, Description:FIELD OF THE INVENTION
The present invention relates to High strength cold rolled steel Sheet with high yield ratio and having excellent Bendablility, improved phosphatability and reduced spring back effect and a method for producing the same. More particularly, the present invention is directed to provide cold rolled steel sheet having composition in terms of mass percent comprising: 0.06wt % to 0.1wt % of C; 1.4 wt% to 2.0 wt% of Mn;0.2 wt% to 0.5 wt% of Si; 0.02 wt% to 0.08 wt% of Al; 0.03 wt% or less of P; 0.05 wt% to 0.1 wt% of Nb; 0.05 wt% to 0.1 wt % of Ti; Up to 0.006wt% of N; and balance as Fe and incidental impurities, having yield strength of 850 MPa or more, Tensile strength of 1050 MPa or less and Yield ratio of 0.85 or more wherein the ratio (Ti+Nb)/C must be in a range from 1.2 to 3 and the steel microstructure constitutes in terms of area fraction having at least one from bainite and pearlite phase less than 15% and ferrite phase more than 85% along with precipitates distributed in ferrite matrix. Such High strength steel Sheet with high yield ratio is suitable for application in the area of Automotive Reinforcements, Automotive Front Cross member, B pillar, seat & assembly, Seat rail and similar 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 with the objective of weight reduction. Improving fuel efficiency and passenger safety are the other major aspects in favor of the use of advance high strength steel. However, the approach of vehicle weight reduction achieved by using thinner gauge High strength Sheet with high yield ratio has a difficulty. As we move to higher yield strength level of 850 MPa, drawability, phosphatability and Bendablility deteriorates and it becomes hard to form the steel to desired shape of automotive component with suitable surface property. In addition problem of spring back associated with high strength steel is another concern for automotive manufacture. 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 to a temperature above Ms (Martensite start temperature).However, the steel described in European patent application number EP1 553202A1 suffers from poor Bendablility, high spring back, inferior phosphatability due to higher Al and Si weight percent.

United States patent application number US2016/0177427A1 describes manufacturing of high yield ratio high strength cold rolled steel sheet and production method thereof. The steel sheet attains high yield ratio 0.8 or more by controlling the macrostructure of said steel sheet comprising up to 7% of martensite along with retained austenite. The Mn and Si weight percent of said steel sheet as described in US2016/0177427A1 is remarkably high imparting desired strength of 1000 MPa or more. However, in attempt of achieving the specified strength, the steel sheet mentioned in US2016/0177427A1 suffers from poor surface property and rather poor Bendablility along with high spring back post forming.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide High strength cold rolled steel sheet with high yield ratio having excellent Bendablility, phosphatability with reduced spring back characteristics and a method for producing the same.

A further object of the present invention is directed to provide said High strength cold rolled steel sheet with high yield ratio having selective composition and processing steps/parameters through continuous annealing route to achieve yield strength of 850 MPa or more with reduced spring back after forming.

A still further object of the present invention is directed to provide said High strength sheet with high yield ratio where the microstructural constituents of steel comprises in terms of area ratioat least one from bainite and pearlite phase less than 15%, ferrite phase more than 85% along with precipitates distributed in ferrite matrix.

A still further object of the present invention is directed to a method of producing said High strength sheet with high yield ratio wherein the annealing of said cold rolled steel sheet is carried out between Ac1 and Ac1+ 60oC temperatures resulting in yield ratio more than 0.85 and hole expansion ratio 40 % or more.

Yet another object of the present invention is directed to provide high strength high yield ratio cold rolled steel sheet having good Bendablility, surface quality and phosphatability including phosphate crystal size ranging from 2 µm to 4 µm and phosphate coating weight in the range from 1.5 g/m2 to 2.5 g/m2.

A still further object of the present invention is directed to provide High strength cold rolled steel sheets wherein the steel has minimum Yield Strength of 850 MPa, YS/UTS ratio of 0.85 or more, uniform elongation more than 7% and no visible cracks appear at bend surface after subjecting the steel sheet to bending at 90°.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to provide High yield ratio high strength cold rolled steel sheet comprising:
a steel composition having:
0.06wt % to 0.1wt % of C;
1.4 wt% to 2.0 wt% of Mn;
0.2 wt% to 0.5 wt% of Si;
0.02 wt% to 0.08 wt% of Al;
0.03 wt% or less of P;
0.05 wt% to 0.1 wt% of Nb;
0.05 wt% to 0.1 wt % of Ti;
up to 0.006wt% of N;
and balance as Fe and incidental impurities wherein (Ti+Nb)/C must be in a range from 1.2 to 3 and having yield strength of 850 MPa or more, Tensile strength of 1050 MPa or less and Yield ratio of 0.85 or more and the microstructural constituents of steel comprises in terms of area ratio at least one from bainite and pearlite phase less than 15% and ferrite phase more than 85%.

A further aspect of the present invention is directed to provide a high yield ratio cold rolled steel sheet comprising, in mass%, one or more of: Cu: 0.01% to 0.2%, Ni: 0.01% to 0.2 %, Cr: 0.01% to 0.2%, V: 0.003% to 0.03, W: 0.005% to 0.1 %, and B: 0.0005% to 0.003%, Mo: 0.01 to 0.2 %.

A still further aspect of the present invention is directed to a high yield ratio cold rolled steel sheet including in mass % at least one element selected from the group comprising of Sc, Co, Zn, Sn, Ca, Hf, and Zr such that each element weight percent is 0.03% or less.

Another aspect of the present invention is directed to a high yield ratio cold rolled steel sheet having yield strength in the range of 850 MPa to 950 MPa, Tensile strength in the range of 950 MPa to 1050 MPa and Yield ratio in the range of 0.85 to 0.95.

A further aspect of the present invention is directed to a high yield ratio cold rolled steel sheet having uniform elongation more than 7% and No visible cracks at bend surface after subjecting the steel sheet to bending at 90° and observing at 40X magnification.

A further aspect of the present invention is directed to a process for the manufacture of high yield ratio cold rolled steel sheet as described above comprising:
a. providing selective steel composition comprising:
0.06wt % to 0.1wt % of C;
1.4 wt% to 2.0 wt% of Mn;
0.2 wt% to 0.5 wt% of Si;
0.02 wt% to 0.08 wt% of Al;
0.03 wt% or less of P;
0.05 wt% to 0.1 wt% of Nb;
0.05 wt% to 0.1 wt % of Ti;
Up to 0.006wt% of N;
and balance as Fe and incidental impurities wherein (Ti+Nb)/C is maintained in a range from 1.2 to 3

b) Carrying out steel sheet manufacturing including hot rolling, pickling, cold reduction and continuous annealing such as to produce said steel sheet having yield strength of 850 MPa or more, Tensile strength of 1050 MPa or less and Yield ratio of 0.85 or more .

A still further aspect of the present invention is directed to said process comprising:
i) Hot rolling of said steel slab keeping Finishing Temperature 840°C to 920°C and hot coiled with average ROT (Run out table) cooling rate before coiling in the range from 9°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 still further aspect of the present invention is directed to a process comprising:
a. soaking said steel at temperature 700°C to 760°C for high yield ratio steel with residence time from 35 to 75 sec.
b. slow cooling further said steel at temperature 600°C to 660°C with slow cooling rate 1 °C/Sec to 4 °C/Sec;
c. rapid cooling of said steel at rapid cooling rate of 30 °C/Sec to 70 °C/Sec up to temperature range from 420°C to 480 °C;
d. Overaging the said steel in the range from 300°C to 380°C for 150 sec or more;
e. Providing skin passing elongation (Temper rolling) on said steel surface in the range from 0.3% to 1%.

Yet another aspect of the present invention is directed to said process for producing said steel sheet having improved Phosphatability, good hole expansion ratio and reduced spring back post forming, wherein the above process steps are selectively controlled such as to achieve anyone or more of:
i) Yield strength 850 MPa or more;
ii) Tensile Strength at least 930 MPa with YS/TS ratio 0.85 or more
iii) Hole expansion Ratio 40 % or more with aging resistance of more than 6 months;
iv) Phosphate crystal size 5 µm or less and coating weight in the range from1.5 g/m2 to 2.5 g/m2; and
v) The microstructural constituents of steel comprises in terms of area ratio, at least one from bainite and pearlite phase less than 15%, ferrite phase more than 85% along with precipitates distributed in ferrite matrix .

The above and other objects and advantages of the present invention are described hereunder in greater details with reference to following exemplary samples/trials.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to 850 MPa Yield strength level High strength cold rolled steel sheet with high yield ratio wherein the steel has minimum Tensile Strength = 930 MPa, YS/UTS ratio of said steel sheet is greater than 0.85, uniform elongation more than 7% and No visible cracks at bend surface after subjecting the steel sheet to bending at 90°and observing at 40X magnification.

The cold rolled steel sheet according to present invention is having composition in terms of mass percent comprising: 0.06wt % to 0.1wt % of C; 1.4 wt% to 2.0 wt% of Mn; 0.2 wt% to 0.5 wt% of Si; 0.02 wt% to 0.08 wt% of Al; 0.03 wt% or less of P; 0.05 wt% to 0.1 wt% of Nb; 0.05 wt% to 0.1 wt % of Ti; Up to 0.006wt% of N; and balance as Fe and incidental impurities, having yield strength of 850 MPa or more, Tensile strength of 1050 MPa or less and Yield ratio of 0.85 or more wherein the ratio (Ti+Nb)/C must be in a range from 1.2 to 3 and the microstructural constituents of steel comprises in terms of area ratio at least one from bainite and pearlite phase less than 15%, ferrite phase more than 85% along with precipitates distributed in ferrite matrix .

The high strength high yield ratio cold rolled steel sheet according to present invention further comprises, in mass%, one or more of: Cu: 0.01% to 0.2%, Ni: 0.01% to 0.2 %, Cr: 0.01% to 0.2%, V: 0.003% to 0.03, W: 0.005% to 0.1 %, and B: 0.0005% to 0.003%, Mo: 0.01 to 0.2 %.

According to a further aspect of the present invention directed to 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, Co, Zn, Sn, Ca, Hf, and Zr such that each element weight percent is 0.03% or less.

Importantly also said High strength cold rolled steel sheets with high yield ratiohaving a minimum Yield Strength = 850 MPa, YS/UTS ratio of said steel sheet is greater than 0.85, uniform elongation more than 7% and No visible cracks at bend surface after subjecting the steel sheet to bending at 90°.

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 1020°C to check surface defects like rolled in scale. During hot rolling finishing mill temperature range of 840°C to 920°C and run out table cooling rate from finishing mill to coiler of more than 90C/sec was maintained to achieve coiling temperature in the 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. High strength Sheet with high yield ratio with minimum 850 MPa yield Strength as per present invention was annealed between Ac1 and Ac1+60°C temperatures to achieve yield ratio more than 0.85.

Annealing time in the range from 35 to 75 seconds gives desired results for High strength steel Sheet with high yield ratio. At soaking section temperature intercritical annealing results in ferrite and austenite microstructure which later transforms to (Ferrite + Bainite)or (Ferrite + Pearlite) or (Ferrite + Pearlite + Bainite) microstructure based on the cooling rate from slow cooling section to rapid cooling section. After soaking section steel strip passes through slow cooling section at cooling rate in the rage from1 to 4 °C/sec. Slow cooling section temperature of 600°C to 660°C was maintained. Following slow cooling section, annealed strip sheet is rapid cooled at cooling rate 25°C/sec or more up to rapid cooling section temperature of 420 °C or more to avoid martensite formation and attain the desired yield strength of 850 MPa or more. After rapid cooling section annealed strip is over aged keeping the over aging section temperature of 300°C-380°C to temper the transformed strengthening Bainite/Pearlite phase. Subsequently, Skin-pass elongation (Temper rolling) in the range from 0.3 % to 1 % is applied to avoid yield point elongation. In addition following relation was fulfilled in favor of chemical composition and during annealing in order to achieve Yield Strength 850 MPa or more;
1.2= (Ti+Nb)/C = 3
Furthermore, high strength cold rolled 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. Moreover, Following are the function of specific chemical composition with weight percent range of elements added to present inventive steel:

Carbon (0.06-0.1 wt %) – Carbon ranging from 0.05% to 0.1 % is used for increasing the tensile strength of the material. To achieve the minimum tensile strength of 930 MPa or more minimum 0.06% of carbon is required. Excessive amount of carbon increases the tensile strength significantly and reduces the ductility. In addition, excess Carbon increases the Martensite phase fraction reducing the yield ratio which ultimately leads to high spring back defect. Hence the upper limit of carbon to maintained to 0.1% to achieve the desired properties. Also, Carbon equivalent less than 0.28% results in better Weldability.
Manganese (1.4-2 Wt%) – Manganese acts as a solid solution strengthening element imparting the desired strength to steel. To achieve the minimum strength level of 930 MPa, minimum 1.4% of Mn is required, however addition of excess Mn deteriorates the surface property by forming complex Manganese oxide during annealing resulting in poor phosphatability. Hence the upper limit should be maintained to 2 %.

Niobium (0.05-0.1 wt %)–Niobium increases yield strength by formation of NbC and Nb[C,N] precipitates. The strengthening effect depends on the size distribution and amount of niobium carbide and carbonitride present. Niobium is also effective in grain refiner; addition of niobium give combined effect of precipitation strengthening and grain refinement thereby increase the strength by 20 to 30MPa per 0.01% of niobium.

Niobium delays the Recrystallization and recovery of cold worked structure. When niobium content is ranges from 0.05 to 0.1wt% the annealing temperature to be set above700°C for Recrystallization. However, if processed above 760°Cthen strengthening by precipitation hardening will be reduced resulting in reduced yield strength and yield ratio.
Minimum niobium required for strengthening is 0.05% to achieve minimum yield strength of850 MPa by precipitation strengthening and maximum niobium to be 0.1 to achieve uniform elongation more than 7%.

Titanium (0.05-0.1 wt %)-Titanium in the low carbon steels forms carbides and nitrides to provide grain refinement and precipitation strengthening. It also forms sulphide to control S. For effective strengthening using titanium, minimum addition of 0.05% is required to achieve Yield strength of 850 MPa minimum. Titanium is an effective grain refiner as it retards austenite grain growth by formation of titanium nitride. Excess addition of Ti above 0.1 wt% does not have any effect on strengthening as the Ti will be present as free Ti and not in carbide or nitride form. Hence, to achieve the desired yield strength in the range from 850 to 1000 MPa and to prevent accelerated aging titanium level should be maintained between 0.05-0.1wt%.

Aluminum (0.02-0.08wt %) – Aluminum acts as a deoxidizing agent but when present more than 0.08% it generates inclusion resulting in Edge sliver defect. Therefore the aluminum present should be 0.08% or less. Minimum Al of 0.02% is mandatory as an indication of effective deoxidation for Al killed steel.

Nitrogen (0- 0.006 wt% max) –Nitrogen present in the high strength steel containing titanium and niobium increase strength by formation of nitride precipitates. However, when present more than 0.006%, it results in the presence of free nitrogen leading to yield point elongation and poor ageing property. Accordingly, nitrogen should be maintained at 0.006% maximum.

Silicon (0.2 to 0.5 wt %) - Si is an effective solid solution element and must be present in an amount more than 0.2% to have any noticeable effect. However, excess addition leads to formation of complex silicon oxide hampering surface coating and phosphating property. Accordingly, upper level is restricted to 0.5 wt%.

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.

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] X100
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.

Method for evaluating Bendablility: To evaluate Bendablility, steel sheet is bend to 90 degree using bending die. The bend surface is observed under SEM at 40 X to observe any micro crack if present. Steel sheet showing micro crack at bend surface at said magnification does not satisfy the 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 = (Ti+Nb)/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,
wt% Mn,
wt% S,
wt% P,
wt% Si,
wt% Al,
wt% N,
wt% Ti,
wt% Nb,
wt% Cr,
wt% Other Elements,
wt% Eq1=
(Nb+Ti)/C Remarks
1 0.07 1.53 0.004 0.013 0.36 0.05 0.006 0.08 0.036 0.015 Ni:0.01,Cu:0.02,W:0.003, B: 0.0008, Mo: 0.0015 1.65 Ex.
2 0.09 2 0.004 0.013 0.45 0.06 0.002 0.08 0.09 0.03 Sc:0.003,Zn:0.002, Co:0.002, Hf: 0.003 1.88 Ex.
3 0.083 3.1 0.003 0.01 0.2 0.03 0.003 0.002 0.03 - - 0.4 Comp.
4 0.13 2.4 0.001 0.01 0.98 0.03 0.002 0.03 -
Mo:0.2,B:0.001

* Ex. - Present inventive example, Comp.- Comparative Examples
** Shaded and underlined boxes indicates “outside the appropriate range
*** Eq1 = (Ti+Nb)/C

Table 2:
Steel.
No FT,
0C ROT cooling
rate, 0C/sec SS,
0C Annealing Time, 0C SCS,
0C RCS,
0C RCS
Cooling Rate,
0C OAS,
0C Remarks Fea
Area % Pearlite +Bainite +Precipit-ate Area % Ms+RA Area %
1A 880 10 720 40 620 430 32 320 Ex 87 13 0
1B 890 12 730 45 630 440 40 330 Ex 86 14 0
1C 870 9 740 50 640 450 50 340 Ex 86.5 13.5 0
1D 830 8 760 75 650 490 20 390 Comp 89 11 0
1E 880 10 780 90 670 480 25 400 Comp 90 10 0
2A 900 13 760 35 660 470 50 380 Ex 88 12 0
2B 910 15 700 60 650 460 70 340 Ex 88.5 11.5 0
3 870 14.2 820 162 700 360 25 260 Comp 55 2 43
4 900 12.1 800 300 700 250 20 230 Comp 48 17 35

Fea: ferrite,Ms :Martensite, RA: Retained Austenite
Note: Steel marked as 1A, 1B, 1C, 1D and 1E have the same chemical composition as steel number 1, 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.
Steel having Martensite as a microstructural constituent does not satisfy the scope of present invention.
** Shaded and underlined boxes indicate “outside the appropriate range.
Steel having Pearlite /bainite area % more than 15 does not comply with the scope of present invention.

* Ex. - Present inventive example, Comp. - Comparative Examples
* FT- hot finish rolling temperature ,ROT- Run out table at hot strip mill , SS- soaking section temperature ,SCS- Slow cooling section temperature , RCS- Rapid cooling section temperature, OAS- Overaging section temperature , SPM- Skin pass elongation ** Shaded boxes indicates “outside the appropriate range”

Table 3:
Steel. No YS,
MPa
UTS,
MPa YS/UTS Total El% Uniform Elongation
Hole Expansion,% Phosphatability Remark Bendablility Remarks Remarks
1A 870 963 0.90 15.3 9 42 O O Ex.
1B 864 951 0.91 13.6 8 45 O O Ex.
1C 877 939 0.93 14.3 8.5 50 O O Ex.
1D 781 892 0.88 7 5 55 O ? Comp
1E 786 905 0.87 8 6 55 O ? Comp
2A 862 953 0.90 12.6 9 45 O O Ex.
2B 920 996 0.92 11.3 7 40 O O Ex.
3 723 1021 0.71 10.8 5 35 ? ? Comp
4 750 1023 0.73 11.2 6 30 ? ? Comp

* Shaded and underlined boxes indicates “outside the appropriate range”
** Steel with phosphatability and aging remark as “O” fulfill both phosphatability and accelerated aging resistance requirement.
**Steels with phosphatability remark “?” do not comply with the phosphatability requirement as the phosphate crystal size after zinc phosphate chemical conversion coating is >4µm and phosphate coating weight is >3 g/mm2 for these steel sheets which is undesirable for coating and painting on steel surface.
**Steels with bendability remark “?” do not fulfill the bendability test at 90° with 1.5t bending radius.
It can be appreciated from Table 1 to Table 3 that steel sheets remarked as “Ex.” are satisfying all the scopes of present invention and exhibits excellent phosphatability having phosphate crystal size =4µm and phosphate coating weight 1.5-2.5 g/m2 post zinc phosphate chemical conversion coating along with yield ratio of atleast 0.85, YS=850 MPa, UTS =1050 MPa along with good aging resistance. Whereas, Steel remarked as “Comp.” from Table 1 to Table 3 doesn’t comply with atleast one of the scope of the present invention and does not conform with minimum one or more of the end product attributes as described in the scope of the invention.

Example 1: Steel sheet “1” as listed in table 1 has chemical composition as per the scope of invention .However Steel “1” was processed through different hot rolling and annealing conditions as listed in table 2 and numbered as 1A, 1B, 1C and 1D. Steel 1A with a rapid cooling section end temperature of 430 0C and cooling rate of = 30 0C/sec confirms Ferrite area % > 85 and atleast one from Pearlite and Bainite < 15 %. Consequently , steel 1A satisfies the scope of the invention with YS of 870 MPa and YS/UTS ratio of 0.9 as listed in table 3.Similar observation can be made for Steel 1B having cooling section end temperature of 440 0C and cooling rate of = 30 0C/sec confirms Ferrite area % > 85 and atleast one from Pearlite and Bainite < 15 %. Hence, steel 1B satisfies the scope of the invention with YS of 864 MPa and YS/UTS ratio of 0.91 as listed in table 3. In addition, Steel 1A and 1B shows excellent phosphatability with phosphate crystal size =4µm and phosphate coating weight 1.5-2.5 g/m2 post zinc phosphate chemical conversion coating and marked as “O”. Steel 1A and 1B also exhibits excellent bending property (marked “O”) with no cracks observed at 40X magnification on bend surface post 900 Vbend.
However, steel sheet “1” when processed with higher RCS temperature of 4800C or more and lower cooling rate of = 30 0C as in case of Steel sheet “1D” and “1E” (as listed in Table 2) resulted in lower YS value of 781 MPa and 786 MPa respectively as listed in table 3 for Steel 1D, and 1E with yield ratio of <0.85 which does not confirm to the scope of present invention. Also, Bendablility of steel 1D and 1E is found to be rather poor and marked as “?” in table 3.

Example 2: Steel sheet “2” as listed in table 1 has chemical composition as per the scope of invention having C, Mn ,Si, Ti ,Nb and other compositional elements within the specified range . Steel “2” was processed through different hot rolling and annealing conditions as listed in table 2 and numbered as 2A and 2B. Bothe 2A and 2B Steel are processed as per the scope of present invention keeping soaking temperature in the range from 700°C to 760°C, rapid cooling section end temperature in the range from range from 420°C to 480 °with cooling rate of =30 0C/sec. Both steel i.e. 2A and 2B confirms the microstructural features as described in the scope with Ferrite area fraction > 85 % and atleast one from bainite and Pearlite < 15 % with no martensite/retained austenite in microstructure. As a result, steel 2A and 2B satisfies the desired properties claimedas a part of invention having YS >850 MPa 8, YS/UTS ratio of atleast 0.85and UTS < 1050 MPa . In addition, Steel 2A and 2B shows excellent phosphatability with phosphate crystal size =4µm and phosphate coating weight in the range from 1.5-2.5 g/m2 post zinc phosphate chemical conversion coating and marked as “O”. Steel 1A and 1B also exhibits excellent bending property as well (marked “O”) with no cracks observed at 40X magnification on bend surface post 900 Vbend.

Example 3: Steel sheet “3”as listed in table 1 has chemical composition out of the range fromthe scope of present invention having atleast one compositional element selected from C, Mn, Si, Ti, Nb and other elements outside the specified range . Also , As listed in table 2, Steel “3” is annealedsoaking section temperature of 820 0C with annealing time of 162 seconds which are outside the range as listed in scope of present invention. In addition, Steel “3” is processed at very low RCS and OAS temperature of 360 0C and 260 0C respectively that is below Maretensite start temperature for given composition. As a consequence, steel 3 shows lower Ferrite fraction of 55 % along with high Martensite+ Retained austenite area fraction of 43 % leading to lower YS of 723 MPa and Yield ratio of 0.71. Due to higher Marteniste/Retained Austenite phase fraction in microstructure, steel 3 shows rather poor Hole expansion (35%) and Bendablility. Steel 3 also has 3.1 % Mn in composition resulting in poor phosphatability due to formation of excess MnO type complex oxide during annealing. A poor aging resistance of steel 3 is attributed to low Ti (0.002 wt %) in composition as listed in table 2. Such low Ti level in composition is not sufficient to fix excess free N leading to poor aging resistance which is marked as “?” in table 3.

It is thus possible by way of the present invention to provide High strength cold rolled steel Sheet with high yield ratio and having excellent Bendablility, improved phosphatability and reduced spring back effect and a method for producing the same. The cold rolled steel sheet is having composition in terms of mass percent comprising: 0.06wt % to 0.1wt % of C; 1.4 wt% to 2.0 wt% of Mn;0.2 wt% to 0.5 wt% of Si; 0.02 wt% to 0.08 wt% of Al; 0.03 wt% or less of P; 0.05 wt% to 0.1 wt% of Nb; 0.05 wt% to 0.1 wt % of Ti; Up to 0.006wt% of N; and balance as Fe and incidental impurities, which is processed through selective process steps with selected process parameters in order to achieve yield strength of 850 MPa or more, Tensile strength in the range of 930MPa to 1050 MPa and Yield ratio of 0.85 or more, uniform elongation more than 7% and no visible cracks appear at bend surface after subjecting the steel sheet to bending at 90°, wherein the ratio (Ti+Nb)/C must be in a range from 1.2 to 3 and the steel microstructure constitutes in terms of area fraction having at least one from bainite and pearlite phase less than 15% and ferrite phase more than 85% along with precipitates distributed in ferrite matrix. The steel sheet is having excellent surface quality and phosphatability including phosphate crystal size ranging from 2 µm to 4 µm and phosphate coating weight in the range from 1.5 g/m2 to 2.5 g/m2. Such High strength steel Sheet with high yield ratio is suitable for application in the area of Automotive Reinforcements, Automotive Front Cross member, B pillar, seat & assembly, Seat rail and similar automotive applications.