EP0215874B1 - Hoops for continuous casting rolls - Google Patents

Abstract

PCT No. PCT/FR86/00086 Sec. 371 Date Nov. 14, 1986 Sec. 102(e) Date Nov. 14, 1986 PCT Filed Mar. 14, 1986 PCT Pub. No. WO86/05423 PCT Pub. Date Sep. 25, 1986.The invention relates to hoops for rollers for the continuous casting of aluminium, characterized in that they are in the form of forged cylindrical casings which have undergone heat treatment and machining and consist of an alloy steel having the following composition in percentages by weight: C: 0.30 to 0.36; Mn: 0.30 to 0.60; Si: 0.15 to 0.45; Ni: Less than 0.40; Cr: 2.80 to 3.40; Mo: 0.85 to 1.25; V: 0.10 to 0.30; S:</=0.020; P:</=0.020; Cu:</=0.30, the remainder being substantially iron and residual impurities.

Description

The invention relates to hoops of continuous casting rolls of aluminum alloy, these hoops having an improved lifetime compared to the hoops made of traditional steels, and retaining good productivity in the casting machine.

In the production of continuously cast aluminum, the molten metal is directly cast at a temperature of approximately 680 ° C, between a pair of cooled rollers rotating in opposite directions, which fulfill the double function of solidification of the strip metal and rolling. of this metal.

It is understood that the cylinders cannot rotate at a very high speed since the alloy must be given time to solidify on contact with the roller and the possibility of undergoing a certain rolling.

To obtain maximum productivity from the installation, the rollers must be cooled efficiently and have good thermal conductivity in order to promote the evacuation of calories. These rollers are produced in two parts a central core comprising cooling means and a hoop which is mounted on this core and comes directly into contact with the cast aluminum. Rolls of this type are already known from patent application FR-A-2 217 098.

This hoop constitutes the part of the roller receiving the most intense stresses and, after a certain number of operating hours, must be revised due to the thermal cracks which have developed.

The first characteristic that is required of a material, to produce frets of a continuous casting machine, is good thermal conductivity. However, these frets are subjected, in operation, to a certain number of constraints, of mechanical origin, namely: hooping, bending, torsion. These constraints impose a minimum of mechanical strength and toughness.

The main stress is thermal cycling which causes plastic fatigue of the surface, initiation and propagation of a network of micro-cracks.

This deterioration requires periodic reconditioning of the hoop, an operation which consists in eliminating, by machining, the thickness of deteriorated metal.

• C: 0,53 à 0,57; Mn 0,70 à 0,90; Si: 0,20 à 0,40;
• Ni: 0,40 à 0,70; Cr: 0,90 à 1,30; Mo: 0,40 à 0,60;
• V: 0,10 à 0,20 avec une teneur en S _< 0,020 et une teneur en P _< 0,020. Cette technique antérieure est connue du déposant.

The alloy steels commonly used until now for the manufacture of these hoops corresponded to the following weight composition in%:

• C: 0.53 to 0.57; Mn 0.70 to 0.90; If: 0.20 to 0.40;
• Ni: 0.40 to 0.70; Cr: 0.90 to 1.30; Mo: 0.40-0.60;
• V: 0.10 to 0.20 with an S content _< 0.020 and a P content _< 0.020. This prior technique is known to the applicant.

These steels favor the thermal conductivity properties and therefore the productivity of the installations, but in return, have a limited resistance to thermal cracking, which results in a shorter operating time requiring more frequent re-machining and therefore greater consumption of frets.

• C: 0,53 à 0,58; Mn 0,40 à 1; Si: 0,1 à 0,2; Ni: 0,45 à 0,55; Cr: 1,5 à 3,0; Mo: 0,8 à 1,2; V: 0,3 à 0,5 avec S _< 0,020 et P < 0,020. Cette technique antérieure est connue du déposant.

Steels are also known for aluminum continuous casting hoops having the following composition by weight in%:

• C: 0.53-0.58; Mn 0.40 to 1; If: 0.1 to 0.2; Ni: 0.45 to 0.55; Cr: 1.5 to 3.0; Mo: 0.8 to 1.2; V: 0.3 to 0.5 with S _< 0.020 and P <0.020. This prior technique is known to the applicant.

These steels have good mechanical strength and operating time properties, but they do not make it possible to obtain productivity results comparable to those obtained by the steels described above.

The object of the present invention is to provide new continuous aluminum casting hoops which, while preserving a high productivity of the installation linked to good thermal conductivity, provide a service life at least comparable to the best products currently used, this thanks to the use of an alloy steel having a high resistance to fatigue of thermal origin.

• C: 0,30 à 0,36; Mn 0,30 à 0,60; Si: 0,15 à 0,45;
• Ni inférieur à 0,40; Cr: 2,80 à 3,40; Mo: 0,85 à 1,25;
• V: 0,10 à 0,30; S _< 0,020; P _< 0,020; Cu _< 0,30 ; le reste étant essentiellement du fer et des impuretés résiduelles.

The subject of the invention is therefore a hoop of a continuous casting roll of aluminum, characterized in that it is in the form of a forged cylindrical casing, having undergone a heat treatment and machined, of an alloy steel having the composition following, as a percentage by weight:

• C: 0.30 to 0.36; Mn 0.30 to 0.60; If: 0.15 to 0.45;
• Ni less than 0.40; Cr: 2.80 to 3.40; Mo: 0.85 to 1.25;
• V: 0.10 to 0.30; S _< 0.020; P _< 0.020; Cu _< 0.30; the remainder being essentially iron and residual impurities.

• C: 0,31 à 0,35; Mn 0,30 à 0,50; Si: 0,15 à 0,35;
• Ni inférieur à 0,25; Cr: 2,80 à 3,40; Mo: 0,90 à 1,10; V: 0,13 à 0,20;
• S _< 0,020; P < 0,020; Cu < 0,30; le reste étant essentiellement du fer et des impuretés résiduelles.

According to a preferred embodiment, the composition is as follows:

• C: 0.31 to 0.35; Mn 0.30 to 0.50; If: 0.15 to 0.35;
• Ni less than 0.25; Cr: 2.80 to 3.40; Mo: 0.90 to 1.10; V: 0.13 to 0.20;
• S _< 0.020; P <0.020; Cu <0.30; the remainder being essentially iron and residual impurities.

• - la Fig. 1 est une vue schématique dure installation de coulée continue horizontale;
• - la Fig. 2 est une vue schématique en élévation latérale et en coupe partielle d'une partie de l'installation de la Fig. 1;
• - la Fig. 3 est un diagramme illustrant le cycle des contraintes en fonction de l'allongement subi par le métal de la frette;
• - la Fig. 4 représente des histogrammes illustrant le nombre et la profondeur des fissures pour quatre nuances d'acier, deux selon la technique antérieure et deux selon l'invention.

The invention is set out below in detail with reference to the accompanying drawings, in which;

• - Fig. 1 is a schematic view of the horizontal continuous casting installation;
• - Fig. 2 is a schematic view in side elevation and in partial section of part of the installation of FIG. 1;
• - Fig. 3 is a diagram illustrating the stress cycle as a function of the elongation undergone by the metal of the hoop;
• - Fig. 4 represents histograms illustrating the number and the depth of the cracks for four steel grades, two according to the prior art and two according to the invention.

The principle of light alloy continuous casting shown in Figs. 1 and 2, relates to so-called horizontal casting. The aluminum, melted in an oven which is not shown, is maintained at constant level in a supply channel 1 and introduced, by means of a nozzle 2, between two rollers 3, at a temperature of around 680 ° C. . The rollers (or cylinders) 3 are rotated in opposite directions with a spacing which determines the thickness of the solidified sheet 4. The rolling grip constitutes a continuous ingot mold in which, in contact with the cold cylinders, the aluminum of casting solidifies, while it is driven by the rotation of the cylinders.

• Une âme 5 qui est un cylindre en acier dans lequel sont ménagés des canaux longitudinaux 6, d'amenée et sortie d'eau par les tourillons 7. Ces canaux alimentent, par des canaux radiaux 8, des rigoles périphériques 9 et une frette 10 qui est montée sur l'âme, de telle sorte quelle est en contact direct avec le fluide de refroidissement qui circule dans les rigoles 9. Cette frette constitue la partie consommable du rouleau. Son rôle premier est d'extraire les calories de l'alliage en solidification. On conçoit que la productivité de la machine de coulée soit directement liée au transfert des calories au travers de la frette.

Each roller has a cooling circuit traversed by a fluid, generally by water. Each roll is produced in two parts, namely:

• A core 5 which is a steel cylinder in which are formed longitudinal channels 6, supply and outlet of water by the pins 7. These channels supply, by radial channels 8, peripheral channels 9 and a hoop 10 which is mounted on the core, so that it is in direct contact with the cooling fluid which circulates in the channels 9. This hoop constitutes the consumable part of the roller. Its primary role is to extract the calories from the solidifying alloy. It is understood that the productivity of the casting machine is directly linked to the transfer of calories through the hoop.

As indicated above, this hoop must have good thermal conductivity, but also appropriate mechanical properties due to the constraints to which it is subjected.

The stress regime, at each point of the frets, is defined by the sum of the stresses of mechanical origin and the stresses of thermal origin due to the cycling of the thermal gradient.

• - le frettage (contraintes statiques) ;
• - le couple d'entraînement qui induit des contraintes de torsion-cisaillement;
• - la pression de laminage qui entraîne la flexion des cylindres, et une répartition de contraintes de compression-cisaillement dans l'emprise des cylindres.

The mechanical stresses originate from:

• - hooping (static constraints);
• - the drive torque which induces torsional-shear stresses;
• - the rolling pressure which causes the bending of the rolls, and a distribution of compression-shear stresses in the grip of the rolls.

• - contrainte circonférentielle de traction: 100 à 250 MPa;
• - contrainte longitudinale en traction: 50 à 150 MPa.

By its principle, the hot hooping operation induces residual stresses in the hoop. These have been evaluated by different approximation formulas, or calculated by the finite element method. A certain number of residual stress measurements were carried out by the X-ray method, on frets of different types. The results corroborate the calculations. Depending on the type of roller and the hooping conditions, the order of magnitude of the residual stresses after hooping is as follows:

• - circumferential tensile stress: 100 to 250 MPa;
• - longitudinal tensile stress: 50 to 150 MPa.

It should be noted that in operation, the frets move slightly. Very quickly, the residual longitudinal stresses will disappear. On the other hand, the circumferential traction constraints, due to tightening, remain.

The operating constraints were also calculated, for a specific type of casting machine.

• - Torsion: the shear stresses due to the drive torque are very low, of the order of MPa. Their effect is therefore quite negligible.
• - Bending: during a rotation, the longitudinal stresses due to bending, go through a maximum in compression, in sheet / cylinder contact, then through a maximum in traction in the opposite position the order of magnitude of these constraints is from: + 70 MPa.
• - The rolling effect which occurs on the alloy sheet, during and just after its solidification, is weak and applies to a material whose flow stress is very low. The Hertz shear stresses generated in the hoop are therefore very modest and can be neglected.

Each rotation of the roller puts its skin in contact with the liquid aluminum, in the contact arc a. This results in a thermal gradient in the thickness of the hoop.

Then, on leaving the contact, the rotation allows the stressed area to cool.

We know that any variation in temperature generates a deformation in volume. The homogeneous variation of temperature, of a free and homogeneous material, involves a variation of volume, but no constraint.

• - la structure est encastrée;
• - la structure n'est pas homogène (d'ou présence de coefficients de dilatation différents);
• - la température n'est pas homogène les fibres souhaiteraient se dilater différentiellement, or elles sont encastrées dans les fibres voisine d'où un état de contraintes, fonction du gradient thermique.

There is a constraint if:

• - the structure is embedded;
• - the structure is not homogeneous (hence the presence of different expansion coefficients);
• - the temperature is not homogeneous, the fibers would like to expand differently, but they are embedded in the neighboring fibers, hence a state of stress, a function of the thermal gradient.

In the case of hoops, the internal face can be considered, at constant temperature, close to that of the cooling water. The external face comes, at each rotation, into contact with the liquid aluminum, then when the sheet comes out of the grip, the hoop is cooled by the ambient air, and by the thermal transfer towards the cooling fluid.

The thermal evolution was studied by the introduction of thermocouples in the thickness of frets. The results of this study inspired the definition of the thermal cycle imposed on the test specimen of the fatigue resistance test of thermal origin which is described below and used in the present description.

• où: E = Module de Young
• a = Coefficient de dilatation
• Δθ = température de la surface - Température interne
• y = Coefficient de Poisson.

Each cycle induces, in the skin of the hoop, a maximum compression stress the value of which can be estimated as a first approximation, assuming the material perfectly elastic, by the relation:

• where: E = Young's modulus
• a = Coefficient of expansion
• Δθ = surface temperature - Internal temperature
• y = Poisson's ratio.

Given the temperature reached at the surface of the hoop, it can be seen that the stress level reached and exceeds the elastic limit of the steel. The heating of the hoop skin, in contact with liquid aluminum, therefore causes plastic deformation of this skin.

The stress cycle is illustrated in Figure 3.

The first heating is represented by the line OA, then by the curve AB, which corresponds to the plastic strain on the stress-strain diagram in Fig. 3.

On cooling, the deformation will tend to cancel out, but the metal will not be able to return to its elastic position since it has undergone, in heating, a deformation in the plastic field in compression. The return to low temperature will cause, in D, the exceeding of the elastic limit in tension and cause plastic deformation, in tension this time, up to point E.

• - une déformation plastique "FB" au chauffage;
• - une déformation plastique "DE" au refroidissement.

The second cycle, starting from E, will cause a new overshoot of the elastic limit in compression in F, and the cycle will continue around EFBD, generating each time:

• - plastic deformation "FB" on heating;
• - plastic deformation "DE" on cooling.

This deformation cycle of thermal origin inevitably generates mechanical fatigue of the surface which results in the initiation, then the propagation of microcracks.

The mechanical stresses, residual hoop stresses and bending stresses are modest. They do not pose a mechanical characteristic problem for steels.

The predominant stresses are those induced by the thermal cycle (which, of course, are superimposed, even modest stresses, of mechanical origin).

The essential property which is required of a continuous casting hoop, in addition to thermal conductivity, is therefore "resistance to thermal fatigue".

If it were possible to decrease the amplitude of the cycle of the temperature gradient, one would decrease the maximum level of the stresses and one would delay the appearance of cracking. This cycle, however, is linked to the process itself.

For a steel, it was previously indicated that the maximum skin stress could be evaluated by the relation:

Increasing the Poisson's ratio would lower the maximum stress. However, this coefficient is more linked to the crystal structure of the alloy than to its composition.

In principle, alloys with a high coefficient of expansion are avoided. However, the differences in expansion coefficient, which may exist between the different types of tool steels, are not significantly enough to make it an essential criterion of choice.

Lowering the modulus of elasticity (Young's Modulus) would make it possible to reduce, all other things being equal, the level of maximum stress. However, if we can very well measure the modulus of elasticity of a steel, the correlation with the composition is quite complex and it has not been the subject of exploitable synthetic work which allows the definition of a steel for a given modulus of elasticity. J.T. LEUKKERI - The elastic module of face- centered cubic transition metal alloys. J. Phy. GBR. 1981-11- (10) p. 1997 - 2005. STANKOVIC D., PAJEVIC M.B. - Determination of some factors influencing the modulars of elasticity of Gray cast Iron. Livarski Vestn. 1981 - 28 - (4). p. 97 - 102.

It is also known that the increase in temperature, as well as that of the stresses up to the level causing micro-plastic deformations, causes a reduction in the modulus of elasticity. VOJTENKO-A.F., SKRIPNIK Yu.D., SOLOVEVA N.G., NADEZHDIN G.N. - Effect of the threeshold of stresses on the Young Modulers, INST. Problem Prochnosti - 1982 no 11 - p. 83-86).

Obtaining a good Young's module cannot be the primary objective of finding a high-performance steel. This research should focus, as a priority, on optimizing resistance to thermal fatigue.

According to a first approach, it follows from the description of the stress cycle that the fatigue resistance of thermal origin appears directly linked to the mechanical characteristics of the steel at the stress temperatures, and in particular, directly linked to the limit of elasticity of the material.

We know the influence of chromium, molybdenum, which pushes back the softening temperatures, of Vanadium which elevates the hot characteristics, the influence of carbon which, by precipitating carbides formed with the preceding elements, will harden the steel .

We know how to use elements such as Nickel, Manganese or Silicon to ensure that the steel has a homogeneous and stable structure with good toughness.

There are thus hot tool steels, used, in particular, for the forging and forging of steels, which have a high elastic limit when hot, good resistance to thermal fatigue and which would seem to be able to be used to constitute frets. continuous casting of aluminum, however, these steels have a too low thermal conductivity, which leads to a more or less significant loss, see incidents, or an impossibility of operation of the machine.

The search for a steel composition optimized for continuous casting hoops consists in using, at best, the alloying elements in order to obtain the best resistance with a thermal conductivity little modified compared to standard steels.

The selection criteria are, of course, the mechanical characteristics of the steel. However, thermal fatigue is a complex mechanism. Besides the effect of the stresses generated by the cycling of the thermal gradient, it integrates the effects of external aggressions, and those of internal modifications of the material, which influence the formation and the development of the network of cracks.

The choice of materials is guided, in addition to their traditional mechanical characteristics, by their behavior under fatigue stresses of thermal origin, through a specific simulation test which will be described below.

Thus, to meet the contradictory criteria in practice of good productivity and maximum longevity, the Applicant's studies have made it possible to determine a steel grade defined above for which an acceptable reduction, from the point of view of industrial exploitation, of the thermal conductivity, however, provides a surprising improvement in resistance to thermal fatigue cracking.

The cast iron frets according to the present invention are produced from a steel grade produced in an electric oven, cast in a pocket where it is refined and degassed, then finally cast in an ingot mold. The ingots are heated to approximately 1200 ° C., drilled to obtain blanks which are themselves forged into tubes of 500 to 1000 mm in diameter. These blanks are then austenitized at around 970 ° C., then quenched and subjected to tempering to give them the required metallurgical quality.

It should be noted that the grades according to the present invention are characterized by a simplicity of implementation during hot transformation and subsidiary treatments for refining the structure in order to obtain the best ductility properties, which is difficult to obtain for grades with higher carbon content or containing more alloying elements.

The blanks thus obtained are then machined to size.

Table 1 below indicates the weight compositions of the various steels tested, for comparison purposes.

After having undergone the metallurgical quality treatments described above, the steels are subjected to the following tests:

First of all, a test aimed at determining the tensile characteristics on broken test pieces at a temperature of 630 ° C. The results of these tests are summarized in Table 11.

Thermal fatigue resistance tests were also carried out on a specific device described below.

This device comprises a finely rectified cylindrical test piece, which is intermittently heated at the surface by high frequency induction and which is permanently cooled internally by a circulation of water.

The definition of the test piece, the power of the generator, the self-test piece coupling, the cooling, made it possible to define a thermal cycle inspired by the temperature measurements recorded on real frets in operation.

Certain types of thermal fatigue tests, such as the Northcott and Baron test, focus only on the superficial aspect of cracking. This criterion is not satisfactory because it does not provide information on the penetration of cracks. The criterion used is therefore the count and the depth of the cracks observed on a specimen section for 10 linear mm on the surface.

The reliability of this test is demonstrated by the excellent correlation observed between the examinations of specimens and the appearance and development of cracks on the real frets made of the same materials.

The test results are plotted on a histogram shown in FIG. 4, where the numbers and the depths of the cracks are shown. The mechanism of cracking is always similar the cracks which were born first, starting from the micro-accidents of surfaces, are those which develop most in depth. Then forms a network of minor cracks of lesser importance. The criterion used is the maximum depth of the main cracks: these are the cracks which determine, on a hoop, the quantity of metal to be removed to renovate its work surface.

The improvement of the fatigue resistance of thermal origin consists in an unexpected reduction of the mesh of the network of the principal cracks and in a reduction in the maximum depths of the cracks, all other things being equal. In the steel constituting the prior art, the frequency of the first initiated cracks is approximately one to two cracks per linear centimeter. These main cracks which penetrate deeply determine the duration of use of the hoop before its repair.

In the steel according to the present invention, the frequency of cracks which are initiated approximately simultaneously is much higher. The distribution of deformations and stresses due to thermal cycling is thereby distributed, which contributes to the lesser penetration of degradation by thermal fatigue.

This is highlighted, Fig. 4 by the results obtained statistically on test pieces, according to the test conditions described above, of thermal cycling between 50 and 630 ° C, representative of the operation of a continuous casting hoop. For the same number of 3,000 cycles, the maximum depth of the cracks, which reaches 2.6 mm with shade 1 according to the prior art does not exceed 0.32 mm with shade 3 according to the present invention.

These favorable results of resistance to thermal fatigue do not cause any significant deterioration in the productivity of continuous castings equipped with the hoops produced according to the grade of the present invention. Thus, a continuous aluminum casting machine equipped with rollers with a diameter of 630 mm made it possible to cast 9,960 tonnes in approximately 4,800 hours.

For a machine fitted with rollers 960 mm in diameter, 3,200 tonnes were poured in 1,000 hours.

Claims (2)

1. Hoop for a roller for the continuous casting of aluminium, characterized in that it is in the form of a forged cylindrical casing which has undergone heat treatment and machining and consists of a steel having the following composition in percentages by weight: C: 0.30 to 0.36; Mn: 0.30 to 0.60; Si: 0.15 to 0.45; Ni: less than 0.40; Cr: 2.80 to 3.40; Mo: 0.85 to 1.25; V: 0.10 to 0.30; S: ≤ 0.020; P: ≤ 0.020; Cu: ≤ 0.30, the remainder being substantially iron and residual impurities.

2. Hoop according to Claim 1, characterized in that the steel of which it is made has the following composition in percentages by weight: C: 0.31 to 0.35; Mn: 0.30 to 0.50; Si: 0.15 to 0.35; Ni: less than 0.25; Cr: 2.80 to 3.40; Mo: 0.90 to 1.10; V: 0.13 to 0.20; S: < 0.020; P: ≤ 0.020; Cu: < 0.30, the remainder being substantially iron and residual impurities.

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