EP0197079B1 - Method for the continuous tempering of sheet metal such as steel and plant for implementing it - Google Patents

Abstract

The sheet metal to be treated (6) is fed to a primary high cooling flux area (4) which is regulated so as to bring the surface temperature of the sheet metal to the temperature range corresponding to the desired structure. The cooling is carried out by passing the sheet metal to a secondary moderated cooling flux area (5), which flux may be regulated, thereby maintaining the surface temperature to the desired level. Plant comprising a primary intense cooling area (4) and a secondary moderated cooling area (5).

Description

The invention relates to a process for the quenching of metal sheets such as steel, making it possible to carry out in the same installation the hard quenching and the soft quenching and an installation for carrying out the process.

The thermal or thermomechanical treatments of steels with little loading of alloying elements make it possible, under certain conditions, to obtain mechanical characteristics equal to and even superior to those of steels which are more highly alloyed in the conventional manner. We have therefore, for the purpose of price competitiveness, developed methods of direct quenching in the hot rolling.

• - la trampe dure qui a pour but l'obtention d'une structure martensitique, et qui nécessite une vitesse de refroidissement maximale jusqu'à une température inférieure à 200°C, pour atteindre le domaine martensitique
• - la trempe douce, ayant pour but l'obtention d'une structure ferritique, perlitique ou bainitique se formant dans une zone de température supérieure à 400°C, nécessite des vitesses de refroidissement pouvant être 10 fois moindres que précédemment dans la zone des températures allant de 800°C (température initiale) à 600°C (témperature finale).

Two types of heat treatment are currently used:

• - the hard tramp which aims to obtain a martensitic structure, and which requires a maximum cooling rate down to a temperature below 200 ° C, to reach the martensitic domain
• - Soft quenching, intended to obtain a ferritic, pearlitic or bainitic structure forming in a temperature zone above 400 ° C, requires cooling rates that can be 10 times lower than previously in the temperature zone ranging from 800 ° C (initial temperature) to 600 ° C (final temperature).

• - pour la trempe dure dans une machine à refroidissement accéléré, telle que celle décrite par exemple dans le brevet français 2.223.096, dans laquelle des rouleaux de guidage font progresser la tôle à traiter dans une enceinte alimentée de chaque côté des faces de la tôle par des admissions d'eau dont le débit permet l'évacuation d'un flux de chaleur de l'ordre de 5 MW/m^2. Egalement, pour la trempe dure, l'article «Use of controlled spray cooling for quenched steels» de N.V. ZIMIN paru dans «Metal Science and Heat Treatment», vol. 19 N° 1/2 janvier/février, pages 117-120, une méthode de refroidissement en plusieurs étapes. On refroidit la surface de la tôle par une pulvérisation vigoureuse d'eau (0,6-0,8 m³/sec m²) jusqu'à la transformation martensitique puis on continue le refroidissement jusqu'au coeur du métal par pulvérisation modérée de l'orde de -0,1 m³/ sec m². C'est une méthode de trempe dure dans laquelle on utilise deux pulvérisateurs de débits différents pour obtenir les deux types de refroidissement. On connaît également de l'article «Bethle- hem installs new plate quench and temper facili- ty» publié dans «Iron and Steel Engineer» vol. 43, N° 10 octobre 1966, pages 169-170, une unité de trempe dure présentant deux zones: une première de refroidissement rapide à haute pression et fort débit et une deuxième zone de refroidissement lent maintenant la surface de la tôle à une basse température comme dans l'article précédent.
• Les unités de trempe ci-dessus décrites ont la possibilité de faire varier le refroidissement en fonction de l'épaisseur des tôles, mais toujours dans des proportions correspondant au seul type de trempe dure. Aucun de ces dispositifs ne permet d'obtenir les deux types de trempe dure et douce,
• - pour la trempe douce par une machine de refroidissement à jets laminaires ou à lame d'eau, telle par exemple celle décrite dans la publication «Transactions I.S.J.» vol. 22, 1982, p. B 245.246 et permettant l'évacuation d'un flux de chaleur de l'ordre'de 0,4 à 0,8 MW/m².

These conditions are achieved in particular in two types of machines:

• - For hard quenching in an accelerated cooling machine, such as that described for example in French patent 2,223,096, in which guide rollers advance the sheet to be treated in an enclosure supplied on each side of the faces of the sheet by water inlets whose flow allows the evacuation of a heat flow of the order of 5 MW / m ^2. Also, for hard quenching, the article "Use of controlled spray cooling for quenched steels" by NV ZIMIN published in “Metal Science and Heat Treatment”, vol. 19 No. 1/2 January / February, pages 117-120, a method of cooling in several stages. The surface of the sheet is cooled by vigorous spraying with water (0.6-0.8 m ³ / sec m ² ) until the martensitic transformation then the cooling is continued to the core of the metal by moderate spraying of the order of -0.1 m ³ / sec m ² . It is a hard quenching method in which two sprayers of different flow rates are used to obtain the two types of cooling. We also know from the article “Bethlehem installs new plate quench and temper facili- ty” published in “Iron and Steel Engineer” vol. 43, No. 10 October 1966, pages 169-170, a hard quenching unit having two zones: a first rapid cooling at high pressure and high flow rate and a second slow cooling zone keeping the surface of the sheet at a low temperature as in the previous article.
• The quenching units described above have the possibility of varying the cooling as a function of the thickness of the sheets, but always in proportions corresponding to the only type of hard quenching. Neither of these devices makes it possible to obtain the two types of hard and gentle quenching,
• - for gentle quenching by a cooling machine with laminar jets or with a water blade, such as for example that described in the publication "ISJ Transactions" vol. 22, 1982, p. B 245.246 and allowing the evacuation of a heat flow of the order of 0.4 to 0.8 MW / m ² .

The characteristics of the sheets subjected to hard or soft quenching being different, it would be interesting to be able, with a single installation and with a reduced energy expenditure, to produce sheets according to one or the other method. Unfortunately, for hydrodynamic reasons, the soft quenching installation cannot in any case remove the heat flux necessary for hard quenching and the hard quenching installation cannot drop to the flow values of soft quenching.

Indeed, the heat flux necessary to cool a sheet is proportional to the thickness of the latter and to the cooling rate.

In the case of gentle quenching, the cooling rate required is approximately 10 ° C / s between 800 ° C and 600 ° C and does not depend on the thickness of the sheet, at least for thicknesses less than 25 mm .

• • environ 100°C/s pour une tôle de 10 mm
  • environ 30°C/s pour une tôle de 30 mm
  • environ 10°C/s pour une tôle de 50 mm.

In the case of hard quenching, the cooling rate is much higher and depends on the thickness of the sheet. Thus between 800 ° C and 200 ° C, we have:

• • about 100 ° C / s for a 10 mm sheet
  • about 30 ° C / s for a 30 mm sheet
  • about 10 ° C / s for a 50 mm sheet.

For thick sheets (above 30 mm), the diffusion of heat in the metal limits the cooling rate.

From the preceding remarks, it follows that, depending on the thickness of the sheet, there may be a ratio of 10 between the heat flux necessary for hard quenching and that necessary for gentle quenching. Whatever the cooling process used in metallurgy, the heat flux in the heat-up regime (for a surface temperature of the metal greater than 400 ° C) varies significantly as the power 0.6 to 0.9 of the water flow. The water flows must therefore vary in a ratio much greater than 10 in order to carry out both the hard quenching and the soft quenching.

The invention provides a quenching method as defined in claim 1 making it possible to carry out, with the same installation, either hard quenching or soft quenching.

The sheet to be treated is scrolled through a primary zone with a high cooling flow so as to bring the surface of the sheet into the temperature range corresponding to the desired structure, cooling is completed by passing through a secondary zone with cooling flow. moderate.

Advantageously, in the case of gentle quenching, the cooling flow and the residence time are chosen so that the super temperature sheet thickness does not fall below 500 ° C in the high-cooling zone and is maintained above 500 ° C in the moderately-cooling zone, the cooling being effected by caléfaction or boiling in film; in the case of hardening the cooling flow and the residence time are chosen so that the surface temperature of the sheet is rapidly close to 100 ° C. so that in the zone with moderate cooling the cooling is carried out by nucleated boiling .

The high cooling flow is around 1.5 MW / m ² in the case of soft quenching and is approximately three times higher in the case of hard quenching, while the moderate cooling flow can vary between 0.2 and 0.8 MW / m ² approximately, in the case of soft quenching, the heat exchange taking place by film boiling or heat-setting, and is approximately three times higher in the case of hard quenching, l 'heat exchange by nucleated boiling.

The installation according to the invention intended for the implementation of this process is defined in claim 5. This comprises a cooling section with high thermal flow followed by a cooling section with moderate thermal flow, the flow of cooling of the first section being adjustable in a ratio of 1 to 3 and that of the second section, in a ratio of 1 to 4, for a given boiling regime (either nucleated boiling, or calefaction) to allow their adaptation to the speeds cooling demanded.

The explanations and figures given below by way of examples will make it possible to understand how the invention can be implemented.

Figure 1 is a schematic representation of a first embodiment of a cooling installation for the implementation of the method according to the invention. Figure 2 shows the cooling curves of a sheet subjected to gentle quenching. Figure 3 shows the cooling curves of a sheet subjected to hard quenching.

According to the method of the invention, the sheet to be treated, leaving the rolling mill at a temperature of the order of 800 ° C, passes through a primary cooling zone capable of evacuating a high thermal flux or intense cooling zone. At least the surface of the sheet is thus brought, at the exit from this zone, into a temperature range corresponding to the desired final martensitic or perlitic-ferritic structure depending on whether it is a question of obtaining a hard quench or a quench sweet. Cooling is continued in a secondary cooling zone capable of evacuating a moderate thermal flux sufficient to maintain the surface temperature of the product substantially at the level corresponding to the desired final structure.

As previously reported, the surface temperature of the sheets at the exit from the cooling zone with high thermal flux does not depend on their thickness but only on the initial temperature and the cooling thermal flux. With regard to gentle quenching, thermal modeling has shown that if a sheet metal at least 12 mm thick is cooled for 3 seconds and whose initial temperature is above 750 ° C with a flow of the order 2 MVII / m ^z , the surface temperature of the sheet does not depend on its thickness and does not drop below 500 ° C and therefore does not reach the areas of hard quench structure formation. On the other hand, the average temperature of the sheet remains above 600 ° C and depends on the thickness.

Cooling can then be completed in the secondary moderate-flow cooling zone (0.8 to 0.2 MW / _M ² depending on the thickness of the sheet), which is adjusted so as to maintain an average cooling rate of 10 ° C / second approximately up to 600 ° C. In this secondary zone, the surface temperature always remains between 600 ° C and 500 ° C.

Cooling is therefore obtained comparable to that obtained by cooling at a constant speed of 10 ° C / s considered necessary for obtaining soft quenching structures.

According to an exemplary embodiment of an installation in accordance with the invention: for a sheet 25 mm thick, the initial temperature of which is 800 ° C., and the final temperature of 600 ° C. with a running speed of 2 m / s, the heat flow for cooling the primary zone being of the order of 2 MW / m ² , the length of the primary zone is 6 m and that of the secondary zone 24 m, resulting in an installation of 30 m total length.

FIG. 3 shows the evolution of the surface, average and core temperatures of a 25 mm thick sheet having passed through the installation as indicated above.

A mild quenching installation, as currently known, would have been 40m long.

To obtain a hard quench, the sheets must be brought into the martensitic domain by very rapid cooling. In addition, hard quenching tests have shown that the sheets should be cooled to an average temperature below 150 ° C, which for thick sheets leads to high cooling times, for example 120 seconds for a sheet of 50 mm thick.

The maximum tolerable temperature difference between the head and the tail of a sheet imposing a minimum speed of the order of 0.1 to 0.2 m / s, the maximum residence time of a sheet moving in the area at intense cooling is 60 seconds.

A thermal modeling shows that, after leaving the first zone of intense cooling adjusted to the maximum of its capacities, the heat flow necessary in the second zone to complete the cooling to 150 ° C hardly exceeds 2 MW / _m ² .

The skin temperature of the sheets leaving the primary zone with a high cooling flow (or intense cooling zone) being close to 100 ° C, the phenomenon of heat build-up, which, in the case of soft quenching, limited the heat exchanges, disappeared and the conventional cooling by sprayed water becomes important and shows an efficiency of the order of 40% as a result of intense vaporization. Thus, for a heat flux of 2 MW / m ² and a cooling efficiency of 40%, the surface flow of water to be produced is 2 kg / m ² . This flow rate is of the same order of magnitude as that required for cooling in the secondary zone with moderate cooling flow and corresponding to the characteristics of conventional gentle quenching installations (approximately 0.8 MW / m ² under heating mode).

The example of installation previously described with a primary zone of 6 m and a secondary zone of 24 m will make it possible to treat sheets of 50 mm thick moving at the speed of 0.2 m / s or 70 mm moving at 0.1 m / s.

FIG. 3 shows the evolution of the surface, average and core temperatures of a sheet 50 mm thick having passed through the installation as indicated above.

FIG. 1 schematically represents an exemplary embodiment of a cooling installation for implementing the method according to the invention. The sheets leaving the rolling mill 1 pass into a leveling installation 2, then into the cooling installation 3. This installation comprises a primary zone 4 with a high cooling flow and a secondary zone 5 with a moderate cooling flow.

The sheet to be treated 6 circulating in the intense cooling zone 4, between upper rollers 7 and lower 8, must have good flatness. In order to obtain the required flatness, the sheet leaving the rolling mill is passed through the leveling installation 2, which has a double advantage: in terms of leveling, straightening the sheet hot, therefore with low energy consumption , and in terms of cooling, better water distribution.

The intense cooling zone consists of elements such as those described in French patent 2,223,096. It comprises a number of pairs of guide rollers between which the sheet passes. These rollers are enclosed in an enclosure 9 having between the rollers flat walls, mutually parallel and with the upper and lower faces of the sheet. These walls allow passage to a blade of cooling water supplied by intakes provided on the casings of some of the rollers and discharged by outlets provided on the casings of other rollers. The adjustment of the water flow allows the cooling flow to be varied in a ratio of 3.

The pairs of guide rollers allow the sheet metal to move, but also help maintain it to prevent accidental deformation during cooling. The thickness of the sheets subjected to the treatment being variable, the upper part of the machine carrying the upper rollers must be mobile to allow the adjustment of the passage between the rollers, also this part of the installation is expensive and it is advantageous to decrease its length. In practice, we limit ourselves to a length of the order of 6 meters.

The moderate cooling zone 5 comprises drive rollers 10 on which the sheet rests and sprayers or sprinkler devices with a water blade or laminar jets provided between the rollers. According to a preferred form, two-dimensional jet spraying devices 11, 12 are used, as described in French patent 2,421,678. These devices consist of hollow bodies, terminated by an elongated slot, supplied with gas under moderate pressure and with water. The water, injected inside the hollow bodies, is ejected by a stream of pressurized gas, through the slot, giving a two-dimensional spray jet. As a result of the good definition of the spray, it is possible to obtain homogeneous cooling over the width of the sheet and similar on both sides with relatively inexpensive equipment and the cooling power of which is easily adjustable in a ratio of 4. This homogeneous and symmetrical cooling avoids deformation of the sheet.

In the example shown, the length of the secondary zone 5 is of the order of 24 meters.

• - d'effectuer une trempe dure au défilé, la zone de refroidissement intense étant réglée au maximum de ses capacités avec des tôles épaisses de 70 mm au plus et pouvant dépasser 10 m de longueur,
• - d'effectuer une trempe dure ou statique de tôles d'épaisseur supérieure à 70 mm, (on fait alors subir à la tôle des mouvements de va-et- vient dans la zone de refroidissement secondaire),
  d'effectuer une trempe douce avec la zone de refroidissement intense réglée pour un flux de refroidissement de 1 à 2 MW/m² et une vitesse de défilement de l'ordre de 2 m/s.

The installation according to the previous description allows:

• - to carry out a hard quenching with the parade, the zone of intense cooling being regulated to the maximum of its capacities with thick sheets of 70 mm at most and being able to exceed 10 m in length,
• - to carry out a hard or static quenching of sheets of thickness greater than 70 mm, (the sheet is then subjected to back and forth movements in the secondary cooling zone),
  perform a gentle quenching with the intense cooling zone set for a cooling flow of 1 to 2 MW / m ² and a running speed of the order of 2 m / s.

In the various examples, the speed of rotation of the rollers is adjustable.

Claims (7)

1. A process for the continuous tempering of sheet steel initially at a high temperature (= 800°C) which permits hard or soft tempering to be achieved selectively in the same installation by passing the sheet at the same speed through two successive zones respectively for intense cooling to a predetermined skin temperature, say about 100°C or 500°C according to the type of tempering required, and then in a second zone for moderate cooling for maintaining the skin temperature previously reached, characterised in that in the first zone intense cooling is obtained by forced longitudinal circulation on each face of the sheet by a stream of water of adjustable flow rate, in the second zone moderate cooling is obtained by transverse spraying of water over each face of the sheet, the flow rate of the sprayed water also being adjustable, the flow rate of water in the two zones being regulated such that the cooling fluxes differ in the same proportion from one to three according to whether hard or soft tempering is required, from a basic fixed flow rate corresponding to a cooling flux of 1.5 MW/m² for the first zone depending on the thickness of the sheet in order to maintain the value of the surface temperature of said sheet, the respective times the sheet remains in the intense and moderate cooling zones being in a ratio of the order of at least 4.

2. A process according to claim 1, characterised in that in the first zone the flow rate of the water is multiplied by 3 when passing from soft tempering to hard tempering whilst the flow of water in the second zone is sensibly regulated to the same value in both cases.

3. A process according to claim 1, characterised in that the flow rate of water in the first zone and in the second zone is sensibly identical in hard tempering.

4. A process according to claim 1 characterised in that the speeds at which the sheet travels as between soft tempering and hard tempering are in a ratio of at least 1 to 10, or about 2 m/s for soft tempering and about 0.2 m/s for hard tempering.

5. A plant for selectively effecting the hard or soft tempering process according to claim 1, characterised in that it comprises a high cooling flux zone (4) and a moderate cooling flux zone (5) comprising rollers having an adjustable speed of rotation (7, 8, 10) on and/or between which the sheet to be treated (6) passes and cooling devices having an adjustable flow rate (9, 11, 12), said devices being constituted in the first zone by an enclosure (9) which contains the rollers (7, 8) and which has flat walls between the rollers parallel to each other and to the upper and lower faces of the sheet to delimit a stream of water of adjustable flow rate, and in the second zone by transverse ramps for cooling by spraying (11, 12) or by sprinkling extending for a distance, parallel to each face of the sheet over the same length as the lower rollers (10).

6. A plant according to claim 5, characterised in that the enclosure of the high cooling flux zone (4) comprises injection devices and extraction devices on the gear-cases of some rollers and parallel to said rollers.

7. A plant according to claim 5, characterised in that the transverse ramps are pneumatic sprayers (11, 12) having a bi-dimensional jet arranged above and below the sheet to be treated (6) and between the rollers (10).

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