GB2092039A - A method of casting steel, particularly steel ingots - Google Patents

Abstract

The method involves pouring a quantity of liquid steel into a metallic ingot mould which is enveloped in a jacket (3, 6, 11) made of a thermally insulating material. After heat exchange and temperature equalisation the ingot, which is still hot (over 1,000 DEG C) due to the insulation provided, is rolled without the need for additional heating. In one embodiment the mould is of such thin walls as to become a part of the finished ingot. <IMAGE>

Description

SPECIFICATION A method of casting steel, particularly steel ingots This invention relates to a method of casting steel, particularly steel ingots for the subsequent production by rolling of blooms, slabs, billets, plate and the like, in which a quantity of molten steel is poured into a metallic mould. The invention also relates to apparatus suitable for carrying out this method.

In a conventional ingot casting process, using cast iron chill moulds, the steel ingot is initially very rapidly chilled and subsequently, after stripping of the mould, it is reheated in soaking or heating furnaces to an even rolling heat in order to enable it to be rolled. This reheating and normalising of the ingots takes place in soaking pits and in ingot-reheating furnaces and demands time, energy and labour.

German Patent Specification OS 28 49 967 describes a method of applying a thermal-equalising process to steel ingots in which after a conventional ingot casting process of the aforementioned kind the chill moulds are stripped as soon as at least half of the steel contained therein has solidified. The steel ingot, which still has a liquid core, is then provided with a heat-insulating envelope so that heat exchange can take place within the ingot itself.

With this known method however energy is also required for reheating the ingots. However, the most serious drawback of this known method resides in that in the course of the internal heat equalisation the already frozen outside walls of the ingot are very strongly heated up by the heat content of the liquid core and tend to bulge outwards due to the high ferrostatic pressure of the liquid ingot core. This causes the ingot to loose its shape and raises considerable problems for the subsequent rolling operation.

The invention aims to avoid the disadvantages appertaining to conventional ingot casting methods.

According to the invention this aim is achieved due to the fact that the ingot mould is enveloped in a jacket of a thermally insulating material and that subsequently, after removal of said jacket, the ingots which have been cast in this manner are rolled at their own residual heat without intermediate reheating.

The insulating jacket provides as far as possible an all-round and even effective thermal protection and screen which greatly deays cooling of the steel ingot. In other words, only a very small fraction of the total heat content in the liquid steel melt is emitted to the exterior enviroment. The quantity of heat contained in the molten steel is thus spread evenly through the steel itself and to the metallic mould. The unit which consists of the mould and the steel which has been poured into said mould assumes a relatively uniform temperature, the steel ingot cools in a substantially more homogeneous manner in a conventional chill-mould ingot casting process. In a highly economical and energy-saving manner the steel ingot may then be directly rolled, either after stripping of the mould or jointly with a thin-walled mould which has become welded thereto.No reheating is required prior to rolling.

The heat content of the liquid steel in the ingot is essentially composed of superheating heat (i.e. heat applied in raising the temperature of the steel above melting point), solidification heat and cooling heat. Only very largely frozen steel ingots can be rolled, for which reason in any case the superheating heat and the major part of the latent heat of solidification must be extracted from the liquid portion of the steel. With thin-walled moulds this takes place due to the fact that these heat quantities are transmitted through the insulating jacket to the enviroment.A very great advantage is achieved, however, if the mass or weight of the mould is selected to provide a thermal capacity which corresponds to the aforementioned heat quantities whereby the steel ingot can be made to approach relatively quickly-but nevertheless whilst thermally insulated- to the highest possible rolling heat temperature. This cuts down the time interval between casting and rolling operations. On the other hand, if a method according to this invention is applied with relatively thin-walled moulds a longer time interval is available between casting and rolling operations which may be profitably used for transporting the ingot in its relatively light-weight jacket to a more distant rolling mill.

The main advantage of the invention resides in that the ingot can be rolled directly from casting heat. Cooling time from pouring temperature (1,550 to 1,600 C) to rolling temperature (1 ,450 to 1,100 C) takes from 5 to 25 hours, or longer, depending on ingot weight. Thus the method of this invention provides adequate time- and temperature reserves to allow the ingots being transported to the rolling mill.

Great advantages are obtained with a mould of which the mass or weight and thus its thermal capacity are so adjusted in relation to the heat quantity contained in the liquid steel which is poured into the mould, particularly in relation to solidification heat, that on receiving the molten steel the mould heats up very quickly, preferably to a temperature above 1,100"C, but below its own liquidus. In contrast with thin-walled moulds such moulds do not weld together with the steel poured into them. Moreover, it is very easy to make such a mould for every individual pouring operation.An ideal combination of re-usable moulds, on the one hand, and optimal utilisation of casting heat for the subsequent rolling process on the other hand is achieved, the vital factor being the realisation that solidification heat and superheating heat are extracted from the liquid steel and absorbed by the mould.

Compared with conventional chill-mould casting (please compare for example "The making, shaping and treating of steel, USS, 1964, page 465) the weight of the mould is substantially less, about only one quarter of that of a conventional chill mould. This means a saving in respect of material and energy.

Furthermore, there is less likelihood of tension cracking in the casting. Considered overall, therefore, the method according to this invention on the one hand allows continued application of normal ingot-casting technology whilst on the other hand it makes working with the mould much earier due to the reduced mould weight and correspondingly less heavy demands on the stripper cranes, and to improved manipulation conditions and improved thermal load application to the mould.

Due to the provision of the heat insulating jacket the moulds achieve a state of relatively homogeneous temperature distribution.

During pouring the temperature at the ladle outlet may be maintained fairly low if relatively large ingots are cast, because the pouring does not take up much time.

The heat-insulating jacket may either be rigidly connected to the mould or it may be a self-supporting jacket, provided with an outer metal casing which surrounds the ingot mould according to the invention, particularly a thinwalled chill mould, laterally and at the bottom thereof. In either case the insulating jacket will be taken off after casting only very shortly prior to the commencement of rolling the ingot in order to maintain thermal insulation and heat exchange within the ingot for as long as possible. If the mould has not become welded to the ingot-which is the case with sufficiently thick-walled moulds- it may be stripped in the usual way. With thin-walled moulds no stripping is necessary. In every respect it is possible to save heat energy and the frozen ingot can be rolled at the highest possible rolling temperature without requiring any further reheating stages.The high temperature of the ingot core makes for improved rolling behavior as compared with reheated ingots which are not so hot in the interior.

When, after pouring, the mould in which the steel has been cast has absorbed the solidification heat of the steel this mould assumes relatively high temperatures. However, this means that the static strength characteristics of the mould no longer apply, and that it is no longer strong enough by itself to take up the ferrostatic pressure of the steel melt, which was poured into the mould and of wn'ic on'y- .he outside has frozen whilst tha interior is still liquid, during the subsequent heat-exchange phase. However this ferrostatic pressure is taken up by the insulating jacket which prevents doming or bulging of the mould.The heat-insulating jacket must therefore be so constructed that it can absorb the ferrostatic pressure- preferably being supported by an outer mould casing, which may be a metal casing, for example. Thus the insulating jacket, besides its thermal insulation function, has the additional function of provide ing external support for the mould.

The layer-thickness of the insulating jacket will be so chosen as to achieve good temperature equalisation in the cast ingot. Furthermore, depending on available plant capacity, it will be thick enough to maintain maximum possible rolling heat for as long as may be required by the local conditions at the works.

For example, if ingot rolling has to be operated on a remote rolling mill the insulating jacket will be relatively thick-walled in order to maintain casting heat during transport of the ingot. This represents a special advantage of the present invention which is particularly valuable for smaller steelworks which do not have rolling capacity for large steel ingots.

The top and bottom regions of the mould will be designed in such a form, in particular dished or prismatic, as to keep losses due to scrap in the subsequent rolling operation as low as possible.

The head pipe which forms in the casting may be sealed off against oxidation by means of a plate placed on top of the molten surface of the ingot and this can be closed up in the subsequent rolling operation.

A method according to this invention and apparatus suitable for its application are hereinafter more particularly described in two examples of execution with reference to the accompanying drawings in which: Figure 1 is a longitudinal section through an ingot mould which is rigidly connected to an insulating jacket, the figure showing also the gripper part of an ingot stripping crane, and Figure 2 is a longitudinal section through a mould with a separate jacket.

Fig. 1 shows a relatively thin-walled mould 1, which is a chill mould having a wallthickness of about 8 cm. The big end is at the bottom and the taper angle (which is greater than 3 ) of preferably 4" is about twice that of conventional, substantially thicker normal chill moulds (about four times as heavy in weight as mould 1). Said mould sits on a base plate 2 having the same wall thickness. However, as in the embodiment shown in Fig. 2, this base plate is not indispensible. The base place 2 shown in Fig. 1 in its turn sits on a substantially larger stool plate 3 which is one part of an insulating jacket and which prevents heat emission towards the bottom.This stool p;ate 3 is supported on a trolley 4 adapted to travel on a track 5 at right angles to the plane of the drawing- the trolley and track being conventional.

The ingot mould 1 (which is preferably coated on its inside with a lubricant before pouring) is rigidly connected to a relatively thick-walled sidewall jacket 6 formed of a thermally insulating material, which, in the lower region thereof has a recess 7 for the stool plate 3 and is further provided with recessed openings 8 which are accessible from the top of the engagement therein of, in each case, one pincer arm 9 of the gripping pincers 10 of a stripper crane (not shown).

The openings 8 are kept clear of material right up to the stripping operation because the reinforced upper collar of the mould 1 provides enough static support to resist ferrostatic pressure. Conveniently, the wall thickness of the mould comprises from 0.2-2% of the largest cross-sectional dimension of the ingot.

Prior to the stripping operation, which is illustrated in Fig. 1, an upper mould cover element 11 has already been removed, see Fig. 1. The stool plate 3, the sidewall jacket 6 and this top cover element 11 together form the complete insulating jacket. The wall thickness of the top cover element 11 is slightly greater than that of the other constitutent parts 3, 6 of the insulation jacket in order to keep heat loss at the top smaller than towards the sides and bottom of the ingot mould.

Conveniently, the overall thickness of the insulation jacket amounts to from 5% to 40% of the largest cross-sectional dimension of the steel ingot.

The sidewall jacket 6 is constructed to satisfy two essential criteria: in the first place, it must be an efficient thermal insulator with adequate heat-resistance and insulation properties; secondly, it must also take up a portion of the ferrostatic pressure. When the steel has been poured into the mould 1, the mould is heated up relatively quickly by the steel and the top of the ingot may be covered with a refractory insulating material immeidately after pouring. The ingot freezes in its peripheral zones adjacent the walls of mould 1, but remains liquid in the core region. In the course of the subsequent heat-exchange and temperature equalisation phase this liquid core transmits its solidification heat to the exterior, and also to the mould 1.The latter, being thermally insulated by the sidewall jacket 6, gets hotter still and may assume temperatures (conveniently above 11 00'C but of course below its melting point) at which it can no longer absorb the ferrostatic pressure without being deformed. For this reason the jacket 6 must be strong enough to absorb this ferrostatic pressure. For example, the outer casing of the sidewall jacket 6 may be made up of several layers of asbestos cord coiled helically in closely adjacent turns in order to prevent outward bulging deformation of the mould 1.

For stripping, the ingot 1 2 is pushed down towards the base plate 2 in the usual manner by means of a ram 13, whilst the mould 1 together with the surrounding sidewall jacket 6 is lifted up by the pincer arms 9.

In contrast with the embodiment of the invention shown in Fig. 1, the sidewall jacket 6 provided in the embodiment according to Fig. 2 is not rigidly attached to the ingot mould 1. Instead, a thin layer 14 of a pourable refractory material, e.g. sand, forming part of the jacket 6, is provided between the rigid sidewall jacket 6 and the ingot mould 1. Also, in this embodiment no base plate 2 is provided. Lastly, the side-wall jacket 6 is no longer constructed in such a way as to be able to take up the ferrostatic pressure but this function is assigned to an outer metal jacket or casing 1 5 which surrounds the sidewall jacket 6 on the outside thereof and with it forms a combined, transportable and re-usable unit. It also protects the material of the jacket 6 from mechanical damage.In order to be able to provide a firm hold for the material of the sidewall jacket 6, this outer casing 1 5 is bent in an L-shaped configiiration in section at the bottom whilst at the top it is provided with loops or hooks 1 6 by means of which the metal casing 1 5 can be lifted upwards together with the jacket 6 secured thereto.

The stool plate 3 is completely enclosed in a metal sump 1 8 which has an upwardly projecting collar 1 9 whereby it overlaps and embraces the metal casing 1 5 on the outside thereof.

Preferably, the temperature of the steel when poured is only slightly above its liquidus temperature and Fig. 2 illustrates the point in time at which pouring has been completed and shortly prior to the application of the top cover element 11. The mould contains the still liquid steel ingot 1 2 on top of which is placed from above a cover plate 1 7 of approximately 5 to 30 mm. thick bright steel plate which may be coated on its underside with a highly refractory foundary varnish. This plate is plunged sufficiently deeply into the molten metal steel to ensure that a small quantity of the steel wells up all round in the gap between said plate and the side walls of the ingot mould so that the plate is marginally sealed and welded to said side walls.

Following heat-exchange and temperature equalisation shortly prior to the start of rolling, the metal casing 1 5 with the side wall jacket 6 attached thereto is pulled up with the aid of the loops 16, which allows the infill layer 14 to flow or trickle down into the sump 1 8 where it is caught, notably by the upstanding collar 19, leaving behind the actual mould 1 which may now be stripped in conventional manner.

In both examples, the mould and insulating jacket are designed to produce an ingot rolling temperature within the range of 1450"C 11 00'C, the ingot however being rolled at the highest possible rolling temperature, the jacket being removed only shortly prior to the commencement of rolling.

In an alternative arrangement, the mould is constructed so that its walls are thin enough to become welded to the steel ingot when the molten steel is poured and obviousiy in this case the mould is not stripped but forms the outer surfaces of the ingot and is rolled with the steel when the latter has solidified.

Claims (18)

1. A method of casting steel, particularly steel ingots for the subsequent production by rolling of blooms, slabs, billets, plate and the like, in which quantity of molten steel is poured into a metallic mould, characterised in that the mould is enveloped by a jacket consisting of a thermally insulating material and that subsequently, after removal of said jacket. the cast steel ingots are rolled at their own residual heat without intermediate reheating.

2. A method according to Claim 1, characterised in that the mass of the ingot mould is adjusted to provide a thermal capacity which corresponds to the amount of heat contained in the quantity of liquid steel which is poured into the mould, particularly to the latent heat of solidification of said steel in such a way that after having received the molten steel the mould heats up very quickly preferably to a temperature above 1,100"C, but below its melting point.

3. A method to Claim 2, characterised in that a relatively thin-walled chill mould is used for casting the ingot.

4. A method according to Claim 1, characterised in that the quantity of liquid steel is poured into a thin-walled steel-box corresponding to the ultimate ingot shape, which is surrounded by a jacket consisting of a thermally insulating refractory material, and in that the molten steel welds to the box walls which ultimately form the ingot surfaces without completely dissolving them.

5. A method according to Claim 4, characterised in that the wall-thickness of the box corresponds to from ().2 to 2% of the largest cross-sectional dimension of the ingot.

6. A method according to Claim 4 or Claim 5, characterised in that the bright metallic inside of the box is coated with a lubricant which decomposes and burns up only oft being wetted by the molten steel.

7. A method according to any one of Claims 1 to 6, characterised in that the temperature of the liquid steel on being poured into the mould is only slightly above the liouidus of the particular steel which is being used.

8. A meihod according to eny one of Claims 1 to 7, characterised in that the heatretaining jacket is removed only shortly prior to the commencement of rolling.

9. A method according to any one of Claims 1 to 8, characterised in that the frozen steel ingot is rolled at the highest possible rolling temperature.

10. A method according to any one of Claims 1 to 9, characterised in that the total layer-thickness of the insulating jacket amounts to from 5 to 40% of the largest cross-sectionai dimension of the steel ingot.

11. A method according to Claim 10, characterised in that the layer thickness and choice of the insulating material, based cn specific thermal conductivity and thermal capacity characteristics of the material, is arranged to establish, at least after several hours a conventional rolling temperature, namely in the range between 1 450 and 1,000'C.

12. A method according to any one of Claims 1 to 11, characterised in that the topside of the ingot is covered with a refractory insulating material immediately after pouring.

13. A method according to any one of Claims 1 to 12, characterised in that after pouring a metallic plate is placed on top of the molten surface of the steel ingot, plunging sufficiently deeply into the molten steel to ensure that a small quantity of molten steel wells up all round in the gap between said plate and the side walls of the ingot mould so that the plate is marginally sealed and welded to said side walls.

14. Apparatus for carrying out a method as claimed in any one of Claims 1 to 13, characterised by the provision of a mould which is rigidly connected to a sidewall-jacket of a thermally insulating material.

15. Apparatus according to Claim 14, characterised in that the side-wall insulating jacket has recesses for the engagement therein of the pincer arms Gf an ingo: stripping crane.

16. Apparatus for carrying out a method as claimed in any one of Claims 1 to 13, characterised by the provision of an ingot mould and a sidewall jacket which are separately manipulable units and which in their mutually assembled state are separated by a gap adapted to be filled with a pourable infill material.

17. Apparatus according to any one of Claim 1 a4 to Ho 6, cl7aN acterised In that the taper a.1gke of Phe mould is greater man s, arid preferably amounts to 4'.

18. A method of casting steel substantially as heieinbefore described wi'rh reference to Fig. 1 or Fig. 2 of the accompanying drawings.

1 r. /tppartus 0 casting reel SUbadl.

tially as hereinbefore described with reference to and as show in Fig. 2 of the accompanying drawings.