the steel substrate forms iron-zinc alloy layer on the steel substrate. A nickel coating is coated on iron-zinc alloy coated 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 nickel coating coated on the iron-zinc alloy coated steel substrate forms a nickel layer.

In an embodiment, the steel substrate is a steel sheet.

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.

The following paragraphs clearly explains the detailed steps or process involved in method of manufacturing coated steel and hot worked coated steel:

The steel substrate used for the coating process such as but not limited to a boron steel. The presence of boron in the steel enhances hardenability of steel. Addition of the boron element in austenitic steels also improves their high-temperature strength. The steel substrate employed for the coating includes by weight percent of: Carbon 0.2 % to 0.25 %; Manganese 1.15 % to 1.4 %; Sulphur less than 0.01 %; Phosphorus less than 0.05 %; Silicon 0.2 % to 0.35 %; Aluminium less than 0.1 %; Copper less than 0.05 %; Chromium 0.15 % to 0.35 %; Nickel less than 0.1 %; Molybdenum less than 0.01 %; Vanadium less than 0.01 %; Niobium less than 0.01 %; Titanium 0.02 % to 0.05 %; Nitrogen less than 50 ppm; Boron 0.002 % to 0.005 %; and the balance being iron optionally along with incidental elements

Steel semi product can be converted into sheet by process such as but not limited to hot rolling. Hot Rolling is a mill process which involves rolling the steel at a high temperature which is above the steel’s recrystallization temperature. When steel is above the recrystallization temperature, it can be shaped and formed easily.

In an exemplary embodiment of the present disclosure, during hot rolling operation to produce steel sheet, the steel semi product such as but not limited to steel slab, steel billet may be heated to final rolling temperature (FRT) and subjected to hot rolling with FRT, so as to obtain a hot-rolled steel product. Later, the rolled product is cooled down to a coiling temperature Tcoil. Coiling the hot-rolled steel product at said coiling temperature Tcoil takes place to obtain a hot rolled steel product such as but not limited to steel sheets having a thickness comprised between 1.8 mm to 5 mm; wherein the coiling temperature Tcoil may be needed to satisfy the condition 450 °C = Tcoil = Tcoil max. and Tcoil max is maximum coiling temperature expressed as Tcoilmax = 650-140*austenite fraction just before the coiling (fy).

In an exemplary embodiment of the present disclosure, the steel substrate for example steel sheet may be subjected for cleaning to remove oil contaminants, grease residue, corrosion or any other foreign entities deposited on the surface of the steel substrate. In the cleaning process, the steel substrate may be 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 in the caustic solution removes oily contaminants on the surface of the steel substrate. The steel substrate may then be rinsed in water, to clean carry overs of the caustic solution on the surface of the steel substrate during washing. Subsequent to rinsing, the steel substrate may be 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. After pickling the steel substrate 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 an embodiment, the cleaned steel substrate may be coated with nickel-copper composite coating. The nickel-copper composite coating may be coated using a process such as but not limited to co-deposition process. During the nickel-copper composite coating by co-deposition process, a coating bath containing a 1M nickel sulphate (NiSO4), a 0.1 M copper sulphate (CuSO4) and a 0.2 M trisodium citrate (Na3C6H5O7) may be prepared. In this coating bath, the cleaned steel substrate is subjected for co-deposition process while making a current density of 20 mA/cm2 for about 10 minutes to 20 minutes at 25 °C. The nickel-copper composite coating may be deposited in one or more layers.

Spectrum Label 1 2 3 4 5
O 4.25 2.69
Si 0.34 0.30 1.44 0.31
Mn 1.18
Fe 2.27 2.55 3.05 5.06 97.22
Ni 66.28 71.16 73.44 72.40 1.29
Cu 26.87 25.99 19.37 22.53
Total 100.00 100.00 100.00 100.00 100.00

Table.1

The composition profile for the nickel-copper composite coating on steel substrate is tabulated in table 1. Figure 1 is an exemplary embodiment of the present disclosure which illustrates cross-sectional elemental view of the nickel-copper composite coating on steel substrate. In an embodiment, the nickel-copper alloy layer on the bare steel substrate includes iron of 2 % to 5 % by weight, nickel 66 % to 74 % by weight and copper 19 % to 27 % by weight and, thickness of the nickel-copper alloy layer ranges from 3 µm and 15 µm.

In yet another embodiment, the steel substrate may be coated with an iron-zinc alloy coating prior to nickel-copper composite coating. For coating the iron-zinc alloy coating on the steel substrate, galvannealing may be carried out. In an embodiment, galvannealing may include annealing of the steel substrate, and then dipping the same in molten zinc metal bath. The molten zinc bath temperature may range from 450-470 °C and the bath has dissolved aluminium of 0.10-0.18 wt.% to enable the formation on thin inhibition layer. The hot dipping time may be about 2-8 seconds. Post hot dipping, the steel substrate may be wiped for controlling the zinc thickness and further annealed in furnace such as but not limited to galvannealing furnace. The temperature of galvannealing may be from 480 °C to 550 °C and the time of galvannealing may be from 5 seconds to 60 seconds.

Figure 2, which is an exemplary embodiment of the present disclosure illustrating the cross-sectional elemental view of the iron-zinc coating on the steel substrate, in accordance with an embodiment of the present disclosure. The iron-zinc alloy coating on the steel substrate forms an iron-zinc alloy layer including iron of 8 % to 15 % by weight and rest being zinc and, thickness of the iron-zinc alloy layer ranges from 6 µm to 15 µm on the steel substrate.

Further, a nickel-copper composite coating is coated on iron-zinc alloy coated steel substrate. The nickel-copper composite coating may be coated on iron-zinc coated steel substrate using a process such as but not limited to co-deposition process. During the nickel-copper composite coating by co-deposition process, a coating bath containing 1 M nickel sulphate (NiSO4), 0.1 M copper sulphate (CuSO4) and 0.2 M trisodium citrate (Na3C6H5O7) is prepared. In this coating bath, the iron-zinc coated steel substrate is subjected for co-deposition process while making a current density of 20 mA/cm2 for about 10 minutes to 20 minutes at 25 °C. The nickel-copper composite coating may be deposited in one or more layers.

Spectrum Label 1 2 3 4 5 6 7 8 9 10
Mn 1.26
Fe 1.32 1.82 6.28 11.16 12.18 11.81 13.51 25.16 97.38 1.66
Ni 60.56 63.81 57.69 0.99 0.50 25.93
Cu 36.17 32.15 8.92 0.82 67.07
Zn 1.63 2.22 27.12 87.03 86.92 88.19 86.49 74.84 0.92 5.34

Table. 2

Table 2 depicts the compositional profile for the nickel-copper composite coating on iron-zinc coated steel substrate. Reference is now made to figure 3 which illustrates the cross-sectional elemental view of nickel-copper composite coating on iron-zinc coated steel, in accordance with an embodiment of the present disclosure. The nickel-copper composite coating on the iron-zinc alloy coated steel forms a nickel-copper alloy layer. The nickel-copper alloy layer on the iron-zinc alloy coated steel substrate includes iron of 1 % to 2 % by weight, nickel of 57 % to 64 % by weight, copper of 8 % to 36 % by weight and zinc of 1 % to 2 % by weight and thickness of the nickel-copper alloy layer ranges from 3 µm and 15 µm.

Subsequently, nickel-copper composite coating coated steel substrate may be subjected to hot working process such as but not limited to hot stamping process. During hot working process, the coated steel substrate may be heated to austenization temperature Tc in between 850 °C-950 °C to obtain a heated substrate. Later, the heated substrate may be transferred to die and punch assembly to perform hot working process in between 750 °C- 850 °C for 3 minute to 8 minute using a punch of 450 kN-550 kN to obtain a hot worked substrate, and finally, the hot worked substrate is cooled to a temperature less than 400 °C to obtain the hot worked coated steel substrate. The cooling process employed may be but not limited to a quenching process.

Furnace temperature/
Austenitizing temperature (oC) Temperature before drawing/
stamping temperature (oC) Austenitizing time
(minute) Furnace atmosphere Punch force
(kN) Quenching time
(s)
O2
(%) Dew point (oC)
850-950 750-850 3 to 8 15 to 21 -50 to -85 450-550 15-25
Table. 3

The heat treatment schedule and parameters followed for hot stamping process of the coated steel is tabulated in table 3, in accordance with an embodiment of the present disclosure.

Spectrum label 1 2 3 4 5 6 7 8 9 10
O 28.00 17.28 17.95 19.81 6.13 5.90 1.66
Mn 0.83 0.71 1.20 0.80 1.14
Fe 47.67 36.98 35.28 56.04 30.78 37.13 78.77 98.43 96.66 98.86
Ni 24.33 40.44 40.22 20.36 53.67 50.68 18.86 2.53
Cu 2.46 6.55 3.79 8.94 4.72
Table. 4

Table 4 indicates the compositional profile for hot worked coated steel with nickel-copper coating. Figure 4 depicts the cross-sectional elemental view of the hot worked coated steel with nickel-copper composite coating. The nickel-copper composite coating on the steel substrate after hot working process forms an iron-nickel-copper solid solution layer on the steel substrate, and an iron-nickel-copper oxide layer on the iron-nickel-copper solid solution layer. During heat treatment process, iron from the steel substrate is diffused to the nickel-copper composite coating which leads to the formation iron-nickel-copper solid solution layer. The iron-nickel-copper solid solution layer is completely miscible and tightly adhering to the steel substrate and prevents the microcrack propagation. An iron-nickel-copper oxide layer forms due to the diffusion of minute amount of oxygen molecules. The heat-treated microstructure clearly shows that the low microcrack severity in case of nickel-copper coated steel. The indicates the advantage of the coating in preventing crack formation and crack propagation during treatment at high temperature.

In an embodiment, the iron-nickel-copper solid solution layer on the iron-zinc alloy coated steel substrate includes iron of 30 % to 78 % by weight, nickel of 18 % to 53 % by weight, copper of 4 % to 8 % by weight, oxygen of 1 % to 6 % by weight and the thickness of the iron-nickel-copper solid solution layer ranges from 2 µm to 15 µm. The iron-nickel-copper oxide layer on the iron-nickel-copper solid solution layer includes iron of 35 % to 56 % by weight, nickel of 20% to 40% by weight, copper of 2 % to 6 % by weight, oxygen of 17 % to 28 % by weight and the thickness of the iron-nickel-copper oxide layer ranges from 1 µm to 5 µm.

Subsequently, nickel-copper composite coating on iron-zinc coated steel substrate may be subjected to hot working process such as but not limited to hot stamping process. During hot working process, the coated steel substrate may be heated to austenization temperature Tc in between 850 °C-950 °C to obtain a heated substrate. Later, the heated substrate may be transferred to die and punch assembly to perform hot working process in between 750 °C- 850 °C for 3 minute to 8 minute using a punch of 450 kN-550 kN to obtain a hot worked substrate, and finally, the hot worked substrate is cooled to a temperature less than 400 °C to obtain the hot worked coated steel substrate. The cooling process employed may be but not limited to a quenching process

Spectrum label 1 2 3 4 5 6 7 8
O 0.42 1.27 1.19
Mn 70.67 76.36 71.74 74.75 83.30 97.56 97.47 11.57
Fe 7.66 7.11 8.16 8.02 5.25 10.46
Ni 1.94 1.26 1.84 1.17 17.05
Cu 18.87 15.28 18.26 16.07 11.45 1.16 60.92
Total 100.00 100.00 100.00 100.00 100.00 100.00 100.00 100.00
Table.5

The compositional profile for the hot worked iron-zinc coated steel with nickel-copper composite coating is given in table 5. Figure 5 illustrates cross-sectional elemental view of the hot worked iron-zinc coated steel with nickel-copper composite coating. The nickel-copper composite coating on the iron-zinc coating steel substrate after hot working process forms an iron rich iron-nickel-copper-zinc solid solution layer on the steel substrate; and a zinc rich iron-nickel-copper-zinc solid solution layer on the iron rich iron-nickel-copper-zinc solid solution layer. During heat treatment process, iron and zinc atoms will diffuse from iron-zinc coated steel substrate into the interface of nickel-copper composite coating to form iron rich iron-nickel-copper-zinc solid solution layer on the steel substrate. Since the temperature of the heat -treatment process well above the melting point of zinc element, the more zinc from iron-zinc coating diffuses deep into the nickel-copper coating to form zinc rich iron-nickel-copper-zinc solid solution layer on the iron rich iron-nickel-copper-zinc solid solution layer. The heat-treated microstructure clearly shows that the microcrack and oxidation severity is much less compared to uncoated steel substrate. The layer morphology after heat treatment process provides maximum protection again microcrack propagation and high temperature oxidation which on the other hand are main cause for reduction of mechanical strength in uncoated steel. Hence, the mechanical strength if he nickel-copper coating on iron-zinc coated steel substrate will be retained even of severe hot working at high temperatures.

In an embodiment, the iron rich iron-nickel-copper-zinc solid solution layer includes iron of 83 % to 98 % by weight, nickel of 5 % to 8 % by weight, copper of 1 % to 2 % by weight, zinc of 1 % to 11 % by weight and the thickness of the iron rich iron-nickel-copper-zinc solid solution layer ranges from 5 µm to 30 µm. The zinc rich iron-nickel-copper-zinc solid solution layer includes iron of 70 % to 76 % by weight, nickel of 7 % to 8 % by weight, copper of 1 % to 2 % by weight, zinc of 16 % to 18 % by weight and the thickness of the zinc rich iron-nickel-copper-zinc solid solution layer ranges from 0.5 µm to 5 µm.

In accordance with another embodiment, a nickel coating is coated on iron-zinc alloy coated steel substrate. The nickel coating may be coated on iron-zinc coated steel substrate using a process such as but not limited to electrodeposition process. During the nickel coating by electrodeposition process, a nickel coating bath including NiSO4.6H2O of 300 g/l, NiCl2.6H2O of 60 g/l and boric acid of 40 g/l to 8 g/l may be used. The nickel bath is maintained at a pH ranging from about 3 to about 4. The electroplating may be carried out by configuring a plate of nickel as anode and the iron-zinc coated steel substrate as cathode, while maintaining a current density from about 300-400 A/m2. The nickel coating may be deposited in one or more layers.

Reference is now made to figure 6 which illustrates the cross-sectional elemental view of nickel coating on iron-zinc coated steel, in accordance with an embodiment of the present disclosure. The nickel coating on the iron-zinc alloy coated steel forms a nickel layer. The nickel layer includes iron of 0.1 % to 15 % by weight, nickel 80 % to 90 % by weight and zinc of 1 % to 10 % by weight and the thickness of nickel layer on the iron-zinc alloy coated steel substrate ranges between 2 µm and 10 µm.

Subsequently, nickel coating coated steel substrate may be subjected to hot working process such as but not limited to hot stamping process. During hot working process, the coated steel substrate may be heated to austenization temperature Tc in between 850 °C-950 °C to obtain a heated substrate. Later, the heated substrate may be transferred to die and punch assembly to perform hot working process in between 750 °C-850 °C for 3 minute to 8 minute using a punch of 450 kN-550 kN to obtain a hot worked substrate, and finally, the hot worked substrate is cooled to a temperature less than 400 °C to obtain the hot worked coated steel substrate. The cooling process employed may be but not limited to a quenching process.

Spectrum Lebel 1 2 3 4 5 6 7 8
O 13.83 18.39
Mn 0.63 1.04 1.50
Fe 0.95 1.14 9.98 23.95 37.84 83.67 98.96 98.50
Ni 1.74 2.85 45.80 45.08 38.79 9.52
Zn 83.49 77.34 44.22 30.97 23.38 6.18
Table. 6

Now reference is made to table 6 which indicates the compositional profile for hot worked coated steel with nickel coating on iron-zinc coated steel. Figure 7 depicts the cross-sectional elemental view of the nickel coating on the iron-zinc coated steel substrate after hot working process. The nickel coating on the steel substrate after hot working process forms an iron rich iron-nickel-zinc layer on steel substrate; and a zinc rich iron-nickel-zinc layer on the iron rich iron-nickel-zinc layer. The use of nickel layer on top of iron-zinc coating ensures inward diffusion of nickel from the nickel-(iron-zinc) interface whereas the iron diffusion into the coating from the steel substrate-(iron-zinc) interface. Due to the enrichment of nickel and iron in the coating, the microcrack formation may be avoided by transformation of zinc rich liquid into nickel or iron rich solids within the coating. The nickel coating exhibits excellent performance in terms of microcrack behaviour and oxidation properties. Due to the presence of nickel coating on the top surface, even hydrogen diffusion is also lowered to a great extent.

In an embodiment, the iron rich iron-nickel-zinc layer includes iron of 10 % to 84 % by weight, nickel of 10 % to 45 % by weight, zinc of 6 % to 44 % by weight and the thickness of the iron rich iron-nickel-zinc layer ranges from 5 µm to 30 µm. The zinc rich iron-nickel-zinc layer includes iron of 1 % to 1.5 % by weight, nickel of 1 % to 3 % by weight, zinc of 77 % to 84 % by weight and the thickness of the zinc rich iron-nickel-zinc layer ranges from 0.5 µm to 5 µm.

Exemplary characterization results:
Cross sectional view for coated steel substrate and hot worked coated steel substrate is carried out using Scanning Electron Microscopy (SEM) technique. Elemental analysis for coated steel substrate and hot worked coated steel substrate is carried out using Energy Dispersive X-Ray Analysis (EDX).

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 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.