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 STEEL SHEET WITH IMPROVED BENDABILITY 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 980 MPa or more involving selective chemical composition and processing through continuous annealing route to achieve the desired microstructure and excellent stretch formability and bendability. The advancement favors generation of cold rolled high strength Galvanized steel sheet having Yield strength of 600 MPa or more, Tensile strength of 980 MPa or more, total elongation of 12% or more, strain hardening coefficient of 0.09 or more, and bend radius less than 1.4t (t-thickness) which makes such steel sheets suitable for automobile applications. The present advancement also concerns achieving better bendability 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-martensitic materials has been utilized where Tempered martensite in ferrite matrix results in bendability. 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 improved bendability, 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 >980 MPa must also exhibit a good strain hardening coefficient (n-value) of atleast 0.09 along with total elongation of no less than 12% and bend radius less than 1.4t (t-thickness).

With utilization of dual phase high strength steel, automobile manufacturers are requiring more high strength materials with UTS 980MPa 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 bendability or press formability when yield ratio increases.. 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.

As a part of prior art, the Chinese patent application number CN102471849A discloses method of manufacturing a cold rolled steel sheet with minimum UTS of 980 MPa and its process for automotive structural component. A high strength automotive steel sheet is disclosed, obtained by balancing bainite and martensite volume fraction percentage. However, the method disclosed in prior art fall short in getting good total elongation and n value due to higher amount of 50% or more martensite phase fraction as strengthening phase.

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 980MPa, and total elongation more than 12% having improved bendability 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 bendability, corrosion resistance and method of manufacturing the same.

A still further object of the present invention is directed to provide cold rolled high strength steel sheet involving 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 provide cold rolled high strength steel sheet having tensile strength 980 MPa or more, a good strain hardening coefficient (n-value) of atleast 0.09 along with high total elongation of no less than 12 %, and Bend radius less than 1.4t

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 980 MPa with composition in terms of weight % comprising:
C: 0.07-0.12 %;
Mn: 1.91- 2.4%;
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; and having selective steel microstructure constituents in terms of area fraction including atleast 50 % ferrite phase preferably in the range of 50 to 70 %and Tempered Martensite from 20 to 40% and in ferrite matrix for excellent Bendability.

A still further aspect of the present invention is directed to said Cold rolled Galvanized steel sheet as above , further comprising by weight % atleast one type of element selected from the group of elements comprising V, and Nb less than 0.08 wt%.

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

Another aspect of the present invention is directed to said cold rolled high strength steel sheets having Yield strength of 600MPa or more, Tensile strength of 980 MPa or more, total elongation of 12% or more, strain hardening coefficient of 0.09 or more and Bend radius less than 1.4t where t is the thickness of sheet.

A further aspect of the present invention is directed to a process for manufacturing the cold rolled high strength steel sheets as described above having tensile strength of atleast 980 MPa comprising the steps of:
a) providing steel having composition in wt% comprising
C: 0.07-0.12 %;
Mn: 1.91- 2.4%;
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;;

and processing the same 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) subjecting said reheated slabs to roughing rolling in roughing mill with roughing mill delivery temperature of 1080°C or less;
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 thereby providing the cold rolled high strength steel sheets having
selective steel microstructure constituents in terms of area fraction including atleast 50% Ferrite phase and Tempered Martensite from 20 to 40% and in ferrite matrix for excellent bendability.

A still further aspect of the present invention is directed to said process further comprising the steps of:
a) Reheating the slab to temperature in the range from 1150°C -1250 °C;
b) Said Reheated slab being subjected to roughing rolling in roughing mill with roughing mill delivery temperature of 1080°C or less preferably in the range of 1020 to 1070 °C ;
c) Said rough rolled steel being subjected to finish rolling with finish mill exit temperature ranging from Ac3 °C to Ac3+100 °C.
d) Coiling the finish rolled steel at with average run out table cooling rate of 10 °C/second or more preferably in the range of 10 to 15 °C/second; and
e) Acid Pickling and Cold rolling the said hot rolled steel sheet with cold reduction of atleast 30% preferably in the range of 35 to 50%.

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 by heating up to soaking section critical temperature range from 760 °C to 810 °C with residence time ranging from 70 to 150 seconds;
b) slow cooling the steel at cooling rate in the rage from 0.2 to 3°C/sec up to a temperature in the range from 680°C to 720 °C after soaking ;
c) rapid cooling the steel from SCS temperature up to a temperature range of 420 °C to 500 °C at a critical cooling rate of 40°C/sec or less preferably in the range of 20 to 35 °C/sec;
d) Over-aging the said steel in the temperature range starting from 380 °C to 420 °C or more with residence time of 250 to 560 seconds wherein
e) Subjecting the over-aged steel to skin pass elongation of 0.20 to 1%.
A still further aspect of the present invention is directed to said process wherein cold rolled steel is subjected to said continuous Galvanizing 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 alloying;
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 drawing and examples:

BRIEF DESCRIPTION OF ACCOMPANYING DRAWING
Figure 1: shows the Microstructure of Cold rolled high strength Galvanized steel sheets (Ferrite-65% and martensite-35%).

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO DRAWING AND EXAMPLES

The present invention relates to cold rolled high strength steel sheet having Tensile strength 980 MPa or more and composition in terms of weight percent comprising:
C: 0.07-0.12 %;
Mn: 1.91- 2.4%;
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; and having selective steel microstructure constituents including atleast 50 % ferrite phase preferably in the range of 50 to 70 %and Tempered Martensite from 20 to 40% as shown in figure 1 and in ferrite matrix for excellent stretch formability.

Cold rolled high strength steel sheet obtained according to present invention having said steel sheet has Yield strength of 600MPa or more, Tensile strength of 980 MPa or more, total elongation of 12% or more, strain hardening coefficient of 0.09 or more, bake hardening index of 30 MPa or more and bend radius less than 1.4t, where t is strip thickness.

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
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 bendability and corrosion resistance according to present invention, its chemical compositions and method of manufacturing are described hereunder with explanation on metallurgical factors deciding the range of constituents in 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: 1.91-2.4 wt %) - Similar to C, Mn is 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>980 MPa, minimum amount of Mn must be at-least 1.91%. 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 2.4 %.

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>980 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 the surface. Hence, the upper Si content 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 is set to 0.3%.

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 or 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 satisfy restricted upto 0.4 wt% for better weldability.

Ti (0.005-0.05 wt %): Ti forms carbide and impart precipitation strengthening to the steel. However, as Mo is already added, any additional content more than 0.05 wt% will add up higher yield strength and poor bendability. Moreover, higher addition of this element will form fine carbides, reducing elongation. Accordingly, upper limit of Ti must be less than 0.05wt%.

V, Ti and Nb (collectively <0.08wt %): V and Nb forms 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 element selecting from V and Nb 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. 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 30% 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 °C/sec up to soaking section temperature.

In Continuous annealing the cold rolled steel sheet by heating up to soaking section critical temperature range from 760 °C to 810 °C with residence time ranging from 70 to 150 seconds then slow cooling the steel at cooling rate in the rage from 0.2 to 3°C/sec up to a temperature in the range from 680°C to 720 °C after soaking, rapid cooling the steel from SCS temperature up to a temperature range of 420 °C to 500 °C at a critical cooling rate of 40°C/sec or less, Over-aging the said steel in the temperature range starting from 380 °C to 420 °C or more with residence time of 250 to 560 seconds wherein steel sheet subjected to skin pass elongation of 0.20 to 1%. Then in Galvanizing line again re-annealing at 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 temperature for 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.

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 rolling of inventive with comparative steel sheets having chemical compositions as per Table 1.
Table 3: CAL Parameters of inventive with comparative steel sheets having chemical compositions as per Table 1.
Table 4: CGL Parameters of inventive with comparative steel sheets having chemical compositions as per Table 1.
Table 5: Mechanical properties, Bend radius and micro structural phase fractions of inventive and comparative steels having chemical composition as per table 1 and being processed as per Table 2, Table 3 and Table 4

Table 1
Chemical Composition in %
Sample No Mn S P Si Al N Ti Cr Other Elements Remarks
1a 0.09 1.95 0.003 0.01 0.24 0.12 0.004 0.025 0.35
Ti-0.02, Mo-0.05 I

1b 0.09 1.95 0.003 0.01 0.24 0.12 0.004 0.025 0.35 Ti-0.02, Mo-0.05 I
2 0.13 2.5 0.004 0.015 0.35 0.05 0.003 0.017 0.6 V-0.02, Mo-0.07 C
3 0.09 3.1 0.01 0.02 0.2 0.035 0.004 0.03 0.2 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, Bendability and improved corrosion resistance without bare spot on the surface. These steels exhibit improved yield strength of 600MPa or more, Tensile strength of 980 MPa or more, total elongation of 12% or more, strain hardening coefficient of 0.09 or more, bake hardening index of 30 MPa or more and bend radius less than 1.4t, where t is strip thickness. 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. 3 to 5 in table 1 has the less carbon % than the scope poor bendability 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 1220 1070 908 570 45
3 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 20% and n value is greater than 0.09 whereas in sample 1b where rapid cooling temperature is 470°C and rapid cooling rate of 42 deg C/sec where martensitic phase % is 45% which resulted as n value is less than required value and zinc coating was Not Good (NG).
In sample 2 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 bend radius.
In Sample 3, where Mn percentage is more than 2.5%, 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. CAL-Continuous Annealing line
Table 3
CAL Parameters
Sample No SS TEMP SS Residence Time SCS Temp RCS TEMP Rapid Cooling Rate OAS Temperature SPM ELONG
1a 780 110 680 444 20 380 0.2
1b 800 80 710 480 30 400 0.3
2 800 80 680 370 50 240 0.4
3 780 110 700 400 45 260 0.4

Table 4
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 800 95 470 42 460 540 0.3
2 800 95 420 50 440 540 0.4
3 780 90 490 10 460 530 0.4

Table 5
Mechanical Prop Product Properties
Sample No YS YPE TS ELONGATION n Value Bend Radius
(t-thickness) Ferrite % Martensite % Zinc Coating Remarks
1a 620 0 1040 14 0.12 1t 80 20 G I
1b 750 0 1130 10 0.08 1.8t 55 45 NG C
2 700 0 1100 11 0.07 2t 55 45 NG C
3 650 0 920 11 0.08 2t 85 15 NG C

*I - Present inventive example, C- Comparative Examples, G- Good, NG- Not Good

Example 2
In sample 1b, 2 and 3 where in sample 3 Elongation is less than 12% and n-value is less than 0.09 which is out of scope of present invention.
In case of sample 1b and 2, n-values are less than 0.09 which is out of scope of current invention; in case of sample 1b where martensite percentage is 45 percent and n value is 0.08 less than 0.09 and has Bend radius 1.8t which is out of scope of current invention.

It is thus possible by way of the present invention to provide 980 MPa Tensile strength level high strength cold rolled steel sheet involving select chemical elements in terms of weight percent: Cold rolled high strength Galvanized steel sheets having tensile strength atleast 980 MPa with composition in terms of weight % comprising: C: 0.07-0.12 %; Mn: 1.91- 2.4%; 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; and having selective steel microstructure constituents including atleast 50 % ferrite phase and Tempered Martensite from 20 to 40% and in ferrite matrix for excellent bendability.

The advancement favors generation of cold rolled high strength steel sheet having Yield strength of 600MPa or more, Tensile strength of 980 MPa or more, total elongation of 12% or more, strain hardening coefficient of 0.09 or more, bake hardening index of 30 MPa or more and bend radius less than 1.4t, where t is strip thickness.
,CLAIMS:We Claim:
1.) Cold rolled high strength Galvanized steel sheets having tensile strength atleast 980 MPa with composition in terms of weight % comprising:
C: 0.07-0.12 %;
Mn: 1.91- 2.4%;
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; and having selective steel microstructure constituents in terms of area fraction including atleast 50 % ferrite phase preferably in the range of 50 to 70 % and Tempered Martensite from 20 to 40% and in ferrite matrix for excellent bendability/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 and Nb less than 0.08 wt%.

3.) Cold rolled high strength Galvanized steel sheet as claimed in claim 1 further comprising in terms of weight % atleast one additive element selected from the group consisting of 0.001% to 0.003% of B, 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 600MPa or more, Tensile strength of 980 MPa or more, total elongation of 12% or more, strain hardening coefficient of 0.09 or more and bend radius less than 1.4t where t is strip thickness.

5.) A process for manufacturing the cold rolled high strength steel sheets as claimed in anyone of claims 1 to 4 having tensile strength of atleast 980 MPa comprising the steps of:
a) providing steel having composition in wt% comprising
C: 0.07-0.12 %;
Mn: 1.91- 2.4%;
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;
and processing the same 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) subjecting said reheated slabs to roughing rolling in roughing mill with roughing mill delivery temperature of 1080°C or less;
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 thereby providing the cold rolled high strength steel sheets having
selective steel microstructure constituents in terms of area fraction including atleast50% Ferrite phase and Tempered Martensite from 20 to 40% and in ferrite matrix for excellent bendability.

6.) The process as claimed in claim 5, comprising the steps of:
f) Reheating the slab to reheating temperature in the range from 1150°C -1250 °C;
g) Said Reheated slab being subjected to roughing rolling in roughing mill with roughing mill delivery temperature of 1080°C or less preferably in the range of 1020 to 1070 °C;
h) Said rough rolled steel being subjected to finish rolling with finish mill exit temperature ranging from Ac3 °C to Ac3+100 °C.
i) Coiling the finish rolled steel at 560-610°C with average run out table cooling rate of 10 °C/second or more preferably in the range of 10 to 15 °C/second; and
j) Acid Pickling the Cold rolling the said hot rolled steel sheet with cold reduction of atleast 30% preferably in the range of 35 to 50 %.
7.) The process as claimed in anyone of claims 5 or 6, wherein said cold rolled steel is subjected to said continuous annealing following the steps comprising;
a) annealing the cold rolled steel sheet by heating up to soaking section critical temperature range from 760 °C to 810 °C with residence time ranging from 70 to 150 seconds;
b) slow cooling the steel at cooling rate in the rage from 0.2 to 3°C/sec up to a temperature in the range from 680°C to 720 °C after soaking;
c) rapid cooling the steel from SCS temperature up to a temperature range of 420 °C to 500 °C at a critical cooling rate of 40°C/sec or less preferably in the range of 20 to 40 °C/sec;
d) Over-aging the said steel in the temperature range starting from 380 °C to 420 °C or more with residence time of 250 to 560 seconds, wherein
e) Subjecting the over-aged steel to skin pass elongation of 0.20 to 1%.
8.) The process as claimed in anyone of claims 5 to 7, wherein cold rolled steel is subjected to continuous Galvanizing following the steps comprising;
f) Re-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;
g) Rapid cooling the steel from SS temperature up to a temperature range of 440 to 470 °C;
h) Zinc bath temperature of the said steel in the range starting from 440 °C to 460 °C;
i) Galvanizing at temperature range of 530 °C to 570°C for Zn-Fe alloying;
j) Subjecting the coated steel to skin pass elongation of 0.2% to 1 %.

Dated this the 19th day of March, 2022
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199