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

1 TITLE OF THE INVENTION :
DEVELOPMENT OF HIGH STRENGTH DOUBLE REDUCED TINPLATED STEEL SHEET FOR TWIST OFF CAP APPLICATION AND A PROCESS TO PRODUCE THE SAME.



2 APPLICANT (S)

Name : JSW STEEL COATED PRODUCTS 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 high strength Tinplated steel sheet for twist off caps applications with Double Cold Reduction and a manufacturing method thereof through continuous annealing Route, wherein the composition of raw material in terms of mass percentage of elements are - C: 0.025-0.07 %; Mn: 0.3- 0.75%; Si: 0–0.03%; P: 0-0.03%; Al: 0.03-0.07%; S: 0.01 % or less; N: 0.006 % or less; Ti: 0.01-0.05% and the balance being Fe and other unavoidable impurities. The advancement favours generation of double reduced high strength tinplated steel wherein the thickness of the said steel is less than 0.18 mm and has Yield strength of 600 -680 MPa, Tensile strength of 620-690 MPa, hardness level (HR30T scale) in the range of 73 to 77 and these properties enables requirement’s for twist off caps application which is pressed around screw threads instead of a flange and can be removed by twisting the cap by hand.

BACKGROUND OF THE INVENTION

Twist off caps is sealing devices used in packaging products like bottles. The caps are pressed around screw threads instead of a flange. It can be removed by twisting the cap by hand, eliminating the need for an opener. The material for the caps are low thickness tin plated steel sheets that have high strength. The high strength is realized by double cold reduction of the continuously annealed steel strip. A high consistency in thickness and mechanical properties throughout the length of strip. The strip is used for high speed component making.
Conventionally produced high strength Tin plated steel has yield strength more than 500 MPa. The double reduction method is used where annealed material is subjected to a secondary reduction of more than 18 % strain harden the material to meet a yield strength greater than 600 MPa.
In order to achieve the required yield strength, the selection of suitable steel chemistry along with higher reduction during secondary rolling was planned. The Double cold reduction mills are designed for producing thinner gauge materials that have a capability of provide a cold reduction of up to 30% maximum.
To facilitate the production of high strength, the tin plated steel strip of yield strength more than 600 MPa, the hot rolled coil chemistries were modified along with continuous annealing cycles. Precipitation hardening mechanism was also applied by addition of Ti which precipitates out the N and forms precipitates with C. A certain level of yield strength and hardness is achieved by restricting the grain growth during annealing. The grain size of the ferrite grains is restricted to a maximum of ASTM-11. Finally, the required Yield strength is achieved by subjecting the annealed material to a secondary reduction of more than 24%.

US9315877B2 reveals a method of production of steel sheet for bottom covers of aerosol cans where thickness of the sheet is between 0.27 to 0.34mm which includes, as chemical composition, C: 0.025 to 0.065 mass %, Mn: 0.10 to 0.28 mass %, P: 0.005 to 0.03 mass %, Al: 0.01 to 0.04 mass %, N: 0.0075 to 0.013 mass %, Si: limited to 0.05 mass % or less, S: limited to 0.009 mass % or less, and the balance consisting of Fe and unavoidable impurities, wherein the yield point, in the rolling direction after aging treatment is in range of 460 to 540 Mpa. This process involves a further process of aging treat leading to high cost of production. Also due to aging treatment the material has yield point elongation, which may lead to defects like stretcher strains.

US8012276B2: The invention provides a steel sheet for hard tinplate and a TFS (tin-free steel) steel sheet each having an excellent formability and a temper grade of T4 to DR9 and it is manufactured from raw materials having the same composition by changing a reduction ratio of temper rolling or double reduce rolling for ultra-low carbon aluminium killed steel C and P contents of which are so regulated as to satisfy a specific formula 1.6×C×104+0.93×P×103?70<1>. Manufacturing method consist of hot rolling a steel containing in terms of percent by mass ,C: 0.0030 to 0.0060%, Si: 0.04% or below, Mn: 0.60% or below, P: 0.005 to 0.03%, S: 0.02% or below, Al: more than 0.005% to 0.1%, N: 0.005% or below, satisfying the following formula <1>, and the balance of Fe and unavoidable impurities, at a finish rolling temperature of an Ar3 temperature or above and a coiling temperature within the range of greater than 650° C and not exceeding 750° C ; cold rolling at a reduction ratio of 85 to 95%; then annealing at a recrystallization temperature or above and temper rolling or double reduce rolling to selectively manufacture a steel sheet having said target hardness level (HR30T) in the range of 61±3 to 76±3, and a sheet thickness of 0.2 mm or below, with a reduction ratio selected according to said target hardness.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed towards high strength tinplated double cold reduced steel strip, wherein the thickness of the said steel is less than 0.18 mm and has Yield strength in the range of 600 to 680 MPa, tensile strength in the range of 620-690 MPa, and hardness level (HR30T scale) in the range of 73 to 77 and a process to produce the same through continuous annealing route.

A still further object of the present invention is directed to provide tin plated a high strength steel strip suitable for the development of twist off cap application and an excellent tin plate coating on a high strength level steel strip that gives good corrosion resistance.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to high strength tinplated steel comprising of

steel composition in terms of weight % :
C: 0.025-0.07 %;
Mn: 0.3- 0.75%;
Si: 0–0.03%;
P: 0-0.03%;
Al: 0.03-0.07%;
S: 0.01 % or less;
N: 0.006 % or less;
and the balance being Fe and other unavoidable impurities; and wherein the selective steel composition includes at least one element from V: 0.002-0.04 % and Ti: 0.01-0.05 %; wherein Ti/N ratio range from 0.6 to 4,
with said tinplated steel having thickness of the said steelless than 0.18 mm preferably in the range of 0.14 mm to 0.17 mm and has yield strength in the range of 600 to 680 MPa, Tensile strength in the range of 620 to 690 MPa, hardness level (HR30T) in the range of 73 to 77.

A further aspect of the present invention is directed to said high strength tinplated steel which is double reduced high strength steel based tinplate having formability for twist off cap with pressed around screw thread fixability and twistability for removal of cap.

A still further aspect of the present invention is directed to said high strength tinplated steel wherein the said steel sheet comprises, in terms of area fraction relative to entire microstructure of steel, 95% or more of ferrite phase, 5 % or less of pearlite phase along with carbide, nitride Sulphides inclusions and having the grains oriented in the rolling direction based on secondary rolling for required anisotropic properties.

A still further aspect of the present invention is directed toa process to manufacturing high strength tinplatesteel as stated above comprising the steps of:
a) reheating the slab having said selective composition to reheating temperature in the range from 1150°C -1250 °C;
b) said reheated slab being subjected to rough rolling in roughing mill with roughing mill delivery temperature in the range of 1040°C to 1080°C. preferably of 1080°C or less;
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 5 °C/second to20 °C/second preferably 10 °C/second or more; and
e) subsequently processingsaid hot rolled coils through pickling coupled with tandem cold rolling mill to remove the oxide layer present in the surface and to provide selective cold reduction of 85% or more.

A still further aspect of the present invention is directed tosaid process comprising a combination of strengthening mechanism including precipitation hardening, and grain size hardening along with strain hardening mechanism to achieve the required strength and hardness without hampering the other properties of the material wherein higher Mn % involved selectively provides for required strength by solid solution hardening, the precipitates formed by alloying elements including titanium and vanadium pin down the grain boundaries of the ferrite grains enhancing the strength by precipitation hardening, the lower soaking temperatures during annealing gives as fine grained structure to the material (ASTM No. 11), and finally the secondary reduction of above 24 % imparting the strength of the material by strain hardening.

A still further aspect of the present invention is directed toprocess 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 690°C to 760 °C with residence time ranging from 50 to 180 seconds;
b) Over-aging Temperature the steel from SS temperature up to a temperature range of 400 to 440 °C;
c) Subjecting to skin pass reduction of above 24%
d) Subjecting to tin deposition through electrolytic process at reflow temperature from 200 to 300°C, such as to finally obtain

the said steel thickness less than 0.18 mm and has Yield strength of 600 -680 MPa, Tensile strength of 620-690 MPa, hardness level (HR30T) in the range of 73 to 77 for desired twist off caps application pressable around screw threads instead of a flange and removable by twisting the cap by hand.

The above and other aspects and advantages of the present invention are described hereunder in details with reference to non-limiting accompanying drawing and examples.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure 1: shows (a) a flow chart indicating the process steps involved and (b) the schematic lauout for implementing the process according to present invention.
Figure 2: shows microstructure of the high strength steel sheet of present invention-(a) at 1000x magnification and (b) at 2000x magnification, having in terms of area fraction relative to entire microstructure of steel, 95% or more of ferrite phase, 5 % or less of pearlite phase along with carbide, nitride and sulphide inclusions.

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

The present invention is directed to produce thicker gauge high strength tinplated steel sheet wherein the thickness of the said steel is less than 0.18 mm and has Yield strength in the range of 600 to 680 MPa, Tensile strength in the range of 620 to 690 MPa, hardness level (HR30T) in the range of 73 to 77 and these properties enables the fabrication requirement for twist off caps application, with chemical composition in terms of weight % comprising:

C: 0.025-0.07 %;
Mn: 0.3- 0.75%;
Si: 0–0.03%;
P: 0-0.03%;
Al: 0.03-0.07%;
S: 0.01 % or less;
N: 0.006 % or less;

and the balance being Fe and other unavoidable impurities; said steel furthercomprising at least one element from V: 0.002 to 0.04 % and Ti: 0.01 to 0.05 %; wherein Ti/N ratio range from 0.6 to 4 for excellent ageing resistance after tin coating.

According to another aspect, the present invention makes use of a combination of strengthening mechanisms that involved solid solution hardening by the higher Mn content, precipitation hardening associated with titanium and or vanadium, grain size hardening associated with a fine grain material (ASTM No.11) associated with boundary pinning of the precipitate at low soaking temperature post deformation and finally strain hardening by secondary rolling to above 24% to achieve the desirable properties.

So to have steel sheet with selective properties, it comprises in terms of area fraction relative to entire microstructure of steel, 95% or more of ferrite phase, 5 % or less of pearlite phase along with carbide, nitride and sulphide inclusions. The grains are oriented in the rolling direction giving it an excellent anisotropic property.

Following abbreviations, terminologies and expressions are used to describe the manner of implementation of the present invention:
SRT -Slab Reheating 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 (%)

According to present invention, the cold rolled high strength tin plated steel strip was produced with good formability and ageing resistance. The said steel had the chemical composition and method of manufacturing as 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 (0.025-0.07 wt.%) : While carbon increases the steel strength, it reduces the cold workability and the deep drawability of the cold rolled steel sheet remarkably and thus the higher limit of the invented cold rolled sheet is set at 0.1 wt%. When carbon is less than 0.02%, the steel shows poor aging characteristics and the carbon level is less than 0.07% which ensures that the steel shows improved r-value i.e. drawability. Within the range from 0.02 to 0.07% the a-ferrite content is highest with least cementite formation. The residual carbon exists as grain boundary Cementite with dispersed precipitate size slightly greater than 1µm. The reduction in solute carbon suppresses the yield point for better ageing resistance.

Manganese (0.3-0.75 wt. %): The Mn content ranges from 0.3 to 0.75 %. The minimum 0.30% Mn content is desirable for the fixing the residual S as MnS inclusion. The residual Mn content of 0.30 to 0.75% contributes to the strength of the steel. Still higher Mn content has an impact on eutectoid composition and temperature.

Titanium (0.01-0.05 wt. %): Advantage of adding Ti is two-fold. Firstly, Ti acts as a strong Nitride former and removes N from the steel to form TiN. The fixing of N ensure freedom from yield point phenomenon. The Ti also forms complex carbo nitrides Ti(C, N) which contributes for precipitation hardening after temper rolling. It is desirable to ensure the ratio of Ti to N to be maintained between 0.6 and 4. Fixing the N partially enables desired ageing guarantee of minimum 12 Months. As the total amount of Ti increases, the ferrite grains become finer because of decrease in TiN precipitate size which increases the yield strength of material. On the other hand, if the ratio of Ti and N ratio is less than 0.6 then it results free N which deteriorate in the ageing property.

Aluminium (0.03-0.07 wt. %): Al content ranges from 0.03 to 0.07 % as it is intended to fix the free N which results unfixed during aging. In addition, less than 0.02% Al, will delay the AlN precipitation, causing the insufficient growth of ferrite grains and deteriorates the {111} texture. An optimized volume fraction of AlN in steel matrix in combination with NbC precipitate that result in the excellent drawability and reduced ?r value, as describes the present invention. The same is achieved by optimize hot rolling and cold rolling parameters to avoid Ostwald ripening phenomenon, which results in coarse precipitate size along with low r-bar values.

Nitrogen (0.006 wt. % or less): The upper limit for N is 0.006%; it is advisable to keep it to minimum level. Higher N content requires higher Al addition to fix the extra N and increase the volume of AlN precipitates, which strengthens the material that ultimately deteriorates the drawing property.

Phosphorus (0-0.03 wt. %): P is an element, which improves the strength at low cost, and the amount of addition thereof varies depending on a target strength level. When added amount exceeds more than 0.03 wt. % the yield strength level increases significantly. In addition, the higher amount of P promotes the formation of surface defects and significantly reduces the corrosion resistance of the base metal. So, maximum limit of P is kept 0.03 %.

Silicon (0-0.03 wt. %): It is an element utilized for increasing the strength of steel. As the silicon content increases the ductility noticeably deteriorates. Since silicon deteriorates tin coating properties by forming SiO2 type of oxides (scale) on surface. It is advantageous to add as low an amount of silicon in the steel as is possible. The added amount of silicon is preferably 0.03 wt% or less.

Vanadium:(0.002 to 0.04 %): This element mainly added to form carbides in steel and in order to play main role in refining the structure and grains of the steel, which reduces its sensitivity to overheating, and increases its strength and toughness, during the heat treatment of steel. Vanadium has a strong affinity with carbon, nitrogen, and oxygen when added preferably in amount 0.04 wt% or less which generates stable compounds along with these elements.

DESCRIPTION OF MANUFACTURING PROCESS

Accompanying Figure 1 shows the flow process chart illustrating the basic steps of the process of present invention. Figure 1 showing (a) the process steps and (b) pictorial overview of schematic layout of processes, including Double cold reduction process where material is subjected to more than 24% reduction after continuous annealing.

To achieve the slab chemistry as described in the scope of the invention, the molten metal from basic oxygen furnace (BOF) is processed through RH degasser and subsequently continuously caster as 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 the finishing mill entry temperature under 1080°C to check surface defects like rolled in scale. During hot rolling, the finishing mill temperature is varied in the range from Ac3 to (Ac3+100) °C. After finish rolling, the run out table cooling rate from finishing mill to coiler of more than 9 0C/sec was maintained to achieve a coiling temperature in the range of 620 to 680 °C. 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 85% or more. Further steps include:

a) Continuous Annealing of the cold rolled steel sheet at soaking section critical temperature range from 690 °C to 760 °C with residence time ranging from 50 to 180 seconds;
b) Over-aging Temperature the steel strip maintained up to a temperature range of 400 to 440°C;
c) Subjecting to secondary cold reduction of above 24 %;
d) Subjecting to tin deposition through electrolytic process at reflow temperature from 230 to 265°C.

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 1 to 5 0C/sec up to soaking section. The soaking section temperature was maintained in the range from 690 to 760 °C, to achieve ferrite and pearlite Phases in the microstructure. The annealing time is kept in the range from 50 to 180 seconds to allow sufficient time for annealed and homogenization of microstructure. Following soaking section, the annealed steel strip passes through the over-aging section to a temperature range of 400 to 440 °C and followed by secondary cold reduction of the annealed material, where the reduction percentage ranges above 24%, in order to get the desired mechanical properties. After the secondary rolling, the said steel is subjected to Tin deposition through the electrolytic process, where pure tin is Anode and Cathode-Sheet and Electrolyte- Stannous + phenyl sulfonic acid. Tin deposited on steel sheet and strong bond is formed due to formation of iron tin alloy at the temperature range from 230 to 265 °C.

Accompanying Figure 2 shows the microstructure of the high strength stee sheet of present invention (a) at 1000x magnification and (b) at 2000x magnification having in terms of area fraction relative to entire microstructure of steel, 95% or more of ferrite phase, 5 % or less of pearlite phase along with carbide, nitride and sulphide inclusions and the grains are oriented in the rolling direction giving it an excellent anisotropic property.

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 example 1:

Table 1- Elemental Compositions in weight % of the inventive steel sheets along with comparative example.
Chemistry
Sample No C Mn S P Si Al N Ti Ti/N Remarks
1 0.025 0.30 0.003 0.01 0.03 0.04 0.005 0.025 5 C
2 0.045 0.35 0.003 0.01 0.015 0.03 0.004 0.015 3.75 I
3 0.050 0.4 0.01 0.015 0.02 0.03 0.004 0.020 5 I
4 0.040 0.45 0.01 0.02 0.025 0.04 0.003 0.035 11.67 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 the steel sheets remarked as “I” are satisfying all the scopes of present invention and improved ageing resistance to avoid problems like earing, fluting marks. The surface of the steels was devoid of any kind of inclusions and scales, which gave a good Tin coating surface without any discontinuities. These steels exhibit improved Yield strength in the range of 600 to 680 MPa, Tensile strength in the range of 620 to 690 MPa, hardness level (HR30T) in the range of 73 to 77. Whereas, the steel remarked as ‘C’ from Table 1 to Table 4 does not comply with at least 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. 1 has low carbon concentration less than 0.03. This lead to low yield strength and required higher reduction of the annealed material to achieve the Yield Strength in the desired range. Similarly, example no. 4 has more Ti. Wt% (>0.03). This may lead to presence of non-metallic surface impurities leading improper coating and low corrosion resistance.

Table 2-hot rolling and cold rolling of inventive steel sample with comparative steel sheets.
Hot Rolling Parameters Cold Rolling Parameters
Sample No SRT, °C Roughing Mill temp, °C FT, °C CT, °C Cold Reduction %


1 1200 1070 870 650 85
2 1210 1050 880 660 85
3 1220 1060 890 570 85
4 1205 1080 860 560 85

*I - Present inventive example, C- Comparative Examples
Note: Steel marked as 2 and 3 have the chemical composition of present inventive steel, and however they are processed at different conditions to validate the claimed process.
* SRT- Slab reheating temperature, FT- hot finish rolling temperature, ROT- Run out table at hot strip mill, CR%- Cold rolling reduction %, SS- soaking section, OAS-Over-Ageing Section, SPM- Skin pass elongation.

Table 3 Tin Coating Parameters of inventive with comparative steel sheets.
CAL and Tin Coating Parameters
Sample No SS TEMP SS Residence Time OAS TEMP Secondary reduction % at DCR Tin induction Reflow Temperature
1 760 95 430 30 250
2 740 95 430 30 250
3 730 95 430 28 250
4 700 90 430 18 250

Table 4 Mechanical properties of comparative steels

Sample No Mechanical Properties Coating Properties Remarks
YIELD STRENGTH TS Elongation % Aging Resistance (Months) Hardness (HR 30T) Tin Coating -
1 560 570 2.8 12 69 G C
2 620 635 1.25 12 75 G I
3 610 620 1.5 12 74 G I
4 580 585 2.5 12 72 G C

*I - Present inventive example, C- Comparative Examples, G- Good, NG- Not Good
In sample 1 and 4 are having yield strength and hardness less than which is out of scope of present invention.

It is thus possible to produce tin plated steel less than 0.18 mm thickness and has Yield strength of 600 -680 MPa, Tensile strength of 620-690 MPa, hardness level (HR30T) in the range of 73 to 77 and good coating/corrosion inhibiting properties by the way of present invention using steel grade with composition by weight %: - C: 0.025-0.07 %; Mn: 0.3- 0.75%; Si: 0–0.03%; P: 0-0.03%; Al: 0.03-0.07%; S: 0.01 % or less; N: 0.006 % or less; and the balance being Fe and other unavoidable impurities and having selective steel with at least one element from V: 0.002-0.04 % and Ti: 0.01-0.05 %; wherein Ti/N ratio range from 0.6 to 4 for excellent for excellent ageing resistance after tin coating and using double cold reduction method with continuous annealing.

The advancement favours generation of double reduced high strength tinplated steel wherein the thickness of the said steel is less than 0.18 mm and has yield strength in the range of 600 to 680 MPa, Tensile strength in the range of 620 to 690 MPa, hardness level (HR30T) in the range of 73 to 77 and these properties enables requirement for twist off caps application which is pressed around screw threads instead of a flange and can be removed by twisting the cap by hand.
, Claims:WE CLAIM:

1. High strength tinplated steel comprising of

steel composition in terms of weight % :
C: 0.025-0.07 %;
Mn: 0.3- 0.75%;
Si: 0–0.03%;
P: 0-0.03%;
Al: 0.03-0.07%;
S: 0.01 % or less;
N: 0.006 % or less;
and the balance being Fe and other unavoidable impurities; and wherein the selective steel composition includes at least one element from V: 0.002-0.04 % and Ti: 0.01-0.05 %;wherein Ti/N ratio range from 0.6 to 4,
with said tinplated steel having thickness of the said steel less than 0.18 mm preferably in the range of 0.14 mm to 0.17 mm and has yield strength in the range of 600 to 680 MPa, Tensile strength in the range of 620 to 690 MPa, hardness level (HR30T) in the range of 73 to 77.

2. The High strength tinplated steel as claimed in claim 1 which is double reduced high strength steel based tinplate having formability for twist off cap with pressed around screw thread fixability and twistability for removal of cap.

3. The High strength tinplated steel as claimed in anyone of claims 1 or 2 wherein the said steel sheet comprises, in terms of area fraction relative to entire microstructure of steel, 95% or more of ferrite phase, 5 % or less of pearlite phase along with carbide, nitride Sulphides inclusions and having the grains oriented in the rolling direction based on secondary rolling for required anisotropic properties.

4. A process to manufacturing high strength tinplatesteel as claimed in anyone of claims 1 to 3 comprising the steps of:
a) reheating the slab having said selective composition to reheating temperature in the range from 1150°C -1250 °C;
b) said reheated slab being subjected to rough rolling in roughing mill with roughing mill delivery temperature in the range of 1040°C to 1080°C preferably of 1080°C or less;
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 5 °C/second to 20 °C/second preferably 10 °C/second or more; and
e) subsequently processing said hot rolled coils through pickling coupled with tandem cold rolling mill to remove the oxide layer present in the surface and to provide selective cold reduction of 85% or more.

5. The process as claimed in claim 4 comprising a combination of strengthening mechanism including precipitation hardening, and grain size hardening along with strain hardening mechanism to achieve the required strength and hardness without hampering the other properties of the material wherein higher Mn % involved selectively provides for required strength by solid solution hardening, the precipitates formed by alloying elements including titanium and vanadium pin down the grain boundaries of the ferrite grains enhancing the strength by precipitation hardening, the lower soaking temperatures during annealing gives as fine grained structure to the material (ASTM No. 11), and finally the secondary reduction of above 24 % imparting the strength of the material by strain hardening.

6. The process as claimed in anyone of claims 4 to 5 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 690°C to 760 °C with residence time ranging from 50 to 180 seconds;
b) Over-aging Temperature the steel from SS temperature up to a temperature range of 400 to 440 °C;
c) Subjecting to skin pass reduction of above 24%;
d) Subjecting to tin deposition through electrolytic process at reflow temperature from 200 to 300°C, such as to finally obtain
the said steel thickness less than 0.18 mm and has Yield strength of 600 -680 MPa, Tensile strength of 620-690 MPa, hardness level (HR30T) in the range of 73 to 77 for desired twist off caps application pressable around screw threads instead of a flange and removable by twisting the cap by hand.

Dated this the 13th day of March, 2024
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199