DESC:FIELD OF THE INVENTION
The present invention relates to 1000 MPa Tensile strength level dual phase cold rolled steel sheet and its method of manufacture with desired characteristics including excellent spot weldability and yield ratio less than 0.60. The development is further directed to dual phase steel having excellent Hole expansion ratio, Phosphatibility and surface properties. The advancement is specifically directed to provide a steel grade suitable for automotive structural parts, pillars and rails, body structures, reinforcements and brackets, bumper-reinforcement beam.

BACKGROUND OF THE INVENTION

It has been observed that in high strength dual phase steel having tensile strength 1000 Mpa or more, it is difficult to get excellent surface finish with uniform phosphate grain size and yield ratio less by the conventional process. It is also observed that there is huge Rolling load shoot up while hot rolling which resulted in poor shape in the size of 2.4 mmX1300 mm.

In JPH11350038A by the combination of the production conditions and the particular steel component has been made 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 is a condition that it is from 0.40 to 0.52. It’s defined in opened patent that Ceq is difficult to ensure the required strength level if it is less than 0.4. However with silicon more than 0.8 will results in high temperatures scale at Hot rolling and results as Rolled in scale and poor phosphatibility crystal size at the end product which is not acceptable.

The present inventions aims at advantageously solving the problems of the prior art described above and an object thereof is to provide a cold rolled steel sheet capable of stably exhibiting uniform phosphate grain size with less coating weight, i.e. being excellent phosphatability, and a method for advantageously manufacturing the steel sheet.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide high strength dual phase cold rolled steel sheet having tensile strength 1000 MPa or more with low yield ratio, excellent phosphatability and hole expansion ratio for automobile application and a process for its manufacture.

A further object of the present invention is directed to provide high strength dual phase cold rolled steel sheet wherein Carbon equivalent less than 0.3 is maintained with Ceq= Carbon Equivalent = [C] + [Si]/30 + [Mn+Cr] /20 + [V]/10 alongwith the desired tensile strength of 1000mpa, with silicon level less than 0.5 Wt % and Phosphorous less than 0.02Wt%, and wherein amount of chromium is added as replacement of higher silicon level which reduces annealing time in order to attain dual phase structure, enhances good surface property and hole expansion ratio.
A still further object of the present invention is directed to provide high strength dual phase cold rolled steel sheet wherein the problem of non uniform phosphate grain size is solved by proper selection of Mn and Si wt % and low yield ratio achieved by proper selection of Micro-alloying with elements like Ti and Nb.
A still further object of the present invention is directed to provide high strength dual phase cold rolled steel sheet wherein the problem of poor shape is resolved through maintaining slab length of 8 m or less and work roll diameter less than 770 mm.

Yet another object of the present invention is directed to provide high strength dual phase cold rolled steel sheet wherein 1000Mpa strength with yield ratio of 0.60 or less having excellent hole expansion ratio and phosphatibility have been achieved with chromium in range of 0.4% to 0.55%, Manganese in range of 1.7% to 2.4% and Silicon 0.2 to 0.5wt% further satisfying (Cr+Mn)/Si ratio 5 to 15.

A still further object of the present invention is directed to provide high strength dual phase cold rolled steel sheet wherein the composition further comprising titanium such that (Al+Ti)/N ratio to be 5 to 20 for achieving bake hardening of 40 MPa or more.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to a dual phase cold rolled steel sheet composition comprising:
0.121wt % to 0.16wt% percent of Carbon;
1.7wt% to 2.4wt% of Manganese;
0.2wt% to 0.5 wt% of Silicon;
0.02 wt% to 0.06wt% of aluminum;
0.02wt% or less of Phosphorous;
0.03wt% to 0.08wt% of Niobium;
0.4wt% to 0.55wt% of Chromium;
Up to 0.006wt% of Nitrogen; and
Balance as Fe and incidental impurities, wherein (Cr+Mn)/Si ratio 5 to 15,
having anyone or more of tensile strength 1000MPa or more , YS/TS ratio upto 0.65 and with hole expansion ratio 25 to 45 preferably atleast 25% and phosphatability including phosphate crystal size 2 to 3.5µm preferably 3.5µm or less and coating weight 2.5 g/m2 or less.

A further aspect of the present invention is directed to a dual phase cold rolled steel sheet composition comprising titanium wherein (Al+Ti)/N ratio to be 5 to 20 for better BH Index.

A still further aspect of the present invention is directed to a dual phase cold rolled steel sheet composition include in mass % at least one element selected from the group comprising of Sn, Ni, Ca, Mg, Co, As, Sb, W, V and Zr such that the total contents thereof is 0.05% or less.

A still further aspect of the present invention is directed to a dual phase cold rolled steel sheet composition wherein (Mn)/(C+S) ratio is in to be 10 or more.

Another aspect of the present invention is directed to a process for the manufacture of dual phase cold rolled steel sheet comprising:
i) providing a selective steel composition for slab generation for desired phosphatibility and hole expansion ratio comprising:
0.121wt % to 0.16wt% percent of Carbon;
1.7wt% to 2.4wt% of Manganese;
0.2wt% to 0.50wt% of Silicon;
0.02 at% to 0.06 wt% aluminium;
0.02wt% or less of Phosphorous;
0.03wt% to 0.08wt% of Niobium;
0.4wt% to 0.55wt% of Chromium;
Up to 0.006wt% of Nitrogen; and
Balance as Fe and incidental impurities such as to maintain (Cr+Mn)/Si ratio 5 to 15, and
ii) carrying out steel sheet manufacturing including hot rolling, pickling, cold reduction and continuous annealing such as to reach to phosphatibility including phosphate crystal size 2 to 3.5 preferably 3.5 µm or less and coating weight 2.5 g/m2 or less and hole expansion ratio 25 to 45 preferably atleast 25%.

Yet another aspect of the present invention is directed to said process comprising:
a) Hot rolling of said steel slab length with 8m or less with Roll diameter to be770mm or less, Finishing Temperature 840 to 900°C preferably 900°C or less and coiled with ROT cooling rate 12 °C/Sec to 16°C/Sec preferably 12°C/Sec or more; and
b) Pickling of said steel to remove oxide layer built on surface of steel sheet and said steel is cold rolled with reduction 35% to 50% preferably 50% or less.

A further aspect of the present invention is directed to said process further comprising:
a. Soaking said steel at temperature 760 to 800°C preferably 800°C or less for Residence time preferably 170 Sec or less;
b. Slow cooling further said steel at temperature 690 to 720°C preferably 720 °C or less with slow cooling rate 0.3°C/Sec to 1°C/Sec preferably 0.3 °C/Sec or more;
c. Rapid cooling of said steel at rapid cooling rate of preferably 11 °C/Sec or more ;
d. overaged the said steel 260°C to 300°C preferably 300°C or less for seconds preferably 570 sec or less;
e. Skin passing of said steel 0.2 to 0.6 preferably 0.6% or less and
f. Wherein Rapid cooling temperature is obtained following
log10 [(SCS-RCS) /20] = 4.6 - 2.2*[%Cr] -1.7* [%Mn].

A still further aspect of the present invention is directed to said process wherein (Mn)/(C+S) ratio is maintained at preferably 10 or more.

Yet another aspect of the present invention is directed to said process for producing said steel sheet having phosphatibility, hole expansion ratio and better shape, wherein the above process steps are selectively controlled such as to achieve anyone or more of:
c) Tensile strength 1000MPa or more with YS /TS ratio 0.65 or less;
d) Bake hardening Index 40Mpa or more;
e) Hole expansion Ratio 25% or more with aging guarantee of 6 months;
f) Phosphate crystal size 3.5µm or less and coating weight 2.5 g/m2 or less; and
g) Volume fraction of 25% martensite or other strengthening phase distributed in soft polygonal ferrite matrix having average ferrite grain size less than 5µm and volume fraction more than 35%.

The objects and advantages of the present invention are described hereunder in greater detail in relation to the following non-limiting exemplary illustrations including the following accompanying figures wherein:

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Figure 1: Showing graphically the effect of (Al+Ti)/N ratio on BH index.
Figure 2: Showing graphically the effect of (Cr+Mn)/Si ratio on Phosphatibility.
Figure 3: Showing graphically the effect of Mn/(C+S) ratio on Hole Expansion Ratio.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

The present invention is directed to provide high strength dual phase cold rolled steel sheet having tensile strength 1000 MPa or more with low yield ratio, excellent phosphatability and hole expansion ratio for automobile application and a process for its manufacture.

Following abbreviations for related terminologies are used to describe the present invention:
RDT- Roughing Mill Delivery Temperature
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 %
YP- Yield Point
YPE- Yield Point Elongation %
El%-Total Elongation at 50mm gauge length sample

With the aim of low yield ratio Dual phase 1000 MPa strength level Non-aging 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 provides require tensile strength significantly. During annealing when the steel strip is hold at two phase region ferrite + austenite, carbon diffuses from ferrite and accommodates more amount of austenite raising the carbon content of austenite region more than nominal base percentage of the element. As carbon is austenite stabilizer and in turn it transforms to martensite because of rapid cooling. The minimum amount of carbon required to achieve strength of 1000mpa is 0.121 and upper limit is restricted to 0.16 considering the other important requirement while component making where hole expansion ratio, spot weldability and formability will be affected by increase in percentage of volume fraction of martensite so percentage of carbon is restricted to 0.160. To obtain the desired tensile strength level optimum range of carbon required 0.121 to 0.16, in addition to achieve hole expansion ratio it should satisfy Mn/(C+S) ratio to be 10 or more as shown in Fig3.

Manganese (Mn: 1.7-2.4 wt %) -Mn improves the Tensile strength of the material ,as manganese is also austenite stabilizer and increases the volume fraction of martensite, DP steel can be produced easily even by a rapid cooling. Also it assists fine dispersion of martensite phase which leads to higher tensile strength and good ductility. Manganese highly beneficial for grain refinement and increases hardenability and decreases Ar3 temperature , ferrite formation start temperature during cooling, Higher manganese contents promotes segregation and undesired banded microstructures, upper limit of manganese is restricted to 2.4 Wt% for present inventive grade .

Silicon (Si: 0.2-0.5wt %) –Addition of silicon in high strength dual phase steel improves ductility and balances the strength .silicon accelerates ferrite recrystallization , increases the strength of Ferrite phase and assists to increase the overall strength. Minimum 0.2 % of silicon exhibits high volume of carbon containing phase, with finer grains and large outline area thus greater surface energy supporting the accelerating ferrite recrystallization, in turn increasing the strength of the material. However higher silicon content causes oxide formation which affects rolling and surface of the material during coating, hence upper limit is restricted to 0.5wt% or less, the ratio of (Cr+Mn)/Si ratio to be 5 to 15 to attain better phosphatibility as per Fig 2.

Chromium (Cr: 0.4-0.55 wt %) – Cr assists Mn in improving strength by improving Mn equivalent. Chromium is Ferrite stabiliser and in present invention is used to reduce and replace silicon, which may cause problems during hot rolling and coating. Chromium also reduces the annealing time in order to achieve dual phase structure and to do so minimum amount of Cr must be >0.4 wt% more preferably >0.5 wt%. However Higher Cr content reduces the workability, Therefore upper limit should be 0.55 wt% or less.

Niobium (Nb: 0.03-0.08 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 0.03 wt%. Nb more than 0.08 wt% unnecessarily adds up to the cost of production and increases YS/UTS ratio. Hence upper limit for present inventive DP grade in 0.08 wt%.

Titanium (Ti: 0.01-0.03 wt %)- Ti acts as precipitate strengthener and retards austenite recrystallization and grain growth , it combine nitrogen forms nitrides at high temperature prevents grain growth and also helps in getting aging resistance. Formability of steel sheet improves by reducing solute N in solution with Ti instead of Al. And so, Amount of Ti preferably added should be 0.01 wt% or more. However, when Ti contents exceeds 0.03 wt%, bake hardening effect will be very less to achieve minimum BH of 40 Mpa, the amount of Ti is made to be 0.03% or less, wherein (Al+Ti)/N ratio to be 5 to 20 to achieve BH index minimum 40 as shown in Fig 1.

Sn,Ca,V,Ni,Mg,Sb,Zr,As,Co and W in the range of 0.002 to 0.05 % - each of from Sn,Ca,V,Ni,Mg,Sb,Zr,As,Co and W act as carbide former and/or nitride former and/or solid solution strengthening elements, however adding each of these elements in an amount more than 0.05 wt% unnecessarily adds up to the cost of the steel.

Description of Process of production–

Slabs of length 8 m or less for better roll ability at hot rolling mill are casted with the chemistry as stated above and hot rolled with slab reheating temperature of below 1220°C to attain roughing mill delivery temperature below 1090°C, to prevent the surface defects like rolled in scale and to prevent buckling shape related defects in tail end of the coil, finishing temperature of 860°C to 900°C cooled in ROT with cooling rate of 12°C /Sec and coiled is set to increase the volume fraction of martensite and processed through pickling coupled with tandem cold rolling mill, to remove the oxide surface present in the surface and cold reduction of 50% or less with the five stands reduction percentage.

Pickled and cold rolled steel strip is processed through continuous annealing line, where electrolytic cleaning 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 is heated 800°C or less with residence time of 160 sec or more where sufficient time is given for the formation of austenite so that volume fraction increases and slow cooled with 3°C/S or less with temperature of 720°C and then rapidly cooled with 11°C/S or more with rapid cooling temperature of 330°C or less where phase change takes place as the rapid cooling rate increases volume fraction of martensite increase and increases the tensile strength directly. Then the steel strip is passed through the overaging section where the temperature is 280°C or less increase in overaging section decreases the tensile strength and passed through skin pass mill with 0.6% of elongation to suppress 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 Galvanized or Galvannealed steel sheets and used as coated product for similar applications.

The experimental trial data on the advanced steel and comparative steel grades are illustrated in the following Table 1 to 4

Table 1-Compositions of the invented DP steel sheets
Table 2- Hot rolling, cold rolling of inventive DP steel
Table 3- Annealing Parameters of inventive steel with different Cycle
Table 4 – Mechanical properties of inventive DP steels

Table 1- Chemical composition of advancement along with comparative steel
I- Inventive, C- Comparative

Steel Number one marked with 1 is inventive steel with low manganese and high Chromium and also satisfying (Cr+Mn)/Si ratio in range of 5 to 20 having good surface and excellent phosphatibility.
Steel number 5 with high Silicon is observed with High temperature scales and affects the surface property

Table 2- Hot rolling parameters and cold rolling reduction percentage of inventive steel
Steel Sample Slab Length (m) Roll Dia FT ROT °C/Sec CR Thk CR Red % Remarks
1 8 763 899 16 1.6 46
1 8 763 899 16 1.6 46
1 8 763 899 16 1.6 46
1 8 763 899 16 1.6 46
2 8 763 840 10 1.8 35.71
3 8 818 887 11 1.8 35.71 Heavy Load, Buckling at Tail End, High Roll dia
4 7 763 860 15 1.6 40
5 10 818 865 11 1.2 52 Heavy Load, Buckling at Tail End, Slab length >8
6 8 763 900 15 1.4 47
7 8 763 905 16 1.2 45
8 8 763 895 15 1.4 47
9 8 763 890 15 1.2 45
10 8 818 850 10 1.4 47 Heavy Load, Buckling at Tail End
11 12 763 850 10 1.2 45

FT – Finishing Mill Temperature, CT- Coiling Temperature,
CR Red –Cold Rolling Reduction %
Slab length and work roll diameter affects the temperature distribution along the length of the slab which affects the rolling ability during hot rolling. It is observed that roll diameter more than 770mm increases the rolling load and unable to control the shape of hot rolled sheet and resulted as buckling as in sample no 3. Also if slab length is more than 8 m then there is huge increase in rolling load at Tail end of hot rolled sheet which results in buckling at tail end as sample no 5.
Steel Numbers marked 1A, 1B, 1C and so forth in table 3 to table 4 are different coils having the same chemistry and Same Hot Rolling Parameters as steel 1 listed in Table 1 and Table 2 however the annealing parameters were varied for these coils to understand the impact of different annealing parameters on final properties.

Table-3 Annealing Parameter of Inventive steel with different Rapid cooling Temperature and cooling Rate
Steel No SS°C SCS°C RCS°C R/CR (°C/Sec) OAS°C Spm
1A 783 693 339 11.26 281 0.3
1B 782 695 284 13.06 271 0.18
1C 782 690 309 12.12 277 0.21
1D 795 705 355 8.9 284 0.32
2 807 685 395 10.35 239 0.11
3 803 687 393 10.5 240 0.1
4 800 691 311 13.51 281 0.23
5 830 - - - 250 0.2
6 782 690 300 17 277 0.21
7 796 692 311 16 295 0.3
8 794 700 297 18 260 0.3
9 770 702 326 16 270 0.41
10 780 687 350 15 320 0.3
11 800 700 380 14 240 0.2

SS- Soaking section, SCS- slow cooling section, RCS- Rapid cooling section, OAS-over aging section, SPM – Skin pass elongation, R/CR- Rapid Cooling Rate
Table 4- Mechanical properties and Surface Remarks of advancement and comparative steels
I- Inventive C- Comparative L- Longitudinal, Inventive Example- Excellent phosphatibility and Low Yield Ratio.
Ceq= Carbon Equivalent = [C] + [Si]/30 + [Mn+Cr] /20 + [V]/10
E1 = log10 [(SCS-RCS) /20] = E2 = 4.6 - 2.2*[%Cr] -1.7* [%Mn].

Example 1 –In steel sample 5 and 7 where carbon wt% are 0.16 and 0.122 where obtained tensile strength are 1123MPa and 1038Mpa as percentage of carbon increases, percentage of martensite also increases whereas in sample 10 where carbon is 0.11wt% obtained tensile strength is 966MPa, carbon is restricted to 0.121wt% and limit the YS/TS ratio 0.6 and higher level of carbon is restricted to 0.16 or less carbon.

Example 2 – In steel sample 4 and 6 where manganese wt % are 1.71 and 2.4 where tensile strength obtained are 1005Mpa and 1044MPa whereas in sample 11 where manganese 1.4Wt% tensile strength obtained is 970MPa, manganese lower limit is kept to 1.7wt% to obtain minimum strength of 1000Mpa and restricted to 2.4 Wt% to obtain better surface property, in sample 3 with manganese 3.1% phosphate crystal size obtained is 8 µm, poor phospatability, so manganese is kept in range of 1.7wt% to 2.4 wt% and to achieve this it should satisfy (Cr+Mn)/Si to be 5 to 20.

Example 3 – In steel sample 6 and 9 where silicon wt% are 0.2% and 0.44% where tensile strength obtained is 1044MPa and 1033MPa,where as whereas in sample 5 where silicon is 0.52 wt% of UTS obtained is very high of 1132Mpa where surface quality for phosphating was found to be poor ,silicon is restricted to 0.5% max.

Example 4 – In steel sample 6 and 8 where niobium wt% are 0.03 and 0.08 where yield strength obtained are 561Mpa and 610MPa as niobium increases yield strength increases , to obtain minimum yield strength of 550MPa niobium at lower limit is 0.03 and to restrict YS/TS ratio niobium at higher level is 0.08%.

Example 5 –In steel sample 1 where Rapid cooling temperature is varied 284°C to 355°C where tensile strength obtained is 1132Mpa and 968MPa, the rapid cooling rate to be 11°C /Sec to achieve tensile strength of 1000MPa or more.

Thus the above results go to confirm the present advancement as discussed above clearly directed to an improved steel chemistry and processing of low yield ratio dual phase steel for automobile structural parts, pillars and rails, bumper-reinforcement beams, door-intrusion beams, wheel and the like automotive components for crash energy absorption.

Phosphatibility 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 grains. Grain size < 3.5 µm is considered as excellent phosphatibility. The phosphate coating weight was measured using the XRF method.

SEM Micrograph obtained for phosphate samples confirms small and uniform Phosphate grain size after Phosphatibility test of continuously annealed Dual Phase steels having grain size 3.5 µm and Coating weight 2.6 g/m2 which indicates good phosphatibility in sample 4 having Mn 1.7 wt % . On the other hand SEM micrograph of sample 2 having Mn 3.1 wt% showing bigger and non-uniform phosphate grain after phosphatibility test was having grain size 8 µm and coating weight 3.1 g/m2 . With uniform and small phosphate grain size after phosphatibility results in excellent paint coating adherence and appearance where as compare to Non uniform and Large phosphate grain size after phosphatibility results in poor painting /coating appearance.

Crystal size results of sample 5 having 8.5µm where level of silicon is 0.5% is not good for phosphatibility as coarser grain size affects the adhesiveness of the paint coating ,where as in sample number 6 where silicon is 0.28% the crystal size is finer 3.3 µm which results for good phosphatibility.

As the scope of present invention intended for excellent surface properties critical for automotive material Mn and Si wt% has been restricted to 2.4 wt% or less and 0.3 wt% or less respectively with satisfying the ratio of (Cr+Mn)/Si to be 5 to 20.

Hole Expansion Test: The hole expansion ratio is a key indicator to evaluate stretch flanging performance of steel sheets, which is usually obtained by hole expanding test using cylindrical or conical punch. According to ISO 16630-2009(Metallic materials -- Sheet and strip -- Hole expanding test).

It is thus possible by way of the present invention to provide high strength(TS 1000MPa min) dual phase cold rolled steel sheet and its method of manufacture with desired characteristics including excellent spot weldability and yield ratio less than 0.60. The steel grade comprising selective composition and processing steps to ensure excellent phosphatibility and hole expansion ratio with aging guarantee of 6 months suitable for automobile application.

,CLAIMS:We Claim:

1. Dual phase cold rolled steel sheet composition comprising:
0.121wt % to 0.16wt% percent of Carbon;
1.7wt% to 2.4wt% of Manganese;
0.2wt% to 0.5 wt% of Silicon;
0.02 wt% to 0.06wt% of aluminum;
0.02wt% or less of Phosphorous;
0.03wt% to 0.08wt% of Niobium;
0.4wt% to 0.55wt% of Chromium;
Up to 0.006wt% of Nitrogen; and
Balance as Fe and incidental impurities, wherein (Cr+Mn)/Si ratio 5 to 15,
having anyone or more of tensile strength 1000MPa or more , YS/TS ratio upto 0.65 and with hole expansion ratio 25 to 45 preferably atleast 25% and phosphatability including phosphate crystal size 2 µm to 3.5 µm preferably 3.5µm or less and coating weight 2.5 g/m2 or less.

2. Dual phase cold rolled steel sheet composition comprising titanium wherein (Al+Ti)/N ratio to be 5 to 20 for better BH Index.

3. Dual phase cold rolled steel sheet composition as claimed in anyone of claims 1 or 2 including in mass % at least one element selected from the group comprising of Sn, Ni, Ca, Mg, Co, As, Sb, W, V and Zr such that the total contents thereof is 0.05% or less.

4. Dual phase cold rolled steel sheet composition as claimed in anyone of claims 1 to 4 wherein (Mn)/(C+S) ratio to be 10 or more.

5. A process for the manufacture of dual phase cold rolled steel sheet as claimed in anyone of claims 1 to 3 comprising:
i) providing a selective steel composition for slab generation for desired phosphatibility and hole expansion ratio comprising:
0.121wt % to 0.16wt% percent of Carbon;
1.7wt% to 2.4wt% of Manganese;
0.2wt% to 0.50wt% of Silicon;
0.02 at% to 0.06 wt% aluminium;
0.02wt% or less of Phosphorous;
0.03wt% to 0.08wt% of Niobium;
0.4wt% to 0.55wt% of Chromium;
Up to 0.006wt% of Nitrogen; and
Balance as Fe and incidental impurities such as to maintain (Cr+Mn)/Si ratio 5 to 15, and
ii) carrying out steel sheet manufacturing including hot rolling, pickling, cold reduction and continuous annealing such as to reach to phosphatibility including phosphate crystal size 2 µm to 3.5 µm preferably 3.5 µm or less and coating weight 2.5 g/m2 or less and hole expansion ratio 25 to 45 preferably atleast 25%.

6. A process as claimed in claim 5 comprising:
a) Hot rolling of said steel slab length with 8m or less with Roll diameter 770mm or less, Finishing Temperature 860°C to 900°C preferably 900°C or less and coiled with ROT cooling rate 12°C/Sec to 15°C/Sec preferably 12°C/Sec or more; and
b) Pickling of said steel to remove oxide layer built on surface of steel sheet and said steel is cold rolled with reduction 35% to 50% preferably 50% or less.

7. A process as claimed in claim 5 or 6 further comprising:
a. Soaking said steel at temperature 760°C to 800°C preferably 800°C or less for Residence time 170 Sec or less;
b. Slow cooling further said steel at temperature 690°C to 720°C preferably 720 °C or less with slow cooling rate 0.3 °C/Sec or more;
c. Rapid cooling of said steel at rapid cooling rate of 11 °C/Sec or more ;
d. overaged the said steel 260°C to 300°C preferably 300°C or less for 570 sec or less;
e. Skin passing of said steel 0.2% to 0.6% preferably 0.6% or less and
f. Wherein Rapid cooling temperature is obtained following
log10 [(SCS-RCS) /20] = 4.6 - 2.2*[%Cr] -1.7* [%Mn].

8. A process as claimed in anyone of claims 5 to 7 wherein (Mn)/(C+S) ratio is maintained 10 or more.

9. A process as claimed in anyone of claims 5 to 8 for said steel sheet having phosphatibility, hole expansion ratio and better shape, wherein the above process steps are selectively controlled such as to achieve anyone or more of:
a) Tensile strength 1000MPa or more with YS /TS ratio 0.65 or less;
b) Bake hardening Index 40Mpa or more;
c) Hole expansion Ratio 25% or more with aging guarantee of 6 months;
d) Phosphate crystal size 3.5µm or less and coating weight 2.5 g/m2 or less; and
e) Volume fraction of 25% martensite or other strengthening phase distributed in soft polygonal ferrite matrix having average ferrite grain size less than 5µm and volume fraction more than 35%.

Dated this the 14th day of September, 2016
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)