DESC:FIELD OF THE INVENTION

The present invention relates to 1000 MPa Tensile strength level dual phase cold rolled steel sheet with high yield ratio and its method of manufacture with desired characteristics including excellent bendability, Hole expansion ratio and surface properties. The advancement is specifically directed toapplication in automotive structural parts, pillars and rails, body structures, reinforcements and brackets, bumper-reinforcement beam.The high strength cold rolled dual phasesteel sheet with high yield ratio according to present invention having composition comprising elements in terms of mass percent: 0.121% to 0.16% of C, Si: 0.2% to 0.5%, Mn: 2% to 2.5%,N: 0.006% or less, Al: 0.02% to 0.06%,Nb: 0.061%-0.1%,Mo:0.25% or less Ca; 0.003% or less and the balance being Fe and other inevitable impurities, wherein the ratio of (Nb+Mo)/C is maintained in a range of 0.5 to 3 for excellent combination of bendability and hole expansion ratio.The steel sheet according to present invention is further having microstructure consisting of (Ferrite + Martensite + Bainite+Precipitates) and has a yield ratio> 0.65 to achieve the desired characteristics of steel sheets for the intended applications.

BACKGROUND OF THE INVENTION

With utilization of dual phase high strength steel automobile manufacturers are incorporating more high strength materials with UTS 980MPa 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 prone to rather poor drawability or press formability when yield ratio increases. Also, the required characteristics of bendability and Hole expansion are poor.

In JPH11350038A by the combination of the production conditions and the particular steel composition 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. It results in cracking while bending, bendability is poor, and shape of strip is very poor due to condition of hot rolling.

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, and advantageously having better Shape, good bendability and excellent flatness with hole expansion ratio and a method for manufacturing the non aging steel sheet through continuous annealing route.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide high strength high yield ratio cold rolled dual phase steel sheet with excellent shape, bendability, surface property, flatness, hole expansion ratio and a process for its manufacture.

A further object of the present invention is directed to provide high strength high yield ratio cold rolled dual phase steel sheet having selective composition wherein the ratio (Nb+Mo)/C is maintained in a range of 0.5 to 3 for desired excellent combination of bendability and hole expansion ratio and comprising of Cr such that 0.4 < (Cr+2Mo)<0.6 for better bendability.

A still further object of the present invention is directed to provide high strength high yield ratio cold rolled dual phase steel sheet having the composition wherein molybdenum is used as Ferrite stabilizer and to reduce and replace silicon, which may cause problems during hot rolling and coating.

A still further object of the present invention is directed to provide high strength high yield ratio cold rolled dual phase steel sheet having UTS = 1000 MPa, YS/UTS ratio of said steel sheet of at least 0.65, bake hardening index of atleast 30 MPa, Hole Expansion Ratio (HER%)= 30 % and no visible crack when bend at 90°,surface without buckling and waviness and aging guarantee of 6 months.

A still further object of the present invention is directed to provide high strength high yield ratio cold rolled dual phase steel sheet produced through continuous annealing route wherein processing parameters are selectively controlled such that microstructure achieved comprises 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%.

A still further object of the present invention is directed to provide high strength high yield ratio cold rolled dual phase steel sheet produced through a process with controlled slab length and roller diameter alongwith selective control on temperature of reheating androllingto check surface defects like rolled in scale and to avoid poor flatness in Tail End 100 m of the coil.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to high strength high yield ratio dual phase cold rolled steel sheet having composition comprising:
0.121wt % to 0.16wt% percent of Carbon;
2 wt% to 2.5 wt% of Manganese;
0.2 wt% to 0.5 wt% of Silicon;
0.02 wt% to 0.06wt% of Aluminum;
0.015wt% or less of Phosphorous;
0.061 wt% to 0.1wt% of Niobium;
0.0 wt% to 0.25wt% of Molybendum;
Up to 0.003wt% of Calcium;
Up to 0.006wt% of Nitrogen; and
balance is Fe and incidental impurities, wherein the ratio (Nb+Mo)/C is maintained in a range of 0.5 to 3, to have tensile strength 1000MPa or more with high yield ratio of atleast 0.65 with excellent bendability.

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

A further aspect of the present invention is directed to said high strength high yield ratio dual phase cold rolled steel sheet further comprising at least one element selected from the group comprising of Sc, V, Co, Cu, Zn, Sn, Ni,Ti, W, V and Zr such that each element weight percent is 0.03% or less.

A still further aspect of the present invention is directed to said high strength high yield ratio dual phase cold rolled steel sheet having the characteristics comprising
i. Tensile strength 1000MPa or more;
ii. Yield Strength at least 650 MPa with YS/TS ration 0.65 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 90° V bend Pass no visible cracks;
vi. surface without buckling and waviness; and
vii. 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%.

Another aspect of the present invention is directed to a process for the manufacture of said high strength high yield ratio dual phase cold rolled steel sheet as described above 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;
2 wt% to 2.5 wt% of Manganese;
0.2 wt% to 0.5 wt% of Silicon;
0.02 wt% to 0.06wt% of Aluminum;
0.015wt% or less of Phosphorous;
0.061 wt% to 0.1wt% of Niobium;
0.0 wt% to 0.25wt% of Molybendum;
Up to 0.003wt% of Calcium;
Up to 0.006wt% of Nitrogen;and
balance is Fe and incidental impurities such as to maintain (Nb+Mo)/C 0.5 to 3,
and
b. carrying out steel sheet manufacturing involving 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 90°.

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, Finishing Temperature 840°C to 900°C and hot coiled with ROT cooling rate in the range of 11°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%.

A further aspect of the present invention is directed to said process further comprising:
a. Soaking said steel at temperature 790°C to 840°C for dual phase steel with residence time from 100 to 170 sec.
b. Slow cooling further said steel at temperature 670°C to 730°C with slow cooling rate 0.2 °C/Sec to 2 °C/Sec;
c. Rapid cooling of said steel at rapid cooling rate of 30°C/Sec to 40 °C/Sec;
d. overaged the said steel 230°C to 300°C for 350 sec or more; and
e. Skin passing of said steel 0.3% to 0.6%.

A still further aspect of the present invention is directed to said process to ensure good 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 1000MPa or more;
ii. Yield Strength at least 650 MPa with YS/TS ratio 0.65 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 90° V bend Pass no visible cracks;
vi. Surface without buckling and waviness; and
vii. 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 in greater details with reference to following non limiting illustrative examples.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ILLUSTRATIVE EXAMPLES
The present invention is directed to provide high strength cold rolled dual phase steel sheets having very 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 1000 MPa strength level Non-aging cold rolled steel sheet, through continuous annealing route, effect of Metallurgical factors affecting the mechanical properties and microstructure are described hereunder in details:

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, carbon has been limited to 0.16wt% or less in present inventive dual phase steel grade.

Manganese (Mn: 2-2.5wt %) -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 (<0.5wt %) 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 Mnwt% increases C equivalent value and deteriorates spot weldability. In addition, Higher Mnwt% 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 2.5wt% for present inventive grade.

Silicon (Si: 0.2-0.5wt %) –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 0.5wt% or less.

Chromium or/ and 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 0.25wt% or less. Addition of chromium should satisfy 0.4 < (Cr+2Mo)<0.6 for better bendability.

Niobium (Nb: 0.061-0.1wt %) - 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 0.061wt%. Nb more than 0.1wt% unnecessarily adds up to the cost of production and increases YS/UTS ratio. Hence, upper limit for present inventive DP grade in 0.1wt%.For Better, Weldability (Nb+Mo)/C to be 0.5 to 3.

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%.

Description of the process of manufacture-
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 lessfor better roll ability at hot rolling mill and for better shape and hot rolled with slab reheating temperature in the range 1180°C to 1240°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 110C/sec to 15 °C/Sec was 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 60%.

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 was maintained in the range of 790 °C to 840 °C based on the final microstructure and property requirement to achieve yield ratio atleast 0.65.

Annealing time in the range of 100 to 170 seconds of high strength steels with 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 0.2 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 30 °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 1000 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 of 350sec or more 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 6months.

The chemical composition, processing parameters and the properties achieved for the inventive grade and comparative grades obtained under various trials according to present invention are presented in the following Tables 1 to 3.

Method for evaluating bending test: Bend test specimen is prepared with 20*50mm as per JIS Z2248 by milling to avoid any burr. To achieve bend radius of 1mm with 90 ° angular die is fixed in hydraulic bend tester, prepared specimen is kept on the die and load is applied, Ram presses the samples and bends it. To judge visible cracks magnifying lens is used to see any visible cracks and later scanning electron microscopy is used to identify any visible crack at 10X to 100X if there is no visible cracks seen and then it declared to be pass and vice versa.
Method of evaluating phosphatability –
Phosphating process gives reasonably hard, electrically non-conducting surface coating of insoluble phosphate. The coating is adjacent and highly adherent to the underlying metal substrate. Also, it is considerably more absorptive than the metal providing metal surface an excellent corrosion resistance and paint ability. The coating is formed on steel surface by top chemical reaction, causing the surface of the steel to incorporate itself to be a part of the corrosion resistant film.

To evaluate phosphatability firstly alkali degreasing was performed on steel sheet at 400 C for 120 sec using FC-E2032 chemical manufactured by NIHON PARKERIZING India Pvt Ltd to the obtained cold rolled steel sheet without any oil/grease on surface. Degreasing was followed by water rinsing and then surface conditioning at room temperature for 30 seconds using PL-Z chemical manufactured by NIHON PARKERIZING India Pvt Ltd. Phosphate treatment using PB-L3020 chemical, manufactured by NIHON PARKERIZING India Pvt was done at 400 C for 90 seconds. Subsequently, the surface after phosphate treatment was observed under a Scanning electron microscope using Secondary Electron image mode. Average grain size was measured assuming circular phosphate crystals. Crystal size < 4µm is considered as excellent for phosphatability. The phosphate coating weight was measured using the XRF method and steel sheet with average coating weight after zinc phosphate chemical conversion coating of 1.5-3 g/m2 is considered having excellent phosphatability.

Table 1 -Elemental Compositions in weight % of the inventive steel sheets along with comparative example and their respective values of Eq1 = (Nb+Mo)/C.
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 phosphatability 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% (Nb+Mo)/C 0.4 < (Cr+2Mo)<0.6
1 0.124 2.42 0.0025 0.015 0.418 0.047 0.0054 0.018 0.085 0.1 0.0025 Cr: 0.25 ,W: 0.004,Cu:0.015 1.5 0.45 Ex.
2 0.12 2.01 0.0029 0.02 0.249 0.042 0.004 0.021 0.075 0.2 0.003 Co:0.004,Cr -0.15 2.3 0.55 Ex.
3 0.11 1.8 0.002 0.011 0.8 0.02 0.006 0.005 0.049 - - Cr -0.015 0.58 0.015 Comp
4 0.16 2.1 0.0025 0.015 0.418 0.047 0.0054 0.018 0.085 0 0 Cr: 0.45 1.5 0.45 Ex.
* Ex. - Present inventive example, Comp.- Comparative Examples
** Shaded and underlined boxes indicates “outside the appropriate range
*** Eq1 = (Nb+Mo)/C

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 8 763 870 13.5 800 134 680 315 35 255 Ex
1B 8 763 865 12.3 810 134 700 315 37 255 Ex
1C 8 763 865 12.7 790 133 700 310 37.4 250 Ex
1D 8 763 860 11.2 830 142 700 385 27 250 Comp
2A 8 763 860 13.5 795 142 685 340 33 260 Ex
2B 8 763 865 13.7 790 133 695 300 37 265 Ex
3 10 818 896 12.5 780 140 687 350 33 320 Comp
4 12 818 865 15 830 150 700 310 32 250 Ex

Note: Steel marked as 1A, 1B, 1C, 1D have 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. Similarly steel number 2A and 2Bhave the same chemical composition as steel number 2.

* 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,% Phosphatability Flatness
Remark Aging Remarks Bendability
No visible Cracks
MPa MPa MPa
1A 789 1096 0.72 13.4 40 40 O O O O
1B 750 1120 0.67 13 45 30 O O O O
1C 755 1130 0.67 12.7 60 35 O O O O
1D 632 938 0.69 16 60 45 O O O O
2A 710 1008 0.7 13 53 40 O O O O
2B 703 1003 0.7 14 35 45 O O O O
3 555 968 0.7 17.9 100 50 ? ? ?
4 678 1180 0.58 12 40 35 O O 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),Flatness and accelerated aging resistance requirement.
**Steels with aging remark “?”do not fulfill the accelerated aging requirement as the YPE observed after accelerated aging test.
Example 1 –Sample 1a to 1d with similar chemistry and processed with different process parameter with soaking section temperature and Rapid cooling section temperature , sample 1A processed with 780°C and RCS with 315°C (cooling rate of 35°C/Sec) where UTS of 1096MPa is achieved whereas in sample 1d where SS temperature is 830°C and RCS with 380°C(Cooling rate of 28°C/Sec) where UTS is 938MPa less than desired property, to achieve UTS 1000MPa or more cooling rate at rapid cooling section to be maintained 30°C/Sec or more.
Example 2 – Sample 1 where Carbon is 0.124% and sample 3 carbon is 0.11% , Mechanical property achieved in each sample are 1096MPa and 938MPa respectively, require carbon percentage to achieve minimum 1000Mpa or more is 0.121% of minimum Carbon is required.
Example 3- Sample 3 where si is 0.8% where coating weight is 3.5g/m2 and crystal size is 5µm , results in poor phosphatability , to achieve better surface si 5 to be maintained between 0.2% to 0.5% so that coating weight can be achieved between 1.5-3g/m2 and crystal size <4µm.

It is thus possible by way of the present invention to provide 1000 MPa Tensile strength level dual phase cold rolled steel sheet with high yield ratio and its method of manufacture with desired characteristics including excellent bendability and Hole expansion ratio and surface properties. The steel sheets according to present invention comprising chemical elements in terms of mass percent: 0.121% to 0.16% of C, Si: 0.2% to 0.5%, Mn: 2% to 2.5%,N: 0.006% or less, Al: 0.02% to 0.06%, Nb: 0.061%-0.1%, Mo:0.25% or less Ca; 0.003% or less and the balance being Fe and other inevitable impurities, wherein (Nb+Mo)/C is in a range of 0.5 to 3 for excellent combination of bendability and hole expansion ratio and contains Cr such that 0.4 <(Cr+2Mo)<0.6 for better bendability. The cold rolled dual phase steel sheet is having minimum 1000 MPa UTS with high yield ratio of > 0.65 and has a microstructure consisting of (Ferrite + Martensite + Bainite+Precipitates) to achieve the desired characteristics to suit intended applications specifically automotive structural parts, pillars and rails, body structures, reinforcements and brackets, bumper-reinforcement beam etc.
,CLAIMS:We Claim:

1. High strength dual phase cold rolled steel sheet having composition comprising:
0.121wt % to 0.16wt% percent of Carbon;
2 wt% to 2.5 wt% of Manganese;
0.2 wt% to 0.5 wt% of Silicon;
0.02 wt% to 0.06wt% of Aluminum;
0.015wt% or less of Phosphorous;
0.061 wt% to 0.1wt% of Niobium;
Up to 0.25wt% of Molybendum;
Up to 0.003wt% of Calcium;
Up to 0.006wt% of Nitrogen; and
balance is Fe and incidental impurities, whereinthe ratio (Nb+Mo)/C is maintained in a range of 0.5 to 3, to have tensile strength 1000MPa or more with high yield ratio of atleast 0.65 with excellent bendability.

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

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

4. High strength high yield ratio dual phase cold rolled steel sheet as claimed in anyone of claims 1 to 3 having the characteristics comprising
ii. Tensile strength 1000MPa or more;
ii. Yield Strength at least 650 MPa with YS/TS ration 0.65 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 90° V bend Pass no visible cracks;
vi. surface without buckling and waviness; and
viii. 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%.

5. A process for the manufacture of high strength dual phase cold rolled steel sheet having high yield ratio as claimed in 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;
2 wt% to 2.5 wt% of Manganese;
0.2 wt% to 0.5 wt% of Silicon;
0.02 wt% to 0.06wt% of Aluminum;
0.015wt% or less of Phosphorous;
0.061 wt% to 0.1wt% of Niobium;
Up to 0.25wt% of Molybdenum;
Up to 0.003wt% of Calcium;
Up to 0.006wt% of Nitrogen; and
balance is Fe and incidental impurities such as to maintain (Nb+Mo)/C 0.5 to 3,
and
b. carrying out steel sheet manufacturing involving 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 90°.

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 840°C to 900°C and hot coiled with ROT cooling rate in the range of 11°C/Sec to 15°C/Sec .
b. 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 anyone of claims 5 or 6 further comprising:
a. Soaking said steel at temperature 790°C to 840°C for dual phase steel with residence time from 100 to 170 sec.
b. Slow cooling further said steel at temperature 670°C to 730°C with slow cooling rate 0.2 °C/Sec to 2 °C/Sec;
c. Rapid cooling of said steel at rapid cooling rate of 30°C/Sec to 40 °C/Sec;
d. overaging the said steel at 230°C to 300°C for 350 sec or more; and
e. Skin passing of said steel 0.3% to 0.6%.

8. A process as claimed in anyone of claims 5 to 7 to ensure good 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 1000MPa or more;
ii. Yield Strength at least 650 MPa with YS/TS ratio 0.65 or
more;
a. Bake hardening Index 30 MPa or more;
b. Hole expansion Ratio 30% or more with aging guarantee of 6 months;
v. Bendability 90° V bend Pass no visible cracks;
vi. Surface without buckling and waviness; and
viii. 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%.

Dated this the 27th day of December, 2017
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)
IN/PA-199