Continuous Casting Tundish Lining Refractories: Practices, Advantages and Disadvantages

 25-Sept-2020

TUNDISH (BACKGROUND)

Continuous casting has been a landmark achievement in the area of steel making. Continuous casting refractories directly control the molten steel in the last stage of liquid steel processing and these are therefore, required to have high stability and special properties. In any continuous casting shop, tundish acts as a buffer vessel between steel ladle and mould. It serves the purpose of reservoir as well as fulfills certain metallurgical functions like floatation of inclusion, control of flow to the moulds, thermal and chemical homogenization etc. Small wonder that over the years, there have been a continuous change in the practices of refractory lining of tundish around the world.

From a mere reservoir and distribution vessel, the tundish started to be viewed as a steel refining vessel and a totally new field in the process of steel making technology emerged known as Tundish Technology. In this article we have briefly mentioned about the developments and practices of tundish lining refractories in their chronological order and also the main features, merits and demerits of different types of refractories used in tundish lining to enable an individual steel maker decide what could be the best for his plant.

TYPES OF TUNDISH LINING REFRACTORIES - Advantages and Disadvantages

There is a host of different tundish lining refractories which can be categorized into 5 major types (also in a roughly chronological order):     

1. Brick Lining
2. Gunnable Tundish Lining
3. Tundish Board Lining
4. Sprayable Tundish Lining
5. Tundish Dry Lining (in-situ formed)

Also Read: Importance of Tundish Design and Flow Modifier Refractories in Steel Making

Brick Lining

With the first commercial introduction of continuous casting in around 1960’s initially the same concept of refractory brick lining technology as used in other metal containing vessels was applied to continuous casting tundish lining. Tundish bricked lining refractories were of high alumina type used in direct contact with liquid steel, after pre-heating. It was essentially an extension of ladle refractory practices to the tundish and continuous casting.

Merits:

• low risk of H picking by molten steel
• no sand
• low inventory
• no investment in equipments
• low washout risk

 Demerits:

·        intensive curing required

·        highly labour intensive

·        poor insulation

·        late stage temperature drop in casting operations due to high thermal conductivity of the brick lining resulting into metal heat loss affecting the metallurgical parameters

·        “Cold Start” not possible

·        large tundish fleet required

·        difficult deskulling (stripping)

·        joints

·        long tundish preparation time

Too many difficulties led some people to opt for a trowellable, and subsequently gunnable, over-lining at some added costs.

Gunnable Tundish Lining

Gunnable refractory lining in tundish is said to have commercially started in Japan to overcome some of the problems of bricked linings. Initially such tundish lining refractories were alumino-silicate based and later converted to magnesite based or basic type to assist with metallurgical practice. In this method the dry refractory powder of the right composition after fluidization is transferred and installed on the tundish wall by using a gunning machine to obtain a monolithic lining. Though it provided a monolithic joint-free structure and relatively improved deskulling but little was gained in the way of preheat times or heat losses due to the relatively high density of the gunned linings. There was still a tendency for the linings to crack and spall during rapid preheat and this also precluded the use of gunnable refractory lining for cold start practices. 

Merits:

• low risk of H picking by molten steel
• no sand
• low inventory
• no joints
• less labour intensive
• relatively easy installation in lesser time
• relatively less difficult to deskull

 Demerits:

·        intensive curing required

·        high wastage because of rebound losses

·        poor insulation

·        “Cold Start” not possible

·        high washout risk

·        low thermal stability

·        high shrinkage causes high stress, subsequent crack formations during operation whereas a low shrinkage can be a barrier for easy deskulling

·        dust problems

·        energy intensive

·        long T/D cycle

·        high costs

·        investment in equipment

Tundish Board Lining

The mid 1970’s saw the introduction of a new type of tundish wear lining; which were board systems comprising low density, highly insulating, disposable, pre-formed, and pre-cured refractory boards. Easy deskull, no equipment investment and the low cost of silica variety also contributed to its run-away popularity among many steel makers. FOSECO’s GARNEX became a household name in Indian continuous casting circles during this time. Initially silica based boards were used which allowed only “cold start” practice. Magnesite based boards were introduced in mid 1980’s to fulfill the requirement of pre-heatability, i.e., a “hot start” practice for low hydrogen considerations in the manufacture of high alloy quality steels. However, the labour intensiveness, presence of joints and sand backing, and breakages etc remained as inherent handicaps of board system.

Merits:

• low risk of H picking (when hot)
• uniform liner shape
• no need to cure
• good insulation
• cold start possible
• easy deskull
• low energy requirement
• short T/D cycle
• no investment in equipments
• low washout risk
• low cost (silica-based board)

 Demerits:

·        joints

·        sand backing

·        hydrogen picking risk (when cold)

·        labour intensive

·        high inventory

·        handling/breakage problem

·        high cost (magnesite-based boards)         

In response to increasing trends for making cleaner steel and also with the advent of Continuous Casting technology a variety of refractories for tundish lining have into the market. However, board system is still popular in countries where labour costs are low and application technologies are not readily available. Tundish board, in particular, developed for energy-saving are finding wide acceptance. 

Sprayable Tundish Lining

Because of some of the above difficulties there was already a push towards automation of the tundish lining system. Meanwhile, advances in machine design and chemical formulation technology in advanced countries led to the development of a “Spray” system, in which a thick slurry could be transported after thorough mixing, and finally deposited onto the tundish after “atomizing” with compressed air. The first robotic application system was commissioned in 1982 which from the later half of the 1980’s started to be widely used in developed countries due to the significant benefits of lower placed density and better control of the lining thickness than gunned linings.

The sprayable tundish lining refractories are mainly MgO and SiO2. The MgO content is usually in the range of 70% to 90% with balance percentage of SiO2. For longer duration of sequence casting higher amount of MgO along with higher thickness of the refractory lining is needed. It was no longer required to transfer the dry powder after fluidization (as required in gunning). This enabled the addition of fibers and other chemicals to the mass and homogeneous mixing and deposition became a reality. The lining could be preheated and the cast taken in a “hot start” fashion, or allowed to cool to room temperature and taken as a “cold start” tundish. While curing, it needs to be controlled to ensure lining integrity and this demands that the tundish permanent lining is ideally below 100 degrees C for satisfactory placement. Wet processes such as sprayable lining with up to 30% water addition by weight and the presence of hoses and spills may create OH and S issues in the steel plant. Even then this spray lining system was able to successfully combine many of the advantages of board and gunning, while eliminating the disadvantages like - joints, sand backing, rebound losses, dust problems, poor insulation etc. 

Merits:

• low risk of H picking
• no joints
• no sand
• low inventory
• less labour intensive
• easy deskull
• good insulation
• “cold start” also possible
• controllable lining thickness
• robotic application for big size tundish (involve large investment)

Demerits:

·        investment in equipments

·        intensive curing required

·        moderate washout risk

·        relatively longer T/D cycle (than boards)

Also Read: Gunning and Spraying - differences in these two methods of tundish wear lining, refractory lining repair and maintenance

Tundish Dry Lining

Dry tundish linings were introduced in Europe probably in 1986. The system differ from all previous processes in the sense that it is applied in a dry powder form and do not require the addition of water to form the tundish working lining. Generally it utilizes a resinous bond (Binder / Catalyst reaction) which is activated by relatively low amounts of heat (around 160^OC). Vibration may or may not be required, depending upon the product being used, but it is essential to use a former and the dry powder is fed in the gap between the tundish permanent lining and the former. The hot air is introduced at approximately 400^OC and the heating cycle takes around 45 minutes with further 30minutes for cooling. Thus a lot time can be saved while on the negative side; the dry system still has lower insulation (due to higher density) and is dependant on crainage in the tundish bay for installation. But one major advantage of dry lining is that because of the absence of water in this system there is no direct adhesion to the permanent tundish lining which ensures good deskulling and prolongs life of tundish lining. Besides, the smooth finish on a dry tundish lining and ability to consistently reproduce lining geometry offers improvements in steel quality and better erosion resistance resulting in the potential to increase sequence lengths.

Merits:

• no joints
• no sand
• low H risk (when hot)
• low inventory
• less labour intensive
• reduced tundish preparing time
• low washout risk
• easy deskull
• uniform liner
• clean environment friendly application
• high sequence possible
• OH and S benefits
• easy, quick installation
• improved steel cleanliness because of lining integrity

Demerits:

·        investment in equipments

·        H risk (when cold)

·        lower insulation

·        crainage dependence

Sequence ‘Continuous Casting’ is increasing and creating a demand for higher performance of refractories. Refractories for continuous casting that are exposed to molten steel, are not only subject to heavy corrosion and abrasion by molten steel, but also have a large effect on quality of the steel and the yield points. So, while there are advantages even in bricked and gunning systems, the disadvantages outweigh the merits. Similarly although there are some demerits in all the systems of board, spray or dry lining, the advantages seem to be more in these systems. Making a choice appears to be difficult amongst the three systems with merits and demerits being almost equally balanced. Therefore, recourse must be taken of other factors like those of steel plant operations, quality of steel, etc when trying to decide between board, spray and dry linings.

       

Importance of Tundish Design and Flow Modifier Refractories in Steel Making | Refractory Industry Guru

 23-Sept-2020

To transfer finished melt steel from a ladle to mould in a continuous casting process, an intermediate vessel is used which is called tundish. The role of tundish is to deliver the molten metal to the moulds evenly and at a designed throughput rate and temperature without causing contamination by inclusions. Inclusion float out, slag vortexing, till end slab volume and residual metal in tundish are a strong function of tundish hydrodynamics. Tundish design as well as flow control devices / modifiers are known to have strong influence on tundish hydrodynamics. 

Read: Continuous Casting Tundish Lining Refractories: Practices, Advantages and Disadvantages 

One of the major functions of steel making tundish is to enhance inclusion floatability and thereby, produce clean steel. For the removal of inclusion through floatation, wall adhesion and agglomeration the flow patterns inside the tundish play an important role, which in turn

Melt flow in any given tundish can be favourably altered by incorporating suitable tundish flow modifiers (TFM) and/or changing the design of the tundish. The flow modifiers play an important role in promoting the floatation of nonmetallic inclusions in steel.

Now-a-days refractory makers are offering customized refractory solution. The new age tundish refractories facilitate temperature homogenization, removal of macro-inclusion, prevention of nozzle clogging etc. inside tundish. To streamline the flow and compress turbulence inside tundish various Flow Control Devices (FCD) are being used in place of traditional FCDs or tundish furniture like Dams, Weirs, Charge Pads, and Side Wall Pads etc. 

The next generation FCDs are popularly known as Tundish Flow Modifier (TFM), Tundish Flow Optimizer (TFO) etc. are precast refractory shapes made of Ultra Low Cement Castables (ULCC) having 85 - 90% alumina. The interior of tundish flow modifiers or flow optimizers as you say it, are designed in such a way that incoming steel gets a churning effect which results into inclusion flotation and subsequent absorption at the tundish powder level. Tundish argon diffusers are also being used to reduce inclusion in steel.

Eventually, it is tundish design from the viewpoint of metal flow and appropriate selection of refractory materials with their right positioning inside tundish that holds the key to the success of subsequent operations in steel making.

Difference between Stainless Steel, Carbon Steel, and Alloy Steel

6-Aug-2020

The basic difference between stainless steel, conventional alloy steel and carbon steel is that Stainless Steel contains a very high percentage of chromium (11 – 26 percent) and nickel (3.5 – 22 percent).

Through varying chromium content and by addition, substitution of other alloying elements like nickel, molybdenum, copper, titanium, aluminium, silicon, niobium, sulfur, selenium etc., resistance to corrosion, oxidation, abrasion, hardness and a variety of other distinct properties are either created or enhanced.

On the other hand, steel that contains carbon up to about 1.7 percent as an essential alloying constituent and has properties and structure made up mostly of the element carbon is better known as Carbon Steel. Most of the steel produced in the world is carbon steel. Unprotected carbon steel rusts when exposed to air or moisture but stainless steel is almost immune to rusting and ordinary corrosion.

WHAT IS CARBON STEEL

Carbon steels are the base metals widely used in manufacturing in nearly every industry, including aerospace, aircraft, automotive, chemical, defense, and precision. Carbon steel’s strength is due to its crystalline structure. Groups of iron and carbon atoms are arranged in a lattice, with the carbon atoms preventing the iron atoms from slipping over each other, which imparts the steel more rigidity.

The addition of an alloy such as titanium or manganese strengthens this structure by adding different atomic sizes to the lattice. This reinforces steel's rigidity by further impeding molecular movement when the metal is subjected to stresses. There are four types of carbon steel based on the amount of carbon present in the alloy.

Stainless Steel Pipes, Board, Kitchen

Low Carbon Steel – Composition of 0.05%-0.29% carbon and up to 0.4% manganese. They are the most common form of steel commonly known as mild steel, a relatively low-cost material, easy to shape (malleable). Low carbon steels provide material properties that are acceptable for many applications.

Medium Carbon Steel – Composition of 0.29%-0.54% carbon, with 0.60%-1.65% manganese. Medium carbon steel is ductile, strong and has good wear resistance. Because of the above properties, they find use in forging, heavy industries, and automotive components.

High Carbon Steel – Composition of 0.55%-0.99% carbon, with 0.30%-0.90% manganese. It is very strong and holds shape memory well, making it ideal for springs and high-strength wires.

Ultra High or Very High Carbon Steel - Composition of 1%-2.1% carbon. Its high carbon content makes it an extremely strong material. Due to its brittleness, this grade requires special handling. However, ultra high carbon steels can be tempered to great hardness and are used for specialized products such as knives, axles etc. Tighter carbon content control for more consistent heat treatment. Steels with carbon content above 2% are considered to be cast iron.

WHAT IS ALLOY STEEL

Stainless Steel utilities

Alloy Steel is an iron based mixture containing manganese greater than 1.65%, silicon over 0.5%, copper above 0.6%, or other minimum quantities of different alloying elements such as chromium, nickel, molybdenum, or tungsten are present, each of which imparts different properties to alloy steel. Alloy Steels are made by combining elements during the smelting process when the iron is still molten. Chromium is added in smaller amounts (0.5-2%) to increase hardenability and larger amounts (4-18%) to increase corrosion resistance. Molybdenum is added in amounts of 0.25-0.40% to increase the strength of the steel. Nickel is added in smaller amounts (2-5%) to increase toughness and in larger amounts (12-20%) to increase corrosion resistance. Silicon is added to steel in smaller amounts (0.2-0.7%) to increase strength and in larger amounts (>2%) to improve its magnetic properties while addition of Sulfur or Lead is done to increase the weldability. 

WHAT IS STAINLESS STEEL

Stainless steel is an alloy developed in the early 1900’s after metallurgists discovered that chromium added iron alloys displayed superior corrosion resistance to carbon steel alloys. The first products using stainless steel were produced in 1908 and the first patents were granted in 1912.

Stainless Steel is a highly durable alloy containing the following major ingredients:

• 10 - 30 per cent of chromium by mass giving excellent corrosion and oxidation resistance to it.
• Rest about more than 50 percent iron.

The corrosion resistance that is unique to stainless steel is the result of a transparent passive film of chromium oxide forms on the surface of the steel and protects it from oxidation. Higher chromium levels increase the corrosion resistance of the steel, but it creases the brittleness of the metal, making it hard to work with. There are different categories and types stainless steels. To know more please refer to -

• Categories and Types of Stainless Steel: Compositions and Properties
• Grades and Series of Stainless Steel - Compositions and Uses

Pyrometric Cone Equivalent (PCE) Test in ORTON or SEGAR to Determine Refractoriness of a Refractory Material

5-Aug-2020

The thermal softening behaviour of a refractory material technically known as ‘Refractoriness’, is determined by Pyrometric Cone Equivalent (PCE) test. In other words, by Pyrometric Cone Equivalent (PCE) of a refractory material we come to know about its ability to withstand exposure to elevated temperature without undergoing appreciable deformation.   

Refractories due to their chemical complexity and phase change undergo fusion (melt progressively) over a range of temperature. This softening behaviour or refractoriness of any refractory material is determined by PCE test which is done in a PCE furnace by comparing ceramic specimen of known softening behaviour (either in ORTON or SEGAR Cones) with the Cone of the refractory material. Value of PCE Cones in ORTON is followed in British standard while SEGAR is in German standard of testing. To view / download PCE Cone numbers with their temperature please click -

Pyrometric Cone Equivalent, PCE in ORTON and SEGAR used to Determine Refractoriness

PCE Cones are small triangular ceramic prisms of definite dimensions that when set at a slight angle bend over in an arc so that the tip reaches the level of the base at a particular temperature if heated at a certain rate (Refer Figures showing PCE Cones set on plaques before and after firing). The bending of the Cones takes place after the formation of a viscous liquid as a result of fusion of the Cone material. PCE is measured by making a Cone of the refractory and firing it until it bends and comparing it with standard Cone(s). Pyrometric Cone Equivalent or PCE test is a must for the quality control purpose for Refractories and Refractory raw materials.

Grades and Series of Stainless Steels - Compositions and Uses

4-Aug-2020

In continuation of our previous article on stainless steel Categories and Types of Stainless Steels: Compositions and Properties, the key points that will be discussed in short here are (this is the 2^nd and last Part of this article):

• Types of Stainless Steels
• Series of Stainless Steel
• Stainless steel compositions
• Uses of different Series of Stainless Steels, etc.

Martensitic Stainless Steel (Compositions, Properties)

This category is identified by their Martensitic microstructure in hardened condition.   

Martensitic Stainless Steel contain –

• Chromium: 11 – 18%
• Nickel: 4 – 22%
• Carbon: around 1%
• Manganese: 1 – 1.5%
• Silicon: 1 – 2%

Due to higher carbon to chromium content ratio, Martensitic Stainless Steel has an exclusive property that it can be hardened by heat treatment. They are usually less resistant to corrosion than other categories of stainless steels. Hardness, ductility and ability to hold an edge are some important characteristics of Martensitic Stainless Steel.

Martensitic Stainless Steel can be cold worked and machined without much difficulty, especially with low carbon content. Martensitic Stainless Steel possesses good toughness and can be hot-worked easily.

Duplex Stainless Steel (Compositions, Properties)

Duplex Stainless Steel has chemical composition and balanced (Duplex) microstructure of approximately equivalent fractions of ferrite and austenitite phases in annealed condition. Duplex Stainless Steel can be characterized by high chromium (i.e. 19 – 32%) and molybdenum (up to 5%) and lower nickel contents than Austenitic Stainless Steel. Addition of nitrogen causes formation of interstitial solid solution which promotes structural hardening and eventually, an increase in yield strength. Duplex Stainless Steel has about twice the yield and tensile strength compared to those of Austenitic Stainless Steel. The toughness of Duplex Stainless Steel grade is superior to that of Ferritic Stainless Steel grade but inferior to Austenitic Stainless Steel. The two-phased-microstructure of Duplex Stainless Steel promotes high resistance to pitting and also stress corrosion cracking.       

Precipitation Hardening Stainless Steel (Compositions, Properties)

Precipitation Hardening Stainless Steel contains 15 – 18% chromium and is a kind of chromium-nickel alloys.

In annealed condition, they may be either austenitic or Martensitic. They develop high strength with heat treatment. Precipitation hardening Martensitic stainless steel has corrosion resistance comparable to austenitic stainless steel but can be precipitation hardened to even higher strengths than other martensitic grade stainless steels.        

STAINLESS STEELS SERIES (Stainless Steel Grades)       

Stainless Steels are classified into three series:

1. 200 series Stainless Steels
2. 300 series Stainless Steels
3. 400 series Stainless Steels

The following table shows the chemical compositions of the above three Series of Stainless Steels:

Chemical Compositions of 200, 300 and 400 Series of Stainless Steel

Chemical Composition (in percentage)	200 series Stainless Steel	300 series Stainless Steel	400 series Stainless Steel
Manganese	5.5 – 10.0	2.0 (max)	1.5 (max)
Nickel	1.0 – 6.0	6.0 – 22.0	1.5 (max)
Chromium	15.5 – 19.0	16.0 – 26.0	10.5 – 19.0
Iron & Other Minerals	Balance	Balance	Balance

USES OF DIFFERENT SERIES OF STAINLESS STEELS        

The major uses or applications of 200, 300 and 400 Series Stainless Steels

200 Series Stainless Steels

The 200 series stainless steel is mainly used in making utensils, households including kitchen appliances, architecture and decorative items, furniture, tubes, pipes, automobiles, railways and transport industries.

300 Series Stainless Steels

The 300 series stainless steels have got all uses mentioned under 200 series besides, in refineries, petrochemicals, diary farm industries. They are also used in high temperature instruments such as in electric furnaces, pharmaceutical appliances, nuclear appliances, power plants, railway coaches, automobiles etc.

400 Series Stainless Steels

The 400 series stainless steels have some special uses such as in coinage, auto-exhausts, razor blades, power packing in petrochemicals etc. Besides, these are also used in consumer durable and transport industries.      

Also Read

Difference between Stainless Steel, Carbon Steel, and Alloy Steel