EP0307474A1 - Method and cover furnace for heat treatment of metal sheet in rolls - Google Patents

Abstract

The method consists in placing the rolls of metal sheet in stacks with gaps, heating of the rolls and cooling them down under the muffle with forced circulation of a protective gas. The minimum value of the gap between the rolls placed in stacks is chosen with a view to ensuring substantial equality of the heat flows transferred through convection from the protective gas and through radiation from the muffle. The cover furnace for implementing the method comprises a stand with a built-in fan, on which, under the muffle and a heating cover, a stack of rolls separated from each other by convection rings (13, 14, 15) is placed. Each ring (13, 14, 15) comprises radial supporting ribs (16), interconnected by hollow intercostal partitions (17). The height (h) of the radial ribs (16) equals 0.15-0.4 of their length.

Description

Technical field

The present invention relates to iron and non-ferrous metallurgy and relates in particular to a method for the heat treatment of metal strip in coil form in a hood furnace and a hood furnace for its implementation.

The present invention can be used particularly effectively for the heat treatment of coils from cold-rolled, tightly wound thin sheet metal strips and strips made of steel of various types (carbon, alloy, electrical steel) and of non-ferrous metals and alloys.

Underlying state of the art

Currently, the main problem with the production of cold-rolled metal is an improvement in quality and an increase in production output. To a significant extent, this is associated with the heat treatment of cold-rolled metal, which is carried out in bell-type furnaces. The quality improvement of the products produced is determined by the same heating conditions, i.e. by the uniformity of the temperature field in the entire volume of the annealing material. The increase in furnace throughput depends to a considerable extent on the intensification of the annealing furnace heating.

There are currently two alternative ways of improving bell annealer operation. One way is to demolish the existing bell-type furnaces and replace them with new furnaces in which H ₂ (instead of a mixture consisting of 4% H ₂ + 96% N ₂ ) is used as protective gas and a fan with increased performance (up to 100 kW) and a specially developed fan wheel with a diameter of up to 1 m can be used. The other solutions are aimed at a more intensive heating of the end faces by the forced circulation of the protective gas in the furnace chamber under the muffle, the intensification being achieved by an increased protective gas velocity in the convection rings which are arranged between the coils. All known constructions of the rings have a low height, which ensures a high inert gas velocity and consequently a high coefficient of heat dissipation by convection. However, the small height of the convection ring means that the muffle radiation can practically not penetrate into the convection ring and does not reach the end face of the collar.

A method for heat treating metal strip in coil form in a hood furnace (Aptermann VN et al. "Hood furnace", 1964, Metallurgia (Moscow), pp. 8 to 10) is known, which consists in that several metal strip bundles are stacked, heated and be cooled. The hood furnace for carrying out this method contains a stand with a diffuser and a circulation fan, a muffle, a heating hood with gas burners or electrical heating elements and convection rings which are arranged in a stack between the coils.

With the gas burners or electrical heating elements, the hood lining and the muffle and, through the latter, the protective gas, which is fed into the furnace space under the muffle, is heated. The protective gas flows under the action of the circulation fan along the muffle surface, is heated and, when flowing through the convection rings, transfers the heat to the end faces of the collar by convection. The height of the convection rings is usually chosen to be minimal in order to increase the circulating speed of the protective gas at the end faces of the collar and consequently the convective heat emission. Depending on the flow of the protective gas over the corpus of the convection ring decreases the gas temperature, whereby the convection heat flow and the temperature are distributed unevenly on the coil radius. Due to the small height of the convection ring, there is sufficient effective heat transfer from the protective gas to the end faces of the collar. guaranteed by convection, but the heat transfer to it is practically impossible due to the muffle radiation.

Various designs of convection rings are known which are used for intensive heat transfer from the protective gas to the end faces of the collar by convection.

Known are a convection ring (SU, A, 3069I49), which represents a corrugated insert, and a convection ring (SU, A, 3618921), which is designed in the form of separate wedge-shaped inserts which are arranged in two rows on the circumference of the furnace.

The construction of a convection ring (SU, A, 436969) is also known which contains radially extending support ribs and webs lying between the ribs, which are hollow and have a smaller height than the support ribs. Openings are provided on the outside of the webs.

Such a construction of the convection ring is aimed at intensifying the heat transfer to the end faces of the collar by convection. When flowing through the openings on the outside of the ring, the protective gas reaches the hollow webs between the ribs, from which it flows out through systems of openings in the form of individual jets directed towards the end faces of the collar. The horizontal surfaces of the webs cover practically the entire front surface of the bundles, as a result of which the latter are shielded from the heat flow radiated by the muffle.

Disclosure of the invention

The invention has for its object to provide such a method for the heat treatment of metal in coil form in a hood furnace and such a hood furnace for its implementation, in which a uniform temperature field is ensured in the entire volume of the bundle stack while intensifying the heating process.

The object is achieved in that, according to the invention, the minimum space between in a method for heat treatment of metal strip in bundle form in a hood furnace, which includes stacking the bundles with gaps, heating the bundles and cooling them under the muffle with forced circulation of a protective gas the bundles to be stacked is selected based on the condition that the heat flows which are transmitted from the protective gas by convection and from the muffle by radiation are essentially the same.

The method according to the invention makes it possible to achieve a uniform temperature field in the entire metal volume in the furnace while intensifying the heating.

This is achieved by the fact that the heat flow radiated through the muffle penetrates into the areas of the end face where the metal temperature is minimal. Since the convection heat flow from the protective gas to the metal remains constant, the additional radiant heat flow from the muffle allows the total heat flow acting on the end faces of the collar to be increased considerably and the metal heating to be intensified.

In the case when the radiation current from the muffle is smaller than the convection current from the protective gas, a uniform temperature field is not achieved in the entire batch volume and the bundle heating is not intensified.

gewählt, worin.In the preferred embodiment variant of the present invention, the minimum space between the bundles is in one area

chosen where.

D and d - the outside and inside diameters of the bundles are corresponding.

The proposed minimum space allows not only the convection heat flow from the shielding gas, but also the radiant heat flow from the muffle to the face, which intensifies the metal heating. By redistributing the total heat flow over the radius of the metal coil, the temperature field in the coil volume is balanced.

With a smaller gap, the radiation current from the muffle is greatly reduced and the inner part of the bundle practically does not reach, which will lead to an uneven temperature field distribution in the total volume of the metal bundle.

The issue is also solved in that a hood furnace for carrying out the process for heat treatment of coil-shaped metal strip, which contains a stand with a built-in circulation fan, on which bundles of the metal strip are stacked under a muffle and a heating hood, which are separated by convection rings are, each of which is provided with radially extending support ribs and hollow webs lying between the ribs of a smaller height than the support rib height, according to the invention the height of the radially extending support ribs is 0.15 to 0.4 of their length.

With a smaller height of the support ribs, the proportion of radiation from the muffle to the end faces of the collar is significantly reduced, the temperature field in the collar volume becomes uneven and the intensity of the Coil heating decreases.

With a larger height of the support ribs, the ring construction loses rigidity and with an increase in the support rib thickness to improve the ring rigidity, the ring weight is increased.

It is expedient for the radially extending support rings to be hollow, to be closed on the inside of the ring and to be open on the outside.

In the case of such a rib construction, the radiation penetrates from the muffle into the interior of the radially extending supporting ribs and reaches the collar end face via the openings on their horizontal surfaces, as a result of which the metal heating is additionally intensified.

It is also expedient that the rib length is greater than the length of the webs lying between the ribs and that the projecting end parts of the ribs form grooves together with the webs.

The grooves between the ribs enlarge the front surface of the collar, which is affected by radiation from the muffle, which intensifies metal heating.

In one of the embodiment variants of the invention, the webs lying between the ribs have a length of 0.5 to 0.9 of the length of the supporting ribs.

With a longer web length, the radiation from the muffle is practically shielded by the horizontal surfaces of the webs, and the heating is not intensified. With a smaller ridge length, the convection heat flow acting on the inner areas of the collar end faces will decrease significantly, the uniformity of the temperature field on the collar radius being impaired at the same time.

Brief description of the drawings

• Fig. 1 - einen erfindungsgemässen Haubenofen (Längsschnitt);
• Fig. 2 - einen Konvektionsring (im Grundriss);
• Fig. 3 - einen Schnitt nach der Linie III-III in Fig. 2;
• Fig. 4 - ein Diagramm der Abhängigkeit der Koeffizienten der Wärmeübertragung durch Strahlung und Konvektion und der relativen Höhe des Zwischenraums Voneinander bei der Anzahl der Bunde im Stapel n = 3 und 4.

Further objects and advantages of the present invention will become apparent from the following detailed description of an embodiment with reference to the accompanying drawings, in which it shows:

• Fig. 1 - a hood furnace according to the invention (longitudinal section);
• Fig. 2 - a convection ring (in plan);
• Fig. 3 - a section along the line III-III in Fig. 2;
• Fig. 4 - a diagram of the dependence of the coefficients of heat transfer by radiation and convection and the relative height of the space between each other with the number of bundles in the stack n = 3 and 4.

The preferred implementation and execution variant of the invention

gewählt, wobei D und d - der Aussen- und der Innendurchmesser entsprechend sind.,The process for the heat treatment of metal strip in coil form in a hood furnace includes the stacking of the bundles with gaps, the heating of the bundles and their cooling under the muffle with forced circulation of a protective gas. The minimum space between the bundles to be stacked is selected on the basis that the heat flows which are transmitted from the protective gas by convection and from the muffle by radiation are essentially the same. The minimum gap is in a range of

selected, where D and d - the outer and inner diameters are corresponding.,

The hood furnace for carrying out the method according to the invention contains a stand 1 (FIG. 1) with a built-in circulation fan 2 and a guide device 3 and a muffle 4 set up on stand 1, which is sealed with a sand seal 5. Metal band bundles 6, 7, 8 are arranged under the muffle 4. The muffle 4 is covered with a heating hood 9 from above, which is provided with gas burners 10 and a discharge system II for combustion products.

The bundles 6, 7, 8 of the metal strip lying one above the other form a stack which is covered by a screen 12 from above. Convection rings 13, 14, 15 are arranged between the collars 6, 7, 8, each of which is designed in the form of hollow radially extending support ribs 16 (FIGS. 2, 3) which are connected to one another by webs 17 located between the ribs. The height of the webs 17 is less than the height h of the ribs 16, which does not exceed 0.15 to 0.4 of their length. The ribs 16 are closed on the inside and open on the outside, their length exceeding the length of the webs 17 and the protruding end parts of the ribs I6 on the outside forming grooves 18 together with the webs 17. In the preferred embodiment variant of the invention, the length of the webs 17 is 0.5 to 0.9 of the length of the supporting ribs 16. Each rib 16 has openings 19 on the upper and lower horizontal surfaces, through which the end faces of the bundles 6 are heated , 7, 8 intensified and the weight of the rings 13, 14, 15 is reduced. The webs 17 are arranged relative to the ribs 16 so that they are at the same distance from the upper and lower horizontal surfaces of the ribs 16 and, together with the end faces of the bundles 6, 7, 8, spaces 20 for the flow of the protective gas and for heat transfer to the central area of the bundles 6, 7, 8 form by convection. The webs 17 and supporting ribs 17 lying between the ribs are connected to one another in such a way that they have a ring construction with a continuous inner channel 21 for the flow of the protective gas form.

berechnet.Before starting the heat treatment of bundles 6, 7, 8 in the hood furnace, the minimum gap between bundles 6, 7, 8 is determined according to the formula

calculated.

The convection rings I3, I4, 15 of the required height are selected in accordance with the calculated size of the space.

The convection ring 13, on which the collar 6 is arranged, is placed on the support plate of the guide apparatus 3. The convection ring I4 is arranged on the upper end face of the collar 6, the collar 7 etc. on its upper end face. The bundle stack can contain 2 to 4 bundles depending on the height of bundles 6, 7, 8 and the muffle 4. After the stack of bundles 6, 7, 8 has been built up, it is covered with the muffle 4, under which a protective gas, for example a hydrogen-nitrogen mixture (4% H ₂ + 96% N ₂ ), is fed. The muffle 4 is covered with the heating hood 9 from above and the burner 10 is ignited. The heat of combustion is transferred to the muffle 4 and from there to the protective gas (by convection) and to the side surfaces of the collars 6, 7, 8 (by radiation). Under the action of the circulation fan 2, the protective gas flows through the convection rings 13, 14, 15 and gives off the heat to the end faces of the bundles 6, 7, 8 by convection (dashed arrows in FIG. 1).

The radiation passes from the muffle 4 into the interior of the convection ring I3, I4, 15 (solid arrows in FIG. 1) and through the grooves I8 and the opening I9 (FIG. 2) onto the end face of the collars 6, 7, 8.

nimmt der Abschirmungseffekt zu und der Strahlungsstrom von der Muffel 4 wird kleiner. Bei drei Bunden 6, 7, 8 im Stapel (n = 3) vergrössert sich bei sonstigen gleichen Bedingungen das Volumen des durch den Konvektionsring 13, I4, I5 strömenden Schutzgases und der Koeffizient α₁ der Wärmeabgabe vom Schutzgas an die Stirnfläche des Bundes 6, 7, 8 durch Konvektion und der Gesamtkoeffizient der Wärmeabgabe α_Σ = α₁ + α₂ nehmen etwas zu. In diesem Fall sind die Koeffizienten α₁, α₂ der Wärmeabgabe durch Konvektion und durch Strahlung entsprechend bei einem relativen Zwischenraum von 0,175 gleich.From the diagram shown in Fig. 4 it can be seen that with a relative size of the Zwi space of O, I5 the coefficients α ₁ , α _{2 of} the heat given off by convection and by radiation are the same for the number of bundles 6, 7, 8 in the stack n = 4. If there is a gap (height h of rings 13, I4, I5) below

the shielding effect increases and the radiation current from the muffle 4 becomes smaller. With three bundles 6, 7, 8 in the stack (n = 3), the volume of the protective gas flowing through the convection ring 13, I4, I5 and the coefficient α _{1 of} the heat emission from the protective gas to the end face of the collar 6 increase under other identical conditions. 7, 8 by convection and the total coefficient of heat emission α _Σ = α ₁ + α ₂ increase somewhat. In this case, the coefficients α ₁ , α _{2 of} the heat dissipation by convection and by radiation are the same with a relative space of 0.175.

Industrial applicability

The present invention can be applied to heat. handling of coils made of cold-rolled, tightly wound thin strips and strips of steel of various types as well as of non-ferrous metals and alloys.

Claims (6)

1. Process for the heat treatment of metal strip in coil form in a hood furnace, which includes the stacking of the bundles with gaps, the heating of the bundles and their cooling under the muffle with forced circulation of a protective gas, characterized in that the minimum gap between the bundles to be stacked starts is chosen on the condition that the heat flows, which are transmitted from the protective gas by convection and from the muffle by radiation, are essentially the same. 2. The method according to claim 1, characterized in that the minimum space between the bundles in one area

is chosen in which. D and d - the outside and inside diameters of the bundles are corresponding. 3. hood furnace for performing the method for heat treatment of metal strip in coil form according to claim 1, which contains a stand (1) with a built-in circulation fan (2) on which under a muffle (4) and a heating hood (9) bundles (6, 7, 8) of the metal strip, which are separated by convection rings (13, 14, 15), each of which is provided with radially extending support ribs (16) and hollow webs (17) between the ribs of a smaller height than the support ribs. , characterized in that the height (h) of the radially extending support ribs (16) is 0.15 to 0.4 of their length. 4. hood furnace according to claim 3, characterized in that the radially extending support rings (16) are hollow, closed on the inside and open on the outside of the ring (13, 14, I5). 5. hood furnace according to claims 3, 4, characterized in that the length of the ribs (I6) is greater than the length of the webs (I7) lying between the ribs, the outer projecting end parts of the ribs (16) together with the webs ( I7) Form grooves (I8). 6. hood furnace according to claim 5, characterized in that the webs lying between the ribs (I7) have a length of 0.5 to 0.9 of the length of the support ribs (I6).

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