DESC:FIELD OF THE INVENTION

The present invention relates to high yield ratio high strength rephosphorized cold rolled steel sheet having excellent formability, surface quality and a method of producing the same through continuous annealing. More particularly, the present invention is directed to provide Interstitial Free rephosphorized high strength steel sheet with yield strength 220 MPa or more having excellent formability, phosphatability making it suitable for application in automotive door panels, tail gates and wheel arches. Importantly, the steel grade having selective composition comprising Carbon: 0.003wt% or less, Mn: 0.3-0.5 wt%, P: 0.03-0.07 wt%, N: 0.004 wt% or less, Ti: 0.02 to 0.04 wt%, V: 0.01-0.025 wt%, B: 0.0007 to 0.0015 wt%, wherein the ratio (Ti+V+B)/N is selectively maintained more than 9 to make it interstitial free. The steel sheets are produced by hot rolling, cold rolling, continuous annealing, overaging with selective process parameters at each step and finally skin pass elongation of 1.6% or more to achieve Yield strength of 220Mpa or more having yield ratio of 0.75 or less, to achieve minimum yield strength of 220Mpa or more , yield ratio of 0.75 or less with excellent planar anisotropy ratio of 1.5 or more and phosphate crystal size of 4µm or less and phosphate coating weight of 2-3 g/m2 after zinc phosphate chemical conversion treatment on steel surface.

BACKGROUND ART
Automobile manufacturers are opting for manufacturing light and more fuel efficient vehicles 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.
A method for producing high strength Rephosphorised interstial free steel with excellent surface quality and high yield ratio having yield strength of 220Mpa minimum is disclosed in patent EP0691415 A1. The said EP method comprises of manufacturing high strength cold rolled steel sheet with excellent formability. The European Patent method comprises of 0.0005 -0.01% of Carbon, not more than 0.8% silicon,0.5-3.0% of Manganese,0.01 to 0.2% of phosphorous, 0.005-0.1% of titanium and 0.003-0.1%of niobium,0.0001 to 0.002% of boron with cold rolling reduction of 60% or more continuously annealed at 700 to 900°C. The cold rolled steel sheet manufactured following this EP method has resulted in yield strength around 160 to 220Mpa with tensile strength above 350MPa and BH index more than 30 Mpa. However, the method of this EP patent involves a problem of accelerated ageing that means yield strength increases with increase in time after skin passing and also drop the elongation of the materials.

To solve this problem, the present invention targets to fix the interstitial elements like C, N by using V, Ti and B. So in present invention, there is no deterioration in mechanical properties with ageing. The cold rolled sheet obtained is high strength Rephosphorised interstial free steel with excellent surface quality and high yield ratio is manufactured with Manganese 0.3% to 0.5% with combination of titanium and vanadium. The cold rolled sheet has excellent surface quality with good phosphatibility with reduction in content of manganese percentage and increasing the high yield ratio of 0.75 max effective in the weight reduction and increase in efficiency of automotive.
Aim of the present advancement is to provide a high strength Rephosphorised steel continuous annealed cold-rolled steel sheet, which has excellent drawability, with superior aging resistance for one year, and a method of manufacturing the same.
In the present invention the steel grade (HC220P) is produced with yield strength of 220Mpa minimum with very good formability r bar of 1.5 minimum, can be used for critical forming components without cracking as compared to known low carbon Rephosphorised high strength steel. Ageing guarantee of one year, when compared to Low carbon Rephosphorised steel. The yield strength to tensile strength ratio (i.e. yield ratio) of an IF steel sheet greater than 0.65, the thickness of the IF steel sheet can be reduced, which is effective in weight reduction.
OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide high yield ratio high strength rephosphorised interstitial free cold rolled steel sheet having excellent formability, surface quality and a method of producing the same through continuous annealing for application/use in outer panel and the like of an automobile body.

A further object of the present invention is directed to provide high yield ratio high strength rephosphorised interstitial free cold rolled steel sheets having yield strength of 220Mpa minimum with very good formability, r bar of 1.5 minimum, can be used for critical forming components without cracking.

A still further object of the present invention is directed to provide high yield ratio high strength rephosphorised interstitial free cold rolled steel sheets having selective composition and processed with selective parameters through continuous annealing to achieve Yield strength of 220Mpa or more having yield ratio of 0.75 or less with excellent planar anisotropy, better surface quality (phosphatibility) and aging guarantee of 12 months.

A still further object of the present invention is directed to provide high yield ratio high strength rephosphorised interstitial free cold rolled steel sheets having the composition selectively maintained wherein Titanium Vanadium and Boron is added as carbide and nitride former and the ratio (Ti+V+B)/N is maintained more than 9 to make the steel interstitial free and achieve good aging guarantee.

A still further object of the present invention is directed to provide high yield ratio high strength rephosphorised interstitial free cold rolled steel sheets wherein the yield ratio of the IF steel sheet is greater than 0.65, such that the thickness of the IF steel sheet can be reduced, enabling effective in weight reduction of automobile panel components.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to steel composition suitable for producing high strength rephosphorised continuously annealed steel sheet comprising
Carbon: 0.003 or less; Mn: 0.3-0.5; P: 0.03-0.07; N: 0.004 or less; Ti: 0.02 to 0.04; V: 0.01-0.025; B: 0.0007 to 0.0015; and balance is Fe and inevitable impurities, wherein the ratio of (Ti+V+B)/N should be more than 9;
having yield strength 220 MPa or more yield ratio of 0.75 or less with excellent planar anisotropy ratio of 1.5 or more.

A further aspect of the present invention is directed to steel composition including atleast one element , selected from Nb, Cr, Bi, Zr, Hf, Mo and W such that the total contents thereof do not exceed 0.06% in mass.

A still further aspect of the present invention is directed to said steel composition wherein the C, Mn and P content is also selectively provided in relation to selective soaking section temperature, skin pass elongation and centre speed following :
3699[C]+123[Mn]+646.6[P]-0.14{SS}+21{SPM Elon} +0.051{Center Speed} = 6 wherein [X] - Wt% of Element X.
SS- is soaking section temperature in continuous annealing line, Centre Speed- is speed of steel strip inside continuous annealing line in meter per minute and SPM Elon – is Skin pass elongation provided to steel strip after continuous annealing.
.
Another aspect of the present invention is directed to steel sheet which is high strength rephosphorized continuously annealed steel sheet comprising (all in terms of weight %):
Carbon: 0.003 or less; Mn: 0.3-0.5; P: 0.03-0.07; N: 0.004 or less; Ti: 0.02 to 0.04; V: 0.01-0.025; B: 0.0007 to 0.0015; and balance is Fe and inevitable impurities, wherein the ratio of (Ti+V+B)/N should be more than 9, having yield strength 220 MPa or more yield ratio of 0.75 or less with excellent planar anisotropy ratio of 1.5 or more.

Yet another aspect of the present invention is directed to steel sheet comprising atleast one element , selected from Nb, Cr, Bi, Zr, Hf, Mo and W such that the total contents thereof do not exceed 0.06% in mass and with ageing guarantee of atleast 12 months.

A yet further aspect of the present invention is directed to steel sheet which is obtained following continuous annealing and maintaining the element and process condition relation as :3699[C]+123[Mn]+646.6[P]-0.14{SS}+21{SPM Elon} +0.051{Center Speed} = 6 wherein [X] - Wt% of Element X , SS- is soaking section temperature in continuous annealing line, Centre Speed- is speed of steel strip inside continuous annealing line in meter per minute and SPM Elon – is Skin pass elongation provided to steel strip after continuous annealing.

A further aspect of the present invention is directed to said steel sheet having phosphate crystal size of 4µm or less after phosphating chemical conversion treatment; In addition phosphate coating weight should be 2-3 g/m2 after phosphating chemical conversion treatment on said steel sheet.

A still further aspect of the present invention is directed to selective texture components relation of said steel sheet having a mean value of X-ray random intensity for the group of {111} <112> oriented plane is at least 10 and mean value of X ray random intensity for group of {111} <110> plane orientation component is atleast threefold of that of {554} <225> group of planes.

A still further aspect of the present invention is directed to a process for manufacture of steel sheet which is rephosphorized high strength continuously annealed steel sheet comprising:

Providing selective composition having Carbon: 0.003 or less; Mn: 0.3-0.5; P: 0.03-0.07; N: 0.004 or less; Ti: 0.02 to 0.04; V: 0.01-0.025; B: 0.0007 to 0.0015; and balance is Fe and inevitable impurities, wherein the ratio of (Ti+V+B)/N should be more than 9; and

carrying out the steel manufacture including continuous annealing such as to achieve rephosphorized high strength continuously annealed steel sheet having yield strength 220 MPa or more yield ratio of 0.75 or less with excellent planar anisotropy ratio of 1.5 or more.

Another aspect of the present invention is directed to a process for manufacturing said steel sheet comprising:
a.) Reheating the slab with said selective composition to temperature 1200 °C or less;
b.) Reheated slab being roughing rolled in roughing mill with roughing mill delivery temperature of 1050°C or less;
c.) Finish rolling the steel after Roughing rolling at temperature 880 °C or more;
d.) Coiling the finish rolled steel at with run out table cooling rate of 8 °C/second or more; and
e.) Cold rolling the said hot rolled steel sheet with cold reduction of 70% or more.

A still further aspect of the present invention is directed to said process comprising:
a) Annealing the cold rolled steel at soaking section temperature of 760 °C to 820°C with residence time of for 50 to 90 seconds;
b) Slow cooling further said steel at temperature of 720°C or less;
c) Rapid cooling said steel at temperature 480°C or less with cooling rate of 10-30°C / Sec;
d) Overaging said steel at temperature 350°C or more with residence time of 130 to 260 Secs; and
e) Annealed steel sheet is further given a Skin pass elongation of 1.2% to 1.9 %.

A still further aspect of the present invention is directed to process comprising:
a) Annealing the cold rolled steel at soaking section temperature of 760 °C to 820°C;
b) Slow cooling further said steel at temperature of 720°C or less;
c) Rapid cooling said steel at temperature 480°C or less;
d) Overaging said steel at temperature 350°C or more;
e) Annealed steel sheet is further given a Skin pass elongation of 1.2% to 1.9 %; and
f) Centre Speed for said steel sheet in continuous annealing line, wherein the constitutional elements in said steel composition additionally meet the following relation including with respect to SS Temp, SPM elongation and Centre speed,
3699[C] + 123[Mn] + 646.6[P] - 0.14{SS} + 21{SPM Elon} + 0.051{Center Speed} = 6, where [X] is wt% of element X, SS is soaking section temperature, SPM% is skin pass elongation % and Centre Speed is speed of steel strip in meter per minute (mpm) inside continuous annealing furnace.

A still further aspect of the present invention is directed to said process carried out involving selective steel chemistry and process control for ageing guarantee of atleast 12 months.

The above and other objects and advantages of the present invention are described hereunder in greater details with reference to the following accompanying non limiting illustrative drawings and examples.
BRIEF DESCRIPTION OF THE ACCOMPNAYING DRAWINGS
Figure 1: is the optical micrograph of IF steel grade produced according to the present invention having yield strength more than 220MPa showing grain size of 9 with complete ferrite structure.

Figure 2: is the SEM image of invented steel grade showing TiN and VN precipitates.

Figure 3: is the SEM image of invented steel grade showing TiN and BN precipitates.

Figure 4: is the Texture of High strength Interstitial Free Steel according to the present invention showing very strong ?-fiber({111} <112> group of planes) intensity and mean value of X-ray random intensity for the group of {111} <112> oriented plane is atleast 10 and the mean value of X ray random intensity for group of {554} <225> plane orientation component is one third or less of that of {111} <110> group of planes .

Figure 5: Showing the SEM micrograph of inventive steel sheet surface after phosphating treatment (also known as Zinc phosphate chemical conversion coating) showing uniform phosphate crystal grain with grain size of less than 4µm and phosphate coating weight of 2-3 g/m2.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING FIGURES INCLUDING A PREFERRED EMBODIMENT
The present invention relates to High strength cold rolled steel with good formability and ageing resistance used for critical automotive components. Automobile panels are press formed to shape in to components which require high planar anisotropy. Sometimes the material supplied by customer is stored and used when require, there should be no change in the mechanical property which requires ageing guarantee. Ultra low carbon Rephosphorised steel produced according to the present invention has all the above said properties. To make interstitial free steel, alloying elements like titanium and vanadium is added to form carbides and nitrides which increase the formability and ageing guarantee.

Following abbreviations are used to describe the present invention:
SS- Soaking Section
SCS – Slow Cooling Section
RCS-Rapid Cooling Section
OAS Over-ageing section
SPM-Skin Pass Elongation
YPE-Yield Point Elongation
HSM-Hot Strip Mill
UTS-Ultimate Tensile Strength
YS-Yield Strength
El – Elongation
SPM-Skin Pass Mill
FT-Finishing Temperature
CT- Coiling Temperature
mpm – meter per minute
CAL – Continuous Annealing line
Center Speed- Speed of steel strip inside the continuous annealing line in Meter per Minute.

The present invention is directed to provide high-strength cold rolled steel sheet manufactured through continuous annealing route appropriate for use in an outer panel and the like of an automobile body and having a yield strength YS of not less than 220 MPa combined with average r-value of higher than 1.5.

Steel grade according to present invention comprising of in weight percent Carbon 0.003Max, Manganese 0.3-0.5, Phosphorous 0.04-0.06, Titanium-0.02-0.04, Boron-0.0007-0.0015, Nitrogen 0.004max, Vanadium- 0.01-0.025, balance iron and essentially the usual balance impurities. The Titanium, Vanadium and Boron is added as carbide and nitride former. The chemical elements Ti, V and B to Nitrogen ratio to be greater than 9 to achieve good ageing guarantee. Following are the justification for keeping the above said composition-

CARBON (0.003 Max) wt.% – Amount of carbon content increases more than 0.003%,then dissolved carbon remains in solute form up on recrystallization deteriotes R bar, it is preferably maintain 0.003% or less to obtain good R bar value.

MANGANESE (0.3-0.5) wt. %- Manganese acts as solid solution strengthening increases the tensile strength of the steel, its lower limit is set as 0.3. Manganese is effective in fixing the sulphur and for preventing the hot embrittlement of the steel. As manganese lowers planar anisotropy of the cold rolled steel sheet the upper limit is set as 0.5.When manganese kept below 0.5% it will reduce the tensile strength. A further problem arises with increase in manganese content more than 0.5% results as oxides formed on surface becomes manganese rich and film of manganese rich oxide is dissolve in large amount while treating solution during phosphating, which affects the deposition of phosphate crystals and affects the phosphatibility. As shown in Fig.5 SEM image of Phosphated surface having manganese less than 0.5% observed with coating weight of 2.4g/m2 , grain size of 3µm .

Phosphorous (0.03-0.07) wt. %- Phosphorous is main strengthener, important component in the present invention. When phosphorous is less than 0.04% no strengthening effect occurs. Phosphorous has high solid solution strengthenability than Si and Mn. The upper limit of P should be 0.06% to provide good strength, but when phosphorus content is more it segregates in grain boundaries during solidification and r bar deteriotes, so the upper limit is set as 0.06%.

Titanium (0.02-0.04)wt% - Titanium is added to fix carbon nitrogen and sulphur to make steel interstitial free, When titanium is kept below 0.02% it will be insufficient to fix the carbon and nitrogen, on the other hand it is more than 0.04% the formation of iron phosphide (FeTiP) which will deteriorates the r-bar and lead to secondary work embrittlement.

Vanadium (0.01-0.025) wt%- Vanadium is also added to fix the carbon and nitrogen as VC, VN. Vanadium is weak carbide former than titanium, makes the grain size coarser.

Boron (0.0007-0.0015) wt% - Boron reduces solute nitrogen by forming Boron nitride, lower limit of boron should be 0.0007% effective for antiaging property. When amount of boron exceeds 0.0015% causes increased cost and embrittlement of steel due to excess formation of Boron nitride. Due to reduce in solute nitrogen enhances ageing guarantee for one year.

Nitrogen (0.004 max) wt%- Nitrogen is an impurity present in the steel, when titanium, Vanadium and Boron is added to the steel, Nitrogen is fixed as TiN, VN and BN improves ageing guarantee. The ratio ((Ti, V, B) / Nitrogen) ratio to be greater than 9, so solute nitrogen present will be negligible and almost zero, ensuring ageing guarantee of 1 year is obtainable. SEM image as shown in Fig 2 and Fig 3 showing precipitates of TiN, VN and BN With 2-3µm particle size of Titanium Nitride
(Ti+V+B)/N > 9………………. (1)
Based on the experimental data, a relation has been developed to satisfy the scope of the invention:

3699[C]+123[Mn]+646.6[P]-0.14{SS}+21{SPM Elon} +0.051{Center Speed} = 6 to achieve yield strength of 220MPa or more along with yield ration of 0.75 or less ,
Where [X] - Wt% of Element X ,SS is soaking section temperature, SPM% is skin pass elongation % and Centre Speed is speed of steel strip in meter per minute (mpm) inside continuous annealing furnace.

The steel grade further comprising atleast one element, selected from Nb, Cr, Bi, Zr, Hf, Mo and W as carbide forming elements such that the total contents thereof do not exceed 0.06% in mass. Said elements are useful n grain refinement and achieving the desired r-bar value. Any further addition of more than 0.06 of said elements hardens the matrix and reduces the planner anisotropy ratio.
Method of manufacturing:
The high strength interstitial free steel is produced by having steel slab composition with the range of invention as given above and slab reheated at temperature 1200°C or less to achieve roughing mill temperature 1050 °C or less to avoid surface defect called rolled in scale and then hot rolled with finishing temperature of 880°C to 920°C and coiled at temperature above 620°C.The average cooling rate in run out table from finishing mill exit to coiling must be more that 8 0C/sec.

After hot rolling the slab, the material is pickled and cold rolled. In order to ensure the deep drawability of the material, minimum 70 % cold rolling reduction to be done.

After pickled and cold rolled steel strip is processed through continuous annealing line, 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 removes emulsion present on the surface. Cleaned surface passes through the preheating section where the strip is heated and then passes through soaking section where it heated above 820°C or less where complete recrystallization takes and makes the steel softer to get desired properties of UTS 320Mpa or more. When the soaking temperature increases from 780°C to 820°C tensile strength decreases, to achieve tensile strength more than 320MPa soaking temperature is set to 820°C or less. Steel strip soaked at 820°C or less for 50 to 90 seconds. Subsequently steel sheet passes through slow cooling section with temperature of the zone being 720°C or less, to get the yield Strength more than 220Mpa as slow cooling temperature increases yield strength decreases. After slow cooling steel sheet is rapidly cooled to 480°C or less with cooling rate of 10-30°C / Sec cooling followed by overaging at 350°C or more with residence time of 130 to 260 Secs to reduce the solute carbon .Subsequently steel sheet is processed through skin-pass mill with Skin pass elongation of 1.5 % or more to avoid any the yield point elongation. As skin pass elongation of 1.4% to 1.9% increases yield strength 230Mpa to 260Mpa.The continuously annealed skin pass steel shows a typical ASTM grain size of 9 as shown in Fig. 1.

Ageing Test: Processed steel strip is made it to dumbbell shaped tensile test specimen and kept at homogenized oil bath at 100°C for 9 hours and tested again, while testing if yield point elongation observed is 0.3% or less then the shell life or the aging resistance of the steel sheet will be 12 months as per hundys equation. Ageing test is done for 9 hours which is equivalent to 12 month as per hundys equation.

Phosphatability test procedure – Firstly, alkali degreasing was performed 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 phosphatability. The phosphate coating weight was measured using the XRF method.

Complete description of Examples and resulting experimental data defining the scope of present application and comparative steel grades are illustrated through following tables I to IV:

Table I- Compositions of the example sheets along with some comparative examples.

Table II- Hot rolling, cold rolling, annealing parameters of Example and comparative steels

Table III- Mechanical Properties of examples along with Ageing Guarantee and Phosphatability results of example and Comparative steels.

Table IV – Texture components of inventive and comparative steel sheet listing X- ray random intensity of {111} <112>, {111} <110> and {554} <220> group of planes .

Table V – Comparing the value of 3699[C]+123[Mn]+646.6[P]-0.14{SS}+21{SPM Elon} +0.051{Center Speed} for some of inventive and comparative steels.

Table I-
S. No C Mn P N Ti B V (Ti+V+B)/N Other Remarks
1 0.002 0.38 0.049 0.002 0.03 0.0007 0.02 25 Nb:0.02 Ex.
2 0.002 0.38 0.049 0.002 0.03 0.0007 0.02 25 Cr:0.02,Zr-0.01 Ex.
3 0.003 0.38 0.051 0.003 0.024 0.001 0.015 13 Hf:0.02 Ex.
4 0.003 0.37 0.047 0.004 0.025 0.0012 0.018 11 Nb:0.01,Hf:0.01 Ex.
5 0.002 0.38 0.049 0.004 0.03 0.0007 0.018 12 Zr:0.012,Mo:0.01 Ex.
6 0.003 0.37 0.047 0.004 0.025 0.0012 0.018 11 Bi:0.005,W:0.006 Ex.
7 0.003 0.38 0.051 0.003 0.024 0.001 0.015 13 Ex.
8 0.002 0.38 0.049 0.004 0.03 0.0007 0.018 14 Ex.
9 0.002 0.38 0.049 0.004 0.03 0.0007 0.016 13 Ex.
10 0.002 0.38 0.049 0.004 0.03 0.0007 0.016 13 Ex.
11 0.003 0.37 0.047 0.004 0.025 0.0012 0.016 11 Ex.
12 0.003 0.37 0.047 0.004 0.025 0.0012 0.016 11 Ex.
13 0.003 0.37 0.06 0.004 0.025 0.0012 0.016 11 Nb:0.02 , Zr:0.01 Ex.
14 0.003 0.37 0.047 0.004 0.025 0.0012 0.016 11 Ex.
15 0.003 0.37 0.047 0.004 0.025 0.0012 0.016 11 Ex.
16 0.003 0.37 0.047 0.004 0.025 0.0012 0.016 11 Ex.
17 0.002 0.37 0.047 0.004 0.029 0.0008 0.015 11 Ex.
18 0.003 0.38 0.051 0.003 0.024 0.001 0.015 13 Ex.
19 0.003 0.38 0.051 0.003 0.024 0.001 0.015 13 Ex.
20 0.002 0.37 0.047 0.004 0.029 0.0008 0.015 11 Ex.
21 0.003 0.38 0.051 0.003 0.024 0.001 0.015 13 Ex.
22 0.003 0.38 0.051 0.003 0.024 0.001 0.015 13 Ex.
23 0.003 0.38 0.051 0.003 0.024 0.001 0.015 13 Ex.
24 0.003 0.38 0.051 0.003 0.024 0.001 0.015 13 Ex.
25 0.005 1 0.049 0.004 0.02 0.0007 0.012 8 Comp
26 0.006 0.8 0.07 0.004 0.02 0.0012 0.015 9 Comp
27 0.003 0.38 0.051 0.004 0.015 0.0004 0.015 8 Comp
28 0.003 0.38 0.051 0.004 0.01 0.0008 0.003 3.45 Comp
29 0.001 0.2 0.03 0.004 0.02 0.0008 0.006 6.7 Comp
30 0.003 0.3 0.025 0.003 0.024 0.001 0.015 13 Comp
Ex. – Example steel, Comp- Comparative Steel

Table II-
HSM Rolling Cold Rolling Annealing Remarks
Sample No HSM CT HSM FT CR Thick Reduction SS SCS RCS OAS Center Speed SPM El
1 658 912 1 75 781 643 479 381 237 1.8
2 653 912 1 75 790 669 477 373 213 1.8
3 655 913 1 75 770 632 456 367 192 1.9
4 658 908 1 75 783 703 480 392 225 1.81
5 653 913 1 75 774 657 479 375 245 1.8
6 650 913 1 75 768 701 476 379 220 1.9
7 663 909 1 75 780 644 471 353 118 1.8
8 666 904 1 75 800 651 481 370 247 1.8
9 664 911 1.2 73 782 642 481 367 151 1.81
10 656 916 1.2 73 780 682 485 390 215 1.82
11 658 907 1.2 73 792 698 480 399 223 1.81
12 658 907 1.2 73 810 706 493 397 222 1.61
13 657 914 1.2 73 765 698 482 383 225 1.61
14 657 914 1.2 73 779 698 480 383 221 1.8
15 657 911 1.2 73 765 695 472 388 214 1.6
16 657 911 1.2 73 772 702 482 395 225 1.61
17 661 914 1.4 73 778 635 480 383 128 1.83
18 638 908 1.4 73 779 643 481 393 155 1.81
19 638 908 1.4 73 779 644 480 390 149 1.81
20 664 912 1.4 73 780 648 481 377 92 1.81
21 663 910 1.4 73 782 641 479 385 165 1.8
22 652 909 1.4 73 780 635 480 366 114 1.82
23 659 910 1.4 73 775 641 481 385 136 1.61
24 659 910 1.4 73 775 643 481 379 123 1.62
25 663 915 0.8 77 782 692 480 391 205 1.71 High Carbon, High Mn, Low R Bar, Poor Phosphatability
26 663 915 0.8 77 782 692 480 391 205 1.71 High Phosphorous , High Mn, Low R Bar, Poor Phosphatability
27 659 910 1.4 73 775 643 481 379 123 1.62 Low Boron Accelerated Ageing
28 655 913 1 75 770 632 456 367 192 2.2 Low Vanadium ,Accelerated Ageing,
29 660 920 0.8 75 830 720 461 380 100 1.3
Low SPM
30 650 910 1.2 70 800 702 487 375 120 1.2 Low SPM

Table III
Mechanical Properties
Sample No YS YS/TS Elongation r-bar (Mean planner anisotropy ratio) YPE After 12Months of Aging Phosphate Crystal Size, µm Phosphate Coating Weight, g/m2
1 250 0.72 41.4 1.96 0 2 2.3
2 253 0.71 43 2.1 0 2.5 2.1
3 239 0.71 45.3 1.77 0 3 2.5
4 237 0.7 46.8 1.85 0 2.2 2
5 258 0.72 42.4 1.95 0 2.1 2.1
6 248 0.71 44.1 1.67 0
7 231 0.69 45.5 1.78 0
8 256 0.72 41.2 2.02 0
9 244 0.7 44.8 1.94 0
10 248 0.7 40.9 1.88 0
11 253 0.73 44.1 1.4 0
12 232 0.67 45.8 1.7 0
13 242 0.69 44.7 1.58 0
14 250 0.71 44.9 1.58 0
15 245 0.7 43.3 1.53 0
16 240 0.69 43.8 1.53 0
17 235 0.69 46 1.95 0
18 236 0.69 47.5 1.67 0
19 243 0.7 45.8 1.83 0
20 242 0.69 45.7 1.86 0
21 254 0.72 45.3 1.75 0
22 234 0.69 45.3 1.67 0
23 231 0.68 47.2 1.69 0
24 236 0.68 46.3 1.93 0
25 295 0.73 34.1 1.13 0.3 8 3.5
26 290 0.725 35.2 1.2 0.5 7 3.4
27 236 0.68 46.3 1.93 0.5
28 239 0.71 45.3 1.23 1
29 180 0.61 49.1 1.5 0.61
30 193 0.63 47.3 1.52 0

Table IV
Steel Number X ray random intensity of {111}<112> group of planes X ray random intensity of {111}<110> group of planes X ray random intensity of {554}<225> group of planes Remarks
1 21.3 21 6.1 Example
2 24.1 20.5 5.2 Example
5 25.7 20.1 5.9 Example
8 25.1 21.6 4.7 Example
25 6.7 5.2 3.8 Comparative
26 5.1 4.7 3.6 Comparative

Table V
Steel Number C wt% Mn wt % P wt% SS temp , 0C SPM % Centre Speed (mpm) Eq* Remarks
1 0.002 0.38 0.049 781 1.8 237 26.3 Ex.
13 0.003 0.37 0.06 765 1.61 225 33.5 Ex.
29 0.001 0.2 0.03 830 1.3 100 -36.1 Comp
30 0.003 0.3 0.025 800 1.2 120 -16.5 Comp
Where, Eq* = 3699[C]+123[Mn]+646.6[P]-0.14{SS}+21{SPM Elon} +0.051{Center Speed}
Ex. – Example, Comp- Comparative
Example 1- from table I and II it is shown that steel sample number 1 to10 with carbon wt% ranges between 0.002-0.003 and cold reduction of 70% with Soaking temperature 820°C or less shows improved r-bar (?) value of >1.5 while steel sample number 26 with high carbon percentage of 0.006 shows low r-bar (?) value is observed 1.2. As Carbon percentage increases, R bar value decreases drastically so to maintain desired r-bar (?) value of >1.5, C percentage to be limited to 0.003 maximum.

Example 2 – Steel number 25 and 26 IN TABLE I having Mn weight % of 0.8 and 1 respectively shows a poor phosphatability with phosphate crystal size of 8 and 7 µm respectively with higher phosphate coating weigh of 3.5 and 3.4 g/m2 .

Example 3 – steel number 25 and 26 in table IV having value of mean x-Ray random intensity for {111}<112> group of planes less than 10 does not comply with the scope of present invention as the planner anisotropy ratio are 1.13 and 1.2 respectively . Similarly steel number 26 having mean x-Ray random intensity for group of {111}<110> planes less than threefold of X ray random intensity of {554}<225> group of planes resulting in less planner anisotropy ratio of 1.23 .

Example 4 - Steel number 28 in table I having value of (Ti+V+B/N) of 3.45 does not comply with aging guarantee of 12 months as accelerated aging of the material results in YPE of 1%.

Example 5 – Steel number 29 an 30 from table V having value of Eq* = 3699[C]+123[Mn]+646.6[P]-0.14{SS}+21{SPM Elon} +0.051{Center Speed} less than 6 showing low Yield strength of 180 and 193 MPa respectively which is out of the scope of present invention .

It is thus possible by way of the present invention to provide High strength cold rolled steel sheets with good formability, required planar anisotropy and ageing resistance/guarantee suitable for application in critical automotive components. The Ultra low carbon Rephosphorized interstitial free steel according to the invention having alloying elements like titanium and vanadium is added to form carbides and nitrides which increase the formability and ageing guarantee wherein the ratio (Ti+V+B)/N is more than 9 to make it interstitial free. The cold rolled sheet has excellent surface quality with good phosphatability with reduction in content of manganese percentage and keeping the yield ratio of 0.75max effective in the weight reduction and increase in efficiency of automotive. The present invention thus relates to high-strength cold rolled steel sheet manufactured through continuous annealing route appropriate for use in an outer panel and the like of an automobile body and having yield strength of at least 220 MPa combined with average r-value of higher than 1.5.
,CLAIMS:We Claim:

1. Steel composition suitable for producing high strength rephosphorized continuously annealed steel sheet comprising
Carbon: 0.003 or less; Mn: 0.3-0.5; P: 0.03-0.07; N: 0.004 or less; Ti: 0.02 to 0.04; V: 0.01-0.025; B: 0.0007 to 0.0015; and balance is Fe and inevitable impurities, wherein the ratio of (Ti+V+B)/N should be more than 9;
having yield strength 220 MPa or more yield ratio of 0.6 to 0.75 preferably 0.75 or less with planar anisotropy ratio of 1.5 or more.

2. Steel composition as claimed in claim 1 including atleast one element , selected from Nb, Cr, Bi, Zr, Hf, Mo and W such that the total contents thereof do not exceed 0.06% in mass.

3. Steel composition as claimed in anyone of claims 1 or 2 wherein the C,Mn and P content is also selectively provided in relation to selective soaking section temperature, skin pass elongation and centre speed as following :
3699[C]+123[Mn]+646.6[P]-0.14{SS}+21{SPM Elon} +0.051{Center Speed} = 6 , wherein [X] - Wt% of Element X.
.
4. Steel sheet which is high strength rephosphorised continuously annealed steel sheet comprising (all in terms of weight %):
Carbon: 0.003 or less; Mn: 0.3-0.5; P: 0.03-0.07; N: 0.004 or less; Ti: 0.02 to 0.04; V: 0.01-0.025; B: 0.0007 to 0.0015; and balance is Fe and inevitable impurities, wherein the ratio of (Ti+V+B)/N should be more than 9, having yield strength 220 MPa or more yield ratio of 0.75 or less with planar anisotropy ratio of 1.5 or more.

5. Steel sheet according to claim 4 comprising atleast one element , selected from Nb, Cr, Bi, Zr, Hf, Mo and W such that the total contents thereof do not exceed 0.06% in mass and with steel sheet having ageing guarantee of atleast 12 months.

6. Steel sheet according to anyone of claims 4 or 5 which is obtained following continuous annealing and maintaining the element and process condition relation as :3699[C]+123[Mn]+646.6[P]-0.14{SS}+21{SPM Elon} +0.051{Center Speed} = 6 wherein [X] - Wt% of Element X.

7. Steel sheet according to anyone of claims 4 to 6 having phosphate crystal size of 4µm or less after zinc phosphate chemical conversion treatment and phosphate coating weight of 2-3 g/m2.

8. Steel sheet as per anyone of claims 4 to 7 having a mean value of X-ray random intensity for the group of {111} <112> oriented plane at least 10 and mean value of X ray random intensity for group of {111} <110> plane orientation component is atleast threefold of that of {554} <225> group of planes .

9. A process for manufacture of steel sheet which is rephosphorised high strength continuously annealed steel sheet comprising:

providing selective composition having Carbon: 0.003 or less; Mn: 0.3-0.5; P: 0.03-0.07; N: 0.004 or less; Ti: 0.02 to 0.04; V: 0.01-0.025; B: 0.0007 to 0.0015; and balance is Fe and inevitable impurities, wherein the ratio of (Ti+V+B)/N is more than 9; and

Carrying out the steel manufacture including continuous annealing such as to achieve rephosphorized high strength hot rolled ,pickled ,cold reduced and continuously annealed steel sheet having yield strength of 220 MPa or more yield ratio of 0.75 or less with excellent planar anisotropy ratio of 1.5 or more.

10. A process for manufacturing said steel sheet as claimed in claim 9 comprising:
a) Reheating the slab with said selective composition to temperature 1150°C to 1200 °C preferably 1200 °C or less;
b) Reheated slab being roughing rolled in roughing mill with roughing mill delivery temperature of 1050°C or less;
c) Finish rolling the steel after Roughing rolling at temperature 880 °C or more;
d) Coiling the finish rolled steel at with run out table at cooling rate of 8 °C/second or more; and
e) Cold rolling the said hot rolled steel sheet with cold reduction 70% or more.
11. A process as claimed in anyone of claims 9 or 10 comprising:
a) Annealing the cold rolled steel at soaking section temperature of 760 °C to 820°C with residence time of for 50 to 90 seconds;
b) Slow cooling further said steel at temperature of 720°C or less;
c) Rapid cooling said steel to temperature of 480°C or less with cooling rate of 10-30°C / Sec;
d) Overaging said steel at temperature 350°C or more with residence time of 130 to 260 Secs; and
e) Annealed steel sheet is further given a Skin pass elongation of 1.2% to 1.9 %.

12. A process as claimed in anyone of claims 9 to 11 comprising:
a) Annealing the cold rolled steel at soaking section temperature of 760 °C to 820°C;
b) Slow cooling further said steel at temperature of 720°C or less;
c) Rapid cooling said steel up to temperature 480°C or less;
d) Overaging said steel at temperature 350°C or more;
e) Annealed steel sheet is further given a Skin pass elongation of 1.2% to 1.9 %; and
f) Centre Speed for said steel sheet in continuous annealing line, wherein the constitutional elements in said steel composition additionally meet the following relation including with respect to SS Temp, SPM elongation and Centre speed,

3699[C]+123[Mn]+646.6[P]-0.14{SS}+21{SPM Elon} +0.051{Center Speed} = 6, where [X] is wt% of element X, SS is soaking section temperature , SPM% is skin pass elongation % and Centre Speed is speed of steel strip in meter per minute (mpm) inside continuous annealing furnace.

13. A process as claimed in anyone of claims 9 to 12 carried out involving selective steel chemistry and process control for ageing guarantee of atleast 12 months.

Dated this the 10th day of August, 2016
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)