Claims:We Claim:
1. Re-Phosphorized low Carbon high strength cold rolled steel sheet composition
comprising:
0.05wt % to 0.1wt% percent of Carbon;
0.5wt% to 0.99wt% of Manganese;
0.04wt % or less of Silicon;
0.03wt% to 0.06wt% of Aluminum;
0.051wt% to 0.08 wt% of Phosphorous;
Up to 0.006wt% of Nitrogen;
Balance as Fe and incidental impurities, wherein ratio of Alto N are in the range of 10 to 35 having tensile strength above 440 MPa, and having micro structural constituents of said steel comprising 70-95% of ferrite and 5-10% of pearlite.

2. Re-phosphorized low Carbon high strength cold rolled steel sheet composition as claimed in claim 1 comprising B from 0.001 to 0.0030 wt % and the ratio of (Mn + P) /(C) to be 8 to 20.

3. Re-phosphorized low Carbon high strength 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 V, Cr and Ca such that each element weight percent is 0.03% or less.

4. Re-phosphorized low Carbon high strength cold rolled steel sheet composition as claimed in anyone of claims 1 to 3 which is continuously annealed steel and having desired spot weldability, ageing resistance of more than 6 months at room Temperature in the range of 6to12 months and being ductile and resistant to secondary work embrittlement having DBT Temperature less than -50°C free of any cracks.

5. Re-phosphorized low Carbon high strength cold rolled steel sheet composition as claimed in anyone of claims 1 to 4 having selectively:
i. Tensile strength 440 MPa or more;
ii. Yield Strength at least 290 MPa with YS/TS ratio of 0.7 or more;
iii. Aging guarantee of 6 months;
iv. R-Value 1.2 or more;
V. N-Value 0.18 or more.

6. Re-phosphorized low Carbon high strength cold rolled steel sheet composition as claimed in anyone of claims 1 to 5 wherein said steel microstructure comprises carbide/nitride/sulphide precipitates of alloying elements with area fraction less than 2%.

7. A process for the manufacture of cold rolled high strength steel sheet as claimed in anyone of claims 1 to 6 comprising:
a.) providing a selective steel composition for slab generation for desired formability comprising:
0.05wt % to 0.1wt% percent of Carbon;
0.5wt% to 0.99wt% of Manganese;
0.04wt % or less of Silicon;
0.03wt% to 0.06wt% of Aluminum;
0.051wt% to 0.08 wt% of Phosphorous;
Up to 0.006wt% of Nitrogen;
Balance as Fe and incidental impurities such as to maintain Al/N ratio 10 to 35; and
b) working said steel composition including providing liquid steel, processing through RH degasser and subsequently continuously casting to steel sheets.

8. A process as claimed in claim 7 comprising:
i. Hot rolling of said steel slab with slab reheating Temperature 1220°C or less preferably 1160 to 1220°C, Finishing Temperature 860°C to 910°C and hot coiled with ROT cooling rate in the range of 5°C/Sec to 10°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 50% to 75%.

9. A process as claimed in claim 7or 8 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 800°C with residence time in the range from35 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 8°C/Sec to 22 °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%.

10. A process as claimed in anyone of claims 7 to 9 for obtaining 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 440 MPa or more;
ii. Yield Strength at least 290 MPa with YS/TS ratio of 0.7 or more;
iii. Aging guarantee of 6 months;
iv. R-Value 1.2 or more
v. N-Value 0.18 or more
vi. having ductile to brittle transition temperature (DBTT) less than -50 °C;and
vii. having micro structural constituents of said steel comprising 70-95% 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 17th day of April, 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 excellent weldability, ageing resistance, and resistance to secondary work embrittlement and method of manufacturing the same. More particularly, the present invention is directed to provide low carbon high yield ratio cold rolled rephosphorized steel having Tensile strength of at least 440 MPa with chemical composition of steel comprising in terms of mass fraction: 0.05% to 0.1% of C, Si: 0.04% or less, Mn: 0.5% to 0.99 %, N: 0.006% or less, Al:0.03 to 0.06%, P: 0.051 to 0.08 %, and the balance being Fe and other inevitable impurities, wherein ratio of Al to N is in the range of 10 to 35. 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 having excellent weldability, ageing resistance of more than 6 months at room Temperature and secondary work embrittlement having ductile brittle transition (DBT) Temperature less than -50°C, suitable for variety of applications including manufacturing of selective automobile components.

BACKGROUND OF THE INVENTION

In the past decades major developments have been taken 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 said components should have good fatigue resistance and as it is weldable with other components spot weldability is also a requirement.

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 which results in poor weldability, phosphatibility and very less ageing resistance.

There has been thus a need in the related filed to developing High Strength Low Alloy Steel with improved ageing guarantee and better weldability for automotive structural applications. Accelerated ageing prevention is important in this grade, as use of material is after 3 to 4 months after production then due to accelerated ageing yield strength will increases by 30-40 MPa, which results as spring back and bending related problems during the component manufacturing. The present invention targets to produce a steel grade that does not only have higher tensile strength but also has good weldability, ageing resistance and resistance for secondary work embrittlement which is required for certain automotive components. All these desirable properties are achieved by aiming the desirable 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 with better weldability and DBTT ( @- 50 °C) without any crack, processed through continuous annealing process suitable for automotive structural applications.

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 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 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 Al to N is experimentally decided to prevent ageing.

The transition from ductile to brittle can be very rapid and often times 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 an low carbon high strength cold rolled continuously annealed steel sheet having UTS= 440 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 involving selective slow cooling temperature with cooling rate followed by rapid cooling temperature with cooling rate, overaging temperature with residence time.

Yet another object of the present invention is directed to provide rephosphorized low carbon high strength cold rolled steel sheet with low DBTT and improved Spot weldability wherein free in carbon equivalent is 0.13 or less.

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 Al to N ratio is maintained from 10 to 35 to control free N to avoid ageing of the material.

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:
0.05wt % to 0.1wt% percent of Carbon;
0.5wt% to 0.99wt% of Manganese;
0.04wt % or less of Silicon;
0.03wt% to 0.06wt% of Aluminum;
0.051wt% to 0.08 wt% of Phosphorous;
Up to 0.006wt% of Nitrogen;
Balance as Fe and incidental impurities, wherein ratio of Alto N are in the range of 10 to 35having tensile strength above 440 MPa and having micro structural constituents of said steel comprising 70-95% of ferrite and 5-10% of pearlite.

A further aspect of the present invention is directed to said re-phosphorized low Carbon high strength cold rolled steel sheet composition comprising B from 0.001 to 0.0030 wt % and the ratio of (Mn + P) /(C) to be 8 to 20.

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

A still further aspect of the present invention is directed to said re-phosphorized low Carbon high strength cold rolled steel sheet composition which is continuously annealed steel and having desired spot weldability, ageing resistance of more than 6 months at room Temperature in the range of 6 to 12 months and being ductile and resistant to secondary work embrittlement having DBT Temperature less than -50°C free of any cracks.

Another aspect of the present invention is directed to said re-phosphorized low Carbon high strength cold rolled steel sheet composition having selectively:
i. Tensile strength 440 MPa or more;
ii. Yield Strength at least 290 MPa with YS/TS ratio of 0.7 or more;
iii. Aging guarantee of 6 months;
iv. R-Value 1.2 or more,
v. N-Value 0.18 or more.

Yet another aspect of the present invention is directed to said Re-phosphorized low Carbon high strength cold rolled steel sheet composition wherein said steel microstructure comprises carbide/nitride/sulphide precipitates of alloying elements with area fraction less than 2%.

A still further aspect of the present invention is directed to a process for the manufacture of cold rolled high strength steel sheet comprising:
a) providing a selective steel composition for slab generation for desired formability comprising:
0.05wt % to 0.1wt% percent of Carbon;
0.5wt% to 0.99wt% of Manganese;
0.04wt % or less of Silicon;
0.03wt% to 0.06wt% of Aluminum;
0.051wt% to 0.08 wt% of Phosphorous;
Up to 0.006wt% of Nitrogen;
Balance as Fe and incidental impurities such as to maintain Al/N ratio 10 to 35; and
b) working said steel composition including providing liquid steel, processing through RH degasser and subsequently continuously casting to steel sheets.

A still 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 preferably 1160 to 1220, Finishing Temperature 860°C to 910°C and hot coiled with ROT cooling rate in the range of 5°C/Sec to 10°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 50% to 75%.

A still further aspect of the present invention is directed to said process 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 800°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 8°C/Sec to 22 °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 said process for obtaining 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 440 MPa or more;
ii. Yield Strength at least 290 MPa with YS/TS ratio of 0.7 or more;
iii. Aging guarantee of 6 months;
iv. R-Value 1.2 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 70-95% 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 440MPa 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 (0.05-0.1%wt %) – Carbon ranging from 0.05 to 0.1 is used for increasing the tensile strength of the material. To achieve the minimum tensile strength of 440Mpa or more minimum 0.05% 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 0.1% 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 (0.5-0.99 % 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 0.5 % is required which will act as a grain refinement, but upper limit should be maintained to 0.99 % to avoid poor phosphatability. The formation of excess manganese oxide on the surface affects the phosphatability of the material. To control the deterioration of surface property and while achieving desired strength of 440 MPa or more, the ratio of (Mn+P)/(C) to be 8 to 20. If the ratio is higher, the chance of P segregation at grain boundary which result in poor secondary work embrittlement.

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 (0.051-0.08 wt%) –Phosphorus being most effective solid solution strengthening element helps to achieve the desired tensile strength of 440 MPa. However, when the ratio of (Mn+P)/(C) are more than 20, the chance of P segregation at grain boundary which results in poor secondary work embrittlement and DBTT is deteriorated. Hence, Upper limit is set to 0.08 % 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 dissolve oxygen which is harmful for steel cleanliness and will lead to Al to N ratio less than 10 which results as poor ageing resistance.

Nitrogen (0.006 wt% 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 0.006%, it results in Al to N ratio less than 10 and results in deteriorated ageing property so nitrogen should be maintained at 0.006% or less.

Boron (0.001-0.003 wt %): Boron is added optionally 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 V, Cr, and Ca such that each element weight percent is 0.03% or less- Group of Elements such as V, 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 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 910°C and run out table cooling rate from finishing mill to coiler of more than 5 0C/sec was maintained to achieve coiling temperature range of 650 °C 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 50% to 75%.

Following pickling and cold rolling to desired thickness, cold rolled steel strip is processed through comprising:
a. Continuous Annealing Line to get cold rolled closed annealed product. 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 800°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 8°C/Sec to 22 °C/Sec;
e. overaged the said steel in the range from 350°C to 400°C for 130 sec or more;

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 -800 °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 - 710°C was maintained. Following slow cooling section annealed strip sheet been rapid cooled in the range from 8 °C/sec to 22 °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 are 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 (Mn + P) /(C), Al/N 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:
S.No C Wt% Mn Wt% S Wt% P Wt% Eq1 Si Wt% Al Wt% N Wt% Al/N B ppm Other Elements

Remarks
1a 0.081 0.75 0.01 0.069 10.11 0.009 0.046 0.004 11.5 0.0018 V-0.025
EX
1b 0.081 0.75 0.01 0.069 10.11 0.009 0.046 0.004 11.5 0.0022 V-0.025 EX
2 0.05 0.9 0.008 0.075 19.50 0.009 0.05 0.004 12.5 0.002 Cr-0.015 EX
3 0.079 0.6 0.011 0.055 8.29 0.030 0.055 0.0035 15.7 0.0017 Cr-0.018, Ca- 0.003 EX
4 0.08 0.5 0.012 0.04 6.75 0.01 0.03 0.0045 6.7 0.0014 - Comp
5 0.07 0.4 0.011 0.12 7.43 0.02 0.02 0.003 6.7 0.0023 - Comp
* Ex. - Present inventive example, Comp.- Comparative Examples
** Shaded and underlined boxes indicates “outside the appropriate range
*** Eq1 = (Mn+P)/C Ratio

Table 2 :
Hot Rolling Cold Rolling Continuous Annealing
SRT°C FT°C CT°C % of Red Speed mpm SS°C Residence Time(Sec) SCS°C Cooling Rate °C/Sec RCS °C Cooling Rate °C/Sec OAS °C OAS Residence Time(sec) Spm%
1188 911 667 67 190 803 60 698 2.2 401 20 380 206 1.81
1188 911 667 67 155 762 73 650 1.8 403 16 395 253 1.60
1179 901 672 75 145 763 78 674 1.4 401 16 391 271 1.61
1184 910 680 77 190 790 60 680 2.3 406 21 360 208 1.35
1150 890 670 68 180 800 65 670 2 410 19 370 210 1.4
1200 910 630 72 170 780 67 650 1.9 420 17 380 238 1.5

Note: Steel marked as 1a, 1b as in table 1) have the same chemical composition as steel number 1, and however they are processed at different hot rolling, cold rolling and continuous annealing conditions to validate the claimed process.
* Ex. - Present inventive example, Comp- Comparative Examples
* 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-
Mechanical Property Product Property
YS MPa YPE % UTS MPa EL % R Bar n VALUE DBTT Aging Guarantee 6 months Type
330 0 458 35.9 1.45 0.183

Ex
309 0 454 38.5 1.60 0.189
Ex
299 0 445 38.4 1.63 0.187
Ex
310 0 470 37.6 1.56 0.187
Ex
290 0.4 420 40 1.66 0.19
comp
350 0.2 520 35 1.33 0.18
comp
* Shaded and underlined boxes indicates “outside the appropriate range”
** Steel with better DBTT and aging remark as “O” 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 and 1b where carbon is in 0.08wt% with different soaking section temperature of 760°C and 800°C with remaining parameters same observed strength is 458MPa and 454MPa. 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 800°C to desired property of above 440MPa.

Example 2 – In sample no 4 where Phosphorous is 0.04wt% where the tensile strength observed is 420MPa but in sample 5 where phosphorous content is 0.12wt% where tensile strength is higher of about 520MPa but where material is failed in test of DBTT,when the ratio of (Mn+P)/(C) are more than 20, the chance of P segregation at grain boundary which results in poor secondary work embrittlement and DBTT is deteriorated. Hence, Upper limit is set to 0.08 % preferably, so it is necessary to maintain Phosphorous in range of 0.051wt% to 0.08Wt%.

It is thus possible by way of the present invention to provide rephosphorized low carbon high strength cold rolled steel sheet having excellent weldability, ageing resistance, and resistance to secondary work embrittlement and method of manufacturing the same. The cold rolled rephosphorized steel having Tensile strength of at least 440 MPa with high yield ratio and having selective chemical composition wherein ratio of Al to N is in the range of 10 to 35. 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 having excellent weldability, ageing resistance of more than 6 months at room Temperature and secondary work embrittlement having ductile brittle transition(DBT) Temperature less than -50°C, suitable for variety of applications including manufacturing of selective automobile components.