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H5/202 E

The second 265 t RH DEGASSERThe second 265 t RH DEGASSER

at the Beeckerwerth steelplant

of ThyssenKrupp Stahl AGMETALLURGICAL PLANTS and ROLLING MILLS

SECONDARY METALLURGY

Reprint of a paper presented at

ASSOCIATION TECHNIQUE de la SIDERURGIE FRANCAISE,

Paris, December 11-12, 2002.

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The second 265 t RH-TOP degasser at ThyssenKrupp Stahl AG in operation.

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Introduction

Today, secondary metallurgical units represent the versatile

usable connection between melting and casting plants insteelworks.

Future process-improving measures must consequently be

developed and applied to ensure that the secondary metall-

urgical facilities achieve their full extent of availability and

thus a high level of productivity at low operative costs. Fur-

thermore, metallurgical processes must be optimised to

ensure that steel grades to be produced fulfil the continual-

ly increasing demands with regard to steel purity, precision

of chemical composition and low contents of undesired

tramp elements. RH and RH TOP vacuum units use the prin-

ciple of the vacuum circulation process, and are in particu-

lar applied in flat-product steelplants to enable mass pro-duction of deep drawing grades with very low contents of

Carbon under economically favourable conditions. Additio-

nally, RH and RH TOP

vacuum units are also used

for the removal of hydro-

gen, for deoxidation and for

the precise adjustment of

the chemical analysis and

temperature of the molten

steel.

TKS Beeckerwerth works

Oxygensteelworks 2 of ThyssenKrupp Stahl AG was ori-

ginally built and put into operation in 1962. The annualproduction of crude steel at Beeckerwerth steelworks was

5.3 million tons in 2001.

The steelworks is equipped with the following production

facilities: two pig iron desulphurisation units, three conver-

ters and two continuous casters, unit 2 of which was revam-

ped to a vertical bending continuous slab casting machine in

the summer of 2001.

The second 265 t RH degasserat the Beeckerwerth steelplant of

ThyssenKrupp Stahl AG

Heinz Liebig, ThyssenKrupp Stahl AG, Duisburg, Germany

Rainer Dittrich and Dieter Tembergen, SMS Mevac GmbH, Essen, Germany

Fig. 1. 2nd 265 t RH-TOP atThyssenKrupp Stahl AG

Heinz Liebig, ThyssenKrupp Stahl AG, Duisburg,Rainer Dittrich, Managing Director, SMS Mevac GmbH, Essen,Dieter Tembergen, SMS Mevac GmbH, Essen

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A majority of the product is supplied to the automotive indu-

stry. The continuously rising market demand for ultra-low

carbon steel grades could not be met by the existing RH faci-

lity no. 1. Additionally production of vacuum treated steel

was lost after the closure of the Dortmund works, however

the increase in market demand meant that production had to

be increased. The logical consequence was the construction

of a second RH facility.

Project aim

In recent years, the client demands for ultra-low carboncontent lead to a situation that the portion of steel melts that

have to undergo vacuum treatment increased by more than

47%.

The main task of the new RH unit is to decarburise liquid

steel down to carbon contents of less than 0.0020 % for hot

rolled strip. These steels have the highest mechanical requi-

rements, specifically excellent ductile properties. Further-

more, the vacuum facility is required to produce steels with

a hydrogen content below 1.5 ppm for the production of

heavy plates and high carbon steels. The annual production

was planned to be around 2.4 million tonnes of molten steel.Cycle times of less than 26 minutes at the continuous casting

plants had to be taken into account for planning the vacuum

unit, figure 2.

Basic concept and main components

The plant is designed as a RH fast-vessel-exchange plant,

figure 2. Each RH vessel is placed on a transfer car which

moves between the treatment position and the respective

stand-by position.

In the treatment position the vessel is connected to the gas

cooler, the suction line and the vacuum pump system by

means of a hydraulically operated compensator. The alloy

agent supply and the vacuum-tight lance passage are also

connected to the vessel via pneumatically operated com-

pensators. The TOP lance is lowered into the vessel in thetreatment position. In this position there is also a wire-fee-

ding machine and a lance system for taking samples and

temperature / E.M.F. measurements by means of a sleeve

manipulator.

Below the vessel two ladle transfer cars (one for getting

the ladle from the BOF and one for taking it to the conti-

nuous caster) travel on a rail track. These cars move the

steel casting ladle into the treatment position below the RH

vessel. The ladle is lifted out of the ladle transfer car by

means of a mechanical lifting device until the snorkels are

immersed into the steel. When the treatment is completedthe ladle is lowered into the ladle transfer car, serving the

continuous caster.

At each of the stand-by positions, burner lances are instal-

led which can be retracted into the stand-by vessel and

serve to heat-up or keep the vessel hot. Snorkel maintenan-

ce can also be performed simultaneously in these positi-

ons.

A vessel lining and heating system serves to dry and bring

the new vessels to temperature.

A separate repair station is provided for the wrecking, reli-

ning and heating of used vessels.

Fig. 2. Main components.

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Ladle lifting system

Lifting and lowering of the ladle is done by a mechanical lif-

ting system driven by a cable winch, figure 3. The laminated

hook can be swivelled hydraulically for picking up the lad-

les. The lifting system is equipped with two parallel opera-

ting drives. If one drive fails, the lifting system can be ope-

rated solely by the second drive.

The stroke is determined by means of two angle encoders.

The measured values of both angle encoders are constantly

compared to each other. If the values differ by more than 20

mm an alarm is triggered and the lifting and lowering pro-

cess is stopped.

After the ladle transfer car is positioned in the treatment

position, the automatic ladle pick-up and lifting process is

started while the beams are lowered by means of the lifting

gear with the laminated hooks in the retracted position.

The laminated hooks are then swivelled into the release posi-

tion and located below the ladle trunnions. The hook positi-

ons are monitored by limit switches on both the hydraulic

cylinder and the beam. The operator can check the hook posi-

tion by means of two cameras. Once the laminated hooks have

been driven into the bails of the ladle suspension system, the

lifting process is initiated. The correct load pick-up is moni-tored by a device which checks the load difference – separa-

te load recognition on the left and right crane hooks – and the

correct lifting position of the load. In automatic mode the lif-

ting and lowering sequence is started by the operator.

Vacuum system

The vacuum pump has four stages and consists of two low-

vacuum steam ejectors (stages 1 and 2), condenser 1, two

parallel operating medium-vacuum steam ejectors (stages 3a

and 3b), condenser 2 and parallel operating water ring

pumps as the atmospheric stage. The sucked off waste gas –

a high content of CO has to be taken into account during

decarburisation – is conveyed by the water ring pumps

through the flare stack to the CO postcombustion unit (flare

stack burner) where the remaining CO is burned and the

waste gas is expelled into the atmosphere, figure 4a.

In order to save steam at lower cooling water temperatures

and to control the vacuum in the range of 300 to approx. 60

mbar, steam ejectors 3a and 3b are equipped with injector

needles. For vacuum control in the range of 60 to approx. 6mbar, steam ejector 2 is also fitted with an injector needle.

The position of the needle in the tuyere is adjusted by means

of a pneumatic actuator, figure 4b.

The injector needles are driven into a certain position depen-

ding on the pressure in condenser 2. This control process is

superimposed by a vacuum control system which is active

for ejectors 3a and 3b until ejector 2 is switched on. After

ejector 2 is switched on, the vacuum control system takes

effect on the injector needle of ejector 2. The generated pres-

sure measurement is the reference input for this. When ejec-

tor 1 is switched on, the injector needle of ejector 2 is driven

to a defined position. Thus this arrangement ensures the best

preconditions to adapt the pressure reduction in the vessel to

suit the relevant metallurgical requirements, whilst also

taking into account the cost savings derived from reduced

steam consumption, figure 5.

Fig. 3. Mechanical ladle lifting system Fig. 4a and b. Vacuum system and ejector needle.

1 mbar 10 mbar 100 mbar

S t e a m [ t / h

]

20

15

10

5

0

25°C30°C34°C

Fig. 5. Suction capacity of vacuum system and steam consumptionaspects.

a b

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Vacuum alloying system

The purpose of the vacuum lock system is to allow the addi-

tion of alloys to the melt during the vacuum treatment. The

alloying materials are added via a side chute connected to

the vacuum vessel.

The vacuum alloying system consists of four vacuum weig-hing bins with a vacuum vibratory feeder arranged below

them and a vacuum shut-off gate as well as two vacuum

locks each with two feeding hoppers and each with a con-

veying chute arranged below.

The vacuum weighing bins (VB) are generally filled with

the following alloying materials:

- VB 01 Scrap 2 m3

- VB 02 Aluminium 2 m3

- VB 03 Titanium 2 m3

- VB 04 Iron silicon 10 m3

The two vacuum locks each consist of the following com-ponents:

- Large feeding lock 3 m3

- Small feeding lock 0.5 m3

- Vacuum lock 3 m3

- Vibratory feeder 3 m3

The alloys are added to the vessel via a vibratory feeder.

The conveying capacity of each can be adjusted via the

motor speed and is switched on according to the material. To

optimise the time, the pure filling process of the feeding

locks can be started by the operator irrespective of the addi-

tion into the vessel.

Located above these components is a rotary chute whichdistributes the material coming from the day bin system to

the corresponding vacuum weighing bin or feeding hoppers

of the respective vacuum locks. The pipe, which can be rota-

ted 360°, has eight discharge positions for filling and a fur-

ther ninth emergency discharge position in case of wrong

weights or for emptying components, e.g. conveying belts of

the superposed day bin system.

The very short cycle periods of the continuous casting plants

at the Beeckerwerth works dictated the design of this com-

plex vacuum alloy system, figure 6. The system design allo-

ws the realisation of very short alloy addition periods. Bypre-filling the feeding hoppers the treatment process can be

optimally controlled and the alloying materials can be added

at very short intervals.

Fig. 6. Vacuum alloying system

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Vessel exchange system

In order to ensure a high level of availability of the plant,

two vacuum vessels are provided.

Each vessel is installed on its own transfer car so that it can

be driven either into the stand-by position or the treatment

position, figure 7.

In the stand-by positions the suction pipe is closed by a

pneumatically operated refractory lined pan. Through the

TOP lance port the stand-by burner can be driven into the

vessel to a distance of 5,000 mm to the vessel bottom. The

purpose of the burner lance is to heat up vessels in the stand-

by position or to keep them hot.

In the treatment position the suction pipe, alloy pipe and

TOP lance are connected via hydraulically or pneumatically

operated compensators.

Change-over of the vessel from the treatment position to therelevant stand-by position is executed by the operator in

automatic mode. The entire process can be performed during

a sequence between two heats within a period of 4 minutes.

Fig. 7. Vessel exchange plant views

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Process operation of ULC steel grades

Amongst the requirements of the RH process, the foremost

is production of steel grades with very low carbon levels –

interstitial free (IF) steel stabilised by titanium or niobium

and high silicon electric steel, down to 20 ppm carbon.

To conduct the decarburisation process in the RH plant in the

best possible way, the starting conditions regarding the rela-tion of carbon and oxygen content have to be controlled.

The required carbon and oxygen content can be adjusted

easily and consistently by TBM (Thyssen Blowing Metall-

urgy, BOF converter) steelmaking. During tapping Ferro-

manganese is added. After tapping and homogenising by

means of lance argon stirring, the heat is transferred to the

RH unit, where the treatment is immediately started on arri-

val of the ladle. It is common practice to treat up to 7 heats

consecutively, which are sequence cast in one of the two

slab casting machines.

A typical RH treatment pattern is shown in table 1. Shortly

after starting of the RH vacuum treatment the steel begins torecirculate and the decarburisation reaction starts.

The decarburisation reaction is controlled by means of a

computerised pump down program and automatically plot-

ting of set points for lift gas flow rate. The heats have to be

decarburised within 15 minutes and the total RH treatment

time should not exceed 26 minutes.

Use of RH degasser No. 2

The secondary metallurgical treatment in the RH facilityNo.2 is used at ThyssenKrupp Stahl Beeckerwerth for the

following purpose:

■ Production of ULC-IF steel grades with minimum carbon

content

■ Production of heavy plates with minimum hydrogen con-

tent

■ Increase of alloying output of oxygen-affine elements

(e.g. Ti)

■ Production of high carbon steel grades

■ Homogenisation and control of small ranges of analysis,

even in case of large alloying quantities (e.g. for the pro-

duction of electrical grades)

The product mix of RH degasser No. 2 during the first 12

months after hot commissioning is shown in figure 8.

Fig. 8. Product mix of RH No. 2

Alloyed+Micro-alloyed steels

6.3 %High strength steels

5.3 %

Low-carbon steel6.9 %

IF-steel(Ti+Nb-alloyed)

8.9 %

High carbonsteels2.9 %

Cold rolledsteels3.0 % Tin plate

14.3 %

IF-steels (Ti-alloyed)52.5 %

Table 1: Total RH cycle time

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200000

160000

120000

80000

40000

0

P r o d u c t i o n [ t ]

Nov2001

Jan2002

Mar2002

May2002

July2002

Sept2002

Nov2002

69,144

87,086

104,564

118,174

139,129

150,513

140,617

174,187

183,542

181,440171,518

191,553

205,682

planned

achieved

C a r b o n [ p p m ]

50

40

30

20

10

0

Decarburisation time [min]

0 5 10 15 20 25 30

k c - v a l u e [ 1 / m i n ]

0.30

0.25

0.20

0.15

0.10

0.05

0.00

RH facility No. 2 (new)

RH facility No. 1

Carbon content (C1 + C2)/2 [ppm]0 10 20 30 40 50

C2 = C1 e-kt

Production of RH degasser No. 2

The 13 months following hot commissioning have shown

a continuous increase of vacuum capacity of RH degasserNo. 2. Within the first month of being put into operation, the

plant achieved a production rate of over 60,000 tonnes.

After eight months, more than 150,000 tonnes of crude steel

per month were vacuum treated in the RH degasser No. 2.

After 13 months a monthly production of over 200,000 ton-

nes was achieved, figure 9.

First metallurgical results

Decarburisation

After a decarburisation time of 15 minutes, Carbon is redu-

ced to approximately 15 ppm, figure 10. An increase of the

decarburisation time to 22 minutes permits the attainment of

Carbon contents between 10 to 12 ppm.

The relationship between the decarburisation constant (kC-

value) and the Carbon concentration obtained in the RHfacility No. 2 (new plant) compared to the RH facility No.1

is shown in figure 11.

The following points are important in case of the excellent

decarburisation results in the RH facility No.2 (new plant):

■ Degree of vacuum necessary in order to achieve internal

decarburisation by the high suction capacity

■ Carbon concentration of the steel in the vessel is made

close to that of the steel in the ladle by the high circulati-

on rate (snorkel and vessel geometry, lift gas flow rate)

■ Effective reaction area of the surface reaction by use of

strong agitation

■ Minimised Carbon contamination from the skull on the

vessel’s inner wall by improved operation of TOP lance

Fig. 9. Production of RH No. 2 duringthe first 12 months.

Fig. 10. Decarburisation results RH No.2 Fig. 11. Relationship between carbon content and decarburisationrate constant (kc)

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Hydrogen removal

The course of Hydrogen contents during RH vacuum treat-

ment for the production of high Carbon steel grades is

shown in figure 12. The Hydrogen content was determined

with the HYDRIS-system.

Removal of Hydrogen at the RH is quite effective due to its

high diffusion coefficient. The achievable hydrogen contentduring vacuum treatment depends on the low-vacuum tre-

atment time. Initial hydrogen contents have a negligible

influence on the achievable final content. A low-vacuum tre-

atment of 15 minutes makes it possible to achieve hydrogen

contents of less than 1.4 ppm.

Summary

In November 2001, ThyssenKrupp Stahl AG started up at

Beeckerwerth work, a new RH vacuum degassing equip-

ment. The RH degasser, supplied by SMS Mevac GmbH,

has been designed to produce 2.4 Mt/year. The main task of

this unit is to decarburise liquid steel down to 20 ppm for the

production of Ultra Low Carbon steel grades. Other capabi-lities include the reduction of Hydrogen, the decrease in

aluminium consumption and production of Ultra Low Carb-

on-high Si electric grades.

This plant has been designed to meet the targets in short tre-

atment times matching those of sequence casting at the exi-

sting casting machines. After a description of the main RH

degasser components and characteristics, the metallurgical

results achieved after the first 13 months operation were pre-

sented. Within this time the metallurgical targets with res-

pect to decarburisation and hydrogen removal have been

successfully achieved.

H y d r o g e n [ p p m

]

Vacuum treatment time (p < 2 mbar) [min]

3

2

1

000 5 10 15 20 25

Fig. 12. Hydrogen removal.

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SMS Mevac GmbH

Secondary Metallurgy

Bamler Strasse 3 a · 45141 Essen · Germany

Phone: +49 201 6323-0Telefax: +49 201 6323-200

E-Mail: [email protected]

Internet: www.sms-mevac.com 1 0 0 0 / 0 3 / 2 0 0 3 k y

· P r i n t e d i n G e r m a n y

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