DESC:FILED OF THE INVENTION
The present invention relates to 390MPa Tensile strength level high-strength interstitial free cold rolled steel sheet used for Reinforced ‘A’ Pillar Inner Upper LH/RH and Longitudinal beams in automotive body parts and a method of manufacturing the same through continuous annealing route. The invented steel grade having composition in terms of mass percent: 0 to 0.003 wt% percent of Carbon; 0.5 wt% to 0.8wt% of Manganese; 0 to 0.01wt% of sulfur; 0.04wt % to 0.07wt % of Phosphorus; 0.02wt% to 0.06wt% of Aluminum; 0.005wt% to 0.02wt% of niobium; up to 0.004wt% of Nitrogen; and the balance being Fe and inevitable or associated impurities also being present, wherein Manganese to Aluminum is in ratio of 15 to 35, having excellent Phosphatibility. It is also having Ti based on ratio of [Ti -3.4*N-4*C+0.52*Nb-1.5*S]/ [P] from 0.05 to 0.35 for improved cold work embritlement resistance. Carbon equivalent value of less than 0.14 in present inventive provides excellent spot weldability along with improved formability having planar anisotropy ratio (r-bar)>1.5. The slab produced with desired chemistry is hot rolled, cold rolled, continuously annealed and subjected to over aging followed by temper rolling with selected parameters to avoid any stretcher strain after annealing and to achieve yield strength between 220 Mpa or more.

BACKGROUND OF INVENTION
Generally High strength Interstial Free steels it’s difficult to achieve good planar anisotropy (r bar) because of formation of FeTiP precipitates. The phenomenon is known as cold work embrittlement (CWE) or secondary work embrittlement (SWE) which leads to brittle failure at lower temperatures. The generally accepted explanation for this embrittlement is that the grain boundaries are weaker than the grains because the former are depleted of carbon and nitrogen by the stabilisation process by Ti and/or Nb. This weakening can be exacerbated by the segregation, to grain boundaries of elements such as phosphorus that are added to IF steels to improve their strength. Phosphorus is a popular additive because of its high potency as a solid solution strengthener in iron alloys. However, phosphorus also has a reputation as an embrittling element in iron alloys because of the weakening effect it has on grain boundaries.
Granted Indian Patent Number 268352 disclosed an improved interstitial free steel grades for high strength having tensile strength 340 to 440 Mpa, comprising Chemical composition of %C %Mn %5 %P %Si %AI %Ti %Nb N B <0.0035 0.5-0.95 <0.01 0.02-0.065 < 0.015 0.04-0.05 0.05-0.06 0.012-0.02 <30ppm 4-11ppm Physical processing a) Hot charging of slabs to reheat furnace for grades IFHS-390 and IFHS-440. The material produced using the chemistry disclosed in patent 268352 which results in poor weldability, cold work embrittlement and phosphatibility.
The present invention aims to solve the problem of prior art by providing proper selection of Mn/Al ratio for better phosphatibility and Ti composition based on ratio of [Ti -3.4*N-4*C+0.52*Nb-1.5*S] / [P] from 0.05 to 0.35 to improve resistance for cold work embrittlement. Poor Resistance for cold work embrittlement will lead to transition material from ductile to brittle and material may lead to failure during service condition. The overall chemical composition is selected in such way that carbon equivalent less than 0.14 which is suitable for better weldability.
The present invention relates generally to a method for improving formability, Cold Work Embrittlement Resistance, phosphatibility and weldability with high tensile strength of cold rolled continuous annealed steel strip having comprises chemical elements in terms of mass percent: 0 to 0.003 Wt% percent of Carbon; 0.5 Wt% to 0.8Wt% of Manganese; 0 to 0.01Wt% of sulfur; 0.04Wt % to 0.07Wt % of Phosphorus; 0.02wt% to 0.06Wt% of Aluminum; 0.005Wt% to 0.02Wt% of niobium; Up to 0.004Wt% of Nitrogen; and the balance being Fe and inevitable or associated impurities also being present, wherein Manganese to Aluminum in ratio of 15 to 35, having excellent Phosphatibility. Heretofore, it has been conventional to improve the deep drawing properties of High strength steel, cold rolled steel strip by adding titanium and Niobium to the steel. However, when titanium was added in excess in order to obtain the desired deep drawing properties cold rolling reduction above 60% with annealing temperature above 770°C
ROT cooling rate is maintained in range of 10-14°C/Sec produce more intense ? fiber with higher average r value, higher coiling temperature precipitates TiC-TiN Complex structure, it leaves very less Ti to form FeTiP, and improved r value above 1.5 is achieved.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide high strength Interstitial free cold rolled steel sheet having excellent formability and improved Phosphatibility and a method for manufacturing the same through continuous annealing route.

A further object of the present invention is directed to provide high strength Interstitial free cold rolled steel having selective composition along with processing through hot rolling, cold rolling, continuous annealing followed by temper rolling with selective parameters at each stage for improved planar anisotropy to improve formability while processing.

A still further object of the present invention is directed to provide high strength Interstitial free steel sheet with excellent Surface wherein to achieve good phosphatibility, cold rolled steel is continuously annealed with desired chemistry having less manganese and silicon.

A still further object of the present invention is directed to provide high strength Interstitial free steel sheet wherein to achieve good Spot Weldability, cold rolled steel is continuously annealed with desired chemistry having less manganese and silicon with carbon equivalent less than 0.14.

Yet another object of the present invention is directed to provide high strength Interstitial free steel sheet with good r bar value and of producing the same wherein free carbon and nitrogen is fixed with addition of desired amount of titanium and Niobium and soaking temperature of 770°C or more for residence time 50Sec or more.

A still further object of present invention is directed to provide high strength Interstitial free steel sheet with excellent formability and improved Phosphatibility where in to ensure the surface quality 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.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to a steel composition having excellent phosphatibility suitable for producing high strength Interstitial Free steel sheet having desired formability comprising:
0 to 0.003 Wt% percent of Carbon;
0.5 Wt% to 0.8Wt% of Manganese;
0 to 0.01Wt% of sulfur;
0.04Wt % to 0.07Wt % of Phosphorus;
0.02wt% to 0.06Wt% of Aluminum;
0.005Wt% to 0.02Wt% of niobium;
Up to 0.004Wt% of Nitrogen;
and the balance being Fe and inevitable or associated impurities wherein Manganese to Aluminum in ratio of 15 to 35 is maintained for phosphatibility and enabling providing finished steel having anyone or more of :
a) Yield strength from 220-290 Mpa and Tensile Strength from 390-440Mpa with YS/TS ratio less than 0.7;
b) Phosphate grain size <3µ, Coating Weight 2-3g/m2;
c) Rbar greater than 1.5;
d) Texture with fraction of favorably oriented {111} texture are greater than 55 %; and
e) Cold work embrittlement temperature less than -50 °C.

A further aspect of the present invention is directed to a steel composition having excellent phosphatability further comprising Ti, wherein composition of Ti is based on ratio of [Ti -3.4*N-4*C+0.52*Nb-1.5*S] / [P] from 0.05 to 0.35.

A still further aspect of the present invention is directed to a high strength Interstitial Free steel sheet having excellent formability and phosphatibility comprising:
0 to 0.003 Wt% percent of Carbon;
0.5 Wt% to 0.8Wt% of Manganese;
0 to 0.01Wt% of sulfur;
0.04Wt % to 0.07Wt % of Phosphorus;
0.02wt% to 0.06Wt% of Aluminum;
0.005Wt% to 0.02Wt% of niobium;
Up to 0.004Wt% of Nitrogen;
and the balance being Fe and inevitable or associated impurities also being present, wherein Manganese to Aluminum in ratio of 15 to 35.
A still further aspect of the present invention is directed to a high strength Interstitial Free cold rolled continuous annealed steel sheet of claim 3, further comprising Ti, composition of Ti is based on ratio of [Ti -3.4*N-4*C+0.52*Nb-1.5*S] / [P] from 0.05 to 0.35.
Another aspect of the present invention is directed to a high strength Interstitial Free cold rolled continuous annealed steel sheet further comprising at least one element from B, Zr, Bi, V, W, Cr and Mo each by content in the range of 0.0005 to 0.03 %.

Yet another aspect of the present invention is directed to a high strength Interstitial Free cold rolled continuous annealed steel sheet comprising:

a) Yield strength from 220-290 Mpa and Tensile Strength from 390-440Mpa with YS/TS ratio less than 0.7;
b) Phosphate grain size <3µ, Coating Weight 2-3g/m2;
c) Rbar greater than 1.5;
d) Texture with fraction of favorably oriented {111} texture are greater than 55 %; and
e) Cold work embrittlement temperature less than -50 °C.

A further aspect of the present invention is directed to a process for manufacturing high strength Interstitial Free steel sheet having excellent formability and phosphatibility comprising,
a) providing a selective steel composition comprising
0 to 0.003 Wt% percent of Carbon;
0.5 Wt% to 0.8Wt% of Manganese;
0 to 0.01Wt% of sulfur;
0.04Wt % to 0.07Wt % of Phosphorus;
0.02wt% to 0.06Wt% of Aluminum;
0.005Wt% to 0.02Wt% of niobium;
Up to 0.004Wt% of Nitrogen;
and the balance being Fe and inevitable or associated impurities also being present, wherein Manganese to Aluminum in ratio of 15 to 35.

b) Hot rolling of said steel at slab reheating temperature of 1220°C or less; Roughing mill delivery temperature 1060°C or less; Finishing Temperature more than 880°C with ROT cooling rate 10-14°C/Sec; and
c) Pickling of said steel to remove oxide layer built on surface of steel sheet and said steel is cold rolled to thickness less than 40% of initial Thickness.
A still further aspect of the present invention is directed to said method of producing steel sheet further comprising,
a) Soaking said steel at temperature range between 770°C to 800 °C with residence time 50-90 Sec;
b)Slow cooling further said steel at temperature range 640 °C to 690°C with cooling rate of 1°C /Sec to 4 °C/sec;
c) Rapid cooling of said steel at temperature 450°C to 480°C with cooling rate 10°C /Sec to 22°C /Sec;
d) Over ageing of said steel at temperature range between 340 °C to 410 with residence time of 180-330Sec; and
e) Skin passing of said steel between 0.5% to 1.2%.

A still further aspect of the present invention is directed to said method of producing steel sheet further comprising:
a) Soaking said steel at temperature range between 770°C to 800 °C;
b) Slow cooling further said steel at temperature range 640 °C to 690°C;
c) Rapid cooling of said steel at temperature 450°C to 480°C;
d) Over ageing of said steel at temperature range between 340 °C to 400°C;
e) Skin passing of said steel between 0.5% to 1.2%; and
f) Continuous annealing Furnace speed of said steel between 120mpm to 225mpm.

Another aspect of the present invention is directed to said method which is thus controlled for producing said steel sheet comprising anyone or more of:
a) Yield strength from 220-290 Mpa and Tensile Strength from 390-440Mpa with YS/TS ratio less than 0.7;
b) Phosphate grain size <3µ, Coating Weight 2-3g/m2;
c) Rbar greater than 1.5;
d) Texture with fraction of favorably oriented {111} texture are greater than 55 %; and
e) Cold work embrittlement temperature less than -50 °C.

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

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure 1: showing the effect of Ti* Rem /P on Embrittlement crack.
Figure 2: Showing the effect of Mn/Al on Phosphate Crystal Size (µm).
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWINGS AND EXAMPLES

The present invention relates to High strength Interstitial Free steel sheet having excellent formability and improved phosphatibility processed through continuous annealing. More particularly, the steel composition comprises ultra low Carbon 0 to 0.003 Wt% percent of Carbon; 0.5 Wt% to 0.8Wt% of Manganese; 0 to 0.01Wt% of sulfur; 0.04Wt % to 0.07Wt % of Phosphorus; 0.02wt% to 0.06Wt% of Aluminum; 0.005Wt% to 0.02Wt% of niobium; Up to 0.004Wt% of Nitrogen; and the balance being Fe and inevitable or associated impurities also being present, wherein Manganese to Aluminum in ratio of 15 to 35. Slab produced with above chemistry is hot rolled with HSM Finishing Temperature 880 to 920°C winded with ROT cooling rate 10-14°C/Sec to precipitate AlN and then cold rolled to thickness less than 40% of initial thickness. To get excellent formability cold rolled steel is continuously annealed with soaking temperature of 770°C or more with residence time of 90Sec which provides complete recrystallization and forms uniform grains. To fix the free carbon and nitrogen desired amount of titanium and Niobium is added and overaged at 340°C or more with residence time of 180 to 330Sec. To suppress the yield point elongation material is skin-passed with 0.5% or more elongation. The above parameters are set to achieve desired properties of yield strength 290Mpa or less, UTS 440Mpa or less, R bar 1.5 or more. Additionally Cold rolled sheet processed can be coated using galvanizing process and used as Galvannealed and Galvanized steel for similar applications.

Following abbreviations have been used in the description of the invention:
SS- Soaking Section
SCS – Slow Cooling Section
RCS-Rapid Cooling Section
OAS Over-ageing section
YPE-Yield Point Elongation
HSM-Hot Strip Mill
UTS-Ultimate Tensile Strength
YS-Yield Strength
El – Elongation
SPM-Skin Pass Mill
ROT – Run Out Table

Thus the present invention is directed to provide a steel grade comprising 0 to 0.003 Wt% percent of Carbon; 0.5 Wt% to 0.8Wt% of Manganese; 0 to 0.01Wt% of sulfur; 0.04Wt % to 0.07Wt % of Phosphorus; 0.02wt% to 0.06Wt% of Aluminum; 0.005Wt% to 0.02Wt% of niobium; Up to 0.004Wt% of Nitrogen; and the balance being Fe and inevitable or associated impurities also being present, wherein Manganese to Aluminum in ratio of 15 to 35. The Niobium and Titanium is added as carbide forming agent, which makes the steel interstitial free. Following are the justification for maintaining the claimed compositions.
CARBON ( 0.003wt.%Max) – Amount of carbon content increases more than 0.003%,then dissolved carbon remains in solute form up on recrystallization deteriotes R bar, it is preferably maintain 0.003% or less to obtain good R bar value.
MANGANESE (0.5-0.8 wt. %)- Manganese acts as solid solution strengthening increases the tensile strength of the steel, to achieve minimum tensile strength of 390Mpa its lower limit is set as 0.5. As manganese lowers planar anisotropy of the cold rolled steel sheet to achieve minimum R bar of 1.5 upper limit is 0.8 and it also forms oxides at the surface and makes the surface not suitable for phosphatibility, ratio of manganese to aluminum to be maintained in 15 to 35 to avoid the oxide surface and to obtain better phosphating surface .Hence the upper limit is set as 0.8.When manganese kept below 0.5% the desired tensile strength cannot be achieved.
Phosphorous (0.04-0.07 wt %) - Phosphorous has high solid solution strengthenability than Si and Mn. The lower limit of P should be 0.04% to provide good strength, but when phosphorus content is more it segregates in grain boundaries during solidification and r bar deteriotes, so the upper limit is set as 0.07%, phosphorous content increases, R bar detoriate to achieve r bar of minimum 1.5 phosphorous Wt% should be 0.07. To achieve minimum strength of 400Mpa phosphorous Wt% should minimum 0.04%.
Titanium (0.025-0.04 wt%) - Titanium is added to fix carbon nitrogen and sulphur to make steel interstitial free, When titanium is kept below 0.025% it will be insufficient to fix the carbon and nitrogen, forms titanium nitrides, carbide and sulfide, on the other hand it is more than 0.040% the formation of phosphide (FeTiP) which will deteriorates the R bar and lead to secondary work embrittlement. To avoid secondary work embrittlement Ti remaining to be calculated using Ti-[3.4*N+4[C-12/93*[Nb]]+48/32*S], to see available titanium to form FeTiP, ratio of titanium remaining to phosphorous to be 0.05 to 0.40 to be free from secondary work embrittlement at temperature -50°C.
Niobium (0.005-0.02) Wt%- Niobium is also added to fix the dissolved carbon as NbC, niobium is strong carbide former than titanium, makes the grain size finer and increase the yield strength of the steel but also decreases the Rbar as excess of Nb present will make recrystallization of austenite in hot rolling very difficult so its upper limit is restricted to 0.02%. Niobium addition gives beneficial effect of fatigue strength during spot welding [1].
Aluminum (0.02-0.06) Wt% - Aluminum acts as a deoxidizing agent but when present more than 0.02% it generates inclusion which is one of the possible causes for Edge sliver and also affects the formability of the steel, therefore the aluminum present should be 0.06% or less. Very less Al 0.03 or less, will lead to leave free N, which also will deteriotes the Rbar.
Nitrogen (0.004) Wt% Max- Nitrogen is an impurity present in the steel, when titanium is added to the steel, Nitrogen is fixed as TiN improves formability. When more amount of Nitrogen is present in the steel titanium added while contributes to form more TiN leaving the solute carbon which deteriotes the formability.
B, Zr, Bi, V, W, Cr and Mo in the range of 0.005 to 0.03 % - each of from B, Zr, Bi, V, W, Cr and Mo 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 0.03 wt% unnecessarily adds up to the cost of the steel.
Method of Testing Secondary Work Embrittlement

The bend test has been used to evaluate the susceptibility of steels samples to secondary work embrittlement (SCWE). This simple method has been used to quickly compare the steels in present invention .The test method used was similar to that described by Henning (1992). Test specimens 13 mm wide x 100 mm long oriented with the longest dimension transverse to the rolling direction were used. The specimens were bent at room temperature to a zero degree (0t) bend radius. The bend specimens were then opened manually by hand at various temperatures to determine the temperature at which a brittle fracture occurred. A pass rating was given to the specimens that showed metal to metal contact after they were pulled open. The failed specimens had a crack after they were pull open with no metal to metal contact at the inner radius. Tests were performed at each temperature with the temperature varying by 10-degree to -50 degree increments. Close to the transition temperature the increment was reduced to 5 degrees. The specimens were immersed in an alcohol/dry ice bath and were held for 5 min at the specified temperature. The specimens were removed and pulled within two seconds to unbend the specimen. They were then assessed for the pass/fail criteria: whether the specimen still had a metal-metal contact at the inner radius tip.5. To determine the transition temperature the percentage of pass specimens was calculated at each temperature. The pass rate was then drawn for each temperature to better visualize the transition curve. The present invented material had no crack at -50°C but the comparative material had cracked at -30°C.

Method of evaluating phosphatability
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 topo chemical reaction, which causes the surface of the base metal to integrate itself as a part of the corrosion resistant film.
To evaluate phosphatibility 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 < 3µm 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 1.5-3.5 g/m2 is considered having excellent phosphatability.

The composition, process parameters and properties of various inventive and comparative steel grades obtained through trials according to the present invention are presented in the following tables 1 to 3.

Table 1 - Compositions of the invented steel sheets along with some comparative examples.

Table 2 - Hot rolling, cold rolling and annealing parameters of present advancement and comparative steels.

Table 3 - Mechanical properties of present advancement and comparative steels.

Table 1:

I-Inventive, C – Comparative
Ti Remaining - Ti-[3.4*N+4[C-12/93*[Nb]]+48/32*S]

Table 2:
Hot Rolling Cold Rolling Annealing
FT°C ROT Cooling Rate°C/Sec Thk mm CCM Red% SS°C SCS°C RCS°C OAS°C SPM El %
938 14 0.7 75 772 667 477 377 1.00
938 14 0.7 75 760 667 477 377 1.00
937 14 0.9 74 776 651 481 380 1.00
918 13 0.65 76 772 651 482 343 1.00
935 13 1 67 774 685 479 397 0.80
885 10 0.95 76 798 689 471 408 1.10
908 11 0.95 76 815 689 471 408 1.10
931 15 1.2 68 785 652 468 396 1.01
938 17 1 74 776 648 480 384 1.00
932 13 1 74 768 641 478 376 1.00
910 13 0.6 81 797 680 480 400 0.80
932 13 0.7 75 780 650 480 382 1.00
912 13 0.9 74 785 656 480 378 1.00
920 14 1 74 777 654 475 383 0.80
937 14 0.7 75 794 664 482 369 0.55
937 14 0.7 75 794 664 482 369 0.40
909 13 1 74 792 673 453 404 0.80
927 13 0.9 74 772 647 480 374 1.01
930 14 0.6 78 774 649 475 359 1.2
930 14 0.6 78 774 649 475 359 1.3
909 13 1 73.68 792 673 462 404 0.8
909 13 1 73.68 792 673 462 404 0.8

Table 3:

Method of Manufacturing:
Slabs are casted with the chemistry said above and hot rolled with finishing temperature of 880°C to 920°C and then coiled with ROT cooling rate 10-14°C/Sec and processed through pickling coupled with tandem cold rolling mill, to remove the oxide surface present in the surface and cold reduced to thickness less than 40% of initial Thickness.
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 790°C or less where complete recrystallization takes and makes the steel softer to get desired properties of UTS 440Mpa or less and R bar of 1.5 or more. Cold reduced steel strip is annealed at soaking temperature 770 °C or more with residence time of 90sec maximum to achieve minimum tensile strength of 390 Mpa. To achieve tensile strength of minimum 400Mpa soaking section temperature should be 800 °C or less. Steel strip soaked at 770°C or more with residence time of 50-95 Sec , sheet passes through slow cooling section with 640°C and above with cooling rate of 1-4°C/Sec to get the yield Strength less than 270Mpa or less, and rapidly cooled to in range of 450°C to 480°C with cooling rate of 22°C /Sec and overaged at 340°C or more to reduce the solute carbon and then processed through skin-pass mill with 0.5% or more elongation to eliminate the stretcher strain, skin pass elongation of 0.5% to 1.2% increases yield strength increases from 220Mpa to 290Mpa, when skin passed at 1.2% or more, dislocation starts and yield strength starts increasing; To achieve yield strength of 290Mpa or less skin pass elongation to be maintained 1.2% or less. Additionally Cold rolled sheet processed can be coated using galvanizing process and used as Galvannealed and Galvanized steel for similar applications.

Example 1 – In sample 1a where carbon weight % 0.003 and soaking section temperature of 773°C, tensile strength of 428Mpa is observed, whereas in sample 1b where Soaking section temperature is 760°C, tensile strength of 445Mpa which is out of scope of present invention SS temperature to 770 or more to maintain tensile strength of 440Mpa max and Rbar minimum of 1.5, in sample 5a where SS temperature is 798°C where tensile strength observed is 395Mpa and in sample 5 b keeping all parameters same and varying SS temperature 815°C tensile strength observed is 370MPa soaking section temperature to be in range of 770°C to 800°C and carbon to maintained 0.003max to get desired R bar of 1.5Min and {111} texture more than 55%.

Example 2 – In sample 6 and 3 where manganese weight percentage is 0.5wt% and 0.8wt%, tensile strength observed is 394MPa and 428Mpa respectively. Whereas in sample 17 where manganese is 0.82wt% where tensile strength is 470Mpa out of desired value, in sample 9 manganese is 0.42wt % and tensile strength observed is 361MPa , to achieve desired value of 390Mpa to 440Mpa , manganese to be in range of 0.5Wt% to 0.8Wt%.

Example 3 –In sample 11 and 13a where titanium weight percentage is 0.04wt% and 0.025wt% where yield strength observed is 225Mpa and 265Mpa in desired range of invention, in sample 4 where titanium is 0.045wt% where yield strength is 295Mpa out of scope and in sample 9 where titanium is 0.005wt%, yield strength observed 200Mpa out of scope of invention, titanium to be maintained in range of 0.025wt% to 0.04wt%.

Example 4 – In sample 8 and 7 where phosphorous weight percentage is 0.04wt% and 0.07Wt% were 395MPa and 435MPa of tensile strength is observed respectively, and where ratio of titanium remaining to phosphorous is 0.21% and 0.38% and tested value of secondary work embrittlement shows no crack at -50°C, whereas in sample 9 phosphorous weight percentage is 0.03wt% tensile strength is 361Mpa but the ratio of Ti remaining to Phosphorous is -0.21 where tested value of secondary work embrittlement at -30°C shows crack , so phosphorous to be maintained in range of 0.04 to 0.07 to attain the desired tensile strength and to material to have better secondary work embrittlement Ratio of Ti*remaining to Phosphorus to be 0.05 to 0.40 as shown in Fig. 1.

Example 5 –In sample 2 and 11 where niobium is 0.005 and 0.02wt%, yield strength observed is 237Mpa and 283Mpa in desired range, where in sample 17 where niobium is 0.025wt%, yield strength observed is 349MPa out of scope, to achieve yield strength in range of 220Mpa to 290Mpa and Rbar greater than 1.5 niobium to be maintained in range of 0.005wt% to 0.02wt%.

Example 6 – In sample 13 a) where spm elongation is 0.55% and yield strength observed is 225Mpa , where in sample 13 b) keeping all other parameters same and varying the skin pass elongation to 0.4% yield strength observed is 215Mpa out of scope of invention, minimum skin pass elongation to be maintained is 0.5% to achieve 220MPa minimum. In sample 16 a) where skin pass elongation is 1.2% and yield strength observed is 288Mpa and in sample 16b ) keeping all parameters same and varying the skin-pass elongation 1.3% , yield strength observed is 312MPa, Skin Pass elongation to be in range of 0.5% to 1.2% to get desired range.

Example 7 – In sample 2 where ratio of Manganese to aluminum is 16 and tested result of phosphate grain size 3µm and coating weight 2.2g/m2 and in sample 10 where ratio of manganese to aluminum is 32, tested result of phosphating grain size is 2.8 µm and coating weight is 2.3 g/m2.Whereas in sample 9 where ratio of Mn to Al is 11, tested result of phosphatibility shows grain size of 6 µm and coating weight of 4g/m2, poor phosphatibility , to achieve good phosphatibility ratio of Manganese to aluminum to be 15 to 35 as shown in Fig. 2.

It is thus possible by way of the present invention to provide 390MPa Tensile strength level high-strength interstitial free cold rolled steel sheet used for Reinforced ‘A’ Pillar Inner Upper LH/RH and Longitudinal beams in automotive body parts. Said steel comprising chemical elements in terms of mass percent: 0 to 0.003 Wt% percent of Carbon; 0.5 Wt% to 0.8Wt% of Manganese; 0 to 0.01Wt% of sulfur; 0.04Wt % to 0.07Wt % of Phosphorus; 0.02wt% to 0.06Wt% of Aluminum; 0.005Wt% to 0.02Wt% of niobium; Up to 0.004Wt% of Nitrogen; and the balance being Fe and inevitable or associated impurities also being present, wherein Manganese to Aluminum in ratio of 15 to 35, having excellent Phosphatibility. It also having Ti based on ratio of [Ti -3.4*N-4*C+0.52*Nb-1.5*S] / [P] from 0.05 to 0.35 for improved cold work embritlement resistance. Carbon equivalent value of less than 0.14 in present inventive provides excellent spot weldability along with improved formability having planar anisotropy ratio (r-bar)>1.5. The slab produced with desired chemistry is hot rolled with finishing mill temperature of 880°C-920°C then coiled with ROT cooling rate 10-14°C/Sec. Cold rolled to thickness less than 40% of initial Thickness then heated with a heating rate of 3-10°C/sec to soaking section temperature of 770°C-800°C, slow cooled at 2-10 °C/sec to slow cooling temperature of 640 °C -690 °C with cooling rate of 1 -4°C/Sec, Rapid cooled at 8-22 °C/sec to rapid cooling section temperature of 450°C to 480°C, over aging at temperature of 340°C to 410°C for 180-330 seconds. In addition, temper rolling of 0.5% or more is provided to avoid any stretcher strain after annealing and to achieve yield strength of 220 Mpa or more.

,CLAIMS:We Claim:
1. A steel composition having excellent phosphatability suitable for producing high strength Interstitial Free steel sheet having desired formability comprising:
0 to 0.003 Wt% percent of Carbon;
0.5 Wt% to 0.8Wt% of Manganese;
0 to 0.01Wt% of sulfur;
0.04Wt % to 0.07Wt % of Phosphorus;
0.02wt% to 0.06Wt% of Aluminum;
0.005Wt% to 0.02Wt% of niobium;
Up to 0.004Wt% of Nitrogen;
and the balance being Fe and inevitable or associated impurities wherein Manganese to Aluminum in ratio of 15 to 35is maintained for phosphatability and enabling providing finished steel having anyone or more of :
a) Yield strength from 220-290 Mpa and Tensile Strength from 390-440Mpa with YS/TS ratio less than 0.7;
b) Phosphate grain size <3µ, Coating Weight 2-3g/m2;
c) Rbar greater than 1.5;
d) Texture with fraction of favorably oriented {111} texture are greater than 55 %; and
e) Cold work embrittlement temperature less than -50 °C.

2. A steel composition having excellent phosphatability as claimed in claim 1 further comprising Ti, wherein composition of Ti is based on ratio of [Ti -3.4*N-4*C+0.52*Nb-1.5*S] / [P] from 0.05 to 0.35.

3. A high strength Interstitial Free steel sheet having excellent formability and phosphatibility comprising:
0 to 0.003 Wt% percent of Carbon;
0.5 Wt% to 0.8Wt% of Manganese;
0 to 0.01Wt% of sulfur;
0.04Wt % to 0.07Wt % of Phosphorus;
0.02wt% to 0.06Wt% of Aluminum;
0.005Wt% to 0.02Wt% of niobium;
Up to 0.004Wt% of Nitrogen;
and the balance being Fe and inevitable or associated impurities also being present, wherein Manganese to Aluminum in ratio of 15 to 35.
4.A high strength Interstitial Free cold rolled continuous annealed steel sheet of claim 3, further comprising Ti, composition of Ti is based on ratio of [Ti -3.4*N-4*C+0.52*Nb-1.5*S] / [P] from 0.05 to 0.35 .
5.A high strength Interstitial Free cold rolled continuous annealed steel sheet of anyone of claims 3 or 4, further comprising at least one element from B, Zr, Bi, V, W, Cr and Mo each by content in the range of 0.0005 to 0.03 %.
6. A high strength Interstitial Free cold rolled continuous annealed steel sheet of anyone of claims 3 to 5 comprising:
a) Yield strength from 220-290 Mpa and Tensile Strength from 390-440Mpa with YS/TS ratio less than 0.7;
b) Phosphate grain size <3µ, Coating Weight 2-3g/m2;
c) Rbar greater than 1.5;
d) Texture with fraction of favorably oriented {111} texture are greater than 55 %; and
e) Cold work embrittlement temperature less than -50 °C.

7. A process for manufacturing high strength Interstitial Free steel sheet having excellent formability and phosphatibility as claimed in claim 3 comprising:
a) providing a selective steel composition comprising:
0 to 0.003 Wt% percent of Carbon;
0.5 Wt% to 0.8Wt% of Manganese;
0 to 0.01Wt% of sulfur;
0.04Wt % to 0.07Wt % of Phosphorus;
0.02wt% to 0.06Wt% of Aluminum;
0.005Wt% to 0.02Wt% of niobium;
Up to 0.004Wt% of Nitrogen;
and the balance being Fe and inevitable or associated impurities also being present, wherein Manganese to Aluminum in ratio of 15 to 35.

b) Hot rolling of said steel at slab reheating temperature of 1220°C or less; Roughing mill delivery temperature 1060°C or less; Finishing Temperature more than 880°C with ROT cooling rate 10-14°C/Sec; and
c) Pickling of said steel to remove oxide layer built on surface of steel sheet and said steel is cold rolled to thickness less than 40% of initial Thickness.
8. The method of producing steel sheet as claimed in claim 7, further comprising:
a) Soaking said steel at temperature range between 770°C to 800 °C with residence time 50-90 Sec;
b)Slow cooling further said steel at temperature range 640 °C to 690°C with cooling rate of 1°C /Sec to 4 °C/sec;
c) Rapid cooling of said steel at temperature 450°C to 480°C with cooling rate 10°C /Sec to 22°C /Sec;
d) Over ageing of said steel at temperature range between 340 °C to 410 with residence time of 180-330Sec; and
e) Skin passing of said steel between 0.5% to 1.2%.

9. The method of producing steel sheet as claimed in anyone of claims 7 to 8, further comprising:
a) Soaking said steel at temperature range between 770°C to 800 °C;
b) Slow cooling further said steel at temperature range 640 °C to 690°C;
c) Rapid cooling of said steel at temperature 450°C to 480°C;
d) Over ageing of said steel at temperature range between 340 °C to 400°C;
e) Skin passing of said steel between 0.5% to 1.2%; and
f) Continuous annealing Furnace speed of said steel between 120mpm to 225mpm.
10) The method of producing steel sheet as claimed in anyone of claims 7 to 9 which is thus controlled for producing said steel sheet comprising anyone or more of:
a) Yield strength from 220-290 Mpa and Tensile Strength from 390-440Mpa with YS/TS ratio less than 0.7;
b) Phosphate grain size <3µm, Coating Weight 2-3g/m2;
c) R-bar greater than 1.5;
d) Texture with fraction of favorably oriented {111} texture are greater than 55 %; and
e) Cold work embrittlement temperature less than -50 °C.

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