GRAIN-ORIENTED STEEL

ACIER A GRAINS ORIENTES

English Abstract

ABSTRACT OF THE DISCLOSURE
A silicon steel for producing grain oriented sheet or
strip for magnetic applications comprises apart from iron
and incidental impurities between 0.02% and 0.06% by weight
of carbon, between 0.02% and 0.035% of sulphur, up to 0.01%
of nitrogen, between 2.0% 4.0% silicon, 0.06% to 0.10%
manganese together with at least one carbide or nitride
former in a quantity sufficient to combine with the carbon or
the nitrogen present in the melt.

Claims

The embodiments of the invention in which an exclusive
property or privilege is claimed are defined as follows:
1. A method for producing a silicon steel for magne-
tic applications including inoculating a steel melt produced by
basic oxygen or open hearth refining with manganese together
with at least one carbide or nitride former, the quantity of
former added being such that during subsequent processing of
steel derived from the melt, the carbides or nitrides form pre-
cipitates effective to promote preferential growth of some
grains within the steel.
2. A method as claimed in claim 1, wherein the steel
derived from the melt comprises apart from iron and incidental
impurities, between 0.02% and 0.06% by weight of carbon,
between 0.02% and 0.035% by weight of sulphur, up to 0.01%
by weight of nitrogen between 2.0% and 4.0% of silicon and
between 0.06% and 0.10% of manganese.
3. A method as claimed in claim 2, wherein the carbon
concentration of the steel lies within the range 0.02% to
0.035%.
4. A method as claimed in claim 1, wherein the melt
is inoculated with sufficient former to combine with the available
carbon or nitrogen.
5. A method as claimed in claim 4, wherein the quanti-
ty of precipitate in the steel is raised by increasing the car-
bon or nitrogen concentration together with the quantity of
former inoculated.
6. A method as claimed in claim 1, wherein the former
is selected to combine predominantly with nitrogen.
7. A method as claimed in claim 6, wherein the former

is titanium or a compound of titanium.
8. A method as claimed in claim 6, wherein the former
is vanadium.
9. A method as claimed in claim 8, wherein the vanadium
is inoculated to produce concentration of up to 0.1%.
10. A method as claimed in claim 9, wherein the vana-
dium, is inoculated to produce a concentration of 0.065%.
11. A method as claimed in claim 1, wherein the former
is selected to combine predominantly with carbon.
12. A method as claimed in claim 11, wherein the former
is vanadium.
13. A method as claimed in claim 1, wherein a slab of
the steel is heated to a temperature between 1350 and 1400°C to
enable the manganese as sulphide, and the carbide or nitride to
enter into solution prior to hot rolling, the hot band is annealed
then cold rolled to substantially final gauge and decarburise
annealed.
14. A method as claimed in claim 13, wherein hot rolling
is at a temperature between 900° and 1395°C.
15. A method as claimed in claim 13, wherein the
thickness of the hot band lies between 1.5 mm to 3.0 mm.
16. A method as claimed in claim 13, wherein the hot
band is annealed at a temperature betwee 900°C and 950°C.
17. A method as claimed in claim 13, wherein the
decarburise anneal is effective to reduce carbon concentration
to below 0.005%.

18. A method as claimed in claim 17, wherein the
decarburise anneal is at temperature between 950°C and 1050°C.
19. A method as claimed in claim 13, wherein the hot
band is cold rolled to substantially final gauge in a single
reduction.
20. A method as claimed in claim 1, wherein a slab of
the steel is heated to a temperature between 1350 and 1400°C
to enable the manganese as sulphide, and the carbide or nitride
to enter into solution, the slab is hot rolled to strip which is
annealed before primary cold reduction to strip of greater than
final gauge, the primary cold reduced strip being subject to a
further anneal before final cold reduction to substantially final
gauge.
21. A method as claimed in claim 20, wherein the hot
band is annealed at a temperature between 900°C and 950°C.
22. A method as claimed in claim 20, wherein the
primary cold reduction is to substantially double final gauge.
23. A method as claimed in claim 20, wherein the
further anneal is at a temperature of about 900°C.
24. A method as claimed in claim 20, wherein the strip
of final gauge is decarburise annealed to reduce carbon to below
0.005%.
25. A method as claimed in claim 24, wherein the
decarburise anneal is at a temperature between 800 to 850°C.
26. A method as claimed in claim 24, wherein the
decarburise anneal is in atmosphere of wet hydrogen.
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27. A method as claimed in claim 24, wherein the
decarburise anneal is a wet nitrogen-hydrogen atmosphere.
28. A method as claimed in claim 24, wherein the
decarburised strip is annealed at a temperature between 1150°C and
1200°C to permit preferential grain growth.
29. A method as claimed in claim 28, wherein the strip
is coated with magnesia and box annealed for 24 hours.
30. A silicon containing steel for magnetic applications
comprising apart from iron and incidental impurities, between 0.02%
and 0.06% by weight of carbon, between 0.02% and 0.03% by weight
of sulphur, up to 0.01% by weight of nitrogen, between 2.0% and
4.0% by weight of silicon, between 0.06% and 0.10% by weight of
manganese and at least one carbide or nitride former in a quantity
sufficient to combine with the carbon or nitrogen present in the
melt.
31. A silicon containing steel as claimed in claim 30,
wherein the former is vanadium.
32. A silicon containing steel as claimed in claim 31,
wherein the vanadium is present in a quantity of up to 0.10%
by weight.
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Description

~737~
This invention relates to the production of steel
for electromagnetic applications, that is to say steel intended
to form a magnetic circuit in electric machines.
he invention is paxticularly concerned with grain
oriented electrical steels which are deliberately inoculated
with silicon as a means of reducing electrical conductivity
to inhibit losses arising from eddy currents produced when
the steel is excited and to impart to the s~eel other desir-
able properties. Such silicon steels conventionally contain
silicon in concentrations within the range 2.0 - 4.0/0 by weight
in contrast to non-silicon steels in which the silicon concen-
tration lies generally well below 2.0%.
Normally steels for electromagnetic appLications
are produced from suitable general purpose steels which have
been refined in any basic oxygen or open hearth process and
which typically contain carbon in concentrations up to 0.035%
nitrogen up to 0,01% with the remainder being iron except
~or other incidental impurities.
For producing grain oriented silicon steels, the
basic steel melt is inoculated with silicon, to raise the
concentration within the range 2.0 - 4.0 %, and in one
conventional process is additionally inoculated with manganese.
The added manyanese combines during solidification with
sulphur, typically present in concentrations of about Q.029%
~ to form manganese sulphide, which during subsequent reheating
; of a ~lab produced from the melt enters into solution and
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737~
becomes precipi-tated later as a fine dispersion in hot
rolled strip derived from the slab. It is believed that
the manganese sulphide is effective during subsequent
processing to promote preferential growth of some grains
within the steel in order to obtain large grains having
predominantly ~110)/00~ miller indices and to inhibit the
- growth of the remaining grains.
One disadvantage arising from the use of a
dispersion of precipitated manganese sulphide as a grain growth
inhibitor arises from its limited solubility in steel and this
limit has placed considerable restriction on the quantity of
dispersed precipitate and therefore on the extent of prefer-
ential grain growth which can be obtained by conventional
routes for producing silicon steels-using the manganese
sulphide route. While some attempts have been made to improve
preferential grain growth by substituting for manganese
sulphide precipitates having a higher solubility, these
alternatives have met with limited success.
The present invention according to its broadest
aspect is based on increasing the concentration of grain
growth inhibitors in the production of silicon steels by
9upplementing rather than by substituting for any manganese
sulphide or the like already present to the limit of its
solubility. The invention is based on the discovery that
grain growth can be selectively inhibited by more than
one precipitate present in the steel and that such

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supplementary preclpitates may be formed with the carbon and/or
with the nitrogen which already is present in the steel by
adding to the steel suitable carbide or nitride formers.
Suitable formers are those which produce carbides or nitrides
that can be su~stantially removed during subsequent processing
in order to a~Toid degrading magnetic properties. By the same
token the former ltself should be such tha~ if left in the
steel after final processing it does not degrade m~gnetic
characteri stics ~
According to one aspect of the present invention a
silicon steel for producing grain oriented sheet or strip
for magnetic applications comprises apart from iron and
incidental impuritles between 0.02% to 0.06% by weight of
carbon between 0.02% and 0.035% of sulphur up to 0.01% of
nltrogen as well as be~ween 2.0% to 4% of silicon and 0~06%
to 0.10% manganese, together with at least one carbide or
; nitride former in a quantity sufficient to combine with at
least the carbon or the nltrogen as the case may be which is
present in the melt.
One suitable steel containing carbon is a concentration
within the range 0.02% to 0.035% by weight~
In the case where the level of carbides or nitrides is
insufficient to promote adequate preferential grain grow~h,
the concentration can be lncreased by raising ~he carbon and~or
the nitrogen level of the steel optionally together wi~h the
: concentration of carbide or nitride former.
~, In a preferred embodiment the former is one which
pre~erentially combines with the nitrogen in the steel and
ldeally is an element whlch in the~form of nitride can either
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7~7~
be easily removed during subsequent processing or is one which
does not degrade the magnetic characteristics of the electrical
steel subsequently produced.
One suitable nitride forming elemen~ is vanadium which
may be inoculated to produce a concen-tration o~ up to 0.1%
by weight although other elements such as tltanium or
compounds of titanium may be included in the steel which is
reduced to sheet or strip by any process well known in the art.
A suitable carbide former is for example niobium or the like.
In one conventional process a steel ingot containing
vanadium in a quanti~y sufficiently to react with available
nitrogen is reduced to slab which is heated at a temperature
between 1350C to 1400C to permit manganese sulphide and
the vanadium nitride to enter into solution. The slabs are
subsequently hot rolled to strip which is annealed at a
temperature within the range 900C to 950C for up to four
minutes before primary cold reduction in which the annealed
hot band is cold rolled to about double final gauge.
In the conventional process, the primary cold reduced
material is subject to an intermediate anneal for up to four
minutes at a temperature of about 900C before final cold
rolling to substantially final gauge. This is followed by a
decarburising anneal effective to reduce contained carbon
to a concentration below 0.005%.
A suitable decarburising anneal is obtained by strand
heating oE the strip at a temperature within the range 800C to
8S0C in an atmosphere o~ wet hydrogen, although alternative
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37~
reducing atmospheres for example wet nitrogen/hydrogen mixtures
may be ~sed~ Decarburised strip is coated with magnesla
and colled for box annealing at 1150C to 1200C over a
period of some 24 hours in order to achleve the required
preferential grain growth and magnetic characteristicsD
It has now been found that the presence o vanadium or
other suitable carbid~ or nitride formers enables the
primary cold reduction as well as the subsequent intermediate
anneal of the convention~l proceSS to be dispensed with while
at least maintaining the magnetic charac~eristics of the
electrical steel or strip produced.
Accordingly, according to a further aspect of the
present invention a process for producing grain oriented
electrical sheet or strip comprises producing a slab
containing apart form iron and incidental impurities, between
0.02% to 0.06% by weight of carbon, between 0.02% to 0.035%
sulphur~ up to 0.01% nltrogen as well as between 2.0% to
4.0% of added silicon a~d 0.06~ to 0.10% of manganese
together with a carbon or a nitride former in a quantity
sufficient to combine with at least the carbon or the
nitrogen in the steel~ heating the slab at a temperature
wlthin the range 1350C to 1400C prior to hot rolling at
between 1300C to 900C, annealing the hot band between 900C
~nd 950C and cold roll-lng substantially to final gauge
before decarburising annealing the cold reduced strip.
Conveniently the slab is reduced to hct band having
a thickness lying within the range 1.5 to 3.0m~.
For nitride precipitates the decarburising anneal
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37~
should reduce carbon to 0.005/O or less although higher carbon
levels can be tolerated at this stage and may even be beneficial
provided that any excess over 0.005% is removed during sub-
sequent processing. For carbide precipitates the removal
of carbon at this stage should be controlled such that suffi-
cient carbide is retained to act as a grain growth inhibitor
in addition to manganese sulphide present. Carbon should then
be subse~uently removed to less than 0.005%. As in conven-
- tional prac~ice decarburised annealed strip is coated with
magnesia before being coiled for bo~ annealing at about
1200C over a period of some 24 hoursO
In the single stage process of the invention the
decarburising anneal conventiently is effect~d at a temperature
above that normally adopted in the two stage process of the
prior art. Suitably decarburising anneal occurs at a tempera-
ture within the range 950C to 1050C although a temperature
above 850C has been found practicable. Prior to decarburising
anneal the cold reduction following the hot band anneal is to
~o substantially final gauge.
In one embodiment of the invention which is now
particularly described by way of example, steel derived from
an open hearth refining process is tapped in an unkilled
condition from the furnace into a ladle. At this stage the
steel which has a typical composition of 0.03% carbon, 0.029%
sulphur, and 0.0065% nitrogen is inoculated with manganese
and with silicon to raise the concentration of these elements
to 0.085% and 3O19% _ _
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~7371
respectively. The silicon an~ the manganese may be added at
a later stage either in elemental form or in the form of a
sultable alloy such as ferrosilicon.
In addition to manganese the melt is additionally
inoculated with van~dium to raise i-ts concentra-tion in the
final ingot to about 0.065% although concentrations of up
to 0.1% may be used.
In the production of the sheet or strip, slabs produced
from the ingot contalning both manganese and vanadium as potential
: graln growth inhibitors are reheated at 1400C for hot
rolling which produces hot band at a delivery temperature of
950C and at a nominal thickness of about 200 milIimetres. The
hot band which now contains a dispersed precipitate of
manganese sulphide as~,well as vanadium nitride as grain
growth i~hibitors is annealed at 910C for about two minutes
and is then cold roll~d in a single reduction to substantially
~inal gauge which typically is of the order of 0.35 mm for
~tandard steel grades.
Ater cold reduction the strip is decarburised annealed
in an atmosphere of wet hydrogen for four minutes and at a
temperature of 1050 prior ~o coating and box annealing in
con~en~ional manner.
A typlcsl electricaL steel produced by the method of ~-
the in~ention displays a core loss o~ 1.12 watts per kilogratn
at 1.5 tesls and 50 cycles per second excitation. A typical
magnetic permeability at one k~loamp per meter is 1.79 tesla
and these flgures compare well with silicon grain or~nted
electrical 6teels produced by the longer and costlier process
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737~
involving two stage cold re~iuctionO
In an ~lternative embodim~nt of the invention, a billet
or slab ~as produced of a steel o similar compositions but
with a carbon content of about 0.05%. When processed by a
route and under conditions similar ~o these as herein
described strip material of substantially the same magnetic
characteri stics was produced .
It will be appreclated however tha~ while the invention
has been described with reference to a shortened process
involving one stage of cold reduction only, the longer route
may equally be employed should it be convenient to do so.
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