WO2002072893A1

WO2002072893A1 - Method for heating slabs and system for carrying out said method

Info

Publication number: WO2002072893A1
Application number: PCT/FR2002/000515
Authority: WO
Inventors: Jean-Paul Baumal; Jérôme Muller; Armand Danda
Original assignee: Usinor
Priority date: 2001-03-12
Filing date: 2002-02-12
Publication date: 2002-09-19
Also published as: FR2821774B1; FR2821774A1

Abstract

The invention relates to a method for heating slabs in a system (6) comprising an enclosed space (2) provided with heating means having a first series of longitudinal support means (7) for the slab (4) disposed at the entrance thereof, followed by a second series of longitudinal support means (8) for the slab (4) which are transversally offset in relation to the first series of support means (7). The inventive method comprises the following steps: the slab (4) is introduced into the heated enclosed space (2) on the first series of support means (7), then the slab (4) is displaced in the enclosed space (2) until it reaches the second support means (8), whereupon the slab (4) is transferred onto the second series of support means (8) which are offset, whereupon it is displaced further until it reaches the exit of the enclosed space and where, prior to transfer, the zones of the lower surface of the slab (4) which are to be brought into contact with the second support means (8) are cooled down in order to avoid the formation of defects. The invention also relates to a system for carrying out (6) said method.

Description

Slab reheating process and installation for implementation

The present invention relates to a method for reheating slabs and an installation for implementing this method, more particularly intended for reheating steel slabs prior to the hot rolling operation.

In the steel sheet production cycle, the steel from the production is poured in the form of parallelepipedal blocks which are called slabs. These slabs have a large thickness and, in order to reduce them in order to obtain sheets of lesser thickness, a hot rolling operation is carried out which requires that the slab be previously heated in an oven.

In a manner known per se, the slabs are heated in horizontal continuous ovens comprising, at one end, an opening intended to introduce the cold slabs into the oven, and, at the other end, an opening intended to take out the hot oven slabs. The first end is called "loading side" and the second end is called "diverting side".

The oven itself is functionally divided, according to its length, into two zones which, starting from the charging side to go to the charging side, are a heating zone and an equalization zone.

In the heating zone, the slabs are gradually brought to an average temperature which is close to the target temperature for rolling, but the lower and upper skins of the slabs are significantly hotter than the core.

In the equalization zone, the heating is continued with the main aim of homogenizing the temperature in the thickness of the slab without increasing the skin temperature.

During the passage of the furnace, the slabs are supported by beams. The longitudinal members are beams extending along the length of the furnace and arranged parallel to each other.

In known manner, the longitudinal members consist of a tube traversed by a circulation of cooling water, the tube comprising, along the generator intended to receive the slabs, either a slide or pads of refractory steel.

In the so-called pushing ovens, all the side members are fixed, the slabs forming a continuous bed which advances each time a new slab is introduced. by pushing it horizontally. Thus, the slabs slide on the rails of the side members.

In the so-called mobile beam ovens, some beams are fixed and have only a support function, other beams are mobile and are intended to ensure the movement of the slabs. The mobile longitudinal members are driven in a cyclic movement in which they remain parallel to themselves. This movement comprises a first phase during which they are raised so as to raise the slabs, a second phase during which they are moved horizontally so as to advance the slabs, a third phase where they are lowered so as to deposit the slabs on the fixed beams a little further in the oven, and a final phase which brings them back to their initial position.

In general, the fixed and mobile beams are arranged alternately according to the width of the oven. In all cases, the side members which are cooled disturb the heating of the lower skin of the slabs by a thermal conduction effect and by a shadow effect. These disturbances generate temperature irregularities which, on rolling, result in thickness irregularities on the strips obtained. In order to remedy this drawback, it has been proposed to use ovens comprising two series of beams, a first in the heating zone and a second, offset transversely with respect to the first, in the equalization zone. These ovens are called offset beam ovens.

In these ovens, the areas of the slab which are in contact with the beams in the heating area are no longer in the equalization area. This results in a significant improvement in the temperature uniformity of the slab, which very significantly attenuates the thickness irregularities observed on the strips. Some ovens are even provided with three series of beams, offset from one another to further reduce heterogeneity. However, it was then found that the areas of the lower surface of the slabs which are brought into contact with the studs or the slides of the second series of beams, or even of the third, are very often deteriorated by the subsequent appearance of defects which may be internal, such as cracks, or which may be external, such as surface defects. The object of the invention was therefore to provide a method and an installation for reheating slabs making it possible to obtain reheated slabs which do not have excessive thermal heterogeneities and which also do not have internal and / or external faults. To this end, a first object of the invention is constituted by a method of reheating slabs in an installation comprising an enclosure equipped with heating means at the entrance of which is a first series of longitudinal means for supporting the slab. , followed by a second series of longitudinal support means for the slab offset transversely with respect to said first series of support means, said method comprising the steps consisting in:

- Introducing a slab into said heated enclosure, on said first series of support means, then - moving the slab in the enclosure until reaching said second series of support means, then

- Transfer the slab onto said second series of offset support means, then continue to move it until it leaves the enclosure, characterized in that, before said transfer, the areas of the underside of the slab are cooled. which will be brought into contact with the second series of offset support means, so as to avoid the formation of defects.

In a preferred embodiment, the cooling is carried out by placing means forming a heat shield in the enclosure, near the underside of the slabs and upstream of the second series of support means.

In another preferred embodiment, the support means of the second series consist of longitudinal members provided on their upper faces with studs or slides and the means forming a heat shield are the extensions of these longitudinal members devoid of studs or slides.

In another preferred embodiment, the heated slab is made of austenitic stainless steel and the areas of the underside of the slab are cooled until their temperature is less than or equal to approximately 1200 ° C. A second object of the invention consists of an installation for implementing the method according to the invention in these different variants, said installation comprising an enclosure, means for heating the enclosure, means for moving and transferring the slabs through the enclosure which comprises at its entry a first series of longitudinal means of support or displacement of the slab followed by a second series of longitudinal means of support or displacement offset transversely with respect to said first series, characterized in that it further comprises means for cooling the underside of the slabs placed in the extension of the second series of support or displacement means, below the level of the first series of support or displacement means, the cooling means and the first series of support or displacement means partially overlapping over a length L In a preferred embodiment, the covering length L represents at least 8% of the length of the enclosure.

In another preferred embodiment, the overlap length L represents at most 12% of the length of the enclosure.

In another preferred embodiment, said longitudinal support means of the first and second series are hollow longitudinal members substantially parallel to each other and provided on their upper faces with solid studs made of refractory material, and said cooling means are extensions side members of the second series of support means, these extensions being devoid of studs. In another preferred embodiment, the side members of the second series of support means comprise a first zone devoid of studs forming said cooling means, followed by a second zone provided with studs and a third zone provided with studs, the second zone having a greater quantity of studs per unit of length than that of the third zone.

Other advantages of the invention will appear better during the detailed description of a particular embodiment, made with reference to the accompanying drawings in which: - Figure 1 shows a schematic top view of an oven of the prior art,

FIG. 2 represents the thermal profile of half of the underside of a slab subjected to a process of the prior art,

FIG. 3 represents an embodiment of an installation according to the invention,

FIG. 4 represents the thermal profile of half of the underside of a slab subjected to the method according to the invention,

- Figure 5 shows the temperature difference ΔT of the underside of a slab subjected to the method according to the invention, compared to a slab subjected to reheating according to the prior art, as a function of the length L of the overlap area. The invention being directly linked to the arrangement of the side members, in order to make it clear, we will first recall what is the arrangement of the side members in an oven with offset side members, whether the side members are fixed or mobile. If we consider Figure 1 which shows an oven with movable beams 1 of the prior art, we see that it consists of an enclosure 2 divided into two zones A and B. Zone A located on the charging side of the slab comprises a first series of longitudinal support means which take the form of hollow longitudinal members 3 of refractory steel arranged parallel to each other. These beams 3 are crossed by a stream of cooling water and carry cylindrical studs of refractory steel regularly spaced (not shown). The slab 4 is placed on the beams 3 and advances by displacement of the beams.

Zone B downstream of zone A comprises longitudinal members 5 identical to the longitudinal members 3 but offset transversely relative to the latter.

Zone A is here placed in the furnace heating zone, as well as the start of zone B, equalization taking place after the transfer, in the end of zone B. In ovens with three series of offset beams, the transfer of the first series of beams to the second generally takes place in the heating zone and the transfer of the second series of beams to the third can take place in the heating zone or in the equalization zone.

FIG. 2 is a curve of the thermal profile of half of the lower surface of a slab circulating in zone A of such an oven before its transfer to zone B. The curve indicates the skin temperature of the points included in a half of the slab according to their position in the width of the LF oven. This curve shows significant variations in temperature which oscillates, in this example, between 1100 ° C and 1245 ° C. The low points correspond to the zones which have been in contact with the first series of beams 3 and which will be exposed to thermal radiation after the transfer to the beams 5, while the high points correspond to the zones fully exposed to thermal radiation and which are brought into contact with the studs of the side members 5, at the time of transfer.

If we now consider Figure 3 which represents an installation according to the invention, we can see that it consists of an enclosure 2 identical to that of the device of the prior art, divided into three zones A ', B ¹ and C. Zone A 'located on the charging side and the zone B' which follows it, correspond to zone A in FIG. 1. Zone C, located on the charging side, corresponds to zone B of the figurel.

The oven comprises two series of beams 7 and 8. The first series of beams 7 (fixed or mobile) extends along the zones A 'and B', said beams being intended in a known manner to receive the slabs.

The second series of beams 8, fixed or mobile, is offset from the first series and extends along the zones B 'and C. These beams 8 are divided into two parts, a part 8a extending along zone C, comprising studs or slides and intended to receive slabs, and a part 8b, extending along zone B ′, not comprising studs or slides. This part 8b is never in contact with the slabs. Its function is to create a shadow zone which generates a cooling of the lower skin of the slabs before it comes into contact with the part 8a of the slides.

With this arrangement and provided that the area B ′ has an adapted length, the inventors have found that the surface defects were eliminated completely or partially. For this to be so, it suffices that the skin temperature of the slabs in the zone which will come into contact with the side members of zone C of the furnace is sufficiently low, without being too low. This condition depends on the nature of the metal from which the slabs are made.

In the case of austenitic stainless steel, this maximum temperature is approximately 1200 ° C. In the general case, a person skilled in the art knows how to determine this temperature, for example by tests. To obtain the result, the length of the part 8b of the longitudinal members must be chosen as a function of the characteristics of the furnace, which the person skilled in the art knows how to do, by tests or by using simulation models, known in themselves.

The overlapping zone B ′ located after the zone A ′ comprises the second part of the longitudinal members 7 provided with studs, but also cooling means which take the form of longitudinal members 8 devoid of studs, located below the level of the longitudinal members 7 and offset transversely relative to them. This overlap zone has a length L less than the total length of each spar 7.

Zone C located after zone B 'only includes the extension of the side members 8 but which are provided with studs (not shown).

The number of studs per unit of length is greater at the start of this zone than at the end, because this is where the lower surface of the slab will first come into contact with the studs of the longitudinal members 8. This is so at that time that the temperature difference is greatest between the bottom surface of the slab 4 and the studs. By increasing their density, the pressure exerted on this surface is therefore reduced when it is the most fragile, thus avoiding damaging it. The zones A 'and B' are here located in the heating zone of the furnace, while the zone C is situated in the equalization zone, but one could perfectly conceive that these three zones A ', B' and C are distributed differently from the heating and equalization zones.

It can therefore be seen that in the overlap zone B ′, there is no direct contact between the longitudinal members 8 and the slab 4. This part of the longitudinal members 8 without studs makes it possible to obtain cooling of the lower surface of the slab 4 by a shadow effect.

Figure 4 shows, by way of example, three superimposed thermal profiles. Profile a corresponds to the profile of Figure 2, obtained by applying the teaching of the prior art. By comparison, the profile b corresponds to the profile obtained using the device of FIG. 3 for an overlap area 2 meters long and the profile c for an overlap area 3 meters long. These profiles are measured each time just before the transfer of the slab 4 from the first series of beams 7 to the second series of beams 8.

The treated slab is made of an austenitic stainless steel identical to that used to trace the profile of FIG. 2.

The difference E2m existing between the profiles a and b in the zones to be brought into contact with the longitudinal members 8 provided with studs has been shown by means of an arrow and the difference E3m existing between the profiles a and c in the same areas. It can be seen that the device according to the invention makes it possible to lower the temperature of these zones by 45 ° C. for a covering length of 2 m and by 58 ° C. for a covering length of 3 m.

The enclosure 2 has, in this specific example, a length of 30 meters, and it is also measured that a covering length of 2.40 m, or 8% of the length of the enclosure, makes it possible to pass between- below the defect formation temperature which is 1200 ° C for the austenitic stainless steel used.

FIG. 5 shows, by way of illustration, the curve connecting the temperature difference of the lower surface of a slab subjected to the method according to the invention with respect to a slab reheated in a conventional oven, as a function of the length of the overlap area. It should be noted that the lower surface of the slab 4 should not be cooled too much, as this would cause cooling at the heart of this slab, thus creating thermal heterogeneity going against the object of the present invention.

This is why the length of the overlap zone is limited. In the particular case of austenitic stainless steel used by way of example, the length of the overlap zone must not exceed 12% of the length of the enclosure.

It goes without saying that the device and the method according to the invention are not limited to this type of steels or metals and that the intervals of the covering lengths will have to be adapted by the person skilled in the art to the materials treated. Furthermore, the length of this overlap area also varies depending on the furnace to be fitted and must be determined taking into account the geometry of the furnace, the slab support system, the distance at which the longitudinal beam offset is placed and operating conditions of the oven.

By taking into account these various constituent elements of the furnace, as well as the temperature of formation of defects of the steel or of the metal to be heated, the person skilled in the art is able to determine the range of covering length necessary for the correct carrying out the method according to the invention.

The process according to the invention, the essential characteristic of which consists in cooling the areas of the underside of the slab which will be brought into contact with the second series of offset support means is preferably implemented using an installation according to the invention but could of course be implemented by means of any suitable installation making it possible to obtain sufficient local cooling to avoid the formation of cracks and / or appearance defects.

In the case of ovens comprising several series of offset beams, thus involving several transfers of the slab to be heated, it is also possible to implement the invention for each of these transfers.

Claims

1. Method for reheating slabs in an installation (6) comprising an enclosure (2) equipped with heating means, at the entrance of which is a first series of longitudinal support means (7) for the slab (4) , followed by a second series of longitudinal support means (8) of the slab (4) offset transversely with respect to said first series of support means (7), said method comprising the steps consisting in:

- introduce a slab (4) into said heated enclosure (2), on said first series of support means (7), then - move the slab (4) in the enclosure (2) until reaching said second series of support means (8), then

- transfer the slab (4) to said second series of offset support means (8), then continue to move it until the outlet of the enclosure (2) characterized in that, before said transfer, cooling is carried out areas of the underside of the slab (4) which will be brought into contact with the second series of offset support means (8), so as to avoid the formation of defects.

2. Method according to claim 1, characterized in that the cooling is carried out by having means forming a heat shield in the enclosure (2), near the underside of the slabs (4) and upstream of the second series of support means (8).

3. Method according to claim 2, characterized in that the support means of the second series consist of beams (8) provided on their upper faces with studs or slides and in that the means forming a heat shield are the extensions of these beams (8) devoid of studs or slides.

4. Method according to any one of claims 1 to 3, characterized in that the heated slab (4) is made of austenitic stainless steel and in that the said zones of the underside of the slab (4) are cooled to 'their temperature is less than or equal to about 1200 ° C.

5. Installation (6) for implementing the method according to any one of claims 1 to 4 comprising an enclosure (2), means for heating the enclosure (2), means for moving and transferring the slabs (4) through the enclosure (2) which comprises at its entry a first series of longitudinal means for supporting or moving the slab (7) followed by a second series of longitudinal support or displacement means (8) offset transversely relative to said first series (7), characterized in that it also comprises means for cooling the face bottom of the slabs (4) placed in the extension of the second series of support or displacement means (8), below the level of the first series of support or displacement means (7), the cooling means and the first series of support or displacement means (7) partially overlapping over a length L.

6. Installation according to claim 5, characterized in that the covering length L represents at least 8% of the length of the enclosure (2).

7. Installation according to claim 6, further characterized in that the overlap length L represents at most 12% of the length of the enclosure (2). 8. Installation according to any one of claims 5 to 7, characterized in that said longitudinal support means of the first and second series (7,

8) are hollow longitudinal members substantially parallel to each other and provided on their upper faces with solid studs made of refractory material, and in that said cooling means are extensions of the longitudinal members of the second series of support means (8), these extensions being devoid of studs.

9. Installation according to claim 8, characterized in that the side members of the second series of support means (8) comprise a first zone devoid of studs forming said cooling means, followed by a second zone provided with studs and a third zone provided with studs, the second zone having a quantity of studs per unit of length greater than that of the third zone.