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 :
LOW CARBON COLD-ROLLED NON-ORIENTED ELECTRICAL STEEL SHEET HAVING IMPROVED WATT LOSS AND MANUFACTURING METHOD THEREOF.


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 a low carbon cold rolled non-oriented semi-finished electrical steel sheet and method of manufacturing the same in industrial process. The method of producing the semi-finished electrical steel sheet having selective composition include processing through thin slab caster route and subjecting to selective hot rolling, cold reduction and intermediate electric batch annealing and final annealing to achieve improved watt loss and ageing resistance and free of surface defects, which has excellent magnetic properties in terms of watt loss at 1.5T, 50 Hz and magnetic flux density at 5000A/m, suitable for application in laminations of rotating electrical machines.

BACKGROUND OF THE INVENTION
In recent years, there has been increasing trend on electric vehicle for reduction in CO2 emissions. Efforts are being made to improve the efficiency and reduce the size of electrical motors or generators. Non-oriented electrical steels are widely used in making the laminated cores of electrical motors or generators. Therefore, there is an increased demand for non-oriented electrical steel with excellent magnetic properties to produce more efficient electric motors or generators along with size reduction. In non-oriented electrical steel in terms of magnetic properties, core loss and magnetic flux density are the main parameters. Core loss is energy lost in magnetizing and demagnetizing cycle, therefore the lower core loss is preferred. Magnetic flux density relates to the output power or torque of a motor, therefore higher magnetic flux density is desirable. In order to reduce the core loss, the main alloying additions are Silicon (Si), and Manganese (Mn), which increases the specific resistivity of steel. At the same time, higher alloying additions deteriorate the magnetic flux density and complexity in production are also increased. One of the most common defect encountered in high Silicon material is generation of surface defects like sliver and holes in final cold rolled annealed sheet. This defect impairs the quality of final annealed sheets and renders it unsuitable for making punched laminations for iron cores or motors or generators. If this defect gets passed on to an iron core, adversely affects the efficiency of the electrical equipment. Therefore all the electrical equipment manufacturers demands that this defect should not be allowed in the sheet.
Non-oriented electrical steel sheets can be divided into two categories. Fully-Finished grades and Semi-Finished, Fully-Finished grades are delivered in the finished condition, continuously annealed and sometimes varnished and on other hand semi-processed grades are given the final annealing treatment to develop their magnetic properties by the user.

As a part of prior art, the Indian patent publication number IN339778 discloses method of manufacturing a non-oriented electrical steel without corrugated defect which has an excellent magnetism, and a manufacturing method thereof. This invention discloses the method to achieve excellent magnetism and absence of shape related defect. However this method has not mentioned about how to produce the material without surface defect.

In another prior art, the Indian patent publication number IN379683A1 discloses method of manufacturing a non-oriented electrical steel sheet with the average flux density in the rolling direction and the direction perpendicular to rolling is 1.75 T or greater. Apart from the alloying additions of the method utilizes control over the non-magnetic inclusion AlN precipitation size distribution, method of hot rolling and annealing at a temperature between 750ºC and the Ac1 transformation temperature and the method of self-annealing at a coil take up temperature of 780ºC or greater. This invention enables to achieve the good magnetic properties but higher coiling temperature leads to more scale generation on hot rolled coil surface. This excess scale causes a lower picking line speeds to remove it, thus adversely affecting the productivity. This excess scale if not removed completely from the hot rolled coils during pickling, then it may get embedded in steel matrix during cold rolling and eventually lead to surface defects like rolled in scales and holes.

In yet another prior art, the Indian patent publication number IN202117023657A discloses a method to stably improve the magnetic flux density by utilizing induction heating and radiant heating during final annealing of a non-oriented electrical steel sheet. This method employs hot rolling a slab of predetermined chemical composition with finishing temperature below the transformation temperature of ferrite to austenite phase, with or without performing hot rolled sheet annealing and having the recrystallization ratio of 80% or less before cold rolling. This method also utilizes the control of average heating rate from 600°C to 720°C is 50°C/s or higher and from 720°C to 760°C is 5°C/s or higher. This invention describes how to achieve good magnetic properties but does not clarify about the possible surface defect which may arise and how to mitigate the same.

Therefore, the present invention discloses a method to industrially produce a low carbon non-oriented electrical steel sheet having excellent magnetic properties, ageing resistance with improved surface quality, which is suitable for use in making laminated cores of electrical equipment, for example, motors or generators.

OBJECTS OF THE INVENTION
The basic objective of the present invention is to provide a cold rolled non-oriented electrical steel sheet having excellent magnetic properties with improved surface quality and method of manufacturing the same.

A further object of present invention is directed to a process to manufacture said cold rolled non-oriented electrical steel sheet with improved surface quality and excellent magnetic properties that can be accomplished with advantage of easy operation, lower cost and better productivity by controlling the chemistry of steel melt, hot rolling process, with or without hot coil annealing, cold rolling and final annealing with or without suitable insulation coating.

In order to achieve the above objectives, the technical solution offered by the present invention is as follows:

SOLUTION OF THE PROBLEM
The present invention provide the solution to problem is to optimize the chemistry of the steel melt of a continuous casted slab, control the hot rolling parameters with or without hot coil annealing, cold rolling and final annealing with or without application of a suitable insulation coating. The present invention made above provides a Low Carbon cold rolled semi-finished electrical steel sheet having improved watt loss has a composition comprising, by mass %: C: 0.02-0.05 %; Mn: 0.1-0.3 %; Si: 0.25–0.5%; Al: 0-0.01%; S: 0.01 % or less;N: 0.007-0.01 %; Ca: 0.001- 0.003 %; and the balance being Fe and other unavoidable impurities; wherein [Ca] / [S] ratio is in a range of 0.1 to 0.7 to avoid elongated sulfide-type of inclusions. The cold rolled steel sheet is subjected to a hot rolling, Coiling of hot rolled coil at coiling temperature range from 640 to 680 °C, 1st cold reduction in the range of 50 to 60%, Intermediate Electric batch Annealing at Hot Spot temperature of 710°C or less with Soaking Time of 6-10 hours and 2nd reduction % range from 40-50% to get desired final thickness in the range of 0.2 to 0.7 and final annealing at furnace hot spot temperature 745°C or less with soaking time of 17 hours to 19 hours to achieve excellent magnetic properties in terms of watt loss at 1.5T, 50Hz W15/50, ranging from 3 to 6 Watts/Kg, Magnetic flux density, B50, ranging from 1.6 to 1.75 T at 5000 A/m.

SUMMARY OF THE INVENTION

The basic aspect of the present invention is directed to a Low Carbon cold rolled non-oriented semi-finished electrical steel sheet having improved watt loss and surface quality has a composition comprising, by mass %
C: 0.02-0.05 %;
Mn: 0.1-0.3 %;
Si: 0.25–0.5%;
Al: 0-0.01%;
S: 0.01 % or less;
N: 0.007-0.01 %;
Ca: 0.001- 0.003 %;
and the balance being Fe and other unavoidable impurities; wherein [Ca] / [S] ratio is in a range of 0.1 to 0.7, free of sulfide-type of inclusions.

A further aspect of the present invention is directed to said low Carbon cold rolled non-oriented semi-finished electrical steel sheet wherein the composition optionally comprising any one or more of the element by mass % selected from the group consisting of: Ti: less than 0.005%, P: less than 0.02, Sb: less than 0.2% and Cr: less than 0.05%.

A still further aspect of the present invention is directed to said low Carbon cold rolled non-oriented semi-finished electrical steel sheet having a thickness in the range of 0.2 – 0.70 mm; wherein the non-oriented electrical steel sheet have watt loss at 1.5T, 50 Hz, W15/50, ranging from 3 to 6 Watts/Kg, Magnetic flux density, B50, ranging from 1.6 to 1.75 T at 5000 A/m.

A still further aspect of the present invention is directed to aprocess for producing Low Carbon cold rolled non-oriented semi-finished electrical steel sheet as claimed in claims 1 to 3 produced through electric batch annealing route comprising
(i) Providing molten steel prepared through Electric Arc Furnace having composition comprising
C: 0.02-0.05 %; Mn: 0.1-0.3 %; Si: 0.25–0.5%; Al: 0-0.01%;
S: 0.01 % or less; N: 0.007-0.01 %; Ca: 0.001- 0.003 %; and the balance being Fe and other unavoidable impurities; wherein [Ca] / [S] ratio is in a range of 0.1 to 0.7,
(ii) Cast in thin slab caster with slab thickness of 50-55mm;
(iii) hot rolling with hot strip mill(HSM) finishing temperature 880 to 910 °C and HSM coiling temperature of 660 °C or more preferably 640 to 680 °C;
(iv) acid pickling hot rolled strips in acid medium (HCl) having concentrations between 4-20% ; followed by
(v) cold rolling with desired cold reduction% in stages and subjecting to annealing.

A still further aspect of the present invention is directed to said process wherein cold rolling comprising a 1st cold reduction % range from 50 -60%, followed by intermediate electric batch Annealing at hot spot temperature of 710°C or less with Soaking Time of 6-10 hours; and a 2nd reduction % range from 40-50% to get desired final thickness in the range of 0.2 to 0.7 mm.

Another aspect of the present invention is directed to said process carried out and involving combination of Hot strip mill coiling temperature 640 °C or more and electric batch annealing furnace hot spot temperature 745°C or less with soaking time of 17 hours to 19 hours to achieve desired watt loss.

Yet another aspect of the present invention is directed to said process wherein electric batch annealing of a coil of cold rolled steel strip is heated at a slow heating rate of about 40-45 0C /Hour in an electric batch annealing furnace in 100% Hydrogen atmosphere to maintain a uniform temperature throughout the coil from edge to core.

A further aspect of the present invention is directed to said process wherein after the electric batch annealing, subjecting the sheet to an optimum skin pass elongation of 4-5 % to get the desired dislocation density required for improved watt loss.

A still further aspect of the present invention is directed to said process wherein the annealed steel sheet is first cooled slowly and then rapidly to minimize the residual thermal stresses which impairs the magnetic properties in final cold rolled and annealed sheet.

A still further aspect of the present invention is directed to said process wherein the cold rolled annealed sheet is further optionally coated with a suitable insulation coating and coiled in exit pay off reels.

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

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING EXAMPLES

The present invention relates to cold rolled non-oriented electrical steel sheet having improved watt loss and surface quality and composition in terms of mass % comprising:
C: 0.02-0.05 %;
Mn: 0.1-0.3 %;
Si: 0.25–0.5%;
Al: 0-0.01%;
S: 0.01 % or less;
N: 0.007-0.01 %;
Ca: 0.001- 0.003 %;
and the balance being Fe and other unavoidable impurities; wherein [Ca] / [S] ratio is in a range of 0.1 to 0.7 to avoid elongated sulfide-type of inclusions.
.
The low Carbon cold rolled semi-finished electrical steel sheet having a thickness in the range of 0.2 – 0.70 mm; wherein the non-oriented electrical steel sheet have watt loss at 1.5T, 50 Hz, W15/50, ranging from 3 to 6 Watts/Kg, Magnetic flux density, B50, ranging from 1.6 to 1.75 T at 5000 A/m..
.
Following abbreviations, terminologies and expressions are used to describe the manner of implementation of the present invention:
SRT -Slab Reheating Temperature
RMX- Roughing Mill Exit Temperature
FT-Finishing Temperature
CT- Coiling Temperature
W15/50- Watt loss in W/kg at 1.5T, 50Hz
B50- Magnetic flux density in T at 5000 A/m

A cold rolled non-oriented electrical steel sheet having improved surface quality and excellent magnetic properties according to the present invention, its chemical composition and method of manufacturing are described hereunder with explanation on metallurgical factors governing the ranges of the constituents in a composition according to a preferred embodiment wherein all the elements are in weight % as follows:

Carbon (0.02 – 0.05 wt%) - Carbon is a harmful element for watt loss which forms carbide precipitate in ferrite matrix. It also causes the magnetic ageing, thus it is set to be 0.02 – 0.05%, more preferably 0.02 – 0.03%. The lower the carbon the better, however it is extremely difficult to reduce the carbon to less than 0.02% in electric Arc furnace.
Manganese (0.1 – 0.3 wt%) – Manganese improves the specific resistivity of the steel and thus helps to reduce the watt losses. It is also effective in coarsening the sulphide precipitate rendering it harmless to the magnetic properties, mainly watt losses. The manganese should be in excess quantity to the stotiometic ratio of [Mn]/[S] to form coarser precipitates therefore, the minimum amount inpresent invention is set to 0.1% for fixing Sulphur. If the Manganese addition is more than 0.4 %, it deteriorates the magnetic flux density by forming carbonitride precipitates. Therefore, in the present invention range is fixed as 0.4%. More preferably it is 0.3 % maximum.
Silicon (0.25 – 0.5 wt%) – Silicon is the most important alloying element for electrical steels. It increases the specific resistivity of steel and decreases watt loss.
The minimum limit specified in the present invention is set to 0.25 %, below that the watt loss reduction is not sufficient to achieve the desired watt losses. However when silicon content is high, i.e. more than 0.5 %, thin slab casting with hot rolling becomes difficult, leading to frequent cobble at Hot rolling mill, also, higher silicon deteriorates the magnetic flux density therefore, the maximum limit is set to 0.5%. More preferably, Si may be contained in amount 0.25 to 0.5 %.
Aluminium (0 – 0.01 wt%) – Similar to silicon, Aluminium also increases the specific resistivity of steel, thereby helps to reduce the watt losses. For this purpose the minimum level is set to 0. However if Aluminium exceeds 0.01 %, it leads to formation of alumina inclusions that results in poor watt loss. Therefore the maximum limit is set to 0.01 %. More preferably, Al may be contained in amount 0.005%.
Sulphur ( 0.01 wt% or less) – Sulphur forms fine precipitates of [M][S] form, where [M] is metallic element present in steel, the most common sulphide forming elements are Fe, Mn, Cu, Ti and Ca etc. These fine precipitates adversely affects the watt loss. Above 0.01 %, the effect becomes very prominent, therefore the maximum limit is set to 0.01%. The minimum level is not specified here, because the lower amount of Sulphur is preferred.
Nitrogen (0.0071 to 0.01 wt%) – Nitrogen forms fine metal nitride precipitates with Ti, Al, Fe etc. These fine precipitates restricts the grain grown by pinning the grain during annealing which adversely affects the watt loss, therefore the maximum limit is set to 0.01%. The minimum level is the limitations of electric Arc furnace as it is not possible to maintain less than 0.007 wt%. Hence, Lower limit of N was kept more than 0.0071%.
Titanium (0.003 wt% or less) – Titanium is a strong nitride and carbide forming element. The fine precipitates of titanium restricts the grain grown by pinning the grain during annealing which adversely affects the watt loss, therefore the maximum limit is set to 0.003%. The minimum level is not specified here, because the lower amount of titanium is preferred.
[Ca] / [S] 0.1 to 0.7– Calcium is added to improve the oxidation resistance of steel. For this effect to become prominent, it should be in elemental form. Copper is prone to form CaS precipitate, which consumes the elemental Ca present in the steel. Therefore stronger sluphide precipitate forming elements are required to be added in steel, like Ca etc. Maintaining [Ca] / [S] minimum 0.1 ensures enough elemental Ca is present in steel to provide the oxidation resistance. [Ca] / [S] more than 0.7 is not considered significant because it will incur additional alloying cost therefore the maximum ratio is set to 0.7 and Ca content is restricted to 0.003 %.
Calcium (0.001 – 0.003 wt% ) – Calcium is added in steel for inclusion modification, it makes the sulphide precipitate coarse and non-deformable stable sulphide which does not hinders the grain growth during recrystallization. For this inclusion modification effect to be noticeable, it must be added more than 0.0001%. However, if it added in excess, it deteriorates the watt loss, therefore the maximum limit is set to 0.003 %.

Description of the process of manufacturing:

Molten steel of the above stated composition is prepared through Electric Arc Furnace and thin slab caster with slab thickness of 50-55mm combined with hot rolling at HSM finishing temperature 880 to 910 °C and HSM coiling temperature of 640 °C or more preferably 640 to 680 °C.

The hot rolled steel strip is then coiled while at a temperature exceeding >6400C (640-680 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, wherein said steel 1st cold reduction % range from 50 -60%, followed by intermediate electric batch Annealing at hot spot temperature of 710°C or less with Soaking Time of 6-10 hours and 2nd reduction % range from 40-50% to get desired final thickness in the range of 0.2 to 0.7 mm. Then, 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 40-45 0C /Hour in an electric batch annealing furnace in 100% Hydrogen atmosphere to maintain a uniform temperature throughout the coil from edge to core as H2 has very high heat conductivity. In addition, 100% H2 atmosphere is beneficial in avoiding graphitization and achieving excellent surface reflectivity 98% or more. The electric batch annealing furnace hot spot temperature 745°C to 735 °C with soaking time of 17 hours to 19 hours to achieve desired watt loss. Soaking Time is selectively chosen as the higher soaking time than 19 hours would cause lower hardness during annealing process which is required from the customers; where as a low soaking time reduces the hardness increases along with high yield strength.
After the electric batch annealing an optimum skin pass elongation of 4-5 % is used mainly to get the desired dislocation density required for improved watt loss.
The annealed steel sheet is first cooled slowly and then rapidly to minimize the residual thermal stresses which impairs the magnetic properties in final cold rolled and annealed sheet.
The cold rolled annealed sheet can further optionally coated with a suitable insulation coating and coiled in exit pay off reels. Samples are drawn from the final annealed coils to evaluate the achieved magnetic properties. Online inspection using automated camera setup is utilized to capture any surface defect present in the steel sheet.
The complete description of steel according to the present invention and comparative steel grades are illustrated in following table 1 to table 3 and the weight % ranges of constituents and selective process parameters according to the present invention are validated through following examples 1 & 2:

Table 1: Elemental Composition in weight % of the inventive steel sheets along with comparative example.
Table 2: Hot rolling and cold rolling parameters of inventive steel having chemical compositions as per Table 1 alongwith comparative steel sheets.
Table 3: Batch annealing parameters, magnetic properties and surface defect observation of inventive with comparative steel sheets having chemical compositions as per Table 1 and being processed as per table 2 and table 3.
Table 1
Chemical Composition in Wt %
Sample No C MN S Ca SI AL N Ti [Ca]/[S] Other Elements Remarks
1 0.03 0.25 0.003 0.002 0.3 0.002 0.0074 0.0012 0.67 Sb-0.009 I
Cr-0.02
2 0.021 0.27 0.0031 0.001 0.4 0.003 0.009 0.0012 0.33 Sb-0.0010 I
Cr-0.04
3 0.05 0.3 0.003 0.002 0.5 0.003 0.01 0.0012 0.67 Sb-0.02 I
Cr-0.02
4 0.025 0.2 0.01 0.0025 0.25 0.004 0.0071 0.0011 0.25 Sb-0.013 I
Cr-0.03
5 0.06 0.4 0.002 0.005 0.1 0.02 0.0026 0.012 2.5 C
6 0.0023 0.06 0.003 0.0002 0.8 0.015 0.0017 0.0015 0.06 C

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

Example 1
It can be observed from table 1 to table 3 that steel sheets samples marked as “I” are inventive steel, satisfying all the scopes of present invention. The watt loss values are within the prescribed range of 3 to 6 W/Kg at 1.5T, 50 Hz and magnetic flux density from 1.63 to 1.75 T at 5000 A/m. Whereas, steel samples marked as “C” are comparative examples where at least one of the elements of the present invention scope is not complied and does not meet at least one of the end product quality attributes. For example in samples 5 to 6does not meet the prescribed [Ca]/[S] range of 0.1 – 0.7 and results in having surface defect sliver present in the final annealed condition.
Table 2

Rolling Parameter
Sample No FT (°C) CT (°C) CR Thk (mm) 1st Reduction % BAF Soaking Temp 2nd Reduction % No of cold rolling operation Remarks
1 890 650 0.35 50 710 40 1 I
2 880 650 0.35 52 680 45 1 I
3 880 660 0.5 55 690 50 1 I
4 910 680 0.5 60 670 40 1 I
5 876 687 0.5 0 0 81.5 1 C
6 864 689 0.5 0 0 80.8 1 C

*I - Present inventive example, C- Comparative Examples

The hot coil finishing and coiling temperature are also maintained within the prescribed range. The sample sheet numbers 1 and 2 are in 0.35 mm final thickness. Whereas, sample sheet numbers 3 to 6 are in 0.50 mm final thickness. The 1st reduction % at cold rolling mill was from 50 to 60 % and 2nd reduction % after electric batch annealing was from 40 to 50.

Example 2
The magnetic properties achieved and surface defect observation remarks for different samples from 1 to 4 are mentioned in table 3, which meets the prescribed range as mentioned in scope of this invention. Whereas for comparative example sample 5 to 6 which does not meet the prescribed range of [Ca]/[S] between 0.1 – 0.7 as per scope of this invention, end up having surface defect, sliver present in the final annealed condition. The magnetic properties of comparative samples are within prescribed range but the surface quality does not meet the requirement therefore this invention provides the solution which is both cost effective and easy to operate.

Table 3

Sample No Electric BAF Parameters Magnetic Properties Surface defect, Sliver remark (Y/N) Remarks
Heating rate (°C/S) Soaking temperature (°C) Soaking time (S) CR Thk (mm) Watt loss, W15/50 (W/Kg) B50 (T) Hardness (HV1)
1 15.6 745 17 0.35 3.89 1.71 166 N I
2 12.6 735 18 0.35 3.39 1.66 202 N I
3 11.5 740 19 0.5 5.25 1.68 156 N I
4 14.1 745 19 0.5 4.42 1.72 157 N I
5 16.3 720 14 0.5 6.38 1.72 161 Y C
6 12.7 710 20 0.5 7.85 1.66 199 Y C

Therefore, the present advancement favors the production of cold rolled non-oriented electrical steel sheet in thickness 0.20 to 0.70 mm, having the watt loss W15/50, ranging from 3 to 6 Watts/Kg at 1.5T, 50 Hz, Magnetic flux density, B50, ranging from 1.63 to 1.75 T at 5000 A/m with improved resistance surface defects.

, Claims:We Claim:
1. A Low Carbon cold rolled non-oriented semi-finished electrical steel sheet having improved watt loss and surface quality has a composition comprising, by mass %
C: 0.02-0.05 %;
Mn: 0.1-0.3 %;
Si: 0.25–0.5%;
Al: 0-0.01%;
S: 0.01 % or less;
N: 0.007-0.01 %;
Ca: 0.001- 0.003 %;
and the balance being Fe and other unavoidable impurities; wherein [Ca] / [S] ratio is in a range of 0.1 to 0.7 free of sulfide-type of inclusions.

2. The Low Carbon cold rolled non-oriented semi-finished electrical steel sheet as claimed in claim 1 wherein the composition optionally comprising any one or more of the element by mass % selected from the group consisting of: Ti: less than 0.005%, P: less than 0.02, Sb: less than 0.2% and Cr: less than 0.05%.

3. The Low Carbon cold rolled non-oriented semi-finished electrical steel sheet as claimed in anyone of claims 1 and 2, having a thickness in the range of 0.2 – 0.70 mm; wherein the non-oriented electrical steel sheet have watt loss at 1.5T, 50 Hz, W15/50, ranging from 3 to 6 Watts/Kg, Magnetic flux density, B50, ranging from 1.6 to 1.75 T at 5000 A/m.

4. A process for producing Low Carbon cold rolled non-oriented semi-finished electrical steel sheet as claimed in claims 1 to 3 produced through electric batch annealing route comprising
(i) providing molten steel prepared through Electric Arc Furnace having composition comprising
C: 0.02-0.05 %; Mn: 0.1-0.3 %; Si: 0.25–0.5%; Al: 0-0.01%;
S: 0.01 % or less; N: 0.007-0.01 %; Ca: 0.001- 0.003 %; and the balance being Fe and other unavoidable impurities; wherein [Ca] / [S] ratio is in a range of 0.1 to 0.7,
(ii) cast in thin slab caster with slab thickness of 50-55mm;
(iii) hot rolling with hot strip mill(HSM) finishing temperature 880 to 910 °C and HSM coiling temperature of 660 °C or more preferably 640 to 680 °C;
(iv) acid pickling hot rolled strips in acid medium (HCl) having concentrations between 4-20% ; followed by
(v) cold rolling with desired cold reduction% in stages and subjecting to annealing.

5. The process as claimed in claim 4, wherein cold rolling comprising a 1st cold reduction % range from 50 -60%, followed by intermediate electric batch Annealing at hot spot temperature of 710°C or less with Soaking Time of 6-10 hours; and a 2nd reduction % range from 40-50% to get desired final thickness in the range of 0.2 to 0.7 mm.

6. The process as claimed in anyone of claim from 4 or 5 carried out and involving combination of Hot strip mill coiling temperature 640 °C or more and electric batch annealing furnace hot spot temperature 745°C or less with soaking time of 17 hours to 19 hours to achieve desired watt loss.

7. The process as claimed in anyone of claims 4 to 6 wherein electric batch annealing of a coil of cold rolled steel strip is heated at a slow heating rate of about 40-45 0C /Hour in an electric batch annealing furnace in 100% Hydrogen atmosphere to maintain a uniform temperature throughout the coil from edge to core.

8. The process as claimed in anyone of claims 4 to 7 wherein after the electric batch annealing, subjecting the sheet to an optimum skin pass elongation of 4-5 % to get the desired dislocation density required for improved watt loss.

9. The process as claimed in anyone of claims 4 to 8 wherein the annealed steel sheet is first cooled slowly and then rapidly to minimize the residual thermal stresses which impairs the magnetic properties in final cold rolled and annealed sheet.

10. The process as claimed in anyone of claims 4 to 9 wherein the cold rolled annealed sheet is further optionally coated with a suitable insulation coating and coiled in exit pay off reels.

Dated this the 28th day of July, 2023
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199