6 Steels: Metallurgy and Applicationsto a thickness of 50 mm, passes through a tunnel furnace to make the tempera-
ture uniform and then passes through a five-stand finishing train. With the ISP
Online strip production) process as practised at Arvedi, 4 the slab is squeezed
while the core is liquid to a thickness of 43 mm. It passes through a three-stand
roughing sequence to reduce the thickness to 15-20 mm and then passes through
a four-stand finishing mill. An advantage of thin slab casting is the reduction in
cost compared with the more conventional process, but at present, the surface
quality of the product does not match that of the conventional process. So far,
however, only a relatively small number of steel works have been equipped with
the thin slab casting process and these are fed with steel from electric arc furnaces
produced mainly by melting steel scrap. The steel may, therefore, contain rela-
tively high residual elements which tend to limit quality.
Further developments are under way to enable the usual hot-rolled strip gauges
to be cast directly. These processes are known as thin strip casting processes. So
far, no steel works has been equipped with thin snip casting, but the expectation
is that the elimination of the finish hot-rolling sequence will lead to a further
reduction in cost. It is likely, however, that the material will not be suitable
for direct use, but will require subsequent cold rolling and annealing for usable
properties to be obtained.
Other means of reducing steel costs have been developed, including ferritic
rolling, the combining of pickling and cold rolling into a single process, the
combining of continuous annealing, temper rolling and inspection into a single
process and the combining of cold rolling and continuous annealing. Only ferritic
hot rolling, however, has a major impact on the metallurgy of the complete
process.
Underlying metallurgical principles
Low-carbon strip steels are based primarily on ferrite microstructures and are
almost invariably subjected to cold-forming operations in order to achieve
specific shapes, e.g. automotive body panels. They are produced in the hot-rolled
condition in thicknesses down to about 2 mm but they are used primarily in the
cold-reduced and annealed state where the gauges can extend down to 0.16 mm
for tinplate grades, e.g. for packaging applications. The basic consideration in
the development of microstructure in strip and low-alloy steels is the Fe-C
phase diagram, a portion of which is shown in Figure 1.3. At temperatures
above about 900"C, a steel containing about 0.05% C will consist of a single
phase called austenite or y iron which has a face-centred cubic crystal structure,
as illustrated in Figure 1.4(a), and all the carbon will be in solid solution. On
cooling slowly, the material reaches a phase boundary at which a separate iron
phase, called ferrite or ot iron, begins to form. This phase contains a low carbon
content and has a body-centred cubic structure, as illustrated in Figure 1.4(b). As
cooling continues, the formation of further low-carbon ferrite leads to a carbon
enrichment of the untransformed austenite. This process continues on cooling
down to a temperature of 723~ the eutectoid temperature, at which stage the
remaining austenite will have been enriched in carbon to a level of about 0.8%.