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 ELECTRICAL STEEL SHEET HAVING IMPROVED SURFACE QUALITY 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 cold-rolled non-oriented electrical steel sheet having excellent watt loss and surface quality, and a method of manufacturing the same. More particularly, the present invention relates to an electrical steel sheet having desired suppressed surface defects including holes and residual scales therein, that is, excellent in surface properties and suitable for applications requiring high surface properties.

BACKGROUND OF THE INVENTION

There is an increased demand for cold-rolled non-oriented electrical steel with excellent magnetic properties along with thickness reduction to produce more efficient electric motors or generators. In cold-rolled non-oriented electrical steel, core loss or watt loss and magnetic flux density are the main parameters. Core loss is energy lost in the 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 desired.

Slitting, punching and shearing are performed on the cold-rolled non-oriented electrical steel. Therefore, when the surface properties are poor, not only does the yield decrease as with ordinary steel sheets, but also roll damage when coating the insulating film with a roll coater. This causes problems such as knife blade spillage during slitting and punch and die damage during punching. The most common defects encountered in non-oriented electrical steel are the generation holes and scabs in the final cold-rolled annealed sheet. This defect gets generated due to the formation of a sticky scale of iron-silicon oxides (Fayalite, Fe2SiO4) by oxidation of the surface layer of a slab in a reheating furnace. This oxidation product is in the liquid phase at around 1170 °C and penetrates inside the FeO layer along the grain boundaries. This has an anchoring effect, and Fe2SiO4 adheres to the scale metal interface. Due to this, it doesn’t get removed completely in subsequent descaling in the hot rolling process. This unremoved scale eventually gets embedded in the hot-rolled sheet and during subsequent cold rolling, it causes discontinuous holes or scabs. This defect impairs the quality of final annealed sheets and renders them 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 demand that this defect should not be allowed in the sheet.

As a surface defect frequently occurs in a non-oriented electrical steel sheet, efforts have been made to reduce these surface defects by either changing the composition or the method of manufacturing. For instance, Japanese Patent Application 10147849 describes a composition to obtain a non-oriented silicon steel sheet small in scale defects caused by hot rolling and excellent in surface properties by specifying the composition composed of C, Si, Al, S, Mn and Fe and regulating MnO/SiO2 as oxides in the steel.

The present inventors had conducted extensive research on the effects of chemical composition to improve the surface properties of the cold-rolled non-oriented electrical steel. Many alloying elements have been studied to improve the surface defects in the cold-rolled non-oriented electrical steel. Cu was identified to improve the surface properties in the old-rolled non-oriented electrical steel.

In prior arts, for instance, Japanese Patent Application 200484053, Japanese Patent 4349340 and Japanese Patent Application 2010229519, it has been shown that Cu is included in a steel melt to increase the tensile strength of steel sheet. The percentage of Cu added is very high, which is above 1%. There is a problem that high Cu-containing electrical steel is prone to develop surface craks during reheating of the slabs. If proper heating rate is not maintained then it may lead to slab fracture inside reheating furnace or during roughing rolling due to embrittlement effect of molten Cu. This may lead to serious breakdown in process of hot rolling. The surface defects peculiar to Cu-containing steel also occurs, and the yield of the product may be reduced.

The inventions described in the above prior arts solve surface defects peculiar to Cu-containing non-oriented electrical steel. The inventions described in the above prior arts solve the space factor-related surface properties but are silent regarding the surface defects like holes, residual scale etc. on the sheet. In the inventions, the above problem is not solved.

Therefore, the present invention is made in view of the said problem. The present invention discloses a chemical composition where a low weight percentage of Cu is used to industrially produce a cold-rolled non-oriented electrical steel sheet having excellent magnetic properties with improved surface quality, which is suitable for use in making laminated cores of electrical equipment, for example, motors or generators. There is no additional process step required to improve the mechanical properties and alloying with a low percentage of Cu will incur reduced cost of the steel. By adding a small quantity of Cu and controlling the ratio of [Cu]/[S], non-oriented electrical steel sheets having excellent surface properties and magnetic properties can be obtained.

OBJECTS OF THE INVENTION

The basic object of the present invention is to solve the above problems of prior arts by providing a cold-rolled non-oriented electrical steel sheet having low watt loss, good magnetic flux density, suppresses surface defects, and increases the yield of the product.

A further object of the present invention is to provide a cold-rolled non-oriented electrical steel sheet having excellent magnetic properties with improved surface quality and a method of manufacturing the same.

It is yet another object of the present invention to manufacture a cold-rolled non-oriented electrical steel with improved surface quality and excellent magnetic properties that can be accomplished which has the advantage of easy operation, lower cost, and better productivity by controlling the chemistry of steel melt.

A further object is directed to conduct intensive research to suppress surface defects of non oriented electrical steel sheet, and resultantly the present inventors also have obtained excellent magnetic properties by maintaining a low amount of Cu and controlling the ratio of Cu/S between 2.5 to 8.0.

SUMMARY OF THE INVENTION
The basic aspect of the present invention is directed to provide a cold-rolled non-oriented electrical steel sheet having desired suppressed surface defects including holes and residual scales therein, comprises the following chemical components by weight %: C: 0.0005 - 0.005 %; Mn: 0.2 - 2.0 %; Si: 1.5 – 3.5%; Al: 0.15 - 1.2%; P: 0.001 - 0.20 %; S: 0.0005 - 0.008 %; N: 0.0005 - 0.007 %; Cu: 0.0005 – 0.05%; Ti: 0.0005 - 0.003%; and the balance are Fe and other unavoidable impurities and wherein weight % [Cu] / [S] ratio is in a range of 2.5 to 8.

In one aspect of the present invention, the cold-rolled non-oriented electrical steel sheet composition optionally comprises by wt % Sb: 0.005 - 0.20%; Ca: 0.0001 % to 0.003 %; and B: 0.0002 to 0.003 % or a combination thereof.

In another aspect of the present invention, the cold-rolled non-oriented electrical steel sheet having a thickness in the range of 0.15 – 0.70 mm; watt loss at 50Hz and 1.5T, W15/50 is in the range of 2.0 to 5.3 Watts/Kg for thickness 0.35 to 0.70 mm; watt loss at 400Hz and 1.0T, W10/400 is in the range 11 – 20 W/kg for thickness less than 0.35 mm; and magnetic flux density at 5000 A/m, B50 is in the range of 1.62 to 1.75 T for thickness 0.15 – 0.70 mm.

In another aspect of the present invention, cold-rolled non-oriented electrical steel sheet is free of holes on the surface of the sheet.

A further aspect of the present invention is directed to provide a method to manufacture the cold-rolled non-oriented electrical steel sheet having desired suppressed surface defects including holes and residual scales therein comprising the following steps. Step 1 includes
subjecting to continuous casting steel composition including following chemical components and weight %: C: 0.0005 - 0.005 %; Mn: 0.2 - 2.0 %;Si: 1.5 – 3.5%;Al: 0.15 - 1.2%;P: 0.001 - 0.20 %;S: 0.0005 - 0.008 %;N: 0.0005 - 0.007 %;Cu: 0.0005 – 0.05%;Ti: 0.0005 - 0.003%; and the balance are Fe and other unavoidable impurities and wherein weight % [Cu] / [S] ratio is in a range of 2.5 to 8 and obtaining therefrom continuous casted slabs;
Step 2 includes the continuous casting slab entering the slab reheating furnace for heating and heat preservation wherein the slab hot charging temperature is 300 °C or more, then reheating said slab to a reheating temperature in the range from 1100 °C to1250°C. Step 3 includes, roughing rolling the reheated slab wherein the roughing rolling temperature is in the range of 1060 °C to 900°C to form a rough rolled substrate. Step 4 includes hot rolling the rough rolled substrate wherein the finishing rolling temperature is in the range of 850 °C to 950 °C to form a hot rolled substrate. Step 5 includes coiling the hot-rolled substrate wherein the coiling temperature is in the range of 600 °C to 700 °C to form a hot-rolled coil. Step 6 includes pickling the hot-rolled coil in a continuous line with concentrations of acid HCl from 2 to 18% to remove the surface scale to form hot-rolled pickled coil. Optionally, hot band annealing of hot-rolled pickled coil carried out at a temperature in the range of 600 °C to 750 °C. Step 7 includes a cold rolling of the hot- rolled pickled coil or the pickled hot-rolled annealed coil to form a cold-rolled coil. Step 8 includes subjecting the cold-rolled coil to final annealing. The temperature rise is maintained at the rate of 5-45 °C/s up to soaking temperature. The soaking temperature is 800 °C to 1100 °C with residence time ranging from 10 to 100 seconds. Optionally, subjecting the annealed coil to a suitable insulation coating.

In another aspect of the present invention, the method to manufacturing the cold-rolled non-oriented electrical steel sheet comprising cold rolling performed either once or twice with intermediate annealing in between and the cold-rolled steel sheet is then subjected to final annealing.

DETAILED DESCRIPTION OF THE INVENTION

The present invention discloses a cold-rolled non-oriented electrical steel sheet with improved surface quality, comprises the following chemical components by weight %:C: 0.0005 - 0.005 %;Mn: 0.2 - 2.0 %;Si: 1.5 - 3.5%;Al: 0.15 - 1.2%;P: 0.001 - 0.20 %;S: 0.0005 - 0.008 %;N: 0.0005 - 0.007 %; Cu: 0.0005 - 0.05%;Ti: 0.0005 - 0.003%; and the balance are Fe and other unavoidable impurities and satisfies the following relation: weight % [Cu] / [S] ratio is in a range of 2.5 to 8.

According to the present invention, a cold-rolled non-oriented electrical steel sheet having good magnetic properties and surface properties can be obtained by appropriately controlling the steel composition.

By using such a non-oriented electrical steel sheet according to the present invention, it is possible to provide a non-oriented electrical steel sheet suitable for a motor laminations that can be used stably without deformation or breakage during punching operation. The soft magnetic laminated cores thus made using such a non-oriented electrical steel will result in improved efficiency and durable service life of a motor or generator. Such energy-saving effects can contribute to the creation of a future society with lesser impact on global greenhouse emission.

Hereinafter, the steel composition is described hereunder with an 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 % are demonstrated.

Carbon (0.0005 – 0.005 wt.%) - Carbon is a harmful element for watt loss which forms carbide precipitate in a ferrite matrix. If the C content exceeds, carbides such as cementite and e-carbide precipitate, and due to this the magnetic property deterioration may become remarkable. It also causes magnetic ageing. To further improve the magnetic properties, particularly to improve the watt loss, the upper limit of the C content is preferably 0.005%. The lower the carbon the better, however, it is extremely difficult to reduce the carbon to less than 0.0005% in industrial processes. Therefore, the C content is set to be 0.0005 – 0.005%, more preferably 0.0005 – 0.003%.

Manganese (0.2 – 2.0 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 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. Therefore, the present invention range is fixed at 2.0%. More preferably it is 1.0 % maximum and even more preferably it is 0.80 % maximum.

Silicon (1.5 – 3.5 wt.%) – Silicon is the most important alloying element for electrical steel. It increases the specific resistivity of steel and decreases watt loss. The minimum limit specified in the present invention is set to 1.5 %, 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 3.5 %, cold rolling becomes difficult, leading to frequent strip breakages in a cold rolling mill. Also, higher silicon deteriorates the magnetic flux density, therefore the maximum limit is set to 3.5%. More preferably, Si may be contained in amounts of 1.5 to 3.0 %.

Aluminum (0.15 – 1.2 wt%) – Similar to silicon, aluminum also increases the specific resistivity of steel, thereby helping to reduce the watt losses. For this purpose, the minimum level is set to 0.15 %. However, if aluminum exceeds 1.2 %, rolling becomes difficult and magnetic flux density is adversely affected. Therefore, the maximum limit is set to 1.2 %. More preferably, Al may be contained in amounts 0.2 to 0.8%.

Phosphorous (0.001 - 0.20 %) – Phosphorus is an element that is used to optimize the mechanical strength and hardness of the cold-rolled non-oriented electrical steel sheet. It also increases the specific resistivity of the steel and improves 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 level is set to 0.20 %. More preferably it is 0.10 % or less, and even more preferably it is 0.08 % or less.

Sulphur (0.0005 - 0.008 %) – S is an unavoidable impurity and does not need to be added. 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 adversely affect the watt loss. If the S content exceeds 0.03%, coarse Mn and Cu-containing sulfides are formed, the toughness of the steel deteriorates, and there is a risk of fracture during cold rolling. Above 0.008 %, the effect becomes very prominent, therefore, the maximum limit is set to 0.008%. The minimum level specified here is 0.0005% because a lower amount of sulphur than 0.0005 is extremely difficult to produce in industrial processes.

Nitrogen (0.0005 - 0.007 %) – Nitrogen forms fine metal nitride precipitates with Ti, Al, Fe, etc. These fine precipitates restrict the grain growth by pinning the grain during annealing which adversely affects the watt loss, therefore the maximum limit is set to 0.007%. The minimum level specified here is 0.0005% because a lower amount of Nitrogen than 0.0005% is extremely difficult to produce in industrial processes.

Titanium (0.0005 - 0.003%) – Titanium is a strong nitride and carbide forming element. The fine precipitates of titanium restrict the grain growth by pinning the grain during annealing which adversely affects the watt loss, therefore the maximum limit is set to 0.003%. The minimum level specified here is 0.0005% because in the industrial process achieving less than 0.0005% Ti is extremely difficult.

[Cu]/[S] 2.5 to 8.0 and Copper (0.0005 - 0.05 wt%) – Cu is an essential element in the present invention.Copper is added to improve the oxidation resistance of steel. Copper preferentially segregates to the grain boundaries and scale metal interface. This enrichment of Copper decreases the adhesion effect of the Fe2SiO4 layer. When the adhesion effect of scale to metal interface is decreased, it becomes easier to remove it under descaling condition. The descaling method employed here is high-pressure water from descaling nozzles. This high-pressure water impingement on the scale surface breaks the layer and oxide scale gets removed which is also aided by decreased adhesion of scale to metal interface. For this decrease in scale to metal surface adhesion effect to become prominent, Cu should be in elemental form. Copper is prone to form CuS precipitate, which consumes the elemental copper present in the steel. Therefore stronger sulphide precipitate forming elements are required to be added in steel, like Mn, Ca etc. Maintaining [Cu] / [S] minimum 2.5 ensures enough elemental copper is present in steel to provide the decreased adhesion effect of oxide scale and metal interface. [Cu] / [S] more than 8 is not considered significant because it will incur additional alloying cost therefore the maximum ratio is set to 8 and copper content is restricted to 0.05 %. However, how this decrease in scale to metal adhesion effect diminished above [Cu] / [S] more than 8, is not clear, but this phenomenon is observed during production trials.

Antimony (0.005 - 0.2 wt%) – Antimony addition helps to improve the final recrystallization texture of cold-rolled non-oriented electrical steels. It preferentially segregates to the grain boundaries and promotes the cube fiber and goss texture which is beneficial for the magnetic properties and suppresses the formation of gamma fiber which is harmful to the magnetic properties. This improves the magnetic flux density. However, an amount above 0.2 %, the effect of improvement is saturated and also it deteriorates the ductility 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 – 0.15 %. Antimony also improves the oxidation resistance of steel.

Calcium (0.0001 – 0.003 wt% ) – Calcium is added to steel for inclusion modification. It makes the sulphide precipitate coarse and non-deformable stable sulphide 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 watt loss, therefore the maximum limit is set to 0.003 %.

Boron (0.0002 – 0.003 wt%) – B is an optional additive element and is not an essential element in the present invention. However, the Boron fixes the nitrogen in preference to aluminum by forming Boron Nitride (BN). Thus, it suppresses the formation of harmful fine AlN precipitate during the hot rolling of steel. When it is added in excess, it segregates to the grain boundaries and deteriorates the magnetic properties. Therefore, it is to be contained in the range of 0.0005 to 0.003 %.

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 Temperature
CT- Coiling Temperature
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 the cold rolled non-oriented electrical steel is described as following.

1. Casting Process: First, a steel melts is processed 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 to achieve the chemistry ranges of the molten steel as described in the scope of the invention. Subsequently, the steel melt is cast into slabs through a continuous casting machine.

2. Hot Rolling Process: The casted slabs having the 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 during the hot rolling process which deteriorates the watt loss. More preferably it is maintained at 1200 °C maximum and even more preferably 1100 to 1170 °C. Below 1100 °C the rolling loads are higher. Then the slab is subjected to rough hot rolling to obtain a steel sheet. Considering the achievement of good watt loss along with magnetic flux density, the roughing mill exit temperature is kept as 1060 °C to 900 °C. Subjecting the steel sheet to finishing rolling temperature 850 °C to 950 °C. The finish rolling temperature as a more preferable range is 925°C to 870°C. The coiling temperature is 600°C to 700°C. Coiling temperature of more than 700°C results in excessive scale formation on hot-rolled coils and the pickling process becomes difficult lower than 600°C results in finer hot band grain size which is not desirable for achieving good watt loss.

3. Cold rolling process:The hot-rolled steel sheet obtained by the above hot rolling process is usually subjected to cold rolling after removing the scale formed on the steel sheet surface during hot rolling by pickling. 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 in temperature range of 600 °C to 750 °C. If the hot coil annealing temperature is too low, then complete recrystallization is not obtained and improvement of magnetic properties is not sufficient, therefore it is defined as 600 °C minimum. A higher hot coil annealing temperature reduces the yield strength of the material by coarsening the average grain diameters. This adversely affects the burr height formation during the punching of laminations during core manufacturing. Therefore, the maximum temperature of hot coil annealing is set as 750°C in the present invention. The soaking time is not particularly limited, it depends on the thickness of hot-rolled coils and the method of annealing, either in a batch furnace or in a continuous furnace. Both processes suffice for the improvement of magnetic properties. The hot-rolled coils after annealing are then cold rolled 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 final cold rolled thickness is in the range of 0.15 to 0.70 mm.

4. Finish annealing process: The cold-rolled full hard steel sheets are then finally annealed in a continuous annealing line in a temperature ranging from 800°C to 1100°C for a time between 10 seconds to 100 seconds. This annealing temperature and time is depending on the thickness, Silicon content, and watt loss requirement of the final product. The heating rate of the steel sheet up to the soaking temperature is from 5°C to 45°C per second. This heating rate depends on the type of furnace 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 texture favorable 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 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 an automated camera setup is utilized to capture any surface defect present in the steel sheet.

The properties of the cold rolled non-oriented electrical steel is described as following:

1. Thickness: In order to reduce the iron loss under high frequency conditions, the thinner the plate thickness, the better. Therefore, the plate thickness is set to 0.15 to 0.70 mm. On the other hand, excessively thinning of cold rolled sheets significantly reduces productivity, so the plate thickness is set to 0.15 mm or more.
2. Surface Defects: Since the non-oriented electrical steel sheet according to the present invention is constituted as described above, the surface defects like scabs or holes are eliminated, and therefore it is suitable for use in applications requiring good surface properties.
3. Magnetic Properties measured as per BIS 649: The watt loss at 50Hz and 1.5T, W15/50 is in the range of 2.0 to 5.3 Watts/Kg for thickness 0.35 to 0.70 mm. The watt loss at 400Hz and 1.0T, W10/400 is in the range 11 – 20 W/kg for thickness less than 0.35 mm. The magnetic flux density at 5000 A/m, B50 is in the range of 1.62 to 1.75 T for thickness 0.15 – 0.70 mm.

Hereinafter, the present invention will be described specifically by way of examples.

Examples

A slab having a steel composition shown in Table 1 below was continuous casted in 220 mm thickness. The slabs are then hot charged into slab reheating furnace, maintaining the slab charging temperature above 300°C. The slabs after reheating are rough rolled in hot strip mill keeping the roughing mill exit temperature between 900 – 1060°C and further rolled in the finishing mill in thickness 2.0 – 3.0 mm keeping the finishing temperature 850 – 950 °C. The hot rolled coils are then optionally annealed after picking. Then it is cold rolled into a cold- rolled steel sheet having a thickness of 0.15 mm to 0.70 mm. The cold rolled sheets are then final annealed and optionally coated with a suitable insulation coating. The final annealing condition has no influence on generation of holes defect in annealed sheet. This treatment is given to develop the desired magnetic properties in final annealed condition. The surface of the steel sheet was observed using an automated camera system to check for surface related defects such as holes and other defects. The magnetic properties include the watt loss at 50Hz and 1.5T (W15/50) for thickness 0.35 – 0.70 mm, & watt loss at 400 Hz and 1.0 T (W10/400) for thickness less than 0.35mm and magnetic flux density at 5000 A/m (B50) was also measured. Table 1 below shows the evaluation results of the surface properties and magnetic properties.

Table 1: Chemical Compositions in weight %, surface and magnetic properties.

Table 1

Sample No. Chemical Composition in Wt % Properties
C Mn S P Si Al N Ti Cu [Cu]/[S] Other Element Thickness (mm) W15/50 (W/Kg) W10/400 (W/Kg B50 (T) Surface defect, holes remark (Y/N)
1 0.0016 0.28 0.0043 0.011 2.63 0.55 0.0021 0.0009 0.012 2.8 B-0.0004 0.20 2.33 14.10 1.62 N
2 0.0019
0.30 0.0036 0.011 2.75 0.54 0.0027 0.0011 0.009 2.5 B-0.0004 0.25 2.36 15.75 1.64 N
3 0.0015 0.35 0.0043 0.014 2.62 0.54 0.0015 0.0009 0.013 3.0 B-0.0005 0.30 2.38 16.40 1.66 N
4 0.0021 0.27 0.0031 0.018 2.70 0.52 0.0019 0.0012 0.010 3.2 B-0.0004 0.35 2.39 NA 1.66 N
5 0.0036 0.36 0.0045 0.035 1.61 0.29 0.0017 0.0008 0.020 4.4 B-0.0002 0.50 5.25 NA 1.68 N
6 0.0028 0.34 0.0039 0.031 1.61 0.28 0.0022 0.0017 0.010 2.6 B-0.0004 0.50 3.42 NA 1.72 N
7 0.0033 0.31 0.0040 0.043 1.83 0.32 0.0023 0.0013 0.030 7.5 Sb-0.010
B-0.0004 0.50 3.40 NA 1.73 N
8 0.0016 0.33 0.0022 0.018 2.65 0.52 0.0024 0.0011 0.010 4.7 B-0.0003 0.50 2.78 NA 1.67 N
9 0.0016 0.33 0.0022 0.018 2.65 0.52 0.0024 0.0011 0.010 4.7 B-0.0003 0.65 3.23 NA 1.68 N
10 0.0022 0.37 0.0031 0.040 1.78 0.29 0.0015 0.0012 0.007 2.4 B-0.0002 0.35 2.80 NA 1.69 Y
11 0.0025 0.41 0.0014 0.042 1.73 0.27 0.0013 0.0011 0.013 9.1 B-0.0003 0.50 3.38 NA 1.72 Y
12 0.0023 0.37 0.0047 0.017 2.62 0.52 0.0026 0.0012 0.009 1.9 B-0.0003 0.50 2.85 NA 1.66 Y
13 0.0023 0.26 0.0031 0.012 2.64 0.55 0.0017 0.0012 0.007 2.3 B-0.0004 0.65 3.39 NA 1.68 Y

*Underline boxes indicates “outside the appropriate range”

The watt loss values are within the prescribed range of 2.0 to 5.3 W/Kg at 1.5T, 50 Hz for sheets with thickness 0.35 to 0.70 mm & 11 to 20 W/kg at 1.0T, 400Hz for sheet less than 0.35 mm and magnetic flux density from 1.62 to 1.75 T at 5000 A/m for 0.15 – 0.70 mm thickness. 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, samples 8 to 11 do not meet the prescribed [Cu]/[S] range of 2.5 – 8.0 and result in having surface defect holes present in the final annealed condition.

The sample sheet numbers 1 to 3 are in range 0.15 to 0.30 mm final thickness, 4 and 10 are in 0.35 mm final thickness. Whereas sample sheet numbers 5 to 7 and 11 to 12 are in 0.50 mm final thickness.

The magnetic properties achieved and surface defect observation remarks for different samples from 1 to 9 are mentioned in table 1, which meets the prescribed range as mentioned in the scope of this invention. Whereas for comparative examples sample 10 to 13 which does not meet the prescribed range of weight% [Cu]/[S] between 2.5 – 8.0 as per the scope of this invention, end up having a surface defect and holes present in the final annealed condition. The magnetic properties of comparative samples are within the prescribed range, but the surface quality does not meet the requirement, therefore, this invention provides a solution to manufacture non-oriented electrical steel sheets with excellent magnetic and surface properties which is both cost-effective and easy to operate.

The steel samples numbers 9 and 13 are of 0.65 mm final CR thickness, where sample number 9 is an example of inventive steel meeting all the prescribed ranges of elements as mentioned in the scope of this invention. Whereas steel sample number 13 which does not meet the prescribed range of weight% [Cu]/[S] between 2.5 – 8.0 as per the scope of this invention, end up having a surface defect and holes present in the final annealed condition. The lower thicknesses like 0.50 mm or less are more prone to hole defects due to a higher degree of cold reduction, but even in 0.65 mm thickness where [Cu]/[S] ratio is outside the scope of the present inventive range, end up having a surface defect. The magnetic properties of comparative samples are within the prescribed range, but the surface quality does not meet the requirement, therefore, this substantiates that the present invention provides the solution of manufacturing cold-rolled non-oriented electrical steel sheets with improved resistance to the surface defect which is both cost effective and easy to operate.

Therefore, the present advancement favors the production of cold-rolled non-oriented electrical steel sheets in thickness 0.15 to 0.70 mm, having the watt loss W15/50 is 2.0 - 5.3 Watts/Kg at 1.5T, 50 Hz for thickness 0.35 to 0.70 mm. For thickness less than 0.35 mm, watt loss W10/400 is 11 – 20 Watts/Kg at 1.0T, 400 Hz. Magnetic flux density, B50 ranging from 1.62 to 1.75 T at 5000 A/m for thickness 0.15 – 0.70 mm with improved resistance surface defects.
, Claims:We Claim:

1. A cold-rolled non-oriented electrical steel sheet having desired suppressed surface defects including holes and residual scales therein comprising the following chemical components by weight %:
C: 0.0005 - 0.005 %;
Mn: 0.2 - 2.0 %;
Si: 1.5 – 3.5%;
Al: 0.15 - 1.2%;
P: 0.001 - 0.20 %;
S: 0.0005 - 0.008 %;
N: 0.0005 - 0.007 %;
Cu: 0.0005 – 0.05%;
Ti: 0.0005 - 0.003%; and the balance are Fe and other unavoidable impurities and wherein weight % [Cu] / [S] ratio is in a range of 2.5 to 8.

2. The cold-rolled non-oriented electrical steel sheet as claimed in claim 1, optionally including by wt% Sb: 0.005 - 0.20%; Ca: 0.0001 % to 0.003 %; and B: 0.0002 to 0.003 % or a combination thereof.

3. The cold-rolled non-oriented electrical steel sheet as claimed in anyone of claims 1 and 2, having a thickness in the range of 0.15 – 0.70 mm;
watt loss at 50Hz and 1.5T, W15/50 is in the range of 2.0 to 5.3 Watts/Kg for thickness 0.35 to 0.70 mm;
watt loss at 400Hz and 1.0T, W10/400 is in the range 11 – 20 W/kg for thickness less than 0.35 mm; and
magnetic flux density at 5000 A/m, B50 is in the range of 1.62 to 1.75 T for thickness 0.15 – 0.70 mm.

4. The cold-rolled non-oriented electrical steel sheet as claimed in anyone of claims 1 to 3, which is free of holes on the surface of the sheet.

5. A method to manufacture the cold-rolled non-oriented electrical steel sheet having desired suppressed surface defects including holes and residual scales therein as claimed in claims 1 to 4, comprising the following steps:
subjecting to continuous casting steel composition including following chemical components and weight %:
C: 0.0005 - 0.005 %;
Mn: 0.2 - 2.0 %;
Si: 1.5 – 3.5%;
Al: 0.15 - 1.2%;
P: 0.001 - 0.20 %;
S: 0.0005 - 0.008 %;
N: 0.0005 - 0.007 %;
Cu: 0.0005 – 0.05%;
Ti: 0.0005 - 0.003%; and the balance are Fe and other unavoidable impurities and wherein weight % [Cu] / [S] ratio is in a range of 2.5 to 8 and obtaining therefrom continuous casted slabs;
the continuous casting slab enters the slab reheating furnace for heating and heat preservation, wherein the slab hot charging temperature is atleast 300 °C, then reheating said slab to a reheating temperature in the range from 1100 °C -1250 °C;
roughing rolling the reheated slab, wherein the roughing rolling temperature is in the range of 1060 °C to 900°C to form a rough rolled substrate;
hot rolling the rough rolled substrate, wherein the finishing rolling temperature is in the range of 850 °C to 950 °C to form hot rolled substrate;
coiling the hot-rolled substrate, wherein the coiling temperature is in the range of 600 °C to 700 °C to form a hot-rolled coil;
pickling the hot-rolled coil in a continuous line with concentrations of acid HCl from 2 to 18% to remove the surface scale to form hot-rolled pickled coil;
cold rolling the hot- rolled pickled coil or the pickled hot-rolled annealed coil to form a cold-rolled coil;
subjecting the cold-rolled coil to final annealing, wherein the temperature is maintained at the rate of 5-45 °C/s up to soaking temperature, soaking temperature is 800 °C to 1100 °C with residence time ranging from 10 to 100 seconds to thereby produce the annealed coils of said cold-rolled non-oriented electrical steel sheet having desired suppressed surface defects including holes and residual scales therein.

6. The method as claimed in claim 5 comprising optionally, hot band annealing of hot-rolled pickled coil at a temperature in the range of 600 °C to 750 °C; and
subjecting the annealed coil to a suitable insulation coating.

7. The method as claimed in anyone of claims 5 to 6, wherein the cold rolling is performed either once or twice with intermediate annealing in between and the cold-rolled steel sheet is then subjected to final annealing.

Dated this the 28th day of June, 2022
Anjan Sen
Of Anjan Sen & Associates
(Applicants Agent)
IN/PA-199