EP0005611A1

EP0005611A1 - Method of treating ingot moulds

Info

Publication number: EP0005611A1
Application number: EP79300793A
Authority: EP
Inventors: Robert George Stafford
Original assignee: Slag Reduction Co Ltd
Current assignee: Slag Reduction Co Ltd
Priority date: 1978-05-23
Filing date: 1979-05-09
Publication date: 1979-11-28
Also published as: ZA792125B; JPS54155933A; GB2021457A; AU4692179A

Abstract

To overcome various problems associated with moulding of steel, the moulds (5) are shot peened in between casts using an air-blast system which incorporates a compressor (4), a shot recovery and supply system (2), a dust collector and control section (3, 3') and a peeningicleaning station (1) which has a nozzle assembly (9) arranged to traverse the length of a mould (5) whilst rotating and simultaneously firing shot at the interior mould surfaces to achieve a peening effect.

Description

The invention relates to the cleaning and surface treatment of the interior of ingot moulds, and more particularly, to such moulds which are used in the production of steel.
Ingot moulds for use in steel production in what is known as an "uphill teeming" method are one- piece, elongate and generally have a substantially square cross-section. Such ingot moulds are generally referred to as "bottle shaped.". In order to provide for easy release of the ingots when the mould is turned over, the opposite facing interior walls of the mould diverge slightly from one another towards the open, normally upper, end so that the interior of the mould is generally frustum-shaped. The bottom end of the mould is usually smoothly curved, but has a small inlet through which molten steel is introduced into the mould cavity in the teeming process. In order to provide a lubrication effect and to aid later release of the solidified ingot a teeming flux is provided on the top of the molten steel so as to coat the walls and the surface of the steel as the level of the steel rises in the ingot.
It will be appreciated that in such a method the moulds are severely thermally stressed each time they are used. This thermal stressing causes moulds to develop what is known as "crazing" after prolonged use, i.e., after about fifteen casts. Although this crazing is usually an irregular pattern of fine surface fractures, the fractures may extend up to ¾ inch into the mould surface, and in severe cases, the side of the mould itself can be split right through to the outside.
The crazing can cause ingots to stick within the mould and, furthermore, causes surface imperfections which are either amplified during subsequent rolling processes, to the detriment of the finished steel, or which have to be removed prior to the rolling process to prevent such imperfections. Because normal cleaning of the moulds is by a wire brush technique which does not enable all surface encrustations to be removed, the crazing can be camouflaged'so that the degree of crazing that has occurred cannot be properly seen and, of course, the greater the degree of crazing the more likely an ingot is to stick in the mould. Investigations have shown that one of the causes of the stickers may be due to the non-uniform transfer of heat around the crazing where a heat build-up locally gives rise to incipient welding between the mould wall and the steel.
In order to overcome the problem of crazing moulds can be ground or milled internally, but after moulds have had further use they again craze and thus the grinding or milling process has to be repeated at frequent intervals. This is particularly difficult in the bottle shaped moulds used in uphill teeming. The grinding or milling processes are lengthy operations which cannot be combined with the normal cleaning which has to take place in a fixed time in order to obtain a suitable "turn around" time during the steel production process, and thus they adversely effect the economics of producing steel in ingots.
It has previously been proposed to clean moulds by sand blasting, but such a process is severe and the surface integrity of the moulds thus cleaned may be destroyed thus ruining the mould for further use, and creating the usual dust problems associated with sand blasting.
In order to overcome these problems and in accordance with the present invention ingot moulds are shot peened in between casts using an air-blast system, in order to maintain a clean work-hardened surface at the mould cavity walls and thus reduce the possibility: of crazing.
Apparatus for carrying out this method preferably comprises a rotary blast head having a pair of blast nozzles, the rotary head being supported for traversing the interior of a mould to be peened.
Preferably, before an ingot mould is used for the first time it will be shot blasted in order to produce a burnished, work-hardened surface, and after subsequent casting operations is again shot blasted in order to maintain a high surface integrity on the mould cavity walls.
The work hardening of the mould surface walls prevents or significantly reduces crazing by neutralizing thermal stressing in the mould walls, and of course also provides a better surface finish within the mould to enable easy release of the ingots which reduces wasted time and to enable a reduction in surface imperfections which in turn increases the quality of the steel emerging from subsequent rolling processes.
By utilising an air blast shot peening method every part of the mould wall can be reached by the stream of shot in order to ensure a suitable degree of peening all over the mould cavity wall surfaces.
Investigations have shown that the peening of ingot moulds according to the invention also serves to clean the interior surfaces so that, by replacing conventional cleaning methods with an air blast shot peening method, additional advantages can be achieved without any further delay in the turn around time during production.
Preferably, steel grain is used in the peening method. Steel grain is particulate non-spherical steel shot recovered from steel slag.
The use of such steel grain enables the cleaning and peening processes to be carried out simultaneously with the same material. Depending on the degree of peening which is required in a particular installation, spherical steel shot may be used as the abrasive/peening agent and being of a consistant hardness value gives improved results but is considerably more expensive. Chilled iron grit has been found to be too angular and is destructive of the surface integrity of the moulds.
If the method is to be used on moulds which have already had significant use then a larger grade of abrasive may be used initially in order to provide sufficient kinetic energy for the shot or grain to remove heavy deposits accumulated over the life of the mould. For example, an S.550 grade of steel grain has been used initially in such circumstances, and thereafter the top size of abrasive in the mix reduced to S.330, thereby improving the number of impacts.per pound of shot.
The extent of peening/cleaning can be adjusted by variations of time, nozzle angle, speed of rotation, traverse speed of the rotary head, blasting pressure, abrasive size and mix, and flow of shot into the blast hose. From one production site to another the conditions and treatment of the moulds will vary requiring corresponding change to variables in the peening process.
It will be appreciated that the degree of peening depends specifically on the actual area of the mould wall blasted by the grain/shot and therefore the coverage of the mould wall by the grain/shot has to be established. 90% coverage is usually achieved within a factor of 0-8 units of time while a further 10 units would be required to achieve 100%. With a machine which can blast 1,000 square feet per hour at 100% coverage, i.e. saturation using 216 lb/min. of shot S.70-S.330 mix, a mould having an internal wall area of about 50 square feet can be cleaned to saturation in four minutes. Correspondingly, a one minute blasting operation would produce 50% saturation. 100% coverage is usually achieved with 10 lb. of mixed abrasive per square foot.
Having established approximate coverage one then needs to determine the intensity of shot peening produced and this is done with a standard testing system which measures the arc height produced on a spring steel strip called an Almen strip. The strip is placed in a block and mounted in the blast stream which causes the surface to curve and this is measured on a gauge block calibrated in thousandths of an inch. Almen Arc heights of .0035"-.0055" were produced in a one minute blasting operation and .0057"-.0075" were produced in a two minute blasting operation.
The degree of peening is affected by the coverage and this may be less than 50% to achieve clean moulds, so that the degree of peening would be at the degree of coverage. The operation, however, will preferably be done in between each cast and a mould lasting 100 lives would enjoy a degree of peening pro rata to the level of the coverage and this is cumulative.
In view of this, it is not envisaged that it is necessary to use a premium i.e., table rolled, peening shot since peening of components is usually done only once. The scope to rework the surface permits normal steel shot and for this development steel grain was used which offers a range of hardness values on one hand and being softer than chilled iron grit is less severe in blasting according to the surface profile.
Although the shot peening may be carried out with a machine which includes a vacuum nozzle surrounding the nozzle for the shot, so that the shot, particles of slag removed from the surface of the mould, used abrasives and any other dirt can be removed directly from the surface of the mould as the shot peening process proceeds, it is preferable for the various elements to be removed separately from the bottom of the mould, but simultaneously.
One example of apparatus for carrying out the method according to present invention will now be described with reference to the accompanying drawings in whicb:-

Figure 1 is a general layout of the apparatus; and,
Figure 2 is an axial section through the blast nozzle and supporting lance.

The apparatus generally comprises a peening/ cleaning station 1, a shot recovery and supply section 2, a dust collector and control section 3, 3^t and a compressor 4.
At the peening/cleaning station 1 a mould 5 can be seen, the mould 5 being supported on a bogie 6 having wheels 7 which are arranged to run on a -track 80 The use of a bogie 6 for transporting the ingot moulds between various operational sites is well known and is used with the present apparatus in order to reduce cranage requirements in the steel making plant.
Steel grain for peening and cleaning is delivered through a rotary nozzle assembly 9 having a pair of nozzles 10 which, in the present example are disposed at an angle of 45° to the vertical. However, this angle may be varied if required. Alterations in the nozzle angle affect significantly the area of concentration of the blast pattern; for example at 90° the area of concentration is about 1 inch wide and at 45° 2 to 3" wide. The nozzle assembly 9 is supported for rotation on a tubular rigid lance 11 and by means of a pair of supporting arms 12, 13. The lance 11 is arranged so as to be slidable along its axis relative to the arm 12, but is fixed axially in relation to the arm 13 which is arranged to be movable on guide rods 14 relative to the arm 12 either by means of a pneumatic piston and cylinder arrangement (not shown) or by a linear electric motor. Alternatively, a rotating screw actuator may be used.
To prevent the shot or grain from flying out of the open top end of the mould 5 a cover 15 is provided to sit on the top of the mould. In order to prevent a build-up of pressure within the mould during treatment air can be continuously removed through a suitable outlet (not shown) built in to the cover 15, the air being removed at the same rate as it is supplied to the mould for the peening. Expended shot and debris from the interior of the mould are removed to the recovery and supply section 2 through the bottom of the mould through a funnel 16 and connected conduit 17 under a partial vacuum. A hose 18 which carries the blast material in an air stream is fed to an inlet housing 19 at the top of the lance 11.
The nozzle assembly 9 will now be described in further detail with reference to Figure 2. As has been previously mentioned, compressed air which carries with it a suitable abrasive,,i.e. spherical steel shot or grain, enters through an inlet manifold 19 and passes down through the lance 11 to the nozzle assembly 9. At the same time, compressed air enters through a conduit 20 to drive a small pneumatic motor 21 to cause a gear wheel 22 to rotate. The motor 21 is mounted on a housing 23 which contains the gear wheel 22 and an annular gear ring 24 to which is attached the rotary head 25 of the nozzle assembly 9. The annular gear ring 24 is attached to a housing 26 which is supported on.an inner cylindrical duct 26', for rotation, by means of bearings 27. At the lower end of the rotary head 25 of the nozzle assembly a pair of ½'' diameter tungsten carbide lined nozzles 10 are mounted in a holder 28 which is screw-threaded into a support 29 attached to the housing 26.
It will be appreciated that the stream of abrasive/peening material is fed from the inlet manifold 19 through the lance 11 and then down through the interior of the nozzle assembly 9 to the rotating .nozzles 10 which are disposed at an angle of approximately 45°C to the axis of the lance and nozzle assembly. In order to rotate the nozzles 10 about the central axis compressed air is fed to the motor 21 which in turn rotates the gear wheel 22 to rotate the annular gear 24 and thus cause rotation of the nozzles via the housing 26 support member 29 and holder 28.
In order to supply a stream of shot or grain to the flexible hose 18 the recovery/supply section 2 comprises a pressure feed hopper 30 which is pressurised through an air feed line 31 which passes from the compressor 4 and through a control box 3, both of which are shown only diagrammatically.
The air feed line 31 supplies compressed air to the feed hopper 30 via an air diffuser 32 in order to prevent wear on the side walls of the hopper 30 from swirling steel shot or grain. A special, adjustable feed valve 33 is disposed at the bottom of the feed hopper to selectively direct the stream of abrasive material to the hose 18. The feed valve is adjustable so that when it is required to use different abrasives at different stages in the peening of the mould, changeover can be accomplished quickly. It is, of course, important to ensure that the correct amount of abrasive, depending on its size, is fed to the stream of air which enters the feed valve 33 through a conduit 34 from the compressor 4.
A dump valve 35 is provided to enable the supply of abrasive, recovered from the bottom of the mould 5, from a reclaimer 36 which serves to separate the abrasive (shot or grain) and the debris from the mould. The reclaimer 36 has an inlet manifold 37 to which the conduit 17 is connected to feed material from the bottom of the mould 5. The abrasive, dust and debris enter from the manifold 37. into a cyclone section 38 -in which the abrasive, dust and debris spiral downwards. The inlet manifold and cyclone are preferably rubber lined to resist abrasion. At the bottom of the cyclone 38 the air is drawn inwards through a narrow annulus (not shown) to separate the dust from the abrasive and larger debris. The dust-laden air is then drawn out of the cyclone 38 through a hose 39 to the dust collector section 3. The clean abrasive and larger particles of debris fall onto a pneumatically operated vibrating screen 40 which allows the abrasive to pass through into a storage hopper 41 but retains the debris to pass into a waste container (not shown). The abrasive retained in the hopper 41 is fed, intermittently, by means of the dump valve 35 into the pressure feed hopper 30.
The dust which passes out of the reclaimer 36 to the dust collector section 3 through the hose 39 contains dust which results from debris on the interior of the mould and also finely broken-down abrasive. The dust is fed from the hose 39 to a cyclone 42 which removes practically all the dust from the air, the dust being passed to a dust collection chamber 45 and the last traces of dust in the air being removed in a bag filter unit 44 as the air is drawn through it by a pump unit 45 which exhausts the air from the bag filter unit 44 through an outlet duct 46.
In use, a mould, on its associated bogie 6,is moved to the peening/cleaning station and the lance 11, together with the cover 15, positioned as shown in Figure 1. A stream of abrasive is fed via the hose 18 and the lance 11 to the nozzles 10 and the nozzles rotated by means of compressed air fed through the line 20, the supply of abrasive being controlled by actuation of the feed valve 33, carried out electrically. As the peening/cleaning proceeds so the lance 11 is gradually lowered automatically by means of the arm 13 moving downwards on the guide rod 14, whilst debris and recovered abrasive are removed through the funnel 16 and duct 17 to the reclaimer 36. After the nozzles reach the bottom of the mould the supply of abrasive can be switched off and the nozzles withdrawn upwards whilst blasting just compressed air, in order to blow out any residue. In some circumstances it may be more suitable to blast from bottom to top, and/or to carry out a second pass.
The degree of peening which is practicable varies between mould types according to the metallurgical composition and time available. The new generation of moulds, i_oe. spheroidal graphite, quazi flake (costing up to 30% more than conventional haematite moulds) will benefit particularly from this treatment.
In the installation illustrated the shot recovery and supply section 2 and the dust collector and control section 3, 3' are provided by a Vacublast PV 10 N machine specially adapted for the purpose. The compressor comprises a diesel-powered APE-Belliss compressor having a capacity of from 8.5 to 17 m³/min. The steel grain used as the abrasive is recovered from steel slag, has a hardness of approximately 30 Rockwell C and is sold by Appleby Abrasives Limited. The abrasive size will vary from application to application but is preferably within the range S .330-S.550. The apparatus shown has been designed to treat tapered ingot moulds having a nominal ingot weight of 6.48 tonnes, the moulds having a height of about 24 metres and a width of nearly 1 metre.
The method of the invention is not limited to use in uphill teeming methods and all types of ingot moulds used in the manufacture of steel may be treated with this method with corresponding benefits. The arrangement of the Vacublast machine and rotary head may be standardized but to cope with variations from one works to another, alterations would have to be made to the peening/cleaning station and method of shot recovery which in some cases would be independent.

Claims

1. A method of treating ingot moulds characterised in that the moulds (5) are shot peened in between casts using an air-blast system, in order to maintain a clean work-hardened mould cavity surface.

2. A method according to claim 1, characterised in that the moulds (5) are shot peened after each casting operation.

3. A method according to claim 1 or claim 2, characterised in that before an ingot mould (5) is used for the first time, it is shot blasted in order to produce a burnished, work hardened surface.

4. A method according to any of the preceding claims, characterised in that the shot peening-is carried out using steel grain.

5. A method according to any one of the preceding claims, characterised in that the shot is continuously recovered from the mould (5) during the shot peening process and recycled.

6. A method according to any cf the preceding claims, wherein the shot peening process is also used as the only process for cleaning the moulds (5).

7. A method according to any of the preceding claims, characterised in that the shot is fired from a nozzle (10) rotating within the mould (5), the nozzle (10) simultaneously traversing the length of the mould (5)0