HIGH-SPEED WIRE ROD ROLLING APPARATUS AND METHOD

Abstract

A wire rod rolling plant constituted by three distinct, independent rolling units (10, 20, 30), wherein there are provided four rolling stands, arranged in oval-round-oval-round sequence, in the first two rolling units, and there are provided at least two rolling stands, in oval-round sequence, in the third rolling unit. One motor (12, 22) is provided in the first two rolling units (10, 20) for actuating the two round steps arranged in even positions and one motor (11, 21), independent from the first one, is provided for actuating the two oval steps arranged in odd positions. Two motors (31, 32), mutually independent, which actuate each rolling step respectively are provided in the third rolling unit.

Description

FIELD OF THE INVENTION

The present invention relates to a wire rod hot rolling apparatus and method thereof, and in particular is applied to the so-called fast rolling blocks or finish blocks or Wire-Rod-Blocks, arranged downstream of the rolling mill.

STATE OF THE ART

Wire rod rolling plants are known, comprising a wire rod block which traditionally consists of a sequence of 8-10 rolling stands with pairs of overhangingly mounted rings having rotation axes mutually and alternatively offset by 90° along the pass-line.

In conventional type blocks, the rolling stands are mechanically restrained to one another by one or more motion transmission shafts which, by means of bevel gear speed shifts, are controlled by a single motor or by several motors connected to one another along a mechanical axis. They are normally synchronous, water-cooled motors, with powers in excess from 2500 kW to over 6000 kW.

The disadvantage of such a situation is that the rolling block is rigid from the point of view of speed ratios existing between subsequent rolling rings.

The rolling sequence used in the blocks is of the oval-round type, in which the odd rolling stands (i.e. those in position 1, 3, 5 etc.) reduce the wire rod from round to oval shape, while the even rolling stands (i.e. those in position 2, 4, 6 etc.) reduce the wire rod from oval to round shape. In modern wire rod rolling mills, the wire rod blocks generally consist of a 10-step oval-round sequence, i.e. 5 oval and 5 round in total.

In some specific rolling plants, one or two round-round calibrating rolling stands may be added downstream of the main sequence.

Problems related to changes of diameter of the product may occur in the plants described above.

The system of speed shift, having defined shift ratios, requires that for each change of final diameter of the wire rod corresponding changes must be made to the inlet sections and the calibration process sections of the rings of all the rolling stands of the block, i.e. all the rolling rings must be replaced.

This requires more or less time to change the cylinders in the rolling stands, during which the entire rolling plant is out of service.

In case of these fixed ratio speed shift systems, the diameters of the rolling rings of a subsequent series of grooves must differ from one another by only small values, approximately +/−0.5 mm, otherwise the longitudinal tension, i.e. the draw, on the wire rod being rolled cannot be controlled, and consequently causes rolling product out of tolerance.

An adjustment of the section reduction ratio of the wire rod for each rolling step is not possible because this would imply, in plants of the prior art, the use of a finish block or wire rod block provided with different, appropriately selected shift ratios, e.g. by means of the so-called “change-over gears”. The prior art has attempted to suggest several solutions to this problem. Document U.S. Pat. No. 7,523,632 describes a block having 1 or 2 motors with variable ratios by means of mechanical speed shift.

Given the variety of finished products that it must be possible to produce by means of the wire rod rolling plant, such a solution poses some limits concerning the complication of the mechanical transmission arranged on the wire rod block. These transmissions must have tens of ratios and are extremely costly and cumbersome.

Document EP0875302A2 describes a finish block for wire rod rolling mill having rolling stands with all mutually independent motors which are selected and controlled separately by means of a common process control system. Furthermore, a separate control device of the motors may be arranged upstream of each drive unit. The rolling stands have pairs of rollers or rolling rings arranged in series and provided with screw-down mechanisms and mill guides. The solution suggested in this document implies an extreme complication to the electric part of the plant, given the significant number of motors needed. For example, 10 motors and 10 drives respectively are needed in a 10-step wire rod block. Furthermore, the excessive fractioning of the motors leads to dimensioning the total installed power considerably in excess.

It is thus felt the need to make a wire rod rolling apparatus which allows to overcome the aforesaid drawbacks.

SUMMARY OF THE INVENTION

It is the main object of the present invention to make a wire rod rolling apparatus which allows to vary the reduction percentage of the rolled product by operating on the motor rpm only, regardless of the diameter of the rolling rings and without the aid of mechanical shifts.

Another object of the present invention is to control with extreme accuracy the rolling draw between the rolling stands so as to considerably improve end product tolerances.

Another object of the present invention is to make a rolling apparatus which allows to obtain a high productivity also for small sections of 5.5-6.5/7 mm of diameter or equivalent at a rolling speed equal to 120-150 m/sec and more.

A further object of the invention is to provide a wire rod rolling method by means of the use of the aforesaid apparatus.

The present invention, thus, intends to reach the objects indicated above by making a wire rod rolling apparatus, defining a rolling direction, comprising two or more rolling units, arranged in sequence in the rolling direction, each rolling unit comprising two or more rolling stands and at least one first and one second motor for actuating the two or more rolling stands, each of said rolling stands having a pair of rolling rings with respective either oval or round section gauge, the rolling stands being arranged along the rolling direction so as to alternatively have in succession a rolling stand with oval section gauge followed by a rolling stand with a round section gauge, characterized in that in at least one of said rolling units the respective first motor actuates the rolling stands with oval section gauge and the respective second motor actuates the rolling stands with round section gauge in a manner independent from the first motor.

The object of the invention is thus reached by an arrangement of the rolling stands, in at least one of the rolling units of the apparatus, having separate pass-line drives, i.e. the drive of the oval gauge section rolling stands is separate from that of the round gauge section rolling stands, for each rolling unit which constitutes the wire rod rolling apparatus. In other words, by considering each rolling unit separately, all the oval gauge section rolling stands of such a unit have a same common motor, while all the round gauge section rolling stands have a second common motor, different from the one of the oval gauge section rolling stands.

The invention applies to various arrangements of the rolling units, which by way of non-limiting example, may have a 4+4+2 or 4+4+3 type layout or even a 6+4 or 6+5 type layout.

Advantageously, the apparatus of the invention also allows to vary the global reduction degree of the wire rod in the finish block in a simple, effective manner by means of the simultaneous free selection of the diameter of the rolling rings in the various rolling stands.

These advantages are obtained by appropriately fractioning the single rolling steps in distinct and independent rolling units, each constituted by a given number of rolling stands, e.g. considering a 4+4+2 combination; i.e. a combination where the first rolling unit has four rolling stands, two of which with oval gauge and two with round gauge arranged alternatively; the second unit has four rolling stands, two of which with oval gauge and two with round gauge arranged alternatively; and the third unit has two rolling stands, the first of which with oval gauge and the second with round gauge; each of which rolling units is provided with a first control motor which works on the odd rolling stands, which are generally those with oval gauge, and a second control motor, independent from the first, which works on the even rolling stands, which are generally those with a round gauge.

In this manner, while significantly containing the number of motors with respect to the solutions of the prior art, it is possible to operate on the relative speed of the round gauge rolling rings with respect to the oval gauge rolling rings, varying at will the draw between rolling stands on the wire rod and thus very accurately controlling the dimensional tolerances of the finished product.

BRIEF DESCRIPTION OF THE FIGURES

Further features and advantages of the present invention will be apparent in the light of the detailed description of a preferred, but not exclusive, embodiment of a wire rod rolling apparatus, illustrated by way of non-limitative example, with the aid of the accompanying drawings, wherein:

FIG. 1 shows a kinematic diagram of the rolling apparatus of the invention;

FIG. 2 shows a kinematic diagram of a first variant of the apparatus of the invention;

FIG. 3 shows a kinematic diagram of a second variant of the apparatus of the invention;

FIG. 4 shows a kinematic diagram of a third variant of the apparatus of the invention;

FIG. 5 shows a kinematic diagram of a fourth variant of the apparatus of the invention;

FIG. 6 shows a technological diagram of the rolling apparatus of the invention.

In the various embodiments, the same reference numbers are used to indicate the same components of the apparatus in the various figures.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

In particular, FIG. 1 shows a kinematic diagram of the wire rod rolling apparatus 1 according to the invention, hereinafter also defined as finish block or conventionally called wire rod block.

Three distinct and mutually independent rolling units 10, 20, 30 are provided therein. The first unit 10, considered in the rolling direction, carries out the first four rolling steps, with a rolling sequence in which the gauge section is oval-round-oval-round. Hereinafter, for the sake of brevity, the terms “round or oval” means gauge with round or oval section.

One motor is provided for actuating the two round steps, arranged in even positions, and one motor, independent from the first, is provided to actuate the two oval steps which occupy the odd positions. The second unit 20 carries out four rolling steps with an oval-round-oval-round rolling sequence. One motor is provided for actuating the two round steps, arranged in even positions, and one motor, independent from the first, is provided to actuate the two oval steps which occupy the odd positions. The third unit 30, also called finishing/calibrating unit, carries out at least two rolling steps with an oval-round rolling sequence. Two mutually independent motors respectively actuate each step of this unit. Preferably, the motors are air-cooled for the sake of constructive simplicity. Alternatively, medium voltage MV motors may be used. Low voltage LV motors may also be used because they guarantee higher reliability due to the absence of brushes.

The rolling process which is carried out by means of this first embodiment of the invention follows the following steps. The product being rolled, which defines the pass-line A and has a rolling advancement sense indicated by the arrow of line A, is passed in the first rolling unit 10, which comprises four rolling stands 71, 72, 73, 74, the rolling rings of which have respective oval S1, round S2, oval S3, round S4 calibration in succession (conventionally indicated with the sequence of letters O-R-O-R). This unit 10 is controlled by two motors of power equal to 1500 kW. One motor 11 actuates the oval section odd rolling stands 71 and 73 and the other motor 12 actuates the round section even rolling stands 72 and 74. The product being rolled crosses in succession the second rolling unit 20, which comprises four rolling stands 75, 76, 77, 78, the rolling rings of which have oval S5, round S6, oval S7, round S8 (O-R-O-R) calibration in succession. This unit 20 is controlled by two motors of power equal to 1500 kW. One motor 21 actuates both oval section odd rolling stands 75 and 77 and the other motor 22 actuates both round section odd rolling stands 76 and 78.

The product being rolled crosses subsequently the third rolling unit 30 which comprises two rolling stands 79, 80, the rolling rings of which have respective oval S9 and round S10 (O-R) calibration in succession. This unit 30 is controlled by two motors of power equal to 1300 kW. One motor 31 actuates the oval section odd rolling stand 79 and the other motor 32 actuates the round section even rolling stand 80.

Advantageously, in the alternative variant of the apparatus in FIG. 3 which corresponds in all its elements in the part of the first two rolling units 10 and 20 to the variant in FIG. 1, the finishing/calibrating unit 30′ provides a further round rolling step 81 provided with its own motor 33 independent from motors 31, 32. The three rolling steps are thus arranged in oval-round-round (O-R-R) sequence. Advantageously, the last round rolling step 81 carries out a low section reduction for the purpose of the limiting the ovalization of the product at the outlet of the finishing/calibrating unit 30′ allowing to obtain an “improved rod” with narrow tolerance.

Advantageously, the rolling centre distance of the last two rolling stands 80 and 81 is as small as possible to limit the rotation of the wire rod in the passage from the second to the third rolling stand, guaranteeing that the action of the last rolling stand 81 actually eliminates the residual ovalization of the rolled product, squeezing the wire rod according to the longer axis of the section, result which is instead not guaranteed with greater centre distances.

It is worth noting that the last rolling step of the product is carried out in a rolling stand chosen from the three units 10, 20, 30 or 30′ according to the final section of the rolled product (the diameter of which varies from 5.5 to 20 mm). For example, with reference to the first variant in FIG. 1, in which the unit 30 has two rolling steps O-R, the following situation may occur:

a) For small diameter rolled products (comprised from 5.5 mm to 8 mm), the last rolling step is carried out in the rolling stand 80 with round gauge in the third unit 30. In particular, for diameters from 7 to 8 mm, the first two rolling stands 75, 76 of the second unit 20 are bypassed upstream.
b) For intermediate diameters (from 8.5 mm to 10.5 mm), the last rolling step is carried out on the round gauge of the rolling stand 78 of the second unit 20. The first two rolling stands 75, 76 of the second unit 20 are bypassed upstream.
c) For large diameters (from 11 mm to 13 mm), the last rolling step is carried out on the first round gauge of the rolling stand 76 of the second unit 20 and the last rolling stand 74 of the first unit 10 and the first rolling stand 75 of the second unit 20 are bypassed upstream.
d) For larger diameters (from 13.5 mm to 20 mm), the last rolling step is carried on the second rolling stand 74 with round gauge of the first unit 10.

In a further advantageous variant of the invention, illustrated in FIG. 2, the wire rod rolling apparatus comprises ten rolling stands, i.e. the same total number of rolling stands as the main variant described above, but the rolling chambers are incorporated in two rolling units 110, 120 (6+4 configuration). The first rolling unit groups the first six rolling stands 171, 172, 173, 174, 175, 176, arranged in such a sequence that the respective gauges are in succession oval S1, round S2, oval S3, round S4, oval S5 and round S6 in the rolling direction. In this first rolling unit, two mutually independent motors 111, 112 are provided for actuating the six first rolling stands. One motor actuates the three oval section gauge rolling stands 171, 173, 175 and the second motor independently actuates the three round section gauge rolling stands 172, 174, 176.

The second rolling unit groups the four subsequent rolling stands 177, 178, 179, 180 arranged in such a sequence that the respective gauges are in succession oval S7, round S8, oval S9, round S10 in direction of the rolling sense, and two mutually independent second motors 121, 122 are provided for actuating these four rolling stands 177, 178, 179, 180.

Also this variant of the apparatus of the invention may be integrated with a further round section gauge rolling stand 181, as shown in FIG. 4, which is incorporated in the second rolling unit 120′, which thus consists of five rolling stands (6+5 configuration). The last rolling stand 181 is actuated by a third motor 123 independent from the two motors 121, 122.

Advantageously, as also shown in FIG. 4, the rolling centre distance of the last two rolling stands 180 and 181 is as small as possible to limit the wire rod rotation. A further variant of the apparatus 1 of the invention is shown in FIG. 5. This variant is shown in a particular application of the 4+4+2 rolling stand configuration, but it is understood that it is equally applicable also to the other rolling stand configurations described above. For the sake of brevity, the components which have already been illustrated above in the context of the description of FIG. 1 are not described again. The first rolling unit 10 comprises a kinematic connection mechanism 115, e.g. of the mechanical toothed wheel type, which can connect the transmission device of the motor 11 to the motion transmission device of the motor 12. Thus, in case of need, all the rolling stands of the unit 10 can be mechanically interconnected and work with a fixed draw between rolling stands. Such a solution may be adopted, for example, in case of maintenance or breakage of one of the two motors 11 or 12.

A kinematic connection mechanism 116, equal to the previous one 115, may be arranged between the motion transmissions of the two motors 21 and 22 of the second rolling unit 20 to carry out the same functions described for the first unit 10.

Therefore, in accordance with the variant shown in FIG. 5, the rolling unit can work with motors at independent rpm, with the kinematic connections 115 and 116 disengaged, or with the motors mechanically connected and fixed speed ratios between rolling units.

The elements 11′, 12′, 21′, 22′, illustrated in FIG. 5 with the dashed lines, represent alternative arrangements of the motors 11, 12, 21, 22, illustrated with solid lines.

The use of independent drives between the rolling stands in even position and those in odd positions in the two or three rolling units, according to the configuration considered, as shown above, allows to obtain the following advantages:

i) Reduction of installation and maintenance costs determined by the use of low voltage motors LV instead of medium voltage MV motors, quantifiable in about 40% with respect to a traditional solution.
ii) Reduction of total installed power, quantifiable in terms of a 10% saving with respect to a traditional finish block.
iii) Saving in ring stock management. Indeed, when a rolling channel is worn it is no longer necessary to grind all the rings of the same block, i.e. to carry out so-called family re-working, but it is sufficient to intervene on the number of rpm of the corresponding motor, with a saving of costs quantifiable in about 50% with respect to a traditional solution.
iv) Possibility of fitting rings of different diameter between the various rolling stands.
v) Optimization of draw management between rolling stands, it being possible to singularly act on the single ring overspeed.
vi) Possibility of constantly obtaining finished product with small dimensional tolerances, i.e. finished product shape errors reduced by 50% with respect to a traditional solution on the final diameter.
vii) Possibility of reducing the gripper family.
viii) Possibility of varying the reduction percentage of the section in single steps.

In order to reach high rolling speeds it is advantageous to adopt the technological diagram shown in FIG. 6. This diagram is described in an application thereof with the arrangement of rolling stands corresponding to the variant in FIG. 1 and the respective description, but the diagram is directly applicable also to the variant of the apparatus described above and illustrated in the other FIGS. 2-5.

The rolling product coming from the rolling train arranged upstream (not shown in the FIGS.) moves along the pass-line A and passes firstly in a prebox 2 for controlling the inlet temperature in the finishing apparatus; subsequently an equalizing duct 3 is provided. The rolling product has a temperature of approximately 900° C. and moves at a speed equal to 16% of the maximum outlet speed.

Exiting from the first rolling unit 10, the rolling product passes through a water box 4 for a further thermal treatment and is passed through the second rolling unit 20, in which it enters at a temperature of approximately 900° C. and a maximum speed equal to 42% of the maximum outlet speed.

At the rolling unit 30 outlet, the product is passed in a water box 9 for a further check of the winding temperature which takes the temperature to a value from approximately 850° C. to approximately 950° C., before the turns fall onto the mat.

The water cooling elements (the so-called water boxes), arranged between the rolling units, allow to advantageously obtain a better temperature control of the rolled product at inlet and at outlet of each units at high rolling speed.

By virtue of the fractioning of the finishing block into three distinct units or sections 10, 20, 30 and the cooling which is carried out between the same units, the rolling is in fact “shifted downstream” of the rolling mill, the total length of the line being equal and the temperature of the rolled item is abated in various intermediate steps with the advantage that:

i) The temperature increase of the rolled product which occurs at high rolling speeds is contained.
ii) The rolled product is stiffer and thus better withstands peak loads generated by the frictions that the rolled product develops at high speeds: in this manner safe introductions of the rolled product between the various operative units is guaranteed even at very high rolling speeds (up to 120, 130, 140 or 150 m/sec).
iii) The rolled product vibrates less during the transit towards the wrapping or laying head and also inside the water boxes, with less risk of catching.
iv) The waste caused by the tail product portion, typically out of tolerance, is reduced, with consequent increase of process efficiency by effect of a better draw control between the units which determines a better shape quality of the section of the final part of the rolled product.

In this manner, the apparatus of the invention further has the advantages given by the solutions of the prior art with independent motors for each rolling stand, i.e. a ring management flexibility, a free setting of the product draw and a consequent improvement of the tolerances of the wire rod combined with a better possibility of controlling rolling temperatures.

Claims

1. Wire rod rolling apparatus, defining a rolling direction, comprising two or more rolling units, arranged in sequence in the rolling direction, each rolling unit comprising two or more rolling stands and at least one first motor and at least one second motor for actuating the two or more rolling stands, each of said rolling stands having a pair of rolling rings with respective either oval or round section gauge, the rolling stands being arranged along the rolling direction so as to display in succession a rolling stand with oval section gauge followed by a rolling stand with a round section gauge, characterized in that in at least one of said rolling units the respective first motor actuates the rolling stands with oval section gauge and the respective second motor actuates the rolling stands with round section gauge.

2. Apparatus according to claim 1, wherein in each rolling unit the respective first motor actuates the rolling stands with oval section gauge and the respective second motor actuates the rolling stands with round section gauge independently from the first motor.

3. Apparatus according to claim 2 comprising three rolling units wherein a first rolling unit comprises four first rolling stands arranged, in sequence so that the respective gauges are oval, round, oval and round in the rolling direction, and a respective first motor and a respective second motor,

wherein a second rolling unit comprises four second rolling stands arranged in sequence so that the respective gauges are oval round, oval and round in the rolling direction, and a respective first motor and a respective second motor,

wherein a third rolling unit comprises at least two third rolling stands arranged in sequence so that the respective gauges are oval, round in the rolling direction, and a respective first motor and a respective second motor.

4. Apparatus according to claim 3, wherein the third rolling unit comprises a further third rolling stand having a pair of rolling rings with round gauge, arranged downstream of said at least two third rolling stands and actuated by a third motor.

5. Apparatus according to claim 2 comprising two rolling units, wherein a first rolling unit comprises six first rolling stands arranged in sequence so that the respective gauges are oval, round, oval, round, oval, round in the rolling direction, and a respective first and a respective second motor,

wherein a second rolling unit comprises four second rolling stands arranged in sequence so that the respective gauges are oval, round, oval, round in the rolling direction, and a respective first and a respective second motor.

6. Apparatus according to claim 5, wherein the second rolling unit comprises a fifth rolling stand having a pair of rolling rings with round gauge, arranged downstream of said four second rolling stands and actuated by a third motor.

7. Apparatus according to claim 3, wherein in at least one of the rolling units there are provided kinematic coupling means, that can be engaged and disengaged, between transmissions of the first motors and transmissions of the second motors, so as to cinematically couple even rolling stands with odd rolling stands of said at least one rolling unit with one of said first and second motors, in particular operating conditions.

8. Apparatus according to claim 5, wherein in at least one of the rolling units there are provided kinematic coupling means, that can be engaged and disengaged, between transmissions of the first motors and transmissions of the second motors, so as to cinematically couple even rolling stands with odd rolling stands of said at least one rolling unit with one of said first and second motors, in particular operating conditions.

9. Apparatus according to 3, wherein a water box is provided between the first rolling unit and the second rolling unit.

10. Apparatus according to claim 5, wherein a water box is provided between the first rolling unit and the second rolling unit.

11. Apparatus according to claim 3, wherein at least one water box and at least one first equalizing channel are provided between the second rolling unit and the third rolling unit.

12. Apparatus according to claim 5, wherein at least one water box and at least one first equalizing channel are provided between the second rolling unit and the third rolling unit.

13. Apparatus according to claim 11, wherein a prebox and a second equalizing channel are provided upstream of the first rolling unit.

14. Apparatus according to claim 12, wherein a prebox and a second equalizing channel are provided upstream of the first rolling unit.

15. Wire rod rolling plant comprising a wire rod rolling apparatus, defining a rolling direction, comprising two or more rolling units, arranged in sequence in the rolling direction, each rolling unit comprising two or more rolling stands and at least one first motor and at least one second motor for actuating the two or more rolling stands, each of said rolling stands having a pair of rolling rings with respective either oval or round section gauge, the rolling stands being arranged along the rolling direction so as to display in succession a rolling stand with oval section gauge followed by a rolling stand with a round section gauge, wherein in at least one of said rolling units the respective first motor actuates the rolling stands with oval section gauge and the respective second motor actuates the rolling stands with round section gauge.

16. Wire rod rolling method by means of a wire rod rolling apparatus, defining a rolling direction, comprising three rolling units, arranged in sequence in the rolling direction, the method comprising the following steps:

each rolling unit comprising two or more rolling stands and at least one first motor and at least one second motor for actuating the two or more rolling stands,

each of said rolling stands having a pair of rolling rings with respective either oval or round section gauge,

the rolling stands being arranged along the rolling direction so as to display in succession a rolling stand with oval section gauge followed by a rolling stand with a round section gauge,

wherein in each rolling unit the respective first motor actuates the rolling stands with oval section gauge and the respective second motor actuates the rolling stands with round section gauge independently from the first motor,

wherein a first rolling unit comprises four first rolling stands arranged in sequence so that the respective gauges are oval, round, oval and round in the rolling direction, and a respective first motor and a respective second motor,

wherein a second rolling unit comprises four second rolling stands arranged in sequence so that the respective gauges are oval, round, oval and round in the rolling direction, and a respective first motor and a respective second motor,

wherein a third rolling unit comprises at least two third rolling stands arranged in sequence so that the respective gauges are oval, round in the rolling direction, and a respective first motor and a respective second motor;

a) rolling the product in first rolling unit with an oval, round, oval, round rolling sequence,

b) rolling the product in the second rolling unit with an oval, round, oval, round rolling sequence,

c) rolling the product in the third rolling unit with an oval, round rolling sequence,

17. Method according to claim 16, wherein the rolling of the product in the third rolling unit includes a further rolling pass with round gauge.

18. Method according to claim 16, wherein:

a thermal treatment is provided before step a) by means of a prebox and first temperature equalizing channel upstream of the first rolling unit;

between step a) and step b) a thermal treatment of the rolled product by means of a first water box is provided,

between step b) and step c) a thermal treatment of the rolled product by means of at least one second water box and at least one second temperature equalizing channel is provided,

after step c) a final cooling of the rolled product is provided until a temperature comprised between 850° C. and 900° C. is reached.