Claims:We Claim:
1. High Strength Low alloy cold rolled steel sheet comprising composition (wt %):
0.08 to 0.10% of C;
1.4 to1.9 % of Mn;
up to 0.004 % of S;
0.04% or less of Si;
0.04 to 0.08% of Al;
0.01 to0.02 % of P;
0.06to 0.1 % of Nb;
0.04 to 0.08 % of Ti;
Up to 0.006% of N;
Balance as Fe and incidental impurities,
with the ratio of Ti to N is in the range of 7 to 20, and
having yield strength 650 MPa or more, with the micro structural constituents of said steel comprising of 55-60% of ferrite and 41-45% of pearlite/bainite.

2. High Strength Low alloy cold rolled steel sheet as claimed in claims 1 comprising V from 0.01 to 0.02 wt % and B from 5 to 15 ppm.

3. High Strength Low alloy cold rolled steel sheet as claimed in anyone of claims 1 to 2 including in mass % at least one element selected from the group comprising of Sc, Co, Zn, Sn, Ni, Zn, Ca and Cu such that each element weight percent is 0.02% or less.

4. High Strength Low alloy cold rolled steel sheet as claimed in anyone of claims 1 to 3 wherein the micro structural constituents of said steel comprises of 55-60% of polygonal ferrite with average ferrite grain diameter less than 6 micron, 41-45 % of islands of pearlite and with average size less than 2 micron and distributed as network at grain boundary of the polygonal ferrite, and with carbide and nitride precipitates of alloying elements.

5. High Strength Low alloy cold rolled steel sheet as claimed in anyone of claims 1 to 4 having UTS>720 MPa or 720-850 MPa, ,YS/UTS>0.8 with ferrite area fraction in the range of 55-60 %, Pearlite/Bainite area fraction in the range of 41-45 % and Martensite Area %<1 and excellent hole expansion ratio >35 %.

6. A process for the manufacture of cold rolled High Strength Low alloy steel sheet as claimed in anyone of claims 1 to 5 comprising:
a) Providing a selective steel composition for slab generation for desired bendability, stretch flanging and weldability comprising:
0.08 to 0.1% of C;
1.4 to1.9 % of Mn;
upto 0.004% of S;
0.04% or less of Si;
0.04 to 0.08% of Al;
0.01 to0.02 % of P;
0.06to 0.1 % of Nb;
0.04 to 0.08 % of Ti;
Up to 0.006% of N;
Balance as Fe and incidental impurities, and maintaining ratio of Ti to N in the range of 7 to 20 and
b) Carrying out steel sheet manufacturing including hot rolling, pickling, cold reduction and continuous annealing such as to reach to the desired micro structural constituents of said steel comprising of 55-60% of ferrite and 41-45% of pearlite/bainite.

7. A process as claimed in anyone of claims 6 comprising:
i. Hot rolling of said steel slab with slab reheating Temperature 1220°C or less, Finishing Temperature 850°C to 910°C and hot coiled with ROT cooling rate in the range of 9°C/Sec to 14°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%.
8. A process as claimed in anyone of claims 6 to 7 comprising:
a) Heating the cold rolled steel in continues annealing line up to soaking temperature with a heating rate in the range from 1.5 to 5 0C/sec.
b) Soaking said steel in continuous annealing line at temperature 730°C to 790°C with residence time in the range from80 to 100 sec.
c) Slow cooling further said steel at temperature 610°C to 670°C with slow cooling rate in the range from 0.6 °C/Sec to 3°C/Sec;
d) Rapid cooling section Temperature 400-480°C for steel at rapid cooling rate in the range from10°C/Sec to 40 °C/Sec;
e) overaged the said steel in the range from 290°C to 340°C for 260 to 330 sec
f) Skin passing of overaged steel in the range from 0.6% to 1.2%.

9. A process as claimed in anyone of claims 6 to 8 for said steel sheet having excellent Stretch flanging, Bendability and weldability, wherein the above process steps are selectively controlled such as to achieve anyone or more of:
i. Tensile strength 720-850 MPa;
ii. Yield Strength 650-750 MPa with YS/TS ratio of 0.8 or more ;
iii. Hole expansion Ratio 35 % or more.
iv. Bendability V bend with R/t<1.4 no visual cracks at 25X.

Dated this the 24th day of December, 2018
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199

, Description:FIELD OF THE INVENTION
The present invention is directed to provide 650 MPa yield strength level high strength low alloy (HSLA) cold rolled steel sheet with high yield ratio having the chemical composition in terms of weight % comprising: 0.08 to 0.1% of C, Si: 0.04% or less, Mn: 1.4 to 1.9%, N: 0.006% or less, Al:0.04 to 0.08 % , P: 0.010 to 0.02 % , Ti: 0.04 to 0.08%, N:0.006% or less, Nb: 0.06 to 0.1%, V: 0.01-0.02% and the balance being Fe and other inevitable impurities, wherein ratio of Ti to N is in the range of 7 to 20.The micro structural constituents of said steel consisting 55-60% of polygonal ferrite with average ferrite grain diameter less than 6 micron, 41-45% of islands of pearlite and/or bainite with average size less than 2 micron and distributed as network at grain boundary of the polygonal ferrite, with balance being carbide and nitride precipitates of alloying elements. Cold rolled HSLA steel described in present invention has an excellent Bendability, stretch flanging and weldability comprising with good hole expansion ratio (HER) of =35%.

BACK GROUND OF THE INVENTION
The major challenge in front of automobile manufacturers is to fulfil the strict environmental norms for global environment conservation along with passenger safety. In addition, light weighing of an automotive structure to improve fuel efficiency and CO2 emission are the other challenges. To cater these requirements automakers are opting for incorporation of thinner gauge high strength low alloy steels in the structural and body components such as pillars,rocker-panels and reinforcements, cross beams, Body structures, brackets, various seating components ,suspension parts, door intrusion beams. Moreover, apart from weight reduction by thinner gauge high strength steel, high yield ratio is required in order to increase the impact resistance.
Apart from surface condition, forming of high strength low alloy steelsis a key characteristic which governs its application in automotive component. Formability is indicated by stretch flangeability for high strength low alloy steelsin addition to tensile properties for automotive parts. It plays a vital role mainly for complicated auto body parts which are under heavy deformation condition. Stretch flangeability is measured in terms of hole expansion ratio which is significantly influenced by microstructure and their distribution for high strength steel. Homogeneous distribution of second phase along with reduced difference in strength of soft and hard phase governs the stretch flangeability. Martensite being the hardest phase among other phases in steel tends to reduce the stretch flangeability due to increased differential hardness between ferrite and martensite. As the carbon weight % in steel increases, hardness of martensite increases and hole expansion reduces. Also, increased phase fraction of martensite tends to reduce the hole expansion ratio.
Spring back phenomenon is other setback associated with high strength low alloy steelsapplication in automotive .Spring back is an observable fact that occurs during forming whenever the Component is withdrawn from tool set. Spring back happens due to relaxation in elastic behavior is not uniform and the shape of the pressing will not be exactly match as the shape of the punch that has been used in its manufacture.
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.

OBJECTS OF THE INVENTION
The basic object of the present invention is directed to provide high strength low alloy cold rolled steel sheet having excellent bendability, weldability and stretch flanging suitable for automobile application and method of manufacturing the same.
A further object of the present invention is directed to provide said high strength low alloy cold rolled steel sheet having selective low alloy composition and processing steps through continuous annealing route to ensure excellent surface properties after zinc phosphate chemical conversion coating treatment with good hole expansion ratio (HER) of =35%.
A still further object of the present invention is directed to provide said high strength low alloy cold rolled steel sheet which is selectively processed to have micro structural constituents of said steel consistingof 55-60% of polygonal ferrite with average ferrite grain diameter less than 6 micron, 41-45 % of islands of pearlite and with average size less than 2 micron and distributed as network at grain boundary of the polygonal ferrite, and with carbide and nitride precipitates of alloying elements to attain the desired properties.

Another object of the present invention is directed to provide said high strength low alloy cold rolled steel sheet wherein apart from weight reduction by thinner gauge high strength steel, high yield ratio is maintained in order to increase the impact resistance.

A further object of the present invention is directed to provide said high strength low alloy cold rolled steel sheet wherein homogeneous distribution of second phase along with reduced difference in strength of soft and hard phase determines the improved stretch flangeability.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to High Strength Low alloy(HSLA)cold rolled steel sheet comprising composition (wt %):
0.08 to 0.10% of C;
1.4 to1.9 % of Mn;
upto 0.004 % of S;
0.04% or less of Si;
0.04 to 0.08% of Al;
0.01 to0.02 % of P;
0.06 to 0.1 % of Nb;
0.04 to 0.08 % of Ti;
Up to 0.006% of N;
Balance as Fe and incidental impurities,
with the ratio of Ti to N is in the range of 7 to 20, and
having yield strength 650 MPa or more, with the micro structural constituents of said steel comprising of 55-60% of ferrite and 41-45% of pearlite/bainite.

A further aspect of the present invention is directed to said High Strength Low alloy cold rolled steel sheet comprising V from 0.01 to 0.02 wt % and B from 5 to 15 ppm.

A still further aspect of the present invention is directed to said High Strength Low alloy cold rolled steel sheet including in weight % at least one element selected from the group comprising of Sc, Co, Zn, Sn, Ni, Zn, Ca and Cu such that each element weight percent is 0.02% or less.

A still further aspect of the present invention is directed to said High Strength Low alloy cold rolled steel sheet wherein the micro structural constituents of said steel comprises of 55-60% of polygonal ferrite with average ferrite grain diameter less than 6 micron, 40-45 % of islands of pearlite and with average size less than 2 micron and distributed as network at grain boundary of the polygonal ferrite, and with carbide and nitride precipitates of alloying elements.

A still further aspect of the present invention is directed to said High Strength Low alloy cold rolled steel sheet having UTS>720 MPa or 720-850 MPa, ,YS/UTS>0.8 with ferrite area fraction in the range of 55-60 %, Pearlite/Bainite area fraction in the range of 40-45 % and Martensite Area %<1 and excellent hole expansion ratio >35 %.

A still further aspect of the present invention is directed to a process for the manufacture of cold rolled High Strength Low alloy steel sheet as described above comprising:
a) Providing a selective steel composition for slab generation for desired bendability, stretch flanging and weldability comprising:
0.08 to 0.1% of C;
1.4 to1.9 % of Mn;
Up to 0.004% of S;
0.04% or less of Si;
0.04 to 0.08% of Al;
0.01 to0.02 % of P;
0.06 to 0.1 % of Nb;
0.04 to 0.08 % of Ti;
Up to 0.006% of N;
Balance as Fe and incidental impurities, and maintaining ratio of Ti to N in the range of 7 to 20 and
b) Carrying out steel sheet manufacturing including hot rolling, pickling, cold reduction and continuous annealing such as to reach to the desired micro structural constituents of said steel comprising of 55-60% of ferrite and 40-45% of pearlite/bainite.
Yet another aspect of the present invention is directed to said process comprising:
i. Hot rolling of said steel slab with slab reheating Temperature 1220°C or less, Finishing Temperature 850°C to 910°C and hot coiled with ROT cooling rate in the range of 9°C/Sec to 14°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 saidprocess comprising:
a) Heating the cold rolled steel in continues annealing line up to soaking temperature with a heating rate in the range from 1.5 to 5 0C/sec.
b) Soaking said steel in continuous annealing line at temperature 730°C to 790°C with residence time in the range from80 to 100 sec.
c) Slow cooling further said steel at temperature 610°C to 670°C with slow cooling rate in the range from 0.6 °C/Sec to 3°C/Sec;
d) Rapid cooling section Temperature 400-480°C for steel at rapid cooling rate in the range from10°C/Sec to 40 °C/Sec;
e) overaged the said steel in the range from 290°C to 340°C for 260 to 330 sec
f) Skin passing of overaged steel in the range from 0.6% to 1.2%.

A still further aspect of the present invention is directed to said process for said steel sheet having excellent Stretch flanging, Bendability and weldability, wherein the above process steps are selectively controlled such as to achieve anyone or more of:
i. Tensile strength 720-850 MPa;
ii. Yield Strength 650-750 MPa with YS/TS ratio of 0.8 or more ;
iii. Hole expansion Ratio 35 % or more.
iv. Bendability V bend with R/t<1.4 no visual cracks at 25X.

The above and other objects and advantages of the present invention are described hereunder in greater details with reference to accompanying illustrative examples.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING EXAMPLES

Present invention relates to 650 MPa Yield strength low carbon high yield ratio cold rolled steel sheet whereas the chemical composition of steel comprises in terms of mass fraction: 0.08 to 0.1% of C; 1.4 to 1.9% of Mn; 0.04%or less of Si; 0.04 to 0.08 % of Al; 0.010-0.02 % of P; 0.06 to 0.1 % of Nb; 0.01 to 0.02% of V; Up to 0.006% of N; 0.04 to 0.08 % of Ti; and the balance being Fe and other inevitable impurities, whereas ratio of Ti to N is in the range of 7 to 20. The micro structural constituents of said steel consisting 55-60 % of polygonal ferrite with average ferrite grain diameter less than 6 micron, 40-45% of islands of pearlite and/or bainite with average size less than 3 micron and distributed as network at grain boundary of the polygonal ferrite, less than 1% of island of martensite with balance being carbide and nitride precipitates of alloying elements.Cold rolled HSLA steel described in present invention has an excellent Bendability, stretch flanging and weldability with good hole expansion ratio (HER %) of =50%.

650 MPa Yield strength level High Strength Low alloy cold rolled steel sheet having selective composition wherein the effect of Metallurgical factors affecting the mechanical and surface properties are described hereunder in details–
Carbon (0.08- 0.10 wt.%) – Carbon ranging from 0.08 to 0.1% is used for increasing the yield strength of the material. To achieve the minimum yield strength of 650MPa and TS atleast720 MPa minimum 0.08% of carbon is required as C forms TiC, Ti[C,N] VC, VN and NbC With Ti and Nb which increases the Yield and Tensile strength by restricting dislocation movement and grain size refinement. However, Excessive amount of carbon increases the yield strength significantly and reduces the ductility and weldability. Hence the upper limit of carbon is maintained in to 0.1% to achieve the desired properties. In addition, higher carbon results in higher amount of second phase (Pearlite/ Bainite/Martensite) and also increases the hardness of second phase results in poor hole expansion.
Manganese (1.4-1.9 wt.%) – Manganese acts as a solid solution strengthening, increase in manganese content increases the tensile strength, the drastic increase in tensile strength by addition of manganese happens not only because of solid solution strengthening but also by ferrite grain refinement. To achieve the minimum strength level 1.4 % is required which will act as a grain refinement. However, upper limit should be maintained to 1.9 % to avoid higher Tensile strength than required limit, which could create problems in forming process.
Silicon (0.04 wt.% or less) - Silicon deteriorates plating /surface properties by forming SiO2 type of oxide (Scale). It is advantageous to add as low an amount of silicon in the steel as is possible. The controlled amount of silicon is preferably being 0.04wt% or less.
Phosphorus (0.010-0.02wt.%)–Phosphorus being most effective and economical solid solution strengthening element helps to achieve the desired yield strength and tensile strength at low cost. However, 0.010 % Minimum P% Is required to achieve 720 Min Tensile strength, when the P content exceeds 0.02%, weldability is deteriorated. High P also causes poor plating property during painting process. Hence, Upper limit is set to 0.02 % preferably.
Niobium (0.06 to 0.10 wt.%) –Niobium increases yield strength by formation of very fine nano metric precipitates such as NbC and Nb[C, N].The strength of steel is mainly governed by the amount, size and distribution of these precipitates. Niobium is also effective in grain refinement; addition of niobium give combined effect of precipitation strengthening and grain refinement thereby increase the strength by 20 to 30 Mpa per 0.01% of niobium addition. Nb in ferrite inhibits the formation excess pearlite by fixing C,hence helps to achieve higher fraction of ferrite. In this way it helps to improve hole expansion ratio of steel.
However, Higher Niobium addition delays the recrystallization and recovery of cold worked steel. When niobium content ranges from 0.06 to 0.1weight%, the annealing temperature can be kept below 790°C for full recrystallization with given continuous annealing time. Annealing with higher temperature above 790 °C with given Nb range may coarsen and dissolve the precipitate resulting is poor strength. In addition, excess Nb will cause higher amount of hard NbC precipitate in ferrite matrix which may deteriorate the hole expansion ratio.
Aluminum (0.02-0.08wt.%) – Aluminum acts as a deoxidizing agent and available in an amount ranging from 0.020 to 0.08 % as an indication of good deoxidation during steel making. Although, when present more than 0.08% it generates inclusion which is one of the possible causes for Al2O3 type slivers (Non-metallic inclusion).Therefore the aluminum present should be 0.08% or less. Very less Al of 0.02% or less will lead to higher dissolve oxygen which is harmful for steel cleanliness.
Nitrogen (0.006 wt.% or less ) –Nitrogen present in the high strength low alloy steel containing the titanium, Vanadium and niobium increase strength by formation of nitride precipitates.When present more than 0.006% with given level of Ti and Nb, it may lead to presence of free nitrogen in steel matrix resulting in yield point elongation and poor ageing property . Hence nitrogen level is restricted to 0.006weight %or less.
Titanium (0.04 to 0.08 wt.%)-Titanium in combination of N should be present in amount such that the ratio of Ti/ N should be in a range from 7 to 20. Ti in steel the low carbon steels forms carbides and nitrides to provide grain refinement and precipitation strengthening. In addition, it acts to control sulphide inclusion by forming complex titanium sulphides.For effective strengthening, minimum titanium level of 0.04% is required to achieve desired yield strength of atleast650Mpa. Titanium as a grain refiner retards austenite grain growth by formation of titanium nitride. Small percentage of titanium first forms titanium nitride, by increasing the level further it forms titanium-carbo-sulphides which in turn provideshape control of titanium-carbo-sulphides whichprovides good surface.
The formation of titanium carbide occurs after the formation of titanium carbosulphides which helps in precipitation strengthening .The minimum requirement of titanium is 0.04% derived from minimum Ti Required to prevent accelerated ageing,as per Ti/ N (Min)= 47/14*N (wt %) to fix 0.006% of Nitrogen as well as to provide strength to the sheet, but when it exceeds the 0.08% the strength of material increases drastically. To achieve the desired yield strength of 650 MPa with yield ratio of 0.8 and also to prevent accelerated ageing in the material titanium level should be maintained between 0.04-0.08 weight percent.
As Ti/N also determines the variation in the yield strength, the ratio of Ti/N has to control between 7 to 20, to achieve minimum yield strength of 650 MPa with yield ratio of 0.80 or more.
Vanadium (0.01 to 0.02 wt.%) Vanadium is strong Nitride and carbide former element, the strength of steel is mainly governed by the amount, and size and distribution of these precipitates, addition of niobium give combined effect of precipitation strengthening and grain refinement thereby increase the strength by 10 to 20Mpa per 0.01% of Vanadium addition. Minimum Vanadium level of 0.01% is required to achieve desired yield strength of atleast650Mpa as well as it provides non ageing guaranty for High Strength low carbon steel sheet.
Boron (0.0005 to 0.0015 wt.%) Boron reduces solute nitrogen by forming Boron Nitride; lower limit of boron should be 0.0005 wt% effective for anti-agingproperty. Boron also strengthens the grain boundaries. However, when amount of boron exceeds 0.0015% causes edge crack during hot rolling and embrittlement of steel due to excess amount of solute boron.
Group of elements from Sc, Co, Zn, Sn, Ni, Zn, Ca and Cu such that each element weight percent is 0.02% or less-Group of Elements such as Sc, Co, Zn, Sn, Ni, Zn, Ca and Cu 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 0.02wt% unnecessarily adds up to the cost of the steel.
Steel Microstructure: High Strength Low alloy cold rolled steel sheet having micro structural constituents comprises 55-60% of polygonal ferrite with average ferrite grain diameter less than 6 micron, 41-45% of islands of pearlite and/or bainite with average size less than 3 micron and distributed as network at grain boundary of the polygonal ferrite, less than 1% of island of martensite with balance being carbide and nitride precipitates of alloying elements. The fraction of martensite has been kept below 1 % to improve the hole expansion as hard martensite tends to deteriorate hole expansion ratio. Minimum 41 % of pearlite is needed to achieve a desired strength of 650 MPa. Higher pearlite more than 45% reduces hole expansion ratio and also deteriorates the drawability. Precipitates such as TiN,TiS, VN, VC,NbC, Nb(CN) form due addition of Ti and Nb and adds up to strength due to gain refinement and precipitation strengthening. Minimum 55% of ferrite helps to achieve good elongation as well as hole expansion.
Hot Rolling: Slab reheating temperature (SRT) in the range from 1150 °C to 1220 °C is suggested as in this range all Carbides of Nb and Ti can be dissolved which can later be precipitated out. Also at this range of reheating temperature, the hot rolling of slabs will be easier and the load at rolling mill will be in within control limit. Higher reheating temperature may lead to excess scale formation deteriorating the surface property where as lower SRT may lead to excess rolling load than mill capability.
Hot rolling finishing temperature (FT) in the range from 850 0C to 9100C is suggested in the present invention. Keeping FT below 850 0C will cause excessive rolling load andmay damage the rolls and mill as well as two phase rolling leading to uneven or mix grain microstructure. FT >9100C may result excessive grain growth, to keep grain size small we keep FT less than910 0C.It is advisable to roll the steel below dynamic recrystallization end temperature (Tnr), hence the upper limit of Ft is kept below 910 0C.
Post hot roiling the run out table (ROT) cooling rate in the range from 9 °C/sec to 14°C/Sec is necessary for present invention as higher cooling rate may lead to lower coiling temperature and lead to martensite formation .Lower cooling rate than 9 °C lead to higher coiling temperature than 590 °C and may lead to coarse ferrite structure.

Complete Description of Process:
To achieve Slab chemistry as described in scope of the invention, liquid steel 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 below 1220°C intended to control roughing mill delivery temperature under 1060°C and finishing mill entry temperature under 1030°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 80C/sec was maintained to achieve coiling temperature range of 550 °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 40 to 60%.
Following pickling and cold rolling to desired thickness,cold rolled steel strip is processed through continuous annealing line to get cold rolled closed annealed product.
While processing the cold rolled steel strip through continuous annealing line, processing steps comprises: electrolytic cleaning which removes rolling emulsion present on the surface. Electrolytic cleaned surface passes through the preheating and heating section inside continuous annealing line where the strip is heated at the rate of 1.5-5°C/sec up to soaking temperature maintained in the range from 730-790°C. Annealing time at soaking section ranges from70-130 seconds which gives desired results forhigh Strength Low alloy cold rolled steel.At soaking section temperature intercritical annealing results in ferrite and austenite microstructure which later transforms to ferrite + pearlite or Ferrite+ Bainite + Martensite 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 of less than 3°C/sec.Slow cooling section temperature of 610°Cto670°C was maintained. Following slow cooling section annealed strip sheet been rapid cooled in the range from10°C/sec to 40°C/sec up to rapid cooling section temperature of 400°C to 480°C or more to avoid martensite formation. After rapid cooling section annealed strip was over aged keeping the over aging section temperature in the range from 290°C to 340°C to avoid yield point elongation. After over aging Skin-pass elongation (Temper rolling) in the range of 0.6 % to 1.2% was applied to avoid yield point elongation.
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

Complete description of Inventive steel and comparative steel grades are illustrated in the following Table 1 to Table 4:
Table 1:Elemental Compositions of the inventive steel sheets along with comparative example and their respective values of Ti/N ratio.
S.NO C Mn P SI AL N TI NB V B Ti/N ratio Others Remarks
1 0.081 1.43 0.011 0.025 0.06 0.0055 0.062 0.064 0.018 0.0008 11.3 Sc:0.002,S:0.003, Ca:0.003, Hf:0.004 Ex
2 0.12 1.4 0.017 0.025 0.065 0.0059 0.053 0.085 0.02 0.0003 9.0 Ca:0.003,Cu:0.005, S:0.002, Comp
3 0.088 1.44 0.017 0.025 0.055 0.0059 0.053 0.065 0.011 0.001 9.0 REM:0.0028 Ex
4 0.092 1.97 0.011 0.016 0.06 0.0047 0.058 0.062 0.018 0.0005 12.3 S:0.002,Zn:0.003, Ca:0.003, S:0.007 Comp
5 0.086 1.28 0.012 0.025 0.06 0.0055 0.062 0.074 0.023 0.0017 11.3 Cr 0.005, S:0.010 Comp
6 0.092 1.47 0.011 0.016 0.06 0.0047 0.058 0.062 0.018 0.0012 12.3 Cu:0.001, Ni:0.005,Mo:0.008,S:0.002 Ex
7 0.086 1.43 0.003 0.025 0.069 0.006 0.041 0.069 0.021 0.0006 6.8 Mo:0.003,Cr:0.008, Ca 0.003, S:0.003, Comp
8 0.088 1.4 0.017 0.025 0.055 0.0035 0.077 0.065 0.02 0.0008 22.0 Cu:0.01 , Ni:0.01,Mo:0.008, ,S:0.001 Comp
9 0.1 1.44 0.011 0.018 0.051 0.0053 0.055 0.055 0.01 0.0001 10.4 Mo:0.3Cr:0.3, Ca 0.003 Comp
10 0.088 1.4 0.017 0.025 0.055 0.0058 0.053 0.115 0.015 0.0008 9.1 Mo:0.003,Cr:0.003, S:0.003 Comp
11 0.092 1.47 0.011 0.016 0.058 0.0047 0.058 0.068 0.013 0.0015 12.3 Cu:0.01 , Ni:0.01,Mo:0.008,S:0.001 Ex
12 0.086 1.43 0.007 0.025 0.06 0.0059 0.062 0.064 0.019 0.0009 10.5 Cu:0.01 , Ni:0.01, Mg:0.003 ,Mo:0.008, Comp
13 0.088 1.4 0.035 0.025 0.075 0.006 0.053 0.072 0.022 0.0019 8.8 Ca:0.003,Cu:0.005, S:0.001 Comp
14 0.096 1.46 0.013 0.023 0.063 0.0045 0.05 0.069 0.017 0.0012 11.1 S:0.003, Sn:0.002,Cr:0.015,Zr:0.003,Co:0.003 Ex

* Ex - Present inventive example, Comp- Comparative Examples.
** Underlined and shaded boxes indicates “outside the appropriate range”
Steels having Ti/Nratiooutside the range of 7 to 20do not comply with the scope of the invention.
Table 2:Hot rolling, cold rolling, continuous annealing parameters of inventive andcomparative steel sheets having chemical compositions as per Table 1.
Steel SRT, FT, ROT CT, 0C Cold SS AT , SCS Temp, 0C RCS Cooling Rate, RCS Temp, 0C OAS Temp, 0C SPM% Remarks
No.

0C

0C

Cooling Redn Temp , sec

0C/sec


rate, ,% 0C
0C/sec
1 1200 885 12.5 565 53.6 743 95 667 29.08 455 311 1.04 Ex
2 1180 875 11.1 550 58.5 765 92 658 23.5 470 325 1.20 Comp
3A 1210 880 12.8 570 50.8 780 90 660 21.9 460 301 1.17 Ex
3B 1185 890 10.2 581 52.7 770 88 651 8 471 333 1.02 Comp
3C 1215 887 12.8 575 55.8 765 94 666 42 478 315 1.21 Comp
4 1190 890 11.5 579 51.5 768 98 661 24.3 459 333 1.18 Comp
5 1195 888 14.8 577 52.1 755 89 670 27.2 469 320 1.02 Comp
6A 1188 860 12.6 565 50.8 762 85 669 22.8 462 331 1.03 Ex
6B 1195 865 7.9 571 52.8 756 89 648 20 452 314 1.19 Comp
7 1200 880 11.6 580 58.1 740 88 665 19.5 460 320 1.09 Comp
8 1195 890 9.8 579 51.5 768 81 652 28.1 475 312 1.12 Comp
9 1180 888 10.5 585 52.8 771 88 669 15.7 470 332 1.18 Comp
10 1215 868 13.2 572 49.7 745 89 662 21.6 467 321 1.02 Comp
11A 1201 871 11.4 565 58.1 735 85 654 30.4 441 310 1.00 Ex
11B 1190 885 11.5 555 51.9 750 83 651 28.5 450 375 1.13 Comp
12 1215 881 13.7 575 51 770 93 665 28.4 468 328 1.01 Comp
13 1185 899 10.5 588 57 755 91 656 22.8 466 330 1.02 Comp
14 1170 885 13.1 580 55.4 778 82 659 26.5 472 338 1.10 Ex

* SRT: Slab reheating temperature, FT: Hot rolling finishing temperature, ROT: Run out table between finishing and coiling at hot rolling,ColdRedn: Cold rolling reduction, SS: Soaking section temperature in Continuous annealing line, AT: Annealing Time, SCS: Slow cooling section temperature, RCS: Rapid cooling section temperature, OAS: Over aging section temperature, SPM: Skin pass elongation.
Table 3:Mechanical property, micro structural phase fractions, Avg. grain size and HER % value of steel sheets having chemical composition as per Table 1 and processed as per Table 2.
Steel YS, MPa UTS, MPa El%, YS/UTS HER% Ferrite area % Average Ferrite Grain Size,µm Pearlite /bainitearea % Martensite area % Remarks
No. 50gl


1 654 722 14.6 0.91 38.5 58.5 5.42 41.5 0 Ex
2 760 862 9.1 0.88 25.9 46.1 5.01 53.9 0 Comp
3A 675 739 13.8 0.91 36.9 56.9 5.64 43.1 0 Ex
3B 631 710 16.4 0.89 39.1 61.5 7.53 38.5 0 Comp
3C 754 855 9.6 0.88 26.3 47.8 5.28 47.2 5 Comp
4 730 860 12.6 0.85 27.7 52.0 5.31 48.0 0 Comp
5 642 715 14.8 0.90 38.1 59.5 6.22 40.5 0 Comp
6A 658 747 14.0 0.88 37.5 57.2 5.31 42.8 0 Ex
6B 638 710 16.0 0.90 38.9 61.4 6.58 35.6 3 Comp
7 640 729 15.9 0.88 37.5 60.8 6.31 39.2 0 Comp
8 757 821 10.3 0.92 26.1 46.7 5.11 53.3 0 Comp
9 645 740 15.1 0.87 37.2 60.0 6.00 40.0 0 Comp
10 755 798 9.77 0.95 27.5 47.0 5.14 53.0 0 Comp
11A 694 774 13.1 0.90 36.8 55.4 5.52 44.6 0 Ex
11B 660 715 15.6 0.92 37.7 58.1 5.83 41.9 0 Comp
12 641 722 15.0 0.89 38.2 60.2 5.91 39.8 0 Comp
13 733 851 10.8 0.86 32.1 51.4 5.25 48.6 0 Comp
14 681 763 13.5 0.89 35.6 55.8 5.66 44.2 0 Ex

*Underlined and shaded boxes indicates “outside the appropriate range”
*HER =Hole expansion ratio
*Steel sheets having YS/UTS ratio <0.80 does not conform to the scope of the present invention.
* Steel having hole expansion ratio less than 35 % does not comply with the scope of present invention.
** Steel microstructure having pearlite/bainite phase fraction 41-45 % and martensite phase fraction >1% does not comply with the scope the present invention.
Table 4:Stretch flanging, Bendability and Weldability, properties of steel sheets having chemical composition as per Table 1 and processed as per Table 2 and property as Table 3.
Steel Stretch flanging Bendability weldability Remarks
No.
1 Good Good Good Ex
2 Poor Poor Poor Comp
3A Good Good Good Ex
3B Good Good Good Comp
3C Poor Poor Poor Comp
4 Poor Poor Poor Comp
5 Good Good Good Comp
6A Good Good Good Ex
6B Good Good Good Comp
7 Good Good Good Comp
8 Poor Poor Good Comp
9 Good Good Good Comp
10 Poor Poor Good Comp
11A Good Good Good Ex
11B Good Good Good Comp
12 Good Good Poor Comp
13 Good Poor Poor Comp
14 Good Good Good Ex

* * Steel with Hole expansion ratio<35 % does not comply with the scope of invention as the stretchflangeabilityis poor for these steel grades.
* Steel having Pearlite fraction in the range of41-45 % and Martensite fraction > 1% does not comply with the scope of invention.
Cold rolled sheet marked as poor bendability when visual cracks observed at 25 X magnification after V bend test at R/t =1.4, where t is thickness and r is bending radius.
Example 1 : As listed in Table 1, Steel sample number 1,3A,6A,11A and 14 (Marked as Ex. ) have chemical composition range as per the scope of present invention with Ti/N ratio in the range from 7 to 20. Low Ti/N ratio from the scope of present invention like Ti/N ratio is 6.8, we could not able to achieve required YS due to insufficient precipitation strengthening and grain boundary strengthening, but in case where Ti/N ratio is more than 20, we achieve mechanical property out ofscope of present invention. In case if Ti/N is in scope of present invention, Steel sample number11A and 14, property of sample is in scope of present invention. All the experimental (Marked as Ex.)steel samples having B in the range of 5 to 15 ppm are within scope of present invention because B form the Nitride at the grain boundary which prevent the grain coarsening and higher B from inventive range reduces the hot ductility of the steel. Steel samples processed in hot rolling mill with varying process parameter and then it processedat continuous annealing line as per the scope of present invention.It should be noted that continuous annealing linehave soaking section,slow cooling section and overaging section. The steel sample number which is out of present invention is marked as Comp sample.

Example 2 : As given in table 2 , Steel sample number 1,3A,6A,11A and 14 is processed in Hot rolling mill with varying Runout table(ROT) cooling rate from 9 to 14, High ROT cooling leads to form High martensite and less ROT cooling rate form more ferritic/pearlitic structure and bigger ferrite grain size than present invention. Soaking section temperature in the range from 7300C to 7900C with soaking time in the range from 80-100 seconds, Slow cooling section temperature in the range from 6100C to 6700C, and rapid cooling section temperature in the range from 400-480 with cooling rate in the range from 10-400C/sec, steel sample number 3C having RCL cooling rate is 420C/sec, form martensite phase, so the maximum rapid cooling rate is 40 0C/secto avoid martensite formation. And Over ageing section temperature in the range of 290-3400C with residence time 260-330 sec, steel sample number 11Bhaving higher OAS temperature forms tempered martensite and hence Tensile strength cannot achieve, which is not in our scope.

As far as the mechanical properties are concerned ,steel sample number 1,3A,6A,11A and 14 satisfies all the scope of present invention having , YS >650 MPaor 650-750 MPa,UTS>720 MPa or 720-850 MPa, ,YS/UTS>0.8 with ferrite area fraction in the range of 55-60 %, Pearlite/Bainite area fraction in the range of 41-45 % and Martensite Area %<1 Excellent hole expansion ratio >35 % for inventive steel samples is attributed to higher fraction (55-60%) of fine polygonal ferrite matrix along with reduced fraction of hard matensite phase.

For surface critical properties, Steel sample number 1,3A,6A,11A and 14 comply with all the scope of present invention as given in table 4 having good stretch flanging ability, bendability, weldability

Example 3: Steel sample number 2, 4, 5, 7, 8, 9, 10, 12, 13 has chemical composition range outside the scope of the present invention with C, Mn, Nb, P, V,Ti/N ratio being outside the specified range. This Steel sample number annealed as per the specified range of present invention,however due to Higher C, Mn, Nb, P, Ti/N ratio and, does not comply with the mechanical property. Steel sample number 2,4,8,10,12, 13 showing poor Stretch flanging,Bendability, weldability due to higher Mn %, Higher RCS Cooling rate and Higher Martensite % are out of present invention scope.