LU84666A1 - PROCESS AND PLANT FOR HEAT TREATMENT OF ALLOYED STEEL BARS 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

1 PROCESS AND INSTALLATION OF HEAT TREATMENT

DE RARRE5 IN ALLOY STEEL READY TO REPLACE

The invention lies in the field of heat treatments of the 5 metal bars at the end of rolling and more specifically relates to the direct production of quenching structures (bainite, martensite) in the useful Dartie of acipr bars, intended in particular for construction mechanical.

The steel industry currently delivers alloy steels with improved machinability which exhibit, in the quenched and tempered state, 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.

In addition, the economy of austenization can be achieved by quenching the bars in the "hot" rolling itself. It is even possible to obtain a more advantageous austenitic state at the end of the rolling than that resulting from a conventional austenitization due to a more complete dissolution of the additives at preheating, with an austenitic grain size. adaptable to the desired hardenability, and a state of hardening of the austenite, more favorable at the time of transformation V - * a ·

The reasons why these bar treatments are hardly developed so far are mainly due to the control of the cooling 50 in the rolling hot. You should first know that the mechanical industries are mainly interested in bars of large diameter (5Γ — 15G mm) in semi-hard shades, which are practically the only ones that allow C to obtain at the heart of these bars resistances of the order of LODON / mm ^. Obtaining a core quenching structure for bars of such large diameter requires relatively hardening steel (42CD4, for example) and a high quenching rate. The risks of spotting are then significant, even if, the duration of the treatment being not limited, it is possible to adopt the 38 least harsh quenching medium compatible with the diameter of the bar

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* t I * and the hardenability of the steel grade.

The situation is further complicated for parade hardening just after rolling. It should be remembered that the most important heat exchange coefficients that we know to achieve practically on read-5 bars are currently of the order of lfi ^ W / m ^. ° C (or 10 ^ * kcal / m ^ .h. ° C), and that it would take approximately 20 seconds to pass through a quenching medium characterized by this efficiency so that the average temperature of a bar of 60 mm in diameter goes from 900 to 4 ° C. (Remember that the average temperature in the section of a bar of radius R is defined by 10 -JTjfR2 ".e (r) .dr where0 (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 for its part would 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).

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.

To this end, the subject of the invention is a treatment method consisting in subjecting the rolled bar to hot rolling, a stepped quench comprising the following three successive stages: a) just after rolling, the bar is subjected during the parade with the action of an extremely severe quenching medium, having a coefficient 4 2 of heat exchange of the order of 10 W / m, ° C so as to bring its average temperature to 600-550 ° C in a minimum time, 50 b) the bar is then allowed to cool naturally in air - or in a similarly slightly cooling medium - until it reaches. substantially to thermal rehomogenization in the section around 550 ° C.

"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 to 30 ° C. or below in a compatible time with the bainitic transformation kinetics of the steel grade 1 constituting the bar.

1 58 I The subject of the invention is also an installation for the implementation of the process comprising, for severe forced cooling of step a), one or more boxes with a blade of water traversed by the moving product. and in which the circulation of the product and the water layer are collinear.

Advantageously, and in isolation or in combination with the aforementioned means, the installation comprises, for the more moderate forced cooling of the etaoe. c), oscillating ramnes with water spray, or a pool of water or oil.

More preferably, the installation comprises, for the thermal homogenization of step b), an air cooler with displacement with "pilgrim steps" on which transit the bars arranged parallel to each other.

To clearly understand the invention, the three successive stages are repeated below in detail with reference to the accompanying drawing plates 15 in which: FIG. 1 represents a diagram of transformation into continuous cooling of a mechanical steel of the 42CD4 type (Γ, 42λ of C, 0.7¾ of Mn, 1¾ of Cr, 0.20¾ of Mo) after an austenization towards E5C-9G0 ° C giving the austenitic structure a size of qrain 20 of the order of 20 / im; - Figure 2 schematically shows an embodiment of an installation for implementing the invention; - Figure 3 schematically shows another embodiment of such an installation; FIG. A schematically represents examples of processing conditions obtained on three different diameters of 42CD4 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 42CD4 steel bars treated according to a complete variant of the method according to the invention.

The first step of the process, denoted a), has two objectives: - to form a surface annular layer of martensite, - to avoid the formation of perlite in most of the section of the bar.

To illustrate these two objectives, FIG. 1 shows, by way of example only intended to better understand the technical field 38 * of the invention, a diagram of a usaoe current in thermal treatments.

Imiques, which represents on a temperature-time graph, the different micrographic constituents resulting, according to the cooling laws considered, from the transformation K -> a of austenite austenite identified 1. Thus fast cooling laws lead to 5 the instantaneous formation of martensite 2 as soon as the temperature of the metal drops below the temperature Ms of the steel (31 ° C. in the example chosen). In contrast, very slow cooling laws lead to the formation of ferrite 3 and perlite 4 at temperatures above 6CPDC. Finally, intermediate cooling laws lead to the formation of bainite 5 and of a volume fraction which is all the more important in martensite as the end of cooling is accelerated.

It should be remembered that, as a general rule, a returned martensite has better tenacity 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, it will be sought, in order to obtain good mechanical characteristics and improve the homogeneity of the structures in the section, to obtain in most of the latter. structures formed at the lowest possible temperatures. To guide in the choice of the cooling cycle, the 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 A ^ of the steel where the transformation begins} a (8DC ° 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 only determined for the continuous laws used to establish the slide. We cannot therefore in all rigor 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 rapidly below, from 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 rt 55 only very slowly reaching the average temperature at the end of quenching. However, the observation of a diagram such as that of Figure 1 suq-: manages a few remarks. Ferrite-perlite will not be formed in this case 58 ^ iParticul ier if all the points of the section see their temperature 5 pass from 800 to 65Ü ° C in less than 150 seconds. Furthermore, a metastability range of austenite 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 towards 575 ° C without significant transformation of I 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.

To return to the two objectives which have been assigned to the 1st stage of bar cooling, it is indicated that the thickness of surface martensite will be greater the higher the quenching severity and the bar diameter. A heat exchange coefficient of 10 W / m. ° C gives, for example on one. 15 bar φ = 60 mm in diameter, a layer of martensite thick • pr - 5 mm ΧΔ 4.2

- a heat exchange coefficient of the order of 10 W / m. ° C

avoids the formation of perlite on bars of diameter up to 150 mm laminated in shade 42CD4 corresponding to the diagram in FIG. 1. The quench times between 900 ° C. and an average temperature of 60 ° C. would be then 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 also be 25 if necessary than an increase in the hardenability of the steel (addition of manganese, molybdenum, boron ... ).

As regards now the 2nd stage, or 2nd phase of the process, noted b), the natural thermal rehomogenization with air of the bar around 55 ° C. generally lasts from 1 to 2 minutes and allows: 2 © - at the core from the bar, to see the temperature drop below 6üC, 0C without noticeable 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 a minute above 5CiC ° C.

25 It goes without saying that, for certain grades that are less soothing than the 42CD4 presented, it will be possible to seek to obtain, at the heart of the bar, during this 2nd step a large volume fraction of fine perlite formed 28, around 625-65L ° C. The mechanical characteristics of these fine perlites, I in particular those hardened by microprecipitation, can prove to be satisfactory for certain applications and have the advantage of not requiring any final income. For these less restrictive applications than those requiring the use of quenched quenching structures, the 2nd stage of natural cooling will be exceptionally extended to room temperature. In this case, step c) is therefore deleted.

Finally, on the subject of this 3rd stage of the treatment, noted c), one must, after the thermal homogenization of the previous phase, and if one is looking for · essentially martensitic structures at heart, subject the bar to a second cooling forced allowing the core temperature to drop below 300 ° C in a time compatible with the kinetics of bainitic transformation 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 sufficient to accelerate the cooling between 550 and 4CD ° C. For particularly hardening grades and / or small diameters, simple natural or slightly accelerated cooling will be sufficient. For example, in the case of grade 42CD4 presented in FIG. 1, it is sufficient to cool to more than 4 or 5 ° C / s between 550 and 3P0DC to obtain an essentially martensitic structure with a core of closed off.

In all cases, the superficial annular layer of martensite obtained in the 1st stage of the treatment and self-returning in the 2nd stage has a double role in the 3rd stage of forced cooling: 25 - it considerably reduces the risks 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".

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

The installation according to the invention follows from the method described above. It preferably includes: - for the 1st stage: at least one modular, high thermal efficiency water blade box, of the type described in French patent application nD 79/14383, where the product is scrolled is wrapped * I „15 · 1 in a sheet of water circulating at a sustained speed collinearly to the I product. Optionally, the water box can be replaced by an equivalent means 1, such as a water spraying device, having a comparable cooling efficiency.

I 5 for the 2nd stage: a "pilgrim's step" cooler allowing 1 natural rehomogenization for 1 to 2 minutes of one or more bars in parallel, 1 - for the 3rd stage: - either a device of oscillating ramps 1 with water spraying installed on the second part of the cooler precede I 10 - either a pool of water or oil supplied with bars by the cooler and allowing continuous exit of these bars ( inclined plane I for example).

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

I Thus, at the level of the first cooling step, we saw 9 the necessary cooling times according to the diameter of the bars I for a coefficient of heat exchange of the device of thirty of the order 9 ή 2 9 20 of 10 W / m. ° C. Knowing the exit speeds in rolling and the 9 temperatures at the end of rolling, we can deduce the lengths of 9. quenching required.

9 At the level of the 2nd stage, one can either seek a maximum self-income 9 of the peripheral layer by letting the heart transform into 1 25 perlite, or avoid as much as possible the transformation at heart if the shade and 9 the diameter treated allow.

9 Finally, at the 3rd stage if we are trying to obtain essentially martensitic structures, the cooling mode will be all the more severe the larger the bar diameter and the kinetics of transformation into superior bainite fast (importance of the hardenability of the steel considered between 6CG and 300 ° C). In any case, as already stated, it is undesirable to continue this cooling phase below 25 ° C (core temperature). A case ! limit 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 9C0 ° C, crossing of a first blade box of water from 9L0 to 6L [, 0C, natural cooling 58 • in 1 to 2 minutes, crossing a second box with a blade of water from 6Cf- to

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jjfl y »- - 14D0-3G0oC before the final income. Another borderline case is that where, pearlitic structures at the core being admitted, the 2nd and 3rd stages are natural air cooling on a conventional cooler.

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

In Figure 2, the bar 20 comes out of the last caqe of the rolling mill 21 to enter one or two boxes 22 with circulation of a water blade of the type described in French patent application No. 79/14383. This bar is extracted at regular speed from the water boxes using the 10 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 at mobile side members moving in "no pilgrim" manner to ensure the bar translation and slow rotation on itself. The cooler 29 has three zones, 15 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 it is possible possibly to 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.

FIG. 3 corresponds very exactly to the description of FIG. 25 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, we have sought, by means of an installation such as that shown schematically in FIG. 3, to obtain at the heart of a bar of 6L · mm in diameter a mechanical resistance of 1GGC N / mm and a Charpy resilience V of 6L · 3 / cn ^ on steel type 42CD4.

FIG. 4, on a temperature-time diagram, represents the transformation conditions y- * a along the thermal laws obtained at 55 core of 42CD4 steel bars with respective diameters of 60 mm for the law marked "41", of 1GC mm for the law marked "42", and 150 mm for the law marked "43". These thermal laws correspond to the following treatments: 5 * !. - - a passaqe in a box with a blade of water characterized by a coefficient Λ 2 of average exchange of the order of 10 W / m. ° C, so that the average temperature of each bar rapidly goes from 900 ° C to 6L'0DC (step a) according to the invention).

5 - then natural air cooling to ambient (step b). The subsequent forced cooling - step c) - was intentionally eliminated in this case.

FIG. 4 shows: - the thermal cycles 41, 42 and 43 from 800 ° C (temperature of the steel), - the line, marked 44, from the start of transformation of the austenite, - the line, marked 45, corresponding to 25 £ of transformation F · * <* j - line, marked 46, corresponding to 50% of transformation, 15 - line, marked 47, corresponding to 75 S of transformation, - 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) 3 2 20 characterized by a heat exchange coefficient of 10 W / m. ° 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 3L0 ° C (martensite).

25 For the 60 mm diameter bar of FIG. 4, the bainitic transformation conditions were carried out substantially from the heart to -4 mm from the skin, the annular peripheral layer consisting of 15 self-returning martensite. A subsequent return of one hour at 5GC, 0C out of 50 this bar made it possible to obtain a mechanical resistance approximately 2 uniform throughout the section and close to 1000 h '/ mm. However, the 2 Charpy V resilience measured at the heart of the bar was only 40 3 / cm at I. ambient temperature.

By opting for a complete treatment according to the invention comprising the same first 55 cooling step as previously, a step b) corresponding to 1 minute of natural cooling towards 6G0-55C, oC, and a step c) corresponding to quenching up to at 1DC ° C characterized by a 3 2 average exchange coefficient of the order of 10 W / m, ° C, we follow, at heart a bar of φ 60 mm, the law 41 of Figure 5. The transformation is Γ · 'ίο then fully martensitic over the entire section, (a final oil quenching, significantly less effective, would still have led to 70% martensite at heart). For ICO bars and 150 mm in diameter, almost three quarters of the core transformation takes place below 5 ALÜ ° C with almost 50% martensite. In all cases, an income of 7 hours at 600 ° C. made it possible to obtain a resistance of the order of 1000 N / mm 2 and a Charpy V è ambient resilience greater than 6C 3 / cm.

It is clear, from the above examples, that the method according to the invention allows great processing flexibility depending on the diameter 10 of the bars to be treated and the level of mechanical characteristics desired.

It also makes it possible to reduce the use of addition elements in steel as much as possible, lending itself well, in its most general version, to the use of economical grades having only a "pearlitic nose" (duration d incubation of the transformation V - * a towards 15 6C0 C) relatively clear and a slow transformation kinetics in the field of superior bainite (shades with Mn-R, etc ...).

In addition, the process according to the invention also lends itself well to the recovery of rolling operations completed at low temperature (around 85 ° C.), which make it possible to keep a hardened austenite and not recrystallized at the time of processing. > 'a. 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.

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Claims (1)

10 By a pool of water or oil. I 9) Installation according to claim 8, characterized in that the I swimming pool has two inclined planes to respectively ensure the introduction of the progressive extraction of the bars to be cooled. ! 10) Installation according to any one of claims 5 to 9, I. 15 characterized in that it comprises a thermal income station I bars placed after the cooler. ^