Yin R. Metallurgical Process Engineering
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218 Metallurgical Process Engineering
and devices) with different kinds
of
connections(connectors). The steel manufac-
turing processes could not be separated from the three elements, namely flow,
process network and order.
It
was also pointed out that the steel manufacturing
process is an open, irreversible, and far from equilibrium dissipative process. As a
dissipative structure, the dynamic running entire system
of
steel manufacturing
process should not be studied with theory based on isolated system and equilib-
rium.
It
must be studied and analyzed with theories
of
dissipation, synergism and
system sciences
(of
course, knowledges
of
metallurgy, materials science and me-
chanics must also be included). Inprevious chapters, discussions and studies were
more on flow and order
of
the manufacturing process. Network structure and its
framework were also involved in the discussions, but not expanded enough.
Based on above analysis and studies, the structure and mode
of
steel plant will be
studied by means
of
discussing the functions
of
process network in metallurgical
manufacturing process.
8.1 Relation between Process Functions' Evolution and the
Process Structure in a Steel Plant
The procedures and devices are important components in the system
of
a manu-
facturing process. They are the composition units
of
sub-layers in the system
of
manufacturing process.
8.1.1 Positions andfunctions
of
procedures in a process
From the perspective
of
static structure
of
process network frame, procedures and
devices, which basically have three functions (see Fig. 8.1), are the nodes in a
network structure:
1. Receiving (input), transfer or heredity(output) functions.A certain procedure
or device is always openly receiving mass flow, energy flow and information flow
(input flow collection) from outside
of
the manufacturing process network or
from its previous procedure or device, and is openly transferring mass flow, en-
ergy flow and information flow (output flow collection) to its next procedure or
device. Therefore, procedure or device has receiving, transfor and output func-
tions.
2. Processing, transforming or even mutation functions. Manufacturing process
system is composed
of
lots
of
procedures and devices with different functions,
and each process or device has its own unique function which processes the mass
flow and energy flow in the manufacturing process by consuming energy and ma-
terials, thus making them under conversion, transformation or even "mutation".
It
should be said that, processing, conversion, transforming and mutation are fun-
damental functions
of
procedures and devices in the manufacturing process, and
Chapter 8 The Structure and Mode
of
Steel Plant Process
219
are the basic reason why procedures and devices could co-exist in the manufactur-
ing process framework.
3. Adaptation function to changes
of
external circumstances . Manufacturing
process is an open, irreversible, far from equilibrium dissipative structure. Due to
its openness the conditions
of
raw materials and energy are fluctuating and the
market demands for products are changing, therefore, the manufacturing process
should be put at a relatively optimized state by use
of
the adaptive ability (to
changes
of
circumstances)
of
each procedure and device. It is helpful to minimize
the dissipation
of
operation, such as minimizing material consumption, energy
consumption, processing time or even positioned space
of
the manufacturing
process.
Of
course, it must be also recognized that in the manufacturing process, the
above-mentioned functions
of
each procedure or device are not independent, but
mutually correlated, coordinated and conditioned.
III
IV
Fig. 8.1 Functions of procedures and devices
I - Rcceiving, transfer or heredity;
IT
- Proccssing, transforming, conversion or mutation ;
TIl
-Ad
aptation to surroudings, IV- Output, transfer or heredity
8.1.2 Evolution
of
procedures'functions
of
steel manufacturing
process
In the steel metallurgical production process, the function of procedure refers to
the behaviors and the major role it plays in the manufacturing process, particu-
larly the processing, converting, transforming and even mutation for mass flow
and energy flow. In steel manufacturing process, functions
of
many procedures
are multi-component, and some functions could be realized among a number
of
procedure. Therefore, people also propose a topic to what extent a function or
several functions would be realized in different procedures and about the com-
plementary, mutually replacing and optimized matching relations among these
functions, and also to study the topics about the combination and integration
of
the functions among different procedures.
220
Metallurgical Process Engineering
Here, it is going to seek to analyze and generalize the main functions
of
related
procedures in the steel manufacturing process from the perspective
of
process
engineering (Yin, 1993).
A. Functions of the
raw
material
yard
I. Storage and classification
of
the raw materials and fuels;
2. Physical blending, stabilizing and adjusting the quality
of
materials;
3. Matching-orderly output
of
the materials and energy;
4. Receiving the raw materials and fuels as a "buffer" seasonally;
5. As an environmental function to avoid various dust flying and control the
leakage
of
the unhealthy water, etc.
B. Functions of sintering
I. Pelletizing and adjusting the size and property
of
the particles;
2. Slightly prereduction and the chemical composition change, and new mineral
phases formation;
3. Treatment for the iron-containing waste or
SOF
slag, etc.;
4. Desulfurization
of
the flue gases and dioxin control;
5. Recovering the low exergy waste heat.
C. Blast f
urnace
(BF)
After gradual development, SF has had the following basic functions.
1. Reactor
of
oxide reduction and carburization
.Bf
is mainly characterized
by its shaft process. With the progressing
of
raw material preparation, the de-
velopment
of
coking and pulverized coal injection, the improving
of
hot blast
system, the developing and application
of
bell-less top, high pressure blast op-
eration and new generation
of
charging system, particularly with the SF
enlargement technology and the prolonging
of
SF
campaign, the utilization
coefficient
of
SFs
with different volumes has been stabilized at
2
~4
t/(m
3
od)
.
It
is the most efficient minerals oxide reduction device so far. The flexible selec-
tions
of
daily production between I
OOO
~
12000 t per unit SF could be made
now. On account
of
the irreplaceability
of
coke column on which the shaft
process is established, and the requirements on its smooth operation at high
temperature, the oxide reduction in
SF
process is always accompanied by hot
metal carburization at varied degrees.
2. Generator and continuous supplier
of
liquid metal.
SF
is the first step in
the manufacturing process to produce liquid metals with minerals-oxides as the
ferrous resources.
It
is the basis on which converter relies for existence. Al-
though
SF
process is a moving bed in shaft furnace moved slowly, the genera-
tion
of
hot metal is continuous. Particularly due to the extension
of
SF cam-
paign (already up to
12
~20
years)
and the deve
lopment
of
multi-hole
Chapter 8 The Structure and Mode
of
Steel Plant Process
221
tapping, the function
of
BF as hot metal continuous supplier could be fully
displayed with longer time limitation.
3. Energy
conversion
reactor
. BF
process
is accompanied by
bulky
energy
consumption
with
conversion
.
It
characterizes the
conversion
of
chemical
energy
of
coke, coal, natural gas and heavy oil into sensible heat and
chemical energy
of
BF outputs (they are all energy carriers, including hot
metal, BF slag and gas). With the progress
of
process
technologies, BF heat
efficiency has
been
increasing
gradually. The developing and application
of
blast
furnace top
pressure
recovery turbine
unit
(TRT)
power
generation
technologies and implementation
of
other
residual heat, surplus energy re-
cycling technologies, the function
of
BF
of
energy conversion has become
increasingly prominent.
4. Controller and regulator
of
metallurgical quality. Generally, the influences
of
BF process on metallurgical quality
of
products are mainly reflected in control-
of
Sand
Si contents in hot metal and hot metal tapping temperature. In consid-
eration
of
in-depth knowledge
of
BF process and different S
content
re-
quirements by various steel products,
under
reasonable cost conditions, the
optimized control values
of
Sand
Si contents in
hot
metal have
been
clear
and
could
be regulated. While hot metal tapping
temperature
would
have
significant influences on possible
subsequent
hot metal
pretreatment
and
hot metal carriage, particularly it prominently influences on the quality
of
sequence
treatment
desulphurization-disiliconization-dephosphorization
.
Therefore, the quality
of
hot metal
plays
an important fundamental role in
regulating the metallurgical quality
of
steel products.
D. Hot metal pretreatment
Hot metal
pretreatment
process is the
new
procedure based on in-depth
study on thermodynamics and kinetics
of
metallurgical process. From the
perspective
of
process engineering, its actual functions have already ex-
tended beyond its original quality control functions; it has the following
main functions.
I. Regulator
of
metallurgical loads and quality among procedures. Generally,
hot metal pretreatment is divided into two categories: hot metal desulphurization
and hot metal disiliconization, desulphurization, and dephosphorization. Both
would impact on the metallurgical loads
of
BF and BOF processes, and thus
directly impact on metallurgical quality
of
steel products. Owing to the good
thermodynamics and kinetics conditions for sulfur removal, hot metal pretreat-
ment has high efficiency with its economic value. Table 8.1 shows a comparison
of
costs spent on removing
lkg
sulfur with different procedures (Hubbard,
1990).
222
Metallurgical Process Engineering
Table 8.1 Cost to remove Ikg sulfur
by different procedures
Procedure
Desulphurization Desulphurization
by hot Desulphurization in Desulphurization
inBF
metal pretreatment BOF in ladle
Cost needed to remove
27 10.5 177 64
Ikg sulfur, USD
Of
course, for the steel product quality, hot metal desulphurization only helps to
control the sulfur content in final products in an economic and effective way, and it is
not able to completely and effectively control the shape and distribution
of
sulfide
inclusions in steel. Hot metal disiliconization and dephosphorization are
of
great sig-
nificance in reducing BOF's dephosphorization metallurgical loads even by using iron
ores with higher phosphorus contents.
It is effective way in steadily producing ultra-
low phosphorus content steels with phosphorus content
of
below 0.004%. Hot metal
disiliconization, desulphurization, and dephosphorization could reduce the slagging
loads
of
BOF, to remove carbon quickly and reduce ferromanganese consumption by
utilizing directly charged Mn-enriched ores, thereby to shorten the smelting cycle
of
large BOFs. Therefore, 100 percent hot metal desulphurization, disiliconization, and
dephosphorization could constitute a platform
of
producing low-cost clean steels in a
highly efficient way.
2. Energy regulator.Hot metal pretreatment can regulate chemical energy
of
hot metal by moderate disiliconization, providing economic and rational precon-
ditions for alleviating steelmaking slag loads, moderately using scraps and con-
trolling proper BOF tapping temperature.
3. Continuous operation coordinator- buffer
of
BF
-BOF
section. As BF has a
continuous reaction and a batch tapping, while BOF has a batch operation based on
certain cycle, the connection-buffer relation
of
BF-BOF
was once realized by mixer
for a period
of
time. The establishment
of
hot metal pretreatment procedure had led
to the abandonment
of
mixer, while large hot metal carrying vessels (torpedo cars,
hot metal receiving ladle, etc.) have gradually become coordinators and buffers
of
continuous operation between BF
-BOF
based on the optimization and matching
of
such important parameters as hot metal composition, operation time, temperature.
E. Oxygen
converter
(BOF)
Through
30
~40
years for BOF evolution and the replacement
of
open hearth by it, now
it has become the major steelmaking equipment from hot metal. With the development
of
related procedures like BF, hot metal pretreatment, secondary metallurgy and par-
ticularly continuous casting, BOF has alreadytaken the following major functions:
1. Fast and high efficient carbon remover.
It
is still the major task
of
BOF
steelmaking to quickly remove carbon from hot metal with more than 4% C
content. Owing to the technical progress in vessel design, oxygen lance design
and combined blowing, BOF's oxygen supply rate has been up to more than 3
Chapter 8 The Structure and Mode
of
Steel Plant Process
223
Nm
3
/(t • min), its oxygen blowing time could be shortened to
12
~
16 min per heat,
which is the fastest decarburizer.
If
the hot metal is pretreated by desulphurization,
disiliconization, and dephosphorization, slag-less smelting might be adopted. In
this case, even for BOF with capacity more than 200 t, the oxygen supply rate
might reach
3.8
~5
m
3
/(t • min) and the oxygen blown time
of
one heat could be
shortened to
9
~
12 min. That is
of
practical economic value for large BOF to be
coordinated with high speed slab caster. Large BOF (for instance, with more than
120 t volume), in effective combination with vacuum treating devices, could pro-
duce advanced low carbon steels, ultra-low carbon steels and ultra-low sulfur
steels efficiently.
It
is one
of
the fundamental technologies, on which a large mod-
em, highly efficient plate/strip steel plant is established.
2. Fast heating up vessel. The fast oxidizing reactions
of
elements like C, Si, P,
and Fe
of
hot metal in BOF lead to the fast heating up
of
metal bath, normally the
metal bath can be heated up from
1280
~
1350 'C to
1640
~
1680 'C quickly within
12
~
16 min, and besides that, it is still capable
of
melting certain quantity
of
scraps (for instance, not more than 25%).
3. Energy conversion reactor. BOF has become a reactor with energy conversion
function, relying on the physical heat and chemical energy
of
hot metal, the rational
oxygen supplying technique, and the recovering
of
BOF gas and steam. A well oper-
ated BOF can recover BOF gas
100
~1O8
Nm
3
/t and steam 40 kg/t, thus basically
compensating the energy consumed by the whole BOF heat, sometimes it could even
make steels with "excess energy". As far as only BOF is concerned, a large BOF
might be able to recover energy
5
~
10 kgce per tonne
of
steel.
4. Optimum de-phosphorizer. Owing to BOF's thermodynamic and kinetic
conditions, the moderate dephosphorization operation in a BOF is an optimized
and low-cost one. When used as a hot metal pre-treater, when Si content in hot
metal is reduced to 0.18% and under proper smelting system, BOF can prioritize
dephosphorization process in the metal bath with high carbon content. BOF can
also treat hot metal with different phosphorus contents. On the other hand, it is
also suitable to make ultra-low phosphor steel. To take an overall view
of
the
dephosphorization indexes
of
different smelting devices, BOF is still a optimum
de-phosphorizer. And desulphurization and disiliconization, should no longer be
the tasks
of
modem BOF.
F. Electric arc furnace (EAF)
With the
development
of
high-power,
ultra-high-power
EAFs and the exis-
tence
of
the relative technologies,
such
as water-cooled wall, water-cooled
roof,
eccentric
bottom tapping, oxy-fuel burner,
door
oxygen
lance, scrap
preheating
and partial
hot
metal charging, particularly elimination
of
reduc-
tion period
of
EAF smelting by
their
combination
with
secondary metallurgy,
the tap-to-tap time
of
EAF has been reduced to 60 min or even
shorter
224
Metallurgical Process Engineering
(Fig.8.2), thus the coordinating-buffering with sequence casting could be
achieved. The EAF
-fully
continuous casting production system under the
principle
of
one after another had been practiced. This is the key reason why
the overall advantages
ofEAFminimill
process could be fully displayed.
Secondary(ladle) metallurgy Water-cooled
furna
ce wall
.----
~
vo1t
a
g
e
!Lon
g
arc operation
j Computer control
Oxygen '
,F
oa
~
i
~
g
slag
blowing
:
~
o
l
c
d
roof
/Oxygen burner
~pping
Ladle furnace
.----
EHT
,
Pr
eh
e~
ting
in scrap bucket
:
L
~r
g
c
'5~
furnace
C/O lance
:
B
ott~m
stirring K-ES(DANARC,KORFARC)
i I Conti; uous scrap preheating
: i i
Tw
i
n
~s
h e
l1
furnace
iii
i
~
h:
n E
~
F
I I
':
I Central shaft E
i\F
IIII'
~
: :
::
i :Shar
E~
F
"with fingers
: I I I I : .Continuous charging shaft,DAN ARC PLUS
:
iii
: : :: CONA RC
I I I I I I I
.---------..
I I : :
:::
~RC
,MSP
,
I:!:
~
RC
, 35min
--_======
200kW'h/,
I.Okg/!
1965 1970 1975 1980 1985 1990
Year
1995 2000 2005
Fig.8.2
Development
of
modern EAF smelting technologies
Under these conditions, EAF functions are mainly reflected in the following
aspects.
l. Fast scrap melting unit. All the mini-mills are the plants with 100 percent
continuous casting system, in which caster is the core procedure for the dynamic-
orderly operation
of
the whole plant. In order to ensure the continuous operation
of
sequence casting in such plant, EAF functions must be optimized. To eliminate
the reduction period, to strengthen scrap melting function, and to shorten power-
on smelting time and auxiliary time by use
of
the new technologies shown in Fig.
8.2, the crucial functions
-scrap
fast melting and liquid steel fast heating-up are
succeeded. The power-on smelting time
of
modem EAFs has been shortened to
40
~50
min against the basic target
of
EAF smelting cycle in 60 min. Some EAFs
have advanced moving ahead towards the target
of
smelting 36 heats per day. In
this course, the EAF function
of
scrap fast melting has become increasingly
prominent.
2. Moderate remover
of
phosphorus and carbon.
It
is not only feasible, but also
economic to moderately remove phosphorus and carbon in ferrous materials
of
scrap and pig iron with suitable smelting technique in the oxidizing and heating-
up conditions
of
EAF fasting melting.
Chapter 8 The Structure and Mode
of
Steel Plant Process
225
3. Energy conversion reactor. EAF's energy conversion function is mainly re-
flected in
eff
ectively converting electric energy and chemical energy into thermal
energy, realizing fast melting, fast heating-up and correct control
of
tapping tem-
perature. As EAF emits bulky high temperature gas in the course
of
strengthened
energy supply and consuming a great quantity
of
chemical energy and oxygen, to
recover the energy
of
the hot gas has become one
of
the important goals
of
EAF
as an energy converter.
G. Secondary metallurgical equ
ipment
The functions
of
secondary meta
l1
urgy come partly from the se
para
ted
fu
nct
ions
of
BOF and EAF, partly from newly developed fu
nct
ions (vac-
uum treatment, powder injection, etc .) after in-depth research and partly
from the requirements by the continuous
operation
of a steelmaking plant
un
der
the production system
of
ful1y contin uous casting. In modern stee l
plants, almost 100% liquid steels are treated by various secondary metalIurgical
devices. Secondary metallurgical devices have become indispensable procedure in
steel manufacturing process and its main functions include:
1. High-efficient refining reactor. Most refining functions
of
traditional steel-
making furnaces have been largely replaced by secondary metalIurgy at higher
level and with stronger ability. For instance, to deeply remove the elements like C,
0,
H, N, S, P, trim the chemical composition in steels, carry out precise alloying
and homogenize composition, remove or control the shapes
of
non-metalIic inclu-
sions, regulate or homogenize the liquid steel temperatures. Table 8.2 shows the
evolution
of
refining function
of
secondary meta
lIurgy.
T
abl
e 8.2 Evolution
of
refining function
of
secondary metallurgy
Procedures
Classification of secondary metallurgy functions
De-
Alloy-
Homo-
Decarbu
- Deoxi-
Desulp
huri-
Inclusion
Inclusions Composition Heating
modifi
Year
gassing ing genization ri
zation
dation
zation
floating
up
trimming
up
cation
End
of
0 0 0
I
%Os
Endof
0 0 0 0 0 0 0
to.
1970s
Endof
0 0 0 0 0 0 0 0 0
to.
1980s
End
of
0 0 0 0 0 0 0 0 0 0
1990s
Remarks: 0 commonly used functions:
to.
functions under development.
2. Coordinating-buffering unit. In the coordinated operation between steelmak-
ing and continuous casting, one
of
the important functions
of
secondary rnetallur-
226 Metallurgical Process Engineering
gical devices is to coordinating-buffering, realizing longer time continuous opera-
tion
of
steelmaking furnace and caster. This coordination and buffer function is
reflected in the control
of
important technical parameters like mass flow, tempera-
ture, quality and time. The coordinating-buffering function
of
secondary metal-
lurgical devices is indispensable for implementing advanced technologies, such as
the long route hotlinkage between steelmaking furnace and rolIing milI, the de-
velopment
of
fully continuous casting and thin slab continuous casting-rolling.
3. Efficiency multiplier. Under numerous production conditions, the secondary
metallurgy function as efficiency multiplier is appeared in enhancing the produc-
tivity
of
the whole manufacturing process, increasing product quality and opti-
mizing cost.
It
is caused by replacing the reduction phase
of
EAF smelting, by
releasing BOF from decarburization (low carbon content stage), de-sulfurization
and de-gassing, and by shortening smelting cycle.
H. Continuous casting(CC)
The steel continuous casting had replaced the processes ingot teeming and bloom-
ing, its functions include:
1. Efficient solidification processing. Because caster is cooled by a high-speed
water flow, the temperature gradient between metal (solidified shelI) and mould
walI stays at a fairly high level (in contrast, in the course
of
ingot casting, the
temperature gradient between mould wall and solidified shell is gradually lower-
ing). Thus continuous caster characterizes as highly efficient solidification.
Of
course, what more manifests the characteristic
of
efficiency is the fact that cast
slab section is nearer to final steel product section than ingot, and the casting is
continuous.
2. Optimized forming vessel. The advantages
of
caster as a forming vessel lie
first in the fact that it could get a rational and economic slab section,which is coordi-
nated and matched with final product of hot rolIingmill much more. And owing to its
long length and heavy piece weight of unit slab, the blooming-breakdown procedure
could be eliminated. Meantimes, this solidifying way would help to realize mecha-
nized, continuous and automatic operation. The near net shape continuous casting-
direct rolling technology is about the further optimizationofforming, and it promotes
a new generation
of
metalIurgicalprocesses.
3. Terminal metallurgical reactor. With the maturity
of
caster process and
equipment, the role
of
metallurgical reactor is manifesting
itself
more and
more clearly. The caster has both the connotation as chemical metallurgy and
the function
of
improving metal-physical process, both correlate with the
quality
of
steel. From the perspective
of
chemical metalIurgy, the stream sealing
technique has prevented liquid steel from re-oxidizing, tundish metallurgy mould
flux behavior have provided further cleaning, and composition control for steel.
Soft-reduction with liquid core and dynomic control
of
secondary cooling have
Chapter 8 The Structure and Mode
of
Steel Plant Process
227
noticeable effects on reducing segregation and dendrite columnar crystal zone in
the slab. Concerning metal-physical process, mould powder and mould oscillation
techniques have provided important conditions for gaining good surface quality.
Electromagnetic processing and rapid solidification technologies have provided
good control for improving slab structures.
4. Energy saving device. On the one hand, continuous casting is energy
saving relative to teeming- soaking
-blo
oming process. As compared with
this process, continuous casting process saves energy
of
6
.
3
~
8.4
GJ/t and
increas es productivity by
1
O
%
~
15%. On the other hand, its energy saving
function is also reflected in the mass flow high temperature linkage of
steelmaki ng
-secon
dary metallurgy
-con
tinuous casting- hot rolling.
It
is
reflected not only in reducing energy consumption of metal reheating and
yie ld increasing, but also in the overall energy-reduce effect brought by
process optimization and fluent transportation from S F through to fina l
products. Table 8.3 shows a comparison
of
energy consumption among dif-
ferent steelmaking- rolling processes.
Table 8.3 Comparison
of
influences on energy consumption
by different steelmaking-rolling processes
Passing procedures
Fuel
No. Process
Surface
consumption
Steel
Ingot teem- Reheat-
for rolling
making ing
Soaking Blooming CC
condi-
ing
Rolling
/GJ ' ( I
tioning
I
IC
-CCR
0 0 0 0 0 0 0 2.01
2
IC- DR
0 0 0 0 0 0.920
3
CC
-CCR
0 0 0 0 0
1.34
4
CC
-HCR
0 0 0 0 0.878
5
C
-DHCR
0 0 0 0 0.334
6 CC
-DR
0 0
ETC
0
I. Reheating furnace
In the steel metallurgical process, changes have also taken place in the func-
tions
of
reheating (soaking) furnace, and to some extent, these functions have
been different from the functions
of
the operation coordinated with break-
down and blooming. On the whole, reheating furnace's heating-up function
has been relatively alleviated, while its function
of
linking-buffering-
coordinating has become increasingly prominent, reheating furnace's main
functions include:
1. Heating and energy conversion unit. Reheating furnace's fundamental func-