DESC:FIELD OF THE INVENTION

The present invention relates to high Strength cold rolled continuous annealed steel sheet obtained of Carbon Manganese steel having 470Mpa tensile strength level, exceptional spot weldability, good fatigue resistance and phosphatibility, primarily proposed for Rocker panels, cross car beams and the like of an automobile body and method of manufacturing the same. The slab having composition comprising 0.14wt% to 0.2Wt% percent of Carbon, 0.7wt% to 1.3Wt% of Manganese,0.1 to 0.30Wt% of Silicon,0.02Wt% or less of Phosphorous, Up to 0.007Wt% of Nitrogen and Balance as Fe and incidental impurities wherein Manganese to silicon ratio to be 3 to 11, is hot rolled with finishing temperature of 860°C to 900°C and winded with run out table cooling rate of 13°C/sec to 15°C/sec. The Hot rolled product is pickled and cold rolled, continuous annealed followed by overaging with selected parameters and temper rolling in the range of 1.2% to 2.0% to suppress yield point in material. The process parameters are set to achieve the Tensile strength of 470Mpa minimum, yield strength 295Mpa -395 Mpa with elongation of 28Min and satisfying the relation 1.54Mn-0.89Al-0.07Si>1.

BACKGROUND OF THE INVENTION

In the past decades major developments are taken for the use of high strength steel in the transport vehicles to reduce the weight by decreasing the thickness of the material and increase in tensile strength of the material. Cold rolled high strength carbon manganese based steel is used at rocker panel, chassis, cross member. The material used for said components should have good fatigue resistance and as it is weldable with other components spot weldability is also a requirement. As all components are painted after final part to get the good aesthetic look, material should have good phosphatability. The present invention of steel does not only have higher tensile strength 470Mpa but also has good Fatigue resistance which is required for certain automotive components with Spot weldability and phosphatability. All these desirable properties are achieved by aiming the desirable chemistry and processing using the desired process parameters.
The present invention relates to a method for producing the cold rolled high strength steel containing C % of 0.14% – 0.2%, 0.7 to 1.3% of Manganese, Silicon 0.1% to 0.30% respectively and strengthening mechanism using Solid solution strengthening (carbon, manganese and silicon). In hot rolled product the by keeping the coiling temperature above 570°C moderate increase in tensile strength occurs.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide High strength cold rolled Steel sheet having good Fatigue resistance with excellent spot weldability, and a method for manufacturing the same through continuous annealing route.

A further object of the present invention is directed to provide High Strength Cold Rolled steel sheet having selective composition along with processing through hot rolling, cold rolling, continuous annealing followed by temper rolling with selective parameters at each stage to ensure to attain the desirable properties with fatigue resistance and spot weldability.

A still further object of the present invention is directed to provide High Strength steel sheet with Fatigue resistance wherein to achieve desired property, cold rolled steel is continuously annealed with selected soaking temperature to ensure complete recrystallization and slow cooling temperature with cooling rate and rapid cooling temperature with cooling rate , overaging temperature with residence time.

Yet another object of the present invention is directed to provide High Strength steel sheet with Fatigue resistance and Spot weldability of producing the same wherein free in carbon equivalent is 0.18 Minimum.

A still further object of the present invention is directed to provide High Strength steel sheet with Fatigue resistance wherein to maintain good phosphatability ratio of manganese to silicon to be maintained from 3 to 11 to control the oxides formed on the surface, so that when the invented material is Phosphated, coating weight of 2.5(g/m2) is achieved, indicative of good phosphatability.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to a steel composition for producing high strength Carbon Manganese Steel Sheets through continuous annealed route and excellent phosphatability comprising:
0.14Wt % to 0.2Wt% percent of Carbon;
0.7wt% to 1.3Wt% of Manganese;
0.12wt% to 0.35Wt% of Silicon;
0.02Wt% or less of Phosphorous;
Up to 0.007Wt% of Nitrogen; and
Balance as Fe and incidental impurities, wherein Manganese to silicon ratio to be 3 to 11,
suitable for generation of steel sheet having anyone or more of
a) Tensile strength from 470Mpa to 550Mpa;
b) Elongation greater than 28%;
c) ASTM Grain size of 11 to 13;
d) Volume fraction of 75% or more ferrite and 25% or less Pearlite;
e) Phosphatibility, phosphate coating weight to be 2 to 4µm and crystal size 1.5 to 2.5g/m2; and
f) Fatigue endurance Limit greater than 0.45.

A further aspect of the present invention is directed to said steel composition wherein manganese, silicon and aluminum to be 1.54Mn-0.89Al-0.07Si>1;

A further aspect of the present invention is directed to a high strength Carbon Manganese cold rolled continuous annealed steel sheet Steel comprising:
0.14Wt % to 0.2Wt% percent of Carbon;
0.7wt% to 1.3Wt% of Manganese;
0.12wt% to 0.35Wt% of Silicon;
0.02Wt% or less of Phosphorous;
Up to 0.007Wt% of Nitrogen; and
Balance as Fe and incidental impurities, Wherein silicon such that Manganese to silicon ratio to be 3 to 11;having
a) Tensile strength from 470Mpa to 550Mpa;
b) Elongation greater than 28%;
c) ASTM Grain size of 11 to 13;
d) Volume fraction of 75% or more ferrite and 25% or less Pearlite;
e) Phosphatibility, phosphate coating weight to be 2 to 4µm and crystal size 1.5 to 2.5g/m2; and
f) Fatigue endurance Limit greater than 0.45.

A still further aspect of the present invention is directed to said high strength Carbon Manganese cold rolled continuous annealed steel sheet further includes in mass % at least one element selected from the group consisting of Bi, Ni, Ca, Co, Nb, Ti, Cr, W, Mo, V, Zr and Hf such that each element shall be in range of wt % of 0.002 to 0.05%.

A still further aspect of the present invention is directed to said high strength Carbon Manganese cold rolled continuous annealed steel sheet comprising manganese, silicon and aluminum to be 1.54Mn-0.89Al-0.07Si>1.

A still further aspect of the present invention is directed to said high strength Carbon Manganese cold rolled continuous annealed steel sheet further comprising copper from 0.12Wt% to 0.15Wt%.

Another aspect of the present invention is directed to a process for the manufacture of steel sheet comprising:
i) providing a selective steel composition for slab generation comprising:
0.14Wt % to 0.17Wt% percent of Carbon;
0.7Wt% to 0.9Wt% of Manganese;
0.15 Wt% to 0.3Wt% of Silicon;
0.02Wt% or less of Phosphorous;
0.12Wt% to 0.13Wt % of Copper;
Up to 0.007Wt% of Nitrogen;
and the balance being Fe and inevitable or associated impurities also being present, wherein manganese, silicon and aluminum tosatisfy 1.54Mn-0.89Al-0.07Si>1; and
ii) Carrying out steel sheet manufacturing including hot rolled, pickled, cold reduced and continuous annealing such as to reach to Fatigue resistance and Spot weldability.

Yet another aspect of the present invention is directed to said process comprising:
a) Hot rolling of said steel sheet with Finishing Temperature 860°C to 900°C and run out table cooling rate of 13°C/sec to 15°C/sec and
b) Pickling of said steel to remove oxide layer built on surface of steel sheet and said steel is cold rolled to thickness less than 60% of initial Thickness.

A further aspect of the present invention is directed to said process further comprising:
a. Soaking said steel at temperature range between 770°C to 800°C with residence time of 90Sec or less;
b. Slow cooling further said steel at temperature range of 680°C to 710°C with cooling rate of 5°C/Sec or less;
c. Rapid cooling of said steel at temperature 480°C with cooling rate of 30°C/Sec or less ;
d. overaged the said steel at 400°C or less with residence time of 300 secs or less; and
e. Skin passing of said steel range of 1.2% to 2%.

A still further aspect of the present invention is directed to said process selectively carried out such as for generation of said steel sheet having Fatigue resistance and Spot weldability, wherein the process is controlled such as to achieve anyone or more of:
a) Tensile strength from 470Mpa to 550Mpa;
b) Elongation greater than 28%;
c) ASTM Grain size of 11 to 13;
d) Volume fraction of 75% or more ferrite and 25% or less Pearlite;
e) Phosphatibility, phosphate coating weight to be 2 to 4µm and crystal size 1.5 to 2.5g/m2; and
f) Fatigue endurance Limit greater than 0.45.

The above and other objects and advantages are described hereunder in greater details with reference to the following accompanying drawing and examples.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING

Figure 1: Shows the Microstructure of High Strength Cold rolled steel according to present invention, annealed below 800°C having 80% of Ferrite and 20% of Pearlite with ASTM Grain Size of 12.

Figure 2: Shows test specimen dimensions for fatigue testing.

Figure 3: Shows effect of Mn/Si ratio on Phosphate Grain Size according to present invention MN/Si ratio to be in range of 3 to 11 to achieve Phosphate grain size < 3µm.
Figure 4: Shows effect of E1 on Tensile Strength, E1 -1.54Mn-0.89Al-0.07Si>1 to achieve tensile strength of 470MPa Minimum E1 to be greater than 1.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING FIGURES AND EXAMPLES INCLUDING A PREFERRED EMBODIMENT
The present invention relates to high Strength cold rolled continuous annealed steel sheet obtained of Carbon Manganese steel having 470Mpa tensile strength level, exceptional spot weldability, good fatigue resistance and phosphatibility, primarily proposed for Rocker panels, cross car beams and the like of an automobile body and method of manufacturing the same.
Steel of present invention having composition comprising by weight percent Carbon 0.14-0.20, Manganese 0.7-1.3, Phosphorous 0.02 or less, Silicon-0.12-0.35, Copper-0.12-0.15, Nitrogen -0.007max, Aluminum- 0.025-0.05 balance iron and essentially the usual balance impurities.

Following abbreviations have been used to describe the present invention:
Abbreviations
SS- Soaking Section
SCS – Slow Cooling Section
RCS -Rapid Cooling Section
OAS - Over-ageing section
UTS-Ultimate Tensile Strength in MPa
YS-Yield Strength in MPa
El% – Total Elongation
SPM % -Skin Pass Elongation in %
FT-Finishing Temperature
CT- Coiling Temperature

The basic principles and justification to arrive at the said desired composition defining the range of constituents are as follows:

Carbon (0.14-0.20%wt %) – Carbon ranging from 0.14 to 0.20 is used for increasing the tensile strength of the material. To achieve the minimum tensile strength of 470Mpa or more minimum 0.14% of carbon is required. Excessive amount of carbon increases the tensile strength significantly and also increases carbon equivalent value which detoriate the spot weldability, deteriotes the cold rolling properties hence the upper limit of carbon to maintained in the range of 0.14 to 0.20% to achieve the desired properties. Carbon equivalent is less than 0.23% resulting in better weldability.

Manganese (0.7-1.3% wt%) – Manganese acts as a solid solution strengthener, increase in manganese content increases the tensile strength, the drastic increase in tensile strength by addition of manganese happens not only because of solid solution strengthening but also by ferrite grain refinement. To achieve the minimum strength level 0.7% is required which will act as a grain refinement, but upper limit should be maintained to 1.3% to achieve the desired range. If manganese contents go beyond 1.3% Tensile strength level of steel will increase more than 550 Mpa which will be out of the range, also affects the surface by formation of manganese oxide on the surface which affects the phosphatibility of the material. So it is desired to maintain manganese level at 0.7% to 1.3%.

Silicon (0.12 – 0.30wt%) - Silicon is strengthening element to increase the tensile strength to achieve minimum tensile strength of 470Mpa silicon should be minimum 0.12%.When the percentage of silicon increases more than 0.30% increases carbon equivalent and affects the weldability of the materials, higher silicon affects during coating by forming oxides on the surface. Silicon is a ferrite stabilizer is hardens the materials more, increases tensile strength more than 480Mpa, desired range of silicon is 0.12% -0.30%, whereas to maintain good phosphatability ratio of manganese to silicon to be maintained from 3 to 11 to control the oxides formed on the surface, where the invented material is Phosphated and coating weight achieved is 2.5(g/m2) .In order to achieve the good phosphatibility coating weight to be 1.5 (g/m2) to 3 (g/m2) so coating weight achieved suggests material has good phosphatibility,

Copper 0.12-0.15wt% -Copper acts as a precipitation strengthener and increases the tensile strength to achieve tensile strength of 470Mpa or more minimum copper required is 0.12%, it also resists corrosion and does not affects the weldability of the materials instead it increases formability of the material moderately .Copper percentage is restricted to 0.15% or less, at higher side the material becomes very hard and starts cracking during the processing.

Bi, Ni, Ca, Co, Nb, Ti, Cr, W, Mo, V, Zr and Hf in the range of 0.002 to 0.05 % - each of from Bi, Ni, Ca, Co, Nb, Ti, Cr, W, Mo, V, Zr and Hf act as carbide former and/or nitride former and/or solid solution strengthening elements, however adding each of these elements in an amount more than 0.05 wt% unnecessarily adds up to the cost of the steel.

Method of Manufacturing:
Slabs are casted with the chemistry said above and hot rolled with slab reheating temperature of below 1190°C to attain roughing mill delivery temperature below 1040°C, to prevent the surface defects like rolled in scale, finishing temperature of 860°C to 900°C and then coiled at run out table cooling rate of 13°c/sec to 15°c/sec and processed through pickling coupled with tandem cold rolling mill, to remove the oxide surface present in the surface and cold reduction of 70% or less with the five stands reduction percentage is distributed from 20-30 % from 1st stand to forth stand, fifth stand used for creating roughness with max 5 % reduction, if this pattern is followed heat generated will be less and the cooling will also take place with generation of heat band being eliminated with the combination of emulsion concentration more than 3.6.

After pickled and cold rolled steel strip is processed through continuous annealing line, where electrolytic cleaning removes emulsion present on the surface. Cleaned surface passes through the preheating section where the strip is heated and then passes through soaking section where it is heated above 770°C or more where complete recrystallization takes and makes the steel softer to get desired properties of UTS 470Mpa or more. As material annealed at 800 or less in sample 4 shows grain size of 12 ASTM which shows volume fraction of 80% ferrite and 20% of Pearlite. Accompanying Figure 1 shows the Microstructure of High Strength Cold rolled steel according to present invention, annealed below 800°C having 80% of Ferrite and 20% of Pearlite with ASTM Grain Size of 12. Steel strip soaked at 800°C or less, sheet passes through slow cooling section with 680°C with cooling rate of 5°C /Sec or less and above to get the yield Strength less than 395 Mpa or less, rapidly cooled to 480°C with cooling rate of 9°C/Sec to 30°C/Sec and overaged at 380°C or more to reduce the solute carbon then processed through skin-pass mill with 1.5% or more elongation to eliminate the yield point elongation. Additionally Cold rolled sheet processed can be coated using galvanizing process and used as Galvannealed and Galvanized steel for similar applications.

Method of evaluating phosphatibility:
Phosphating process can be defined as the treatment of a metal surface so as to give a reasonably hard, electrically non-conducting surface coating of insoluble phosphate which is contiguous and highly adherent to the underlying metal and is considerably more absorptive than the metal which provides excellent corrosion resistance and paint ability to steel surface .The coating is formed as a result of a topochemical reaction, which causes the surface of the base metal to integrate itself as a part of the corrosion resistant film.

To evaluate phosphatability firstly alkali degreasing was performed on steel sheet at 400 C for 120 sec using FC-E2032 chemical manufactured by NIHON PARKERIZING India Pvt Ltd to the obtained cold rolled steel sheet without any oil/grease on surface. Degreasing was followed by water rinsing and then surface conditioning at room temperature for 30 seconds using PL-Z chemical manufactured by NIHON PARKERIZING India Pvt Ltd. Phosphate treatment using PB-L3020 chemical, manufactured by NIHON PARKERIZING India Pvt was done at 400 C for 90 seconds. Subsequently, the surface after phosphate treatment was observed under a Scanning electron microscope using Secondary Electron image mode. Average grain size was measured assuming circular phosphate crystals. Crystal size < 4µm is considered as excellent for phosphatability. The phosphate coating weight was measured using the XRF method and steel sheet with average coating weight after zinc phosphate chemical conversion coating of 1.5-2.5 g/m2 is considered having excellent phosphatability.

Method for fatigue testing:

Endurance limit is used to describe a property of materials: the amplitude (or range) of cyclic stress that can be applied to the material without causing fatigue failure. The sample for fatigue test is prepared as given in Figure 2 and bending moment is calculated and set to 1.81Nm and speed is set 1500mpm and cycle is set to be 10^7 , the testing starts with given load sample starts vibrating and it stops when it reaches set cycles or when samples cracks. Fatigue tester provides fatigue limit value and the fatigue strength value based on the representation of graphs and endurance limit of the material is calculated. For the present steel tested endurance limit is 0.45-0.5, it always good to have higher endurance limit to prevent material fatigue failure under service condition.

The composition, process parameters and properties of samples obtained under various experimental trials carried out according to the present invention along with some comparative results are illustrated in the following tables I to III.

Table I- Compositions of the invented steel sheets.
Table II- Hot rolling, cold rolling, annealing parameters.
Table III – Mechanical properties of respective steels.

Table I:

CE –Carbon equivalent, [C] + [Si]/30 + [Mn+Cu+Cr] /20 + [Ni]/60 + [Mo]/15+ [V]/10+ 5*B =0.25

In sample 13 and 14 where carbon equivalent is greater than 0.26 and 0.30 were poor weldability is observed in testing whereas in all inventive steel carbon equivalent is less than 0.25 where good weldability is observed.

Table II:

Hot Rolling Cold Rolling Annealing
S.no FT ROT Cooling Rate °C/Sec CR % Thickness SS °C SCS °C RCS °C OAS °C Spm
1 878 14 62 1.9 784 694 486 393 1.56
2 890 15 60 1.92 799 710 508 379 1.6
3 886 14 61 1.55 784 687 488 400 1.6
4 872 14 61 1.55 771 682 485 390 1.6
5 881 15 60 1.9 778 684 487 393 1.6
5 881 15 60 1.9 778 684 487 393 2.3
5 881 15 60 1.9 765 684 487 393 1.6
6 878 14 61 1.38 779 680 483 392 2
7a 881 14 61 1.55 810 689 471 362 1.6
7b 881 14 61 1.55 825 690 465 380 1.6
8a 877 13 61 1.38 787 679 473 397 1.2
8b 877 13 61 1.38 787 679 473 397 1
9 860 13 61 1.55 780 680 485 390 1.2
10 867 12 61 1.55 785 670 470 390 1.4
11 860 13 61 1.38 770 650 480 390 1.5
12 890 15 61 1.55 790 680 490 400 1.3
13 850 14 62 1.5 782 650 486 380 1.4
14 870 13 62 1.5 780 660 470 370 1.6

Table III:

E1 = 1.54Mn-0.89Al-0.07Si>1 to achieve 470Mpa min Tensile strength

Example 1- In sample no 7 where carbon is in 0.14wt% and in sample 5a its 0.2Wt% and tensile strengths are 492 Mpa(79% Ferrite and 21% Pearlite) and 548 MPa whereas in steel sample 14 carbon wt% 0.22 having tensile strength of 600Mpa exceeding the required range, and phase volume fraction of ferrite and Pearlite are 70% and 30% respectively, so carbon is restricted to 0.2Wt%, but in sample 9 carbon is 0.13wt% where tensile strength is 460Mpa marginally lower than the desired tensile strength, hence carbon is restricted to 0.14 wt% to 0.20Wt% , volume fraction of ferrite to 75% or more and Pearlite to be 25 % or less.

Example 2 – In sample no 5a and 6 where manganese is 0.7 and 1.3Wt% the tensile strength observed is 548Mpa and 492MPa,whereas sample 11 having manganese of 1.4Wt% with tensile strength of 565Mpa and in sample 10 where manganese is 0.5 Wt% tensile strength is 440MPa , to achieve desired properties manganese to be 0.7wt to 1.3Wt%. As shown in Figure 4 E 1 >1, E1 = 1.54Mn-0.89Al-0.07Si>1 to achieve minimum tensile strength of 470Mpa.

Example 3 –In sample 6 and 5a where silicon wt% are 0.12 % and 0.3% where tensile strength are 523Mpa and 548Mpa , whereas in sample 13 silicon wt% is 0.35 and its tensile strength observed is 559Mpa , so silicon is restricted to 0.3wt% , in sample 10 where silicon is 0.025% ,tensile strength is 440Mpa where strengthening is poor , silicon to be maintained 0.12 Wt% to 0.3wt%.As well sample 10 as poor Phosphatbility with coating weight of 3.5 g/m2 and crystal size of 8 µm , its Manganese to silicon ratio is 20 , whereas in all invented steel sample Mn/Si ratio is 3 to 11 where phosphate grain size is less than 3.5 µm and coating weight less than 3 g/m2 as shown in Figure 3.

Example 4 –In sample 3 and 4where copper wt% 0.12 and 0.15 where tensile strength is 510Mpa and 500Mpa whereas sample 9 where copper is 0.04% tensile strength is 460Mpa out of scope of present invention , copper to be maintained in range of 0.12Wt% to 0.15Wt%.

Example 5- In sample no 5 keeping all parameter samples and varying ss temperature in 5c where SS temperature is below 770 °C, tensile strength observed is 560Mpa so soaking temperature is 770°C or more. Whereas in sample 7a and 7b again only soaking temperature is varied, ss temperature 825°C observed with 465Mpa, soaking temperature to be in range of 770°C to 800 °C.

Example 6 –In sample 8a and 8bwhere all parameter are kept same and varying spm elongation 1.2% and 1% where yield strength is 305Mpa and 2850Mpa to achieve minimum yield strength spm elongation to be maintained 1.2%.whereas in sample 5a and 5b spm is varied 1.6 and 2.3% , ys observed is 356Mpa and 400MPa , desired yield strength range is 295Mpa to 395Mpa to achieve desired strength spm elongation to be in range of 1.2% to 2%.

It is thus possible by way of the present invention to provide High Strength cold rolled continuous annealed Carbon Manganese steel sheet having tensile strength 470Mpa (min), yield strength 295Mpa -395 Mpa with elongation of 28Min, along with excellent fatigue strength, spot weldability and phosphatibility suitable for automobile body parts.

,CLAIMS:We Claim:

1. Steel composition for producing high strength Carbon Manganese Steel Sheets through continuous annealed route and excellent phosphatibility comprising:
0.14Wt % to 0.2Wt% percent of Carbon;
0.7wt% to 1.3Wt% of Manganese;
0.12wt% to 0.35Wt% of Silicon;
0.02Wt% or less of Phosphorous;
Up to 0.007Wt% of Nitrogen; and
Balance as Fe and incidental impurities, wherein Manganese to silicon ratio to be 3 to 11, Suitable for generation of steel sheet having anyone or more of
a) Tensile strength from 470Mpa to 550Mpa;
b) Elongation greater than 28%;
c) ASTM Grain size of 11 to 13;
d) Volume fraction of 75% or more ferrite and 25% or less Pearlite;
e) Phosphatibility, phosphate coating weight to be 2 to 4µm and crystal size 1.5 to 2.5g/m2; and
f) Fatigue endurance Limit greater than 0.45.

2. Steel composition as claimed in claim 1 wherein manganese, silicon and aluminum to be 1.54Mn-0.89Al-0.07Si>1.

3. A high strength Carbon Manganese cold rolled continuous annealed steel sheet Steel comprising:
0.14Wt % to 0.2Wt% percent of Carbon;
0.7wt% to 1.3Wt% of Manganese;
0.12wt% to 0.35Wt% of Silicon;
0.02Wt% or less of Phosphorous;
Up to 0.007Wt% of Nitrogen; and
Balance as Fe and incidental impurities, wherein silicon such that Manganese to silicon ratio to be 3 to 11.
having a) Tensile strength from 470Mpa to 550Mpa;
b) Elongation greater than 28%;
c) ASTM Grain size of 11 to 13;
d) Volume fraction of 75% or more ferrite and 25% or less Pearlite;
e) Phosphatibility, phosphate coating weight to be 2 to 4µm and crystal size 1.5 to 2.5g/m2; and
f) Fatigue endurance Limit greater than 0.45.

4.A high strength Carbon Manganese cold rolled continuous annealed steel sheet of claim 3further includes in mass % at least one element selected from the group consisting of Bi, Ni, Ca, Co, Nb, Ti, Cr, W, Mo, V, Zr and Hf such that each element shall be in range of wt % of 0.002 to 0.05%.

5. A high strength Carbon Manganese cold rolled continuous annealed steel sheet of anyone of claims 3 or 4 comprising manganese, silicon and aluminum to be 1.54Mn-0.89Al-0.07Si>1.

6. A high strength Carbon Manganese cold rolled continuous annealed steel sheet of anyone of claims 3to 5further comprising copper from 0.12Wt% to 0.15Wt%.

7. A process for the manufacture of steel sheets comprising:
i) providing a selective steel composition for slab generation comprising:
0.14Wt % to 0.17Wt% percent of Carbon;
0.7Wt% to 0.9Wt% of Manganese;
0.15 Wt% to 0.3Wt% of Silicon;
0.02Wt% or less of Phosphorous;
0.12Wt% to 0.13Wt % of Copper;
Up to 0.007Wt% of Nitrogen;
and the balance being Fe and inevitable or associated impurities also being present, wherein manganese, silicon and aluminum to satisfy 1.54Mn-0.89Al-0.07Si>1; and
ii) Carrying out steel sheet manufacturing including hot rolled, pickled, cold reduced and continuous annealing such as to reach to Fatigue resistance and Spot weldability.

8. A process as claimed in claim 7 comprising:
a) Hot rolling of said steel sheet with Finishing Temperature 860°C to 900°C and run out table cooling rate of 13°C/sec to 15°C/sec and
b) Pickling of said steel to remove oxide layer built on surface of steel sheet and said steel is cold rolled to thickness less than 60% of initial Thickness.

9. A process as claimed in claim 7 or 8, further comprising:
a. Soaking said steel at temperature range between 770°C to 800°C with residence time of 90Sec or less;
b. Slow cooling further said steel at temperature range of 680°C to 710°C with cooling rate of 5°C/Sec or less;
c. Rapid cooling of said steel at temperature 480°C with cooling rate of 30°C/Sec or less ;
d. overaged the said steel at 400°C or less with residence time of 300 secs or less; and
e. Skin passing of said steel range of 1.2% to 2%.

10. A process as claimed in anyone of claims 7 to 9 selectively carried out such as for generation of said steel sheet having Fatigue resistance and Spot weldability, wherein the process is controlled such as to achieve anyone or more of:

a) Tensile strength from 470Mpa to 550Mpa;
b) Elongation greater than 28%;
c) ASTM Grain size of 11 to 13;
d) Volume fraction of 75% or more ferrite and 25% or less Pearlite;
e) Phosphatibility, phosphate coating weight to be 2 to 4µm and crystal size 1.5 to 2.5g/m2; and
f) Fatigue endurance Limit greater than 0.45.

Dated this the 24th day of August, 2016
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)