Full resolution (JPEG) - On this page / på denna sida - Second part - IX. Mining Industry and Metal Production - 2. The Iron and Steel Industry. By the late Prof. J. G. Wiborgh - Ingot iron (Steel)
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749
When the percentage of carbon is low and the temperature high, the iron
has a tendency to take up protoxide of iron from the slag, which causes
red-shortness, and which must be removed by an addition of ferro-manganese or
silico-ferro-manganese.
Frequently ferro-manganese with a percentage of iron under 0*4o, is used
for iron and silico-ferro-manganese for steel.
This addition is generally made in the furnace just before the iron is
tapped, but on account of the acid slag, a large portion of the manganese is
oxidized and goes into the slag. If, instead, this addition is made in the ladle,
a smaller quantity of manganese is required, but the mixture is frequently less
complete.
When the open-hearth metal is to be used for castings, silico-ferro-manganese
is always added in order to obtain a solid product free from blowholes.
The tapping hole is closed during the process by a fire stone surrounded on
all sides by quartz, and when the iron is to be tapped the stone is pushed into
the furnace, whereby a sufficiently wide opening is made for the iron very quickly
to run out into the ladle placed under the tapping hole. From this ladle the
iron is poured into the moulds in the ordinary manner.
For special steel, an addition of chrome or nickle is used, of the latter
metal 0*5 to 3 according to the purpose for which it is to be used.
After the tapping the hearth is carefully cleaned of molten iron, and
cavities in the bottom of the hearth are fixed and smoothed over by filling the
same with quartz. For such repairs, 1 up to 6 hectoliters of the quartz are
required, depending upon how far the hearth has been burned out.
As soon as the mass is applied, the furnace is heated to such a
temperature that the quartz begins to fuse, and the furnace is ready for a new
charge. On account of these constant, small repairs, which only take ’/»to
S,U of an hour each, the same furnace bottom can be used for a long time,
half a year or more. The furnace bottom is affected the most when the
temperature is low and the slag is rich in iron, as when a steel low in
silicon is to be produced. During the process of refining, there is plenty of
time for taking samples of the steel, and for ascertaining the hardness of the
metal by means of forging tests. These tests are the same as in the Bessemer
process.
The time required for refining a charge varies considerably, from seven to
twelve hours, according to the different proportions of pig iron and wrought iron
scrap used for the charge. The consumption of fuel is 250 to 300 kilogr. of
fossil coal or 30 to 40 hectoliters of wood or stumps per ton of ingot.
The basic open-hearth process is carried out in about the same manner as
the acid. In order to protect the basic material of which the hearth is built, from
being affected by silicic and phosphoric acid, which are formed in melting the
pig iron, lime is added both at the charging of the furnace and during the process
of melting. Generally slaked lime is used, for raw lime causes a violent bubbling
and swelling when the carbonic oxide escapes. Also here, ore is used to hasten
the refining, though in smaller quantities, because all substances which, on being
oxidized, form acids, such as silicon, phosphorus, carbon, and sulphur, oxidize
more easily; a basic slag and a basic lining being at hand.
Manganese, however, which forms a basic oxide, is oxidized more slowly
than in the acid process and remains partly in the iron. For this same reason
less iron is oxidized, wherefore it is usual to add in the beginning, besides lime,
some iron ore, to make the slag sufficiently rich in iron. This ore ought,
of course, preferably to be calciferous. The more silicon and phosphorus the
charge contains from the beginning, the more lime is required, but at the same
time the volume of slag increases, which makes the refining more difficult. The
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