Title of Invention

PROCESS TECHNOLOGY FOR PRODUCTION OF SEMI PROCESSED ELECTRICAL STEEL WITH CORE LOSS VALUE OF 4.5 WATT/KG (MAX)

Abstract

This invention relates to a process for the production of semi-processed electrical steel, which comprise the following steps: (A) Preparing a steel composition in a LD converter having the following specific alloy chemistry. (a) %C: 0.040 max. (b) %Mn: 0.25-0.45 (c) %P: 0.20 max. (d) %S: 0.006 max. (e) %Si: 0.5-0.70 (f) %AI: 0.80-0.10 (B) Subjecting the above steel to Vacuum Arc Refining (VAR) and Argon rinsing so as to achieve a nitrogen level in the steel of less than 80 ppm and sulphur less than 0.006%; (C) Casting the steel of step 2 into slabs of desired shapes and sizes at a casting speed ranging between 0.7 to 0.8 meter per minute in the hot condition and; (D) Rolling the so achieved slabs into hot bands of less than 2.7 mm thickness followed by finish rolling and cooling the said bands. (E) Pickling of hat bands in HCI medium followed by cold rolling of hot bends to 0.53 ±0.1 mm thickness (F) Annealing of cold rolled coils through either batch step or as a continuous process with designed annealing parameters (G) Temper rolling of annealed coils with 2-3% reduction (H) Decarburization annealing of stamped sheets with designed annealing parameters

Full Text

FIELD OF THE INVENTION This invention relates to the process for the production of semi-processed electrical steel. This invention more particularly relates to the process for the production of semi- processed electrical steel with core loss value of 4.5 watt/kg (max.) at 1.5 Tesla magnetic induction and 50 Hz frequency. PRIOR ART AND DRAWBACKS It is already known to produce semi processed electrical steel which are a variety of electrical steel which is supplied to the customers in the form of temper rolled sheets/coils. These sheets/coils are punched into stampings of desired shape which are the subjected to decarburization annealing treatment for developing desired magnetic properties. It has, however, so far not been possible to achieve core loss of 4.5 watt/kg (max.) at 1.5 Tesla and 50 Hz after decarburization annealing. OBJECT OF THE INVENTION It is an object of the invention to propose for the production of semi-processed electrical steel. It is a further object of the invention to propose for the production of semi- processed electrical steel with core loss value of 4.5 watt/kg (max.) at 1.5 Tesla and 50 Hz. These and other objects will be apparent from the following paragraphs. BACKGROUND OF THE INVENTION In our investigations, we have found that our objectives can be achieved by producing a specific chemistry steel and subjecting to specific hot strip rolling conditions. It has also been observed that further steps include trimming, pickling, cold rolling and annealing and tempering are to be carried out with carefully controlled conditions. BRIEF STATEMENT OF THE INVENTION Thus, according to this invention, there is proposed a process for the production of semi-processed electrical steel with core loss value of 4.5 watt/kg (max.) at 1.5 Tesla and 50 Hz, characterized in that it has a chemical composition which contains in weight- C: %0.040 max., Mn: 0.25-0.45, P: 0.20 max., S: 0.006 max., Si: 0.5-0.70 and Al: 0.80- 0.10, which comprise the following steps: (A) Preparing a steel composition in a LD converter having the following specific alloy chemistry: (a) %C: 0.040 max. (b) %Mn: 0.25-0.45 (c) %P: 0.20 max. (d) %S: 0.006 max. (e) %Si: 0.5-0.70 (f) %AI: 0.08-0.10 where low silicon (0.5-0.7%) results in lower hardness in the tempered rolled coils; Sulphur and nitrogen levels maintained to minimum to achieve low core loss property; (B) Subjecting the above steel to Vacuum Arc Refining (VAR) and Argon rinsing so as to achieve a nitrogen level in the steel of less than 80 ppm and sulphur less than 0.006%; (C) Casting the steel of step 2 into slabs of desired shapes and sizes at a casting speed ranging between 0.7 to 0.8 meter per minute in the hot condition to produce defect free slabs and; (D) Rolling the so achieved slabs into hot bands of less than 2.7 mm thickness followed by finish rolling and cooling the said bands. (E) Pickling of hat bands in HCI medium followed by cold rolling of hot bends to 0.53 ±0.1 mm thickness (F) Annealing of cold rolled coils through either batch step or as a continuous process with designed annealing parameters (G) Temper rolling of annealed coils with 2-3% reduction (H) Decarburization annealing of stamped sheets with designed annealing parameters The steel composition includes low silicon (0.5-0.7%) which results in lower hardness in the tempered rolled coils. Lower hardness is beneficial in sheets because it improves the life of dies during punching of laminations from tempered rolled coils. Sulphur and nitrogen levels should be minimum to achieve low core loss property. That is why vacuum arc refining and argon rinsing were carried out to minimize the sulphur and nitrogen contents. It is preferable to keep the vacuum level in the VAR at a level of 150 mbar. Vacuum level in VAR was maintained at 150mbar to achieve sulphur level of 0.006% max. Casting speed was maintained at 0.7-0.8 meter per minute to produce defect free slabs. The casting is preferably carried out at hot tundish temperature of about 1560°C ± 10°C. Temperature of molten steel was maintained at 1560+10°C to ensure production of slabs with minimum inclusions and defects. It is ideal to hot roll slabs into hot bands of 2.5 to 2.7 mm thickness for carrying out subsequent steps efficiently. The thickness of hot bands was maintained at 2.5-2.7 mm to ensure smooth cold rolling of hot bands of such high silicon steel to 0.53 mm thickness. The finish rolling of the bands is carried out at a temperature between 860 ± 10°C. The coiling of the finished roll band is preferably carried out at temperature in the range of 720 ±10°C. Finish rolling and coiling temperatures were maintained at 860+10°C and 720+10°C respectively to ensure formation of coarse ferrite grains (average size: 15-20 µm) in hot bands. Pickling is carried out using dilute HCI acid and cold rolling is carried out to obtain cold rolled bands of 0.53 ±0.1 mm thickness. Pickling in HCI acid is carried out to ensure removal of scale from the surface of hot bands. Hot bands were cold rolled to 0.53+0.1 mm thickness to achieve desired thickness of 0.5 mm in the coils after temper rolling. It is possible to carry out the annealing either as a batch step or as a continuous process and both are successful. The batch annealing steps is carried out in a batch-annealing furnace under the following conditions: Annealing Temperature 620 ± 10°C. Soaking duration 10-12 hrs. depending upon charge weight Type of cooling Furnace cooling Similarly, the continuous annealing is carried out in a continuous annealing furnace under the following conditions: Annealing temperature 820 ± 10°C. Total residence time in annealing furnace ~ 6.0 mins. The annealed are temper rolled to achieve a cold reduction of 2 to 3%. Annealing carried out in batch annealing furnace / continuous annealing furnace with designed annealing parameters to evolve strain free fresh ferrite grains of -12 µm size . Also, the annealing parameters were designed to achieve a hardness of 50-55 HRB after annealing. Temper rolling with 2-3% cold reduction helped in storing sufficient energy in the coils to achieve abnormal grain growth after decarburization annealing. It is important to note that the decarburization annealing practice should be as follows. Annealing Temperature 800°C. Total annealing duration 180 minutes Soaking time 60 minutes Furnace atmosphere Decarburizing to ensure reduction in % C to 0.0002-0.004 Decarburization annealing with designed parameters helped in removal of carbon content to 0.003% level and to achieve abnormal grain growth which resulted in evolution of ferrite grains with average grain size of around 100 µm. We have tested the properties of the steel produced by the above process and have found core loss of 4.5 watt/kg (max.) at 1.5 Tesla and 50 Hz. after decarburization annealing. The core loss property is evaluated with the help of a Digital Iron loss Tester at 1.5 Tesla and 50 Hz frequency. Alternatively, core loss property can also be measured with the help of Epstein Tester. In this case, large sizes of test specimens are needed. ADVANTAGE OF THE INVENTION The innovative features of this invention include the designing of special steel composition as well as process parameters to achieve core loss property of 4.5 watt/kg max at 1.5 Tesla and 50 Hz. It may be mentioned here that all the processing steps starting from hot strip rolling to decarburization annealing are very critical to achieve desired core loss property. The operating parameter for each processing step has to be carefully designed to facilitate formation of coarse ferrite grains during decarburization annealing. Because of such coarse ferrite grains, core loss value of 4.5 watt/kg could be achieved even in steel having 0.5-0.7% silicon. Different embodiments of the invention are possible to achieve the best process of performance and to obtain the product as stated above to meet the object of the invention. It will be understood that skilled persons with many modifications, variations and adaptations may carry out the invention into practice without departing from its spirit or exceeding the scope of claims in describing the invention for the purpose of illustration. WE CLAIM: 1. A process for the production of semi-processed electrical steel with core loss value of 4.5 watt/kg (max.) at 1.5 Tesla and 50 Hz, characterized in that it has a chemical composition which contains in weight- C: %0.040 max., Mn. 0.25-0.45, P: 0.20 max., S: 0.006 max., Si: 0.5-0.70 and Al: 0.80-0.10, which comprise the following steps: (A) Preparing a steel composition in a LD converter having the following specific alloy chemistry: (a) %C: 0.040 max. (b) %Mn: 0.25-0.45 (c) %P: 0.20 max. (d) %S: 0.006 max. (e) %Si: 0.5-0.70 (f) %AI: 0.08-0.10 where low silicon (0.5-0.7%) results in lower hardness in the tempered rolled coils; Sulphur and nitrogen levels maintained to minimum to achieve low core loss property; (B) Subjecting the above steel to Vacuum Arc Refining (VAR) and Argon rinsing so as to achieve a nitrogen level in the steel of less than 80 ppm and sulphur less than 0.006%; (C) Casting the steel of step 2 into slabs of desired shapes and sizes at a casting speed ranging between 0.7 to 0.8 meter per minute in the hot condition to produce defect free slabs and; (D) Rolling the so achieved slabs into hot bands of less than 2.7 mm thickness followed by finish rolling and cooling the said bands. (E) Pickling of hat bands in HCI medium followed by cold rolling of hot bends to 0.53 ± 0.1 mm thickness (F) Annealing of cold rolled coils through either batch step or as a continuous process with designed annealing parameters (G) Temper rolling of annealed coils with 2-3% reduction (H) Decarburization annealing of stamped sheets with designed annealing parameters 2. A process as claimed in claim 1, wherein it is preferable to keep the vacuum level in the VAR at a level of 150 mbar to achieve suphur level of 0.006% max.. 3. A process as claimed in claim 1, wherein the casting is preferably carried out at hot tundish temperature of about 1560°C ± 10°C to achieve minimum inclusions and defects. 4. A process as claimed in claim 1 and 2, wherein it is ideal to hot roll slabs into hot bands of 2.5 to 2.7 mm thickness for carrying out subsequent steps efficiently. 5. A process as claimed in the previous claims, wherein the finish rolling of the bands is carried out at a temperature between 860 ± 10°C. 6. A process as claimed in the previous claims, wherein the coiling of the finished roll band is preferably carried out at temperature in the range of 720 ± 10°C to ensure formation of coarse ferrite grains (average size: 15-20 µm) in hot bands. 7. A process as claimed in claim 6, wherein pickling is carried out using dilute HCI acid to remove scale from the surface of hot bands and cold rolling is carried out to obtain cold rolled bands of 0.53 ±0.1 mm thickness. 8. A process as claimed in the previous claims, wherein the annealing either as a batch step or as a continuous process. 9. A process as claimed in claim 8, wherein the batch annealing steps is carried out in a batch-annealing furnace under the following conditions parameters : Annealing Temperature 620 ± 10°C. Soaking duration 10-12 hrs. depending upon charge weight Type of cooling Furnace cooling to ensure strain free fresh ferrite grains of -12 µrn size and to achieve a hardness of 50-55 HRB after annealing. 10. A process as claimed in claim 8, wherein similarly, the continuous annealing is carried out in a continuous annealing furnace under the following conditions: Annealing temperature 820 ± 10°C. Total residence time in annealing furnace ~ 6.0 mins. 11. A process as claimed in the previous claims, wherein the annealed are temper rolled to achieve a cold reduction of 2 to 3%. 12. A process as claimed in the previous claims, wherein the decarburization annealing practice should be as follows: Annealing Temperature 800°C. Total annealing duration 180 minutes Soaking time 60 minutes Furnace atmosphere Decarburizing to ensure reduction in % C to 0.0002-0.004 13. A process for the production of semi-processed electrical steel substantially as herein described. This invention relates to a process for the production of semi-processed electrical steel, which comprise the following steps: (A) Preparing a steel composition in a LD converter having the following specific alloy chemistry. (a) %C: 0.040 max. (b) %Mn: 0.25-0.45 (c) %P: 0.20 max. (d) %S: 0.006 max. (e) %Si: 0.5-0.70 (f) %AI: 0.80-0.10 (B) Subjecting the above steel to Vacuum Arc Refining (VAR) and Argon rinsing so as to achieve a nitrogen level in the steel of less than 80 ppm and sulphur less than 0.006%; (C) Casting the steel of step 2 into slabs of desired shapes and sizes at a casting speed ranging between 0.7 to 0.8 meter per minute in the hot condition and; (D) Rolling the so achieved slabs into hot bands of less than 2.7 mm thickness followed by finish rolling and cooling the said bands. (E) Pickling of hat bands in HCI medium followed by cold rolling of hot bends to 0.53 ±0.1 mm thickness (F) Annealing of cold rolled coils through either batch step or as a continuous process with designed annealing parameters (G) Temper rolling of annealed coils with 2-3% reduction (H) Decarburization annealing of stamped sheets with designed annealing parameters

Documents:

152-kol-2003-abstract.pdf

152-kol-2003-claims.pdf

152-kol-2003-correspondence.pdf

152-kol-2003-description (complete).pdf

152-kol-2003-examination report.pdf

152-kol-2003-form 1.pdf

152-kol-2003-form 13.pdf

152-kol-2003-form 18.pdf

152-kol-2003-form 2.pdf

152-kol-2003-form 26.pdf

152-KOL-2003-FORM 27.pdf

152-kol-2003-form 3.pdf

152-KOL-2003-FORM-27.pdf

152-kol-2003-granted-abstract.pdf

152-kol-2003-granted-claims.pdf

152-kol-2003-granted-correspondence.pdf

152-kol-2003-granted-description (complete).pdf

152-kol-2003-granted-examination report.pdf

152-kol-2003-granted-form 1.pdf

152-kol-2003-granted-form 13.pdf

152-kol-2003-granted-form 18.pdf

152-kol-2003-granted-form 2.pdf

152-kol-2003-granted-form 26.pdf

152-kol-2003-granted-form 3.pdf

152-kol-2003-granted-pa.pdf

152-kol-2003-granted-reply to examination report.pdf

152-kol-2003-granted-specification.pdf

152-kol-2003-pa.pdf

152-kol-2003-specification.pdf