DESC:FIELD OF THE INVENTION
Present invention relates to 600 MPa Tensile strength level low carbon high yield ratio cold rolled, continuous annealed or galvannealed steel sheet, having composition in terms of mass fraction: 0.05% to 0.1% of C, Si: 0.04% or less, Mn: 1.0% to 1.5 %,N: 0.006% or less, Al:0.02 to 0.06 % , P: 0.02 to 0.08 %, Nb: 0.01% to 0.04%,Ti:0.02 to 0.04% and the balance being Fe and other inevitable impurities, whereas ratio of P to C is in the range of 0.2 to 1.2 and the ratio of (Mn+Si) /(C+P) must be less than 20.The micro structural constituents of said steel consisting 70-90% of polygonal ferrite with average ferrite grain diameter less than 10 micron, 5-20% of islands of pearlite and/or bainite with average size less than 5 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 steel described in present invention has an excellent phosphatability, stretch flangeability, low temperature toughness property and galvannealing property 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 =50 %, DBTT less than -50 0C , UTS x Elongation >14000 and UTS x HER % >32000.

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. However, As far as the high strength steel are concerned, Phosphatability and Galvanizing property are the other major requirement to provide good corrosion resistance to the steel surface. While galvanizing the high strength steel surface condition is a key aspect to be concerned about as higher alloying addition to achieve high strength such as Mn and Si, high strength steels are susceptible to surface oxidation which hampers the coating property. Due to formation of patches of surface oxides, formation of bare spots (areas without zinc coating) after galvannealing treatment is the vital concern. In addition, these surface oxides hampers the zinc phosphate chemical conversion coating during Phosphating treatment results in poor phosphatability.

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 (>0.7) results in lower spring back due to reduced elastic relaxation.

As a part of prior art Indian patent application number 870-KOL-2012 aims to produce a high yield ratio type high strength steel sheet. Strength is achieved by addition of higher amount of Mn and Si to the steel composition along with Cu as listed in inventive examples. Also a two step heating process after cold rolling has been specified as a part of invention to reduce banded structure and improve drawability. However due to higher Mn and Si weight % along with Cu and two step heating, patent application number 870-KOL-2012 are susceptible to rather poor galvanizing and phosphatability due to formation of excess oxides during heating multiple times. Also due to martensite being present as strengthening phase, hole expansion will be rather poor.

Indian patent number 276963 describes Methods for Manufacturing High Strength Hot-Dip Galvanized Steel Sheet and High Strength Hot-Dip Galvannealed Steel Sheet having minimum tensile strength of 590 MPa. However, with inclusion of 0.7 to 1.8 % of Si in the chemical composition, Indian patent number 276963 is prone to rather poor zinc coating property and powdering property due to excess SiO2 type scale formation during hot rolling /annealing. Also, it is susceptible to formation of rolled in scale embedded in steel during hot rolling which cannot be removed during pickling.

In the lights of above requirement, the present invention aims to solve the problem of prior art by providing a high yield ratio high strength steel sheet with 600 MPa tensile strength level having good Phosphating property, galvannealing property along with superior stretch flangeability.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide low carbon high strength high yield ratio cold rolled steel sheet with excellent Phosphating property, galvannealing property along with superior stretch flangeability and aging resistance and a process for its manufacture.

A further object of the present invention is directed to provide low carbon high strength high yield ratio cold rolled steel sheet having the properties of Tensile strength 600 MPa or more; Yield Strength at least 430 MPa with YS/TS ratio of 0.7 or more ; and Hole expansion Ratio 50 % or more.

A still further object of the present invention is directed to provide low carbon high strength high yield ratio cold rolled steel sheet selectively processed to have micro structural constituents of said steel consisting 70-90% of polygonal ferrite with average ferrite grain diameter less than 10 micron,5-20% of islands of pearlite and/or bainite with average size less than 5 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 with area fraction less than 2%.

A still further object of the present invention is directed to provide low carbon high strength high yield ratio cold rolled steel sheet would have excellent phosphatability, stretch flangeability, low temperature toughness property and galvannealing property 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 =50 %, DBTT less than -50 0C , UTS x Elongation >14000 and UTS x HER % >32000.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to re-phosphorized low Carbon high strength cold rolled steel sheet composition comprising:
0.05 wt % to 0.1wt% percent of Carbon;
1.0wt% to 1.5wt% of Manganese;
0.04wt % or less of Silicon;
0.02 wt% to 0.06wt% of Aluminum;
0.02 wt% to 0.08 wt% of Phosphorous;
0.01 wt% to 0.04 wt% of Niobium;
0.02wt% to 0.04wt % of Titanium;
Up to 0.006wt% of Nitrogen; and
balance is Fe and incidental impurities, having tensile strength 600 MPa or more, UTS x Elongation of atleast 14000 and UTS x HER % of atleast 32000,wherein ratio of P to C are in the range of 0.2 to 1.2 and the ratio of (Mn+Si) /(C+P) must be less than 20, additionally the micro structural constituents of said steel consisting 70-90% of ferrite and 5-20% 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 said re-phosphorized low Carbon high strength cold rolled steel sheet composition wherein the ratio of Ti/N is in the range of 4 to 20 to achieve the desired minimum yield strength of 430 MPa with yield ratio of 0.7 or more and also to prevent accelerated ageing.

A still further aspect of the present invention is directed to said re-phosphorized low Carbon high strength cold rolled steel sheet composition further comprises B from 0.001 to 0.0030 wt %.

A still further aspect of the present invention is directed to said re-phosphorized low Carbon high strength cold rolled steel sheet composition further including in mass % at least one element selected from the group consisting of Sc, Co, Zn, Sn, V, Ni, Cu, Zn, Cr, Mo, Ca, W, Hf and Zr such that each element weight percent is 0.03% or less.

Another aspect of the present invention is directed to said re-phosphorized low Carbon high strength cold rolled steel sheet wherein micro structural constituents of said steel consisting of 70-90% polygonal ferrite with average ferrite grain diameter less than 10 micron, 5-20% of islands of pearlite and/or bainite with average size less than 5 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 with area fraction is less than 2 %.

Yet another aspect of the present invention is directed to a process for the manufacture of re-phosphorised low carbon high strength cold rolled steel sheet comprising:
a.) providing a selective steel composition for slab generation for desired formability and coatability comprising:
0.05 wt % to 0.1wt% percent of Carbon;
1.0 wt% to 1.5 wt% of Manganese;
0.04 wt % or less of Silicon;
0.02 wt% to 0.06wt% of Aluminum;
0.02 wt% to 0.08 wt% of Phosphorous;
0.01 wt% to 0.04 wt% of Niobium;
0.02wt% to 0.04wt % of Titanium;
Up to 0.006wt% of Nitrogen; and
balance is Fe and incidental impurities such as to maintain P/C ratio 0.2 to 1.2, and the ratio of (Mn+Si) /(C+P) must be less than 20, and
b) Carrying out steel sheet manufacturing including hot rolling, pickling, cold reduction and continuous annealing or Galva 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/m2.

A further 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 of 40% to 70%.

A still 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 810°C with residence time in the range from45 to100 sec;
c. Slow cooling further said steel at temperature 650°C to 710°C with slow cooling rate in the range from 0.5 °C/Sec to 3°C/Sec;
d. Rapid cooling of said steel at rapid cooling rate in the range from 15°C/Sec to 45 °C/Sec;
e. overaging the said steel in the range from 350°C to 450°C for 150 sec or more;
f. Skin passing of overaged steel in the range from 0.6% to 1.6%.

A still further aspect of the present invention is directed to said process wherein hot rolled, pickled and cold rolled steel is processed alternatively in continuous galvanizing line for zinc coating on said steel surface comprising the steps of:
a. Heating the cold rolled steel in continuous galvanizing 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 galvanizing line at temperature 730°C to 800°C with residence time in the range from 15 to 50 sec.
c. Rapid cooling of said steel at rapid cooling rate in the range from 10 to 30°C/Sec;
d. Applying a zinc coating (Galvanizing) to the said steel sheet at a temperature in the range from 450°C to 500°C.
e. Galvannealing the hot dip galvanized sheet in an inline galvanizing vacuum furnace (GVF) at an annealing temperature of 500-550 °C.
f. Providing skin pass elongation to the galvannealed steel sheet in the range from 0.5 to 1.4 %.

A still further aspect of the present invention is directed to said process for producing said steel sheet having excellent Phosphatability, hole expansion ratio and better shape, wherein the above process steps are selectively controlled such as to achieve anyone or more of:
i. Tensile strength 600 MPa or more;
ii. Yield Strength at least 430 MPa with YS/TS ratio of 0.7 or more ;
iii. Hole expansion Ratio 50 % or more.

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

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING EXAMPLES
Present invention relates to 600 MPa Tensile strength level low carbon high yield ratio cold rolled rephosphorized , continuous annealed or galvannealed steel sheet wherein the chemical composition of steel comprises in terms of mass fraction: 0.05% to 0.1% of C, Si: 0.04% or less, Mn: 1.0% to 1.5 %,N: 0.006% or less, Al:0.02 to 0.06 % , P: 0.02 to 0.08 %, Nb: 0.01% to 0.04%,Ti:0.02 to 0.04% and the balance being Fe and other inevitable impurities, wherein ratio of P to C is in the range of 0.2 to 1.2 and the ratio of (Mn+Si) /(C+P) must be less than 20. The micro structural constituents of said steel consisting of 70-90% of polygonal ferrite with average ferrite grain diameter less than 10 micron, 5-20% of islands of pearlite and/or bainite with average size less than 5 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 steel described in present invention has an excellent phosphatability and galvannealing property 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 along with good hole expansion ratio of =50 %.
600 MPa Tensile strength level low carbon high yield ratio cold rolled rephosphorized steel sheets and low carbon high yield ratio rephosphorized galvannealed steel sheet produced according to present invention wherein effect of Metallurgical factors affecting the mechanical and surface properties as described hereunder in details–

Carbon (0.05-0.1wt %) – 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 600MPa and YS atleast 430 MPa minimum 0.05% of carbon is required as C forms TiC,Ti[C,N] 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 tensile 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 (1-1.5 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 % is required which will act as a grain refinement. However, upper limit should be maintained to 1.5 % to avoid poor phosphatability and poor galvannealing property. The formation of excess manganese oxide on the surface affects the phosphatability of the material. Also, it causes bare spots during galvannealing. To control the deterioration of surface property and while achieving desired strength of 600 MPa or more ,the ratio of (Mn+Si)/(C+P) to be target less than 20 . If the ratio is higher, the chance of oxide formation is more and in turn poor phosphatability and poor Zinc coating results.
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 be 0.04 wt% or less.
Phosphorus (0.02-0.08 wt%)–Phosphorus being most effective and economical solid solution strengthening element helps to achieve the desired tensile strength of 600 MPa at low cost. However, when the P content exceeds 0.1%, DBTT is deteriorated. Very high P also causes poor plating property during galvannealing. Hence, Upper limit is set to 0.08 % preferably.
Niobium (0.01-0.04wt %) –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 30mpa 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 workedsteel. When niobium content ranges from 0.01 to 0.04weigh %, the annealing temperature can be kept below 800°C for full recrystallization with given continuous annealing time. Annealing with higher temperature above 800 °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 (0.02-0.06wt%) – Aluminum acts as a deoxidizing agent and available in an amount ranging from 0.02 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 (0.006wt% or less ) –Nitrogen present in the high strength steel containing the titanium 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 0.006% or less.
Titanium (0.02-0.04wt %)-Titanium in combination of N should be present in amount such that the ratio of Ti/ N should be in a range from 4 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 0.02% is required to achieve desired tensile strength of atleast 600Mpa. 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 carbosulphides 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(Min)=47/14*N (Wt %) to fix 0.006% of Nitrogen. But when it exceeds the 0.04% the strength of material increases drastically. To achieve the desired yield strength of 430 MPa with yield ratio of 0.7 and also to prevent accelerated ageing in the material titanium level should be maintained between 0.02-0.04 weight percent.
As TiN also determines the variation in the yield strength, the ratio of Ti/N has to controlled between 4 to 20, to achieve minimum yield strength of 430MPa with yield ratio of 0.7 or more.
Boron (0.001-0.003wt %): Boron is added optionally in order to fix N and BN in present inventive grade. However, higher B > 0.003 should be avoiding as it creates problem during casting. Also higher B results in edge cracking during hot rolling.
Group of elements from Sc, Co, Zn, Sn, V, Ni, Cu, Zn, Cr, Mo, Ca, W, 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, 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: in thepresent cold rolled / Galvannealed steel, the micro structural constituents comprises 70-90% of polygonal ferrite with average ferrite grain diameter less than 10 micron, 5-20% of islands of pearlite and/or bainite with average size less than 5 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 5 % of pearlite is needed to achieve a desired strength of 600 MPa. Higher pearlite more than 20% reduces hole expansion ratio and also deteriorates the drawability. Precipitates such as TiN,TiS, NbC, Nb(CN) form due addition of Ti and Nb and adds up to strength due to gain refinement and precipitation strengthening. Minimum 70% of ferrite helps to achieve good elongation as well as hole expansion.
Hot Rolling: Slab reheating temperature (SRT) in the range from 1150 0C to 1220 0C 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 be 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 910 0C is suggested in the present invention. Keeping FT below 850 0C will cause excessive rolling load and may damage the rolls and mill. FT >920 0C may result excessive grain growth and two phase rolling leading to uneven or mix grain microstructure. Also ,to get fine microstructure post hot rolling , 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 600 °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 540 °C to 600 °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 75%.
Following pickling and cold rolling to desired thickness, cold rolled steel strip is processed through either of two different routes comprising:
1. Continuous Annealing Line to get cold rolled closed annealed product.
2 Continuous Galvanizing line for hot-dip galvanization to provide a zinc coating on the steel sheet and galvannealing for providing an alloying treatment to the coated zinc.
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 of1.5-50C/sec up to soaking section temperature maintained in the range from 760°C -810 °C. Annealing time at soaking section ranges from 70-140 seconds which gives desired results for present high strength rephosphorized grade. At soaking section temperature intercritical annealing results in ferrite and austenite microstructure which later transforms to ferrite + pearlite or Ferrite+ Bainite microstructure based on the cooling rate from slow cooling section to rapid cooling section inside continuous annealing line. After soaking section steel strip passes through slow cooling section at cooling rate of less than 3°C/sec .Slow cooling section temperature of 650 °C -710°C was maintained. Following slow cooling section annealed strip sheet been rapid cooled in the range from 15°C/sec to 45°C/sec up to rapid cooling section temperature of 400°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 350°C -450°C to avoid yield point elongation. After over aging Skin-pass elongation (Temper rolling) in the range of 0.6 % to 1.6% was applied to avoid yield point elongation.
While processing the cold rolled steel strip through continuous galvanizing line to produce galvannealed steel, 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 galvanizing line where the strip is heated at the rate of 1.5-5 0C/sec up to soaking section temperature maintained in the range from 730°C -800 °C. Annealing time at soaking section ranges from15-50 seconds which gives desired results for present high strength rephosphorized grade. At soaking section temperature intercritical annealing results in ferrite and austenite microstructure which later transforms to ferrite + pearlite or Ferrite+ Bainite microstructure based on the cooling rate from soaking section to rapid cooling section inside continuous galvanizing line. After soaking section steel strip sheet is rapid cooled at cooling rate in the range from 8°C/sec to 20°C/sec up to rapid cooling section temperature of 450°C or more to avoid martensite formation. After rapid cooling section annealed strip passes though molten zinc bath to apply zinc coating on annealed steel surface .Molten zinc bath temperature is maintained in the range from 450°C-500°C.After hot zinc bath zinc coated steel strip being processed through an inline galvanizing vacuum furnace (GVF). Galvanizing vacuum furnace temperature is maintained in the range from 500°C -550 °C to let the base metal iron diffuse into the zinc forming a zinc-iron alloy coating and to get galvannealed steel strip. Galvannealed steel strip is then provided skin pass elongation in the range from 0.5 to 1.4 %.The total coating weight on both the surfaces ranges from 30 -80 g/m2.
The high yield ratio high strength rephosphorized cold rolled annealed / galvannealed steel sheet with excellent phosphatability, palatability and stretch flangeability is obtained by processing through either of the above two routes.
Anti-Powdering property of galvannealed coating: The hot dip galvannealing coating process results a uniform, firmly adherent, zinc coating on both sides of the steel surface. After the zinc pot coating the steel strip passes through a galvannealing vacuum furnace to let the base metal iron diffuse into the zinc forming a zinc-iron alloy coating. The coating generally has 8 – 13% of iron. Coating damage or powdering is phenomenon when galvannealed steel sheet are subjected to forming application. For good formability of galvannealed product, zinc-iron alloy coating must have good anti-powdering and anti flaking property.
To evaluate anti powdering property of galvannealed steel sheet V-bend test is carried out. Punch with V shape and 600V angle has been utilized. Firstly GA steel sample is cleaned with ultrasonic cleaner in Toluene + Alcohol 1:1 solution for the period of 3 minutes minimum.Test piece is set in the die groove, keeping testing surface on top. Two test pieces is utilized one each for top surface and bottom surfaces .Flat GA steel strip is then punched in V die cavity with 10 tons of load.Punch is then lifted and test piece is removed from the die. Subsequently Sample is placed on flat die and recovery bend is applied. Scotch tape is then attached on inner radius of the sample (Inside side of V-bend).Pressure is applied on the tape to ensure the Zn dust is adhered to scotch tape.The width to which zinc dust has adhered is measured after attaching the tape removed from the test piece to a white paper sheet. The width of zinc dust adherence gives the severity of powdering. Zinc dust width =6 mm is considered good for steel for automotive drawing application. GA Steel sheet having zinc dust width on tape > 6 mm is considered having bad anti-powdering property and are out of the scope of this invention.
Method of evaluating phosphatability –
Phosphating is a treatment on metal surface that 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 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 1.5-2.5 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 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 (Mn+Si)/(C+P),Ti/N, and P/C.
Table 2- Hot rolling, cold rolling, continuous annealing / Continuous Galvannealing parameters of inventive and comparative steel sheets having chemical compositions as per table 1.
Table 3–Mechanical property, Micro structural phase fractions and HER % value of steel sheets having chemical composition as per table 1 and processed as per table 2 .
Table4 – Coating property, Phosphatability property, DBTT values and anti powdering properties of steel sheets having chemical composition as per table 1 and processed as per table 2

Table 1:
Steel No. C Mn Si P Al N Nb Ti B Ti/N Others (Mn+Si)/(C+P ) P/C Remarks
1 0.082 1.3 0.03 0.046 0.05 0.005 0.025 0.037 0.002 7.4 Cu:0.01 , Ni:0.01, Mg:0.003 ,Mo:0.008,
Ca:0.003,V:0.005 10.4 0.56 Ex
2 0.095 1.23 0.02 0.06 0.04 0.004 0.03 0.026 0.0015 6.5 W:0.003 , Sn:0.002,Cr:0.015,Zr:0.003,Co:0.003 8.1 0.63 Ex
3 0.075 1.5 0.025 0.07 0.035 0.0032 0.03 0.05 0.001 16 Sc:0.002,Zn:0.003, Ca:0.003, Hf:0.004 10.5 0.93 Ex
4 0.08 1.4 0.03 0.065 0.04 0.004 0.04 0.03 0.001 7.5 Ca:0.003 9.86 0.81 Ex
5 0.12 2.22 0.34 0.007 0.045 0.0034 0.055 0.089 - 26.2 V:0.045,Cu:0.09,
Ni:0.1 21.9 0.06 Comp
6 0.064 2.6 0.5 0.009 0.042 0.0067 0.063 0.12 - 17.9 REM:0.0028 42.5 0.14 Comp
7 0.18 1.5 0.5 0.02 0.05 0.005 0.04 0.04 0.002 8 Mo:0.3,Cr:0.3 9 10 Comp

* Ex - Present inventive example, Comp- Comparative Examples.
** Underlined and shaded boxes indicates “outside the appropriate range”
Steels having Ti/N ratio outside the range of 4 to 20 does not comply with the scope of the invention
Steels having (Mn+Si)/(C+P) ratio = 20 does not comply with the scope of the invention.
* Steel having P/C ratio outside the range from 0.2 to 1.2 are marked as comp examples and does not comply with the scope of invention.

Table 2:
Steel
No. Final Product Type SRT,
0C FT,
0C ROT
Cooling
rate,
0C/sec CT, 0C Cold
Redn
,% SS
Temp ,
0C AT ,
sec SCS Temp, 0C RCS Cooling Rate,
0C/sec RCS Temp, 0C OAS Temp, 0C Zinc Pot Temperature,
0C GVF Temperature,
0C SPM% Remarks
1 CR 1200 895 11.5 580 55.5 773 66 688 38 409 365 - - 1.4 Ex
GA 1190 870 10.8 580 56.0 777 45 - 15.9 466 - 461 530 0.9 Ex
2 CR 1200 885 12.1 565 60.0 785 59 700 35.8 430 379 - - 1.2 Ex
GA 1190 890 13.4 565 60.0 760 39 - 16.1 470 - 470 540 1.1 Ex
3 CR 1210 880 12.5 570 64.2 799 56 692 40 430 379 - - 1.5 Ex
GA 1185 880 12.3 570 64.2 750 35 - 18.1 450 - 470 540 1 Ex
4 CR 1190 870 13.2 560 60 790 60 700 35 450 400 - - 1.2 Ex
GA 1200 890 13.8 560 60 790 40 - 17.5 450 - 460 530 1.2 Ex
5 GA 1250 890 7.5 640 55 800 45 - 12.1 487 - 470 530 1.1 Comp.
CR 1260 870 6.4 680 55 770 95 700 38.5 360 340 - - 0.8 Comp.
6 CR 1230 880 9.7 600 60 820 74 690 12.5 475 404 - - 1 Comp.
GA 1220 890 10.6 620 55 800 40 - 15.5 496 - 490 500 0.8 Comp.
7 CR 1200 890 30 570 50 790 70 700 45 360 340 - - 0.6 Comp
GA 1180 900 12.4 570 50 800 40 - 20.3 480 - 470 520 0.5 Comp

* Note: Steel with final product type as CR refers to Cold rolled annealed product and are processed in continuous annealing line (CAL) after cold rolling .Steel marked as GA refers to Galvannealed product and processed in continuous galvanizing line where zinc coating is applied on steel surface followed by galvannealing in galvannealing vacuum furnace.
** SRT: Slab reheating temperature, FT: Hot rolling finishing temperature, ROT: Run out table between finishing and coiling at hot rolling, Cold Redn: 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, GVF: Galvannealing vacuum furnace temperature, SPM: Skin pass elongation
*** Continuous galvanizing line does not have slow cooling and overaging section, rather the steel marked as GA are processed through continuous galvanizing line (CGL) where steel strip is immersed in zinc pot maintained in the range from 4500C to 500 0C post Rapid cooling section of CGL followed by annealing in galvannealing vacuum furnace maintained at a temperature range from 5000C -550 0C. Skin pass elongation is provided post galvannealing.
Table 3:
Steel
No. Final Product Type YS , MPa UTS, MPa El%,
50gl YS/UTS HER% UTS x El% UTS x HER% Ferrite area % Average Ferrite Grain Size ,µm Pearlite /bainite area % Martensite area % Average Pearlite/
Bainite/
Martensite
Island size
,µm Remarks
1 CR 484 619 26.9 0.78 65 16651 40235 86 6 12.5 0 3 Ex.
GA 456 600 27 0.76 70 16200 42000 87 7.4 11.5 0 2.6 Ex.
2 CR 451 618 27.5 0.73 63 16995 38934 85 6.8 14 0 2.5 Ex.
GA 485 631 27.3 0.77 57 17226 35967 85.5 7 13.5 0 2.5 Ex.
3 CR 471 610 26.3 0.77 65 16043 39650 87.5 7.5 11.5 0 1.9 Ex.
GA 489 623 28.1 0.78 59 17506 36757 84 7.2 15.0 0 2.2 Ex.
4 CR 490 636 27.1 0.77 74 17235 47064 87 6.5 12 0 3.0 Ex.
GA 487 649 26.5 0.75 75 17198 48675 88 7 10.5 0 2.5 Ex.
5 GA 512 798 17.1 0.64 29 13646 23142 64 5.4 35 0 2.1 Comp.
CR 483 816 16.5 0.59 25 13464 20400 64 5.1 0 36 3.0 Comp.
6 CR 451 653 21.2 0.69 40 13844 26120 78 7.6 21 0 3.5 Comp.
GA 479 669 20.4 0.72 37 13648 24753 76 6.5 23 0 3.9 Comp.
7 CR 457 834 17.8 0.55 28 14845 23352 63 3.8 0 37 2.4 Comp
GA 489 772 18.2 0.63 35 14050 27020 65 4.5 35 0 3.1 Comp

*Underlined and shaded boxes indicates “outside the appropriate range”
*HER =Hole expansion ratio
*Steel sheets having YS/UTS ratio <0.7 does not conform with the scope of the present invention.
* Steel having hole expansion ratio less than 50 % does not comply with the scope of present invention.
* Steel sheet having UTS x El% <14000, and UTSxHER %<32000 does not comply with the scope of present invention.
** Steel microstructure having pearlite/bainite phase fraction >20 %and martensite phase fraction >1% does not comply with the scope the present invention.

Table 4 :
Steel
No. Final Product Type Bare spots Observed YES/NO Surface
after Galva-
annealing Phosphatability
Remark DBTT,
0C Anti-Powdering Property Remarks
1 CR - - Good -70 - Ex.
GA NO Good Good -60 Good Ex.
2 CR - - Good -70 - Ex.
GA NO Good Good -60 Good Ex.
3 CR - - Good -70 - Ex.
GA NO Good Good -60 Good Ex.
4 CR - - Good -60 - Ex.
GA NO Good Good -70 Good Ex.
5 GA YES Poor Poor -20 Poor Comp.
CR - - Poor -20 - Comp.
6 CR - - Poor -30 - Comp.
GA YES Poor Poor -10 Poor Comp.
7 CR Poor -30 - Comp.
GA YES Poor Poor -30 Poor Comp.

* steels with phosphatability remark poor does not fulfill the phosphatability requirement as the phosphate crystal size after zinc phosphate chemical conversion coating is >4µm and phosphate coating weight is >2.5 g/mm2 for these steel sheets which is harmful for plating and painting on steel surface.
* Steel with Hole expansion ratio<50 % does not comply with the scope of invention as the stretchflangeability is poor for these steel grades.
* Steel having Pearlite fraction > 30 % and Martensite fraction > 1% does not comply with the scope of invention.
** Surface after galvannealing marked as “poor“denotes the appearance of bare spots after galvannealing treatment. This is attributed to formation of patches of surface oxides during hot rolling/annealing resulting in poor coating property.
*GA Steel marked as poor in anti powdering property does not comply with V bend test and zinc dust width in scotch tape after V bend is > 6mm for these steels.
** Steel having DBTT (Ductile to brittle transition temperature >-50 0Cdoes not comply with the scope of present invention .To determine the DBTT , steel strip of 100mm length and 20mm width Is V bend from the middle and dipped for 20 minutes in Ethanol bath maintained at temperature up to -70 0C . Subsequently the sample is straightened and hammered for deformation .Steel sheet showing crack at bend portion post hammering is considered as brittle at given bath temperature. Steel sheet of present invention does not undergo any brittle transformation up to -50 0C.

Example 1 : As listed in Table 1, Steel number 1 to 4 (Marked as Ex. ) have chemical composition range as per the scope of present invention with (Mn+Si)/(C+P) ratio of atleast 20 and P/C ratio in the range from 0.2-1.2 .Steel 1 to 4 is processed at continuous annealing line (Marked as CR in Table 2 to 4)and Continuous Galvanizing line (Marked as GA in table 2 to 4) as per the scope of present invention. It should be noted that continuous galvanizing line does not have Slow cooling section and overaging section, hence the steel marked as GA are processed with different annealing and rapid cooling condition. Steel marked as CR is rapid cooled from Slow cooling section (SCS) where as Steel marked as GA are rapid cooled directly from Soaking Section (SS) up to rapid cooling end temperature maintained above 450 0C.Steel marked as GA refers to Galvannealed product and processed in continuous galvanizing line where zinc coating is applied on steel surface followed by galvannealing in galvannealing vacuum furnace.

As given in table 2 , steel 1 to 4 is processed with Soaking section temperature in the range from 760 0C to 810 0C with soaking time in the range from 70-140 seconds, Slow cooling section temperature in the range from 650 0C to 710 0C, rapid cooling rate in the range from 15-45 0C/sec with rapid cooling end temperature above 400 0C to avoid martensite formation. For GA product the soaking section temperature is maintained in the range from 730 0C -800 0C with soaking time of 15-50 seconds. Rapid cooling rate in the range from 8- 20 0C/sec with rapid cooling end temperature above 450 0C was maintained. Molten Zinc bath temperature in the range from 450 0C -500 0C along with Galvanizing vacuum furnace temperature in the range from 500°C -550 °C is set for GA product.
As far as the mechanical properties are concerned, steel number 1 to 4 satisfies all the scope of present invention having UTS>600 MPa, YS >430 MPa, UTSxEl%>14000 ,HER%>50 % UTS x HER% > 32000 ,YS/UTS>0.7 with ferrite area fraction > 80 %, Pearlite/Bainite area fraction < 20 % and Martensite Area % < 1 % for both CR and GA product as listed in Table 3.

Excellent hole expansion ratio >50 % for inventive steel sheets is attributed to higher fraction (80%) of fine polygonal ferrite matrix along with reduced fraction of hard matensite phase.

For surface critical properties, Steel number 1 to 4 comply with all the scope of present invention as given in table 4 having good phosphatability post zinc phosphate chemical conversion coating , No bare spots post galvannealing and good Anti powdering properties post forming. The DBTT temperature of inventive steel is below -50 0C complying to good resistance to brittle transformation at lower temperatures.
Surface after galvannealing marked as “good” denotes the absence of bare spots after galvannealing treatment. Bare spots results due to formation of patches of excess surface oxides during hot rolling/annealing resulting in poor coating property.

Example 2 : Steel No 6 has chemical composition range outside the scope of the present invention with Mn and Si, N and Ti being outside the specified range. In addition, steel number 6 has the value of (Mn+Si)/(C+P) >20 and P/C of 0.06 which are outside the specified range as per present invention. Steel Number 6 annealed and galvanized as per the specified range of present invention ,however due to Higher Mn and Si weight %, steel 6 does not comply with the phosphatability and Zinc coating requirement showing poor phosphatability and bare spots post zinc coating as given in table 4. Steel 6 also has poor anti powdering property post forming due to excess Manganese and Silicon oxide formation during annealing.

Example 3 :Steel No 7 has chemical composition range outside the scope of the present invention with C, Mn and Si, Mo, Cr being outside the specified range. In addition, steel number 7 has the value of P/C =10 which is outside the specified range from 0.2 to 1.2 as per present invention. While annealing Steel Number 7 for CR product , the RCS temperature is kept to 360 0C (Table 2) which is below the martensite start temperature for given composition. As a result, steel number 7 shows significant amount of Martensite area fraction in the final microstructure as given in table 3. The mechanical property of steel7 corresponds to typical dual phase steel with UTS > 780 MPa having yield ratio <0.6 as given in table 3. Due to higher martensite area fraction, steel 7 shows poor hole expansion ratio of 28 % for CR product as higher hardness difference between high carbon Martensite and clean ferrite makes the ferrite –Martensite interface weak location for crack generation.

In addition, due to Higher Mn and Si weight % of Steel7, it does not comply with the phosphatability and Zinc coat ability requirement showing poor phosphatability and bare spots post galvannealing as given in table 4. Steel 7 also has poor anti powdering property post forming due to excess Manganese and Silicon oxide formation during galvannealing.

It is thus possible by way of the present invention to provide 600 MPa Tensile strength level low carbon high yield ratio cold rolled rephosphorized, continuous annealed or galvannealed steel sheet having selective chemical composition and processed through selective steps with controlled parameters to achieve desired strength and yield ratio as well as micro structural constituents of said steel consisting 70-90% of polygonal ferrite with average ferrite grain diameter less than 10 micron,5-20% of islands of pearlite and/or bainite with average size less than 5 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 steel described in present invention has an excellent phosphatability, stretch flangeability, low temperature toughness property and galvannealing property 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 =50 %, DBTT less than -50 0C , UTS x Elongation >14000 and UTS x HER % >32000 .

,CLAIMS:We Claim:
1. Re-phosphorized low Carbon high strength cold rolled steel sheet composition comprising:
0.05 wt % to 0.1wt% percent of Carbon;
1.0wt% to 1.5wt% of Manganese;
0.04wt % or less of Silicon;
0.02 wt% to 0.06wt% of Aluminum;
0.02 wt% to 0.08 wt% of Phosphorous;
0.01 wt% to 0.04 wt% of Niobium;
0.02wt% to 0.04wt % of Titanium;
Up to 0.006wt% of Nitrogen; and
balance is Fe and incidental impurities, having tensile strength 600 MPa or more, UTS x Elongation of atleast 14000 and UTS x HER % of atleast 32000 ,wherein ratio of P to C are in the range of 0.2 to 1.2 and the ratio of (Mn+Si)/(C+P) must be less than 20, additionally the micro structural constituents of said steel consisting of 70-90% of ferrite and 5-20% of pearlite and/or bainite and less than 1% of Martensite with balance being carbide and nitride precipitates of alloying elements.

2. Re-phosphorized low Carbon high strength cold rolled steel sheet composition as claimed in claim 1 wherein the ratio of Ti/N is in the range of 4 to 20 to achieve the desired minimum yield strength of 430 MPa with yield ratio of 0.7 or more and also to prevent accelerated ageing.

3. Re-phosphoresced low Carbon high strength cold rolled steel sheet composition as claimed in anyone of claims 1 or 2 further comprises B from 0.001 to 0.0030 wt %.

4. Re-phosphorized low Carbon high strength cold rolled steel sheet composition as claimed in anyone of claims 1 to 3 further including in mass % at least one element selected from the group comprising of Sc, Co, Zn, Sn, V, Ni, Cu, Zn, Cr, Mo, Ca, W, Hf and Zr such that each element weight percent is 0.03% or less.

5. Re-phosphorized low Carbon high strength cold rolled steel sheet as claimed in anyone of claims 1 to 4, wherein the micro structural constituents of said steel consisting 70-90% of polygonal ferrite with average ferrite grain diameter less than 10 micron, 5-20% of islands of pearlite and/or bainite with average size less than 5 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 with area fraction less than 2%.

6. A process for the manufacture of re-phosphorized low Carbon high strength cold rolled steel sheet as claimed in anyone of claims 1 to 5 comprising:
a.) providing a selective steel composition for slab generation for desired formability and coatability comprising:
0.05 wt % to 0.1wt% percent of Carbon;
1.0 wt% to 1.5 wt% of Manganese;
0.04 wt % or less of Silicon;
0.02 wt% to 0.06wt% of Aluminum;
0.02 wt% to 0.08 wt% of Phosphorous;
0.01 wt% to 0.04 wt% of Niobium;
0.02 wt% to 0.04wt% of Titanium;
Up to 0.006wt% of Nitrogen; and
balance is Fe and incidental impurities such as to maintain P/C ratio 0.2 to 1.2, and the ratio of (Mn+Si) /(C+P) must be less than 20, and
b) Carrying out steel sheet manufacturing including hot rolling, pickling, cold reduction and continuous annealing or Galva 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/m2.

7. A process as claimed in claim 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 40% to 70%.

8. A process as claimed in anyone of claims 5 or 6 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 810°C with residence time in the range from 45 to100 sec.
c. Slow cooling further said steel at temperature 650°C to 710°C with slow cooling rate in the range from 0.5 °C/Sec to 3°C/Sec;
d. Rapid cooling of said steel at rapid cooling rate in the range from 15°C/Sec to 45 °C/Sec;
e. Overaging the said steel in the range from 350°C to 450°C for 150 sec or more;
f. Skin passing of overaged steel in the range from 0.6% to 1.6%.
9. A process as claimed in anyone of claims 6 or 7 where hot rolled, pickled and cold rolled steel processed alternatively in continuous galvanizing line for zinc coating on said steel surface where as the processing steps comprising:
a. Heating the cold rolled steel in continuous galvanizing 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 galvanizing line at temperature 730°C to 800°C with residence time in the range from 15 to 50 sec.
c. Rapid cooling of said steel at rapid cooling rate in the range from 10 to 30°C/Sec;
d. Applying a zinc coating (Galvanizing) to the said steel sheet at a temperature in the range from 450°C to 500°C.
e. Galvannealing the hot dip galvanized sheet in an inline galvanizing vacuum furnace (GVF) at an annealing temperature of 500-550°C.
f. Providing skin pass elongation to the galvannealed steel sheet in the range from 0.5 to 1.4 %.

10. A process as claimed in anyone of claims 6 to 9 for producing said steel sheet having excellent Phosphatability, hole expansion ratio and better shape, wherein the above process steps are selectively controlled such as to achieve anyone or more of:
i. Tensile strength 600 MPa or more;
ii. Yield Strength at least 430 MPa with YS/TS ratio of 0.7 or more ;
iii. Hole expansion Ratio 50 % or more.

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