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 MULTIPHASE STEEL SHEET WITH IMPROVED STRETCH-FLANGEABILITY AND SURFACE FINISH FOR EXPOSED PANEL APPLICATION.



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 590 MPa or more involving select chemical elements in terms of weight C: 0.06-0.1%; Mn: 1.2-2.0 %; Si: 0.3–0.8%; Al: 0.02-0.1%; S: 0.005 % or less; N: 0.005 % or less; Ti: 0.005-0.05%; and the balance being Fe and other unavoidable impurities; wherein [Mn] / [Si] ratio is in a range of 1.5 to 4 and having selective steel microstructure constituents including at-least 10% Bainite phase, 70-85% Ferrite and Retained Austenite from 3 to 8% for stretch flangebility and processing to achieve the desired microstructure and the hole expansion property. The advancement favors generation of cold rolled high strength steel sheet having Yield strength of 350 MPa or more, Tensile strength of 590 MPa or more, total elongation of 25% or more, strain hardening coefficient of 0.2 or more and hole expansion ratio of 60% or more and better surface finish which makes such steel sheets suitable for automobile exposed panel applications.

BACKGROUND OF THE INVENTION
There is an increased focus on light weighting, reliability in product performance, and passenger safety in the automotive industry. A stringent emission standard has prompted automakers to speed up their programs to reduce vehicle weight for higher fuel efficiency and better environmental performance.Steel sheets with strength greater than 590 MPa can be used in place of conventional ones in order to achieve this. Furthermore, increasing the strength of steel through higher alloying results in poor formability, surface finish and stretch-flangeability. Therefore, high strength steel sheet should not be used for exposed body parts in automotive vehicles that require high formability and stretching, as well as excellent surface finish. The conventional use of high strength steels has increased in recent years in order to reduce vehicle weight, improve dent resistance, and reduce vehicle weight.
In automotive body parts with complex profiles, high strength steel's formability is a major limitation. Better formability can be achieved by improving the strain hardening coefficient. It has been found that ferrite-Bainite materials provide excellent stretch-flangeability due to bainite, retained austenite, and ferrite matrix.
However, the optimum deployment of said steel can only be achieved by right combination of retained austenite and bainite phase fractions and their distribution. In order to achieve the said improvedformability and Better Surface Finish, Si, Mn and Al wherein [Mn] / [Si] ratio is in a range of 1.5 to 4 and having selective steel microstructure constituents including atleast 10% bainite phase, 70-85% Ferrite and Retained Austenite from 3 to 8% for improved formability and surface finish for exposed panel application.
To avoid crack generation during press forming with better surface finish, the high strength steel sheet having Yield strength of 350 MPa to 450 MPa, Tensile strength of 590 MPa to 720 MPa, total elongation of 25% to 35%, strain hardening coefficient of 0.2 to 0.3 and Hole Expansion Ratio of 60% to 90%.

As a part of prior art Indian patent application number 870-KOL-2012 aims to produce a high yield ratio type high strength steel sheet. Strength is achieved by addition of higher amount of Mn and Si to the steel composition along with Cu as listed in inventive examples. Also a two-step heating process after cold rolling has been specified as a part of invention to reduce banded structure and improve drawability. However due to higher Mn and Si weight % along with Cu and two step heating, patent application number 870-KOL-2012 are susceptible to rather poor galvanizing and phosphatability due to formation of excess oxides during heating multiple times. Also due to martensite being present as strengthening phase, hole expansion will be rather poor.

Indian patent number 276963 describes Methods for Manufacturing High Strength Hot-Dip Galvanized Steel Sheet and High Strength Hot-Dip Galvannealed Steel Sheet having minimum tensile strength of 590 MPa. However, with inclusion of 0.7 to 1.8 % of Si in the chemical composition, Indian patent number 276963 is prone to rather poor zinc coating property and powdering property due to excess SiO2 type scale formation during hot rolling /annealing. Also, it is susceptible to formation of rolled in scale embedded in steel during hot rolling which cannot be removed during pickling and it will not be suitable for exposed panel applications.

In the lights of above requirement, the present invention aims to solve the problem of prior art by providing a high yield ratio high strength steel sheet with 600 MPa tensile strength level having good surface finish with color test reading more than 65%, along with superior stretch flangeability and excellent surface finish suitable for exposed panel of automotive segment.
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 590 MPa, and total elongation more than 25% 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 590 MPa or more, a good strain hardening coefficient (n-value) of atleast 0.2 along with high total elongation of more than 25 %, and Hole Expansion ratio more than 60%.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to provide cold rolled high strength steel sheet having Tensile strength 590 MPa or more and comprising of composition in terms of weight percent:
C: 0.06-0.1 %;
Mn: 1.2-2.0 %;
Si: 0.3–0.8%;
Al: 0.02-0.1%
S: 0.005 % or less;
N: 0.005 % or less
Ti: 0.005-0.05%; and
the balance being Fe and other unavoidable impurities; wherein [Mn] / [Si] ratio is in a range of 1.5 to 4 and having selective steel microstructure constituents including atleast 10% bainite phase, 70-85% Ferrite and Retained Austenite from 3 to 8% for improved formability and surface finish for exposed panel application with color test value more than 60%.

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 Nb, Mo and V in amount less than 0.05 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 0.001 % to 0.003 % Ca and 0.0005 to 0.003 % B.

Another aspect of the present invention is directed to said cold rolled high strength steel sheets havingYield strength of 350 MPa to 450 MPa, Tensile strength of 590 MPa to 720 MPa, total elongation of 25% to 35%, strain hardening coefficient of 0.2 to 0.3 and Hole Expansion Ratio of 60% to 90%.

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 at least 590 MPa comprising the steps of:
a) providing steel having composition comprising
C: 0.06-0.1 %;
Mn: 1.2-2.0 %;
Si: 0.3–0.8%;
Al: 0.02-0.1%
S: 0.005 % or less;
N: 0.005 % or less
Ti: 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 1180°C -1250°C;
c) subjecting said reheated slabs to roughing rolling in roughing mill with roughing mill delivery temperature of 1060°C or less;
d) subjecting said rough rolled steel to finish rolling with finish mill exit temperature ranging from Ac3°C to Ac3+100 °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 outreheating said slab to reheating temperature in the range from 1180°C -1250°C;
b. Roughing rolling said slab, wherein the roughing rolling temperature is 1060°C or less to form a rough rolled substrate;
c. Finishing rolling said rough rolled substrate, wherein the finishing rolling temperature is between Ac3°C to Ac3+100 °C;
d. Cooling, Coiling, Cold Rolling and Continuous annealing said substrate to form a cold rolled sheet;

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 5-8 0C/sec up to soaking section critical temperature range from 800 °C to 850 °C with residence time ranging from 70 to 150 seconds;
b) slow cooling(SCS) the steel at cooling rate in the range from 3 to 5°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 440 °C to 510 °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 0.20 to 1.4%;

Another aspect of the present invention is directed to said cold rolled high strength steel sheets having cold rolled high strength Galvanized steel sheet which is selectively controlled to have steel sheet with yield strength of 350 MPa to 450 MPa, Tensile strength of 590 MPa to 720 MPa, total elongation of 25% to 35%, strain hardening coefficient of 0.2 to 0.3 and Hole Expansion Ratio of 60% to 90%, and having microstructure atleast 10% bainite phase, 70-85% Ferrite and Retained Austenite from 3 to 8% for improved formability and surface finish for exposed panel application.

The above and other objects and advantages of the present invention are described hereunder with reference to non-limiting accompanying drawing and 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 590 MPa or more and composition in terms of weight percent comprising:
C: 0.06-0.1 %;
Mn: 1.2-2.0 %;
Si: 0.3–0.8%;
Al: 0.02-0.1%
S: 0.005 % or less;
N: 0.005 % or less
Ti: 0.005-0.05%; and
the balance being Fe and other unavoidable impurities; wherein [Mn] / [Si] ratio is in a range of 1.5 to 4 and having selective steel microstructure constituents including atleast 10% bainite phase, 70-85% Ferrite and Retained Austenite from 3 to 8% for improved formability and surface finish for exposed panel application.

Cold rolled high strength steel sheet with 350 MPa to 450 MPa, Tensile strength of 590 MPa to 720 MPa, total elongation of 25% to 35%, strain hardening coefficient of 0.2 to 0.3 and Hole Expansion Ratio of 60% to 90%, and having microstructure atleast 10% bainite phase, 70-85% Ferrite and Retained Austenite from 3 to 8% for improved formability and surface finish for exposed panel application.
Accompanying Figure 1 shows the micrograph of the invented steel grade showing the distribution of phase fractions.

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:
C: 0.06-0.1 %;
Mn: 1.2-2.0 %;
Si: 0.3–0.8%;
Al: 0.02-0.1%
S: 0.005 % or less;
N: 0.005 % or less
Ti: 0.005-0.05%; and the balance being Fe.

The technical reasoning for selecting the concentration of elements in composition are as follows:

Carbon (C: 0.06-0.1wt %) – 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 0.06weight % of C is required. More preferably, the amount of carbon must be more than 0.07 to make austenite stable and to effectively lower the Martensite start temperature. On the other hand, increasing the carbon content above 0.1 % 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 0.1 %.

Manganese (Mn: 1.2-2.0 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>590 MPa, minimum amount of Mn must be atleast 1.2%. 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: 0.3–0.8 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>590 MPa, minimum amount of Si must be atleast 0.3 wt%. 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%.
[Mn] / [Si] ratio is in a range of 1.5 to 4: Mn/Si ratio shall be more than 1.5 in order to get preferential oxidation of Mn during continuous annealing, resulted in better phosphatibility. But with increasing Mn/Si ratio, its increases cost of steel as well as creates bluish surface which is not suitable for exposed panel applications so maximum limit of Mn/Si restricted to 4.

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 deoxidation, 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%.

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.

Ti(Ti: 0.005-0.05 wt %) - Ti 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 Ti which should be added is 0.005wt%. Ti 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%.

Nb, V and Mo (collectively <0.05 wt%): Nb, V and Mo forms carbide and impart precipitation strengthening to the steel and any additional content more than 0.05 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 Nb, V and Mo must be less than 0.05 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.

Color Test:
To evaluate surface finish, color reader is used to measures the color and whiteness of steel sheet in solid form. It is utilized in order to control color from raw materials through the process of manufacturing. The Color Test value varies from 0-100%. Higher the Color Test value is more suitable for Exposed panel application. For Exposed panel Application Color Test Value shall be more than 60%.

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 1060°C and finishing mill entry temperature under 1060°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 80C/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 5-8 0C/sec up to soaking section critical temperature range from 800 °C to 850 °C with residence time ranging from 70 to 150 seconds; slow cooling(SCS) the steel at cooling rate in the range from 3 to 5°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 440 °C to 510 °C at a critical cooling rate of 20°C/sec or less. Over-aging the said steel in the temperature range starting from 390 °C to420 °C or more with residence time of 250 to 560 seconds wherein, subjecting the over-aged steel to skin pass elongation of 0.20 to 1.4%.

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 Ti [Mn]/[Si] Other Elements Remarks
1a 0.07 1.8 0.003 0.021 0.7 0.06 0.004 0.03 2.57 Ca-0.001, -0.003, I
Nb-0.02, V-0.01
1b 0.07 1.8 0.003 0.021 0.7 0.06 0.004 0.03 2.57 B-0.002 I
2 0.1 2.0 0.004 0.015 0.6 0.04 0.004 0.03 3.33 Ca-0.0015 I
3 0.09 1.3 0.003 0.01 0.7 0.06 0.005 0.04 1.85 V-0.04 I
4 0.12 1.1 0.003 0.02 0.15 0.04 0.004 0.08 7.33 C
5 0.05 2.5 0.004 0.015 0.25 0.05 0.003 0.017 10 V-0.02, Cr-0.01 C
6 0.09 2.1 0.01 0.02 0.2 0.035 0.004 0.03 10.5 C

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

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>0.2, Total elongation > 25%, HER %> 60, UTS =590 MPa and Yield strength more than 350 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. 5 in table 1 has the less carbon %, higher Mn % and lower Si wt% than the scope and does not comply with required ratio of Mn/Si ratio and has poor n-value, Total Elongation and Hole Expansion Ratio and also not suitable for exposed panel applications having color test value less than 50%.

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


1a 1205 1040 903 558 40
1b 1205 1035 903 558 59
2 1200 1060 895 580 45
3 1190 1020 900 620 40
4 1250 1050 910 550 42
5 1220 1030 908 570 45
6 1260 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 450°C where retained austenite formed is 4% and n value is greater than 0.22 whereas in sample 1b where rapid cooling temperature is 380°C where retained austenite formed is 0% and n value is 0.16 less than required value and also HER% 50 less than the required value of HER.
In sample 3 where carbon percentage is 0.09% and Mn% is 1.3%, whereSS temperature is more than 840 °Cand OAS temperature is less than 380 where percentage of bainite formed is less than desired level of 10% with martensite 12% and results in Tensile strength more 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 3 450 20 394 327 0.3
1b 800 95 685 2 380 42 330 327 1.5
2 850 140 700 5 480 10 400 260 0.8
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 % Color Test (%) Remarks
1a 420 0 690 28 0.22 65 75 20 4 2 65 I
1b 500 0 750 20 0.16 50 65 0 0 35 40 C
2 400 0 640 29 0.25 68 80 15 4 1 70 I
3 450 0 760 23 0.21 48 74 8 6 12 45 C
4 470 0 900 12 0.09 26 70 0 0 30 40 C
5 480 0 830 13 0.12 32 90 0 0 10 45 C
6 480 0 805 12 0.09 33 65 0 0 35 40 C

*I - Present inventive example, C- Comparative Examples, E-Exposed Panel, UNE- Unexposed Panel
Example 2
In case of sample 1b, 4, 5 and 6, n values is less than 0.2 which is out of scope of current invention, in case of sample 1b where retained austenite is 0 and martensite percentage is 35 percent and n value is 0.16 less than 0.2 and has poor HER % less than 60% 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 Tensile strength of 590 MPa or more involving select chemical elements in terms of weight C: 0.06-0.1%; Mn: 1.2-2.0 %; Si: 0.3–0.8%; Al: 0.02-0.1%; S: 0.005 % or less; N: 0.005 % or less; Ti: 0.005-0.05%; and the balance being Fe and other unavoidable impurities; wherein [Mn] / [Si] ratio is in a range of 1.5 to 4 and having selective steel microstructure constituents including at-least 10% Bainite phase, 70-85% Ferrite and Retained Austenite from 3 to 8% as shown in accompanying Figure 1 for improved surface finish suitable for exposed panel application with color test value more than 60%.

The advancement favors generation of cold rolled high strength steel sheet having Yield strength of 350 MPa or more, Tensile strength of 590 MPa or more, total elongation of 25% or more, strain hardening coefficient of 0.2 or more and Hole Expansion ratio 60% or more.

, Claims:We Claim:
1.) Cold rolled high strength steel sheet having Tensile strength 590 MPa or more comprising of steel composition in terms of weight percent:
C: 0.06-0.1 %;
Mn: 1.2-2.0 %;
Si: 0.3–0.8%;
Al: 0.02-0.1%
S: 0.005 % or less;
N: 0.005 % or less
Ti: 0.005-0.05%; and
the balance being Fe and other unavoidable impuritieswherein [Mn] / [Si] ratio is in a range of 1.5 to 4 and having selective steel microstructure constituents including atleast 10% bainite phase, 70-85% Ferrite and Retained Austenite from 3 to 8% for improved formability and surface finish for exposed panel application with color test value more than 60%.

2.) Cold rolled high strength steel sheet as claimed in claim1, comprising atleast one type of element selected from the group of elements consisting of Nb, Mo and V in amount less than 0.05 wt%.

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

4) Cold rolled high strength steel sheets as claimed in anyone of claims 1 to 3 having Yield strength of 350 MPa to 450 MPa, Tensile strength of 590 MPa to 720 MPa, total elongation of 25% to 35%, strain hardening coefficient of 0.2 to 0.3 and Hole Expansion Ratio of 60% to 90%.

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 590 MPa comprising the steps of:
a) providing steel having composition comprising
C: 0.06-0.1 %;
Mn: 1.2-2.0 %;
Si: 0.3–0.8%;
Al: 0.02-0.1%
S: 0.005 % or less;
N: 0.005 % or less
Ti: 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 1180°C -1250°C;
c) subjecting said reheated slabs to roughing rolling in roughing mill with roughing mill delivery temperature of 1060°C or less;
d) subjecting said rough rolled steel to finish rolling with finish mill exit temperature ranging from Ac3°C to Ac3+100 °C;
e) Subjecting said hot rolled sheet to cooling, coiling, cold rolling and continuous annealing.
6.) The process for manufacturing cold rolled high strength steel sheet as claimed in claim 5, 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 5-8 0C/sec up to soaking section critical temperature range from 800 °C to 850 °C with residence time ranging from 70 to 150 seconds;
b) slow cooling(SCS) the steel at cooling rate in the range from 3 to 5°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 440 °C to 510 °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 0.20 to 1.4%.

7.) The process for manufacturingcold rolled high strength steel sheet as claimed in anyone of claims 5 or 6 for producing cold rolled high strength Galvanized steel sheet which is selectively controlled including following processing stages as above to have steel sheet with 350 MPa to 450 MPa, Tensile strength of 590 MPa to 720 MPa, total elongation of 25% to 35%, strain hardening coefficient of 0.2 to 0.3 and Hole Expansion Ratio of 60% to 90%, and having microstructure atleast 10% bainite phase, 70-85% Ferrite and Retained Austenite from 3 to 8% for improved formability and surface finish for exposed panel application.

Dated this the 1st day of December, 2022
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199