Claims:We Claim:
1. High strength interstitial free steel having excellent formability comprising:
Carbon (0.003)Max, Mn-0.4-0.5, P 0.05-0.06, Si up to 0.02, Ti 0.04 to 0.05, Nb 0.01-0.02 , B 0.001 to 0.002 ,Al up to 0.05, N up to 0.004 in wt% and having anisotropy ratio (R-bar: 1.6 min) with tensile strength 400 Mpa or more, having complex precipitate of Titanium carbo-nitride less than 4µm and Average ASTM grain Size 9 and above .
2. High strength interstitial free steel having excellent formability as claimed in claim 1 having yield strength 230MPa or more having Average ASTM grain Size 9 and above .

3. High strength interstitial free steel having excellent formability as claimed in anyone of claims 1 or 2 comprising carbon 0.003Max, Manganese 0.4-0.45, Phosphorous 0.05-0.06,Silicon-0.02 or less ,Titanium-0.04-0.05,Niobium-0.01-0.02,Boron-0.001-0.002,Nitrogen-0.001- 0.004,Aluminium 0.03-0.05 in wt % and balance iron.

4. A process for manufacture of High strength interstitial free steel having excellent formability as claimed in anyone of claims 1 to 3 comprising:

v) providing steel slab with selective composition comprising of Carbon (0.003)Max,Mn-0.4-0.5, P 0.05-0.06, Si up to 0.02, Ti 0.04 to 0.05, Nb 0.01-0.02 , B 0.001 to 0.002 ,Al up to 0.05, N up to 0.004 in wt% and balance iron;
vi) subjecting the said steel slabs to hot rolling with controlled coiling temperature of above 6000C ;
vii) cold rolling reduction of 75% to 85%;
viii) annealing the cold rolled steel maintaining core temperature in the range of 6700 to 6950C, such as to provide for steel with r-value greater than 1.6 and strength above 400 Mpa.

5. A process as claimed in claim 4 wherein said hot rolling is carried out with finishing temperature in the range of 8800 to 9200C.

6. A process as claimed in anyone of claims 4 or 5 which is carried out with carbon wt% ranges between 0.002-0.003 and cold reduction of 75% wherein core temperature above 670°C and above is maintained for R-value of >1.6 and with higher core temperature of 690 results in 1.65R-value.

7. A process as claimed in anyone of claims 4 to 6 wherein HSM coiling temperature is maintained in the range of 600°C - 640°C,CCM with reduction between 75 to 80%.

8. A process as claimed in anyone of claims 4 to 7 comprising:
- Hot rolling the steel at a Finishing Temperature 880 to 910 °C and coiling temperature of 600°C - 640°C.
9. A process as claimed in anyone of claims 4 to 8, wherein combination of said steel composition and hot spot temperature was maintained with P: 0.05-0.06 with Nb: 0.01-0.02 and 690°C respectively, to achieve UTS 400 Mpa or more; and
combination of said steel composition and cold spot temperature was maintained as P: 0.05-0.06 with Nb: 0.01-0.02 and 670°C respectively, to achieve the minimum UTS 400 Mpa or more.
10. A process as claimed in anyone of claims 4 to 9 wherein the combination of cold working deformation and coiling temperature is maintained as 75 to 80% and 600 or above to achieve R-bar 1.6 or more.

Dated this the 18th day of February, 2016
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)

, Description:FIELD OF THE INVENTION
The present invention relates to high strength interstitial free(IF) steel having excellent formability, and a method for manufacturing the same through batch annealing route. More particularly, the present invention is directed to provide interstitial free high strength steel sheet with strength above 400Mpa having excellent formability which are suitable for inner panel for automobile longitudinal structural component. Importantly, the ultra-low carbon (0.003max),IF steel according to the present invention is processed involving CCM reduction more than 75 % with controlled HSM Coiling temperature and batch annealed at 690-670 °C to achieve excellent anisotropy ratio (R-bar: 1.6 min) with tensile strength 400Mpa or more. The optimum skin pass elongation used to get yield strength 230MPa or more.

BACKGROUND OF THE INVENTION

Interstitial Free (IF) Steels, initially introduced in Japan in the 70’s, has been widely known worldwide as the best affordable high quality steel material for deep drawing applications and is utilized in a wide range of applications from automobile components to household appliances. IF steels are generally considered as aluminum-killed steel with an extra-low carbon content, nominally 0.005%, in which the residual carbon is combined with niobium, titanium, or some similar element with a strong affinity for carbon. Carbon is small enough to fit into the interstices of a primarily iron matrix, making it an "interstitial element" in steel. If the steel alloy has an ultralow carbon level (typically less than 50 parts per million), most of these gaps will not be occupied and, as such, can be called interstitial-free (IF) steel. These primarily ferric (iron), very formable IF steels are extra-deep-drawing steel (EDDS). Achieving this low carbon level does not occur using conventional steel processing. Instead, the molten steel must be put under a vacuum that decarburizes it by removing carbon monoxide, as well as other gases like hydrogen and nitrogen. This process is called vacuum degassing. Microalloying elements such as niobium or titanium are added to these steels to stabilize the interstitial carbon and nitrogen atoms as carbides and nitrides. However, when Ti and or Nb is used in excess, they do not combine with either C or N but stayed in solid solution making the steel expensive and affecting their mechanical properties.

Ultra low carbon cold rolled steel sheets are found to have improved formability with higher elongation and plastic strain ratio. The reduction in the amounts of both interstitial and alloying elements in the matrix leads to a measurable difference in the texture development and improved deep drawability. Hot rolling parameters, including slab reheating temperature, finishing temperature, coiling temperature, hot rolling speed and reduction have strong effects on the deep drawability. These effects have been related to size and distribution of the precipitates and the grain size of the hot band. The effects of steel composition and processing are deeply interwoven and one parameter often cannot be isolated from interaction with another element or processing step. Due to its ultra low carbon content, IF steels also show good formability. Interstitial Free steel is marked by a distinguished higher values of uni-axial elongation, normal anisotropy (r-value) and strain hardening exponential (n-value) than an ordinary low carbon steel.

Auto manufacturers now face the need to use thinner steel sheets without significantly compromising the rigidity and stiffness of the automobile. The trend in the automotive steel industry is thus shifting towards using higher strength thin gauge sheet steels. The steel industries thus need to develop high strength IF steel sheets with excellent formability.

Generally, in case of High strength Interstial Free steels, it is difficult to achieve good planar anisotropy (r bar) because of formation of FeTiP precipitates. 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 it leads to formation of phosphide (FeTiP)which will deteriorates the R bar and lead to secondary work embrittlement . Also any excess of Nb present will make recrystallization of austenite in hot rolling very difficult. The present invention is thus directed to solve the problems of prior art by developing a method for improving good r bar with high tensile strength of cold rolled batch annealed IF steel strip containing 0.003 max percent carbon with alloying of Ti, Nb and 0.01-0.02%, B in selective proportion wherein Boron is added to steel to prevent secondary work embrittlement.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide high strength interstitial free steel having excellent formability, and method for manufacturing the same through batch annealing route.

A further object of the present invention is directed to provide high strength interstitial free steel with excellent formability having composition comprising ultra-low carbon (0.003max) alloyed with Nb, Ti and B in selective amounts, to achieve excellent anisotropy ratio (R-bar: 1.6 min) with tensile strength 400Mpa or more.

A still further object of the present invention is directed to provide high strength interstitial free steel having excellent formability which is suitable for inner panel for automobile longitudinal structural component.

A still further object of the present invention is directed to provide high strength interstitial free steel having excellent formability involving a method for improving good r bar with high tensile strength of cold rolled batch annealed steel strip.

A still further object of the present invention is directed to provide high strength interstitial free steel having excellent formability wherein Titanium and Niobium is added as carbide and nitride former and Boron is added to the steel to prevent secondary work embrittlement.

A still further object of the present invention is directed to provide high strength interstitial free steel having excellent formability produced involving a method wherein coiling temperature kept above 600°C produce more intense ? fiber with higher average r value, and higher coiling temperature precipitates Ti4C2S2, leaving very less Ti to form FeTiP, favoring improved r value above 1.6.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to provide high strength interstitial free steel having excellent formability comprising:
Carbon (0.003)Max,Mn-0.4-0.5, P 0.05-0.06, Si up to 0.02, Ti 0.04 to 0.05, Nb 0.01-0.02 , B 0.001 to 0.002 ,Al up to 0.05, N up to 0.004 in wt% and having anisotropy ratio (R-bar: 1.6 min) with tensile strength 400 MPa or more.

A further aspect of the present invention is directed to said high strength interstitial free steel having excellent formability having yield strength 230MPa or more.

A still further aspect of the present invention is directed to said high strength interstitial free steel having excellent formability comprising carbon 0.003Max, Manganese 0.4-0.45, Phosphorous 0.05-0.06,Silicon-<0.02, Titanium-0.04-0.05, Niobium-0.01-0.02, Boron-0.001-0.002, Nitrogen-0.001- 0.004,Aluminium 0.03-0.05 in wt% and balance iron.

Another aspect of the present invention is directed to a process for manufacture of High strength interstitial free steel having excellent formability as described above comprising:

i) providing steel slab with selective composition comprising of Carbon (0.003)Max,Mn-0.4-0.5, P 0.05-0.06, Si up to 0.02, Ti 0.04 to 0.05, Nb 0.01-0.02 , B 0.001 to 0.002 ,Al up to 0.05, N up to 0.004 in wt% and balance iron;
ii) subjecting the said steel slabs to hot rolling with controlled coiling temperature of above 6000C ;
iii) cold rolling reduction of 75% to 85%;
iv) annealing the cold rolled steel maintaining core temperature in the range of 6700 to 6950C , such as to provide for steel with r-value greater than 1.6 and strength above 400 Mpa.

A further aspect of the present invention is directed to said process wherein said hot rolling is carried out with finishing temperature in the range of 8800 to 9200C.

A still further aspect of the present invention is directed to said process which is carried out with carbon wt% ranges between 0.002-0.003 and cold reduction of 75% wherein core temperature above 670°C and above is maintained for R-value of >1.6 and with higher core temperature of 690 results in 1.65R-value.

Yet another aspect of the present invention is directed to said process wherein HSM coiling temperature is maintained in the range of 600°C - 640°C, CCM with reduction between 75 to 80%.

A further aspect of the present invention is directed to said process comprising:
- Hot rolling the steel at a Finishing Temperature 880 to 910 °C and coiling temperature of 600°C - 640°C.

A still further aspect of the present invention is directed to said process, wherein combination of said steel composition and hot spot temperature was maintained with P: 0.05-0.06 with Nb: 0.01-0.02 and 690°C respectively, to achieve UTS 400 Mpa or more; and
combination of said steel composition and cold spot temperature was maintained as P: 0.05-0.06 with Nb: 0.01-0.02 and 670°C respectively, to achieve the minimum UTS 400 Mpa or more.

A still further aspect of the present invention is directed to said process wherein the combination of cold working deformation and coiling temperature is maintained as 75 to 80% and 600 or above to achieve R-bar 1.6 or more.

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

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

Figure 1 : shows the SEM image of Ti(CN) precipitates in cold rolled and batch annealed sample.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWINGS AND EXAMPLES

The present invention relates generally to high strength interstitial free steel with excellent formability and a method for producing the same with provision for improving good r bar with high tensile strength of cold rolled batch annealed steel strip containing between 0.003 max percent carbon with alloying of Ti 0.04-0.05% and Nb -0.01-0.02%, B-0.001-0.002.

Boron is added to steel to prevent secondary work embrittlement, involving cold rolling reduction above 75% with annealing temperature above 670°C.Coiling temperature is kept above 600°C produce more intense ? fiber with higher average r value, higher coiling temperature precipitates Ti4C2S2 and Ti (CN) it leaves very less Ti to form FeTiP, improved r value above 1.6 is achieved. Average Grain Size observed is in between 9 to 11 to achieve yield strength 230 Mpa and above.

Steel of present invention consist of carbon 0.003Max, Manganese 0.4-0.45, Phosphorous 0.05-0.06, Silicon-0.02 or less, Titanium-0.04-0.05, Niobium-0.01-0.02, Boron-0.001-0.002, Nitrogen-0.001- 0.004, Aluminium 0.03-0.05; balance iron and essentially the usual balance impurities.

The Titanium and Niobium is added as carbide and nitride former. Boron is added to the steel to prevent secondary work embrittlement.

Following are the technical justification for maintaining the above said composition in the IF steel according to the present invention for desired strength and formability-

Carbon ( 0.003wt.%Max) – when amount of carbon content increases more than 0.003%, then dissolved carbon remains in solute form which on recrystallization deteriorates R bar; hence it is preferably to maintain carbon 0.003% or less to obtain good R bar value.

Manganese (0.4-0.5) wt. %- Manganese acts as solid solution strengthening agent that increases the tensile strength of the steel, its lower limit is set as 0.4.As manganese lowers planar anisotropy of the cold rolled steel sheet, the upper limit is set as 0.45.When manganese kept below 0.4% it will not act as strengthening element but will only act to produce MnS that prevents hot shortness.

Phosphorous (0.05-0.06) wt.%- Phosphorous has high solid solution strengthenability than Si and Mn , effectively improves r bar. The lower limit of P should be 0.05% to provide good strength, but when phosphorus content is more it segregates in the grain boundaries on solidification and deteriorates the required properties, so the upper limit is set as 0.06%.

Silicon (0.02 wt % or less) – Silicon is an element used for increasing the strength of steel. As the silicon content increases the ductility and r-value noticeably deteriorated .Since silicon deteriorates plating/surface properties as well by forming SiO2 type of oxide (Scale), it is advantageous to add as low an amount of silicon in the steel as is possible, the added amount of silicon is preferably 0.02wt% or less.

Titanium (0.04-0.05)wt% - Titanium is added to fix carbon, nitrogen and sulphur to make steel interstitial free. When titanium is kept below 0.04%, it will be insufficient to fix the carbon and nitrogen, and on the other hand if it is more than 0.05%, the formation of phosphide (FeTiP) occurs which deteriorates the R bar and lead to secondary work embrittlement.

Niobium (0.01-0.02) 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 improves the strength of the steel, but has no effect on R bar. Excess of Nb present will make recrystallization of austenite in hot rolling very difficult, so its upper limit is restricted to 0.02%.

Boron (0.001-0.002) Wt% - Boron is added to steel to prevent secondary work embrittlement, to do so minimum amount of boron to be added should be 0.001%; when amount of boron exceeds 0.002%, it delays recrystallization increasing load during rolling and deteriotes quality of the annealed steel.

Aluminum (<0.05) Wt% - Aluminum acts as an deoxidizing agent but when present in amounts more than 0.05wt%, it generates inclusion and also affects the formability of the steel, therefore the aluminum present should be 0.05wt% or less.

Nitrogen (<0.004) Wt%- Nitrogen is an impurity present in the steel, when titanium is added to the steel, Nitrogen is fixed as TiN which improves formability. When more amount of Nitrogen is present in the steel, titanium added contribute to form more TiN leaving the solute carbon which deteriorates the formability.

Manufacturing Method

The high strength interstitial free steel is produced by having steel slab composition comprising carbon 0.003Max, Manganese 0.4-0.45, Phosphorous 0.05-0.06, Silicon 0.02 or less, Titanium-0.04-0.05, Niobium-0.01-0.02, Boron-0.001-0.002, Nitrogen-0.001- 0.004, Aluminum 0.03-0.05 in wt%; balance iron and essentially the usual balance impurities, and hot rolled with finishing temperature of 880°C to 920°C and coiled at temperature above 600°C.

Coiling temperature after hot rolling influences the r value, when coiling temperature is above 600°C, Ferrite + pearlite structure is formed more preferable for hot rolling sheet structure as the hardness of the phases are similar uniform cold rolling is done. Moreover as the coiling temperature is increased, R bar is increased significantly, as above 600°C Ti4C2S2 and Ti (CN) leaving less Ti to form FeTiP, which strongly relates with R Bar. The precipitate size of Ti (CN) is less than 4 µm as shown in SEM micrograph in Figure 1.

Cold rolling reduction of (75% -85%) improves r bar, when processed below 75% and the r bar value deteriotes drastically.

Cold rolled steel is annealed at core temperature between 670°C to 695°C, as the annealing temperature is increased the R bar value increases, but when annealed more than 695°C tensile strength decreases to obtain strength above 400 Mpa the core temperature is set between 670°C to 695°C, to obtain minimum r bar of 1.6 the lower limit of core temperature is set to 670°C.

Complete description of Inventive steel and comparative steel grades are illustrated in the following tables I to II.

Table I- Compositions of the invented steel sheets along with some comparative examples.
Sample No C Mn Si P S Al N B Ti Nb Remarks Reason
1 0.003 0.41 0.02 0.056 0.008 0.04 0.0034 0.001 0.042 0.017 I
2 0.003 0.41 0.02 0.056 0.008 0.04 0.0034 0.001 0.042 0.017 I
3 0.003 0.41 0.02 0.056 0.008 0.04 0.0034 0.001 0.042 0.017 I
4 0.003 0.41 0.02 0.056 0.008 0.04 0.0034 0.001 0.042 0.017 I
5 0.002 0.43 0.006 0.054 0.008 0.04 0.0028 0.001 0.047 0.015 I
6 0.002 0.43 0.006 0.054 0.008 0.04 0.0028 0.001 0.047 0.015 I
7 0.002 0.43 0.006 0.054 0.008 0.04 0.0028 0.001 0.047 0.015 I
8 0.002 0.43 0.006 0.054 0.008 0.04 0.0028 0.001 0.047 0.015 I
9 0.004 0.42 0.006 0.052 0.008 0.04 0.0034 0.001 0.048 0.014 C high C , less R bar
10 0.003 0.56 0.006 0.054 0.008 0.038 0.0035 0.001 0.054 0.018 C low CT, less R Bar
11 0.003 0.56 0.006 0.054 0.008 0.038 0.0035 0.001 0.054 0.018 C More Mn, Less R Bar
12 0.0025 0.042 0.006 0.057 0.008 0.04 0.0034 0.001 0.052 0.015 C low% red, Less R Bar
13 0.002 0.43 0.006 0.055 0.008 0.04 0.0034 0.0012 0.052 0.012 C low% red, Less R Bar
14 0.0025 0.042 0.006 0.057 0.008 0.04 0.0034 0.001 0.052 0.015 C low% red, Less R Bar
15 0.002 0.4 0.006 0.06 0.008 0.039 0.0034 0.0014 0.043 0.014 I
16 0.002 0.4 0.006 0.06 0.008 0.039 0.0034 0.0014 0.043 0.014 I
17 0.002 0.4 0.006 0.05 0.008 0.03 0.0038 0.0014 0.041 0.014 I
18 0.003 0.4 0.006 0.059 0.008 0.039 0.0032 0.002 0.044 0.014 I
19 0.0045 0.45 0.006 0.056 0.008 0.037 0.0035 0.001 0.05 0.011 C High C, Less R Bar
20 0.002 0.43 0.008 0.052 0.008 0.028 0.0042 0.0016 0.04 0.02 C less Ti, less Al, Less R Bar
21 0.003 0.44 0.006 0.056 0.008 0.04 0.0035 0.0013 0.04 0.016 I
22 0.003 0.44 0.006 0.056 0.008 0.04 0.0035 0.0013 0.04 0.016 I
23 0.003 0.45 0.006 0.056 0.008 0.05 0.0035 0.0013 0.04 0.016 I
24 0.0035 0.6 0.006 0.063 0.008 0.04 0.0035 0.0013 0.055 0.016 C High P, Less Rbar
25 0.003 0.5 0.008 0.055 0.008 0.055 0.0036 0.0012 0.050 0.025 C High Core Temp, Less UTS
26 0.003 0.5 0.008 0.055 0.008 0.055 0.0036 0.0012 0.050 0.013 C High Core Temp, Less UTS
27 0.003 0.5 0.008 0.055 0.008 0.055 0.0036 0.0012 0.050 0.013 C HIGH Core Temp, Less UTS
28 0.002 0.35 0.01 0.047 0.006 0.05 0.0036 0.001 0.05 0.01 C less Mn, Less UTS
29 0.002 0.3 0.008 0.04 0.004 0.047 0.004 0.0025 0.055 0.01 C High Core Temperature, Less Mn, Less TS
30 0.004 0.3 0.007 0.045 0.011 0.046 0.005 0.0005 0.045 0.005 C Less Mn, Low TS

Table II- Hot rolling, cold rolling and annealing parameters along with the mechanical properties of respective sample steel sheets.

Sample No HSM Rolling Cold Rolling Mechanical Properties
CT FT CR Thick Reduction BAF Cycle Core Temp SPM El YS UTS Elongation R Bar
1 625 912 0.9 78 710/680 678 1 250 414 39.2 1.627
2 609 913 0.9 78 710/690 682 1 235 401 37.5 1.634
3 630 915 0.9 82 710/690 683 1 261 400 38.4 1.65
4 620 915 0.9 79 710/680 675 1.2 253 420 38.9 1.62
5 626 933 0.9 77 710/680 672 0.6 230 401 40.1 1.61
6 615 923 0.9 75 710/680 670 1 245 414 37.1 1.6
7 614 924 0.9 80 710/690 686 1 252 425 39.3 1.65
8 631 923 0.9 75 710/690 671 1.2 250 404 38.8 1.6
9 660 916 0.9 75 710/680 684 1 260 412 37.2 1.45
10 560 920 0.9 78 710/690 692 1 245 402 39 1.4
11 590 908 0.9 78 710/680 678 1 252 402 39.2 1.45
12 624 902 0.9 70 710/690 694 1 242 400 39.8 1.52
13 625 910 1 68 710/680 686 1 240 410 39.5 1.45
14 615 911 0.9 65 710/690 698 1 239 395 39.6 1.4
15 628 925 1 78 700/680 672 1.2 232 412 37.6 1.601
16 639 923 0.9 80 700/680 679 1.2 238 415 37.8 1.625
17 631 923 0.7 78 700/680 685 1.2 234 402 38.2 1.67
18 626 920 0.7 79 700/680 680 1.2 271 425 37.5 1.61
19 627 915 0.9 76 700/680 675 1 265 428 36.5 1.48
20 620 920 0.7 75 710/690 684 1.2 230 404 38.3 1.34
21 632 914 1.6 77 710/690 689 1 248 411 39.2 1.63
22 625 920 0.8 77 710/690 690 1 250 405 39 1.62
23 620 918 1.6 77 710/690 693 1 255 400 38.8 1.6
24 628 935 0.8 77 680/660 665 1 242 410 37.4 1.5
25 638 932 0.7 78 720/700 712 1.2 270 390 40.1 1.6
26 630 920 0.7 78 720/700 708 1.2 265 392 39.8 1.62
27 628 935 0.7 78 720/700 698 1.2 260 397 39.5 1.62
28 702 915 1 80 740/720 714 0.7 225 371 42 1.56
29 714 917 1.6 73 740/720 709 0.7 205 355 41.1 1.7
30 718 908 1 77 710/680 702 0.7 245 382 42 1.45

Example 1- from table I and II it is shown that steel sample number 1 to 8 with carbon wt% ranges between 0.002-0.003 and cold reduction of 75% with core temperature above 670°C and above shows improved R-value of >1.6 whereas steel sample number 22 to 23 with higher core temperature up to 690 shows improved 1.65 R-value, samples 25 to 28 with higher core temperature more than 690 shows less tensile strength than required , hence core temperature to be maintained between 670°C to 690°C

Example 2- The effect of coiling temperature is justified by taking examples of 10 to 11 in table II. For sample number 10 and 11 with chemical composition and other parameters are kept same, coiling temperature is varied 560°C to 590°C, where drop in r bar to 1.4 is observed. Hence it is concluded to keep coiling temperature above 600°C.

Example 3 –The impact of cold rolling reduction on Planar anisotropy (R Bar) is justified by taking examples of 12 to 14 with C of about 0.0025% with reduction of 65 to 70 R bar of 1.52 is observed. Hence it is required to keep cold rolling reduction above 75% to achieve R bar above 1.6.

Example 4 –The steel sample number 25 to 27 with sample chemical composition, keeping all other parameters same and varying only annealing temperature it is observed that drop in tensile strength up to 390Mpa, hence annealing temperature is restricted up to <690°C to achieve the tensile strength 400 Mpa and above.

Example 5- Impact of carbon 0.005 wt% on R bar of cold rolled batch annealed steel has been illustrated in table I and table II. R Bar observed is less than expected in steel sample no 19 with carbon wt% of 0.004 while keeping the other parameters the same. Hence it is required to keep the carbon wt% less than or equal to 0.003 to achieve excellent R bar.

Example 6- Impact of Manganese less than 0.4 on tensile strength is illustrated in table I and table II. Tensile strength observed in sample no 28 to 30 is less than 390Mpa with manganese less than 0.4% with other parameters same , hence it is required to keep the Manganese wt% 0.4 % to 0.5%.

It is thus possible by way of the present invention to provide high strength Interstitial Free(IF) steel sheet with strength above 400Mpa having excellent formability. More particularly, the ultra-low carbon (0.003max) IF steel is processed with CCM reduction of more than 75 % and controlled HSM Coiling temperature and batch annealed at 690-670 °C to achieve excellent anisotropy ratio (R-bar: 1.6 min) with tensile strength 400Mpa or more. The optimum skin pass elongation used to get yield strength 230MPa or more, which are suitable for inner panel for automobile longitudinal structural component.