Note: Descriptions are shown in the official language in which they were submitted.
1357
This invention relates to a method of mal;in~ steel
from solid ferrous metal charges~ particularly scrap iron,
solid pig iron9 ferrous pellets, iron spon~e, or any
mi~ture of these, in a through-blo~n converter which is
provided with means for blowing o ygen at the top of the
surface of the melt and with means for injectin~ carbonaceous
fuels beneath the surface of tlle melt.
The production of steel from scrap metal ~ithout using
molten pig iron is nowadays frequently carried out in
electric arc ~urnaces but the economic viability of this
method depends very largely on current electrical energy
costs. By comparison ~ith an electric arc furnace~ the
conventional converter method where o~ygen is blo~n on top
of, or through the melt, is faster and cheaper. Mo~ever~
for making steel in a converter it has hitherto been
necessary to use liquid pig iron to which iron scrap is
added at an appropriate rate in relation l~rith its
composition as a cooling agent in the course of the refining
process. Normally the scrap proportion will amount up to
30,S of the steel ~eight.
~ :`
rrhere are however kno~n methods of producing steel in
a converter which per~it the abo~e~mentioned scrap ercen-
tage being increased. The requisite heat is applied to the
melt by preheating the scrap and by injecting carbonaceous
~uels into the melt. The thermal efficiency of these fuels
essentially determines the cost efficiency of th s ty~e of
process.
German Patents 50S 966 and 537 7t31, respective
dating back as far as tne years 1924 and 1929, already
describe a method of smelting ore in a converter wherein
coal dust and o~ygen? or o~ygcn-enriclled air, are
introduced into the melt. ~videntl the introduction o~
the fuels and the transmission of the required a~ount of
heat to the melt ~roved difficult because the patent of
-
~ 57
addition No. 537 781 suggests that the process according to
Patent 508 966 be undertaken in an electrically heated converter.
U.S. Patent No. 41 95,985 relating to a method of
applying heat in steel making in a converter describes a con-
venient mode of working aimed at enabling the scrap percentage
in the converter to be increased as may be desired so as ultimate-
ly to produce a steel melt solely from a solid ferrous charge.
However, according to the method proposed in this patent applica-
tion the scrap is introduced into the converter and preheated
for approximately 10 minutes. During this preheating phase
; oxygen tuyeres in the converter bottom operate as burners. After
preheating, molten pig may be added to the converter charge, or
else the work may be continued without an addition of such pig
iron. ~s soon as a melt has formed in the converter, ground
carbonaceous fuel, mainly coke or coke dust, and oxygen are
`injected through the tuyeres beneath the surface level of the
-` melt, the injected jet of fuel being sheathed in a protective
envelope of hydrocarbon gas medium. At the same time about the
same amount of oxygen as that which is injected beneath the
2Q melt surface is blown in a downwardly directed stream through
s ~
the free converter space on to the top of the melt surface.
In the practical application of this method it was
found that i~f the molten pig iron percentage relative to steel
weight is reduced substantially, even to the extent of using
exclusively solid ferrous charges, the melting down time
required for the solid charge tends to grow disproportionately
longer. This entails a corresponding prolongation of charge-
sequence time. Furthermore, energy consumption fluctuates in
the melting phase and mean energy consumption is higher. Con-
sequently it is evident that this known method cannot be applied
as reliably and economically without liquid pig addition as with
a minimum addition of
molten pig to each charge. A longer charge cycle must be ex-
pected and there are fluctuations in thermal efficiency of the
fuels which are used in the scrap-melting phase.
It is the aim of the present invention to provide a
steel-making method whereby steel can be reliably produced from
solid ferrous charges without an addition of molten pig iron
but with a high degree of energy economy, that is to say, at the
same kind of energyconsumption rate as normal when using a mini-
mum liquid pig iron charge addition, in relatively quick charge
cycles and therefore in a more cost efficient way than hereto-
fore possible.
Accordingly, the present invention provides a method
for producing steel from solid, iron containing metallic pieces
in a blown oxygen converter equipped with submerged injection
devices for oxygen and pulverized fuel, said method comprising
the steps of: introducing a charge of solid, iron containing
metallic pieces into a said converter. preheating said charge;
contacting said preheated charge in said converter with a quan-
tity of molten steel to produce a batch; smelting said batch for
a sufficient period of time to fuse the entire batch and convert
the same into a steel product; tapping a first portion of said
steel product from the converter for further processing; tapping
a second portion of said steel product from the converter and
retaining the same separately from said converter; and subse-
quently reintroducing said retained second portion of steel pro-
duct into the converter and using the same as said quantity of
molten steel during said contacting step, said second portion
and therefore said quantity of molten steel being of sufficient
size to substantially reduce the length of said smelting time
period.
In one example of carrying out a method in accordance
with the invention use may be made of a through-blown converter
~ _
~,`.i,~
57
such as is described in U.S. Patent No. 4,195,985. This conver-
ter has oxygen-injecting tuyeres disposed beneath the surface
level of the melt through which oxygen-containing fining agents,
preferably conventional oxygen, hydrocarbon-sheathed for tuyere
protection, can be injected into the melt. At the same time the
converter is provided with oxygen injection means above the sur-
face level of the melt through which oxygen can be directed at
the top surface of the melt. These oxygen injection means may
be jet nozzles or tuyeres, or they may be water-cooled lances
of conventional kind. Between 20~ and 80~ of the total amount
of oxygen is supplied from above by one or more nozzles or tu-
yeres which throughout a
,.
- 4a -
5~7
substantial part of the fining process operate as iree jets
blowing into a gas-containing space. lhe converter is
further provided wi-th jet no~les for injectin~ carbon-
containing fuels below the melt surface. Suitable fuels for
this purpose are cole~ lignite coke, graphite, various
quality coal and mi~tures of these.
The solid ferrous metal char~es, particularl~ scrap
iron, solid pig iron, pellets, iron sponge or any mixture
of these are initially preiheated in the above described
converter. During this preheating phase, the tuyeres benea-th
the melt surface are operated as burners, ~uelled by liquid
and/or gaseous hydrocarbons, preferably natural ~as or oil.
Surprisin~ly it ~ras found that the melting do~m time
and the thermal efficiency of the a~lied fuels ~`or melting
the scrap remain approximately constant with an increase of
the char~e amount of solid ferrous metals in a 30 tonne
converter up to 30,~ of the melt~ in a 60 tonne converter
up to approximately 20~' o~ thc melt and in a 200 tonne
converter up to about 10,' of the melt.
Ilith lOOC~ solid charges (e g. scrap iron) ~elting do
time, that is to say, the tim_ followin~ the ~reheatinO
phase until a melt is for~ed in the re~ion of the jets at
the converter bottom, is prolon~ed more than had been
e~pected and effective use of the energy of applied ~uels
fluctuates rather widely during this phase, bein, on
a~rerage lo~ter tllan in co~arative ~elts containin~ a liquid
cllarge percenta2e. Probably an e-i~lanation of this
e~tended meltin~ c'o~m phase and ttle artlr increased energ-~r
consumption ~then wor.;ing ~itnout a liquid char e addition
consists in that the transition fro~l prelleated scrap to a
liquid sum? of ade~uate volurle ~;herein tile carbon-
containin~ fue7s can be disscl;ed and burnt to ~a~e C0 and
liber2te heat~ tends to ~-ary o~rin~r to the relatively ~; c'el~
fluctuatin~ operative condl-tiolls in tihis process ~llase.
~.~5~D~57
Varyin~ operative conditions ma~ be caused, for e~am~le, by
different lcinds of scrap, the ?acl~ing in the converter~ the
de~ree of o~idation achieved in the preheatin~ phase of the
solid charge and the resultin~ modification in heat
transmission. `~Ioreover~ fluctuations must be e~pected
durin~ meltin~ down and in the initial formation of the
melt in the converter, and its position relative to the
tuyeres.
~ Io~Yever, according to the present invention, from
lOp to 30~'~ more steel is produced in the specified ConVQrter
than the desired tap~ing weight, i.e. the al~ount of steel
desi~nated for further proccssing in the steelworl;s For
e~ample, approcimately 120 tonnos of st~el are re~ined in a
100 tonne converter but only 100 tonnes of this melt are
tapped off for further processin~.
~ fter the main melt has been tapped off~ the
additionally ~roducccl ~uan1;ity of steel, amountin~ to
10',' - 30Co of the convorter contents~ i9 dra:ined into a
holding vessel, e.~. a suitable ladle acting as a holcling
or au~iliary ladle9 In this holding ladle the steel is
held for a certain time and then added to tlle ne~.t rnelt
after the ~reheating phase of tne ne~t soli~ c~arg~e.
It is important ~or the pra~tical application as ~rell
as from the point of view of an econor~icall~ efficient modus
operandl of the new process that the liquid steel wllich i9
intended to be used in the ne.:t char~e be drained from tlle
converter and re-introduction aftcr the scra~p iron o~ tllc
neJt charO~e ~as been char~ed and preheatecl in tile converterO
Leavin~ a resiclual amount of molten stesl inside the
ccnverter would en-tail tlro vi-tal dra~rbaclcs: a re~aincler of
liquid steel in the converter makes it ~mpossible to
~reheat the ne~s o]lo~rin~ sol:d scrap iron cha ~e b~r me-ans
of the b-lrncrs in the con~erter bottom. ln t~at case ~t
least vwice tlle amount OL liquid stoel wou~cl be needed as
~5~57
compared with what is recuired wnen operatlng with preheated
scrap char~es. Furthermore, it is alwavs ha~arclous to adcl
a scrap iron charge to a steel melt~ especiall~J ~hen therc
is a lar~e quantity of thc latter, becaus~ normally the
operation of introducin~ the solid scrap into the converter
involves manipulations at the converter and also because
additional reactions may then occur bet~een the scrap ancl
the molten steel.
The molten steel remainder which is to be recycled
in the converter, i.e. the additional lG~ of 30'o of steel
~lich were overproduced in the precedin~ charge cycle, mav
be re-carbonised to achieve a carbon contcnt wllich is higher
tllan l~ot preferably bet~een 2Co and 3~ hilst such
carbllrisation ent~ils coolin~ of the steel in the holding
ladle~ the lo~erin~ of free~ temperature for the hlgller
carbon melt on the other hand reduces tihe risl; of the
molten remainder in tlle holding ladle Pree~in~ partiall~,~
~hsn comin~ into contact with the solid scrap iron on
bein~ re-char~ed into the convertor, In the final result
the injection of o~:~gen~ with the consequent combus-tion o~
the carbon contained in the melt~ produces energ~ ~ich
contributes to the desired fast heating up of the sump in
the converter and increases the amount of li~uid melt in
the converter.
ln order to achieve optimum thermal efficiency in the
preheating of the solid ferrous metal charge in the
converter it is desirable that no residual amount of s~eel
should remain in the converter at tne end of a charge c~cle.
I-Io~ever, if thc steel is drained completel~,~ from the
convarter a certain amount oi molten slag ~ill inevitabl~;
be carried alon~ with it. Therefore there is a rlsl; of
uncontrolled rcactlons betueen this sla~ hich is rich in
lron o~ides and the carburised stcel in tlle holding ladle.
For this reason, according to tlle present invent on,
i7
deo;idising agents, e.g. silicon, in percentages rznging
from 0.1 to 1.5,~ are preferably added to the residual melt
and this largely suppresses such undesirable slag/steel
reactions.
It is found particularly economical to acld carbon,
either ~holly or partly, to the resiclual amount of molten
steel while this i9 still in the converter and before it
is tapped into the holding ladle. Recarburization is thus
easily achievecl by the injection of carbon-containing fuels
in finely ground form, e.g. coke, through the e.{isting
bottom tuyeres.
.
Rccarburization of the residual melt in the eonverter
has speeial advantagcs when the stcel is rcfined by
application of the so-callod two-slag process. Accordin~
to this t~ro-slag process, the first slag is dra~n off as
completely as possible after the main refining periocl.
This is the slag which contains the major part of those
elements which are to be removed in the course of the
refining process~ sucll as phosphorous and sulphur. During
the after-blowing, or finishing phase a second slag is
~ormed in the converter with an iron-o~ide content which
is nor~ally be-tween 1~-, and 20c,:,. If no~ carbon is inJected
into the residual amount of steel which is to be recycled
in the ne~t char~e of the converter~ this will at the same
time reduce the iron oxide content of this secondary sla~
l~lich is still contained inside the converter. The sla~
becomes viscous and crumbl~ or friable, ancd can in this
condltion be more easily held bacl inside tha converter
~;hen the remaincl0r o~ the steel is tap~ed off into the
holdin~ ladle. This sla~ is le~t in the converter as a
source ol lime for the ne t .mel-t. This method also cuts
clo~n on lime consum~tion lJilil S t a~ the sama time improving
thermal efficiency inasmuch as there is less lime to be
lleated u~ a 4 the be.gillning of the crcle~ It was ~urther
57
found that such partly crumbly slag consistency improves
thermal efficiency of the system as a ~hole in the
preheating phase by comparison with a molten slag.
Probably this is due to the fact that liquid slag tends to
free~e more readily on coming into contact Wit;l the solid
metallic charge, e.g. scra~ iron, and thus detracts ~rom a
ready heat e~change. Besides~ as already mentioned, it is
very difficult to drain the residual amount of steel
completely from the converter ~.hile the sla~ has a thinly
liquid consistency. On the other hand3 residual amounts of
steel inside the converter clearly entail a deterioration in
thermal efficiency of applied fuels during the proheat~n~
~hase as previously mentioned.
It is fully within the scope of this invention to adapt
its essential characteristic aspects, namoly tho recyclin~
of a residual amount of suxplus molten steol in the ne~t
charge~ to tlle speci~ic operative parameters of different
steelworl;s. This eovers varlations within ver~ id~ limits
in respect o~ the quan-tit~ of re-cyeled steel, time of
addition and allo~ing percentages, partic~larly in regard to
carbon and silicon, ~hilst maintaining the basic inventive
principle of producing steel ~ithout tne aid of molten plg
iron.
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