GB2026914A

GB2026914A - Castine blooms

Info

Publication number: GB2026914A
Application number: GB7926439A
Authority: GB
Original assignee: Creusot Loire SA
Current assignee: Creusot Loire SA
Priority date: 1978-08-04
Filing date: 1979-07-30
Publication date: 1980-02-13
Also published as: ES482396A1; IT1119040B; GB2026914B; IT7968472A0; DE2931438A1; DE2931438C2

Abstract

A bloom 6 is made by bottom casting in a mould 4 and during pouring and solidification of the bloom the upper face of the bloom is covered with an exothermic or insulating medium which slows down or prevents the cooling of the upper face so that cooling and solidification of the bloom proceeds from the bottom upwards. The ratio <IMAGE> is equal to at most 1.3 and for a bloom for producing a flat forging the ratio H DIVIDED D is preferably at most 0.8 while for a bloom for producing a hollow forging the ratio H DIVIDED D is preferably at most 1.2. The bloom may be compressed, drilled to remove segregation from the centre, and forged to produce a hollow article. <IMAGE>

Description

SPECIFICATION Improvements in and relating to blooms The present invention relates to a process for the manufacture of a bloom and also to a bloom obtained by this process, such blooms being particularly but not exclusively for use in the manufacture of flat or hollow forgings.

Hitherto, all forgings, for example discs, flanges, tubes, ferrules, shafts, rotors and flat forgings, have generally been produced from a conventional bloom having a well-known morphology, namely a body in which the ratio H (Height of bloom D Diameter is greater than 1.3 and can be 2, a large head, constituting 15% to 30% of the total weight of bloom, and, in general, a foot.

This morphology of the conventional bloom is not aways well adapted to the morphology of the final forging to be obtained, and, although it is very suitable for forgings obtained by drawing, such as, for example, shafts and rotors, it has proved not to be suitable for the manufacture of a certain number of other forgings and more particularly flat forgings and hollowforgings.

The present invention relates to a new process for the manufacture of a bloom, which process makes it possible to obtain a bloom having a morphology which is suitable for the forging to be obtained, in particular forflatforgings and hollowforgings.

Furthermore, blooms obtained by this process can have better characteristics (satisfactory internal state in respect of ultrasound, purity in terms of inclusions, degree of segregation of carbon and of other elements, and the like) than those of the conventional bloom, and they additionally make it possible to achieve a significant saving in terms of metal and conversion costs.

According to the invention there is provided a process for the manufacture of a bloom, comprising bottom casting the liquid material in a mould without a refractory lining and without a head or foot, wherein the ratio H Height of the bloom D Mean diameter is equal to at most 1.3, and, during the bottom casting and solidification of the bloom, the upper face of the bloom is covered with an exothermic or insulating medium which slows down or prevents the cooling of the upper face.

The mould used can be a conventional mould which is not particular to this type of process. It can be used both in bottom casting and in top casting for conventional blooms and advantageously has neither an internal nor an external refractory lining nor analogous insulating means for preventing heat losses through the walls.

Advantageously, no heating means, such as means using the Joule effect, an electric arc, induction heating means, a burner or any other external source of heat, is used at the upper part of the bloom.

By virtue of this process, there is therefore obtained simultaneous cooling through the bottom and through the side walls of the bloom. These phenomena are taken into consideration in defining the initial bloom, whilst also taking account of the final morphology of the forging to be obtained, namely a hollow forging or a flat forging.

Advantageously in the manufacture of a bloom intended for producing a flat forging, the ratio H (Height of bloom D Mean diameter is equal to at most 0.8. In this case, it may be preferable to accelerate the cooling of the lower face of the bloom after casting. H Advantageously in the manufacture of a bloom intended for producing a hollow forging, the ratio D iS preferably equal to at most 1.2. Furthermore, for this type of bloom, the latter can advantageously be withdrawn from the mould before it has completely solidified, in order to shorten the solidification time as much as possible.

According to another aspect of the invention there is provided a bloom, for example for use in producing a flat forging or a hollow forging, which is manufactured by the process described above. The bloom may be round or polygonal in section, with or without undulation, cylindrical or in the shape of a truncated cone.

According to a further aspect of the invention there is provided a process of manufacture of a flat forging from a bloom as described above, wherein the cast and solidified bloom is subjected to a compression operation by means of forging.

According to a further aspect of the invention there is provided a process of manufacture of a hollow forging from a bloom as described above, wherein the cast and solidified bloom is successively subjected to a possible compression operation and a drilling operation before it is converted into the desired final shape by subjecting it to a forging operation.

To properly appreciate the advance brought about by the present invention, it is necessary to compare the morphology, after solidification, of a bloom manufactured according to the present invention with the morphology of a bloom of known type, which has been produced by top casting with a foot and a head, and in which the ratio is greater than 1.3 for the part forming the body of the bloom.

It must be recalled, in this context, that conventional blooms are polygonal and are produced by top H casting and not by bottom casting, and that they possess three parts, namely a body, in which the ratio D iS generally greater than 1.3 and can reach 2 in certain cases, a large head, the cropped weight of which represents 15 to 30% of that of the body, and, most frequently, a more or less conical foot, the maximum diameter of which is less than the diameter of the bloom, and the cropped weight of which can reach 10 to 15% of the weight of the body.

In a conventional bloom of this type, the head and the foot are cropped after blooming, this representing a relatively large loss of metal.

Nevertheless, such a bloom is very suitable for finally producing forgings by drawing, such as shafts and rotors. On the other hand, it is not very suitable for the manufacture of flat forgings or hollow forgings.

In the conventional bloom, centre line shrinkage at the top surface is avoided because the hot liquid metal of the head permanently fills the central hollow which would otherwise tend to form because of the shrinkage due to the gradual solidification of the metal.

However, avoidance of centre line shrinkages in this way is not without disadvantage. In fact, this permanent supply of liquid metal originating from the head slows down the solidification of the top of the bloom. Moreover, this supply of liquid metal is richer in elements other than iron (carbon, phosphorus, sulphur and the like) than the metal which has already solidified. As a result, the top of the body of the conventional bloom has a segregation which starts fairly near the wall. The top of the bloom thus possesses a fairly wide segregation pocket. More precisely, considering only the carbon content, the curves of equal carbon content deviate more from the axis of the bloom in the upper third of the body of the bloom than in the lower two thirds. The absence of centre line shrinkage is thus at the expense of an increased segregation at the top.It must be pointed out that all the means which tend to lengthen the solidification time, by insulating or lagging the bloom during its solidification, for example on the side walls, increase the degree of segregation.

The invention will be more fully understood from the following description of embodiments thereof, given by way of example only, with reference to the accompanying drawings.

In the drawings: Figure 1 schematically represents the bottom casting of a bloom according to the invention; Figure 2 shows diagrammatically the steps in the production of a flat forging from a conventional bloom and from a bloom according to the invention; Figure 3 is a vertical half-section through a conventional bloom; Figure 4 is a vertical half-section through a bloom according to the present invention and for use in producing the same hollow forging as the bloom of Figure 3; Figure shows diagrammatically the steps in the forging of a ferrule from the conventional bloom of Figure 3 and from the bloom according to the invention of Figure 4;; Figure 6 is a vertical half-section through a blank hollow forging produced from the conventional bloom of Figure 3, showing the 10% and 20% curves of equal degree of segregation of carbon, the bloom not having been subjected to prior compression; and Figure 7 is a vertical half-section through a blank hollow forging produced from the bloom of Figure 4, showing the 10% and 20% curves of equal degree of segregation of carbon, the bloom not having been subjected to prior compression.

Two embodiments of the process according to the invention will now be described by way of non-limiting examples, the first embodiment relating to the production of a bloom intended for manufacturing a flat forging and the second embodiment relating to the production of a bloom intended for manufacturing a hollow forging.

Figure 1 shows a bottom casting installation comprising a liquid steel feed, comprising a funnel 1 and a vertical pipe 2, and a bottom casting plate 3 of high thickness, for example of the order of 50 cm. A channel 4 is made in the bottom casting plate 3. The walls of the channel 4 are advantageously made of a refractory material, the channel 4, as seen in the drawing, opens out at the base of a mould 5 which has the shape of a truncated cone. After having optionally subjected the liquid steel to a degassing operation, or any other suitable treatment carried out in the ladle, it is possible to cast a bloom 6, which, in the embodiment shown, is intended for the manufacture of a flat forging and therefore possesses a small mean ratio H/D (height/mean diameter), which is of the order of 0.6 in this embodiment. The upper face of the bloom 6 has been covered with a layer 7 of an exothermic or insulating powder of the "vermiculite" type.

After the mould 5 has been filled, the lower face of the bloom 6 is subjected to accelerated cooling because it is in contact with the large mass of metal constituting the bottom casting plate 3 and because it has large relative dimensions, and cooling of the upper face of the bloom 6 is virtually elminated because of the layer 7 of exothermic product, but no supply of heat of an electrical or other nature is required.

Consequently, the solidification front of the bloom propagates predominantly verticaliy, from bottom to top, conventional cooling through the side walls, with horizontal propagation, being relatively slight because of the small height of the bloom, despite the total absence of insulation on the side walls.

A bloom 6 manufactured in this way has considerable advantages: - it does not exhibit centre line shrinkage, that is to say that the upper face of the bloom possesses a very slight concavity. As a result, the forging can be placed directly under the press after heating, without complicated handling, which makes it possible to save time.

- the uniform solidification associated with this process imparts to the bloom an excellent internal state in respect of ultrasound.

- by virtue of the bottom casting, of the small height of the bloom and of the ascending vertical advance of the solidification front, the inclusions gradually rise to the upper surface of the bloom and can then easily be removed therefrom; the purity, in terms of inclusions, of the bloom thus obtained is therefore remarkable.

- in particular by virtue of the absence of a head, the solidification time is shortened and this causes a reduction in the degree of segregation and hence a better homogeneity of the mechanical characteristics of the resulting bloom. Furthermore, as in the case of the impurities, the maximum segregations are thrown up to the surface of the bloom and can therefore easily be removed.

- the weight of the bloom is very similar to that of the body of a conventional bloom required for the manufacture of an identical forging, and this leads to a saving, in terms of metal, which can range up to 35%.

A bloom manufactured by the above process according to the invention can thus have a morphology which is well suited to the final forging which it is desired to obtain. As a result, the conversion operations by means of forging are considerably simplified, compared with the conventional series of manufacturing steps, and this manifests itself in appreciable savings.

Figure 2 schematically represents, on the left-hand side, the main steps in the manufacture, by forging, of a flat disc from a conventional bloom, and, on the right-hand side, the main steps in the manufacture of the same flat disc from a bloom as described above according to the invention.

As shown in Figure 2, when starting from the conventional bloom, the following operations are required: A:blooniing (light forging intended to make the bloom cylindrical), B:cropping the head and foot, C:placing in the upright position, D:pre-compression, E:compression.

In contrast, when starting from a bloom as described above according to the invention, only operation E (compression) is required, operations A, B, C and D being omitted.

A second embodiment of the process according to the invention, which relates to the production of a bloom intended for manufacturing a hollow forging, will now be described, still by way of a non-limiting example. In order to gain a good understanding of the execution and the advantages of this second example, it is important to compare it with the casting of a conventional bloom.

A conventional bloom, shown in Figure 3, has a body 8, a head 9 and a foot 10 with the following characteristics.

Total weight: 30 tons.

Weightofthe body 8: 24 tons.

Height (H) of the body 8: 2.2 metres = 2,200 millimetres.

Mean diameter (D) of the body 8: 1,315 millimetres.

Hd= 1.673.

Weightofthe head 9: 5 tons.

Weight ofthe foot 10: 1 ton.

This bloom was produced by top casting. The continuous lines represent the profile of the bloom in the liquid state, and the dashes 11 and 11' respectively represent, after shrinkage due to solidification, the side profile of the body of the bloom and the upper profile of the head 9. The profile S represents the contour of the transverse section of the bloom.

A bloom manufactured according to the present invention, and hence according to Figure 1, but with a greater ratio H' represented is in ure 4. It greater ratio HD ,is represented in Figure 4. is possesses the following characteristics: Total weight: 21.5 tons.

Height (H'): 1.525 metres = 1,525 millimetres.

Diameter (D'): 1,500 millimetres.

D,1.016.

This bloom was produced according to the invention by bottom casting. As soon as casting into the mould has started, the liquid metal is covered with an insulating "cushion"; then, as soon as casting has stopped, its upper face, which is still liquid, is covered with a layer of an exothermic or insulating product. The dashes 12 represent the side profile of the bloom and the dashes 12' represent its upper face, with a centre line shrinkage 12", after shrinkage due to the solidification of the metal. It is noted that the primary centre line shrinkage is very large, which is not a disadvantage; indeed, in the intended application, it is an advantage, as will be seen below.

The transverse profile of this bloom, which is not shown, is similar to the profile S of the bloom of Figure 3, but with a slightly larger diameter.

Figure 5 schematically represents, on the left-hand side, the steps in the manufacture, by forging, of a ferrule from the conventional bloom of Figure 3, and, on the right-hand side, the steps in the manufacture of the same ferrule from the bloom of Figure 4 according to the invention.

As shown, when starting from the conventional bloom of Figure 3, the following operations are required: a: blooming, b:cropping the head and foot, c: placing in the upright position, d:compression, e: drilling, f: drawing on a mandrel, g:shaping on a beak-iron.

In contrast, when starting from the bloom of Figure 4 according to the invention, operations a, b, c and possibly d are omitted.

The advantages of the method of manufacture according to the invention, starting from the bloom of Figure 4, are as follows: 1. No head orfoot.

2. No operation involving blooming, cropping the head and foot and placing the bloom in the upright position.

3. Much better morphology of the blank.

In order to gain a good understanding of this latter advantage, reference must be made to Figures 6 and 7.

The two Figures together represent two blanks 13 and 14 after simulation of a 400 millimetre drilling along the axis, the blank 13 of Figure 6 having been produced from the bloom of Figure 3 and the blank 14 of Figure 7 having been produced from the bottom of Figure 4 according to the invention, both blanks not having been subjected to compression.

Figures 6 and 7 are vertical half-sections of the two blanks and both include a plot of the 10% and 20% curves of equal degree of segregation of carbon. It is known that the segregations of other elements, such as phosphorus or sulphur, follow those of carbon fairly closely.

Figure 6 clearly shows that, in the blank 13, the segregation pocket of carbon (at a degree of segregation of 10% and even 20%) extends well beyond the 400 millimetre drilling. The curve 15 represents the degree of segregation of 10% and the curve 16 represents the degree of segregation of 20%. This segregation pocket thus involves a large part of the blank 13.

In contrast, Figure 7 shows that, in the blank 14, the centre line shrinkage 19 and the major part of the segregated zone, delimited by the curves 17 (for the degree of segregation of 10%) and 18 (for 20%), are removed by the drilling 20 of 400 millimetres diameter.

It is of value to note that, under the action of compression, by forging before drilling the bloom, the flow of segregated material into the zone delimited by the curve 17 will be displaced and will partially fill the hollow left by the centre line shrinkage 19. Consequently, the major part of the segregations will be removed during drilling.

Thus, it is seen that one of the main advantages of the process according to the invention, both in the case of blooms for flat forgings and in the case of blooms for hollow forgings, is that the morphology of the resulting bloom is well adapted to the morphology of the blank which is suitable for the forging to be obtained.

In fact, in a bloom manufactured according to the invention, the absence of a head, in conjunction with the use of an exothermic powder over the entire upper face of the bloom during its cooling, has the result that: (a) the segregation is localised in the central zone around the axis of the bloom.

(b) the central hollow of the centre line shrinkage does not exist for blooms having a ratio H which is equal to at most 0.8, and it develops increasingly in depth if the ratio HD increases from 0.8 to 1.3, but always remains narrow and located along the axis of the bloom. Now, this narrqw hollow, and also the large central segregation which accompanies it, are not troublesome since this type of bloom, with a ratioof between 0.8 and 1.3, is intended for producing hollow forgings, and since substantially all the hollow and the segregation will be removed in the drill core which will have to be removed at a certain stage in the manufacture of the hollow forging.

Furthermore, in a bloom manufactured according to the present invention, the absence of a foot, together with a ratiowhich is always moderate (because it is equal to at most 1.3), has the result that the vertical solidification, starting from the base, is relatively substantial, which, combined with the horizontal solidification, accelerates the resulting rate of solidification and thus shortens the solidification time. Now, it is known that a reduction in the solidification time automatically results in a reduction in the degree of segregation.

Withdrawal of the bloom from the mould may advantageously be carried out before solidification is complete, and this accelerates both the solidification which advances from the walls and the solidification which advances from the bottom of the bloom, the upper face always remaining under the thermal protection of an exothermic or insulating material. The segregation is thus concentrated even more around the axis and at the top of the bloom, and the degree of segregation is reduced by the reduction in the solidification time.

Finally, since it is produced by bottom casting, a bloom according to the invention has all the advantages inherent in this method of casting, namely: (a) the absence of macro-inclusions in the foot.

(b) the skin of the bloom is less subject to cracking.

(c) the simplicity of the procedure, compared with the conventional method of producing a bloom, by top casting, with a head, joins and a dummy base.

The process according to the invention thus exhibits both an economic advantage (through the absence of a head and a foot on the bloom and through a simpler conversion of the bloom) and an advantage in terms of the quality of the resulting hollow forging.

Of course, it is possible to envisage variants and improvements in detail, and also to envisage the use of equivalent means, without going outside the scope of the invention.

Claims

1. A process for the manufacture of a bloom, comprising bottom casting the liquid material in a mould without a refractory lining and without a head or foot, wherein the ratio H Height of the bloom D Mean diameter is equal to at most 1.3, and, during the bottom casting and solidification of the bloom, the upper face of the bloom is covered with an exothermic or insulating medium which slows down or prevents the cooling of the upper face.

2. A process for the manufacture of a bloom intended for the production of a flat forging, according to claim 1, wherein the ratio H Height of the bloom D Mean diameter is equal to at most 0.8.

3. A process for the manufacture of a bloom according to claim 2, wherein cooling of the lower face of the bloom, after casting, is accelerated.

4. A process for the manufacture of a bloom intended for the production of a hollow forging, according to claim 1, wherein the ratio H (Height of the bloom D Mean diameter is equal to at most 1.2.

5. A process for the manufacture of a bloom according to claim 4, wherein cooling of the bloom is accelerated, to shorten the total solidification time, by withdrawing the bloom from the mould before it has completely solidified.

6. A bloom manufactured by a process according to any one of claims 1 to 5.

7. A process for the manufacture of a flat forging from a bloom manufactured by a process according to any one of claims 1, 2 and 3, wherein the cast and solidified bloom is subjected to a compression operation by means of forging.

8. A process for the manufacture of a hollow forging from a bloom manufactured by a process according to any one of claims 1,4 and 5, wherein the cast and solidified bloom is successively subjected to a possible compression operation and to a drilling operation before being subjected to a forging operation to convert it into the desired final shape.

9. A process for the manufacture of a bloom according to claim 1, substantially as herein described with reference to the accompanying drawings.

10. A process for the manufacture of a forging according to either claim 7 or claim 8, substantially as herein described with reference to Figures 1 and 2 or Figures 1 and 3 to 5 of the accompanying drawings.

11. A bloom according to claim 6 substantially as herein described with reference to the accompanying drawings.