DESC:FIELD OF THE INVENTION

The Present invention relates to cold rolled high strength steel sheet having excellent strain hardening property and method of manufacturing the same. More particularly, the present invention is directed to provide 780MPa Tensile strength level high strength cold rolled steel sheet having selective chemical composition wherein elements in terms of weight percent comprising: 0.121% to 0.2 % of C, 0.9-1.5 % of Si , 1.5% to 2.2 % of Mn,N: 0.005% or less, 1 % or less of Al, 0.005 % or less of S , 0.005% to 0.05 % of Ti, 0.005 % to 0.05 % of Nband the balance being Fe and other inevitable impurities, wherein[Mn+Si+Al] % / [C] % is in a range from 12 to 27and steel microstructure constituents relative to the whole microstructure of said steel satisfy a selective relation in terms of area fraction to achieve the desired strain hardening index and formability property. Cold rolled high strength steel sheet of present invention has Yield strength of 450MPa or more, Tensile strength of 780 MPa or more, total elongation of 23% or more, strain hardening coefficient of 0.19 or more and bake hardening index of 40 MPa or more.Through improving the strain hardening coefficient better formability is achieved utilizing Transformation induced plasticity (TRIP) phenomenon whereby retained austenite in ferrite-Bainite matrix can be transformed to martensite post forming resulting better formability. However, the optimum deployment of said TRIP steel is achieved by right combination of retained austenite, bainite and martensite phase fraction and distribution experimentally established by way of the present invention.

BACKGROUND OF THE INVENTION

There is an increased attention to light weighing, reliability in product performance and passenger safety in automotive segment. To boot, the stringent emission norms has initiated automakers speed up programs to reduce vehicle weight for better environmental performance. One solution to cater such requirement is to reduce the automotive body weight. This can be achieved by incorporating thinner high strength steel sheet having strength more than 780MPa in place of conventional one. Conversely, Increasing strength of steel through higher amount of alloying results in rather inferior drawability and poor surface appearance.As a result, High strength steel sheet are infeasible to be used for automotive body parts requiring high amount of drawability and superior surface appearance. 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 one 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, Transformation induced plasticity (TRIP) phenomenon has been utilized where retained austenite in ferrite-Bainite matrix can be transformed to martensite post forming resulting better formability. However, the optimum deployment of said TRIP steel can only be achieved by right combination of retained austenite, bainite and martensite phase fraction and distribution. In addition, in order to achieve the said TRIP phenomenon, the surface property deteriorates caused by higher amount of Si and Mn required retaining austenite 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 0.19 along with high total elongation of no less than 22 %. At the same time the yield ratio must be below 0.6 for better formability.

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, 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.

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 strength of 780 MPa, having improved strain hardening properties along with excellent surface properties and a process for its manufacture.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide cold rolled high strength steel sheet having excellent strain hardening property, surface quality and method of manufacturing the same.

A still further object of the present invention is directed to providecold rolled high strength steel sheet having selective composition comprising in terms of weight percent comprising: 0.121% to 0.2 % of C, 0.9-1.5 % of Si , 1.5% to 2.2 % of Mn , N: 0.005% or less, 1 % or less of Al , 0.005 % or less of S , 0.005 % to 0.05 % of Ti, 0.005 % to 0.05 % of Nb and the balance being Fe and other inevitable impurities, wherein [Mn+Si+Al] % / [C] % is in a range from 12 to 27 and produced through selective processing steps whereby steel microstructure constituents comprising ferrite, bainite, retained austenite and martensite, relative to the whole microstructure of said steel satisfy a specific relation in terms of area fraction to achieve the desired strain hardening index and formability property.

A still further object of the present invention is directed to providecold rolled high strength steel sheet having strength 780 MPa or more, a good strain hardening coefficient (n-value) of atleast 0.19 along with high total elongation of no less than 22 %,and a yield ratio below 0.6 for better formability to suit automobile application.

A still further object of the present invention is directed to providecold rolled high strength steel sheethaving excellent strain hardening property, surface quality that ensure avoiding crack generation during press forming.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to cold rolled high strength steel sheets having tensile strength atleast 780 MPa with composition in terms of weight % comprising:
C: 0.121-0.2 %;
Mn: 1.5-2.0 %;
Si: 0.9 -1.5%;
Al: =1%;
S: 0.005 % or less;
N:0.005 % or less
Ti: 0.005-0.05%;
Nb: 0.005-0.05;
and the balance being Fe and other unavoidable impurities; wherein ratio of [Mn+Si+Al] % / [C] % is in the range from 12 to 27 and steel microstructure constituents relative to the whole microstructure of said steel are in a relation satisfying;
3 = [Fea + ?RA] / [Ms+ Ba] = 7;
where, Fea: Total area fraction of Ferrite,?RA: Total area fraction of Retained Austenite, Ms: Total area fraction of Martensite, Ba: Total area fraction of Bainite.

A 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 V, Zr, Hf, W 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 additionally comprising in terms of weight % atleast one element selected from the group consisting of 0.002 % to 0.2 % Cu, 0.002 % to 0.2 % Ni, 0.002 to 0.3 wt % Mo and less than 0.01 % Ca.

Another aspect of the present invention is directed to said cold rolled high strength steel sheets having Yield strength of 450MPa or more, Tensile strength of 780 MPa or more, total elongation of 23% or more, strain hardening coefficient of 0.19 or more and bake hardening index of 40 MPa or more.

Yet another aspect of the present invention is directed to said cold rolled high strength steel sheet comprising in terms of area fraction relative to entire microstructure of steel, 60% or more of ferrite phase, 2 % or less of martensite phase, 10 % or more of retained Austenite phase and balance is Bainite phase along with carbide, nitride and sulphide precipitates.

A further aspect of the present invention is directed to a process for manufacturing the cold rolled high strength steel sheets as described above comprising the steps of:
a.) Providing steel slab having composition as given above comprisingprocessing Heat from basic oxygen furnace (BOF) through RH degasser and subsequently continuously cast;
b.) Reheating the slab having said composition to reheating temperature in the range from 1150°C -1250 °C ;
c.) Said Reheated slab being subjected to roughing rolling in roughing mill with roughing mill delivery temperature of 1080°C or less;
d.) Said rough rolled steel being subjected to finish rolling with finish mill exit temperature ranging from Ac3 °C to Ac3+100 °C.
e.) Coiling the finish rolled steel at with average run out table cooling rate of 9 °C/second or more; and
f.) Acid Pickling the Cold rolling the said hot rolled steel sheet with cold reduction of atleast 40%.

A still further aspect of the present invention is directed to saidprocess for manufacturing cold rolled steel sheet, wherein cold rolled steel is subjected to continuous annealing following the steps comprising:
a) Annealing the cold rolled steel sheet at soaking section critical temperature range from Ac1+20 °C to Ac3+20 °C with residence time ranging from 70 to 150 seconds;
b) Slow cooling the steel up to a temperature in the range from Ac1 - 60 °C to Ac1 °C after soaking ;
c) Rapid cooling the steel from SCS temperature up to a temperature range of T1 °C or more at a critical cooling rate of V1 °C/sec or less wherein ,
V1 =18.0- 4[%C]-1.8[%Mn]-2.2[%Si]+10[%Ti+%Nb],and
T1 = 550 - 324[C%] - 32.4 [Mn%] - 10.8[Si%]-10.8[Mo%],
where [%X] denotes weight % of element X in said steel sheet.
d) Overaging the said steel in the temperature range starting from T1°Cor less to T2 °C or more with residence time of 250 to 560 seconds wherein,
T2= 555 - 70[C%] - 35 [Mn%] - 75[Si%] - 40[Mo%];
e) Subjecting the overaged steel to skin pass elongation of 0.2% to 1%.

The above and other objects and advantages of the present invention are described hereunder in details with reference to accompanying examples:

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ILLUSTRATIVE EXAMPLES INCLUDING A PREFERRED EMBODIMENT

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: 0.121% to 0.2 % of C, 0.9-1.5 % of Si , 1.5% to 2.2 % of Mn , N: 0.005% or less, 1 % or less of Al , 0.005 % or less of S , 0.005 % to 0.05 % of Ti, 0.005 % to 0.05 % of Nb and the balance being Fe and other inevitable impurities, wherein [Mn+Si+Al] % / [C] % is in a range from 12 to 27 and steel microstructure constituents relative to the whole microstructure of said steel are in a relation satisfying;
3 = [Fea + ?RA] / [Ms+ Ba] = 7;
Where, Fea: Total area fraction of Ferrite, ?RA: Total area fraction of Retained Austenite, Ms: Total area fraction of Martensite, Ba: Total area fraction of Bainite. Cold rolled high strength steel sheet of present invention has Yield strength of 450MPa or more, Tensile strength of 780 MPa or more, total elongation of 23% or more, strain hardening coefficient of 0.19 or more and bake hardening index of 40 MPa or more.

Following abbreviations, terminologies and expressions are used to describe the manner of implementation of the present invention:
Abbreviations
CAL – Continuous annealing line
YPE- Yield Point Elongation
SS- Soaking Section
SCS – Slow Cooling Section
RCS -Rapid Cooling Section
OAS - Over-ageing section
UTS-Ultimate Tensile Strength in MPa
YS-Yield Strength in MPa
El% – Total Elongation in %
SPM % -Skin Pass Elongation in %
SRT-Slab Reheating Temperature
FT-Finishing Temperature
CT- Coiling Temperature
%Fea- Ferrite area %
% Ms – Martensite area %
?RA: Total area fraction of Retained Austenite
Ba: Total area fraction of Bainite
BH- Bake hardening
ROT= Run out Table at Hot rolling
CR%- Cold reduction %
n-value – Strain hardening coefficient at strain range from 10% up to uniform elongation
Ac1 & Ac3 – Lower And Upper Critical temperatures in iron-carbon phase transformation diagram.
V1 = Maximum Cooling rate permissible to cool steel sheet from SCS to up to RCS temperature, where
V1=18.0- 4[%C]-1.8[%Mn]-2.2[%Si] + 10[%Ti+%Nb]
T1 = Minimum allowable RCS temperature up to what steel sheet can be cooled from SCS. Where,
T1=550-324[C%]-32.4[Mn%]-10.8[Si%]-10.8[Mo%]
T2 = Minimum over aging section temperature up to what steel sheet can be hold from RCS to OAS section end.Where,
T2=555 - 70[C%] - 35 [Mn%] - 75[Si%] - 40[Mo%]

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

Carbon (C: 0.121-0.2 wt %)–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 TRIP phenomenon atleast 0.1 weight % of C is required. More preferably, the amount of carbon must be more than 0.12 to make austenite stable and to effectively lower the Martensite start temperature. Keeping carbon above 0.12 also avoids the peritectic contraction during solidification at continuous casting, thereby preventing the risk of slab cracking during solidification. On the other hand, increasing the carbon content above 0.2 % deteriorates the weldability and hole expansion ratio. 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 0.2 %.

Manganese (Mn: 1.5- 2.0wt %)- Similar toC, 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>780MPa, 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: 0.9-1.5 wt %) –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 atleast 0.9 wt%. However, increasing Si level 1.5% does not cause any significant effect to inhibit cementite formation. In addition, adding excess Si deteriorates the phosphatability and strain hardenability. Hence, the upper Si content must not be more than 1.5 %.

Aluminium (less than 1.0 wt %) –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 moreMn 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 deoxidation, the soluble aluminum (Al Sol.) preferably be atleast 0.02 %. Al also adds on to fix harmful dissolve N to form AlN. Increasing Al level above 1 wt% to replace Si causes problems during continuous casting and adds up to the cost of production. Accordingly, upper limit is set to 1.0 %.

Titanium (Ti: 0.005-0.05 wt %)- Ti acts as a nitride forming element to fix solute N in steel thus helps in getting aging resistance. Formability of steel sheet improves by reducing solute N in solution with Ti instead of Al. And so, amount of Ti preferably added should be 0.005 wt% or more. However, when Ti contents exceeds 0.05 wt%, the effects are saturated, therefore the amount of Ti is made to be 0.05% or less. In addition, when Ti is added in excess of the amount required for reducing solid solution N, excessive TiC may form, which inhibits the bake hardening properties and stable formation of austenite, which is not preferable .

Niobium (Nb: 0.005-0.05 wt %) –Nb as carbide former strengthens the ferrite matrix by formation nano sized NbC precipitates. For achieving the said benefits, minimum amount of Nb must be above 0.005 %. Nb also refines the grain size and improves the strength. However, excess addition of Nb results in coarse carbide formation which reduces the elongation. Also, excessive Nb addition result in lower carbon fraction in austenite and reduces its stability. Accordingly, upper limit of Nb is set 0.05 %.

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. Excessive dissolve nitrogen needs additional Ti to be added to fix it as TiN and cost of production increases. In addition, to achieve good aging resistance, upper limit of N must be 0.005 wt % or less.

V, Zr, Hf, W less than 0.04 wt%: V, Zr, Hf, W forms carbide and impart precipitation strengthening to the steel. However as Ti and Nb are already added, 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 atleast one of the element selecting from V, Zr, Hf, W must be less than 0.04 wt%.

Atleast one selected from ( 0.002 % to 0.2 % Cu, 0.002 % to 0.2 % Ni, 0.002 to 0.3 wt % Mo and less than 0.01 % Ca ):Cu , Ni and Mo are solid solution strengthening elements and adds up to strength of the steel. Addition of these elements also helps in improving corrosion resistance as well. However, to get any noticeable effect minimum amount must be 0.002 wt% or more. Adding excess amount increases the cost of production and reduces drawability. Accordingly, upper limit of Cu and Ni is set as 0.2 wt% maximum. Addition of Mo more than 0.3 wt% may promote the formation of martensite which reduces the strain hardenability. Consequently, the upper limit of Mo is 0.3 wt% in present inventive steel. Ca addition in steel helps in removing S effectively thereby avoiding the effect of edge cracking and poor fatigue due to S. However, any additional content more than 0.01 wt% does not have any significant effect.

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 casted. Special measures are taken to hot roll resulted slabs by keeping slab reheating temperature in the range of 1150°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 °C to 590 °C. Hot rolled coils were subsequently processed through pickling coupled with tandem cold rolling mill to remove the oxide surface present in the surface and to provide a cold reduction of 40% 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 temperature. Soaking section temperature was maintained in the range from Ac1+20 °C to Ac3+20 °C to achieve retained austenite in final microstructure. Annealing time is kept in the range from 70 to 150 seconds to allow sufficient time for annealed and homogenization of austenite microstructure. After soaking section steel strip passes through slow cooling section at cooling rate in the rage from 0.2 to 3 °C/sec. Slow cooling section temperature is kept in the range from Ac1 - 60 °C to Ac1 °C to avoid any pearlite formation during cooling. Following slow cooling section, annealed strip sheet passes through rapid cooling section at cooling rate of V1 °C/sec or less and cooled up to rapid cooling section temperature of T1 °C or more. This is to keep martensite area fraction in microstructure less than 2 %. Subsequent to RCS, annealed strip passes through over aging section(OAS) where rapid cooled steel strip is over aged keeping the over aging section temperature of T2 °C or more to allow bainite transformation. Over aged steel sheet is then provided with skin-pass elongation in the range from 0.2 % to 1 % to avoid yield point elongation.

Furthermore, Cold rolled high strength steel sheet described in present invention can be processed through continuous galvanizing route for zinc coating to produce GA/GI steel sheets and used as coated product for similar applications.

Method of evaluating phosphatability:
Phosphating process provides a hard, non-conducting surface coating of insoluble phosphate to metal surface. The adherent and contagious coating layer provides excellent paint ability and corrosion resistance to steel surface.

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 <6 µm 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 1.5-3 g/m2 is considered having excellent phosphatability.

Method of evaluating bake hardening in a tensile test:
Tensile test specimen as per JIS Z2241 No.5 with 50mm gauge length 25mm width was and prepared across the rolling direction of steel sheet. Tensile test specimen was then strained to 2% at strain rate of about 0.008/second and then heated at 1700C for 20 minutes. Heated sample was then subjected to tensile test. Bake hardening index was then evaluated by measuring thedifference between the initial strength at 2% strain before bake hardening and final yield strength (at lower yield point) after heating at 1700C for 20 minutes.

Complete description of Inventive steel and comparative steel grades are illustrated in following table 1 to table 3 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 and their respective values of Eq1 = (Mn+Si+Al)/C.
Table 2- Hot rolling, cold rolling, critical temperatures (Ac1-Ac3), and annealing parameters of inventive and comparative steel sheets having chemical compositions as per table 1.
Table 3-Mechanical properties, surface phosphatability properties and micro structural phase fractions of inventive and comparative steels having chemical composition as per table 1 and being processed as per table 2.

Table 1:
Steel .No C Mn S Si Al N Ti Nb Mo Other
Elements Eq1=
(Mn+Si+Al)/C V1,
0C/sec T1,
0C T2
,0C Remarks
1 0.18 1.7 0.003 1.4 0.06 0.003 0.02 0.02 0.04 Ca:0.005 , V:0.008,
Cu:0.05 17.6 11.5 420 376 I
2 0.16 1.8 0.004 1.45 0.1 0.004 0.02 0.015 Ni:0.05,
Hf: 0.005 , W:0.005 20.9 11.3 424 372 I
3 0.175 1.63 0.002 1.43 0.05 0.005 0.017 0.019 0.03 17.8 11.6 424 377 I
4 0.19 1.55 0.003 1.3 0.4 0.003 0.015 0.025 0.1 Cu:0.04 17.1 12.0 420 386 I
5 0.09 2.1 0.01 0.5 0.035 0.005 _ _ V:0.05,
Cr: 0.5 29.3 12.8 447 438 C
6 0.08 1.9 0.009 0.7 0.046 0.004 _ _ V:0.06,
Cr: 0.7 33.1 12.7 455 430 C
7 0.08 2.5 0.003 0.3 0.03 0.003 0.02 0.03 35.4 13.0 440 439 C
8 0.08 2.9 0.003 0.3 0.03 0.003 0.02 0.015 0.2 Cr: 0.5 40.4 12.2 419 417 C
9 0.06 1.2 0.005 0,2 0.05 0.006 _ 0.03 0.1 24.2 15.5 486 490 C
*I - Present inventive example, C- Comparative Examples
Where,
* Eq1= [Mn+Si+Al] % / [C] %, V1=18.0- 4[%C]-1.8[%Mn]-2.2[%Si]+10[%Ti+%Nb]
T1=550 - 324[C%] - 32.4 [Mn%] - 10.8[Si%]-10.8[Mo%]
T2=555 - 70[C%] - 35 [Mn%] - 75[Si%] - 40[Mo%]
** Shaded and underline boxes indicates “outside the appropriate range”
** Steel having value of {[Mn+Si+Al] % / [C] %} >27 does not comply with scope of the present invention resulting in rather poor phosphatability and n value.

Example 1
It can be appreciated from Table 1 to Table 3 that steel sheets remarked as “I” are satisfying all the scopes of present invention and exhibits excellent strain hardening property and phosphatability. These steels exhibits improved n value>0.19 , phosphate crystal size =6µm and phosphate coats weight 1.5-3 g/m2 post zinc phosphate chemical conversion coating ,yield ratio of =0.6 , BH index =40MPa and UTS =780 MPa. Whereas, Steel remarked as “C” from Table 1 to Table 3 doesn’t comply with atleast 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. 5 to 8 in table 1 has the value of [Mn+Si+Al] % / [C] % outside the range from 12 to 27.Consequently steel 5 to 8 does not comply with scope of the present inventionwith lower n value and poor elongation.

Table 2:
Steel No. SRT, FT, ROT CT, CR% , Ac1, Ac3, SS , Annealing Time, SCS , SCS to RCS cooling RCS, OAS , OAS SPM% Remark
0C 0C Cooling 0C 0C 0C 0C 0C sec 0C rate 0C 0C Time,
Rate, ,0C/sec sec
0C
1A 1200 910 10.9 570 50 746 840 820 105 700 24.62 450 395 561 0.2 I
1B 1200 910 12.1 570 50 746 840 820 113 710 45 320 300 390 0.3 C
2A 1180 900 10.5 565 47 745 832 800 103 700 28.41 455 390 393 0.2 I
2B 1180 900 11.6 555 47 745 832 800 103 700 43.75 320 280 430 0.3 C
3A 1210 915 10.7 565 50 747 823 790 110 695 24.62 450 395 436 0.3 I
3B 1210 910 11.2 560 50 747 823 790 110 695 34.09 340 300 436 0.2 C
4 1190 900 11.7 560 50 744 859 840 110 710 25.57 450 400 425 0.3 I
5 1200 900 12.8 530 50 724 819 820 95 690 30.3 380 315 415 0.2 C
6 1200 895 13.1 535 50 731 841 800 95 680 30.78 360 320 440 0.4 C
7 1210 910 9.3 560 50 705 791 810 110 700 32.32 380 350 310 0.4 C
8 1190 915 10.2 570 50 709 827 820 110 710 40.77 310 280 460 0.4 C
9 1200 900 11.2 560 47 704 865 800 103 700 31 420 380 420 0.5 C

*I - Present inventive example, C- Comparative Examples
Note: Steel marked as 1A & 1B, have the same chemical composition as steel number 1, however they are processed at different continuous annealing conditions to validate the claimed process. Similarly steel number 2A and 2B have the same chemical composition as steel number 2 and so forth.
* 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- Overaging section , SPM- Skin pass elongation
** Shaded and underline boxes indicates “outside the appropriate range”
** Comparative Steels having SCS to RCS cooling rate more than V1 0C/sec do not comply with the scope of the present invention as these steels tends to form more martensite due to higher cooling rate at CAL. Consequently, these steels do not comply with minimum n-value requirement of 0.19.
** Comparative Steels having RCS temperature under T1 0C do not comply with the scope of the present invention. These steels are likely to form more martensite phase due to lower RCS temperatures. Consequently, they do not comply with minimum total elongation requirement of 22%.
** Comparative Steels having OAS temperature below T2 0C do not fulfill the scope of the present invention. These steels are likely to be overaged below martensite start temperature. Consequently, they show poor formability with n value lower than 0.19.

Example 2: Steel sample no. 1 as listed in table 1 has chemical composition as per the scope of present invention.However Steel No 1 is processed through two different annealing conditions listed in table 2 as “1A and 1B”. Steel 1A with a rapid cooling section temperature of 450 0C and rapid cooling rate from SCS to RCS of 24.620C/sec is confirming to condition of RCS > T1 0C with rapid cooling rate < V1 0C/sec. In addition, Steel 1A is also satisfies the relation 3 = [Fea + ?RA] / [Ms+ Ba] = 7. Thus, steel number 1A assures the scope of the invention with UTS of 854 MPa and n-value of 0.21 as listed in Table 3. Contrary to that, steel number 1B is processed with RCS temperature of 320 0C which is far below the calculated T1 temperature (420 0C) as per table 1. In addition, steel number 1B is cooled at a high rapid cooling rate of 450C/sec from SCS to RCS, higher than calculated maximum cooling rate V1 0C/sec. In consequence, steel no. 1B has rather poor n-value and elongation owing to higher martensite area fraction (28%) as a microstructural constituent. Moreover, as listed in table 2, steel 1B is over aged at 320 0C, significantly below the calculated minimum temperature T2 (372 0C). As a result, steel 1B has rather lower bainite area fraction.

Similar conclusion can be made for steel No. 2A and 2B. Steel no. 2A is processed meeting all the annealing condition as per the scope of present invention with RCS>T1 0C, OAS>T2 0C and rapid cooling rate 780MPa, n value > 0.19 along with total elongation > 22%.
Steel 2B in contrast is processed with RCS V1 0C/sec resulting in poor n-value and elongation. Furthermore, steel 2B does not satisfy the relation {3 = [Fea + ?RA] / [Ms+ Ba] = 7} attributed to formation of excess martensite.
Similar to steel 1B and 2B, poor n value and low elongation of steel 3B can also be concluded.

Table 3:

Steel.
No Fea % Ba % ?RA % Ms% Eq2 YS UTS YS/UTS Total
Elongation
%,
n-value
(10-Ul%) BH Index,
MPa Phosphat-
-ability
Remark Aging Remarks Remarks
1A 64 21 14 1 3.5 490 854 0.57 25.4 0.21 70 O O I
1B 64 6 2 28 1.9 512 903 0.57 15.2 0.12 61 O O C
2A 68 19 13 0 4.3 467 853 0.55 25.8 0.21 65 O O I
2B 67 4 3 26 2.3 497 889 0.56 16.6 0.13 57 O O C
3A 69 17 14 0 4.9 495 841 0.59 26.1 0.22 67 O O I
3B 65 4 3 28 2.1 507 918 0.55 15.8 0.122 56 O O C
4 64 21 15 0 3.8 471 849 0.55 26.8 0.214 76 O O I
5 70 16 2 12 2.6 451 721 0.63 20.1 0.125 95 ? ? C
6 71 9 3 17 2.8 491 732 0.67 18.5 0.13 109 ? ? C
7 65 24 0 11 1.9 703 837 0.84 14.2 0.13 39 ? O C
8 55 7 0 38 1.2 721 981 0.73 13.7 0.12 35 ? O C
9 89 11 0 0 9 371 574 0.65 27.3 0.15 97 O ? C
*I - Present inventive example, C- Comparative Examples
** Shaded and underline boxes indicates “outside the scope of the invention.
** Eq2= [Fea+ ?RA] / [Ms+ Ba] where, Fea: Total area fraction of Ferrite,?RA: Total area fraction of Retained Austenite, Ms: Total area fraction of Martensite, Ba: Total area fraction of Bainite.
** Steel sheet having the value of Eq2 outside the range from 3 to 7 does not comply with the scope of the invention.While, Steel having value of Eq2 <3 does not meet the terms of minimum n-value requirement. Whereas, steel having Eq2 >7 does not meet the minimum strength requirement of 780 MPa.
**Steels with phosphatability remark “?” do not meet the terms of phosphatability requirement as the phosphate crystal size after zinc phosphate chemical conversion coating is >6 µm and zinc phosphate coating weight is >3 g/mm2.
**Steels with aging remark “?” do not fulfill the accelerated aging requirement as the YPE observed after accelerated aging test.

It is thus possible by way of the present invention to provide 780 MPa Tensile strength level high strength cold rolled steel sheet comprises chemical elements in terms of weight percent: 0.121% to 0.2 % of C, 0.9-1.5 % of Si , 1.5% to 2.2 % of Mn , N: 0.005% or less, 1 % or less of Al , 0.005 % or less of S , 0.005 % to 0.05 % of Ti, 0.005 % to 0.05 % of Nb and the balance being Fe and other inevitable impurities, wherein [Mn+Si+Al] % / [C] % is in a range from 12 to 27 and steel microstructure constituents relative to the whole microstructure of said steel are in a relation satisfying;
3 = [Fea + ?RA] / [Ms+ Ba] = 7;
Where, Fea: Total area fraction of Ferrite, ?RA: Total area fraction of Retained Austenite, Ms: Total area fraction of Martensite, Ba: Total area fraction of Bainite. Cold rolled high strength steel sheet of present invention has Yield strength of 450MPa or more, Tensile strength of 780 MPa or more, total elongation of 23% or more, strain hardening coefficient of 0.19 or more and bake hardening index of 40 MPa or more, making such steel sheets suitable for automobile applications.

,CLAIMS:We Claim:

1. Cold rolled high strength steel sheets having tensile strength atleast 780 MPa with composition in terms of weight % comprising:
C: 0.121-0.2 %;
Mn: 1.5-2.0 %;
Si: 0.9 -1.5%;
Al: =1%;
S: 0.005 % or less;
N:0.005 % or less
Ti: 0.005-0.05%;
Nb: 0.005-0.05;
and the balance being Fe and other unavoidable impurities; wherein ratio of [Mn+Si+Al] % / [C] % is in the range from 12 to 27 and steel microstructure constituents relative to the whole microstructure of said steel are in a relation satisfying;
3 = [Fea + ?RA] / [Ms+ Ba] = 7;
where, Fea: Total area fraction of Ferrite,?RA: Total area fraction of Retained Austenite, Ms: Total area fraction of Martensite, Ba: Total area fraction of Bainite.

2. Cold rolled high strength steel sheetsas claimed in claim 1 comprising atleast one type of element selected from the group of elements consisting of V, Zr, Hf, W in amount less than 0.04 wt%.

3. Cold rolled high strength steel sheets as claimed in anyone of claims 1 or 2, additionally comprising in terms of weight % atleast one element selected from the group consisting of 0.002 % to 0.2 % Cu, 0.002 % to 0.2 % Ni, 0.002 to 0.3 wt % Mo and less than 0.01 % Ca.

4.Cold rolled high strength steel sheets as claimed in anyone of claims 1 to 3 having Yield strength of 450MPa or more, Tensile strength of 780 MPa or more, total elongation of 23% or more, strain hardening coefficient of 0.19 or more and bake hardening index of 40 MPa or more.

5.Cold rolled high strength steel sheets as claimed in anyone of claims 1 to 4, comprising in terms of area fraction relative to entire microstructure of steel, 60% or more of ferrite phase, 2 % or less of martensite phase, 10 % or more of retained Austenite phase and balance is Bainite phase along with carbide, nitride and sulphide precipitates.

6. A process for the manufacturing of cold rolled high strength steel sheetsas claimed in claims 1 to 5, comprising the steps of:
i. Producing steel having composition as claimed in claims 1 to 3,processing Heat from basic oxygen furnace (BOF) through RH degasser and continuously cast in slabs;
ii. subjectingsaid slabs having said composition to reheating at a temperature in the range from 1150°C to 1250 °C;
iii. subjecting said reheated slab to roughing rolling in roughing mill with roughing mill delivery temperature of 1080°C or less;
iv. subjecting said rough rolled steel to finish rolling with finish mill exit temperature ranging from Ac3 °C to Ac3+100 °C;
v. Coiling the finish rolled steel with average run out table cooling rate of 9 °C/second or more; and
vi. Acid Pickling and Cold rolling the said hot rolled steel sheet with cold reduction of atleast 40%.

7.A process for the manufacturing of cold rolled high strength steel sheet as claimed in claim 6, wherein said cold rolled steel sheet obtained at step(vi) is subjected to continuous annealing following the steps comprising;
i. Annealing the cold rolled steel sheet at soaking section critical temperature range from Ac1+20 °C to Ac3+20 °C with residence time ranging from 70 to 150 seconds;

ii. Slow cooling the steel up to a temperature in the range from Ac1 - 60 °C to Ac1 °C after soaking;

iii. Rapid cooling the steel from SCS temperature up to a temperature range of T1 °C or more at a critical cooling rate of V1 °C/sec or less wherein,
a. V1 =18.0- 4[%C]-1.8[%Mn]-2.2[%Si]+10[%Ti+%Nb],
b. T1 = 550 - 324[C%] - 32.4 [Mn%] - 10.8[Si%]-10.8[Mo%],
Where [%X] denotes weight % of element X in said steel sheet;

iv. Overaging the said steel in the temperature range starting from T1 °C or less to T2 °C or more with residence time of 250 to 560 seconds wherein,T2= 555 - 70[C%] - 35 [Mn%] - 75[Si%] - 40[Mo%];

v. Subjecting the overaged steel to skin pass elongation of 0.2% to 1%.
Dated this the 24th day of April, 2018
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)
IN/PA-199