Method of producing hollow steel ingot and apparatus therefor

The casting apparatus for a hollow steel ingot is constructed such that: at least three concentric pipes are provided in the central portion of an ordinary mold for a steel ingot; a core is formed by filling up a space formed between the first outer-most pipe having the largest diameter and the second pipe disposed inwardly of the first pipe with granular refractory material wherein zircon or chromite sand is bound by a binder such as an organic resin; a double pipe is disposed inwardly of said core and forming a gas flow course for cooling the core; and pouring gates formed through the stool for feeding molten steel at an intermediate portion between the inner wall of said mold and the core. The method for producing a hollow steel ingot by use of the casting apparatus as described above comprises: blowing the cooling gas from above into the third inner-most pipe having the least diameter, passing the gas through the second pipe from below and discharging it to above; cooling the molten steel fed into the mold and brought into contact with the outer surface of the first pipe by cooling the core through the inner wall of the second pipe; and controlling the cooling conditions for the core such as the thickness of the cylindrical refractory material of the core, the cross-sectional area of the cooling gas flow course and the thickness of the second pipe so that the finally solidifying position of the molten steel fed can be set at a position which is apart from the core side at a distance of 20 to 50% of the wall thickness of the hollow steel ingot to be formed.

FIELD OF THE INVENTION 
The present invention relates to a method of producing hollow steel ingot 
and an apparatus therefor, and more particularly, to a method of producing 
a large hollow steel ingot with sound interior quality for use as a 
forging material for a large cylindrical body and the like and an 
apparatus therefore. 
DESCRIPTION OF THE PRIOR ART 
It is readily understandable that, in general, as the material steel ingot 
in the case of manufacturing a cylindrical forging such as a pressure 
vessel material, the use of a hollow steel ingot is by for more efficient 
than the use of an ordinary steel ingot. However, heretofore, the 
technique in manufacturing hollow steel ingots, particularly large hollow 
steel ingots have not been established, and important cylindrical forgings 
have been manufactured by the use of big-end up solid steel ingot of 
polygonal cross section through machining after being subjected to a 
complicated process comprising the steps of: 
grip forging; 
upsetting; 
solid forging; 
upsetting and punching; 
bore enlarging; and 
mandrelling. 
Consequently, lowered forging yield and increased heating expenses lead to 
increased forging costs, thus resulting in very high manufacturing costs. 
In contrast with this, in the use of a hollow steel ingot, machining can 
be applicable to it immediately after undergoing a few steps such as bore 
enlarging and mandrelling, thus enabling expect improved yield and lowered 
forging cost to a considerable extent. 
Heretofore, a variety of methods of producing hollow steel ingots having 
advantages as described above have been proposed, and typical methods 
shown below are included therein. 
(a) A method wherein a water-cooled core rotatable in the central portion 
of a mold, molten steel is fed into a portion between the mold and the 
core, and, after a solidifying wall contacting the core has grown to a 
certain extent, the core is progressively elevated to be drawn. 
(b) A method of providing a metal core or a sand-mold core in the central 
portion of a mold. 
(c) A method by centrifugal casting. 
(d) A method wherein a metal plate core being a circle or deformed shape in 
horizontal cross-section is provided at the central portion of a mold, the 
interior of said metal plate core is made hollow and cooling water or 
cooling gas is blown thereinto, radiant heat absorbing substance is put or 
stuffed thereinto, thereby controlling the solidifying conditions of 
molten steel. (Japanese Patent Application Publication No. 28898/75). 
However, there have been encountered problems in complicated manufacture 
and installation of the core, unsatisfactory surface quality of the hollow 
ingot products, increased segregation of the interior quality of the ingot 
due to insufficient cooling and the like. Particularly, many problems are 
encountered in the production of hollow ingots larger in weight than 100 
tons, and satisfactory products have not been reliably obtained. 
BRIEF SUMMARY OF THE INVENTION 
One object of the present invention is to obviate the abovementioned 
disadvantages of the prior art in the production of hollow steel ingots 
and provide a method of producing hollow steel ingots excellent in the 
qualities of the surface and interior. 
Another object of the present invention is to provide an apparatus for 
casting hollow steel ingots, wherein said apparatus has construction of 
core meeting four requirements shown below, the core is not damaged by 
static pressure of molten steel fed into the mold, and the core is 
efficiently cooled during producing hollow steel ingots. 
(a) Arrangements for producing and installing cores and easily made. 
(b) Cooling of the core can be effected quick and suitably controlled. 
(c) The core can be readily taken out after the steel ingot has been 
solidified. 
(d) No cracking is caused to the interior of the steel ingot due to 
solidifying and shrinking stress, and construction of the core is suitable 
for obtaining a steel ingot product having excellent surface quality. 
The most important problem in producing hollow steel ingots resides in the 
construction of the mold, and particularly, in the construction of the 
core. Study on the problems of the mold including the core made by the 
present invention has led to the conclusion that the above-mentioned four 
items are vital. 
Said objects, other objects and advantages of the present invention will 
hereafter be made evident in conjunction with the detailed description of 
the present preferred embodiment of the invention illustrated in the 
accompanying drawings. However, it is to be understood that the drawings 
are merely illustrative but not limitative in the scope of the present 
invention. 
The aforesaid objects of the present invention can be achieved by the 
present invention having the following gist. 
The gist of the present invention resides in that 
A method of producing a hollow steel ingot, wherein a cylindrical core is 
disposed in the central portion of a cast iron mold installed on a stool 
and molten steel is fed into said mold by bottom pouring, characterized in 
that said core comprises a cylindrical refractory member formed of 
granular refractory material and steel pipes covering the outer and inner 
surfaces of said cylindrical refractory member, the inner surface of said 
core is cooled by a gas stream, and the finally solidifying position of 
said molten steel fed is restricted to a distance of 20 to 50% of the wall 
thickness of the steel ingot from the side of said core, and 
a casting apparatus for producing a hollow steel ingot, wherein said 
apparatus comprises a cast iron mold installed on a stool, a cylindrical 
core disposed in the central portion of said mold and pouring gates formed 
through the stool for feeding molten steel at intermediate portions 
between the inner wall of said mold and the core, wherein, said apparatus 
comprises: 
a first steel pipe disposed in the central portion of said mold; 
a second steel pipe provided inside said first steel pipe and 
concentrically therewith; 
a core formed of granular refractory material filled up in a space formed 
between said first and second steel pipes; 
a third steel pipe provided inside said second steel pipe and 
concentrically therewith; and 
a gas flow course for cooling the core descending into said third steel 
pipe from above and then ascending inside said second steel pipe.

DETAILED DESCRIPTION OF THE INVENTION 
Description will hereunder be given of the method of producing a hollow 
steel ingot and the apparatus therefor according to the present invention 
in conjunction with the embodiments thereof. 
FIG. 1 is a longitudinal section showing the casting apparatus for 
producing hollow steel ingots according to the present invention. Namely, 
said casting apparatus comprises a casting mold 2 installed on a stool 1, 
a core 3 disposed in the central portion of the casting mold 2, a gas flow 
course 4 for cooling the core provided further inside the core 3, and 
pouring gates 14 formed through the stool 1 for feeding molten steel 9 at 
intermediate portions between the mold 2 and the core 3. Said core 3 and 
the gas flow course 4 for cooling the core are fixed at the upper end of 
the mold 2 through a support fitting 5 so as not to be lifted up into 
molten steel. 
The core 3 comprises a first steel pipe 6 disposed in the central portion 
of the casting mold 2, a second steel pipe 7 provided inside said first 
steel pipe 6 and concentrically therewith, and granular refractory 
material 8 such as molding sand filled up in the space formed between the 
first and second steel pipes, and the outer surface of the first steel 
pipe 6 is brought into direct contact with molten steel 9 fed into the 
mold 2. 
The gas flow course 4 for cooling the core provided inside the core 3 by 
utilizing the inner surface of the second steel pipe 7 and the third steel 
pipe 10 . More specifically, a gap 11 is provided between the lower end of 
the third steel pipe 10 and the stool 1. The gas 12 for cooling the core 
is introduced from above the third steel pipe 10, descends through the 
third steel pipe 10, passes through the gap 11 between the third steel 
pipe 10 and the stool 1, ascends through a space formed between the second 
and third steel pipes 6 and 7, and discharged upward, thereby generally 
forming the gas flow course 4 for cooling the interior of the second steel 
pipe 6. 
The followings are the problems to be encountered in producing the hollow 
steel ingots by use of the casting apparatus with the above arrangement 
according to the present invention. 
(a) The thickness of the first steel pipe 
(b) Construction of the second steel pipe 
(c) Type and structure of the refractory material of the core 
(d) Means for cooling the core 
(e) The relationship between the thickness of the refractory material of 
the core and the finally solidifying position of molten steel 
Description will hereunder be given of the above-mentioned problems. 
(a) The Thickness of the First Steel Pipe 
Since the first steel pipe 6 used as the core 3 comes into direct contact 
with molten steel fed into the mold 2, it needs to have resistance to 
melting loss. Hence, it is preferable to use a low-carbon steel pipe 
having a higher melting point than that of molten steel 9 to be cast. 
Further, the first steel pipe 6 needs to be removed from the surface of 
the hollow steel ingot produced as the scales by heating for 5 to 10 hours 
during forging, and the resistance to melting loss is required as 
described above, and hence, the thickness of the first steel pipe 6 may be 
5 to 20 mm, preferably 8 to 10 mm. 
(b) Construction of the Second Steel Pipe 
It is desirable that the thickness of the second steel pipe 7 is as thin as 
possible from the viewpoint of cooling effect by use of gas cooling. On 
the other hand, it needs to have mechanical strength sufficient to bear 
the static pressure of molten steel 9 and prevent the collapse of the 
core. From this reason, particularly, with the casting apparatus for 
producing large hollow steel ingots, it is preferable to provide a 
plurality of reinforcing ribs 15 between the first steel pipe 6 and the 
second steel pipe 7 in the radial direction as shown in FIG. 3. The number 
of the reinforcing ribs depends on the thickness of the second steel pipe 
7, and at least 4 to 6 reinforcing ribs are preferable as shown in FIG. 3. 
Said reinforcing ribs 15 are required to merely support the second steel 
pipe 7 and the third steel pipe 10 in the radial direction, and therefore, 
the reinforcing ribs need not to be fixed by welding and the like. 
The results of experiments conducted by the present inventors have been 
shown that the upper limit of the elevated temperature of the second steel 
pipe when the hollow steel ingot was produced is 780.degree. C. Therefore, 
assuming that the maximum temperature is 800.degree. C., the required 
thickness of the second steel pipe 7 for bearing the static pressure of 
molten steel has been calculated. As the result, it has been found that, 
in order to obtain the thickness of the second steel pipe, the following 
formulae (1) and (2) may be satisfied for buckling breaking and the 
following formula (3) may be satisfied for compressive breaking. 
EQU t.gtoreq.0.030[H/(n.sup.2 -1)].sup.1/3 .multidot.R (1) 
wherein n.gtoreq.2, equation (1) is satisfied for buckling 
EQU t.gtoreq.0.020H.sup.1/3 .multidot.R (2) 
wherein n.gtoreq.1, equation (2) is satisfied for buckling 
EQU t.gtoreq.0.0047HR (3) 
for compressive braking equation (3) is satisfied wherein, 
t: the thickness (cm) of the second steel pipe, 
H: the height (m) of the hollow steel ingot, 
n: the number of the reinforcing ribs 
R: the inner radius (cm) of the second steel pipe 
(c) Type and Structure of the Refractory Material of the Core 
The refractory material 8 filled up in the space formed between the first 
steel pipe 6 and the second steel pipe 7 needs to be satisfactorily high 
refractoriness and not to cause seizure so that shrinkage of the hollow 
steel ingot due to solidification can be absorbed and the removing of the 
second steel pipe 7 can be facilitated after the completion of 
solidification. 
When seizure is caused, it is difficult to separate the hollow steel ingot 
from the core and the hollow steel ingot cannot be subjected to forging. 
In this respect, granular refractory materials having refractoriness of 
1100.degree. C. and more such as zircon sand, silica sand and chromite 
sand are combined by an organic binder such as furan resin and urethane 
resin are useful. Particularly useful is the refractory material in which 
zircon sand and chromite sand are combined by a furan resin. 
Table 1 shows one embodiment of the chemical composition and grain sizes of 
silica sand, zircon sand and chromite sand, which are usable in the 
present invention. 
TABLE 1 
__________________________________________________________________________ 
Refractory 
Chemical composition (Weight %) 
material Ignition 
Mean grain 
Type SiO.sub.2 
ZrO.sub.2 
Cr.sub.2 O.sub.3 
Al.sub.2 O.sub.3 
Fe.sub.2 O.sub.3 
CaO 
MgO 
TiO.sub.2 
P.sub.2 O.sub.5 
loss fineness number 
__________________________________________________________________________ 
Silica 
sand 98.3 
-- -- 0.90 
0.17 
0.15 
0.09 
-- -- 0.03 108 
Zircon 
sand 33.6 
65.8 
-- 0.3 0.05 
-- -- 0.3 
0.01 
-- 111 
Chromite 
sand 1.6 -- 45.3 
14.7 
25.1 
-- 10.1 
0.6 
-- -- 116 
__________________________________________________________________________ 
The term "grain fineness number" in Table 1 refers to the value digitally 
indicating the particle size distribution of the sand grain groups 
prescribed in JIS Z-2602. 
It is one of the features of the present invention that the granular 
refractory material such as a silica sand, zircon sand or chromite sand is 
used as the refractory material 8 of the core 3 and the organic resin is 
used as the binder as described above. According to the present invention, 
said core does not come into direct contact with molten steel 9, however, 
is heated to a high temperature by the molten steel 9 and completely 
burned up by using the organic binder upon feeding of the molten steel 9. 
Consequently, no seizure takes place with the granular refractory material 
and the knock-out work due to the removing of the second steel pipe 7 
becomes very easy when the hollow steel ingot is drawn. 
(d) Means for Cooling the Core 
As shown in FIGS. 1 and 2, the core 3 according to the present invention is 
cooled from inside. The purposes of cooling the core include restricting 
the finally solidifying position of molten steel to a distance of 20 to 
50% of the wall thickness of the steel ingot from the side of said core 3, 
preventing the second steel pipe 7 from being heated, decreased in 
strength and finally deformed to a considerable extent by the heat of 
casting the steel ingot, and contributing to smooth heat radiation from 
the interior of steel ingot to avoid sintering of the refractory material. 
As the means of cooling, natural convection, spray cooling, gas cooling and 
the like are conceivable, and it is preferable to adopt such type of 
cooling that in which the coefficient of heat-transfer are selected over a 
wide range and industrially easily workable means such as air or nitrogen 
gas stream is used. If the flow rate of the gas stream is set within a 
range from 0.5 to 5 m/sec, preferably 0.8 to 2 m/sec, then the temperature 
of the second steel pipe 7 can be kept at about 780.degree. C. maximum, 
thereby enabling to avoid the breaking thereof. 
Furthermore, as the result of calculating the heat transfer, it has been 
found that, in order to prevent the temperature of the cooling gas 12 in 
the flow course 4 from being raised so as to ensure the cooling effect, 
the cross-sectional area S of the flow course 4 should satisfy the 
following formula (4). 
EQU S&gt;5.9 HR (4) 
wherein, 
S: the cross-sectional area (cm.sup.2) of the cooling PG,14 gas flow 
course 
H: the height (m) of the hollow steel ingot, 
R: the inner radius (cm) of the second steel pipe 
(e) The Relationship Betweeen the Thickness of the Core Refractory Material 
and the Finally Solidifying Position of Molten Steel 
The thickness of the granular refractory material 8 filled up in the space 
formed between the first steel pipe 6 and the second steel pipe 7 is 
determined as follows depending upon the three conditions including the 
quality of the refractory material, the method of cooling the second steel 
pipe 7 and the finally solidifying position of the steel ingot. 
FIG. 4 is a partial cross-sectional view showing the relationship between 
the mold 2, the core refractory material 8 and the finally solidifying 
position of molten steel 9 during production of the hollow steel ingot. 
Now, assume that the thickness of molten metal 9 is T, the thickness of 
the refractory material 8 is D, the inner radius of the hollow steel ingot 
is R, the finally solidifying position of molten steel is a distance d 
apart from the core and d=xT, then said finally solidifying position will 
be (1=x)T apart from the inner wall of the mold 2. 
The relationship between the thickness D of the refractory material and the 
finally solidifying position of molten steel can be obtained as follows by 
the calculation of heat-transfer in the case of the solidification wherein 
the temperature of molten steel is 1500.degree. C. and that of the cooling 
gas is 20.degree. C. 
##EQU1## 
wherein a=T/R, .alpha. is the coefficient of heat-transfer and k is the 
thermal conductivity. The values of .alpha. and k are substantially as 
follows: 
.alpha.: 100 to 1500 Kcal/m.sup.2 .multidot.h.multidot..degree.C. in the 
case of gas cooling 
k: 0.30 Kcal/m.multidot.h.multidot..degree.C. for chromite sand, 0.26 for 
zircon sand and 0.20 for silica sand. 
The present inventors produced the hollow steel ingots having the finally 
solidifying position of x=0.05.about.0.5 at last by varying the cooling 
rate on the core side, using hollow steel ingots having various shapes, 
subjected said hollow steel ingots to forging from inside and outside, and 
thereafter, conducted the flaw detecting dyeing test on said hollow steel 
ingots. FIG. 5 show the results of the test. In FIG. 5, the index of 
defect F is the index for indicating the magnitude of defect, and the 
products having F larger than 3 are not usable. As apparent from FIG. 5, 
in the case the finally solidifying position is not spaced apart from the 
core side at a distance of 20% or more of the thickness T of the steel 
ingot, no matter what shape the hollow steel ingot may have, it is not 
usable. Namely, if the cooling at the inner surface of the core 3 is 
unsatisfactory and the finally solidifying position is not spaced apart 
from the core side a distance of 0.2 of the wall thickness T of the steel 
ingot, there is a high possibility of that defects may appear when the 
hollow steel ingot product is subjected to inner surface finishing. 
However, the more the finally solidifying position approaches the center 
of the wall thickness T of the hollow steel ingot, i.e. x=0.5, the less 
the forced cooling is needed. 
Consequently, it was found that the finally solidifying position should be 
limited to the formula of 0.2.ltoreq..times..ltoreq.0.5. If this limiting 
requirement is substituted for the formula (5) and the upper and lower 
limits of the coefficient of heat-transfer .alpha. by gas cooling is made 
to be 100.ltoreq..alpha. (Kcal/m.sup.2 
.multidot.h.multidot..degree.C.).ltoreq.1500, then, the upper and lower 
limits of the thickness D of the core refractory material 8 can be 
calculated as follows: 
(a) when the inner radius of the steel ingot is R&lt;0.5 m 
##EQU2## 
(b) 0.5.ltoreq.R&lt;1.0 
##EQU3## 
(c) 1.0.ltoreq.R&lt;2.0 
##EQU4## 
It has been already described, that the thermal conductivity k 
(Kcal/m.multidot.h.multidot..degree.C.) obtainable through the above 
formula varies depending upon the types of the refractory material used, 
and, as far as the types of the refractory material used in the present 
invention are concerned, the thermal conductivities are 0.20 for silica 
sand, 0.26 for zircon sand and 0.30 for chromite sand. 
The present inventors, in the production of the hollow steel ingots various 
in inner diameter R by use of the casting apparatus according to the 
present invention as shown in FIG. 1, used the cores 3 having the 
thickness of the refractory material 8 calculated by the above-mentioned 
formulae (6), (7) and (8), fed molten steel under gas cooling, cut the 
hollow steel ingot after cooling, and determined the finally solidifying 
position by etching for macro-inspection. The results have satisfactorily 
coincided with the result of calculation, thereby enabling to prove the 
reasonableness of the calculating method adopted by the present inventors. 
As described above, if the thickness (D) of the refractory material is 
selected, then it is possible to set the finally solidifying position of 
the steel ingot which is apart from the core side at a distance of 20 to 
50% of the wall thickness (T) of the steel ingot. In order to set the 
finally solidifying position deeply, it is necessary to make the wall 
thickness (T) of the steel ingot as thin as possible. However, as viewed 
from the workability in filling the refractory material between the first 
steel pipe 6 and second steel pipe 7 and the fact that, when the layer of 
refractory material is too thin, the refractory material is sintered with 
the result that the core becomes difficult to be removed after the steel 
ingot has been solidified and also the shrinkages cannot be absorbed to 
thereby cause cracking, the thickness of 20 mm or more is practically 
necessary. 
From the results of tests in practical casting and the conclusion derived 
from the formulae (6), (7) and (8), 100 mm is given as the maximum 
thickness of the refractory material. Consequently, the range from 20 to 
100 mm is preferable for the thickness of the core refractory material of 
the ordinary hollow steel ingots. 
EXAMPLE 1 
Using a Cr-Mo steel containing in weight 0.08% C, 0.06% Si, 0.38% Mn, 2.05% 
Cr, 0.96% Mo, 0.011% P and 0.004% S, a 20 ton hollow steel ingot was 
produced according to the present invention. 
The principal dimensions of the casting apparatus are as follows: 
______________________________________ 
Outer diameter of the first steel pipe 6 of the core 
500mm 
Outer diameter of the second steel pipe 7 of the core 
440mm 
Thickness (t) of the second steel pipe 7 of the core 
10mm 
Height (H) of the hollow steel ingot 
1530mm 
Height of the mold 1800mm 
Inner diameter of the lower end of the mold 
1417mm 
Inner diameter of the upper end of the mold 
1600mm 
Number (n) of the reinforcing ribs 
1 
Thickness (D) of the core refractory material 
20mm 
______________________________________ 
Using the casting apparatus of the above-mentioned specification according 
to the present invention and the core refractory material in which 
chromite sand was bound by furan resin, molten steel was fed from bottom 
with air for cooling the core being supplied at the flow rate of 4 m/sec. 
The temperature of molten steel at the time of casting was 1595.degree. C. 
and the required casting time was 9 min. 
The 20 ton hollow steel ingot thus obtained according to the present 
invention was cut along the plane passing through the diameter of said 
steel ingot and studies were made on the state of segregation of C, the 
finally solidifying position of the molten steel and the structure of the 
steel adjacent the finally solidifying position. 
As for the state of segregation of C at the cross-section of said hollow 
steel ingot, as shown in FIG. 6, the maximum segregation rate was about 
20% even immediately beneath the feeder head, and this value is lower than 
the maximum segregation rate of the solid steel ingot having the weight 
equal thereto, thereby ensuring the excellent interior quality. 
Furthermore, from FIG. 6 showing the state of segregation of C, it has been 
proven that the finally solidifying position of molten steel of said 
hollow steel ingot is set substantially at the center plane of the steel 
ingot equalling to x.apprxeq.0.5 in FIG. 4, i.e. at a position of 
substantially 50% of the wall thickness T of the steel ingot from the core 
side. 
Additionally, porous parts never appear in said hollow steel ingot which 
are liable to appear in the finally solidifying positions of ordinary 
solid steel ingots due to high rate of solidification and high 
concentration of the dissolved substances, thereby enabling to obtain a 
highly sound hollow steel ingot. 
EXAMPLE 2 
Using a Cr-Mo steel containing in weight 0.12% C, 0.07% Si, 0.45% Mn, 5.02% 
Cr, 0.59% Mo, 0.011% P and 0.005% S, a 45 ton hollow steel ingot was 
produced according to the present invention. 
The principal dimensions of the casting apparatus in this case are as 
follows: 
______________________________________ 
Outer diameter of the first steel pipe 6 of the core 
700mm 
Outer diameter of the second steel pipe 7 of the core 
600mm 
Thickness (t) of the second steel pipe 7 of the core 
10mm 
Height (H) of the hollow steel ingot 
2320mm 
Height of the mold 2718mm 
Inner diameter of the lower end of the mold 
1706mm 
Inner diameter of the upper end of the mold 
1995mm 
Number (n) of the reinforcing ribs 
4 
Thickness (D) of the core refractory material 
40mm 
______________________________________ 
Using the casting apparatus of the above-mentioned specification according 
to the present invention and the core refractory material in which 
chromite sand was bound by urethane resin, molten steel was fed according 
to the present invention with nitrogen gas for cooling the core being 
supplied at the flow rate of 2 m/sec. The temperature of molten steel at 
the time of casting was 1595.degree. C. and the required casting time was 
14.5 min. 
The 45 ton hollow steel ingot thus obtained according to the present 
invention, in the same manner as in the Example 1, was cut along the plane 
passing through the diameter of said steel ingot and studies were made on 
the state of segregation of C, the finally solidifying position of the 
molten steel and the structure of the steel adjacent the finally 
solidifying position. 
As for the state of segregation of C at the cross-section of said hollow 
steel ingot, as shown in FIG. 7, the maximum segregation ratio was about 
20% even immediately beneath the feeder head, thereby ensuring excellent 
interior quality. The maximum ratio is defined as a ratio percentage of a 
difference between the maximum content of an element and the ladle 
analysis of the element to the ladle analysis content of the element. 
Furthermore, from FIG. 7 showing the state of segregation of C, it has been 
proven that the finally solidifying position of molten steel of said 
hollow steel ingot is set at x.apprxeq.0.45 in FIG. 4, i.e. at a position 
of substantially 45% of the wall thickness T of the steel ingot from the 
core side. 
Additionally, little porous parts appear in the vicinity of the finally 
solidifying position, thereby enabling to obtain a highly sound hollow 
steel ingot. 
EXAMPLE 3 
Using a low alloy steel containing in weight 0.2% C, 0.35% Si, 1.40% Mn, 
0.75% Ni, 0.11% Cr, 0.54% Mo, 0.008% P and 0.002% S, a 200 ton hollow 
steel ingot was produced according to the present invention. 
The principal dimensions of the casting apparatus in this case are as 
follows: 
______________________________________ 
Outer diameter of the first steel pipe 6 of the core 
1000mm 
Outer diameter of the second steel pipe 7 of the core 
800mm 
Thickness (t) of the second steel pipe 7 of the core 
20mm 
Height (H) of the hollow steel ingot 
2942mm 
Inner diameter of the lower end of the mold 
3100mm 
Inner diameter of the upper end of the mold 
3500mm 
Number (n) of the reinforcing ribs 
8 
Thickness (D) of the core refractory material 
80mm 
______________________________________ 
Using the casting apparatus of the above mentioned specification according 
to the present invention and the core refractory material in which silica 
sand is bound by furan resin, molten steel was fed according to the 
present invention with nitrogen gas for cooling the core being supplied at 
the flow rate of 0.8 m/sec. The temperature of molten steel at the time of 
casting was 1595.degree. C. and the required casting time was 35 min. 
The 200 ton hollow steel ingot thus obtained according to the present 
invention, in the same manner as in the Example 1, was cut and studies 
were made on the state of segregation of C, the finally solidifying 
position of the molten steel and the structure of the steel adjacent the 
finally solidifying position. 
As for the state of segregation of C at the cross-section of said hollow 
steel ingot, the maximum segregation ratio was about 30% immediately 
beneath the feeder head. And, although there was seen such a tendency 
similar to the conventional solid steel ingots that the maximum 
segregation rate increases with the increase in the weight of steel ingot, 
said 200 ton hollow steel ingot has by far less maximum segregation rate 
than 40 to 42% of the conventional solid steel ingots having the weight 
equalling thereto, thereby enabling to obtain a hollow steel ingot having 
excellent interior equality. 
Furthermore, it has been proven that the finally solidifying position of 
molten steel is set at x.apprxeq.0.35, i.e. at a position of substantially 
35% of the wall thickness T of the steel ingot from the core side. 
Additionally, sparcely distributed shrinkage cavities each having a 
diameter of 2 to 3 mm appear in the vicinity of the finally solidifying 
position, and accordingly, the porous part were less significant than the 
conventional solid steel ingots having the weight equalling thereto, 
thereby enabling to obtain a highly sound 200 ton hollow steel ingot. 
As is apparent from the above-described examples, the following advantages 
have been attained by use of the method of producing a hollow steel ingot 
and an apparatus therefor according to the present invention. 
(a) The finally solidifying position of the hollow steel ingots produced 
according to the present invention is set at a position 0.2 to 0.5 of the 
wall thickness T of the steel ingot apart from the side surface of the 
core, thus enabling to obtain hollow steel ingots having highly sound 
interior and surface qualities. 
(b) The most proper thickness of the core can be readily calculated 
depending on the shapes and dimensions of the hollow steel ingots to be 
produced, so that the selection of types of the refractory material and 
the arrangement for producing the core may be very easily made. 
(c) The cores can be easily removed after the steel ingot has been 
solidified, the inner surface of the steel ingot is clean because the 
inner surfaces of the core are covered by the steel pipes, and the 
remaining steel pipes can be readily removed as scales by heating before 
the hollow steel ingot is forged. 
(d) According to the present invention, air or other inert gases are used 
for cooling the inner surface of the core, and hence, the work of feeding 
molten steel is performed very safely as compared with other methods of 
cooling and the cooling effect is comparatively high as well. 
In addition, it has been found that, in comparison between the hollow steel 
ingot produced according to the present invention and the conventional 
solid steel ingot in the costs of material, of heating and of forging all 
of which are required from the materials for producing the forgings having 
the shapes and weights identical with each other, use of the hollow steel 
ingot according to the present invention is by far more advantageous in 
every respect as shown in Table 2. 
TABLE 2 
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Solid steel ingot 
Hollow steel ingot 
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Material cost rate 
100 85 
Heating cost rate 
100 50 
Forging cost rate 
100 70 
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Further, the result of comparison is shown in Table 3 which has been made 
between said hollow steel ingot and said solid steel ingot in the weight 
of steel ingot required for obtaining the forgings equalling in weight to 
each other and also in the time required for solidification. 
TABLE 3 
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Required weight Solidifying time 
Forging of ingot steel (t) 
of steel ingot (h) 
Solid Hollow Solid Hollow 
Weight (t) 
steel ingot 
steel ingot 
steel ingot 
steel ingot 
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15 25 20 7 3 
35 60 45 10 3.5 
70 120 90 14 8 
110 175 140 23 10 
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As apparent from Tables 2 and 3, the advantage of using a hollow steel 
ingot for producing a hollow forging is profound, particularly in 
producing a large forging. 
It should be understood that the embodiments or examples described in the 
item of the detailed description of the present invention are intended to 
illustrate the technical contents of the present invention only and not 
limit the invention to the specific forms to be confined in a narrow 
sense. All such variations and modifications as are in accord with the 
principles described are meant to fall within the scope of the appended 
claims without departing from the spirit of the invention.