Description:TECHNICAL FIELD
The present disclosure relates to the field of metallurgy. Particularly, the present disclosure relates to aluminium-silicon based hot-dip coatings, methods of depositing them on steel substrates, and steel substrates obtained therefrom.

BACKGROUND OF THE DISCLOSURE
In recent years, there has been an emphasis on the development of high-quality hot forming grade steel for automotive applications. For the manufacture of hot formed grade steel parts, a prior coating on the steel blank is a prerequisite not only to protect the steel surface from high temperature oxidation but also to improve its corrosion resistance during service. The Al-Si based coating possesses desired high temperature properties and it is feasible to produce these coatings in an industrial continuous hot dip process line. For the last two decades, there has been a substantial effort in the development of Al-Si based hot dip coating on steel substrate for various special applications, such as roof walls of a building, automobile exhaust and fuel tank, heat equipment parts and more recently, ultra-high strength automobile components produced by hot forming (also known as hot stamping) method etc.

In the hot forming process, the steel blank is heated to austenitization temperature in the range of 750-950oC. The components are isothermally held at those temperatures for 3-10 minutes followed by quick transfer to water-cooled closed dies for forming and subsequent quenching operations. During austenitization at high temperatures, oxide scale formation and decarburization takes place on the surface of an uncoated steel sheet. To prevent this surface oxidation and decarburization, a protective Al-Si based coating is provided on the steel surface prior to hot forming operation. The Al-Si based coating generally provides excellent high temperature properties like resistance to oxidation, spalling and resistance to crack formation during hot forming operation. The required high temperature properties of Al-Si based coatings depend on several process parameters including coating bath composition, bath temperature, dipping time, strip entry temperature, intermetallic phases formed at the interface between substrate and coating during hot dip process and kinetics of growth of AlxFeySiz intermetallics during austenitization. To obtain a good quality Al-Si based coating for high temperature performance, it is preferable to have compact coating with small grain size, formation of compact and stable thin protective oxide layer on the top surface to avoid internal oxidation of coating, controlled formation of intermetallic phases at the substrate-coating interface during dipping and minimization of void formation in the coating. The voids in the coating are generally formed during the diffusion process and act as nucleation sites for crack formation during hot forming operation, leading to flaking of coating in subsequent hot forming operation. The corrosion resistance of hot dip Al-Si based coatings is influenced by phases formed on the surface of the coating and its distribution, grain size, surface appearance and morphology.

In the batch hot dip process, a fluxing agent is coated on cold rolled steel sheet/strip to prevent its surface oxidation during the transfer process after pickling and, prior to hot dipping in the molten alloy bath. A fluxing agent on the strip surface improves the wettability of the steel substrate in the molten alloy bath. This additional requirement of fluxing agents often leads to an increase in production time and cost.

There are many approaches explored in the art for the development of hot dip Al-Si based coatings mainly for corrosion resistance and hot forming applications. For example, US 2016/0312342 A1 and CN 101736248 A disclose coatings with improved corrosion resistance through the addition of many elements including Mg, Ca, Sr and rare earth into the Al-Si based molten alloy bath. US 4,891,274A discloses the production of heat resistant and adherent Al-Si based hot dip coatings, wherein nickel or nickel alloy layer(s) are deposited on the substrate prior to Al-Si based hot dip coating to prevent non-coated bare spots and pinholes. US 2011/0300407A1 and JP 4700543B2 disclose steel substrates coated with Al-15 wt.%Si for hot forming application, wherein the occupancy ratio of Fe3Al and FeAl intermetallic compound layers in the coating was controlled to impart good resistance against crack and corrosion.

Several studies have highlighted the formation of intermetallic layers at the substrate-coating interface and cracking mechanisms of Al-Si based coatings. During austenitization of the steel substrate at high temperature, the growth of these intermetallic layers takes place at a very fast rate through solid-state diffusion-controlled mechanisms as well as new intermetallic phases are formed due to interactive reactions between these layers. However, the formation of voids in the coating is expected due to the brittle nature of these intermetallic layers and differential thermal expansion between the intermetallic layers and upper part of the coating. This results in primary crack initiation and its inward propagation in the direction perpendicular to the substrate surface. This eventually leads to secondary crack generation and propagation parallel to the substrate surface leading to the delamination of the coating. Increased tendency of cracking and delamination in an Al-Si based coating during hot forming operation decreases the corrosion resistance by exposing the steel surface to oxidizing and corrosive environment. Therefore, apart from resistance to high temperature oxidation, the resistance to void formation and delamination are critical requirements for a good quality coating for hot forming application and this poses challenges in manufacturing the same.

Thus, there is a need in the art to provide an improved coating composition and processing method to produce Al-Si based coated steel sheet with superior high temperature oxidation resistance and formability. The present disclosure attempts to address said need. Compared to conventional Al-Si based coatings, the present invention envisages controlling the formation of desirable phases and their distribution on the steel surface as well as within the coating during the hot dip deposition, without employing any prior fluxing agent. The processing method in the invention also aims to overcome the cracking and delamination issues in the coating by suppression of void formation.

STATEMENT OF THE DISCLOSURE
The present disclosure relates to an aluminium-silicon (Al-Si) alloy coating on a steel substrate, comprising 5-9 wt.% Si, 0.5-4.0 wt.% magnesium (Mg), 0.1-1.5 wt.% copper (Cu), 0.002-0.2 wt.% scandium (Sc), 0.002-0.2 wt.% strontium (Sr) and the balance being Al.

The present disclosure also relates to a coated steel substrate comprising the Al-Si-Mg-Cu-Sr-Sc alloy coating.

The present disclosure relates to a method for preparing a coated steel substrate comprising the Al-Si-Mg-Cu-Sr-Sc alloy coating, said method comprising steps of: a) heating a steel substrate to an annealing temperature of about 750-850? to obtain an annealed substrate; b) cooling the annealed substrate to a strip entry temperature (SET) of about 650-750? to obtain a cooled substrate; and c) dipping the cooled substrate in an Al-Si-Mg-Cu-Sc-Sr alloy bath at a temperature of about 640-660? to obtain the coated steel substrate; wherein said steps (a)-(c) are carried out at a dew point of +10?.

The present disclosure also relates to a coated steel substrate obtained by the methods of the present disclosure.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES
Figure 1 shows a schematic of an exemplary annealing and hot dipping schedule employed in the methods of preparing a coated steel substrate according to the present disclosure.

Figure 2 shows the effect of varying the coating composition on the microstructure of the coating.

Figure 3 shows a photograph of an Al-Si-Mg-Cu-Sc-Sr coated steel sheet substrate showing no bare spots.

Figure 4 shows an optical micrograph (panel (a)) and a SEM micrograph (panel (b)) of the Al-Si-Mg-Cu-Sc-Sr coating.

Figure 5 shows an elemental mapping through WDS of a hot dip coated specimen.

Figure 6 shows the time-temperature schedule used for the heat treatment of a hot dip coated specimen.

Figure 7 shows optical micrographs of the coated specimen heat treated at 850? (panel (a)), 900? (panel (b)) and 950? (panel (c)) for 60 s.

Figure 8 shows the sequence of hot bending test of the coated sample. Panel (a) shows the coated sample before heating. Panel (b) shows the coated sample during the hot bending test. Panel (c) shows the coated sample after hot bending followed by cooling to room temperature.

Figure 9 shows the coated sample after hot bending followed by water quenching to room temperature. Panel (a) shows the inner surface and Panel (b) shows the outer surface.

Figure 10 shows optical micrographs of the coating in transverse section after hot bending and water quenching.

Figure 11 shows SEM micrographs of the coating cross section after hot bending and water quenching.

Figure 12 shows the high temperature deformation test of the hot dip coating. Panel (a) shows the high temperature deformation schedule. Panel (b) shows the visual examination of the coating after high temperature deformation.

DETAILED DESCRIPTION OF THE DISCLOSURE
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. The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results. Throughout this specification, the word “comprise”, or variations such as “comprises” or “comprising” or “containing” or “has” or “having”, or “including but not limited to” wherever used, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

Reference throughout this specification to “some embodiments”, “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment may be included in at least one embodiment of the present disclosure. Thus, the appearances of the phrases “in some embodiments”, “in one embodiment” or “in an embodiment” in various places throughout this specification may not necessarily all refer to the same embodiment. It is appreciated that certain features of the disclosure, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the disclosure, which are, for brevity, described in the context of a single embodiment, may also be provided separately or in any suitable sub-combination.

The term “about” as used herein encompasses variations of +/-5% and more preferably +/-2.5%, as such variations are appropriate for practicing the present invention.

The present disclosure provides an aluminium-silicon (Al-Si) alloy coating on a steel substrate, comprising 5-9 wt.% Si, 0.5-4.0 wt.% magnesium (Mg), 0.1-1.5 wt.% copper (Cu), 0.002-0.2 wt.% scandium (Sc), 0.002-0.2 wt.% strontium (Sr) and the balance being Al. The coating composition range of elements is shown in Table 1.

Table 1: Elemental composition of the developed hot dip coating
Elements Si Mg Cu Sc Sr Al
Wt.% 5.0-9.0 0.5-4.0 0.1-1.5 0.002-0.2 0.002-0.2 Bal.

In some embodiments, the Al-Si alloy coating comprises 6.5-7.5 wt.% Si, 1.5-2.5 wt.% Mg, 0.4-0.8 wt.% Cu, 0.08-1.2 wt.% Sc, 0.08-1.2 wt.% Sr and the balance being Al. In an exemplary embodiment, the Al-Si alloy coating comprises 7 wt.% Si, 2 wt.% Mg, 0.6 wt.% Cu, 0.1 wt.% Sc, 0.1 wt.% Sr and the balance being Al.

The importance of lower and upper threshold values for each element is described below.

Silicon (Si): The workability of aluminium-based coating improves with the addition of silicon as it reduces the rate of growth of aluminium rich intermetallic layers that form at the substrate/coating interface. However, addition of excessively high amount of silicon in liquid aluminium bath may reduce the coating lifetime due to the generation of defects (voids and cracks) at the substrate/coating interface during high temperature applications. Keeping in view the beneficial effect of silicon in aluminide coating, the amount of silicon in the coating is maintained between 5-9 wt.%.

Magnesium (Mg): The amount of magnesium in the coating is maintained between 0.5-4.0 wt.%. The addition of Mg in the Al-Si based coating allows the formation of a Mg2Si phase that increases the corrosion resistance, precipitation strengthening as well as formation of protective Mg-Si oxides on the coating surface during hot forming operation.

Copper (Cu): The amount of copper in the coating is maintained between 0.1-1.5 wt.%. The formation of a Cu-Si phase at the grain boundaries provides beneficial effect as it fills the voids generated at the grain boundaries mainly due to shrinkage during hot dipping process. Also, the presence of Cu in the coating acts as a capillary agent to fill voids due to the growth of intermetallics during the hot forming operation.

Scandium (Sc): The amount of scandium in the coating is maintained between 0.002-0.2 wt.%. Scandium added to Al-Si coating forms Al3Sc precipitates, which impart precipitation strengthening and increase the high temperature formability of the coating.

Strontium (Sr): The amount of strontium in the coating is maintained between 0.002-0.2 wt.%. Strontium gives rise to modification of eutectic Si morphology and provides a more refined coating microstructure and thereby, improves the coating strength.

The inventors found that the presence of both Sr and Sc provides a refinement in grain sizes of the coating microstructure. This refinement in grain size, in turn, increases the hardness value of the coating. Taken together, the refinement and modification of eutectic Si due to Sr combined with precipitation strengthening effect of Sc increase the overall hardness of the coating as well as improves the hot forming behaviour of the coating.

The present disclosure further provides a method for preparing a coated steel substrate comprising the Al-Si-Mg-Cu-Sc-Sr alloy coatings as described herein. The method broadly comprises the steps of annealing a steel substrate, cooling the annealed substrate to a strip entry temperature (SET), hot dipping the substrate cooled to the SET in an Al-Si-Mg-Cu-Sc-Sr alloy bath, and cooling the dipped substrate to room temperature, wherein each of these steps are carried out at a dew point of +10?. FIG. 1 shows an exemplary schematic of the annealing and hot dipping process conditions. The inventors found that by maintaining the dew point +10? at each step, the requirement of coating the steel substrate with fluxing agents to prevent surface oxidation can be eliminated. This significantly reduces the cost and time required for preparing hot-forming grade steel substrates.

In some embodiments, a method for preparing a coated steel substrate comprises steps of: a) heating a steel substrate to an annealing temperature of about 750-850? to obtain an annealed substrate; b) cooling the annealed substrate to a strip entry temperature (SET) of about 650-750? to obtain a cooled substrate; and c) dipping the cooled substrate in an Al-Si-Mg-Cu-Sc-Sr alloy bath at a temperature of about 640-660? to obtain the coated steel substrate; wherein each of steps (a)-(c) are carried out at a dew point of +10?.

In some embodiments, the steel substrate is heated to the annealing temperature of about 750-850? at a heating rate of about 10?/s and the steel substrate is annealed at said annealing temperature for about 10-120 seconds. In some embodiments, the annealing temperature is about 770-820? or about 775-800?, including values and ranges thereof. In some embodiments, the annealing temperature is about 750, 755, 760, 765, 770, 775, 780, 785, 790, 795, 800, 805, 810, 815, 820, 825, 830, 835, 840, 845, or 850 ?. In some embodiments, the duration of annealing is about 10-120 seconds, about 20-100 seconds, about 30-90 seconds, or about 40-80 seconds, including values and ranges thereof.

The annealed steel substrate is then cooled to a strip entry temperature (SET). The SET regulates the formation of interfacial intermetallic layer/s at the substrate-coating interface during dipping. In some embodiments, the strip entry temperature (SET) is about 650-750, 675-725, 680-720, or 690-710 ?, including values and ranges thereof. In some embodiments, the SET is about 650, 660, 670, 680, 690, 700, 710, 720, 730, 740, or 750 ?.

In some embodiments, the annealed steel substrate is cooled to the SET of about 650-750? at a cooling rate of about 10?/s.

The annealed substrate cooled to the SET is dipped in an Al-7Si-2Mg-0.6Cu-0.1Sc-0.1Sr liquid alloy bath maintained at 650±10? with immersion duration of 5-30s. The dipping time controls the formation and growth of interfacial intermetallic layers. In some embodiments, the dipping step (where the annealed substrate cooled to the SET is dipped in the Al-Si alloy coating bath) is carried out for an immersion duration of about 5-30 seconds, 5-25 seconds, 5-20 seconds, 5-15 seconds, or about 5-10 seconds.

In some embodiments, the dipping step is followed by a step of nitrogen gas jet wiping to control the thickness of the coating to about 25-35 µm.

After the dipping step and the optional nitrogen gas jet wiping step, the steel substrate coated with the Al-Si alloy coating is cooled to a room temperature. In some embodiments, the term “room temperature” refers to the temperatures of about 15-40?, 15-35?, 15-30?, 15-25?, 15-20?, 20-40?, 20-35?, 20-30?, 25-40?, 25-35?, or 25-30?, including values and ranges thereof. In some embodiments, cooling of the coated steel substrate to a room temperature is carried out at a cooling rate of 10?/s.

The steps of annealing, cooling the annealed substrate to the SET, hot dipping the substrate in the Al-Si alloy bath, nitrogen gas jet wiping, and cooling the substrate to the room temperature are all carried out at a dew point of +10?. In some embodiment, the dew point of +10? is maintained by using a mixture of nitrogen and 5-10 vol.% hydrogen gas. The inventors have found that when the above steps are performed at a dew point of +10?, the surface oxidation of the steel substrate prior to dipping is suppressed and a good quality coating free from any bare spots on the coated surface is obtained.

Prior to subjecting a steel substrate to annealing, the steel substrate such as a full hard cold-rolled steel sheet substrate, is first cleaned in an alkaline solution followed by pickling and then rinsing in distilled water. The steel substrate is then dried, cleaned in alcohol, and then subjected to the steps described above.

The present disclosure also provides a coated steel substrate obtained by the methods described herein. The coated steel substrate of the present disclosure comprises the Al-Si-Mg-Cu-Sc-Sr alloy coating as described herein. In some embodiments, the steel substrates have a composition similar to hot forming grade steel. In some embodiments, the steel substrate is a full hard cold-rolled steel sheet.

ADVANTAGES
The Al-Si-Mg-Cu-Sc-Sr alloy coatings of the present disclosure exhibit enhanced strength and corrosion properties compared to binary coating comprising aluminium and silicon. The present coatings also exhibit excellent adhesion and high temperature stability to sustain hot deformation that is required for the industrial hot forming/stamping process. These properties of the coating are mainly due to the elemental composition of the coating containing Al, Si, Mg, Cu, Sc, and Sr. Mg not only forms protective Mg-Si oxides on the top surface of the coating but also improves the corrosion performance. The role of copper in the coating is to act as a capillary agent to fill voids formed within the coating during the hot forming operation. Scandium and strontium refine and modify the coating microstructure and thereby improves the coating strength compared to other existing Al-Si coating.

Taken together, the present disclosure provides a good quality adherent hot dip Al-Si-Mg-Cu-Sc-Sr coating with refined coating microstructure and improved strength. The present hot dip Al-Si-Mg-Cu-Sc-Sr coating exhibits excellent high temperature oxidation resistance, excellent formability and anti-delamination behavior during hot bending operation. The process of preparing a steel substrate comprising the Al-Si-Mg-Cu-Sc-Sr coating disclosed herein by controlling the dew point is industrially feasible without any prior fluxing agent. This reduces the additional cost of fluxing agent and leads to increased productivity at industrial scale, as the present coating can be produced through continuous hot dip coating line. The coating composition and the method of preparing a steel substrate with the coating have been designed to provide a high coating strength and corrosion resistance. Further, the addition of Sc in specific amounts improves the strength of the coating by precipitation hardening and high temperature formability.

It is to be understood that the foregoing descriptive matter is illustrative of the disclosure and not a limitation. While considerable emphasis has been placed herein on the particular features of this disclosure, it will be appreciated that various modifications can be made, and that many changes can be made in the preferred embodiments without departing from the principles of the disclosure. Those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein. Similarly, additional embodiments and features of the present disclosure will be apparent to one of ordinary skill in art based upon description provided herein.

Descriptions of well-known/conventional methods/steps and techniques are omitted so as to not unnecessarily obscure the embodiments herein. Further, the disclosure herein provides for examples illustrating the above-described embodiments, and in order to illustrate the embodiments of the present disclosure certain aspects have been employed. The examples used herein for such illustration are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the following examples should not be construed as limiting the scope of the embodiments herein.

EXAMPLES

Example 1: The effect of alloy composition on microstructure and hardness
Four different compositions as shown in Table 2 were chosen to illustrate the effect of additions of alloying elements such as Sr, Cu and Sc on hardness and microstructure. Hardness properties of the as-cast composition are shown in Table 3 and the as-cast microstructure is shown in Figure 2.

Table 2: Different compositions chosen for a comparative study
Coating composition (Wt.%) Al Si Mg Cu Sr Sc
1 bal 7 --- --- --- ---
2 bal 7 2 --- 0.1 ---
3 bal 7 2 0.6 0.1 ---
4 bal 7 2 0.6 0.1 0.1

The microstructure of Composition-1 (Al-7%Si) mainly consists of coarse grains aluminium rich matrix in white contrast and a flake-like hard silicon rich phase in grey contrast. With alloying additions (Compositions 2-4), fine grain microstructure can be obtained as shown in Figure 2. The refinement in grain sizes observed in microstructure is more prominent in Composition-4 (containing Sr along with Sc) in comparison to other alloys. Consequently, for Composition-4, the hardness value was found to be the highest. Thus, it appears that the grain size refinement effect of Sr combined with precipitation strengthening effect of Sc has increased the overall hardness of Composition-4. Accordingly, Compostion-4 is expected to perform better in comparison to other coating compositions.

Table 3: Variation of hardness based on the alloy composition
Alloy Coating Composition (wt.%) Hardness (HV)
1 Al-7%Si 34
2 Al-7%Si -2%Mg-0.1Sr 59
3 Al-7%Si-2%Mg-0.1%Sr-0.6%Cu 68
4 Al-7%Si-2%Mg-0.1%Sr-0.6%Cu-0.1%Sc 78

Example 2: Preparation of a steel substrate comprising an Al-Si-Mg-Cu-Sc-Sr hot dip coating
Cold rolled steel sheet with a composition comprising Fe, 0.09%C, 1.35% Mn, 0.03% Si, 0.04% Cr, 0.04% Al, 0.01% P, and 0.007% S was prepared and annealed at the temperature of 780?. As compared to the conventional hot dip process, no wet or dry fluxing agent was used to prevent surface oxidation. Instead, the dew point was controlled and maintained at +10? using N2 and 5-10 vol.%H2 to prevent any surface oxidation. It was observed that a dew point of +10? resulted in a good quality coating with no bare spots. The process illustrated in Figure 1 was followed to obtain an Al-Si-Mg-Cu-Sc-Sr hot dip coating on the sheet and the coated steel sheet is shown in Figure 3. A smooth, continuous and adherent coating of 25-35 µm thickness is obtained that exhibits no visible bare spots on the coated top surface.

Example 3: Optical and secondary electron (SE) micrograph studies of the coating
Cold rolled steel sheet with a composition comprising Fe, 0.09% C, 1.35% Mn, 0.03% Si, 0.04% Cr, 0.04% Al, 0.01% P, and 0.007% S was prepared and annealed at the temperature of 780? with a dew point of +10?. The annealing and hot dipping schedule illustrated in Figure 1 was adopted to obtain an Al-Si-Mg-Cu-Sc-Sr coating on the steel sheet. The hot dipping experiment was performed by maintaining strip entry temperature (SET) of 700?, dipping time (DT) of 5s and the bath temperature of 650±10?.
The optical and secondary electron (SE) micrographs of the coating are shown in Figure 4. Optical micrograph shows that the microstructure of the coating mainly consists of grains of ?-aluminium in bright contrast (Figure 4, panel (a)). The presence of other phases in the microstructure can also be seen, particularly at the grain boundaries. Optical and SE micrographs also indicate the presence of a continuous interfacial layer at the interface between substrate and the coating in grey contrast. The SE micrograph, at high magnification, reveals the presence of two distinct continuous layers at the interface between substrate and coating (Figure 4, panel (b)). The thickness of the interfacial layer is in the range of 4-6 µm.
The cross-sectional elemental mapping of the coating was carried out through wavelength dispersive spectroscopy (WDS) with a purpose to observe the various phases present in the coated microstructure (Figure 5). WDS analysis of Fe and Si (Figure 5) clearly demonstrates the variation of intensity of Fe and Si at the interface. WDS analysis indicates the presence of two distinct Fe-Al-Si rich intermetallic compounds at the interface between substrate and coating. A high concentration of Fe and comparatively low concentration of Si in the first Fe-Al-Si interfacial layer adherent with the substrate can be seen. However, in the second Fe-Al-Si based interfacial layer, little away from the interface, the intensity of Fe concentration reduces, where Si intensity becomes more prominent. WDS analysis also indicates that Sc is segregated along the periphery of Mg-Si based phases. Sr was mainly present in the coating away from the interface.

Example 4: Effects of heat treatment on the coated steel substrates
Cold rolled steel sheet with a composition comprising Fe, 0.09% C, 1.35% Mn, 0.03% Si, 0.04% Cr, 0.04% Al, 0.01% P, and 0.007% S was prepared and annealed at the temperature of 780? with a dew point of +10?. The annealing and hot dipping schedule illustrated in Figure 1 was adopted to obtain an Al-Si-Mg-Cu-Sc-Sr coating on the sheet. The hot dipping experiment was performed by maintaining a strip entry temperature (SET) of 700? and dipping time (DT) of 5 s. A 25-30 ?m thickness smooth, adherent continuous coating was obtained with visibly no bare spots. The heat treatment of the hot dip coated specimen was carried out to assess the high temperature behaviour of the coating. The heat treatment experiments were performed by heating the coated specimen at three different austenitization temperatures of 850?, 900? and 950? for the duration of 60 s, as per the schedule shown in Figure 6.
The optical micrographs of heat-treated specimens are shown in Figure 7 (panel (a): 850?, panel (b): 900?, and panel (c): 950?). A continuous layer of the coating was found to remain intact after the annealing treatment. However, the microstructure of the coating is drastically different from that of as-dipped coating microstructure. The as-dipped coating microstructure is changed due to the growth of Fe-Al-Si based intermetallic phases in the coating, which is mainly attributed to the solid-state diffusion of iron from the steel substrate into the coating. The presence of mainly two different morphologies of Fe-Al-Si based intermetallic phases in the coating was observed, which was more prominent at high annealing temperatures. A decrease in coating layer thickness with an increase in annealing temperature was also observed, which might be due to the loss of the coated layer.

Example 5: Hot bending test of the coated substrate
Cold rolled steel sheet with a composition comprising Fe, 0.09% C, 1.35% Mn, 0.03% Si, 0.04% Cr, 0.04% Al, 0.01% P, and 0.007% S was prepared and annealed at temperature of 780? with a dew point of +10?. The annealing and hot dipping schedule illustrated in Figure 1 is adopted to obtain an Al-Si-Mg-Cu-Sc-Sr coating on the sheet. The hot dipping experiment was performed by maintaining the strip entry temperature (SET) of 700? and dipping time (DT) of 5 s. A 25-30 µm thick smooth, adherent continuous coating was obtained with visibly no bare spots. The hot bending test of the heat-treated coated specimen was carried out to assess the high temperature formability of the coating. The samples were first heated to the austenitization temperature (900? and 930?) and held for 120 s. Thereafter, specimens were taken out of the furnace and immediately hot bending test was carried out, as illustrated sequentially in Figure 8. Panel “a” of Figure 8 shows the coated sample before the hot bending test. The coated sample was twisted at high temperature to an angle of more than 90o (Figure 8, panel (b)) followed by immediate quenching in cold water. The bent sample after quenching (Figure 8, panel (c)) shows that the coating is intact and undamaged after the hot bending test.
A visual examination of the bent specimens immediately after water quenching reveals that the coating is intact and undamaged after the hot bending test (Figure 9). Panel (a), Figure 9 shows the inner surface (compression) of the bent sample after water quenching and panel (b) of Figure 9 shows the outer surface (tension) of the bent sample after water quenching. It indicates that the coating has good adhesion and strength to sustain bending at high temperature.
Optical micrographs of the coating in transverse section taken from the bended location after hot bending test and water quenching are shown in the Figure 10. The low magnification optical micrograph (Figure 10, panel (a)) confirms the presence of adherent continuous layer of coating on both sides (inner and outer sides) of the substrate. The higher magnification optical micrographs (Figure 10, panels (b) and (c)) taken from the inner side of the coated substrate reveal the continuous layer of coating almost free from defects/cracks after bending. This is mainly due to the generation of compressive force in the coating on the inner side of the bended substrate. Micrographs also indicate that the growth of the Fe-Al-Si based intermetallic throughout the cross section of the coating is almost completed. However, the presence of overlay Al-Si based coating can also be seen at some of the locations towards the top surface of the coating.
However, the optical micrographs at higher magnification (Figure 10, panels (d) and (e)) taken from the outer side of the coated substrate reveal the presence of many cracks in the Fe-Al-Si based intermetallic layer close to the bended location. This is mainly due to the presence of hard and brittle Fe-Al-Si based intermetallic layers, which experiences the tensile nature of forces in the outer side of the coated substrate during bending. The presence of cracks in the intermetallic layer is expected due to the differential thermal expansion behavior of intermetallic than the substrate steel. The presence of overlay Al-Si based coating was absent due to the growth of the Fe-Al-Si based intermetallic layer throughout the cross section of the coating. The SEM micrographs of the hot bend test coated specimen, at both sides of the substrate surfaces along the cross-section, are shown in Figure 11. SEM micrographs further confirm a continuous layer of coating almost free from any defects after the bend test on the inner side of the bended substrate. However, outer side of the coated specimen reveals the presence of cracks in the intermetallic layer perpendicular to substrate surface.

Example 6: High temperature tensile deformation test of the coated substrate
Cold rolled steel sheet with a composition comprising Fe, 0.09% C, 1.35% Mn, 0.03% Si, 0.04% Cr, 0.04% Al, 0.01% P, and 0.007% S was prepared and annealed at temperature of 780? with a dew point of +10?. The annealing and hot dipping schedule illustrated in Figure 1 is adopted to obtain an Al-Si-Mg-Cu-Sc-Sr coating on the sheet. The hot dipping experiment was performed by maintaining strip entry temperature (SET) of 700? and dipping time (DT) of 5s. A 25-30 µm thick smooth, adherent continuous coating was obtained with visibly no bare spots.
The high temperature tensile deformation test of the coated specimen was also carried out in Gleeble 3800 thermo-mechanical process simulator to further assess the high temperature formability of the coating. The samples were first heated to the austenitization temperature of 900? (sample 1) and 930? (sample 2) and held for 3 mins. Thereafter, the specimens were subjected to tensile deformation at 850? up to a strain of 20% at a constant strain rate of 0.5 s-1, followed by air quenching. The heat treatment schedule is illustrated in Figure 12, panel (a). A visual examination of the hot deformed specimens, as shown in Figure 12, panel (b) reveals that the coating is intact and undamaged. It indicates that the coating has good adhesion and strength to sustain high temperature deformation that is required for the industrial hot stamping process.

Claims:We Claim:
1. An aluminium-silicon (Al-Si) alloy coating on a steel substrate, comprising 5-9 wt.% Si, 0.5-4.0 wt.% magnesium (Mg), 0.1-1.5 wt.% copper (Cu), 0.002-0.2 wt.% scandium (Sc), 0.002-0.2 wt.% strontium (Sr) and the balance being Al.
2. The Al-Si alloy coating as claimed in claim 1, comprising 6.5-7.5 wt.% Si, 1.5-2.5 wt.% Mg, 0.4-0.8 wt.% Cu, 0.08-1.2 wt.% Sc, 0.08-1.2 wt.% Sr and the balance being Al.
3. The Al-Si alloy coating as claimed in claim 1 or 2, comprising 7 wt.% Si, 2 wt.% Mg, 0.6 wt.% Cu, 0.1 wt.% Sc, 0.1 wt.% Sr and the balance being Al.
4. A coated steel substrate comprising the Al-Si alloy coating as claimed in any one of claims 1-3.
5. The coated steel substrate as claimed in claim 4, wherein said steel substrate has a composition similar to hot forming grade steel.
6. A method for preparing a coated steel substrate as claimed in claim 4 or 5, comprising steps of:
a. heating a steel substrate to an annealing temperature of about 750-850? to obtain an annealed substrate;
b. cooling the annealed substrate to a strip entry temperature (SET) of about 650-750? to obtain a cooled substrate;
c. dipping the cooled substrate in an Al-Si-Mg-Cu-Sc-Sr alloy bath at a temperature of about 640-660? to obtain the coated steel substrate;
wherein said steps (a)-(c) are carried out at a dew point of +10?.
7. The method as claimed in claim 6, wherein said heating is carried out at a heating rate of about 10?/s and the steel substrate is annealed at said annealing temperature for about 10-120 seconds.
8. The method as claimed in claim 6 or 7, wherein said cooling is carried out at a cooling rate of about 10?/s.
9. The method as claimed in any one of claims 6-8, wherein said dipping is carried out for a duration of about 5-30 seconds.
10. The method as claimed in any one of claims 6-9, comprising a step of nitrogen gas jet wiping after said dipping to control thickness of the coating to about 25-35 µm.
11. The method as claimed in any one of claims 6-10, comprising a second cooling step after said dipping where the coated steel substrate is cooled to room temperature at a cooling rate of about 10?/s.
12. The method as claimed in claim 10 or 11, wherein the step of nitrogen gas jet wiping and the second cooling step are carried out at a dew point of +10?.
13. A coated steel substrate obtained by the method as claimed in any one of claims 6-12.