DESC: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 :
COLD ROLLED HIGH STRENGTH GALVANIZED STEEL SHEET WITH IMPROVED CORROSION RESISTANCE AND METHOD OF MANUFACTURING THE SAME.



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 cold rolled high strength Galvanized steel sheet with improved corrosion resistance and method of manufacturing the same. The Galvanized steel sheets have Tensile strength of 780 MPa or more involving selective chemical composition and processing through continuous annealing route to achieve the desired microstructure and excellent stretch formability, bendability and hole expansion ratio. The advancement favors generation of cold rolled high strength Galvanized steel sheet having Yield strength of 450 MPa or more, Tensile strength of 780 MPa or more, total elongation of 15% or more, strain hardening coefficient (n-value) of 0.12 or more, hole expansion ratio of 30 % or more and bake hardening index 30 or more which makes such steel sheets suitable for automobile applications. The present advancement also concerns achieving better corrosion resistance with uniform coating thickness.

BACKGROUND OF THE INVENTION
Formability is a major limitation which restricts the application of high strength steel in automotive body parts having complex profile. Through improving the strain hardening coefficient better formability can be achieved. To facilitate, ferrite-martensiticMaterials has been utilized whereTempered martensite in ferrite matrix results in excellent hole expansion.However, the optimum deployment of said steel can only be achieved by right combination of retained austenite, ferrite and martensite phase fractions and their distribution. In order to achieve the said high Hole Expansion Ratio, Si and Al are added to get the desired amount of retained austenite and Ferrite at room temperature.

To avoid crack generation during press forming, the high strength steel sheet having strength >780 MPa must also exhibit a good strain hardening coefficient (n-value) of atleast 0.12 along with total elongation of no less than 15%and Hole Expansion ratio from 30 to 50%.

With utilization of dual phase high strength steel, automobile manufacturers are requiring more high strength materials with UTS 780MPa or more with high yield ratio in their reinforcement, structural components and pillars for light weighing, improving fuel efficiency and to satisfy the norms of future legislation concerning emission and fuel consumption.

However, high strength dual phase steels are rather prone to poor drawability or press formability when yield ratio increases. Also the bendability and Hole expansion are poor. There had been thus a need to improve these properties of steel along with required high strength and corrosion resistivity for application in automobile components.

Japanese patent application number JP2005336526A discloses a High strength steel sheet having excellent workability and its production method comprising 50% or more of tempered martensite as a major phase component. A good combination of strength, ductility and stretch flange formability has been claimed as a part of invention by virtue of keeping high space factor of sintered martensite and retained austenite. However, due to presence of high proportion of hard martensite, the n-value deteriorates and material does not perform well in actual press forming due to poor strain hardening. In addition, due to excess P weight % in composition which is added to impart strength may result in poor elongation and temper embrittlement.

The present invention thus attempts to overcome the above mentioned problems and limitations of the prior art by way of providing a high-strength cold-rolled steel sheet with minimum tensile strength of 780MPa, and total elongation more than 15% having improved corrosion resistance and a process for its manufacture.

OBJECTS OF THE INVENTION
The basic object of the present invention is directed to provide cold rolled high strength steel sheet having excellent hole expansion ratio, corrosion resistance and method of manufacturing the same.

A still further object of the present invention is directed to providecold rolled high strength steel sheetinvolving selective composition and processing to achieve the desired microstructure and the stretch formability property.

A still further object of the present invention is directed to providecold rolled high strength steel sheet having tensile strength 780 MPa or more, a good strain hardening coefficient (n-value) of atleast 0.12 along with high total elongation of no less than 15 %, and Hole Expansion ration more than 30%.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to provide cold rolled high strength Galvanized steel sheets having tensile strength atleast 780 MPa with composition in terms of weight % comprising:
C: 0.07-0.12 %;
Mn: 1.5- 1.9%;
Si: 0.1–0.35%;
Al: 0.1-0.3%
S: 0.005 % or less;
N: 0.005 % or less
Ti: 0.005-0.05%;
Mo: 0.02-0.1%;
Cr: 0.21-0.4%;
and the balance being Fe and other unavoidable impurities; wherein [Al] / [Si] ratio is in a range of 0.4 to 2 and having selective steel microstructure constituents including atleast60% Ferrite phase and Tempered Martensite from 10 to 40% and in ferrite matrix for excellent stretch formability.

A still further aspect of the present invention is directed to saidcold rolled high strengthGalvanized steel sheet further comprising atleast one type of element selected from the group of elements comprising V, Nb, Ti, W in amounts less than 0.08 by weight %.

A still further aspect of the present invention is directed to said cold rolled high strength Galvanized steel sheet further comprising in terms of weight % atleast one additive element selected from the group consisting of 0.001% to 0.003% of B, 0.002 % to 0.2 %, Cu, 0.002 % to 0.2 % Ni, and less than 0.005 % Ca;

Another aspect of the present invention is directed to saidcold rolled high strength Galvanized steel sheets having Yield strength of 450MPa or more, Tensile strength of 780 MPa or more, total elongation of 15% or more, strain hardening coefficient of 0.12 or more and bake hardening index of 30 MPa or more.

A further aspect of the present invention is directed to a process for manufacturing the cold rolled high strength Galvanized steel sheets as described above having tensile strength of atleast780 MPa comprising the steps of:
a) providing steel having composition comprising
C: 0.07-0.12 %;
Mn: 1.5- 1.9%;
Si: 0.1–0.35%;
Al: 0.1-0.3%
S: 0.005 % or less;
N: 0.005 % or less
Ti: 0.005-0.05%;
Mo: 0.02-0.1%;
Cr: 0.21-0.4%;
and the balance being Fe and other unavoidable impurities; wherein [Al] / [Si] ratio is in a range of 0.4 to 2 and
involving processing through Heat from basic oxygen furnace (BOF) and RH degasser and subsequently continuously casting into slabs and reheating said slabs having said composition to reheating temperature in the range from 1190°C -1250 °C;
b) (i) subjecting said reheated slabs to roughing rolling in roughing mill with roughing mill delivery temperature of 1080°C or less;
(ii)Said rough rolled steel being subjected to finish rolling with finish mill exit temperature ranging from Ac3°C to Ac3+100 °C;
c) cold rolling;followed by
d) continuous annealing;
e) Galvanizing and skin pass rolling.
so as to have selective steel microstructure constituents including atleast 60% Ferrite phase and Tempered Martensite from 10 to 40% and in ferrite matrix for excellent stretch formability.

A still further aspect of the present invention is directed to said process further comprising the steps of:
a) Coiling the finish rolled steel with average run out table cooling rate of 10 °C/second or more; and
b) Acid Pickling and Cold rolling the said hot rolled steel sheet with cold reduction of atleast 30%.

A still further aspect of the present invention is directed to said process wherein cold rolled steel is subjected to said continuous annealing following the steps comprising:
a) Annealing the cold rolled steel sheet at soaking section critical temperature range from Ac1+20 °C to Ac3+20 °C with residence time ranging from 70 to 150 seconds;
b) Rapid cooling the steel from SS temperature up to a temperature range of 440 to 470 °C;
c) Zinc bath temperature of the said steel in the range starting from 440 °C to 460 °C;
d) Galvanizing at temperature range of 530-570°C for Zn-Fe alloy layering/coating on surface;
e) Subjecting the coated steel to skin pass elongation of 0.2% to 1%.

The above objects and advantages of the present invention are described hereunder in details with reference to non-limiting accompanying examples:

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING

Figure 1: shows the micrograph of the invented steel grade showing the distribution of phase fractions.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING EXAMPLE AND DRAWING

The Present invention relates to cold rolled high strength Galvanized steel sheet having Tensile strength 780 MPa or more and composition in terms of weight percent comprising:
C: 0.07-0.12 %;
Mn: 1.5- 1.9%;
Si: 0.1–0.35%;
Al: 0.1-0.3%
S: 0.005 % or less;
N: 0.005 % or less
Ti: 0.005-0.05%;
Mo: 0.02-0.1%;
Cr: 0.21-0.4%;
and the balance being Fe and other unavoidable impurities; wherein [Al] / [Si] ratio is in a range of 0.4 to 2 and having selective steel microstructure constituents including atleast 60% Ferrite phase and Tempered Martensite from 10 to 40% and in ferrite matrix for excellent stretch formability.

Cold rolled high strength steel sheet obtained according to present invention having Yield strength of 450MPa or more, Tensile strength of 780 MPa or more, total elongation of 15% or more, strain hardening coefficient of 0.12 or more, Hole Expansion ratio 30% or more and bake hardening index of 30 MPa or more.

Following abbreviations, terminologies and expressions are used to describe the manner of implementation of the present invention:
CGL – Continuous Galvanizing Line
RCS -Rapid cooling section
CS - Center Speed
SRT -Slab Reheating Temperature
FET- Finishing Mill Entry Temperature
FT-Finishing Temperature
CT- Coiling Temperature
HER–Hole Expansion Ratio
Ac1 & Ac3 – Critical temperatures in iron-carbide diagram
El – Elongation (%)
UTS - Ultimate Tensile Strength (MPa)
YS - Yield Strength (MPa)
SPM - Skin Pass Elongation (%)

A cold rolled high strength Galvanized steel sheet with improved corrosion resistance according to present invention is disclosed, and its chemical compositions and method of manufacturing are described hereunder with explanation on metallurgical factors deciding the range of constituentsin a composition according to a preferred embodiment wherein all the elements are in weight % as follows:

Carbon (C: 0.07-0.12wt %) – Carbon effectively increases the hardenability and strength of steel. It also lowers the transformation temperature; hence more austenite forms during soaking of steel. In addition, Carbon also lowers the martensite finish temperature, which stabilize austenite phase at room temperature. However, to utilize the dual phase phenomenon at least 0.07 weight % of C is required. More preferably, the amount of carbon must be more than 0.08 to make austenite stable and to effectively lower the Martensite start temperature. On the other hand, increasing the carbon content above 0.12 % deteriorates the weldability and fatigue resistance. Also, with higher carbon, the austenite becomes too stable to be transformed to martensite during forming which results in poor strain hardenability. With these limitations, upper limit of carbon is 0.12 %.

Manganese (Mn: 1.5-1.9wt %) - Similar to C, Mn is also an austenite stabilizer.Mn increases the hardenability of steel by lowering Ms Temperature. It also assists in partitioning of C more to austenite and hence makes Austenite more stabilized. In order to attain the desired amount of solid solution strengthening to achieve UTS>780 MPa, minimum amount of Mn must be atleast 1.5%. However, an increase in manganese concentration restricts the fraction of ferrite that can form. Higher Mn weight percent may also lead to higher martensite fraction resulting in lower strain hardening effect. Hence, the upper limit of Mn is 1.9 %.

Silicon (Si: 0.1–0.35wt %) –Si suppresses precipitation of cementite, therefore, it helps in enrichment of carbon in austenite and make it more stable. Si as a solid solution strengthening element strengthens the ferrite matrix. To attain that effect and to get minimum UTS>780 MPa, minimum amount of Si must be atleast0.1wt%. However, increasing Si level more than 0.35% does not cause any significant effect to inhibit cementite formation. In addition, adding excess Si deteriorates the corrosion resistance by creating bare spot on thesurface. Hence, the upper limit of Sicontent must not be more than 0.35 %.

Aluminium (Al: 0.1-0.3 wt %)-Like Si, Al also suppresses the cementite precipitation and in this way it can be used as a replacement for Si. However, Al does not strengthen the ferrite matrix, hence more Mn need to be added to achieve the desired strength level. Al also acts as a deoxidizer during steel making process to kill dissolved oxygen. It also improves corrosion resistance by preferential oxidation before zinc coating and for which Al shall be at least 0.1 % by weight. Al also adds on to fix harmful dissolve N to form AlN. Increasing Al level above 0.3wt% to replace Si causes longitudinal cracks while casting. Accordingly, upper limit of Al is set to 0.3%. [Al]/[Si] promotes preferential oxidation before siliconoxide formation and for which Al/Si shall be at least 0.4. But with increasing Al/Si ratio, its increases cost of steel as well as creates slab cracking while casting so maxiumum limit of Al/Si restricted to 2.

Nitrogen (N: 0.005 wt% or less) – N is present in steel as an impurity and should be present at minimum amount to avoid aging. In addition, to achieve good aging resistance, upper limit of N must be 0.005 wt % or less.

Molybdenum and Chromium (Mo: 0.02 to 0.1 and Cr: 0.21-0.4 wt %) – Mo assists Mn in improving strength by improving Mn equivalent. Molybdenum is Ferrite stabilizer and in present invention is used to reduce and replace silicon, which may cause problems during hot rolling and coating. Molybdenum also reduces the annealing time in order to achieve dual phase structure. However, higher Mo content reduces the workability. Therefore upper limit should be 0.1wt% or less. Addition of chromium should restricted upto 0.4 wt% for better weldability.

V, Nb, Ti and W (collectively <0.08wt %):V, Nb, Ti and Wforms carbide and impart precipitation strengthening to the steel. However as Mo is already added, any additional content more than 0.08 of each element will add up to cost of production. Moreover, higher addition of this element will form coarser carbides, reducing elongation. Formation of excess carbide also leads to lower carbon fraction in austenite and reduces its stability. Accordingly, upper limit of at least one of the elements selecting from V, Nb, Ti and W must be less than 0.08wt%.

Description of the process of manufacturing:

To achieve Slab chemistry as described in scope of the invention, Heat from basic oxygen furnace (BOF) is processed through RH degasser and subsequently continuously casted into slabs.Special measures are taken to hot roll resulted slabs by keeping slab reheating temperature in the range of 1190°C to 1250°C intended to control roughing mill delivery temperature under 1080°C and finishing mill entry temperature under 1080°C to check surface defects like rolled in scale. During hot rolling, finishing mill temperature is varied in the range from Ac3 °C to Ac3+100 °C. After finish rolling, Run out table cooling rate from finishing mill to coiler of more than 9 0C/sec was maintained to achieve coiling temperature range of 530 to 630 °C, to avoid ID collapse after coil winding, it is held for 120sec at mandrel. Hot rolled coils were subsequently processed through pickling coupled with tandem cold rolling mill to remove the oxide surface present in the surface and to provide selective cold reduction of 35% or more.

Subsequent to pickling and cold rolling to desired thickness, cold rolled steel strip are processed through continuous annealing line where electrolytic cleaning removes rolling emulsion present on the surface. Cleaned surface passes through the preheating and heating section where the strip is heated at the rate of 0.5-5 0C/sec up to soaking section temperature. Soaking section temperature was maintained in the range from 760 °C to 810 °C to achieve retained austenite in final microstructure. Annealing time is kept in the range from 70 to 150 seconds to allow sufficient time for annealed and homogenization of austenite microstructure. Following soaking section, annealed strip sheet passes through rapid cooling section at cooling rate of 40°C/Sec or less and cooled up to rapid cooling section temperature of 440 °C or more. This is to keep martensite area fraction in microstructure less than 40 %. Subsequent to RCS, annealed strip passes through zinc pot where rapid cooled steel strip is coated with zinc at zinc pot temperature of 440-470 Deg C. The Galvanized steel sheet is passed through galva annealing furnace at temperature of 530 deg C or more temperaturefor creating iron-zinc (Fe-Zn) diffusion on the surface results in three hard, dark-grey Fe-Zn alloy layers. Galvannealed steel sheet is then provided with skin-pass elongation in the range from 0.20 to 1 % to avoid yield point elongation.

Method of evaluating hole expansion ratio: The hole expansion ratio (HER %) is significant to assess the stretch flangeability of steel sheets. It is acquired by the hole expansion test utilizing conical or cylindrical punch in forming test machine. Whole expansion tests were performed as per ISO 16630-2009 utilizing forming test machine. Samples having a pouched hole of 10mm diameter were used for the test. Conical punch having an angle of 600 and cylinder diameter 50 mm was used. The punching speed of the conical punch during hole expansion was 0.3 mm/s. The conical punch was moved up against the sample with 10mm hole until the small crack appeared at the edge of hole and detected by optical instrument. The final average diameter of the hole after the small crack appeared was determined by measuring in two directions. Test were repeated for four to five times for each steel numbers and average HER% was taken with the following standard equation -
HER% = [(Df- Do)/ D0] X100
Where Do = Initial hole diameter, Df = final hole diameter before crack.

Complete description of steel according to the present advancement and comparative steel grades are illustrated in following table 1 to table 4 and the weight percent range of constituents and the selective process parameters according to the invention are validated through following examples 1 & 2:

Table 1: Elemental Compositions in weight % of the inventive steel sheets along with comparative example.
Table 2: Hot rolling and cold rollingof inventive with comparative steel sheets having chemical compositions as per Table 1.
Table 3:CGL(Continuousgalvanizing Line) Parameters of inventive with comparative steel sheets having chemical compositions as per Table 1.
Table 4:Mechanical properties, HER % and micro structural phase fractions of inventive and comparative steels having chemical composition as per table 1 and being processed as per Table 2 and Table 3.

Table 1
Chemical Composition in %
Sample No C Mn S P Si Al N Ti Cr [Al]/[Si] Other Elements Remarks
1a 0.09 1.6 0.003 0.021 0.2 0.14 0.004 0.025 0.35 0.7
V-0.02, Mo-0.07 I

1b 0.09 1.6 0.003 0.021 0.2 0.14 0.004 0.025 0.35 0.7 V-0.02, Mo-0.07 I
2 0.11 1.8 0.003 0.01 0.2 0.25 0.004 0.02 0.3 1.25 Nb-0.02 I
3 0.07 1.5 0.004 0.015 0.15 0.1 0.005 0.03 0.35 0.67 V-0.02 I
4 0.14 1.7 0.004 0.01 0.45 0.06 0.004 0.06 0.45 0.13 Nb-0.04 C
5 0.13 1.63 0.004 0.015 0.35 0.05 0.003 0.017 0.6 0.14 V-0.02, Mo-0.07 C
6 0.09 2.1 0.01 0.02 0.2 0.035 0.004 0.03 0.2 0.175 Mo-0.18 C

*I - Present inventive example, C- Comparative Examples
*Underline boxes indicates “outside the appropriate range”

Example 1
It can be appreciated from Table 1 to Table 4 that steel sheets remarked as “I” are satisfying all the scopes of present invention and exhibits excellent strain hardening property, HER% and improved corrosion resistance without bare spot on the surface. These steels exhibits improved n value>0.12, HER %>30, UTS =780MPa and Yield strength more than 450 MPa. Whereas, Steel remarked as ‘C’ from Table 1 to Table 4 doesn’t comply with atleast one of the scope of the present invention and does not conform with minimum one or more of the end product attributes as mentioned in the scope of the present invention. For example steel no. 4 to 5 in table 1 has the higher carbon % and Steel no. 6 has the high Mn wt% than the scope and does not comply with required ratio of Al/Si ratio and has poor Hole Expansion Ratio and less n value.

Table 2

Hot Rolling Parameters Cold Rolling Parameters
Sample No SRT°C Roughing Mill temp°C FT°C CT°C Cold Reduction %


1a 1205 1075 903 558 40
1b 1205 1075 903 558 59
2 1200 1060 895 580 45
3 1190 1070 900 620 40
4 1250 1050 910 550 42
5 1220 1070 908 570 45
6 1210 1080 910 560 50

*I - Present inventive example, C- Comparative Examples
Note: Steel marked as 1a and 1b have the same chemical composition as steel number 1, and however they are processed at different conditions to validate the claimed process. In sample no 1a and 1b the Rapid cooling temperature is varied in 1a temperature is 444°C where martensite formed is 18% and n value is greater than 0.12 whereas in sample 1b where rapid cooling temperature is 470°C and rapid cooling rate of 42 deg C/sec where martensitic phase % is 42% which resulted as n value is less than required value and zinc coating was Not Good (NG).
In sample no. 4where carbon percentage is more than 0.12%, Si percentage is more than 0.35 and Cr percentage is more than 0.4and also does not comply with required ratio of Al/Si ratio and has poor coating property.
In sample no. 5 where carbon percentage is more than 0.12%, where RCS temperature is less than 440 and where percentage of martensite formed is more than desired level and results in less n value and Hole Expansion Ratio.
In Sample 6, where Mn percentage is more than 1.9%, where RCS temperature is more than 470, so we get martensite percentage less than desired limit.
* SRT- Slab reheating temperature,FT- hot finish rolling temperature ,ROT- Run out table at hot strip mill , CR%- Cold rolling reduction % , SS- soaking section , RCS- Rapid cooling section, GVF- Galvalume Furnace, SPM- Skin pass elongation.

Table 3
CGL Parameters
Sample No SS TEMP SS Residence Time RCS TEMP RCS Cooling Rate°C/Sec Zinc Pot Temp. GVF Temp SPM ELONG
1a 780 95 444 30 440 530 0.4
1b 820 95 470 42 460 540 0.3
2 800 100 460 35 440 530 0.4
3 770 120 440 30 460 540 0.3
4 840 60 480 45 490 500 0.2
5 800 95 420 50 440 540 0.4
6 780 90 490 10 460 530 0.4

Table 4
Mechanical Prop Product Properties
Sample No YS YPE TS ELONGATION Yield Ratio n Value HER % Ferrite % Martensite % Zinc Coating Remarks
1a 500 0 840 20 0.65 0.16 45 82 18 G I
1b 450 0 850 13 0.53 0.1 26 58 42 NG C
2 470 0 860 18 0.54 0.15 40 80 20 G I
3 480 0 880 17 0.54 0.16 30 75 25 G I
4 550 0 900 13 0.61 0.09 25 55 45 NG C
5 480 0 776 13 0.61 0.09 25 55 45 NG C
6 650 0 805 12 0.8 0.09 33 91 9 NG C

*I - Present inventive example, C- Comparative Examples, G- Good, NG- Not Good
Example 2:
In sample 1b, 4, 5 and 6 where Elongation is less than 15%, n-value is less than 0.12, Poor zinc coating which is out of scope of present invention.
In case of sample 1b, 4, 5 and 6, n-valuesare less than 0.12 which is out of scope of current invention; incase of sample 4 and 5 where martensite percentage is more than 40 percent and n value is 0.1 less than 0.12 and has poor HER % less than 30% which is out of scope of current invention.

It is thus possible by way of the present invention to provide 780 MPa Tensile strength level high strength cold rolled steel sheet involving selectchemical elements in terms of weight percent:C: 0.07-0.12 %; Mn: 1.5- 1.9%; Si: 0.1–0.35%; Al: 0.1-0.3%;S: 0.005 % or less; N: 0.005 % or less Ti: 0.005-0.05%; Mo: 0.02-0.1% and Cr: 0.21-0.4%; and the balance being Fe and other unavoidable impurities; wherein [Al] / [Si] ratio is in a range of 0.4 to 2 and having selective steel microstructure constituents including atleast 60 % ferrite phase and Tempered Martensite from 10 to 40% and in ferrite matrix for excellent stretch formability as shown in accompanying Figure 1.

The advancement favors generation of cold rolled high strength steel sheet having Yield strength of 450 MPa or more, Tensile strength of 780 MPa or more, total elongation of 15% or more, strain hardening coefficient of 0.12 or more, Hole Expansion ratio 30% or more and bake hardening index more than 30 MPa.

,CLAIMS:We Claim:
1. Cold rolled high strength Galvanized steel sheets having tensile strength atleast 780 MPa with composition in terms of weight % comprising:
C: 0.07-0.12 %;
Mn: 1.5- 1.9%;
Si: 0.1–0.35%;
Al: 0.1-0.3%
S: 0.005 % or less;
N: 0.005 % or less
Ti: 0.005-0.05%;
Mo: 0.02-0.1%;
and the balance being Fe and other unavoidable impurities; wherein [Al] / [Si] ratio is in a range of 0.4 to 2 and having selective steel microstructure constituents including atleast 60 % ferrite phase and Tempered Martensite from 10 to 40% and in ferrite matrix for excellent stretch formability.
2.) Cold rolled high strength Galvanized steel sheet as claimed in claim 1, further comprising by weight % atleast one type of element selected from the group of elements comprising V, Nb, Ti, W less than 0.08 wt%.

3.) Cold rolled high strength Galvanized steel sheet as claimed inclaim 1 further comprising in terms of weight % atleast one additive element selected from the group consisting of 0.001% to 0.003% of B, 0.002 % to 0.2 % Cu, 0.002 % to 0.2 % Ni, and less than 0.005 % Ca.

4) Cold rolled high strength Galvanized steel sheets as claimed in anyone of claims 1 to 3 having Yield strength of 450MPa or more, Tensile strength of 780 MPa or more, total elongation of 15% or more, strain hardening coefficient of 0.12 or more,Hole Expansion ratio 30% or more and bake hardening index of 30 MPa or more
5.) A process formanufacturing cold rolled high strength Galvanized steel sheet as claimed in anyone ofclaims 1 to 4, having tensile strength at least 780MPa comprising the steps of:
a) providing steel having composition comprising
C: 0.07-0.12 %;
Mn: 1.5- 1.9%;
Si: 0.1–0.35%;
Al: 0.1-0.3%
S: 0.005 % or less;
N: 0.005 % or less
Ti: 0.005-0.05%;
Mo: 0.02-0.1%;
Cr: 0.21-0.4%;
and the balance being Fe and other unavoidable impurities; wherein [Al] / [Si] ratio is in a range of 0.4 to 2 and
involving processing through Heat from basic oxygen furnace (BOF) and RH degasser and subsequently continuously casting into slabs and reheating said slabs having said composition to reheating temperature in the range from 1190°C -1250 °C;
b) (i)subjecting said reheated slabs to roughing rolling in roughing mill with roughing mill delivery temperature of 1080°C or less;
(ii)Said rough rolled steel being subjected to finish rolling with finish mill exit temperature ranging from Ac3°C to Ac3+100 °C;
c) cold rolling; followed by
d) continuous annealing;
e) Galvanizing and skin pass rolling.
so as to have selective steel microstructure constituents including atleast 60% Ferrite phase and Tempered Martensite from 10 to 40% and in ferrite matrix for excellent stretch formability.

6) The process for manufacturing cold rolled high strength Galvanized steel sheetas claimed in claim 5 comprising
a)Coiling the finish rolled steel with average run out table cooling rate of 10 °C/second or more; and
b) Acid Pickling the Cold rolling the said hot rolled steel sheet with cold reduction of atleast 30%.

7.) The process for manufacturing cold rolled high strength Galvanized steel sheet as claimed in anyone ofclaims5 or 6, wherein cold rolled steel is subjected to continuous annealing following the steps comprising :

a)Annealing the cold rolled steel sheet at soaking section critical temperature range from Ac1+20 °C to Ac3+20 °C with residence time ranging from 70 to 150 seconds;

b) Rapid cooling the steel from SS temperature up to a temperature range of 440 to 470 °C;

c)Zinc bath temperature of the said steel in the range starting from 440°Cto 460 °C;

d)Galvanizing at temperature range of 530 °C to 570°C for Zn-Fe alloying;

e) Subjecting the coated steel to skin pass elongation of 0.2% to 1 %

8.) The process for manufacturing cold rolled high strength Galvanized steel sheet as claimed in anyone of claims 5 to 7 which is selectively controlled to have steel sheet withYield strength of 450MPa or more, Tensile strength of 780 MPa or more, total elongation of 15% or more, strain hardening coefficient of 0.12 or more,Hole Expansion ratio 30% or more and bake hardening index of 30 MPa or more; and having microstructure in terms of area fractioncomprising, 60% or more of ferrite phase, 20 % or more of martensite phase along with carbide, nitride and sulphide precipitates.

Dated this the 10th Day of March, 2021
Anjan Sen
Of Anjan Sen & Associates
(Applicants’ Agent)
IN/PA-199