Field of the Invention
The present invention relates to production of electrical steel in general, and in
particular to a method of producing low silicon low carbon non-oriented electrical
steel sheets. Such steel is used in laminated form as core material of small (less
than 1 HP) and medium (1-10 HP) sized electrical motors.
Background of the Invention
An electric motor is composed of a stator and a rotor. The stator is composed of
wire made of high conductivity material e.g. copper. The stator wire carries the
magnetizing current and it is wound on the rotor (rotating core). The core is
generally made in laminated form typically using high silicon steel sheets. The
steel used in the core should possess good magnetic properties like (a) low core
loss and (b) high permeability. Core loss is the loss of power in the form of heat
in the rotor during the running of the motor. Permeability is the ability of the
rotor material to acquire high magnetic induction at a given magnetic field for a
given volume of the rotor. Generally, low core loss and high permeability are
desirable properties of the steel sheets used in the rotor.
For large electric motors low watt-loss is desirable as these are more difficult to
cool compared to smaller motors. For small motors permeability is more
important in order to reduce the size of the motor. Hence for rotors of smaller
motors, high permeability combined with moderate watt loss and low cost is
highly desirable. Generally, 0.4 to 3.5% silicon is added to electrical steels.
Silicon reduces core-loss in steel but it decreases the permeability or saturation
induction of the steel. Also, high silicon steels are costly due to higher alloying
additions and higher processing cost. Decreasing the silicon content in the steel
is expected to reduce the overall cost of the rotor in addition to imparting higher
permeability.

Electrical steels in the form of sheets and strips constitute essential components
in electrical equipment used for generation, distribution and utilization of
electrical energy. In AC appliances such as transformer, motor and generators, it
is necessary to utilize core material or lamination, which possesses high
permeability and low core loss. Based on crystallographic texture and resulting
magnetic properties, electrical steel strips are classified into two types (i) Cold
Rolled Non Oriented (CRNO) electrical steel (ii) Cold Rolled Grain Oriented
(CRGO) electric steel. Non-oriented electrical steel has a crystallographic texture
containing almost all-possible orientations (random orientation) distributed within
the grains resulting in same magnetic properties in all directions. These are
essentially required in the cores of rotating electrical machines like motors and
generators where the magnetic flux has to flow in all directions along its path.
CRNO electrical steels can be supplied as (i) Fully Processed (FP) strips (ii) Semi
Processed (SP) strips. In case of fully processed material, final magnetic
properties are achieved through annealing at the supplies / steel producers' end
and can be readily used by the equipment manufacturer for punching
laminations. Semi-processed electrical steels are increasingly being used
replacing conventional fully processed CRNO due to their cost effectiveness.
These materials need decarburizatton annealing treatment at customers' end for
developing final magnetic properties Semi processed electrical steels are finished
to the final thickness by the steel producer / supplier and final magnetic
properties in the punched lamination are achieved though decarburizatton
annealing / stress relief annealing at the customers' end. Semi processed coils
are generally temper rolled after cold rolling and annealing to improve
punchability, facilitate strain induced grain growth during decarburizatton
annealing and to obtain desired amount of surface roupjhness to prevent sticking
of the lamination during decarburizatton annealing. Semi-processed silicon
bearing steel is well established due to Hs favourable effect on core loss
properties but due to higher alloying content, it affects the permeability of the
steel sheet, thus reducing the scope for its application in small motor & rotor
component.
In the present invention it is aimed to develop steel with tow amount of Si by
selecting a suitable chemistry and process parameters resulting in a combination
of moderate watt toss with high permeability as required by application to
smaller and medium sized motors. This is because smaller motors are easier to
cool after service than larger motors and smaller motors are generally used
intermittently.
Description of the Invention
The chemistry of electrical steel was selected based on the magnetic and
mechanical properties required by the users. Alloying additions are genrally made
to increase the electrical resistivity of the material and thus to decrease the eddy
current component of the core loss. The addition of silicon to electrical steel
decreases the coercive force (He), thereby reducing the hysteresis loss. It also
increases the electrical resistivity and thus decreases the eddy current loss. In
conventional non-oriented steel, it is used in the range of > 0.4% depending on
the end-use, however, in the present invention the chemistry was selected with
Si <0.40%. The manganese was added to increase the resistivity thereby
decreasing the eddy current loss. It improved the mechanical properties and
hence improved punchability. Carbon in the form of Fa3C is detrimental for
magnetic properties as it increases the core loss. Also, carbon held in solution
during cooling after annealing leads to precipitation of carbides during service
over a long period of time resulting into increase in watt loss. This phenomenon
is known as magnetic ageing. Thus for best magnetic behaviour of the material,
carbon in the lamination preferably should not exceed the level of 30 ppm after
decarburisation annealing.
In this invention, two different composition are included: (i) low carbon (LC) and
(ii) ultra low carbon (ULC). While the LC variety contain maximum 0.05 wt%
carbon the ULC variety contains 0.0035wt% carbon. In this steel, aluminum was
controlled as low as possible in view of its deleterious effect on magnetic
properties. Sulphur in the form of MnS inhibits the grain growth during annealing
and thus detrimental for magnetic properties. Sulphur was controlled to a low
level since it reduces the efficacy of manganese forming MnS.
In the present invention, semi-processed low-Si electrical steels have been
developed by judicious selection of chemistry and optimizing the process
parameter.
According to one objective of the present invention it is proposed to reduce Si
from the conventional electrical steel for improving permeability and processing
cost.
Another objective of the invention is to adopt commercial production through the
sequential step of LD-Slab caster - hot strip rolling - cold rolling - batch
annealing - temper rolling mill.
A further objective of the invention is to achieve the required hardness by
judicious selection of chemistry and percentage of temper rolling.
A still further objective of the invention is to achieve strain induced grain growth
during decarburisation annealing LOW CARBON (LC) and annealing ULTRA LOW
CARBON (ULC) steel laminations by optimizing the %reduction during temper
rolling.
Yet another objective of the invention is to achieve low core loss value by (a)
reduction in carbon from the matrix and grain growth during decarburisation
annealing in case of (LC) low Si steel and (b) by annealing ULC low Si steel for
motor lamination applications.
According to the invention there is provided a method of producing low silicon
electrical steel to be formed as electrical lamination sheet comprising the steps of
producing a liquid steel of composition as in weight % of Carbon - 0.0035 to
0.050, Manganese - 0.40 (max), Silicon - 0.20 to 0.30, Sulphur - 0.010,
Phosphorus 0.80 (max), Aluminium 0.003 (max) and Nitrogen - 60 ppm (max),
continuous casting into slab of 210 mm thickness in LD - slab caster, hot rolling
the slab to 2.3 mm thickness preceeded by charging the slab in a reheating
furnace, cold rolling the slab to form 0.5 mm - 1 mm thickness sheet, annealing
the steel sheet in batch annealed furnace at 560°C - 700°C, temper rolling the
sheet with 6 to 9% deformation and finally annealing/decarburisation annealing
in a furnace at 740°C to 780°C.
-5-
Detailed Description of the Invention
The low silicon electrical steel sheets of two varieties were developed: (i) low
carbon and (ii) ultra low carbon. The steels were made in a LD converter. The
ultra low carbon steel was further routed through vacuum degasser to reduce
the carbon content.
Cold rolled sheet of 0.5 mm were commercially processed through LD-Slab caster
- hot strip rolling - cold - rolling - batch annealing temper rolling. The steel was
continuously cast in 210 mm slabs. The slabs were charged in reheating furnace
and rolled to 2.3 mm to 1 mm thickness with proper hot rolling parameters. After
cold rolling to 0.5 mm, the steel sheet was annealed in (Batch Annealed Furnace)
BAF at 560 - 700°C followed by appropriate amount of temper rolling (6-9%) to
achieve the required hardness and roughness in the steel sheet products and to
facilitate strain-induced grain growth during decarburization annealing (LC steel)
or annealing (ULC steel).
The magnetic properties of LC and ULC low-Si electrical steels are shown in the
following Table 2.
Grade 1 steel sheet is produced by the sequential steps of
(HR-» CR (0.5 -lmm) ? Batch Annealing at 560 - 700°C ? Temper rolling (6-9%) at
room temperature? Decarburisation annealing in a commercial furnace (740 - 780°C).
Grade 2 steel sheet is produced by the sequential steps of
(HR-» CR (0.5-lmm) ? Batch Annealing at 560 - 700°C ? Temper roling (6-9%) at
room temperature ? Annealing in a laboratory furnace (780°C)
Grades 3 steel sheet is produced by the sequential steps of
(HR? CR (0.5mm) ? Batch Annealing at 640°C ? Temper rolling (6-9%) at room
temperature ? Annealing in a commercial furnace (740 - 780°C).
The magnetic induction value of this grade was found to be much superior to
equivalent fully processed conventional CRNO grade, which is enumerated below
as an illustrative example in Tables 3 and 4.
Comparative improved magnetic induction properties of the present invention
over conventional silicon steel. The relevant values are given in Table 3 and 4.
The present invention should not be read in a restrictive manner as many
modifications, adaptations and changes are possible within the scope of the
invention as encampused in the appended clams.
WE CLAIM:
1. A method of producing low silicon electrical steel sheet to be formed as
electrical lamination sheet comprising the steps of producing a liquid steel of
composition in weight % of Carbon - 0.0035 to 0.050, Manganese - 0.40 (max),
Silicon - 0.20 to 0.30, Sulphur - 0.010, Phosphorus 0.08 (max), Aluminium 0.003
(max) and Nitrogen - 60 ppm (max), continuous casting into slab of 210 mm
thickness in LD - slab caster, hot rolling the slab at 900°C to 1200°C to 2.3 mm
thickness preceeded by charging the slab in a reheating furnace, cold rolling the
slab to form 0.5-1 mm thickness sheet, annealing the steel sheet in batch
annealed furnace at 640°C - 680°C, temper rolling the sheet at room
temperature with 6 to 9% deformation and finally annealing/decarburisation
annealing in a furnace at 740°C to 780°C.
2. A method as claimed in claim 1 wherein low carbon, low silicon steel sheet
for lamination is formed by deforming the casted slab through the sequential
steps of Hot rolling ? cold rolling to 0.5 mm thickness ? batch annealing at
680°C ? temper rolling to 6-8% deformation ? decarburisation annealing in a
commercial furnace at 780°C .
3. A method as claimed in claim 1 wherein the ultralow carbon low silicon
steel sheet for lamination is formed by deforming the casted slab through the
sequencial steps of hot rolling ? cold rolling to thickness of 0.5mm ? batch
annealing at 560-700°C ? temper rolling to 6-8% deformation ? annealing in a
laboratory furnace at 740°C.
4. A method as claimed in the preceeding claims wherein the LC and ULC
low silicon steel has core loss (watt/kg) of 4.62 to 5.7, permeability 3000 to 3157
and ASTM GSN 2 to 3 for 0.50 mm thick sheet to represent magnetic induction
value to be much superior to equivalent fully processed conventional Cold Rolled
Non Oriented (CRNO) electrical steel.
5. A method as claimed in the preceeding claims wherein the LC and ULC
steel for achieving an induction value requires much less magnetizing field than
the conventional fully processed CRNO steel (Si ~ 0.4 to 0.6 % wt).
6. A method as claimed in the preceeding claims wherein the LC and ULC
steel produces superior induction than the conventional CRNO steel at a
particular magnetizing field.
7. A method of producing low silicon electrical steel wherein in the steel
composition silicon is maintained below 0.40% as different from the conventional
steel where silicon is more than 0.40%.
8. A low carbon steel composition in weight % of C - 0.050 (max), Mn -
0.40 (max), Si - 0.20 to 0.30, S - 0.010, P - 0.08 (max), Al - 0.003 (max), N2 -
60 ppm (max), and
an ultra low carbon steel composition in weight % of C-0.0035 (max), Mn-
0.40 (max), Si - 0.20 to 0.30, S - 0.010, P - 0.08 (max), Al - 0.003 (max) and
N2 - 60 ppm (max), produced by the method as claimed in claim 1.

This invention relates to a method of producing silicon free electrical steel to be
formed as an electrical lamination sheet comprising the steps of producing a
liquid steel of composition in weight % of Carbon - 0.0035 to 0.050, Manganese
- 0.40 (max), Silicon - 0.20 to 0.30, Sulphur - 0.010, Phosphorus 0.08 (max),
Aluminium 0.003 (max) and Nitrogen - 60 ppm (max), continuous casting into
slab of 210 mm thickness in LD - slab caster, hot rolling the slab at 900° -
1200°C to 2.3 mm thickness preceeded by charging the slab in a reheating
furnace, cold rolling the slab to form 0.5 to 1 mm thickness sheet, annealing the
steel sheet in batch annealed furnace at 640°C - 680°C, temper rolling the sheet
at room temperature with 6 to 9% deformation and finally
annealing/decarburisation annealing in a furnace at 740°C to 780°C.