Claims:We Claim:
1.Hot dip coating composition for corrosion resistant coating of steel substrates comprising:
Zn-Al-Mg coating composition comprising:
4% to 6% by weight of aluminium;
4% to 6% by weight of magnesium; and 88% to 92% by weight of zinc as other major constituent,
wherein the hot-dip zinc-aluminium-magnesium coating layer contains 45% to 55%by volume of MgZn2 phase estimated using Rietveld refinement method of X-ray diffraction plots and with Mg/Al ratio is in the range of 0.85 to 0.95,
suitable for generating corrosion resistant coating on steel substrates having corrosion resistance greater than 3000 hours in range of at least about 3000 to 4000 hours in salt spray test (SST) conforming to (ASTM B117).

2. A hot dip coated steel comprising a steel substrate and a zinc-aluminium-magnesium coating formed thereon obtained of the hot dip coating composition as claimed in claim 1, comprising zinc-aluminium-magnesium coating layer having constituent elements, based on the total weight of the zinc-aluminium-magnesium coating layer:
4% to 6% by weight of aluminium;
4% to 6% by weight of magnesium; and 88% to 92% by weight of zinc as other major constituent and unavoidable impurities, wherein the hot-dip zinc-aluminium-magnesium coating layer on said steel substrate contains 45% to 55% by volume of MgZn2 phase estimated using Rietveld refinement method of X-ray diffraction plots;
the Mg/Al ratio is in the range of 0.85 to 0.95;
and have corrosion resistance greater than 3000 hours in range of at least about 3000 to 4000 hoursin salt spray test (SST) conforming to (ASTM B117).

3. The hot dip coated steel as claimed in claim 2, wherein the zinc-aluminium-magnesium coating layer thickness is in the range of 10 to 30 µm and having high hardness in the range of 140-175 HV.

4. The hot dip coated steel as claimed in anyone of the claims 2 to 3, wherein the spot weldability of hot dip coated steel is in the current range of 6 to 12 kA without expulsion.

5. The hot dip coated steel as claimed in anyone of the claims 2 to 4, wherein the zinc-aluminium-magnesium coating layer shows good adherence to the steel without any peel off when tested by drop weight method.

6. A method of manufacturing hot dip coated steel as claimed in anyone of the claims 2 to 5 comprising the steps of:

providing hot dip bath composition comprising 4% to 6% by weight of aluminium; 4 to 6% by weight of magnesium; and 88% to 92% by weight of zinc as other major constituent and unavoidable impurities;

immersing in said hot dip bath composition annealed steel for coating,

transferring the thus coated steel from the hot dip bath and subjecting to gas purging and cooling to thereby obtain coating layer on the steel containing 45% to 55% by volume of MgZn2 phase and
the Mg/Al ratio is in the range of 0.85 to 0.95; and
coiling the coated steel.

7. The method of manufacturing hot dip coated steel as claimed in claim 6, wherein the said coated steel is transferred from hot dip bath to a non-oxidizing atmosphere for gas purge cooling, said non-oxidizing atmosphere having a dew point temperature in the range of -20°C to -30°C and non-oxidizing atmosphere is 5% hydrogen + 95% nitrogen.

8. The method of manufacturing hot dip coated steel as claimed in anyone of the claims 6 to 7, the temperature of hot dip bath is 440 - 460°C, the temperature of the steel is 470-490°C, the immersion time of the steel substrate in the coating solution is 2-4 seconds; the steel sheet is cooled at a speed of 10-15°C / s; the coating thickness is controlled using air knife with wiping speed of 50 mm/s.

Dated this the 29th day of October, 2020
Anjan Sen
Of Anjan sen & Associates
(Applicant’s Agent)
IN/PA-199

, Description:FORM 2
THE PATENT ACT 1970
(39 OF 1970)
&
The Patent Rules, 2003
COMPLETE SPECIFICATION
(See Section 10 and Rule 13)

1 TITLE OF THE INVENTION :
ZINC-ALUMINIUM-MAGNESIUM BASED HOT-DIP COATING COMPOSITION AND COATED STEEL HAVING EXCELLENT CORROSION RESISTANCE, ADHERENCE AND WELDABILITY OBTAINED THEREOF.



2 APPLICANT (S)

Name : JSW STEEL LIMITED.

Nationality : An Indian Company incorporated under the Companies Act, 1956.

Address : JSW CENTRE,
BANDRA KURLA COMPLEX,
BANDRA(EAST),
MUMBAI-400051,
MAHARASHTRA,INDIA.



3 PREAMBLE TO THE DESCRIPTION

COMPLETE

The following specification particularly describes the invention and the manner in which it is to be performed.

FIELD OF THE INVENTION
The present invention relates to the technical field of metallurgical corrosion prevention, in particular to a zinc-aluminium-magnesium coated steel with excellent corrosion resistance, good adherence and higher weldability and a method of manufacturing the same.

BACKGROUND OF THE INVENTION

Zinc-based coatings are widely used coated steel products due to the good combination of barrier and galvanic protection provided by Zn to the steel. Zn-based coatings are deposited using hot-dip galvanizing, electrode position and physical vapour deposition techniques. However, hot-dip galvanizing is primarily produced by steel industries for mass production of coated steel products. The quest to further improve the corrosion resistance has led to the development of different coating compositions other than conventional galvanized steel i.e. Zn-Al, Zn-Al-Mg etc. Other elements like Pb, Sb, Ti, Sn, La, Ce, Ni, Mn, Sr are also added in small quantities in Zn-bath to improve corrosion resistance and other properties. However, it will be difficult to maintain the bath with many elements and it can increase the cost of production. Zn-Al-Mg coatings have attracted the industries due to its highest corrosion resistance among all the hot-dip Zn-based coatings. Microstructure of Zn-Al-Mg coatings consists of Zn, Zn/MgZn2 binary eutectic, Zn/Al/MgZn2 ternary eutectic phases. Out of all the phases, MgZn2plays most important role in protection of steel against corrosion. It has been shown that Zn/MgZn2 undergo preferential dissolution in the initial stage of the corrosion resulting in release of Mg+2 cations. These cations form dense corrosion products causing improvement in corrosion resistance. Therefore, increasing the fraction of MgZn2 phase can improve the corrosion resistance to a large extent. It has been reported that higher content of Mg in Zn-Al-Mg coating increases the total fraction of MgZn2 phase in the coating.Since, Mg is lighter and highly reactive, Mg addition beyond a certain limit, causes excessive oxidation and dross generation. Therefore, controlling of Zn-bath becomes difficult. The dross can get adhered to the coated surface resulting in the defective coated sheet. Addition of Al can diminish the oxidation of bath containing Mg to some extent. In addition, Al provides barrier protection to the coating resulting in improved corrosion resistance. However, the higher addition of Al causes formation of brittle intermetallic layer which is not good for formability application. Therefore, it is clear that further optimization of Zn-Al-Mg coating composition by maintaining the ratio of Mg to Al can lead to the improvement in the properties.

In patent no. KR101665912B1, Zn-Al-Mg coating comprises of 2.0 to 3.0% of Al, 2.5 to 5.0% of Mg, balance Zn and inevitable impurities has been reported. The salt spray test (SST) life of the zinc aluminium magnesium coating has ? 1500 hours coated. The cooling rate after dipping has been used in the range of 3 to 8.6°C/s. However, slow cooling rate causes increase in the fraction of single phase Zn and thereby reduction in corrosion resistance. Moreover, slow cooling rate also leads to coarsening of grains resulting in lower corrosion resistance of Zn-Al-Mg coating as compared to that obtained with high cooling rate. Further, if Al content is less than Mg, the bath oxidation and dross generation become a serious problem in continuous galvanizing line.
.
Hence, there is need to develop a better corrosion resistant coated steel along with other properties like hardness, adherence and weldability. The better corrosion resistance coated steel sheets will provide higher service life in real life applications such as building materials, civil engineering, and vehicle bodies.

OBJECT OF THE INVENTION

It is an object of the present invention to overcome the disadvantages of the prior art and to provide an improved zinc-aluminium-magnesium hot dip coated steel and a method of hot dip coating ensuring excellent anticorrosive property for metallic surfaces comprising zinc-aluminium-magnesium coated steel, The corrosion resistance of the zinc-aluminium-magnesium coated steel, in particular, can be improved by this method, and excellent adhesion can be achieved.

The primary object of the present invention is to provide a hot-dip coating bath and a hot-dip zinc-aluminium-magnesium coated steel sheet produced thereby.

The main object of the present invention is to provide a hot dip coating bath composition with slightly higher aluminium than magnesium and remainder zinc as major content, in zinc-aluminium-magnesium coated steel production method and coated steel obtained thereof.

A further advantageous object of the present invention is directed to a zinc-aluminium-magnesium coated steel which has excellent corrosion resistance, good adhesion, increased hardness and higher weldability.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to provide hot dip coating composition for corrosion resistant coating of steel substrates comprising:
Zn-Al-Mg coating composition comprising:
4% to 6% by weight of aluminium;
4% to 6% by weight of magnesium; and
88% to 92% by weight of zinc as other major constituent,
wherein the hot-dip zinc-aluminium-magnesium coating layer contains 45% to 55% by volume of MgZn2 phase estimated using Rietveld refinement method of X-ray diffraction plots and with Mg/Al ratio is in the range of 0.85 to 0.95, suitable for generating corrosion resistant coating on steel substrates having corrosion resistance greater than 3000 hours in range of at least about 3000 to 4000 hours in salt spray test (SST) conforming to(ASTM B117).

A further aspect of the present invention is directed to a hot dip coated steel comprising a steel substrate and a zinc-aluminium-magnesium coating formed thereon, involving the zinc-aluminium-magnesium coating composition comprising the following constituent elements, based on the total weight of the zinc-aluminium-magnesium coating layer: 4% to 6% by weight of aluminium; 4% to 6% by weight of magnesium; and 88% to 92% by weight of zinc as other major constituent and unavoidable impurities. The hot-dip zinc-aluminium-magnesium coating layer contains 45% to 55% by volume of MgZn2 phase estimated using Rietveld refinement method of X-ray diffraction plot. The Mg/Al ratio is in the range of 0.85 to 0.95. The hot dip coated steel have -1000 to -1050 mV corrosion potential versus a saturated calomel electrode using potentio-dynamic polarization tests in a 3.5% NaCl solution (ASTM G102-89) and have corrosion resistance in the range of 3000 to 4000 hours in salt spray test (SST) conforming to (ASTM B117).

A still further aspect of the present invention is directed to the zinc-aluminium-magnesium coating having high hardness which is in the range of 140-175 HV.

A still further aspect of the present invention is directed to the zinc-aluminium-magnesium coating layer thickness which is in the range of 10 to 30 µm.

A further aspect of the present invention is directed to the hot dip coated steel having spot weldability in the current range of 6 to 12 kA without expulsion.

A still further aspect of the present invention is directed to the hot-dip zinc-aluminium-magnesium coating layer which shows good adherence without any peel off when tested by drop weight method.

Another aspect of the present invention is directed to a method of manufacturing hot dip coated steel. The method comprising following the steps of. Firstly, a hot dip bath is prepared comprising 4% to 6% by weight of aluminium; 4 to 6% by weight of magnesium; and 88% to 92% by weight of zinc as other major constituent and unavoidable impurities. The hot-dip zinc-aluminium-magnesium coating layer contains 45% to 55% by volume of MgZn2 phase. The Mg/Al ratio is in the range of 0.85 to 0.95. Secondly, a hot dip coated steel is prepared by firstly annealing the steel, secondly immersing in the hot dip bath for coating, thirdly adjusting the amount of coating layer on the steel using air knife and lastly cooling the steel.

Another aspect of the present invention is directed to method, wherein the steel substrate is transferred from hot dip bath to a non-oxidizing atmosphere for gas purge cooling. The non-oxidizing atmosphere has a dew point temperature in the range of -20°C to -30°C and non-oxidizing atmosphere is 5% hydrogen + 95% nitrogen.

Yet another aspect of the present invention is directed to method, wherein the temperature of hot dip bath is 440 - 460°C. The temperature of the steel substrate is 470-490°C. The immersion time of the steel substrate in the coating solution is 2-4 seconds. The steel substrate is cooled at a speed of 10-15°C / s. The coating thickness is controlled using air knife with wiping speed of 50 mm/s.

The above and other objects and advantages of the present invention are described hereunder in greater details with reference to following accompanying non limiting illustrative drawings.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

FIG. 1 Illustrates annealing cycles of steel for hot dip coating according to present invention.

Skilled artisans appreciate that elements in the figures are illustrated for simplicity and clarity to help to improve understanding of embodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

The accompanying figure together with the detailed description below forms a part of the specification and serves to further illustrate various embodiments and to explain the various principles and advantages all in accordance with the present invention.

The present invention is now discussed in more detail referring to the drawings that accompany the present application.

The present invention deals with the development of a novel hot dip bath composition comprising of Zn, Al and Mg with some impurities to be used as a hot dip galvanized coating on steel sheets.

A further aspect of the present invention is directed to a hot dip coated steel comprising a steel substrate and a zinc-aluminium-magnesium coating formed thereon, wherein the zinc-aluminium-magnesium coating layer is obtained involving said hot dip bath composition comprising the following constituent elements, based on the total weight of the zinc-aluminium-magnesium coating layer: 4% to 6% by weight of aluminium; 4% to 6% by weight of magnesium; and 88% to 92% zinc and other unavoidable impurities. The hot-dip zinc-aluminium-magnesium coating layer contains 45% to 55% by volume of MgZn2 phase estimated using Rietveld refinement method of X-ray diffraction plot. The Mg/Al ratio is in the range of 0.85 to 0.95. The hot dip coated steel have -1000 to -1050 mV corrosion potential versus a saturated calomel electrode using potentio-dynamic polarization plots in a 3.5% NaCl solution (ASTM G102-89).The zinc-aluminium-magnesium coating layer results in high corrosion life (appearance of 5% red rust) of at least about 3000 to 4000 hours during salt spray test (SST) in 5% NaCl solution. The zinc-aluminium-magnesium coating layer thickness which is in the range of 10 to 30 µm. The hot dip coated steel having spot weldability in the current range of 6 to 12 kA without expulsion. The zinc-aluminium-magnesium coating layer coating is easily spot weldable in a wide range of current (6kA-12kA) without expulsion making it suitable for many applications. The hot-dip zinc-aluminium-magnesium coating layer which shows good adherence without any peel off when tested by drop weight method. Adhesion ability of the zinc-aluminium-magnesium coating layer analysed using the drop weight method shows no peel-off indicating good adhesion.

The reason why the content of Mg in the coating bath is set to 4% to 6% by weight is that a coated steel sheet excellent in corrosion resistance and workability can be obtained in this range. It results in phase fraction of MgZn2 in the optimum range of 45-55% which helps in developing good corrosion, adhesion and weldability simultaneously.

The zinc-aluminium-magnesium coating layer may contain low amounts of impurities. The impurities are not greater than 0.15 wt. %.

A method of manufacturing hot dip coated steel is disclosed. The method comprising following the steps of. Firstly, a hot dip bath is prepared comprising 4% to 6% by weight of aluminium; 4 to 6% by weight of magnesium; and 88% to 92% by weight of zinc and other unavoidable impurities. The hot-dip zinc-aluminium-magnesium coating layer contains 45% to 55% by volume of MgZn2 phase. The Mg/Al ratio is in the range of 0.85 to 0.95. Secondly, a hot dip coated steel is prepared by firstly annealing the steel, secondly immersing in the hot dip bath for coating, thirdly transferring the steel from the hot dip bath for gas purging and cooling and fourthly adjusting the amount of coating layer on the steel and lastly coiling the steel. The steel is transferred from hot dip bath to a non-oxidizing atmosphere for gas purge cooling. The non-oxidizing atmosphere has a dew point temperature in the range of -20°C to -30°C and non-oxidizing atmosphere is 5% hydrogen + 95% nitrogen. The temperature of hot dip bath is 440 - 460°C. The temperature of the steel is 490°C. The immersion time of the steel substrate in the coating solution is 2-4 seconds. The steel sheet is cooled at a speed of 10-15°C / s. The coating thickness is controlled using air knife with wiping speed of 50 mm/s.

Since it has hot dip coated steel excellent corrosion resistance good adherence and higher weldability, it is useful for applications such as building materials, civil engineering, and vehicle bodies.

In this document, relational terms such as first and second, top and bottom, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms "comprises," "comprising," or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article or composition that comprises a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article or composition. An element proceeded by "comprises...a" does not, without more constraints, preclude the existence of additional identical elements in the process, method, article or composition that comprises the element.

In the present specification, when a part “includes” a certain component, it means that the component may further include other components, not to exclude other components, unless specifically stated otherwise.

Although not defined differently, all terms including technical terms and scientific terms used herein have the same meaning as those generally understood by those skilled in the art to which the present invention pertains. Commonly used dictionary-defined terms are additionally interpreted as having meanings consistent with related technical documents and currently disclosed contents, and are not interpreted as ideal or very formal meanings unless defined.

Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art to which the present invention pertains can easily practice. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein.

In addition, unless otherwise specified, % means weight%.

Advantages and features of the present invention, and methods for achieving them will be clarified with reference to embodiments described below in detail together with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but may be implemented in various different forms, and only the embodiments allow the disclosure of the present invention to be complete, and are conventional in the art to which the present invention pertains. It is provided to fully inform the knowledgeable person of the scope of the invention, and the invention is only defined by the scope of the claims. The same reference numerals refer to the same components throughout the specification.

Thus, in some embodiments, well-known techniques are not specifically described to avoid obscuring the present invention. Unless otherwise defined, all terms (including technical and scientific terms) used in the present specification may be used as meanings commonly understood by those skilled in the art to which the present invention pertains.

The present invention is described further hereinafter by reference to a series of examples.

Experiments that were actually performed are now described by way of following examples.

Experiment 1: Evaluation of zinc-aluminium-magnesium hot dip coated steel

The compositions combinations have Mg/Al ratios <0.85, 0.85-0.95 and > 0.95 were experimented to produce hot-dip coating. The hot-dip coating was carried out for the 3 selected compositions on a low carbon steel substrate. The galvanizing grade steel samples were cut from the cold-rolled coils. The corners of the rectangular samples (11 cm x 22 cm) were rounded for smooth entry in the bath. Before coating, the samples were cleaned with acetone and annealed in the atmosphere of 95% N2 - 5% H2 to avoid oxidation wherein the dew point was kept in the range of -30°C to -20°C. The annealing cycle used during experiments is shown in accompanying FIG. 1.The sample was annealed before coating by heating to 790°C at the rate of 5.5°C/s followed by cooling to 480°C at the cooling rate of 33°C/s. The sample was soaked at the temperature of 480°C for 10 s and subsequently dipped in to the bath for 2 s. After dipping, the sample was passed through the air-knife for controlling the coating thickness followed by cooling to room temperature at the rate of 12°C/s.

The process parameters used during the hot-dip coating is presented in Table 1.
Table 1: Process parameters used during hot-dip coating
Bath temperature Annealing medium Dew point Cooling Rate
460-465°C 95% N2 - 5% H2 -30°C to -20°C 12°C/s

The samples were cooled to 490 OC which is 25 OC higher than the bath temperature to compensate the heat loss taken place before strip entry. The bath temperature was kept at 460 OC with the dipping time of 2 seconds. The dipping and wiping speeds were kept 50 mm/s. The air knife was used to control the coating thickness. The bath composition used for hot dip coating was maintained at 4-6 wt.% Al, 4-6% Mg and remaining Zn. In order to understand the effect of Mg and Al content, Mg to Al ratio (Mg/Al) was kept in 3 ranges <0.85, 0.85-0.95 and > 0.95 and studied for corrosion, adhesion and weldability. The thickness of the coated surface was analysed using scanning electron microscope (FE-SEM, FEI)after polishing the cross-sectional surface. Coating thickness has been measured to be in the rage of 10-30 µm. Phase identification have been carried out with the help of X-ray diffraction analysis of the coated samples using Cu Ka1 (?=1.5406 Å) source in the 2?range of 10º -90º (Panalytical X-ray diffractometer).The phase fractions of different phases in all the experiments were estimated using Rietveld refinement method. The fraction of MgZn2 has been found to be in the range of 45%-55%, when Mg/Al is maintained between 0.85-0.95which is an ideal range.

The micro-hardness of the coating was estimated using Zwick/Roell ZHµ hardness tester at the load of 50 g with dwell time of 10s. The micro-hardness of coating having Mg/Al between 0.85-0.95 is in the range of 140-175 HV. The higher hardness will lead to better scratch resistance and easy handling of the coated sheet.

Salt spray test (SST) was performed to evaluate corrosion resistance according to ASTM B117 standard. The samples were exposed to a 5% NaCl (pH 6.5-7) fog in a salt spray chamber maintained at 35°C. The appearance of 5% red rust due to salt spray exposure is taken as the SST life of the sample. The coating having Mg/Al between 0.85-0.95 shown up to 3500 hours of SST life without any red rust. This is due to the combination of process parameters and coating composition which resulted in optimum fraction of MgZn2phase.

In order to study the weldability behaviour of Zn-Al-Mg coatings, spot welding in butt joint form was performed using spot welding machine (Amada Miyachi IS800, electrode ISO -5821- Type G) using electrode diameter of 6 mm. Electrode force of 6 kN was applied during welding. Weld time, hold time, squeeze time and clamping time were kept as 20, 15, 25 and 15 cycles, respectively. In order to determine the ease of spot welding, welding current was varied from 4 to 12 kA. The presented coating having Mg/Al between 0.85-0.95 Mg were easily welded using welding current in the wide range of 5-12 kA without expulsion.

In order to understand the possibility of developed coating in applications involving high formability requirements, adhesion ability was also measured using drop weight method and no peel-off or removal of coating material was observed after the test. It suggests that after coating if forming is done the coating will not undergo peeling-off.

The results of phase fractions, properties (weldability, adhesion) and operational issues are compared in Table 2.

Table 2. Comparison of properties with different ratios of Mg/Al
Mg/Al Phase fraction of MgZn2 SST Life (hours) Micro-hardness
(HV) Spot welding current (kA) Bath oxidation/Dross generation Adhesion
< 0.85 < 25% < 2000 <120 6-12 Low Good
0.85-0.95 > 45% >3500 >140 6-12 Low Good
>0.95 > 55% <2000 >140 7-12 High Reduces

Table 2 shows the effect of Mg/Al ratios on various properties of the coated steel. The phase fraction of MgZn2 reduces significantly (< 25%) when Mg/Al ratio is kept below 0.85 whereas with increase in Mg/Al ratio (0.85 -0.95), phase fraction of MgZn2 increases (45-55%). However, further increase of phase fraction of MgZn2 by increasing the Mg/Al ratio (>0.95) leads to crack formation. Since, MgZn2 is a hard phase, its higher phase fraction has also resulted in increase of the hardness of coated surface which can improve its scratch resistance and benefit in easy handling. The salt spray test (SST) life of coating with Mg/Al ratio (0.85-0.95) is greater than 3000 hours. However, SST life decreases (<2000 hours) when the Mg/Al ratio is kept below 0.85 due to lower fraction of MgZn2 whereas the decrease in SST life in case of coating with high Mg/Al ratio >0.95 is due to the crack formation. Coated samples can be spot welded in the wide current range of 6-12 kA provided the Mg/Al ratio is below 0.95. This can enable the present coating to be used in variety of applications involving low current requirement. However, the welding current range reduces when Mg/Al ratio is higher than 0.95.