END USER FOCUS: Tundish refractory opportunities

By Alex Feytis
Published: Tuesday, 23 March 2010

A vital element in continuous casting, the tundish has become one of the biggest cost centres in the process, using refractory minerals such as magnesite, andalusite, bauxite, olivine and dunite

In continuous casting the transformation of molten metal into solid the control of steel flow rates is accomplished by the tundish. This refractory-lined distributor, is placed over the mould, receives molten steel from the furnace ladle, and acts as a buffer vessel between the steel ladle and the mould.

The tundish’s key control parameter consists in matching the flow of liquid steel into the mould with the withdrawal speed of the strand out of the mould (see figure), and this is where refractories have their key application.

The tundish’s main duty has always been the continuous production of steel in the continuous casting process, serving the purpose of a reservoir. But over the years, the device has been assigned further tasks, and has undergone sophisticated change.

Today, the tundish is multitasking. In addition to its original function, the device now has to separate non-metallic inclusions, calm the flow, and balance the temperature to prevent the reoxidation of the steel.

As a consequence, the tundish has increasingly gained importance from being a mere reservoir to being viewed as a “metallurgical vessel”, becoming one of the most important areas of refractory application.

As one of the biggest single cost centres in the continuous casting process, tundish refractories mainly comprise andalusite and magnesia, in addition to aluminas, alumina carbon, zircon silicates and low cement castables (LCC).


Tundish with permanent lining before spraying (left) and with the wear
lining above after spraying (right)


Lining and dams

The tundish has to resist corrosion, thermal shock, and high temperatures ranging between 1,520º-1,600ºC. The most important tundish components for refractory applications are the permanent lining, the wear lining and dams  including the weir and the baffle, since they contain the most refractory materials.

Permanent lining. The permanent lining, primarily a safety lining and the base of the wear lining, is used for embedding well blocks and nozzles as well as lock nozzle sets and the submerged entry nozzles (SEN). The requirements placed on the permanent lining include high thermal shock resistance, high chemical resistance to ladle slag, tundish powder and steel melts, low sintering tendency with the wear lining, and high mechanical stress (tilting/removal of skull).

It is possible to brick or cast monolithic permanent lining the permanent lining which is mainly made of bauxite or andalusite based LCC, microsilica, and high alumina cement (4-6%).

Usually the quantity of andalusite can range around 40-50%. A monolithic permanent lining is distinguished by the following characteristics: lining without any joints, thus easier to remove skull, high strength and stability, high thermal shock resistance, shorter time required for lining work and easy repair. The permanent lining can be used in 500 to 1,000 heats and therefore has to be changed once or twice a year.

Wear lining. The wear lining is a temporary lining located above the permanent lining. There is a host of different tundish lining practices which can be categorised into five major types: bricked, gunnable, board, sprayable and dry.

The wear lining is mainly made of magnesia, with olivine or dunite. The average ratio is 3:1 but the MgO content can vary from 60% to 100% for high quality steels with olivine increasing from 0% to over 40%.

The thickness of the wear lining depends on two things. First, the number of ladles in the sequence: the more ladles, the more centimetres of lining. For instance, in an average plant, for a sequences of up to 12 ladles, wear thickness will be 3-4cm. Second, it also depends on the steel and slag activity against the magnesia refractory. It is used in three to 50 heats a few hours of utilisation.

Dams, weir, baffle. As shown on the schematic, these different types of dams are used to control the flow and purify the steel before it goes to the mould as steel at about 1,550ºC has similar flow ability than water. The flow from the tundish to the slab must be stable.

These dams have a similar composition to the permanent lining, being mainly made of andalusite based LCC. Like the permanent lining, they are supposed to last between 500 and 1,000 heats.

Simplified tundish showing use of industrial minerals





Magnesia debate

As explained above, andalusite or bauxite magnesia and olivine or dunite - are the main refractories raw materials used in the tundish. Except for olivine, their prices have fluctuated significantly since the beginning of the recession, therefore impacting refractory material prices. An industry source told IM that after reaching prices at $400-750/tonne FOB last year, dead burned magnesia prices used for refractories dropped to around $300-342/tonne FOB at the beginning of 2010. As a consequence, refractory prices decreased in parallel, dropping by about 12% during the same period.

But the trend seems to have made a U-turn since then. “Magnesia prices are now starting to increase again,” explained Jean Martin, marketing director of French refractories producer Daussan Group, who expects the impact on the market to appear next summer. Therefore, it is quite likely that prices of related refractories will augment during H2 2010.

The magnesia/olivine or dunite balance of the wear lining’s composition usually depends on the type of steel which is made. Until now, the trend was to use up to 85-100% MgO with less dunite or olivine for high quality steel such as tool steel, diminishing to up to 60% MgO for a lower quality such as flat steel.

However, the subject is still a hot debate in the industry. According to some recent studies, the MgO content could be reduced in profit of a higher content of olivine or dunite - with no effect on the end product’s quality. A source of worries for magnesite producers, it comes as good news for refractories producers as olivine prices, mainly sourced in Norway - are at present reported at €110/tonne ($150/tonne), allowing them to reduce their production costs.

Bauxite versus andalusite

Technically, using bauxite or andalusite for the permanent lining and the dams in the tundish does not make a huge difference. In this application, the properties are quite similar and the decision to use one or the other will mainly depend on the raw material cost. According to various sources in the industry, bauxite prices (Guyana, refractory grade) are at present flirting with $600/tonne while andalusite is about $570/tonne FOB.

Depending on the size of the tundish, about 3-7 tonnes/tundish of andalusite is needed. It will therefore come as no surprise that refractories manufacturers very often decide to choose andalusite over bauxite. “As bauxite prices are quite high at the moment, we are going back to andalusite,” commented Martin.

Outlook

For the long term, the future of these refractory materials seems to be bright as the steel industry is on a firm road to recovery. “Steel is performing fairly well. Most of the plants are working at utilisation rates between 70 to 90%,” commented a source from the refractory industry.

Some steel producers already report full sales for H1 2010, but that may not be the trend for H2. The recovery, slow and laborious, is not expected before early 2011. “That will depend as well on raw material price control, which is not easy with other competing markets increasing with almost double digit growth,” added the source.

In the short term, low demand is expected from steel end markets from H2 2010, which if compounded by a forecast increase in MgO prices may result in increased refractories prices within the next six months.