EP2616561B1 - Martensitic stainless steel machineability optimization - Google Patents

Martensitic stainless steel machineability optimization Download PDF

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Publication number
EP2616561B1
EP2616561B1 EP11773051.5A EP11773051A EP2616561B1 EP 2616561 B1 EP2616561 B1 EP 2616561B1 EP 11773051 A EP11773051 A EP 11773051A EP 2616561 B1 EP2616561 B1 EP 2616561B1
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EP
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Prior art keywords
temperature
steel
fabricating
cooling
martensitic stainless
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German (de)
French (fr)
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EP2616561A1 (en
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Jean-François Laurent CHABOT
Laurent Ferrer
Pascal Charles Emile Thoison
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Safran Aircraft Engines SAS
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SNECMA SAS
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    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/19—Hardening; Quenching with or without subsequent tempering by interrupted quenching
    • C21D1/22—Martempering
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite

Definitions

  • the percentages of composition are percentages by weight unless otherwise specified.
  • a stainless martensitic steel is a steel with a chromium content greater than 10.5%, and whose structure is essentially martensitic (ie the amount of alphagenic elements is sufficiently high compared to that of the elements Gammagens - see explanations below).
  • This half-product is then pre-cut into sub-elements which are shaped (for example by forging or rolling) in order to give them a shape approximating their final shape.
  • Each sub-element thus becomes a part with overthicknesses (called part in the rough state) with respect to the final dimensional dimensions of use.
  • the objective of the phase (A) is to homogenize the microstructure within the part, and to re-dissolve soluble particles at this temperature by recrystallization.
  • Phase (B) has as its primary objective a maximum transformation of austenite to martensite within the steel part.
  • transformations of the martensitic microstructure do not occur simultaneously at any point in the room, but gradually from its surface to its core.
  • the change in crystallographic volume that accompanies these transformations therefore generates internal stresses and, at the end of quenching (because of the low temperatures then reached), limits the relaxations of these stresses.
  • the second objective is to minimize the risk of quenching taps, that is to say the appearance of cracks on the surface of the workpiece by the release of residual stresses in the steel in a weak metallurgical martensitic state.
  • phase (C) a treatment of income
  • T max is substantially equal to the nominal temperature M F end of martensitic transformation of the steel, ie from 150 to 200 ° C for a martensitic stainless steel.
  • T min is 20 to 28 ° C depending on the chemical composition. It then remains in the steel a residual austenite rate that could not be transformed.
  • Phase (C) - first treatment of income - of this quality heat treatment aims on the one hand a transformation of fresh martensite into martensite revenue (more stable and more tenacious) and on the other hand a destabilization of the residual austenite from previous phases.
  • phase (D) - cooling of the first income - of this quality heat treatment aims to transform the residual austenite into martensite.
  • the hottest part of the room must also be cooled down to a temperature within the temperature range [T max ; T min ].
  • phase (E) - second treatment of income - of this heat treatment of quality aims at the transformation of the new fresh martensite into martensite revenue (more stable and tenacious) aiming to reach the best compromise in the mechanical properties of the steel.
  • phase (F) - cooling of the second income - of this quality heat treatment brings the raw room to room temperature.
  • the documents FR 2 920 784 and FR 2 893 954 disclose the manufacture of a martensitic stainless steel by austenitization followed by tempering and two incomes.
  • the present invention aims to provide a manufacturing method that improves the machinability of these steels.
  • thermomechanical treatments such as forging, rolling
  • This blank is then intended to be machined to give it its final shape after performing the heat treatment quality.
  • the blank in this steel is heated to a temperature above the austenization temperature T AUS , and the workpiece is maintained at this temperature until the entire workpiece is at a temperature of temperature above the austenization temperature T AUS (austenization of steel).
  • the steel is then quenched sufficiently fast so that the austenite does not turn into a ferrito-pearlitic structure (see explanations and figure 3 below).
  • the majority of the volume of the steel part is likely to turn into martensite, since the austenite can only be transformed into martensite if it has not previously been transformed into a ferrito-pearlitic structure.
  • the austenization of the steel and then its quenching corresponds to treatment 1 on the figure 1 .
  • the steel solidifies gradually during its cooling. This solidification takes place by growth of dendrites 10, as illustrated in FIG. figure 2 .
  • the dendrites 10, corresponding to the first solidified grains are by definition richer in alphagenes elements while the interdendritic regions 20 are richer in elements, gamma (application of the rule known segments on the phase diagram).
  • An alphagene element is an element that favors a ferritic type structure (structures that are more stable at low temperature: bainite, ferrite-pearlite, martensite).
  • a gammagenic element is an element that favors an austenitic structure (stable structure at high temperature: austenite). There is therefore segregation between dendrites 10 and interdendritic regions 20.
  • the figure 3 is a known temperature (T) - time (t) diagram for a steel according to the invention when it is cooled from a temperature above the austenitic temperature T AUS .
  • Curves D and F mark the beginning and the end of the austenite transformation (region A) in a ferrito-pearlitic structure (FP region). This transformation takes place, partially or fully, when the cooling curve C that following the ingot passes respectively in the region between the D and F curves or in the FP region. It does not take place when the cooling curve C is entirely in the region A, as illustrated in FIG. figure 3 .
  • curves D, F, M S , and M F in solid lines are valid for structures richer in alphagenic elements (that is to say in the dendrites of steel), whereas the same curves in lines dotted d ', F', M S , and M F are valid for structures richer in gammagene elements (that is to say in the interdendritic spaces of steel).
  • austenite transformation curves in the ferrito-pearlitic structure in the case of interdendritic spaces are shifted to the right with respect to the austenite transformation curves into a ferrito-pearlitic structure in the dendrites (curves D and F). It takes more time at a given temperature to transform the austenite into a ferritic-pearlitic structure in the case of interdendritic spaces than in the case of dendrites.
  • the cooling of the steel during quenching after austenisation follows curve C of the figure 3 .
  • the steel goes below the temperature of martensitic transformation end in cooling M F interdendritic spaces. Due to the cooling process, the skin temperature of the room is lower than the temperature in the heart of the room, which is its hottest part.
  • This heating is effected for example by placing the room in an environment (preheated oven or heating chamber) where there is a temperature at least equal to the maximum temperature T max .
  • a first income of the steel is then made by continuing to heat it up to a temperature T R , which is lower than the austenitic temperature T AUS .
  • This income makes it possible to stabilize the fresh martensitic crystallographic phase by, for example, precipitating carbides within the martensite and thus imparting more resilience to the martensite of the steel.
  • This first income treatment corresponds to step 2 in figure 1 .
  • the steel is then cooled until the hottest part of the steel reaches the maximum temperature T max which is lower than the martensitic transformation end temperature in cooling M F 'of the interdendritic spaces, and is then heated immediately. steel.
  • the steel is then immediately subjected to a second treatment of income, substantially identical to the first treatment of income, then allowing the steel to cool to room temperature T A.
  • This second income treatment corresponds to step 3 in figure 1 .
  • the inventors have carried out machinability tests on stainless martensitic steels having undergone the process of the invention. They compared the results of these tests to the results of machinability tests on austenized steels followed by quenching and two incomes but where the minimum temperature of the hottest part of the part is simply less than the martensitic transformation end temperature in cooling M F of the dendrites, and the steel is not immediately warmed between tempering and first income, or between first income and second income.
  • the wear of the machining plates per meter of machined steel is divided by about 10 (11 mm to 1.3 mm cutting speed of 120 m / min compared to a steel manufactured according to a method of the prior art.
  • the power required for machining is further divided by more than two compared to a steel manufactured according to a method of the prior art.
  • the surface condition of the steel after machining is also improved.
  • the results can be explained as follows: as indicated above, the martensitic transformation end temperature in cooling M F 'of the interdendritic regions is less than the martensitic transformation end of cooling temperature M F dendrites. Now we have seen that during the cooling of steel, this steel solidifies into a microstructure which is an alternation of dendrites and interdendritic regions ( figure 2 ). Thus, when the temperature drops below the martensitic transformation end temperature in M F cooling of the dendrites, the dendrites have become martensite, while the interdendritic regions have not yet been transformed into martensite.
  • zones in all the steel ie the interdendritic regions
  • residual austenite Part of this residual austenite will be transformed at the next first income stage into fresh martensite.
  • the other part of this residual austenite will be located only at the most segregated points of the material (for example, in the most concentrated interdendritic spaces).
  • the new fresh martensite stabilizes but another portion of the remaining residual austenite continues to turn into fresh martensitic in these most segregated areas.
  • Steel therefore has a structural heterogeneity with harder grains corresponding to fresh martensite in a softer matrix. It is this heterogeneity that is responsible for the bad machinability of steel, the harder grains using platelets and blocking their advance.
  • the maximum temperature T max that reaches the hottest part of the steel before being reheated is between 20 ° C and 75 ° C.
  • Such a temperature T m is lower than the martensitic transformation end temperature in cooling M F 'interdendritic spaces.
  • this maximum temperature T max is between 28 ° C and 35 ° C.
  • step ( ⁇ ) In order to determine when the hottest part of the steel reaches the maximum temperature T max , it is possible for example, in step ( ⁇ ), to measure the skin temperature of the steel and to use abacuses to deduce the temperature of the hottest part of the steel.
  • the temperature gradient between the surface of the steel and the hottest part of the steel is as small as possible in order to reduce the gap between the end temperature of the steel martensitic transformation in M F cooling of dendrites and martensitic transformation end temperature in cooling M F 'interdendritic spaces. Indeed, by reducing this gap, the constraints in the room are then lower, and we gain in productivity.
  • the threshold duration d s depends on the geometry of the part.
  • the length of s is at least 15 minutes (min) to a minimum dimension of the part of 50 mm, 30 min to a minimum dimension of the part of 100 mm, 45 min to a minimum dimension of the workpiece 150 mm, and so on.
  • d s (15 min) ⁇ ⁇ minimum dimension (in mm) ⁇ / 50.
  • the steel can for example be placed in an oven where a temperature of between T min and MF' prevails.
  • the steel can be thermally insulated from the outside environment, for example by placing it in a blanket.
  • At least one expansion of the steel is performed at a temperature below the temperature of income T R at which the first income and the second income have been made.
  • This relaxation corresponds to step 4 in figure 1 . It allows the relaxation of residual stresses within the steel, and improves the service life.
  • the ESR process consists in placing a steel ingot in a crucible in which a slag (mineral mixture, for example lime, fluoride, magnesia, alumina, spath) has been poured in such a way that the lower end of the ingot quenches in the slag . Then an electric current is passed into the ingot, which serves as an electrode. This stream liquefies the slag and melts the lower end of this electrode which is in contact with the slag. The molten steel of this electrode passes through the slag in the form of fine droplets, to solidify below the layer of supernatant slag, into a new ingot that grows gradually.
  • a slag mineral mixture, for example lime, fluoride, magnesia, alumina, spath
  • the slag acts, inter alia, as a filter which extracts the inclusions from the steel droplets, so that the steel of this new ingot located below the slag layer contains fewer inclusions than the initial ingot (electrode). .
  • This operation is carried out at atmospheric pressure and air.
  • the VAR process consists in melting in a crucible under a high vacuum the steel ingot, which serves as an electrode.
  • the ingot / electrode is melted by establishing an electric arc between the end of the ingot / electrode and the top of the secondary ingot which is formed by melting the ingot / electrode.
  • the secondary ingot solidifies in contact with the walls of the crucible and the inclusions float on the surface of the secondary ingot, and may subsequently be removed. A secondary ingot of greater purity than the initial ingot / electrode is thus obtained.
  • the steel undergoes, before step (1), a reflow.
  • reflow is chosen from a group comprising ESR slag remelting or VAR vacuum arc remelting.
  • step (1) a homogenization treatment of the steel is carried out.
  • the inventors have found that satisfactory results are obtained when the ingot is subjected in this oven to a homogenization treatment during a holding time t after the temperature of the most The cold of this ingot has reached a homogenization temperature T, this time t being equal to at least one hour, and the homogenization temperature T varying between a lower temperature T inf and the burn temperature of this steel.
  • the temperature T inf is approximately equal to 900 ° C.
  • the burn temperature of a steel is defined as the temperature in the raw state of solidification at which the grain boundaries in the steel transform (or even liquefy), and is greater than T Inf . This time t of maintaining the steel in the furnace therefore varies inversely with this homogenization temperature T.
  • the homogenization temperature T is 950 ° C., and the corresponding holding time t is equal to 70 hours.
  • the homogenization temperature T is 1250C which is slightly lower than burn temperature, then the corresponding holding time t is equal to 10 hours.
  • the maximum temperature T max is lower than the martensitic transformation end temperature in cooling M F of the dendrites in the steel, and in steps (1) and (2) it is ensured that steel remains at or below the maximum temperature T max for as short a time as possible.

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Description

La présente invention concerne un procédé de fabrication d'un acier martensitique inoxydable comportant les étapes de traitement thermique suivantes :

  1. (1) On chauffe l'acier à une température supérieure à la température d'austénisation TAUS de l'acier, puis on trempe l'acier jusqu'à ce que la partie la plus chaude de l'acier soit inférieure ou égale à une température maximale Tmax, et supérieure ou égale à une température minimale Tmin, la vitesse de refroidissement étant suffisamment rapide pour que l'austénite ne se transforme pas en structure ferrito-perlitique.
  2. (2) On effectue un premier revenu de l'acier suivi d'un refroidissement jusqu'à ce que la partie la plus chaude de l'acier soit inférieure ou égale à la température maximale Tmax, et supérieure ou égale à la température minimale Tmin.
  3. (3) On effectue un second revenu de l'acier suivi d'un refroidissement jusqu'à température ambiante TA.
La température ambiante est égale à la température de la pièce où le procédé est réalisé.The present invention relates to a method for manufacturing a stainless martensitic steel comprising the following heat treatment steps:
  1. (1) The steel is heated to a temperature above the austenization temperature T AUS of the steel, and then the steel is quenched until the hottest part of the steel is less than or equal to a maximum temperature T max , and greater than or equal to a minimum temperature T min , the cooling rate being fast enough so that the austenite does not turn into a ferrito-pearlitic structure.
  2. (2) A first steel income is followed by cooling until the hottest part of the steel is less than or equal to the maximum temperature T max and greater than or equal to the minimum temperature T min .
  3. (3) A second steel income is followed by cooling to room temperature T A.
The ambient temperature is equal to the temperature of the room where the process is carried out.

Dans la présente invention, les pourcentages de composition sont des pourcentages massiques, à moins qu'il en soit précisé autrement.In the present invention, the percentages of composition are percentages by weight unless otherwise specified.

Un acier martensitique inoxydable est un acier dont la teneur en chrome est supérieure à 10,5%, et dont la structure est essentiellement martensitique (c'est-à-dire que la quantité en éléments alphagènes est suffisamment élevée par rapport à celle des éléments gammagènes - Voir explications ci-dessous).A stainless martensitic steel is a steel with a chromium content greater than 10.5%, and whose structure is essentially martensitic (ie the amount of alphagenic elements is sufficiently high compared to that of the elements Gammagens - see explanations below).

On part d'un demi-produit sous une forme quelconque, par exemple sous une forme de billettes ou de barres de cet acier.We start from a half-product in any form, for example in the form of billets or bars of this steel.

Ce demi-produit est ensuite prédécoupé en sous-éléments qui sont mis en forme (par exemple par forgeage ou laminage) afin de leur conférer une forme se rapprochant de leur forme finale. Chaque sous-élément devient ainsi une pièce avec des surépaisseurs (appelée pièce à l'état brut) par rapport aux côtes dimensionnelles finales d'utilisation.This half-product is then pre-cut into sub-elements which are shaped (for example by forging or rolling) in order to give them a shape approximating their final shape. Each sub-element thus becomes a part with overthicknesses (called part in the rough state) with respect to the final dimensional dimensions of use.

Cette pièce à l'état brut avec des surépaisseurs est destinée ensuite à être usinée afin de lui donner sa forme finale (pièce finale).This piece in the raw state with overthickness is then intended to be machined to give it its final shape (final piece).

Dans le cas où les pièces finales doivent posséder une grande précision dimensionnelle (comme par exemple dans l'aéronautique), ces pièces à l'état brut doivent subir un traitement thermique (traitement thermique de qualité) avant cet usinage. Ce traitement thermique de qualité ne peut pas être effectué après cet usinage, car il conduit à des changements dimensionnels qu'il est difficile de prévoir pour des pièces de géométrie complexe.In the case where the final parts must have a high dimensional accuracy (as for example in aeronautics), these parts in the raw state must undergo a heat treatment (heat treatment quality) before this machining. This quality heat treatment can not be performed after this machining, because it leads to dimensional changes that are difficult to predict for parts of complex geometry.

Ce traitement thermique de qualité qui permet d'ajuster très finement les propriétés de la pièce en acier par des transformations métallurgiques comprend six phases majeures :

  1. (A) une austénisation, c'est-à-dire un chauffage au-dessus de la température à laquelle la microstructure de l'acier s'est transformée en austénite (température austénitique TAUS)
  2. (B) suivie d'une trempe,
  3. (C) suivie d'un premier traitement de revenu,
  4. (D) suivi d'un refroidissement
  5. (E) suivi d'un second traitement de revenu
  6. (F) suivi d'un refroidissement.
This quality heat treatment, which allows the properties of the steel part to be very finely tuned by metallurgical transformations, comprises six major phases:
  1. (A) austenization, that is, heating above the temperature at which the microstructure of the steel has turned into austenite (austenitic temperature T AUS )
  2. (B) followed by quenching,
  3. (C) followed by a first income treatment,
  4. (D) followed by cooling
  5. (E) followed by a second income treatment
  6. (F) followed by cooling.

La phase (A) a pour objectif d'homogénéiser la microstructure au sein de la pièce, et de remettre en solution de particules solubles à cette température par recristallisation.The objective of the phase (A) is to homogenize the microstructure within the part, and to re-dissolve soluble particles at this temperature by recrystallization.

La phase (B) a pour objectif premier une transformation maximale de l'austénite en martensite au sein de la pièce en acier. Cependant, les transformations de la microstructure martensitique ne se font pas simultanément en tout point de la pièce, mais graduellement de sa surface vers son coeur. Le changement de volume cristallographique qui accompagne ces transformations engendre donc des contraintes internes et, en fin de trempe (à cause des basses températures atteintes alors), limite les relaxations de ces contraintes. Le second objectif est de minimiser le risque de tapures de trempe, c'est-à-dire l'apparition de fissures en surface de la pièce de par la libération de contraintes résiduelles dans l'acier dans un état métallurgique martensitique peu tenace. Pour atteindre ces deux objectifs antinomiques, il est habituel de commencer à réchauffer la pièce par un traitement de revenu (phase (C)) lorsque sa partie la plus chaude s'est refroidie jusqu'à une température dans une fourchette avec une température maximale Tmax et une température minimale Tmin pour éviter les tapures. La température Tmax est sensiblement égale à la température nominale MF de fin de transformation martensitique de l'acier, soit de 150 à 200°C pour un acier martensitique inoxydable. La température Tmin est de 20 à 28°C suivant la composition chimique. Il reste alors dans l'acier un taux d'austénite résiduelle qui n'aura pas pu être transformé.Phase (B) has as its primary objective a maximum transformation of austenite to martensite within the steel part. However, transformations of the martensitic microstructure do not occur simultaneously at any point in the room, but gradually from its surface to its core. The change in crystallographic volume that accompanies these transformations therefore generates internal stresses and, at the end of quenching (because of the low temperatures then reached), limits the relaxations of these stresses. The second objective is to minimize the risk of quenching taps, that is to say the appearance of cracks on the surface of the workpiece by the release of residual stresses in the steel in a weak metallurgical martensitic state. To achieve these two antinomic objectives, it is usual to start warming the room by a treatment of income (phase (C)) when its hottest part has cooled down to a temperature in a range with a maximum temperature T max and a minimum temperature T min to avoid taps. The temperature T max is substantially equal to the nominal temperature M F end of martensitic transformation of the steel, ie from 150 to 200 ° C for a martensitic stainless steel. The temperature T min is 20 to 28 ° C depending on the chemical composition. It then remains in the steel a residual austenite rate that could not be transformed.

La phase (C) - premier traitement de revenu - de ce traitement thermique de qualité a pour objectif d'une part une transformation de la martensite fraîche en martensite revenue (plus stable et plus tenace) et d'autre part une déstabilisation de l'austénite résiduelle issue des phases antérieures.Phase (C) - first treatment of income - of this quality heat treatment aims on the one hand a transformation of fresh martensite into martensite revenue (more stable and more tenacious) and on the other hand a destabilization of the residual austenite from previous phases.

La phase (D) - refroidissement du premier revenu - de ce traitement thermique de qualité a pour objectif de transformer l'austénite résiduelle en martensite. La partie la plus chaude de la pièce doit être aussi refroidie jusqu'à une température dans la fourchette de températures [Tmax ; Tmin].The phase (D) - cooling of the first income - of this quality heat treatment aims to transform the residual austenite into martensite. The hottest part of the room must also be cooled down to a temperature within the temperature range [T max ; T min ].

La phase (E) - second traitement de revenu - de ce traitement thermique de qualité a pour objectif la transformation de la nouvelle martensite fraîche en martensite revenue (plus stable et plus tenace) visant à atteindre le meilleur compromis dans les propriétés mécaniques de l'acier.The phase (E) - second treatment of income - of this heat treatment of quality aims at the transformation of the new fresh martensite into martensite revenue (more stable and tenacious) aiming to reach the best compromise in the mechanical properties of the steel.

La phase (F) - refroidissement du second revenu - de ce traitement thermique de qualité ramène la pièce brute à température ambiante.The phase (F) - cooling of the second income - of this quality heat treatment brings the raw room to room temperature.

Les documents FR 2 920 784 et FR 2 893 954 divulguent la fabrication d'un acier inoxydable martensitique par une austénitisation suivie d'une trempe et de deux revenus.The documents FR 2 920 784 and FR 2 893 954 disclose the manufacture of a martensitic stainless steel by austenitization followed by tempering and two incomes.

Durant l'usinage des pièces, malgré ce traitement thermique de qualité, on observe actuellement une grande dispersion dans l'usinabilité de lots de pièces formées dans un acier résultant d'un tel procédé de fabrication. Il en découle des variations importantes dans l'usure des plaquettes d'usinage, et des variations importantes dans les puissances nécessaires à fournir par le dispositif d'usinage pour parvenir à usiner ces pièces en acier. La conséquence est une consommation trop importante, dispersée et imprévisible de plaquettes d'usinage, une perte de cadence dans l'usinage de lots de pièces, et une dispersion dans les états de surface obtenus, avec dans certains cas de moins bons états de surface usinée des pièces.During the machining of parts, despite this quality heat treatment, there is currently a large dispersion in the machinability of batches of parts formed in a steel resulting from such a manufacturing process. This results in significant variations in the wear of the machining wafers, and significant variations in the power required to provide the machining device to achieve machining these steel parts. The consequence is an excessive, dispersed and unpredictable consumption of machining plates, a loss of cadence in the machining of batches of parts, and a dispersion in the obtained surface states, with in certain cases of poorer surface finishes. machined parts.

La présente invention vise à proposer un procédé de fabrication qui permette d'améliorer l'usinabilité de ces aciers.The present invention aims to provide a manufacturing method that improves the machinability of these steels.

Ce but est atteint grâce au fait que, la température maximale Tmax est inférieure ou égale à la température de fin de transformation martensitique en refroidissement MF des espaces interdendritiques dans l'acier, et en ce que, à la fin de chacune des étapes (1) et (2), on effectue la sous-étape suivante :

  • (ω) Dès que la température de la partie la plus chaude de l'acier atteint la température maximale Tmax, on réchauffe l'acier immédiatement.
This object is achieved by virtue of the fact that, the maximum temperature T max is less than or equal to the martensitic transformation end temperature in cooling M F of the interdendritic spaces in the steel, and in that at the end of each step (1) and (2), the following sub-step is carried out:
  • (ω) As soon as the temperature of the hottest part of the steel reaches the maximum temperature T max , the steel is heated immediately.

Grâce à ces dispositions, on obtient une moindre usure des plaquettes d'usinage par unité de longueur usinée, et une moindre puissance requise pour l'usinage. L'état de surface de l'acier après usinage est également amélioré (plus faibles tailles des stries causées par la plaquette d'usinage sur la surface). Ainsi, on diminue le coût du procédé.With these provisions, we obtain a lower wear of the machining plates per unit of machined length, and less power required for machining. The surface condition of the steel after machining is also improved (smaller streak sizes caused by the machining wafer on the surface). Thus, the cost of the process is reduced.

L'invention sera bien comprise et ses avantages apparaîtront mieux, à la lecture de la description détaillée qui suit, d'un mode de réalisation représenté à titre d'exemple non limitatif. La description se réfère aux dessins annexés sur lesquels :

  • la figure 1 montre schématiquement les traitements thermiques du procédé selon l'invention,
  • la figure 2 est un schéma illustrant les dendrites et les régions interdendritiques,
  • la figure 3 montre schématiquement un diagramme temps-température pour un acier utilisé dans le procédé selon l'invention.
The invention will be better understood and its advantages will appear better on reading the detailed description which follows, of an embodiment shown by way of non-limiting example. The description refers to the accompanying drawings in which:
  • the figure 1 schematically shows the heat treatments of the process according to the invention,
  • the figure 2 is a diagram illustrating dendrites and interdendritic regions,
  • the figure 3 schematically shows a time-temperature diagram for a steel used in the process according to the invention.

Dans le procédé selon l'invention, on part d'une pièce brute avec des surépaisseurs qui a subi une succession de traitements thermomécaniques (tels que le forgeage, le laminage) afin de lui conférer une forme la plus proche possible de sa forme finale.In the process according to the invention, starting from a blank with thicknesses that has undergone a succession of thermomechanical treatments (such as forging, rolling) to give it a shape as close as possible to its final shape.

Cette pièce brute est destinée ensuite à être usinée afin de lui donner sa forme finale après avoir réalisé le traitement thermique de qualité.This blank is then intended to be machined to give it its final shape after performing the heat treatment quality.

On chauffe la pièce brute en cet acier jusqu'à une température supérieure à la température d'austénisation TAUS, et on maintient la pièce à cette température jusqu'à ce que la totalité de la pièce soit à une température supérieure à la température d'austénisation TAUS (austénisation de l'acier).The blank in this steel is heated to a temperature above the austenization temperature T AUS , and the workpiece is maintained at this temperature until the entire workpiece is at a temperature of temperature above the austenization temperature T AUS (austenization of steel).

On effectue ensuite une trempe de l'acier suffisamment rapide pour que l'austénite ne se transforme pas en structure ferrito-perlitique (voir explications et figure 3 ci-dessous). Ainsi, la majorité du volume de la pièce en acier est susceptible de se transformer en martensite, puisque l'austénite ne peut se transformer en martensite que si elle n'a pas au préalable été transformée en structure ferrito-perlitique.The steel is then quenched sufficiently fast so that the austenite does not turn into a ferrito-pearlitic structure (see explanations and figure 3 below). Thus, the majority of the volume of the steel part is likely to turn into martensite, since the austenite can only be transformed into martensite if it has not previously been transformed into a ferrito-pearlitic structure.

Enfin on termine par les deux revenus successifs pour affiner les propriétés de l'acier.Finally we end with the two successive incomes to refine the properties of steel.

L'austénisation de l'acier puis sa trempe correspondent au traitement 1 sur la figure 1.The austenization of the steel and then its quenching corresponds to treatment 1 on the figure 1 .

On décrit ci-dessous différentes transformations métallurgiques susceptibles de se produire au sein d'un acier selon l'invention au cours de son refroidissement depuis la température austénitique.Various metallurgical transformations that may occur in a steel according to the invention during its cooling from the austenitic temperature are described below.

En amont de la chaine industrielle, durant les opérations d'élaboration et la réalisation du dernier lingot, l'acier se solidifie progressivement pendant son refroidissement. Cette solidification s'effectue par croissance de dendrites 10, comme illustré en figure 2. En accord avec le diagramme de phases des aciers martensitiques inoxydables, les dendrites 10, correspondant aux premiers grains solidifiés sont par définition plus riches en éléments alphagènes tandis que les régions interdendritiques 20 sont plus riches en éléments, gammagènes (application de la règle connue des segments sur le diagramme de phases). Un élément alphagène est un élément qui favorise une structure de type ferritique (structures plus stables à basse température : bainite, ferrite-perlite, martensite). Un élément gammagènes est un élément qui favorise une structure austénitique (structure stable à haute température : austénite). Il se produit donc une ségrégation entre dendrites 10 et régions interdendritiques 20.Upstream of the industrial chain, during the development operations and the realization of the last ingot, the steel solidifies gradually during its cooling. This solidification takes place by growth of dendrites 10, as illustrated in FIG. figure 2 . In agreement with the phase diagram of stainless steels, the dendrites 10, corresponding to the first solidified grains are by definition richer in alphagenes elements while the interdendritic regions 20 are richer in elements, gamma (application of the rule known segments on the phase diagram). An alphagene element is an element that favors a ferritic type structure (structures that are more stable at low temperature: bainite, ferrite-pearlite, martensite). A gammagenic element is an element that favors an austenitic structure (stable structure at high temperature: austenite). There is therefore segregation between dendrites 10 and interdendritic regions 20.

La figure 3 est un diagramme température (T) - temps (t) connu pour un acier selon l'invention lorsqu'on le refroidit depuis une température supérieure à la température austénitique TAUS. Les courbes D et F marquent le début et la fin de la transformation d'austénite (région A) en structure ferrito-perlitique (région FP). Cette transformation s'effectue, partiellement ou pleinement, lorsque la courbe de refroidissement C que suit le lingot passe respectivement dans la région entre les courbes D et F ou dans la région FP. Elle ne s'effectue pas lorsque la courbe de refroidissement C se situe entièrement dans la région A, comme illustré en figure 3.The figure 3 is a known temperature (T) - time (t) diagram for a steel according to the invention when it is cooled from a temperature above the austenitic temperature T AUS . Curves D and F mark the beginning and the end of the austenite transformation (region A) in a ferrito-pearlitic structure (FP region). This transformation takes place, partially or fully, when the cooling curve C that following the ingot passes respectively in the region between the D and F curves or in the FP region. It does not take place when the cooling curve C is entirely in the region A, as illustrated in FIG. figure 3 .

Lorsque la courbe de refroidissement C passe en dessous de la température de début de transformation martensitique en refroidissement MS (droite MS sur la figure 3), la majorité de l'austénite restant dans l'acier commence à se transformer en martensite. Lorsque la courbe de refroidissement passe en dessous de la température de fin de transformation martensitique en refroidissement MF (droite MF sur la figure 3), la majorité de l'austénite restant dans l'acier s'est transformée en martensite, appelée martensite fraiche.When the cooling curve C falls below the martensitic transformation start temperature in cooling M S (straight line M S on the figure 3 ), the majority of the remaining austenite in the steel starts to turn into martensite. When the cooling curve falls below the martensitic transformation end temperature in cooling M F (straight M F on the figure 3 ), the majority of the remaining austenite in the steel has turned into martensite, called fresh martensite.

Sur la figure 3, les courbes D, F, MS, et MF en traits pleins sont valables pour des structures plus riches en éléments alphagènes (c'est-à-dire dans les dendrites de l'acier), tandis que les mêmes courbes en traits pointillés D', F', MS, et MF sont valables pour des structures plus riches en éléments gammagènes (c'est-à-dire dans les espaces interdendritiques de l'acier).On the figure 3 curves D, F, M S , and M F in solid lines are valid for structures richer in alphagenic elements (that is to say in the dendrites of steel), whereas the same curves in lines dotted d ', F', M S , and M F are valid for structures richer in gammagene elements (that is to say in the interdendritic spaces of steel).

On note que les courbes de transformation d'austénite en structure ferrito-perlitique dans le cas des espaces interdendritiques (courbes D'et F') sont décalées vers la droite par rapport aux courbes de transformation d'austénite en structure ferrito-perlitique dans le cas des dendrites (courbes D et F). Il faut donc plus de temps à une température donnée pour transformer l'austénite en structure ferrito-perlitique dans le cas des espaces interdendritiques que dans le cas des dendrites.It is noted that the austenite transformation curves in the ferrito-pearlitic structure in the case of interdendritic spaces (curves D'and F ') are shifted to the right with respect to the austenite transformation curves into a ferrito-pearlitic structure in the dendrites (curves D and F). It takes more time at a given temperature to transform the austenite into a ferritic-pearlitic structure in the case of interdendritic spaces than in the case of dendrites.

On note que les courbes de transformation d'austénite en martensite dans le cas des espaces interdendritiques (droites MS', et MF') sont décalées vers le bas par rapport aux courbes de transformation d'austénite en martensite dans le cas des dendrites (droites MS et MF). La transformation d'austénite en martensite s'effectue donc à des températures plus basses dans le cas des espaces interdendritiques que dans le cas des dendrites.We note that the transformation curves of austenite to martensite in the case of interdendritic spaces (straight lines M S 'and M F ') are shifted downwards with respect to the austenite transformation curves in martensite in the case of dendrites. (straight lines M S and M F ). The transformation of austenite into martensite is therefore carried out at lower temperatures in the case of interdendritic spaces than in the case of dendrites.

Dans le procédé selon l'invention, le refroidissement de l'acier durant la trempe après austénisation (traitement qui correspond à l'étape 1 en figure 1) suit la courbe C de la figure 3. Ainsi, l'acier passe en dessous de la température de fin de transformation martensitique en refroidissement MF des espaces interdendritiques. De par le processus de refroidissement, la température de peau de la pièce est inférieure à la température au coeur de la pièce, qui est sa partie la plus chaude.In the process according to the invention, the cooling of the steel during quenching after austenisation (treatment which corresponds to step 1 in figure 1 ) follows curve C of the figure 3 . Thus, the steel goes below the temperature of martensitic transformation end in cooling M F interdendritic spaces. Due to the cooling process, the skin temperature of the room is lower than the temperature in the heart of the room, which is its hottest part.

Dès que la température de la partie la plus chaude de la pièce atteint une température maximale Tmax. qui est donc inférieure à la température de fin de transformation martensitique en refroidissement MF' des espaces interdendritiques, on réchauffe la pièce.As soon as the temperature of the hottest part of the room reaches a maximum temperature T max . which is therefore lower than the temperature of martensitic transformation end cooling M F 'interdendritic spaces, the room is heated.

Ce réchauffement s'effectue par exemple en plaçant la pièce dans un environnement (four préchauffé ou enceinte calorifique) où règne une température au moins égale à la température maximale Tmax.This heating is effected for example by placing the room in an environment (preheated oven or heating chamber) where there is a temperature at least equal to the maximum temperature T max .

On effectue ensuite un premier revenu de l'acier en continuant à le réchauffer jusqu'à une température TR, qui est inférieure à la température austénitique TAUS. Ce revenu permet de stabiliser la phase cristallographique martensitique fraîche en faisant par exemple précipiter des carbures au sein de la martensite et donc de conférer plus de résilience à la martensite de l'acier.A first income of the steel is then made by continuing to heat it up to a temperature T R , which is lower than the austenitic temperature T AUS . This income makes it possible to stabilize the fresh martensitic crystallographic phase by, for example, precipitating carbides within the martensite and thus imparting more resilience to the martensite of the steel.

Ce premier traitement de revenu correspond à l'étape 2 en figure 1.This first income treatment corresponds to step 2 in figure 1 .

On refroidit ensuite l'acier jusqu'à ce que la partie la plus chaude de l'acier atteigne la température maximale Tmax qui est inférieure à la température de fin de transformation martensitique en refroidissement MF' des espaces interdendritiques, puis on réchauffe immédiatement l'acier.The steel is then cooled until the hottest part of the steel reaches the maximum temperature T max which is lower than the martensitic transformation end temperature in cooling M F 'of the interdendritic spaces, and is then heated immediately. steel.

On fait ensuite subir immédiatement à l'acier un second traitement de revenu, sensiblement identique au premier traitement de revenu, en laissant ensuite l'acier refroidir jusqu'à température ambiante TA.The steel is then immediately subjected to a second treatment of income, substantially identical to the first treatment of income, then allowing the steel to cool to room temperature T A.

Ce second traitement de revenu correspond à l'étape 3 en figure 1.This second income treatment corresponds to step 3 in figure 1 .

Les inventeurs ont réalisé des essais d'usinabilité sur des aciers martensitiques inoxydables ayant subi le procédé de l'invention. Ils ont comparé les résultats de ces essais aux résultats d'essais d'usinabilité sur des aciers ayant subi une austénisation suivie d'une trempe et de deux revenus mais où la température minimale de la partie la plus chaude de la pièce est simplement inférieure à la température de fin de transformation martensitique en refroidissement MF des dendrites, et où l'acier n'est pas immédiatement réchauffé entre la trempe et le premier revenu, ou entre le premier revenu et le second revenu.The inventors have carried out machinability tests on stainless martensitic steels having undergone the process of the invention. They compared the results of these tests to the results of machinability tests on austenized steels followed by quenching and two incomes but where the minimum temperature of the hottest part of the part is simply less than the martensitic transformation end temperature in cooling M F of the dendrites, and the steel is not immediately warmed between tempering and first income, or between first income and second income.

La composition des aciers Z12CNDV12 est la suivante (norme DMD0242-20 indice E) :

  • C (0,10 à 0,17%) - Si (<0,30%) - Mn (0,5 à 0,9%) - Cr (11 à 12,5%) - Ni (2 à 3%) - Mo (1,50 à 2,00%) - V (0,25 à 0,40%) - N2 (0,010 à 0,050%) - Cu (<0,5%) - S (<0,015%) - P (<0,025%) et satisfaisant le critère 4,5 ≤ ( Cr - 40×C - 2×Mn - 4×Ni + 6×Si + 4×Mo + 11×V - 30×N) < 9.
The composition of the Z12CNDV12 steels is the following (standard DMD0242-20 index E):
  • C (0.10 to 0.17%) - If (<0.30%) - Mn (0.5 to 0.9%) - Cr (11 to 12.5%) - Ni (2 to 3%) - Mo (1.50 to 2.00%) - V (0.25 to 0.40%) - N 2 (0.010 to 0.050%) - Cu (<0.5%) - S (<0.015%) - P (<0.025%) and meeting the criterion 4.5 ≤ (Cr - 40 × C - 2 × Mn - 4 × Ni + 6 × Si + 4 × Mo + 11 × V - 30 × N) <9.

Les inventeurs ont constaté qu'avec un acier fabriqué selon le procédé de l'invention, l'usure des plaquettes d'usinage par mètre d'acier usinée est divisée par environ 10 (passage de 11 mm à 1,3 mm) pour une vitesse de coupe de 120 m/min comparé à un acier fabriqué selon un procédé de l'art antérieur. La puissance requise pour l'usinage est en outre divisée par plus de deux comparé à un acier fabriqué selon un procédé de l'art antérieur. L'état de surface de l'acier après usinage est également amélioré.The inventors have found that with a steel manufactured according to the method of the invention, the wear of the machining plates per meter of machined steel is divided by about 10 (11 mm to 1.3 mm cutting speed of 120 m / min compared to a steel manufactured according to a method of the prior art. The power required for machining is further divided by more than two compared to a steel manufactured according to a method of the prior art. The surface condition of the steel after machining is also improved.

En particulier, avec une température maximale Tmax comprise entre 28°C et 35°C, l'usure des plaquettes d'usinage par unité de longueur d'acier usinée est divisée par 15, et la puissance requise pour l'usinage divisée par 2,5. Une température maximale Tmax comprise entre 20°C et 75°C donne aussi de bons résultats.In particular, with a maximum temperature T max of between 28 ° C and 35 ° C, the wear of the machining plates per unit length of machined steel is divided by 15, and the power required for the machining divided by 2.5. A maximum temperature T max of between 20 ° C and 75 ° C also gives good results.

Lorsque la température maximale Tmax est au delà de 90°C (et jusqu'à 180°C) les résultats en usinage sont les plus mauvais.When the maximum temperature T max is above 90 ° C (and up to 180 ° C) the machining results are the worst.

On retrouve des résultats moyens (intermédiaires entre bons et mauvais) lorsqu'on réchauffe l'acier dès que la partie la plus chaude de la pièce atteint une température au delà de 180°C (et jusqu'à 300°C).Average results (intermediate between good and bad) are found when heating the steel as soon as the hottest part of the room reaches a temperature above 180 ° C (and up to 300 ° C).

Selon les inventeurs, les résultats peuvent s'expliquer de la façon suivante : comme indiqué ci-dessus, la température de fin de transformation martensitique en refroidissement MF' des régions interdendritiques est inférieure à la température de fin de transformation martensitique en refroidissement MF des dendrites. Or on a vu que lors du refroidissement de l'acier, cet acier se solidifie en une microstructure qui est une alternance de dendrites et de régions interdendritiques (figure 2). Ainsi, lorsque la température descend en dessous de la température de fin de transformation martensitique en refroidissement MF des dendrites, les dendrites ont fini de se transformer en martensite, alors que les régions interdendritiques n'ont pas encore fini se transformer en martensite. Donc, si l'on réchauffe l'acier dès qu'il a atteint la température de fin de transformation martensitique en refroidissement MF des dendrites, des zones dans tout l'acier (à savoir les régions interdendritiques) contiennent de l'austénite résiduelle. Une partie de cette austénite résiduelle se transformera lors de l'étape du premier revenu suivant en martensite fraiche. L'autre partie de cette austénite résiduelle sera localisé uniquement au niveau des points les plus ségrégés de la matière (par exemple, au niveau des espaces interdendritiques les plus concentrés).According to the inventors, the results can be explained as follows: as indicated above, the martensitic transformation end temperature in cooling M F 'of the interdendritic regions is less than the martensitic transformation end of cooling temperature M F dendrites. Now we have seen that during the cooling of steel, this steel solidifies into a microstructure which is an alternation of dendrites and interdendritic regions ( figure 2 ). Thus, when the temperature drops below the martensitic transformation end temperature in M F cooling of the dendrites, the dendrites have become martensite, while the interdendritic regions have not yet been transformed into martensite. Thus, if the steel is warmed up as soon as it has reached the end of martensitic transformation temperature in cooling M F of the dendrites, zones in all the steel (ie the interdendritic regions) contain residual austenite. Part of this residual austenite will be transformed at the next first income stage into fresh martensite. The other part of this residual austenite will be located only at the most segregated points of the material (for example, in the most concentrated interdendritic spaces).

Au cours du second revenu, la nouvelle martensite fraîche se stabilise mais une autre partie du restant de l'austénite résiduelle continue à se transformer en martensitique fraîche dans ces endroits les plus ségrégés. L'acier présente donc une hétérogénéité de structure avec des grains plus durs correspondant à la martensite fraîche dans une matrice plus douce. C'est cette hétérogénéité qui est responsable de la mauvaise usinabilité de l'acier, les grains plus durs usant les plaquettes et bloquant leur avance.In the second income, the new fresh martensite stabilizes but another portion of the remaining residual austenite continues to turn into fresh martensitic in these most segregated areas. Steel therefore has a structural heterogeneity with harder grains corresponding to fresh martensite in a softer matrix. It is this heterogeneity that is responsible for the bad machinability of steel, the harder grains using platelets and blocking their advance.

A l'inverse, si l'on réchauffe l'acier dès que la partie la plus chaude de la pièce atteint une température élevée (comprise entre 180°C et 300°C), on conserve de l'austénite résiduelle, qui donne au final un comportement moyen lors de l'usinage ultérieur.On the other hand, if the steel is warmed up as soon as the hottest part of the part reaches a high temperature (between 180 ° C and 300 ° C), residual austenite is left, which gives final average behavior during subsequent machining.

On comprend donc pourquoi le refroidissement de l'acier jusqu'à la température de fin de transformation martensitique en refroidissement MF' des régions interdendritiques, puis le réchauffement immédiat de l'acier dès qu'il a atteint cette température MF, permettent d'obtenir une microstructure plus homogène au sein de l'acier.It is thus clear why the cooling of the steel up to the end of martensitic transformation temperature in cooling M F 'of the interdendritic regions, then the immediate warming of the steel as soon as it has reached this temperature M F , make it possible to to obtain a more homogeneous microstructure within the steel.

Par exemple, la température maximale Tmax qu'atteint la partie la plus chaude de l'acier avant d'être réchauffée est comprise entre 20°C et 75°C. Une telle température Tm est inférieure à la température de fin de transformation martensitique en refroidissement MF' des espaces interdendritiques.For example, the maximum temperature T max that reaches the hottest part of the steel before being reheated is between 20 ° C and 75 ° C. Such a temperature T m is lower than the martensitic transformation end temperature in cooling M F 'interdendritic spaces.

Par exemple, cette température maximale Tmax est comprise entre 28°C et 35°C.For example, this maximum temperature T max is between 28 ° C and 35 ° C.

Afin de déterminer quand la partie la plus chaude de l'acier atteint la température maximale Tmax, on peut par exemple, à l'étape (ω), mesurer la température de peau de l'acier et utiliser des abaques pour en déduire la température de la partie la plus chaude de l'acier.In order to determine when the hottest part of the steel reaches the maximum temperature T max , it is possible for example, in step (ω), to measure the skin temperature of the steel and to use abacuses to deduce the temperature of the hottest part of the steel.

Par ailleurs, il est avantageux que le gradient de température entre la surface de l'acier et la partie la plus chaude de l'acier soit le plus faible possible, afin de réduire l'écart entre la température de fin de transformation martensitique en refroidissement MF des dendrites et la température de fin de transformation martensitique en refroidissement MF' des espaces interdendritiques. En effet, en réduisant cet écart, les contraintes au sein de la pièce sont alors moindres, et on gagne en productivité.Furthermore, it is advantageous for the temperature gradient between the surface of the steel and the hottest part of the steel to be as small as possible in order to reduce the gap between the end temperature of the steel martensitic transformation in M F cooling of dendrites and martensitic transformation end temperature in cooling M F 'interdendritic spaces. Indeed, by reducing this gap, the constraints in the room are then lower, and we gain in productivity.

Ainsi, avantageusement, dans chacune des étapes (1) et (2), on effectue la sous-étape suivante avant la sous-étape (ω):

  • (ψ) Dès que la température de la partie la plus chaude de l'acier atteint une température seuil Ts inférieure à la température de début de transformation martensitique en refroidissement MS des dendrites dans ledit acier, et supérieure à la température de fin de transformation martensitique en refroidissement MF' des espaces interdendritiques, on maintient l'acier dans un environnement où règne sensiblement une température comprise entre la température minimale Tmin et la température MF' pendant une durée seuil ds de façon à réduire le gradient de température entre la surface de l'acier et la partie la plus chaude de l'acier.
Thus, advantageously, in each of the steps (1) and (2), the following sub-step is carried out before the substep (ω):
  • (ψ) As soon as the temperature of the hottest part of the steel reaches a threshold temperature T s less than the martensitic transformation start temperature in cooling M S of the dendrites in said steel, and greater than the end temperature of martensitic transformation in cooling M F 'interdendritic spaces, the steel is maintained in an environment where substantially a temperature between the minimum temperature T min and the temperature M F ' prevails for a threshold time d s so as to reduce the gradient of temperature between the surface of the steel and the hottest part of the steel.

La durée seuil ds dépend de la géométrie de la pièce. La durée ds est au minimum de 15 minutes (min) pour une dimension minimale de la pièce de 50 mm, de 30 min pour une dimension minimal de la pièce de 100 mm, de 45 min pour une dimension minimale de la pièce de 150 mm, et ainsi de suite. Pour une dimension minimale de la pièce comprise entre ces valeurs, on peut par exemple en déduire la durée ds par extrapolation avec la formule : ds = (15 min) × {dimension minimale (en mm)}/50.The threshold duration d s depends on the geometry of the part. The length of s is at least 15 minutes (min) to a minimum dimension of the part of 50 mm, 30 min to a minimum dimension of the part of 100 mm, 45 min to a minimum dimension of the workpiece 150 mm, and so on. For a minimum dimension of the part between these values, it is possible for example to deduce the duration d s by extrapolation with the formula: d s = (15 min) × {minimum dimension (in mm)} / 50.

Pour maintenir l'acier dans un environnement où règne sensiblement la température comprise entre la température minimale Tmin et la température MF', on peut par exemple placer l'acier dans un four où règne une température comprise entre Tmin et MF'.In order to maintain the steel in an environment where the temperature between the minimum temperature T min and the temperature MF 'substantially prevails, the steel can for example be placed in an oven where a temperature of between T min and MF' prevails.

Alternativement, on peut isoler thermiquement l'acier de l'environnement extérieur, par exemple en le plaçant dans une couverture.Alternatively, the steel can be thermally insulated from the outside environment, for example by placing it in a blanket.

Avantageusement, après le second revenu, on effectue au moins une détente de l'acier à une température inférieure aux températures de revenu TR auxquelles le premier revenu et le second revenu ont été effectués.Advantageously, after the second income, at least one expansion of the steel is performed at a temperature below the temperature of income T R at which the first income and the second income have been made.

Cette détente correspond à l'étape 4 en figure 1. Elle permet la relaxation de contraintes résiduelles au sein de l'acier, et en améliore la durée de vie.This relaxation corresponds to step 4 in figure 1 . It allows the relaxation of residual stresses within the steel, and improves the service life.

Afin d'améliorer la tenue en fatigue des aciers selon l'invention, on cherche à augmenter la propreté inclusionnaire de l'acier, c'est-à-dire à diminuer la quantité d'inclusions indésirables (certaines phases alliées, oxydes, carbures, composés intermétalliques) présentes dans l'acier. En effet, ces inclusions agissent comme des sites d'amorces de fissures qui conduisent, sous sollicitation cyclique, à une ruine prématurée de l'acier.In order to improve the fatigue strength of the steels according to the invention, it is sought to increase the inclusion cleanliness of the steel, that is to say to reduce the amount of undesirable inclusions (certain alloy phases, oxides, carbides intermetallic compounds) present in steel. Indeed, these inclusions act as crack initiation sites which lead, under cyclic stress, to premature failure of the steel.

On connaît des procédés pour améliorer la propreté inclusionnaire, notamment un procédé de refusion tel que la refusion sous laitier ou ESR (Electro Slag Refusion), ou la refusion par arc sous vide ou VAR (Vacuum Arc Remelting). Ces procédés sont connus, et seul leur fonctionnement global est rappelé ci-après.Processes for improving the inclusion cleanliness are known, including a reflow process such as slag remelting or ESR (Electro Slag Refusion), or vacuum arc reflow or VAR (Vacuum Arc Remelting). These methods are known, and only their overall operation is recalled hereinafter.

Le procédé ESR consiste à placer un lingot en acier dans un creuset dans lequel on a versé un laitier (mélange minéral, par exemple chaux, fluorures, magnésie, alumine, spath) de telle sorte que l'extrémité inférieure du lingot trempe dans le laitier. Puis on fait passer un courant électrique dans le lingot, qui sert d'électrode. Ce courant liquéfie le laitier et fait fondre l'extrémité inférieure de cette électrode qui est en contact avec le laitier. L'acier fondu de cette électrode traverse le laitier sous forme de fines gouttelettes, pour se solidifier en dessous de la couche de laitier qui surnage, en un nouveau lingot qui croît ainsi progressivement. Le laitier agit, entre autres comme un filtre qui extrait les inclusions des gouttelettes d'acier, de telle sorte que l'acier de ce nouveau lingot situé en dessous de la couche de laitier contient moins d'inclusions que le lingot initial (électrode). Cette opération s'effectue à la pression atmosphérique et à l'air.The ESR process consists in placing a steel ingot in a crucible in which a slag (mineral mixture, for example lime, fluoride, magnesia, alumina, spath) has been poured in such a way that the lower end of the ingot quenches in the slag . Then an electric current is passed into the ingot, which serves as an electrode. This stream liquefies the slag and melts the lower end of this electrode which is in contact with the slag. The molten steel of this electrode passes through the slag in the form of fine droplets, to solidify below the layer of supernatant slag, into a new ingot that grows gradually. The slag acts, inter alia, as a filter which extracts the inclusions from the steel droplets, so that the steel of this new ingot located below the slag layer contains fewer inclusions than the initial ingot (electrode). . This operation is carried out at atmospheric pressure and air.

Le procédé VAR consiste à fondre dans un creuset sous un vide poussé le lingot d'acier, qui sert d'électrode. Le lingot/électrode est fondu par l'établissement d'un arc électrique entre l'extrémité du lingot/électrode et le sommet du lingot secondaire qui se forme par fusion du lingot/électrode. Le lingot secondaire se solidifie au contact des parois du creuset et les inclusions flottent à la surface du lingot secondaire, et peuvent ultérieurement être éliminées. On obtient donc un lingot secondaire d'une plus grande pureté que le lingot/électrode initial.The VAR process consists in melting in a crucible under a high vacuum the steel ingot, which serves as an electrode. The ingot / electrode is melted by establishing an electric arc between the end of the ingot / electrode and the top of the secondary ingot which is formed by melting the ingot / electrode. The secondary ingot solidifies in contact with the walls of the crucible and the inclusions float on the surface of the secondary ingot, and may subsequently be removed. A secondary ingot of greater purity than the initial ingot / electrode is thus obtained.

Avantageusement, l'acier subit, avant l'étape (1), une refusion.Advantageously, the steel undergoes, before step (1), a reflow.

Par exemple la refusion est choisie dans un groupe comprenant la refusion sous laitier ESR ou la refusion par arc sous vide VAR.For example, reflow is chosen from a group comprising ESR slag remelting or VAR vacuum arc remelting.

Avantageusement, avant l'étape (1), on effectue un traitement d'homogénéisation de l'acier.Advantageously, before step (1), a homogenization treatment of the steel is carried out.

En effet, lors de cette homogénéisation, il se produit une diffusion des éléments d'alliage des zones à forte concentration vers les zones à faible concentration. On permet alors une réduction de l'intensité des ségrégations en éléments alphagènes dans les dendrites 10, et une réduction de l'intensité des ségrégations en éléments gammagènes dans les régions interdendritiques 20. La réduction de l'intensité des ségrégations en ces éléments gammagènes a notamment pour conséquence un rapprochement de la température de fin de transformation martensitique en refroidissement MF des dendrites et de la température de fin de transformation martensitique en refroidissement MF' des espaces interdendritiques, ainsi qu'une moindre différence structurale entre les dendrites 10 et les régions interdendritiques 20.Indeed, during this homogenization, there is a diffusion of the alloying elements of high concentration areas to low concentration areas. The intensity of the segregations into alphagenic elements in the dendrites 10 is then reduced, and the intensity of the segregations in gamma elements in the interdendritic regions 20 is reduced. The reduction of the segregation intensity in these gammagenic elements has been reduced. in particular, as a consequence of bringing the temperature of the end of martensitic transformation into cooling M F of the dendrites and of the end of martensitic transformation temperature into cooling M F 'of the interdendritic spaces, as well as a lesser structural difference between the dendrites 10 and interdendritic regions 20.

En ce qui concerne les particularités du traitement d'homogénéisation, les inventeurs ont trouvé que des résultats satisfaisants sont obtenus lorsque le lingot est soumis dans ce four à un traitement d'homogénéisation pendant un temps de maintien t après que la température du point le plus froid de ce lingot a atteint une température d'homogénéisation T, ce temps t étant égal à au moins une heure, et la température d'homogénéisation T variant entre une température inférieure Tinf et la température de brûlure de cet acier.With regard to the particularities of the homogenization treatment, the inventors have found that satisfactory results are obtained when the ingot is subjected in this oven to a homogenization treatment during a holding time t after the temperature of the most The cold of this ingot has reached a homogenization temperature T, this time t being equal to at least one hour, and the homogenization temperature T varying between a lower temperature T inf and the burn temperature of this steel.

La température Tinf est environ égale à 900°C. La température de brûlure d'un acier est définie comme la température à l'état brut de solidification à laquelle les joints de grains dans l'acier se transforment (voire se liquéfient), et est supérieure à TInf. Ce temps t de maintien de l'acier dans le four varie donc inversement à cette température d'homogénéisation T.The temperature T inf is approximately equal to 900 ° C. The burn temperature of a steel is defined as the temperature in the raw state of solidification at which the grain boundaries in the steel transform (or even liquefy), and is greater than T Inf . This time t of maintaining the steel in the furnace therefore varies inversely with this homogenization temperature T.

Par exemple, dans le cas d'un acier martensitique inoxydable Z12CNDV12 (norme AFNOR) utilisé par les inventeurs dans les essais, la température d'homogénéisation T est 950°C, et le temps de maintien t correspondant est égal à 70 heures. Lorsque la température d'homogénéisation T est de 1250C qui est légèrement inférieure à la température de brûlure, alors le temps de maintien t correspondant est égal à 10 heures.For example, in the case of a stainless martensitic steel Z12CNDV12 (AFNOR standard) used by the inventors in the tests, the homogenization temperature T is 950 ° C., and the corresponding holding time t is equal to 70 hours. When the homogenization temperature T is 1250C which is slightly lower than burn temperature, then the corresponding holding time t is equal to 10 hours.

Selon un autre mode de réalisation de l'invention, il est possible, afin d'améliorer l'usinabilité des aciers martensitiques inoxydables, d'effectuer un traitement d'homogénéisation de l'acier tel que décrit ci-dessus, puis d'effectuer les étapes (1), (2) et (3) selon l'art antérieur sans effectuer la sous-étape (ω). Dans ce mode de réalisation, la température maximale Tmax est inférieure à la température de fin de transformation martensitique en refroidissement MF des dendrites dans l'acier, et, dans les étapes (1) et (2) on fait en sorte que l'acier reste à une température égale ou inférieure à la température maximale Tmax pendant un temps le plus court possible.According to another embodiment of the invention, it is possible, in order to improve the machinability of the stainless martensitic steels, to carry out a homogenization treatment of the steel as described above, and then to perform steps (1), (2) and (3) according to the prior art without performing the substep (ω). In this embodiment, the maximum temperature T max is lower than the martensitic transformation end temperature in cooling M F of the dendrites in the steel, and in steps (1) and (2) it is ensured that steel remains at or below the maximum temperature T max for as short a time as possible.

Claims (10)

  1. A method of fabricating a martensitic stainless steel including the following heat treatment steps:
    (1) heating the steel to a temperature higher than the austenizing temperature TAUS of the steel, then quenching the steel until the hottest portion of the steel is at a temperature less than or equal to a maximum temperature Tmax, and greater than or equal to a minimum temperature Tmin, the rate of cooling being sufficiently fast for the austenite not to transform into a ferrito-perlitic structure;
    (2) performing a first anneal on the steel followed by cooling until the hottest portion of the steel is at a temperature less than or equal to said maximum temperature Tmax and greater than or equal to said minimum temperature Tmin; and
    (3) performing a second anneal of the steel followed by cooling to ambient temperature TA;
    said method being characterized in that said maximum temperature Tmax is less than or equal to the temperature MF' for the end of martensitic transformation on cooling of inter-dendritic spaces in said steel, and in that, at the end of each of the steps (1) and (2), the following substep is performed:
    (ω) as soon as the temperature of the hottest portion of the steel reaches said maximum temperature Tmax, the steel is immediately heated once more.
  2. A method of fabricating a martensitic stainless steel according to claim 1, characterized in that said maximum temperature Tmax lies in the range 20°C to 75°C.
  3. A method of fabricating a martensitic stainless steel according to claim 2, characterized in that said maximum temperature Tmax lies in the range or equals 28°C to 35°C.
  4. A method of fabricating a martensitic stainless steel according to any one of claims 1 to 3, characterized in that, in step (ω), the temperature of the skin of the steel is measured and charts are used to deduce therefrom the temperature of the hottest portion of the steel.
  5. A method of fabricating a martensitic stainless steel according to any one of claims 1 to 4, characterized in that after step (3), said steel is subjected to relaxation at least once at a temperature lower than the annealing temperatures at which the first anneal of step (2) and the second anneal of step (3) were performed.
  6. A method of fabricating a martensitic stainless steel according to any one of claims 1 to 5, characterized in that, in each of steps (1) and (2), the following substep is performed before the substep (ω):
    (ψ) as soon as the temperature of the hottest portion of the steel reaches a threshold temperature Ts lower than the temperature MS for the start of martensitic transformation on cooling of dendrites in said steel, and higher than the temperature MF' for the end of martensitic transformation on cooling of inter-dendritic spaces, the steel is maintained in an environment in which there substantially exists a temperature lying between the minimum temperature Tmin and the temperature MF' for a threshold duration ds so as to reduce the temperature gradient between the surface of the steel and the hottest portion of the steel.
  7. A method of fabricating a martensitic stainless steel according to claim 6, characterized in that, in step (ψ), the steel is placed in an oven in which there exists a temperature lying in the range said minimum temperature Tmin to said temperature MF' .
  8. A method of fabricating a martensitic stainless steel according to any one of claims 1 to 7, characterized in that prior to step (1), said steel is subjected to remelting.
  9. A method of fabricating a martensitic stainless steel according to any one of claims 1 to 8, characterized in that prior to step (1), homogenization treatment is performed on said steel.
  10. A method of fabricating a martensitic stainless steel according to any one of claims 1 to 9, characterized in that the composition of said steel is C (0.10% to 0.17%) - Si (<0.3%) - Mn (0.5% to 0.9%) - Cr (11% to 12.5%) - Ni (2% to 3%) - Mo (1.5% to 2%) - V (0.25% to 0.4%) - N2 (0.01% to 0.05%) - Cu (<0.5%) - S (<0.0150) - P (<0.025%), and satisfying the following criterion: 4.5 ≤ (Cr - 40xC - 2×Mn - 4xNi + 6xSi + 4×Mo + 11×V - 30xN) < 9
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