Claims:We Claim:

1) Cold rolled high strength steel sheet having Tensile strength 780 MPa or more with desired formability and Stretch-flangeability comprising of composition in terms of weight percent:
C: 0.12-0.18 %;
Mn: 1.5-2.0 %;
Si: 0.5–0.8%;
Al: 0.02-0.1%
S: 0.005 % or less;
N: 0.005 % or less
Nb: 0.005-0.05%; and
the balance being Fe and other unavoidable impurities and wherein [Al] / [Si] ratio is in a range of 0.05 to 0.12 and having selective steel microstructure constituents including atleast 20% bainite phase, 60-75% Ferrite and Retained Austenite from 5 to 10% .

2) The Cold rolled high strength steel sheet as claimed in claim1, comprising atleast one type of element selected from the group of elements consisting of Ti, V, and Cr in amount less than 0.04 wt%.

3)The Cold rolled high strength steel sheet as claimed in anyone of claims 1 or 2 comprising in terms of weight % atleast one element selected from the group consisting of 0.001 % to 0.003 % Ca, 0.002 % to 0.2 % Ni and 0.0005 to 0.003 % B.

4)The Cold rolled high strength steel sheets as claimed in anyone of claims 1 to 6 having Yield strength of 400 MPa or more preferably in the range of 400 to 530MPa, Tensile strength of 780 MPa or more preferably in the range of 780 to 900 MPa , total elongation of 20% or more preferably in the range of 20 to 30%, strain hardening coefficient of 0.18 or more preferably in the range of 0.18 to 0.23 and Hole Expansion Ratio of 40% or more preferably in the range of 40 to 50%.
5) A process for manufacturing cold rolled high strength steel sheet as claimed in anyone of claims 1 to 4, having tensile strength at least 780MPa with desired formability and Stretch-flangeability comprising the steps of:
a) providing steel having composition comprising
C: 0.12-0.18 %;
Mn: 1.5-2.0 %;
Si: 0.5–0.8%;
Al: 0.02-0.1%
S: 0.005 % or less;
N: 0.005 % or less
Nb: 0.005-0.05%; and the balance being Fe and other unavoidable impurities; involving processing through Heat from basic oxygen furnace (BOF) and RH degasser and subsequently continuously casting into slabs;
b) reheating said slabs having said composition to reheating temperature in the range from 1190°C -1250°C;
c) subjecting said reheated slabs to roughing rolling in roughing mill with roughing mill delivery temperature of 1080°C or less preferably1000°C to 1080°C;
d) subjecting said rough rolled steel to finish rolling with finish mill exit temperature ranging from Ac3°C to Ac3+100 °C in the range of 860 to 920°C;
e) subjecting said slabs reheating, roughing rolling in roughing mill, hot rolling hot rolled sheet to cooling, coiling, cold rolling and continuous annealing maintainingsoaking section critical temperature range from 760 °C to 800 °C.

6) The process for manufacturing cold rolled high strength steel sheet as claimed in claim 5 comprising the steps of:
a) carrying out said Coiling the finish rolled steel at 530 to 630°C after being cooled with average run out table cooling rate of 9°C/second or more;
b) carrying outs said Cold rolling of said hot rolled steel sheet with cold reduction of at-least 35% after acid pickling;
c) Subjecting said cold rolled sheet to continuous annealing with selective parameters followed by skin passing.

7) The process for manufacturing cold rolled high strength steel sheet as claimed in anyone of claims 5 or 6, wherein cold rolled steel is subjected to continuous annealing following the steps comprising:
a) annealing the cold rolled steel sheet by heating at the rate of 0.5-5 0C/sec up to soaking section critical temperature range from 760 °C to 800 °C with residence time ranging from 70 to 150 seconds;
b) slow cooling(SCS) the steel at cooling rate in the range from 0.2 to 3°C/sec up to a temperature in the range from 680°C to 720 °C after soaking ;
c) rapid cooling the steel from SCS temperature up to a temperature range of 420 °C to 480 °C at a critical cooling rate of 20°C/sec or less.
d) Over-aging the said steel in the temperature range starting from 390 °C to 420 °C or more with residence time of 250 to 560 seconds wherein,
e) Subjecting the over-aged steel to skin pass elongation of 0.30 to 1.4%.

Dated this the 22nd day of September, 2021
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199
, Description:FORM 2
THE PATENT ACT 1970
(39 OF 1970)
&
The Patent Rules, 2003
COMPLETE SPECIFICATION
(See Section 10 and Rule 13)

1 TITLE OF THE INVENTION :
COLD ROLLED HIGH STRENGTH STEEL SHEET WITH IMPROVED FORMABILITY AND METHOD OF ITS MANUFACTURING.



2 APPLICANT (S)

Name : JSW STEEL LIMITED.

Nationality : An Indian Company incorporated under the Companies Act, 1956.

Address : JSW CENTRE,
BANDRA KURLA COMPLEX,
BANDRA(EAST),
MUMBAI-400051,
MAHARASHTRA,INDIA.



3 PREAMBLE TO THE DESCRIPTION

COMPLETE

The following specification particularly describes the invention and the manner in which it is to be performed.

FIELD OF THE INVENTION
The Present invention relates to cold rolled high strength steel sheet having improved formability and excellent strain hardening property and method of manufacturing the same. The steel sheets have Tensile strength of 780 MPa or more involving selective chemical composition and processing to achieve the desired microstructure and the stretch formability property. The advancement favors generation of cold rolled high strength steel sheet having Yield strength of 400MPa or more, Tensile strength of 780 MPa or more, total elongation of 20% or more, strain hardening coefficient of 0.18 or more and hole expansion ratio of 40% or more and crash resistance which makes such steel sheets suitable for automobile applications.

BACKGROUND OF THE INVENTION
There is an increased attention to light weighing, reliability in product performance and passenger safety in automotive segment. The stringent emission norms have triggered the automakers to speed up the programs to reduce vehicle weight for higher fuel efficiency and better environmental performance. This can be achieved by incorporating thinner high strength steel sheet having strength more than 780 MPa in place of conventional one. Conversely, increasing strength of steel through higher amount of alloying results in poor formability and Stretch-flangeability. As a result, high strength steel sheet arenot recommended to be used for automotive body parts requiring high amount of formability and stretching. In recent years, many high strength steels have been tried conventionally with the aim of reducing the vehicle weight and increasing the strength of the auto body to ensure safety.

Formability is a major limitation which restricts the application of high strength steel in automotive body parts having complex profile. Through improving the strain hardening coefficient better formability can be achieved. To facilitate, ferrite-Bainite Materials has been utilized where bainite, retained austenite in ferrite matrix results in excellent Stretch-flangeability. However, the optimum deployment of said steel can only be achieved by right combination of retained austenite and bainitephase fractions and their distribution. In order to achieve the said improvedformability, Si and Al are added to get the desired amount of retained austenite and Bainite at room temperature.

To avoid crack generation during press forming, the high strength steel sheet having strength >780 MPa must also exhibit a good strain hardening coefficient (n-value) of atleast 0.18 along with total elongation of no less than 20% and Hole Expansion ratio from 40 to 60%.

As a part of prior art, the Indian patent application number 3164/MUM/2012 discloses method of manufacturing a cold rolled steel sheet with minimum UTS of 780 MPa along with good elongation for automotive structural component. A high strength automotive steel sheet is disclosed is obtained by keeping increased Mn weight % and selective heating and cooling strategy. However the method disclosed in prior art fall short in getting good strain hardening property due to higher amount of martensite phase fraction as strengthening phase and high yield ratio of >0.6. In addition, the phosphatability gets deteriorated due to higher Mn in composition.

Japanese patent application number JP2005336526A discloses a High strength steel sheet having excellent workability and its production method comprising 50% or more of tempered martensite as a major phase component. A good combination of strength, ductility and stretch flange formability has been claimed as a part of invention by virtue of keeping high space factor of sintered martensite and retained austenite. However, due to presence of high proportion of hard martensite, the ‘n’ value deteriorates and material does not perform well in actual press forming due to poor strain hardening. In addition, due to excess P weight % in composition which is added to impart strength may result in poor elongation and temper embrittlement.
Chinese Patent Application Number CN1330786Cdiscloses complex phase steel plate with the tensile strength of 780MPa, which comprises the chemical constituents of the weight percent: 0.13 to 0.17% of C, 0.20 to 0.40% of Si, 1.30 to 1.50% of Mn, 0.02 to 0.03% of Nb and Fe as the rest. The complex phase steel plate has a complex phase structure of granular bainite and martensite, wherein the volume ratio of granular bainite is from 90 to 95%, and the volume ratio of martensite is from 10 to 5%. The yield strength is from 525 to 550MPa, the tensile strength is from 785 to 795MPa, and the total elongation percentage after fracture is from 20 to 25%.

The present invention thus attempts to overcome the above mentioned problems and limitations of the prior art by way of providing a high-strength cold-rolled steel sheet with minimum tensile strength of 780MPa, and total elongation more than 20% having improved strain hardening properties along with improved hole expansion ratio which is the major problem in any dual phase high strength steel sheet.

OBJECTS OF THE INVENTION
The basic object of the present invention is directed to provide cold rolled high strength steel sheet having excellent formability, strain hardening property, hole expansion ratio, surface quality and method of manufacturing the same.

A still further object of the present invention is directed to provide cold rolled high strength steel sheet involving selective composition to achieve the desired microstructure and the Stretch-flangeability property.

A still further object of the present invention is directed to provide cold rolled high strength steel sheet having tensile strength 780 MPa or more, a good strain hardening coefficient (n-value) of atleast 0.18 along with high total elongation of no less than 20 %, and Hole Expansion ratio more than 40%.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to provide Cold rolled high strength steel sheets having tensile strength 780 MPa or more with desired formability and Stretch-flangeabilitycomprising of composition in terms of weight %:
C: 0.12-0.18 %;
Mn: 1.5-2.0 %;
Si: 0.5–0.8%;
Al: 0.02-0.1%
S: 0.005 % or less;
N: 0.005 % or less
Nb: 0.005-0.05%; and
the balance being Fe and other unavoidable impurities and wherein [Al] / [Si] ratio is in a range of 0.05 to 0.12 and having selective steel microstructure constituents including atleast 20% Bainite phase, 60-75% Ferrite and Retained Austenite from 5 to 10% .

A still further aspect of the present invention is directed to said cold rolled high strength steel sheets wherein said steel microstructure constituents comprise including atleast 20% Bainite phase, 60-75% Ferrite and Retained Austenite from 5 to 10% balance being martensite phase along with carbide, nitride and sulphide.

A still further aspect of the present invention is directed to said Cold rolled high strength steel sheet comprising atleast one type of element selected from the group of elements consisting of Ti, Va, and Cr in amount less than 0.04 wt%.

A still further aspect of the present invention is directed to said cold rolled high strength steel sheet comprising in terms of weight % atleast one element selected from the group consisting of 0.001 % to 0.003 % Ca, 0.002 % to 0.2 % Ni and 0.0005 to 0.003 % B.

Another aspect of the present invention is directed to said cold rolled high strength steel sheets having Yield strength of 400 MPa or more preferably in the range of to 400 to 530 MPa, Tensile strength of 780 MPa or more preferably in the range of 780 to 900 MPa total elongation of 20% or more preferably in the range of 20 to 30%, strain hardening coefficient of 0.18 or more preferably in the range of 0.18 to 0.23and Hole Expansion Ratio of 40% or more preferably in the range of 40 to 50%.

A further aspect of the present invention is directed to a process for manufacturing the cold rolled high strength steel sheets as described above having tensile strength of atleast 780 MPa with desired formability and Stretch-flangeability comprising the steps of:
a) providing steel having composition comprising
C: 0.12-0.18 %;
Mn: 1.5-2.0 %;
Si: 0.5–0.8%;
Al: 0.02-0.1%
S: 0.005 % or less;
N: 0.005 % or less
Nb: 0.005-0.05%; and the balance being Fe and other unavoidable impurities; involving processing through Heat from basic oxygen furnace (BOF) and RH degasser and subsequently continuously casting into slabs; and
b) reheating said slabs having said composition to reheating temperature in the range from 1190°C -1250°C;
c) subjecting said reheated slabs to roughing rolling in roughing mill with roughing mill delivery temperature of 1080°C or less preferably1000°C to 1080°C;
d) subjecting said rough rolled steel to finish rolling with finish mill exit temperature ranging from Ac3°C to Ac3+100 °C in the range of 860 to 920°C;
e) subjecting said hot rolled sheet to cooling, coiling, cold rolling and continuous annealing.

A still further aspect of the present invention is directed to said process further comprising the steps of:
a) carrying out said Coiling the finish rolled steel at 530 to 630°C after being cooled with average run out table cooling rate of 9°C/second or more;
b) carrying outs said Cold rolling of said hot rolled steel sheet with cold reduction of at-least 35% after acid pickling;
c) Subjecting said cold rolled sheet to continuous annealing with selective parameters followed by skin passing.

A still further aspect of the present invention is directed to said process wherein cold rolled steel is subjected to said continuous annealing following the steps comprising:
a. annealing the cold rolled steel sheet by heating at the rate of 0.5-5 0C/sec up to soaking section critical temperature range from 760 °C to 800 °C with residence time ranging from 70 to 150 seconds;
b. slow cooling(SCS) the steel at cooling rate in the range from 0.2 to 3°C/sec up to a temperature in the range from 680°C to 720 °C after soaking ;
c. rapid cooling the steel from SCS temperature up to a temperature range of 420 °C to 480 °C at a critical cooling rate of 20°C/sec or less.
d. Overaging the said steel in the temperature range starting from 390 °C to 420 °C or more with residence time of 250 to 560 seconds wherein,
e. Subjecting the over-aged steel to skin pass elongation of 0.30 to 1.4%.

The above and other objects and advantages of the present invention are described hereunder with reference to non-limiting accompanying examples:

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1: shows the micrograph of the invented steel grade showing the distribution of phase fractions.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING EXAMPLE AND DRAWING
The Present invention relates to cold rolled high strength steel sheet having Tensile strength 780 MPa or more and composition in terms of weight percent comprising:
C: 0.12-0.18 %;
Mn: 1.5-2.0 %;
Si: 0.5–0.8%;
Al: 0.02-0.1%
S: 0.005 % or less;
N: 0.005 % or less
Nb: 0.005-0.05%; and
the balance being Fe and other unavoidable impurities; wherein [Al] / [Si] ratio is in a range of 0.05 to 0.12 and having selective steel microstructure constituents including atleast 20% Bainite phase, 60-75% Ferrite and Retained Austenite from 5 to 10% for improved formability and Stretch-flangeability.

Cold rolled high strength steel sheet obtained according to present invention having Yield strength of 400 MPa or more, Tensile strength of 780 MPa or more, total elongation of 20% or more, strain hardening coefficient of 0.18 or more and Hole Expansion Ratio of 40% or more.

Following abbreviations, terminologies and expressions are used to describe the manner of implementation of the present invention:
CAL – Continuous annealing Line
RCS -Rapid cooling section
OAS- Over-aging Section
CS - Center Speed
SRT -Slab Reheating Temperature
FET- Finishing Mill Entry Temperature
FT-Finishing Temperature
CT- Coiling Temperature
HER–Hole Expansion Ratio
Ac1 & Ac3 – Critical temperatures in iron-carbide diagram
El – Elongation (%)
UTS - Ultimate Tensile Strength (MPa)
YS - Yield Strength (MPa)
SPM - Skin Pass Elongation (%)

A Cold rolled High strength steel sheet having excellent formability and strain hardening property according to present invention, its chemical compositions and method of manufacturing are described hereunder with explanation on metallurgical factors deciding the range of constituents in a composition according to a preferred embodiment wherein all the elements are in weight % as follows:

Carbon (C: 0.12-0.18wt %) – Carbon effectively increases the hardenability and strength of steel. It also lowers the transformation temperature; hence more austenite forms during soaking of steel. In addition, Carbon also lowers the martensite finish temperature, which stabilize austenite phase at room temperature. However, to utilize the COMPLEX phenomenon at least 0.12 weight % of C is required. More preferably, the amount of carbon must be more than 0.13 to make austenite stable and to effectively lower the Martensite start temperature. On the other hand, increasing the carbon content above 0.18 % deteriorates the weldability and fatigue resistance. Also, with higher carbon, the austenite becomes too stable to be transformed to martensite during forming which results in poor strain hardenability. With these limitations, upper limit of carbon is 0.18 %.

Manganese (Mn: 1.5-2.0 wt %) - Similar to C, Mn is an austenite stabilizer .Mn increases the hardenability of steel by lowering Ms Temperature. It also assists in partitioning of C more to austenite and hence makes Austenite more stabilized. In order to attain the desired amount of solid solution strengthening to achieve UTS>780 MPa, minimum amount of Mn must be atleast 1.5%. However, an increase in manganese concentration restricts the fraction of bainite that can form. Higher Mn weight percent may also lead to higher martensite fraction resulting in lower strain hardening effect. Hence, the upper limit of Mn is 2.0 %.

Silicon (Si: 0.5–0.8wt %) –Si suppresses precipitation of cementite, therefore, it helps in enrichment of carbon in austenite and make it more stable.Si as a solid solution strengthening element strengthens the ferrite matrix. To attain that effect and to get minimum UTS>780 MPa, minimum amount of Si must be atleast0.5wt%. However, increasing Si level more than 0.8% does not cause any significant effect to inhibit cementite formation. In addition, adding excess Si deteriorates the surface. Hence, the upper Si content must not be more than 0.8%.

Aluminium (Al: 0.02-0.1wt%)-Like Si, Al also suppresses the cementite precipitation and in this way it can be used as a replacement for Si. However, Al does not strengthen the ferrite matrix, hence more Mn need to be added to achieve the desired strength level. Al also acts as a deoxidizer during steel making process to kill dissolved oxygen. To achieve adequate de-oxidation, the soluble aluminum (Al Sol.) preferably be at least 0.02 %. Al also adds on to fix harmful dissolve N to form AlN. Increasing Al level above 0.1wt% to replace Si causes longitudinal cracks while casting. Accordingly, upper limit is set to 0.1%.
[Al]/[Si] promotes preferential oxidation for better phosphatability before silicon oxide formation and for which Al/Si shall be at least 0.05. But with increasing Al/Si ratio, its increases cost of steel as well as creates slab cracking while casting so maximum limit of Al/Si restricted to 0.12.

Nitrogen (N: 0.005 wt% or less) – N is present in steel as an impurity and should be present at minimum amount to avoid aging. In addition, to achieve good aging resistance, upper limit of N must be 0.005 wt % or less.

Niobium (Nb: 0.005-0.05 wt %) - Niobium has a notable role on grain size development in conjunction with carbon enrichment, transformation mechanism of the austenite followed by nucleation of martensite which makes controlling the process parameter much easier, which further improve the mechanical property. To attain the explained benefits minimum amount of Nb which should be added is 0.005wt%. Nb more than 0.05 wt% unnecessarily adds up to the cost of production and increases yield strength. Hence, upper limit for present inventive grade in 0.05wt%.

Ti, V and Cr (collectively <0.04 wt%): Ti, V and Cr forms carbide and impart precipitation strengthening to the steel and any additional content more than 0.04 of each element will add up to cost of production. Moreover, higher addition of this element will form coarser carbides, reducing elongation. Formation of excess carbide also leads to lower carbon fraction in austenite and reduces its stability. Accordingly, upper limit of at least one of the element selecting from Ti, V and Cr must be less than 0.04 wt%.
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] X100
Where Do = Initial hole diameter, Df = final hole diameter before crack.
Description of the process of manufacturing:

To achieve Slab chemistry as described in scope of the invention, Heat from basic oxygen furnace (BOF) is processed through RH degasser and subsequently continuously castedSpecial measures are taken to hot roll resulted slabs by keeping slab reheating temperature in the range of 1190°C to 1250°C intended to control roughing mill delivery temperature under 1080°C and finishing mill entry temperature under 1080°C to check surface defects like rolled in scale. During hot rolling, finishing mill temperature is varied in the range from Ac3 °C to Ac3+100 °C. After finish rolling, Run out table cooling rate from finishing mill to coiler of more than 9 0C/sec was maintained to achieve coiling temperature range of 530 to 630 °C, to avoid ID collapse after coil winding, it is held for 300sec at mandrel. 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 selective cold reduction of 35% or more.

Subsequent to pickling and cold rolling to desired thickness, cold rolled steel strip are processed through continuous annealing line where electrolytic cleaning removes rolling emulsion present on the surface. Cleaned surface passes through the preheating and heating section where the strip is heated at the rate of 0.5-5 0C/sec up to soaking section critical temperature range from 760 °C to 800 °C with residence time ranging from 70 to 150 seconds, slow cooling(SCS) the steel at cooling rate in the range from 0.2 to 3°C/sec up to a temperature in the range from 680°C to 720 °C after soaking, rapid cooling the steel from SCS temperature up to a temperature range of 420 °C to 480 °C at a critical cooling rate of 20°C/sec or less, Overaging the said steel in the temperature range starting from 390 °C to 420 °C or more with residence time of 250 to 560 seconds wherein and Subjecting the over-aged steel to skin pass elongation of 0.30 to 1.4% to avoid yield point elongation.

Complete description of steel according to the present advancement and comparative steel grades are illustrated in following table 1 to table 4 and the weight percent range of constituents and the selective process parameters according to the invention are validated through following examples 1 & 2:

Table 1: Elemental Compositions in weight % of the inventive steel sheets along with comparative example.
Table 2: Hot rolling and cold rolling of inventive with comparative steel sheets having chemical compositions as per Table 1.
Table 3: CAL Parameters of inventive with comparative steel sheets having chemical compositions as per Table 1.

Table 4: Mechanical properties, n-Value, HER % and micro structural phase fractions of inventive and comparative steels having chemical composition as per table 1 and being processed as per Table 2 and Table 3.

Table 1

Chemical Composition in %
Sample No C Mn S P Si Al N Nb [Al]/[Si] Other Elements Remarks
1a 0.17 1.8 0.003 0.021 0.5 0.06 0.004 0.045 0.12 Ca-0.002, -0.003, I
Ti-0.02, V-0.01
1b 0.13 1.9 0.003 0.021 0.8 0.06 0.004 0.01 0.075 B-0.002 I
2 0.12 1.9 0.004 0.015 0.6 0.04 0.004 0.03 0.067 Ca-0.0015 I
3 0.14 1.5 0.003 0.01 0.7 0.06 0.005 0.04 0.085 V-0.04 I
4 0.12 1.4 0.003 0.02 0.15 0.04 0.004 0.08 0.26 C
5 0.19 1.63 0.004 0.015 1.43 0.05 0.003 0.017 0.034 V-0.02, Cr-0.01 C
6 0.09 2.1 0.01 0.02 0.2 0.035 0.004 0.03 0.175 C

*I - Present inventive example, C- Comparative Examples
*Underline boxes indicates “outside the appropriate range”

The inventive steel grade according to present invention having yield strength of 400 MPa or more, Tensile strength of 780 MPa or more, total elongation of 20% or more, strain hardening coefficient of 0.18 or more and hole expansion ratio of 40% or more.

Example 1

It can be appreciated from Table 1 to Table 4 that steel sheets remarked as “I” are satisfying all the scopes of present invention and exhibits excellent strain hardening property,Total elongation % and HER%. These steels exhibit improved n value>0.18, Total elongation > 20%, HER %> 40, UTS =780MPa and Yield strength more than 400 MPa. Whereas, Steel remarked as 4, 5 and 6 from Table 1 to Table 4 doesn’t comply with at-least one of the scope of the present invention and does not conform with minimum one or more of the end product attributes as mentioned in the scope of the present invention. For example steel no. 6 in table 1 has the less carbon %, higher Mn % and lower Si wt% than the scope and does not comply with required ratio of Al/Si ratio and has poor n-value, Total Elongation and Hole Expansion Ratio.

Table 2
Hot Rolling Parameters Cold Rolling Parameters
Sample No SRT°C Roughing Mill temp°C FT°C CT°C Cold Reduction %


1a 1205 1075 903 558 40
1b 1205 1075 903 558 59
2 1200 1060 895 580 45
3 1190 1070 900 620 40
4 1250 1050 910 550 42
5 1220 1070 908 570 45
6 1210 1080 910 560 50
*I - Present inventive example, C- Comparative Examples
Note: Steel marked as 1a and 1b have different chemical composition within present scope of chemical composition as steel number 1, and however they are processed at different conditions to validate the claimed process. In sample no 1a and 1b the Rapid cooling temperature is varied in 1a temperature is 443°C where retained austenite formed is 6% and n value is greater than 0.19 whereas in sample 1b where rapid cooling temperature is 380°C where retained austenite formed is 0% and n value is 0.10 less than required value and also HER% 36 less than the required value of HER.
In sample 3 where carbon percentage is 0.14% and Mn% is 1.5%, whereSStemperature is more than 800 °Cand OAS temperature is less than 380 where percentage of bainite formed is less than desired level of 20% and results in Tensile strength less than the desired value.
* SRT- Slab reheating temperature, FT- hot finish rolling temperature ,ROT- Run out table at hot strip mill , CR%- Cold rolling reduction % , SS- soaking section, SCS- Slow cooling section , RCS- Rapid cooling section , OAS- Over-aging section, SPM- Skin pass elongation.

Table 3
CAL Parameters
Sample No SS TEMP SS Residence Time SCS TEMP SCS Cooling Rate, °C/Sec RCS TEMP RCS Cooling Rate, °C/Sec OAS TEMPERATURE OAS Residence Time SPM ELONG
1a 800 95 685 2 443 20 394 327 0.3
1b 800 95 685 2 380 42 330 327 0.3
2 760 140 700 3 480 10 400 260 0.2
3 840 80 690 2 420 30 380 350 0.3
4 810 90 670 5 380 45 340 300 0.2
5 830 113 690 1.67 460 25 410 393 0.4

6 810 113 700 1.32 380 33.5 350 390 0.4

Table 4
Mechanical Prop Product Properties
Sample No YS YPE TS ELONGATION n-Value HER % Ferrite % Bainite % Retained Austenite % Martensite % Remarks
1a 480 0 830 24 0.19 45 65 27 6 2 I
1b 400 0 850 13 0.1 36 80 0 0 20 C
2 450 0 840 25 0.2 42 64 29 6 1 I
3 450 0 760 23 0.21 48 74 18 6 2 C
4 470 0 900 12 0.09 26 70 0 0 30 C
5 480 0 830 13 0.12 32 90 0 0 10 C
6 480 0 805 12 0.09 33 65 0 0 35 C

*I - Present inventive example, C- Comparative Examples
Example 2

In sample 1b, 2 and 3 where in sample 3 Tensile strength is 760 MPa and bainite is 18%.
In case of sample 1b, 4, 5 and 6, n values is less than 0.18 which is out of scope of current invention, in case of sample 1b where retained austenite is 0 and martensite percentage is 20 percent and n value is 0.1 less than 0.14 and has poor HER % less than 40% which is out of scope of current invention. Sample 4, 5 and 6 are also having lower hole expansion ratio, n-value and Total Elongation because of high martensite % and no Bainite phase which is out of specification of present invention.

It is thus possible by way of the present invention to provide 780 MPa Tensile strength level high strength cold rolled steel sheet involving select chemical elements in terms of weight percent: C: 0.12-0.18 %; Mn: 1.5-2.0 %; Si: 0.5–0.8%; Al: 0.02-0.1% S: 0.005 % or less; N: 0.005 % or less Nb: 0.005-0.05%; and the balance being Fe and other unavoidable impurities; wherein [Al] / [Si] ratio is in a range of 0.05 to 0.12 and having selective steel microstructure constituents including atleast 20% bainite phase, 60-75% Ferrite and Retained Austenite from 5 to 10% as shown in accompanying Figure 1 for improved formability and Stretch-flangeability

The advancement favors generation of cold rolled high strength steel sheet having Yield strength of 400MPa or more, Tensile strength of 780 MPa or more, total elongation of 20% or more, strain hardening coefficient of 0.18 or more and Hole Expansion ratio 40% or more.