Claims:We Claim:

1. High Strength Low alloy cold rolled steel sheet composition comprising (wt %):
0.06to 0.1% of C;
1.0to1.7 % of Mn;
0.04% or less of Si;
0.02 to 0.06% of Al;
0.02 % or less of P;
0.04 to 0.1 % of Nb;
0.01 to 0.04% of V;
Up to 0.006% of N;
Balance as Fe and incidental impurities, having yield strength 550 MPa or more, wherein ratio of V to N is in the range of 3.6 to 20, additionally the micro structural constituents of said steel comprising of 60-69% of ferrite and 30-39% of pearlite and/or bainite and less than 1% of Martensite with balance being carbide and nitride precipitates of alloying elements.

2. High Strength Low alloy cold rolled steel sheet composition as claimed in anyone of claims 1 comprises Ti from 0.02 to 0.08wt % such that ratio of (Ti+V)/N is in the range of 10 to 40 for ageing resistance.

3. High strength low alloy cold rolled steel sheet as claimed in anyone of claims 1 or 2 wherein:
Si is in the range of 0 to 0.04%; and
Pis in the range of 0 to 0.02%.

4. High Strength Low alloy cold rolled steel sheet composition 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, Cu, Zn, Cr, Mo, Ca, W, Hf and Zrwith each element weight percent being 0.03% or less.

5. A process for the manufacture of cold rolled High Strength Low alloy steel sheet as claimed in anyone of claims 1 to 3 comprising:
a.) providing a selective steel composition comprising:
0.06to 0.1% of C;
1.0 to 1.7 % of Mn;
0.04%or less of Si;
0.02 to 0.06 % of Al;
0.02 % or less of P;
0.04 to 0.1 % of Nb;
0.01 to 0.04% of V;
Up to 0.006% of N;
Balance as Fe and incidental impurities, whereas ratio of V to N is in the range of 3.6 to 20, which is continuously cast into slab 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 2.5 g/m2and such as to provide sheet steel with micro structural constituents comprising of 60-69% of ferrite and 30-39% of pearlite and/or bainite and less than 1% of Martensite with balance being carbide and nitride precipitates of alloying elements.

6. A process as claimed in claim 4 comprising:
i. Hot rolling of said steel slab with slab reheating Temperature 1220°C or less preferably 1180 to 12000C, 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 40% to 70%.

7. A process as claimed in claim 4 or 5 further 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 760°C to 820°C with residence time in the range from70 to130sec.
c. Slow cooling further said steel at temperature 610°C to 690°C with slow cooling rate in the range from 0.5 °C/Sec to 4°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 340°C to 400°C for 250 to 440 sec
f. Skin passing of overaged steel in the range from 0.6% to 1.8%.

8. A process as claimed in anyone of claims 5 to 6 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 620-720 MPa;
ii. Yield Strength 550-650 MPa with YS/TS ratio of 0.8 or more ;
iii. Hole expansion Ratio 40 % or more;
iv. Bendability;
v. Weldablity; and
having micro structural constituents of said steel consisting of 60-69% polygonal ferrite with average ferrite grain diameter less than 7 micron, 30-39% 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.

Dated this the 14th day of August, 2018
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s agent)
IN/PA-199

, Description:FIELD OF THE INVENTION

Present invention relates to high strength low alloy (HSLA) having high yield ratio cold rolled steel sheet having yield strength 550 MPa or morewherein the chemical composition of steel comprises in terms of mass fraction: 0.06 to 0.1% of C, Si: 0.04% or less, Mn: 1.0 to 1.7%, N: 0.006% or less, Al:0.02 to 0.06 % , P: 0.02 % or less, Nb: 0.04 to 0.1%,V: 0.01-0.04% and the balance being Fe and other inevitable impurities, whereas ratio of V to N is in the range of 3.6 to 20. Said steel is selectively processed to have micro structural constituents with selective proportion and distribution comprising polygonal ferrite with average ferrite grain diameter less than 7micron, 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 to achive the desired properties including excellent Bendability, stretch flanging and weldability and a phosphate crystal size of 4 µm or less and phosphate coating weight of 1.5-2.5 g/m2 after zinc phosphate chemical conversion coating treatment with good hole expansion ratio (HER %) of =40%.

BACKGROUND ART

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 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 steel is a key characteristic which governs its application in automotive component. Formability is indicated by stretch flangeability for high strength steel in 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 steel application 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. The most important fact that affects spring back is yield ratio. A higher yield ratio(>0.8) results in lower spring back due to reduced elastic relaxation.

In JPH11350038A by the combination of the production conditions and the particular steel composition it is possible to improve ductility and stretch flange formability in 980MPa grade high-tensile steel, with carbon equivalent Ceq = C + Mn / 6 + Si / 24 defined by the content of Mn and Si is a condition that it is from 0.40 to 0.52. It’s defined in opened patent that Ceq is difficult to ensure the required strength level if it is less than 0.4. It results in cracking while bending, bendability is poor, and shape of strip is very poor due to condition of hot rolling.

The present inventions aims at advantageously solving the problems of the prior art described above and an object thereof is to provide a cold rolled steel sheet capable of stably exhibiting uniform phosphate grain size with less coating weight, and advantageously having better Shape, good bendability, weldability and excellent hole expansion ratio and a method for manufacturing the non aging steel sheet through continuous annealing route.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide high strength 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 propertiesafter zinc phosphate chemical conversion coating treatment with good hole expansion ratio (HER %) of =40%.

A still further object of the present inventionis directed to provide said high strength low alloy cold rolled steel sheet which is selectively processed to have micro structural constituents of said steel consisting 60-69% of polygonal ferrite with average ferrite grain diameter less than 7micron,30-39% 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 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.
Yet another object of the present invention is directed to provide said high strength low alloy cold rolled steel sheet wherein higher yield ratio(>0.8) results in lower spring back due to reduced elastic relaxation.

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 toprovideHigh Strength Low alloy(HSLA) cold rolled steel sheet composition comprising (wt %):
0.06to 0.1% of C;
1.0to1.7 % of Mn;
0.04% or less of Si;
0.02 to 0.06% of Al;
0.02 % or less of P;
0.04 to 0.1 % of Nb;
0.01 to 0.04% of V;
Up to 0.006% of N;
Balance as Fe and incidental impurities, having yield strength 550 MPa or more, wherein ratio of V to N is in the range of 3.6 to 20, additionally the micro structural constituents of said steel comprising of 60-69% of polygonal ferrite and 30-39% of pearlite and/or bainite and less than 1% of Martensite with balance being carbide and nitride precipitates of alloying elements.

A further aspect of the present invention is directed to provide said High Strength Low alloy cold rolled steel sheet composition comprising Ti from 0.02 to 0.08wt % such that ratio of (Ti+V)/N is in the range of 10 to 40 for ageing resistance.

A still further aspect of the present invention is directed to said High strength low alloy cold rolled steel sheet wherein:
Si is in the range of 0 to 0.04%; and
P is in the range of 0 to 0.02%.

A still further aspect of the present invention is directed to said High Strength Low alloy cold rolled steel sheet composition including in mass % at least one element selected from the group comprising of Sc, Co, Zn, Sn, Ni, Cu, Zn, Cr, Mo, Ca, W, Hf and Zr with each element weight percent being 0.03% or less.

Another aspect of the present invention is directed to a process for the manufacture of cold rolled High Strength Low alloy steel sheet comprising:
a.) providing a selective steel composition comprising:
0.06to 0.1% of C;
1.0 to 1.7 % of Mn;
0.04%or less of Si;
0.02 to 0.06 % of Al;
0.02 % or less of P;
0.04 to 0.1 % of Nb;
0.01 to 0.04% of V;
Up to 0.006% of N;
Balance as Fe and incidental impurities, wherein ratio of V to N is in the range of 3.6 to 20, said steel continuously cast into slab 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 2.5 g/m2and such as to provide sheet steel with micro structural constituents comprising of 60-69% of ferrite and 30-39% of pearlite and/or bainite and less than 1% of Martensite with balance being carbide and nitride precipitates of alloying elements.

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 lesspreferably 1180 to 12000C, 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 40% to 70%.

A further aspect of the present invention is directed to said process further 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 760°C to 820°C with residence time in the range from70 to130sec.
c. Slow cooling further said steel at temperature 610°C to 690°C with slow cooling rate in the range from 0.5 °C/Sec to 4°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 340°C to 400°C for 250 to 440 sec
f. Skin passing of overaged steel in the range from 0.6% to 1.8%.

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 620-720 MPa;
ii. Yield Strength 550-650 MPa with YS/TS ratio of 0.8 or more ;
iii. Hole expansion Ratio 40 % or more;
iv. Bendability;
v. Weldablity; and
having micro structural constituents of said steel consisting of 60-69% polygonal ferrite with average ferrite grain diameter less than 7 micron, 30-39% 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 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 EXAMPLES:

Present invention relates to high strength low alloy(HSLA) cold rolled steel sheet having yield strength 550 MPa or more with high yield ratio wherein the chemical composition of steel comprises in terms of mass fraction: 0.06 to 0.1% of C; 1.0 to 1.7 % of Mn; 0.04%or less of Si; 0.02 to 0.06 % of Al; 0.02 % or less of P; 0.04 to 0.1 % of Nb; 0.01 to 0.04% of V; Up to 0.006% of N;, and the balance being Fe and other inevitable impurities, whereas ratio of V to N is in the range of 3.6 to 20 and the ratio of (Ti+V)/N must be in the range of 10-40. The micro structural constituents of said steel consisting 60-69 % of polygonal ferrite with average ferrite grain diameter less than 7 micron, 30-39% 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 excellent Bendability, stretch flanging and weldability comprising a phosphate crystal size of 4 µm or less and phosphate coating weight of 1.5-2.5 g/m2 after zinc phosphate chemical conversion coating treatment with good hole expansion ratio (HER %) of =40%.

The effect of Metallurgical factors affecting the mechanical and surface properties to decide the composition of 550 MPa Yield strength level High Strength Low alloy cold rolled steel sheet,are described hereunder in details–
Carbon (0.06-0.1%wt %) – Carbon ranging from 0.06 to 0.1% is used for increasing the yield strength of the material. To achieve the minimum yield strength of 550MPa and TS atleast 620 MPa minimum 0.06% 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 (1-1.7 % Weight%) – 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 % is required which will act as a grain refinement. However, upper limit should be maintained to 1.7 % to avoid higher Tensile strength than required limit, which could create problems in forming process.

Silicon (0.04 weight % 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 (0.02 weight % or less)–Phosphorus being most effective and economical solid solution strengthening element helps to achieve the desired yield strength and tensile strength at low cost. However, 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 (0.04 to 0.1weight %) –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 0.04 to 0.1weight%, the annealing temperature can be kept below 820°C for full recrystallization with given continuous annealing time. Annealing with higher temperature above 820 °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 (0.02-0.06 weight %) – Aluminum acts as a deoxidizing agent and available in an amount ranging from 0.020 to 0.06 % as an indication of good deoxidation during steel making. Although, when present more than 0.06% it generates inclusion which is one of the possible causes for Al2O3 type slivers (Non metallic inclusion) .Therefore the aluminum present should be 0.06% 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 weight % or less ) –Nitrogen present in the high strength 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 (0.02 to 0.08weight %)-Titanium in combination of N should be present in amount such that the ratio of (Ti+V)/ N should be in a range from 10 to 40. 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 0.02% is required to achieve desired yield strength of atleast 550Mpa. 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 provide sulphide shape control advantageous in providing good surface.

The formation of titanium carbide occurs after the formation of titanium carbo-sulphides which helps in precipitation strengthening .The minimum requirement of titanium is 0.02% derived from minimum Ti required to prevent accelerated ageing,as per (Ti+V)/ N (Min)=(47+51)/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 550 MPa with yield ratio of 0.8 and also to prevent accelerated ageing in the material titanium level should be maintained between 0.02-0.08 weight%.
As (Ti+V)/N also determines the variation in the yield strength ,the ratio of (Ti+V)/N has to controlled between 10 to 40, to achieve minimum yield strength of 550 MPa with yield ratio of 0.80 or more.

Vanadium (0.01 to 0.04 weight%) 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 atleast 550Mpa as well as it provides non ageing guaranty for High Strength low carbon steel sheet. The ratio of V/N has to be in the range of 3.6 to 20 to achieve Yield Strength and non ageing property to High strength low carbon steel.

Group of elements from Sc, Co, Zn, Sn, Ni, Cu, Cr, Mo, Ca, W,B,Hf and Zr such that each element weight percent is 0.03% or less-Group of Elements such as Sc, Co, Zn, Sn, V, Ni, Cu, Zn, Cr, Mo, Ca, W,B,Hf 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.

Steel Microstructure: High Strength Low alloy cold rolled steel sheet having micro structural constituents comprises 60-69% of polygonal ferrite with average ferrite grain diameter less than 7 micron, 30-39% 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 30 % of pearlite is needed to achieve a desired strength of 600 MPa. Higher pearlite more than 40 % 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 60% 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 rolling 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 of manufacturing:
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 70%.
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 1.5-50C/sec up to soaking section temperature maintained in the range from 760 °C -820 °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°Cto690°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 340°C to 400°C to avoid yield point elongation. After over aging Skin-pass elongation (Temper rolling) in the range of 0.8 % to 1.6% was applied to avoid yield point elongation.

Method of evaluating phosphatability –
Phosphating is a treatment on metal surface with provides a hard and non conducting zinc phosphate coating which is insoluble. It gives better paint ability, adherence and corrosion resistance to metal surface used for automotive application.
To evaluate phosphatability firstly alkali degreasing was performed on steel sheet at 4000C 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 4000C 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 1.5-2.5 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]X 100
Where Do = Initial hole diameter, Df=final hole diameter

Complete descriptions of inventive steel and comparative steel grades obtained under various trials by way of the present invention 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 V/N ratio and (Ti+V)/N ratio.
Table 2- Hot rolling, cold rolling, continuous annealing parameters of inventive andcomparative steel sheets having chemical compositions as per table 1.
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.
Table4 – Stretch flanging, Bendability and Weldability, properties of steel sheets having chemical composition as per table 1 and processed as per table 2.
Table 1:

Steel No. C Mn P SI AL N TI NB V V/N ratio (Ti+V)/N ratio Others Remarks
1 0.1 1.44 0.011 0.018 0.06 0.005 0.055 0.066 0.018 3.6 14.4 Sc:0.002,Zn:0.003, Ca:0.003, Hf:0.004 Ex
2 0.12 1.5 0.013 0.023 0.053 0.0045 0.05 0.069 0.022 4.9 16.0 Ca:0.003,Cu:0.005 Comp
3 0.088 1.1 0.017 0.025 0.055 0.0047 0.053 0.065 0.02 4.3 15.5 REM:0.0028 Ex
4 0.096 1.8 0.013 0.023 0.053 0.0045 0.05 0.069 0.022 4.9 16.0 Sn:0.002,Zn:0.003, Ca:0.003, B:0.004 Comp
5 0.065 1.47 0.011 0.022 0.058 0.005 0.05 0.045 0.019 3.8 13.8 Cr 0.005, Mo: 0.020 Ex
6 0.08 1.44 0.011 0.018 0.06 0.005 0.055 0.11 0.017 3.4 14.4 Cu:0.01 , Ni:0.01,Mo:0.008 Comp
7 0.085 1.47 0.011 0.022 0.058 0.0028 0.05 0.064 0.01 3.6 21.4 Mo:0.3,Cr:0.3, Ca 0.003, B 0.004 Ex
8 0.1 1.44 0.011 0.018 0.06 0.0035 0.055 0.066 0.042 12.0 27.7 Cu:0.01 , Ni:0.01,Mo:0.008, Zn: 0.020 Comp
9 0.088 1.1 0.017 0.025 0.055 0.0047 0.053 0.065 0.02 4.3 15.5 Mo:0.3,Cr:0.3, Ca 0.003, B 0.004 Ex
10 0.14 2.24 0.014 0.432 0.055 0.005 0.013 0.071 0.002 0.4 3 Mo:0.003,Cr:0.003 Comp
11 0.065 1.47 0.011 0.022 0.058 0.0025 0.05 0.045 0.055 22.0 42 Cu:0.01 , Ni:0.01,Mo:0.008 Comp
12 0.065 1.47 0.011 0.022 0.058 0.0028 0.078 0.064 0.039 13.9 41.8 Cu:0.01 , Ni:0.01, Mg:0.003 ,Mo:0.008, Comp
13 0.088 1.1 0.017 0.025 0.055 0.004 0.022 0.065 0.015 3.8 9.3 Ca:0.003,Cu:0.005 Comp
14 0.092 1.47 0.011 0.016 0.06 0.0047 0.058 0.062 0.018 3.8 16.2 W: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 V/N ratio outside the range of 3.6 to 20 do not comply with the scope of the invention
Steels having (Ti+V)/Nratiooutside the range of 10 to 40do not comply with the scope of the invention.

Table 2:
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 1180 875 11.5 555 55.6 792 105 659 27.08 460 383 1.272 Ex
2 1190 870 10.2 560 60.5 795 98 655 25.5 455 385 1.215 Comp
3A 1200 880 11.8 570 52.8 780 92 672 21.9 470 375 1.165 Ex
3B 1195 890 10.2 581 52.7 790 95 681 9 461 380 1.201 Comp
3C 1205 888 11.8 578 56.8 788 99 685 42 480 375 1.111 Comp
4 1190 895 12.5 565 58 782 101 664 24.1 489 393 1.181 Comp
5A 1195 885 13.8 570 59.6 795 89 675 28.1 469 380 1.215 Ex
5B 1180 860 15.6 590 61 792 100 679 25.8 472 371 1.028 Comp
6 1190 865 11.9 580 55.8 786 79 648 20 452 384 1.176 Comp
7A 1205 880 10.6 585 60.2 791 70 660 18.5 465 390 1.201 Ex
7B 1195 890 9.8 579 58.5 788 90 670 25.1 470 420 1.215 Comp
8 1200 885 9.5 575 58.8 780 89 670 15.7 475 387 1.176 Comp
9 1210 870 10.2 580 60.7 775 80 662 25.6 482 378 1.0182 Ex
10 1200 875 11.4 565 68 788 130 694 35.4 341 250 0.326 Comp
11 1195 885 11.5 585 65 780 90 671 30.5 450 395 1.125 Comp
12 1210 880 12.7 575 51 790 93 665 28.4 468 385 1.012 Comp
13 1185 895 10.5 588 66 795 91 680 22.8 466 380 1.015 Comp
14 1170 885 13.1 580 55.4 778 82 659 26.5 472 394 1.2014 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:

Steel YS , MPa UTS, MPa El%, YS/UTS HER% UTS x El% UTS x HER% Ferrite area % Average Ferrite Grain Size ,µm Pearlite /bainite area % Martensite area % Remarks
No. 50GL



1 630 708 16.9 0.89 40.5 11965 28674 61.7 6.42 38.3 0 Ex
2 668 743 14.7 0.90 34.1 10922 25336 57 5.72 43 0 Comp
3A 591 675 17.5 0.88 45.6 11813 30780 65.5 6.64 34.5 0 Ex
3B 545 610 20.1 0.89 37.3 12261 28853 70.1 8.51 29.9 0 Comp
3C 680 750 15.5 0.91 33.2 11625 24900 55.8 5.58 36.2 8 Comp
4 691 765 14.1 0.90 30.4 10787 23256 53 5.31 47 0 Comp
5A 555 642 19.8 0.86 47.6 12712 30559 68.5 6.92 31.5 0 Ex
5B 651 725 14.8 0.90 32.1 10730 23273 58.7 5.9 38.3 3 Comp
6 669 740 13.9 0.90 34.2 10286 25308 56.4 5.58 43.6 0 Comp
7A 580 680 16.5 0.85 43.4 11220 29512 66.4 6.41 33.6 0 Ex
7B 551 615 19.5 0.89 36.1 12090 28582 69 6.81 31 0 Comp
8 651 720 15.1 0.90 32.3 10872 23256 59.3 5.91 40.7 0 Comp
9 598 679 17.1 0.88 42.2 11611 28654 64.4 6.57 35.6 0 Ex
10 614 969 19.2 0.63 27.5 18605 26648 51 6.25 1 48 Comp
11 655 710 14.3 0.92 31.7 10153 22507 58.1 6.14 40.9 1 Comp
12 693 763 14 0.91 30.8 10682 23500 52.2 5.26 47.8 0 Comp
13 540 605 21.1 0.89 38 12766 29101 71.4 8.61 28.6 0 Comp
14 601 700 16.3 0.86 42.1 11410 29470 64.1 6.52 35.9 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 40 % does not comply with the scope of present invention.
* Steel sheet having UTS x El% <11000, andUTSxHER %< 28000 does not comply with the scope of present invention as this provide poor bendability and stretch flangibility in the materials.
** Steel microstructure having pearlite/bainite phase fraction 30-39 %and martensite phase fraction >1% does not comply with the scope the present invention.
Table 4:
Steel Stretch flanging Phosphatability Bendability weldability Remarks
No. Remark
1 Good Good Good Good Ex
2 Poor Good Poor Good Comp
3A Good Good Good Good Ex
3B Poor Good Good Good Comp
3C Poor Poor Poor Good Comp
4 Poor Poor Poor Poor Comp
5A Good Good Good Good Ex
5B Poor Poor Poor Good Comp
6 Poor Good Poor Good Comp
7A Good Good Good Good Ex
7B Poor Good Good Good Comp
8 Poor Good Poor Good Comp
9 Good Good Good Good Ex
10 Poor Poor Poor Poor Comp
11 Poor Good Good Good Comp
12 Poor Poor Poor Poor Comp
13 Poor Good Good Good Comp
14 Good Good Good Good Ex
* Steel with Hole expansion ratio<40 % does not comply with the scope of invention as the stretchflange-abilityis poor for these steel grades.
* Steel having Pearlite fraction in the range of 30-39 % 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 1,3,5,7,9 and 14 (Marked as Ex. ) have chemical composition range as per the scope of present invention with V/N ratio in the range of 3.6 to 20 and (Ti+V)/N ratio in the range from 10 to 40 . Low V/N ratio from the scope of present invention like V/N ratio is 0.40 we could not able to achieve required YS due to insufficient precipitation strengthening and grain boundary strengthening, but in case where V/N ratio is more than 20, we achieve mechanical property out of scope of present invention. In case if (Ti+V)/N is in scope of present invention, Steel sample number 14, property of sample is in scope of present invention. All the experimental (Marked as Ex.) steel samples processed in hot rolling mill with varying process parameter and then it is 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,5A,7A,9 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/peralitic structure than present invention. Soaking section temperature in the range from 760 0C to 820 0C with soaking time in the range from 70-130 seconds, Slow cooling section temperature in the range from 6100C to 6900C, and rapid cooling section temperature in the range from 400-480 with cooling rate in the range from 10-40 0C/sec,maximum rapid cooling rate is 40 0C/secto avoid martensite formation. And Over ageing section temperature in the range of 340-400 0C with residence time 250-440 sec, high OAS temperature forms tempered martensite and hence Tensile strength cannot achieve.
As far as the mechanical properties are concerned,steel sample number 1,3,5,7,9 and 14 satisfies all the scope of present invention having , YS >550 MPa or 550-650 MPa,UTS>620 MPa, UTS x El% >11000,HER%>40 % UTS x HER%> 28000 ,YS/UTS>0.8 with ferrite area fraction in the range of 60-69 %, Pearlite/Bainite area fraction in the range of 30-39 % and Martensite Area %<1 Excellent hole expansion ratio >40 % for inventive steel sheets is attributed to higher fraction (60 to 69 %) of fine polygonal ferrite matrix along with reduced fraction of hard matensite phase.
For surface critical properties, Steel sample number 1,3A,5A,7A,9 and 14 comply with all the scope of present invention as given in table 4 having good phosphatability with zinc phosphate chemical conversion coating.

Example 3 : Steel sample number 2,4, 6,8,10,11,12,13 has chemical composition range outside the scope of the present invention with C, Mn, Nb,V, V/N ratio and (Ti+V)/N 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,V, V/N ratio and (Ti+V)/N weight %, does not comply with the mechanical property.Steel sample number 3C, 4, 5B, 10,12 showing poor Stretch flanging, Phosphatability, Bendability, weldability due to higher Mn %, Higher RCS Cooling rate and Higher Martensite % are outside the scope of present invention.