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 EXTRA DEEP DRAWING STEEL SHEETS WITH IMPROVED SURFACE FINISH AND PRODUCTION PROCESS THEREOF THROUGH EAF ROUTE.



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 Extra Deep Drawing Steel Sheets and production method thereof through electric arc furnace and electric batch annealing route. This extra deep drawing steel sheet has excellent formability and an adequate surface finish, making it suitable for outer panels for automobiles.

Importantly, the EDD steel having selective composition comprising low Carbon C: 0.025-0.065 wt%, Mn: 0.1-0.2 wt%, N: 0.007 to 0.01 wt%, P= 0.015 wt%, Si=0.05 wt%, Ca: 0.001-0.003 wt%, Al: 0.03-0.05 wt%, B: 0.001- 0.0035 wt%; and balance being Fe and other unavoidable impurities, which is aluminum killed steel having specified B/N ratio between 0.12 to 0.35. This steel produced through Electric Arc Furnace and thin slab caster with slab thickness of 50-55mm combined with hot rolling at temperatures of 880 to 910°C and 630°C to 650°C for HSM finishing to precipitate AlN before electric batch annealing to prevent orange peel.Selective B/N ratio results in very little remaining N in the solid solution, helping to achieve excellent plastic anisotropy ratios (r-bar-1.7 min) and without orange peel on the surface. Selective Ca is added to modify sulphide and oxide inclusions to get a surface free of laminations and steel suitable for exposed panels on automobile bodies and critical components.

BACKGROUND OF THE INVENTION
Generally, it is difficult to produce steel with N wt % less than 0.005 through Electric Arc Furnace, which results in poor draw-ability and orange peel after forming. Moreover, high N wt% makes material less resistant to aging after annealing and skin pass. Ageing in CRCA materials results in a decrease in elongation and a rise in yield strength, resulting in poor formability. Thus, steel sheets with low yield strength, higher r-bar with improved surface finish are required for automotive outer panel applications at Lower CO2 emissions.

As a prior art regarding a low carbon steel sheet with good formability, patent application number 1089/MUM/2015discloses a method of manufacturing Extra deep drawing steel sheet having excellent formability with adequate surface finish suitable for outer panel for automobiles. It discloses composition comprising C: 0.025-0.065 wt%, Mn: 0.12-0.18 wt%, N: 0 to 0.004 wt%, P= 0.015 wt%, Silicon=0.015 wt%, Cr: 0.01-0.02 wt%, Al: 0.03-0.05 wt%, and balance iron, which is aluminium killed steel having specified Al/N atomic ratio between 8 to, sheets produced through CCM reduction between 75 to 80%, HSM Finishing Temperature 880 to 910°C and HSM Coiling temperature 530 to 550°C to avoid AlN precipitation before electric batch annealing. Advantageously, selective Al/N ratio ensures very less remaining Al in the solid solution which assist in achieving excellent plastic anisotropy ratio (r-bar- 1.9 min) value.
However, batch annealed steel sheet obtained according to the process of patent application number 1089/MUM/2015has a major disadvantages is that it produced through Blast Furnace (BF) and Basic Oxygen Furnace (BOF) conventional steel making, continuous casting and Hot Rolling route. The BOF route can provide N wt% less than 0.004 but it generates more Green House gas.
The BF-BOF process accounts for 75% of world steel production while the EAF method is used to produce 25% of world total steel production but The amount of CO2 emission of per ton crude steel from the BF-BOF steelmaking and EAF steelmaking is 1149.7 and 46.6 kg, respectively.

The present invention relates in general to an extra deep drawing steel sheet having excellent formability with superior surface finish and a method for improving the drawing property of steel strip and more predominantly to a method for improving the drawing properties of low carbon Al killed cold rolled batch annealed steel strip containing between 0.025 and 0.065 wt % carbon through Electric Arc Furnace (EAF) and Thin Slab caster and Hot rolling route for lower CO2 emission in atmosphere.

The present invention is thus directed to achieve a significant improvement over the prior art as far as the drawability, good ageing property, high r-value and surface free from orange peel with Lower CO2 emissions. In present invention, B has been used to stabilize N and to orange peel. The mechanism for the same has been described herein after N in present invention has been fixed only by B as BN by controlling the B/N ratio precisely making such steel sheets suitable for outer panel for automobiles where drawability is the major concern without affecting environment.

OBJECTS OF THE INVENTION

The basic object of the present invention is directed to provide Extra Deep Drawing (EDD) steel sheet having excellent formability and surface quality suitable for outer panel application in automobiles, and a method for manufacturing the same through Electric Arc Furnace (EAF) ,Thin Slab caster ,hot rolling and Electric batch annealing route.

A further object of the present invention is directed to provide Extra Deep Drawing (EDD) steel sheet having improved drawability, good ageing property, high r-value and surface free from orange peel making it suitable for exposed panel of the automobile body and critical components which need excellent drawability.

A still further object of the present invention is directed to provide Extra Deep Drawing steel sheet having excellent formability and surface quality and a method of its production wherein low carbon Al killed cold rolled steel strip containing boron as a nitride former is obtained having an r-value greater than 1.7 and yield strength of less than 180 MPa produced through Electric Arc Furnace without a vacuum degassing operation or a decarburizing operation andwith a carbon content between 0.025 wt% and 0.065 wt% and N content more than 70 ppm.

A still further object of the present invention is directed to provide Extra Deep Drawing steel sheet having excellent formability and surface quality wherein Ca is used to stabilize oxygen and to modify shape of oxide inclusions.

A still further object of the present invention is directed to provide Extra Deep Drawing steel sheet having excellent formability and surface quality wherein no orange peel arises in the resultant steel sheet during the press forming operation due to improved planar anisotropy value (r-bar) and noyield point elongation due to ageing.

A still further object of the present invention is directed to provide Extra Deep Drawing steel sheet having excellent formability and surface quality wherein N is fixed by B and Al by controlling the HSM coiling Temperature and Batch Annealing Soaking Temperature and time precisely.

A still further object of the present invention is directed to provide Extra Deep Drawing steel sheet having excellent formability and surface quality wherein B/N ratio range is kept between 0.12 to 0.35 to achieve a higher r-value of at least 1.7.

A still further object of the present invention is directed to provide Extra Deep Drawing steel sheet having excellent formability and surface quality wherein coiling temperature is selectively maintained to AlN controlled precipitation before electric batch annealing and sufficient time is provided to precipitate out AlN during annealing.

A still further object of the present invention is directed to provide Extra Deep Drawing steel sheet having excellent formability and surface quality wherein reheating temperature is kept high (~1200 0C) along with selective coiling temperature to keep BN selective precipitations.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to cold rolled extra deep drawing steel sheets having formability with defect free surface finish having composition comprising
C: 0.025-0.065 wt%,
Mn: 0.1-0.2 wt%,
N: 0.007 to 0.01 wt%,
P= 0.015 wt%,
Si=0.05 wt%,
Ca: 0.001-0.003 wt%,
Al: 0.03-0.05 wt%,
B: 0.001- 0.0035 wt%
And balance being Fe and other unavoidable impurities, and having specified B/N ratio between 0.12 to 0.35.

A further aspect of the present invention is cold rolled extra deep drawing steel sheets which is aluminum killed steel obtained through Electric Arc Furnace having enhanced planar anisotropy ratio involving r-bar value of 1.7 min and yield strength of less than 180 MPa with ageing resistance of more than 6 months and free from orange peel surface defects.
Importantly, said cold rolled extra deep drawing steel sheet is suitable for automobile exposed panel applications.

Yet another aspect of the present invention is directed to a process for producing cold rolled extra deep drawing steel sheets through Electric batch annealing furnace route having steel composition
C: 0.025-0.065 wt%,
Mn: 0.1-0.2 wt%,
N: 0.007 to 0.01 wt%,
P= 0.015 wt%,
Si=0.05 wt%,
Ca: 0.001-0.003 wt%,
Al: 0.03-0.05 wt%,
B: 0.001- 0.0035 wt%,
and balance being Fe and other unavoidable impurities, having specified B/N ratio between 0.12 to 0.35and subjecting to processing for aluminum killed steel with formability and defect free surface finish produced through Electric Arc Furnace, involving HSM finishing temperature 880 to 910 °C and HSM coiling temperature of 620 °C or more preferably 630 to 650 °C, followed by cold rolling and electric batch annealing.

A still further aspect of the present invention is said process involving combination of Hot strip mill coiling temperature 620 °C or more and electric batch annealing furnace hot spot temperature 710°C or less with soaking time of 10 hours to 12 hours to achieve strip surface free from orange peel.

Another aspect of the present invention is directed to said process as above carried out in combination of B/N ratio 0.12 or more, HSM coiling temperature 630 °C or more such as to achieve yield point elongations zero even after 1 year of ageing.

A further aspect of the present invention is directed to said process comprising
(i) providing selective molten steel composition having
C: 0.025-0.065 wt%,
Mn: 0.1-0.2 wt%,
N: 0.007 to 0.01 wt%,
P= 0.015 wt%,
Si=0.05 wt%,
Ca: 0.001-0.003 wt%,
Al: 0.03-0.05 wt%,
B: 0.001- 0.0035 wt%;
and balance being Fe and other unavoidable impurities, Prepared without vacuum degassing and killed in the conventional manner;
(ii) casting the steel into thin slabs casting and rolling route with slab thickness of 50-55 mm;
(iii) Subjecting the thin slabs to hot rolling into hot rolled strip with maintaining hot roll finishing temperature of at least 8800C and preferably 880-910 0C.
(iv) subjecting the hot rolled steel strip to coiling while at a temperature exceeding >6200C, preferably 630-650 0C ;
(v) Subjecting the hot rolled and acid pickled strips to cold rolling with a minimum cold reduction of 75 percent, preferably 75-80 % to achieve higher plastic strain ratio post annealing;
(vi) Subjecting the cold rolled steel strip to electric batch annealing comprising
(a) heating a coil of cold rolled steel strip at a slow heating rate of about 40-45 0C /Hour in a electric batch annealing Electric furnace in 100% Hydrogen atmosphere to maintain a uniform temperature throughout the coil from edge to core;
(b) following annealing Soaking temperature 710-730 °C for soaking Time for 10-12 hours to achieve a higher r –bar value of 1.7 and above;
(c) Subjecting the electric batch annealed steel sheet to an optimum skin pass elongation of 0.6-1.2 % to get the yield strength less than 180 MPa, to avoid orange peel.

The above and other objects and advantages of the present invention are described hereunder in greater details with reference to the following accompanying non limiting illustrative drawings and examples.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
Figure 1: shows the flow chart illustrating the steps of the process for producing EDD steel sheet according to present invention.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING DRAWINGS

The present invention is directed to provide extra deep drawing steel sheet having excellent formability with adequate surface finish suitable for outer panel for automobiles.

A method in accordance with the present invention produces low carbon Al killed cold rolled steel strip containing B as a nitride former in place of titanium and having an r-value greater than 1.7 and yield strength of less than 180 MPa produced through Electric Arc Furnace (EAF) and Thin Slab caster and hot rolling route and Electric batch annealing route. The method comprises controlling the rolling and annealing parameters as described more fully below.

The EDD steel according to the present invention is having excellent drawability, good ageing property, high r-value and surface free from orange peel is concerned. In present invention, B has been used to stabilize N and to avoid orange peel. The mechanism for the same has been described later. N in present invention has been fixed only by B as BN by controlling the B/N ratio precisely.

In the present invention, a “steel sheet” stands for a cold-rolled steel sheet of which sheet thickness is 1.6 mm or less and all the steel component composition percentage is in weight %.

EDD Steel of present invention having composition comprising C: 0.025-0.065 wt%, Mn: 0.1-0.2 wt%, N: 0.007 to 0.01 wt%, P= 0.015 wt%, Si=0.05 wt%, Ca: 0.001-0.003 wt%, Al: 0.03-0.05 wt%, B: 0.001- 0.0035 wt% and balance being Fe and other unavoidable impurities, which is aluminum killed steel produced through Electric Arc Furnace and having specified B/N ratio between 0.12 to 0.35. The boron is added as nitride forming agent. Boron addition also adds up in getting excellent surface properties which is free from orange peel. Following are the justification for keeping the compositions as given above:

CARBON: ( 0.025-0.065 wt.%) – While carbon increases the steel strength, it reduces the cold workability , r value and the deep drawability of the cold rolled steel sheet remarkably and thus the higher limit of the cold rolled sheet is set at 0.1 wt%. For improving the r-value i.e. drawability it desirable to reduce the C level less than 0.05 wt%. Lowering the carbon content below 0.025 wt% results in poor ageing property since below 0.025 wt% steel is in complete a-ferrite region in Iron-Cementite phase diagram resulting no cementite formation. Consequently more free carbon will be available in steel matrix which deteriorates the ageing property. Keeping low carbon <0.025 % also requires vacuum degassing the molten steel which add more cost of production. To avoid that the C level is maintained between 0.025-0.065 wt%.

MANGANESE: (0.1-0.2) wt% - Manganese is essential for avoiding the red shortness caused by Sulphur. However, it hardens the steel and lowers the drawability. The r-value of a cold rolled steel sheet drops in addition. Thus its upper limit is kept at 0.2%. It is desirable to control the manganese content below 0.2% for the present grade where strength is not particularly required. Keeping the Mn content below 0.1 wt % could make steel susceptible to hot shortness due to unbound Sulphur.

Al: (0.03-0.05) wt% - Major advantage in low carbon grade is that it scavenges off the N by forming AlN precipitate which improves drawability by reducing interstitial N and by promoting the formation of {111} type of texture during electric batch annealing hence facilitating the drawability and high r-values. Availability of Al ensures sufficient AlN precipitation during electric batch annealing to improve ageing properties. In addition, higher Al content of more than 0.06 % may deteriorate the cleanliness of the steel during casting due to formation of unavoidable oxide Al2O3 inclusions which deteriorates the surface properties. Thus the upper value is limited to 0.06%. When better drawability is required it is desirable to restrict the acid soluble aluminum content to less than 0.05%.

NITROGEN: (0.007-0.01 wt %) - Normally, nitrogen is limited in an amount between 0.0015% and 0.006% in Al-killed steel. But in Electric Arc furnace it is difficult to control N wt % less than 0.006 wt% which deteriorates the ageing property and generate problem of orange peel. In present invention we resolved the problem of orange peel due to very high N wt % in the range of 0.007 to 0.01 by boron addition in steel making. Thus it is advantageous to keep the Nitrogen level below 0.01 wt% in order to avoid orange peel issue during drawing and restricted use of boron addition. Hence B/N wt% ratio range is kept between 0.12 to 0.35 to achieve a higher r-value of more than 1.7 and surface free from orange peel.

Boron: (0.001-0.0035wt %) - Advantage of adding boron is twofold, firstly B is strongnitride and carbide former so it helps in scavenging off nitrogen and carbon and stabilizes it by forming boron carbide and nitride (B(CN)). Secondly it improves the surface property of batch annealed steel by eliminating the Orange peel which is a major surface defect during forming. The upper limit is 0.0035 wt% as higher than that will restrict the grain growth and achieving high r-value will not be feasible. Hence for achieving the higher r-value more than 1.7, it’s required to keep B content less than 0.0035wt%. Keeping the Boron content more than 0.001 wt% is desired to avoid orange peel by maintaining B/N ratio from 0.12 minimum. B/N ratio maximum maintained as 0.35 to avoid poor rbar value.

COILING TEMPERATURE (620 to 650°C) - Coiling temperature is kept between 630°C to 650°C to controlled AlN precipitation before electric batch annealing as sufficient time is provided to precipitate out AlN during annealing hence no prior precipitation is needed. Also reheating temperature is kept high~12000C along with higher coiling temperature to keep controlled BN precipitation.

Manufacturing Method

Molten steel of the above noted composition is prepared through Electric Arc Furnace and thin slab caster with slab thickness of 50-55mm combined with hot rolling at temperatures of 880 to 910°C and 630°C to 650°C.

The hot rolled steel strip is then coiled while at a temperature exceeding >6200C (630-650 0C preferably). After hot rolling strips are pickled in acid medium (HCl) having concentrations between 4-20% and cold rolled with a minimum cold reduction of 75 percent (preferably 75-80 %) to achieve higher plastic strain ratio post annealing.
Following cold rolling, the steel strip is batch annealed. In electric batch annealing, a coil of cold rolled steel strip is heated at a slow heating rate of about 40-450C /Hour in an electric batch annealing furnace in 100% Hydrogen atmosphere to maintain a uniform temperature throughout the coil from edge to core as H2has very high heat conductivity. In addition, 100% H2 atmosphere is beneficial in avoiding graphitization and achieving excellent surface reflectivity 98% or more. Annealing cycle of Soaking temperature 710-730 °C for soaking Time for 10-12 hours to achieve a higher r –bar value of 1.7 and Soaking Time is so chosen as the higher soaking time than 12 hours would cause orange peel during annealing process which is a major surface defect in electric batch annealing route; where as a low soaking time reduces the drawing properties along with high yield strength.
After the electric batch annealing an optimum skin pass elongation of 0.6-1.0 % is used mainly to get the yield strength less than 180 MPa. The occurrence of any yield point elongation is avoided by keeping skin pass elongation more than 0.6 %.
Accompanying Figure 1 shows the flow chart with details of steps of production of EDD steel sheet according to present invention.

Complete characteristics and advantageous features of extra deep drawing steel sheet according to the present invention having excellent formability with superior surface finish and comparative steel grades are illustrated in the accompanying table I to III and also through following illustrative examples 1-6.

Table I- Compositions of the invented steel sheets along with some comparative examples.
Chemical Composition (Wt %)
Sample No C Mn S P Si Al B N B/N Ratio Ca Remarks
1 0.049 0.1 0.007 0.013 0.004 0.04 0.001 0.007 0.14 0.0013 Invention
2 0.049 0.18 0.007 0.013 0.004 0.04 0.0015 0.0075 0.20 0.003 Invention
3 0.029 0.2 0.007 0.013 0.004 0.04 0.0012 0.008 0.15 0.003 Invention
4 0.059 0.15 0.007 0.013 0.004 0.04 0.003 0.01 0.30 0.0013 Invention
5 0.07 0.25 0.006 0.013 0.02 0.06 0.0001 0.0037 0.03 0 Comparative Steel
6 0.02 0.17 0.007 0.005 0.019 0.06 0.0001 0.0034 0.03 0 Comparative Steel
7 0.05 0.35 0.004 0.019 0.02 0.05 0.0001 0.0042 0.02 0 Comparative Steel

Table II- Hot rolling, cold rolling, annealing parameters along with the mechanical properties of respective steel sheets.
HSM Rolling Cold Rolling CR Mechanical Properties
Sample No HSM FT HSM CT Cold rolled Thick BAF Cycle Soaking Time SPM Elongation YS
(MPa) UTS
(MPa
) Elongation % r -bar n value
1 890 645 0.8 710 10 0.8 165 294 45.8 1.9 0.23
2 890 630 0.8 720 11 1.2 185 298 46.7 1.8 0.228
3 890 620 0.8 720 12 0.8 159 295 47.8 2 0.226
4 890 650 0.8 730 10 0.8 148 293 47.9 1.9 0.234
5 890 550 0.7 740 9 0.7 180 304 46.3 1.6 0.23
6 890 560 0.7 720 13 0.8 184 293 45.4 1.65 0.223
7 890 560 0.7 680 8 0.8 185 299 41.6 1.5 0.165

*HSM- hot strip mill, *FT-Finishing Temperature, *CT-Coiling temperature

Table III – Ageing properties of respective steels.
sample no Angle After Heating 6 hrs 1000C (equivalent to 6-months ageing ) After Heating 12 hrs 1000C (equivalent to 1-year ageing ) Surface Remarks
YS YPE TS Aging YS YPE TS Aging
MPa % MPa
1 90° 168 0 310 OK 170 0 310 OK Free from Orange Peel
0° 160 0 317 OK 162 0 316 OK
3 90° 160 0 312 OK 162 0 306 OK Free from Orange Peel
0° 155 0 319 OK 158 0 317 OK
4 90° 170 0 307 OK 172 0 304 OK Free from orange Peel
0° 165 0 318 OK 170 0 317 OK
5 90° 180 0.9 304 YPE Observed 278 0.9 312 YPE Observed Orange Peel
0° 0.8 314 YPE Observed 275 0.8 314 YPE Observed
6 90° 0.9 320 YPE Observed 300 2.1 323 YPE Observed Orange Peel
0° 0.8 322 YPE Observed 296 1.9 325 YPE Observed

Example 1- from table I and II above it can be found that steel sample number 1 to 4 with N wt% ranges between 0.007-0.01 and B wt% ranges between 0.001 to 0.003% having B/N ratio of 0.14 to 0.3shows improved r-value of >1.7 with low yield strength (<180MPa) whereas steel sample number 5 to 7 with lower N wt% content of 0.0034 to 0.0042 wt% without B addition and lower coiling Temperature of less than 600 Deg c have reduced r-value of about 1.6 along with high yield strength, ageing issue and orange peel observed.

Example 2- The impact of B/N ratio can be observed from steel sample number 1 to 4 where the ratio is between 0.14 to 0.3, the outcome is high r-value with low yield strength due to less soluteN content in steel matrix. Steel sample number 5to 7 with lowB/N ratio of ranging from 0.02 to 0.03 shows less r-value with higher yield strength as compared to sample 1 to 4. This can be concluded from the fact that low B/N ratio leads to higher solute N content inside the steel matrix hence higher strength due to precipitate strengthening and ageing effect, which reduces the drawability.

Example 3 – keeping the carbon level below 0.065wt% is justified by taking the examples of steel sample number 5 in table I and II. For sample number 5 with Carbon wt% of 0.07. A remarkable drop in r-value is observed along with increase in yield strength for sample 5. Hence it is concluded to keep the carbon level between 0.025-0.065 as keeping the level below 0.025wt% deteriorates the ageing property.

Example 4- Impact of carbon wt% of less than 0.025 wt% on ageing property of cold rolled batch annealed steel has been illustrated in table I and table III. Yield point elongation is observed in steel sample no 6 with carbon wt% of 0.02 while keeping the other parameters the same. Hence it is claimed to keep the carbon wt% more than 0.025 to achieve ageing guarantee of 1 year.
, Claims:We Claim:
1. Cold rolled extra deep drawing steel sheets having formability with defect free surface finish having composition comprising
C: 0.025-0.065 wt%,
Mn: 0.1-0.2 wt%,
N: 0.007 to 0.01 wt%,
P= 0.015 wt%,
Si=0.05 wt%,
Ca: 0.001-0.003 wt%,
Al: 0.03-0.05 wt%,
B: 0.001- 0.0035 wt%
And balance being Fe and other unavoidable impurities, and having specified B/N ratio between 0.12 to 0.35.
2. Cold rolled extra deep drawing steel sheets as claimed in claim 1 which is aluminum killed steel obtained through Electric Arc Furnace having enhanced planar anisotropy ratio involving r-bar value of 1.7 min and yield strength of less than 180 MPa with ageing resistance of more than 6 months and free from orange peel surface defects suitable for automobile exposed panel applications.
3. A process for producing Cold rolled extra deep drawing steel sheets as claimed in claims 1 to 2 through electric batch annealing route comprising
Providing molten steel having composition
C: 0.025-0.065 wt%,
Mn: 0.1-0.2 wt%,
N: 0.007 to 0.01 wt%,
P= 0.015 wt%,
Si=0.05 wt%,
Ca: 0.001-0.003 wt%,
Al: 0.03-0.05 wt%,
B: 0.001- 0.0035 wt%
and balance being Fe and other unavoidable impurities having B/N wt% ratio between 0.12 to 0.35, and subjecting to processing for aluminum killed steel with formability and defect free surface finish involving HSM finishing temperature 880 to 910 0C and HSM coiling temperature of 6200 C or more preferably 6300 to 6500C, followed by cold rolling and electric batch annealing.
4. A process as claimed in claim 3 carried out involving combination of Hot strip mill coiling temperature 620 °C or more and electric batch annealing furnace hot spot temperature 710°C or less with soaking time of 10 hours to 12 hours to achieve strip surface free from orange peel.
5. A process as claimed in anyone of claims 3 to 4 carried out in combination of B/N ratio 0.12 or more, HSM coiling temperature 630 °C or more such as to achieve yield point elongations zero even after 1 year of ageing.
6. A process as claimed in anyone of claims 3 to 5 comprising
A. providing selective molten steel composition having
C: 0.025-0.065 wt%,
Mn: 0.1-0.2 wt%,
N: 0.007 to 0.01 wt%,
P= 0.015 wt%,
Si=0.05 wt%,
Ca: 0.001-0.003 wt%,
Al: 0.03-0.05 wt%,
B: 0.001- 0.0035 wt%;
And balance being Fe and other unavoidable impurities, Prepared without vacuum degassing and killed in the conventional manner;
B. casting the steel into thin slabs casting and rolling route with slab thickness of 50-55 mm;
C. subjecting the thin slabs to hot rolling into hot rolled strip with maintaining hot roll finishing temperature of at least 8800C and preferably 880-910 0C.
D. subjecting the hot rolled steel strip to coiling while at a temperature exceeding >6200C, preferably 630-650 0C ;
E. subjecting the hot rolled and acid pickled strips to cold rolling with a minimum cold reduction of 75 percent, preferably 75-80 % to achieve higher plastic strain ratio post annealing;
F. subjecting the cold rolled steel strip to electric batch annealing comprising
(i) heating a coil of cold rolled steel strip at a slow heating rate of about 40-45 0C /Hour in a electric batch annealing Electric furnace in 100% Hydrogen atmosphere to maintain a uniform temperature throughout the coil from edge to core;
(ii) following annealing Soaking temperature 710-730 °C for soaking Time for 10-12 hours to achieve a higher r –bar value of 1.7 and above;
(iii) Subjecting the batch annealed steel sheet to an optimum skin pass elongation of 0.6-1.2 % to get the yield strength less than 180 MPa, to avoid orange peel.

Dated this the 6th day of April, 2023
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199