Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The invention relates to a method of producing steel
ingots, especially ingots which are to be rolled, to ingots so
produced and to rolled steel product made from the ingots.
In a known process for making ingots, unkilled steel
is cast into an ingot mould in one single casting action, and
after a certain period of time the remaining liquid contents of
the ingot mould are fully killed by the addition of deoxidizing
agent and are subsequently homogenized by the injection of a
gas. In Dutch Patent Application No. 70.12060 (corresponding
to British Patent Specification No. 1,304,826) the predecessors
in law of the present applicants suggested a similar method, in
which however, preference was given to a mode of performance in
which the heat flow away from within the central part of the
mould is reduced by applying an i'nsulating layer (a so-called
"hot-top") locally against the inner side of the ingot mould
and/or by sprinkling an insulating powdery substance on top of
the ingot. By these measures it is intended to keep liquid the
upper portion of the ingot as much as possible over the entire
cross section, in order to counteract the formation of a shrin-
kage cavity. The dirt which is present in the top of the ingot
after solidification (also caused by and coming from the hot
top) must be eliminated from the steel when rolled to slabs,
which affects the yield of slabs rolled from the ingot unsatis-
factorily. Also applying heat insulating material to the top
end of the inner surface of the ingot mould, and the sprinkling
of insulating and/or exothermic substance on top of the ingot
both tend further to increase the cost of making the ingot.
It is an object of the invention at least partially to
eliminate the abov ~entioned drawbacks, and particularly to in-
crease the slab yield while reducing the cost.
According to the invention there is provided a methodof producing steel ingots in which unkilled steel is cast into
an ingot mould in a single casting operation, and after a period
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of time the remaining liquid contents of the mould are fully
killed by the addition of deoxidizing agent and are subsequent-
ly subjected to homogenization by injection of a gas in which
method the ingot mould is an uncovered mould and the contents
of the mould are, after the said homogenization, force-cooled
and at least partially solidified from the upper surface of the
steel downwards by means of cooling fluid. sy an "uncovered"
mould we mean a mould in which special measures to reduce heat
loss at the top, for instance provision of a hot top, are not
taken.
Preferably the cooling fluid is a liquid directly
applied to the upper surface of the steel, e.g. by spraying.
With this new method, it has been found possible to
increase the slab yield in a particular application in the ap-
plicant's works from 81% to 86%.
It is remarked that British Specification No. 1,346,987
shows a method in which an ingot mould is filled by pouring in
one single action without using a hot top, and in which also the
top of the ingot is force-cooled. In that case, however, the
amount of deoxidizing substance added is such that only the
steel in the top of the ingot mould is killed, whereas inten-
tionally the steel in the lower part of the ingot mould is kept
unkilled. The deoxidizing substance supplied is not homogenized
through the melt by the injection of a gas, since it is the spe-
cial intention with this known method that the continuing forma-
tion of gas in the lower part of the ingot ensures that the
ingot solidifies with a convex upper surface because of trapped
gas bubbles. A drawback of this known method is that the de-
oxidizing substance is distributed unequally through the ingot,
with the result that upon rolling materials of different quali-
ties are produced from one and the same ingot.
Preferably in the method of the present invention the
said homogenization is performed for a period of 20 to 30 seconds
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by the injection of air from a lance which is introduced down-
wardly into the liquid steel to near the bottom of the ingot
mould, and water is thereafter sprayed onto the upper surface
of the steel for a period of 15 to 30 minutes.
An important advantage obtainable with the method of
the invention is that, compared to the known method first de-
scribed above, the cast ingot needs substantially less time for
solidifying in the ingot mould. Also it has been found that the
ingot may be placed in the pit furnace while warmer, and also a
shorter time is required in the pit furnace until it has suffi-
ciently equalized in temperature in order that it may be slab
rolled. These factors can result in substantial savings in
investment in ingot moulds and in costs of pit furnace opera-
tion.
The following comparative example illustrates the
invention.
EXAMPLE
A number of ingots of 20 tons steel each were cast and
subsequently slab rolled by the known method first described a-
bove and by a method embcdying the invention respectively. The
tests were performed with a standard low alloy low carbon un-
killed steel.
According to the known method, this steel was cast in
ingot mould provided with a hot top and was caused to rim
against the mould for 4 minutes. Thereupon a sufficient quanti-
ty of aluminium ribbon was injected into the remaining melt in
order to kill it. By bubbling of air from a pipe which was
moved downwardly to near the bottom of the mould the melt was
homogenized for 24 seconds.
It was found that the ingots cast in this manner had
to cool down for 300 minutes in the mould before the moulds
could be stripped. A further cooling period of 30 minutes was
then necessary before the ingots could be placed in the pit
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furnace. Equalization of temperature for 480 minutes in the
pit furnace was found to be necessary in order that the ingots
could be rolled to slabs. After the slab rolling the slab yield
was found to be 81~.
Steel of the same quality was cast in single and un-
interrupted casting operatings into similar ingot moulds, which
however, were not provided with hot tops. The treatment was
similar to that described above with the exception that after
the injection of aluminium ribbon to fully kill the still-molten
contents and the homogenization, water was sprayed onto the up-
per surface of the steel for 20 minutes to force-cool and at
least partially solidify the melt downwardly. This time it was
found that after a "covered" cooling time of 95 minutes the
ingots could be stripped. After a "naked" cooling time of 65
- minutes these ingots could be placed in pit furnaces in which
after 300 minutes they had sufficiently equalized in temperature
to be rolled into slabs. Twenty of such slab rolled ingots were
found to have a slab yield of 86%, as compared with the 81%
yield of the slab rolled ingots produced according to the known
method.
