A.5 Emerging techniques
A.5.1 Efficient use of energy
A.5.1.2 Efficient energy use in thin slab casting and directly connected rolling
Direct strand reduction processes, which link a thin slab caster and a following hot rolling mill, are commercially available from different plant manufacturers. Several concepts have been developed, e.g. CSP ("Compact Strip Production") by SMS, ISP ("In-line Strip Production") by MDH or CONROLL ("CONtinuous thin slab casting and ROLLing") by VAI.
These concepts, combining continuous casting and hot rolling, take advantage of the heat contained in the cast steel. Thin slabs are cast with a thickness of about 50mm (CSP), 60mm (ISP, directly soft reduced to about 45mm) and 75-130mm (CONROLL) and are then further reduced in rolling stands [52, 51, 72, 73]. In the following, the above-mentioned three concepts are briefly presented. All of them have in common, that they aim to minimise the number of process steps from liquid steel to the coiled hot strip, while still achieving a good surface quality. The steps required to produce hot rolled coil with these technologies are [52, 134, 140]:
• Casting
• Soaking
• Forming
• Cooling
• Reeling
Another process, which directly links casting with hot rolling is the TSP-process (Tippins Samsung Process) by Tippins Incorporated. This technology combines a thin slab caster (strand thickness about 100-125mm) with a Steckel-mill. Cf. i.a. [137, 183, 184] for more detailed information on this process.
A.5.1.2.1 CSP - Compact Strip Production
Figure A.5-2 presents a diagrammatic view of the CSP process.
Coiler Cooling line
Rolling mill Soaking furnace
Mould Ladle Tundish
CSP.ds4
Figure A.5-2: Diagrammatic view of a CSP plant
Source: [73]
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The basic features of the CSP technology are: A continuous caster with a specially designed mould shape and vertical strand guide producing a 50mm thin slab, which is cut into individual lengths according to the required coil weight. These slabs enter an equalising furnace (e.g. roller-hearth furnace) at a constant speed. After equalisation, the thin slab is accelerated to the considerably higher rolling-mill entry speed and is fed to the mill after passing through a high-pressure descaling facility [50]. The energy requirement for soaking within the CSP process is limited to compensation of idle running losses within the roller hearth furnace. It amounts to about 25kWh/t [134]. Figure A.5-3 presents an energy balance for a single strand CSP plant.
Casting
Solidification region
1550°
Steel in mould
100% =1311 kj/kg = 364.16 kWh/t
18.3% Mould cooling
Figure A.5-3: Energy balance for a single-strand CSP plant
Source: [50]
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Products that have already been produced with the CSP technology show a thickness range from 25mm down to 1mm for many steel grades (e.g. next to carbon steel also micro-alloyed steel and Si-alloyed steel for electro-plate; cf. [134] for a detailed list of steel grades, that have been processed by CSP technology). CSP plants allow a production of hot strip of about 1-1.5 million tons per strand per year [147]. For a more detailed covering of the CSP-technology cf.
i.a. [49, 130, 134].
A.5.1.2.2 ISP - In-line Strip Production
Figure A.5-4 shows a diagrammatic view of an ISP plant. ISP plants have production capacities of about 1.5 million tons hot strip per strand per year [139].
Mould Ladle Tundish
Coiler Cooling line Rolling mill
Coilbox Furnace
HRM (hot reduction mill)
ISP.ds4
Figure A.5-4: Diagrammatic view of an ISP plant
Source: [73]
The main steps of the ISP process are cast-rolling with liquid core (liquid core reduction), cast-rolling with solid core, intermediate heating, transfer bar coiling and finish rolling. It is possible to roll the hot strip down to a thickness of about 1.0mm with this process, also satisfying deep-drawing requirements [139]. For intermediate heating in the ISP process, often induction furnaces are used. Depending on the strip thickness to be achieved and the rolling technology (e.g. austenitic or ferritic rolling, cf. Figure A.5-5 and Figure A.5-6), the required temperature elevation differs considerably.
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800 900 1000 1100 1200 1300 1400 1500
Temperature [°C]
Distance 700
150 K
3.0 mm 1.0 mm Temperature control by induction heating saves up to 56 kWh/t
isptemp2.ds4
Figure A.5-5: Flexible temperature control by induction heating
Source: [139]
800 900 1000 1100 1200 1300 1400 1500
Temperature [°C]
Distance 700
Micro-structure 1.0 mm
austenitic
1.0 mm ferritic
α+γ α
γ
ISP features austenitic and ferritic rolling
isptemp.ds4
Figure A.5-6: Flexible temperature control yields new rolling technologies
Source: [139]
An induction furnace allows a flexible control of the temperature elevation and may save energy up to 56kWh/t [139]. Figure A.5-7 shows three different plant concepts employing the ISP technology.
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290m
Ipsco, USA
180 m
Posco, Republic of Korea
376 m
Saldanha, South Africa
ISPconce.ds4
Figure A.5-7: Different plant concepts using the ISP technology
Source: [73]
A.5.1.2.3 CONROLL - Continuous thin slab casting and Rolling
The CONROLL plant aims to produce high quality hot strip by a continuously operating mill.
CONROLL plants have capacities of about 1.5Mt/y per strand [51]. According to plant manufacturers’ information, in order to achieve an excellent surface quality, a minimum mould thickness of 90 to 100mm is necessary [51]. Directly after casting follows a soft reduction of the strand, while the core is still liquid. After equalising the strand in a roller hearth furnace, rolling is carried out in two steps. The main data of a case study is presented in Table A.5-2.
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Table A.5-2: Main technological data of a CONROLL case study
Plant configuration
Meltshop: 2 EAF, 1 AOD, 1 L MF
CONROLL plant: Thin slab caster, walking beam furnace, roughing stand, 6 finishing stands Casting thickness: 75-130mm
Max. heat size: 135t
Width range: 635-1283mm Thickness hot strip: 1.8-12.7mm Processed steel grades
Low carbon, peritectic carbon, medium carbon, high carbon HSLA grades, alloyed grades, stainless grades, silicon grades Source: [52]
The main advantages of this process are energy savings and an increase in productivity [52].
Figure A.5-8 shows a comparison of the time-temperature course between cold charging and hot charging in a conventional plant and direct charging in a CONROLL plant [37].
0 5 10 15 20 25 30 35 40 45 50 55
500 1000 1500 2000
Continuous Casting
Hot charging
Furnace
Slab storage
Hot strip mill
Conroll.ds4
Time [h]
T [°C]
Final product
Final product Conroll
Final product
Figure A.5-8: Course of temperature and time for several plant configurations
Source: [37]