[0001] This invention relates to a cast-iron ingot mould for use in the production of steel
ingots in the steel industry, and to a method of manufacturing such an ingot mould.
[0002] In the manufacture of rolled steel products, the starting material is often steel
ingots which have been obtained by casting liquid steel into a cast-iron ingot mould
and allowing the steel to solidify at least partly in the ingot mould. Such an ingot
mould is a thick-walled hollow cast-iron body open at both ends and with a slightly
tapering shape so that the ingot mould can be stripped from the cast steel ingot.
During casting of the steel ingot, the ingot mould is located on a heavy metal ingot
plate, which defines the bottom of the ingot. The foot of the ingot mould contacts
the plate.
[0003] During solidification of the steel a substantial part of the heat present in the
steel is transmitted to the ingot mould and to the ingot plate, both of which are
thus subjected to high thermal stresses. During the subsequent stripping of the mould
from the solidified steel ingot, the ingot mould is further subjected to severe mechanical
stresses, which in turn are followed by a thermal load as the ingot mould cools down
after stripping. All this results in that ingot moulds are subject to damage and wear,
which limit their useful life. This life-span is an important cost-determining factor
for the final steel product.
[0004] With ingot moulds of good quality, 100 castings per ingot mould are considered normal
for a life-span. Nevertheless it is found that with ingot moulds of otherwise very
good quality there now and then occurs the phenomenon of broken feet. This consists
in the breakage away of pieces of material from the mould on the inside of the mould
foot during stripping. Unlike other forms of damage which can limit, the life-span
of the ingot mould, it appears that broken feet often occur during the first castings
made in the ingot mould. If they do not occur then, there is a significantly lower
chance that they will occur again during further use of the ingot mould. It further
appears that the probability of broken feet occurring is greater at a higher temperature
of the ingot mould foot. This for instance may be the case if during casting use is
made of ingot plates which have not been completely cooled down.
[0005] It is supposed that the phenomenon of broken feet is influenced by the susceptibility
of the ingot mould material to welding to the solidifying steel, and is also influenced
by the strength and brittleness of the ingot mould material in the foot region. Therefore
there has been a general aim to select the composition of the ingot mould so that
a stronger and less brittle material is obtained.
[0006] It is generally known that the presence of phosphorus in cast-iron has a weakening
effect on the metal and in particular increases the brittleness. Therefore there is
a general tendency to keep down the percentage of phosphorus in the cast-iron used
for ingot moulds. Phosphorus percentages of less than 0.1% by weight are sought. Nevertheless
it has not been found possible to counteract successfully the phenomenon of broken
feet by further reduction of the phosphorus content.
[0007] German Offenlegungsschrift 1 758 706 describes a liquid cast-iron composition for
the production of ingot mould, which contains:

[0008] Before casting, this liquid cast iron is enriched with substantial quantities of
silicon, either in a metallic form or as a metal alloy of silicon, in order to achieve
a silicon content in the cast ingot mould which is between 0.4 and 0.5% higher than
the Mn content. This means that in general the Si content in the ingot mould should
be between 1.2 and 2.4%.
[0009] It is known that alloying cast-iron with silicon is expensive and requires a careful
control in order to prevent the formation of graphite.
[0010] The present invention has the object of improving the quality of cast-iron ingot
moulds, in particular ingot moulds which are to be used for the production of heavy
ingots in the weight range of 24 tons and above, and is especially concerned at minimizing
the problem of broken feet, especially by the use of a process which is of low cost
and is easily carried out.
[0011] It has now been found that, to improve the quality of ingot moulds and to increase
their life-span, especially with regard to the ingot mould feet, the analysis of the
cast-iron is of great importance. More particularly it has been found that good results
can only be achieved with an analysis in which the margins for the varying alloying
elements and for carbon are narrower and more specific than previously disclosed,
for example in the German specification mentioned above. It has been found, surprisingly,
that the phenomenon of broken feet in an ingot mould can be counteracted very effectively
by raising the percentage of phosphorus instead of by lowering it.
[0012] The invention therefore consists in a cast-iron ingot mould with an analysis (by
weight) of the cast-iron of:

rest Fe and usual impurities, wherein the Si-content is smaller than the Mn-content.
[0013] Preferably the sulphur content is between 0.015 and 0.25%.
[0014] It should be regarded as a special advantage of these ingot moulds of the invention
that the Si-content need not be increased, and is in fact kept smaller than the Mn-content.
[0015] Especially good results are obtained if the Si-content is between 0.55 and 0.75%
and the Mn content 0.74 and 0.95% but alternatively the Si-content may be between
1.0 and 1.15% and the Mn-content between 1.1 and 1.3%.
[0016] A phosphorus content between 0.14 and 0.195% is unusually high for a cast-iron analysis.
Nevertheless it has been found that this high phosphorus percentage definitely improves
the life-span of ingot moulds of the above new composition, particularly by effectively
counteracting the phenomenon of broken feet.
[0017] Best results have been obtained if the P content is between the narrow limits of
0.16 and 0.18%.
[0018] As explained already the main effect of the increased P-content is to lower the risk
of broken feet. Since this phenomenon of broken feet is restricted to the inside of
the mould foot, where it can occur with moulds of otherwise outstanding quality and
long useful life, it is in many cases not necessary to increase the phosphorus content
throughout the entire mould. Where this is possible in production conditions, and
where it is desirable for other technical reasons, it is recommended that outside
the region at the mould foot the P-content is about 0.06% lower than in the mould
foot itself.
[0019] The invention relates to not only the ingot mould, but also to a method for the manufacture
of the ingot moulds according to the present invention as described above. According
to this method, before casting of the ingot mould a cast-iron composition of the analysis
(by weight):

is enriched with P by the addition of suitable amounts of an iron-phosphorus alloy.
Compared with a method where a pig iron with high phosphorus content is produced in
a blast furnace, this new method has clear advantages. These consist in that firstly
no abnormal production scheme has to be used in operating the blast furnace, and secondly
the addition of a phosphorus alloy to the cast-iron makes possible a much more accurate
adjustment of the phosphorus content within the desired limits.
[0020] It has been found that use of ingot moulds according to the invention can reduce
the percentage of broken feet to only a fraction of the previously usual percentage.
This effects a substantial reduction in the costs of the use of ingot moulds and of
repair of ingot moulds with broken feet, which in turn results into a substantial
lowering of the cost of the cast steel ingots. In addition the cast steel ingots manufactured
with the ingot moulds of the invention can have a smoother surface near the ingot
mould foot, which provides advantages in the further conversion of the steel ingots
into rolled products.
[0021] During tests in the applicant's works two series each of 100 ingot moulds were compared
under similar conditions. One series had been manufactured to have a composition throughout
as described above according to the invention. In the other series the phosphorus
percentage was selected at the conventional level, i.e. about 0.06 % lower.
[0022] The same criteria were used for judging
(i) the general condition of these ingot moulds,
(ii) their life-span until they were no more regarded suitable for further use, and
(iii) especially the occurrence of broken feet.
[0023] On average it was found that the ingot moulds according to the invention had a 10%
increase in life-span, while the work needed and costs incurred for maintenance of
these ingot moulds were reduced by about 50%.
1. Cast-iron ingot mould for use in the production of steel ingots, wherein the cast-iron
consists of iron, carbon, silicon, manganese, sulphur, phosphorus and usual impurities,
characterized in that at least at a region at the mould foot, the amounts of these
elements by weight are, in combination:-

rest iron and usual impurities, and in that the Si-content is smaller than the Mn-content.
2. Ingot mould according to claim 1 wherein the S-content is between 0.015 and 0.025%.
3. Ingot mould according to claim 1 or claim 2 wherein the Si-content is between 0.55
and 0.75% and the Mn-content between 0.74 and 0.95%.
4. Ingot mould according to claim 1 or claim 2 wherein the Si-content is between 1.0
and 1.15% and the Mn-content between 1.1 and 1.3%.
5. Ingot mould according to any one of the preceding claims wherein the P-content
is between 0.16 and 0.18%.
6. Ingot mould according to any one of the preceding claims wherein outside the said
region at the mould foot the cast-iron of the ingot mould has a P-content about 0.06%
lower than in the said region at the mould foot.
7. Method of manufacturing an ingot mould according to claim 1 wherein before casting
the mould a cast-iron composition consisting of, by weight:

rest iron and usual impurities, is enriched with P by the addition of suitable amounts
of an iron-phosphorus alloy and is then used to cast the ingot mould.