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 STEEL SHEET FOR TINPLATE HAVING EXCELLENT AGEING RESISTANCE AND METHOD FOR ITS MANUFACTURING.



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 steel sheet for tinplate having excellent ageing resistance and Target hardness level (HR30T) in the range of 54 to 64 with composition in terms of weight % comprising: C: 0.02-0.06 %;Mn: 0.1- 0.5%;Si: 0–0.03%; P: 0-0.02%; Al: 0.02-0.06%; S: 0.01 % or less; N: 0.005 % or less; and the balance being Fe and other unavoidable impurities; and having atleast one element selected from V: 0.002-0.03 % and Ti: 0.003-0.01 %; wherein Ti/N ratio range from 0.6 to 4 for excellent ageing resistance after tin coating.
The advancement favors generation of cold rolled high strength steel sheet having cold rolled steel sheet for tinplate wherein the said steel sheet has yield strength of 250-350 MPa, ageing resistance of 9 months and more, Tensile strength of 300-400 MPa, total elongation of 30% or more and hardness level (HR30T) in the range of 54 to 64.

BACKGROUND OF THE INVENTION

Tin coating and wrinkling is a major limitation which restricts the application of tin coated steel sheet in Oil CAN having deep drawing due to aging nature of steel sheet after temper rolling. Wrinkling is one of the major defects observed during deep drawing process due to ageing nature of sheet metal.

Through improving the ageing resistance better formability can be achieved. To facilitate, ferrite Materials has been utilized where interstitial elements like C, N and S fixed using Ti and or V, Ca and B in ferrite matrix results in excellent ageing resistance and uniform tin coating. However, the optimum deployment of said steel can only be achieved by right combination of ferrite and pearlite phase fractions and their distribution. In order to achieve the said improved ageing resistance, Ti and Ca are added to get surface free from graphitization and non-metallic inclusions on the surface which provides better tin coating.

To avoid wrinkling during deep drawing, cold rolled steel sheet for tinplate wherein the said steel sheet has yield strength of 250-350 MPa, ageing resistance of 9 months and more, Tensile strength of 300-400 MPa, total elongation of 30% or more and hardness level (HR30T) in the range of 54 to 64.

Aging phenomenon is known to be reappearance of yield point in which strength of metal increases and ductility decreases and a low value of strain rate sensitivity on heating at a relatively low temperature after cold-working. This is due to the diffusion of C and N atoms to the dislocations during the aging to form new solute atmospheres anchoring the dislocations. Nitrogen plays a more important role in the strain-aging of iron than carbon because it has a higher solubility and diffusion coefficient and produces less complete precipitation during slow cooling.

It is thus important to eliminate aging in deep drawing steels because the reappearance of yield point can lead to difficulties with surface markings or stretcher strains due to localized heterogeneous deformation.

US4374682 disclosed a process for producing a cold rolled steel strip having excellent workability by a short-time continuous annealing, which process is constituted by hot rolling a low carbon steel slab of steel having a carbon content of from about 0.003 to 0.08% into a steel strip; cold rolling the hot rolled steel strip; and subjecting the steel strip to said short time annealing and then cooled steel strip to an over-ageing treatment, with selected parameters. Cold rolled steel strip is widely used for cold forming articles such as press-formed automobile parts, and as such the strip is required to have an excellent press-forming property. Thus this prior art targets a shorter annealing time to be more effective in preventing dissolving of carbides formed in the hot rolled steel strip, thus shortening the subsequent over-ageing treatment. Low carbon having C: 0.003 to 0.04% is manufactured without alloying elements wherein little solute carbon will be left to cause yield point elongation after ageing.
EP0857794A1 discloses process to obtain the excellent deep drawability and excellent anti-aging properties by eliminating over ageing section and with 0.015% -0.15%of Carbon, 1% or less silicon, 0.1% to 1.0% of Manganese, with combination of Ti and Nb 0.001% to 0.03% and by adding Boron less than 50ppm, Chromium in range of 0.05 to 1.00% with annealing at 800°C and temper rolling with 0.8% to achieving ageing index less than 40Mpa.
Ageing index with 40Mpa or less will cause the problem at the time of final component making using press forming, high chance of material to crack if yield strength is 210, tested after production, after period of time yield strength will be 250 Mpa which will cause a problem during press forming and also out of the desirable property.
Problem to be solved by the Invention
The Present invention aims to solve the problem of the prior arts by providing the good ageing resistance for 9 months and also change in yield strength after 9 months within 20Mpa with alloying of vanadium, Ti, Ca and B with temper rolling of 1.0 % or more with process consisting of over ageing section Temperature 400 to 450 °C to suppress the yield point elongation ensuring ageing guarantee for 9 months at 30°C. The Ratio of Ti/N to be minimum 0.6 to ensure the ageing guarantee and also providing difference in yield strength after processed condition and after 6 hrs of heated at 100 °C and tested condition is around 10 to 20 Mpa.
The continuous annealing has such a serious disadvantage that no satisfactory ageing resistance can be given to Al-killed steel, and thus the remarkable quality and less production cycle time of the continuous anneal¬ing have not been fully obtained in the case of the Al-¬killed steel up to now because of less AlN precipitation which is required to fix N.

OBJECTS OF THE INVENTION
The basic object of the present invention is directed to provide Cold rolled steel sheet for tinplate having excellent aging resistance and method of manufacturing the same

A still further object of the present invention is directed to provide cold rolled steel sheet for tinplate selective composition and processing to achieve the desired hardness and tin coating property.

A still further object of the present invention is directed to provide cold steel sheet for tinplate has yield strength of 250-350 MPa, ageing resistance of 9 months and more, Tensile strength of 300-400 MPa, total elongation of 30% or more and hardness level (HR30T) in the range of 54 to 64.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to provide cold rolled steel sheet for tinplate having ageing resistance of atleast 9 months and Target hardness level (HR30T) in the range of 54 to 64 with composition in terms of weight % comprising:
C: 0.02-0.06 %;
Mn: 0.1- 0.5%;
Si: 0–0.03%;
P: 0-0.02%;
Al: 0.02-0.06%;
S: 0.01 % or less;
N: 0.005 % or less;
and the balance being Fe and including atleast one element selected from V: 0.002-0.03 % and Ti: 0.003-0.01 %; wherein Ti/N ratio range from 0.6 to 4 for desired ageing resistance after tin coating;

A still further aspect of the present invention is directed to said cold rolled steel sheet for tinplate comprising in terms of weight % one additive element selected from the group consisting of 0.0005% to 0.003% of B, and less than 0.003 % Ca.

Another aspect of the present invention is directed to said cold rolled steel sheet for tinplate wherein the said steel sheet has Yield strength of 250-350 MPa, ageing resistance of 9 months and more, Tensile strength of 300-400 MPa, total elongation of 30% or more preferably in the range of 35 to 45%.

Yet another aspect of the present invention is directed to said cold rolled steel sheet for tinplate 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 and sulphide precipitates.

A further aspect of the present invention is directed to a process for manufacturing the Cold rolled steel sheet for tinplate having ageing resistance of atleast 9 months and Target hardness level (HR30T) in the range of 54 to 64, comprising
(a) providing steel with composition in terms of weight % comprising:
C: 0.02-0.06 %;
Mn: 0.1- 0.5%;
Si: 0–0.03%;
P: 0-0.02%;
Al: 0.02-0.06%;
S: 0.01 % or less;
N: 0.005 % or less;
and the balance being Fe and other unavoidable impurities; and having selective steel with atleast one element from V: 0.002-0.03 % and Ti: 0.003-0.01 %; wherein Ti/N ratio range from 0.6 to 4 for desired ageing resistance after tin coating and
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) subjecting said reheated slabs to roughing rolling in roughing mill with roughing mill delivery temperature of 1080°C or less preferably in the range of 1010 to 1060 °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)cold rolling; followed by
e) continuous annealing;
f) skin pass rolling;
g) Tin Coating;
so as to have selective steel sheet 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 precipitates for desired stretch formability with ageing resistance.

A still further aspect of the present invention is directed to said process comprising the steps of:
a) Reheating the slab having a composition as per any of 3 to reheating 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 1010 to1060°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; and
e) Acid Pickling the Cold rolling the said hot rolled steel sheet with cold reduction of atleast 70%.

A still further aspect of the present invention is directed to said process wherein Cold rolled steel sheet for tinplate, 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 Ar3 °C to Ar3+100 °C with residence time ranging from 40 to 200 seconds;
b) Over-aging Temperature the steel from SS temperature up to a temperature range of 400 to 450 °C;
c) Subjecting to skin pass elongation of 1 % to 3 %;
d) Subjecting to tin deposition through electrolytic process at reflow temperature from 200 to 300°C.

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 THE ACCOMPANYING DRAWINGS
Figure 1: shows the Microstructure of Cold rolled Tin Coated steel sheets having by area fraction Ferrite-96% and Pearlite-4%.

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

The present invention relates to Cold rolled steel sheet for tinplate having excellent ageing resistance and Target hardness level (HR30T) in the range of 54 to 64 with composition in terms of weight % comprising:
C: 0.02-0.06 %;
Mn: 0.1- 0.5%;
Si: 0–0.03%;
P: 0-0.02%;
Al: 0.02-0.06%;
S: 0.01 % or less;
N: 0.005 % or less;
and the balance being Fe and other unavoidable impurities; and having selective steel with atleast one element from V: 0.002-0.03 % and Ti: 0.003-0.01 %;
wherein Ti/N ratio range from 0.6 to 4 for excellent ageing resistance of 9 months and more after tin coating.
Cold rolled steel sheet for tinplate wherein the said steel sheet has Yield strength of 250-350 MPa, ageing resistance of 9 months and more, Tensile strength of 300-400 MPa, total elongation of 30% or more and hardness level (HR30T) in the range of 54 to 64.

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 (%)

A cold rolled steel sheet for tinplate with improved ageing resistance and surface tin coating 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 (0.02-0.06 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 cold rolled sheet is set at 0.1 wt%. For improving the r-value i.e. drawability it desirable to reduce the C level less than 0.06 wt%. Lowering the carbon content below 0.02 wt% results in poor ageing property since below 0.02 wt% steel is in complete a-ferrite region in Iron-Cementite phase diagram resulting no cementite formation. Consequently more free carbon will be available in steel matrix which deteriorates the ageing property. Cementite formed in grain boundaries its precipitate size should be greater than 1µm which in helps in reducing solute carbon and suppress the YPE for better ageing resistance. Keeping low carbon <0.02 % also requires vacuum degassing the molten steel which add more cost of production. To avoid YPE and to have yield strength within the desired limit, the level of Carbon is between 0.02-0.06 wt percent.
Mn (0.1-0.5) wt. % - The Mn ranges from 0.1 to 0.5 % is most desirable for the given IF grade provided that S remains in the favourable range of 0.008wt% or less. Keeping the Mn level 0.1wt% with higher sulphur content may result in surface irregularities such as red shortness, edge crack sliver etc. here keeping the level to minimum is advisable for providing a soft material, however, the lower limit is 0.1% as below it the surface property deteriorates and embrittlement may occur due to hot brittleness as the amount of Mn is insufficient to fix S. More than 0.5 %Mn makes the steel hard and the deep draw ability lower.
Vanadium 0.0020 to 0.03%: Vanadium is strong carbide former, reduces solute carbon in the mild steel and forms Vanadium Carbide, as it forms coarser carbide particles it restricts the yield strength within 350Mpa and promotes the ageing resistance. To achieve stretcher strain free material minimum vanadium to be added is 0.002 and to restrict yield strength maximum vanadium is to be 0.03. To achieve the desired properties vanadium is restricted to 0.002 to 0.03 more preferably 0.005 to 0.02.

Ti (0.003-0.01wt%)–Advantage of adding Ti is two-fold. Firstly, Ti acts as a strong Nitride former forming TiN and complex carbo nitrides Ti(C, N) helpful in fixing C and N in order to avoid aging problem in low carbon steel sheet after temper rolling.
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 and higher Ti/N ratio results in higher yield strength so kept Ti/N ration max as 4.
Al (0.02-0.06) wt. % – Al wt% ranges from 0.02-0.06 is intended for fixing free N which results in aging if left unfixed. 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 results in excellent drawability and reduced ?r value as describes the present invention. 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 (0.005wt% or less) -The upper limit for N is 0.005%; it is advisable to keep it to minimum level. Higher N content requires to higher Al addition to fix extra N and increase the volume of AlN precipitates which strengthens the material, ultimately deteriorating the drawing property.
P (0-0.02) wt%: It is the most important strengthening element in present invention as the name itself implies as an Interstitial free high strength rephosphorized steel. P is as 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 0.02wt% the yield strength level increases significantly. In addition, higher amount of P promotes formation surface defects after tin coating. So, maximum limit of P is kept 0.02 %.

Si (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.

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 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 70% or more. Some salient parameters are as follows:
a) Annealing the cold rolled steel sheet at soaking section critical temperature range from Ar3 °C to Ar3+100 °C with residence time ranging from 40 to 200 seconds;
b) Over-aging Temperature the steel from SS temperature up to a temperature range of 400 to 450 °C;
c) Subjecting to skin pass elongation of 1 % to 3 %
d) Subjecting to tin deposition through electrolytic process at reflow temperature from 200 to 300°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 1-5 0C/sec up to soaking section temperature. Soaking section temperature was maintained in the range from Ar3 °C to Ar3+100 °C to achieve ferrite and pearlite
Phases in microstructure. Annealing time is kept in the range from 40 to 200 seconds to allow sufficient time for annealed and homogenization of austenite microstructure. Following soaking section, annealed strip sheet passes through Over-aging section to a temperature range of 400 to 450 °C and followed by double stand skin pass mill where skin pass elongation ranges from 1 to 3% in order to get desired mechanical properties. After Skin pass, said steel is subjected to Tin deposition through 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 temperature ranges from 200 to 300 °C. Resulting microstructure is shown in accompanying Figure 1 illustrating the Microstructure of Cold rolled Tin Coated steel sheets having by area fraction Ferrite-96% and Pearlite-4%.

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 trough 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: Tin Coating Parameters of inventive with comparative steel sheets having chemical compositions as per Table 1.
Table 4:Mechanical properties of 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 V Ti/N Other Elements Remarks
1 0.03 0.1 0.003 0.01 0.03 0.04 0.003 0.005 0.01 1.67 I
Ca: 0.0015
2 0.05 0.4 0.003 0.01 0.02 0.03 0.005 0.01 0.005 2 B: 0.001 I
3 0.002 0.15 0.01 0.015 0.03 0.03 0.003 0.025 0.008 8.3 Nb: 0.007 C
4 0.004 0.2 0.01 0.02 0.04 0.04 0.004 0.065 0.005 16.2 Nb:0.003 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 improved ageing resistance to avoid wrinkling problem and improved Tin coating without bare spot on the surface. These steels exhibits improved yield strength of 250-350 MPa, ageing resistance of 9 months and more, Tensile strength of 300-400 MPa, total elongation of 30% or more and hardness level (HR30T) in the range of 54 to 64.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 has the ultra-low carbon wt% less than 0.02 and higher Ti wt% (>0.01 %) and example steel no. 3 and 4 has does not comply with required ratio of Ti/N ratio and has poor Tin coating surface.

Table 2

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


1 1205 1060 900 650 80
2 1205 1070 890 660 90
3 1220 1070 908 570 70
4 1210 1080 910 560 75

*I - Present inventive example, C- Comparative Examples
Note: Steel marked as 1 and 2have the chemical composition of present inventive steel, and however they are processed at different conditions to validate the claimed process. In sample no 1 and 2 double reduction skin pass elongation varied as 1.8% and 2.5% which resulted YS more than 250 MPa and hardness more than 54 HR.
* 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
CAL and Tin Coating Parameters
Sample No SS TEMP SS Residence Time OAS TEMP Double Reduction Skin Pass Tin Coating Temperature
1 750 95 410 1.8 200
2 770 95 420 2.5 250
3 780 95 350 0.5 180
4 820 90 360 0.8 320

Table 4
Mechanical Prop Coating Properties
Sample No YS TS Elongation % Aging Resistance (Months) Hardness (HR 30T) Tin Coating Cost Remarks
1 280 330 38 9 56 G Low I
2 250 350 40 12 60 G Low I
3 130 310 48 12 35 NG High C
4 140 320 54 12 40 NG High C
*I - Present inventive example, C- Comparative Examples,
G- Good- 0% bare spot on surface, NG- Not Good- > 0% bare spot on surface
Example 2
In sample 3 and 4 where in sample 3 is having ys less than 250 MPa and hardness is less than less than 45 HR and poor coating property which is out of scope of present invention.
In case of sample 3 and 4, Tin coating property is also not goodwhich is out of scope of current invention. As per Table 4, Sample 3 and 4 gives good aging resistance but at higher cost.

It is thus possible by way of the present invention to provide Cold rolled steel sheet for tinplate having excellent ageing resistance and Target hardness level (HR30T) in the range of 54 to 64 with composition in terms of weight % comprising:
C: 0.02-0.06 %;Mn: 0.1- 0.5%;Si: 0–0.03%;P: 0-0.02%;Al: 0.02-0.06%;S: 0.01 % or less;N: 0.005 % or less;and the balance being Fe and other unavoidable impurities; and having selective steel with atleast one element from V: 0.002-0.03 % and Ti: 0.003-0.01 %;wherein Ti/N ratio range from 0.6 to 4 for excellent for excellent ageing resistance after tin coating.

The advancement favors generation of cold rolled high strength steel sheet having
cold rolled steel sheet for tinplate Yield strength of 250-350 MPa, ageing resistance of 9 months and more, Tensile strength of 300-400 MPa, total elongation of 30% or more and hardness level (HR30T) in the range of 54 to 64.

,CLAIMS:We Claim:

1. Cold rolled steel sheet for tinplate having ageing resistance of atleast 9 months and Target hardness level (HR30T) in the range of 54 to 64 with composition in terms of weight % comprising:
C: 0.02-0.06 %;
Mn: 0.1- 0.5%;
Si: 0–0.03%;
P: 0-0.02%;
Al: 0.02-0.06%;
S: 0.01 % or less;
N: 0.005 % or less;
and the balance being Fe and including atleast one element from V: 0.002-0.03 % and Ti: 0.003-0.01 %;
wherein Ti/N ratio range from 0.6 to 4 for desired ageing resistance after tin coating.

2.) The Cold rolled steel sheet for tinplate of claim 1 further comprising in terms of weight % at-least one additive element selected from the group consisting of 0.0005% to 0.003% of B, and less than 0.003 % Ca;

3) The Cold rolled steel sheet for tinplate wherein the said steel sheet as claimed in anyone of claims 1 or 2 having Yield strength of 250-350 MPa, Tensile strength of 300-400 MPa and total elongation of 30% or more preferably in the range of 35 to 45%

4.) The Cold rolled steel sheet for tinplate per claim 1 to 3 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 and sulphide precipitates.

5) A process for manufacturing the Cold rolled steel sheet for tinplate as claimed in anyone of claimed in anyone of claims 1 to 4 having ageing resistance of atleast 9 months and Target hardness level (HR30T) in the range of 54 to 64 comprising :

a) providing selective steel composition in terms of weight % comprising:
C: 0.02-0.06 %;
Mn: 0.1- 0.5%;
Si: 0–0.03%;
P: 0-0.02%;
Al: 0.02-0.06%;
S: 0.01 % or less;
N: 0.005 % or less;
and the balance being Fe and including atleast one element from V: 0.002-0.03 % and Ti: 0.003-0.01 % and wherein Ti/N ratio range from 0.6 to 4 for desired ageing resistance after tin coating and 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) subjecting said reheated slabs to roughing rolling in roughing mill with roughing mill delivery temperature of 1080°C or less preferably in the range of 1010 to 1060 °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) cold rolling; followed by
e) continuous annealing;
f) skin pass rolling; and
g) Tin Coating,

such as to have selective steel sheet 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 supplied precipitates for desired stretch formability with ageing resistance.

6.) The process for manufacturing Cold rolled steel sheet for tinplate as claimed in claim 5, comprising the steps of:
a)Reheating the slab having a composition as per claim 5 to reheating 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 1010 to1060°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; and
e) Acid Pickling the Cold rolling the said hot rolled steel sheet with cold reduction of atleast 70%.

7.) The process for manufacturing Cold rolled steel sheet for tinplate of claim 5 to 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 Ar3 °C to Ar3+100 °C with residence time ranging from 40 to 200 seconds;
b) Over-aging Temperature the steel from SS temperature up to a temperature range of 400 to 450 °C;
c) Subjecting to skin pass elongation of 1 % to 3 %;
d) Subjecting to tin deposition through electrolytic process at reflow temperature from 200 to 300°C.

Dated this the 4th day of April, 2022
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199