CA 02397524 2002-07-16
Ag;~lomerates containing iron and at least one further element of groups 5 or
6 of the
periodic s, s
The present invention relates to agglomerates containing iron and at least one
further
element of groups 5 or 6 of the periodic system, their use, and a method for
producing them.
As further element, one may in particular consider molybdenum and tungsten.
From DE-A - 196 22 097 agglomerates are known which are formed of an
iron/molybdenum
alloy having 60 to 80% by weight of molybdenum and are used as alloying agents
for metal
melts containing iron and molybdenum.
Molybdenum is used f.i. as an alloying element for producing high-strength
structural steels
containing molybdenum, alloyed cast iron types as well as molybdenum-
containing, rust-
resisting, acid-resisting and heat-resisting steels and nickel base alloys.
When producing molybdenum-containing alloys, steels and cast iron types, for
economic
reasons the greater part of the necessary molybdenum alloying contribution is
added to the
melt either in the form of revert scrap containing molybdenum or in the form
of briquetted
molybdenum trioxide (Mo03).
Adding molybdenum in an oxidic form is possible because in liquid steel the
iron acts as a
reducing agent and thus, the Mo03 is transformed into metallic molybdenum.
However, this
way of adding molybdenum is difficult in terms of manipulation. Attention has
to be paid to
a deep penetration of the Mo03 into the melt, given that at the temperatures
of the liquid
steel Mo03 vaporizes very easily and/or is set in the slag, whereby
insufficient immersion of
the Mo03 may cause great losses in yield.
In the course of a so-called secondary metallurgical aftertreatment, finishing
the smelting of
the above steels, for reducing the detrimental gas contents (oxygen,
nitrogen), for the exact
setting of the desired casting temperature and the final analysis of the
steel, the fine setting
of the molybdenum content is therefore effected with lumpy so-called
ferromolybdenum.
Ferromolybdenum is an iron/molybdenum alloy usually having 60-80% by weight of
molybdenum and produced by way of a metallothermal process. The metallothermal
production according to the thermite burning process is complex, given that
the metals iron
and molybdenum have to be melted on and together. The use of expensive
reducing agents
such as aluminium or ferrosilicon is required. The process may be automated
only to a
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limited extent. This results in a higher market price of the ferromolybdenum
as compared to
the molybdenum trioxide (Mo03).
A disadvantage of ferromolybdenum produced according to the thermite process
is the
relatively high lump density (f.i. about 8.8 g/cm3 in standard FeMo70),
resulting in that when
alloying f.i. steel melts (density about 7.5 g/cm3), the material sinks to the
bottom of the
melting vessel where it forms depositions difficult to dissolve which only
come off in the
subsequent melts. Dissolving such ferromolybdenum lumps in the liquid steel
bath is
additionally made more difficult by the high melting point of the material,
which in the case
of a usual commercial FeMo70 quality is about 1950°C. The temperatures
in the steel bath
are significantly below this level so that now the dissolution of the FeMo
parts can only be
effected by way of diffusion processes which, accordingly, require long
periods of time.
The dissolution of ferromolybdenum produced according to the thermite process
is basically
carned out according to the following mechanism:
The alloy lumps immerging into the liquid melt sink to the bottom of the
treater. This is
caused by the high density of the parts, which is higher than that of the
liquid steel. An outer
layer of solidified steel forms on the lumps, which layer results from the
quench effect of the
immerged cold FeMo lump. Due to the heat transition from the melt to the alloy
lump, the
outer layer subsequently gets dissolved again. However, given that the melting
point of the
alloy lumps is above the temperature of the liquid steel bath, the alloy lumps
can only
dissolve by diffusion of iron from the steel bath into the boundary layer of
the melt and the
alloy lump and by the reduction of the melting point associated therewith.
According to the above DE-A-196 22 097, agglomerates are produced from an
iron/molybdenum blend by briquetting, wherein the iron/molybdenum blend is
obtained by
reducing a fine-grained molybdenum-trioxide/iron-oxide blend with hydrogen-
containing
gas. Briquetting is carried out by adding a binding agent such as water glass
in order to
improve the grain binding. Agglomerates having a lump density higher than 3.5
g/cm3 are
formed therein.
Disadvantages of this process are on the one hand the use of binding agents
which introduce
detrimental tramp elements such as silicon, sulfur, and hydrogen into the
steel, and on the
other hand the poor lump densities and resistances of the material which are
available with
this method and which lead to great losses of molybdenum to the slag.
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US-A - 5,954,857 describes the production of briquets consisting of molybdenum
oxide with
NaOH as binding agent. When introducing these briquets into liquid steel
melts, the
molybdenum oxide is reduced to the metallic molybdenum by the liquid iron,
wherein iron
oxide is formed. Disadvantages of this process are the danger of losing
molybdenum oxide
by absorption in the slag which is on the surface of the liquid steel, and the
losses of iron
occurring in the reduction of the molybdenum oxide.
From US-A - 4,400,207 a method for producing metal alloys is known according
to which
molybdenum oxide, f.i., is mixed with a fine ferrosilicon powder in the
stoichiometric ratio.
As a binding agent, up to 5% bentonite are admixed, and the mixture then is
briquetted.
When introducing these briquets into steel melts, the contained ferrosilicon
acts as a
reducing agent for the molybdenum oxide which passes over to the steel melt in
a metallic
form.
A disadvantage thereof is the formation of silicon oxide as a reaction product
which has to
be set in the slag, which in the steel-making processes used today is only
possible when
taking additional measures.
The invention has as its object to provide agglomerates containing iron and at
least one
further element of groups S or 6 of the periodic system and having an improved
dissolubility
in metal melts, in order to keep the costs of treating the melt low. In
particular, the
agglomerates should not sink to the bottom of a metal melt and should have,
furthermore, a
sufficient resistance in view of storage and transport. Moreover, the quality
of the metal melt
should not be prejudiced by tramp elements being in the agglomerate and acting
as binding
agents, f.i., and a loss of molybdenum and iron should be avoided.
According to the invention, this object is achieved insofar as the
agglomerates have a
porosity in the range of 20 to 65% by volume, particularly of 30 to 45% by
volume.
The agglomerates according to the invention have a porosity and, by that, a
lump density
which on the one hand allows the penetration of a slag cover on a metal melt
and allows the
agglomerates to penetrate into the metal melt. On the other hand, the
inventive porosity of
the agglomerates results in that capillary action fills the pores of the
agglomerates with metal
melt and in that the thereby occurring enlargement of the boundary surface
between the
metal melt and the agglomerate rapidly dissolves the regions filled with metal
melt. Here,
dissolving means the melting of the agglomerates and the homogeneous
distribution of the
components of the agglomerates in the metal melt.
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The dissolution process of the inventive agglomerates in a metal melt can be
described as
follows:
After penetration of the agglomerates through the slag cover on the melting
bath and
immersion into the melt, a boundary layer of solidified steel forms on the
surface of the
agglomerates, which steel results from the quench effect of the cold
agglomerates. This
boundary layer is much thinner than the layer that forms when using ferro-
alloys produced
with the thermite process, given that the thermal capacity of the agglomerates
is lower due to
high porosity.
Even though the density of the agglomerates is lower than that of the liquid
steel, they
immerge deep into the melt because of the kinetic energy of the parts which
have to cover a
corresponding height of fall before impinging on the steel bath.
After dissolution of the outer zone, the liquid steel penetrates into the
pores of the
agglomerates. The thereby produced large boundary surface between the
agglomerate and
the melt leads to a rapid warming and diffusion of iron in this boundary
layer, which
eventually causes the dissolution of the agglomerates. In addition, the gas
included in the
pores of the agglomerates expands because of the rapid warming and enters into
the metal
melt. The thereby generated turbulent flow on the surface of the agglomerates
causes the
rapid reduction of the existing concentration gradients on alloying agents
between the
boundary surface and the melt, which leads to an increase in the diffusion
rate that depends,
according to Fick's law, on the concentration gradients.
A high dissolution rate means the saving of time and costs in the production
of alloyed metal
melts.
According to a preferred embodiment, the inventive agglomerates contain as
further element
molybdenum in an amount of 45 to 85% by weight, preferably of 60 to 80% by
weight. The
lump density of these agglomerates is preferably 4.2 to 6.3 g/cm3,
particularly preferred 4.5
to S .7 g/cm3.
According to another preferred embodiment, the agglomerates contain as further
element
tungsten in an amount of 60 to 90% by weight, preferably of 70 to 85% by
weight. Their
lump density is preferably 4.7 to 8.4 g/cm3, particularly preferred 5.8 to 7.4
g/cm3.
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The present invention also relates to the use of the agglomerates for
producing alloyed metal
melts, especially molybdenum-alloyed and/or tungsten-alloyed metal melts.
The invention further relates to a process for producing the agglomerates,
wherein iron oxide
and oxides of at least one further element of groups 5 or 6 of the periodic
system are reduced
to the respective metals.
US-A - 3,865,573 relates to a process for producing molybdenum powder and/or
ferromolybdenum, wherein molybdenum oxide and/or a blend of molybdenum oxide
and
iron oxide are reduced in a two-stage fluidized-bed process.
US-A - 4,045,216 describes a process for producing directly reduced molybdenum-
oxide
pellets, based on the two-stage reduction of molybdenum-oxide pellets in an
hydrogen-
containing atmosphere. As reduction aggregate, a shaft furnace is used which
is traversed in
counterflow by the product and the reducing gas. In this process, pellets
having a very low
density and abrasion resistance are produced.
The process according to the invention is characterized in that the reduced
metals are
compacted, especially briquetted, without adding any binding agents and in
that the thereby
formed compacted products are sintered.
Sintering is effected preferably at temperatures from 1000 to 1400°C,
in air or preferably in
an inert-gas atmosphere, for 15 to 60 minutes. At the inventive sintering
temperatures,
mainly the iron contained in the agglomerates acts as sinter-active phase and
as a binder for
the particles contained in the agglomerates. Thereby the agglomerates are
prevented from
becoming too dense during the sintering process, which would have a negative
effect on
their dissolution in metal melts.
In the following, the invention will be explained in more detail by means of
three exemplary
embodiments and Figs. 1-6.
Example 1
A powder mixture consisting of 74% molybdenum, 21% iron and 5% oxidic
contaminations
such as silica, aluminium oxide and calcium oxide and produced by reducing a
mixture of
oxides of technical purity of both metals in an hydrogen atmosphere was
compacted to
agglomerates having a diameter of 60 mm and a height of 40 mm in a compacting
press.
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These pressed parts were sintered for different periods of time in a
laboratory sintering
furnace in a nitrogen atmosphere at 1170°C. After having cooled the
parts and withdrawn
them from the sintering furnace, samples were taken from the parts, and the
porosity was
measured.
Table 1 below shows the porosities of FeMo agglomerates as a function of the
sintering
period and the resulting lump density. Here, the porosity was measured with an
Hg
porosimeter. By comparison, the density and porosity of a conventional FeMo
agglomerate
is indicated (Comparative Example).
Table 1
Sintering periodDensity [g/cm3] Porosity
at
1170C
Sam 1e 1 15 4,15 42,4
Sam 1e 2 25 4,3 39,7
Sam 1e 3 45 5,48 23,1
Sam 1e 4 60 6,0 -
Com arative Exam 8,0 0
1e
Fig. 1 shows the pore size distribution of FeMo agglomerates produced with the
process
according to the invention. The particle size of the agglomerates was in a
range of 2 to 4
mm. The measurements were taken by means of an Hg porosimeter at an Hg column
pressure of 200 mm.
The curve numbered 1 represents the pore size distribution of the FeMo
agglomerates
referred to as sample 1 in the above table after sintering at 1170°C.
The molybdenum
content of these agglomerates was 74%. The curve numbered 2 represents the
pore size
distribution of the FeMo agglomerates according to sample 2. Finally, the
curve numbered 3
represents the pore size distribution of the agglomerates according to sample
3. It can be
seen from this that the mere choice of different sintering parameters
(temperature and period
of time) makes it possible to vary the number of the pores and the
distribution of the pore
size within a wide range.
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Agglomerates produced according to the inventive method and corresponding to
the material
referred to as sample 1 in table 1 were dissolved in a steel melt in a
laboratory electric-arc
furnace (see Example 2).
Fig. 2 shows, in an exemplary manner, the dissolution rate of an inventive
FeMo
agglomerate as compared to standard FeMo (produced by way of a silicothermal
process).
The curves were recorded when smelting a high-speed-steel quality (S-6-5-2,
1.3343) with a
molybdenum content of 5%. The composition of the steel produced in the
experiment is
indicated in table 2 below.
Table 2
S-6-5-2, % b wei ht
1.3343
C 0,9
Cr 4,1
Mo 5
V 1,8
W 6,4
Fe ~ rest
Data specification of the experimental electric-arc furnace:
Electrical data: 3-phase; power max. 200 kw
voltages: 52 / 63,5 / 75 / 86,5 / 90 / 110 / 120 / 150 v
Electrodes: graphite Q~ 100 mm, automatic control
Furnace crucibles: infeed with magnesite, with a casting nose
effective volume about 1001
The size of the experimental melt was 300 kg. The melt was used in a three-
phase electric-
arc furnace as a set-up charge, that is, the steel composition was set to a
pure-iron melt by
adding ferro-alloys in a corresponding amount. As a first step, all of the
alloying elements
except Mo were added and set according to the target analysis. For protection
against
reoxidation, the steel bath was covered with a calcium aluminate slag.
In a first experimental melt, the molybdenum content was set by adding
ferromolybdenum
having a grain size of 5-50 mm and produced according to the thermite process.
After having
added the FeMo, samples were taken from the melt at short intervals. A second
melt was
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produced in the same way except that here, the inventive agglomerates were
used to set the
molybdenum content. It could be seen that the inventive agglomerates
(represented in Fig. 2
by the broken line) dissolved much faster than standard FeMo (represented in
Fig. 2 by the
unbroken line).
The significant advantage of the agglomerates according to the invention is
that they
dissolve faster in steel melts than standard FeMo, which results in the saving
of time and, by
that, costs for the user.
Example 2
In a large-scale application experiment, the dissolution behaviour of the
inventive
agglomerates was compared with that of usual commercial ferromolybdenum
produced
according to the thermite process.
Agglomerates produced with the inventive method and corresponding to the
material
referred to as sample 1 in table 1 were dissolved in a steel melt in a steel
ladle having a
charge weight of about 190 t, and the dissolution rate was compared to that of
ferromolybdenum produced according to the thermite process. Table 4 indicates
the
composition of the produced steel.
Table 4
Element % b wei t
C <0,2
Si 0,1
Mn 1,2
Cr 0,25
V 0,02
Mo - _ ~0'S
During the experiments, the steel bath was protected against reoxidation by a
calcium
aluminate slag, and for better homogenization, the melt was washed with Ar by
means of a
fireproof lance introduced from above into the melt.
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A total of six experiments was carned out, two charges of them with usual
commercial
ferromolybdenum having a grain size of 5-50 mm and four charges of them with
the
agglomerates according to the invention. The alloying agent was added via a
slide from a
bunker system. The samples were taken by means of an automated sublance system
at
intervals of about 20 s.
The experimental parameters are summed up in table 5.
Table 5
Charge Number 39999 40000 40300 40301 40324 40348
FeMo FeMo A 1. A I. A 1. A 1.
St. St. 1 2 3 4
LD converter
LD tapping time 11:24 12:16 12:46 13:35 11:16 09:34
Mo content % 0,064 0,074 0,012 0,066 0,075 0,087
in LD
Calc. weightt 190,8 184,2 192,8 182,7 192,8 189,9
LD
TN
Temp. arrivalC 1616 1627 1628 1604 1640 1627
FeMo additionkg 1000 1000 1000 1000 1000 1000
Mo content kg 681,7 681,7 724 703 743 743
Gas stirringmin 17 14 14 14 15 15
time
Gas flow Nl/min 925 922 763 765 900 922
de-S
Continuous
casting
plant
Start cont.time 13:28 14:31 15:07 16:09 14:00 15:48
casting
Charge weightt 191,1 I 81,7 191,7 183,1 190,1 192,3
Mo content % 0,493 0,622 0,481 0,497 0,629 0,49
1
Moyield % 98,6 94,1 96,1 99,9 95,8 95,6
1
Mo content % 0,488 0,637 0,482 0,501 0,625 0,492
2
Mo field % 97,4 96,7 96,3 100,8 95,1 96,1
2
As to Fig. 3, it can be seen that the inventive agglomerates dissolve much
faster, yielding
more molybdenum. From the curves relating to standard FeMo it can be seen that
even after
periods of treating the melt of about 10 min, less than 80% of the added
molybdenum has
dissolved in the melt. In practice, this means that such a melt has to be
heated up once again
in a pan furnace in order to obtain a commercial molybdenum yield, which,
however,
requires higher treatment costs.
Example 3
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Agglomerates produced with the inventive method and corresponding to the
material
referred to as sample 1 in table 1 were dissolved in a steel melt in a steel
ladle having a
charge weight of about 90 t, and the dissolution rate was compared to that of
ferromolybdenum produced according to the thermite process.
Table 6 indicates the chemical composition of the produced steel.
Table 6
Elements % b wei t
C 0,02
Si 0,5
Mn 1,5
P <p,04
S <0,0055
Cr 17
Ni 11
Mo 2,0
Al <0,007
<0,03
Four charges of the steel, each having a melting weight of about 90 t, were
produced. In the
ladle washing station, FeMo produced according to the thermite process was
added to two
charges and the inventive agglomerates were added to two charges. The added
quantities can
be seen in table 7. After the addition, samples were taken from the melt at
regular intervals
so as to be able to examine the increase in molybdenum content.
Table 7
Experiment FeMo additionForm of FeMo
k
E 1 347 standard
E2 414 standard
E3 250 a lomerates
E4 350 a lomerates
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Additionally, slag samples and samples from the cold rolled strip produced
from the steel
were taken during the experiments so as to be able to study a possible effect
on the degree of
purity of the produced steel, caused by the use of the inventive agglomerates.
Fig. 4 shows a comparison of the dissolution rates of the ferromolybdenum
produced
according to the thermite process vs. those of the inventive agglomerates. It
can be seen that
also in Example 3, the inventive agglomerates dissolve faster in steel than
the standard
FeMo.
The examinations of the degree of purity of the product produced did not show
any
significant changes caused by the use of the inventive agglomerates for
producing
molybdenum-alloyed steels.
Referring to exemplary applications in steel melts, Figs. 5 and 6 show further
examples of
the dissolution rates of inventive FeMo agglomerates as compared to standard
FeMo.
