Method, Device and System for the Heat Treatment of a Moving Metal Strip

Abstract

The invention relates to a method for the heat treatment of a running metal strip, which comprises: a heating step for heating the strip; a step for cooling the heated strip; and conductive heat transfer from at least one segment of the strip which is being cooled to at least one segment of the strip which is being heated, so that at least part of each of said cooling and heating steps is carried out on the strip. The invention also relates to a heat transmission device for implementing said method and having at least one thermally conductive solid element, such as for example a roll, and to a system for the heat treatment of a running metal strip that incorporates such a device.

Description

The present invention relates to a method, device and system for the heat treatment of a moving metal strip.

In the metallurgical field, it is generally known by persons skilled in the art how to heat treat metal strips in order for example to modify the crystalline structure of the metal to improve its mechanical or other characteristics. Particularly advantageously, such a method can be effected continuously, by circulating the metal strip through a plurality of zones at different temperatures. This makes it possible for example to integrate the heat treatment of the metal strip in a continuous production line, with certain advantages of economic efficiency.

One type of heat treatment method is the one known as annealing. In an annealing method the metal is heated in order to reach temperatures ranging for example from 500° C. to 1100° C. and then cooled in order to modify the crystalline structure of the metal. One drawback of such a method, as well as other heat treatment methods, is its high consumption of energy. In steel production, it is frequently necessary to anneal sheets after prior cooling, for example in the case of cold rolling. In conventional continuous annealing installations, the heating of the sheet is obtained by passing it in front of radiant tubes in which fume gases circulate coming from the combustion of a fuel and air. In these installations, provision has already been made for recovering heat from the fumes leaving the radiant tubes in order to preheat the combustion air. However, given the losses of heat through the fumes and the leakages in the chamber of the annealing installation, the heat consumed is, despite this recovery, around 1.7 times the heat found in the metal sheet, which corresponds to an efficiency of 60%.

Typically, for annealing at 800° C., 50 kg of CO2/t of steel is produced, if the fuel gas is methane. Given that, after the thermal cycle, the temperature of the steel returns to its initial temperature, that is to say the one before annealing, the heat consumed is situated entirely in the atmosphere, and/or in the cooling agent.

Though insulating the hot parts of the installation and improving the efficiency of the fume recuperators makes it possible to improve the global efficiency, it is extremely difficult to radically reduce the energy consumption without affecting the very basis of the heating and cooling system.

Provision has also been made to improve the efficiency of the cooling of steel objects, such as tubes, subjected to continuous annealing and then cooling in several steps. To do this, the cooling gas is blown in a cascade onto the tubes, from one cooling step to the previous one, as described in the international patent application WO 00/25076, This method, although efficient in theory, cannot be put into industrial practice on sheet-metal annealing lines with a high heating capacity, more than around 40 t/hour. It is in fact impossible to effectively collect the gas flows heated and cooled successively in the various sections of the cascade.

For this reason, it has also been proposed to use regenerative methods, where at least some of the heat removed from the metal strip during cooling thereof is used for preheating. Such a regenerative method, described in the international application WO 2004/063402 and forming the closest prior art, comprises heating of the strip, cooling of the heated strip, and transfer of heat from at least one segment of the strip being cooled to at least one segment of the strip being heated, so as to effect at least some of each of the said cooling and heating of the strip.

However, in this prior art, the said heat transfer is effected by circulation of a heat-transfer gas. This has the drawbacks of offering only a reduced heat transfer rate and requiring a supplementary addition of energy in order to actuate the circulation of the heat-transfer gas. By increasing the rate of circulation of the gas beyond a certain point, any gain in transfer of heat is more than compensated for by the additional work necessary for making the gas circulate more quickly.

The problem to be solved is therefore the reduction in the energy consumption in a method of heat treatment of a moving metal strip.

In the present invention, this problem is solved by effecting the said transfer of heat mainly by conduction. In this way, the heat is transmitted very effectively without requiring a high supplementary addition of energy in the form of work. The transfer heat by conduction is the most effective form of heat transfer.

Preferably, the said heat transfer is effected from a plurality of segments of the strip being heated to a plurality of segments of the strip being cooled in reverse order in the direction of travel of the strip. In this way it is possible to effect a high transfer of heat while maintaining moderate temperature gradients, and therefore avoiding internal tensions and deformations in the metal strip.

Preferably, in the said step of heating the strip, the strip is also heated by a source of heat external to the strip. In this way a thermal differential serving to impel the said heat transfer is created between the strip being cooled and the strip being heated.

Preferably, the said heat transfer is effected by means of at least one heat-conducting solid element in contact with a segment of the strip being heated and a segment of the strip being cooled. In this way, the heat conduction between the segment of the strip being heated and the segment of the strip being cooled is provided by the said solid element.

Preferably, the said at least one heat-conducting solid element is in the form of a roller, preferably metal. Such a roller can provide continuous contact, and therefore good conduction of heat, with the two segments of the moving strip.

Preferably, the segment of the strip being cooled is in contact with the said roller at a contact angle of at least 20′, preferably at least 30°. With such a contact angle, it is possible to offer a good contact surface between roller and strip, and therefore good heat transfer.

Preferably, the segment of the strip being heated is in contact with the said roller at a contact angle of at least 20°, preferably at least 30°.

Preferably, the temperature difference between a segment of metal strip being cooled and a segment of strip being heated between which at least some of the said heat transfer takes place by conduction is at least 200° C. and/or below 500° C. Such a temperature difference would allow effective heat transfer, without causing excessive thermal shock in the metal strip.

The present invention also relates to a heat transmission device for simultaneously heating a moving metal strip upstream of an additional heating zone and cooling it downstream of the said additional heating zone. In order to effect an efficient transfer of heat without requiring significant addition of energy in the form of work, the device comprises at least one heat-conducting solid element intended to be in contact with the said metal strip both upstream and downstream of the main heating zone, so as to transfer heat by conduction between at least one segment of the metal strip downstream and at least one segment of the metal strip upstream.

Preferably, the device comprises a series of several solid heat conducting elements, for example five, for successively contacting the said metal strip both upstream and, in reverse order in the direction of travel of the strip, downstream of the main heating zone, so as to transfer heat by conduction between segments of the metal strip downstream and segments of the metal strip upstream. In this way it is possible to provide progressive heating of the strip being heated and just as progressive cooling of the strip being cooled, in order to avoid thermal shocks while ensuring significant transfer of heat.

Preferably, the device also comprises at least one deflector roller in order to define a contact angle, preferably at least 20°, between the said metal strip upstream and/or downstream of the furnace and the said heat-conducting solid element in the form of a roller.

The present invention also relates to a system for the continuous heat treatment, in particular annealing, of a moving metal strip comprising a main heating zone and a heat-transmission device according to the invention.

Details concerning the invention are described below, illustratively, but not restrictively, referring to the drawings.

FIG. 1 shows a diagram of a prior method,

FIG. 2 shows a diagram of a method according to an embodiment of the invention,

FIG. 3 shows a heat treatment system according to an embodiment of the invention,

FIG. 4 shows a heat treatment system according to an alternative embodiment of the invention,

FIG. 5 shows a heat transmission device according to an embodiment of the invention, and

FIG. 6 shows curves for heating and cooling the metal strip that can be obtained with the heat transmission device of FIG. 5.

In FIG. 1, a conventional method for the continuous annealing of a moving steel strip is illustrated schematically. After cleaning 1 of the strips, the latter is heated from 30° C. to 800° C. in a heating step 2 in a radiant tube furnace. This specifies an energy addition Q of 210 kW per tonne of steel, in the form of natural gas, producing by its combustion 50 kg of CO2 and 80 mg of NOx per tonne of steel.

Next, in order to discharge a heat Q′ in the cooling 3 of the strip to 450° C., 2 m3 of water per tonne of steel is specified, as well as a supplementary addition of energy W in the form of 20 kW of electricity per tonne of steel in order to circulate the cooling fluid or fluids.

The cost and the environmental impact of this conventional method are therefore not insignificant.

In FIG. 2, an embodiment of the method of the present invention is shown schematically. As in the conventional method in FIG. 1, the steel strip is heated after cleaning 1. However, in this method, the heating is divided into a preheating step 2a in which the steel strip is preheated from 30° C. to 450° C., and a main heating step 2b in a radiant-tube furnace, in which the strip is heated from 450° C. to 800° C. The heat Q′ transferred to the strip in the preheating strip 2a comes from the cooling 3 of the same strip from 800° C. to 450° C. and is transmitted by conduction. With this method, an energy addition Q′ in the radiant-tube furnace suffices, reducing the consumption of natural gas to an equivalent of 120 kW per tonne of steel, in this way producing only 30 kg of CO2 and 45 mg of NOx per tonne of steel. Moreover, the cooling can take place without consumption of water and without the need to carry out work for circulating a cooling fluid. The cost and environmental impact of this method according to the invention are therefore substantially less.

FIG. 3 shows a system 4 for the continuous annealing of a moving steel strip 5, according to one embodiment of the invention. This system 4 comprising a device 6 for transmitting heat by conduction for the preheating 2a and cooling 3 of the strip 5, and a furnace 7 with radiant tubes 8 for the additional heating 2b of the strip 5. In the embodiment shown in this FIG. 3, the furnace 7 with radiant tubes 8 is of the vertical type. However, the alternative of a horizontal arrangement of the furnace 7 with radiant tubes 8 can also be considered, as illustrated in FIG. 4.

The heat-transmission device 6 is illustrated in greater detail in FIG. 5. The strip 5 enters the device 6 through the entry opening 9 and passes through the said device 6 in the direction 10 as far as the furnace 7 while being preheated. After the main heating 2b, the strip 5 leaves the furnace again and passes through the device 6 in the opposite direction 11 as far as the exit opening 12 while cooling.

The device 6 comprises an alignment of seven heat-conducting rollers 6 and two alignments of six deflector rollers 14, one on each side of the alignment of conductive rollers 13. In the embodiment illustrated, both the conductive rollers 13 and the deflector rollers 14 have a diameter of 800 mm. However, alternating diameters for each roller, as well as arrangements with different provisions and numbers of rollers could be envisaged by a person skilled in the art according to circumstances. The conductive rollers 13 must have a diameter capable of providing a good contact surface with the strip 5 with a relatively low speed of rotation, while avoiding a plastic deformation of the strip 5. The deflector rollers 14 must also have a diameter avoiding plastic deformation of the strip 5. According to the geometric and mechanical parameters of the strip 5, the conductive 13 and deflector 14 rollers may therefore have diameters situated, for example, in a range between 400 and 1600 mm.

Because of the thermal expansion of the strip 5, the speed of the strip 5 during cooling is normally greater than its speed during heating. Several solutions can be envisaged by persons skilled in the art for preventing partial slipping of the band 5 on a conductive roller 13. For example, the conductive roller 13 could have an angularly variable radius making it possible to adjust the effective radius of the conductive roller 13 to the speed of the strip 5 on each side of the conductive roller 13. Another solution that can be envisaged is that the conductive roller 13 is divided into radial segments, having a certain freedom of angular movement relative to one another.

When a strip 5 passes through the device 6 both during preheating and during cooling, the deflector rollers 14 hold the segments 5a of the strip 5 during preheating and the segments 5b of the strip 5 during cooling simultaneously in contact with the conductive rollers 13 at contact angles α. Different contact angles α can be envisaged by persons skilled in the art according to circumstances. Each conductive roller 13 thus transfers heat by conduction from a segment 5b of the strip 5 during cooling to a segment 5a of the strip during heating. As the strip 5 passes through the device 6 in opposite directions 10, 11 during preheating and during cooling, the strip 5 contacts the conductive rollers 13 in reverse order in its travel during heating and during cooling. This conduction of heat will therefore be effected between the last segment 5b of the strip 5 during cooling and the first segment 5a of the strip 5 during preheating, between the penultimate segment 5b of the strip 5 during cooling and the second segment 5a of the strip 5 during preheating, and so on. In this way, the temperatures of the strip 5 during cooling and during preheating follow respectively the curves 15 and 16 along the device 6, as illustrated in FIG. 6. As can be appreciated in this figure, this configuration allows progressive preheating and cooling of the strip 5. The stages 17 correspond to the temperatures of the conductive rollers 13, each of them being intermediate to those of the segments 5a and 5b with which the respective conductive roller 13 is in contact.

Table 1 presents the parameters of an embodiment of the heat treatment method of the invention in the device 6 described above with a strip 5 with a thickness of 1 mm, a width of 1500 mm and a travel speed of 150 m/min for a production of 106 tonnes per hour.

Although the present invention has been described with reference to specific example embodiments, it is obvious that various modifications and changes can be made to these examples without departing from the general scope of the invention as defined by the claims. Consequently, the description and drawings must be considered in an illustrative rather than restrictive sense.

REFERENCES OF THE FIGURES

• 1 Cleaning
• 2 Heating
• 2a Preheating
• 2b Main heating
• 3 Cooling
• 4 Continuous annealing system
• 5 Steel strip
• 5a Segment of strip during preheating
• 5b Segment of strip during cooling
• 6 Heat-transmission device
• 7 Radiant-tube furnace
• 8 Radiant tubes
• 9 Entry opening
• 10 Direction of travel of strip during preheating
• 11 Direction of travel of strip during cooling
• 12 Exit opening
• 13 Conductive roller
• 14 Deflector roller
• 15 Preheating curve
• 16 Cooling curve
• 17 Temperatures of conductive rollers

TABLE 1
Operating parameters of the device 6
Position of conductive roller 13	1st	2nd	3rd	4th	5th	6th	7th
Contact angle α with segment 5a [°]	31.4	62.9	62.9	62.9	62.9	62.9	31.4
Coefficient of contact with segment 5a [%]	16.3	16.3	16.3	16.3	16.3	16.3	16.3
Contact surface with segment 5a [m2]	0.0358	0.0716	0.0716	0.0716	0.0716	0.0716	0.0358
Contact angle α with segment 5b [°]	35.7	71.5	71.5	71.5	71.5	71.5	35.7
Coefficient of contact with segment 5b [%]	16.3	16.3	16.3	16.3	16.3	16.3	16.3
Contact surface with segment 5b [m2]	0.0407	0.0813	0.0813	0.0813	0.0813	0.0813	0.0407
Initial temperature of segment 5a [° C.]	30	64.3	138.1	210.6	286.3	353.8	415.4
Exit temperature of segment 5a [° C.]	64.3	138.1	210.6	286.3	353.8	415.4	442.2
Heat transmitted to strip 5 [kcal/h]	425775	914931	898273	880617	840611	805057	363114
Initial temperature of segment 5b [° C.]	482.2	549.7	614.2	675.6	730.4	778.7	800.0
Exit temperature of segment 5b [° C.]	449.8	482.2	549.7	614.2	675.6	730.4	778.7
Heat received from strip 5 [kcal/h]	453947	973283	953595	927897	886571	847638	384988

Claims

1. A method for the heat treatment of a moving metal strip, comprising:

heating of the strip,

cooling of the heated strip, and

transfer of heat from at least one segment of the strip being cooled to at least one segment of the strip being heated, so as to effect at least part of each of the said cooling and heating of the strip,

and characterised in that the said heat transfer takes place mainly by conduction.

2. A method according to claim 1, wherein the said heat transfer is effected from a plurality of segments of the strip being heated to a plurality of segments of the strip being cooled in reverse order in the direction of travel of the strip.

3. A method according to claim 1, wherein, in the said step of heating the strip, the strip is also heated by a heat source external to the strip.

4. A method according to claim 1, wherein the said heat transfer is effected by means of at least one heat-conducting solid element in contact with a segment of the strip being heated and a segment of the strip being cooled.

5. A method according to claim 4, wherein the said at least one heat-conductive solid element is in the form of a roller, preferably metal.

6. A method according to claim 5, wherein the segment of the strip being cooled is in contact with the said roller at a contact angle of at least 20°, preferably at least 30°.

7. A method according to claim 5, wherein the segment of the strip being heated is in contact with the said roller at a contact angle of at least 20°, preferably at least 30°.

8. A method according to claim 1, wherein the difference in temperature between a segment of the metal strip being cooled and a segment of the strip being heated between which at least part of the heat transfer takes place by conduction is at least 200° C.

9. A method according to claim 1, wherein the difference in temperature between a segment of the metal strip being cooled and a segment of the strip being heated between which at least part of the said heat transfer takes place by conduction is below 500° C.

10. A heat transmission device for simultaneously heating a moving metal strip upstream of a main heating zone and cooling it downstream of the said main heating zone, characterised in that it comprises at least one heat-conductive solid element that is in contact with the said metal strip both upstream and downstream of the main heating zone, so as to transfer heat by conduction between at least one segment of the metal strip downstream and at least one segment of the metal strip upstream.

11. A device according to claim 10, comprising a series of several heat-conductive solid elements for successively contacting the said metal strip upstream and, in reverse order in the direction of travel of the strip, downstream of the main heating zone, so as to transfer heat by conduction between segments of the metal strip downstream and segments of the metal strip upstream.

12. A device according to claim 10, wherein said at least one heat-conductive solid element is in the form of a roller, preferably metal.

13. A device according to claim 10, also comprising at least one deflector roller in order to define a contact angle between the said metal strip upstream and/or downstream of the main heating zone and the said heat-conductive solid element in the form of a roller.

14. A system for the continuous heat treatment, in particular annealing, of a moving metal strip, comprising a main heating zone and a heat transmission device according to claim 10.

15. A method according to claim 6, wherein the segment of the strip being heated is in contact with the said roller at a contact angle of at least 20°, preferably at least 30°.

16. A device according to claim 11, wherein said at least one heat-conductive solid element is in the form of a roller, preferably metal.

17. A device according to claim 11, also comprising at least one deflector roller in order to define a contact angle between the said metal strip upstream and/or downstream of the main heating zone and the said heat-conductive solid element in the form of a roller.

18. A device according to claim 12, also comprising at least one deflector roller in order to define a contact angle between the said metal strip upstream and/or downstream of the main heating zone and the said heat-conductive solid element in the form of a roller.

19. A system for the continuous heat treatment, in particular annealing, of a moving metal strip, comprising a main heating zone and a heat transmission device according to claim 11.

20. A system for the continuous heat treatment, in particular annealing, of a moving metal strip, comprising a main heating zone and a heat transmission device according to claim 12.