DESC:FIELD OF THE INVENTION
The present invention relates to Nb-Cr stabilized cold rolled interstitial free-steel sheets with improved planar anisotropy and excellent surface finish and a method of producing the same. More particularly, the present invention is directed to extra deep drawing Interstitial free steel sheet having excellent formability suitable for automobile and like applications The Nb-Cr stabilized cold rolled interstitial free-steel sheets are EDD IF cold rolled steel involve selective chemistry of the steel composition including optimum Al and N weight % along with selected operative conditions including coiling temperature leading to optimum AlN/NbC volume fraction, targeted to achieving excellent formability with improved average anisotropy ratio (r-bar =2), and reduced ?r value of 0.4 or less, work hardening exponent n value > 0.235 and excellent surface finish which can be used for critical drawing use suitable for automobile inner or outer body panel and similar applications.

BACKGROUND OF THE INVENTION
The components of automobile body prepared by press forming of cold rolled steel sheets should have required drawing properties avoiding crack formation or stretcher strain marks while also have desired surface quality to ensure desired finish and durability of coatability/paintability.
The automobile component as shown under accompanying Figure 1 illustrates a critically drawn part which requires excellent drawability along with adequate aging resistance of steel sheet prior to drawing. The prerequisite of average anisotropy ratio (r-bar) for such automobile components is more than 2 and n value >0.23. Consequence of having r-bar value below 2 and n value <0.23 can be judged by the cracking which originates during critical drawing process revealed as cracked component in above figure.
In addition, r-bar =2 along with YS <160 MPa is a very critical requirement for IF based cold rolled EDDQ materials such as DC06 (European Standard) for excellent drawability.
As a part of prior art European patent specification EP0120976 B1 discloses to a method of producing cold rolled steel sheets used for automotive exterior plate and the like and adapted for deep drawing C=0.015 wt%, Mn= 0.4 wt% , P= 0.03 wt% ,0.005 -0.1 wt% , N=0.01 wt%. A basic stoichiometry -0.015=Ti-(48/32) S + (48/14) N <0.004 has been claimed as a part of invention for minimum amount of Ti in slab required to fix N and S. The main drawback of the steel grade obtained according to this prior art is that it does not comply with any aging guarantee as the C level is very high up to 0.015 wt%. Also no alloying element has been claimed to fix C, and Ti added is only sufficient to fix N and S which is derived from stoichiometry and not from experiment. It is found that teachings of European patent specification EP 0120976 B1 suffers from poor drawability as would obviously lead to a r-bar value less than 1.8 along with aging problem as upper N level of 0.01 wt% claimed is also very high.
Indian Patent Application number 859/KOL/2012 describes a cold-rolled steel sheet having excellent bending formability, comprising a composition including in mass%, C: 0.005 % to 0.030 %; Si: 0.05 % or less; Mn: 0.10 % to 0.35 %; P: 0.025 % or less; S: 0.015 % or less; N: 0.01 % or less with specific Si to Mn ratio less than 0.5 with 50% cementite precipitate present in ferrite matrix. However, the cold rolled steel sheet described in 859/KOL/2012 suffers from low aging due to cementite phase and lack of complete ferritic matrix makes it susceptible to aging. Also drawability and texture suffers due to high Carbon percentage.
Another Indian Patent Application number 2736/MUM/2011 describes steel sheet having excellent workability with C=0.01% , Si =0.2 wt% , Mn =0.5wt% N=0.01 wt% Ti -0.02 -0.1 wt% B <0.003 % along with Ti4C2S2 having particle size >10 nm dispersed in to steel at a volume fraction of 0.005 to 0.5 % . However, the cold rolled steel sheet described in 2736/MUM/2011 may suffer from poor drawability in actual press forming due to high C wt% as far as the interstitial free cold rolled steel are concerned. Added amount of Ti as described in patent number 2736/MUM/2011 is sufficient to fix C, N and remaining S, however additional alloying such as high Mn and Si wt% along with B, Mo, Cu Ni, Pb, Sb and other alloying as described in 2736/MUM/2011 remain free in solution and hardens the ferrite matrix. Hence the steel grade obtained according to application number 2736/MUM/2011 suffers from problems of poor drawability and inferior texture.
United States Patent number US4551182 reveals a process for producing deep-drawing cold rolled steel strips comprising 0.0014-0.0031% C, 0.010-0.100% Al 0.019-0.052% Ti, 0.00l9-0.0038% N with the balance being iron and unavoidable impurities .With this technique adequate elongation and r-bar value can be obtained however it will not be sufficient for actual press formability as required for IF based EDDQ steel material. Cracking may occur as use of Ti only to fix C and N may result in higher ?r value which is not good for press formability.
Detailed explanation of Problem to be solved with present invention:
The automobile component as shown in accompanying Figure 1 is critically drawn part which requires excellent drawability along with adequate aging resistance of steel sheet prior to drawing. However, the figure typically represents consequence of having average anisotropy ratio r-bar value below 2 and work hardening exponent n value <0.23 which is apparent from the cracking which arises during critical drawing process revealed as cracked component in figure 1.

OBJECTS OF THE INVENTION
The basic object of the present invention is directed to provide a Nb-Cr stabilized interstitial free cold rolled steel sheets for automobile applications and method for manufacturing the same through continuous annealing route for improved planar anisotropy and excellent surface quality.
Another object of the present advancement is directed to developing improved chemistry and processing which would satisfies the r-bar requirement of more than 2, ?r <0.4 and n value > 0.23 along with desired ageing guarantee in order to avoid any cracking concern as described above during drawing operation.
Yet another object of the present invention is directed to a Nb-Cr stabilized interstitial free cold rolled steel sheets and its processing involving selective Coiling temperature, Soaking Temperature, controlled NbC and AlN precipitate size as well as Volume fraction of AlN to NbC maintained in steel matrix to achieve desired r-bar, ?r value and improved texture with desired fraction of favorably oriented {111} grains for improved drawability.
A further object of the present invention is directed to providing an EDD IF cold rolled steel sheet developed through Continuous annealing route wherein optimized amount of Al, Nb and Cr is maintained in composition to fix C and N and finishing and coiling temperatures in hot rolling as well as the soaking temperature in annealing are selectively controlled so as to attain desired AlN/NbC, Nb (C,N) precipitate size and volume fraction vis-à-vis the desired drawability and aging resistance properties of the cold rolled sheet.
SUMMARY OF THE INVENTION
Thus according to the basic aspect of the present invention there is provided a steel composition for processing crack resistant for extra deep drawing of continuous annealed Nb-Cr stabilized IF steels comprising in terms of weight %:-
(In wt %) (In wt %)
C: 0.003wt% or less Al: 0.025-0.05
Mn: 0.08-0.2 N: 0.004 or less (more preferably 0.003wt%or less )
Cr: 0.01-0.03 Nb: 0.013-0.02

having, [Nb] - 7.75([C]-0.098[Cr])-1.77 (7.5[N]-[Al]) = 0,
and the balance being Fe and other unavoidable impurities wherein the constitutional elements and SS Temp satisfying the following relation as hereunder:
8.1 [Nb] +4.2 [Cr] + 4.4[Al] +0.00185 [SS Temp] = 140 [C] + 300 [N],
whereas rolled continuously annealed steel having the average precipitate size of NbC and /or Nb(C, N) in steel matrix in the range of 7-40nm, average precipitate Size of AlN in the range of 80–500 nm and ratio of volume fraction of AlN to NbC in steel matrix in the range of 7 to 30.
Preferably, said steel composition for processing crack resistant for extra deep drawing of continuous annealed Nb-Cr stabilized IF steels comprises P, Si, S, Cu, Ni, Zn, B, V, Ti, Ca, Zr, W, Mo, Hf in less than 0.1 weight %.
According to another aspect of the present invention there is provided Nb-Cr stabilized cold rolled interstitial free-steel sheets comprising:
r-bar value =2, ?r value = 0.4, YS less than 180 MPa, UTS less than 310 MPa which is obtained of cold rolled continuous annealing of steel composition comprising in terms of weight %:-
(In wt %) (In wt %)
C: 0.003 wt.% or less Al: 0.025-0.05
Mn: 0.08-0.2 N: 0.004 or less (more preferably 0.003 wt.% or less)
Cr: 0.01-0.03 Nb: 0.013-0.02

Having, [Nb] - 7.75([C]-0.098[Cr])-1.77 (7.5[N]-[Al]) = 0
And the balance being Fe and other unavoidable impurities wherein the constitutional elements and SS Temp satisfying the following relation as hereunder:
8.1 [Nb] +4.2 [Cr] + 4.4[Al] +0.00185 [SS Temp] = 140 [C] + 300 [N]
According to another preferred aspect of the present invention there is provided Nb-Cr stabilized cold rolled interstitial free-steel sheets having the average precipitate size of NbC and /or Nb(C, N) in steel matrix in the range of 7-40nm, Average precipitate Size of AlN in the range of 80–500 nm and ratio of Volume fraction of AlN to NbC in steel matrix in the range of 7 to 30.
Preferably said process for producing Nb-Cr stabilized cold rolled interstitial free-steel sheets comprises:
i) providing selective steel composition comprising in terms of weight %:-
(In wt %) (In wt %)
C: 0.003wt% or less Al: 0.025-0.05
Mn: 0.08-0.2 N: 0.004 or less (more preferably 0.003wt%or less )
Cr: 0.01-0.03 Nb: 0.013-0.02

Having, [Nb] - 7.75([C]-0.098[Cr])-1.77 (7.5[N]-[Al]) = 0
And the balance being Fe and other unavoidable impurities wherein the constitutional elements and SS Temp satisfying the following relation as hereunder:
8.1 [Nb] +4.2 [Cr] + 4.4[Al] +0.00185 [SS Temp] = 140 [C] + 300 [N]
ii) Processing crack resistant for extra deep drawing said Nb-Cr stabilized IF steels
Including cold rolled continuously annealed steel sheet having r-bar value =2, ?r value = 0.4, YS less than 180 MPa, UTS less than 310 MPa.

According to another preferred aspect of the present invention there is provided a process for producing Nb-Cr stabilized cold rolled interstitial free-steel sheets comprising:
i) hot rolling and continuous annealing carried out keeping the hot rolling finishing temperature in the range of T1-20 0C to T1 + 200C and coiling hot rolled sheet in the range of T2-80 0C to T2-30 0C where
T1 = 910- 203 C1/2 -30 Mn+ 11Cr+ 44.7 Si-400 Al -15.2 Ni+31.5 Mo-3 +20 Cu-700P-400Ti
T2=723-10.7Mn+16.9Cr+6.38W-16.9Ni
ii) After cold rolling cold rolled sheet continuously annealed at a temperature ranging from T2+ 60 0C to T1-60 0C;
iii) Subjecting the thus annealed sheet to skin pass elongation of (YS/300) % to (YS/150) % .wherein YS – is the Yield Strength of material when stretcher strain is suppressed completely.

According to another preferred aspect said process is provided comprising providing texture with fraction of favorably oriented {111} grains are >60 % involving coiling temperature T2-800C or more, Soaking Temperature T2+70 °C or more, NbC precipitate size 7-40nm, Volume fraction of AlN to NbC in steel matrix in the range of 7 to 30.
Preferably said process comprises restricting the total weight % of unavoidable impurities and trace elements including P, Si, S, Cu, Ni, Zn, B, V, Ti, Ca , Zr, W, Mo, Hf less than 0.1 weight %.
Preferably said process comprises hot scarfing of slabs after casting to avoid surface defects.
More preferably in said process wherein after cold rolling cold rolled sheet is continuously annealed at a temperature ranging from T2+ 60 ?C to T1-60 ?C along with slow cooling temperature of T2-40 ?C or more.
The objects and advantages of the present invention are described hereunder in greater details with reference to following accompanying illustrative drawings and examples.
BRIEF DESCRIPTION OF THE ACCOMPNAYING DRAWINGS
Figure 1: A photo image illustrating the Cracking problem in extra deep drawn automotive component made of steel sheet having r-bar value < 2.
Figure 2 : shows Orientation Distribution Function (ODF) Ø2 0-900 for Steel number 1 showing strong ?-fibre intensity at Ø2-450 of the steel of the present invention steel mainly responsible for higher r-bar values.
Figure 3: shows EBSD image of steel of the present invention showing strong {111} texture confirming > 60% favorably oriented {111} grains.
Figure 4: is the Graphical plot of Ratio of AlN /NbC precipitate volume fraction vs. r-bar Values.
Figure 5: is the Graphical Plot of E1 vs. E2 for the steel of the present invention and comparative steels and their impact on respective r-bar values wherein steels having E1< E2 the r-bar value are out of the scope of present invention.
Figure 6: is the Graphical Plot of E3 vs. ?r value for inventive and comparative steels wherein steels having E3<0, the ?r value are out of the scope of present invention.
DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWINGS AND A PREFERRED EMBODIMENT
As discussed hereinbefore the present invention thus provides for Nb-Cr stabilized cold rolled interstitial free-steel sheets with improved planar anisotropy and excellent surface finish and a method of producing the same.
Steel of present invention include a select composition comprising C-0.003 wt% or less, wherein having specified Cr-0.01 to 0.03 wt%, Mn-0.08-0.2 wt%, Al-0.025 to 0.05 wt%, N- 0 to 0.004 wt%, Nb-0.013 to 0.02 wt%, with [Nb] - 7.75([C]-0.098[Cr])-1.77 (7.5[N]-[Al]) = 0 wherein Aluminum and Nb are indented to stabilize the Nitrogen and Carbon respectively to form Aluminum nitrides and Niobium carbides along with Niobium carbonitrides to make steel interstitial free. A specified ratio of AlN to NbC volume fraction along with size range of NbC/ Nb (CN) has been established as a part of invention.
Following abbreviations and symbols are used herein to describe the present invention:
SS Temp – Soaking section temperature in 0C
SCS- Slow cooling section Temperature in 0C
RCS – Rapid cooling section Temperature in 0C
OAS- Over aging section Temperature in 0C
SPM % - Skin pass elongation (temper rolling)
FT- Hot finish rolling temperature in 0C
CT- Hot coiling temperature in 0C
IF- Intestinal free
CAL- Continuous annealing line
YS – Yield Strength in MPa
UTS – Tensile strength in MPa
El% - Total elongation in %
r- bar – Average anisotropy ratio
?r- Difference in anisotropy

As discussed hereinbefore the invention is further and better illustrated by way of the following equations developed by way of the present advancement:
E1=8.1 [Nb] +4.2[Cr] +4.4[Al] + 0.00185 [SS Temp] &
E2=140 [C] + 300 [N],
where scope of the present invention will be satisfied only if E1=E2.
E3 = [Nb] - 7.75([C]-0.098[Cr])-1.77 (7.5[N]-[Al]) = 0
T1 = 910- 203 C1/2 -30 Mn+ 11Cr + 44.7 Si-400 Al -15.2 Ni+31.5 Mo-3 +20 Cu-700P-400Ti
T2=723-10.7Mn+16.9Cr+6.38W-16.9Ni
Note -All the chemical elements are in terms of their weight %.

With the aim of developing a Non-aging, Extra deep drawing cold rolled IF steel sheets by continuous annealing route according to present invention, effect of Metallurgical factors affecting the mechanical properties and drawability are described below in detail.
Carbon (0.003wt% or less) - C present as intestinal solute is a major element deteriorating the drawability as well as increasing the strength of steel. It is well known that lower the C wt% softer will be the material and aging property and drawability will be better. Also reducing the C wt% reduces the amount of alloying addition (Nb, Ti, Cr) to fix the same ultimately reducing the production cost. Also C in steel as an interstitial element prevents the formation of {111} texture, deteriorating the drawability. Hence the C wt% is restricted to 0.003wt%.
Mn (0.08-0.2) wt. % - The Mn ranges from 0.08 to 0.2 % is most desirable for the given IF grade provided that S remains in the favourable range of 0.008wt% or less. Keeping the Mn level 0.1wt% with higher sulphur content may result in surface irregularities such as red shortness, edge crack sliver etc. here keeping the level to minimum is advisable for providing a soft material, however, the lower limit is 0.08% as below it the surface property deteriorates and embrittlement may occur due to hot brittleness as the amount of Mn is insufficient to fix S . More than 0.2%Mn makes the steel hard and the deep draw ability lower.
Nb (0.013-0.02 wt%)–Advantage of adding Nb is twofold. Firstly, Nb acts as a strong Carbide former forming NbC and complex carbo nitrides Nb(C, N) helpful in fixing C. Secondly Nb is the most effective element for reducing planner anisotropy(?r, ?YS, ?El ) which is attributed to relatively small grain size of hot bands for Nb added IF steel .It has also been found that the optimum precipitate size of NbC must be between 8-30 nm for improved texture and less ?r value. Also the volume fraction of NbC and Nb(C,N) combined must be in the range of 0.01-0.04 % .
Nb helps in reducing Anisotropies of r value significantly by forming NbC and Nb (C, N). This can be seen by taking an example of Ti only IF steel having r-value of around 2.2 in transverse direction (r90), 1.8 in rolling direction (r0) and 1.25 in diagonal direction (r45) giving the anisotropy (?r=( r90+r0 -2r45)/2) of 0.75 where as for Nb added steel the ?r ranges from 0.1-0.4 .
Minimum amount of Nb required to fix C in combination with Cr is 0.01wt% keeping the other elements such as C, N and S and Cr within controlled limit as described in present invention. Nb levels more than 0.02wt% will unnecessarily add up to the production cost. The recrystallization temperature increase with added Nb hence YS increases for same annealing temperature. R-value also decreases with free Nb in solution due to solid solution strengthening.
Al (0.025-0.05) wt. % – Al wt% ranges from 0.025-0.05 is intended for fixing free N which results in aging if left unfixed. In addition less than 0.025 % Al will delay the AlN precipitation causing the insufficient growth of ferrite grains and deteriorates the {111} texture. An optimized volume fraction of AlN in steel matrix in combination with NbC precipitate results in excellent drawability and reduced ?r value as describes the present invention. Same is achieved by optimize hot rolling and cold rolling parameters to avoid Ostwald ripening phenomenon which results in coarse precipitate size along with low r-bar values.
Nitrogen (0.004wt% or less) -The upper limit for N is 0.004%; it is advisable to keep it to minimum level. Higher N content requires to higher Al addition to fix extra N and increase the volume of AlN precipitates which strengthens the material, ultimately deteriorating the drawing property.
Chromium (0.01-0.03) wt% - Chromium forms chromium carbide (23/27Cr7C3) helps in scavenging off solute carbon. In present invention a minimum 0.01 wt% of Cr must be added to get any advantage of Cr addition to fix any remaining C and to avoid room temperature aging. The use of Cr to stabilize C is also effective to slow down the recrystallization rate and assist the formation of {111} textures to achieve good r-bar values. However, excess Cr in solution unnecessarily adds up in cost of production as well as reduces drawability. Hence the upper limit is set to 0.03 wt%.
P, Si, S, Cu, Ni, Zn, B, V, Ti, Ca, Zr, W, Mo, Hf (less than 0.1 weight %) –In present invention these elements are not added intentionally and are present due to restriction of steel making process and/or as trace element carried over in hot metal or from scrap charging during basic oxygen furnace blowing. The total concentration of these elements must be reduced to minimum level as it tends to harden the steel matrix. For present invention the level must below 0.1 weight %.
Hot Rolling and CAL Annealing Parameters selectively attained based on trials according to the present invention–
To achieve desired precipitate size and volume fraction of AlN / NbC cast slab must be hot rolled keeping the Finishing temperature in the range of T1 –20 0C to T1 + 200C where
T1 = 910- 203 C1/2 -30 Mn+ 11Cr + 44.7 Si-400 Al -15.2 Ni+31.5 Mo-3 +20 Cu-700P-400Ti.
and Hot rolling coiling temperature must be kept in the range of T2-80 0C to T2-30 0C, where
T2=723-10.7Mn+16.9Cr+6.38W-16.9Ni.
Hot rolled steel is pickled in acidic medium followed by cold rolling with 75% or more reduction to achieve the desired texture and r-bar values. Cold rolled sheet must be Continuously annealed at a temperature ranging from T2+ 60 0C to T1-60 0C followed by annealing an optimum skin pass elongation of (YS/300) % to (YS/150) % must be provided to suppress stretcher strain, where YS – is the Yield Strength of material when stretcher strain is suppressed completely.
Equations developed through trials to achieve the desired properties of the steel grades/sheets-
In addition following relationships has been derived and must be satisfied for the combination of alloying elements claimed to fulfill the scope of the invention –
[Nb] - 7.75([C]-0.098[Cr])-1.77 (7.5[N]-[Al]) = 0 ------------ (i)
8.1 [Nb] + 4.2[Cr] + 4.4[Al] + 0.00185[SS Temp] = 140 [C] + 300 [N] ------------- (ii)
The relation specified above is resultant of experimental data. Inventors have taken trials on different compositional ranges and annealing parameters such as soaking temperature and come across that main parameters affecting the r-bar value as well as room temperature aging are amount of C, N and to fix that sufficient and optimized amount of Al, Nb and Cr must be added. By taking these in to consideration an experimental relationship was derived. Left hand side of equation (ii) gives minimum soaking temperature along with optimized amount of Nb, Al and Cr which must be added to counter the negative effect of C and N to achieve r-bar value of at least 2 which is very critical value for extra deep drawing applications and to meet the European specification such as (DC06) and other IF based EDDQ cold rolled steel sheet. Compliance of equation (i) and (ii) are shown in figure 6 and figure 5 respectively .Components made up with steel chemistry which does not satisfying equation (i) and (ii) results in cracking during actual deep drawing application due to poor anisotropy ratio.
Critical Precipitate Sizes and Volume fractions – It has been found that NbC /Nb(CN) along with AlN precipitate and their optimum volume fraction responsible for improved texture with favorable {111} oriented grains > 60 % along with excellent r- bar values of 2 or more and reduced ?r value of 0.4 or less . The impact of precipitate size and volume fraction has been listed out in table 2 where AlN /NbC volume fraction ratio ranging between 7-30 results in excellent drawing properties where as the lower ratio hampers the r-bar value. Also average NbC size of less than 6 nm is not favorable for r-bar as well as ?r value as described with the help of experimental examples given in Table 2. The control of size and volume fraction of critical precipitates as described are only achieved when equation (i) and (ii) are satisfied along with keeping the hot rolling and annealing parameters as described in scope of the invention. Effect of AlN to NbC volume fraction is shown in Figure 4 where an optimum volume fraction ratio in steel matrix gives excellent r-bar value.
Average NbC size has been controlled by controlling the Nb and C weight %, coiling temperature along with soaking section temperature. Excess Nb and C will cause larger excess volume fraction and Large NbC or Nb (CN) precipitate size which hampers the drawability by strengthening the steel matrix. An optimum volume fraction must be present for the development of favorable {111} texture which has been claimed as a part of present invention. As evident from steel number 9 and 10 with very low NbC volume fraction and very low Coiling temperature along with low SS temperature resulting in poor r-bar value and ?r value. The optimized NbC precipitate range as claimed in invention increases the recrystallization temperature of the steel and inhibits the tendency of initial {001}<110> oriented grains and due to higher temperature annealing > 800 0C favors the formation of {111}<112> texture which in turn improves the drawability (r-bar and ?r values). AlN (Forms during coiling stage of hot rolling) being the larger precipitate than NbC occupies larger volume fraction. However when N level is too high> 0.005 along with higher Al weight % , higher AlN volume fraction may hamper the drawability due to hard matrix . Very high coiling temperature may also leads to higher AlN precipitate size and volume fraction. Hence optimum Al and N weight % along with selected coiling temperature leads to optimum AlN/NbC volume fraction. Lower AlN/NbC volume fraction will result in unfixed C due to less NbC and Higher AlN/NbC may result in unfixed N due to less AlN. Hence the optimum AlN /NbC volume fraction is chosen.

Details of processing steps-
Continuously cast slabs having the composition as described above is reheated keeping slab reheating furnace delivery temperature below 1200°C intended to achieve roughing mill delivery temperature under 1050°C and finishing mill entry temperature under 1010°C to check surface defects like rolled in scale. Hot mill finishing temperature of T1-20°C to T1+20°C and coiling temperature of T2-80°C to T1-30°C was employed. Hot rolled coiled later processed through pickling coupled with tandem cold rolling mill, to remove the oxide surface present in the surface and to provide cold reduction of 78% or more .
Following pickling and cold rolling to desired thickness, cold rolled steel strip being processed through continuous annealing line, where electrolytic cleaning removes rolling emulsion present on the surface. Cleaned surface passes through the preheating and heating section where the strip is heated at the rate of 1-10 °C/sec to soaking section temperature maintained at T2+60°C to T1-60°C for different Yield strength requirements. Annealing Residence time 40-130 seconds gives desired results for present inventive grades. At soaking section complete recrystallization results softer steel to get desired properties of UTS 310Mpa or less. After soaking section steel strip passes through slow cooling section at cooling rate of 0.5- 5 °C/sec .Slow cooling section temperature of T2-40°C to T2+10 °C was maintained. Following slow cooling section annealed strip sheet been rapid cooled at 8-30 °C/sec up to 450 °C or more. After rapid cooling section annealed strip was over aged keeping the over aging section temperature of 350 °C or more. After over aging and furnace cooling, an optimum skin pass elongation of (YS/300) % to (YS/150) % is provided to avoid any stretcher strain; Whereas YS is the Yield Strength of material when stretcher strain is suppressed completely. Additionally Cold rolled sheet described in present invention can be used for zinc coating using galvanizing process to produce Galvannealed and Galvanized steel and used as coated product for similar applications.
Complete description of Nb-Cr stabilized cold rolled interstitial free steel and comparative steel grade are illustrated by way of the following table 1 to 3.
Table 1 - Compositions of the invented steel sheets along with some comparative examples.
Table 2 –Precipitate sizes and volume fraction of key precipitates in steel matrix as described in scope of invention along with Inventive equations which must be satisfied in order to accomplish the scope of the invention .
Table 3- Hot rolling and CAL annealing parameter along with Mechanical properties of inventive and comparative steels.

Table 1:
Steel No. C Mn Al N Nb Cr Other Remarks
1 0.001 0.18 0.05 0.0023 0.015 0.03 S-0.008 ,P-0.01 I
2 0.002 0.15 0.03 0.002 0.018 0.013 S-0.005 ,P-0.011 I
3 0.015 0.2 0.023 0.0023 0.013 0.02 S-0.012 ,P-0.015 C
4 0.003 0.28 0.04 0.003 0.01 0.02 Cu-0.2,Ni-0.1,Ti-0.05 C
5 0.005 0.5 0.01 0.004 0.01 0.06 B-0.003,Ni-0.1 ,V-0.03,Si-0.2 C
6 0.0023 0.2 0.05 0.0023 0.02 0.015 S-0.003 ,P-0.012 ,Ti-0.003 I
7 0.002 0.15 0.03 0.001 0.014 0.014 S-0.008 ,P-0.01 I
8 0.002 0.17 0.035 0.002 0.018 0.02 S-0.006 ,P-0.01 I
9 0.009 0.5 0.028 0.002 0.005 0.011 Ti-0.06,Sb-0.01 ,Zr-0.04 ,P-0.03 C
10 0.005 0.2 0.028 0.004 0.003 0.01 Ti-0.04,Pb-0.004,Sn-0.005 ,Co-0.02 C
11 0.002 0.4 0.028 0.005 0.01 0.01 Hf-0.03 , Mo-0.04,W-0.01,Ti-0.04 C
12 0.001 0.11 0.05 0.002 0.018 0.025 S-0.007 ,P-0.013,Si-0.008 I
13 0.002 0.15 0.05 0.002 0.014 0.023 S-0.004 ,P-0.008,Si-0.01 I
14 0.002 0.17 0.025 0.003 0.017 0.015 S-0.006,P-0.014,Si-0.005 I
15 0.01 0.18 0.005 0.005 0.07 0.03 Mo-0.05,W-0.01,B-0.003,Si-0.15 C
16 0.01 0.4 0.004 0.006 0.001 0.003 Zr-0.03 , V-0.04, Cu-0.2,Ti-0.04,Si-0.2 C
17 0.002 0.15 0.045 0.0023 0.015 0.01 S-0.006,P-0.014,Si-0.005 I
18 0.002 0.2 0.04 0.002 0.013 0.01 S-0.004,P-0.01,Si-trace I
19 0.002 0.1 0.05 0.0035 0.014 0.01 S-0.008,P-0.013,Ti-0.01 I

Remarks * - I- Nb-Cr stabilized cold rolled interstitial free steel of the invention , C- Comparative-
Steels marked as C are comparative examples having one or more element out of the scope of present invention, such as steel 15, 16 and 17 having higher carbon weight % along with total additional elements quantity such as Si, Ti, Zr, Cu, Mo, B, P > 0.06 weight% resulting either in lower AlN/NbC volume fraction ratio or lower NbC precipitate size and volume fraction, the impact of the same is listed in table 2 resulting in lower-bar value, higher ?r value and poor drawability.

Table 2 :
Steel No. NbC+ Nb(C,N) Volume % Average NbC Size(nm) AlN volume % Size of Average AlN (nm) AlN (Vol%)/ NbC Vol% E1 E2 E3 Remarks
1 0.015 19 0.15 120 10.0 2.00 0.83 0.088 I
2 0.02 17 0.17 132 8.5 1.85 0.88 0.039 I
3 0.018 41 0.05 81 2.8 1.73 2.79 -0.078 C
4 0.007 5 0 0 0 1.78 1.32 0.033 C
5 0.01 6 0.12 151 12.0 1.80 1.90 -0.019 C
6 0.017 12 0.19 165 11.2 2.02 1.01 0.077 I
7 0.018 17 0.16 187 8.9 1.75 0.58 0.049 I
8 0.02 16 0.2 196 10.0 1.92 0.88 0.053 I
9 0.005 4 0 0 0 1.60 1.86 -0.033 C
10 0.002 5 0 0 0 1.62 1.76 -0.024 C
11 0.008 6 0 0 0 1.64 1.78 -0.015 C
12 0.008 13 0.18 165 22.5 1.99 0.74 0.091 I
13 0.012 22 0.15 151 12.5 1.94 0.88 0.078 I
14 0.018 19 0.2 300 11.1 1.85 1.18 0.017 I
15 0.05 34 0.04 76 0.8 2.20 2.90 -0.042 C
16 0 0 0 0 0 1.56 3.20 -0.147 C
17 0.013 15 0.2 179 15.4 1.89 0.97 0.056 I
18 0.016 15 0.17 185 10.6 1.85 0.88 0.049 I
19 0.012 13 0.22 250 18.3 1.89 1.18 0.055 I

E1=8.1 [Nb] +4.2[Cr] + 4.4[Al] + 0.00185[SS Temp], E2=140 [C] + 300 [N], where as the scope of the invention will be satisfied only if E1=E2.
E3 = [Nb] - 7.75([C]-0.098[Cr])-1.77 (7.5[N]-[Al]) must be = 0 as per the scope of the invention.
Table 3:
Steel No. FT, 0C CT, 0C SS Temp,0C YS
(MPa) UTS (MPa) El% r-bar n-value ?r
Remarks Results After Forming
1 912 657 830 145 304 49.3 2.49 0.26 0.2 I Good
2 905 664 821 157 310 47.6 2.24 0.248 0.2 I Good
3 915 663 780 190 307 43 1.3 0.21 0.7 C Cracking occurred
4 916 660 780 210 334 41.1 1.2 0.2 0.4 C Cracking occurred
5 900 640 760 201 326 42.1 1.21 0.21 0.4 C Cracking occurred
6 908 653 830 159 310 49.8 2 0.24 0.23 I Good
7 910 654 780 150 296 50.2 2.47 0.251 0.31 I Good
8 910 654 830 156 291 52.1 2.3 0.25 0.26 I Good
9 890 540 750 211 339 41.5 1.24 0.19 0.7 C Cracking occurred
10 870 610 771 200 329 43.6 1.31 0.2 0.7 C Cracking occurred
11 901 550 753 217 325 41.9 1.25 0.19 0.8 C Cracking occurred
12 919 664 819 147 301 50.5 2.4 0.25 0.3 I Good
13 918 661 818 151 299 51.7 2.21 0.247 0.27 I Good
14 918 670 830 149 298 51.9 2.3 0.25 0.2 I Good
15 890 720 800 231 347 41.1 1.1 0.19 0.9 C Cracking occurred
16 880 540 821 229 351 42.3 1.2 0.19 0.8 C Cracking occurred
17 912 661 827 140 295 53 2.4 0.247 0.35 I Good
18 912 661 823 139 291 52.1 2.38 0.25 0.2 I Good
19 914 660 820 151 305 50.3 2.2 0.249 0.3 I Good

* Underlined values are out of the scope of present invention
* Steel Numbers not satisfying either of FT - T1 -20 0C to T1 + 200C , CT- T2-80 0C to T2-30 0C or SS Temperature of T2+60 0C to T1-600C are Violating the scope of the invention and marked as Comparative (C), where,
T1 = 910- 203 C1/2 -30 Mn+ 11Cr + 44.7 Si-400 Al -15.2 Ni+31.5 Mo-3 +20 Cu-700P-400Ti
T2=723-10.7Mn+16.9Cr+6.38W-16.9Ni.
Comparative steel examples marked as “C” performed poorly during extra deep drawing application as described in Figure 1. Severe cracking occurred due to poor r-bar value and higher ?r values where as steel numbers marked as “I” performed very well in same drawing condition.
Example 1 – Steel number 3, 4, 15 and 16 in table 2 having AlN volume % /NbC(volume%) less than the value claimed in the scope of the present invention results in higher ?r values along with poor drawability due to lower r-bar values .
Example 2 – Steel number 9, 10, 11 with low coiling temperature and soaking temperatures. Also the amount of additional alloying elements such as Ti, Zr,Mo,W,Hf is > 0.04 wt% resulting in lower volume fraction and size of NbC /Nb(C,N). As a result ?r values are high along with poor drawability due to lower r-bar and high Yield strength values.
Example 3- Steel number 3, 10, 16 does not satisfy the specified equations
8.1[Nb] +4.2 [Cr] + 4.4[Al] +0.00185 [SS Temp] = 140 [C] + 300 [N] and
[Nb] - 7.75([C]-0.098[Cr])-1.77 (7.5[N]-[Al]) = 0
Hence these steels are outside the scope of present invention. These steels suffer with poor drawability due to higher Yield strength and poor r-bar values.
Example 4-Steel Number 11 having CAL SS temperature less than the specified range of T2 + 600C to T1 -60 0C and Hot Coiling temperature less than specified range T2-800C to T2-30 0C results in lower NbC precipitate size of 6nm along with lower r-bar value and higher ?r value.
Example 5-Confirmation of improved planner anisotropy ratio (r-bar) and excellent drawability of present inventive grade is represented with orientation distribution function (ODF) Ø2-0-900 and Electron back scattered diffraction (EBSD) image in Fig 2.1 and Figure 2.2 respectively. Figure 2.1 illustrates very strong ?-fiber intensity at (ODF) Ø2-450 which is primarily responsible for excellent drawability and r-bar =2. Also as represented with EBSD image in Fig.2.2 a very high fraction >60% of favorably oriented {111} texture which is most favorable as far as drawability is concerned is a prime feature of present inventive grade.
Accompanying Figure 3 shows EBSD image of the steel of the present invention showing strong {111} texture confirming > 60% favorably oriented {111} grains.
Accompanying Figure 4 shows a graphical plot of Ratio of AlN /NbC precipitate volume fraction vs. r-bar Values.
Accompanying Figure 5 shows the graphical plot of E1 vs. E2 for the steel of the present invention and comparative steels and their impact on respective r-bar values wherein steels having E1< E2 the r-bar value are out of the scope of present invention.
Accompanying Figure 6 shows the graphical Plot of E3 vs. ?r value for the steel of the present invention and comparative steels wherein steels having E3<0, the ?r value are out of the scope of present invention.
It is thus possible by way of the present invention to provide Nb-Cr stabilized cold rolled interstitial free-steel sheets with improved planar anisotropy and excellent surface finish and a method of producing the same. The non aging extra deep drawing Interstitial free steel sheet as demonstrated above required to have selective composition and produced through continuous annealing route with selective process parameters such as to attain excellent drawability suitable for automobile body components. Thus, the extra deep drawing IF cold rolled steel according to the present invention with selective Al and N weight % along with selected coiling temperature provide for desired AlN and NbC precipitate size and optimum AlN/ NbC, Nb (C,N) volume fraction ratio for the target end characteristics. Moreover, the advanced non aging extra deep drawing Interstitial free steel sheet , as would be well apparent from the above studies and results could be developed through continuous annealing route having excellent formability with advanced average anisotropy ratio (r-bar =2), and reduced ?r value of 0.4 or less, n value > 0.235, required fraction of favorably oriented {111} grains and excellent surface quality which can be advantageously used for critical drawing operation suitable for automobile inner or outer body panel and similar applications.

,CLAIMS:We Claim:

1. Steel composition for processing crack resistant for extra deep drawing of continuous annealed Nb-Cr stabilized IF steels comprising in terms of weight %:-
(In wt %) (In wt %)
C: 0.003wt% or less Al: 0.025-0.05
Mn: 0.08-0.2 N: 0.004 or less (more preferably 0.003wt%or less )
Cr: 0.01-0.03 Nb: 0.013-0.02

having, [Nb] - 7.75([C]-0.098[Cr])-1.77 (7.5[N]-[Al]) = 0,
and the balance being Fe and other unavoidable impurities wherein the constitutional elements and SS Temp satisfying the following relation as hereunder:
8.1 [Nb] +4.2 [Cr] + 4.4[Al] +0.00185 [SS Temp] = 140 [C] + 300 [N],
whereas rolled continuously annealed steel having the average precipitate size of NbC and /or Nb(C, N) in steel matrix in the range of 7-40nm, average precipitate Size of AlN in the range of 80–500 nm and ratio of volume fraction of AlN to NbC in steel matrix in the range of 7 to 30.
2. Steel composition for processing crack resistant for extra deep drawing of continuous annealed Nb-Cr stabilized IF steels as claimed in claim 1 comprising P, Si, S, Cu, Ni, Zn, B, V, Ti, Ca, Zr, W, Mo, Hf in less than 0.1 weight %.
3. Nb-Cr stabilized cold rolled interstitial free-steel sheets comprising:
r-bar value =2, ?r value = 0.4 , YS less than 180 MPa , UTS less than 310 MPa which is obtained of cold rolled continuous annealing of steel composition comprising in terms of weight %:-
(In wt %) (In wt %)
C: 0.003wt% or less Al: 0.025-0.05
Mn: 0.08-0.2 N: 0.004 or less (more preferably 0.003wt%or less )
Cr: 0.01-0.03 Nb: 0.013-0.02
Where, [Nb] - 7.75([C]-0.098[Cr])-1.77 (7.5[N]-[Al]) = 0
And the balance being Fe and other unavoidable impurities wherein the constitutional elements and SS Temp satisfying the following relation as hereunder:
8.1 [Nb] +4.2 [Cr] + 4.4[Al] +0.00185 [SS Temp] = 140 [C] + 300 [N]
4. A process for producing Nb-Cr stabilized cold rolled interstitial free-steel sheets as claimed in anyone of claims 2 or 3 comprising:
i) Providing selective steel composition comprising in terms of weight %:-
(In wt %) (In wt %)
C: 0.003wt% or less Al: 0.025-0.05
Mn: 0.08-0.2 N: 0.004 or less (more preferably 0.003wt%or less )
Cr: 0.01-0.03 Nb: 0.013-0.02

Where, [Nb] - 7.75([C]-0.098[Cr])-1.77 (7.5[N]-[Al]) = 0
and the balance being Fe and other unavoidable impurities wherein the constitutional elements and SS Temp satisfying the following relation as hereunder:
8.1 [Nb] +4.2 [Cr] + 4.4[Al] +0.00185 [SS Temp] = 140 [C] + 300 [N]
ii) Processing crack resistant for extra deep drawing said Nb-Cr stabilized IF steels
Including cold rolled continuously annealed steel sheet having r-bar value =2, ?r value = 0.4, YS less than 180 MPa , UTS less than 310 MPa.
5. A process for producing Nb-Cr stabilized cold rolled interstitial free-steel sheets as claimed in claim 4 comprising:
i) hot rolling and continuous annealing carried out keeping the hot rolling finishing temperature in the range of T1-20 0C to T1 + 200C and coiling hot rolled sheet in the range of T2-80 0C to T2-30 0C where
T1 = 910- 203 C1/2 -30 Mn+ 11Cr+ 44.7 Si-400 Al -15.2 Ni+31.5 Mo-3 +20 Cu-700P-400Ti
T2=723-10.7Mn+16.9Cr+6.38W-16.9Ni
ii) After cold rolling cold rolled sheet continuously annealed at a temperature ranging from T2+ 60 0C to T1-60 0C;
iii) Subjecting the thus annealed sheet to skin pass elongation of (YS/300) % to (YS/150) % .wherein YS – is the Yield Strength of material when stretcher strain is suppressed completely.
6. A process as claimed in anyone of claims 4 or 5 comprising providing texture with fraction of favorably oriented {111} grains are >60 % involving coiling temperature T2-800C or more, Soaking Temperature T2+70 °C or more, NbC precipitate size 7-40nm, Volume fraction of AlN to NbC in steel matrix in the range of 7 to 30.
7. A process as claimed in anyone of claims 4 to 6 comprising restricting the total weight % of unavoidable impurities and trace elements including P, Si ,S, Cu, Ni, Zn, B, V, Ti, Ca , Zr, W, Mo, Hf less than 0.1 weight %.
8. A process as claimed in anyone of claims 4 to 7 comprising hot scarfing of slabs after casting to avoid surface defects.
9. A process as claimed in anyone of claims 4 to 8 wherein after cold rolling cold rolled sheet continuously annealed at a temperature ranging from T2+ 60 ?C to T1-60 ?C along with slow cooling temperature of T2-40 ?C or more.

Dated this the 2nd day of August, 2016
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)