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 NON-ORIENTED THIN GAUGE ELECTRICAL STEEL SHEET 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 implemented.

FIELD OF INVENTION
The present invention relates to cold-rolled non-oriented thin gauge electrical steel sheet for use as core laminations of motors, generators and drive motors of automobiles in Electric Vehicles (EVs) or Hybrid Electric Vehicles (HEVs). More particularly, the present invention relates to a thin gauge non-oriented electrical steel sheet having desired magnetic properties, yield strength more than 400 MPa and specific magnetic properties, in addition to the prevention of strip breakage during cold rolling and a method of its manufacturing.

BACKGROUND OF THE INVENTION
In recent years, there is an increasing concern to reduce the greenhouse gases, which demand that the appliances and machineries should have better efficiency and lower energy consumption. Thin gauge non-oriented electrical steels is widely used in as core material in various appliances and machineries motors or generators. Compressor motors of refrigerators and air conditioners are typical examples where higher efficiency motors are required to bring down the electrical power consumption, thus proportionally reducing the greenhouse emission associated with it. In the case of automobiles, vehicles consisting of an electrical drive motors are becoming a popular choice over conventional fuel based automobiles. The core of drive motors which are used in Electric Vehicles (EVs) or Hybrid Electric vehicles (HEVs) are manufacture from thin gauge (thickness ≤ 0.35 mm) non-oriented electrical steels. These motors usually operates at higher frequencies and rpm to achieve better operational efficiency. Hence, it is of great importance that the raw materials used in making core of such drive motors should have lower losses at higher frequencies, typically at 400Hz. The drive motors laminated core consists of two parts, one is stator which is stationary and other is rotor which rotates at high rpm. Since these drive motors runs at higher rpm, this puts up the requirement of having adequate yield strength to sustain the resultant centrifugal forces without causing any deformation in the core. The stator laminated core requires excellent magnetic properties, that is, lower core losses and higher magnetic flux density whereas rotor core material requires to have higher yield strength along with excellent magnetic properties. Providing different non oriented electrical steel sheet for stator and rotor parts are not preferred choice for lamination core makers, it is expected that same steel sheet should meet the characteristics required by both stator and rotor. Therefore, the thin gauge non-oriented electrical steel should have lower core losses and higher magnetic flux density at high frequency as well as higher yield strength.
The present inventors have done extensive research to meet the above stated requirements in non-oriented electrical steel material. Many alloying combination and ratios were studied to attain the desired magnetic properties and yield strength requirements. The present invention describes the methods and techniques to manufacture the same.
One way to reduce the core losses in electrical steel is by increasing the Silicon and Aluminium content, these alloying additions increase the bulk resistivity of steel and due to this eddy current losses are decreased thereby reducing the core loss. Another way is to reduce the thickness, because eddy current losses in core material is proportional to square of thickness. This thickness reduction is very effective in reducing the eddy current losses at higher frequencies, i.e. at 400Hz to 1.0 KHz. Therefore, the thinner sheets of non-oriented electrical steels are termed as thin gauge non-oriented electrical steel. But, when the Silicon and Aluminium combined is more than 5.0%, the magnetic flux density deteriorates. Also, the cold rolling to thinner thickness becomes difficult due to the brittleness caused by high Silicon content and frequent strip breakage can occur during cold rolling. This impacts the productivity of cold rolling mill and the yield of the final output material. The present invention discloses a method to manufacture thin gauge non-oriented electrical steel with desired magnetic properties,yield strength and strip breakage during cold rolling is avoided.
In prior arts, for instance in Indian Patent (Nippon patent) number 466856, it was disclosed in the method that following the procedure of the invention gives the desired excellent magnetic properties and tensile strength of 600 MPa or more. But this invention is silent about yield strength, the start of deformation or yielding in material is governed by yield strength. If the material yield strength is not adequate to withstand the centrifugal forces caused by high rpm revolution then yielding in rotor core may occur which can eventually cause to alter the small air gap between stator and rotor and thus the free revolution of rotor may be hindered which directly affects the performance during service of such motor.
In another prior arts, for instance in Indian Patent 503017 (JFE Patent) alloying element Zn in a specific range is prescribed. This is difficult to maintain in steel bath due to low melting point and lower vapour pressure of Zn. The microstructure also contains significant amount of non-recrystallized grains which adversely affects the core losses. Addition of Cr increases the tensile strength but do not improve the magnetic flux density. This disclosure is also silent about the achievement of minimum level of yield strength requirement.
In yet another prior arts, for Indian Patent 502382, uses siliconizing process using Silane (SiCl4) gas. This gas is toxic and possess explosion hazard. Thickness control during vapour deposition requires extreme care. For diffusion of deposited silicon requires higher annealing temperature and time, also the deposited silicon layer lacks the preferred texture for better magnetic properties.
In yet another prior arts, for Indian Patent 456574, costly alloying additions like Cr, Ni, Mg are prescribed. Carbide forming elements such as Nb, V and Ti are used which increases the tensile strength but significantly deteriorate the magnetic properties. Different annealing treatment are prescribed for rotor and stator material. This implies that the material manufactured for rotor application cannot be used for stator and vice versa.
To solve the above problems of prior arts, the present inventors conducted extensive study to provide a method to manufacture non-oriented thin gauge electrical steel having desired magnetic properties, yield strength more than 400 MPa and strip breakage during cold rolling is avoided.

OBJECTS OF THE INVENTION
The basic object of the present invention is directed to providecold-rolled non-oriented thin gauge electrical steel sheet for use as core laminations of motors, generators and drive motors of automobiles in Electric Vehicles (EVs) or Hybrid Electric Vehicles (HEVs) and manufacturing method thereof.
A further object of the present inventionis directed to providesaid cold-rolled non-oriented thin gauge electrical steel sheetcomprising a final thickness in the range of 0.10 to 0.35 mm having Watt loss at 50Hz and 1.5T, W15/50 ≤ 3.5 Watts/kg and Watt loss at 400Hz and 1.0T, W10/400 is ≤ 20 Watts/kg and magnetic flux density at 5000 A/m, B50 is in the range of 1.60 to 1.73 T and yield strength of 400 MPa minimum.
A further object of the present inventionis directed to providesaid cold-rolled non-oriented thin gauge electrical steel sheet for which strip breakage during cold rolling is avoided.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to a cold-rolled non-oriented thin gauge electrical steel sheet comprising of steel composition by weight % comprising:
C: 0.0005 to 0.01 %;
Mn: 0.2 to 2.0 %;
Si: 2.0 to 4.0%;
Al: 0.5 to 1.2%;
P: 0.001 to 0.20 %;
S: 0.0005 to 0.008 %;
Cu: 0.0005 to 0.10 %;
N: 0.0005 to 0.007 %;
Ti: 0.0005 to 0.003%; and the balance are Fe and other unavoidable impurities and wherein weight % [Mn+Cu]/[S] ≥ 200 and [Si] + [Al] is maintained between ≥ 3.0% and ≤ 5.0%; for desired core loss and magnetic flux density at thickness ≤ 0.35 mm and with yield strength of more than 400 MPa .
A further aspect of the present invention is directed to said cold-rolled non-oriented thin gauge electrical steel sheet wherein the steel composition includes elements in wt.%. Sb: 0.005 to 0.20%; Ca: 0.0001 % to 0.003 %; or a combination thereof.
A still further aspect of the present invention is directed to said cold-rolled non-oriented thin gauge electrical steel sheet which has an average grain size “d” of 50 – 200 micrometers in final annealed condition which comply the relation as
0.39×T_f-〖2.5×√T〗_f×t-400 ≤d ≤0.39×T_f-〖2.5×√T〗_f×t-330
Where Tf is the final annealing temperature in Kelvin, t is the final sheet thickness in mm and d is the average grain size in µm.
A still further aspect of the present invention is directed to said cold-rolled non-oriented thin gauge electrical steel sheet comprising a final thickness in the range of 0.10 to 0.35 mm having Watt loss at 50Hz and 1.5T, W15/50 ≤ 3.5 Watts/kg and Watt loss at 400Hz and 1.0T, W10/400 is ≤ 20 Watts/kg and magnetic flux density at 5000 A/m, B50 is in the range of 1.60 to 1.73 T and yield strength of 400 MPa minimum.
A still further aspect of the present invention is directed to a method of manufacturing cold-rolled non-oriented thin gauge electrical steel sheet as described above comprising
steel making through BOF and RH vacuum degassing route followed by continuous casting the steel to a slab;
reheating the slab with hot charging temperature of at least 300 °C to a temperature in the range of 1100 to 1250 oC, and
subjecting reheated slab to a step of rough rolling whereinend of rough rolling the slab has temperature in the range 1060 to 900 oC;
subjecting the rough rolled steel to a step of finish rolling to achieve a final temperature of 800 to 950 oC; and
coiling the hot finish rolled sheet at a temperature range of 600 to 700 oC and cooled to room temperature.
Another aspect of the present invention is directed to said method wherein hot rolled coil is pickled with 2 to 18% hydrochloric acid to remove the scalesand thereafter the pickled steel hot band is optionally subjected to annealing of the hot-rolled pickled coil by soaking at a temperature in the range of 700 °C to 1000 °C for 10 to 180 seconds in the case of continuous annealing or 6 to 40 hours in the case of box annealing.
Yet another aspect of the present invention is directed tosaid method wherein the steel sheet thus obtained is cold rolled once or twice with intermediate annealing in between, and the cold-rolled steel sheet is then subjected to final annealing; said intermediate annealing when applicable is performed by soaking in a temperature range 750°C to 1050°C for 10 to 180 seconds in case of continuous annealing and 6 to 40 hours in case of box annealing.
A further aspect of the present invention is directed tosaid method wherein cold rolling reduction in first cold rolling is controlled within 50 to 80% and after intermediate annealing the reduction should be less than 80%.
A still further aspect of the present invention is directed tosaid method wherein said steel sheet after first cold rolling and intermediate annealing is subject to edge trimming as a vital step to avoid strip breakage during next cold rolling.
Another aspect of the present invention is directed tosaid method wherein steel sheet thus obtained is subject to final annealing where the temperature raise rate is maintained at the rate of 5 to 45 °C/s up to a soaking temperature of 800 °C to 1100 °C and the residence time at soaking temperature is ranging from 10 to 100 seconds to thereby produce the annealed coils of said cold-rolled non-oriented thin gauge electrical steel sheet of the thickness in the range 0.10 to 0.35 mm.
A further aspect of the present invention is directed tosaid method wherein the steel sheet after final annealing is coated with a suitable coating on both top and bottom surface to enhance the surface insulation resistivity.
The above and other aspects of the present invention are described in further details with reference to following non-limiting illustrative drawings and examples.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS:
[Fig. 1]: It is an image of edge cracks initiated from saw edges generated after first cold rolling of steel.
[Fig. 2]: It is an image of microstructure of inventive steel thus obtained as well as of comparative example. The images are not true to scale and only used for representative comparison purpose.
[Fig. 3]: It is schematic representation of process route followed for inventive example steel.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO ACCOMPANYING DRAWINGS AND EXAMPLES
The present invention discloses a method of manufacturing thin gauge cold rolled non-oriented electrical steel sheet comprising of following chemical composition by weight percent as C: 0.0005 to 0.01%; Mn: 0.2 to 2.0%; Si: 2.0 to 4.0%; Al: 0.5 to 1.2%; P: 0.001 to 0.20%; S: 0.0005 to 0.008%; N: 0.0005 to 0.007%; Cu: 0.0005 to 0.10%; Ti: 0.0005 to 0.003%; Sb or Sn: 0.0005 to0.20%; and the balance are Fe and other unavoidable impurities and satisfies the following relation: weight % [Mn+Cu]/[S] ≥ 200.
According to the present invention, a steel of the chosen composition with the specified range of alloying elements and impurity level the desired thin section of the sheets could be fabricated that finally meets the target magnetic and mechanical properties. The chosen composition in the steel can be obtained by appropriately alloying the elements during steel making. By using such electrical steel sheet according to the present invention it is possible to provide a thin gauge cold rolled non-oriented electrical steel sheet suitable for use as a core lamination for motors, generators and drive motors of automobiles in Electric Vehicles (EV) or Hybrid Electric Vehicles (HEVs) including both rotor and stator core laminations.
Hereinafter the chemical composition of the steel is described with the reason of limiting the specified ranges of each constituent and the metallurgical significance thereof. According to the preferred embodiment wherein all the elements were demonstrated in weight percent.
Carbon (0.0005 – 0.01 wt %) - Carbon is a harmful element for electrical steel sheets as it increases core loss by carbide precipitate. If the carbon content exceeds 0.01%, carbides such as cementite and alloy carbides precipitate, and due to this the core loss deterioration may become significant during service. To further improve the magnetic properties, particularly to improve the core loss, the upper limit of the C content is preferably 0.005%. The lower limit of carbon is not particularly limited, but, it increases the production time to bring Carbon less than 0.0005% in industrial processes. Therefore, the carbon content is set as 0.0005 – 0.01%, more preferably 0.001 – 0.005%.
Manganese (0.2 – 2.0 wt %) – Manganese improves the specific resistivity of the steel and thus helps to reduce the core losses. Manganese also improves the yield strength by solid solution strengthening. 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 stoichiometric ratio of [Mn]/[S] to form coarser precipitates therefore, the minimum amount in the present invention is set to 0.2% for fixing Sulphur. If the manganese addition is more than 2.0 %, it deteriorates the magnetic flux density by forming carbonitride precipitates and cold rolling becomes difficult due to increased yield strength. Therefore, the maximum Manganese content is set as 2.0%. More preferably it is 1.2 % maximum and even more preferably it is 1.0 % maximum.
Silicon (2.0 – 4.0 wt %) – Silicon is a vital alloying element for electrical steel. It increases the specific resistivity of steel and decreases core loss. The minimum limit specified in the present invention is set to 2.0 %, below that the core loss reduction is not sufficient to achieve the desired watt losses. However when silicon content is high, i.e. more than 4.0 %, cold rolling becomes difficult, causing frequent strip breakages during cold rolling operation. Silicon if exceeds 4.0% then it remarkably deteriorates the magnetic flux density, therefore the maximum limit is set to 4.0%. More preferably, Si may be maintained in amounts of 2.0 to 3.5 %. Even more preferably 2.5 to 3.5%.
Aluminum (0.50 – 1.2 wt %) – Like silicon, Aluminum also increases the specific resistivity of steel, thereby reducing the core losses. To achieve this effect, the minimum level is set to 0.5 %. However, if Aluminum exceeds 1.2 %, magnetic flux density is adversely affected. Therefore, the maximum limit is set to 1.2 %. More preferably, Al may be contained in amounts 0.6 to 1.0%.
[Si] + [Al] ≥ 3.0% and ≤ 5.0% – The total amount of Si and Al combined is set as 3.0% minimum to achieve the desired reduction in core losses by increasing the specific resistivity of steel. When [Si] + [Al] combined is less than 3.0% the reduction in core losses is not significant as required for present invention. When [Si] + [Al] combined is more than 5.0% the cold rolling of the steel becomes difficult, resulting in frequent breakages during cold rolling, also it deteriorates the magnetic flux density significantly.
Phosphorous (0.001 - 0.20 wt %) – Phosphorous increases the specific resistivity of the steel and improves favorable texture by preferential segregation to the grain boundaries. However, when it is above 0.20 %, the steel becomes brittle and cold rolling becomes difficult. Therefore, the maximum Phosphorous content is set to 0.20 %. More preferably it is 0.10 % or less, and even more preferably it is 0.06 % or less.
Sulphur (0.0005 - 0.008 wt %) – S is a harmful element for electrical steel sheets. Sulphur forms fine precipitates of [M][S] form, where [M] is a metallic element present in steel. The most common sulphide forming elements are Fe, Mn, Cu, Ti and Ca, etc. These fine precipitates deteriorates the core loss. When sulphur exceeds 0.008 %, the core loss deterioration effect becomes remarkable, therefore, the maximum limit is set to 0.008%. The minimum level is not particularly limited as specified here is 0.0005% because a lower amount of sulphur than 0.0005% is extremely difficult to obtain in industrial processes.
Nitrogen (0.0005 - 0.007 wt %) – Nitrogen is a harmful element for electrical steel sheets. It forms fine metal nitride precipitates with Ti, Al, Fe, etc. These fine precipitates restrict the grain growth by pinning the grains during annealing which adversely affects the core losses, therefore the maximum limit is set to 0.007%. The minimum level is not particularly limited as specified here is 0.0005% because a lower amount of Nitrogen than 0.0005% is extremely difficult to obtain in industrial processes.
Titanium (0.0005 - 0.003 wt %) – Titanium is a harmful element for magnetic properties of electrical steel. Ti is a strong nitride and carbide forming element. The fine precipitates of titanium restrict the grain growth by pinning the grains during annealing which deteriorates the core loss, therefore the maximum limit is set to 0.003%. The minimum level is not particularly limited but as specified here is 0.0005% because in the industrial process achieving less than 0.0005% Ti is difficult.
Antimony (0.005 - 0.2 wt %) – Antimony addition helps to improve the favorable recrystallization texture of cold-rolled non-oriented electrical steels. It preferentially segregates to the grain boundaries and promotes the theta fiber and Goss texture which is beneficial for the magnetic properties and suppresses the formation of gamma fiber which is harmful for the magnetic properties. However, an amount above 0.2 %, the effect of improvement is saturated and also it deteriorates the cold rolling ability of steel. Therefore, the maximum limit is restricted to 0.2 %. When the amount of Antimony is less than 0.005 % the improvement is not noticeable, therefore, the preferable range is 0.005 to 0.20% and even more preferably 0.04 – 0.18 %.
Calcium (0.0001 – 0.003 wt %) – Calcium is added to steel for sulphide inclusion morphology modification. It makes the sulphide precipitate globular and non-deformable which does not hinder the grain growth during recrystallization. For this inclusion modification effect to be noticeable, it must be added more than 0.0001%. However, if it is added in excess, it deteriorates the core loss, therefore the maximum limit is set to 0.003 %.
[Mn+Cu]/[S] ≥ 200 and Copper (0.005 -0.10 wt %) – Copper is added to improve the yield strength and oxidation resistance of steel. It also combines with Sulphur along with Manganese to form coarse sulphide inclusion. This prevents formation of fine sulphide inclusion which can act as a grain growth pinning agent during recrystallization annealing. Therefore stronger sulphide precipitate forming elements are required to be added in steel, like Mn, Ca etc. Maintaining [Mn+Cu] / [S] > 200 ensures Sulphur content is controlled in heat to less than 80ppm and enough Manganese and Copper is present in steel to combine with free Sulphur. Higher limit of this ratio is not particularly limited but minimum 200 need to be assured. Copper more than 0.1% is not considered significant because it will incur additional alloying cost therefore the maximum limit is set to 0.1% and minimum copper content is set to 0.005 %, more preferably 0.01%.
Grain size 50 - 200µm – Fully recrystallized grains are preferred after final annealing for lower core losses. When the average grain size is less than 50µm the reduction in core losses are not as per the target of the present invention. Therefore the average recrystallized grain size is set as more than 50µm. The maximum limit of average grain size is set as 200µm as the core losses start to increase rapidly due to reduction in eddy current losses at higher frequencies. Grain size more than 200µm also deteriorated the required yield strength of the sheets. The grain size evolution during final annealing is depending on the annealing temperature and soaking time for a particular sheet thickness. Therefore the relationship is established for optimum range of grain size with respect to annealing temperature and sheet thickness which is shown as
0.39×T_f-〖2.5×√T〗_f×t-400 ≤d ≤0.39×T_f-〖2.5×√T〗_f×t-330
Hereinafter, the manufacturing method of the non-oriented electrical steel sheet of this invention is demonstrated in detail. Following abbreviations, terminologies and expressions are used to describe the manner of implementation of the present invention:
ACL – Annealing and coating Line
SRT – Slab Reheating Temperature
RMX – Roughing Mill Exit Temperature
FT – Finishing Rolling Exit Temperature
CT – Coiling Temperature
1st CR% – First cold rolling reduction
2nd CR% – Second cold rolling reduction
HR thk – Hot rolled coil thickness
Final thk – Final thickness after second cold reduction
W15/50- Watt loss in W/kg at 1.5T, 50Hz
W10/400- Watt loss in W/kg at 1.0T, 400Hz
B50- Magnetic flux density in T at 5000 A/m
The method of manufacturing thin gauge cold rolled non-oriented electrical steel sheets are describes as follows.
Steel making and Casting Process: First, the primary steel is made through Basic Oxygen Converter (BOF). The molten steel is decarburized in the converter, was taken out into the ladle, and the ladle was moved to the RH type vacuum degasser. Vacuum decarburization was performed with an RH vacuum degassing apparatus and then alloying additions were made (in RH or LF ) to achieve the required chemistry ranges of the steel bath as described in the scope of the invention. Subsequently, the steel melt is cast into slabs through a continuous casting machine.
Hot Rolling Process: The continuous casted slabs having the desired chemical composition of the present invention are then hot charged into the reheating furnace of the hot strip mill. The minimum hot charging temperature is defined as 300°C. If the slabs of high silicon are allowed to cool down to room temperature, it is prone to develop micro-cracks, which during further heating in the reheating furnace, may get opened up and fracture inside the furnace. Therefore care must be taken to ensure the minimum slab charging temperature is above 300 °C. Before hot rolling the slab having the above steel composition is heated in the temperature range of 1100 to 1250 °C. For a reheating temperature of more than 1250 °C, there is a possibility of dissolution of AlN, MnS, etc. precipitates and re-precipitation in finer size distribution during the hot rolling process which deteriorates the core loss. More preferably it is maintained at 1200 °C maximum and even more preferably 1100 to 1170 °C. Below 1100 °C the hot rolling loads becomes high. The reheated slabs is then subjected to rough hot rolling to obtain a steel plate. Considering the achievement of low core loss and high magnetic flux density, the roughing mill exit temperature is kept as 1060 °C to 900 °C. Subjecting the steel plate to finishing rolling in form of hot rolled strip in tandem rolling stands with finish rolling exit temperature as 800 °C to 950 °C. The finish rolling temperature as a more preferable range is 850°C to 930°C. The finishing mill exit thickness i.e. hot coil thickness is maintained between 1.80 to 3.0 mm. In thickness less than 1.80 mm, controlling the shape or waviness of the strip becomes difficult and for thickness more than 3.0 mm the degree of cold rolling reduction increases which adversely affect the favorable texture development after final annealing which deteriorates the magnetic properties. The coiling temperature is 600°C to 700°C. Coiling temperature when exceed 700°C, results in excessive scale formation on hot-rolled coils and the pickling process becomes difficult. Coiling temperature lower than 600°C results in finer hot band grain size which is not desirable for achieving good core loss.
Cold Rolling Process: The hot-rolled steel strip coil obtained by the above hot rolling process is subjected to cold rolling after removing the scale formed on the steel sheet surface during hot rolling by pickling process. The hot rolled coil was then optionally annealed to improve the magnetic properties further. When hot-rolled sheet optional annealing is performed on the hot-rolled steel sheet, it can be pickled either before or after hot-rolled sheet annealing. The hot-rolled coils which are optionally annealed after a pickling process are subjected to annealing by soaking in temperature range of 700 °C to 1000 °C for 10 to 180 seconds in case of continuous annealing and 6 to 40 hours in case of box annealing. The other conditions of hot band annealing process is not particularly limited. If the hot coil annealing temperature is too low, then complete recrystallization adequate grain growth is not obtained and improvement of magnetic properties is not sufficient, therefore it is defined as 700 °C minimum. A higher hot coil annealing temperature increases the resultant recrystallized grains size excessively, which reduces the yield strength of the material by coarsening the average grain diameters. Therefore, the maximum temperature of hot coil annealing is set as 1000°C in the present invention. The adoption of hot coil annealing method as box annealing or continuous annealing both works well as both of the processes suffice for the improvement of magnetic properties. The hot-rolled coils after annealing are then subjected to cold rolling to achieve the final thickness. This cold rolling process to achieve the final thickness can occur once or more than once. If it is more than once then it is implied that intermediate annealing is performed in between the consecutive cold rolling processes. The intermediate annealing if required is performed by soaking in a temperature range 750°C to 1050°C for 10 to 180 seconds in case of continuous annealing and 6 to 40 hours in case of box annealing. The other conditions of intermediate annealing process is not particularly limited. The cold rolling reduction in first cold rolling to be controlled within 50 to 80% and after intermediate annealing it should be less than 80% so that sufficient amount of shear bands are formed after final reduction which will promote favorable theta fiber and Goss texture components after final annealing. Thus improving the magnetic flux density and reduction of core losses. More than 80% cold reduction in final cold rolling process tends flatten the grains along the rolling direction instead of shear banding which promotes unfavorable gamma and alpha fiber components which deteriorates the magnetic flux density and increases the core loss. Edge trimming is required to be done after intermediate annealing to remove the rough or saw edges generated during first cold rolling which may lead to initiation of crack during second cold rolling and strip breakage. This is a vital step to avoid strip breakage during cold rolling. The final cold rolled thickness is in the range of 0.10 – 0.35 mm.
Final Annealing Process: The final cold rolled sheet is then subjected to final annealing and coating process in Annealing and Coating Line (ACL). The temperature raise rate is maintained at the rate of 5-45 °C/s up to soaking temperature. The soaking temperature of final annealing is kept between 800 – 1100°C and holding time at annealing temperature is kept as 10 – 100 Seconds. If the soaking temperature is less than 800°C, then complete recrystallization do not occur and average grain size becomes less than 50 µm therefore the core losses reduction is not sufficient. Whereas when soaking temperature exceeds 1100°C, excessive grain growth take place and average grain size becomes more than 200 µm, this leads to lower yield strength and core loss increases due to increase in eddy current losses at higher frequencies. The average heating rate up to the soaking temperature is kept between 5 to 45°C per seconds. This heating rate depends on the type of furnace used and the heating method employed. In the present invention, the continuous annealing line heating method employs a combination of radiant tube heating and electrical heating element radiation heating. A higher heating rate up to the soaking temperature is preferred as it improves the favorable texture for better magnetic properties. The annealed steel sheet is first cooled slowly and then rapidly to minimize the residual thermal stresses which impair the magnetic properties in the final cold-rolled and annealed sheet. The cold-rolled annealed sheet can further optionally be coated on both surfaces with a suitable insulation coating, as per the requirement of customers, which then dried and baked at a suitable temperature to get the final desired thin gauge non-oriented electrical steel sheet. The sheets thus produced have required core loss, magnetic flux density and yield strength as specified in the present invention.
Hereinafter, the present invention will be described specifically by way of examples.
Examples
A steel melt having a chemical composition as given in Table 1 is made through steel making process from converter and further vacuum refined in RH degasser. The steel melt thus produced having chemical composition as prescribed in the present invention were then casted into slabs through a continuous casting route. The slabs thus obtained were then charged in hot strip mill reheating furnace for hot rolling operation. Care was taken during charging of slabs so that the temperature does not fall below 300°C. The slabs were heated in furnace to a temperature between 1100 – 1250°C. The slabs after heating were discharged from the furnace for hot rolling followed by a high pressure water jet descaling, the slabs were then moved though roller table to the roughing mill. Roughing reductions were given in the slab such that after last roughing reduction pass exit temperature was maintained between 900 to 1060°C. The steel plate after roughing was then sent to the finishing mill for further reduction in thickness. The finishing mill exit temperature was controlled between 800 to 950°C and the target exit thickness was maintained as 2.0 mm. After finishing rolling the coils were coiled at coiling temperature between 600 to 700°C.

The hot rolled coils thus produced were pickled with concentrations of acid HCl from 2 to 18% to remove the surface scale. Here conditions of pickling operation such as speed, temperature and other parameters are not particularly limited and known conditions of pickling to obtain a pickled surface can be used. The hot rolled pickled coils were then subjected to hot band annealing treatment. The hot band annealing soaking temperature was kept in the range of 700 °C to 1000 °C for 6 to 40 hours in of box annealing furnace. The other conditions of hot band annealing process is not particularly limited. Hot band annealing is recommended as it further improves the magnetic flux density.

The hot band annealed coils were then subjected to cold rolling. The cold rolling was done in two steps. In the first step the cold rolling, the total cold reduction was controlled between 50 to 75%. After first step cold rolling rough and saw edges developed on both edges. The coils were then subjected to intermediate annealing by soaking in a temperature range 750°C to 1050°C for 10 to 180 seconds in case of continuous annealing and 6 to 40 hours in case of box annealing. The other conditions of intermediate annealing process is not particularly limited. The sheet which developed rough/saw edges during first step cold rolling was then subjected to second step cold rolling after intermediate annealing process. During second cold rolling process the sheet breakages occurred frequently. This breakages happened due to crack initiated from rough/saw edges during rolling tension and propagated across width. The saw edges thus generated after first cold rolling is shown in fig 1. In order to overcome this frequent breakages, the intermediate annealed coils were subjected to edge trimming to remove the rough/saw edges. Then the coils were sent for second cold rolling reduction, the cold rolling tensions were also reduced to prevent excessive stress on the edges. After adopting this technique the strip breakage did not occur and final cold rolled sheet were successfully rolled in thickness of 0.15 to 0.35 mm which is listed in table 2. The cold rolling reduction in second step was restricted to less than 80% to develop sufficient shear bands in the microstructure. The grains with favorable theta or Goss texture nucleate from these shear bands which improves the magnetic properties after final recrystallization annealing. A cold rolling reduction higher than 80% lead to reduction in proportion of favorable theta and Goss texture and unfavorable gamma texture proportion increases. The cold rolled sheet thus obtained after second step cold rolling were then subjected to final recrystallization annealing and coating process.

The final recrystallization annealing and coating is performed in production Annealing and Coating Line (ACL). The conditions of final annealing parameters are presented in table 2. The annealed sheet is then coated on both surfaces with a suitable coating to enhance the surface insulation resistivity. After this process a sample is drawn to evaluate the electromagnetic and mechanical properties. Microstructural characterization were done on the sample to estimate the corresponding average grain size. The microstructure of inventive steel thus obtained as well as of comparative example is shown in fig 2. The images are not true to scale and only used for representative comparison purpose. These properties results are listed in table 2.
The process flow of inventive steel example is shown in fig 3. In the above process flow, the edge trimming is a vital step to avoid strip breakage during second cold rolling process. As it is evident from figure 1 that during second cold rolling the edge cracks generated from saw edges and eventually lead to strip breakage. If the edge trimming operation is performed after intermediate annealing which was done after first cold rolling then edge crack did not initiate and successful second cold rolling is done.

Table 1: Steel chemical composition (Wt%)
No C Mn Si Al P S Cu N Ti Sb Ca [Mn+Cu]/[S] [Si] + [Al] Remarks
1 0.003 1.16 2.96 0.57 0.018 0.0025 0.007 0.0016 0.0019 0.103 0.0010 466.8 3.53 Inventive example
2 0.0042 1.19 3.01 0.57 0.018 0.0024 0.007 0.0017 0.0017 0.104 0.0012 498.8 3.58 Inventive example
3 0.0043 1.28 2.98 0.55 0.018 0.00235 0.007 0.0015 0.0018 0.112 0.0014 547.7 3.53 Inventive example
4 0.0042 1.25 2.99 0.55 0.018 0.0024 0.008 0.0017 0.0016 0.106 0.0013 524.2 3.54 Inventive example
5 0.0044 1.15 3.18 0.65 0.017 0.0028 0.007 0.0013 0.0014 0.145 0.0011 413.2 3.83 Inventive example
6 0.0043 1.24 3.18 0.64 0.016 0.0027 0.007 0.0015 0.0012 0.142 0.0011 461.6 3.82 Inventive example
7 0.0043 1.17 3.21 0.65 0.017 0.0028 0.007 0.0016 0.0013 0.146 0.0012 420.4 3.86 Inventive example
8 0.0038 1.12 2.93 0.56 0.017 0.0032 0.007 0.0018 0.0014 0.110 0.0014 352.2 3.49 Inventive example
9 0.0025 0.35 1.74 0.29 0.041 0.0043 0.006 0.0017 0.0013 --- 0.0001 81.9 2.04 Comparative example
10 0.0017 0.25 2.244 0.37 0.012 0.0048 0.006 0.0031 0.0009 --- 0.0001 53.6 2.62 Comparative example
Balance Fe and other unavoidable impurities

Table 2: Electromagnetic and mechanical properties
No HR Thk (mm) 1st CR % Edge Trimming after Intermediate annealing 2nd CR % Strip Breakage Final Thk (mm) Final Annealing temperature Tf(°C) W15/50 (W/kg) W10/400 (W/kg) B50 (T) YS (MPa) Avg. Grain size ‘d’ (µm) Remarks
1a 2.0 65 Y 64.3 N 0.25 1033 1.93 12.05 1.66 422 122.0 Inventive example
1b 2.0 60 Y 68.8 N 0.25 1043 1.93 11.96 1.648 430 133.0 Inventive example
1c 2.0 65 N --- Y Strip broke during second cold rolling Comparative example
2a 2.0 65 Y 64.3 N 0.25 1035 1.95 12.27 1.65 417 129.0 Inventive example
2b 2.0 60 Y 68.8 N 0.25 1046 1.96 11.98 1.653 429 130.0 Inventive example
3a 2.0 65 Y 64.3 N 0.25 1035 1.94 12.29 1.66 418 133.0 Inventive example
3b 2.0 60 Y 68.8 N 0.25 1046 1.95 12.21 1.653 433 128.0 Inventive example
4 2.0 65 Y 64.3 N 0.25 1035 1.93 12.06 1.66 412 138.0 Inventive example
5a 2.0 55 Y 72.2 N 0.25 1046 1.90 11.84 1.635 460 118.0 Inventive example
5b 2.0 55 N --- Y Strip broke during second cold rolling Comparative example
6 2.0 55 Y 72.2 N 0.25 1046 1.94 12.09 1.629 454 125.0 Inventive example
7 2.0 55 Y 72.2 N 0.25 1046 1.91 11.87 1.633 447 128.0 Inventive example
8 2.0 60 Y 68.8 N 0.25 1050 2.06 12.67 1.66 426 130.0 Inventive example
9 2.0 82.5 N *--- N 0.35 988 2.90 21.81 1.70 267 79.0 Comparative example
10 2.0 82.5 N *--- N 0.35 1040 2.71 20.62 1.68 285 88.4 Comparative example
* Final thickness achieved in 1st cold rolling.

The watt loss at 1.5T, 50Hz and 1.0T, 400Hz are within the prescribed range for the inventive examples mentioned above. Whereas steel samples other than invention examples where at least one of the elements of the present invention scope does not comply and does not meet at least one of the end product quality attributes. For example, steel no. 9 and 10 does not meet the prescribed Al content, Si + Al content together and [Mn + Cu]/[S] is also lower than 200. Therefore, for comparative example number 9 and 10 both watt loss and Yield strength values are not achieved. For comparative examples 1C and 5b where edge trimming after first intermediate annealing were not done resulted in strip breakages which were not processed further through final annealing process. The inventive example sheets were edge trimmed after intermediate annealing to remove the saw or rough edges formed during 1st cold rolling reduction therefore processed successfully through final annealing process. The average grain size is within the prescribed range. The final annealed and coated sheets thus produced have yield strength at room temperature more the 400 MPa which is suitable for making core lamination of motors, generators and drive motors of automobiles in Electric Vehicles (EVs) or Hybrid Electric Vehicles (HEVs) which is easy to produce and strip breakage during cold rolling is avoided.
, Claims:We Claim
A cold-rolled non-oriented thin gauge electrical steel sheet comprising of steel composition by weight % comprising:
C: 0.0005 to 0.01 %;
Mn: 0.2 to 2.0 %;
Si: 2.0 to 4.0%;
Al: 0.5 to 1.2%;
P: 0.001 to 0.20 %;
S: 0.0005 to 0.008 %;
Cu: 0.0005 to 0.10 %;
N: 0.0005 to 0.007 %;
Ti: 0.0005 to 0.003%; and the balance are Fe and other unavoidable impurities and wherein weight % [Mn+Cu]/[S] ≥ 200 and [Si] + [Al] is maintained between ≥ 3.0% and ≤ 5.0%; for desired core loss and magnetic flux density at thickness ≤ 0.35 mm and with yield strength of more than 400 MPa .
The cold-rolled non-oriented thin gauge electrical steel sheet asclaimed in claim 1, wherein the steel composition includes elements in wt.%. Sb: 0.005 to 0.20%; Ca: 0.0001 % to 0.003 %; or a combination thereof.
The cold-rolled non-oriented thin gauge electrical steel sheet as claimed in anyone of claims 1 or 2 has an average grain size “d” of 50 – 200 micrometers in final annealed condition which comply the relation as
0.39×T_f-〖2.5×√T〗_f×t-400 ≤d ≤0.39×T_f-〖2.5×√T〗_f×t-330
where Tf is the final annealing temperature in Kelvin, t is the final sheet thickness in mm and d is the average grain size in µm.
The cold-rolled non-oriented thin gauge electrical steel sheet as claimed in anyone of claims 1 to 3, comprising a final thickness in the range of 0.10 to 0.35 mm having Watt loss at 50Hz and 1.5T, W15/50 ≤ 3.5 Watts/kg and Watt loss at 400Hz and 1.0T, W10/400 is ≤ 20 Watts/kg and magnetic flux density at 5000 A/m, B50 is in the range of 1.60 to 1.73 T and yield strength of 400 MPa minimum.

A method of manufacturing cold-rolled non-oriented thin gauge electrical steel sheet as claimed in anyone of claims 1 to 4 comprising

steel making through BOF and RH vacuum degassing route followed by continuous casting the steel to a slab;
reheating the slab with hot charging temperature of at least 300 °C to a temperature in the range of 1100 to 1250 °C; and
subjectingreheated slab to a step of rough rolling whereinend of rough rolling the slab has temperature in the range 1060 to 900 °C;
subjecting the rough rolled steel to a step of finish rolling to achieve a final temperature of 800 to 950 °C; and
coiling the hot finish rolled sheet at a temperature range of 600 to 700 °C and cooled to room temperature.
The method as claimed in claim 5, wherein hot rolled coil is pickled with 2 to 18% hydrochloric acid to remove the scales and thereafter the pickled steel hot band is optionally subjected to annealing of the hot-rolled pickled coil by soaking at a temperature in the range of 700 °C to 1000 °C for 10 to 180 seconds in the case of continuous annealing or 6 to 40 hours in the case of box annealing.
The method as claimed in anyone of claims 5 or 6 wherein the steel sheet thus obtained is cold rolled once or twice with intermediate annealing in between, and the cold-rolled steel sheet is then subjected to final annealing;said intermediate annealing when applicable is performed by soaking in a temperature range 750°C to 1050°C for 10 to 180 seconds in case of continuous annealing and 6 to 40 hours in case of box annealing.
The method as claimed anyone of claims 5 to 7 wherein cold rolling reduction in first cold rolling is controlled within 50 to 80% and after intermediate annealing the reduction should be less than 80%.
The method as claimed in anyone of claims 5 to 8 wherein said steel sheet after first cold rolling and intermediate annealing is subject to edge trimming as a vital step to avoid strip breakage during next cold rolling.
The method as claimed in anyone of claims 5 to 9 wherein steel sheet thus obtained is subject to final annealing where the temperature raise rate is maintained at the rate of 5 to 45 °C/s up to a soaking temperature of 800 °C to 1100 °C and the residence time at soaking temperature is ranging from 10 to 100 seconds to thereby produce the annealed coils of said cold-rolled non-oriented thin gauge electrical steel sheet of the thickness in the range 0.10 to 0.35 mm.
The method as claimed in anyone of claims 5 to 10 wherein the steel sheet after final annealing is coated with a suitable coating on both top and bottom surface to enhance the surface insulation resistivity.

Dated this the 22nd day of January, 2025
Anjan Sen
Of Anjan Sen & Associates
(Applicant’s Agent)
IN/PA-199