Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BLENDED CHARGE FOR STEEL PRODUCTION
FIELD OF THE INVENTION
The invention is related to the production of
-i- iron and steel and particularly to the production of steel
in electric furnaces.
BACKGROUND OF THE INVENTION
It is known that steel can be produced in an
electric arc furnace which is initially charged material
with scrap metal and a material in the form of iron-ore
pellets cast with iron amounting to 0.5 to 5.0 ton per ton
of scrap. The use of a charge containing pellets of a non-
regulated content of an iron-carbon alloy (as pig iron) and
i~ iron oxide leads to a great range of carbon concentration
upon melting (0.2 to 2.6%) which hinders metal refining.
This increases the time of melting and sharply deteriorates
the quality of steels and their chemical stability.
Also, the known charge, due to the variable
chemical composition, has limited uses. Its utilization
falls within the field of high-carbon steel production
(i.e., 1.25% of carbon or more) by melting.
It is a technical advantage of this invention 1)
,i to reduce the time of melting, 2) to improve the quality of
s steel and 3) to increase the range of metals obtained by
electric furnace melting.
SUMMARY OF THE INVENTION
These advantages are achieved by utilization of
~i a blended charge in steel production. The blended charge
is composed of an iron-carbon alloy (50 to 95% by weight)
and an oxide material (5 to 50% by weight). The oxide
material contains free oxides of metals which have an
-~ affinity for oxygen that is equal to or less than the
affinity of carbon for oxygen when the metal oxides are
present in amounts in excess of 0.25% by weight. As a
metallic component, iron-carbon alloys having 0.2 to 4.5%
by weight of carbon are used, for instance, conversion pig
iron. The oxide material may comprise oxidized flux-
bearing and flux-free iron-ore materials, i.e., agglomerate
or pellets of raw ores and their waste products, scale,
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oxidized metal scrap of a chip type, fragmented metal waste
; and solid oxidizers obtained by agglomeration of flue dust
~, and sludge from metallurgical processes.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The oxide material is constituted, for example,
.7
by the oxides of iron and manganese in valence states II or
III (MnO, MnO2, Mn2O3 and Mn3O4) and also oxides of alloying
`- elements chrome, nickel, molybdenum, tungsten and cobalt,
-/ provided that these elements are consistent with the steel
;~ 10 grade under production. The affinity of these metals for
~ oxygen in the condition of a steelmaking bath i5 lower than
;i- for carbon. This provides, in the process of melting,
,
their reduction to metal by carbon existing in the metallic
component of the steelmaking bath. The blended charge is
formed with an iron-carbon alloy ranging from 50 to 95% by
i' weight and 5 to 50% oxide material.
i The utilization of a blended charge in which the
iron-carbon alloy component amounts to more than 95% and
the oxide material is less than 5% results in partial
oxidation of silicon and other highly active elements
during bath melting due to the lack of oxygen in the
charge. This does not allow the proper conduct of the
oxidizing period during melting and hinders the removal of
phosphorus and oxidation of carbon. The makeup of oxygen
to oxidize the remaining content of silicon, phosphorus and
."
carbon increases in time both the oxidizing period and the
entire melting cycle thus producing the metal of a lower
quality and thereby generally decreasing the effectiveness
of electromelting. The amount of carbon in a metal tends
to increase by the end of melting in a bath if the
composition is as given above. This makes the oxidizing
period longer and demands extra oxygen.
` The use of a blended charge containing less than
50% of an iron-carbon alloy and, respectively, more than
50% of oxide materials results in a low carbon
concentration in the bath during melting. This hinders
further bath heating and obtaining the target temperature
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of metal at tapping. Besides, an excessive portion of
oxide materials in the blended charge brings about a sharp
cooling of the metallic bath and increases thereby the time
~;~of melting and power waste, thus producing metal of a lower
S grade with regard to the content of gas and inclusion
components.
The required relationship between an iron-carbon
alloy and oxide material components in the blended charge
amounting to 50 to 95~ and 5 to 50%, respectively, provides
top technical and economic results in melting and obtention
of steel of a high quality for various steel grades. It
also allows to carry out complete oxidation of silicon and
other highly active elements of a vanadium and titanium
-type (in case they compose metallic constituents of the
blended charge) and thereby giving an opportunity for
5'carbon and phosphorus oxidation at early stages. As the
oxide material component of the blended charge grows and
exceeds 5%, the amount of oxygen in the charge proves to be
sufficient to oxidize a portion of the carbon. The
il20 emission of gaseous products in this reaction l) enhances
<`the transfer of heat and material in a bath, 2) accelerates
the formation of a fluid slag stage, 3) intensifies slag-
foaming and arc-shielding, 4) improves the conditions of
metal heating, 5) accelerates carbon and phosphorus
oxidation and 6) makes it easier to conduct slag flushing
.and phosphorus removal.
If the relationship between iron-carbon alloys
and oxide material components falls within the weight
ranges 70 to 80% and 20 to 30~, respectively, the amount of
oxygen proves to be sufficient to carry out 1) complete
oxidation of all alloy admixtures (including carbon,
silicon, manganese) and 2) overall reduction of iron oxides
to the metallic condition.
~<~This relationship 1) provides for a maximum
possible bath boiling, 2) eliminates the influence of the
blend on the chemical composition of the metallic bath, 3)
creates favorable conditions for metal refining and 4
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decreases the time of melting. This composition of the
; blended charge is optimal. If the relationship between
iron-carbon alloys and oxide material components is over
?::` 70:30, a part of the iron oxides that were not used in
oxidation of alloy admixtures moves into the slag
increasing its oxygen content and accelerating the
dissolving of lime. This increases the refining and
~ foaming characteristics of the slag, makes the removal of
~?,~ carbon, phosphorus, sulphur and gases deeper and also
combines the periods of melting and oxidation. Thus, it
results in the general decrease of the melting cycle. A
, gradual decrease of the iron-carbon alloy and respective
increase of oxide materials in the blended charge is
accompanied by a continuous decrease of iron supplied to
15 the metallic bath from the blended charge. Beginning from
the definite relationship between the charge constituents,
the amount of iron that is reduced from iron oxides does
not make up for the decrease of metal coming from the iron-
carbon alloy. As a result, the output of hot metal
20 decreases. Along with this decrease, the amount of slag
and cooling of the bath increases. Thus, a higher amount
of oxide materials in the blended charge (over 50%) leads
to unjustified slag growth and excessive waste on heating,
~ decomposition and melting of oxide materials and the
i~; 25 feasible output generally decreases.
The charge composed of a higher amount of iron-
carbon alloy and a lower portion of oxide materials can be
used in the production of medium-carbon and high-carbon
c steels. The blended charge constituted by a lower iron-
30 carbon alloy content and a higher content of oxide
materials might be employed in a lower-carbon steel
production, as well as, in the production of special low-
carbon steel like stainless electrotechnical steel used in
automobile sheet production, etc. Additionally, the
35 blended charge having a high volume of oxide materials can
be used as a coolant in an oxygen converter process, and
also can be used in electric furnaces melting metallized
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pellets leaving a portion of the metal from the previous
~;~, melt in the furnace ("swamp"). In the latter case, melting
of the initial charge is accompanied by a local overheating
of metal in the area coming under the arcs and also by the
increase in hearth wear. This charge is characterized by
; the combination of a maximum cooling effect and purity.
;-- Under these conditions, the blended charge can be
substituted for scrap as a coolant and also used in the
production of steels with specific properties.
Free metal oxides that have a chemical affinity
for oxygen (as much as and/or lower than carbon) in an
amount of metal oxide of at leas~ 0.25% by weight create
conditions for complete reduction of iron oxides and other
~,elements to metal and pure admixture-free metal arriving in
'15 the bath. Dilution of the metal bath with a pure molten
metal decreases the concentration of unwanted elements
'~which negatively influences the properties of steel and
~lalso increases the output of hot metal.
-~On the other hand, the oxygen in the oxide
materials oxidizes the alloys of pig iron such as silicon,
vanadium, titanium and other highly reactive elements
first, and then the carbon. Mixing of the bath by the
generated gaseous products of carbon oxidation intensifies
heat and mass exchanges in the bath. The mixing effect
starts from the melting of the first charged portion and
continues through the entire melting period. This allows
the formation of an active, fluid, highly foaming slag at
the end of the melting period making it possible to
terminate the arcs and to provide normal flushing which
improves the conditions of dephosphorization and
desulphurization of metal and bath degassing. The
consequences of this are expressed in partial coincidence
of the oxidation and melting periods, in the reduction of
the melting cycle and in improvement of the quality of
metal.
~The occurrence of silicon in major iron-carbon
`~alloys, particularly in pig irons, leads to its oxidation
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and formation of a silicon oxide that provides an acid slag
with low basicity and yield during melting. Such slag
weakly comes off the bath, has low foaming properties,
hinders dephosphorization of the molten metal, has a
negative impact on the furnace lining and delays carbon
oxidation. The choice of free metal oxide content in
excess of 0.25% by weight is required by 1) the conditions
of an active fluid slag formation at the end of melting and
2) production of a given carbon content on deoxidation
having regard for the iron-carbon alloy makeup and its
relative portion in the blended charge.
,5'^ The large amount of hot fluid slag produced by
the end of the melting stage accelerates dephosphorization
~-and desulphurization producing the metal with low
concentration of phosphorus, sulfur and chrome. Due to
this, the oxidation period is partially combined with
melting which makes further conduct of this stage easier
`and reduces it to the adjustment of the carbon content by
~`the addition of small solid oxidizers (i.e., agglomerate)
-20 and heating of the metal to a required temperature.
;~Accelerated formation of a hot slag and metal allows steady
'arc combustion and sharply reduces the noise load to 10 to
12 minutes from the beginning of melting.,
-~If the content of carbon and other admixtures is
minimal both in iron-carbon alloy (0.2%) and in the blended
charge, the free metal oxide content amounting to 0.25% or
more is sufficient to conduct complete oxidation of a small
amount of silicon, manganese and other admixtures.
A lower oxide content is unsuitable due to the
lack of oxygen required in the oxidation of alloy
~`admixtures. A higher oxide content is inexpedient because
of the increase of heat flow used in melting and due to a
~large quantity of evolving slag.
`~The largest possible concentration of free metal
oxides corresponds to a maximal silicon and carbon content
in the iron-carbon alloy (pig iron) and its maximum portion
(95%) in the proposed blended charge. These conditions
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2125~1~
provide 1) a complete oxidation of silicon, 2) formation of
~:a hot fluid slag possessing maximal foaming and increased
refining properties with regard to phosphorus and sulphur
,and also a high oxidizing potential. As a result, it is
`~5 possible to achieve simultaneous and concurrent oxidation
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of phosphorus and carbon and the removal of sulphur. In
.other words, it is possible to combine melting and
'oxidation periods. The melting is characterized by a
minimal duration and the output of a high-quality metal.
~" 10 The composition of various exemplary oxide
materials is given in Tables lA and lB.
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~` Example
- Production of electric steel containing carbon
(not more than 0.035%), silicon (3%), copper (within 0.3 to
0.6%) was carried out in a 100 metric ton arc furnace. The
-~ 5 metal charge included blended charge (13 to 35 tons), slab
` waste (20 to 25 tons), rolled crop ends (16 to 45 tons) and
scrap (amounting to 8 to 36 tons). The blended charge
material was charged in two portions; first 70% and then
30%. As slag-forming materials, we used lime (2 to 3.5
~` 10 tons) agglomerate (2.5 to 4 tons) and fluorspar (0.3 to 0.5
ton). Oxygen was delivered through a free lance, total
consumption per melt being within 1,200 to 1,600 cubic
meters.
The blended charge was made in pig iron pouring
` 15 machines where various oxide materials were poured out
, . .
together with pig iron. Large melts were carried out using
iron-carbon pellets produced at Mikhailovsky and Lebedinsky
Preparation Plants as an oxide component. Also,
~¦ agglomerate, scale, agglomerated dust and sludge and a mix
of various solid oxides were used.
The molten metal contained carbon (0.1 to 1.0%),
manganese (0.05 to 0.20%), phosphorus (0.007 to 0.016%),
,j sulphur (0.018 to 0.025%), chrome and nickel (less than
0.05%), all percentages by weight. After melting, the bath
was decarburized and heated, copper alloyed, oxidized,
alloyed with silicon and tapped. Then ladle treatment and
continuous teeming was carried out. See Table 2 (reporting
melting carried out on the suggested and familiar blends)
and Table 3 (reporting ready production grading).
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Referring to Tables 2 and 3, blend composition 1
is a prior art prototype. Blend compositions 2 to 10 are
~' according to this invention. Table 2 shows that a blended
charge according to this invention can cut down the period
of melting from 20 to 60 minutes. It reduced power flow
~-i wasted on melting by 5 to 15% and also increased the output
'~ of hot steel preferred compositions.
Referring to Table 3, the best results correspond
to the preferred blended charge. The products made of
steel melted from the preferred blended charge are
characterized by excellent electromagnetic properties in
`~ comparison with the prior art prototype. The amount of
quality production thereby exceeds by 80% as compared with
50% when employing the prototype. Apart from
electrotechnical steel, the blended charge was tested in
the production of plain carbon steels. The tests proved
the possibility to use the new blended charge in the
l~l production of various steel grades.
', Having thus described our invention with the
1 20 detail and particularity required by the Patent Laws, what
is desired to be protected by Letters Patent is set forth
in the following claims.
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