Cast iron especially suited for ingot molds

This is provided an ingot mold formed of a cast iron consisting essentially of 3.7 to 4.0% C, not more than 1.6% Si, 0.40 to 0.80% Mn, 0.10 to 0.45% P, not more than 0.10% S, 0.020 to 0.050% Mg, the balance Fe with normally appearing impurities. The elements are adjusted to provide a specific carbon equivalent in the range of 3.2 to 3.6% calculated as C.sub.eqv =%C+0.65% Si+0.35% P-35% Mg. In addition, the ingot mold structure of the present invention contains an amount of not more than 5% by volume of carbide, an amount of not more than 25% by volume of ferrite and at least two-thirds of the total graphite volume being spheroidal graphite with the balance pearlite.

The present invention relates to a cast iron especially suited for ingot 
molds, which possesses good resistance to deterioration in connection with 
thermal cycling thus prolonging the achievable time of use. 
It is always a problem when casting ingots into molds to prevent crack 
initiation in the mold material in one way or another. The crack 
initiation is primarily a result of the deterioration of ductility that is 
a result of the fact that the structure is negatively affected during the 
thermal cycling with repeated exposure of the interior surface of the mold 
under oxidation ambient in connection with stripping the ingot from the 
mold. Various methods have been proposed for the purpose of improving the 
lifetime of such ingot molds one of which residing in changing the 
analysis of the ingot mold material, another residing in changing the 
design of the ingot mold. These proposals, however, have not yet been 
successful for various reasons. 
British Pat. No. 1,218,035, for example, discloses a cast iron for ingot 
molds where the iron by inoculation has been affected to appear with a 
structure wherein vermicular graphite is distributed in a mainly pearlitic 
matrix at the same time as phosphorus and sulfur are present in certain 
low amounts. This which differs from commonly used cast iron, also did not 
result in increased resistance against thermal fatigue. 
With the foregoing in mind it is an object of the invention to provide a 
cast iron that is more suited for ingot molds than those cast iron 
materials proposed to date. The lifetime of an ingot mold primarily 
depends on the properties of the material, from which the mold is 
produced. The following properties are desirable with an ingot mold 
material: 
1. High strength and toughness at elevated temperatures and good thermal 
conductivity, which means good resistance to thermal shocks, thermal 
cycling and oxidation. 
2. Insignificant shrinkage during solidification and good workability. 
Extensive studies of the relations between the above properties and the 
analysis and structure of the cast iron have been conducted, which 
surprisingly have shown that it ought to be possible to have the 
consituents balanced against a certain carbon equivalent in a suitable 
manner for the purpose of reaching an optimum of the material properties 
related above. 
According to the present invention there is provided a cast iron containing 
3.7 to 4.0% C, not more than 1.6% Si, 0.4 to 0.80% Mn, 0.010 to 0.045 P, 
not more than 0.010% S, 0.020-0.050% Mg and the balance Fe with normally 
appearing impurities, the said elements being balanced against a specific 
carbon equivalent in the range 3.2 to 3.6% calculated as C.sub.ekv. 
=%C+0.65%Si+0.35%P-35%Mg. 
According to a preferred embodiment of the invention there is provided a 
cast iron containing 3.7 to 4.0% C, not more than 1.3% Si, 0.40 to 0.70 
Mn, 0.010 to 0.040% P, not more than 0.010% S, 0.020 to 0.040% Mg and the 
balance Fe with normal impurities, the said elements being balanced 
against a specific carbon equivalent in the range 3.3 to 3.6%. 
According to another preferred embodiment of the invention there is 
provided a cast iron containing 3.7 to 3.9% C, not more than 1.1% Si, 0.45 
to 0.60% Mn, 0.015 to 0.050% P, not more than 0.010% S, 0.020 to 0.040% Mg 
and the balance Fe and normal impurities, the said elements being balanced 
against a specific carbon equivalent in the range 3.3 to 3.6%. 
The cast iron shall in all these cases be produced such that its structure 
contains carbide less than 5% of volume, ferrite not more than 25% of 
volume, graphite being spheroidal to a dominant amount, preferable at 
least 2/3 of total volume of graphite and the balance being pearlite. 
The results of laboratory tests and full scale tests of the cast iron of 
the invention have shown that longitudinal and transverse cracks have 
almost entirely been eliminated as a reason for scrapping. As a 
consequence thereof this new material has shown to result in a lifetime 
that amounts to 1.25 to 1.75 times that of previously used ingot mold 
materials. 
The cast iron of the present invention has a very good resistance to 
thermal fatigue. This has been achievable by optimizing its analysis as 
related above for the purpose of reaching a maximum of high-temperature 
strength and ductility. 
In the Table I below is set out some compositions of castings of irons in 
accordance with the invention and some compositions beyond the scope of 
the invention, which have been subjected to hot tensile tests. 
TABLE I 
______________________________________ 
Chemical analysis of test materials 
Charge No. 
C Si Mn P Mg C.sub.ekv. 
______________________________________ 
6.28222 3.70 0.82 0.78 0.042 0.028 3.3 
6.28170 3.91 0.83 0.77 0.042 0.031 3.4 
6.53777 3.82 1.51 0.65 0.012 0.038 3.5 
6.28214 3.64 1.68 0.78 0.044 0.031 3.7 
6.28192 4.00 1.10 0.81 0.042 0.029 3.7 
6.28162 3.88 0.97 0.01 0.065 0.019 3.9 
6.28167 3.94 0.89 0.79 0.037 0.017 3.9 
6.28251 3.92 0.89 0.78 0.025 0.016 3.9 
6.28160 3.92 0.97 0.02 0.024 0.016 4.0 
6.28168 3.97 0.95 0.79 0.072 0.018 4.0 
6.28197 3.99 1.68 0.78 0.044 0.028 4.1 
______________________________________ 
Melts for testing purposes were produced in an acid high-frequency 
induction furnace in which sufficient raw materials such as iron, 
ferrosilicon, Mn-metal and FeP had been added. The melt was then 
inoculated with FeSiMg for obtaining nodular graphite and the melt was 
poured at about 1330.degree. C. 
Test bars were then produced from the melt, which were subjected to 
hardness tests and tensile tests in a Gleeble-machine. In connection 
therewith said test bars were heated to a choosen test temperature 
(300.degree.-1100.degree. C.), was maintained 100 seconds at that 
temperature and then tensile tested at a constant speed of 25 mm/sec., 
whereby obtained values for area reduction (.psi.) and ultimate strength 
(.sigma..sub.B) were registered.

It is essential that the constituents of the cast iron are present in 
amounts such as to give a carbon equivalent within the ranges stated. 
Presence of carbon highly contributes to prevent shrinkage during 
solidification and simultaneously give the cast iron good castability. In 
view thereof carbon should be present in an amount of at least 3.7 weight 
percent. The maximum carbon content should be 4.0% and preferably less 
than 3.9%, since hot-ductility and strength otherwise might decrease too 
markedly. In FIG. 1 is illustrated values that have been registered after 
a comparison between three different alloys with varying carbon content. 
As can be gathered therefrom a decreased ductility is the result of an 
analysis, when carbon content has not been adequately optimized against 
the other constituents. 
Silicon might be present in a maximum amount of 1.6% but preferably should 
be present in an amount less than 1.3% and most preferably in an amount 
less than 1.1%. Higher silicon contents should be avoided since silicon, 
like carbon, will cause a decrease of hot-ductility and strength if not 
being adequately optimized. Cast irons containing low silicon amounts have 
a more clearly tendency of pearlite formation, which means improved 
ductility at temperatures above 700.degree. C. A most rapid pearlite 
transformation is desirable since the two-phase structure 
austenite-ferrite causes a deterioration of the ductility. FIGS. 2 and 3 
show the influence of C, Si and C+S on strength properties. As can be 
gathered therefrom too high silicon amounts, if not adequately optimized, 
have markedly decreased the strength properties. 
Presence of manganese improves ductility and strength and should, 
therefore, appear in the cast iron in amounts of at least 0.40% and not 
more than 0.80%. Since manganese stabilizers pearlite formation and 
decreases the carbon activity manganese will advantageously reduce 
graphite growth at thermal cycling. Manganese content, however, should not 
exceed 0.70% and should preferable amount of 0.45% to 0.60% having regard 
to internal oxidation and cementite formation during solidification. 
Phosphorus ought to be present in an amount of at least 0.010% and should 
preferably amount to at least 0.015% since presence of phosphorus 
increases the strength. The phosphorus content, however, should be 
optimized in relation to the elements C, Si and Mg. FIGS. 3 and 4 show 
that unbalanced phosphorus causes a decrease of the burning limit, i.e. 
the limit when ductility abruptly decreases. Phosphorus could be present 
in amounts up to 0.045 but ought to be less than 0.040% and, if silicon 
content is high, preferably should be lower than 0.030%. 
The sulphur may be present in about same contents as normally used, which 
means contents up to a maximum of 0.010%. 
Magnesium affects the graphite formation. A successively increasing 
magnesium content causes changes of the graphite from lamellar to 
vermicular structure and finally to nodular structure. It is essential 
that a sufficiently high magnesium content is maintained so as to obtain 
fully nodular graphite. This graphite formation has been found to be 
necessary in cast iron for ingot molds with regard to crack initiation. 
Hence, magnesium content should be a value between 0.020 and 0.050%, 
preferably between 0.020 and 0.040%. Presence of magnesium also 
contributes to improve hot ductility properties and stabilize pearlite. 
FIG. 5 shows ductility values for two test samples, one of which contains 
magnesium at an amount that has not been adequately optimized. A clear 
decrease of the ductility is a visible result thereof. 
It is essential that a matrix structure suitable for ingot mold production 
is present in the cast iron. Laboratory studies and full scale studies of 
the material here under consideration have shown that the present cast 
iron has improved structure stability. The present cast iron shall be 
produced such that its carbide amount not exceeds 5 percent of volume, 
ferrite not more than 25% of volume, graphite is nodularized to a dominant 
part, preferably to at least 2/3 of total graphite volume and the balance 
being pearlite. The speed at which the internal oxidation and the change 
of structure occurs is determined of the speed of decarburization and 
crack initiation. As can be gathered of the speed of decarburization and 
crack initiation. As can be gathered from FIGS. 6 and 7 the nodular 
graphite gives less decarburization depth and hence also decreased 
possibilities for crack initiation. In order that the present cast iron 
simultaneously shall obtain sufficiently high strength it is necessary to 
limit the ferrite content. This is achievable primarily by optimizing the 
manganese content in the manner previously related. From the aspect of 
physical properties it is simultaneously important to adequately optimize 
the content of phosphorus. Carbon and silicon both cause an increased 
phosphorus activity. When both these elements are present in higher 
amounts within the ranges stated it must consequently be controlled that 
the content of phosphorus is low enough so as to avoid decrease of 
hot-ductility at high temperatures. 
The results of using ingot molds produced from prior art cast irons (nos. 
163-186) and results of using ingot molds produced from a cast iron of the 
present invention (nos. 901-907) have indicated that a considerable 
improvement of the durability of the mold has been found achievable. In 
Table II below actual material analysis have been listed. As regards 
graphite formation as appearing in the structure it shall be noticed that 
designation numbers I, III and VI correspond to flaked graphite, 
vermicular graphite and nodular graphite respectively. Hence, mould sample 
no. 163 is indicated to comprise a graphite structure type III--VI 
distribution 14-1, which means that graphite is present in nodular form to 
an amount of 1/15 whereas the balance of graphite has vermicular 
configuration. 
The results of full scale testing have been indicated in Table III and in 
each specific case the reason for scrapping has been indicated by codes. 
Codes 3, 4, 6 and 7 are directly coupled to the ingot mould material per 
se whereas the other codes refer to scrapping, which primarily occurs from 
the handling of the ingot molds. As regards code no. 3, it has been 
indicated after how many charges vertically extending cracks have been 
observed. The results can be summarized as follows: 
1. Longitudinally and transversely extending cracks have mainly been 
eliminated as a reason for scrapping the molds. 
2. The durability of the mold has been improved at an order of 1.25-1.7 
times, which has resulted in decreased consumption mould material/to 
steel. 
As an example it can be mentioned that steel consumption decreased from 
14.9 to 9.7 kilos ingot mold for each ton steel produced with an ingot 
mould indicated Sandvik 27", which is the mould design referred to in 
Table III. 
TABLE II 
__________________________________________________________________________ 
Charges 
Mold before Graphite Pear- 
Fer- 
Car- 
type scrap- 
Analysis Distr. 
Total 
lite 
rite 
bide 
No. ped % C % C-ek. 
% Si 
% Mn 
% P % S % Mg 
Type 
% % % % % 
__________________________________________________________________________ 
Sandvik 
27" 
163 60 III-VI 
14-1 
15 18 65 2 
165 41 3.92 
4.0 1.02 
0.33 
0.031 
0.002 
0.018 
III 15 15 18 65 2 
166 48 III 15 15 18 65 2 
183 86 3.92 
3.6 0.42 
0.05 
0.027 
0.005 
0.017 
III-VI 
13-2 
15 15 67 3 
184 42 
185 57 3.89 
4.3 1.23 
0.70 
0.012 
0.005 
-- I -- 20 65 15 -- 
186 57 
901 83 III-VI 
2-13 
15 64 20 1 
3.82 
3.5 1.03 
0.55 
0.028 
0.006 
0.027 
902 94 III-VI 
2-13 
15 63 20 2 
903 96 III-VI 
4-11 
15 56 25 4 
4.01 
3.5 0.86 
0.44 
0.029 
0.007 
0.030 
904 90 III-VI 
5-10 
15 57 25 3 
905 111 III-VI 
2-13 
15 65 15 5 
3.79 
3.4 1.10 
0.57 
0.029 
0.007 
0.032 
906 117 III-VI 
2-13 
15 64,5 
20 0.5 
907 113 3.90 
3.4 0.96 
0.50 
0.030 
0.005 
0.035 
III-VI 
2-13 
15 64,5 
20 0.5 
__________________________________________________________________________ 
TABLE III 
__________________________________________________________________________ 
FIELD TESTING 
Mold 
Number 
Reasons 
TEST INGOT MOLDS 
type 
of for xx Ageing indications 
No. Charges 
scrapping 
1 2 3 4 5 6 7 8 
__________________________________________________________________________ 
SANDVIK 27" 
163 60 3 21ch II 2mm 
165 41 3 15ch I 2mm 
166 48 3 28ch I 
183 86 3 21ch III 12/8mm 
184 42 3 21ch I 
185 57 3 57ch 26ch I 
186 57 3 26ch I 
901 83 1 + 2 
57ch 
83ch 
0 II-III 
4/5mm 
80ch 
902 94 2 + 5 
-- 94ch 
0 0 12mm 
II-III 
6/6.5mm 
80ch 
903 96 2 + 5 
80ch 
96ch 
80ch 
0 5mm 
II-III 
4/5mm 
80ch 
904 90 2 + 5 
90ch 
90ch 
0 0 12mm 
III 4/9mm 
80ch 
905 111 2 102ch 
-- 0 0 II-III 
5/1mm 
80ch 
906 117 2 -- -- 0 0 II-III 
5/1mm 
80ch 
907 113 2 -- -- 0 0 II 4/3mm 
60ch 
__________________________________________________________________________ 
.sup.xx ageing indications and/or scrapped 
Code 
.sup.1 Erosion cavities from stream of steel 
.sup.2 Ingot stuck in the moulds remmed out 
.sup.3 Vertical cracks 
.sup.4 Horisontal cracks 
.sup.5 "Stickers" owing to the bottom having bent up and steel having 
solidified under the mould 
.sup.6 Crazing of the inner surface: I = inconsiderable, II = smooth 
surface where the pattern of crazing is evident, III = crazing beginning 
to come out of the grain boundaries, IV = considerable crazingcracks 
propagated from the surface 
.sup.7 Outside bending ( = thermal deformation) measured in mm from a 
straight ruler 
.sup.8 Burnt inner surface