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 :
COLD ROLLED STEEL SHEET FOR TINPLATING FREE OF COIL BREAK DEFECT AND METHOD FOR ITS PRODUCTION.



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 Coil break defect free cold rolled steel sheet for tinplatingapplication and method of producing the same. Selected Steel Grade having the composition in terms of weight % comprising: C: 0.03-0.08 %; Mn: 0.1-0.45%; Si: 0–0.03%; P: 0-0.015%; Al: 0.02-0.06%; S: 0-0.01 %; N: 0-0.0055 %; and the balance being Fe and other unavoidable impurities.
The advancement favors minimization of Coil break defect in cold rolled steel sheet for tinplate through batch annealing furnace which is very susceptible to this defect and resulting in yield loss and lower productivity at intermediate stages, the said steel sheet has yield strength of 250-325 MPa, Tensile strength of 310-380 MPa, total elongation of 35% or more and hardness level (HR30T) in the range of 54 to 60.

BACKGROUND OF THE INVENTION

Coil break lines are most common defect for cold rolled sheets produced through batch annealing furnace and finally used for Tinplating.
This defect due to its severe impact on product performance and aesthetic look not acceptable to Tinplating customers and hence resulting in high level of rejections. Along with rejections it also leads to productivity loss especially at double cold reduction mill where speed needs to be reduced drastically to lower levels.
During investigation it was observed that effect of air cooling rate during batch annealing production cycle was the major contributing factors along with minor factors of tension at double cold rolled mill, tension setting at electrolytic cleaning line.
Electrolytic line tension setting was optimized along with double cold reduction mill uncoiling tension and a number of trials scheduled with different air cooling parameters in batch annealing cycle.

What are coil breaks and why are they objectionable?
Coil Breaks are surface defects in form of creases which appear as line defects and transverse to the rolling direction and usually extending across the width of the steel. Coil breaks can be narrow, irregularly changing deformation lines which are mainly observed with low carbon steel strips processing. They can appear in many metallurgical manufacturing processes, such as pickling, galvanizing, cold rolling, processing on slitting or cutting lines, etc. They are considered as serious surface defects leading to aesthetical as well as performance related concerns with the end product.
KR20040059355A
A method for preventing coil break from forming on low carbon steel is provided to prevent formation of the coil break by simultaneously performing under spray and track spray from a time point when a top part of strip arrives at coiling thermometer, thereby cooling the top part of the strip. In a method for manufacturing hot rolled low carbon steel sheet in a hot rolling mill comprising a down coiler(1) for coiling hot rolled low carbon steel sheet that is hot finish rolled, pinch rolls(2) installed at an input side part of the down coiler, a coiling thermometer(3) installed at an input side of the pinch rolls, and a table roller(4) for moving the hot rolled steel sheet, the method for preventing coil break from forming on low carbon steel comprises a step of cooling a back surface of the hot rolled steel sheet transferred between the pinch rolls and the coiling thermometer; and a step of cooling a front surface of the hot rolled steel sheet, where cooling conditions of the back and front surfaces of the hot rolled steel sheet are set to satisfy the hot rolling conditions that the steel sheet is hot rolled up to a finish rolling temperature of 800 °C at a coiling temperature of 400 °C or less, the steel sheet is hot rolled up to a finish rolling temperature of 750 °C at a coiling temperature of 500 °C or less, and the steel sheet is hot rolled up to a finish rolling temperature of 700 °Cat a coiling temperature of 600 °C or less.

EP2532450A1
A cold-rolling method for preventing coil break steel, characterized in that the coil break strip steel has Si content=2.3wt%, and at the beginning of cold-rolling, temperature of inlet strip steel is above 45°C; during the cold-rolling process, emulsion liquid is sputtered to the strip steel, flow rate of the emulsion liquid is 3500L/mm at the inlet in rolling direction, flow rate of the emulsion liquid is 1500~4000L/min at outlet in rolling direction, and temperature of the strip steel is ensured being above 45°C under the precondition to guarantee technological lubrication. The cold-rolling method of the invention might prevent fracture of head portion and tail portion of the strip steel, raise rate of finished products and production efficiency. During the cold-rolling process: for first pass of rolling, reduction ratio is 28%, a rearward unit tension is 10N/mm2 and a forward unit tension is 80N/mm2; for middle passes of rolling, reduction ratios are 18~30%, a rearward unit tension is 40~150N/mm2 and a forward unit tension is 60~350N/mm2; for finish pass of rolling, reduction ratio is 23%, a rearward unit tension is 90N/mm2 and a forward unit tension is 190N/mm2.

EP0435968A1
A process is described for manufacturing an undressed steel strip free of winding folds and having a low elastic limit and a constant elongation at the elastic limit over time. Traditionally, winding wrinkles in hot strip are avoided by sufficient alloying or straightening of the strip to prevent elongation at the yield point. On the other hand, the steel strip described is manufactured by adding a minimum quantity of titanium by alloy and with a low winding temperature of between 550 and 400°C. The quantity of titanium added per alloy is chosen so as to be equal to the quantity stoichiometrically necessary to bind the nitrogen but nevertheless less than that necessary to bind the carbon and the nitrogen. In addition, the wrinkle-free and age-resistant steel used for the hot-dip galvanized strips is a vacuum steel with a titanium content of 0.02 to 0.04% or an LD steel with a titanium content of 0.03 to 0.08%.

318/MUM/2014 (Title: Low carbon coil break free hot rolled steel sheets and a process for its production.)
This invention relates to providing low carbon coil break free hot rolled (HR) steel sheet in the thickness range of 1.5-8.0 mm with minimum yield point phenomenon and a process for its production through thin slab caster route followed by controlled hot rolling, having composition comprising C: 0.04-0.08wt%; Mn: up to 0.4wt%; Si: up to 0.03 wt%;Al < 0.05wt%;Nb: 0.002-0.01wt%;N: < 120 ppm; Ca: 0-50 ppm; and balance is iron with S & P in amounts of allowable impurities along with selective alloying element Nb: 0.002-0.01wt% for fixing free N and C. Typical applications of the invented steel grade include direct sheet application in automotive and other general engineering application including machinery, white good application, BIW application etc.

Problem to be solved by the Invention
The Present invention propose a novel method to minimize coil break line defect with an aim to produce nil coil break line defects of cold rolled steel coils produced through batch annealing furnace for final usages in Tinplating application.

OBJECTS OF THE INVENTION
The basic object of the present invention is directed to provide a cold rolled and closed annealed (CRCA) steel sheet for tinplating application with minimum defect or defect free with respect to the specific Coil break lines during cold rolling.

A further object of the present invention is directed to provide Cold rolled steel sheet for tinplate selective composition and processing to achieve the desired mechanical properties including hardness and tin coating property with new established annealing cycle parameters.

A still further object of the present invention is to fine tune process parameters at Batch annealing cooling rates, electrolytic cleaning line, double cold reduction mill in order to improve productivity by increasing higher speed processing and an increase in yield by minimizing rejections due to Coil break defect.

SUMMARYOF THE INVENTION

The basic aspect of the present invention is directed to cold rolled steel sheet for tinplate having coil break defect free with composition in terms of weight % comprising:
C: 0.03-0.08 %;
Mn: 0.1- 0.45%;
Si: 0–0.03%;
P: 0-0.015%;
Al: 0.02-0.06%;
S: 0-0.01;
N: 0-0.0055 %;
and the balance being Fe and other unavoidable impurities.
where Al/N ratio is above 2 preferably in the range of 2.2 to 3.0 maintained for ageing resistance after tin coating and maintain ductility.

A further aspect of the present invention is directed to said cold rolled steel sheet for tinplate having Yield strength of 250-340 MPa, ageing resistance of 6 months and more, Tensile strength of 310-380 MPa, total elongation of 32% or more and hardness level (HR30T) in the range of 54 to 60 which provide for minimized coil break line defects and related improved productivity and final yield.

A still further aspect of the present invention is directed to a process to manufacturing Cold rolled steel sheet for tinplate as given above, comprising the steps of:
reheating the slab having a select composition of

C: 0.03-0.08 %;
Mn: 0.1- 0.45%;
Si: 0–0.03%;
P: 0-0.015%;
Al: 0.02-0.06%;
S: 0-0.01;
N: 0-0.0055 %;
and the balance being Fe and other unavoidable impurities.
where Al/N ratio is above 2 preferably in the range of 2.2 to 3.0,

in the range from 1170°C -1230 °C;
said Reheated slab being subjected to roughing rolling in roughing mill with roughing mill delivery temperature of 1070°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.
coiling the finish rolled steel at with average run out tablecooling rate of 10 °C/second or more; and
acid Pickling the Cold rolling the said hot rolled steel sheet with cold reduction of at least 75%.

A still further aspect of the present invention is directed to said process for manufacturing Cold rolled steel sheet for tinplate, wherein said cold rolled steel sheet for tin plate is subjected to :
batch annealing wherein
first annealing of the cold rolled steel sheet is carried out at heating and soaking section critical temperature range from Ar3 °C to Ar3+100 °C;
thereafter, cooling in different steps followed by air cooling temperature lower than 340 °C which was observed as best suitable operating temperature in order to minimize coil break lines defects in cold rolled and annealed sheet;
subjecting to skin pass elongation or double cold reduction of 1 % to 3 %, finally
subjecting to tin deposition through electrolytic process at reflow temperature from 200 to 300°C.

A further aspect of the present invention is directed to said process for manufacturing Cold rolled steel sheet for tinplate wherein said step of double cold reduction is carried out of 1.5 to 2.5 %.

A further aspect of the present invention is directed to saidprocess for manufacturing Cold rolled steel sheet for tinplate wherein
the said steel sheet having Yield strength of 250-340 MPa, ageing resistance of 6 months and more, Tensile strength of 310-380 MPa, total elongation of 32% or more and hardness level (HR30T) in the range of 54 to 60 is suitable for being subjected to batch annealing, double cold rolling and tin coating with a minimized coil break line defects and improved productivity and final yield.

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

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: Process layout for Cold Rolling Mill.
Figure 2: a flow chart showing the process steps indicating single reduction step. Material is subjected to 1-3% reduction after annealing instead of more than 25% reduction in case of double reduction mode.
Figure 3: shows the micrograph illustrating the microstructure with presence of 92 to 97% of ferrite phase, 3 to 7 % of pearlite phase.
Figure 4:(a) General Appearance of Coil Break Lines in existing steel sheets, (b) Digital Photograph of Coil break free steel sheets according to present invention.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWINGS

The Present invention relates to method of producing coil break defect free cold rolled steel sheet for tinplating single reduced category which was resulting in yield loss & productivity due to rejections at intermediate stages at Batch annealing, Double cold reduction and finally at Electrolytic Tinning line. Selected Grade was with Temper T3 with target hardness level (HR30T) in the range of 54 to 60 with composition in terms of weight % comprising:
C: 0.03-0.08 %;
Mn: 0.1- 0.45%;
Si: 0–0.03%;
P: 0-0.015%;
Al: 0.02-0.06%;
S: 0-0.01;
N: 0-0.0055 %;
and the balance being Fe and other unavoidable impurities; and having selective steel with element from Al: 0.02-0.06%
where Al/N ratio kept above 2 for contributing in ageing resistance after tin coating at prescribed higher temperature and maintaining corresponding ductility.
Cold rolled steel sheet for tinplate where the said steel sheet has Yield strength of 250-325 MPa, Tensile strength of 310-380 MPa, total elongation of 32% or more and hardness level (HR30T) in the range of 54 to 60.

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)
DCR–Double cold reduction mill (Skin Pass Elongation)

Temper rolling
After annealing, the steel strip is in a very soft condition and it is given a very light rolling treatment, usually in a two-stand mill operated without strip lubrication. This is known as “temper rolling” or “skin pass rolling” or “double cold reduction”. This process defines the final gauge and imparts mechanical properties appropriate to the end use. It also improves the strip shape and produces the desired surface finish on the strip. The overall reduction is of the order of 1 to 3%

A cold rolled steel sheet for tinplate with cold break defect free 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 where all the elements are in weight % as follows:

Carbon (0.03-0.08 wt%) - While carbon increases the steel strength, it reduces the cold workability and the 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. draw ability it desirable to reduce the C level less than 0.08 wt%. Lowering the carbon content below 0.03 wt% results in poor ageing property since below 0.03 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 0.03-0.08 wt percent.

Mn (0.1-0.45) wt. % - The Mn ranges from 0.1-0.45 % is most desirable for the given low carbon steel grade provided that S remains in the favorable range of 0.010wt% 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.

Al (0.02-0.06) wt. % – Al wt% ranges from 0.02-0.06 is intended for combining with N and 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 (0-0.015) wt%:When added amount exceeds more than 0.02 wt% the yield strength level increases significantly. In addition, higher amount of P promotes formation of surface defects after tin coating and also increases corrosion resistance of tinplate. So, maximum limit of P is kept 0.015 %.

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 Ladle heating furnace and subsequently continuously casted. Special measures are taken to hot roll resulted slabs by keeping slab reheating temperature in the range of 1170°C to 1230°C intended to control roughing mill delivery temperature under 1070°C and finishing mill entry temperature under 1070°C to restrict formation of surface defects like rolled in scales. 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 75% or more.

Accompanying Figure 1 shows the Process layout for Cold Rolling Mill. Accompanying Figure 2 shows a flow chart showing the process steps indicating single reduction step. Material is subjected to 1-3% reduction after annealing instead of more than 25% reduction in case of double reduction mode.

In Annealing section, the cold rolled steel sheet was subjected for batch annealing and during annealing cycle air cooling temperature kept at varied at different levels 390°C, 360 °C and <340°C in order to check intensity and formation of coil break lines defect at double cold reduction mill entry where strip is further subjected to elongation of 1 % to 3 %.
In further process step, strip is subjected to tin deposition through electrolytic process at a reflow temperature from 200 to 300°C.

Subsequent to pickling and cold rolling to desired thickness, cold rolled steel strip are first cleaned at electrolytic cleaning line then annealed at batch annealing for further processing at electrolytic tinning line where initially at electrolytic cleaning section removes rolling emulsion present on the surface, and finally cleaned surface passes through the Quench and Reflow Section for tin coating followed by passivation and oiling.

After annealing, followed by double cold reduction mill where elongation ranges from 1 to 3% in order to get desired mechanical properties. After double cold reduction, said steel is subjected to Tin deposition through Electrolytic process where pure tin acts as 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.

Process conditions to achieve coil free defects are mentioned below:
Product Quality
Tin Grade Hardness (HR30T) Y.S (Mpa)
T-3/T-57 57±3 275±30

Process parameters adopted at Batch Annealing Furnace
Heating Soaking Furnace Cooling Air Cooling Water Cooling
Temperature (°C) / Time (hours)
600-580/7 600-580/13 550/2 350-330/4 110-70/14
Furnace cooling (from 580 °C to 550 °C); Air cooling (from 550 °C to 330 °C); Water cooling (from 330 °C to 70 °C)

Tension Level maintained in the Process
@ DCR At the time of Coil break line defect issue 18-25 KN
In present invention case reduced to 20-30%
@ ECL At the time of Coil break line defect issue 9.5-12.5 KN
In present invention case 10.8 -12.0 KN

Accompanying Figure 3 shows the micrograph illustrating the microstructure with presence of 92 to 97% of ferrite phase, 3 to 7 % of pearlite phase in coil break defect free steel sheet of present invention.

Accompanying Figure 4 shows (a) General Appearance of Coil Break Lines in existing steel sheets, (b) Digital Photograph of Coil break free steel sheets according to present invention.

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 parameters of inventive with comparative steel sheets having chemical compositions as per Table 1.
Table 3: Annealing parameters along with air cooling experimental values, Tin Coating Parameters of inventive for steel sheets 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 BAF ID Stack positon C MN S P Si AL N AL/N Remarks
1 Coil 1 Bottom 0.042 0.15 0.01 0.012 0.02 0.02 0.007 2.86 I
Coil 2 2nd Bottom 0.052 0.23 0.01 0.013 0.02 0.021 0.0071 2.96
2 Coil 1 Bottom 0.038 0.31 0.01 0.011 0.015 0.02 0.0074 2.7 I
Coil 2 2nd Bottom 0.048 0.25 0.01 0.014 0.018 0.018 0.0075 2.4
3 Coil 1 Bottom 0.055 0.49 0.01 0.02 0.038 0.03 0.0058 5.17 C
Coil 2 2nd Bottom 0.027 0.5 0.01 0.018 0.04 0.041 0.008 5.13
4 Coil 1 Bottom 0.063 0.48 0.01 0.016 0.032 0.052 0.005 10.4 C
Coil 2 2nd Bottom 0.024 0.54 0.01 0.017 0.033 0.052 0.005 6.4

*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 exhibit improved yield strength of 250-340 MPa, ageing resistance of 6 months and more, Tensile strength of 310-380 MPa, total elongation of 32% or more and hardness level (HR30T) in the range of 54 to 60. Whereas, Steel remarked as ‘C’ from Table 1 to Table 4 doesn’t 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. example steel no. 3 and 4 has does not comply with required ratio of Al/N ratio and has poor Tin coating surface.

Table 2
Rolling Parameters
Sample No BAF ID Stack position SRT°C RM°C FT°C CT°C Cold Reduction%
1 Coil 1 Bottom 1205 1060 900 590 82
Coil 2 2nd Bottom 1200 1045 880 580 85
2 Coil 1 Bottom 1210 1053 910 610 87
Coil 2 2nd Bottom 1210 1060 920 610 80
3 Coil 1 Bottom 1225 1072 790 570 70
Coil 2 2nd Bottom 1220 1070 800 560 72
4 Coil 1 Bottom 1220 1065 810 630 68
Coil 2 2nd Bottom 1228 1070 825 610 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 1.6% and 2.4% which resulted YS more than 250 MPa and hardness more than 54 HR.

*SRT- Slab reheating temperature,RM- Roughing Mill temperature, FT- hot finish rolling temperature,ROT- Run out table at hot strip mill, CR%- Cold rolling reduction %.

Table 3
BAF and Tin Coating Parameters
Sample No BAF ID Stack position Thickness (mm) Heating temperature Air Cooling TEMP Double Reduction Skin Pass Tin Coating Temperature
1 coil 1 bottom 0.27 580 <340 1.9 235
coil 2 2nd bottom 0.265 580 <340 2.2 239
2 coil 1 bottom 0.27 580 <340 2.4 238
coil 2 2nd bottom 0.27 580 <340 2.3 230
3 coil 1 bottom 0.24 580 360 1.5 242
coil 2 2nd bottom 0.25 580 360 1.8 241
4 coil 1 bottom 0.285 580 390 0.7 238
coil 2 2nd bottom 0.28 580 390 0.9 245

Table 4-Mechanical and coating properties
Mechanical Properties Coating Properties
Sample No BAF ID Stack pos YS, MPa TS, MPa Elongation % Hardness (HR 30T) Tin Coating Cost Remarks
1 coil 1 bottom 285 321 36 60 G Low I
coil 2 2nd bottom 261 334 38 58 G Low
2 coil 1 bottom 310 372 35 61 G Low I
coil 2 2nd bottom 290 345 34 59 G Low
3 coil 1 bottom 248 322 42 47 NG High C
coil 2 2nd bottom 237 301 41 45 NG High
4 coil 1 bottom 311 387 39 46 NG High C
coil 2 2nd bottom 295 371 40 49 NG High

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

In sample 3 and 4, where in sample 3 is having Yield strength less than 250 MPa and hardness is less than less than 54 HR30T 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 provideCold rolled steel sheet having thickness in the range of 0.27 mm through continuous annealing route for tinplate having excellent ageing resistance and Target hardness level (HR30T) in the range of 54 to 60 with composition in terms of weight % comprising:
C: 0.03-0.08 %; Mn: 0.1-0.45%; Si: 0–0.03%; P: 0-0.015%; Al: 0.02-0.06%; S: 0-0.01 %; N: 0-0.0055 %; and the balance being Fe and other unavoidable impuritieswhere Al/N ratio range from 1.4-1.8for excellent ageing resistance after tin coating.

The advancement favors generation of cold rolled higher thickness tinplated sheet having thickness in the range of 0.27 mm withYield strength of 250-340 MPa, ageing resistance of 6 months and more, Tensile strength of 310-380 MPa, total elongation of 32% or more and hardness level (HR30T) in the range of 54 to 60.

, Claims:We Claim:

1. Cold rolled steel sheet for tinplate having coil break defect free with composition in terms of weight % comprising:
C: 0.03-0.08 %;
Mn: 0.1- 0.45%;
Si: 0–0.03%;
P: 0-0.015%;
Al: 0.02-0.06%;
S: 0-0.01;
N: 0-0.0055 %;
and the balance being Fe and other unavoidable impurities.
where Al/N ratio is above 2 preferably in the range of 2.2 to 3.0 kept above for ageing resistance after tin coating and maintain ductility.

2.The cold rolled steel sheet for tinplate as claimed in claim 1 having Yield strength of 250-340 MPa, ageing resistance of 6 months and more, Tensile strength of 310-380 MPa, total elongation of 32% or more and hardness level (HR30T) in the range of 54 to 60 which provide for minimized coil break line defects and related improved productivity and final yield.

3. A process for manufacturing Cold rolled steel sheet for tinplate as claimed in claim 1, comprising the steps of:
reheating the slab having a select composition of

C: 0.03-0.08 %;
Mn: 0.1- 0.45%;
Si: 0–0.03%;
P: 0-0.015%;
Al: 0.02-0.06%;
S: 0-0.01;
N: 0-0.0055 %;
and the balance being Fe and other unavoidable impurities.
where Al/N ratio is above 2 preferably in the range of 2.2 to 3.0,

in the range from 1170°C -1230 °C;
said Reheated slab being subjected to roughing rolling in roughing mill with roughing mill delivery temperature of 1070°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;
coiling the finish rolled steel at with average run out tablecooling rate of 10 °C/second or more; and
acid Pickling the Cold rolling the said hot rolled steel sheet with cold reduction of at least 75%.

4. The process for manufacturing Cold rolled steel sheet for tinplate as claimed in claim 3, wherein said cold rolled steel sheet for tin plate is subjected to :
batch annealing wherein
first annealing of the cold rolled steel sheet is carried out at heating and soaking section critical temperature range from Ar3 °C to Ar3+100 °C.
thereafter, cooling in different steps followed by air cooling temperature lower than 340 °C which was observed as best suitable operating temperature in order to minimize coil break lines defects in cold rolled and annealed sheet;
subjecting to skin pass elongation or double cold reduction of 1 % to 3 %, finally
subjecting to tin deposition through electrolytic process at reflow temperature from 200 to 300°C.

5. The process for manufacturing Cold rolled steel sheet for tinplate as claimed in anyone of claims 3 to 4 wherein said step of double cold reduction is carried out of 1.5 to 2.5 %.

6.The process for manufacturing Cold rolled steel sheet for tinplate as claimed in anyone of claims 3 to 5 wherein
the said steel sheet having Yield strength of 250-340 MPa, ageing resistance of 6 months and more, Tensile strength of 310-380 MPa, total elongation of 32% or more and hardness level (HR30T) in the range of 54 to 60 is suitable for being subjected to batch annealing, double cold rolling and tin coating with a minimized coil break line defects and improved productivity and final yield.

Dated this the 8th day of May, 2023
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199