Steel Designers Manual - TheBestFriend.org

This material is copyright - all rights reserved. Reproduced under licence from The Steel Construction Institute on 12/2/2007
To buy a hardcopy version of this document call 01344 872775 or go to http://shop.steelbiz.org/
Steel Designers' Manual - 6th Edition (2003)
234 Applied metallurgy of steel
lead to the formation of iron; the molten iron is tapped continuously from the
bottom of the blast furnace. The molten metal at this stage is approximately 90%
to 95% iron, the remainder being impurities, which have to he removed or reduced
to acceptable levels at the next stage, that of steelmaking. This material is fed
together with recovered scrap iron or steel into the steelmaking furnace, the
common types of which are known as either a basic oxygen furnace or an electric
arc furnace. In the basic oxygen furnace oxygen is blown on to the molten metal by
a water-cooled lance. In the electric arc furnace heat is produced by an arc between
electrodes over the metal surface and the molten metal itself conducting electricity.
Chemical reactions take place following additions of selected materials to the
molten metal, which lead to the reduction of the impurities and to the achievement
of the required controlled chemical composition of the steel. The impurities are
reduced by addition of elements which combine and float out to the surface of the
molten metal in the slag or dross waste material on the surface. Deoxidation or
killing of the steel takes place in the final stages before the furnace is tapped. Older
steel manufacturing practice was to tap the steel from the furnace into ladies and
then pour the molten steel into large moulds to produce ingots. These ingots would
normally be allowed to solidify and cool before reprocessing at a later stage by
rolling into the required product form. Modern steelmaking practice has now moved
much more to a process known as continuous casting, in which molten steel is poured
at a steady rate into a mould to form a continuous solid strand from which lengths
of semi-finished product are cut for subsequent processing. Semi-finished products
take the form of slabs, billets or blooms. Continuous casting has the advantage of
eliminating the reheating and first stage rolling required in the ingot production
route, and is generally more efficient, but ingot production is still required for some
product forms.
6.4.2 Casting and forging
If the final product form is a casting the liquid steel is poured direct into a mould
of the required geometry and shape. Steel castings provide a versatile way of achieving
the required finished product, particularly where either many items of the same
type are required and/or complex geometries are involved. Special skills are
required in the design and manufacture of the moulds in order to ensure that good
quality castings are obtained with the required mechanical properties and freedom
from significant imperfections or defects. High-integrity castings for structural applications
have been successfully supplied for critical components in bridges, such as
the major cable saddles for suspension bridges, cast node and tubular sections for
offshore structures, and the pump bowl casings for pressurized water reactor
systems. The size of component which can be made in cast form is limited to a
maximum of some 30–50 t however, and only a small proportion of total steel production
is completed as castings for direct application.