DESC:Present invention relates 340MPa Tensile strength level high-strength interstitial free continuous annealed cold rolled steel sheet and its process of manufacture directed to achieving excellent formability and surface property appropriate for Body parts ,Rail and side members, Auto Front Pillars, Auto Door Inners and the like of automotive parts. The advancement is further directed to a rephosphorized high-strength IF steel having excellent Phosphatability and surface properties applicable for automotive Exposed body parts. Additionally, the advancement also targets chemical composition and processing conditions for excellent formability. In order to achieve improved texture with mean value of X-ray random intensity ratio of group of {111} <110> to {001} <110> orientation must be atleast 3.5 .Mean value of X ray random intensity for {111} <112> orientation component must be atleast 3 multiple than that of {554} <225> orientation component in order to achieve planer anisotropy ratio of = 1.5 and yield ratio of =0.65.

BACKGROUND OF THE INVENTION
Automobile manufacturers are incorporating more high strength materials in their vehicle body as well as structural components for light weighing ,improving fuel efficiency and to fulfill the norms of future legislation concerning emission and fuel consumption. This forces the development of materials having excellent drawability along with superior strength to fulfill the passenger safety norms.
However high strength steels are susceptible to rather poor drawability and surface property due to higher solid solution element addition such as Mn, Si etc. This in turn hampers the coating property on steel surface. Also, drawability is poor due to poor texture orientations associated with harden matrix due to solid solution strengthening.
Aim of the present advancement is to provide a high strength steel with 340MPa strength level with superior drawability, Phosphatability, improved texture and accelerated aging resistance suitable for automotive body structure and like of other applications.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide high strength interstitial free cold rolled steel sheet having tensile strength 340MPa or more with excellent drawability and phosphatability and a process for manufacturing the same through continuous annealing route.

A further object of the present invention is directed to provide high strength interstitial free cold rolled steel sheet for excellent drawability and phosphatability wherein improved surface property is achieved by controlled low Mn and Si content suitable for automotive exposed body parts.

A still further object of the present invention is directed to provide high strength interstitial free cold rolled steel sheet for excellent drawability and phosphatability wherein to achieve r-bar value =1.5 and desired UTS of = 340 MPa ,value of [C] wt% + [Mn+Si] wt% must be in the range of 0.06wt% to 0.12wt%.

Yet another object of the present invention is directed to provide high strength interstitial free cold rolled steel sheet for excellent drawability and phosphatability wherein to achieve improved texture with mean value of X-ray random intensity ratio of group of {111} <110> to {001} <110> orientation must be atleast 3.5 and mean value of X ray random intensity for {111} <112> orientation component must be atleast 3 multiple than that of {554} <225> orientation component in order to achieve planer anisotropy ratio of = 1.5 and yield ratio of =0.65.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to a steel composition for producing cold rolled continuously annealed steel sheet with tensile strength of 340 MPa or more comprising in terms of weight %:
(In weight %) (In weight %)
C: =0.003 N: 0.004 or less
Mn: 0.35-0.7 V: 0.005-0.02
Si: = 0.03 P: 0.035-0.07

and balance being Fe and other unavoidable impurities wherein,
[C]% + [Mn+Si]%/6 is in the range of 0.06wt% to 0.12 wt% suitable for generating steel sheet having mean value of X-ray random intensity ratio of group of {111} <110> to {001} <110> orientation atleast 3.5,also mean value of X ray random intensity for {111} <112> orientation component atleast 3 multiple of {554} <225>orientation component.

A further aspect of the present invention is directed to a steel composition further comprising 0.002 to 0.025 %by mass % atleast one type of element selected from the group comprising Nb, Zr, Mg, Cr, Mo, W, Hf, Co, Ni, Cu, Zn, Sc, Ca, Pb and Sn.
A still further aspect of the present invention is directed to a steel composition further comprising by weight % atleast one from Ti: 0.02% to 0.05 % and B: 0.0005% to 0.002%.

A still further aspect of the present invention is directed to a steel composition having said steel composition further related to soaking section temperature:
0.0032*SS -24[C]-0.1 [Mn]-1.7[P]-0.2[Si]-70[N] =1.9
Where, SS is soaking section temperature in 0C and [M] is weight % of element M;
such as to favor achieving steel having anyone or more of
UTS =340 MPa, YS/UTS ratio =0.65, mean planer anisotropy ratio (r-bar) of =1.5, bake hardening index of less than 10 MPa, and no yield point elongation after accelerated aging for atleast 6 months.

A still further aspect of the present invention is directed to a cold rolled continuously annealed steel sheet with tensile strength of 340 MPa or more comprising in terms of weight %:
(In weight %) (In weight %)
C: =0.003 N: 0.004 or less
Mn: 0.35-0.7 V: 0.005-0.02
Si: = 0.03 P: 0.035-0.07

and balance being Fe and other unavoidable impurities wherein,
[C]% + [Mn+Si]%/6is in the range of 0.06wt% to 0.12 wt% and having mean value of X-ray random intensity ratio of group of {111} <110> to {001} <110> orientation atleast 3.5,also mean value of X ray random intensity for {111} <112> orientation component atleast 3 multiple of {554} <225>orientation component.

A still further aspect of the present invention is directed to a cold rolled steel sheet further comprising 0.002% to 0.025 % by mass % atleast one type of element selected from the group comprising Nb, Zr, Mg, Cr, Mo, W, Hf, Co, Ni, Cu, Zn, Sc, Ca, Pb and Sn.

Another aspect of the present invention is directed to a cold rolled steel sheet according to claim 5 or 6, further comprising by weight % atleast one from Ti: 0.02% to 0.05 % and B: 0.0005% to 0.002%.

Yet another aspect of the present invention is directed to a cold rolled steel sheet having said steel composition further related to soaking section temperature:
0.0032*SS -24[C]-0.1 [Mn]-1.7[P]-0.2[Si]-70[N] =1.9
Where, SS is soaking section temperature in 0C and [M] is weight % of element M.

A further aspect of the present invention is directed to a said Steel sheet, having UTS =340 MPa, YS/UTS ratio =0.65, mean planer anisotropy ratio (r-bar) of =1.5, bake hardening index of less than 10 MPa, and no yield point elongation after accelerated aging for atleast 6 months.

A still further aspect of the present invention is directed to said Steel sheet having a phosphate crystal size of 4 µm or less and phosphate coating weight of 1.5-2.5 g/m2 after zinc phosphate chemical conversion coating treatment on said steel sheet surface.

A further aspect of the present invention is directed to a process for manufacturing cold rolled steel sheet, comprising the steps of:
a) providing steel composition and producing steel slabs comprising in terms of weight %:
(In weight %) (In weight %)
C: =0.003 N: 0.004 or less
Mn: 0.35-0.7 V: 0.005-0.02
Si: = 0.03 P: 0.035-0.07

and balance being Fe and other unavoidable impurities wherein,
[C]% + [Mn+Si]%/6 is in the range of 0.06wt% to 0.12wt%;
b)Reheating the slab to reheating temperature of 1160 °C -1220 °C;
c)Said Reheated slab being roughing rolled in roughing mill with roughing mill delivery temperature of 1060°C or less ;
d)Said rough rolled steel being subjected to finish rolling after at temperature range of 860°C to 920°C;
e)Coiling the finish rolled steel at with run out table cooling rate of 8 °C/second or more; and
f)Cold rolling the said hot rolled steel sheet with cold reduction of 65% or more.

A still further aspect of the present invention is directed to said process wherein the steel composition involved is further related to soaking section temperature:
0.0032*SS -24[C]-0.1 [Mn]-1.7[P]-0.2[Si]-70[N] =1.9
where, SS is soaking section temperature in 0C and [M] is weight % of element M.

A still further aspect of the present invention is directed to a process for manufacturing cold rolled steel sheet comprising:
a) Annealing at soaking section temperature range of 770 °C to 830°C with residence time of for 50 to 140 seconds;
b) Slow cooling the steel up to a temperature range of 660°C to 720°C after soaking ;
c) Rapid cooling the steel up to a temperature range of 420 °C to 500°C with cooling rate of 10°C / second to 30°C / second;
d) Overaging said steel at temperature range of 320°C to 400°C with residence time of 100 seconds to 300 seconds; and
e) Subjecting the overaged steel to skin pass elongation of 0.6% to 1.2 %.

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

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1: shows the Orientation distribution function at ?2-450 section showing X-Ray random intensities for different plane orientations for inventive steel “A”.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWING AND EXAMPLES INCLUDING A PREFERRED EMBODIMENT
The present invention relates to cold rolled continuously annealed interstitial free steel sheet having tensile strength of 340 MPa or more with excellent drawability and phosphatability and a method of manufacturing the same.

Cold rolled steel sheet described in scope of present invention consist in terms of weight %, C=0.003%, Si: =0.03% , Mn :0.35%-0.7% , P: 0.035%-0.07% , N =0.004%, V: 0.005%-0.02% and the balance being Fe and unavoidable impurities . Vanadium is added as carbide and nitride formers. Additionally, atleast one type of element selected from the group comprising Nb, Zr, Cr, Mg, Mo, W, Hf, Co, Ni, Cu, Zn, Ca, Pb and Sn such that each element by content in the range of 0.002 to 0.025 % in terms of weight %. Also, steel further including atleast one from Ti: 0.02% to 0.05 % and B: 0.0005% to 0.002%.

The abbreviations used to describe the present invention are as follows:
SS – Soaking section in continuous annealing furnace
SCS – Slow cooling section in continuous annealing furnace
RCS- Rapid cooling section in continuous annealing furnace
OAS- Over aging section in continuous annealing furnace
YS – Yield strength in MPa
UTS- Ultimate tensile strength in MPa
BH – Bake hardening index in MPa
SPM %– Skin passes elongation in %
YPE- Yield Point Elongation %
El%-Total Elongation at 50mm gauge length sample
CCM%- Cold rolling reduction %
r-bar - Mean planer anisotropy ratio

With the endeavor of developing cold rolled Interstitial free high strength steel with atleast 340 MPa tensile strength level by continuous annealing route, effect of Metallurgical factors affecting the mechanical properties and drawability are described in detail.

Carbon (=0.003 wt%)- 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 .Furthermore, reducing the C wt% reduces the amount of alloying addition (Nb, Ti, V) 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% or less.

Manganese (0.35 wt% to 0.7 wt%): Manganese is an effective solid solution strengthening element which facilitates achieving the desired level of Tensile strength of 340 MPa or more. Effect on Yield strength is not significant hence Mn helps in achieving desired yield ratio of 0.7 or less. However, to achieve any noticeable strengthening effect the minimum level should be more than 0.35 wt%, and therefore its lower limit should preferably be atleast 0.35 wt%. Keeping the Mn wt% on higher side significantly hampers the drawing property (r-bar value) and phosphatability of interstitial free high strength rephosphorized steel hence upper limit is restricted to 0.7 wt% for present steel.

Phosphorus(0.035wt% to 0.07wt%)- Phosphorus is most important strengthening element in present invention as the name itself implies as an Interstitial free high strength rephosphorized steel. P is as an element which improves the strength at low cost, and the amount of addition thereof varies depending on a target strength level. For the strength level of more than 340 MPa as described in scope of present invention the minimum amount of P should be more than 0.035 wt%. However when added amount exceeds more than 0.07 wt% the yield strength level increases significantly. In addition, higher amount of P promotes secondary working embrittlement and increases the ductile to brittle transition temperature (DBTT).

Silicon(=0.03wt%)– Silicon is an element utilized for increasing the strength of steel. As the silicon content increases the ductility and r-value noticeably deteriorated .Since silicon deteriorates plating / phosphatability properties by forming SiO2 type of oxides (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 0.03 wt% or less.

Titanium(0.02 wt% to 0.05 wt% ) - Titanium is added to fix carbon nitrogen and Sulphur to make steel interstitial free. Keeping Titanium level below 0.02 wt% will be insufficient to fix the carbon and nitrogen; on the other hand if it is more than 0.05 wt% the formation of phosphate (FeTiP) will deteriorates the r- bar value and lead to secondary work embrittlement.

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 Ti addition to fix extra N and increase the volume of TiN precipitates which strengthens the material, ultimately deteriorating the drawing property.

Vanadium (0.005wt% -0.02 wt%)- Vanadium has been added with intension to fix solute Carbon as VC .Vanadium is weak carbide former than titanium, makes the grain size coarser. It has been reported that Ti and V added steel has increased Elongation and r bar of the steel than titanium only added steel. To achieve the said advantage scope of V level is from 0.005wt% -0.02 wt% in present inventive steel.

Boron (0.0005wt% to 0.002wt%) - Boron reduces solute nitrogen by forming Boron Nitride, lower limit of boron should be 0.0005 wt% effective for anti aging property. Boron also strengthens the grain boundaries. However, when amount of boron exceeds 0.002 wt% causes and edge crack during hot rolling and embrittlement of steel due to excess amount of solute boron.

0.06 = [C] wt% + [Mn+Si] /6 wt% = 0.12 wt%- The value of [C] wt% + [Mn+Si]/6 wt% has a major impact on planner anisotropy ratio and phosphatability . To achieve r-bar value =1.5 and desired UTS of = 340 MPa, value of [C] wt% + [Mn+Si]/6 wt% must be in the range of 0.06wt% to 0.12wt%. In spite of having tensile strength value more than 340 MPa, inventor of present invention have ensured that value of [C] wt% + [Mn+Si] wt%/6 must be less than 0.12 wt%. This has been achieved by keeping C =0.003 wt%, Si level = 0.03 wt% and Mn level =0.7 wt%.

Atleast one from the group of Nb, Zr, Cr, Mg, Mo, W, Hf, Co, Ni, Cu, Zn, Sc, Ca, Pb and Sn – Present inventive steel further including atleast one from Nb, Zr, Cr, Mg, Mo, W, Hf, Co, Ni, Cu, Zn, Ca, Pb and Sn in a range of 0.002wt%-0.025wt%. These elements are including as they form carbide and/or nitride and/or sulphide which supports the aging resistance and low BH index. However amount more than 0.025 wt% add up to the cost of production and also reduces the drawability.

Method of manufacturing

To achieve Slab chemistry as described in scope of the invention Heat from basic oxygen furnace (BOF) is processed through RH degasser and subsequently continuously casted. Special measure have been taken to hot roll resulted slabs by keeping slab reheating temperature in the range of 1160°C to 1220°C intended to control roughing mill delivery temperature under 1060°C and finishing mill entry temperature under 1020°C to check surface defects like rolled in scale. During hot rolling finishing mill temperature range of 860°C to 910°C and run out table cooling rate from finishing mill to coiler of more than 80C/sec was maintained to achieve coiling temperature range of 620 °C to 670 °C .Hot rolled coils were subsequently processed through pickling coupled with tandem cold rolling mill to remove the oxide surface present in the surface and to provide cold reduction of 65% or more .

Following pickling and cold rolling to desired thickness, cold rolled steel sheet 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 0C/sec to soaking section temperature maintained at 770 °C -830 °C. Annealing time of 50-140 seconds gives desired results for present interstitial free 340MPa tensile strength grade. After soaking steel strip passes through slow cooling section at cooling rate of less than 3°C/sec. Slow cooling section temperature in the range of 660 °C - 720°C was maintained. Following slow cooling section annealed strip sheet been rapid cooled at 10 °C/sec or more up to rapid cooling section temperature maintained in the range of 420-500 0C. After rapid cooling section annealed strip was over aged keeping the over aging section temperature in the range of 320°C -400 °C for 100 to 300 seconds. Subsequent to over aging steel strip is given a skin-pass elongation (temper rolling) in the range of 0.6 % to 1% to avoid yield point elongation and to improve flatness of steel strip. In addition in order to achieve improved texture and drawability, mean value of X-ray random intensity ratio of group of {111} <110> to {001} <110> orientation must be atleast 3.5 generated due to controlled chemistry, hot rolling and annealing. Also, mean value of X ray random intensity for {111} <112> orientation component must be atleast 3 multiple than that of {554} <225> orientation component in order to achieve planer anisotropy ratio of = 1.5 and yield ratio of =0.65. In addition following relation must be fulfilled in favor of chemical composition and during annealing –

0.0032*SS -24[C]-0.1 [Mn]-1.71[P]-0.2[Si]-70[N] =1.9

Where, SS is soaking section temperature in 0C and [M] is weight % of element M.
Furthermore, Cold rolled continuous annealed steel sheet described in present invention can be processed through continuous galvanizing route for zinc coating to produce GA/GI steel sheets and used as coated product for similar applications.

Method of evaluating phosphatability –

Phosphating process can be defined as the treatment of a metal surface so as to give a reasonably hard, electrically non-conducting surface coating of insoluble phosphate which is contiguous and highly adherent to the underlying metal and is considerably more absorptive than the metal which provides excellent corrosion resistance and paint ability to steel surface .The coating is formed as a result of a topochemical reaction, which causes the surface of the base metal to integrate itself as a part of the corrosion resistant film. [1]
[1] T.S.N. Sankara Narayanan, SURFACE PRETREATMENT BY PHOSPHATE CONVERSION COATINGS, Rev.Adv.Mater.Sci. 9 (2005) 130-177.

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

Method of evaluating bake hardening in a tensile test: present inventive steel characterized by its low bake hardenability with bake hardening index of less than 10 MPa .To evaluate the same tensile test specimen as per JIS Z2241 No.5 with 50mm gauge length 25mm width was prepared across the rolling direction of steel sheet. Tensile test specimen was then strained to 2% at strain rate of about 0.008/second and then heated at 1700C for 20 minutes. Heated sample was then subjected to tensile test. Bake hardening index was then evaluated by measuring the difference between the initial strength at 2% strain before bake hardening and final yield strength (at lower yield point) after heating at 1700C for 20 minutes.

Method of evaluating accelerated aging resistance: tensile test specimen as per JIS Z2241 No.5 with 50mm gauge length 25mm width was prepared across the rolling direction of steel sheet. Prepared specimen was kept in an oil bath at 1000C for 3 and 6 hours respectively. Subsequently the specimen was tested in tensile test machine with strain rate of 0.008/second. Specimen which showed YPE>0.15% after the test are be prone to accelerated aging and does not confirm to the scope of invention.

Complete description of Inventive steel and comparative steel grades are illustrated in the following tables I and V:

Table I- Elemental Compositions of the inventive steel sheets along with comparative example and their respective values of [C]% + [Mn+Si]% / 6 …Eq1.
Table II- Hot rolling, cold rolling, annealing parameters and the value of 0.0032*SS-24[C]-0.1 [Mn]-1.7[P]-0.2[Si]-70[N]…Eq 2, of inventive and comparative steel sheets.
Table III-Mechanical properties along with BH index, phosphate crystal size and phosphate coating weight of inventive and comparative steel sheets.
TABLE IV –Room temperature ageing properties of invented and comparative steels.
Table-V –Mean value of X-ray random intensity ratio of group of {111} <110> to {001} <110> orientation along with mean value of X ray random intensity for {111} <112> and {554} <225> orientation component for Inventive and comparative steel.

Table I:

S.No Steel No. C,
wt% Mn
,wt% Si,
wt% P,
wt% N,
wt% Ti,
wt% B,
wt% V,
wt% Other Elements,
wt% Eq1 Remarks
1 A 0.001 0.35 0.005 0.04 0.003 0.036 0.0012 0.01 Mo:0.01,Nb:0.015,
Cr : 0.01 0.060 Ex.
2 B 0.002 0.65 0.01 0.065 0.003 0.04 0.0015 0.018 Ni:0.01,Hf:0.005, Nb:0.005, W:0.005 0.112 EX.
3 C 0.003 0.45 0.02 0.05 0.003 0.045 0.001 0.015 Ca:0.005,Zr:0.003, Co:0.003, Cu:0.005 0.081 Ex.
4 D 0.01 1 0.2 0.06 0.005 0.01 - - Nb: 0.03 0.210 Comp
5 E 0.003 0.1 0.02 0.01 0.005 0.005 - 0.005 Nb:0.02 0.023 Comp
6 F 0.006 1 0.4 0.05 0.004 0.01 - - Mo: 0.1 0.239 Comp
7 G 0.004 0.8 0.02 0.12 0.005 - 0.0015 0.01 Nb-0.01,Sn-0.002 0.141 Comp
* All compositional elements are in terms of weight %
* Ex. - Present inventive example, Comp.- Comparative Examples
** Underlined and shaded boxes indicates “outside the appropriate range”
*** Eq1 = [C] + [Mn+Si]/6
Note: steel A, B and C are marked inventive examples as their composition well within the range of scope of the invention, also the value of Eq1 is inside the range of 0.06 to 0.12.
However, Steel D, E, F and G are marked as comparative examples as atleast one of their composition is outside the range of scope of the invention, also the value of Eq1 is outside the range of 0.06 to 0.12 .

Table II:
S.No Steel No. FT,0C Cooling rate after Finish Rolling ,0C/sec CT,0C Cold reduction ,% SS Temperature,0C Soaking Time , sec SCS Temperature,0C RCS Temperature,0C OAS Temperature,0C SPM ,% Eq2 Remarks
1 A 910 11.2 650 79 810 100 691 471 377 1 2.25 Ex.
2 B 900 12.3 643 75 805 90 660 450 381 0.7 2.14 EX.
3 C 890 11.8 660 75 810 75 710 480 375 0.9 2.18 Ex.
4 D 890 11.6 610 70 760 60 714 430 341 1.2 1.60 Comp
5 E 920 10.4 720 80 820 108 705 492 395 0.6 2.17 Comp
6 F 900 12.2 670 75 770 90 689 460 340 0.9 1.77 Comp
7 G 900 9.4 630 75 770 85 680 470 360 1 1.73 Comp
* Ex. - Present inventive example, Comp.- Comparative Examples
* FT- hot finish rolling temperature , CT- Hot coiling temperature , SS- soaking section ,SCS- Slow cooling section , RCS- Rapid cooling section , OAS- Overaging section , SPM- Skin pass elongation
** Underlined and shaded boxes indicates “outside the appropriate range”
*** Eq2 = 0.0032*SS -24[C]-0.1 [Mn]-1.7[P]-0.2[Si]-70[N]
Note: steel A, B and C are marked inventive examples as their hot and cold rolling parameters are well within the range of scope of the invention, also the value of Eq2 is =1.9 resulting in good ? value, excellent phosphatability and improved {111}<110> and {111} <112> texture as listed in subsequent tables.
However, Steel D, F and G are marked as comparative examples as the value of Eq2 is <1.9 resulting in poor ? value, poor phosphatability and low ratio of X-ray intensity for {111}<110> to {001}<110> orientations < 3.5 as listed in subsequent tables.

Table III
S.No Steel No. YS,MPa UTS,MPa YS/UTS ? BH Index,
MPa Total EL% Phosphate Crystal Size, µm Phosphate Coating weight , g/m2 Remarks
1 A 215 354 0.61 1.8 2 45.6 2.5 1.8 Ex.
2 B 252 398 0.63 1.7 5 41 3 2.2 EX.
3 C 213 361 0.59 1.8 3 45 3 1.9 Ex.
4 D 300 410 0.73 1.1 40 30 6 2.9 Comp
5 E 156 288 0.54 1.8 45 49 5 2.9 Comp
6 F 296 410 0.72 1.14 35 31 5.5 2.8 Comp
7 G 305 432 0.71 1.2 37 32 6 2.9 Comp
Ex. – inventive steel, Comp. - Comparative steel
* Underlined and shaded boxes indicate “outside the appropriate range”.
*** ? is mean planer anisotropy ratio .
YS- Yield Strength, UTS- Tensile strength
* Steel sheets having YS/UTS ratio >0.65 does not conform with the scope of the present invention, also steels with phosphate crystal size after zinc phosphate chemical conversion coating >4µm and phosphate coating weight >3g/mm2 are suggests poor phosphatability and marked as comparative .steel sheets with BH index more than 10 are marked as examples as it does not confirm to the scope of invention and are prone to room temperature aging as listed in table IV.
* Steel E processed at very high coiling temperature of 720 0C during hot rolling shows poor phosphatability with phosphate crystal size of 5µm and phosphate coating weight of 2.9 g/m2. High coiling temperature resulted in severe oxide formation on steel surface resulting in poor phosphatability.

Table IV
Steel
Number Initial YS,
(MPa) After 3 hours aging, at 100 0C YPE,
% After 6 hours aging , at 100 0 C YPE, % Remarks
LYP
(MPa) UYP
(MPa) YS LYP
(MPa) UYP
(MPa) YS
A 215 219 0 223 0 Inventive steel- Room temperature Aging pass
B 252 256 0 259 0 Inventive steel -Room temperature Aging pass
C 213 214 0 217 0 Inventive steel -Room temperature Aging pass
D 300 312 331 331 0.8 319 343 343 1.2 Comparative, Room temperature aging fail, YPE 1.2 % for 6 months of aging
E 156 168 174 174 0.75 169 189 189 1 Comparative, Room temperature aging fail, YPE 1 for 6 months of aging
F 296 298 319 319 0.7 308 328 328 0.9 Comparative, Room temperature aging fail, YPE 0.9 for 6 months of aging
* LYP – Lower yield Point, UYP – Upper Yield Point. , YPE- yield point elongation
** To simulate the room temperature aging tensile test specimen of 50mm gauge length and 25 mm width was immersed in oil bath which was homogeneously maintained. Specimen was kept inside the bath at 100 0C for 3 hours and 6 hours to simulate 3months and 6months aging respectively. Specimens were then subjected to tensile test at strain rate of 0.008/sec .Steels with no YPE after tensile test fulfills the scope of the invention and comply with atleast six months aging guarantee.
Note: steel A, B and C are marked inventive examples as their composition well within the range of scope of the invention resulting in aging resistance of atleast 6 months as No YPE observed after 6 hours of accelerated aging. However steel D, E and F are prone to aging as YPE is observed after accelerated aging of these steel sheets as listed in table IV.

TABLE V:
Steel Number Ratio of {111} <110> to {001} <110> component group X-ray intensity X ray random intensity for {111} <112> component group X ray random intensity for {554} <225> component group Ratio of X-ray intensity for group of {111} <112> to {554} <225> orientation Remarks
A 7 13.2 1.9 6.9 Ex.
B 6 10.5 1.4 7.5 Ex.
C 6.9 11.7 2.1 5.6 Ex.
D 2.5 6 2.9 2.1 Comp.
F 2.1 5.8 3.1 1.9 Comp.

Note: Table V listing a mean value of X-ray random intensity ratio of group of {111} <110> to {001} <110> orientation components. Steel A,B and C having X–ray random intensity ratio of {111} <110> to {001} <110> texture component more than 3.5 are mentioned as inventive example . Therefore A,B and C are complying with the r-bar value of = 1.5 and YS/UTS ratio of 0.65 or less .Whereas comparative steel D and F with X-ray intensity ratio of {111} <110> to {001} <110> orientation component lower than 3.5 does not comply with ? (Drawability) requirement . Also inventive steel A ,B and C complying with mean value of X ray random intensity of {111} <112> orientation component atleast 3 multiple than that of {554} <225> orientation components where steel D and F does not.
* Low X–ray random intensity ratio of {111} <110> to {001} <110> texture component for steel D and F is attributed to value Eq2 = 0.0032*SS -24[C]-0.1 [Mn]-1.7[P]-0.2[Si]-70[N] less than 1.9 as listed in table II.
* Orientation distribution function at ?2-450 section showing X-Ray random intensities for different plane orientations for inventive steel “A” is shown in Figure 1.

It is thus possible by way of the present invention to provide cold rolled continuously annealed interstitial free steel sheet having tensile strength of 340 MPa or more with excellent drawability and phosphatability wherein by selectively controlling the composition and process parameters attributes like YS/UTS ratio =0.65, mean planer anisotropy ratio (r-bar) of =1.5, bake hardening index of less than 10 MPa, and no yield point elongation after accelerated aging for atleast 6 months and a phosphate crystal size of 4 µm or less and phosphate coating weight of 1.5-2.5 g/m2 after zinc phosphate chemical conversion coating treatment on said steel sheet surface is advantageously achieved to suit intended automotive application.

,CLAIMS:We Claim:
1) A steel composition for producing cold rolled continuously annealed steel sheet with tensile strength of 340 MPa or more comprising in terms of weight %:
(In weight %) (In weight %)
C: =0.003 N: 0.004 or less
Mn: 0.35-0.7 V: 0.005-0.02
Si: = 0.03 P: 0.035-0.07
And balance being Fe and other unavoidable impurities, wherein
[C]% + [Mn+Si]%/6 is in the range of 0.06wt% to 0.12 wt% suitable for generating steel sheet having mean value of X-ray random intensity ratio of group of {111} <110> to {001} <110> orientation atleast 3.5, also mean value of X ray random intensity for {111} <112> orientation component atleast 3 multiple of {554} <225>orientation component.

2. A steel composition according to claim 1, further comprising 0.002 to 0.025 %by mass % atleast one type of element selected from the group comprising Nb, Zr, Mg, Cr, Mo, W, Hf, Co, Ni, Cu, Zn,Sc, Ca, Pb and Sn.

3. A steel composition according to claim 1 and 2, further comprising by weight % atleast one from Ti: 0.02% to 0.05 % and B: 0.0005% to 0.002%.

4. A steel composition as claimed in anyone of claims 1 to 3 having steel composition further related to soaking section temperature:
0.0032*SS -24[C]-0.1 [Mn]-1.7[P]-0.2[Si]-70[N] =1.9
Where, SS is soaking section temperature in 0C and [M] is weight % of element M; such as to favor achieving steel having anyone or more of
UTS =340 MPa, YS/UTS ratio =0.65, mean planer anisotropy ratio (r-bar) of =1.5, bake hardening index of less than 10 MPa, and no yield point elongation after accelerated aging for atleast 6 months.

5. A cold rolled continuously annealed steel sheet with tensile strength of 340 MPa or more comprising in terms of weight %:
(In weight %) (In weight %)
C: =0.003 N: 0.004 or less
Mn: 0.35-0.7 V: 0.005-0.02
Si: = 0.03 P: 0.035-0.07

And balance being Fe and other unavoidable impurities wherein,
[C]% + [Mn+Si]%/6is in the range of 0.06wt% to 0.12 wt% and having mean value of X-ray random intensity ratio of group of {111} <110> to {001} <110> orientation atleast 3.5,also mean value of X ray random intensity for {111} <112> orientation component atleast 3 multiple of {554} <225>orientation component.

6. A cold rolled steel sheet according to claim 5, further comprising 0.002 to 0.025 %by mass % atleast one type of element selected from the group comprising Nb, Zr, Mg, Cr, Mo, W, Hf, Co, Ni, Cu, Zn, Sc, Ca, Pb and Sn .

7. A cold rolled steel sheet according to claim 5 or 6, further comprising by weight % atleast one from Ti: 0.02% to 0.05 % and B: 0.0005% to 0.002%.

8. A cold rolled steel sheet as claimed in anyone of claims 5 to 7 having steel composition further related to soaking section temperature:
0.0032*SS -24[C]-0.1 [Mn]-1.7[P]-0.2[Si]-70[N] =1.9
Where, SS is soaking section temperature in 0C and [M] is weight % of element M.

9. Steel sheet as per claim 5 to 8, having UTS =340 MPa, YS/UTS ratio =0.65, mean planer anisotropy ratio (r-bar) of =1.5, bake hardening index of less than 10 MPa, and no yield point elongation after accelerated aging for atleast 6 months.

10. Steel sheet as per claim 5 to 9 having a phosphate crystal size of 4 µm or less and phosphate coating weight of 1.5-2.5 g/m2 after zinc phosphate chemical conversion coating treatment on said steel sheet surface.

11. A process for manufacturing cold rolled steel sheet, comprising the steps of:
a) Providing steel composition and producing steel slabs comprising in terms of weight %:
(In weight %) (In weight %)
C: =0.003 N: 0.004 or less
Mn: 0.35-0.7 V: 0.005-0.02
Si: = 0.03 P: 0.035-0.07

And balance being Fe and other unavoidable impurities wherein,
[C]% + [Mn+Si] %/6 is in the range of 0.06wt% to 0.12wt%
b)Reheating the slab to reheating temperature of 1160 °C -1220 °C;
c)Said Reheated slab being roughing rolled in roughing mill with roughing mill delivery temperature of 1060°C or less ;
d) Said rough rolled steel being subjected to finish rolling after at temperature range of 860°C to 920°C;
e)Coiling the finish rolled steel at with run out table cooling rate of 8 °C/second or more; and
f) Cold rolling the said hot rolled steel sheet with cold reduction of 65% or more.

12. A process as claimed in claim 11 wherein the steel composition involved is further related to soaking section temperature:
0.0032*SS -24[C]-0.1 [Mn]-1.7[P]-0.2[Si]-70[N] =1.9
Where, SS is soaking section temperature in 0C and [M] is weight % of element M.

13. A process for manufacturing cold rolled steel sheet as per claim 11 to 12 comprising:
a) Annealing at soaking section temperature range of 770 °C to 830°C with residence time of for 50 to 140 seconds;
b) Slow cooling the steel up to a temperature range of 660°C to 720°C after soaking ;
c) Rapid cooling the steel up to a temperature range of420 °C to 500°C with cooling rate of 10°C / second to 30°C / second;
d) Overaging said steel at temperature range of 320°C to 400°Cwith residence time of 100 seconds to 300 seconds; and
e) Subjecting the overaged steel to skin pass elongation of 0.6% to 1.2 %.

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