CLIAMS:We Claim:
1. Interstitial free (IF) steel sheet composition suitable for outer skin panel application for automobile body comprising
C: 0.001-0.0025 wt%,
Mn: 0.1-0.18wt%,
Al-0.02-0.05wt%,
Ti-0.06-0.075wt%,
N≤0.004 wt%,
P≤0.015wt%,
Si ≤0.015wt%,
Nb≤0.005wt%,
B≤0.0005wt%, and
balance iron,
having R-bar ≥2.0 and enabling formability and free of surface defects.

2. Interstitial free (IF) steel sheets as claimed in claim 1 having yield strength 150 MPa or less , ultimate tensile strength 300 MPa or less.

3. Interstitial free (IF) steel sheets as claimed in anyone of claims 1 or 2, which is free of any surface defects including temper color, sticker mark, sliver seamlines, wrinkling in particular scuodo components.

4. A process for production of Interstitial Free (IF) steel sheets as claimed in anyone of claims 1 to 3 comprising:

(i) providing the selective Interstitial free (IF) steel sheet composition comprising
C: 0.001-0.0025 wt%,
Mn: 0.1-0.18wt%,
Al-0.02-0.05wt%,
Ti-0.06-0.075wt%,
N≤0.004 wt%,
P≤0.015wt%,
Si ≤0.015wt%,
Nb≤0.005wt%,
B≤0.0005wt%, and
balance iron and other unavoidable impurities,
(ii) subjecting to hot rolling , then cold rolling with reduction percentage of 75% to 85% and annealed at 7200C to 7400C ;
(iii) preferably, carrying out hot scarfing of as cast slabs for removal of top surface upto 100mm thereby removing the surface defects;
(iv) subjecting to hot rolling under finishing temperature of atleast 8900C, coiling at atleast 7000C;and
(v) cold rolling with selective reduction percentage of 80% to 85%.

5. A process as claimed in claim 4 comprising alkaline electrolytic cleaning of cold rolled coils involving high current density maintaining electrolytic cleaning speed 250 mpm or less.

6. A process as claimed in claim 5 comprising carrying out batch annealing after cleaning and then stacking only at top two positions and annealing at high temperatures in controlled hydrogen atmosphere and selectively maintaining the cold spot and hot spot temperatures to achieve desired surface defect free steel sheets with improved formability.

7. A process as claimed in anyone of claims 5 to 6 wherein said ECL speed is maintained from 120 to 250mpm maximum to ensure temper colour free surface.

8. A process as claimed in anyone of claims 6 or 7 wherein annealing cycle of the material is differed by holding at 550°C for 4 hours to evaporate the emulsion residual present at the surface.

9. A process as claimed in anyone of claims 6 to 8 wherein after holding at 550°C, the coils are heated for 6 hours to reach the hot spot temperature of 740°C and maintained for 14 hours.
10. A process as claimed in anyone of claims 6 to 9 wherein the combination of cold working deformation cold spot temperature are maintained as 80 to 85% and 720°C respectively or cold working deformation hot spot temperature as 80 to 85% and 740°C respectively , to achieve the minimum R-bar 2.0.

11. A process as claimed in anyone of claims 5 to 10 wherein the steel material is skin passed with 0.3% to 0.7% elongation to maintain yield strength below 150Mpa.

12. A process as claimed in anyone of claims 5 to 11 wherein the core temperature is maintained above 720°C to get the desired tensile strength properties of 300Mpa or less.

Dated this the 17th day of February, 2015
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)

,TagSPECI:FIELD OF THE INVENTION

The present invention relates to interstitial free(IF) steel sheets with improved formability and surface appearance suitable for automobile outer panel applications and method for manufacturing the same through batch annealing route. More particularly, the IF steel produced according to the present invention is having selective composition and is cold rolled with reduction between 75-85 % and batch annealed at temperature between 720-740 °C to ensure improved R-bar 2.0 or more, yield strength 150 Mpa or less , ultimate tensile strength 300 Mpa or less. Hot scarfing is used after casting to avoid any surface defects like slivers and optimum electrocleaning line(ECL) speed is applied to avoid temper color free surface after batch annealing. Moreover, wrap to wrap stickiness for strip thickness less than 0.8 mm while processing through batch annealing route have been resolved by loading these materials only at defined positions in a stack. Also, the optimum skin pass elongation is used to get yield strength less than 150 MPa and to achieve desire surface roughness. Thus the cold rolled IF steel sheet developed through batch annealing route according to the present invention having excellent formability and surface finish which can be used for exposed panel of the automobile body and critical component which needs excellent draw ability.

BACKGROUND OF THE INVENTION

It is well known that the steel sheets for producing body outer panel of automobiles requires high formability, reasonably good tensile strength and defect free surface. Interstitial Free(IF) steels with very low carbon and nitrogen content as well as alloying with Nb and Ti provides the required ductility and formability of such intended applications. IF Steel produced with very low amounts of interstitial elements (primarily carbon and nitrogen) with small amounts of titanium or niobium added to tie up the remaining interstitial atoms. Without free interstitial elements, these steels are very ductile and soft, will not age or bake harden, and will not form strain (Lüder’s) lines during forming due to the absence of YPE (yield point elongation).Commercially available Cold rolled sheets/strips of the relevant grades produced through batch annealing has the surface defects like slivers or temper colour. It is observed that wrap to wrap stickiness is bigger challenges in the strip thickness less than 0.8 mm while processing through batch annealing route.

Pending Indian patent application no. 1306/KOL/2006 by Tata Steel Limited disclosed a method of making batch annealed Ti stabilized Interstitial Free steel strips with improved drawability by increasing the annealing temperature by 660°C to 750°C and simultaneously increasing cold rolling reduction to improve rm value to 2.3. This invention is directed to provide Ti-stabilized interstitial free (IF) steel sheets by optimizing of Cold rolled Reduction and Annealing temperature to increase the formability wherein Cold rolling reduction amounts to a maximum of 80%. According to this prior art, there arises a problem of sticker marks when processed at high temperature 690°C-750°C by stacking the coil in batch annealing furnace in all position and also there arises a problem of wrinkling in particular Scudo component, if steel sheet is produced at relatively low cold spot temperature 720 °C or less due to low R-bar of 2 or less. The material produced using these process parameters are also not suitable for skin panel material for automotive applications as there arises a problem of temper colour on edges of strip also.

There has been thus a need in the related filed to developing interstitial free steel with improved formability while also eliminate the surface defects observed in existing IF steel grades to make it suitable for automobile outer panel application.
The present invention aims to solve the problem of prior art by providing surface defect free product of interstitial free steel sheets with improved formability through batch annealing route suitable for skin panel applications.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide Interstitial Free(IF) steel sheets with improved formability and free of any surface defects like slivers, temper color , wrap to warp stickiness, sticker marks etc produced through batch annealing route by involving controlled process parameters.
A further object of the present invention is directed to provide Interstitial Free(IF) steel sheets with improved formability and free of any surface defects wherein IF steel of selective composition is used to cold rolled with reduction between 75-85 % and batch annealed at temperature between 720-740 °C to ensure improved R-bar 2.0 or more, yield strength 150 Mpa or less , ultimate tensile strength 300 Mpa or less.
A still further object of the present invention is directed to provide Interstitial Free(IF) steel sheets with improved formability and free of any surface defects wherein Hot scarfing is used after casting to avoid any surface defects like slivers or seamlines and optimum electrocleaning line(ECL) speed is applied to avoid temper color free surface after batch annealing.
A still further object of the present invention is directed to provide Interstitial Free(IF) steel sheets with improved formability and free of any surface defects wherein Sticker Mark free surface is achieved by selectively Stacking in Top 2 Position in batch annealing furnace.
A still further object of the present invention is directed to provide Interstitial Free(IF) steel sheets with improved formability and free of any surface defects wherein titanium is added to stabilize the carbon and nitrogen to form Titanium nitrides and carbides to the make steel interstitial free.
A still further object of the present invention is directed to provide Interstitial Free(IF) steel sheets with improved formability and free of any surface defects wherein cold rolling with reduction percentage of 80% to 85% improves planar anisotropy.
A still further object of the present invention is directed to provide Interstitial Free(IF) steel sheets with improved formability and free of any surface defects wherein optimum skin pass elongation is used to get yield strength less than 150 MPa and to achieve desire surface roughness.
A still further object of the present invention is directed to provide Interstitial Free(IF) steel sheets with improved formability and free of any surface defects wherein the core temperature is maintained above 720°C during annealing to get the desired properties of 300Mpa or less.
A still further object of the present invention is directed to provide Interstitial Free(IF) steel sheets with improved formability and free of any surface defects wherein annealing cycle of the material is deferred by holding at 550°C for 4 hours to evaporate the emulsion residual present at the surface.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to Interstitial free (IF) steel sheet composition suitable for outer skin panel application for automobile body comprising
C: 0.001-0.0025 wt%,
Mn: 0.1-0.18wt%,
Al-0.02-0.05wt%,
Ti-0.06-0.075wt%,
N≤0.004 wt%,
P≤0.015wt%,
Si ≤0.015wt%,
Nb≤0.005wt%,
B≤0.0005wt%, and
balance iron,
having R-bar ≥2.0 and enabling formability and free of surface defects.

A further aspect of the present invention is directed to Interstitial free (IF) steel sheets having yield strength 150 MPa or less , ultimate tensile strength 300 MPa or less.
A still further aspect of the present invention is directed to Interstitial free (IF) steel sheets which is free of any surface defects including temper color, sticker mark, sliver seamlines, wrinkling in particular scuodo components.
Yet another aspect of the present invention is directed to a process for production of Interstitial Free (IF) steel sheets comprising:

(i) providing the selective Interstitial free (IF) steel sheet composition comprising
C: 0.001-0.0025 wt%,
Mn: 0.1-0.18wt%,
Al-0.02-0.05wt%,
Ti-0.06-0.075wt%,
N≤0.004 wt%,
P≤0.015wt%,
Si ≤0.015wt%,
Nb≤0.005wt%,
B≤0.0005wt%, and
balance iron and other unavoidable impurities,
(ii) subjecting to hot rolling , then cold rolling with reduction percentage of 75% to 85% and annealed at 7200C to 7400C ;
(iii) preferably, carrying out hot scarfing of as cast slabs for removal of top surface upto 100mm thereby removing the surface defects;
(iv) subjecting to hot rolling under finishing temperature of atleast 8900C, coiling at atleast 7000C;and
(v) cold rolling with selective reduction percentage of 80% to 85%.

A further aspect of the present invention is directed to said process comprising alkaline electrolytic cleaning of cold rolled coils involving high current density maintaining electrolytic cleaning speed 250 mpm or less.

A still further aspect of the present invention is directed to a process comprising carrying out batch annealing after cleaning and then stacking only at top two positions and annealing at high temperatures in controlled hydrogen atmosphere and selectively maintaining the cold spot and hot spot temperatures to achieve desired surface defect free steel sheets with improved formability.

A still further aspect of the present invention is directed to a process wherein said ECL speed is maintained from 120 to 250mpm maximum to ensure temper colour free surface.
A still further aspect of the present invention is directed to a process wherein annealing cycle of the material is differed by holding at 550°C for 4 hours to evaporate the emulsion residual present at the surface.
A still further aspect of the present invention is directed to a process wherein after holding at 550°C, the coils are heated for 6 hours to reach the hot spot temperature of 740°C and maintained for 14 hours.
Another aspect of the present invention is directed to a process wherein the combination of cold working deformation cold spot temperature are maintained as 80 to 85% and 720°C respectively or cold working deformation hot spot temperature as 80 to 85% and 740°C respectively , to achieve the minimum R-bar 2.0.
Yet another aspect of the present invention is directed to a process wherein the steel material is skin passed with 0.3% to 0.7% elongation to maintain yield strength below 150Mpa.
A still further aspect of the present invention is directed to a process wherein the core temperature is maintained above 720°C to get the desired tensile strength properties of 300Mpa or less.

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

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: is the graphical presentation of the annealing cycle for batch annealing process for producing IF steel according to the present invention.
Figure 2: is the photographic image of the Scudo Component showing wrinkle free surface.
Figure 3: shows graphically the impact of Cold Rolling Reduction on R bar.
Figure 4: shows graphically the effect of Skin Pass Elongation on Yield Strength .
Figure 5: shows graphically the variation of Tensile Strength Vs BAF Core Temperature.
Figure 6: shows graphically the variation of Yield strength Vs BAF Core Temperature.
Figure 7: shows graphically the effect of Phosphorous content in IF steel on R Bar.
Figure 8: shows graphically the effect of Phosphorous content in IF steel on Tensile Strength.
Figure 9: shows graphically the effect of silicon content in IF steel on R bar.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS
The present invention is to provide Interstitial Free(IF) steel sheets with improved formability and free of any surface defects suitable for automobile outer panel application. This is obtained by changing the process parameters maintaining the ECL speed at 250mpm max to eliminate temper colour, in addition to that while annealing the material is holded at 550°C for 4 hours to evaporate the emulsion residue. At high temperature annealing problem of sticker mark arises in the surface which is solved by stacking the coils at top two positions.
The IF Steel of present invention having the composition on wt % basis comprising C-0.001-0.0025,Mn-0.1-0.18, Al-0.02-0.05, Ti-0.06-0.075, N-0.004 or less, P-0.015 or less, Si -0.015 or less and balance iron and the usual impurities. Titanium is added to stabilize the carbon and nitrogen to form Titanium nitrides and carbides to the make steel interstitial free.
Carbon(0.001-0.0025)wt% -if amount of Carbon present is more than 0.0025wt%, solute carbon will remain during recrystallization, which deteriorate the planar anisotropy R bar.
Mn (0.1-0.18) wt. % Max- The Mn ranges from 0.1 to 0.18 % for the given IF grade. Here keeping the level to minimum is advisable for providing a soft material, however, the lower Limit is 0.1% and below it the surface property deteriorates with hot brittleness as the amount of Mn is insufficient to fix S . More than 0.18% Mn makes the steel hard and lowers the deep drawability.

Al (0.02-0.05) wt. % Max –Al in solution ranges from 0.02-0.05 is intended for ordinary Al killed steel. Less than 0.02 % Al will delay the AlN precipitation causing the insufficient growth of ferrite grains and deteriorates the {111} texture. With adequate precipitation, ferrite grain size becomes fine. On the other hand, greater than 0.10 % Sol. Al makes final product hard, declining the drawability also adding extra cost of production.

Nitrogen -The upper limit for N is 0.005%, it is advisable to keep it to minimum level as higher N content leads to higher AlN precipitates, hardens the material and deteriorates the drawing properties.

Titanium (0.06-0.075) wt. % Max- The minimum amount of titanium required if only Ti is used for full stabilization, based on a stoichiometric approach, is (Tither &Stuart, 1995):

Tistab = 4C+3.42N+1.5S

Based on above equation and proposed chemistry for IF grade the minimum amount of Ti required to be added to stabilize C, N and S is 0.036%, but excess titanium of 0.024 is added, total of 0.06min is set as lower limit for obtaining good r bar. Further addition of Titanium more than 0.075 will increase the cost of the materials.

Nb :(0.005wt% or less)
In present invention Nb in limited upto 0.005 % as added Ti is well sufficient to fix C and N and also Nb added Steels have higher recrystallization temperature (750°C -800°C ) as compared to Ti only IF steels. As Low temperature formation of NbC precipitates retards the grain growth during annealing, resulting in finer grain size and Higher Yield strength as compared to Ti-only stabilized IF steels . Hence Nb is restricted to 0.005 wt%.

B: 0.0005wt% or less
B should be less than 0.0005 which has no effect on the material but when more than 0.0005 delays recrystallization increase load during rolling and deteriorates quality of the annealed steel, affects the formability of the steels and reduces plan anisotropy. As Ti has highest affinity for N than Al and B, i.e. TiN will form first followed by AlN and BN. Hence To avoid any Interstitial B we are keeping the level below 0.0005% (5 ppm).

Manufacturing Method
Interstitial free steel is produced by having steel slab having above stated composition is first hot rolled, then cold rolled with reduction percentage of 75% to 85% and finally annealed at 720°C -740°C.
After the steel slab is casted with desired composition, hot scarfing is done that is removal of the top surface up to 100mm using a scarf so that surface defects like sliver seamlines can be removed.
Slab is hot rolled with finishing temperature of 890°C and above, coiled at 700°C or above, cold rolled with reduction percentage of 80% to 85% to improve planar anisotropy. When cold rolled with less than 80% desirable r bar value cannot be obtained, when rolled above 85% will cause production loss.
Electrolytic cleaning(ECL) with selected Speed (250 mpm or less):
Cold rolled coils processed in cold rolling mills have a thin film of residual rolling oil on the surface after rolling. The alkaline cleaning of cold rolled coils by the use of high current density (HCD) ensures totally clean surface of strip oil and dirt by chemical action before annealing and that cleanliness depends upon ECL Speed. Oil residual on the strip surface increases with increase in ECL Speed which dissociates by chemical reaction with H2 in batch annealing and form moisture that oxidize the edges of strip of the coils and at further de-oxidation with H2 leave temper color (whitish mark) on edge of the strip. That Temper color can be completely eliminated by reducing ECL Speed 250 mpm or less as shown in examples 1-7.
Batch annealing:
After electrolytic cleaning, cold rolled coils are batch annealed, stacked only at top two positions to prevent the sticker marks as it is soft material and annealed at high annealing temperature above 720°C. Annealing cycle of the material is differed by holding at 550°C for 4 hours to evaporate the emulsion residual present at the surface. The cold spot of the material reaches to 400 °C at the end of holding temperature and emulsion evaporation temperature lies between 300-400 °C and also it is observed that dew point of H2 atmosphere reduced -60°C as shown in accompanying Figure 1, this is because of water vapor formation due to emulsion and H2 reaction. To ensure fast H2 and emulsion reaction and fast moisture elimination from inside the inner cover, the hydrogen flow rate is maximized to 40m3/hrs, to avoid oxidation on the strip edges which ensures no whitish temper color. After holding at 550°C it is heated for 6 hours to reach the hot spot temperature of 740°C and maintained for 14 hours detailed annealing cycle is shown in accompanying Figure 1. The problem of wrinkling in particular in Scudo component as shown in Figure 2 if steel sheet produced at relatively low cold spot temperature 720 °C or less due to low R bar 2 or less.

Complete details of the invention and the comparative steels are given in following Table I and Table II wherein
Table I provides the composition of the invented steel and comparative steel; and
Table II provides Process parameters of hot rolling and cold rolling with mechanical properties of inventive and comparative steel.

Table I:
Chemical Composition (Wt %) HSM Rolling Parameters
Sample No C Mn S P Si Al N Ti HSM FT HSM CT Remarks
1 0.002 0.16 0.008 0.008 0.004 0.05 0.0034 0.075 910 705 Invention
2 0.002 0.17 0.008 0.008 0.004 0.05 0.0028 0.067 910 705 Invention
3 0.001 0.12 0.007 0.004 0.003 0.045 0.0034 0.062 910 705 Invention
4 0.001 0.12 0.007 0.004 0.003 0.045 0.0034 0.062 910 705 Invention
5 0.002 0.18 0.008 0.01 0.006 0.043 0.0031 0.069 910 705 Invention
6 0.002 0.18 0.008 0.01 0.006 0.043 0.0031 0.069 910 705 Comparative Steel
7 0.002 0.18 0.008 0.01 0.006 0.043 0.0031 0.069 910 705 Invention
8 0.0025 0.17 0.008 0.008 0.004 0.05 0.0028 0.067 910 705 Comparative Steel
9 0.002 0.18 0.008 0.01 0.006 0.043 0.0031 0.069 910 705 Comparative Steel
10 0.0025 0.17 0.008 0.008 0.004 0.05 0.0028 0.067 910 705 Invention
11 0.002 0.16 0.008 0.008 0.004 0.05 0.0034 0.075 910 705 Comparative Steel
12 0.002 0.16 0.008 0.008 0.004 0.05 0.0034 0.075 910 705 Comparative Steel
13 0.002 0.12 0.007 0.004 0.003 0.045 0.0034 0.062 910 705 Comparative Steel
14 0.002 0.12 0.007 0.004 0.003 0.045 0.0034 0.062 910 705 Comparative Steel
15 0.002 0.15 0.008 0.01 0.005 0.05 0.0029 0.066 910 705 Invention
16 0.002 0.12 0.007 0.004 0.003 0.045 0.0034 0.062 910 705 Invention
17 0.003 0.14 0.008 0.006 0.007 0.04 0.0037 0.06 910 705 Comparative Steel
18 0.002 0.15 0.008 0.01 0.005 0.05 0.0029 0.066 910 705 Invention
19 0.003 0.14 0.008 0.006 0.007 0.04 0.0037 0.06 910 705 Invention
20 0.002 0.15 0.008 0.01 0.005 0.05 0.0029 0.066 910 705 Invention
21 0.002 0.12 0.007 0.004 0.003 0.045 0.0034 0.062 910 705 Invention
22 0.002 0.15 0.008 0.008 0.006 0.05 0.0034 0.069 910 705 Invention
23 0.003 0.2 0.008 0.014 0.008 0.035 0.0027 0.067 910 705 Comparative Steel
24 0.003 0.17 0.007 0.018 0.005 0.04 0.0035 0.069 910 705 Comparative Steel
25 0.003 0.17 0.007 0.015 0.005 0.055 0.0028 0.07 910 705 Comparative Steel
26 0.003 0.15 0.008 0.004 0.006 0.033 0.0047 0.063 910 705 Comparative Steel
27 0.002 0.17 0.006 0.013 0.005 0.037 0.004 0.055 910 705 Comparative Steel
28 0.004 0.15 0.006 0.015 0.004 0.034 0.0032 0.062 910 705 Comparative Steel
29 0.002 0.17 0.007 0.014 0.018 0.05 0.0028 0.065 910 705 Comparative Steel

Table II:
CRM Parameters Mechanical Properties
Sample No Thk per_red ECL Speed Temper Color BAF Cycle Stack Pos Sticker SPM Elong Yield Strength TS Rbar
1 0.75 83.33 120 No 740-720 4 No 0.5 131 296 2.345
2 0.75 83.33 160 No 740-720 5 No 0.5 122 296 2.4
3 0.75 83.33 180 No 740-720 5 No 0.5 110 288 2.197
4 0.75 83.33 230 No 740-720 4 No 0.5 127 277 2.371
5 0.75 83.33 250 No 740-720 3 No 0.5 138 292 2.377
6 0.75 83.33 220 No 740-720 2 Yes 0.5 135 299 2.335
7 0.75 83.33 230 No 740-720 5 No 0.5 149 295 2.32
8 0.75 83.33 350 Yes 740-720 3 No 0.5 146 285 2.12
9 0.75 83.33 200 No 740-720 2 Yes 0.5 135 295 2.428
10 0.75 83.33 230 No 740-720 4 No 0.5 133 293 2.334
11 0.75 83.33 180 No 740-720 5 No 0.5 120 298 2.379
12 0.75 83.33 230 No 740-720 5 No 0.8 155 295 2.4
13 0.75 83.33 280 Yes 740-720 4 No 0.7 114 281 2.302
14 0.75 83.33 360 Yes 740-720 5 No 0.5 111 274 2.415
15 0.75 83.33 240 No 740-720 5 No 0.5 117 288 2.344
16 0.75 83.33 240 No 740-720 4 No 0.5 112 280 2.423
17 0.75 83.33 380 Yes 740-720 5 No 0.5 112 281 2.423
18 0.75 83.33 240 No 740-720 4 No 0.5 121 290 2.363
19 0.75 83.33 240 No 740-720 5 No 0.5 117 289 2.301
20 0.75 83.33 240 No 740-720 4 No 0.5 121 293 2.191
21 0.75 83.33 240 No 740-720 5 No 0.5 119 282 2.4
22 1.6 73.33 200 No 740-720 1 No 0.5 145 288 2.035
23 0.7 78.13 240 No 740-720 4 No 0.5 160 314 1.96
24 0.7 78.13 240 No 740-720 3 No 0.5 152 328 1.95
25 0.7 78.13 240 No 740-720 4 No 0.5 144 325 1.91
26 1 80 240 No 740-720 5 No 0.5 155 312 1.9
27 0.7 80.56 240 No 740-720 5 No 0.5 150 313 1.93
28 0.7 78.13 240 No 740-720 2 No 0.5 159 316 1.88
29 0.75 78.57 240 No 740-720 3 No 0.5 157 325 2.053

Example 1: illustrates the effect of ECL speed on color temper and in Table I and Table II it is shown that sample number 1 to 8 where ECL speed is maintained from 120 to 250mpm Max to ensure temper colour free surface, when processed above 250mpm occurrence of temper colour is visible as shown in sample number 8,13,14 and 16. When processed below 120 mpm speed will cause production loss.
Example 2: Stacking position in batch annealing at top two position shows no occurrence of sticker mark as shown in examples of 1 to 5. When stacked in below position sticker mark is clearly visible as shown in sample number 6.
Example 3: The impact of cold rolling is shown is sample number 28 when cold rolled at 78% the plan anisotropy obtained is 1.88 which is below the desired property, to obtain above 2.0 cold rolling reductions to be maintained above 80% as shown in Figure 3.
Example 4: The material is skin passed with 0.3% to 0.7% elongation to maintain yield strength below 150Mpa.when skin passed above 0.7% there in increase in yield strength as shown in sample number 12 as shown in Figure 4.
Example 5: The core temperature is maintained above 720°C to get the desired properties of 300Mpa or less as shown in Figure 5 and when annealed below 720°C the Tensile strength will get increased and same changes in the case of yield strength is shown in Figure 6.
Example 6: The effect of Phosphorous on tensile strength and planar anisotropy by taking sample number 24, phosphorous is limited to 0.015% max to obtain good R bar, when increased more than 0.015% it will detoriate the formability as show in Figure 7 but in case of tensile strength increase in P content more than 0.015% will increase the tensile strength as shown in Figure 8.
Example 7: The effect of silicon on R bar is shown in Figure 9 as the silicon increases the R bar decreases and to obtain R bar of 2.0 and above, silicon level to be maintained 0.015 or less.

It is thus possible by way of the present invention to provide IF steel sheets produced according to the present invention having selective composition and which is cold rolled with reduction between 80-85 % and batch annealed at temperature between 720-740 °C to ensure improved R-bar 2.0 or more, yield strength 150 MPa or less , ultimate tensile strength 300 MPa or less ensuring excellent formability and defect free surface which can be used for exposed panel of the automobile body and critical component which needs excellent draw ability.

We Claim:
1. Interstitial free (IF) steel sheet composition suitable for outer skin panel application for automobile body comprising
C: 0.001-0.0025 wt%,
Mn: 0.1-0.18wt%,
Al-0.02-0.05wt%,
Ti-0.06-0.075wt%,
N≤0.004 wt%,
P≤0.015wt%,
Si ≤0.015wt%,
Nb≤0.005wt%,
B≤0.0005wt%, and
balance iron,
having R-bar ≥2.0 and enabling formability and free of surface defects.

2. Interstitial free (IF) steel sheets as claimed in claim 1 having yield strength 150 MPa or less , ultimate tensile strength 300 MPa or less.

3. Interstitial free (IF) steel sheets as claimed in anyone of claims 1 or 2, which is free of any surface defects including temper color, sticker mark, sliver seamlines, wrinkling in particular scuodo components.

4. A process for production of Interstitial Free (IF) steel sheets as claimed in anyone of claims 1 to 3 comprising:

(i) providing the selective Interstitial free (IF) steel sheet composition comprising
C: 0.001-0.0025 wt%,
Mn: 0.1-0.18wt%,
Al-0.02-0.05wt%,
Ti-0.06-0.075wt%,
N≤0.004 wt%,
P≤0.015wt%,
Si ≤0.015wt%,
Nb≤0.005wt%,
B≤0.0005wt%, and
balance iron and other unavoidable impurities,
(ii) subjecting to hot rolling , then cold rolling with reduction percentage of 75% to 85% and annealed at 7200C to 7400C ;
(iii) preferably, carrying out hot scarfing of as cast slabs for removal of top surface upto 100mm thereby removing the surface defects;
(iv) subjecting to hot rolling under finishing temperature of atleast 8900C, coiling at atleast 7000C;and
(v) cold rolling with selective reduction percentage of 80% to 85%.

5. A process as claimed in claim 4 comprising alkaline electrolytic cleaning of cold rolled coils involving high current density maintaining electrolytic cleaning speed 250 mpm or less.

6. A process as claimed in claim 5 comprising carrying out batch annealing after cleaning and then stacking only at top two positions and annealing at high temperatures in controlled hydrogen atmosphere and selectively maintaining the cold spot and hot spot temperatures to achieve desired surface defect free steel sheets with improved formability.

7. A process as claimed in anyone of claims 5 to 6 wherein said ECL speed is maintained from 120 to 250mpm maximum to ensure temper colour free surface.

8. A process as claimed in anyone of claims 6 or 7 wherein annealing cycle of the material is differed by holding at 550°C for 4 hours to evaporate the emulsion residual present at the surface.

9. A process as claimed in anyone of claims 6 to 8 wherein after holding at 550°C, the coils are heated for 6 hours to reach the hot spot temperature of 740°C and maintained for 14 hours.
10. A process as claimed in anyone of claims 6 to 9 wherein the combination of cold working deformation cold spot temperature are maintained as 80 to 85% and 720°C respectively or cold working deformation hot spot temperature as 80 to 85% and 740°C respectively , to achieve the minimum R-bar 2.0.

11. A process as claimed in anyone of claims 5 to 10 wherein the steel material is skin passed with 0.3% to 0.7% elongation to maintain yield strength below 150Mpa.

12. A process as claimed in anyone of claims 5 to 11 wherein the core temperature is maintained above 720°C to get the desired tensile strength properties of 300Mpa or less.

Dated this the 17th day of February, 2015
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)

ABSTRACT
TITLE: INTERSTITIAL FREE (IF) STEEL SHEET COMPOSITION SUITABLE FOR OUTER SKIN PANEL APPLICATION FOR AUTOMOBILE BODY.

The present invention relates to interstitial free(IF) steel sheets with improved formability and surface appearance suitable for automobile outer panel applications and a process for its production through batch annealing route. More particularly, the IF steel produced according to the present invention is having selective composition and is cold rolled with reduction between 75-85 % and batch annealed at temperature between 720-740 °C to ensure improved R-bar 2.0 or more, yield strength 150 Mpa or less , ultimate tensile strength 300 Mpa or less. Hot scarfing is used after casting to avoid any surface defects like slivers and optimum electrocleaning line(ECL) speed is applied to avoid temper color free surface after batch annealing. Moreover, wrap to wrap stickiness for strip thickness less than 0.8 mm while processing through batch annealing route have been resolved by loading these materials only at defined positions in a stack.