application of sustainable improvement in continuous casting...
TRANSCRIPT
Application of Sustainable Improvement in Continuous Casting for Steel
Construction
PT KRAKATAU POSCO
BY
YUSUF NUGROHO*
JUNG YEUNG-KANG**
SYNOPSIS:
PT. Krakatau Posco is an integrated steel factory in South East Asia. Specifically located in
Cilegon, Indonesia. At the beginning of production, the target of this company is to stabilize
operation and achieve production targets. However, currently PTKP has achieved a stable
operation and started to build national energy, especially in the construction sector. One of them
is steel that applied for the elevated high ways. To produce this steelgrade, clean steel and high
performance of the continuous casting machine is needed. In the beginning of production various
obstacles have been experienced such as longitudinal cracks that was found on slab surface. In
order to obtain stability and quality, some improvement were applied. They are mold powder
modification and molten steel flow control in the mold. From this paper, it will further discuss
about these improvements. By doing all those improvements, the quality issue especially for
longitudinal crack can be removed during the production of steel grade for steel construction
application
Keywords: Improvement, Continuous Casting, Longitudinal Crack, Mold Powder.
* Engineer, Continuous Casting Plant – Krakatau Posco, Cilegon, Indonesia
** Global Engineer, Continuous Casting Plant – Krakatau Posco, Cilegon, Indonesia
1. Introduction
1.1 General process
PT. Krakatau Posco is an Integrated Steel Mill that has single line process. It means that
process is started from the raw material until become a finished product and all of the main
facilities are just available one part machine that interconnected each process to produce the steel
without any spare facility during abnormal process. The products of PT Krakatau Posco are steel
slab and plate. The iron making process in PT Krakatau Posco is using Blast Furnace technology.
Ore and other material are fed into Blast Furnace to be processed into molten iron or pig iron.
The molten iron will be processed become molten steel in steelmaking plant. The technologies
that are used in steelmaking plant are Kanvara Reactor, Converter, Ladle Furnace, and RH
(Rheinstahl Heraeus) Degasser. The main purpose of steelmaking plant besides converting the
iron into the steel is also to adjusting the steel composition based on customer order. The next
process after treating molten steel in Steelmaking Plant is casting process. This casting process is
done in Continuous Casting Plant. In Continuous Casting Plant, molten steel from Steelmaking
Plant will be solidified to be teel slab with certain dimension.
Currently as the results until August 2018, daily charge per day of PT. Krakatau Posco is
31.0 charges per day (every charge has 300 tons molten steel) and 2 million tons of crude steel
has been produced. The general specification of PT Krakatau Posco can be shown at the table
below:
Table 1: PT.KP specification
Facilities Clasification
Blast Furnace BF 3800 m3
Pretreatment KR x 2 ea
BOF 300 tons/ch x 1 ea
Refining equipment LF x 1 ea, RH x 1 ea
Continuous casting 1 ea slab caster (2 strands)
Plate and sheet mill Plate mill x 1 ea
Capacity 3.0 M.Ton
1.2 Continuous Casting Process
To make high quality of steel construction plate a good quality slab is needed. Therefore
continuous casting machine has a very important role to make basic materials which will be
rolled later on plate rolling mill. To make high quality slab, some technologies and facilities are
needed to support the production and ensure the product quality. The main facilities that are used
in continuous casting plant such as Ladle, Ladle Turret, Shroud Nozzle, Tundish, Submerged
Entry Nozzle (SEN), Mold, Segment, and Torch Cutting Machine (TCM). Generally the process
in continuous casting plant can be seen in Figure 1. The flow of molten steel is firstly started
from ladle to tundish using shroud nozzle as guidance, next through submerged entry nozzle
(SEN) the molten steel will flow inside mold. In the mold and segment the molten steel will be
solidified and then this solidified steel will be cut using torch cutting machine (TCM) become a
slab.
Figure 1: Schematic of Continuous Casting Process
Ladle is used for transporting molten steel from Steel Making Plant to Continuous
Casting Plant. Molten steel from ladle will flow to tundish through shroud nozzle that the
function is to avoid re-oxidation. After come into the tundish, then molten steel will flow into the
mold. This molten steel flow from ladle to tundish through submerged entry nozzle (SEN) which
function is to avoid re-oxidation and make a double roll flow inside mold. Then mold flux will
also be used when the molten steel is already inside of mold. The function of this mold flux is to
avoid re-oxidation of molten steel by surrounding air, avoiding thermal lost, catch the inclusion,
and also act as lubricant between molten steel that is solidified first (shell) and mold wall. The
molten steel that has got primary cooling from mold cooling then come to the segment. There are
total 16 segments to treat hot strand with secondary spray cooling and roll gap. The roll gap of
segment is one of the important parameter that can affect the quality of slab that is produced.
In the last part of casting machine there is Torch Cutting Machine (TCM) which function
is to cut the slab with a certain length using oxygen and natural gas. This device will cut the slab
with certain length according to customer order. General specification of Continuous Casting
Plant PT Krakatau Posco can be seen in Table 2.
Table 2: PT.KP Slab Caster Main Specification
Caster Type Vertical Bending
Capacity 3 million ton/ year
Radius 95000 mm
Strand 2
Slab Size :
Thickness 230 mm
Width 1100-2300 mm
Casting Speed Max 2.1 m/min
Metallurgical Length 38.9 m
Secondary cooling Air Mist
Soft Reduction Yes
EMS Yes
2. Problem Identification
Currently PT Krakatau Posco is involved in the construction of the Jakarta-Cikampek toll
road. In that project, a steel structure is needed to support the elevated highway construction.
Steel structure is needed due to it can be reduce dead load, more economic foundations, simple
erection procedures, shorter execution time and also faster and easier rehabilitation. The steel
structure has code specifications JS-SM520B and JS-SM570TMC that will be use to make web
and bottom flanges for steel box girder structure (the part of elevated highway). This product
must has good mechanical properties like proper strength, flexibility and high thoughness to
sustain the high load from vehicles that pass and from structure it self. Plate application and
Mechanical properties can be shown on picture and below
Figure 2. PT KP plates for elevated high way
structure
Figure 3. Steel box girder to support elevated
high way
Table 3: Mechanical properties of product
Spec Code Yield Point (N/mm
2)
Tensile Strength
(N/mm2)
Elongation
16 or under Over 16-40 100 or under Thickness (mm) % min
SM520B 365 355 520~540
<5 19
≥5<16 15
≥16<50 19
SM570 460 450 570~720
<5 19
≥5<16 26
≥16<50 20
PT Krakatau
Posco’s plates
In the process, during producing these steel grades, a lot of longitudinal cracks were
found on the slab surface. Longitudinal cracks are one of many problems that can be occurs on
continuous casting product. Generally, the initial formation of longitudinal crack was occurs in
the mold. They can be caused by a variety of sources such as including non-uniform heat transfer,
mold level fluctuations, inadequate taper, excessive taper, or mold powder used that not suitable
characteristic.
Figure 4. Crack occured on slab Figure 5. Crack occurred on the middle
of plate
If this condition was ignored, longitudinal crack will be very dangerous for product
quality. Even though it is only a small longitudinal crack, then slab was rolled by plate rolling
mill, it will make a crack along the plate. An in the end, the plate must be rejected because it is
not in accordance with the quality product standards.
Base on this problem, test plan was arranged. First of all is measuring the flow of molten
steel in the mold. After several measurements, it turns out that when the steel is produced the
flow obtained is a single roll. This condition make molten steel temperature inside mold was do
not homogeneous and the shell that has been formed will pulls by each other and make initial of
crack. Then substitution of mold powder is also considered to make molten steel lubrication was
getting better in the mold so that a homogeneous temperature is formed on the width side of
mold.
Position 600 ~ 1000mm from right
Length 200~ 1000 mm
Width 2000 mm
Width 1~3 mm
3. Sustainable Improvement
3.1 High Standard Operation Control
Standard operation has significan role to produce high quality product, from molten steel
to be slab and plate rolled mill. More detailed operating standards will affect to the product
results especially for cleans steel product. For example is chemical composition content, the
accuracy of the composition is very advanced. Oxygen bubbling free on RH vaccum will make
oxygen content inside of molten steel to be very low and reduce inclusion built up during
continuous casting process. The general standard operation from steelmaking to continuous
casting can be shown on the table below
Table 4: Standard operation procedure
Process Item Detail Operation Method
KR [S] After KR Treatment [S]: <20 ppm
Scrap standard usage : +skull #2 max 10 ton
BOF
Lead Time Top Ch ≤85 min, Base Ch ≥65 min
Bottom Bubling ≥ 80 % bubbling
Tapping
Alloying (Aim for target value)
BAP Bottom bubbling (≥5min)
CaO : 1.2 ton
Flourspar : 70kg
2nd
Refining
Holding time ≥20min
LF Treatment LF only for heating
RH Treatment
Ensure molten homogenized before alloying
RH <2 torr following Sec. Ref Code (H3)
Circulation with Ar
Avoid RH Oxygen blowing
Ti input only in RH
Composition Adjust to aim for target composition
CCP
Bell type Shroud nozzle, Ar+NG Mix gas must apply
Scarfing Input scarfing code and inform to finishing to hold the
slab wheather there is abnormality
Check Scarfing Conduct Check Scarfing for representative slab
From the table above, generally all treatment methods were applied to make cleans steel
before cast at continuous casting machine. Lead time was applied to give optimum time from
treatment in every charge. Then bottom bubbling applied to make homogeneous temperature and
molten steel composition. In every charge minimum holding time was adjusted to give a change
for inclusion floating up and catch by slag. RH OB was avoided to prevent inclusion built up
after RH treatment. During cast on continuous casting machine also has standar procedure for
using shoud nozzle bell type, that has to imers to molten steel on the tundish before pouring
conducted. And for final result, details checking were needed to early detection of slab surface
quality in every representative slab. This procedure has purpose to make sure there is no deffect
before rolling at plate rolling mill.
3.1 Electromagnetic Stirring System (EMS) Modification
Currently, the quality of the continuous casting product has been paid more and more
attention. With the stress on the production of high quality slab, there are higher requirements for
the microstructure and the composition homogenization of the casting product. The chemical
composition, solidification conditions and the nature of the liquid steel flow in the mold affects
to the surface quality and the inner structure of the cast product. Base on this, electromagnetic
stirrer system was needed to control the molten steel flow inside the mold with the specific
current applied.
During steel for construction application production, especially for JS-SM520B and JS-
SM570TMC project with the slab dimension 230 mm thickness and 2000~2300 mm of width,
the current that applied on the EMS system is approximately 50~100 amphere. But with this
current supplay, the result is not satisfied. Target flow in the mold should be double roll but
actual flow is only single roll. This condition make flow inside of mold did not optimum and can
cause the surface deffect. The schematic of EMS used was shown on the picture below.
Figure 6. Molten steel flow scheme on mold before improvement conducted
From the scheme above, molten steel flow was only single roll. The flow of molten steel was
directly to the side and inside of strand, so make mold surface was cold due to did not get new
supplay of molten steel from the tundish.
Base on the problem that already mentioned above, the improvement was arranged to
make the flow of molten steel get the optimum scheme. Several test were conducted with the
variation of current supply applied. Finally the optimum current can be obtained with the current
value range 10~20 amphere applied. The scheme of molten steel inside can be shown on the
picture figure 7 below.
Figure 7. Double roll obtained after inprovement conducted
3.2 Mold Powder Modification
Continuous casting mold powder is used primarily to facilitate the passage of steel
through the mold. Mold powder plays an important role and to be one of the most critical and
influential factors in the continuous casting of liquid steel.
Figure 8. Schematic diagram of mold powder layer in the mold
During casting, the powder melts on the steel surface, forming a layer of liquid mold slag.
Subsequently, the mold slag infiltrates between the steel shell and the mold creating a thin slag
film which solidifies into glassy and crystalline phases. The properties of the slag film dictate the
main functions of strand lubrication and mold heat transfer. The formation of crystals is
favorable for a homogeneous and controlled (horizontal) heat transfer during casting, which is
Mold Flux
Sintered Layer
◀Molten Metal
◀Solidified Shell
◀ Liquid Film
◀ Solidified Film
Mold Slag Pool
H2(g)
CO(g)
(a) low [H] content (b) high [H] content
Mold Flux
Sintered Layer
◀Molten Metal
◀Solidified Shell
◀ Liquid Film
◀ Solidified Film
Mold Slag Pool
H2(g)
CO(g)
Mold Flux
Sintered Layer
◀Molten Metal
◀Solidified Shell
◀ Liquid Film
◀ Solidified Film
Mold Slag Pool
H2(g)H2(g)
CO(g)CO(g)
(a) low [H] content (b) high [H] content(a) low [H] content (b) high [H] content
required to prevent the formation of surface cracks. Mold powder has some main function, they
are; molten steel lubrication in the mold, covering the molten steel surface and prevent re-
oxydation, catch the inclusion inside of mold, and also heat up and loose some carbon by react
with oxygen that can make temperature inside mold will be homogeneous.
To decrease and prevent slab surface deffect especially longitudinal crack that occur
during produce JS-SM520B and JS-SM570TMC project, proper mold powder with the specific
specification was needed. Base on this, mold powder test plan with high basicity was arranged.
The purpose of using high-basicity mold powder are to provide strand lubrication and to control
mold heat transfer in the horizontal direction between the developing steel shell and the water-
cooled copper mold. Powder specification comparison will be shown on Table 5 below.
Table 5: Mold powder specification comparison
Convensional Type D9-S Type
Basicity 1.28 1.65
SiO2 31.02 26.4
CaO 31.50 43.8
Melting point 1224 1221
Viscocity 1.67 1.50
4. Improvement Result
Finally after sustainable improvement that have been applied, stable operation and high
quality of product can be achieved. By controlling standard procedure from steelmaking until
continuous casting plant cleanless steel can be produce and achieve product requirment Table 6.
Molten steel flow has getting better with schematic double roll flow in the mold. Mold powder
has also significant influence to decrease slab surface deffect especially for longitudinal crack.
The comparison table during mold powder trial test can be shown on Table 7 below. From this
table can be shown that with those improvements are very effective to decrease slab deffect ratio
especially for longitudinal crack occurence deffect.
Table 6: JS-SM520B and JS-SM570TMC Operation result
Total
Charge
Converter Secondary Refining CCP
L/D
B/B
Open
Ratio
Heavy
Treatment
RH Holding
Time
(≥ 20
min)
NG+Ar
Purging
/Shroud
Bell
Typed
Close
Start
(≥30
ton)
No
RH-OB
No
Coolant
Circulation
after
alloying
(≥4min)
13 100% 100% 100% 92% 100% 100% 100% /
100% 100%
Table 7: Longitudinal crack base on mold powder type
1.3
0.0
Convensional High basicity
LC ratio
(%)
Basicity
(poise)
1.28
1.65
5. Conclusion
Base on improvements result that were mentioned before, continuous casting machine
has significant role to produce high quality of slab. With high quality of slab, plate specification
such as dimension and mechanical properties can be produced by plate rolling mills. And in the
end, with steel for construction product PT Krakatau Posco can play a major role for national
infrastructure development.
6. Reference
R. Tachibana, M. Kodama, T. Chino, N. Shibata, T. Nozaki and S. Okano: Tetsu-to-Hagané, 62 (1976),
S88.
Masahito HANAO, ISIJ International, Vol. 53 (2013), No. 4, pp. 648–654