Department of Metallurgical Engineering

Gandhi Institute of Engineering and Technology, Gunupur

BIKASH RANJAN PRADHAN11MET018

By

Introduction

Classification

Equipments

Materials required

Process

Defects

Modern developments

Advantages

Disadvantages

Earlier 1950s steel was poured into stationary moulds to form ‘ingots’ since then “continuous casting” has evolved to achieve improved yield, quality, productivity & cost efficiency

Continuous casting may be defined as teeming of liquid metal in short mould with a false bottom through which partially solidified ingot

(billet ,bloom or slab) is continuously withdrawn at the same rate at which metal is poured in the mould.

Which are subsequently rolled to get finished products.

Three main types of machines are in use in practice namely

Vertical mould

Vertical mould with bending

Curved mould or S-type

This type of mould is very

tall and hence needs either a

tall shop or a large pit to

accommodate the equipments.

The problem is acute if high

casting speed are employed

& consequence longer cooling

zone is required.

Used for large and medium sections

It is easy to repair and restart the machine.

It is a modification over the vertical design to reduce the overall height of the machine.

The roller apron and pinch rolls are similar to those in vertical machine.

After the product emerges from the pinch rolls it is bent to obtain the discharge horizontal.

A horizontal set of straightening rolls becomes necessary.

Heavy sections being difficult to bend .

30% height is saved by this design.

This is the latest design now almost universally adopted for continuous casting of almost all sections like billets, blooms and slabs.

Characteristics-

The mould is itself curved mould.

The strands comes out of the mould in curvilinear fashion with a fixed radius.

It is bent before entire cross-section is solidified.

The curved strand is in fact straightened after it is fully solidified.

Curved mould(s-type)

Ladle

Tundish

Mould

Dummy bar

Roller apron

Withdrawal rolls

Bending and straightening rolls

Cooling sprays

Cutting devices

Auxiliary electrical and/or mechanical gears to help run the machine smoothly.

Ladle & turret-

Steel from EAF or BOF is tapped into a ladle and taken to the continuous casting platform.The ladle is raised into a turret that rotate the ladle into the casting position above the tundish

Tundish-

Shroud

Turbo blower

Mono block Stopper rod

SEN

It is a container which collects liquid steel from ladle through shroud.

Here a turbo blower is fixed which converts the turbulantflow of liquid steel to laminar flow.

Then by opening the stopper rod metal flows through the submerged entry nozzle.

Mould

It is made of copper, copper & silver(cu 99.95%) due to its high thermal conductivity.

Nickel plate is used inside the mould to protect the mould wall from eroding.

Mould of 220mm to 300mm thick and 2500mm wide 900mm height is used.

Now a days the mould width is decreased at any time when required during casting.

4500 -5000 ltrs/min of water is flow through the mould.

Life of the mould is about 1000 heats.

Dummy bar-

To begin with, the bottom of the mould is temporarily

Closed by what is known as dummy bar.

It lead the product through the roller apron and withdrawal rolls.

The head of dummy bar is supposed to nearly close the bottom of the mould and whatever gap is left is packed with asbestos to obtain a temporarily fully closed bottom of the mould.

Withdrawal rolls-

These are one or two pairs of rolls meant to finally grip the ingot and pull it out at a pre fixed rate without deforming the product.

The pressure exerted by rolls on product should neither be excessive to cause deformation nor less to allow slip.

Bending & straightening rolls-

Single bending roller is used in vertical mould with horizontal discharge machine.

In s-type machine multi point bending rollers are used.

A straightening roller table is provided for straighten the product.

Cutting devices-

Torch cutter with 4 torches(two for normal cutting & two for sample cutting in line)

Oxy-fuel cutter is used , where LPG or acetylene are used as fuel.

Liquid steel

Asbestos

Fluxes

Chiller

Steel rods

Tundish preparation Tundish and sen preheating

Dummy bar insertion

Mould setting

Dummy bar sealing

Ladle & tundishin casting position

Teeming

Start the machine Cutting

To start a cast the mould bottom is sealed by a steel dummy bar which is held in place hydraulically by the straightener withdrawal units.

The gap between the mould bottom and the dummy bar head is filled by asbestos. After that chiller & steel rods are provided for rapid solidification of the steel and it stick to the dummy bar head.

The steel is partially solidified in mould and producing a steel strand with a solid outer shell and a liquid core.

In this primary cooling area once the steel shell has a sufficient thickness about 10-20mm, the straightenerwithdrawal units are started and proceeds to withdraw the partially solidified strand out of the mould along with the dummy bar. Liquid steel continues to pour into the mould to replenish the withdrawn steel at an equal rate.

The withdrawal rate depends on the cross-section, grade, quality of steel being produced and may vary from 30 cm to 100cm per minute.

Casting time is typically 45 minutes to 90 minutes per heat to avoid excessive ladle heat losses.

Open exiting the mould the strands enters a roller containment section and secondary cooling chamber in which the solidifying strand is sprayed with water to promote solidification. This area preserves cast shape integrity and product quality.

Once the strand is fully solidified and has passed through the straightener withdrawal units the dummy bar is disconnected. Following the straightener the strand is cut into individual pieces of the following as-cast products: slabs, blooms, billets, rounds or beam blanks depending on machine design.

Mould stroke in the range of 5-10 mm.

Frequency of stroke , which is 150-200/min

Casting speed normally in the range of 2m/min

Negative strip time .1-.2 sec

Total cycle time of the order of .3-.4 sec

Positive strip time .15-.3% of cycle time

Powder consumption in the range of .17-.40kg/m2 of strand area.

Break out Edge cracks Longitudinal cracks Primary scales Random surface defects arising due to fluctuations of

surfaces Variations in width Non-uniform thickness and flatness Non-uniform microstructure in either direction Segregation Slag particle sticking the surface- entrapment of lubricant

slag particle at the surface.

Remote adjustable mould

Air mist cooling

Dynamic spray cooling

BOPS

AMLC

Sprue, runner, riser etc. are not used. Hence , no waste metal this leads to casting yield.

Process is automatic

Product has good consistent soundness.

Mechanical properties are very high and very reproducible.

Not suitable for small quantity production.

Continuous and efficient cooling of mould is required, else, centre line shrinkage develops.

Requires large floor space.

Continuous casting has evolved from a batch process into a sophisticated continuous process. This transformation has occurred through understanding principle of mechanical design , heat transfer , steel metallurgical properties and stress strain relationships, to produce a product with excellent shape and quality.

In recent year the process has been optimized through careful integration of electro-mechanical sensors, computer-controls, & planning to provide a highly automated system designed for new millenia.