EP0090749B1 - Process for cooling metallurgical workpieces with minimized deformation - Google Patents

Abstract

1. Method of cooling basically long or flat metallurgical products (1) having a substantially constant thickness with a view to minimizing the geometric distortions without altering the use properties, characterised in that substantially half the lateral surface is covered by a layer (2) which is insulating or a poorer conductor of heat than the product to be cooled (1) before the covered surface (2) of the product is brought into contact with an aqueous cooling medium (4).

Description

This invention relates to an economical method of cooling metallurgical products which minimizes deformations thereof, while preserving their properties of use.

The cooling of metallurgical products is involved in a large number of operations such as casting and solidification, after hot transformation on various tools (rolling mills, spinning presses, etc.) or even in the case of metallurgical quenching .

For reasons of productivity (in particular for the cooling of the products continuously in the process), and / or of metallurgical quality (solidification, quenching), it is often necessary that these coolings are carried out at high speeds, for example by spraying of water under high pressure or two-phase spraying (air-water). We know that the heat density extracted from the products can reach 1 to 5 MW / m ² for water flows of the order of 20 kg / m ² / sec., Or 1,200 I / mi _n. / _M2

• a) la pulvérisation d'eau à fort débit (> 2000 I/ min./m²) et sous forte pression (p > 1,5 MPa ou 15 bar) entraîne des consommations d'eau importantes et nécessite des équipements onéreux (surpresseur de très forte capacité).

However, these methods have the following drawbacks:

• a) spraying water at a high flow rate (> 2000 I / min./m ² ) and under high pressure (p> 1.5 MPa or 15 bar) results in significant water consumption and requires expensive equipment (booster pump) very high capacity).

• b) dans la pulvérisation pneumatique d'eau et d'air sous pression, ces deux fluides sont introduits simultanément dans les buses à pulvérisation pneumatique à faible débit d'eau, ou séparément par soufflage d'air comprimé en sortie de buse de pulvérisation mécanique d'eau, à débit élevé.

In addition, it is not suitable for cooling thin products, in particular that of thin sheets or strips of light alloys because the impact force of the jet causes significant deformation of these products;

• b) in the pneumatic spraying of water and pressurized air, these two fluids are introduced simultaneously into the pneumatic spray nozzles with low water flow, or separately by blowing compressed air at the outlet of the mechanical spray nozzle of water, at high flow rate.

Pneumatic spraying has the disadvantage of being expensive because of the price of the nozzles (ten times higher than that of the mechanical nozzles) and the high cost of investment and operation of the air compressors. In addition, this system is dangerous because of the risk of bursting of the compressed air lines and is excessively noisy.

• - Les déformations ou distorsions des produits refroidis imposent souvent leur redressage ou leur remise en forme à l'aide de moyens puissants tels que presses, planeuses à rouleaux, bancs de traction, etc.... opérations qui, à leur tour, induisent dans ceux-ci des contraintes internes néfastes, par exemple: distorsions lors d'un usinage chimique ou mécanique ultérieur.
• - L'utilisation de dispositifs spéciaux limitant ou supprimant les distorsions lors du refroidissement, par exemple, lors de la trempe, tels que cadres, brides, trempe sous presse, etc.... est d'une certaine efficacité, mais cependant limitée, car, de toute façon, et malgré toutes les précautions prises, le niveau de contraintes internes reste élevé.

In addition, with high cooling rates, the products are the seat of a level of high internal stresses if they are thick, or very significant deformations if they are thin, these two effects being harmful:

• - The deformations or distortions of the cooled products often require their straightening or their reshaping using powerful means such as presses, levelers with rollers, traction benches, etc. operations which, in turn, induce in these harmful internal stresses, for example: distortions during subsequent chemical or mechanical machining.
• - The use of special devices limiting or eliminating distortions during cooling, for example, during quenching, such as frames, flanges, press quenching, etc. is of a certain efficiency, but however limited, because, in any case, and despite all the precautions taken, the level of internal constraints remains high.

A method used to reduce the internal stresses of metallurgical products is described in patent FR 2286880. This however relates to the cooling of the product of non-regular section, such as rails, the uniformity of cooling of which is obtained by using a insulating coating on the parts which cool the fastest.

On the contrary, the method according to the invention which eliminates or at least minimizes distortions, aims to cool the metallurgical product of substantially constant thickness with a cooling speed as large as possible and a different cooling mode between approximately two halves adjacent to its outer surface, each of them extending in the long direction or the main plane of the product.

Thus, for a rectangular bar, different cooling is applied between the two adjacent side faces of the product and the two opposite faces. For a tube, different cooling is applied between its two opposite faces. Of course, the method can be generalized to other stamped, extruded, forged or molded metallurgical products provided that they are not self-clamped by their shape and / or their dimensions during cooling.

• - dans le cas d'un refroidissement par immersion dans un milieu refroidisseur, par enduction de la moitié environ de la surface externe par un revêtement constitué d'un produit isolant ou nettement moins conducteur de la chaleur que l'alliage à refroidir (exemples: placage acier/alliage d'AI, poteyage isolant et réfractaire) avant immersion;
• - dans le cas du refroidissement par aspersion ou pulvérisation, par la différence de l'état de surface entre deux moitiés considérées, l'une d'elle étant revêtue, avant arrosage du produit, d'un enduit peu ou pas conducteur de la chaleur.

According to the present invention, these objects are achieved by means of the measures indicated in the characterizing part of claim 1. Thus, the differentiated cooling is ensured by the following means:

• - in the case of cooling by immersion in a cooling medium, by coating approximately half of the external surface with a coating consisting of an insulating product or clearly less conductive of heat than the alloy to be cooled (examples: steel / AI alloy plating, insulating and refractory lining) before immersion;
• - in the case of cooling by sprinkling or spraying, by the difference in the surface state between two halves considered, one of them being coated, before spraying the product, with a coating which is little or not heat conductive .

In particular, it has been found, surprisingly, that the unilateral cooling by conventional spraying or spraying of half of the surface of the product, the latter being previously coated with an insulating coating or poor conductor of heat, leads to higher cooling rates for thin products than for uncoated products without significant distortion. The coolant generally used consists essentially of water, possibly supplemented with certain additives (anti-corrosion, anti-foaming agent, etc.).

The coating can be carried out at any stage of the production range, prior to cooling provided that the latter remains adherent and retains its effectiveness during the range (for example, the product can be coated before heating in solution. which precedes quenching).

It is preferable, in all cases, that the watering be done through the lower part of the products, the long direction thereof being substantially horizontal.

As indicated, the process applies mainly, but not exclusively, to long metal products (wires, bars, tubes, profiles) or flat (strips, sheets, flat) of relatively small transverse dimensions and of substantially constant thicknesses.

For AI and aluminum alloys, the thickness of the flat products is preferably less than or equal to 15 mm (which corresponds substantially to a diameter of 30 mm for a solid round bar).

For copper and copper alloys, the thickness is preferably less than or equal to 35 mm (or to a diameter of approximately 70 mm).

For steels or ferrous alloys, the thickness is preferably less than or equal to 8 mm (or to a diameter of about 15 mm).

Two products will be considered to have equivalent cross sections if the external lateral surface / volume ratio is the same.

The method is particularly applicable to the quenching of high strength AI alloys for which the characteristics of use (mechanical and / or corrosion resistance) are practically not modified compared to conventional quenching with, however, very small deformations.

When cooling is ensured by spraying or sprinkling using nozzles, it is important that the impact zones of the jets cover the entire cooled surface of the product, either directly or indirectly by trickling, rebounds, etc. ... The surface water deficits remain below 2000 I / min. / M ² and, preferably, 8001 / min. / M2; the nozzles are placed at a distance of less than 15 cm from the product surface, and the injection pressure is less than 1.5 MPa (15 bars) and preferably 1 MPa (10 bars).

• - Produits minces (voir dimensions ci-dessus): absence de distorsions notables ou meilleure planéité (ou rectitude) et contraintes résiduelles faibles après refroidissement même rapide, élimination ou simplification des opérations de planage, dressage ou remise en forme ultérieure (par exemple traction ou compression contrôlées des alliages d'AI avant revenu ou maturation).
• - Pour les produits de dimensions quelconques:
  • conservation des propriétés optimales (résistance mécanique, tenue à la corrosion) à l'état d'utilisation:
  • diminution des consommations ou volumes de fluide de refroidissement mises en jeu;
  • meilleure fiabilité ou répétabilité des opérations de refroidissement
  • possibilité de contrôle et de modulation des vitesses de refroidissement des produits.

The process according to the invention offers the following advantages over conventional cooling methods:

• - Thin products (see dimensions above): absence of significant distortions or better flatness (or straightness) and low residual stresses after even rapid cooling, elimination or simplification of leveling operations, dressing or subsequent shaping (for example traction or controlled compression of AI alloys before tempering or maturing).
• - For products of any size:
  • conservation of optimal properties (mechanical resistance, corrosion resistance) in the state of use:
  • reduction in consumption or volumes of cooling fluid involved;
  • better reliability or repeatability of cooling operations
  • possibility of controlling and modulating product cooling rates.

The invention will be better understood using the following drawings and examples:

• (a) barre ronde
• (b) tôle
• (c) tube cylindrique
• (d) cornière (côté intérieur)
• (e) profilé quelconque

munis d'un revêtement isolant (2), sur environ la moitié de leur surface extérieure et refroidis soit à l'aide de buses de pulvérisation (3), soit par immersion dans un milieu refroidisseur (4) contenu dans le bac (5).Figure 1 shows an end view of products (1)

• (a) round bar
• (b) sheet metal
• (c) cylindrical tube
• (d) angle iron (interior side)
• (e) any profile

provided with an insulating coating (2), on approximately half of their external surface and cooled either by means of spray nozzles (3), or by immersion in a cooling medium (4) contained in the tank (5) .

FIG. 2 represents the characteristics of deformation after cooling: deflection (f) and tile (t) of a rectangular flat product, initially planar.

Example 1:

Three sheets of 400x400x10 mm in aluminum alloys (7075) were quenched by vertical immersion in cold (20 ° C) or hot (60 ° C) water after dissolving for 4 h at 470 ° C in an oven with ventilated air.

The sheets A and B, uncoated, were soaked in water tanks at temperatures respectively equal to 20 ° C and 60 ° C. The sheet (C) was quenched by immersion in water at 60 ° C after pickling the surfaces and coating a single large surface by means of a continuous insulating layer of low thermal effusivity, essentially consisting of a mixture of sodium silicate in solution in water, barium sulphate and titanium oxide.

Table 1 below gives the quenching rates measured between 400 and 250 ° C at 1.5 mm under each surface by means of thermocouples placed in the center of the large faces of the sheets as well as the macroscopic deformations measured after quenching (maximum arrow the along the horizontal axis - maximum tile along the vertical axis of the sheets), and the mechanical longitudinal tensile characteristics at mid-thickness in the center of the sheets, measured after tempering in the T6 state (24 h at 120 ° VS). It is noted, in particular, that the sheet (C) quenched according to the invention by asymmetric cooling, is provided with both ca mechanical tensile properties equivalent to those of sheets most rapidly symmetrically hardened in cold water (current practice) and hardening deformation significantly lower than other sheets including sheet (B) symmetrically hardened with speeds of yet relatively slow cooling.

Example 2:

Thin sheets of dimension 630x350x2 mm (total thickness) made of aluminum alloys (7475) plated with 70 µm aluminum alloy (7072) on each side, were quenched after dissolving for 30 minutes at 475 ° C , either by vertical immersion in a water tank at 18 ° C according to the usual practice, or by spraying water at 18 ° C on the only static horizontal lower surface of a sheet coated on its only inner surface with a layer consisting of the same coating based on sodium silicate as in Example 1.

Spraying was provided over the entire lower horizontal surface of the sheets by adjacent jets with solid cone jets, with an angle of about 60 °, delivering a surface flow of 1451 / min./m ² under a pressure of 3 bars (0, 3 MPa).

Table II below gives the deformations of the sheets after quenching, the surface electrical conductivity of the sheets five days after quenching (which is all the lower in the quenched-ripened state as the quenching is faster), the speed of average cooling of the sheets in the critical quenching range (400-250 ° C) and the mechanical characteristics on tensile test pieces treated by tempering - T76 (6 h at 107 ° C + 16 h at 160 ° C) after stripping of the coating and controlled traction of the sheets before tempering.

It is found that the quenched sheets according to the invention have significantly lower residual deformations than those obtained by symmetrical quenching, while retaining substantially equal mechanical properties and although the surface flow rates used here are relatively low.

Example 3:

We have dipped, by horizontal spraying on parade or static, sheets of dimension 650x350x4 mm in aluminum alloy (2024) after dissolving for 30 minutes at 495 ° C in a ventilated air oven.

The quenching installation included a 700 mm long quenching window, made up of spraying booms situated on either side of a carriage intended to support and translate the sheets with a modular passage speed, so as to simulate the horizontal quenching at the parade in passage oven outlet. The spray nozzles consisted of sprayers in full cone jets, angle 60 °, supplied with a flow of water at 15 ° C pressurized to 6.5 bars. A sheet was quenched by symmetrical spraying on the two uncoated faces, according to usual practice. The other sheets were quenched by horizontal unilateral spraying on the underside with the same surface flow with coating of the surface to be sprayed with an insulating coating based on sodium silicate.

Table III below gives the test conditions, the average cooling rates measured between 400 and 250 ° C. by thermocouples in the center of the sheets, the deformations of the sheets after quenching (longitudinal deflection - transverse tile) as well as the sensitivity to intergranular corrosion of the hardened sheets and stripped of the coating evaluated according to the immersion test in solution of sodium chloride and distilled water according to standard AIR 9048.

It can be seen that the asymmetrical quenching of the coated sheet metal according to the invention improves both the quenching rates (approximately x2) and the resistance to intergranular corrosion of the alloy as well as the flatness of the sheets while increasing the productivity of the installation (x 3) and by reducing the necessary water consumption (: 2 approximately) compared to the symmetrical quenching of bare sheets.

Example 4:

We quenched by mechanical spraying at a pass at 25 cm / s, after dissolving for 1 hour at 500 ° C, angles of section 50x50x5 mm and length 600 mm made of aluminum alloy (6061).

Two angles were soaked in water at 18 ° C, either by symmetrical spraying (on both sides) or unilateral (on their underside) by means of longitudinal spray bars with nozzles 100 mm apart from the top of the angles, so as to directly cool the entire angle iron from both sides or from the underside alone, previously coated with a uniform layer of sodium silicate (Na, 0: 3.3 SiO.) in solution in water at density 1.08 (see Figure 1 d).

The nozzles gave jets with a full cone, angle 60 °, with a surface flow of 5801 / min./m ² under a water pressure of 6 bars (0.6 M Pa).

Table IV below gives the test conditions as well as the cooling rates and the average longitudinal deformations (arrows) after quenching. The results show that the angle iron quenched by unilateral spraying on the coated surface according to the invention has a better compromise between cooling speed and straightness than the angle angles cooled by conventional symmetrical cooling by spraying in the stream or by immersion.

The coating and the unilateral quenching significantly increase the productivity (x 3) with acceptable quenching deformations.

Claims (11)

1. Method of cooling basically long or flat metallurgical products (1) having a substantially constant thickness with a view to minimizing the geometric distortions without altering the use properties, characterised in that substantially half the lateral surface is covered by a layer (2) which is insulating or a poorer conductor of heat than the product to be cooled (1) before the covered surface (2) of the product is brought into contact with an aqueous cooling medium (4).

2. Method according to claim 1, characterised in that cooling is effected by immersion in a fluid (or mixture of fluids) or a suspension.

3. Method according to claim 1, characterised in that cooling of the covering product is effected by sprinkling or spraying with a fluid or mixture of fluids from nozzles.

4. Method according to one of claims 1 to 3, characterised in that the longitudinal direction of the product (1) is substantially horizontal.

5. Method according to one of claims 1 to 4, characterised in that the product is cooled on its one or more than one lower surface.

6. Method according to one of claims 1 to 5, characterised in that the product (1) is a product of heat treated aluminium alloy the cross-section of which is equivalent to that of a flat product the thickness of which is equal to or less than 15 mm.

7. Method according to one of claims 1 to 5, characterised in that the product (1) is a product of copper or copper alloy the cross-section of which is equivalent to that of a flat product, the thickness of which is less than or equal to 35 mm.

8. Method according to one of claims 1 to 5, characterised in that the product (1) is that of an iron alloy having a cross-section equivalent to that of a flat product having a thickness equal to or less than 8 mm.

9. Method according to one of claims 3 to 8, characterised in that the output per surface area of the nozzles is less than 2000 I/min. m², preferably 800 I/min. m^2.

10. Method according to one of claims 6 to 9, characterised in that the supply pressure of the nozzles is less than 1.5 MPa (15 bars) and preferably less than 1 MPa (10 bars).

11. Method according to one of claims 3 to 10, characterised in that the distance of the nozzles from the product is less than or equal to 15 cm.

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