Monitor Coatings, a UK-based Castolin Eutectic company, first developed a unique, ceramic composite coating technology, XuperCoat ™, which has produced best-in-class results in terms of mould life time, cast product quality and caster operating costs.
In continuous casting, the surface of the mould plates is paramount to the quality of the steel being cast. The HVOF-deposited XuperCoat ™ has allowed the mould to increase its service life between 6 and 10 times over the standard Copper or Copper Chrome Mould. This enables longer sequences between mould exchanges which improves the cost per ton of steel and a higher caster availability, whilst reducing maintenance costs on the mould. In addition, plants report evidence of fewer sticking incidents during casting. This also helps improve quality of the yields. The reduction in wear is an advantage during mould refurbishing and can extend the copper life by 4-6 times and also prolong the life of the exit support foot rolls and guides. Complete mould assemblies often give a campaign life of upwards 250,000 tons between restorations and are used in the casting of carbon and stainless steel products across Europe, Asia and the USA. Furthermore, with increasing environment restrictions on hexavalent chromium compounds XuperCoat ™ is the most efficient reliable alternative for steel casters moving away from electroplating solutions. The coating inside of bloom and billet moulds with XuperCoat is the latest development by Monitor Coatings for this technology.
Typical Wear Attacks in the Steel Casting Process
Continuous caster components are exposed to high temperatures, corrosive environments and wear. The primary function of the mould in the casting process is to control the rate of solidification and the shape of the strand. 60% of the heat exchange with the solidifying steel occurs in the top half of the mould and controlled heat transfer is critical to the rate and homogeneity of steel solidification. In the lower half of the mould wear by the solid steel shell becomes even more significant. The friction developed between the mould wall and the solidifying steel shell is minimized by the combined effect of molten flux or lubricant addition and oscillating mould vibration.
As a result of intensive mould wear, the following features degrade dramatically the product quality:
- Deteriorating coating layers lead to pick-up of copper from the mould wall which is then trapped on the hot liquid metal surface and forms star cracks which may result in costly coil breaks in the following rolling processes. Manual inspections including surface scarfing on cooled-down slabs are required to detect those cracks and cost days and energy for slab re-heating. In the case of a continuous casting/rolling process, manual inspections are even impossible.
- As mould wear appears typically first in the corners, the slab transforms increasingly into a W-shape which leads eventually to rejections due to mismatch with required shape standards. It may also cause longitudinal corner cracks which bear the risk of coil breaks in downstream rolling processes.
- Mould wear can be one reason for improper heat exchange causing the steel shell to stick to the inner copper mould surface and to tear. This results in transversal and edge cracks and in the worst case in a sticker breakout. This most detrimental incident is not only very costly but is also a serious safety hazard for the operators of the casting machine.
XuperCoat™ Technology for improved Steel Quality and reduced Operating Costs
XuperCoat™ is the next generation hard-metal composite coating using HVOF techniques capable of manipulating the coating composition to give an optimum microstructure design. This goes beyond the simple dual-phase alloys of tungsten carbide and cobalt to composite architectures. Significant performance improvements in coating properties have been achieved by changes in size, shape and distribution of the phases to produce ultra-fine-grained materials. Despite being very dense coatings, even tungsten-based hard-metal coatings do not ultimately meet the corrosion- and abrasion-requirements of specific steel industry applications. Micro-porosity (pore size smaller than 5µm) in the coatings can lead to crack initiation. To combat this phenomenon, specialist coatings are also used to densify the underlying coating, forming a physical barrier between the component and the working environment. Unique, thermo-chemically formed ceramic coatings are subsequently formed, where a metal oxide bond is established not only between the particulate materials, grains or powders used to form the coating, but also between the coating and the substrate.
Monitor Coatings’ customers reported yield increases in cast steel on the same mould in the range of 2 to 8 fold compared to electroplating solutions. The actual mould’s life time increase varies dependent on the mode of operation and the nature of the product. Some plants operate thin slab casters or produce stainless steel, which tend to run on higher casting speeds and requires different coating setups compared to the low-speed traditional slab casters.
In addition to increased yield with no operation disturbances noted, even on high casting speed, customers identified additional benefits of the application of XuperCoat™ mould coatings, such as:
Improved product quality:
- Less star and corner crack defects
- Less strand shape issues
- Less stickers and sticker breakouts
Reduced operating cost:
- Lower consumption of copper mouldS
- Lower consumption of mould flux powdeR
- Better economy of scale due to less wear-related enforced stoppages
Castolin Eutectic will be present from May 8 till 11, 2017 on AISTECH in Nashville, USA and from June 26 till 29, 2017 on the European Continuous Caster Conference in Vienna, Austria.