Claims:We Claim

1. Cold rolled high strength steel sheets having tensile strength atleast 690 MPa with composition in terms of weight % comprising:
C: 0.12 - 0.16 %;
Mn: 1.0 – 1.49 %;
Si: 0.81 -1.2%;
S: 0.005 % or less;
N: 0.005 % or less;
Ti: 0.005-0.05%;
Nb: 0.005 - 0.04%;
and the balance being Fe and other unavoidable impurities; wherein [Mn] / [Si] ratio is in a range of1 to 2 and steel microstructure constituents comprising atleast 30 % of bainite, atleast 10 % of retained austenite phase and balance is ferrite phase along with carbide, nitride and sulphide precipitates andhaving strain hardening coefficient of 0.2 or more.

2. Cold rolled high strength steel sheets as claimed in claim 1 comprising

ODF random intensity of ?-fiber texture of (111)[1-10], (111)[2-31], (111)[1-2 1] and (111)[1-32] greater than 2 for excellent stretch formability including total elongation of 25% or more, strain hardening coefficient of 0.2 or more and bake hardening index of 40 MPa or more and Yield strength of 410 MPa or more.

3. Cold rolled high strength steel sheets as claimed in anyone of claims 1 or 2 having tensile strength in the range of 690 to 800 MPa, strain hardening coefficient in the range of 0.2 to 0.3, total elongation in the range of 25to 35%, bake hardening index in the range of 40to 80 MPa and Yield strength in the range of 410 to 510 MPa.

4. Cold rolled high strength steel sheet according to anyone of claims 1 to 3 , further comprising at least one type of element selected from the group of elements consisting in terms of weight % of V, Zr, Hf, W and Cr in amount less than 0.04 wt%.

5.Cold rolled steel high strength sheet according to anyone of claims 1 to 4, further comprising in terms of weight % atleast one element selected from the group comprising of 0.002 to 0.2 % Cu, 0.002 to 0.2 % Ni and 0.002 to 0.3 wt % Mo.

6. A process for manufacture of cold rolled high strength steel sheet of Claim 1 to 5, comprising the steps of:
a) providing steel slab having said selective composition and involving processing Heat from basic oxygen furnace (BOF) through RH degasser and subsequently continuously casting following casting speed in the range of 0.5 to 1.4 mpm such as to avoid longitudinal cracks on slab surface;
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 1010 to 1100°C preferably 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 to achieve coiling temperature in the range of 530 to 590C; and
f) acid Pickling the Cold rolling the said hot rolled steel sheet with cold reduction of at-least 40 %
g) subjecting the cold rolled steel strip to continuous annealing,andfurthercarrying out transformation induced plasticity (TRIP) for achieving desired retained austenite phase and balance is ferrite phase along with carbide, nitride and sulphide precipitates.

7.A process to manufacturing cold rolled high strength steel sheet of Claim 6 comprising the steps of:

a) Providing steel slab having composition as given above comprising processing Heat from basic oxygen furnace (BOF) through RH degasser and subsequently continuously cast having casting speed <= [1.6-(0.02)x superheat] to avoid longitudinal cracks on slab surface;
b) Reheating the slab having said composition to reheating temperature in the range from 1190°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 at-least 40 %.

8. A process for manufacturing cold rolled steel sheet of claim 6 to 7, wherein cold rolled steel is subjected to said continuous annealing following the steps comprising;
g) Annealing the cold rolled steel sheet at soaking section critical temperature range from 770 to 830 °C with residence time ranging from 70 to 150 seconds;
h) Slow cooling the steel up to a temperature in the range from 670 to 710 °C after soaking ;
i) Rapid cooling the steel from SCS temperature up to a temperature range of 420to 500 °C at a critical cooling rate of 40°C/sec or less wherein ,
j) Overaging the said steel in the temperature range starting from 370to 420°C with residence time of 240 to 360 seconds wherein,
k) Subjecting the over-aged steel to skin pass elongation of 0.20 to 1%.

9. A process for manufacturing cold rolled steel sheet of claim 6 to 8 which is selectively carried out such that said steel sheet has Yield strength of 410 MPa or more, Tensile strength of 690 MPa or more, total elongation of 25% or more, strain hardening coefficient of 0.2 or more, Hole expansion ratio more than 40% and bake hardening index of 40 MPa or more.

10. A process for manufacturing cold rolled steel sheet of claim 6 to 9 which is selectively carried out such that the said steel sheet comprises, 30% or more of Bainite, 10 % or more of retained austenite phase and balance is ferrite phase along with carbide, nitride and sulphide precipitates.

Dated this the 19th day of May, 2018
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)
IN/PA-199
, Description:FIELD OF THE INVENTION

The present invention relates to high strength cold rolled TRIP steel sheet having tensile strength of 690MPa level and chemical composition in terms of weight percentcomprising: 0.12 to 0.16 % of C, 0.81 to 1.2 % of Si , 1.0 to 1.49 % of Mn, 0.005% or less of N, 0.1 % or less of Al, 0.005% or less of S , 0.005to 0.05% of Ti, 0.005 to 0.04% of Nb and the balance being Fe and other inevitable impurities, wherein [Mn] / [Si] ratio is in the range of 1 to 2, subsequently continuously cast having casting speed = [1.6 – (0.02 x superheat)] to avoid longitudinal cracks on slab surface and steel microstructure consisting of 30% or more of bainite, 10% or more of retained austenite phase and balance is ferrite phase along with carbide, nitride and sulphide precipitates and preferred ODF random intensity of ?-fiber texture of (111)[1-10], (111)[2-31], (111)[1-21] and (111)[1-32] greater than 2 for excellent stretch formability and hole expansion ratio. Cold rolled high strength steel sheet of present invention has Yield strength of 410MPa or more, Tensile strength of 690 MPa or more, total elongation of 25% or more, strain hardening coefficient of 0.2 or more and bake hardening index of 40 MPa or more.

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 forced the automakers to reduce vehicle weight for better fuel efficiency and 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 690MPa in place of590 MPa steel. 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 undesirable 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 automotive 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. Further, to achieve the said TRIP phenomenon, the surface property deterioration caused by higher amount of Si and Mn require retaining austenite at room temperature.

To avoid crack generation during press forming, the high strength steel sheet having tensile strength >690 MPa must also exhibit a good strain hardening coefficient (n-value) of atleast 0.2 along with high total elongation of no less than 25%. At the same time the yield ratio must be below 0.65 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 690MPa along with good elongation for automotive structural component. A high strength automotive steel sheet is disclosed is obtained by keeping increased Mnwt% 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 >0.60. In addition, the phosphatability gets deteriorated due to higher Mn content.

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 wt% in composition which is added to impart strength may result in poor elongation and temper embrittlement.
The present invention has been made in the light of the above mentioned problems. The objective of the present invention is to provide a method of manufacturing a high-strength cold-rolled steel sheet with minimum strength of 690 MPa, having improved strain hardening properties along with excellent surface properties to avoid longitudinal crack in slab while continuous casting.

OBJECTS OF THE INVENTION

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

A further object of the present invention is directed to provide high strength cold rolled TRIP steel sheet having selective chemical composition comprising ofchemical elements in terms of weight percent:0.12 to 0.16 % of C, 0.81 to 1.2% of Si , 1.0 to 1.49% of Mn, 0.005% or less of N, 0.1 % or less of Al , 0.005 % or less of S , 0.005 to 0.05 % of Ti, 0.005 to 0.04 % of Nb and the balance being Fe and other inevitable impurities, wherein [Mn] / [Si] ratio is in a range of 1 to 2, subsequently continuously cast having casting speed =[1.6 – (0.02 x superheat)] to avoid longitudinal cracks on slab surface.

A still further object of the present invention is directed to provide high strength cold rolled TRIP steel sheet having selective chemical composition and processing steps to achieve steel microstructure consisting of 30 % or more of bainite, 10 % or more of retained austenite phase and balance is ferrite phase along with carbide, nitride and sulphide precipitates and preferred ODF random intensity of ?-fiber texture of (111)[1-10], (111)[2-31], (111)[1-2 1] and (111)[1-32] greater than 2 for excellent stretch formability and hole Expansion ratio.

Another object of the present invention is directed to provide high strength cold rolled TRIP steel sheet having selective chemical composition and properties comprising Yield strength of 410MPa or more, Tensile strength of 690 MPa or more, total elongation of 25% or more, strain hardening coefficient of 0.2 or more, Hole Expansion ratio more than 40% and bake hardening index of 40 MPa or more.

Yet another object of the present invention is directed to provide high strength cold rolled TRIP steel sheet wherein Transformation induced plasticity (TRIP) phenomenon has been utilized where retained austenite in ferrite-bainite matrix that is transformed to martensite post forming resulting better formability by the optimum deployment of right combination of retained austenite, bainite and martensite phase fraction and distribution.

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 provide Cold rolled high strength steel sheets having tensile strength at least 690 MPa with composition in terms of weight % comprising:
C: 0.12 - 0.16 %;
Mn: 1.0 – 1.49 %;
Si: 0.81 -1.2%;
S: 0.005 % or less;
N: 0.005 % or less;
Ti: 0.005-0.05%;
Nb: 0.005 - 0.04%;
and the balance being Fe and other unavoidable impurities; wherein [Mn] / [Si] ratio is in a range of1 to 2 and steel microstructure constituents comprising atleast 30 % of bainite, atleast 10 % of retained austenite phase and balance is ferrite phase along with carbide, nitride and sulphide precipitates and having strain hardening coefficient of 0.2 or more.

A further aspect of the present invention is directed to provide Cold rolled high strength steel sheets comprising

ODF random intensity of ?-fiber texture of (111)[1-10], (111)[2-31], (111)[1-2 1] and (111)[1-32] greater than 2 for excellent stretch formability including total elongation of 25% or more, strain hardening coefficient of 0.2 or more and bake hardening index of 40 MPa or more and Yield strength of 410 MPa or more.

A still further aspect of the present invention is directed to provide Cold rolled high strength steel sheets having tensile strength in the range of 690 to 800 MPa, strain hardening coefficient in the range of 0.2 to 0.3, total elongation in the range of 25 to 35%, bake hardening index in the range of 40 to 80 MPa and Yield strength in the range of 410 to 510 MPa.

Another aspect of the present invention is directed to provide Cold rolled high strength steel sheet further comprising at least one type of element selected from the group of elements consisting in terms of weight % of V, Zr, Hf, W and Cr in amount less than 0.04 wt%.

Yet another aspect of the present invention is directed to provide Cold rolled steel high strength sheet further comprising in terms of weight % atleast one element selected from the group comprising of 0.002 to 0.2 % Cu, 0.002 to 0.2 % Ni and 0.002 to 0.3 wt % Mo.

A further aspect of the present invention is directed to a process for manufacture of cold rolled high strength steel sheet as described above, comprising the steps of:
a) providing steel slab having said selective composition and involving processing Heat from basic oxygen furnace (BOF) through RH degasser and subsequently continuously casting following casting speed in the range of 0.5 to 1.4 mpm such as to avoid longitudinal cracks on slab surface;
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 1010 to 1100 °C preferably 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 to achieve coiling temperature in the range of 530 to 590C; and
f) acid Pickling the Cold rolling the said hot rolled steel sheet with cold reduction of at-least 40 %
g) subjecting the cold rolled steel strip to continuous annealing, andfurthercarrying out transformation induced plasticity (TRIP) for achieving desired retained austenite phase and balance is ferrite phase along with carbide, nitride and sulphide precipitates.

A still further aspect of the present invention is directed to providesaid process to manufacturing cold rolled high strength steel sheet comprising the steps of:

a) Providing steel slab having composition as given above comprising processing Heat from basic oxygen furnace (BOF) through RH degasser and subsequently continuously cast having casting speed = [1.6-(0.02)x superheat] to avoid longitudinal cracks on slab surface;
b) Reheating the slab having said composition to reheating temperature in the range from 1190°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 at-least 40 %.

A still further aspect of the present invention is directed toa process for manufacturing cold rolled steel sheet, wherein cold rolled steel is subjected to said continuous annealing following the steps comprising;
a) Annealing the cold rolled steel sheet at soaking section critical temperature range from 770 to 830 °C with residence time ranging from 70 to 150 seconds;
b) Slow cooling the steel up to a temperature in the range from 670 to 710 °C after soaking ;
c) Rapid cooling the steel from SCS temperature up to a temperature range of 420to 500 °C at a critical cooling rate of 40°C/sec or less wherein ,
d) Overaging the said steel in the temperature range starting from 370to 420°C with residence time of 240 to 360 seconds wherein,
e) Subjecting the over-aged steel to skin pass elongation of 0.20 to 1%.

Another aspect of the present invention is directed to said process for manufacturing cold rolled steel sheet which is selectively carried out such that said steel sheet has Yield strength of 410 MPa or more, Tensile strength of 690 MPa or more, total elongation of 25% or more, strain hardening coefficient of 0.2 or more, Hole expansion ratio more than 40% and bake hardening index of 40 MPa or more.

Yet another aspect of the present invention is directed to said process for manufacturing cold rolled steel sheet which is selectively carried out such that the said steel sheet comprises, 30% or more of Bainite, 10 % or more of retained austenite phase and balance is ferrite phase along with carbide, nitride and sulphide precipitates.

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

Abbreviations used to describe the invention herein are as follows:

Ac1 & Ac3 – Critical temperatures in iron-carbide diagram
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
BH- Bake hardening
ROT= Run out Table at Hot rolling
CR%- Cold reduction %
HER%- Hole Expansion Ratio %
n-value – Strain hardening coefficient at strain range from 10% up to uniform elongation

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

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

The Cold rolled High strength steel sheet having excellent strain hardening property according to present invention will be explained in detail in provisions to constituent compositions and method of manufacturing. All the chemical composition elements are in weight %.

Carbon (C: 0.12-0.16wt%): 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 at least 0.1 wt% of C is required. More preferably, the amount of carbon must be more than 0.12wt% to make austenite stable and to effectively lower the Martensite star temperature. Keeping carbon above 0.12wt% 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 0.16 % deteriorates the weldability and hole expansion ration. 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 0.16 %.

Manganese (Mn: 1.0-1.49wt%): Similar to carbon, 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>690 MPa, minimum amount of Mn must be atleast 1.0%.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 1.49 %.

Silicon (Si: 0.81-1.2wt%): 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>690 MPa, minimum amount of Si must be atleast 0.81wt%. However, increasing Si level 1.2wt% does not cause any significant effect to inhibit cementite formation. In addition, adding excess Si deteriorates the surface quality after continuous casting. Hence, the upper Si content must not be more than 1.2wt%.

Aluminium (less than 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 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 0.1wt% to replace Si causes problems during continuous casting and adds up to the cost of production. Accordingly, upper limit is set to 0.10 %.

Titanium (Ti: 0.005-0.05 wt%): Ti acts as a nitrides 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: 0.005-0.04wt%):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 0.04 %.

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 and Cr less than 0.04 wt%: V, Zr, Hf, W and Cr 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 and Cr must be less than 0.04 wt%.

Atleast one from (0.002 % to 0.2 % Cu, 0.002 % to 0.2 % Ni and 0.002 to 0.3 wt % Mo): 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.

Description of the process of manufacture:

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 measure are taken to hot roll resulted slabs by keeping slab reheating temperature in the range 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 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 40 °C/sec or less and cooled up to rapid cooling section temperature of 420 °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 370°C more to allow bainite transformation. Over aged steel sheet is then provided with skin-pass elongation in the range of 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. Ltd.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 the difference 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:

Table 1-Elemental Compositions in weight % of the inventive steel sheets along with comparative example and their respective values of Eq1 = Mn/Si.
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 Ageing 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 Eq1= Eq2
Casting Speed-[1.6-0.02xSuperheat] Remarks
Elements Mn/Si Casting Speed
1 0.16 1.49 0.003 1.1 0.06 0.003 0.024 0.024 0.005 Ca:0.005 , V:0.008, 1.4 1.2 -0.2 I
Cu:0.05
2 0.12 1 0.004 0.9 0.08 0.004 0.05 0.03 0.001 Cr-0.03, W-0.002, V-0.005, Zr-0.004 1.1 1 0 I
3 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 1.21 1.4 0.3 C
4 0.09 2.1 0.01 0.5 0.035 0.005 0.003 0.03 - V:0.05, Cr: 0.5 4.20 1.2 0.4 C
5 0.08 1.9 0.009 0.7 0.046 0.004 0.04 - - 2.71 1.2 0.5 C
6 0.08 2.5 0.003 0.3 0.03 0.003 0.02 0.03 - - 8.33 1.4 0.1 C
7 0.06 0.9 0.005 0.2 0.05 0.006 _ 0.03 0.1 - 4.50 1.6 0.6 C

*I - Present inventive example, C- Comparative Examples
Where,
** Shaded and underline boxes indicates “outside the appropriate range”
** Steel having value of Eq1= { [Mn]/[Si] %} >2does not comply with scope of the present invention resulting in rather poor phosphatability and n value.
Eq2-Casting Speed-[1.6-0.02XSuperheat] >0 does not comply with scope of the present invention resulting in longitudinal cracks on surface

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>0.2 , 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, HER%>=40 and UTS =690 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. 4 to 7 in table 1 has the value of [Mn]/[Si] outside the range from 1 to 2. Consequently steel 4 to 7 does not comply with scope of the present invention with lower n value and poor elongation. For example steel no. 3 to 7 in table 1 has the value of Eq2- Casting Speed-[1.6-0.02*superheat]<=0 outside the range greater than zero. Consequently steel 3 to 7 does not comply with scope of the present invention with longitudinal cracks on slab surface.

Table 2
Steel No. SRT, FT, ROT CT, CR, Ac1, Ac3, SS , Annealing Time, SCS , SCS to RCS cooling OAS
°C SPM% Remark
°C °C Cooling °C % °C °C °C Sec °C rate
Rate, ,0C/sec
0C
1A 1200 870 10.9 570 42 739 805 770 70 670 20 390 0.2 I
1B 1200 890 12.1 570 40 739 805 820 110 700 34 400 0.3 I
1C 1220 900 12.1 570 40 739 805 820 110 700 45 390 0.2 C
2A 1250 880 10.5 545 43 745 832 800 80 690 25 370 0.2 I
2B 1180 900 11.6 620 50 745 832 800 90 710 30 360 0.3 C
3 1210 915 10.7 565 50 747 823 790 100 695 30 340 0.3 C
4 1200 900 12.8 530 50 724 819 820 95 690 50 320 0.3 C
5 1200 895 13.1 535 50 731 841 800 95 680 42 350 0.2 C
6 1210 910 9.3 560 50 705 791 810 110 700 30 280 0.4 C
7 1200 900 11.2 560 47 704 865 800 103 700 20 380 0.4 C

*I - Present inventive example, C- Comparative Examples
Note: Steel marked as 1A, 1B& 1C, 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 400C/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 0.2.
** Comparative Steels having RCS temperature under 4200C 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 25 %.
** Comparative Steels having OAS temperature below 3700C 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 0.2.

Example 2: Steel sheet no. 1 as listed in table 1 has chemical composition as per the scope of present invention.However Steel No 1 is processed through three different annealing conditions listed in table 2 as “1A, 1B and 1C”. Steel 1A with a rapid cooling section temperature of 450 0C and rapid cooling rate from SCS to RCS of 200C/sec is confirming to condition of RCS >4200C with rapid cooling rate <400C/sec . Thus, steel number 1A assures the scope of the invention with UTS of 755 MPa, Total Elongation 26.2 and n-value of 0.21 as listed in Table 3. Contrary to that, steel number 1C is processed with RCS temperature of 3800C less than 420 °C. In addition, steel number 1C is cooled at a high rapid cooling rate of 450C/sec from SCS to RCS, higher than cooling rate 400C/sec. In consequence, steel no. 1C has rather poor n-value and elongation owing to higher martensite 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 >4200C, OAS >3700C and rapid cooling rate <400C/sec. accordingly, Steel 2A has strength > 690 MPa, n value > 0.2along with total elongation > 30%.

Steel 2B in contrast is processed with RCS <5000C, OAS <3700C and rapid cooling rate <400C/sec resulting in poor n-value and elongation.

Similar to steel 1B and 2B, poor n value and low elongation of steel 3 can also be concluded.

Table 3
Steel. YS UTS YS/UTS Total n-value BH Index, HER % Longitudinal Cracks Phosphat- Aging Remarks Remarks
No Elongation (10-Ul%) MPa -ability
%, Remark
1A 471 755 0.62 26.2 0.21 70 55 O O O I
1B 460 748 0.61 27.5 0.22 61 56 O O O I
1C 460 831 0.55 21.7 0.16 60 35 O O O C
2A 437 700 0.62 30.2 0.23 65 60 O O O I
2B 451 683 0.66 27.9 0.16 57 38 O O O C
3 495 841 0.59 26.1 0.18 67 30 ? O O C
4 451 721 0.63 20.1 0.125 95 28 ? ? ? C
5 491 732 0.67 18.5 0.13 109 34 ? ? ? C
6 703 837 0.84 14.2 0.13 39 38 ? ? O C
7 371 574 0.65 27.3 0.15 97 39 ? ? ? C

*I - Present inventive example, C- Comparative Examples
** Shaded and underline boxes indicates “outside the scope of the invention.
**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.