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 ANISOTROPY 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 anisotropy and Target hardness level (HR30T) in the range of 48 to 56 with composition in terms of weight % comprising: 0.0005-0.003 %;Mn: 0.1- 0.5%;Si: 0–0.03%;P: 0.01-0.03%; Al: 0.02-0.06%;S: 0.01 % or less;N: 0.005 % or less;Cr: 0.01-0.04 %;and the balance being Fe and other unavoidable impurities; and having selective steel with atleast one element from Ti: 0.01-0.08 % and Nb: 0.01-0.03% wherein Nb/Cr range from 1to 3 for excellent 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 200-250 MPa, planner anisotropy ?r less than 0.04, Tensile strength of 270-350 MPa, total elongation of 35% or more and hardness level (HR30T) in the range of 48 to 56.

BACKGROUND OF THE INVENTION
Tin coating and earing is a major limitation which restricts the application of tin coated steel sheet in COCA COLA CAN having extra deep drawing due to anisotropic nature of steel sheet. Earing is one of the major defects observed during deep drawing process due to anisotropic nature of sheet metal. Earing is defined as formation of waviness on uppermost portion of deep drawn cup.

Through improving the anisotropy better formability can be achieved. To facilitate, ferriteMaterials has been utilized where interstitial elements like C, N and S fixed using Ti and or Nb and Cr in ferrite matrix results in excellent anisotropy 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 saidimproved anisotropy, Cr and Al are added to get surface free from graphitization and non-metallic inclusions on the surface which provides better tin coating.

To avoid earing during deep drawing, cold rolled steel sheet for tinplate wherein the said steel sheet has Yield strength of 200-250 MPa, planner anisotropy ?r less than 0.04, Tensile strength of 270-350 MPa, total elongation of 35% or more and hardness level (HR30T) in the range of 48 to 56.

As a part of prior art European patent specification EP0120976 B1 discloses to a method of producing cold rolled steel sheets used for automotive exterior plate and the like and adapted for deep drawing C=0.015 wt%, Mn= 0.4 wt% , P= 0.03 wt%,0.005 -0.1 wt% , N=0.01 wt%. A basic stoichiometry -0.015=Ti-(48/32) S + (48/14) N <0.004 has been claimed as a part of invention for minimum amount of Ti in slab required to fix N and S. The main drawback of the steel grade obtained according to this prior art is that it does not comply with any aging guarantee as the C level is very high up to 0.015 wt%. Also no alloying element has been claimed to fix C, and Ti added is only sufficient to fix N and S which is derived from stoichiometry and not from experiment. It is found that teachings of European patent specification EP 0120976 B1 suffers from poor drawability as would obviously lead to a r-bar value less than 1.8 along with aging problem as upper N level of 0.01 wt% claimed is also very high.
Indian Patent Application number 859/KOL/2012 describes a cold-rolled steel sheet having excellent bending formability, comprising a composition including in mass%, C: 0.005 % to 0.030 %; Si: 0.05 % or less; Mn: 0.10 % to 0.35 %; P: 0.025 % or less; S: 0.015 % or less; N: 0.01 % or less with specific Si to Mn ratio less than 0.5 with 50% cementite precipitate present in ferrite matrix. However, the cold rolled steel sheet described in 859/KOL/2012 suffers from low aging due to cementite phase and lack of complete ferritic matrix makes it susceptible to aging. Also drawability and texture suffers due to high Carbon percentage.
Another Indian Patent Application number 2736/MUM/2011 describes steel sheet having excellent workability with C=0.01% , Si =0.2 wt% , Mn =0.5wt% N=0.01 wt% Ti -0.02 -0.1 wt% B <0.003 % along with Ti4C2S2 having particle size >10 nm dispersed in to steel at a volume fraction of 0.005 to 0.5 % . However, the cold rolled steel sheet described in 2736/MUM/2011 may suffer from poor drawability in actual press forming due to high C wt% as far as the interstitial free cold rolled steel are concerned. Added amount of Ti as described in patent number 2736/MUM/2011 is sufficient to fix C, N and remaining S, however additional alloying such as high Mn and Si wt% along with B, Mo, Cu Ni, Pb, Sb and other alloying as described in 2736/MUM/2011 remain free in solution and hardens the ferrite matrix. Hence the steel grade obtained according to application number 2736/MUM/2011 suffers from problems of poor drawability and inferior texture.
United States Patent number US4551182 reveals a process for producing deep-drawing cold rolled steel strips comprising 0.0014-0.0031% C, 0.010-0.100% Al 0.019-0.052% Ti, 0.00l9-0.0038% N with the balance being iron and unavoidable impurities .With this technique adequate elongation and r-bar value can be obtained however it will not be sufficient for actual press formability as required for IF based EDDQ steel material. Cracking may occur as use of Ti only to fix C and N may result in higher ?r value which is not good for press formability.

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

A still further object of the present invention is directed to provideCold 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 providecold steel sheet for tinplate has Yield strength of 200-250 MPa, Tensile strength of 270-350 MPa, total elongation of 35% or more and hardness level (HR30T) in the range of 48 to 56.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to provideCold rolled steel sheet for tinplate having excellent anisotropy and Target hardness level (HR30T) in the range of 48 to 56comprising of steel composition in terms of weight % comprising:
C: 0.0005-0.003 %;
Mn: 0.1- 0.5%;
Si: 0–0.03%;
P: 0.01-0.02%;
Al: 0.03-0.06%;
S: 0.01 % or less;
N: 0.005 % or less;
Cr: 0.01-0.04 %;
and the balance being Fe; and which includes atleast one element from Ti: 0.01-0.08 % and Nb: 0.01-0.03% and wherein Nb/Cr range from 1 to 3 suitable for tin coated tinplate;

A still further aspect of the present invention is directed to said cold rolled steel sheet for tinplate comprising in terms of weight % atleast one additive element selected from the group comprising 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 steel sheet for tinplate wherein the said steel sheet having Yield strength of 200-250 MPa, planner anisotropy ?r less than 0.04, Tensile strength of 270-350 MPa, total elongation of 35% or more and hardness level (HR30T) in the range of 48 to 56.

Yet another aspect of the present invention is directed to said cold rolled steel sheet for tinplatewherein the said steel sheet comprises, in terms of area fraction relative to entire microstructure of steel, 99% or more of ferrite phase, 1 % 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 as described above having excellent anisotropy comprising the steps of:
(a)providing steel having composition in terms of wt% comprising
C: 0.0005-0.003 %;
Mn: 0.1- 0.5%;
Si: 0–0.03%;
P: 0.01-0.02%;
Al: 0.03-0.06%;
S: 0.01 % or less;
N: 0.005 % or less
Cr: 0.01-0.04 %;
and the balance being Fe; and which includes atleast one element from Ti: 0.01-0.08 % and Nb: 0.01-0.03% wherein Nb/Cr range from 1 to 3 for excellent tin coating and
involving processing through Heat from basic oxygen furnace (BOF) and RH degasser and subsequently continuously casting into slabs and reheating said slabs having said composition to reheating temperature in the range from 1190°C -1250 °C;
b)subjecting said reheated slabs to roughing rolling in roughing mill with roughing mill delivery temperature of 1080°C or lesspreferably in the range of 1010 to 1060°C;
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)skin pass rolling;
f)Tin Coating;
so as to have selective steel sheet having microstructure constituents including atleast 99 % Ferrite phase and 1 % or less of pearlite phase along with carbide as shown in figure 1 and nitride and sulphide precipitates for excellent stretch formability.

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 temperature 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) coiling the finish rolled steel at with average run out table cooling rate of 10 to 15 °C/second preferably about 10 °C/second or more; and
e) acid pickling and cold rolling the said hot rolled steel sheet with cold reduction of atleast 80%.

A still further aspect of the present invention is directed to said process for manufacturing 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+50 °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 2 %;
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 greater details with reference to non-limiting accompanying drawing and examples:

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1: shows the Microstructure of Cold rolled Tin Coated steel sheets (Ferrite-99% and Pearlite-1%).

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 and Target hardness level (HR30T) in the range of 51±3 to 53±3 with composition in terms of weight % comprising:
C: 0.0005-0.003 %;
Mn: 0.1- 0.5%;
Si: 0–0.03%;
P: 0.01-0.02%;
Al: 0.03-0.06%;
S: 0.01 % or less;
N: 0.005 % or less;
Cr: 0.01-0.04 %;
and the balance being Fe and other unavoidable impurities; and having selective steel with atleast one element from Ti: 0.01-0.08 % and Nb: 0.01-0.03% wherein Nb/Cr range from 1to 3 for excellent tin coating.

Cold rolled steel sheet for tinplate wherein the said steel sheet has Yield strength of 200-250 MPa, Tensile strength of 270-350 MPa, total elongation of 35% or more, andhardness level (HR30T) in the range of 51±3 to 53±3.

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 anisotropy 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 constituentsin a composition according to a preferred embodiment wherein all the elements are in weight % as follows:

Carbon (0.0005-0.003wt% or less) - C present as intestinal solute is a major element deteriorating the drawability as well as increasing the strength of steel. It is well known that lower the C wt% softer will be the material and aging property and drawability will be better. Also reducing the C wt% reduces the amount of alloying addition (Nb and Ti) to fix the same ultimately reducing the production cost. Also C in steel as an interstitial element prevents the formation of {111} texture, deteriorating the drawability. Hence the C wt% is restricted to 0.003wt%.
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.
Nb (0.01-0.03 wt%)–Advantage of adding Nb is two-fold. Firstly, Nb acts as a strong Carbide former forming NbC and complex carbo nitrides Nb(C, N) helpful in fixing C and avoids graphitization to improve Tin Coat-ability. Secondly Nb is the most effective element for reducing planner anisotropy (?r, ?YS, ?El ) which is attributed to relatively small grain size of hot bands for Nb added IF steel. It has also been found that the optimum precipitate size of NbC must be between 8-30 nm for improved texture and less ?r value. Also the volume fraction of NbC and Nb(C,N) combined must be in the range of 0.01-0.04 % .
Nb helps in reducing Anisotropies of r value significantly by forming NbC and Nb (C, N). This can be seen by taking an example of Ti only IF steel having r-value of around 2.2 in transverse direction (r90), 1.8 in rolling direction (r0) and 1.25 in diagonal direction (r45) giving the anisotropy (?r=( r90+r0 -2r45)/2) of 0.75 whereas for Nb added steel the ?r ranges from 0.1-0.4 .
Minimum amount of Nb required to fix C in combination with Cr is 0.01wt% keeping the other elements such as C, N and S and Cr within controlled limit as described in present invention. Nb levels more than 0.03 wt% will unnecessarily add up to the production cost. The recrystallization temperature increase with added Nb hence YS increases for same annealing temperature. R-value also decreases with free Nb in solution due to solid solution strengthening.
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 (0.01-0.02) wt%: It is themost important strengthening element in present inventionas 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. For the strength level of more than 270 MPa as described in scope of present invention the minimum amount of P should be more than 0.01 wt%. However, when added amount exceeds more than 0.03 wt% 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%: Itis an element utilized for increasing the strength of steel. As the silicon content increases the ductility and r-value noticeably deteriorates. Since silicon deteriorates tin coatingproperties 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.
Cr (0.01-0.04) wt% - Chromium forms chromium carbide (23/27Cr7C3) helps in scavenging off solute carbon. In present invention a minimum 0.01 wt% of Cr must be added to get any advantage of Cr addition to fix any remaining C and to avoid graphitization on surface and improve tin coating. The use of Cr to stabilize C is also effective to slow down the recrystallization rate and assist the formation of {111} textures to achieve good r-bar values. However, excess Cr in solution unnecessarily adds up in cost of production as well as reduces drawability. Hence the upper limit is set to 0.04 wt%. It is required to maintain Nb/Cr ratio as 1 in order to get preferential carbide formation by Nb over Cr but higher Nb/Cr ratio add extra cost and also increase yield strength of material due to fine precipitate of NbC so maximum limit of Nb/C kept less than 3.

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 600 to 660 °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 80% or more.

Subsequent to pickling and cold rolling to desired thickness, cold rolled steel strip are processed through continuous annealing line where electrolytic cleaning removes rolling emulsion present on the surface. Cleaned surface passes through the preheating and heating section where the strip is heated at the rate of 0.5-5 0C/sec up to soaking section temperature. Soaking section temperature was maintained in the range from Ar3 °C to Ar3+50 °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 2% 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. Accompanying Figure 1 shows the Microstructure of Cold rolled Tin Coated steel sheets having phases by area % :Ferrite-99% and Pearlite-1%.

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

Table 1: Elemental Compositions in weight % of the inventive steel sheets along with comparative example.
Table 2: Hot rolling and cold rollingof inventive with comparative steel sheets having chemical compositions as per Table 1.
Table 3: 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 Cr Nb/Cr Other Elements Remarks
1 0.003 0.1 0.003 0.01 0.03 0.04 0.003 0.01 0.02 1.5
Nb:0.03 I
Ca: 0.0015
2 0.001 0.4 0.003 0.01 0.02 0.03 0.005 0.06 0.01 1 Nb: 0.01 Ca: 0.0015, B:0.001 I
3 0.002 0.15 0.01 0.015 0.03 0.03 0.003 0.025 0.008 0.875 Nb: 0.007 C
4 0.004 0.2 0.01 0.02 0.04 0.025 0.004 0.065 0.005 0.6 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 anisotropy property (?r) to avoid earing problem and improved Tin coating without bare spot on the surface. These steels exhibits improved ?r< 0.04, Hardness>50 and YS> 200 MPa. Whereas, Steel remarked as ‘C’ from Table 1 to Table 4 doesn’t comply with atleast one of the scope of the present invention and does not conform with minimum one or more of the end product attributes as mentioned in the scope of the present invention. For example steel no. 5 in table 1 has the high carbon % and Si wt% and example steel no. 3 has less Nb and Cr wt% than the scope and does not comply Tin coating , yield strength andanisotropy property.

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 920 640 80
2 1205 1070 910 630 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 1.5% and 2% which resulted YS more than 200 MPa and hardness more than 50 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
1 710 95 410 1.5 200
2 740 95 420 2 250
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 % ?r Hardness (HR 30T) Tin Coating Remarks
1 200 330 45 -0.02 52 G I
2 230 350 46 -0.03 53 G I
3 130 310 48 0.05 35 NG C
4 140 320 54 0.06 40 NG C

*I - Present inventive example, C- Comparative Examples, G- Good- 0% bare spot on surface, NG- Not Good- > 0% bare spot on surface
Bare spot- Uncoated tin coated surface
Example 2

In sample 3 and 4where in sample 3 ?r is greater than 0.04 and hardness is less than less than 45 HR and Yield strength is less than 200 MPa 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.

It is thus possible by way of the present invention to provideCold rolled steel sheet for tinplate having excellent anisotropy and Target hardness level (HR30T) in the range of 51±3 to 53±3 with composition in terms of weight % comprising: 0.0005-0.003 %;Mn: 0.1- 0.5%;Si: 0–0.03%;P: 0.01-0.03%;Al: 0.02-0.06%; S: 0.01 % or less;N: 0.005 % or less;Cr: 0.01-0.04 %;and the balance being Fe and other unavoidable impurities; and having selective steel with atleast one element from Ti: 0.01-0.08 % and Nb: 0.01-0.03% wherein Nb/Cr range from 1 to 3 for excellent 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 200-250 MPa, planner anisotropy ?r less than 0.04, Tensile strength of 270-350 MPa, total elongation of 35% or more and hardness level (HR30T) in the range of 48 to 56.
,CLAIMS:We Claim:

1. Cold rolled steel sheet for tinplate having excellent anisotropy and Target hardness level (HR30T) in the range of 48 to 56 comprising of steel composition in terms of weight % comprising:
C: 0.0005-0.003 %;
Mn: 0.1- 0.5%;
Si: 0–0.03%;
P: 0.01-0.03%;
Al: 0.02-0.06%;
S: 0.01 % or less;
N: 0.005 % or less;
Cr: 0.01-0.04 %;
and the balance being Fe; and which includes atleast one element from Ti: 0.01-0.08 % and Nb: 0.01-0.03% and wherein Nb/Cr range from 1 to 3 suitable for tin coated tinplate.

2.)The Cold rolled steel sheet for tinplate of claim 1 comprising in terms of weight % atleast one additive element selected from the group comprising of 0.001% to 0.003% of B, and less than 0.005 % Ca.

3) The Cold rolled steel sheet for tinplate as claimed in anyone of claims 1 to 2 having Yield strength of 200-250 MPa, planner anisotropy ?r less than 0.04, Tensile strength of 270-350 MPa, total elongation of 35% or more and hardness level (HR30T) in the range of 48 to 56.

4.)The Cold rolled steel sheet for tinplate as claimed in anyone of claims 1 to 3 wherein the said steel sheet comprises, in terms of area fraction relative to entire microstructure of steel, 99% or more of ferrite phase, 1 % 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 claims 1 to 4 comprising the steps of:
a) providing steel having composition comprising
in terms of weight % comprising:
C: 0.0005-0.003 %;
Mn: 0.1- 0.5%;
Si: 0–0.03%;
P: 0.01-0.03%;
Al: 0.02-0.06%;
S: 0.01 % or less;
N: 0.005 % or less;
Cr: 0.01-0.04 %;
and the balance being Fe; and which includes atleast one element from Ti: 0.01-0.08 % and Nb: 0.01-0.03% and wherein Nb/Cr range from 1 to 3 and
involving processing through Heat from basic oxygen furnace (BOF) and RH degasser and subsequently continuously casting into slabs and reheating said slabs having said composition to reheating temperature in the range from 1190°C -1250 °C;
b) 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;
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) skin pass rolling;
f) Tin Coating;

so as to have selective steel sheet with microstructure constituents including atleast 99 % Ferrite phase and 1 % or less of pearlite phase along with carbide, nitride and sulphide precipitates for excellent stretch formability.

6) The process to manufacturing Cold rolled steel sheet for tinplate of claim 5, 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 temperature 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) coiling the finish rolled steel at with average run out table cooling rate of 10to 15°C/second preferably about 10 °C/second or more; and
e) acid pickling the Cold rolling the said hot rolled steel sheet with cold reduction of atleast 80%.

7.)The process for manufacturing Cold rolled steel sheet for tinplate as claimed in anyone of claims 5to 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+50 °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 0.8% to 2 %
d) subjecting to tin deposition through electrolytic process at reflow temperature from 200 to 300°C

Dated this the 2nd day of April, 2022
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199