Claims:We Claim:

1. A single reduced tin-plated steel of hardness grade TH435 having the hardness HR30Tm within 63 to 69 comprising of chemical composition in percentage by weight:

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;
at least one of V: 0.002-0.03 % and Ti: 0.015-0.025 %;
and the balance of Fe, wherein Ti/N ratio range from 0.6 to 4 and having yield strength in the range of 385 -485 Mpa, tensile strength in the range of 450-500 MPa, total elongation in the range of 15% to 25% and yield point elongation (YPEL) to a maximum of 6%.

2. The single reduced tin-plated steel of hardness grade TH435 as claimed in claim 1, wherein the internal microstructure comprises of ferrite matrix with cementite perlite phase distributed on the grain boundaries and ferrite matrix has ASTM grain number ranging from 11 to 12.

3. The single reduced tin-plated steel of hardness grade TH435 as claimed in claim 1, having ageing resistance of 6 months.

4. The single reduced tin-plated steel of hardness grade TH435 as claimed in claim 1, having thickness in the range 0.16mm to 0.28mm.

5. The method of manufacturing single reduced tin-plated steel of hardness grade TH435 according to claim 1, comprising the following steps:
obtaining molten steel comprising of 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;
at least one of V: 0.002-0.03 % and Ti: 0.015-0.025 %;
and the balance of Fe, wherein Ti/N ratio range from 0.6 to 4;
refining and continuously casting the molten steel to obtain a solid slab;
carrying out hot continuous rolling on the solid slab to obtain a hot rolled steel;
pickling and cold rolling the hot rolled steel to obtain a cold-rolled steel;
continuously annealing the cold-rolled steel to obtain an annealed steel, wherein during the continuous annealing, the annealing soaking temperature is 700-715 ?, and the annealing soaking time is 40 to 200s and cold-rolled steel is over-aged at the temperature from 700- 400 ?;
skin pass elongation of the annealed steel to obtain steel strip; wherein the reduction is 2-3%; and
electro tinning on the steel strip to obtain the tinned plate steel of hardness grade TH435, wherein the reflow temperature is controlled to 230 to 265 ?.

6. The method of manufacturing single reduced tin-plated steel of hardness grade TH435 according to claim 5, wherein the hot continuous rolling on the solid slab to obtain a hot rolled steel comprising the following steps:
the solid slab is reheated to a temperature of 1150°C -1250 °C;
the reheated slab is subjected to roughing rolling in roughing mill with roughing mill delivery temperature in the range of 980 to 1080°C;
the rough rolled steel is subjected to finish rolling with finish mill exit temperature ranging from Ac3 °C to Ac3+100 °C.;
the finish rolled steel is cooled at an average run-out table cooling rate in the range of 10 to 30°C/second;
the finished rolled steel is then acid pickled to form the hot rolled steel with a cold reduction in the range of 85 to 95%.

Dated this the 17th day of February, 2022
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199

, 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 :
SINGLE REDUCED TIN-PLATED STEEL WITH HARDNESS GRADE OF TH435 AND MANUFACTURING METHOD THEREOF.



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 cold-rolled low carbon tin-plated steel and more particularly to single reduced tin plated steel with properties of grade TH435 and a method of manufacturing thereof. This steel is having good formability suitable for making tinplate CAN components for packaging from thin sheets without facing issues like cracking during can making operations and isfree from the impact of further strain aging after the production of which affects the formability of the steel adversely and restricts the yield point elongation to a maximum of 6%.

BACKGROUND OF THE INVENTION
Difficulty in producing tin plates with high strength properties with a combination of good formability that is essential for making CAN components like crown corks, 3-piece CAN body, easy open ends and twist off caps restricts the use of thinner gauge materials for CAN making creating a major hindering factor in down gauging of Tinplate CAN making which along with being profitable to CAN makers can lead lighter sustainable food packaging solution as a substitute of plastic.

To facilitate the production of high strength tin-plated steel four types of strengthening mechanisms are used. The first being the solid solution strengthening which is brought about by addition of Mn in the steel chemistry. The precipitation hardening is facilitated by use of Ti which precipitates out the N. 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 YS is achieved by the process of strain aging which occurs when steel is heated up to a temperature of 240 degCelsius while processing it in Electrolytic tinning line. The added Titanium in the steel also helps to prevent further aging of the steel by fixing the Nitrogen in the steel which has a detrimental effect on the formability of steel and leads to issues like cracking and fluting mark during CAN making. They also help in getting a surface free from graphitization and non-metallic inclusions surface which provides better tin coating.

For making tinplate CAN components for packaging from thin sheets without facing issues like cracking during can making operations. Also the formed CANs need to withstand the pressure exerted on its walls by the contents inside it. Tin-plated steel with properties of grade TH435 having yield strength of 385-480 MPa, Tensile strength of 450-500 MPa, total elongation in the range of 15 to 25% more and hardness level (HR30T) in the range of 63 to 69 is most suitable for the mentioned application.

The steel needs to be free from the impact of further strain aging after the production of which affects the formability of the steel adversely. Aging phenomenon is known to be the reappearance of yield point phenomenon 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.

Although the present invention uses the mechanism of strain aging to achieve the desired properties, further aging steel after production is eliminated by addition of Ti to the composition of the steel which in turn restricts the yield point elongation to a maximum of 6% . This leads to elimination of issues like cracking, earring formation and markings or stretcher strains due to localized heterogeneous deformation during CAN making

US3095361A discloses a process for producing a cold-rolled steel strip having higher hardness by a short-time continuous annealing and by cold reducing the annealed strip by 30% or more to impart the the strip with hardness of at least 65 HRT. which process is constituted by hot rolling a low carbon steel slab of steel; 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. The annealed soft steel is subjected to at least 30 % cold reduction to achieve the desired hardness of more than 65 HR30T. The strip has excellent stiffness which is used for making crown cork, CAN ends and bodies. Thus this prior art targets a shorter annealing time followed by cold reduction of more than 30 % to increase the strength by the method of strain hardening. Due to a cold reduction of more than 30% there is no further aging of the material due to a high dislocation density in the material. Although the strength of the strip steel obtained by this method is high, the high reduction rate in the double cold reduction stage results in a low elongation of the strip steel, not suitable for applications requiring certain formability. Also there is an increase in production cost due to the secondary rolling process.
WO2008/102006A1 discloses process for producing cold-rolled steel with high hardness and strength by short time annealing steel comprising of C : 0.0005 - 0.004 wt.% , Mn : 0.050 - 0.300 wt.% , Alsoi : 0.010 - 0.100 wt.%, N : 0.0005 - 0.0050 wt.%, S : 0- 0.020 wt.% , P: 0 - 0.020 wt.%, Cr: 0.001-0.1wt.%, Cu: 0 - 0.050 wt.% , Si : 0 - 0.020 wt.%, Ni : 0 - 0.050 wt.%, Ti : 0.01 - 0.085 wt.% further optionally comprising one or more of the micro-alloying elements. The higher strength steel substrate is prepared by cold reduction of the hot rolled coil by at least 75%, further by annealing the material with a soaking temperature of 600-640°C and finally the annealed strip is subjected to skin pass elongation of below 1 % before tin coating. As the annealing takes place below recrystallization temperature the microstructure obtained by the process comprises un recrystallized grains. The process of recovery reduces the dislocation density but still the dislocation density in the material is higher. Although the strength of the material is higher the un-recrystallized grains give a non-uniform property to the material. Also elongation percentage being considerably low the material is not suitable for applications requiring high formability.
Generally, the current tin plating is done on interstitial free steel. It are preferred due to good formability. However, the processing requires RH degassing and double reduction. Due to this, interstitial frees have a significantly high cost of production as compared to normal low carbon steels. Hence there is a need to replace interstitial free steel as raw material to eliminate the process of secondary reduction.

The present invention aims to solve the problem of the prior arts by achieving the required yield strength with an elongation of at least 15%. The steel is free from the effect of further aging as it is alloyed with Ti. The process involves producing the steel through continuous annealing route with a soaking Temperature of 700 to 715 °C. Addition of Mn provides the solid solution strength to the material, while Ti/N ratio of minimum 0.6 ensures the ageing guarantee as well as the forms precipitates that pin down the grains and prevent them from growing giving a fine grained ferrite microstructure. This helps the material to reach a certain YS before being processed at the electrolytic tinning line. The tinning process makes use of the temperature of up to 240 deg celcius used in the reflow section of the tinning line to strain age the material and reach the required YS.
The strain aged steel has a Yield point elongation of 4-6% which has a minimum effect on the formability. The added Titanium helps in providing no difference in yield strength after 6 hrs minutes of heating the produced steel at 100°C. This guarantees no increase in yield point elongation due to further aging for 1 year in 30°C. The method is simple and devoid of any complexity of production that are mentioned in the prior arts. Also due to proper annealing and non-involvement of secondary reduction of more than 15% after annealing the microstructure produced is uniform with properly recrystallized ferrite grains that improves the formability of the material significantly. The process also does not involve using Interstitial free steel as raw material as mentioned in some of the methods, this leads to low cost of production.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide tin-plated steel with properties of high strength tinplate grade TH435.

A still further object of the present invention is directed to provide tin-plated steel sheet with good formability.

A still further object of the present invention is directed to restrict the yield point elongation of the steel to a maximum of 6% and does not undergo strain age hardening.

A still further object of the present invention is to provide a method for manufacturing single reduced tin plated steel.

A still further object of the present invention is directed to provide an excellent coating property to the tin-plated steel giving it a good corrosion resistance which enables it to be used in food packaging applications.

SUMMARY OF THE INVENTION

The present invention relates to a single reduced tin-plated steel of hardness grade TH435 having the hardness HR30Tm within 63 to 69 comprising of chemical composition in percentage by weight:
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;
at least one of V: 0.002-0.03 % and Ti: 0.015-0.025 %;
and the balance of Fe, wherein Ti/N ratio range from 0.6 to 4and having yield strength in the range of 385-485 Mpa, tensile strength in the range of 450-500 MPa, total elongation in the range of 15% to 25% and the yield point elongation (YPEL) to a maximum of 6%.

The single reduced tin-plated steel of hardness grade TH435, wherein the internal microstructure comprises of ferrite matrix with cementite perlite phase distributed on the grain boundaries and ferrite matrix has ASTM grain number ranging from 11 to 12.

The single reduced tin-plated steel of hardness grade TH435 having ageing resistance of 6 months.
The single reduced tin-plated steel of hardness grade TH435, having thickness in the range 0.16mm to 0.28mm.

Another aspect of the present invention relates to the method of manufacturing single reduced tin-plated steel of hardness grade TH435 comprising the following steps: firstly obtaining molten steel comprising of 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;at least one of V: 0.002-0.03 % and Ti: 0.015-0.025 %;and the balance of Fe, wherein Ti/N ratio range from 0.6 to 4; secondly, refining and continuously casting the molten steel to obtain a solid slab; thirdly, carrying out hot continuous rolling on the solid slab to obtain hot rolled steel; fourthly, pickling and cold rolling the hot rolled steel to obtain cold-rolled steel; fifthly, continuously annealing the cold-rolled steel to obtain an annealed steel; wherein during the continuous annealing, the annealing soaking temperature is 700-715 ? and the annealing soaking time is 40 to 200s; further, the cold-rolled steel is over-aged at the temperature from 700- 400 °C; sixthly, skin pass elongation of the annealed steel to obtain steel strip; wherein the reduction is 2-3%; and lastly, electro-tinning on the steel strip to obtain the tinned plate steel of hardness grade TH435, wherein the reflow temperature is controlled to 230 to 265 ?.

Another aspect of the present invention is directed to said method wherein the step of the hot continuous rolling on the solid slab to obtain a hot rolled steel comprising the following steps: firstly, the solid slab is reheated to a temperature of 1150°C -1250 °C, secondly, the reheated slab is subjected to roughing rolling in a roughing mill with a roughing mill delivery temperature in the range of 980- 1080°C ;thirdly, the rough rolled steel is subjected to finish rolling with finish mill exit temperature ranging from Ac3 °C to Ac3+100 °C; fourthly, the finished rolled steel is cooled at an average run out table cooling rate in the range of 10 to 30°C/second; and lastly, the finished rolled steel is then acid pickled to form the hot rolled steel with cold reduction in the range of 85 to 95%.

The above objects and advantages of the present invention are described here in detail with reference to non-limiting accompanying drawings and examples.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure 1: depicts a flow chart showing the process steps indicating single reduction step wherein material is subjected to 2-3% reduction after annealing instead of more than 25% reduction in case of double reduction mode.
Figure 2: shows the micrograph illustrating the microstructure with presence of 92 to 97% of ferrite phase, 3 to 7 % or less of pearlite phase along with carbide, nitride, and sulphide precipitates.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

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 (%)

The present invention relates to a single reduced tin-plated steel of hardness grade TH435 having the hardness HR30Tm within 63 to 69 comprising the following chemical composition in percentage by weight:
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;
at least one of V: 0.002-0.03 % and Ti: 0.015-0.025 %;
and the balance of Fe, wherein Ti/N ratio range from 0.6 to 4.

The single reduced tin-plated steel has the thickness in the range 0.16mm to 0.28mm

According to the present invention, the cold-rolled steel produced for tin plating high strength for tinplate with good formability, ageing resistance and surface tin coating, its chemical compositions, and method of manufacturing is 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 (0.025-0.040 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 is desirable to reduce the C level to less than 0.06 wt%. Lowering the carbon content below 0.02 wt% results in poor ageing properties since below 0.02 wt% steel is in complete a-ferrite region in the Iron-Cementite phase diagram resulting in no cementite formation. Consequently more free carbon will be available in the steel matrix which deteriorates the ageing property. Cementite formed in grain boundaries its precipitate size should be greater than 1µm which helps in reducing solute carbon and suppress the yield point elongation for better ageing resistance. To avoid Yield Point Elongation and to have yield strength within the desired limit, the level of Carbon is between 0.02-0.06 wt percent.
Mn (0.3-0.5) wt. % - The Mn ranges from 0.3 to 0.5 % is most desirable given 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. The lower limit of Mn is kept at 0.3% to ensure the surface properties deteriorate 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.
Ti (0.015-0.025 wt.%)– The 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 problems in low carbon steel sheets after temper rolling.
It is desirable that the ratio of Ti and N shall be 0.6 to 4 to fix the N partially and get the desired ageing guarantee of min 12 Months. As the total amount of Ti increases, the ferrite grains become finer because of a decrease in TiN precipitate size which increases the yield strength of the material. On the other hand, if the ratio of Ti and N ratio is less than 0.6 then it results in free N which deteriorates in the ageing property.
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 described in the present invention. The same is achieved by optimizing 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 a minimum level. Higher N content requires a 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. %: -. P is an element that improves the strength at low cost, and the amount of addition thereof varies depending on a target strength level. When the added amount exceeds more than 0.02 wt.% the yield strength level increases significantly. In addition, a higher amount of P promotes the formation of surface defects and significantly reduces the corrosion resistance of the base metal. So, the maximum limit of P is kept at 0.02 %.
Si (0-0.03) wt.%: Itis 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 the 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.
The single reduced tin-plated steel has a tensile strength in the range of 450-500 MPa, total elongation in the range of 15% to 25%, and the yield point elongation (YPEL) to a maximum of 6% to restrict strain aging. The strain aging appears due to the strain aging process during the electro-tin plating process. The further aging after tin plating is also restricted.

Another aspect of the present invention related to the method of manufacturing single reduced tin-plated steel of hardness grade TH435 as illustrated in accompanying Figure 1 comprising the following steps: Firstly using the chemical composition to obtain molten steel. Secondly, refining and continuously casting the molten steel to obtain a solid slab. Heat from the basic oxygen furnace (BOF) is processed through RH degasser and subsequently continuously cast. Thirdly, carrying out hot continuous rolling on the solid slab to obtain hot rolled steel. Fourthly, pickling and cold rolling the hot rolled steel to obtain cold-rolled steel. Fifthly, continuously annealing the cold-rolled steel to obtain an annealed steel. During the continuous annealing, the annealing soaking temperature is 700-715 ? and the annealing soaking time is 40 to 200s. Subsequent to pickling and cold rolling to desired thickness, cold-rolled steel strips are processed through a continuous annealing line where electrolytic cleaning removes the rolling emulsion present on the surface. The 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 700 to 715 °C to achieve ferrite and pearlite phases in microstructure. Annealing time is kept in the range of 40 to 200 seconds to allow sufficient time for annealing and homogenization of microstructure. Further, the cold-rolled steel is over-aged at the temperature from soaking section temperature up to a temperature range of 400 to 430 °C. Sixthly, skin pass elongation of the annealed steel to obtain steel strip; wherein the reduction is 2-3%. Lastly, electro-tinning on the steel strip to obtain the tinned plate steel of hardness grade TH435, wherein the reflow temperature is controlled to 230 to 265 ?. The steel is subjected to Tin deposition through an electrolytic process where pure tin is Anode and Cathode-Sheet and Electrolyte- Stannous + phenyl sulfonic acid. The deposited tin is melted and a strong bond is formed due to the formation of the iron tin alloy at temperatures ranging from 230 to 265 °C. Due to involvement of temperature more than 200° C steel gets hardened by strain aging mechanism while processing at electrolytic tinning line and desired mechanical properties are achieved.

Another aspect of the invention includes the method of the hot continuous rolling on the solid slab to obtain a hot rolled steel comprising the following steps. Firstly, the solid slab is reheated to a temperature of 1150°C -1250 °C, Secondly, the reheated slab is subjected to roughing rolling in a roughing mill with a roughing mill delivery temperature of 980-1080°C. Thirdly, the rough rolled steel is subjected to finish rolling with finish mill exit temperature ranging from Ac3 °C to Ac3+100 °C. Fourthly, the finish rolled steel is cooled at an average run-out table cooling rate of 10 °C/second to 30 °C/second and was maintained to achieve a coiling temperature range of 620 to 680 °C. Lastly, the finish rolled steel is then acid pickled to form the hot rolled steel with cold reduction of at least 85%. Hot-rolled coils were subsequently processed through pickling coupled with a tandem cold rolling mill to remove the oxide surface present in the surface.

The tin-plated steel sheet uses strain aging mechanism to achieve the required strength and hardness without hampering the elongation percentage of the material. Higher Mn % improves the strength by solid solution hardening, the precipitates formed by alloying elements like 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 the skin pass elongation of 2-3 % improves the strength of the material by strain hardening. Thus in this method material is subjected to 2-3% reduction only after annealing instead of more than 25% reduction in case of double reduction mode. The above mentioned mechanisms help the material to certain yield strength before processing in the tinning line. The process makes use of the temperature of up to 240°C used in the reflow section of the tinning line to strain age the material and reach the required Yield strength

a) Higher Mn % improves the strength by solid solution hardening,
b) The precipitates formed by alloying elements like 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), wherein the internal microstructure of the single reduced tinplate comprises in terms of area fraction relative to entire microstructure of steel, 92-97% or more of ferrite phase, 3 to 7 % or less of pearlite phase along with carbide, nitride and sulphide precipitates.
c) and the skin pass elongation of 2-3 % improves the strength of the material by strain hardening.
d) The above mentioned mechanisms help the material to certain yield strength before processing in the tinning line. The process makes use of the temperature of up to 240°C used in the reflow section of the tinning line to strain age the material and reach the required yield strength.

Another aspect of the present invention relates to an internal microstructure of the single reduced tin-plated steel as illustrated in accompanying Figure 2 comprises the following components in percentage by volume:
92 to 97% of ferrite phase, 3 to 7 % or less of pearlite phase along with carbide, nitride, and sulphide precipitates.

EXAMPLES

Table 1 shows the chemical composition (in weight percent) of the single reduced tin-plated steel sheet of the examples of the invention, with the balance being Fe and unavoidable impurities.
Table 1
Chemistry (in weight percent)
Sample No. C MN S P SI AL N Ti Ti/N Remarks
1 0.035 0.3 0.003 0.01 0.03 0.04 0.005 0.025 5 I
2 0.030 0.4 0.003 0.01 0.02 0.03 0.004 0.015 3.75 I
3 0.02 0.3 0.004 0.01 0.03 0.05 0.0025 0.020 4 I
4 0.045 0.5 0.007 0.01 0.02 0.06 0.006 0.024 3.95 I
5 0.050 0.35 0.01 0.015 0.03 0.03 0.007 0.004 0.5 C
6 0.015 0.45 0.01 0.02 0.04 0.04 0.003 0.035 11.67 C

*I - Present inventive example, C- Comparative Examples
*Underline boxes indicates “outside the appropriate range”

Table 2 shows the control parameters of the method of manufacturing of the single reduced tin-plated steel sheet
Table 2

Sample No Hot Rolling Parameters (°C) Cold Rolling Parameters CAL and Tin Coating Parameters
SRT Roughing Mill temp FT CT Cold Reduction % SS TEMP SS Residence Time OAS TEMP Skin Pass Elongation Electro Tinning Reflow Temperature


1 1205 1060 900 650 80 700 95 410 2.4 200
2 1205 1070 890 660 90 715 95 420 2.3 250
3 1200 1060 880 653 92.5 700 95 420 3 245
4 1200 1065 885 648 90 715 95 420 2 240
5 1220 1070 908 570 70 730 95 350 1.8 180
6 1210 1080 910 560 75 750 90 360 1.5 260

*I - Present inventive example, C- Comparative Examples
*Underline boxes indicates “outside the appropriate range”
* 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 shows the mechanical properties of the single reduced tin-plated steel sheet according to the examples of the present invention
Table 3

Sample No Mechanical Properties Coating Properties Salt spray data Cost Remarks
YS TS Elongation % Yield Strength difference after aging test ASTM Grain Number Hardness (HR 30T) Porosity Shelf life test
1 445 467 17 4 Mpa 11 68 5 6 months 6 hrs Low I
2 405 453 20 6 Mpa 11 64 6 6 Months 6 hrs Low I
3 483 502 15 3 Mpa 12 69 6 6 Months 6 hrs Low I
4 386 451 22.5 5 Mpa 11 63 6 6 Months 6 hrs Low I
5 380 467 21.5 5 Mpa 10 59 6 6 months 6 hrs High C
6 360 420 25 3 Mpa 9 55 5 4 Months 3 hrs High C

*I - Present inventive example, C- Comparative Examples, G- Good, NG- Not Good
*Underline boxes indicates “outside the appropriate range”
*Standards referred for Mechanical Properties testing- IS-1993-2018
It can be appreciated from Table 1 to Table 3 that the tin-plated steel samples remarked as “I” were satisfying all the scopes of present invention and improved ageing resistance to avoid problems like wrinkling, earring, stretcher strains and 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 tin-plated steel samples exhibit improved yield strength of 385-450 MPa, tensile strength of 450-500 MPa, total elongation in the range of 15% to 25%, hardness level (HR30T) in the range of 63 to 69 and excellent ageing resistance. Porosity test was done to check the coating quality where the number of pores were observed in the acceptable range of 0-7. The shelf life of the samples were checked by subjecting the samples to ambient conditions. The samples resisted corrosion in ambient conditions for a time period of 6 months. Whereas, tin-plated steel samples remarked as ‘C’ 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, tin-plated steel sample no. 5 has a high carbon% more than 0.05 and less Ti. wt% (<0.01 %). This leads to low aging resistance showing an increase in yield strength after 1 hour of aging test. Similarly, tin-plated steel sample no. 6 has C% of less than 0.02% and more Ti. Wt% (>0.03). This leads to presence of non-metallic surface impurities leading to improper coating and low corrosion resistance. The sample with defects started corroding from the defective site after 3 hours of exposure to salt spray chamber while the good samples did not corrode for 6 hours. Also the shelf life was lower than the other samples by 2months in the ambient conditions. The tin-plated steel sample no. 3 and 4 are having YS less than 385 MPa and hardness is less than 60 HR30T which is out of scope of the present invention. The cost of production in case of sample 5 and 6 was higher due to involvement of higher temperature during annealing process.