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