DESC:FIELD OF THE INVENTION

The present invention relates to ultra high strength Cold Rolled Steel Sheet having excellent formability and bake hardening Index and a method for producing the same. More particularly, the present invention is directed to provide dual phase and Multiphase cold rolled steel sheet having tensile strength of 1180 MPa minimum and composition comprising chemical elements in terms of mass percent: 0.121% to 0.16% of C, Si: 0.2% to 0.5%, Mn: 2% to 3%,N: 0.006% or less, Al: 0.02% to 0.06%, Nb: 0.061%-0.1% Ti: 0.01% to 0.03%, and the balance being Fe and other inevitable impurities, whereas (Mn+Si)/C is in a range of 14 to 28 for excellent combination of phosphatability and hole expansion ratio, produced through continuous annealing route with selective process parameters. Cold rolled steel with minimum 1180 MPa UTS as per present invention has been categorized based on the yield ratio wherein a first category of steel has a Dual phase structure (Ferrite + Martensite+ Bainite+Precipitates) where annealing is carried out between Ac1 and Ac3 temperatures resulting in yield ratio less than 0.65 for the same; and a second category of steel has multiphase structure (Ferrite + Martensite + Retained Austenite+ Bainite+ Precipitates) and has a yield ratio > 0.65. In addition, second category of steel with multiphase structure has been annealed above Ac3 temperature during continuous annealing. Such ultra high strength steel grade is suitable for application in the area of Automotive Reinforcements, Automotive Front Cross member, B pillar, Seat Cross member and the like’s automotive applications.

BACKGROUND OF THE INVENTION
In the perspective of global environmental conservation by reducing exhaust gas emission, modern automotive manufacturers are opting for incorporation of ultra high strength steel in their vehicle. Improving fuel efficiency and passenger safety are the other major aspect in the favor of the use of advance high strength steel. However, the approach of vehicle weight reduction achieved by using thinner gauge AHSS (Advanced high strength steel) has a difficulty. As we move to higher strength level of 1180 MPa, drawability, phosphatability, weldability and bending performance deteriorates and it’s hard to form the steel to desired automotive component with suitable surface property. As a part of prior art European patent application number EP1 553202A1 describes an Ultra-high strength steel sheet having excellent hydrogen embrittlement resistance having a bainitic ferrite microstructure. The trip effect has been achieved by addition of higher amount of Al+ Si and rapid cooling the steel at a temperature above Ms (Martensite start temperature).However, the steel described in European patent application number EP1 553202A1 suffers from poor surface property and phosphatability due to higher Al and Si weight percent and crack while bending due to poor bendability.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide ultra high strength Cold Rolled Steel Sheet having excellent formability, bendability and bake hardening Index, phosphatability and a method for producing the same.

A further object of the present invention is directed to provide said ultra high strength Cold Rolled Steel Sheet having selective composition and processing through continuous annealing route to achieve tensile strength of minimum 1180 MPa with improved hole expansion ratio.

A still further object of the present invention is directed to provide said ultra high strength Cold Rolled Steel Sheet which is produced through a process such that a first category of steel has a Dual phase structure (Ferrite + Martensite+ Bainite+Precipitates) where annealing is carried out between Ac1 and Ac3 temperatures resulting in yield ratio less than 0.65.

A still further object of the present invention is directed to provide said ultra high strength Cold Rolled Steel Sheet which is produced through a process such that a second category of steel having multiphase structure (Ferrite + Martensite + Retained Austenite+ Bainite+ Precipitates) wherein said second category of steel with multiphase structure has been annealed above Ac3 temperature during continuous annealing to have a yield ratio > 0.65.

Yet another object of the present invention is directed to provide ultra high strength Cold Rolled Steel Sheet having good surface quality and phosphatability including phosphate crystal size 2 µm to 5 µm preferably 3.5 µm or less and coating weight 1.5 g/m2 to 3 g/m2 and hole expansion ratio 30 % or more.

A still further object of the present invention is directed to provide ultra high strength Cold Rolled Steel Sheet having where both category of steel would have bake hardening index is atleast 50 MPa for both dual phase and multiphase steel and Hole Expansion Ratio (HER%)= 30 % with aging guarantee of 6 months or more.

A still further object of the present invention is directed to provide ultra high strength Cold Rolled Steel Sheet wherein both said dual phase and multiphase category of steel would have good bendability with V bend of 90° with no visible cracks even at 40X Magnifications.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to provide ultra high strength cold rolled steel sheet composition comprising:
0.121wt % to 0.16wt% percent of Carbon;
2.0 wt% to 3.0 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;
0.01 wt% to 0.03 wt % of Ti
Up to 0.006wt% of Nitrogen;
Balance as Fe and incidental impurities, having tensile strength 1180 MPa or more whereas (Mn+Si)/C must be in a range of 14 to 28.

A further aspect of the present invention is directed to ultra high strength cold rolled steel sheet composition further comprises Cr from 0.31 to 0.55 wt % and/or Mo wt% such that ratio of Cr/Mo to be 1.5 to 3 for good weldability.

A still further aspect of the present invention is directed to ultra high strength cold rolled steel sheet composition wherein (Mn)/(C+S) ratio is maintained 10 or more.

A still further aspect of the present invention is directed to ultra high strength cold rolled steel sheet composition further comprises B from 0.001 to 0.0030 wt %.

Another aspect of the present invention is directed to ultra high strength cold rolled steel sheet composition further including in mass % at least one element selected from the group comprising of Sc, V, Co, Cu, Zn, Sn, Ni, Ca, W, V and Zr such that each element weight percent is 0.03% or less.

Yet another aspect of the present invention is directed to ultra high strength cold rolled steel sheets wherein the steel has minimum UTS = 1180 MPa,YS/UTS ratio of said steel sheet is less than 0.65 for dual phase steel, YS/UTS ratio is more than 0.65 for multiphase steel, bake hardening index is atleast 50 MPa for both dual phase and multiphase steel and Hole Expansion Ratio (HER%)= 30 %.

A further aspect of the present invention is directed to Ultra high strength cold rolled steel sheet, wherein said steel has bendability of 90° V bend with no visible cracks even at 40X.

A still further aspect of the present invention is directed to ultra high strength cold rolled steel sheet wherein microstructure of said steel contains in terms of area ratio, ferrite phase of 10 to 40% , total of area of martensite phase and or/Bainite phase 50% to 80 %, Retained austenite area fraction 10 % or less and the area fraction of cementite and is less than 2 %.

A still further aspect of the present invention is directed to ultra high strength cold rolled steel sheet wherein microstructure of said steel further contains carbide/Nitride/Sulphide precipitates of alloying elements and the area fraction is less than 2%.

Another aspect of the present invention is directed to a process for the manufacture of ultra high strength 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;
2.0 wt% to 3.0 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;
0.01 wt% to 0.03 wt % of Ti
Up to 0.006wt% of Nitrogen;
Balance as Fe and incidental impurities such as to maintain Cr/Mo ratio 1.5 to 3, and
b) carrying out steel sheet manufacturing including hot rolling, pickling, cold reduction and continuous annealing such as to reach to Phosphatability including phosphate crystal size 2 µm to 5 µm preferably 3.5 µm or less and coating weight 1.5 g/m2 to 3.0 g/m2 and hole expansion ratio 30 % or more.

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 55%.

A further aspect of the present invention is directed to said process further comprising:
a. Soaking said steel at temperature 760°C to 800°C for dual phase steel and 800 °C to 850 °C for multi phase steel sheet with residence time from 80 to 170 sec.
b. Slow cooling further said steel at temperature 670°C to 730°C with slow cooling rate 0.3 °C/Sec to 2 °C/Sec;
c. Rapid cooling of said steel at temperature of 340°C or less at rapid cooling rate of 25 °C/Sec to 50 °C/Sec;
d. overaged the said steel 230°C to 300°C for 300 sec or more;
e. Skin passing of said steel 0.2% to 0.6% and
f. Wherein Rapid cooling temperature is obtained by following relation -
log10 [(SCS-RCS) /40] = 5.5- 2.2*[%Cr] -1.7* [%Mn].

A still further aspect of the present invention is directed to said process 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:
iii. Tensile strength 1180 MPa or more;
iv. Yield Strength at least 740 MPa with YS/TS ratio 0.7 or less and Yield strength of at least 860 MPa with YS/TS ratio 0.7 to 0.9.
v. Bake hardening Index 50 MPa or more;
vi. Hole expansion Ratio 30 % or more with aging guarantee of 6 months;
vii. Phosphate crystal size 5 µm or less and coating weight 3 g/m2 or less; and
viii. Volume fraction of 50% or more of martensite or other strengthening phase distributed in soft polygonal ferrite matrix having average ferrite grain size less than 5µm and volume fraction more than 20%.

The above and other objects and advantages of the invention are described hereunder in greater details with reference to following accompanying examples based on experimental trials.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING EXAMPLES

The present invention relates to 1180 MPa Tensile strength level dual phase and Multiphase cold rolled steel sheet having excellent formability and bake hardening Index, phosphatability and a method for producing the same and suitable for automotive components and a process for manufacturing the same.

The present invention relates to 1180 MPa Tensile strength level dual phase and Multiphase cold rolled steel sheet comprises chemical elements in terms of mass percent: 0.121% to 0.16% of C, Si: 0.2% to 0.5%, Mn: 2% to 3%,N: 0.006% or less, Al: 0.02% to 0.06%, Nb: 0.061%-0.1% Ti: 0.01% to 0.03%, and the balance being Fe and other inevitable impurities, whereas (Mn+Si)/C is in a range of 14 to 28 for excellent combination of phosphatability and hole expansion ratio, and wherein the steel composition further satisfies the following relations with Slow cooling section temperature and rapid cooling section temperature in continuous annealing:
log10 [(SCS-RCS) /40] = 5.5- 2.2*[%Cr] -1.7* [%Mn].
(Where, SCS is slow cooling section temperature and RCS is rapid cooling section temperature in continuous annealing and all compositional elements are in weight %).

The ultra high strength cold rolled steel sheet according to the present invention further comprising Cr in the range of 0.31 to 0.55 wt % and/or Mo in wt% such that ratio of Cr/Mo to be 1.5 to 3 for good weldability and also contains B in the range of 0.001 to 0.0030 wt %.

In addition said ultra high strength cold rolled steel sheet composition as described above further including in mass % at least one element selected from the group comprising of Sc, V, Co, Cu, Zn, Sn, Ni, Ca, W, V and Zr such that each element weight percent is 0.03% or less.

Cold rolled steel with minimum 1180 MPa UTS as per present invention has been categorized based on the yield ratio. Selective processing steps are followed to obtain a First category of steel has a Dual phase structure (Ferrite + Martensite /Bainite+ Precipitates) with same chemical composition range as listed above; however annealing has been carried out between Ac1 and Ac3 temperatures for first category of steel and the resultant yield ratio is less than 0.65 for the same.

Also a Second category of steel is produced having same composition but annealed above Ac3 temperature during continuous annealing to obtain multiphase structure (Ferrite + Martensite + Retained Austenite+ Bainite+Precipitates) and a yield ratio > 0.65.

With the aim of achieving Dual phase and multiphase with strength level of 1180 MPa, 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, (Mn+Si)/C is in a range of 14 to 28 for excellent combination of phosphatability and hole expansion ratio, to attain the said ratiocarbon has been limited to 0.16 wt% or less in present inventive dual phase steel grade.

Manganese (Mn: 2.0-3.0 wt %) -Mn significantly improves the hardenability, hence, Dual phase steel can be produced easily even by a simple air cooling. In addition, it assists fine dispersion of martensite phase which leads to higher tensile strength and good ductility. The addition of small amount of Si (<0.5 wt %) gives beneficial effects, the tensile strength further increases without a significant loss of ductility.

However, 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 3 wt% for present inventive grade.

Silicon (Si: 0.2-0.5 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 0.5 wt% 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 (Cr/Mo) ratio 1.5 to 3 for better bendability.

Niobium (Nb: 0.061-0.1 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 0.061wt%. Nb more than 0.1 wt% unnecessarily adds up to the cost of production and increases YS/UTS ratio. Hence, upper limit for present inventive DP grade is 0.1wt%.

Titanium (Ti: 0.005-0.03 wt %)- Ti acts as a nitrides forming element to fix solute N in steel thus 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%, the effects are saturated, therefore the amount of Ti is made to be 0.03% or less. when Ti is added in excess of the amount required for reducing solid solution N, excessive TiC may form, which inhibits the stable formation of second phase, which is not preferable .

Sc, V, Co, Cu, Zn, Sn, Ni, Ca, W, V and Zr in the range of 0.002 to 0.03 % - each of from Sc, V, Co, Cu, Zn, Sn, Ni, Ca, W, V and Zr act as carbide former and/or nitride former and/or sulphide former and/or solid solution strengthening elements, however adding each of these elements in an amount more than 0.03 wt% unnecessarily adds up to the cost of the steel.

Boron (B: 0.001% - 0.003% wt%) – Boron is a nitride former which also strengthen the grain boundary . It also improves hardenability of steel. However Boron content exceeding more than 0.003 wt% cause defects during casting and hot rolling such as edge crack . Hence it is preferable to keep boron below 0.003 wt%.

Complete Description of Process -
To achieve steel slab chemistry as described above, Heat from basic oxygen furnace (BOF) is processed through RH degasser and subsequently continuously casted. Special measure have been taken to hot roll resulted slabs by keeping slab reheating temperature in the range 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 of more than 11 0C/sec was maintained to achieve coiling temperature range of 530 °C to 590 °C. Hot rolled coils were subsequently processed through pickling coupled with tandem cold rolling mill to remove the oxide surface present in the surface and to provide cold reduction of 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 was maintained in the range of 760 °C -850 °C based on the final microstructure and property requirement. First category of Ultra high strength steel with minimum 1180 MPa UTS as per present invention was annealed between Ac1 and Ac3 temperatures (760°C to 800°C ) to achieve yield ratio is less than 0.65 . Second category of Ultra high strength steel with minimum 1180 MPa UTS as per present invention was annealed above Ac3 temperature (800°C to 850°C ) to achieve yield ratio atleast 0.7.

Annealing time in the range of 80 to 170 seconds gives desired results for both categories of ultra high strength steels. 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 0.3 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 25 °C/sec or more up to rapid cooling section temperature of 340 °C or less to avoid pearlite formation and attain the desired strength of 1180 MPa or more. After rapid cooling section annealed strip was over aged keeping the over aging section temperature of 230°C-300°C to temper the transformed strengthening phase (Martensite and/or Bainite). After over aging Skin-pass elongation (Temper rolling) in the range of 0.2 % to 0.6% was applied to avoid yield point elongation. In addition following relation was fulfilled in favor of chemical composition and during annealing in order to achieve Tensile Strength 1180 MPa or more–

log10 [(SCS-RCS) /40] = 5.5- 2.2*[%Cr] -1.7* [%Mn],

Where SCS- Slow cooling section Temperature in 0C, RCS- Rapid cooling section Temperature in 0C, [M] = Elemental composition of element M in wt%.

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 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 < 4 µm 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.

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 was 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 and 25mm width was 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.

Method for evaluating Bending Test: To evaluate bend test a small sample of 25 mm width and 90mm length from annealed sheet is cut where length is in transverse to rolling direction. The sample is V bent keeping the ratio bend radius (r) to thickness of sheet (t) to 1.5. Post bending the bend surface is observed under scanning electron microscope at 40 X. steel having visible crack at 40X magnification does not comply with Bendablility requirement.

The chemical composition, process parameters and properties of the inventive steel grade as well as comparative examples based on experimental trials are presented in the 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)/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 4 – Microstructural constituent’s area fraction of inventive and comparative steel sheets processed at different annealing conditions.

Table 1:
Steel .No C,
wt% Mn,
wt% S,
wt% P,
wt% Si,
wt% Al,
wt% N,
wt% Ti,
wt% B,
wt% Nb,
wt% Cr,
wt% Mo,
wt% Other Elements,
wt% Eq1=
(Mn+Si)/C Remarks
1 0.126 2.5 0.003 0.016 0.425 0.04 0.0054 0.015 0.0025 0.085 0.456 - Zr:0.005, Ni:0.01,Cu:0.02,Ca:0.003,W:0.003 23.2 Ex.
2 0.14 2.3 0.002 0.01 0.24 0.05 0.004 0.02 0.002 0.075 0.35 0.2 Sc:0.003,Zn:0.002, Co:0.002 18.1 Ex.
3 0.083 3.1 0.003 0.01 0.9 0.03 0.006 0.002 - 0.03 - - - 39.8 Comp.
* Ex. - Present inventive example, Comp.- Comparative Examples
** Shaded and underlined boxes indicates “outside the appropriate range
*** Eq1 = (Mn+Si)/C

Table 2 :
Steel.
No Ac3,0C FT,
0C ROT cooling
rate, 0C/sec SS,
0C Annealing Time, 0C SCS,
0C RCS,
0C RCS
Cooling Rate,
0C OAS,
0C Eq2 Eq3 Remarks
1A 801 870 12.4 821 126 690 301 29.7 250 0.99 0.2 Ex
1B 801 860 13.2 780 133 704 307 28.5 254 1.0 0.2 Ex
1C 801 865 12.7 830 120 720 320 27.8 255 1.0 0.2 Ex
1D 801 860 11.2 780 170 680 400 16.6 289 0.85 0.2 Comp
2A 791 860 13.5 825 130 710 292 33.7 243 1.0 0.8 Ex
2B 791 865 13.7 785 126 705 320 31.4 240 0.98 0.8 Ex
3 774 870 14.2 820 162 695 380 19 272 0.9 0.2 Comp

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 2B have 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”
Ac3 – Upper critical temperature in iron carbide diagram
** Eq2 = log10 [(SCS-RCS) /40], Eq3= 5.5- 2.2*[%Cr] -1.7* [%Mn]

Table 3 :
Steel.
No YS,
MPa UTS,
MPa YS/UTS El% BH Index,
MPa Phosphatability Remark Aging Remarks Bending (90°) HER%
1A 860 1210 0.71 12.5 112 O O O 43
1B 763 1193 0.64 12.3 127 O O O 39
1C 843 1190 0.71 12.1 131 O O O 41
1D 790 1147 0.69 10.1 105 ? O ? 27
2A 854 1217 0.70 13.2 140 O O O 45
2B 759 1198 0.63 12.3 127 O O O 36
3 827 1081 0.76 10.8 131 ? ? ? 23
* Ex. - Present inventive example, Comp.- Comparative Examples
* BH- bake hardening index, HER- Hole expansion ratio
*Shaded and underlined boxes indicates “outside the appropriate range”
** Steel with phosphatability, aging and bendability remark as “O” fulfill both phosphatability, accelerated aging resistance bendability requirement.
**Steels with phosphatability and bendability remark as “?” do not comply with the phosphatability ,aging and bendability requirements (cracks are visible at 40X ). For these steels the phosphate crystal size after zinc phosphate chemical conversion coating is >4µm and phosphate coating weight is >3 g/mm2 making it undesirable for coating and painting on steel surface.
**Steels with aging remark “?” do not fulfill the accelerated aging requirement as the YPE observed after accelerated aging test.
Table 4:
Steel.
No Ferrite Martensite area % Bainite Area % Pearlite Area % Retained Austenite Area % Precipitates
Area % Remarks
1A 26 57 12 0 5 <1 Ex
1B 25 65 10 0 0 <1 Ex
1C 25.5 55 12.5 0 7 <1 Ex
1D 27 35 27 11 0 <1 Comp
2A 28 59 9 0 4 <1 Ex
2B 25 62 13 0 0 <1 Ex
3 33 37 22 8 0 <1 Comp
* Ex. - Present inventive example, Comp.- Comparative Examples
* Steel having martensite phase fraction <50 % does not comply with the scope of present invention.
* Steel 1A ,1C and 2A are processed above Ac3 shows retained austenite in microstructure and posses higher yield ratio.

Example 1 Steel number 1 having chemical composition range as per the scope of present invention . Steel 1 has (Mn+Si)/C ratio in the range of 14 to 28 .However, steel 1 processed at continuous annealing line (CAL) at different annealing conditions listed as 1A ,1B,1C and 1D respectively in Table 2. Steel no 1A and 1B is processed at above and below Ac3 (Upper critical temperature) respectively. Both steel is processed at RCS < 340 0C and RCS cooling rate > 25 0C/sec as per the scope of invention. However, As given in table 3, due to higher annealing temperature > Ac3, steel 1A results in higher yield ratio (YS/UTS) >0.7 where as steel 1B being processed below Ac3 results in lower yield ratio of <0.65. Both the steel has UTS>1180. Conversely, Steel 1D having same chemical composition as steel 1A and 1B but processed at higher RCS of 400 0C and reduced cooling rate of 16.6 0C which is outside the scope . As a result , steel 1D shows lower UTS of 1147 MPa as given in Table 3. This is due to the fact that higher RCS results in Lower martensite fraction and higher Bainite /pearlite fraction.

Example 2 : Similar to steel 1, Steel number 2 having chemical composition range as per the scope of present invention has been processed at (CAL) at different annealing conditions listed in Table 2 as 2A ,2B respectively. Steel no 2A and 2B is processed at above and below Ac3 (791 0C) respectively as given in Table 2. Both steel is processed at RCS < 340 0C and RCS cooling rate > 25 0C/sec as per the scope of invention. However, As given in table 3, due to higher annealing temperature > Ac3, steel 2A shows higher yield ratio (YS/UTS) >0.7 where as steel 2B being processed below Ac3 results in lower yield ratio of <0.65. Both the steel has UTS>1180.

Example 3 : Steel number 3 as listed in Table 1 has chemical composition outside the scope of present invention as Mn > 3 % and Si>0.5 % along with low Ti <0.01 % and Nb < 0.06 %. Also, (Mn+Si)/C ratio is outside the range of 14 to 28 for steel 3. In addition, steel 3 is processed at higher RCS temperature of 380 0C and lower rapid cooling rate of 19 0C /sec. Consequently of higher RCS and Low rapid cooling rate steel sheet no. 3 shows poor strength of 1081 MPa which is outside the scope of present invention. Furthermore , Due to higher Mn and Si weight % , steel 3 shows poor phosphatability marked as “?” in table 3 having phosphate crystal size > 4 µm and phosphate coating weight > 3 g/m2 post zinc phosphate chemical conversion coating. Owing to Ti weight % of < 0.01 %, steel no 3 shows poor aging resistance and remarked in Table 3 as “?”.

Example 4 : Table 4 list area fraction of microstructural constituents of inventive and comparative steel . As given in table 4, Steel 1A has 5 % retained austenite along with 57 % martensite and 12 % bainite as microstructural constituent as steel 1A is annealed above Ac3 . Similar observation can be made for steel 1C and 2A as both the steel are annealed above Ac3 and have retained austenite as part of microstructural constituent . As a complex phase steel (Ferrite + Martensite + Bainite+ Retained Austenite+ Precipitate) , steel 1A, 1C and 2A shows higher yield ratio of > 0.7 .Contrary to that , steel 1B And 2B is processed below Ac3 and does not have Retained austenite as a microstructural constituent resulting in lower yield ration of <0.65.

Example 5: steel 1D being processed at higher RCS temperature of 400 0C along with lower Rapid cooling rate of 16.6 0C/sec shows reduced martensite area fraction of 35% along with 11 % pearlite in microstructure . this attribute to reduced UTS of 1147 MPa for steel 1D.

It is thus possible by way of the present invention to provide ultra high strength Cold Rolled Steel Sheet having selective composition and processed through steps with selective parameters through continuous annealing route to ensure UTS 1180MPa or more such that a first category of steel has a Dual phase structure (Ferrite + Martensite+ Bainite+ Precipitates) where annealing is carried out between Ac1 and Ac3 temperatures resulting in yield ratio less than 0.65; and a second category of steel has multiphase structure (Ferrite + Martensite + Retained Austenite+ Bainite+Precipitates) and has a yield ratio > 0.65, said second category of steel with multiphase structure is annealed above Ac3 temperature during continuous annealing. Both category of steel would have bake hardening index atleast 50 MPa for both dual phase and multiphase steel and Hole Expansion Ratio (HER%)= 30 % and bendability of 90° V bend with no visible cracks at 40X with aging guarantee of 6 months or more suitable for Automotive Reinforcements, Automotive Front Cross member, B pillar, Seat Cross member and the like’s automotive applications.

,CLAIMS:We Claim:

1. Ultra high strength cold rolled steel sheet composition comprising:
0.121wt % to 0.16wt% percent of Carbon;
2.0 wt% to 3.0 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;
0.01 wt% to 0.03 wt % of Ti
Up to 0.006wt% of Nitrogen;
Balance as Fe and incidental impurities, having tensile strength 1180 MPa or more whereas (Mn+Si)/C must be in a range of 14 to 28.

2. Ultra high strength cold rolled steel sheet composition as claimed in claim 1 further comprises Cr from 0.31 to 0.55 wt % and/or Mo wt% such that ratio of Cr/Mo to be 1.5 to 3 for good weldability.

3. Ultra high strength cold rolled steel sheet composition as claimed in anyone of claims 1 or 2 wherein (Mn)/(C+S) ratio is maintained 10 or more.

4. Ultra high strength cold rolled steel sheet composition as claimed in anyone of claims 1 to 3 further comprises B from 0.001 to 0.0030 wt %.

5. Ultra high strength cold rolled steel sheet composition as claimed in anyone of claims 1 to 4 further including in mass % at least one element selected from the group comprising of Sc, V, Co, Cu, Zn, Sn, Ni, Ca, W, V and Zr such that each element weight percent is 0.03% or less.

6. Ultra high strength cold rolled steel sheet as claimed in anyone of claims 1 to 5, wherein the steel has minimum UTS = 1180 MPa,YS/UTS ratio of said steel sheet is less than 0.65 for dual phase steel, YS/UTS ratio is more than 0.65 for multiphase steel, bake hardening index is atleast 50 MPa for both dual phase and multiphase steel and Hole Expansion Ratio (HER%)= 30 %.

7. Ultra high strength cold rolled steel sheet as claimed in anyone of claims 1 to 6, wherein said steel has bendability of 90° V bend with no visible cracks at 40X.

8. Ultra high strength cold rolled steel sheet as claimed in anyone of claims 1 to 7, wherein microstructure of said steel contains in terms of area ratio, ferrite phase of 10 to 40% , total of martensite phase and or/Bainite phase 50% to 80 %, Retained austenite 10 % or less and the fraction of cementite less than 3 % .

9. Ultra high strength cold rolled steel sheet as per claim 1 to 8, wherein microstructure of said steel further contains carbide/Nitride/ Sulphide precipitates of alloying elements and the area fraction is less than 2 %.

10. A process for the manufacture of ultra high strength cold rolled steel sheet as claimed in anyone of claims 1 to 9 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;
2.0 wt% to 3.0 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;
0.01 wt% to 0.03 wt % of Ti
Up to 0.006wt% of Nitrogen;
Balance as Fe and incidental impurities such as to maintain Cr/Mo ratio 1.5 to 3, and
ii) carrying out steel sheet manufacturing including hot rolling, pickling, cold reduction and continuous annealing such as to reach to Phosphatability including phosphate crystal size 2 µm to 5 µm preferably 3.5 µm or less and coating weight 1.5 g/m2 to 3.0 g/m2 and hole expansion ratio 30 % or more.

11. A process as claimed in claim 10 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 55%.

12. A process as claimed in claim 10 or 11 further comprising:
a. Soaking said steel at temperature 760°C to 800°C for dual phase steel and 800 °C to 850 °C for multi phase steel sheet with residence time from 80 to 170 sec.
b. Slow cooling further said steel at temperature 670°C to 730°C with slow cooling rate 0.3 °C/Sec to 2 °C/Sec;
c.Rapid cooling of said steel at temperature of 340°C or less at rapid cooling rate of 25 °C/Sec to 50°C/Sec;
d.Overaged the said steel 230°C to 300°C for 300 sec or more;
e.Skin passing of said steel 0.2% to 0.6% and
f.Wherein Rapid cooling temperature is obtained by following relation -
log10 [(SCS-RCS) /40] = 5.5- 2.2*[%Cr] -1.7* [%Mn].

13. A process as claimed in anyone of claims 10 to 12 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 1180 MPa or more;
ii. Yield Strength at least 740 MPa with YS/TS ratio 0.7 or less for dual phase steel and Yield strength of at least 860 MPa with YS/TS ratio 0.7 to 0.9 for multiphase steel.
iii. Bake hardening Index 50 MPa or more;
iv. Hole expansion Ratio 30 % or more with aging guarantee of 6 months;
v. Phosphate crystal size 5 µm or less and coating weight 3 g/m2 or less; and
vi. Volume fraction of 50% or more of 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 2nd day of November, 2017
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)
IN/PA-199