Mönchengladbach, June 20, 2018

Energy efficient production of long products - Continuous mill technology

The worldwide steel industry is seeking strategies for compact and efficient production units utilizing local resource and distribution markets. This development originates from the requirement of a sustainable, energy efficient and cost-optimized steel production process. To meet this market demand SMS developed the CMT® Continuous Mill Technology, process.

Aim of the CMT® process is to combine billet continuous casting directly with the rolling process. Hence, energy consumption and yield losses can be reduced significantly compared to conventional dis-continuous processes. To maximize these and other benefits of the directly linked casting and rolling the casting machine is casting on one strand only with speeds as high as the required entry speed of the rolling mill, hence allowing a real endless production of rolled product with uninterrupted sequences of approx. 23 hours.

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CMT® plant concept

The casting and rolling process steps comprise a significant potential to reduce the energy requirement. The highest potential is clearly to be found in the reduction of the energy amount required for the reheating of cold billets from ambient to rolling temperature. The CMT® process allows, depending on the parameters like cast steel grades and billet section, to immediately roll the hot billet without or with very low reheating energy demand. By applying the direct casting and rolling concept more than 90% of the energy required for reheating of the billets is saved when compared to a conventional production concept.

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Reheating energy required for different casting-rolling concepts
CMT® takes the basic minimill concept a step forward

The development of the CMT® started with the CMT®300 solution, rated for a production of 300.000 tpy. At the base of this concept there is a melt shop with a productivity of 50t/h, and a single-strand continuous caster producing a 130 x 130 mm billet. The cast product is fed endless into the rolling mill at a speed of approx. 6 m/min. By using the INVEX mould technology it is possible to achieve the required casting speeds without sacrificing the quality of the cast product. By eliminating the need of a gas fired reheating furnace a considerable amount of energy can be saved. Additionally the bar head and tail cutting is no longer necessary and the chances of process interruptions caused by the threading of cut to length billets is reduced to a great extent. These two effects increase the yield of the rolling process significantly.

Higher productivities can be covered by increasing the casting speed up to 7m/min or by increasing the cast section to 150 x 150mm.

The CMT®500 is rated, for 500.000 t/y, with a hourly productivity of 75t/h while casting a 165x165mm endless billet. For further increasing the productivity to 100t/h the cast section is increased to 190x190mm or to a “mini-slab”-format of i.e. 165 x 220mm format, achieving a production of 700.000 tons per year.

No. of strandsFormat (mm²)Productivity (t/h)Productivity (t/a)Casting Speed (m/min)
CMT®3001130 x 13050300,0006
CMT®5001165 x 16575500,0006
CMT®7001190 x 190105700,0006

For production capacities in excess of 1.000.000tpy, SMS solution is the CMT®1000 concept. In this case, the CCM is equipped with two or more strands and feeds cut-to-length billets into the rolling mill, in semi-endless mode. In the semi-endless mode, billets are transferred via a cross-transfer, to a hot charging roller table in line with the mill passline. The billets are heated up to rolling temperature and equalized by means of an induction heating system in front of the first mill stand.

This concept has been adopted in various steel plants since 2011; recently in the new mini mill ordered by Moon Iron and Steel Company (MISCO) in Oman. Tung Ho Steel Corp., Taiwan has already ordered and successfully commissioned the second production line implementing this technology.

Depending on the end product characteristics, straight bars – produced on a cooling bed with downstream cut to length and bundling facilities – and/or compact coils, produced on a vertical coil spooler line, the installation of a billet welding machine allows the production of tailor-made product bundles in dimensions and weight.

In a combined casting and rolling process, casting speed must be synchronized with rolling speed to preserve continuous material flow through the plant. Since casting speed cannot be adjusted freely to keep a constant mold level, the latter is controlled by acting on the tundish slide gate. The continuous caster operates in submerged casting mode, using a submerged entry shroud (SES) and mold lubricating powder. The design of the slide gate, SES as well as the refractory materials used is optimised to achieve uninterrupted casting for long time.

In steady state, the temperature of the strand entering the rolling mill ranges between 1150°C (in the center) and 1050°C (on the surface), and therefore is suitable for direct rolling without additional energy Input.

An induction heater is however placed in front of the rolling mill to provide additional heating, or temperature equalization, when the billet temperature drops below the set limit; this is the situation present at the beginning of a casting-rolling sequence.

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Induction heater supplied by SMS Elotherm. Used for direct rolling of 150 x 150 mm billets at Tung Ho, Taiwan

In normal conditions, the cast strand will run endlessly into the rolling mill. Only in case of emergency, a hydraulic shear, placed between caster and mill, will cut the billet separating the two plant areas.

The rolling mill is designed for continuous direct charging of the cast billets. The continuous mill is equipped with the SMS group HL housingless type stands arranged in horizontal and vertical configuration. Mechanical properties of the final product are improved thanks to the thermoprocessing systems installed on the rolling line. The mill is also equipped with the industry leading SMS group HSD® (High Speed Delivery) system for safe and reliable delivery of bars on the cooling bed. After cooling the rolled stock is transported to the finishing area operates for fully automatic cut to commercial length, bundling stations and tying.  

Automation concept

The brain of the CMT® plant is the automation package covering Level-1 equipment control, Level-2 process control and Level-3 production control functionalities specifically designed to meet all the production challenges posed by the specific operational requirements and latest safety requirements. In addition to seamlessly integrating itself with the mechanical equipment to assure high levels of productivity and efficiency, the CMT® automation allows to easily and quickly adapt the production mix to varying market demands with day-by-day adjustments by means of built-in capabilities for plant-wide production planning, scheduling, tracking and pacing.

The peculiar arrangement of CMT® plant requires the Level 1 equipment control logics to grow beyond the usual boundaries of individual areas, in addition to assuring a stable and high speed production for single machines it guarantees a coordinated flow of the material across multiple areas. For meeting the CMT® challenge, in addition to implementing individual automations for high speed casting and rolling, SMS developed a specific control solution to mutually coordinate the speed of both caster and rolling mill to assure a perfect continuity of the material flow in all condition. The above meant the development of inter-area equipment control logics carefully designed to guarantee both the correct speed coordination and the minimization of the material waste during the normal start-up and shutdown sequences.

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Sequences for normal start-up and shut-down procedures

Furthermore, to properly handle emergency situations, specific inter-area equipment control logics implement different strategies for the shut-down depending on the origin of the problem and are designed to assure maximum safety and easy resumption of the production.

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Sequences for emergency shut-down procedures

The Level 2 specifically developed by SMS for the CMT® plant fully exploits the innovation opportunity offered by the process continuity between caster and rolling mill and offers unprecedented levels of seamless integration of process control across all plant areas starting from the scrap yard to the end products dispatching bay.

The strong process coordination attitude of the CMT® Level 2 is immediately visible at the level of the metallurgical database, where a specific centralized tool allows the plant metallurgists to design the melting-refining-casting-rolling process in all of its details, but conceiving and handling it as a whole as the plant philosophy itself clearly requires.  

On the production control the Level 3 functionalities also offers full support for a plant-wide production planning, scheduling, tracking and pacing.

Impact on Capex and Opex

Steel producers benefits from CMT® minimills:

  • Compact plant design with reduced land requirements
  • Compact design reduces cost for foundation and steel work 
  • In-Line transport of billets reduces crane capacities and facilitates lighter steel structure
  • Overall capex reduction of 15-20% compared to a conventional minimill
  • Energy savings - Minimized energy consumption through use of induction heating
  • Higher Yield – zero scale formation (no reheating) and minimized cutting loses (endless Billet)
  • Endless billet or welding of billets
  • Reduced personnel requirements 
  • Lower operational cost
  • Saves up to 20Euro/ton production cost