Description:

FIELD OF INVENTION
The present invention relates to high strength cold rolled galvanized steel coil with high zinc coating and a method of manufacture thereof.

BACKGROUND OF INVENTION
Cold rolled Hot-dip galvanized steel coil is produced by a method in which a cold rolled coil is subjected to recrystallization annealing in an annealing furnace and a hot-dip galvanizing treatment in a continuous hot-dip galvanizing line.
The substrate/ base material of the galvanised product is a mild carbon steel with alloying elements such as silicon (Si), aluminium (Al) and manganese (Mn) mainly present in it. Micro-alloying element like Nb and Ti is added primarily for increasing the base material strength by contributing to fine carbide precipitation and grain-size refinement. Manganese in steel helps in solid-solution strengthening. Silicon is an effective strengthening alloying element in steel. Silicon concentration in steel also plays a major role in the hot dip galvanizing process. Si increases the wettability of molten Zn with steel which leads to uncontrolled reaction between iron and zinc leading to very patchy and dull coatings. Steels containing small amounts of silicon (<0.03 wt%) have a compact and continuous zinc coating which is composed of Γ (gamma, Fe3Zn10), δ (delta, FeZn7), ζ (zeta, FeZn13) and eta (, Zn(Fe)) layers after long duration hot dip galvanizing (or post dip annealing). However, during hot dip galvanizing for shorter durations such as in commercial continuous galvanizing lines, only eta (, solid solution of Fe in Zn with an iron solubility of 0.03 wt%) and Fe2Al5Znx inhibition layer are formed. Steels containing 0.15–0.25 wt% Si also produce a compact and coherent coating. However, Si has negative impact on the quality of the zinc coating when its content is between 0.03 to 0.14 wt% leading to coating discontinuities, non-uniform higher thickness, lack of lustre and impaired adhesion with the base steel. Hence, Si in steel is typically maintained either below 0.03 wt% or beyond 0.14 wt% for better galvanizability of steel strip and good quality industrial coatings.
During cold rolling of steel coil, elongated grain with high internal stress developed. To relieve internal stress and develop an equalized grain, cold rolled steel coil further annealing above recrystallization temperature. The grain size after annealing depends on the recrystallization temperature and time. The strength and ductility of steel coil primarily depends on alloy chemistry and annealing time & temperature.
US20140287263A1 relates to a high-strength hot-dip galvanized steel sheet and the like, which are capable of obtaining high ductility while ensuring high strength with maximum tensile strength of 900 MPa or more. Chemical composition mass %,1)C, 0.075 to 0.400%, Si: 0.01 to 2.00%, Mn: 0.80 to 3.50%, P: 0.0001 to 0.100%, S: 0.0001 to 0.0100%, Al: 0.001 to 2.00%, N, 0.0001 to 0.0100%, and O: 0.0001 to 0.0100% with a balance being constituted of iron and inevitable impurities, 2) one or more of Ti: 0.001 to 0.150%, Nb: 0.001 to 0.100%, and V: 0.001 to 0.300% (Micro-alloying) 3) one or more of Cr: 0.01 to 2.00%, Ni: 0.01 to 2.00%, Cu: 0.01 to 2.00%, Mo: 0.01 to 2.00%, B: 0.0001 to 0.0100%, and W: 0.01 to 2.00% (Other alloying). Thus, the chemical composition comprises micro-alloying and other alloying elements. Furthermore, the annealing temperature after cold rolling is 750oC or more.
US10704132B2 relates to a high-strength hot-dip galvanized steel including a hot-dip galvanized plating layer on a steel sheet base material whose tensile strength is 590 MPa or more wherein the plating layer includes projecting alloy layers being in contact with the steel sheet base material, a number density of the projecting alloy layers is 4 pieces/mm or more. A steel sheet base material containing, 1)C: 0.05 to 0.4 mass %,Si: 0.4 to 3.0 mass %,Mn: 1.0 to 4.0 mass %,P: 0.0001 to 0.1 mass %,S: 0.0001 to 0.01 mass %,Al: 0.005 to 0.1 mass %,N: 0.0005 to 0.01 mass %, and O: 0.0001 to 0.01 mass %, with a balance made up of Fe and inevitable impurities, and having a tensile strength of 590 MPa or more, 2)steel sheet base material further contains one kind or two kinds of: Ti: 0.001 to 0.15 mass %, and Nb: 0.001 to 0.10 mass % (Micro-alloying) 3) steel sheet base material further contains one kind or two kinds or more of: Mo: 0.01 to 2.0 mass %, Cr: 0.01 to 2.0 mass %, Ni: 0.01 to 2.0 mass %, Cu: 0.01 to 2.0 mass %, and B: 0.0001 to 0.01 mass % (Other alloying). Thus, the chemical composition comprises micro-alloying and other alloying elements.
US20110253263A1 relates to a hot dip galvanized steel sheet includes a zinc plating layer which is disposed on a steel sheet containing 0.01% to 0.15% C, 0.001% to 2.0% Si, 0.1% to 3.0% Mn, 0.001% to 1.0% Al, 0.005% to 0.060% P, and 0.01% or less S on a mass basis, the remainder being Fe and unavoidable impurities, and which has a mass per unit area 20 g/m2 to 120 g/m2.
CN108914014A relates to a cold-rolled high-strength hot-dip galvanizing sheet steel and preparation method, which is by weight percentage：C：0.12-0.16%, Mn：0.60-0.90%, Si≤0.05%, P≤0.02%, S≤0.01%, Al：0.02-0.05%, remaining is Fe and other inevitable impurity. The invention uses low manganese ingredient design of low-carbon, steel plate yield strength reaches 500-650MPa after annealing and hot galvanizing. The annealing temperature after cold rolling is 560-600oC. The elongation percentage is only 3-15% and the coating thickness is low i.e. 40-150 g/m2.
JP5504737B2 relates to a high-strength hot-dip galvanized steel strip that is suitable mainly for structural members of automobiles, and in particular, has a high tensile strength TS of 780 MPa or more and having a composition of In mass%, C: 0.05-0.2%, Si: 0.5-2.5%, Mn: 1.5-3.0%, P: 0.001-0.05%, S: 0.0001-0.01%, Al: 0.001-0.1%, N: 0.0005- 0.01% is contained, the remainder has a composition composed of Fe and inevitable impurities, further comprising Cr: 0.01 to 1.5% Ti: 0.0005 to 0.1%, B: 0.0003 to 0.003% Nb: 0.0005 to 0.05% Mo: 0.01 to 1.0%, Ni: 0.01 to 2.0%, Cu: 0.01 to 2.0% (Micro-alloying & other alloying). Thus, the chemical composition comprises micro-alloying and other alloying elements. Furthermore, the annealing temperature after cold rolling is 700oC and above.
There still exists a need to develop a high strength cold rolled galvanized steel coil with high zinc coating having a high tensile strength, high coating thickness, no usage of micro-alloying and other elements in the steel and having favorable mechanical properties and long durability. The present inventors have surprisingly developed an efficient high strength cold rolled galvanized steel coil which ameliorates the aforesaid shortcomings of the prior art.

OBJECTS OF THE INVENTION
It is an object of the present invention to overcome the shortcomings of the prior art.
It is an object of the present invention to provide a high strength hot dip galvanized steel coil.
It is another object of the present invention to provide a high strength hot dip galvanized steel coil without using any micro-alloying element.
It is another object of the present invention to provide a high strength hot dip galvanized steel coil having a high tensile strength, high coating thickness, and having favorable mechanical properties and long durability.
It is yet another object of the present invention to provide a method of preparing a high strength hot dip galvanized steel coil.

SUMMARY OF THE INVENTION
According to an aspect of the present invention there is provided a high strength hot dip galvanized steel coil.
According to another aspect of the present invention there is provided a method for preparing a high strength hot dip galvanized steel coil.

BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1: Overall Process Line diagram
Figure 2: (a) Optical microstructure of Base material after hot dip galvanizing
(b) Zn layer thickness- Optical Microscope
(c) Secondary electron image showing zinc coating-steel interface
Figure 3: Anodic polarization plots for high strength galvanized coils showing active-passive corrosion behavior in 3.5% NaCl solution

Figure 4: Photographs showing the appearance of zinc coated test coupons after 100 hours of salt fog exposure showing no white rust formation.

DETAILED DESCRIPTION OF THE INVENTION
The following description with reference to the accompanying drawings is provided to assist in a comprehensive understanding of exemplary embodiments of the invention. It includes various specific details to assist in that understanding but these are to be regarded as merely exemplary.
Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope of the invention. In addition, descriptions of well-known functions and constructions are omitted for clarity and conciseness.
The terms and words used in the following description and claims are not limited to the bibliographical meanings, but, are merely used by the inventor to enable a clear and consistent understanding of the invention. Accordingly, it should be apparent to those skilled in the art that the following description of exemplary embodiments of the present invention are provided for illustration purpose only and not for the purpose of limiting the scope of the invention as defined by the appended claims and their equivalents.
It is to be understood that the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise.
Features that are described and/or illustrated with respect to one embodiment may be used in the same way or in a similar way in one or more other embodiments and/or in combination with or instead of the features of the other embodiments.
It should be emphasized that the term “comprises/comprising” when used in this specification is taken to specify the presence of stated features, steps or components but does not preclude the presence or addition of one or more other features, steps, components or groups thereof.
The present invention relates to the development of a high strength hot dip galvanized steel coil and a method of manufacturing thereof.
The present invention relates to development of a High strength cold rolled galvanized steel coil (GP350) having yield strength (YS) 350-500 MPa, Upper tensile strength (UTS) 420-570 MPa and High elongation 16-40%. The developed steel has Zn coating between 275 GSM to 600 GSM suitable for use in grain silos, solar panel etc. The developed steels grade possesses high strength primarily by substitutional and interstitial strengthening and grain-size refinement by optimum annealing.
The processing method including steel making, hot rolling, cold rolling and hot-dip galvanizing line (HDGL) annealing & operational parameters have been determined appropriately so as to achieve the desired strength and coating thickness.

The present invention provides a high strength cold rolled galvanized steel coil comprising a Zinc (Zn) coating disposed on a steel base having a composition comprising:
Carbon (C) in the range of 0.050-0.10 wt.%;
Manganese (Mn) in the range of 0.20-1.00 wt.%;
Phosphorous (P) in the range of 0.001- 0.030 wt.%;
Sulphur(S) in the range of 0.001-0.010 wt.%;
Silicon (Si) in the range of 0.01-0.200 wt.%;
Aluminium (Al) in the range of 0.01 to 0.05 wt.%; and
balance being Iron (Fe).

In an embodiment, the said Zn coating has a thickness ranging from 275 GSM to 600 GSM on steel base material thickness of 0.35-2.0 mm.
In an embodiment, the said steel composition does not have any micro-alloying element.
In an embodiment, the said steel has yield strength of 350-500 MPa; tensile strength of 420-570 MPa and total elongation of 16-40%.

The present invention also provides a method for manufacturing a high strength cold rolled galvanized steel coil comprising the steps of:
(i) Producing a base steel by processing heat from basic oxygen furnace (BOF), Ladle refining without vacuum degassing and continuously cast in slabs;
Wherein the said base steel has a composition comprising: Carbon (C) in the range of 0.050-0.10 wt.%; Manganese (Mn) in the range of 0.20-1.00 wt.%, Phosphorous(P) in the range of 0.001-0.03 wt.%; Sulphur(S) in the range of 0.001-0.010 wt.%; Silicon(Si) in the range of 0.01-0.200 wt.%; Aluminium(Al) in the range of 0.01 to 0.05 wt.%; and balance being Iron (Fe);
(ii) Subjecting the cast slabs of step (i) to hot rolling in which rolling completion temperature is 850 to 950° C; thereafter rapidly cooling to 600 to 680° C, and coiling on a coil;
(iii) Acid Pickling and Cold rolling the said hot rolled steel coils of step (ii) with cold reduction of 60% or more;
(iv) Annealing of cold rolled steel coil of step (iii) by heating at a temperature of 600-700oC and then rapid cooling at a temperature range of 450-465oC;
(v) A plating step of performing hot-dip galvanizing on surface of the annealed steel coil of step (iv) to form a plating layer by immersing the steel coil in a galvanizing bath under the conditions of Direct Fire Furnace (DFF) temperature from 600-700oC, Radiant Tube Heating (RTH) temperature from 600-700oC, Radiant Tube Soaking (RTS) temperature from 600-700oC and plating bath temperature: 450-465oC, and effective Zn amount in the plating bath: 275-600 GSM to obtain the high strength cold rolled galvanized steel coil.
The High strength cold rolled galvanized steel of the present invention is denoted as “GP350” grade steel.
Advantages of the invention:
- Achieved steel material shows consistent and favorable mechanical properties suitable for construction sector.
- Lean chemistry w.r.t to C & Mn i.e C< 0.10 & Mn < 1.0%
- Annealing temperature has been kept 600-700oC so, both strength and elongation achieved in combination.
- Achieves a high elongation i.e. minimum 16 % with Yield strength of 350 MPa minimum & Upper tensile strength of 420MPa minimum.
- The material achieved high yield strength without exceeding the minimum limit of elongation.
- No usage of micro-alloying and other elements in the steel resulted into excellent balance of strength and ductility required.
- High Zinc coating 275-600 GSM (i.e. 275 g/m2 to 600 g/m2) for long durability in adverse environment condition.
- Material of high strength having yield strength 350-500 MPa, tensile strength 420-570 MPa, Elongation 16-40% and coating thickness 275-600 GSM
-Material exhibits Corrosion properties:
• Exhibit active-passive corrosion behaviour and revealed very low corrosion rates of 0.3 - 4.0 mpy indicating their superior corrosion resistance in 3.5% NaCl solution using Tafel extrapolation technique the aqueous chloride test medium.
• No white rust formation even after 100-200 hour of exposure in accelerated 5% NaCl salt spray environment.
EXAMPLES:
The following examples are meant to illustrate the present invention. The examples are presented to exemplify the invention and are not to be considered as limiting the scope of the invention.
EXAMPLE 1:
The line diagram for production of hot dip galvanized steel according to present invention is shown in Figure-1.
Table-1: Heat Chemistry of the inventive GP350 grade steels (in wt%)
Heat No. C Mn P S Si Al
2156926 0.082 0.30 0.026 0.002 0.039 0.031
2156927 0.070 0.33 0.020 0.003 0.028 0.028
2250432 0.069 0.31 0.01 0.008 0.036 0.029
2250446 0.064 0.31 0.016 0.006 0.030 0.040
2255242 0.067 0.83 0.020 0.005 0.158 0.028
2255243 0.073 0.91 0.022 0.006 0.197 0.029
2355419 0.08 0.8 0.025 0.007 0.167 0.033

Process route for development of GP350+ 275-600 GSM are:
 Steel Process Route comprising the steps of:
(i) Producing a base steel by processing heat from basic oxygen furnace (BOF), Ladle refining without vacuum degassing and continuously cast in slabs;
(ii) Subjecting the cast slabs of step (i) to hot rolling in which rolling completion temperature is 850 to 950° C; thereafter rapidly cooling to 600 to 680° C, and coiling on a coil;
(iii) Acid Pickling and Cold rolling the said hot rolled steel coils of step (ii) with cold reduction of 60% or more;
(iv) Annealing of cold rolled steel coil of step (iii) by heating at a temperature of 600-700oC and then rapid cooling at a temperature of 450-465oC;
(v) A plating step of performing hot-dip galvanizing on surface of the annealed steel coil of step (iv) to form a plating layer by immersing the steel coil in a galvanizing bath under the conditions of plating bath temperature: 450-465oC, and effective Zn amount in the plating bath: 275-600 GSM to obtain the high strength cold rolled galvanized steel coil.

Process parameters:
 Slab thickness: 220 mm
 Hot rolled coil thickness: 3.00-4.00 mm
 Finish rolling Temperature at 850- 900oC
 Coiling Temperature at 600- 680oC
 Cold rolling thickness 0.50-2.00 mm
 hot-dip galvanizing line (HDGL) Line speed for different coil thickness

Table 2: HDGL Line speed employed for different coil thickness
Thickness of coil mm 0.77 0.95 1.15 1.43 1.55 1.95
Required maximum residence time sec 109 135 163 203 220 276
Line speed mpm 100 80 65 55 50 40
 Hot dip Galvanizing Furnace Strip Temperature:
 Direct Fire Furnace (DFF) : 600-700oC
 Radiant Tube Heating (RTH): 600-700oC
 Radiant Tube Soaking (RTS):600-700oC
 Rapid Jet Cooling (RJC): 450-465oC
 Galvanizing bath Temperature: 450-465oC
 The horizontal distance between DAK lip and the moving strip :20-40 mm
Figure 2 illustrates (a) Optical microstructure of Base material after hot dip galvanizing (b) Zn layer thickness- Optical Microscope (c) Secondary electron image showing zinc coating-steel interface
Comparative examples:
SN HDGL Coil ID Thickness T, mm Annealing Temperature RJC Mechanical Properties Coating GSM
DFF RTH RTS YS MPa UTS MPa %El
1 CGI03339 1.45 540* 580* 620 460 472 523 12* 463
2 CGI03447 0.74 640 720* 720* 460 305* 401* 32 458
3 CGI03460 1.44 534* 580* 580* 460 564 619 6* 465
4 CGI07028/29 1.52 580* 620 620 460 573 601 14* 462
5 CGI07030/31 1.52 580* 620 620 460 561 592 12* 458
Table 3
* Parameter is deviated from designed parameters.
Abbreviations: Direct Fire Furnace (DFF), Radiant Tube Heating (RTH), Radiant Tube Soaking (RTS); Rapid Jet Cooling (RJC); Elongation % (EL%), yield strength (YS), Upper tensile strength (UTS)
As shown in above Table 3, varying temperatures below or beyond the range of 600-700oC results in steel material having low elongation of <16% [Samples 1, 3, 4, 5] or having lower yield strength and lower upper tensile strength (Sample 2).

EXAMPLE 2:

Characterization of the developed steel:
The developed steels grade possesses high strength and high coating thickness (Table-4)
Table-4: Tensile properties of GP350 + 275-600 GSM
SN HDGL Coil ID Heat No YS MPa UTS MPa %El Coating GSM
1. CGI03333 2156927 371 437 28% 357
2. CGI03335 2156927 365 423 36% 455
3. CGI03376 2156926 385 441 24% 495
4. CGI03448 2156927 380 468 24% 456
5. CGI03456 2156927 372 430 20% 469
6. CGI06971 2250432 385 447 24% 604
7. CGI06982 2250446 368 462 24% 666
8. CGI08480 2255242 401 473 32% 602
9. CGI08518 2255242 381 447 28% 639
10. CGI08519 2255243 425 491 24% 637
11. CGI08521 2255243 406 492 24% 626
12. CGI08522 2255242 444 504 20% 668
13. CGI08525 2255243 369 441 32% 663
14. CGI08528 2255243 496 544 16% 679
15. FGI04675 2355419 398 485 24% 277
16. FGI04676 2355419 389 433 28% 296

Corrosion rates of high strength galvanized coils was determined electrochemically through anodic polarization in 3.5% NaCl solution using Tafel extrapolation technique are shown in Table 5. The coil samples were found to exhibit an active-passive corrosion behaviour as depicted in Figure 3 and revealed very low corrosion rates of 0.44-3.29 mpy indicating their superior corrosion resistance in the aqueous chloride test medium.

Figure 4 shows the appearance of zinc coated test coupons sampled from the coils after 100 hours of salt fog exposure in the highly aggressive 5% NaCl solution. The chromate passivated zinc coated test coupons showed no white rust formation after the 100 hour exposure period indicating their superior corrosion resistance in saline environment.
Table 5: Corrosion rates of high strength galvanized coils determined electro-chemically through anodic polarization testing in 3.5% NaCl solution using Tafel extrapolation technique
Coil thickness (mm) Equilibrium Corrosion Potential, Ecorr (mV vs. Ag/AgCl (KCl satd.)) Cathodic Tafel Constant, bc (mV/ decade) Anodic Tafel Constant, ba (mV/ decade) Equilibrium Corrosion Current, Icorr (mA/cm2) Corrosion Rate, CR (mpy)
0.80 -88.805 126.098 231.763 1.755 1.01
1.00 -61.431 89.488 47.62 5.70 3.29
1.20 -4.128 81.792 173.189 4.195 2.42
1.50 -48.253 141.21 55.666 0.759 0.44

It is to be understood that the present invention is susceptible to modifications, changes and adaptations by those skilled in the art. Such modifications, changes, adaptations are intended to be within the scope of the present invention.
, Claims:
1. A high strength cold rolled galvanized steel coil comprising a Zinc (Zn) coating disposed on a steel base having a composition comprising:
Carbon (C) in the range of 0.050-0.10 wt.%;
Manganese (Mn) in the range of 0.20-1.00 wt.%;
Phosphorous (P) in the range of 0.001-0.03 wt.%;
Sulphur(S) in the range of 0.001-0.010 wt.%;
Silicon (Si) in the range of 0.01-0.200 wt.%;
Aluminium (Al) in the range of 0.01 to 0.05 wt.%; and
balance being Iron (Fe).

2. The high strength cold rolled galvanized steel coil as claimed in claim 1, wherein the said Zn coating has a thickness ranging from 275 GSM to 600 GSM on steel base material thickness of 0.35-2.0 mm.

3. The high strength cold rolled galvanized steel coil as claimed in claim 1; wherein the said steel composition does not have any micro-alloying element.

4. The high strength cold rolled galvanized steel coil as claimed in any one of the preceding claims, wherein the said steel has yield strength of 350-500 MPa; tensile strength of 420-570 MPa and total elongation of 16-40%.

5. A method for manufacturing a high strength cold rolled galvanized steel coil comprising the steps of:
(i) Producing a base steel by processing heat from basic oxygen furnace (BOF), Ladle refining without vacuum degassing and continuously cast in slabs;
Wherein the said base steel has a composition comprising: Carbon (C) in the range of 0.050-0.10 wt.%; Manganese (Mn) in the range of 0.20-1.00 wt.%, Phosphorous(P) in the range of 0.001-0.03 wt.%; Sulphur(S) in the range of 0.001-0.010 wt.%; Silicon(Si) in the range of 0.01-0.200 wt.%; Aluminium(Al) in the range of 0.01 to 0.05 wt.%; and balance being Iron (Fe);
(ii) Subjecting the cast slabs of step (i) to hot rolling in which rolling completion temperature is 850 to 950° C; thereafter rapidly cooling to 600 to 680° C, and coiling on a coil;
(iii) Acid Pickling and Cold rolling the said hot rolled steel coils of step (ii) with cold reduction of 60% or more;
(iv) Annealing of cold rolled steel coil of step (iii) by heating at a temperature of 600-700oC and then rapid cooling at a temperature of 450-465oC;
(v) A plating step of performing hot-dip galvanizing on surface of the annealed steel coil of step (iv) to form a plating layer by immersing the steel coil in a galvanizing bath under the conditions of Direct Fire Furnace (DFF) temperature from 600-700oC, Radiant Tube Heating (RTH) temperature from 600-700oC, Radiant Tube Soaking (RTS) temperature from 600-700oC and plating bath temperature: 450-465oC, and effective Zn amount in the plating bath: 275-600 GSM to obtain the high strength cold rolled galvanized steel coil.