FIELD OF THE INVENTION

[001] The present disclosure, in general, relates to hot stamp grade steel and, more particularly, to a method for coating a steel substrate for producing hot stamp grade steel and coated hot stamp grade steel thereof.
BACKGROUND OF THE INVENTION
[002] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
[003] The fuel efficiency of automobiles has gained much interest in the resent past due to environmental regulations. One of most prominent way to reduce fuel consumption of automobile is by weight reduction. However, safety regulations are also to be considered. So, researchers are working to develop very high strength steel such to address this issue for the last decade. Hot formed steel is one of the best solution which can achieve very high strength of 1500 to 2000 Mega-Pascal (Mpa) with considerable lowering in weight. Hot formed steel grades are being used in structural components in an automobile such as side impact beams, bumpers, A and B pillars, roof rails, cross members, and tunnels etc. The hot formed processing of steel involves isothermal heating of the steel above austenitizing temperature followed by stamping and simultaneous cooling to form fully martensitic microstructure in the steel substrate.
[004] Hot stamping process is generally performed in a dry atmosphere with 0.21 atm oxygen partial pressure. This condition imposes an inherent difficulty of oxidation to the hot stamping process. When the steel is heated to austenitizing temperature in such atmosphere almost all the elements in steel substrate tends to oxidize. However, oxidation of Iron (Fe), Carbon (C) and Boron (B) becomes more relevant than other alloying elements. When Fe oxidizes it forms a thick layer of different types of iron oxides, this creates significant issues in the post processing. Being interstitial component of the steel, kinetics of oxidation of C and B is large enough. This results in depletion of C and B from the steel matrix. Modification of the steel chemistry hinders the formation of martensite and eventually high strength is not achieved. So, coating becomes one of the essential aspects of the process to avoid such problems. However, commercial coatings come with an added problem of formation of liquid phases at the austenitizing temperature and eventually results in Liquid Metal Induced Embrittlement (LMIE). It has been observed that if the coating forms a liquid phase in the coating during stamping then the liquid penetrates to the substrate through grain boundaries and results in liquid embrittlement.
[005] Zinc (Zn) based coating such as galvanized and galvannealed coatings are the most used coatings for high strength steel for their corrosion resistance and formability. However, these coatings being Zn rich have very low melting temperatures in comparison to the hot stamping temperature. These coatings generally form different intermetallic phases such as zeta, delta, gamma etc. depending upon the heat treatment condition. Among these phases gamma phase shows highest melting temperature of 782°C. During austenitizing all the phases melts and form large volume of liquid in the coating. However, due to very high temperature Fe diffuses in the coating and at higher time of austenitizing process alpha Fe-Zn solid solution stats to form. However, it requires very high time to form 100% solid alpha Fe-Zn solid solution in the coating. This is not practical for commercial application. So, galvanized and galvannealed coatings are not suitable for hot stamping process as they may cause liquid metal induced embrittlement (LMIE).
[006] Another commercially available coating for hot stamping process is Aluminium (Al) based Al-Si coating, also known as USIBOR. These coating forms different Fe-Al intermetallic phases in the coating during austenitizing. The melting temperature of the Fe-Al intermetallic phases goes as high as 1171°C. The temperatures of the Fe-Al phases are much higher than the austenitizing temperature. Thus, the Al-Si coating does not form liquid in the coating structure during stamping. So, the coating gives very good resistance to Liquid Metal Induced Embrittlement (LMIE). However, the intermetallic phases are very brittle and may peels off from the surface of the substrate during stamping. Also, Al based coating does not provide sacrificial corrosion protection to the steel substrate. However, Al-Si coating is most widely used coating for hot stamping process due to their ability to protect the steel from oxidation and resistance to Liquid Metal Induced Embrittlement (LMIE).
[007] Another type of coating which is gaining interest of the auto-makers is electrodeposited Nickel-Zinc (Ni-Zn) alloy coating. This coating shows Ni-Zn intermetallic phases having 11wt-% Ni, 0.6wt-% Fe. This coating forms different intermetallic phases in the coating during austenitizing with comparatively higher melting temperature to galvanized and glavannealed coating. This coating has been reported to resist Liquid Metal Induced Embrittlement due the high melting temperatures of Ni-Zn phases. Multilayer coating where first plating layer containing 60% by mass or more of Ni and the remainder consisting of Zn and second plating layer containing 10 to 25% by mass of Ni and the remainder consisting of Zn has also been reported. However, electrodeposition of Ni-Zn alloy coating may cause hydrogen embrittlement of the steel substrate. These coatings are also not very much effective to provide sacrificial corrosion protection to the steel substrate due to lower Zn content in the coating in the final microstructure.
[008] Thus, there is a requirement of a method for coating a steel substrate to prevent liquid metal induced embrittlement, hydrogen embrittlement during hot stamping process to produce hot stamp grade steel and also to provide improved protection against corrosion.
OBJECTIVES OF THE INVENTION
[009] It is therefore the object of the invention to overcome the aforementioned and other drawbacks existing in prior art.
[010] A object of the invention is to provide a method for coating a steel substrate for producing hot stamp grade steel.
[011] Another object of the invention is to provide a method for coating a steel substrate to prevent liquid metal induced embrittlement during hot stamping process.
[012] Another object of the invention is to provide a method for coating a steel substrate to prevent hydrogen embrittlement during hot stamping process.
[013] Yet another object of the invention is to provide a method for coating a steel substrate to provide improved protection against corrosion.
[014] Still another object of the invention is to provide a coated steel substrate that prevents liquid metal induced embrittlement during hot stamping process.
[015] Still another object of the invention is to provide a coated steel substrate that prevents hydrogen embrittlement during hot stamping process.
[016] Still another object of the invention is to provide a coated steel substrate that provides improved corrosion resistance.
[017] These and other objects and advantages of the present invention will be apparent to those skilled in the art after a consideration of the following detailed description taken in conjunction with the accompanying drawings in which a preferred form of the present invention is illustrated.
SUMMARY OF THE INVENTION
[018] The present invention provides a method (100) for coating a steel substrate for producing hot stamp grade steel and a coated stamp grade steel thereof.
[019] In accordance with an embodiment, the present disclosure provides a method of coating a steel substrate for producing hot stamp grade steel comprising the steps of coating the steel substrate with Zinc (Zn) to form a galvannealed coated steel substrate; coating the galvannealed coated steel substrate with Nickel (Ni) to form Ni coated galvannealed steel substrate; hot stamping the Ni coated galvannealed steel substrate to form hot stamped steel substrate; and coating the hot stamped steel substrate with Zinc phosphate.
[020] In accordance with said embodiment, coating the substrate with Zinc comprises the step of degreasing the steel substrate with an alkali solution of 70°C; pickling the degreased steel substrate in 10 vol.% sulfuric acid solution at a temperature of 50°C; and coating the degreased and pickled steel substrate with Zn coating and suitable annealing process to form galvannealed coating.
[021] In accordance with said embodiment, Nickel (Ni) is coated on the galvannealed coated steel substrate by electroplating process.
[022] In accordance with said embodiment, coating the hot stamped steel substrate with Zinc phosphate comprises the steps of degreasing the hot stamped steel substrate with an alkali solution; rinsing the degreased hot stamped steel substrate with distilled water; dipping the rinsed hot stamped steel substrate in titanium activation solution; coating with phosphate; rinsing the phosphate coated hot stamped steel substrate; and drying with hot air.
[023] In accordance with another embodiment, the invention provides a coated hot stamp grade steel comprising a steel substrate and a Zinc (Zn), Nickel (Ni) and Zinc phosphate coating on the steel substrate.
[024] Various objects, features, aspects, and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components.
[025] 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 to form a further embodiment of the disclosure.
[026] 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.
BREIF DESCRIPTION OF THE DRAWINGS
[027] The illustrated embodiments of the subject matter will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. The following description is intended only by way of example, and simply illustrates certain selected embodiments of devices, systems, and processes that are consistent with the subject matter as claimed herein, wherein:
Figure 1: illustrates flowchart of an exemplary method to coat a steel substrate, in accordance with an embodiment of the invention;
Figure 2: illustrates flowchart of an exemplary method to coat steel substrate with Zinc to form a galvannealed coated steel substrate, in accordance with an embodiment of the invention;
Figure 3: illustrates flowchart of an exemplary method to coat hot stamped steel substrate with Zinc phosphate., in accordance with an embodiment of the invention;
Figure 4: illustrates a schematic diagram of hot stamping hat profile, in accordance with an exemplary embodiment of the invention;
Figure 5: illustrates binary phase diagram of (a) Fe-Zn and (b) Ni-Zn, in accordance with an exemplary embodiment of the invention;
Figure 6: illustrates ternary phase diagram of Fe-Ni-Zn, in accordance with an exemplary embodiment of the invention;
Figure 7: illustrates microstructure of the steel substrate, in accordance with an exemplary embodiment of the invention;
Figure 8: illustrates cross-sectional image of Ni coated galvannealed (GA) steel for (a) 5 min, (b) 10 min and (c) SEM image with elemental line scan, in accordance with an exemplary embodiment of the invention; and
Figure 9: illustrates digital image of hot stamped sample with 5 minutes Ni coating, in accordance with an embodiment of the present invention.
Figure 10: illustrates CLSM image of cross section of the hot stamped samples with (a) 5 min Ni and (b) 10 min Ni coating on galvannealed (GA) steel, in accordance with an embodiment of the present invention.
Figure 11: illustrates CLSM image of etched cross section of the hot stamped samples with (a) 5 min Ni and (b) 10 min Ni coating on galvannealed (GA) steel, in accordance with an embodiment of the present invention.
Figure 12: illustrates SEM image and EDS analysis (wt.%) of the cross section hot stamped samples with 5 min Ni coating on galvannealed (GA) steel, in accordance with an embodiment of the present invention.
Figure 13: illustrates SEM image and EDS analysis (wt.%) of the cross section hot stamped samples with 10 min Ni coating on galvannealed (GA) steel, in accordance with an embodiment of the present invention.
Figure 14: illustrates schematic representation of coating phase evolution during the hot stamping process, in accordance with an embodiment of the present invention.
Figure 15: illustrates SEM image of phosphate surface, in accordance with an embodiment of the present invention.
[028] 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 INVENTION
[029] While the embodiments of the disclosure are subject to various modifications and alternative forms, specific embodiment thereof have been shown by way of example in the figures and will be described 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 alterative falling within the scope of the disclosure.
[030] As used in the description herein and throughout the claims that follow, the meaning of "a," "an," and "the" includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of "in" includes "in" and "on" unless the context clearly dictates otherwise.
[031] The terms "comprises", "comprising", or any other variations thereof used in the disclosure, are intended to cover a non-exclusive inclusion, such that a device, system, assembly that comprises a list of components does not include only those components but may include other components not expressly listed or inherent to the system, or assembly, or device. In other words, one or more elements in a system or device proceeded by "comprises... a" does not, without more constraints, preclude the existence of other elements or additional elements in the system, apparatus or device.
[032] Hereinafter a method for coating a steel substrate for producing hot stamp grade steel and a coated stamp grade steel thereof is explained in more detail.
[033] Referring to figure 1, which illustrates flowchart of an exemplary method (100) to coat a steel substrate in accordance with the invention. The invention comprises the step of coating the steel substrate (10) with Zinc (Zn) to form a galvannealed coated steel substrate; coating the galvannealed coated steel substrate (20) with Nickel (Ni) to form Ni coated galvannealed steel substrate; hot stamping (30) the Ni coated galvannealed steel substrate to form hot stamped steel substrate; and coating the hot stamped steel substrate (40) with Zinc phosphate.
[034] Referring to figure 2, wherein depicts the process of coating the substrate (10) with Zinc comprises comprising the step of: degreasing the steel substrate (11) with an alkali solution of 70°C; pickling the degreased steel substrate (12) in 10 vol.% sulfuric acid solution at a temperature of 50°C; and coating the degreased and pickled steel substrate (13) with Zn coating and suitable annealing process to form galvannealed coating. Further, the step of coating the degreased and pickled steel substrate (13) with Zn coating comprises dipping in a bath composition consisting of 0.05-0.3 wt.% Aluminum and 99.95-99.7 wt.% Zinc. The step of coating the degreased and pickled steel substrate (13) with Zn coating has a bath temperature in a range of 440-470 °C. The step of coating the degreased and pickled steel substrate (13) with Zn coating has a residence time in a range of 2 to 5 seconds. The step of coating the degreased and pickled steel substrate (13) with Zn coating has a galvannealing temperature in a range of 450 to 550 °C. The step of coating the degreased and pickled steel substrate (13) with Zn coating has a galvannealing time in a range of 5 to 20 seconds.
[035] In accordance with said embodiment, Nickel is coated on the galvannealed coated steel substrate (20) by electroplating process. The process of electroplating has a bath composition comprising of:
? 250-350 gm/l NiSO4.6H2O;
? 40-70 gm/l NiCl2. 6H2O; and
? 30-50 gm/l Boric Acid.
[036] Further, the process of electroplating uses a current in a range of 10-30 ampere, time in a range of 3-20 minute, has a bath temperature in a range of 20-70 °C and has pH in a range of 2-5.
[037] In accordance with said embodiment, the process of hot stamping (30) has a furnace temperature /Austenitizing temperature in a range of 850-950 °C, a temperature before drawing /Stamping temperature in a range of 750-850 °C, an austenetizing time in a range of 3 to 8 minutes, 15 to 21% O2 in furnace, a dew point in a range of -50 to -85 °C in furnace, has a punch force in a range of 450 to 550 kN and has quenching time in a range of 15-25 seconds.
[038] In accordance with said embodiment, coating the hot stamped (40) steel substrate with Zinc phosphate comprises the steps of degreasing the hot stamped (41) steel substrate with an alkali solution; rinsing the degreased hot stamped steel substrate with distilled water (42); dipping the rinsed hot stamped steel substrate in titanium activation solution (43); coating with phosphate (44); rinsing the phosphate coated hot stamped steel substrate (45); and drying with hot air (46).
[039] In accordance with said embodiment, the step of degreasing (41) has a bath concentration of 1.5-3vol.%, time of 80-130 seconds, temperature of 50-70°C and pH in range of 11-13. The step of rinsing with water (42) comprises time in range of 100-150 seconds, temperature in range of 15-35°C and pH in range of 7-7.5. The step of dipping in titanium activation solution (43) comprises a bath concentration of 0.05-0.2, time in range of 50-75 seconds, temperature of 20-35°C and pH in range of 8-9. The step of coating with phosphate (44) has a bath concentration comprising: 0.9 free acid point; 21 total acid point; 4 accelerator; 0.062% Zinc (Zn); 0.064% Nickel (Ni); and 0.047% Manganese (Mn). The step of coating with phosphate (44) has a time in range of 150-200 seconds, temperature in the range of 50-60°C and pH in range of 2.5-3. The step of rinsing with water (45) comprises time in range of 50-60 seconds, temperature in range of 20-35°C and pH in range of 6-7. The step of drying with hot air (46) has a time of 120 seconds and a temperature of 80°C.
[040] In accordance with another embodiment, the invention refers to a coated hot stamp grade steel comprising a steel substrate preferably cold rolled steel substrate; and a coating on the steel substrate as discussed above. The steel substrate comprising Carbon (C) from 0.2-0.25 Wt.%; Manganese (Mn) form 1.1-1.3 Wt.%; Boron (B) form 0.002-.003 Wt.%; Silicon (Si) from 0.2-0.3 Wt.%; Aluminum (Al) from 0.03-0.04 Wt.%; Copper (Cu) from 0.01-0.02 Wt.%; Chromium (Cr) from 0.1-0.2 Wt.%; Nickel (Ni) from 0.02-0.03; Molybdenum (Mo) from 0.001-0.002 Wt.%; Vanadium (V) from 0.001-0.002 Wt.%; Niobium (Nb) from 0.001-0.002 Wt.%; Titanium (Ti) from 0.03-0.035 Wt.%; Nitrogen (N) from 35-40 ppm; Sulfur (S) from 0.001-0.002 Wt.%; and Phosphorus (P) from 0.005-0.0010 Wt.%.
[041] The following example is given by way of illustration of the present disclosure and therefore should not be construed to limit the scope of the present disclosure.
[042] Substrate details: The Boron added cold rolled steel substrate was used for the coating. The chemical composition of the steel substrate is given in the Table 1. The thickness of the substrate used for the present study was 1.5 mm.
Table 1. Substrate composition
Element Amount Elements Amount
C (Wt.%) 0.2-0.25 Mo (Wt.%) 0.001-0.002
Mn (Wt.%) 1.1-1.3 V (Wt.%) 0.001-0.002
B (Wt.%) 0.002-0.003 Nb (Wt.%) 0.001-0.002
Si (Wt.%) 0.2-0.3 Ti (Wt.%) 0.03-0.035
Al (Wt.%) 0.03-0.04 N (ppm) 35-40
Cu (Wt.%) 0.01-0.02 S (Wt.%) 0.001-0.002
Cr (Wt.%) 0.1-0.2 P (Wt.%) 0.005-0.0010
Ni (Wt.%) 0.02-0.03

[043] Zn coating: The cold rolled substrate was coated with Zn coating in conventional hot dipping method and heat treated to form galvannealed (GA) coating. The range of process parameters and parameters for the example shown for the present literature are given in Table 2. The sample surface was prepared by conventional surface preparation process of degreasing and pickling. The alkali solution was heated to 70°C and used for degreasing. The pickling was performed in 10 vol.% sulfuric acid solution at a temperature of 50°C. The degreased and pickled substrate then coated with Zn coating and suitable annealing process to form galvannealed coating.
Table 2. Zn coating parameter
Range Example
Bath composition 0.13 wt.% Al and rest Zn
Bath temperature 460 0C
Residence time 3 s
Galvanncaling temperature 500 0C
Galvanncaling time 10 s

Bath Composition 0.05-0.3 wt. % Al and rest Zn
Bath Temperature 440-470 0C
Residence time 2-5s
Galvanncaling temperature 450-550 0C
Galvanncaling time 5-20s

[044] Nickel electroplating: The freshly prepared Galvannealed (GA) coated steel was coated with Ni by electroplating process. The range and example electroplating process parameters are listed in Table 3. The coated samples were cut and prepared for cross sectional observation. The cross section was observed by optical microscope and Laser Scanning Microscope.
Table 3. Ni electroplating parameters
Range
Bath Composition Current (amp) Time (s) Bath Temp (oC) pH
250-350 gm/1 NiSO4.6H2O
40-70 gm/1 NiCI2.6H2O
30-50 gm/1 Boric Acid 10-30 3-20 20-70 2-5

Example
Bath Composition Current (amp) Time (s) Bath Temp (oC) pH
300 gm/l NiSO4.6H2O
60 gm/l NiCI2.6H2O
40 gm/l Boric Acid 20 5 and 10 35 4

[045] Hot stamping parameters: The hot stamping tests were performed in a pilot facility. The hot stamping parameters are listed in Table 4. The sample was heated with 10 °C/s till the austenitizing temperatures. Then the sample was transferred to the stamping press and stamped at desired temperatures. A schematic of the hat profile of the sample is shown in Fig 4. The different section of the sample was cut and microstructure was prepared for further analysis.

Table 4. Hot stamping parameters
Range
Furnace temperature/Austenitizing temperature (0C) Temperature before drawing / stamping temperature (0C) Austenetizing time (minute) Furnace atmosphere Punch force (kN) Quenching time (s)
O2 % Dew Point (0C)
850-950 750-850 3 to 8 15 to 21 -50 to -85 450-550 15-25

Example
Furnace temperature/Austenitizing temperature (0C) Temperature before drawing / stamping temperature (0C) Austenetizing time (minute) Furnace atmosphere Punch force (kN) Quenching time (s)
O2 (%) Dew Point (0C)
900 780 5 21 -78 500 20

The cross sections were observed under optical and Laser Scanning Microscope for microcrack analysis. The coating was also characterized by Scanning Electron Microscope and Energy Dispersive Spectroscopy.
[046] Post treatment: Phosphating: Zinc Phosphate coating was applied on the hot stamped steel samples. Only degreasing but no pickling was performed before phosphating. The patent range and example phosphating process steps and parameters are listed in Table 5.
Table 5. Post treatment parameters

Range
Steps Process Bath condition / concentration Experimental condition
Time (s) Temperature (0C) pH
1 Degreasing: alkali solution 1.5 – 3 vol. % 80-130 50-70 11-13

3 Rinsing Distilled Water 100-150 15-35 7-7.5

4 Titanium activation solution 0.05-0.2 50-75 20-35 8-9

5 Phosphate coating Free acid point: 0.9
Total acid point: 21 Accelerator: 4
Zn: 0.062%
Ni: 0.064%
Mn: 0.047% 150-200 50-60 2.5-3

6 Rinsing Distilled Water 50-60 20-35 6-7
7 Hot air drying - 120 80 -

Example
Steps Process Bath condition / concentration Experimental condition
Time (s) Temperature (0C) pH
1 Degreasing: alkali solution 2 vol. % 105 60 12.8

3 Rinsing Distilled Water 120 22 7.1

4 Titanium activation solution 0.1 vol. % 60 22 6.2

5 Phosphate coating Free acid point: 0.9
Total acid point: 21 Accelerator: 4
Zn: 0.062%
Ni: 0.064%
Mn: 0.047% 180 50 2.7

6 Rinsing Distilled Water 60 22 6.2
7 Hot air drying - 120 80 -

[047] Result and Discussion
Phase diagram of Ni-Zn-Fe: The binary and ternary phase diagram was calculated by ThermoCalc. TCAL4 database was used for the calculation. The binary phase diagrams are shown in Fig 5. The Fe-Zn binary phase diagram (Fig 5(a)) showed different intermediate phases such as Zeta, Delta, Gamma etc. At Fe rich side of the phase diagram BCC_B2 solid solution was observed. The melting temperatures of the intermediate phase was significantly lower than the hot stamping temperature of nearly 900 °C. The solidus line for Fe rich solid solution ranges from nearly 782 °C to 1539 °C. As observed in the phase diagram Fe composition of more than 68 wt.% would have solid Fe rich solid solution at hot stamping temperature of nearly 900°C
[048] The Ni-Zn binary phase diagram is shown in Fig 5(b). The Ni-ZN phase diagram showed different intermediate phases such as NiZN8, gamma_D83, NiZN¬_LT, BCC_B2. FCC_L12 solid solution was observed in Ni rich side of the phase diagram. At the hot stamping temperature of nearly 900 oC BCC_B2 showed the first solidus line with maximum content of Zn. So, Ni composition of nearly 38 wt.% and more one would find only solid phases at the hot stamping temperature of 900°C as shown in Fig 5(b).
[049] The isothermal section of ternary phase diagram at 900°C of the Fe-Ni-Zn system is shown in Fig 6. Regions with no liquid present at such compositions are shown with highlight. As observed at very high Ni concentration with Zn as high as 58 wt.% one could get solid phase present in the microstructure. However, with moderate or low Ni maximum 40 wt.% Zn could be present without liquid formation in the coating. The binary and ternary phase diagrams helped to identify the compositional regions to avoid liquid formation in the coating before stamping operation.
[050] Characterization of steel substrate: The Boron (B) added steel sample was cut into small pieces and the cross section of the sample was hot mounted. The cross-section sample was prepared following conventional metallographic route and observed under optical microscope. The steel microstructure is shown in Fig 7.
[051] Characterization of the Ni coated galvannealed steel: The galvannealed (GA) steel sheet was electrodeposited by Ni following conventional electrodeposition route. The coated sample was cut and prepared following conventional metallographic route. The cross-section of the coated steel sample was observed under Confocal Laser Scanning Microscope (CLSM). The microstructure is shown in Fig 8. The galvannealed (GA) coating thickness was varied from 7 to 12 µm. The Ni coating thickness was varied from 2 to 10 µm. The SEM image with elemental line scan is shown in Fig 8(c). The elemental analysis showed glavannealed coating over steel substrate and pure Ni coating on galvannealed coating.
[052] Hot stamping of coated samples: Hot stamping of the samples was performed following the process conditions mentioned in Table 4. Digital image of hot stamped sample is shown in Fig 9. The hot stamped steel samples were cut and prepared for microstructural observation as described in Fig 4.
[053] The sample region ‘Y’ shown in Fig 4 goes through the highest severity of deformation. The section goes through stretching, bending and abrasion with the die wall at the same time. So, the samples from region ‘Y’ was characterised under confocal laser scanning microscope (CLSM) and scanning electron microscope (SEM). The microstructures observed under CLSM is shown in Fig 10. The coating was cracked due to high deformation during stamping process. However, the coating did not peel off from the substrate. Microcracks due to liquid metal penetration was not observed in the substrate surface. Similar morphology of coating crack and substrate surface was observed in all the samples with 5 min to 15 min Ni coating on galvanneled steel samples.
[054] The samples were etched with 5% Nital to reveal the microstructural features of the substrate. The etched microstructure observed under CLSM is shown in Fig 11. Fully martensitic microstructure of the steel substrate was observed. The final coating thickness of the hot stamped steel increased with increasing Ni coating thickness.
[055] The cross sections samples were observed under SEM and the elemental analysis of the samples was performed by Energy Dispersive X-Ray Spectroscopic (EDS) technique. The SEM image of 5 min Ni coated hot stamped sample is shown in Fig 12. Cracks were only observed in the coating. Microcracks in the steel substrate were not observed in the sample. Coating thickness of nearly 20 µm was observed. At the top of the coating a layer of zinc oxide was observed with little amount of Ni and Fe. Beneath the oxide layer a Ni-Zn layer was observed with up 1to 10 wt.% of Fe into it. Beneath that a Ni-Zn-Fe ternary layer with Fe of 20 to 40 wt.% was observed. Finally, just above the substrate a layer of Fe rich Fe-Ni-Zn solid solution layer was observed.
[056] The SEM image of 10 min Ni coated hot stamped sample is shown in Fig 13. No microcracks were observed in the steel surface. Coating thickness of nearly 30 µm was observed. At the top of the coating a layer of zinc oxide was observed with little amount of Ni and Fe. However, the oxide layer was much thinner compared to lower thickness of Ni coating samples. Beneath the oxide layer a Ni-Zn layer was observed with up to 11 wt.% of Fe into it. Beneath that a Ni-Zn-Fe ternary layer with Fe up to 40 wt.% was observed. Finally, just above the substrate a layer of Fe rich Fe-Ni-Zn solid solution layer was observed.
[057] Schematic representation of coating phase evolution during the hot stamping process: The mechanism of the phase evolution in the coating is schematically described in Fig 14. The initial coating microstructure at room temperature as shown in Fig 14(a) consisting Ni coating on galvannealed (GA) coating. The galvannealed coating had different layers in the coating. The Fe-Zn gamma phase at the steel coating interface with a body centered cubic crystal structure was observed. On the Fe-Zn gamma phase Fe-Zn delta phase with hexagonal crystal structure was observed. At the top of the galvannelaed coating a thin layer of Fe-Zn zeta phase with base centered monoclinic crystal structure was observed. At the top of the galvannealed coating pure Ni coating with face centered cubic crystal structure was observed. The coated sample was slowly heated at 10 °C /s till the desired austenetizing temperature. As the temperature of the sample was increased Ni, Zn and Fe started to diffuse due to concentration gradients. When the sample reached a temperature between 670 to 780 °C as shown in Fig 14(b), the Fe-Zn zeta and delta phase melted as they had melting temperatures below 670°C. At the Ni interface Ni rich solid solution and at substrate interface Fe rich solid solution was observed. The melting of the Fe-Zn phase assisted diffusion and further nucleation and growth of the phases. When the sample reached the austenetizing temperature as shown in Fig 11(c), The Fe-Zn gamma phase and Ni-Zn gamma phase melted down as they had melting temperatures below or equal to 881 °C. Melting of the phases further assisted the diffusion, nucleation and growth of different phases. Ni rich solid solution grew at the interface of Ni coating and a mixed zone of liquid and Ni rich solid solution formed adjacent to the Ni rich solid solution layer as shown in Fig 14(c). Similarly, the Fe rich solid solution phase grew at the substrate interface and a mixed zone of liquid and Fe rich solid solution formed adjacent to the Fe rich solid solution layer as shown in Fig 14(c). While the sample was kept at the austenitizing temperature for prolonged time as shown in Fig 14(d), large amount of Ni and Fe diffused in coating and the liquid phase was consumed. So, the coating was completely solidified. At the substrate interface a Fe rich solid solution was observed and above it a Ni-Zn-Fe ternary layer with Fe from 20 to 40 wt.% and above it a Ni rich solid solution layer was observed as shown in Fig 14(d). At this time, all the Ni from pure Ni coating was consumed so, pure Ni coating layer was not observed in the coating. It is to be mentioned apart from these phase formations in the coating, a layer of ZnO with small amount of Ni and Fe was observed at the top of the coating surface. This layer was started to form at the time of heating of the sample and continued to grow till the sample was quenched to a lower temperature as shown in Fig 12(b) to Fig 14(d). After the completion of the austenitizing process the sample was transferred to the stamping press and stamped with specified force. The stamping temperature was kept between 750°C to 850°C. At these temperatures, the coating microstructure remained like the microstructure observed at end of austenitizing process. During stamping the sample was die quenched simultaneously. The austenite in the steel at stamping temperature transformed to martensite due to quenching.
[058] Post treatment of the coated samples: The hot stamped sample was phosphate following the method described Above. The phosphate surface of the sample is shown in Fig 15. A uniform phosphate surface was observed on the sample surface in Fig 15(a). The vertical cracks are visible in the coating which was also observed in cross sectional image of the stamped sample. The complete surface was covered with the phosphate coating although different regions of the coating showed different morphology of the phosphate coating as shown in Fig 15(b) to Fig 15(d). Outside the crack elongated crystals were observed. However, smaller cuboidal crystals were observed inside the crack. The phosphate surface showed significant surface morphology which would provide good anchoring for paint.
[059] 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.
[060] 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).
[061] The above description does not provide specific details of manufacture or design of the various components. Those of skill in the art are familiar with such details, and unless departures from those techniques are set out, techniques, known, related art or later developed designs and materials should be employed. Those in the art are capable of choosing suitable manufacturing and design details.
[062] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the present disclosure. It will be appreciated that several of the above-disclosed and other features and functions, or alternatives thereof, may be combined into other systems or applications. Various presently unforeseen or unanticipated alternatives, modifications, variations, or improvements therein may subsequently be made by those skilled in the art without departing from the scope of the present disclosure as encompassed by the following claims.
[063] The claims, as originally presented and as they may be amended, encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others.
[064] 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.

We claim:

1. A method (100) of coating a steel substrate for producing hot stamp grade steel comprising the steps of:
? coating the steel substrate (10) with Zinc (Zn) to form a galvannealed coated steel substrate;
? coating the galvannealed coated steel substrate (20) with Nickel (Ni) to form Ni coated galvannealed steel substrate;
? hot stamping (30) the Ni coated galvannealed steel substrate to form hot stamped steel substrate; and
? coating the hot stamped steel substrate (40) with Zinc phosphate.
2. The method of coating steel substrate as claimed in claim 1, wherein coating the substrate (10) with Zinc comprises the step of:
? degreasing the steel substrate (11) with an alkali solution of 70°C;
? pickling the degreased steel substrate (12) in 10 vol.% sulfuric acid solution at a temperature of 50°C; and
? coating the degreased and pickled steel substrate (13) with Zn coating and suitable annealing process to form galvannealed coating.
3. The method of coating steel substrate as claimed in claim 2, wherein the step of coating the degreased and pickled steel substrate (13) with Zn coating comprises dipping in a bath composition consisting of 0.05-0.3 wt.% Aluminum and 99.95-99.7 wt.% Zinc.
4. The method of coating steel substrate as claimed in claim 2, wherein the step of coating the degreased and pickled steel substrate (13) with Zn coating has a bath temperature in a range of 440-470 °C.
5. The method of coating steel substrate as claimed in claim 2, wherein the step of coating the degreased and pickled steel substrate (13) with Zn coating has a residence time in a range of 2 to 5 seconds.
6. The method of coating steel substrate as claimed in claim 2, wherein the step of coating the degreased and pickled steel substrate (13) with Zn coating has a galvannealing temperature in a range of 450 to 550 °C.
7. The method of coating steel substrate as claimed in claim 2, wherein the step of coating the degreased and pickled steel substrate (13) with Zn coating has a galvannealing time in a range of 5 to 20 seconds.
8. The method of coating a steel substrate as claimed in claim 1, wherein Nickel is coated on the galvannealed coated steel substrate (20) by electroplating process.
9. The method of coating a steel substrate as claimed in claim 8, wherein the process of electroplating has a bath composition comprising of:
? 250-350 gm/l NiSO4.6H2O;
? 40-70 gm/l NiCl2. 6H2O; and
? 30-50 gm/l Boric Acid;
10. The method of coating a steel substrate as claimed in claim 8, wherein the process of electroplating uses a current in a range of 10-30 ampere.
11. The method of coating a steel substrate as claimed in claim 8, wherein the process of electroplating has a time in a range of 3-20 minute.
12. The method of coating a steel substrate as claimed in claim 8, wherein the process of electroplating has a bath temperature in a range of 20-70 °C.
13. The method of coating a steel substrate as claimed in claim 8, wherein the process of electroplating has pH in a range of 2-5.
14. The method of coating a steel substrate as claimed in claim 1, wherein the process of hot stamping (30) has a furnace temperature /Austenitizing temperature in a range of 850-950 °C.
15. The method of coating a steel substrate as claimed in claim 1, wherein the process of hot stamping (30) has a temperature before drawing /Stamping temperature in a range of 750-850 °C.
16. The method of coating a steel substrate as claimed in claim 1, wherein the process of hot stamping (30) has an austenetizing time in a range of 3 to 8 minutes.
17. The method of coating a steel substrate as claimed in claim 1, wherein the process of hot stamping (30) has 15 to 21% O2 in furnace.
18. The method of coating a steel substrate as claimed in claim 1, wherein the process of hot stamping (30) has a dew point in a range of -50 to -85 °C in furnace.
19. The method of coating a steel substrate as claimed in claim 1, wherein the process of hot stamping (30) has a punch force in a range of 450 to 550 kN.
20. The method of coating a steel substrate as claimed in claim 1, wherein the process of hot stamping (30) has quenching time in a range of 15-25 seconds.
21. The method of coating a steel substrate as claimed in claim 1, wherein coating the hot stamped (40) steel substrate with Zinc phosphate comprises the steps of:
? degreasing the hot stamped (41) steel substrate with an alkali solution;
? rinsing the degreased hot stamped steel substrate with distilled water (42);
? dipping the rinsed hot stamped steel substrate in titanium activation solution (43);
? coating with phosphate (44);
? rinsing the phosphate coated hot stamped steel substrate (45); and
? drying with hot air (46).
22. The method of coating a steel substrate as claimed in claim 21, wherein the step of degreasing (41) has a bath concentration of 1.5-3vol.%, time of 80-130 seconds, temperature of 50-70°C and pH in range of 11-13.
23. The method of coating a steel substrate as claimed in claim 21, wherein the step of rinsing with water (42) comprises time in range of 100-150 seconds, temperature in range of 15-35°C and pH in range of 7-7.5.
24. The method of coating a steel substrate as claimed in claim 21, wherein the step of dipping in titanium activation solution (43) comprises a bath concentration of 0.05-0.2, time in range of 50-75 seconds, temperature of 20-35°C and pH in range of 8-9.
25. The method of coating a steel substrate as claimed in claim 21, wherein the step of coating with phosphate (44) has a bath concentration comprising:
? 0.9 free acid point;
? 21 total acid point;
? 4 accelerator;
? 0.062% Zinc (Zn);
? 0.064% Nickel (Ni); and
? 0.047% Manganese (Mn).
26. The method of coating a steel substrate as claimed in claim 21, wherein the step of coating with phosphate (44) has a time in range of 150-200 seconds, temperature in the range of 50-60°C and pH in range of 2.5-3.
27. The method of coating a steel substrate as claimed in claim 21, wherein the step of rinsing with water (45) has a time in range of 50-60 seconds, temperature in range of 20-35°C and pH in range of 6-7.
28. The method of coating a steel substrate as claimed in claim 21, wherein drying with hot air (46) is for 120 seconds and at a temperature of 80°C.
29. A coated hot stamp grade steel comprising:
? a steel substrate; and
? a coating on the steel substrate as claimed in claim 1.
30. The coated hot stamp grade steel as claimed in claim 29, wherein the steel substrate is a cold rolled steel substrate.
31. The coated hot stamp grade steel as claimed in claim 29, wherein the steel substrate comprising:
? Carbon (C) from 0.2-0.25 Wt.%;
? Manganese (Mn) form 1.1-1.3 Wt.%;
? Boron (B) form 0.002-.003 Wt.%;
? Silicon (Si) from 0.2-0.3 Wt.%;
? Aluminum (Al) from 0.03-0.04 Wt.%;
? Copper (Cu) from 0.01-0.02 Wt.%;
? Chromium (Cr) from 0.1-0.2 Wt.%;
? Nickel (Ni) from 0.02-0.03;
? Molybdenum (Mo) from 0.001-0.002 Wt.%;
? Vanadium (V) from 0.001-0.002 Wt.%;
? Niobium (Nb) from 0.001-0.002 Wt.%;
? Titanium (Ti) from 0.03-0.035 Wt.%;
? Nitrogen (N) from 35-40 ppm;
? Sulfur (S) from 0.001-0.002 Wt.%; and
? Phosphorus (P) from 0.005-0.0010 Wt.%.