Claims:We Claim:

1. Re-Phosphorized low carbonhigh strength cold rolled steel sheet having composition comprising:
0.01 wt % to 0.05wt% of Carbon;
0. 2 wt% to 0.5 wt% of Manganese;
0.04 wt % or less of Silicon;
0.02 wt% to 0.06wt% of Aluminum;
0.02 wt% to 0.05 wt% of Phosphorous;
0.01 wt% to 0.03 wt% of Vanadium;
Up to 0.005wt% of Nitrogen;
0.001 to 0.0030 wt % of Boron; and balance Fe and incidental impurities,
wherein ratio of (V+B+Al) to N are in the range of 10 to 35 and the ratio of P/B is in the range of 15 to 40 and having tensile strength 340 MPa or more.

2. Re-Phosphorized low Carbonhigh strength cold rolled steel sheet composition as claimed in claim 1 having micro structural constituents of said steel comprising 80-95% of ferrite and 1-5 % of pearlite,with balance being carbide and nitride precipitates of alloying elements.

3. Re-Phosphorized low Carbonhigh strength cold rolled steel sheet as claimed in anyone of claims 1 or 2 further comprises at least one element selected from the group consisting of Ti, Cr and Ca such that weight percent of each element is 0.03% or less.

4. Re-Phosphorized low Carbonhigh strength cold rolled steel sheet as claimed in anyone of claims 1 to 3 having ductile to brittle transition temperature (DBTT) less or equal to -50 °C.

5. A process for the manufacture of low Carbonhigh strength cold rolled steel sheet as claimed in anyone of claims 1 to 4 comprising:
a) providing a selective steel composition for slab generation for desired formability comprising:
0.01 wt % to 0.05wt% percent of Carbon;
0. 2 wt% to 0.5 wt% of Manganese;
0.04 wt % or less of Silicon;
0.02 wt% to 0.06wt% of Aluminum;
0.02 wt% to 0.05 wt% of Phosphorous;
0.01 wt% to 0.03 wt% of Vanadium;
Up to 0.005wt% of Nitrogen;
0.001 to 0.0030 wt % of Boron; and balance Fe and incidental impurities such as to maintain the ratio of (V+B+Al) to N in the range of 10 to 35 and the ratio of P/B in the range of 15 to 40.
b)processing selectively said slab after reheating through hot rolling, pickling cold rolling, continuous annealing followed by skin pass rolling to achieve desired properties.

6. A process as claimed in claim 5 comprising:
i. Hot rolling of said steel slab with slab reheating Temperature 1220°C or less, Finishing Temperature 880°C to 930°C and hot coiled at a temperature of 600 to 700 with ROT cooling rate in the range of 5°C/Sec to 15°C/Sec .
ii. Pickling of said steel to remove oxide layer built on surface of steel sheet and cold rolling said steel with reduction in the range of 60% to 80%.

7. A process as claimed in claim 5 or 6 further comprising:

(i) 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.
(ii) Soaking said steel in continuous annealing line at temperature 760°C to 820°C with residence time in the range from 35 to 100 sec.
(iii) Slow cooling further said steel at temperature 650°C to 700°C with slow cooling rate in the range from 0.9 °C/Sec to 5°C/Sec;
(iv) Rapid cooling of said steel at rapid cooling rate in the range from 10°C/Sec to 26 °C/Sec;
(v) overaged the said steel in the range from 350°C to 400°C for 130 sec or more;
(vi) Skin passing of overaged steel in the range from 1.3% to 1.8%.

8. A process as claimed in anyone of claims 5 to 7 to produce said steel sheet having excellent weldability and ductile to brittle transition temperature, wherein the above process steps are selectively controlled such as to achieve anyone or more of:
(i) Tensile strength 340MPa or more;
(ii) Yield Strength at least 200 MPa with YS/TS ratio of 0.65 or more;
(iii) Aging guarantee of 6 months;
(iv) R-Value 1.5 or more;
(v) N-Value 0.18 or more;
(vi) having ductile to brittle transition temperature (DBTT) less than -50 °C;
(vii) having micro structural constituents of said steel comprising 80-95% of ferrite and 5-10% of pearlite and carbide /nitride /sulphide precipitates of alloying elements and the area fraction is less than 2%.

Dated this the 26th day of July, 2017
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)

, Description:FIELD OF THE INVENTION

The present invention relates to rephosphorized low carbon high strength cold rolled steel sheet having resistance to secondary work embrittlement and good formability and method of manufacturing the same. More particularly, the present invention is directed to provide low carbon high yield ratio cold rolled rephosphorized steel sheet with chemical composition comprising in terms of weight percent : 0.01% to 0.05% of C, Si: 0.04% or less, Mn: 0.2% to 0.5 %, N: 0.005% or less, Al:0.02 to 0.06%, P: 0.02 to 0.05 %, V: 0.01 to 0.03 %, B 0.001 to 0.003 % and the balance being Fe and other inevitable impurities, wherein ratio of (V+B+Al) to N are in the range of 10 to 35 and having Tensile strength of at least 340 MPa. The micro structural constituents of said steel consisting of 80-95% of ferrite and 1-5 % of pearlite, with balance being carbide and nitride precipitates of alloying elements. Cold rolled steel sheets according to the invention demonstrates resistance to secondary work embrittlement having ductile to brittle transition(DBT) Temperature less than -50°C and better formability with r90–1.5 or more suitable for variety of applications including manufacturing of selective automobile components.

BACKGROUND OF THE INVENTION

In the past decades, major developments have been taken up for the use of high strength steel in the transport vehicles to reduce the weight by decreasing the thickness of the material and increase in tensile strength of the material. Cold rolled high strength carbon manganese based steel is used at rocker panel, chassis, cross member. The material used for such components should not only have good ageing resistance for forming components but also to avoid spring back issue and resistance for secondary work embrittlement to avoid failure of formed components at very low temperature due to ductile to brittle transitions.

Granted Patent CN103131843 relates to automotive structural parts with high strength low alloy steel cold rolled sheet HC340LA obtained through stabilized continuous annealing process, belonging to the field of automotive steel production technology with yield strength minimum of 340Mpa by adding Niobium and titanium for strengthening by reducing the variation in mechanical properties within 30Mpa. According to the said prior art there arises the problem of accelerated ageing as steel strip is coiled at temperature of 565°C to 595°C and after annealing, when aged at 340°C to 380°C. The material produced using the chemistry disclosed in patent CN103131843 results in poor formability for critical automotive parts.

There has been thus a need in the related field to developing High Strength Low Alloy Steel with improved formability for automotive structural applications. The present invention targets to produce a steel grade that not only have higher tensile strength but also has good formability, ageing resistance and resistance for secondary work embrittlement which is required for certain automotive components. All these desirable properties are achieved by ascertaining the selective chemistry and processing using the desired process parameters.

The present invention aims to solve the problem of prior art by providing good ageing resistance for 6 months, prevents accelerated ageing along with better weldability and low ductile to brittle transition temperature(DBTT)(- 50 °C) without any crack, processed through continuous annealing process suitable for automotive structural applications.

Furthermore, it is well known that 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 that is not uniform and the shape of the pressing will not 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 stress, so to avoid the problem, certain level of yield strength to tensile strength ratio is to be maintained. In the present invention the yield ratio is more than 0.65 which help in preventing spring back while forming and ratio of (V+Al+B)/Nis experimentally decided to preventageing.

The transition from ductile to brittle can be very rapid and often it could be very disastrous since there is almost no warning. A Material with very low DBTT provides adequate resistance to low temperature brittleness.The present invention thus makes an attempt to solve the problems of the prior art by providing a low carbon high strength cold rolled continuously annealed steel sheet having UTS= 340 MPa, has superior weldability, excellent aging resistance and with adequate resistance to low temperature brittleness through low DBT Temperature.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide rephosphorized low carbon high strength cold rolled steel sheet having good weldability, ageing resistance and secondary work embrittlement, and a method for manufacturing the same through continuous annealing route.

A further object of the present invention is directed to provide rephosphorized low carbon high strength cold rolled steel sheet having selective composition along with processing through hot rolling, cold rolling, continuous annealing followed by temper rolling with selective parameters at each stage to ensure to attain the desirable properties with ageing resistance and spot weldability.

A still further object of the present invention is directed to provide rephosphorized low carbon high strength cold rolled steel sheet with aging resistance and DBTT wherein to achieve desired property, cold rolled steel is continuously annealed with selected soaking temperature to ensure complete recrystallization and involvingselective slow cooling temperature with cooling rate followed by rapid cooling temperature with cooling rate, overaging temperature with residence time.

A still further object of the present invention is directed to provide rephosphorized low carbon high strength cold rolled steel sheet with excellent ageing resistance and resistance for secondary work embrittlement wherein (V+Al+B)/N ratio is maintained from 10 to 35 to control free N to avoid ageing of the material, as well as phosphorous to Boron(P/B) ratio of 15 to 40 to avoid secondary work embrittlement.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to re-phosphorized low carbon high strength cold rolled steel sheet having composition comprising:
0.01 wt % to 0.05wt% of Carbon;
0. 2 wt% to 0.5 wt% of Manganese;
0.04 wt % or less of Silicon;
0.02 wt% to 0.06wt% of Aluminum;
0.02 wt% to 0.05 wt% of Phosphorous;
0.01 wt% to 0.03 wt% of Vanadium;
Up to 0.005wt% of Nitrogen;
0.001 to 0.0030 wt % of Boron; and balance Fe and incidental impurities,
wherein ratio of (V+B+Al) to N are in the range of 10 to 35 and the ratio of P/B is in the range of 15 to 40 and having tensile strength 340 MPa or more.

A further aspect of the present invention is directed to said re-phosphorized low Carbon high strength cold rolled steel sheet composition having micro structural constituents of said steel comprising 80-95% of ferrite and 1-5 % of pearlite,with balance being carbide and nitride precipitates of alloying elements.

A still further aspect of the present invention is directed to re-phosphorized low Carbon high strength cold rolled steel sheet further comprises at least one element selected from the group consisting of Ti, Cr and Ca such that weight percent of each element 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 having ductile to brittle transition temperature (DBTT) less or equal to -50 °C.

Yet another aspect of the present invention is directed to a process for the manufacture of low Carbonhigh strength cold rolled steel sheet as described above comprising:
a.) providing a selective steel composition for slab generation through continuous casting for desired formability comprising:
0.01 wt % to 0.05wt% percent of Carbon;
0. 2 wt% to 0.5 wt% of Manganese;
0.04 wt % or less of Silicon;
0.02 wt% to 0.06wt% of Aluminum;
0.02 wt% to 0.05 wt% of Phosphorous;
0.01 wt% to 0.03 wt% of Vanadium;
Up to 0.005wt% of Nitrogen;
0.001 to 0.0030 wt % of Boron; and balance Fe and incidental impurities such as to maintain the ratio of (V+B+Al) to N in the range of 10 to 35 and the ratio of P/B in the range of 15 to 40.
b) processing selectively said slab after reheating through hot rolling, pickling cold rolling, continuous annealing followed by skin pass rolling to achieve desired properties.

A further aspect of the present invention is directed to a process comprising:
i. Hot rolling of said steel slab with slab reheating Temperature 1220°C or less, Finishing Temperature 880°C to 930°C and hot coiled at a temperature of 600 to 700 °C with ROT cooling rate in the range of 5°C/Sec to 15°C/Sec .
ii. Pickling of said steel to remove oxide layer built on surface of steel sheet and cold rolling said steel with reduction in the range of 60% to 80%.

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 820°C with residence time in the range from 35 to 100 sec.
c) Slow cooling further said steel at temperature 600°C to 700°C with slow cooling rate in the range from 0.9 °C/Sec to 5°C/Sec;
d) Rapid cooling of said steel at rapid cooling rate in the range from 10°C/Sec to 26 °C/Sec;
e) overaged the said steel in the range from 350°C to 400°C for 130 sec or more;
f) Skin passing of overaged steel in the range from 1.3% to 1.8%.

A still further aspect of the present invention is directed to a process to produce said steel sheet having excellent weldability and ductile to brittle transition temperature, wherein the above process steps are selectively controlled such as to achieve anyone or more of:
i. Tensile strength 340MPa or more;
ii. Yield Strength at least 200 MPa with YS/TS ratio of 0.65 or more;
iii. Aging guarantee of 6 months;
iv. R-Value 1.5 or more
v. N-Value 0.18 or more
vi. having ductile to brittle transition temperature (DBTT) less than -50 °C;
vii. having micro structural constituents of said steel comprising 80-95% of ferrite and 5-10% of pearlite and carbide/nitride/sulphide precipitates of alloying elements and the area fraction is less than 2%.

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

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING EXAMPLES

The present invention is directed to provide rephosphorized low carbon high strength cold rolled steel sheet having good weldability, ageing resistance and secondary work embrittlement, and a method for manufacturing the same through continuous annealing route.

The invention is directed to produce low carbon high yield ratio cold rolled rephosphorized steel sheet having tensile strength of at least 340MPa with high yield ratio or rephosphorized galvannealed steel sheet, wherein the selective composition is decided based on effect of Metallurgical factors affecting the mechanical and surface properties are described hereunder in details –

Carbon (0.01-0.05%wt %) – Carbon ranging from 0.01 to 0.05 is used for increasing the tensile strength of the material. To achieve the minimum tensile strength of 340Mpa or more, minimum 0.01% of carbon is required. Excessive amount of carbon increases the tensile strength significantly reduces the ductility and deteriorates the cold rolling properties hence the upper limit of carbon is maintained in to 0.05% to achieve the desired properties. In addition, higher carbon results in higher amount of second phase (Pearlite/ Bainite) and also increases the hardness of second phase results less R Bar.

Manganese (0.2-0.5 % Wt%) – Manganese acts as a solid solution strengthening and therefore increase in manganese content increases the tensile strength; however, 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 0.2 %Mn is required which will act as a grain refinement, but upper limit should be maintained to 0.5% to avoid poor phosphatability.

Silicon (0.04 or less) - Silicon deteriorates plating /surface properties as well 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 0.04 wt% or less.

Phosphorus (0.02-0.05wt%) –Phosphorus being most effective solid solution strengthening element helps to achieve the desired tensile strength of at least 340MPa.However, the ratio of P/B ratio to be in the range of 15 to 40,since when the ratio is more than40, the chance of P segregation at grain boundary occurs which results in poor secondary work embrittlement and DBTT is deteriorated. Hence,Upper limit is set to 0.05 % preferably.

Aluminum (0.03-0.06 wt%) – Aluminum acts as a deoxidizing agent but 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 0.03% or less will lead to higher dissolved oxygen which is harmful for steel cleanliness and will lead to Al to N ratio less than 10 which results in poor ageing resistance.

Nitrogen (0.005wt% or less ) –Nitrogen present in the high strength steel containing the titanium and niobium increase strength by formation of nitride precipitates but when present more than 0.005%, results in deteriorated ageing property so nitrogen should be maintained at 0.005%or less.

Boron (0.001-0.003 wt %): Boron is added in order to fix N as 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 Ti, Cr, and Ca such that each element weight percent is 0.03% or less- Group of Elements such as Ti, Cr, and Ca 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.

Complete Description of the Process of manufacture:

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 the 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 860°C to 930°C and run out table cooling rate from finishing mill to coiler of more than 5°C/sec was maintained to achieve coiling temperature in the range of 600 to 700°C. Hot rolled coils were subsequently processed through pickling coupled with tandem cold rolling mill to remove the oxide scales present in the surface and to provide cold reduction of 60% to 80%.

Following pickling and cold rolling to desired thickness, cold rolled steel strip is processed through the steps 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 from 35 to 100 sec.
c) Slow cooling further said steel at temperature 650°C to 700°C with slow cooling rate in the range from 0.9 °C/Sec to 5°C/Sec;
d) Rapid cooling of said steel at rapid cooling rate in the range from 10°C/Sec to 26 °C/Sec;
e) overaged the said steel in the range from 350°C to 400°C for 130 sec or more;
f) Skin passing of overaged steel in the range from 1.3% to 1.8%.

While processing the cold rolled steel strip through continuous annealing line, processing steps comprises: electrolytic cleaning which removes rolling emulsion present on the surface. Electrolytic cleaned surface passes through the preheating and heating section inside continuous annealing line where the strip is heated at the rate of 1.5-5 0C/sec up to soaking section temperature maintained in the range from 760 °C -820 °C. Annealing time at soaking section ranges from 35-100 seconds which gives desired results for present high strength re-phosphorised 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 5°C/sec.Slow cooling section temperature of 650 °C - 700°C was maintained. Following slow cooling section annealed strip sheet is rapid cooled in the range from 10 °C/sec to 26 °C/sec up to rapid cooling section temperature of 420°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 -400°C to avoid yield point elongation. After over aging Skin-pass elongation (Temper rolling) in the range of 1.3 % to 1.8% was applied to avoid yield point elongation.

Test for DBTT-Sample Dimension: 13 mm wide x 100 mm long (T direction)
oriented with the longest dimension transverse to the rolling direction.The specimens were bent at room temperature to a zero degree (0°) bend radius.The bend specimens were then opened manually by hand at various temperatures to determine the temperature at which a brittle fracture occurred.

Sample Preparation: The specimens were immersed in an alcohol/dry ice bath and were held for 5 min at the specified temperature. The specimens were removed and pulled within two seconds to unbend the specimen. They were then assessed for the pass/fail criteria: whether the specimen still had a metal-metal contact at the inner radius tip. To determine the transition temperature the percentage of pass specimens was calculated at each temperature.

Complete description of Inventive steel and comparative steel grades as per the trials conducted according to present invention is illustrated in the following table 1 to table 3:

Table 1- Elemental Compositions of the inventive steel sheets along with comparative example and their respective values of (V+B+Al) to N, P/B ratio.

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,DBTT of steel sheets having chemical composition as per table 1 and processed as per table 2.

Table 1:
Sample no C Mn S P SI AL N B V P/B (V+B+Al) /N Other Elements
1 0.022 0.20 0.012 0.028 0.008 0.055 0.0035 0.0018 0.012 15.556 19.7 Ti-0.01
2 0.028 0.34 0.005 0.032 0.032 0.040 0.0042 0.0012 0.018 26.667 14.1
3 0.025 0.48 0.008 0.030 0.010 0.035 0.0030 0.0017 0.015 17.647 17.2 Cr-0.01,Ca-0.0015
4 0.025 0.48 0.008 0.030 0.010 0.035 0.0070 0.0017 0.008 17.647 6.4
5 0.034 0.36 0.010 0.046 0.012 0.060 0.0052 0.0015 0.022 30.667 16.1
6 0.050 0.42 0.010 0.034 0.015 0.035 0.0037 0.0015 0.028 22.667 17.4
7 0.030 0.25 0.010 0.055 0.008 0.040 0.0050 0.0008 0.02 68.750 11.2
8 0.018 0.08 0.008 0.030 0.008 0.032 0.0030 0.0015 0.023 20.000 18.8
9 0.023 0.30 0.010 0.027 0.008 0.040 0.0020 0.0015 0.045 18.000 43.3 Ti-0.01
10 0.035 0.02 0.008 0.015 0.010 0.020 0.0035 0.0020 0.030 7.500 14.9

* Ex. - Present inventive example, Comp.- Comparative Examples
** Shaded and underlined boxes indicates “outside the appropriate range
*** Eq1 = (P/B)Ratio
****Eq2= (V+Al+B)/N ratio
Table 2 :

Note: Steel marked as 1a, 1b 1cas in table 1)have the same chemical composition as steel number 1, and however they are processed at different continuous annealing conditions to validate the claimed process.
* FT- hot finish rolling temperature ,ROT- Run out table at hot strip mill , SS- soaking section ,SCS- Slow cooling section , RCS- Rapid cooling section , OAS- Overaging section , SPM- Skin pass elongation ** Shaded boxes indicates “outside the appropriate range”.

Table 3-
S.no YS YPE UTS EL r-value n VALUE Aging Ductile to brittle temperature
1a 190 0.00 345 39.0 1.80 0.19

1b 253 0.00 363 38.6 1.60 0.185

1c 236 0.00 365 38.8 1.62 0.184

2 251 0.00 361 38.7 1.55 0.185

3 245 0.00 370 36.9 1.67 0.177

4 235 0.50 400 32.0 1.30 0.16

5 255 0 387 37.8 1.52 0.175

6 280 0.00 382 36.4 1.60 0.183

7 220 2.5 335 38 1.88 0.18

8 200 320 41.0 1.85 0.2

9 310 370 32.0 1.30 0.17

10 185 320 40.0 1.90 0.18

* Shaded and underlined boxes indicates “outside the appropriate range”
** Steel with better DBTT and aging remark as “ ”aging resistance for 6 months requirement.
**Steels with aging remark and DBTT “ “ do not fulfill the accelerated aging requirement as the YPE observed after accelerated aging test, DBTT of -50°C

Example 1- In sample no 1a, 1band 1c whereSoaking section temperature is varied from 770°C and 830°C with remaining parameters same observed strength is 375Mpa and 330MPa, processing below 760°C will increase the tensile strength and will not achieve the desired property, so it is restricted to maintain between 760°C to 820°C to desired property of above 340MPa

Example 2 – In sample no 7 where P/B ratio is 42.8 where material failed in test of DBTT,when the ratio of (P)/(B) is more than 40, there is chance of P segregation at grain boundary which results in poor secondary work embrittlement and DBTT is deteriorated. Hence, Upper limit is set to P/B ratio as 40 preferably, whereas in sample no 10 where p is 0.015 % tensile strength observed is 320MPa less than desired value and EQ1 (P/B) ratio is 7.5 to achieve desired property P/B ratio to be 15,limit of P/B ratio is defined as 15 to 40.

Example 3 – In sample no. 4 where spm elongation is 1 % and yield point is not suppressed, obtained YPE is 0.50 whereas in remaining samples spm is varied from 1.3 to 1.8% YPE observed is 0 whereas Vanadium is less than 0.01% and ratio of Eq1 ratio is 6.4 where sample failed in aging test to get aging of 3 to months the ratio of (V+Al+B)/N to maintained minimum of 15 ,in sample where Vanadium is 0.045 % and ratio of (V+Al+B)/N is 43.3 where yield strength observed is 310; so to restrict mechanical properties within desired range of ratio to be maintained within 35, hence range to be 10 to 35.

It is thus possible by way of the present invention to provide re-phosphorized low carbon high strength cold rolled steel sheet having goodformability and resistance to secondary work embrittlement and method of its manufacturing through continuous annealing route. The cold rolled re-phosphorizedsteel having Tensile strength of at least 340 MPa with high yield ratio and having selective chemical composition wherein ratio of (V+B+Al) to N are in the range of 10 to 35 and P/B ratio is in the range of 15 to 40. The micro structural constituents of said steel consisting 80-95% of ferrite and 5-10% of pearlite with balance being carbide and nitride precipitates of alloying elements. Cold rolled steel sheets according to the invention havegood formability, ageing resistance of more than 6 months at room Temperature and secondary work embrittlement having ductile to brittle transition (DBT) Temperature less than -50°C, suitable for variety of applications including manufacturing of selective automobile components.