DESC:FIELD OF THE INVENTION
The present invention relates to 780 MPa Tensile strength level dual phase cold rolled steel sheet with high yield ratio (YS/UTS) and a method of its manufacture through continuous annealing route to achieve desired characteristics including excellent bendability, Hole expansion ratio and surface properties. The dual phase steel grade obtained according to present invention is having Tensile strength of atleast 780MPa with high yield ratio comprising chemical elements in terms of mass percent: 0.121wt % to 0.16wt% percent of Carbon,1.6 wt% to 2.0 wt% of Manganese,0.1 wt% to 0.4 wt% of Silicon, 0.02 wt% to 0.06wt% of Aluminum,0.015wt% or less of Phosphorous,0.02 wt% to 0.06wt% of Niobium, up to 0.25wt% of Molybendum,0.001 wt% to 0.003wt% of Calcium, up to 0.006wt% of Nitrogen and the balance being Fe and other inevitable impurities, wherein (Mn/Si) ratio is in a range of 5 to 15% for excellent combination of bendability and hole expansion ratio. Cold rolled dual phase steel has a microstructure consisting of (Ferrite+Martensite+Bainite+Precipitates) and has a yield ratio > 0.60. The advancement is specifically directed to obtain a steel grade suitable for application in automotive structural parts, pillars and rails, body structures, reinforcements and brackets, bumper-reinforcement beam.

BACKGROUND OF THE INVENTION

With utilization of dual phase high strength steel, automobile manufacturers are requiring more high strength materials with UTS 780MPa or more with high yield ratio in their reinforcement, structural components and pillars for light weighing, improving fuel efficiency and to satisfy the norms of future legislation concerning emission and fuel consumption.

However, high strength dual phase steels are rather prone to poor drawability or press formability when yield ratio increases. Also the bendability and Hole expansion are poor. There had been thus a need to improve these properties of steel along with required high strength for application in automobile components.

In JPH11350038A by the combination of the production conditions and the particular steel component obtained, it is found that it is possible to improve ductility and stretch flange formability in 980MPa grade high-tensile steel, with carbon equivalent Ceq = C + Mn / 6 + Si / 24 defined by the content of Mn and Si with a condition that it is from 0.40 to 0.52. It is defined in opened patent that Ceq is less than 0.4, it is difficult to ensure the required strength level. It results in cracking while bending due to poor bendability, shape of strip is very poor due to condition of hot rolling.

The present inventions aims at solving the problems of the prior art described above advantageously providing a cold rolled steel sheet capable of stably exhibiting uniform phosphate grain size with less coating weight, excellent shape, good bendability and excellent flatness with hole expansion ratio and a method for manufacturing the steel sheet through continuous annealing route with selective parameters.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide dual phase cold rolled steel sheet having Tensile strength 780 MPa or more with high yield strength ratio and a method of its manufacture through continuous annealing route to achieve desired characteristics including excellent bendability, Hole expansion ratio and surface properties.

A further object of the present invention is directed to provide dual phase cold rolled steel sheet having Tensile strength 780 MPa or more with high yield strength ratio having a selective composition wherein (Mn/Si) ratio is in a range of 5 to 15% for excellent combination of bendability and hole expansion ratio.

A still further object of the present invention is directed to provide dual phase cold rolled steel sheet having Tensile strength 780 MPa or more having the microstructure consisting of (Ferrite+Martensite+Bainite+Precipitates) and has a yield ratio > 0.60.

Another object of the present invention is directed to provide dual phase cold rolled steel sheet having Tensile strength 780 MPa or more comprising bake hardening index of atleast 30 MPa and Hole Expansion Ratio (HER%)= 30 % with aging guarantee of 6 months.

A further object of the present invention is directed to provide dual phase cold rolled steel sheet having Tensile strength 780 MPa or more involving a method wherein at soaking section temperature intercritical annealing results in ferrite and austenite microstructure which later transforms to ferrite + martensite or Ferrite+ Martensite + Bainite microstructure based on the cooling rate from slow cooling section to rapid cooling section inside continuous annealing line.

A still further object of the present invention is directed to provide dual phase cold rolled steel sheet having Tensile strength 780 MPa or more wherein the microstructure comprising in terms of area ratio, ferrite phase of 20 to 50%, total of martensite phase and or/bainite phase 40 to 70%, 10% or less of secondary phases and fraction of cementite less than 3%.
SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to provide high strength cold rolled dual phase steel sheets having composition comprising:
0.121wt % to 0.16wt% percent of Carbon;
1.6 wt% to 2.0 wt% of Manganese;
0.1 wt% to 0.4 wt% of Silicon;
0.02 wt% to 0.06wt% of Aluminum;
0.015wt% or less of Phosphorous;
0.02 wt% to 0.06wt% of Niobium;
0.0 wt% to 0.25wt% of Molybendum;
0.001 wt% to 0.003wt% of Calcium;
Up to 0.006wt% of Nitrogen; and
Balance is Fe and incidental impurities,
Wherein Mn/Si ratio is in a range of 5 to 15;
having tensile strength 780 MPa or more, with yield ratio 0.60 or more, and a dual phase microstructure having Martensite phase greater than 20%.

A further aspect of the present invention is directed to provide high strength cold rolled dual phase steel sheet having composition further comprising at least one element selected from the group comprising of V, Co, Ti, W and Zr, with weight percent 0.03% or less.

A still further aspect of the present invention is directed to provide high strength cold rolled dual phase steel sheet having composition further comprising of Cr such that 0.4 < (Cr+2Mo)<0.6.

A still further aspect of the present invention is directed to provide high strength cold rolled dual phase steel sheet having UTS = 780 MPa, YS/UTS ratio of at least 0.60, bake hardening index of at least 30 MPa and Hole Expansion Ratio (HER%)= 30 %.

Another aspect of the present invention is directed to a process for the manufacture of high strength cold rolled dual phase steel sheet comprising:
a. providing a selective steel composition for slab generation for desired formability and bake hardening index comprising:
0.121wt % to 0.16wt% percent of Carbon;
1.6 wt% to 2.0 wt% of Manganese;
0.1 wt% to 0.4 wt% of Silicon;
0.02 wt% to 0.06wt% of Aluminum;
0.015wt% or less of Phosphorous;
0.02 wt% to 0.06wt% of Niobium;
up to 0.25wt% of Molybdenum;
0.001 wt% to 0.003wt% of Calcium;
Up to 0.006wt% of Nitrogen; and
Balance is Fe and incidental impurities; wherein Mn/Si ratio is in the range of 5 to 15; and
ii) carrying out steel sheet manufacturing including hot rolling, pickling, cold reduction and continuous annealing such as to reach to hole expansion ratio 30 % or more and no visible crack when bended at a radius of 3T(where T is the sheet thickness).

Yet another aspect of the present invention is directed to said process comprising:
i. Hot rolling of said steel slab length with 8m or less with Roll diameter 770mm or less, slab reheating temperature 1180-1230 °C, Finishing Temperature 850°C to 910°C and hot coiled with ROT cooling rate in the range of 10°C/Sec to 15°C/Sec.
ii. Pickling of said steel to remove oxide layer built on surface of steel sheet and said steel is cold rolled with reduction of 30% to 60%.

A further aspect of the present invention is directed to said process comprising subjecting to continuous annealing involving:
a. Soaking said steel at temperature 800°C to 840°C for dual phase steel with residence time from 100 to 150 sec.
b. Slow cooling said steel at temperature 670°C to 730°C with slow cooling rate 0.5 °C/Sec to 2 °C/Sec;
c. Rapid cooling of said steel at rapid cooling rate of 28 °C/Sec to 40 °C/Sec to achieve Tensile Strength 780 MPa or more .
d. Over-aging said steel at 230°C to 300°C for 460 sec or less;
e. Skin passing of said steel at 0.2% to 0.6%.

A further aspect of the present invention is directed to said process wherein the above process steps are selectively controlled such as to achieve anyone or more of:
(i) Tensile strength 780 MPa or more;
(ii) Yield Strength at least 500 MPa with YS/TS ratio 0.60 or more.
(iii) Bake hardening Index 30 MPa or more;
(iv) Hole expansion Ratio 30% or more with aging guarantee of 6 months;
(v) obtaining microstructure comprising in terms of area ratio, ferrite phase of 20 to 50%, total of martensite phase and/or bainite phase 40 to 70%, 10% or less of secondary phases and fraction of cementite less than 3%.

The above and other objects and advantages of the present invention are described hereunder with reference to illustrative embodiments.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING ILLUSTRATIVE EXAMPLES

The present invention is directed to provide high strength cold rolled dual phase steel sheets having high Yield Ratio, excellent Shape, bendability and hole expansion ratio suitable for automotive components and a process for manufacturing the same.

With the aim of achieving low yield ratio Dual phase 780MPa strength level Non-aging cold rolled steel sheet, through continuous annealing route, effect of Metallurgical factors affecting the mechanical properties and microstructure are described in detail as follows:

Carbon (C: 0.121-0.16 wt %) - Carbon being the main alloying element improved hardenability significantly. All transformations are noticeably affected and by which the final microstructure and the mechanical properties are controlled. Carbon stabilizes the austenite which leads to the formation of martensite in the case of dual phase steels. However, other requirements such as spot weldability and formability limit the use of carbon to round about 0.16 mass %. To further improve the Hole Expansion ratio, carbon has been limited to 0.16 wt% or less in present inventive dual phase steel grade.

Manganese (Mn: 1.6-2.0 wt %)- Mn significantly improves the hardenability, hence, DP steel can be produced easily even by a simple air cooling. Also it assists fine dispersion of martensite phase which leads to higher tensile strength and good ductility. The addition of small amount of Si (<0.4 wt %) gives beneficial effects, the tensile strength further increases without a significant loss of ductility.

Higher Mn tends to form micro segregations during the steel casting process, i.e. the distribution of Mn in the slab will not be homogeneous. Since Mn lowers the AC1-temperature, the Mn-rich areas will start to transform to austenite prior to the surrounding areas with lower Mn-content. The consequence will be a structure of ferrite with the martensite phase to some extent distributed in bands, a so called banded structure. Also higher Mn wt% increases C equivalent value and deteriorates spot weldability. In addition, higher Mn wt% results in oxidized surface after continuous annealing having somewhat Yellow and Blue in color seriously damaging the surface and coating properties. To avoid the above inadequacy upper limit of Mn has been restricted to 2.0 wt% for present inventive grade.

Silicon (Si: 0.1-0.4 wt %) –Si being a ferrite stabilizer increases the strength of Ferrite phase and assists to increase the overall strength. However higher silicon content causes problems during hot rolling and coating due to formation of oxidized surface commonly known as Scale. For that reason upper limit of Si has been restricted to 0.4wt% or less.

Chromium and/or Molybdenum (Mo: Up To 0.25wt %) – Mo assists Mn in improving strength by improving Mn equivalent. Molybdenum is Ferrite stabilizer and in present invention is used to reduce and replace silicon, which may cause problems during hot rolling and coating. Molybdenum also reduces the annealing time in order to achieve dual phase structure. However, higher Mo content reduces the workability. Therefore upper limit should be 0.25 wt% or less. Addition of chromium should satisfy the condition 0.4 < (Cr+2Mo) <0.6 for better bendability.

Niobium (Nb: 0.02-0.06 wt %) - Niobium has a notable role on grain size development in conjunction with carbon enrichment, transformation mechanism of the austenite followed by nucleation of martensite which makes controlling the process parameter much easier, which further improve the mechanical property. To attain the explained benefits minimum amount of Nb which should be added is 0.02wt%. Nb more than 0.06 wt% unnecessarily adds up to the cost of production and increases YS/UTS ratio. Hence, upper limit for present inventive DP grade is 0.06wt%.

Calcium (Up to 0.003Wt%)– Calcium is used as inclusion modifiers, adding Calcium for reduction of sulphide and oxide inclusion, also modifies shape and size of Inclusion, helps the material not to Crack because of inclusion, calcium is added up to 0.003Wt%.

Complete Description of process:
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 length 8m or less for better roll-ability at hot rolling mill and for better shape and hot rolled with slab reheating temperature in the range of 1180°C to 1230°C intended to control roughing mill delivery temperature under 1080°C and finishing mill entry temperature under 1050°C to check surface defects like rolled in scale. During hot rolling, finishing mill temperature range of 850°C to 910°C and run out table cooling rate from finishing mill to coiler between 100C/sec to 15 °C/Sec are maintained to achieve coiling temperature range of 530 °C to 590 °C to avoid poor flatness in Tail End 100 m of the coil. 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 30% to 50%.
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 0.5-5 0C/sec up to soaking section temperature. Soaking section temperature is maintained in the range of 790°C to 840°C based on the final microstructure and property requirement to achieve yield ratio atleast 0.60.

Annealing time in the range of 100 to 150 seconds gives desired results for 780Mpa or more tensile strength. At soaking section temperature intercritical annealing results in ferrite and austenite microstructure which later transforms to ferrite + martensite or Ferrite+ Martensite + Bainite microstructure based on the cooling rate from slow cooling section to rapid cooling section inside continuous annealing line. After soaking section steel strip passes through slow cooling section at cooling rate in the rage of 0.5 to 2 °C/sec. Slow cooling section temperature of 670 °C to 730°C was maintained. Following slow cooling section annealed strip sheet was rapid cooled at cooling rate 28 °C/sec or more up to rapid cooling section temperature of 360 °C or less to avoid pearlite formation and attain the desired strength of 780 MPa or more with high yield ratio. After rapid cooling section annealed strip was over aged keeping the over aging section temperature of 230°C-300 °C with residence time of 460sec or less, tempering the transformed strengthening phase (Martensite and/or Bainite). After over aging, Skin-pass elongation (Temper rolling) in the range of 0.3 % to 0.6% was applied to avoid yield point elongation.
Furthermore, Cold rolled dual phase 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 bake hardening in a tensile test: Tensile test specimen as per JIS Z2241 No.5 with 50mm gauge length 25mm width was and 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 hole expansion ratio: The hole expansion ratio (HER %) is significant to assess the stretch flangeability of steel sheets. It is acquired by the hole expansion test utilizing conical or cylindrical punch in forming test machine. Whole expansion tests were performed as per ISO 16630-2009 utilizing forming test machine. Samples having a pouched hole of 10mm diameter were used for the test. Conical punch having an angle of 600 and cylinder diameter 50 mm was used. The punching speed of the conical punch during hole expansion was 0.3 mm/s. The conical punch was moved up against the sample with 10mm hole until the small crack appeared at the edge of hole and detected by optical instrument. The final average diameter of the hole after the small crack appeared was determined by measuring in two directions. Test were repeated for four to five times for each steel numbers and average HER% was taken with the following standard equation -
HER% = [(Df - Do) / D0]X 100
Where Do = Initial hole diameter, Df = final hole diameter before crack

Method for evaluating accelerated aging resistance: Tensile test specimen as per JIS Z2241 No.5 with 50mm gauge length 25mm width was and prepared across the rolling direction of steel sheet. To simulate the aging resistance tensile test specimens were immersed in oil bath which was homogeneously maintained at 1000C for 6 hours. Subsequently samples were tested at strain rate of about 0.008/second. Aged samples which showed Yield point elongation after tensile test does not comply with aging resistance of atleast 6 months.

Various trials conducted under the present invention for developing the claimed high strength cold rolled dual phase steel sheet with high yield ratio, the compositions, process parameters and properties achieved are presented in following Tables 1 to 3.

Table 1 -Elemental Compositions in weight % of the inventive steel sheets along with comparative example and their respective values of Eq1 = Mn/Si

Table 2- Hot rolling, cold rolling, annealing parameters of inventive and comparative steel sheets having chemical compositions as per table 1.

Table 3 – Mechanical properties, surface phosphatibility properties, Hole expansion ratio, Accelerated Aging property of inventive and comparative steels having chemical composition as per table 1 and being processed as per table 2.
Table 1:
Steel .No C Mn S P Si Al N Ti Nb Mo Ca Other Elements Eq1= Remarks
Eq2=
wt% wt% wt% wt% wt% wt% wt% wt% wt% wt% Wt% wt% Mn/Si Ratio 0.4 < (Cr+2Mo)<0.6
1 0.122 1.91 0.003 0.011 0.25 0.042 0.0045 0.005 0.06 0 0 Cr-0.46 7.67 0.46 EX
2 0.15 1.7 0.004 0.013 0.13 0.05 0.0054 0.008 0.04 0.2 0 Cr-0.2 13.08 0.6 EX
3 0.17 2.3 0.005 0.019 0.1 0.06 0.0048 0.06 0 0.5 0 0 23 1 com
4 0.13 2 0.004 0.015 0.4 0.04 0.005 0.005 0.02 0.25 0.002 Zr-0.02 5 0.5 EX
5 0.1 1.6 0.003 0.02 0.55 0.06 0.0042 0.01 0.03 0 0 0 2.90909 0 com
6 0.14 1.8 0.004 0.015 0.3 0.04 0.0045 0.006 0.07 0.1 0.001 0 6 0.2 com

* Ex. - Present inventive example, Comp.- Comparative Examples
** Shaded and underlined boxes indicates “outside the appropriate range
*** Eq1 = Mn/Si Ratio
Table 2:
Steel. Slab Length Roll Dia FT ROT cooling SS Annealing Time, 0C SCS RCS RCS OAS Remarks
No 0C rate, 0C/sec 0C 0C 0C Cooling Rate, 0C
m
0C
1A 8 763 865 13.7 820 133 681 363 28 255 Ex
1B 8 763 865 13.5 792 135 693 303 34 265 Ex
2 8 763 880 14.2 810 135 700 363 30 280 Ex
3 10 818 896 12.5 810 132 710 340 34 280 Com
4a 8 818 890 13 780 142 670 380 23 290 Com
4b 8 818 890 13 820 126 680 340 31 260 EX
4c 8 818 890 13 850 126 700 380 30 290 Com
5 8 818 910 12.7 800 132 710 360 32 300 Com
6 8 818 885 12.8 785 131 680 400 26 310 Com

Note: Steel marked as 1A, 1Bhave the same chemical composition as steel number 1, and however they are processed at different hot rolling, cold rolling and continuous annealing conditions to validate the claimed process.

* Ex. - Present inventive example, Comp. - Comparative Examples
* FT- hot finish rolling temperature ,ROT- Run out table at hot strip mill , SS- soaking section ,SCS- Slow cooling section , RCS- Rapid cooling section , OAS- Overaging section , SPM- Skin pass elongation ** Shaded boxes indicates “outside the appropriate range”
Table 3 :
Steel. No YS, UTS, YS/UTS El% BH Index, Hole Expansion,% Flatness Aging Remarks Bendability Phosphatability
MPa MPa MPa Remark No visible Cracks
1A 540 820 0.66 20.2 49 45 O O O(3T) O
1B 670 1003 0.67 16.9 38 36 O O O(3T) O
2 570 830 0.69 21.5 50 34 O O O(3T) O
3 600 940 0.64 16 25 30 ? O O(3T) ?
4a 490 760 0.65 20 45 35 O O O(3T) O
4b 520 840 0.62 21 50 35 O O O(3T) O
4c 450 770 0.56 23 50 40 O O O(3T) O
5 430 750 0.57 26 45 45 O O O(3T) ?
6 610 810 0.75 23 20 25 O O O(3T) O

* Shaded and underlined boxes indicates “outside the appropriate range”
** Steel with Flatness Remark and aging remark as “O” fulfill Bendability (90° V – Bend Test with 3T (Thickness), Flatness and accelerated aging resistance requirement.
**Steels with Phosphatibility and Flatness remark “?” do not fulfill

Example 1- In sample 1a and 1 b where same chemical composition is kept and processed at different soaking temperature 820°C and 792°C and tensile strength achieved is 820 and 1003MPa. Varying soaking section temperature from 790 to 840 provides the desired property of 780MPa min.

Example2-In sample 1 and sample 3 where C % is 0.122 and 0.17% respectively where tensile strength achieved is 820 and 940 Mpa, but Mn/si ratio for sample 1 and 3 are 7.67 and 23 where phosphatabiltiy results were found poor for sample 3 due to higher Mn/Si ratio and in sample 5 where Mn /Si ratio is 2.9 phosphatability is poor, hence to achieve better phosphatability results Mn/Si ratio to be maintained 5 to 15.
Similarly in sample 3 where Ti is 0.06 and in sample 1 Ti is 0.005 where BH index achieved is 49Mpa and 25Mpa to achieve desired BH index of 30Mpa min Ti to be maintained in range of 0.005 to 0.03Max.

Example 3 – In sample 4 a,b and c where same chemical composition is kept and processed at different SS temperature of 780, 820 and 850°C with RCS temperature varying 380 340 370°C where tensile strength achieved were 760Mpa,840Mpa and 800Mpa, but Ys/Ts ratio is 0.56 in sample 4c with SS temperature 850°C, hence to achieve YS/TS ratio >0.6 SS temperature to be restricted to 840°C max; and at lower SS temperature of 760°C, tensile strength achieved is less than 780Mpa, thus minimum soaking temperature required is 790°C ,hence to achieve desired property of 780Mpa minimum and YS/TS ratio of 0.6min soaking section temperature range to 790°C to 840°C.

Example 4-In sample 4 and 6 where Nb are 0.02 and 0.07 and YS/TS ratio are 0.64 and 0.75 ,having higher YS/TS ratio will affect during component manufacturing creating cracks to avoid that YS/TS ratio to be maintained optimum of 0.6 to 0.7 range ,hence niobium greater than 0.06 increase YS which is evident in sample 4 b) where YS is 420Mpa and in sample 6 YS is 610MPa; if Niobium is increased, yield strength is increased drastically; Higher the YS/TS ratio, lower is hole expansion ratio; in sample 6 hole expansion ratio is 25 compare to sample 4b having 35, so to maintain YS/TS ratio niobium to be maintained is range of 0.02wt% to 0.06wt%.

It is thus possible by way of the present invention to provide 780 MPa Tensile strength level dual phase cold rolled steel sheet with high yield strength ratio and its method of manufacture through continuous annealing route to achieve desired characteristics including excellent bendability, Hole expansion ratio, surface property, bake hardening index and with aging resistance of atleast 6 months. The present invention is directed to provide a steel grade having selective composition wherein (Mn/Si) ratio is in a range of 5 to 15% for excellent combination of bendability and hole expansion ratio. Cold rolled steel with minimum 780 MPa UTS produced according to present invention through selective process steps with controlled parameters has microstructure consisting of (Ferrite + Martensite + Bainite+Precipitates) and has a yield ratio > 0.60. The advancement is specifically directed to provide steel sheets suitable for application in automotive structural parts, pillars and rails, body structures, reinforcements and brackets, bumper-reinforcement beam.

,CLAIMS:We Claim:
1. High strength cold rolled dual phase steel sheet composition comprising:
0.121wt % to 0.16wt% percent of Carbon;
1.6 wt% to 2.0 wt% of Manganese;
0.1 wt% to 0.4 wt% of Silicon;
0.02 wt% to 0.06wt% of Aluminum;
0.015wt% or less of Phosphorous;
0.02 wt% to 0.06wt% of Niobium;
0.0 wt% to 0.25wt% of Molybendum;
0.001 wt% to 0.003wt% of Calcium;
Up to 0.006wt% of Nitrogen;
Balance as Fe and incidental impurities, having tensile strength 780 MPa or more with high yield ratio, whereas Mn/Si must be in a range of 5 to 15 and dual phase microstructure having Martensite phase greater than 20%.

2. High strength cold rolled dual phase steel sheet composition as claimed in anyone of claims 1 further including in mass % at least one element selected from the group comprising of V, Co, Ti, W and Zr such that each element weight percent is 0.03% or less.

3. High strength cold rolled dual phase steel sheet composition as claimed in anyone of claims 1 to 2 further comprising of Cr such that 0.4 < (Cr+2Mo)<0.6.

4. High strength cold rolled dual phase Steel sheet as claimed in anyone of claims 1 to 3, having UTS = 780 MPa ,YS/UTS ratio of said steel sheet is atleast 0.60, bake hardening index is atleast 30 MPa and Hole Expansion Ratio (HER%)= 30 %.

5. A process for the manufacture of high strength cold rolled dual phase steel sheet as claimed in anyone of claims 1 to 4 comprising:
a. providing a selective steel composition for slab generation for desired formability and bake hardening index comprising:
0.121wt % to 0.16wt% percent of Carbon;
1.6 wt% to 2.0 wt% of Manganese;
0.1 wt% to 0.4 wt% of Silicon;
0.02 wt% to 0.06wt% of Aluminum;
0.015wt% or less of Phosphorous;
0.02 wt% to 0.06wt% of Niobium;
0.0 wt% to 0.25wt% of Molybendum;
0.001 wt% to 0.003wt% of Calcium;
Up to 0.006wt% of Nitrogen;
Balance as Fe and incidental impurities such as to maintain Mn/Si must be in a range of 5 to 15 and
b) carrying out steel sheet manufacturing including hot rolling, pickling, cold reduction and continuous annealing such as to reach to hole expansion ratio 30 % or more and no visible crack when bend at 3T.
(T-Thickness)

6. A process as claimed in claim 5 comprising:
i. Hot rolling of said steel slab length with 8m or less with Roll diameter 770mm or less, slab reheating temperature 1180-1230 °C, Finishing Temperature 850°C to 910°C and hot coiled with ROT cooling rate in the range of 10°C/Sec to 15°C/Sec;
ii. Pickling of said steel to remove oxide layer built on surface of steel sheet and said steel is cold rolled with reduction 30% to 60%.

7. A process as claimed in claim 5 or 6 further comprising:
I. Soaking said steel at temperature 800°C to 840°C for dual phase steel with residence time from 100 to 150 sec;
II. Slow cooling further said steel at temperature 670°C to 730°C with slow cooling rate 0.5 °C/Sec to 2 °C/Sec;
III. Rapid cooling of said steel at rapid cooling rate of 10 °C/Sec to 15 °C/Sec to achieve Tensile Strength 780 MPa or more and Rapid cooling of said steel at rapid cooling rate 28 °C/Sec to 40 °C/Sec to achieve Tensile Strength 980 MPa or more;
IV. overaged the said steel 230°C to 300°C for 460 sec or less;
V. Skin passing of said steel 0.2% to 0.6%.

8. A process as claimed in anyone of claims 5 to 7 for producing said steel sheet having Phosphatability, hole expansion ratio and better shape, wherein the above process steps are selectively controlled such as to achieve anyone or more of:
I. Tensile strength 780 MPa or more;
II. Yield Strength at least 500 MPa with YS/TS ratio 0.6 or more.
III. Bake hardening Index 30 MPa or more;
IV. Hole expansion Ratio 30% or more with aging guarantee of 6 months;
V. Bendability by way of 3T bend test Pass with no visible cracks;
VI. Microstructure having in terms of area ratio, ferrite phase of 20 to 50%, total of martensite phase and or/bainite phase 40 to 70%, 10% or less of secondary phases and fraction of cementite less than 3%.

Dated this the 10th day of April, 2018
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)
IN/PA-199