Claims:We Claim:
1. High strength cold rolled galvanized steel having steel composition comprising wt.%:
C 0.20 max, Mn 1.35 max, Si 0.50 max, S 0.035 maximum, (Nb+V+Ti) 0.10 max and rest Fe having YS > 410 MPa, UTS > 480MPa, %El >10 in 50 mm gauge length.

2. High strength cold rolled galvanized steelas claimed in claim 1 comprising structural steel having :
C 0.20 max; preferably 0.055-0.065
Mn 1.35 max; preferably 1.00-1.10
Si 0.50 max; preferably 0.15-0.25
S up to 0.035 max; preferably 0.003-0.006
Nb+V+Ti 0.1 max; preferably Nb 0.03-0.035
Al 0.020-0.070; preferably 0.025-0.040
N up to 120 ppm max; preferably 60-80 ppm and balance having
said yield strength > 420 MPa, preferably in the range of 620-650 MPa and UTS > 490 MPa, preferably in the range of 650-680 MPa and %El is >10% preferably in the range of 13-15%

3. High strength cold rolled galvanized steel as claimed in anyone of claims 1 to 2 having said high strength yield strength in the range of 620-650 MPa and UTS 650-680 MPa.

4. High strength cold rolled galvanized steel as claimed in anyone of claims 1 to 3 having said % elongation 13-15%.

5. High strength cold rolled galvanized steel as claimed in anyone of claims 1 to 4 having continuous hot dip galvanized said zinc coating on cold rolled sheet as per 450 GSM standard preferably 510-520 grams/m2

6. A process for producing high strength cold rolled galvanized structural steel comprising:
i)involving selective hot metal steel composition followed by refining to have in wt.%:
C 0.20 max, Mn 1.35 max, Si 0.50 max, S 0.035 maximum, (Nb+V+Ti) 0.10 max and rest Fe ;
ii) casting involving thin slab caster ;
iii) hot rolling and cooling such as to achieve said microstructure of acicular ferrite and quasi polygonal ferrite providing for YS > 450 MPa, UTS > 520MPa and %El > 25for hot rolled coil;
iii) slitting the hot rolled sheet from the edge to remove the edge defects and to get the desired width followed by subjecting to pickling to clean the surface of the steel;
iv) cold rolling of HR steel (3.0-4.5 mm)to reduce the thickness to the level of 1.5-2.5 mm preferably involving approximately 35 to 60 percent reduction.
v) the thus hardened sheet coming from cold roll unit is next subjected to step of galvanising with or without further passivation to protect the zinc layer for added corrosion resistance.

7. A process as claimed in claim 6 comprising:
i)involving selective hot metal steel composition followed by refining to have in wt.%:
C 0.20 max, Mn 1.35 max, Si 0.50 max, S 0.035 maximum, (Nb+V+Ti) 0.10 max and rest Fe
ii) casting involving thin slab caster
iii) hot rolling and cooling such as to achieve said microstructure of acicular ferrite and quasi polygonal ferrite providing for YS > 450 MPa, UTS > 520MPa and %El > 25for hot rolled coil.
iii) slitting the hot rolled sheet from the edge to remove the edge defects and to get the desired width in the range of 5 to 15 mm lesser than the hot rolled coil and thereafter pickling the coil preferably involving HCl to remove the oxide layer and dent to clean the surface of the steel
iv) cold rolling of the steel involving 1 stand 4 high reversible mill to reduce the thickness to the desired level with preferably approximately 38 percent reduction.
v) the fully hardened sheet coming from cold roll unit subjected to step of galvanizing including passed through continuous galvanizing line where the sheet is passed through 4 furnaces namely NOF, RTF, SF and JCF and thereafter the sheet is hot dip galvanized in the zinc pot and preferably the coating was carried out as per GSM 450 grade and desired coating thickness in the level of 450 to 520grams/m2 maintained by air knife pressure.
vi) passivating the thus galvanized sheet involving Chromic acid thereby providing Chromium layer to protect the zinc layer and strengthen the corrosion resistance of the steel.

8. A process as claimed in anyone of claim 6 or 7 comprising:
step of carrying out said steel casting at casting speed of 5-7 m/min;
homogenizing in the temperature range of 1100 to 1700 0C;
descaling before hot rolling;
carrying out final rolling pass below no recrystallization temperature;
involving ultra-fast cooling after finish rolling in combination with standard laminar cooling;
involving coiling temperature in the range of 500-600 0C under slow cooling in coiling.
slitting the hot rolled sheet to remove the edge defects;
pickling of the hot rolled sheet to clean the surface of the sheet;
carrying out cold rolling below recrystallization temperature;
galvanizing of sheet through continuous galvanizing line.

9. A process as claimed in anyone of claims 6 to 8 comprising:
I. chemical composition
C 0.20 max; preferably 0.055-0.065
Mn 1.35 max; preferably 1.00-1.10
Si 0.50 max; preferably 0.15-0.25

S up to 0.035 max; preferably 0.003-0.006
Nb+V+Ti 0.1 max; preferablyNb 0.03-0.035
Al 0.020-0.070; preferably 0.025-0.040
N up to 120 ppm max; preferably 60-80 ppm
slitting the hot rolled sheet to remove the edge defects;
pickling of the hot rolled sheet to clean the surface of the sheet;
carrying out cold rolling below recrystallization temperature;
galvanizing of sheet through continuous galvanizing line.

10. A process as claimed in anyone of claims 6 to 9 comprising:
I. Preparing the liquid steel in CONARC furnace followed by ladle refining furnace
II. Casting the steel slabs in a thin slab caster with slab thickness 55-65 mm;
III. Homogenizing the cast slab in a tunnel furnace at temperature 1080-1120 DegC;
IV. Descaling the slab in a high pressure descaler followed by low pressure descaler to remove the scale formed in tunnel furnace
V. Rolling the slab in a 6 stand tandem rolling mill to the desire final thickness and finish rolling temperature in the range of 870-900 0C;
VI. Subjecting the hot rolled coils to ultra-fast cooling such as to ensure high nucleation rate and fine grain structure followed by laminar cooling for facilitating coiling.
VII. Slitting of the HR coil is done to remove the edge defects and to get required width.
VIII. Pickling of the coil is done to remove the oxide layer from Hot rolled sheet surface by using HCl acid
IX. The hot rolled sheet is cold rolled by mechanically deforming 30-40% below recrystallization temperature to reduce the thickness and increase the hardness of the sheet.
X. Further it is passed through the continuous galvanizing line to do zinc coating on the surface of the cold rolled sheet as per 450 GSM standard.
11. A process as claimed in anyone of claims 6 to 10 wherein said hot metal is treated in a CONARC furnace and further in a ladle refining furnace and thereafter the liquid steel is cast in thin slab caster with casting speed of 4-6 m/min, followed by charging in tunnel furnace and rolling to sheet/strip at FRT 870 to 900 0C and at coiling temperature 500 – 580 0C to obtain desired high strength.
12. A process as claimed in anyone of claims 6 to 11 wherein hot rolled steel manufacture is controlled to produce high strength steel sheet/strip of 4 mm thickness.
13. A process as claimed in anyone of claims 6 to 12 wherein hot rolled process carried out under controlled operating conditions comprising
(i) Casting Speed 4-6 m/min,
(ii) Slab Thickness 55-65 mm,
(iii) Slab Cutting Temp 980-1030 Deg C,
(iv) Homogenization Temp (Tunnel Furnace) 1080-1120 Deg C,
(v) Homogenization Time 8-15 min,
(vi) Finish Rolling Temp 870-900 Deg C,
(vii) Stand wise rolling reduction/time/temp comprising

Parameters F1 F2 F3 F4 F5 F6
Relative Reduction 40-45 45-50 35-40 30-35 25-30 20-25
Interstand time 5-15 4-15 3-10 3-10 2-10 2-10
Stand Entry Temp 1010-990 970-950 940-925 920-910 910-900 900-870

(viii) Coiling Temp 500-580 Deg C,
(ix) Ultrafast Cooling Rate 60-80 Deg C/sec,
(x) Lamellar Cooling rate 10-25 Deg C/sec.

14. A process as claimed in anyone of claims 6 to 13 whereinthe hot rolled coil is pickled using HCl to remove the oxide layer and stains to clean the surface of the steel , by passing the coil through 4 acid tanks with different concentration of HCl acid and finally it is rinsed in water in the 5th tank to prevent any carryover acid on the surface of steel, involving Pickling Details as:
Tank HCl concentration
Tank 1 3-5%
Tank 2 5-9%
Tank 3 9-13%
Tank 4 13-18%

15. A process as claimed in anyone of claims 6 to 14 whereinthe hot rolled pickled coil is further cold rolled using 1 stand 4 high reversible rolling where 7 passes were given to give thickness reduction of 38 % following,
Cold Rolling Detail:
Coil No Size (mm) HR Coil Thickness (mm) Thickness after cold rolling (mm) No. of pass at cold Rolling Reduction at cold rolling (%)
A 2.50*1220 4.00 2.466 7 38.4
B 2.50*1220 4.00 2.466 7 38.4
C 2.50*1220 4.00 2.466 7 38.4

16. A process as claimed in anyone of claims 6 to 15 wherein the cold rolled sheet is passed through the continuous galvanizing line (CGL) comprising:
I. Annealing including the steel strip being passed through 4 types of furnace wherein the strip first passes through NOF furnace where it is heated to 600 C or more to remove emulsion, then the strip is passed through Radiant Tube Furnace (RTF) where the strip is heated radially through tubes to 750C or more, further it is passed through Soaking Furnace (SF) for uniform temperature throughout the strip wherein the temperature maintained is maintained 750 and above and finally the strip is passed through Jet Cooling Furnace (JCF) where the furnace temperature maintained through air blow is 500 C and above;
II. hot dip galvanizing in Zn pot wherein hot dip galvanizing of strip is done to coat a zinc layer which increases the corrosion resistance of the strip, the air knife pressure is used to maintain proper coating of given Zn layer thickness preferably GSM 450 grade, wherein the air knife pressure maintained is around 0.210 Kg/cm2;
III. carrying out stretch levelling to remove the stretcher strain/luder bands wherein the Galvanised steel stretch levelling is done to get desired shape by applying tensile stress & bending stress;
IV. subjecting to passivation wherein Chemical treatment of the Galvanised surface with a superficial layer of Cr is carried out to further protect the zinc coating with
Galvanizing Details maintained as hereunder:
Sample Name A B C
Size 2.50*1220 2.50*1220 2.50*1220
Line Speed (meter/minutes) 32 32.1 32.1
Production output Mt/hour 45.96 46.11 46.11
Strip temperature at Exit in degree centigrade
NOF 681 679 679
RTF 796 821 821
SF 756 758 758
JCB 539 540 540
Air knife pressure Kg/cm2 0.216 0.216 0.216

Dated this the 26th day of November, 2019
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)
IN/PA-199

, Description:FIELD OF THE INVENTION:

Present invention relates to development of low cost high strength coldrolled galvanized steel sheet and a manufacturing method thereof. More particularly the present invention is directed to developing low carbon high strength,galvanized, stretch leveled and chromated steel specifically for structural applicationshaving Zinc coating of 450 GSM (450g/m2)and a method of its productionthrough thin slab caster(TSCR), cold rolling and hot dip galvanizing processing route. The low cost, high strength steelwith good appearance is characterized by high YS 420MPa minimum, UTS 490 MPa minimum and %El 10% minimum at a gauze length of 50 mm. The chemical composition comprises of %C 0.20 max, %Mn 1.35 max, % Si 0.5 max, %S 0.035 maximum, %(Nb+V+Ti) 0.10max, the balance being Fe and the inevitable trace elements.
BACKGROUND OF THE INVENTION
The production of high strength cold rolled galvanized steelis of great interest because of the increasing demand of structural steel in which specific mechanical properties are required for strength in a finished part, usually in loadbearing structuresand for the octagonal tubes for the solar panels. Galvanized steel in present era plays an important role in everyday lives. It has various field usage of construction, power transmission, and wherever long life is needed.
This requires the high strength with good elongation and better surface treatment high grade of the steel which has pushed the development of high strengthgalvanized steelaiming at superior performance and durability to operate in harsh environments.
High strength galvanized structural grades is produced either by “direct pickling and galvanizing” of hot rolled steel or by “pickling and cold rolling of hot rolled steel” followed by galvanizing. But in the first process there are limitations in (i) producing thinner gauze high strength hot rolled steel, (ii) drop in mechanical properties during galvanizing process (iii) difficulty in maintaining shape/dimensional tolerance in a close range. In the 2nd process, one can produce hot rolled steel with comparatively lower strength and further strength increment can be done by heavy cold reduction. So there is no need to produce very high strength and thinner gauze hot rolled steel for further processing. In this process, the hot rolled steel is first trimmed to remove the edge defects and to get the required width. After that pickling of the HR coil is done to remove the oxide layer from the surface using HCl acid. Thepickled coil is then cold rolledbelow its recrystallization temperature to reduce the thickness and imparting hardness/ strength. The fully hardened material goes to continuous galvanizing line (CGL). After emulsion the sheet is passed through Non-ox furnace (NOF) where it is directly heated to remove emulsion, then goes to radiant tube furnace where it is radially heated. After then thesheet goes to soaking furnace (SF) for the uniformity in temperature and at last it is jet cooled with air blow in jet cooling furnace (JCF). The coil coming from the JCF enters the Zinc Pot where steel strip with a layer of Zinc is coated to increase the corrosion resistance of the strip.To achieve flatness, removing stretcher strain and dent, the Stretch Levelling of the galvanized coil is done to get desired shape by applying tensile stress & bending stress. Further the passivation is done where Galvanized surface is chemically treated with chromic acid to attain a superficial layer of chromium.
Hot rolled steel can be produced either by conventional thick slab processing route or by thin slab casting & rolling (TSCR) route. The advantage of thin slab technology is reduction in energy consumption (because of lower slab thickness and elimination of reheating process), with consequent benefits in terms of production costs and pollution reductions, and permits also a reduction in investment costs because of the compact layout. These features make the thin slab technology the preferred choice for new mill construction. Its processing involves thin slab (55-65 mm) continuous casting, temperature equalization (1080-1130 Deg C) at tunnel furnace, thermo mechanical controlled processing (TMCP) followed by accelerated cooling (ACC). By this method the rolling mill is able to produce higher strength microalloyed steel by grain refinement using lower carbon content and thus excellent weldability. Because of the very low slab thickness, it allows producing very thin hot rolled steel with minimum amount of microalloying.
Prior Art:
Some of the prior patents in the related field tried to improve upon the properties of existing grade of cold rolled steel which include the Chinese patent with publication number CN104250703B published in 2017 titled “One kind of cold rolled 340 MPa grade HSLA steel and manufacturing method thereof”. The invented steel was produced through the CSP routewith casting speed 4-4.7m/min and further cold rolled and batch annealed. The Yield strength achieved was 340-460MPa, UTS was 440-520MPaand %Elongation up to 25%.
Another patent with publication numberCN107904485Awas published in 2018 titled “Cold-rolled low alloy and high strength with yield strength of 420 MPa and manufacturing method of cold-rolled low alloy and high-strength steel”. The invented steel has richer chemistry C:0.06 to 0.10 wt. %, Mn:1.25 to 1.70 wt.% and Si: 0.10 to 0.25 wt.%, S:0.020 to 0.050 wt.%, P:0.025 max wt.% and Ti: 0.031 to 0.05 wt.%. It is also produced through conventional casting with the casting speed of 1-1.45m/min. After cold rolling it was continuous annealed.The Yield strength achieved was 420-480MPa, UTS was 490-590MPa and %Elongation up to 19%.
In both the above cited patents the steel was not galvanized.
Another patent with patent number EP3178961A1was published by JFE steel corporation at European Patent Office in 2017 titled “High-strength hot-dip-galvanized steel sheet”. The invented steel sheet has a composition containing, by mass%, C: 0.06% or more and 0.09% or less, Si: 0.30% or less, Mn: 1.7% or more and 2.3% or less, P: 0.001% or more and 0.020% or less, S: 0.010% or less, Mo: 0.05% or more and 0.30% or less, N: 0.005% or less, and Al: 0.01% or more and 0.10% or less, the remainder being Fe and inevitable impurities. The microstructure has a martensite area of 7-25 % and a ferrite area of 50% or more. The tensile strength (TS) of 590 MPa to 690 MPa was achieve. Casting of this grade particularly through TSCR would be very much difficult because of the peritectic carbon range. Also Mnand other microallying are very high leading to commercially unviable. Also it seems to be steel is hot rolled through conventional slab casting route.
The present invention is thus directed to develop a high strength cold rolled galvanized steel sheet. Its processing involves hot rolling of steel sheets through TSCR route with slab thickness of 50-65 mm and casting speed of 4-6m/min. Further the hot rolled coil is slitted, pickled, cold rolled and then finally galvanized with Zinc coating of 450 GSM (450g/m2) to achieve desired microstructure along with specified range of YS, UTS and elongation properties.
OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide high strength cold rolled and galvanized steel for structural application with minimum Yield strength (YS) of 420 MPa, minimum Ultimate tensile strength (UTS) of 490 MPa minimum Elongation of 10% at gauge length 50 mm and a method of its production.
A further aspect of the present invention is directed to providing low carbon high strength cold rolled galvanized steel with better strength and elongation property wherein the steel composition is having Nb, as micro-alloying element for strengthening by grain refinement predominantly to obtain desired fine grained ferrite microstructure with combination of high strength and ductility properties.
A still further object of the present invention is directed to providing low carbon high strength cold rolled galvanized steel sheets with improved strength properties wherein the product obtained through thin slab caster route due to thin section and fast cooling, center line segregation is not a problem which is common in conventional slab caster.
Another aspect of the present invention is directed to providing low carbon high strength cold rolled galvanized steel with sheets with improved strength and ductility properties wherein due to fine grain size and effective use of micro alloying, leaner chemistry (low cost) can be used to produce desirable mechanical properties compared to conventional casting.
Yet anotheraspect of the present invention is directed to providing low carbon high strength cold rolled galvanized steel with improved strength and elongation properties wherein the steel composition is having upper limit of carbon restricted to 0.07% because of poor effect on weldability and its detrimental effect on surface quality due to formation of longitudinal cracks when processed using thin slab caster.
A further object is to produce the said low carbon high strength cold rolled and galvanized steel by slitting the edges of hot rolled coil to remove the edge defects.
A still further object is to produce the said low carbon high strength cold rolled and galvanized steel by pickling the coil to remove the surface oxide scales to improve the surface quality of the steel.
A still further object is to develop the said low carbon high strength cold rolled and galvanized steel by cold rolling the pickledhot rolled coilbelow recrystallization temperature so that desired strength can be achieved.
Yetanother object of the present invention is directed to providing low carbon high strength cold rolled and galvanized steel through hot dip continuous galvanizing line (CGL).
A further object of the present invention is directed to providing low carbon high strength cold rolled and galvanized steel with minimum 450 GSM zinc coating.
SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to provide High strength cold rolled galvanized steel having steel composition comprising wt.%:
C 0.20 max, Mn 1.35 max, Si 0.50 max, S 0.035 maximum, (Nb+V+Ti) 0.10 max and rest Fe having YS > 410 MPa, UTS > 480MPa, %El >10 in 50 mm gauge length.

A further aspect of the present invention is directed to High strength cold rolled galvanized steelcomprising structural steel having :
C 0.20 max; preferably 0.055-0.065
Mn 1.35 max; preferably 1.00-1.10
Si 0.50 max; preferably 0.15-0.25
S up to 0.035 max; preferably 0.003-0.006
Nb+V+Ti 0.1 max; preferably Nb 0.03-0.035
Al 0.020-0.070; preferably 0.025-0.040
N up to 120 ppm max; preferably 60-80 ppm and balance having
said yield strength > 420 MPa, preferably in the range of 620-650 MPa and UTS > 490 MPa, preferably in the range of 650-680 MPa and %El is >10% preferably in the range of 13-15%

A still further aspect of the present invention is directed to said High strength cold rolled galvanized steel having said high strength yield strength in the range of 620-650 MPa and UTS 650-680 MPa.

A still further aspect of the present invention is directed to saidHigh strength cold rolled galvanized steel having said % elongation 13-15%.

A still further aspect of the present invention is directed to saidHigh strength cold rolled galvanized steel having continuous hot dip galvanized said zinc coating on cold rolled sheet as per 450 GSM standard preferably 510-520 grams/m2

Another aspect of the present invention is directed to aprocess for producing high strength cold rolled galvanized structural steel comprising:
i)involving selective hot metal steel composition followed by refining to have in wt.%:
C 0.20 max, Mn 1.35 max, Si 0.50 max, S 0.035 maximum, (Nb+V+Ti) 0.10 max and rest Fe ;
ii) casting involving thin slab caster ;
iii) hot rolling and cooling such as to achieve said microstructure of acicular ferrite and quasi polygonal ferrite providing for YS > 450 MPa, UTS > 520MPa and %El > 25for hot rolled coil;
iii) slitting the hot rolled sheet from the edge to remove the edge defects and to get the desired width followed by subjecting to pickling to clean the surface of the steel;
iv) cold rolling of HR steel (3.0-4.5 mm)to reduce the thickness to the level of 1.5-2.5 mm preferably involving approximately 35 to 60 percent reduction.
v) the thus hardened sheet coming from cold roll unit is next subjected to step of galvanising with or without further passivation to protect the zinc layer for added corrosion resistance.

A further aspect of the present invention is directed to saidprocess comprising:
i)involving selective hot metal steel composition followed by refining to have in wt.%:
C 0.20 max, Mn 1.35 max, Si 0.50 max, S 0.035 maximum, (Nb+V+Ti) 0.10 max and rest Fe
ii) casting involving thin slab caster
iii) hot rolling and cooling such as to achieve said microstructure of acicular ferrite and quasi polygonal ferrite providing for YS > 450 MPa, UTS > 520MPa and %El > 25for hot rolled coil.
iii) slitting the hot rolled sheet from the edge to remove the edge defects and to get the desired width in the range of 5 to 15 mm lesser than the hot rolled coil and thereafter pickling the coil preferably involving HCl to remove the oxide layer and dent to clean the surface of the steel
iv) cold rolling of the steel involving 1 stand 4 high reversible mill to reduce the thickness to the desired level with preferably approximately 38 percent reduction.
v) the fully hardened sheet coming from cold roll unit subjected to step of galvanizing including passed through continuous galvanizing line where the sheet is passed through 4 furnaces namely NOF, RTF, SF and JCF and thereafter the sheet is hot dip galvanized in the zinc pot and preferably the coating was carried out as per GSM 450 grade and desired coating thickness in the level of 450 to 520grams/m2maintained by air knife pressure.
vi) passivating the thus galvanized sheet involving Chromic acid thereby providing Chromium layer to protect the zinc layer and strengthen the corrosion resistance of the steel.

A still further aspect of the present invention is directed to saidprocess comprising:
step of carrying out said steel casting at casting speed of 5-7 m/min;
homogenizing in the temperature range of 1100 to 1700 0C;
descaling before hot rolling;
carrying out final rolling pass below no recrystallization temperature;
involving ultra-fast cooling after finish rolling in combination with standard laminar cooling;
involving coiling temperature in the range of 500-600 0C under slow cooling in coiling.
slitting the hot rolled sheet to remove the edge defects;
pickling of the hot rolled sheet to clean the surface of the sheet;
carrying out cold rolling below recrystallization temperature;
galvanizing of sheet through continuous galvanizing line.

A still further aspect of the present invention is directed to saidprocess comprising:
I. chemical composition
C 0.20 max; preferably 0.055-0.065
Mn 1.35 max; preferably 1.00-1.10
Si 0.50 max; preferably 0.15-0.25

S up to 0.035 max; preferably 0.003-0.006
Nb+V+Ti 0.1 max; preferablyNb 0.03-0.035
Al 0.020-0.070; preferably 0.025-0.040
N up to 120 ppm max; preferably 60-80 ppm
slitting the hot rolled sheet to remove the edge defects;
pickling of the hot rolled sheet to clean the surface of the sheet;
carrying out cold rolling below recrystallization temperature;
galvanizing of sheet through continuous galvanizing line.

Another aspect of the present invention is directed to said process comprising:
I. Preparing the liquid steel in CONARC furnace followed by ladle refining furnace
II. Casting the steel slabs in a thin slab caster with slab thickness 55-65 mm;
III. Homogenizing the cast slab in a tunnel furnace at temperature 1080-1120 DegC;
IV. Descaling the slab in a high pressure descaler followed by low pressure descaler to remove the scale formed in tunnel furnace
V. Rolling the slab in a 6 stand tandem rolling mill to the desire final thickness and finish rolling temperature in the range of 870-900 0C;
VI. Subjecting the hot rolled coils to ultra-fast cooling such as to ensure high nucleation rate and fine grain structure followed by laminar cooling for facilitating coiling.
VII. Slitting of the HR coil is done to remove the edge defects and to get required width.
VIII. Pickling of the coil is done to remove the oxide layer from Hot rolled sheet surface by using HCl acid
IX. The hot rolled sheet is cold rolled by mechanically deforming 30-40% below recrystallization temperature to reduce the thickness and increase the hardness of the sheet.
X. Further it is passed through the continuous galvanizing line to do zinc coating on the surface of the cold rolled sheet as per 450 GSM standard.
Yet another aspect of the present invention is directed to saidprocess wherein said hot metal is treated in a CONARC furnace and further in a ladle refining furnace and thereafter the liquid steel is cast in thin slab caster with casting speed of 4-6 m/min, followed by charging in tunnel furnace and rolling to sheet/strip at FRT 870 to 900 0C and at coiling temperature 500–5800C to obtain desired high strength.
A further aspect of the present invention is directed to saidprocess wherein hot rolled steel manufacture is controlled to produce high strength steel sheet/strip of 4 mm thickness.

A still further aspect of the present invention is directed to saidprocess wherein hot rolled process carried out under controlled operating conditions comprising
(i) Casting Speed 4-6 m/min,
(ii) Slab Thickness 55-65 mm,
(iii) Slab Cutting Temp 980-1030 Deg C,
(iv) Homogenization Temp (Tunnel Furnace) 1080-1120 Deg C,
(v) Homogenization Time 8-15 min,
(vi) Finish Rolling Temp 870-900 Deg C,
(vii) Stand wise rolling reduction/time/temp comprising

Parameters F1 F2 F3 F4 F5 F6
Relative Reduction 40-45 45-50 35-40 30-35 25-30 20-25
Interstand time 5-15 4-15 3-10 3-10 2-10 2-10
Stand Entry Temp 1010-990 970-950 940-925 920-910 910-900 900-870

(viii) Coiling Temp 500-580 Deg C,
(ix) Ultrafast Cooling Rate 60-80 Deg C/sec,
(x) Lamellar Cooling rate 10-25 Deg C/sec.

A still further aspect of the present invention is directed to saidprocess whereinthe hot rolled coil is pickled using HCl to remove the oxide layer and stains to clean the surface of the steel , by passing the coil through 4 acid tanks with different concentration of HCl acid and finally it is rinsed in water in the 5th tank to prevent any carryover acid on the surface of steel, involving Pickling Details as:
Tank HCl concentration
Tank 1 3-5%
Tank 2 5-9%
Tank 3 9-13%
Tank 4 13-18%

A still further aspect of the present invention is directed to saidprocess whereinthe hot rolled pickled coil is further cold rolled using 1 stand 4 high reversible rolling where 7 passes were given to give thickness reduction of 38 % following
Cold Rolling Detail:
Coil No Size (mm) HR Coil Thickness (mm) Thickness after cold rolling (mm) No. of pass at cold Rolling Reduction at cold rolling (%)
A 2.50*1220 4.00 2.466 7 38.4
B 2.50*1220 4.00 2.466 7 38.4
C 2.50*1220 4.00 2.466 7 38.4

A still further aspect of the present invention is directed to said process wherein the cold rolled sheet is passed through the continuous galvanizing line (CGL) comprising:
I. Annealing including the steel strip being passed through 4 types of furnace wherein the strip first passes through NOF furnace where it is heated to 600 C or more to remove emulsion, then the strip is passed through Radiant Tube Furnace (RTF) where the strip is heated radially through tubes to 750C or more, further it is passed through Soaking Furnace (SF) for uniform temperature throughout the strip wherein the temperature maintained is maintained 750 and above and finally the strip is passed through Jet Cooling Furnace (JCF) where the furnace temperature maintained through air blow is 500 C and above;
II. hot dip galvanizing in Zn pot wherein hot dip galvanizing of strip is done to coat a zinc layer which increases the corrosion resistance of the strip, the air knife pressure is used to maintain proper coating of given Zn layer thickness preferably GSM 450 grade, wherein the air knife pressure maintained is around 0.210 Kg/cm2;
III. carrying out stretch levelling to remove the stretcher strain/luder bands wherein the Galvanised steel stretch levelling is done to get desired shape by applying tensile stress & bending stress;
IV. subjecting to passivation wherein Chemical treatment of the Galvanised surface with a superficial layer of Cr is carried out to further protect the zinc coating with
Galvanizing Details maintained as hereunder:
Sample Name A B C
Size 2.50*1220 2.50*1220 2.50*1220
Line Speed (meter/minutes) 32 32.1 32.1
Production output Mt/hour 45.96 46.11 46.11
Strip temperature at Exit in degree centigrade
NOF 681 679 679
RTF 796 821 821
SF 756 758 758
JCB 539 540 540
Air knife pressure Kg/cm2 0.216 0.216 0.216

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

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURE

Figure 1: is the flow chart showing the details of different steps involved in producing the high strength hot rolled steel sheet for the application of cold rolling and galvanization according to the present invention.

Figure 2a-b: is the micrographs of the Microstructure Images of hot rolled sheet taken at 100X and 500X, having typical grain size in the range of around 5-10 microns.

Figure 3: is the flow chart showing the details of different steps involved in producing the high strength cold rolled and hot dip continuous galvanized sheet according to the present invention.

Figure 4a-b: is the micrographs of the Microstructure Images of cold rolledand galvanized sheet taken at 200X and 500X, having elongated grain structure.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWINGS
The present invention is directed to provide low carbon high strength cold rolled hot dip galvanized steel for structural produced through CONARC furnace, thin slab casting processing route, cold rolling and continuous galvanizing route.

Thus according to the present invention, a cost effective composition of low carbon high strength low alloy cold rolled and galvanized steel sheet and a method of making such steel is provided. Steel is processed using a thin slab caster and chemistry consists of low carbon along with micro alloys such as Nb addition. Mechanical properties of hot rolled sheet are YS > 450 MPa, UTS > 520MPa and %El > 25.Further, the hot rolled sheet is pickled and cold rolled and finally hot dip galvanized and passivated using Chromium.Mechanical properties of cold rolled galvanized steel are yield strength > 420MPa, preferably in the range of 620-650 MPa and UTS > 490 MPa, preferably in the range of 650-680 MPa, %El is >10% preferably in the range of 13-15%.
In order to produce the low carbon cold rolled galvanized steel grade with above stated properties, the selective steel composition used for processing through thin slab caster is as follows:
I. C 0.20 max; preferably 0.055-0.065
II. Mn 1.35 max; preferably 1.00-1.10
III. S up to 0.01 max; preferably 0.003-0.006
IV. Si 0.50 max; preferably 0.15-0.25
V. (Nb+V+Ti) 0.10 max; preferably Nb0.03-0.035
VI. Al 0.020-0.070; preferably 0.030-0.040
VII. N up to 120 ppm max; preferably 60-80 ppm

The process route followed was: Electric Arc Furnace (CONARC) ?Ladle Furnace? thin slab Caster? 6 stand hot rolling mill? coiling?slitting?pickling??1stand?4 high cold rolling reversible mill ?Hot dip continuous galvanizing ?passivation; however, any other combination of processes (before caster) which gives steel of same chemistry can also be used.

The detailed consideration for selecting the above chemical composition are as below:

Carbon: Carbon is the most effective and cheap strengthening element in steel by solute strengthening and formation of carbide and /or carbonitrides of Nb. However, it has adverse effect on toughness and weldability. Also it has detrimental effect on surface quality due to formation of longitudinal cracks when processed using thin slab caster with carbon level above 0.08% being particularly prone to surface defects. Present invention is selected %C 0.04-0.07

Manganese: Mn is an important element for solid solution strengthening. Usually in low C steel, with increase in Mn content, toughness of the steel increase Mn also decrease the YS/TS ratio. However upper values are restricted because of its poor effect on weldability, castability in thin slab caster and load during hot rolling apart from cost implication. Centre line segregation is another major issue with increasing % Mn. Range of %Mn 1.0-1.1 is selected in this invention.

Silicon: Si is generally used as a deoxidizer as well as an alloying element. Silicon significantly increases the tensile strength and a lesser extent yield strength and thus lowers the YS/UTS ratio. Upper limit is limited by detrimental effect on surface quality as sticky scale is form on the surface with high Si. In this present invention, Si range is selected 0.15-0.25 %

Niobium: Nb is essential for strengthening by grain refinement & precipitation strengthening. It is one of the main sources of strengthening. Nb increases TNR (temp of no recrystallization) temperature to a great extent and thus prohibiting any recrystallization& grain coarsening during final phases of hot reduction.
It also reduces the rate of recrystallization of austenite during controlled rolling of HSLA Steel to improve grain refinement. Other outstanding effects of niobium, is to lowering the austenite / ferrite transformation temperature by a solute drag effect and thus used as the effective precipitation strengthening element also.

Upper limit is restricted because of its effect on rolling load during hot rolling. Also because of the limitation of higher temperature and retention time at tunnel furnace, maximum Nb is restricted. In this invention Nb range selected is (0.03-0.035)

Nitrogen: Nitrogen is a key element with significant role in formation of nitride and carbonitride precipitates; however upper limit is restricted because of its poor effect on formability and toughness of steel. In our case, N is selected in the range of 60-80 ppm.

Calcium: 0-50 ppm: Steel has to be Calcium treated to counter the harmful effect of Sulphur as well as help in casting.

Accompanying Figure 1 and 3 shows the flow chart illustrating the different steps involved in producing the high strength cold rolled galvanized steel with improved elongation according to the present invention.

The process used for making the product according to an embodiment of the present invention is described in details with the help of following examples:

EXAMPLE:

(i) Hot metal from blast furnace was refined with the help of Electric Arc Furnace (CONARC) and final chemistry adjustments were done in a ladle refining furnace to obtain a selective composition as given above involving micro alloying with Nb. The composition of different steel samples obtained on trial heats are given in following table 1:
Table 1:
Sample ID C % Mn % P% S% Si% Al% N% Nb%
A 0.057 1.025 0.010 0.005 0.243 0.030 0.0071 0.031
B 0.064 1.028 0.010 0.006 0.237 0.033 0.0061 0.030

(ii) Steel was cast using a thin slab caster with slab thickness ranging from 50-65 mm.
(iii) Slab was further homogenized in a Tunnel furnace at temperature >1100 °C but below 1120 °C. Descaler was used after Tunnel Furnace to remove scales.
(iv) Hot rolling with 6 stand reductions was used to reduce the thickness to the required level with finish rolling temperature in the range of 800-880°C. Final rolling pass below TNR (no re-crystallization temperature). The details stand wise parameters for hot rolling are presented in table 2 as follows:

Table 2:
Parameters F1 F2 F3 F4 F5 F6
Relative Reduction 40-45 45-50 35-40 30-35 25-30 20-25
Interstand time 5-15 4-15 3-10 3-10 2-10 2-10
Stand Entry Temp 1010-990 970-950 940-925 920-910 910-900 900-870

(v) After finish rolling, an ultra-fast cooling was used in combination of standard laminar cooling to achieve Coiling temperature 500-550°C. Initial ultra-fast cooling ensures a high nucleation rate thus leading to obtain fine and uniform grain size/structure across the thickness and subsequent laminar cooling of less severity is used to maintain good flatness of sheet.
(vi) Coil was subsequently slow cooled in coil yard.
(vii) The resulting steel grade is subjected to testing and inspection to ascertain attainment of desired properties.

Mechanical properties of hot rolled sheet:
Tensile test was done by following ASTM standards E-8.
The product obtained was free from any surface defects and the mechanical properties observed for different samples are presented in the following table 3:

Table 3:
Sample ID Width (mm) Thickness (mm) CT FT YS UTS % El
YS/UTS
A 1230 4 570 900 477 544 27 0.87
B 1230 4 565 890 475 542 27 0.87

Microstructure of hot rolled sheet:
The Microstructure of samples was found to consist of acicular ferrite and quasi polygonal ferrite and typical grain size is below 10 microns and preferably in the range of 5-10 microns. Accompanying Figure 2a-b shows the Microstructure Images taken at 100X and 500X, having typical grain size in the range of around 5-10 microns.

(viii) The hot rolled sheet wasslitted to remove the edge defects and to get the desired width. Further the coil was pickled using HCl to remove the oxide layer and stains to clean the surface of the steel. The coil passes through 4 acid tanks with different concentration of HCl acid and finally it is rinsed in water in the 5th tank to prevent any carryover acid on the surface of steel.The detailed parameters for pickling are presented in table 4.

Table 4:
Tank HCl concentration
Tank 1 3-5%
Tank 2 5-9%
Tank 3 9-13%
Tank 4 13-18%

(ix) Steel was hot rolled using 4 high 1 stand reversible mill to reduce the thickness to the desired level with approximately 38 percent reduction.The detailed parameter for cold rolling are presented in table 5.

Table 5:
Coil No Final Size (mm) HR Coil Thickness (mm) Thickness after cold rolling (mm) No. of pass at cold Rolling Reduction at cold rolling (%)
A 2.50*1220 4.00 2.466 7 38.4
B 2.50*1220 4.00 2.466 7 38.4
C 2.50*1220 4.00 2.466 7 38.4

(x) The fully hardened sheet coming from cold roll unit is passed through continuous galvanizing line. There the sheet passed through 4 furnaces namely NOF, RTF, SF and JCF. Further the sheet was hot dip galvanized in the zinc pot. The coating was as per GSM 450 grade. The coating thickness was maintained by air knife pressure. The detailed parameters for cold rolling are presented in table 6.

Table 6:
Sample Name A B C
Size 2.50*1220 2.50*1220 2.50*1220
Line Speed (meter/minutes) 32 32.1 32.1
Production output Mt/hour 45.96 46.11 46.11
Strip temperature at Exit in degree centigrade
NOF 681 679 679
RTF 796 821 821
SF 756 758 758
JCB 539 540 540
Air knife pressure Kg/cm2 0.216 0.216 0.216

(xi) The galvanized sheet is further passivated using Chromic acid. Chromium layer protects the zinc layer and strengthen the corrosion resistance of the steel.

Mechanical properties of cold rolled and galvanized sheet:
Tensile test was done by following ASTM standards E-8.
The product obtained was free from any surface defects and the mechanical properties observed for different samples are presented in the following table 7:

Table 7:
Sample Size Thickness*width(mm) Coating (GSM) YS UTS %El (50 GL)
A 2.5*1220 520 647 670 15
B 2.5*1220 510 676 693 13
C 2.5*1220 515 634 654 13
D 2.5*1220 510 627 656 13

Microstructure of cold rolled galvanized sheet:
The Microstructure of samples was found to consist of acicular ferrite. Accompanying Figure 4a-b shows the Microstructure Images of cold rolled galvanized sheet taken at 200X and 500X, having elongated grains.

It is thus possible by way of the present invention to provide low carbon high strength cold rolled hot dip galvanized steel sheet for structural application and a cost effective method of its manufacture through thin slab caster(CSP) processing route followed by hot rolling, coiling, pickling, cold rolling and hot dip galvanizing. The low carbon high strength cold rolled galvanized steel for structural application having zinc layer of GSM 450 standard. The low cost, high strength steel is characterized by high yield strength > 420 MPa, UTS > 490 MPa, and %El is >10% at a gauze length of 50 mm. The chemical composition of said steel comprises %C 0.20 max, %Mn 1.35 max, %S 0.035 maximum, %(Nb+V+Ti) 0.10 Max, the balance being Fe.