EP0090682B1 - Heat treating method and apparatus for making rods of alloy steel ready for use - Google Patents

Abstract

The rods, up to approximately 150 mm in diameter, are used especially for mechanical construction; this treatment, which is carried out at rolling heat, comprises the following three successive stages: a) a severe forced cooling on the run by a sheet of water 2, to lower the average temperature of the section to approximately 600-550 DEG C over a minimum length, for example less than 30 m, b) natural air cooling on a cooling bed 29 for a certain thermal homogenisation of the section and autotempering of the surface martensite, c) a second forced cooling on the cooling bed either by spraying water at 28a or by immersion, at least to an average temperature of the order of 300 DEG C, it being possible for the final cooling to take place in air at 27. This treatment may be supplemented by conventional tempering. <IMAGE>

Description

The invention relates to the field of heat treatment of metal bars at the end of rolling and more particularly relates to the direct production of quenching structures, preferably martensite, and optionally bainite, in the useful part of steel, intended in particular for mechanical construction.

The steel industry currently delivers alloy steels with improved machinability which exhibit, with the quenched and tempered effect, machinability comparable to that of conventional steels of the same composition but in the annealed state. Consequently, the possibility of machining certain parts from bars having the final characteristics of use is of considerable economic interest, in particular due to the elimination of annealing for machining on a blank, of the quenching and tempering treatment for the user, or resuming machining by rectification.

This results in a simplification of the manufacturing cycles, a reduction in the costs of heat treatments and a gain on the execution times.

It is also possible to save austenitization by quenching the bars in the "hot" rolling itself. It is even possible to obtain at the end of the rolling an austenitic state more interesting than that resulting from a conventional austenitization due to a more complete dissolution of the elements of addition to preheating, of an adaptable austenitic grain size to the desired quenchability, and a state of hardening of the austenite, more favorable at the time of the transformation γ → α.

The reasons why these bar treatments are hardly developed so far are mainly due to the control of cooling in the hot rolling. It should first be known that the mechanical industries are mainly interested in bars of large diameter (50 - 150 mm) in semi-hard grades, which are practically the only ones making it possible to obtain at the heart of these bars resistances of the around 1000 N / mm². Obtain a quenching structure at the heart of bars such a large diameter requires relatively hardening steel (42CD4, for example) and a high hardening speed. The risks of scuffing are then significant, even if, the duration of treatment not being limited, it is possible to adopt the least severe quenching medium compatible with the diameter of the bar and the hardenability of the steel grade.

où Θ(r) est la température à la distance "r" de l'axe). Comme la vitesse de laminage de ces barres est couramment de quelques mètres par seconde, il faudrait des longueurs de trempe déraisonnables pour former de la martensite à coeur. La trempe directe en piscine d'eau serait, pour sa part, insuffisamment rapide pour le plus forts diamètres et risquerait de conduire à des cintrages de barre (du fait des dissymétries de traitement), ou à des tapures (du fait de la mise en tension de la couche périphérique fragile de martensite lors du gonflement lié à la transformation ultérieure du coeur).The situation is further complicated for parade hardening just after rolling. It should be remembered that the largest heat exchange coefficients that we know how to achieve practically on smoothed bars are currently of the order of 10⁴W / m². ° C (or 10⁴ kcal / m².h ° C), and that 'It would take approximately 20 seconds to pass through a quenching medium characterized by this efficiency for the average temperature of a 60 mm bar to drop from 900 to 400 ° C. (Remember that the average temperature in the section of a bar of radius R is defined by

where Θ (r) is the temperature at distance "r" from the axis). Since the rolling speed of these bars is commonly a few meters per second, unreasonable quench lengths would be required to form core martensite. Direct quenching in a swimming pool of water would, for its part, be insufficiently rapid for the largest diameters and would risk leading to bar bending (due to the asymmetries of treatment), or to curls (due to the setting in tension of the fragile peripheral layer of martensite during the swelling linked to the subsequent transformation of the heart).

Document DD-119270 describes a quenching treatment interrupted in at least two steps immediately following rolling. A steel bar first undergoes a very rapid quenching bringing its peripheral part to a temperature lower than the martensitic transformation temperature of the steel considered. Then the quenching is interrupted so as to allow the temperatures of the core and the periphery of the product to equalize. Finally the bar is subjected to a second less energetic quenching at the end of which the core of the bar is transformed into martensite. The whole treatment lasts about 1.5 s and is only intended for products with a very small section, for example with a diameter of 10 mm.

The object of the invention is to obtain, without bending or taping, quenching structures in the section of ready-to-use rolled alloy steel bars having large diameters.

• a) juste après le laminage, on soumet la barre au défilé à l'action d'un milieu de trempe extrêmement sévère, présentant un coefficient d'échange thermique de l'ordre de 10⁴W/m².°C de manière à porter sa température moyenne vers 600-550 °C et à former une couche superficielle de martensite tout en évitant la formation de ferrite et de perlite à coeur.
• b) on laisse ensuite refroidir la barre naturellement à l'air - ou dans un milieu faiblement refroidissant analogue - jusqu'à parvenir sensiblement à une réhomogénéisation thermique dans la section vers 550°C, tout en évitant une formation significative de perlite ou de bainite à coeur.
• c) puis on soumet à nouveau la barre à un refroidissement forcé, de sévérité moins forte que celle mise en jeu dans l'étape a) pour que la température du coeur descende en dessous de 300°C environ en un temps compatible avec les cinétiques de transformation martensitique principalement, et bainitique, de la nuance d'acier mise en oeuvre.

To this end, the subject of the invention is a method of heat treatment of alloy steel bars of diameter 50 to 150 mm consisting in subjecting the rolled bar to hot rolling, a stepped quench comprising the following three successive steps:

• a) just after rolling, the bar is subjected to the process of action by an extremely severe quenching medium, having a heat exchange coefficient of the order of 10⁴W / m². ° C so as to bring its temperature average around 600-550 ° C and to form a surface layer of martensite while avoiding the formation of ferrite and perlite at the core.
• b) the bar is then allowed to cool naturally in air - or in a similar weakly cooling medium - until substantially thermal rehomogenization is achieved in the section around 550 ° C., while avoiding significant formation of perlite or bainite to heart.
• c) then the bar is again subjected to forced cooling, of less severe severity than that involved in step a) so that the temperature of the core drops below about 300 ° C. in a time compatible with the kinetics mainly martensitic and bainitic transformation of the steel grade used.

The invention also relates to an installation for implementing the method successively comprising, in the direction of travel of the product, means enabling a forced cooling of the bars in the parade according to step a) ensuring an average heat exchange coefficient greater than 5.10³W / m². ° C, and a chiller with movable beams ensuring a regular translation "with pilgrim steps" and a slow rotation on itself of the bar, said cooler being equipped with forced cooling means.

Advantageously, the cooling means according to step a) consist of at least one water box where the bar is wrapped in a sheet of water flowing collinearly with the bar. The means for the more moderate forced cooling of the stage can be constituted by oscillating ramps with water spraying, or a pool of water or oil.

• la figure 1 représente un diagramme de transformation en refroidissement continu d'un acier de construction mécanique du type 42 CD4 (0,42 % de C, 0,7 % de Mn, 1 % de Cr, 0,20 % de Mo) après une austénitisation vers 850 - 900 °C conférant à la structure austénitique une taille de grain de l'ordre de 20 µm ;
• la figure 2 représente schématiquement un mode de réalisation d'une installation de mise en oeuvre de l'invention ;
• la figure 3 représente schématiquement un autre mode de réalisation d'une telle installation ;
• la figure 4 représente schématiquement des exemples de conditions de transformation sur trois diamètres différents de barres en acier 42 CD4 traitées selon une variante simplifiée de l'invention ;
• la figure 5 représente schématiquement des exemples de conditions de transformation obtenues sur trois diamètres différents de barres en acier 42 CD4 traitées selon une variante complète du procédé selon l'invention.

To clearly understand the invention, the three successive stages are repeated below in detail, with reference to the accompanying drawing plates in which:

• Figure 1 shows a transformation diagram in continuous cooling of a mechanical steel of the 42 CD4 type (0.42% of C, 0.7% of Mn, 1% of Cr, 0.20% of Mo) after an austenitization around 850 - 900 ° C giving the austenitic structure a grain size of the order of 20 μm;
• FIG. 2 schematically represents an embodiment of an installation for implementing the invention;
• FIG. 3 schematically represents another embodiment of such an installation;
• FIG. 4 schematically represents examples of processing conditions on three different diameters of 42 CD4 steel bars treated according to a simplified variant of the invention;
• FIG. 5 schematically represents examples of processing conditions obtained on three different diameters of 42 CD4 steel bars treated according to a complete variant of the method according to the invention.

• former une couche annulaire superficielle de martensite,
• éviter la formation de perlite dans la plus grande partie de la section de la barre.

The first step of the process, denoted a), meets two objectives:

• form a surface annular layer of martensite,
• avoid the formation of perlite in most of the bar section.

To illustrate these two objectives, FIG. 1 shows, by way of example only intended to better understand the technical field of the invention, a diagram, of current use in thermal treatments, which represents on a temperature-time graph , the different micrographic constituents resulting, according to the cooling laws considered, from the transformation γ → α of the metastable austenite identified 1. Thus rapid cooling laws lead to the instantaneous formation of martensite 2 as soon as the temperature of the metal drops below the temperature Ms of the steel (310 ° C. in the example chosen). In contrast, very slow cooling laws lead to the formation of ferrite 3 and perlite 4 at temperatures above 600 ° C. Finally, intermediate cooling laws lead to the formation of bainite 5 and of a volume fraction which is all the more significant in martensite as the end of cooling is accelerated.

It should be remembered that, as a general rule, a returned martensite has better toughness than a returned bainitic structure with the same final level of resistance. In general, if the installation is equipped with a station for a final income, we will seek, in order to obtain good mechanical characteristics and improve the homogeneity of the structures in the section, to obtain, in most of this section, structures formed at the lowest possible temperatures. To guide in the choice of the cooling cycle, transformation diagrams of the type presented in FIG. 1 however have a certain number of limits. First, the shape of the thermal cycle above the temperature A₃ of the steel where the start of transformation γ ⇄ α takes place (800 ° C in the case presented) generally has no influence. We can therefore consider the thermal cycles only from this temperature, as was done in Figure 1. More delicate is the fact that the transformation conditions are determined only for the continuous laws used to establish the diagram . It is therefore not strictly possible to superimpose on this diagram, to predict the final structures, complex thermal cycles such as those recommended by the invention. Thus during a severe quenching interrupted during the parade, a surface layer sees its temperature drop quickly below the point Ms and then rise to the level of the average temperature of the section at the end of quenching. On the contrary, the bar core does not see its temperature drop until after entering the quenching device and only very slowly reach the average temperature at the end of quenching. However, looking at a diagram like the one in Figure 1 suggests some remarks. We will not form ferrite-perlite in this particular case if all the points of the section have their temperature drop from 800 to 650 ° C in less than 150 seconds. Furthermore, an austenite metastability domain between the temperatures (above 600 ° C) of perlite formation and those (below 550 ° C) of bainite formation will allow natural thermal rehomogenization of the bar around 575 ° C without significant transformation of the austenite. Finally, in the case presented in FIG. 1, cooling rates greater than 3 ° C / s between 550 and 300 ° C are necessary to repel the transformations from bainite to more interesting martensite.

• que l'épaisseur de martensite superficielle sera d'autant plus forte que la sévérité de trempe et le diamètre de barre seront élevés. Un coefficient d'échange thermique de 10⁴ w/m² . °C donne, par exemple sur une barre de 0̸ = 60 mm de diamètre, une couche de martensite d'épaisseur $\frac{\text{0̸}}{\text{12}}$
  
  = 5 mm ;
• qu'un coefficient d'échange thermique de l'ordre de 10⁴ W/m² . °C permet d'éviter la formation de perlite sur des barres de diamètre allant jusqu'à 150 mm laminées dans la nuance 42 CD4 correspondant au diagramme de la figure 1. Les durées de trempe entre 900 °C et une température moyenne de 600 °C seraient alors respectivement de 7,5 , 17 et 35 s pour des diamètres de 60, 100 et 150 mm, ce qui permet d'envisager des longueurs de traitement raisonnables. Il faut d'ailleurs noter que, pour les plus forts diamètres, une augmentation du coefficient d'échange thermique serait aussi nécessaire qu'une augmentation de la trempabilité de l'acier (addition de manganèse, molybdène, bore ...).

To return to the two objectives which were assigned to the first stage of cooling the bars, we indicate:

• the greater the thickness of surface martensite, the higher the quenching severity and the bar diameter. A heat exchange coefficient of 10⁴ w / m². ° C gives, for example on a bar of 0̸ = 60 mm in diameter, a layer of thick martensite $\frac{\text{0̸}}{\text{12}}$
  
  = 5 mm;
• a heat exchange coefficient of the order of 10⁴ W / m². ° C avoids the formation of perlite on bars up to 150 mm in diameter rolled in grade 42 CD4 corresponding to the diagram in Figure 1. The quench times between 900 ° C and an average temperature of 600 ° C would then be 7.5, 17 and 35 s respectively for diameters of 60, 100 and 150 mm, which makes it possible to envisage reasonable treatment lengths. It should also be noted that, for larger diameters, an increase in the heat exchange coefficient would be as necessary as an increase in the hardenability of the steel (addition of manganese, molybdenum, boron, etc.).

• au coeur de la barre, de voir la température passer en-dessous de 600 °C sans transformation notable en perlite ou bainite ;
• à la couche annulaire superficielle de martensite, de voir améliorer sa ténacité par un auto-revenu de l'ordre de la minute au-dessus de 500 °C.

With regard now to the 2nd step, or 2nd phase of the process, denoted b), the natural thermal rehomogenization with air of the bar around 550 ° C. generally lasts from 1 to 2 minutes and allows:

• at the heart of the bar, to see the temperature drop below 600 ° C without significant transformation into perlite or bainite;
• to the annular surface layer of martensite, to see its toughness improved by a self-income of the order of per minute above 500 ° C.

Finally, on the subject of the 3rd stage of treatment, noted c), after the thermal homogenization of the previous phase, we are looking for essentially martensitic structures at heart, subjecting the bar to a second forced cooling allowing the temperature of the core to drop below 300 ° C. in a time compatible with the kinetics of martensitic transformation mainly, and possibly bainitic of the steel considered. In fact, the "lower" bainites (those formed at temperatures a little higher than Ms) have, after tempering, mechanical characteristics comparable to that of martensite, so that it is often enough to accelerate the cooling between 550 and 400 ° C. For particularly hardening grades and / or small diameters, simple natural or slightly accelerated cooling will be sufficient. For example, it suffices, in the case of the 42 CD4 grade presented in FIG. 1, cooling to more than 4 or 5 ° C / s between 550 and 300 ° C to obtain an essentially martensitic structure at helm heart.

• elle réduit considérablement les risques de tapures lors d'une transformation martensitique ultérieure plus profonde,
• elle limite les risques de cintrage des barres par dissymétrie du refroidissement, puisqu'elle constitue une sorte de "corset" rigide annulaire.

In all cases, the annular surface layer of martensite obtained in the 1st stage of treatment and self-reappearing in the 2nd stage has a double role in the 3rd stage of forced cooling:

• it considerably reduces the risk of tapures during a deeper subsequent martensitic transformation,
• it limits the risks of bending the bars by dissymmetry of the cooling, since it constitutes a sort of rigid annular "corset".

This dual role makes it possible to envisage, for the 3rd stage of forced cooling, less demanding cooling means as regards the symmetry of revolution of the quenching than those used in the 1st stage.

• pour la 1ère étape : au moins une boîte à lame d'eau, modulaire, à grande efficacité thermique, du type de celle décrite dans la demande de brevet EP-A-0020246, où le produit en défilement est enveloppé dans une nappe d'eau circulant à vitesse soutenue colinéairement au produit. Eventuellement, la boîte à eau peut être remplacée par un moyen équivalent, comme un dispositif de pulvérisation d'eau, ayant une efficacité de refroidissement comparable ;
• pour la 2ème étape : un refroidissoir à "pas de pélerin" permettant la réhomogénéisation naturelle pendant 1 à 2 minutes d'une ou plusieurs barres en parallèle ;
• pour la 3ème étape :

• · soit un dispositif de rampes oscillantes à pulvérisation d'eau installé sur la seconde partie du refroidissoir précédent,
• · soit une piscine d'eau ou d'huile alimentée en barres par le refroidissoir et permettant une sortie continue de ces barres (plan incliné par exemple).

The installation according to the invention follows from the process described above. It preferably includes:

• for the 1st stage: at least one box with a blade of water, modular, with high thermal efficiency, of the type described in patent application EP-A-0020246, in which the moving product is enveloped in a sheet of water circulating at a sustained speed collinearly with the product. Optionally, the water box can be replaced by an equivalent means, such as a water spraying device, having a comparable cooling efficiency;
• for the 2nd stage: a "pilgrim step" cooler allowing natural rehomogenization for 1 to 2 minutes of one or more bars in parallel;
• for the 3rd stage:

• · Either a device of oscillating ramps with water spraying installed on the second part of the previous cooler,
• · Either a swimming pool of water or oil supplied with bars by the cooler and allowing a continuous exit of these bars (inclined plane for example).

The operating modes of the above-described installation depend, at each cooling step, on the bar diameter considered, on the exit speed of the rolling mill, on the hardenability of the steel grade considered and on the desired structures at the heart of the bars.

Thus, at the level of the first cooling step, the necessary cooling times have been seen depending on the diameter of the bars for a heat exchange coefficient of the quenching device of the order of 10⁴ w / m². ° C. Knowing the exit speeds in rolling and the temperatures at the end of rolling, we can deduce the necessary quenching lengths.

At the level of the 2nd stage, the transformation to the core into perlite is avoided as much as possible if the shade and the diameter treated allow it.

Finally, at the level of the 3rd stage, we seek to obtain essentially martensitic structures. Consequently, the cooling mode will be all the more severe the larger the bar diameter and the kinetics of transformation. fast upper bainite mation (importance of the hardenability of the steel considered between 600 and 300 ° C). In all cases, as has already been specified, it is undesirable to continue this cooling phase below 250 ° C. (core temperature). A limiting case is that where the speed of movement of the bar and the hardenability of the grade considered allows the following evolution of the average temperature of the section: end of rolling around 900 ° C., crossing of a first box with blade d water from 900 to 600 ° C, natural cooling in 1 to 2 minutes, passing through a second box with a blade of water from 600 to 400 - 300 ° C before final tempering.

Other characteristics and advantages of the invention will emerge from the description which follows with reference to FIGS. 2, 3, 4 and 5.

In FIG. 2, the bar 20 comes out of the last stand of the rolling mill 21 to enter one or two boxes 22 with circulation of a water blade of the type of those described in patent application EP-A-0020246. This bar is extracted at regular speed from the water boxes using the extractors 23, then slowed down to a stop on a line of braking rollers 24. A tilting device 25 allows the bar to be placed on a cooler 29 with movable beams performing a movement in "no pilgrim" to ensure the bar a slow translation and rotation on itself. The cooler 29 has three zones, marked 26, 27 and 28a. Zone 26 corresponds to the natural cooling of step b) according to the invention. Zone 27 is a natural cooling of evacuation of the bars after the forced cooling phase ensured in zone 28a, but we can possibly start the final income from this zone 27. Finally, zone 28a corresponds to forced cooling of the step c) according to the invention. This cooling is, in this example, ensured by water spraying ramps parallel to the bars and driven in a longitudinal movement back and forth sufficient for the treatment to be uniform along the bars.

Figure 3 corresponds exactly to the description of FIG. 2, with the exception of the forced cooling of step c) which is carried out at 28b by progressive immersion in a pool of water or oil. An advantageous device of inclined planes not shown allows the progressive introduction and extraction of the bars.

By way of example, an attempt has been made, by means of an installation such as that shown diagrammatically in FIG. 3, to obtain at the heart of a 60 mm diameter bar a mechanical resistance of 1000 N / mm² and a Charpy V resilience of 60 J / cm² on a 42 CD4 type steel.

• un passage en boîte à lame d'eau caractérisée par un coefficient d'échange moyen de l'ordre de 10⁴ W/m². °C, de manière que la température moyenne de chaque barre passe rapidement de 900 °C à 600 °C (étape a) selon l'invention) ;
• puis un refroidissement naturel à l'air jusqu'à l'ambiante (étape b). Le refroidissement forcé ultérieur - étape c) - a été volontairement supprimé dans ce cas.

FIG. 4, on a temperature-time diagram, represents the transformation conditions γ → α along the thermal laws obtained at the heart of 42 CD4 steel bars with respective diameters of 60 mm for the law marked "41", of 100 mm for the law marked "42", and 150 mm for the law marked "43". These thermal laws correspond to the following treatments:

• a passage in a water slide box characterized by an average exchange coefficient of the order of 10⁴ W / m². ° C, so that the average temperature of each bar rapidly goes from 900 ° C to 600 ° C (step a) according to the invention);
• then natural air cooling to ambient (step b). The subsequent forced cooling - step c) - was intentionally eliminated in this case.

• les cycles thermiques 41, 42 et 43 à partir de 800 °C (température A₃ de l'acier),
• la ligne, repérée 44, de début de transformation de l'austénite,
• la ligne, repérée 45, correspondant à 25 % de transformation γ → α,
• la ligne, repérée 46, correspondant à 50 % de transformation,
• la ligne, repérée 47, correspondant à 75 % de transformation,
• la ligne, repérée 48, correspondant à la fin de la transformation.

FIG. 4 shows:

• thermal cycles 41, 42 and 43 from 800 ° C (temperature A₃ of the steel),
• the line, marked 44, from the start of transformation of the austenite,
• the line, marked 45, corresponding to 25% of transformation γ → α,
• the line, marked 46, corresponding to 50% transformation,
• the line, marked 47, corresponding to 75% transformation,
• the line, marked 48, corresponding to the end of the transformation.

Figure 5 corresponds to the same representations as Figure 4, but with a 3rd cooling step according to the invention (step c) characterized by a heat exchange coefficient of 10³ W / m². ° C.

The comparison of Figures 4 and 5 shows that the acceleration of the cooling of the 3rd stage made it possible to obtain at the heart of the three diameters considered more than 50% of structures formed below 300 ° C (martensite).

≃ 4 mm de la peau, la couche périphérique annulaire étant constituée de martensite auto-revenue. Un revenu ultérieur d'une heure à 500 °C sur cette barre a permis d'obtenir une résistance mécanique à peu près uniforme dans toute la section et voisine de 1000 N/mm². Cependant, la résilience Charpy V mesurée à coeur de barre n'a été que de 40 J/cm² à température ambiante.For the 60 mm diameter bar in FIG. 4, the martensitic transformation conditions were carried out substantially from the core up to $\frac{\text{0̸}}{\text{15}}$

≃ 4 mm from the skin, the annular peripheral layer consisting of self-returning martensite. A subsequent tempering of one hour at 500 ° C. on this bar made it possible to obtain a roughly uniform mechanical resistance throughout the section and close to 1000 N / mm². However, the Charpy V resilience measured at the core of the bar was only 40 J / cm² at room temperature.

By opting for a complete treatment according to the invention comprising the same first cooling step as above, a step b) corresponding to 1 minute of natural cooling to 600-550 ° C., and a step c) corresponding to quenching up to 100 ° C characterized by an average exchange coefficient of the order of 10³ w / m². ° C, we follow, at the heart of a 0̸ 60 mm bar, the law 41 of Figure 5. The transformation is then entirely martensitic over the entire section, (a final oil quenching, significantly less effective, would have still leads to 70% martensite at heart). For bars of 100 and 150 mm in diameter, almost three-quarters of the core transformation takes place below 400 ° C with almost 50% martensite. In all cases, an hour's income at 600 ° C made it possible to obtain a resistance of the order of 1000 N / mm² and a Charpy V resilience to the ambient greater than 60 J / cm².

It is clear, from the above examples, that the method according to the invention allows great processing flexibility depending on the diameter of the bars to be treated and the level of mechanical characteristics desired. It also makes it possible to minimize the use of addition elements in steel by lending itself well, in its most general version, to the implementation of economic nuances having only a "pearlitic nose" (duration d 'incubation of the transformation γ → α around 600 ° C) relatively clear and slow transformation kinetics in the field of upper bainite (shades of Mn-B, etc.).

In addition, the process according to the invention also lends itself well to the recovery of rolling operations completed at low temperature (around 850 ° C.), which make it possible to keep a hardened austenite which is not recrystallized at the time of the transformation γ → α . The inheritance of the crystalline defects of the hardened austenite will be all the more interesting that one will pass quickly to temperatures where the restoration of austenite is difficult, then to final structures of hardening.

Claims (9)

1. Process for the heat treatment of alloy steel rods of 50 to 150 mm diameter, which are ready for use, especially for mechanical construction, characterised in that it consists in subjecting the rod, at rolling heat, to a stagewise quenching comprising the following three successive stages:

   a) just after the rolling the rod is subjected on the move to a severe forced cooling exhibiting a heat exchange coefficient of the order of 10⁴ W/m² °C so as to lower the mean temperature of the rod section rapidly to approximately 600-550°C and to form a surface martensite layer while avoiding the formation of ferrite and of perlite at the core;

   b) a self-tempering of the surface martensite layer is then induced by allowing the rod to cool naturally in air until it substantially reaches a thermal rehomogenisation in section at about 550°C, while avoiding a significant formation of perlite or of bainite at the core;

   c) the rod is then again subjected to a forced cooling, of lesser severity than that involved in stage a), in order that its core temperature may fall below approximately 300°C over a period which is compatible with the kinetics of conversion which is chiefly martensitic and bainitic of the steel grade used.
2. Plant for implementing the process according to Claim 1, characterised in that it comprises in succession, in the direction of movement of the product:

   - means making it possible to ensure a forced cooling of the rods on the move according to stage a) ensuring a mean heat exchange coefficient higher than 5x10³ W/m² °C;

   - a cooler with movable racks ensuring a steady pilger step movement and a slow rotation of the rod about itself, the said cooler being equipped with means for forced cooling.
3. Plant according to Claim 2, characterised in that the means for cooling the product according to stage a) consist of at least one water box (22) where the rod is enveloped in a sheet of water circulating linearly together with the rod.
4. Plant according to Claim 3, characterised in that the means for cooling the product according to stage a) consist of a water spraying device.
5. Plant according to Claim 2, characterised in that the means for forced cooling with which the cooler is equipped consist of racks for spraying water.
6. Plant according to Claim 5, characterised in that the said racks are driven in a "to and fro" movement parallel to the axis of the rod to be cooled.
7. Plant according to Claim 2, characterised in that the means for forced cooling with which the cooler is equipped consist of a pool of water or of oil.
8. Plant according to Claim 7, characterised in that the pool comprises two inclined planes for ensuring the introduction and the progressive withdrawal, respectively, of the rods to be cooled.
9. Plant according to any one of Claims 2 to 8, characterised in that it comprises a plant unit for thermal tempering of the rods, which is placed after the cooler.

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