Steel making and ingot casting

The raw materials for steelmaking (iron ore and coke) are converted in the blast furnace to molten iron containing about 3% carbon at a rate which varies up to about 8 million tons per annum. (This makes the blast furnace by far the most economic process for producing steel, but economies which have not the capacity to utilize steel production of this magnitude may utilize the direct reduction of iron ore to sponge and powder instead.) Figure 7.3 depicts a typical blast furnace.

The conversion of the iron from the blast furnace to flat products, billet bar and sections has, since World War 11,
undergone a revolutionary change due to the development of oxygen argon lancing, vacuum treatment, continuous casting and rapid in works analysis. Iron from the blast furnace is conveyed while still molten to the basic oxygen converter (BOC or BOF) (see Figure 7.4) where oxygen gas is passed through it reducing its carbon content to approximately that of the specified steel. It may then proceed direct to the pouring ladle, where alloying additions are made and slagging processes undertaken. The molten steel is poured into a lander which conveys it to a water-cooled mould whose base is formed by the previously poured solid metal. This metal is retracted through the mould which may oscillate in a vertical axis. The whole of the blast furnace output may pass through a single mould of this type (Figure 7.5(a)) and the cast metal may be fed directly into a rolling mill which reduces it to plate bar or section as required (Figure 7.5(b)). The process as described is applicable to very large throughputs but there is no reason successive ladle charges should not differ in composition, the different alloys being separated later in the mill
train. Where higher-quality or special steels are required the liquid steel from the BOC may be transferred to a vacuum plant for further treatment prior to pouring13 (see Figure 7.6).

The Dortmund Harder (DH) and Ruhrstahl Heracus (RH) processes transfer the metal from a ladle into a superimposed
vessel. the DH by sucking it up by vacuum. the RH by driving it up by ]pressure of argon. A further improvement has been to
equip a vacuum argon treatment vessel with electric arc heating (Vacuum Arc Deoxidation (VAD))14 and effectively
to produce a secondary steel-making process (see Figure 7.7). Most :special steels. particularly stainless steels. cannot be
decarbui-ized by the BQF because the oxygen which reduces the carbon present also oxidizes chromium. Oxidation may be
prevented by blowing oxygen in a vacuum but there are practical probiems involved. These may be overcome by the
ACID process, in whlch an artificial vacuum (so far as the partial pressure of carbon monoxide is concerned) is produced
by diluting the blown oxygen with argon. This process is carried out in a tiltable vessel" (see Figure 7.8) with base
tuyeres which agitate the vessel contents. With this process removal of carbon, slag reduction, metal dioxidation, desulphuration
and alloying are easily achieved.

Conversion of the molten iron from the blast furnace direct to the finished product in an integrated steelworks not only
gives significant economies in energy and labour but can also provide steel of a quality equal or better than was previously
available from electric arc melting. In the first place, oxygen blown steel is naturally low in nitrogen so that the toughness problems associated with strain ageing are eliminated. However, where, as in austenitic stainless steels, nitrogen has a beneficial effect on tensile strength, controlled amounts may be introduced by replacing some or all of the argon in the AOD process.
The greatest improvement in liquid metal quality is. however, gained by vacuum treatment. Raising and lowering
the steel in the DH or RH vessel very significantly reduces the partial pressures of hydrogen and carbon monoxide in the melt
and the
C + Fe 0- CO + Fe
reaction proceeds until equilibrium between carbon and oxygen is established at a lower oxygen level. Line A in Figure 7.9
represents the oxygen carbon equilibrium at 1013 mbar (1 atm) over the carbon range 0.034.13%: the effect of
reducing the pressure (e.g. by vacuum degassing) to 133.3 and 13.33 mbar appears in curves B and C, respectively. Since
carbon is lost to the system as well as oxygen the theoretical effect on a steel initially at 0.05% C is shown in line 1
connecting the three curves. The actual effect on steels with various vacuum degassing techniques is shown in Pines 2-5.
Vacuum degassing thus effects reduction of the oxygen content to levels less than half those obtained in the best practice
in steels air melted and refined at atmospheric pressure.

Consequently, final residual deoxidation can be effected with much smaller amounts of aluminium or silicon and much
cleaner steel can be produced with (if required) lower carbon Bevels. Vacuum carbon deoxidation is extremely beneficial in
the production of plate with good through-thickness ductile properties because it virtually eliminates planar concentrations
of non-metallic inclusions.

Vacuum degassing techniques. properly appiied to permit carbon deoxidation, are the most economical way of upgrading
steels and, in particular, low-carbon steels. It is further possible to make injections in a stream of inert gas of such
elements as calcium or calcium carbide which can reduce sulphur and phosphorus and to replace the sometimes damaging
inclusion of silicons and aluminium with other inclusions which improve transverse ductility, fatigue and machinability.
Additional advantages are gained by the substitution of continuous for ingot casting. Carbon. sulphur and phosphorus
have been shown to segregate enough to give a concentration ratio of 3 or 3 to 1 between the top and bottom of a iarge
conventionally poured ingot. Silicate inclusions segregate to different parts of the ingot. No longitudinal segregation can
develop in a continuously cast ingot once the casting process has reached equilibrium. The only longitudinal variation of
composition is that between one ladle charge and another, and this is revealed by analysis and may be corrected before
pouring. There will be some compositional variation across the section, and possibly between dendrites. but at the outside
this is unlikely to exceed 5% on either side of the mean. Continuous casting has the further advantage that the cross
section of the ingot can be much better matched to that of the final rolled product than is possible wi:h a conventionally cast

ingot. A large plate requires a large ingot, which in conventional casting must be of large cross section and may be too
large for the available rolls. With continuous casting the size of plate is only limited by handling down the line and the metal
quality is improved by the finer structure achieved by casting a smaller section.
However, it is seldom that the quantity of special and stainless steels warrants production from hot metal in proximity
to the blast furnace. Such steels are generally produced from cold metal in the arc furnace. Here the quality of the
steel is dependent on careful selection of the charge, so that quantities of tramp elements tin, arsenic, antimony, bismuth,
copper, etc., which have serious effects upon ductility and  cannot be reduced by steel making are minimized. Electric arc
furnace steel is usually considered superior to steel made in the BOF, but the application of AOD and VAD can produce
equivalent steel from either.

There are other steel-making processes that will produce very high-quality metal, but all depend on the use of goodquality
raw material. The vacuum high-frequency furnace will produce metal of low oxygen, nitrogen and hydrogen content,
but is not well adapted to a continuous casting process. It is mainly used to produce small quantities of metal as small
ingots (or castings). The highest-quality steel is produced by the consumable vacuum arc or the ESR processes. In both of
these an electrode (or electrodes) is made of steel of the target composition and this electrode is progressively melted by striking an arc between it and a starting pad which is progressively withdrawn (or the electrode is withdrawn). thereby
producing a semi-continuous casting (see Figure 7.10). The metal is refined on passing through the vacuum in the case of
the consumable vacuum arc or through a low melting point slag in the case of the ESR. The solidifying metal is very well
fed from the pool of molten metal and inclusions are dispersed and very substantially reduced in size. The consumable vacuum
arc removes all gaseous or gasifiabie inpurities but does not significantly influence the proportion of non-metallics.

ESR can transfer non-metallic inclusions into the slag but does not remove hydrogen. ESR consumables must therefore be
hydrogen-free.

The influence of these melting processes on such important properties as ductility or fatigue strength is very significant.
The highest property values are not usually improved significantly compared with conventionally melted steel, but the
proportion of values falling below a specific standard is very significantly reduced so that a much higher component performance
may be guaranteed. A small proportion of steel is electron beam or plasma melted.

The proportion of steel that is continuously (or semicontinuously) cast has greatly increased. l5 The large in  tegrated steelworks may have its entire throughput fed to a me stream continuous casting of 1 m x 2 m section16, but batch metal from an electric arc melting shop is often semicontinuously cast in sections down to approximately 10 cm.

Mould costs are reduced and throughputs and metal recovery  considerably increased compared with the older type of
mould. The proportion of wastage at the feeding head and the chances of segregation in the pipe are reduced.
Ingot is still employed where the quantity of steel required is insufficient to justify the use of continuous casting or for very
large forgings, where the cross section of the ingot required to withstand the reduction needed is too large for it to he
continu'ously cast.