EP2488672A1 - Homogenization of martensitic stainless steel after remelting under a layer of slag - Google Patents

Homogenization of martensitic stainless steel after remelting under a layer of slag

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Publication number
EP2488672A1
EP2488672A1 EP10781969A EP10781969A EP2488672A1 EP 2488672 A1 EP2488672 A1 EP 2488672A1 EP 10781969 A EP10781969 A EP 10781969A EP 10781969 A EP10781969 A EP 10781969A EP 2488672 A1 EP2488672 A1 EP 2488672A1
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EP
European Patent Office
Prior art keywords
temperature
ingot
steel
homogenization
slag
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EP10781969A
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German (de)
French (fr)
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EP2488672B1 (en
Inventor
Laurent Ferrer
Patrick Philipson
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Safran Aircraft Engines SAS
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SNECMA SAS
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    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D6/00Heat treatment of ferrous alloys
    • C21D6/002Heat treatment of ferrous alloys containing Cr
    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D6/00Heat treatment of ferrous alloys
    • C21D6/004Heat treatment of ferrous alloys containing Cr and Ni
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22BPRODUCTION AND REFINING OF METALS; PRETREATMENT OF RAW MATERIALS
    • C22B9/00General processes of refining or remelting of metals; Apparatus for electroslag or arc remelting of metals
    • C22B9/16Remelting metals
    • C22B9/18Electroslag remelting

Definitions

  • the present invention relates to a method for manufacturing a stainless martensitic steel comprising a slag remelting step of an ingot of this steel and a cooling step of this ingot
  • a stainless martensitic steel is a steel whose chromium content is greater than 10.5%, and whose structure is essentially martensitic.
  • ESR Electro Slag Refusion
  • the lower end of this electrode being in contact with the slag, melts and 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.
  • Non-destructive ultrasonic testing performed by the inventors, showed that these steels practically had no known hydrogen defects (flakes).
  • the dispersion of the fatigue strength results is therefore due to another undesirable mechanism of premature initiation of cracks in the steel, which leads to its premature failure in fatigue.
  • the present invention aims to provide a manufacturing method that allows to raise these low values, and thus reduce the dispersion of the fatigue strength of stainless martensitic steels, and also to increase its average value in resistance to fatigue.
  • the ingot from the slag remelting is, before the skin temperature of this ingot is less than the martensitic transformation temperature Ms of the steel, placed in an oven whose initial temperature T 0 is then greater than the end of pearlitic transformation temperature in Arl cooling of said steel, this ingot being subjected in this oven to a homogenization treatment for at least one holding time t after the temperature of the coldest point of the ingot has reached a homogenization temperature T, this holding time t being equal to at least one hour, and the homogenization temperature T varying between about 900 ° C and the burn temperature of the steel.
  • FIG. 1 compares fatigue life curves for a steel according to the invention and a steel according to the prior art
  • FIG. 2 shows a fatigue stress curve
  • FIG. 3 is a diagram illustrating dendrites and interdendritic regions
  • FIG. 4 is a photograph taken under the electron microscope of a fracture surface after fatigue, showing the gas phase having initiated this fracture.
  • FIG. 5 schematically shows cooling curves on a time-temperature diagram for a region richer in alphagenic elements and less rich in gamma-ray elements
  • FIG. 6 schematically shows cooling curves on a time-temperature diagram for a region less rich in alpha-gene elements and richer in gammagenic elements.
  • the dendrites 10 corresponding to the first solidified grains, are by definition richer in elements.
  • alphagenes while the interdendritic regions 20 are richer in gamma-like elements (application of the known rule segments on the phase diagram).
  • An alphagene element is one that promotes a ferritic type structure (more stable structures at low temperature, bainite, ferrite-pearlite, martensite).
  • a gamma element is an element that promotes an austenitic structure (stable structure at high temperature). There is therefore segregation between dendrites 10 and interdendritic regions 20.
  • FIG. 5 is a temperature (T) -time (t) diagram known for a region richer in alphagenes and less rich in gamma elements, such as dendrites 10.
  • Curves D and F mark the beginning and the end of the transformation of austenite (region A) into a ferrito-pearlitic structure (FP region). This transformation takes place, partially or fully, when the cooling curve that follows the ingot passes respectively in the region between the D and F curves or in the FP region. It does not occur when the cooling curve is entirely in region A.
  • FIG. 6 is an equivalent diagram for a region richer in gammagenic elements and less rich in alphagenic elements, such as the interdendritic regions 20. It will be noted that with respect to FIG. 5, the curves D and F are shifted to the right, that is to say, it will cool more slowly the ingot to obtain a ferritoperlitic structure.
  • FIG. 5 and 6 shows three cooling curves from an austenitic temperature, corresponding to three cooling rates; fast (curve C1), average (curve C2), slow (curve C3).
  • fast curve C1
  • average curve C2
  • slow curve C3
  • the temperature begins to decrease from an austenitic temperature.
  • the cooling rates of the surface and the heart of the ingot are very close. The only difference is that the surface temperature is lower than that of the core because the surface was the first to cool relative to the core.
  • the dendrites 10 first turn into ferritic structures during cooling (by crossing the curves D and F of FIG. 5). While the interdendritic regions 20 either do not change (in the case of rapid cooling according to the curve C1) or change later, in whole or in part (in the case of average cooling according to the curve C2 or slow according to the curve C3), to temperatures inferior (see Figure 6).
  • the interdendritic regions thus retain a longer austenitic structure.
  • the lighter elements are able to diffuse ferritic structure dendrites towards the interdendritic regions 20 of austenitic or all-part structure and to concentrate during the period of coexistence of the ferritic and austenitic structures.
  • the risk that the solubility of these light elements is exceeded locally in the interdendritic regions is accentuated. When the concentration in light elements exceeds this solubility, it appears then in the steel microscopic gas pockets containing these light elements.
  • the austenite of interdendritic regions tends to locally transform into martensite when the temperature of the steel falls below the martensitic transformation temperature Ms, which is slightly above the temperature. ambient ( Figures 5 and 6).
  • martensite has a threshold of solubility in light elements even lower than other metallurgical structures and that austenite. There is therefore more microscopic gaseous phase within the steel during this martensitic transformation.
  • This zone P is the imprint of the gaseous phase consisting of the light elements, and which is at the origin of the formation of these fissures F which, by propagating and agglomerating, created a zone of macroscopic fracture.
  • the inventors have carried out tests on stainless martensitic steels, and have found that when, immediately after the ESR step, a specific homogenization treatment is carried out on the ingot taken out of the ESR crucible, the formation is reduced. of gaseous phases of light elements.
  • the homogenization treatment also leads to a homogenization of the martensitic transformation temperature Ms.
  • the diffusion of the alloying elements is far from negligible. Moreover, if the temperature gradient makes it possible to have a warmer surface That the center of the ingot, which the conditions of recovery proposed by the inventors allow, the light elements diffuse towards the surface, which reduces their overall content in the steel.
  • 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 temperature T min and the burn temperature of this steel.
  • the temperature T m j n is approximately equal to 900 ° C.
  • the burning temperature of a steel is defined as the temperature in the raw state of solidification at which the grain boundaries in the steel are transformed (or even liquefied), and is greater than T min . 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 1250 ° C. which is slightly lower than the burn temperature, then the corresponding holding time t is equal to 10 hours.
  • the homogenization temperature T is selected from a range selected from the group consisting of the following ranges: 950 ° C to 1270 ° C, 980 ° C to 1250 ° C, 1000 ° C to 1200 ° C.
  • the minimum hold time t is selected from a range selected from the group consisting of the following ranges: 1 hour to 70 hours, 10 hours to 30 hours, 30 hours to 150 hours.
  • the inventors have found that satisfactory results are obtained when the ingot at the outlet of the ESR crucible is placed in an oven whose initial temperature T 0 is greater than the end of pearlitic transformation temperature in cooling Arl of this steel. and when the skin temperature of this ingot remains higher than the martensitic transformation temperature Ms of this steel.
  • the initial temperature T 0 of the oven is lower than the homogenization temperature T
  • the temperature of the oven is, after the ingot has been placed in this oven, increased to a temperature at least equal to the temperature homogenization.
  • the temperature in the center of the ingot therefore remains lower than the skin temperature of the ingot throughout the rise in temperature. This allows a global degassing and more effective ingot.
  • the initial temperature T 0 of the oven may be greater than the homogenization temperature, in which case the oven temperature is simply maintained above this homogenization temperature.
  • the inventors have found that the homogenization treatment is especially necessary when:
  • the maximum dimension of the ingot is less than approximately 910 mm, and the H content of the ingot before slag remelting is greater than 10 ppm, and
  • the maximum dimension of the ingot is greater than about 910 mm and the minimum dimension of the ingot is less than about 1500 mm, and the H content of the ingot before slag remelting is greater than 3 ppm, and
  • the minimum dimension of the ingot is greater than 1500 mm and the H content of the ingot before slag remelting is greater than 10
  • the maximum dimension of the ingot is that of the measurements in its most massive part, and the minimum dimension of the ingot is that of the measures in its least massive part;
  • the concentrations of light elements may be greater (greater than 10 ppm) when the minimum dimension of the ingot or of the deformed ingot is greater than a large size threshold (in this case 1500 mm).
  • a high threshold (1500 mm) for the minimum dimension of the ingot is as follows: when the minimum dimension of the ingot is greater than this threshold, we approach the case of slow cooling (curve C3) in which there is almost no structural difference between dendrites and interdendritic regions during cooling.
  • the cooling rate is sufficiently low so that the temperature is substantially homogeneous between the core of the skin of the ingot, so that the diffusion of light elements to the surface is facilitated, which allows a greater degassing.
  • the core of the ingot is, during the cooling, much hotter than its surface, which favors a diffusion of the light elements towards the core and slows down the degassing.
  • the slag is previously dehydrated before use in the ESR crucible, because it minimizes the amount of hydrogen present in the slag, and thus minimizes the amount of hydrogen that could pass the slag ingot during the ESR process.
  • the inventors have carried out tests on Z12CNDV12 steels produced with the process according to the invention, that is to say with a homogenization carried out immediately after the ingress of the ingot of the ESR crucible according to the following parameters:
  • Test No. 1 skin temperature of the ingot at 250 ° C., put in the oven at 400 ° C., rise of the oven at the homogenization temperature of 1250 ° C., metallurgical maintenance (as soon as the coldest temperature of the ingot reaches the homogenization temperature) of 75h, cooling to room temperature.
  • Test No. 2 Skin temperature of the ingot at 600 ° C., put in the oven at 450 ° C., rise of the furnace at the homogenization temperature of 1000% metallurgical maintenance (as soon as the coldest temperature of the ingot reaches the temperature homogenization) of 120h, cooling to room temperature. The results of these tests are presented below,
  • composition of the Z12CNDV12 steels is the following (standard DMD0242-20 index E):
  • the martensitic transformation temperature Ms measured is 220 ° C.
  • the amount of Hydrogen measured on the ingots before slag remelting varies from 3.5 to 8.5 ppm.
  • Figure 1 qualitatively shows the improvements made by the method according to the invention.
  • the value of the number N of rupture cycles necessary to break a steel specimen subjected to a cyclic stress in tension as a function of the pseudo-alternating stress C is obtained experimentally (this is the stress experienced by the test specimen under imposed deformation. , according to Sncma DMC0401 standard used for these tests).
  • Such a cyclic bias is shown schematically in FIG. 2.
  • the period T represents a cycle.
  • the constraint evolves between a maximum value C ma x and a minimum value C min .
  • the first curve 15 (in fine lines) is (schematically) the average curve obtained for a steel produced according to the prior art.
  • This first average curve CN is surrounded by two curves 16 and 14 in dashed fine lines. These curves 16 and 14 are situated respectively at a distance of +3 ⁇ s ⁇ and -3 at t from the first curve 15, where ⁇ * is the standard deviation of the distribution of the experimental points obtained during these fatigue tests. and ⁇ 3 ⁇ statistically corresponds to a confidence interval of 99.7%.
  • the distance between these two curves 14 and 16 in dashed line is therefore a measure of the dispersion of results.
  • Curve 14 is the limiting factor for dimensioning a part.
  • the second curve 25 (in thick line) is (schematically) the average curve obtained from the results of fatigue tests carried out on a steel produced according to the invention under a load according to FIG. CN average curve is surrounded by two curves 26 and 24 dashed thick line, respectively located at a distance of +3 ⁇ 2 and -3 ⁇ 2 of the second curve 25, ⁇ 2 being the standard deviation of the distribution of points experimental results obtained during these fatigue tests.
  • Curve 24 is the limiting factor for dimensioning a part.
  • the second curve 25 is located above the first curve 15, which means that under fatigue stress at a stress level C, the steel test pieces produced according to the invention break on average to a number N of cycles higher than that where the steel test pieces according to the prior art are broken.
  • the distance between the two curves 26 and 24 in thick dashed line is smaller than the distance between the two curves 16 and 14 in dashed fine lines, which means that the dispersion in fatigue resistance of the developed steel according to the invention is lower than that of a steel according to the prior art.
  • Oligocyclic fatigue means that the bias frequency is of the order of 1 Hz (the frequency being defined as the number of periods T per second).
  • the minimum value of fatigue stress required to break a steel according to the invention is greater than the minimum fatigue stress value M (set at 100%) necessary to break a steel according to the prior art.
  • the carbon content of the stainless martensitic steel is lower than the carbon content below which the steel is hypoeutectoid, for example a content of 0.49%.
  • a low carbon content allows a better diffusion of the alloying elements and a lowering of the temperatures of solution of the primary or noble carbides, which leads to a better homogenization.

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  • Organic Chemistry (AREA)
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Abstract

The present invention relates to a method for producing a martensitic stainless steel that includes a step in which an ingot of the steel is remelted under a layer of slag, followed by a step in which the ingot is cooled. Before the skin temperature of the ingot resulting from the slag remelting step drops below the martensitic transformation temperature Ms of the steel, the ingot is placed in a furnace, the initial temperature T0 of which is then above the cooling‑induced pearlite transformation finish temperature Ar1 of the steel. In the furnace, the ingot is subjected to a homogenization treatment at least for a holding time t after the temperature of the coolest point in the ingot has reached a homogenization temperature T, said holding time t being equal to at least one hour and the homogenization temperature T varying between approximately 900°C and the burning temperature of the steel.

Description

HOMOGENEISATION D'ACIERS MARTENSmQUES INOXYDABLES APRES REFUSION SOUS LAITIER  HOMOGENIZATION OF STAINLESS MARTENSTIC STEELS AFTER REFUSION UNDER DAIRY
La présente invention concerne un procédé de fabrication d'un acier martensitique inoxydable comportant une étape de refusion sous laitier d'un lingot de cet acier puis une étape de refroidissement de ce lingotThe present invention relates to a method for manufacturing a stainless martensitic steel comprising a slag remelting step of an ingot of this steel and a cooling step of this ingot
Dans la présente invention, les pourcentages de composition sont des pourcentages massiques, à moins qu'il en soit précisé autrement In the present invention, 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.  A stainless martensitic steel is a steel whose chromium content is greater than 10.5%, and whose structure is essentially martensitic.
Il est important que la tenue en fatigue d'un tel acier soit la plus élevée possible, afin que la durée de vie de pièces élaborées à partir de cet acier soit maximale.  It is important that the fatigue strength of such a steel is the highest possible, so that the life of parts made from this steel is maximum.
Pour cela, 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.  For this, it seeks 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 the steel. Indeed, these inclusions act as crack initiation sites which lead, under cyclic stress, to premature failure of the steel.
Expérimentalement, on observe une dispersion importante des résultats d'essais en fatigue sur des éprouvettes de test de cet acier, c'est- à-dire que pour chaque niveau de sollicitation en fatigue à déformation imposée, la durée de vie (correspondant au nombre de cycles conduisant à la rupture d'une éprouvette de fatigue dans cet acier) varie sur une plage large. Les inclusions sont responsables des valeurs minimales, dans le sens statistique, de durée de vie en fatigue de l'acier (valeurs basses de la plage).  Experimentally, a considerable dispersion of the results of fatigue tests is observed on test specimens of this steel, that is to say that for each level of fatigue strain imposed strain, the service life (corresponding to the number of cycles leading to the rupture of a fatigue test piece in this steel) varies over a wide range. Inclusions are responsible for the minimum statistical values of steel fatigue life (low values in the range).
Pour diminuer cette dispersion de la tenue en fatigue, c'est-à-dire remonter ces valeurs basses, et également augmenter sa valeur moyenne en tenue à la fatigue, il est nécessaire d'augmenter la propreté inclusionnaire de l'acier. On connaît la technique de refusion sous laitier, ou ESR (Electro Slag Refusion). Dans cette technique, on place le 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 est suffisamment élevé pour chauffer et liquéfier le laitier et pour chauffer l'extrémité inférieure de l'électrode d'acier. L'extrémité inférieure de cette électrode étant en contact avec le laitier, fond et 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. To reduce this dispersion of the fatigue strength, that is to say up these low values, and also increase its average value in resistance to fatigue, it is necessary to increase the inclusion cleanliness of the steel. We know the technique of slag remelting, or ESR (Electro Slag Refusion). In this technique, the steel ingot is placed in a crucible into which a slag (mineral mixture, for example lime, fluoride, magnesia, alumina, spath) has been poured so that the lower end of the ingot quenches in the slag. . Then we do pass an electric current into the ingot, which serves as an electrode. This current is high enough to heat and liquefy the slag and to heat the lower end of the steel electrode. The lower end of this electrode being in contact with the slag, melts and 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.
Bien que la technique de l'ESR permette de réduire la dispersion de la tenue en fatigue dans le cas des aciers martensitiques inoxydables par élimination des inclusions, cette dispersion en terme de durée de vie des pièces reste néanmoins encore trop importante.  Although the ESR technique makes it possible to reduce the dispersion of the fatigue strength in the case of stainless martensitic steels by elimination of inclusions, this dispersion in terms of lifetime of the parts still remains too important.
Des contrôles non-destructifs par ultrasons, effectués par les inventeurs, ont montré que ces aciers ne comportaient pratiquement pas de défauts hydrogènes connus (flocons).  Non-destructive ultrasonic testing, performed by the inventors, showed that these steels practically had no known hydrogen defects (flakes).
La dispersion des résultats de tenue en fatigue, spécifiquement les valeurs basses de la plage de résultats, est donc due à un autre mécanisme indésirable d'amorçage prématuré de fissures dans l'acier, qui conduit à sa rupture prématurée en fatigue.  The dispersion of the fatigue strength results, specifically the low values of the range of results, is therefore due to another undesirable mechanism of premature initiation of cracks in the steel, which leads to its premature failure in fatigue.
La présente invention vise à proposer un procédé de fabrication qui permette de remonter ces valeurs basses, et donc de réduire la dispersion de la tenue en fatigue des aciers martensitiques inoxydables, et également d'augmenter sa valeur moyenne en tenue à la fatigue.  The present invention aims to provide a manufacturing method that allows to raise these low values, and thus reduce the dispersion of the fatigue strength of stainless martensitic steels, and also to increase its average value in resistance to fatigue.
Ce but est atteint grâce au fait que le lingot issu de la refusion sous laitier est, avant que la température de peau de ce lingot soit inférieure à la température de transformation martensitique Ms de l'acier, placé dans un four dont la température initiale T0 est alors supérieure à la température de fin de transformation perlitique en refroidissement Arl dudit acier, ce lingot étant soumis dans ce four à un traitement d'homogénéisation pendant au moins un temps de maintien t après que la température du point le plus froid du lingot a atteint une température d'homogénéisation T, ce temps de maintien t étant égal à au moins une heure, et la température d'homogénéisation T variant entre environ 900°C et la température de brûlure de l'acier. This object is achieved by virtue of the fact that the ingot from the slag remelting is, before the skin temperature of this ingot is less than the martensitic transformation temperature Ms of the steel, placed in an oven whose initial temperature T 0 is then greater than the end of pearlitic transformation temperature in Arl cooling of said steel, this ingot being subjected in this oven to a homogenization treatment for at least one holding time t after the temperature of the coldest point of the ingot has reached a homogenization temperature T, this holding time t being equal to at least one hour, and the homogenization temperature T varying between about 900 ° C and the burn temperature of the steel.
Grâce à ces dispositions, on diminue la formation de phases gazeuses de taille microscopique (non détectables par les moyens de contrôle non destructifs industriels) et constituées d'éléments légers au sein de l'acier, et on évite donc l'amorce prématurée de fissures à partir de ces phases microscopiques qui conduit à la ruine prématurée de l'acier en fatigue.  Thanks to these provisions, it reduces the formation of microscopically sized gas phases (not detectable by industrial non-destructive testing means) and consist of light elements within the steel, and thus avoids the premature crack initiation from these microscopic phases which leads to the premature failure of the steel in fatigue.
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 :  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:
- la figure 1 compare des courbes de durée de vie en fatigue pour un acier selon l'invention et un acier selon l'art antérieur,  FIG. 1 compares fatigue life curves for a steel according to the invention and a steel according to the prior art,
- la figure 2 montre une courbe de sollicitation en fatigue,  FIG. 2 shows a fatigue stress curve,
- la figure 3 est un schéma illustrant les dendrites et les régions interdendritiques,  FIG. 3 is a diagram illustrating dendrites and interdendritic regions,
- la figure 4 est une photographie prise au microscope électronique d'une surface de fracture après fatigue, montrant la phase gazeuse ayant initié cette fracture.  FIG. 4 is a photograph taken under the electron microscope of a fracture surface after fatigue, showing the gas phase having initiated this fracture.
- la figure 5 montre schématiquement des courbes de refroidissement sur un diagramme temps-température pour une région plus riche en éléments alphagènes et moins riche en élément gammagènes,  FIG. 5 schematically shows cooling curves on a time-temperature diagram for a region richer in alphagenic elements and less rich in gamma-ray elements,
- la figure 6 montre schématiquement des courbes de refroidissement sur un diagramme temps-température pour une région moins riche en éléments alphagènes et plus riche en élément gammagènes.  FIG. 6 schematically shows cooling curves on a time-temperature diagram for a region less rich in alpha-gene elements and richer in gammagenic elements.
Au cours du processus d'ESR, l'acier qui a été filtré par le laitier se refroidit et se solidifie progressivement pour former un lingot. Cette solidification intervient pendant le refroidissement et s'effectue par croissance de dendrites 10, comme illustré en figure 3. 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 gammagène est un élément qui favorise une structure austénitique (structure stable à haute température). Il se produit donc une ségrégation entre dendrites 10 et régions interdendritiques 20. During the ESR process, the steel that has been filtered by the slag cools and gradually solidifies to form an ingot. This solidification takes place during cooling and is carried out by growth of dendrites 10, as illustrated in FIG. 3. In accordance with the phase diagram of the stainless martensitic steels, the dendrites 10, corresponding to the first solidified grains, are by definition richer in elements. alphagenes while the interdendritic regions 20 are richer in gamma-like elements (application of the known rule segments on the phase diagram). An alphagene element is one that promotes a ferritic type structure (more stable structures at low temperature, bainite, ferrite-pearlite, martensite). A gamma element is an element that promotes an austenitic structure (stable structure at high temperature). There is therefore segregation between dendrites 10 and interdendritic regions 20.
Cette ségrégation locale de composition chimique se conserve ensuite tout le long de la fabrication, même pendant les opérations ultérieures de mise en forme à chaud. Cette ségrégation se retrouve donc aussi bien sur le lingot brut de solidification que sur le lingot déformé ultérieurement.  This local segregation of chemical composition is then preserved throughout the manufacturing process, even during subsequent hot forming operations. This segregation is therefore found on both the solid ingot of solidification and on the subsequently deformed ingot.
Les inventeurs ont pu montrer que les résultats dépendent du diamètre du lingot issu directement du creuset ESR ou du lingot après déformation à chaud. Cette observation peut s'expliquer par le fait que les vitesses de refroidissement décroissent avec un diamètre croissant. Les figures 5 et 6 illustrent différents scénarii qui peuvent se produire.  The inventors have been able to show that the results depend on the diameter of the ingot coming directly from the ESR crucible or the ingot after hot deformation. This observation can be explained by the fact that the cooling rates decrease with increasing diameter. Figures 5 and 6 illustrate different scenarios that may occur.
La figure 5 est un diagramme température (T) - temps (t) connu pour une région plus riche en éléments alphagènes et moins riche en éléments gammagènes, telle que les dendrites 10. 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 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 se situe entièrement dans la région A.  FIG. 5 is a temperature (T) -time (t) diagram known for a region richer in alphagenes and less rich in gamma elements, such as dendrites 10. Curves D and F mark the beginning and the end of the transformation of austenite (region A) into a ferrito-pearlitic structure (FP region). This transformation takes place, partially or fully, when the cooling curve that follows the ingot passes respectively in the region between the D and F curves or in the FP region. It does not occur when the cooling curve is entirely in region A.
La figure 6 est un diagramme équivalent pour une région plus riche en éléments gammagènes et moins riche en éléments alphagènes, telle que les régions interdendritiques 20. On note que par rapport à la figure 5, les courbes D et F sont décalées vers la droite, c'est-à-dire qu'il faudra refroidir plus lentement le lingot pour obtenir un structure ferrito- perlitique.  FIG. 6 is an equivalent diagram for a region richer in gammagenic elements and less rich in alphagenic elements, such as the interdendritic regions 20. It will be noted that with respect to FIG. 5, the curves D and F are shifted to the right, that is to say, it will cool more slowly the ingot to obtain a ferritoperlitic structure.
Chacune des figures 5 et 6 montre trois courbes de refroidissement depuis une température austénitique, correspondant à trois vitesses de refroidissement ; rapide (courbe Cl), moyenne (courbe C2), lente (courbe C3). Au cours du refroidissement, la température commence à décroître depuis une température austénitique. A l'air, pour les diamètres concernés dans notre cas, les vitesses de refroidissement de la surface et du c ur du lingot sont très proches. La seule différence vient du fait que la température en surface est plus faible que celle du cœur car la surface a été la première à se refroidir par rapport au cœur. Each of Figures 5 and 6 shows three cooling curves from an austenitic temperature, corresponding to three cooling rates; fast (curve C1), average (curve C2), slow (curve C3). During cooling, the temperature begins to decrease from an austenitic temperature. In the air, for the diameters concerned in our case, the cooling rates of the surface and the heart of the ingot are very close. The only difference is that the surface temperature is lower than that of the core because the surface was the first to cool relative to the core.
Dans le cas des refroidissements plus rapide qu'un refroidissement rapide (courbe Cl) (figures 5 et 6), les transformations ferrito-perlitiques ne se font pas.  In the case of faster cooling than rapid cooling (curve C1) (FIGS. 5 and 6), the ferrito-pearlitic transformations do not take place.
Dans le cas d'un refroidissement rapide selon la courbe Cl, les transformations ne sont que partielles, uniquement dans les dendrites (Figure 5).  In the case of rapid cooling according to the curve C1, the transformations are only partial, only in the dendrites (Figure 5).
Dans le cas d'un refroidissement moyen selon la courbe C2, les transformations ne sont que partielles dans les espaces interdendritiques 20 (Figure 6) et quasi-complètes dans les dendrites 10 (Figure 5).  In the case of an average cooling according to the C2 curve, the transformations are only partial in the interdendritic spaces 20 (FIG. 6) and almost completely in the dendrites 10 (FIG. 5).
Dans le cas d'un refroidissement lent selon la courbe C3 et de refroidissements encore plus lents, les transformations sont quasiment complètes à la fois dans les espaces interdendritiques 20 et dans les dendrites 10.  In the case of slow cooling according to the C3 curve and even slower cooling, the transformations are almost complete both in the interdendritic spaces 20 and in the dendrites 10.
Dans le cas de refroidissements rapide (Cl) ou moyen (C2), il y a cohabitation plus ou moins marquée entre des régions ferritiques et des régions austénitiques.  In the case of rapid cooling (Cl) or medium (C2), there is more or less cohabitation between ferritic regions and austenitic regions.
En effet, une fois la matière solidifiée, les dendrites 10 se transforment en premier en structures ferritiques au cours du refroidissement (en traversant les courbes D et F de la figure 5). Tandis que les régions interdendritiques 20 soit ne se transforment pas (cas du refroidissement rapide selon la courbe Cl) soit se transforment ultérieurement, en tout ou partie (cas des refroidissements moyen selon la courbe C2 ou lent selon la courbe C3), à des températures inférieures (voir figure 6).  In fact, once the material has solidified, the dendrites 10 first turn into ferritic structures during cooling (by crossing the curves D and F of FIG. 5). While the interdendritic regions 20 either do not change (in the case of rapid cooling according to the curve C1) or change later, in whole or in part (in the case of average cooling according to the curve C2 or slow according to the curve C3), to temperatures inferior (see Figure 6).
Les régions interdendritiques 20 conservent donc plus longtemps une structure austénitique.  The interdendritic regions thus retain a longer austenitic structure.
Durant ce refroidissement à l'état solide, localement, il y a une hétérogénéité structurale avec cohabitation de microstructure austénitique et de type ferritique. Dans ces conditions, les éléments légers (H, N, O), qui sont davantage solubles dans l'austénite que dans les structures ferritiques, ont donc tendance à se concentrer dans les régions interdendritiques 20. Cette concentration est augmentée par la teneur plus élevée en éléments gammagènes dans les régions interdendritiques 20. Aux températures inférieures à 300°C, les éléments légers ne diffusent plus qu'à des vitesses extrêmement faibles et restent piégés dans leur région. Après transformation en structure ferritique, totale à partielle, des zones interdendritiques 20, la limite de solubilité de ces phases gazeuses est atteinte dans certaines conditions de concentration et ces phases gazeuses forment des poches de gaz (ou d'une substance dans un état physique permettant une grande malléabilité et incompressibilité). During this cooling in the solid state, locally, there is a structural heterogeneity with coexistence of austenitic microstructure and ferritic type. Under these conditions, the light elements (H, N, O), which are more soluble in the austenite than in the structures ferritic, therefore tend to concentrate in the interdendritic regions 20. This concentration is increased by the higher content of gamma-elements in the interdendritic regions 20. At temperatures below 300 ° C, the light elements only diffuse at extremely low speeds and remain trapped in their area. After the interdentitic zones 20 have been converted into a total or partial ferritic structure, the solubility limit of these gaseous phases is reached under certain concentration conditions and these gaseous phases form pockets of gas (or a substance in a physical state allowing great malleability and incompressibility).
Pendant la phase de refroidissement, plus le lingot en sortie d'ES (ou le lingot ultérieurement déformé) a un diamètre important (ou, plus généralement, plus la dimension maximale du lingot est importante) ou plus la vitesse de refroidissement du lingot est faible, plus les éléments légers sont aptes à diffuser des dendrites 10 de structure ferritique vers les régions interdendritiques 20 de structure tout ou partie austénitique et à s'y concentrer pendant la période de cohabitation des structures ferritiques et austénitiques. Le risque que la solubilité en ces éléments légers soit dépassée localement dans les régions interdendritiques est accentué. Lorsque la concentration en éléments légers dépasse cette solubilité, il apparaît alors au sein de l'acier des poches gazeuses microscopiques contenant ces éléments légers.  During the cooling phase, the more the ingot at the ES outlet (or the subsequently deformed ingot) has a large diameter (or, more generally, the larger the dimension of the ingot is large) or the ingot cooling rate is lower , the lighter elements are able to diffuse ferritic structure dendrites towards the interdendritic regions 20 of austenitic or all-part structure and to concentrate during the period of coexistence of the ferritic and austenitic structures. The risk that the solubility of these light elements is exceeded locally in the interdendritic regions is accentuated. When the concentration in light elements exceeds this solubility, it appears then in the steel microscopic gas pockets containing these light elements.
De plus, durant la fin de refroidissement, l'austénite des régions interdendritiques a tendance à se transformer localement en martensite lorsque la température de l'acier passe en dessous de la température de transformation martensitique Ms, qui se situe légèrement au dessus de la température ambiante (Figures 5 et 6). Or la martensite a un seuil de solubilité en éléments légers encore plus faible que les autres structures métallurgiques et que l'austénite. Il apparaît donc davantage de phases gazeuses microscopiques au sein de l'acier durant cette transformation martensitique.  Moreover, during the end of cooling, the austenite of interdendritic regions tends to locally transform into martensite when the temperature of the steel falls below the martensitic transformation temperature Ms, which is slightly above the temperature. ambient (Figures 5 and 6). However, martensite has a threshold of solubility in light elements even lower than other metallurgical structures and that austenite. There is therefore more microscopic gaseous phase within the steel during this martensitic transformation.
Au cours des déformations ultérieures que subit l'acier durant des mises en forme à chaud (par exemple forgeage), ces phases s'aplatissent en forme de feuille.  During subsequent deformations that the steel undergoes during hot forming (eg forging), these phases flatten in sheet form.
Sous une sollicitation en fatigue, ces feuilles agissent comme des sites de concentration de contraintes, qui sont responsables de l'amorce prématurée de fissures en réduisant l'énergie nécessaire à l'amorçage de fissures. Il se produit ainsi une ruine prématurée de l'acier, qui correspond aux valeurs basses des résultats de tenue en fatigue. Under a stress in fatigue, these leaves act as sites of concentration of constraints, which are responsible for the primer premature cracking by reducing the energy required to initiate cracks. There is thus a premature failure of the steel, which corresponds to the low values of the fatigue resistance results.
Ces conclusions sont corroborées par les observations des inventeurs, comme le montre la photographie au microscope électronique de la figure 4.  These conclusions are corroborated by the observations of the inventors, as shown by the electron micrograph of FIG.
Sur cette photographie d'une surface de fracture d'un acier martensitique inoxydable, on distingue une zone sensiblement globulaire P d'où rayonnent des fissures F. Cette zone P est l'empreinte de la phase gazeuse constituée des éléments légers, et qui est à l'origine de la formation de ces fissures F qui, en se propageant et en s'aggloméra nt, ont créé une zone de fracture macroscopique.  In this photograph of a fracture surface of a stainless steel martensitic, there is a substantially globular zone P from which radiates fissures F. This zone P is the imprint of the gaseous phase consisting of the light elements, and which is at the origin of the formation of these fissures F which, by propagating and agglomerating, created a zone of macroscopic fracture.
Les inventeurs ont réalisé des essais sur des aciers martensitiques inoxydables, et ont trouvé que lorsqu'on réalise, immédiatement après l'étape d'ESR, un traitement d'homogénéisation particulier sur le lingot sorti du creuset d'ESR, on diminue la formation de phases gazeuses d'éléments légers.  The inventors have carried out tests on stainless martensitic steels, and have found that when, immediately after the ESR step, a specific homogenization treatment is carried out on the ingot taken out of the ESR crucible, the formation is reduced. of gaseous phases of light elements.
En effet, par diffusion des éléments d'alliage des zones à forte concentration vers les zones à faible concentration, on permet 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 les conséquences suivantes : un moindre décalage vers la droite des courbes D et F de transformation en structure ferrito-perlitique (Figure 6), une moindre différence structurale entre les dendrites 10 et les régions interdendritiques 20, et une moindre différence de solubilité en éléments légers (H, N, O) entre les dendrites et les régions interdendritiques, permettant une meilleure homogénéité en terme de structure (moins de cohabitation de structures austénitiques et ferritiques) et de composition chimique y compris les éléments légers.  Indeed, by diffusion of the alloying elements of the high concentration zones towards the low concentration zones, a reduction of the intensity of the segregations in alphagenes elements in the dendrites 10 is allowed, and a reduction of the intensity of the segregations in gammagens elements in interdendritic regions 20. The reduction of the intensity of segregations in these gammagens has the following consequences: a smaller shift towards the right of the curves D and F of transformation into a ferrito-pearlitic structure (Figure 6), a lower structural difference between the dendrites 10 and the interdendritic regions 20, and a lesser difference in solubility in light elements (H, N, O) between the dendrites and the interdendritic regions, allowing a better homogeneity in terms of structure (less cohabitation of structures austenitic and ferritic) and chemical composition including light elements.
En outre, le traitement d'homogénéisation entraîne aussi une homogénéisation de la température de transformation martensitique Ms.  In addition, the homogenization treatment also leads to a homogenization of the martensitic transformation temperature Ms.
Quand la température de l'acier est à une température supérieure à 300°C, la diffusion des éléments d'alliage est loin d'être négligeable. De plus, si le gradient de température permet d'avoir une surface plus chaude que le centre du lingot, ce que les conditions de reprise proposées par les inventeurs permettent, les éléments légers diffusent vers la surface, ce qui réduit leur teneur globale dans l'acier. When the temperature of the steel is at a temperature above 300 ° C, the diffusion of the alloying elements is far from negligible. Moreover, if the temperature gradient makes it possible to have a warmer surface That the center of the ingot, which the conditions of recovery proposed by the inventors allow, the light elements diffuse towards the surface, which reduces their overall content in the steel.
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 Tmin 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 temperature T min and the burn temperature of this steel.
La température Tmjn 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 Tmin. Ce temps t de maintien de l'acier dans le four varie donc inversement à cette température d'homogénéisation T. The temperature T m j n is approximately equal to 900 ° C. The burning temperature of a steel is defined as the temperature in the raw state of solidification at which the grain boundaries in the steel are transformed (or even liquefied), and is greater than T min . 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 1250°C 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 1250 ° C. which is slightly lower than the burn temperature, then the corresponding holding time t is equal to 10 hours.
Par exemple, la température d'homogénéisation T est choisie dans une plage choisie dans le groupe comprenant les plages suivantes : 950°C à 1270°C, 980°C à 1250°C, 1000°C à 1200°C.  For example, the homogenization temperature T is selected from a range selected from the group consisting of the following ranges: 950 ° C to 1270 ° C, 980 ° C to 1250 ° C, 1000 ° C to 1200 ° C.
Par exemple, le temps de maintien minimal t est choisie dans une plage choisie dans le groupe comprenant les plages suivantes : 1 heure à 70 heures, 10 heures à 30 heures, 30 heures à 150 heures.  For example, the minimum hold time t is selected from a range selected from the group consisting of the following ranges: 1 hour to 70 hours, 10 hours to 30 hours, 30 hours to 150 hours.
Par ailleurs, les inventeurs ont trouvé que des résultats satisfaisants sont obtenus lorsque le lingot en sortie du creuset d'ESR est placé dans un four dont la température initiale T0 est supérieure à la température de fin de transformation perlitique en refroidissement Arl de cet acier, et lorsque la température de peau de ce lingot reste supérieure à la température de transformation martensitique Ms de cet acier. Dans le cas où la température initiale T0 du four est inférieure à la température d'homogénéisation T, la température du four est, après que le lingot ait été placé dans ce four, augmentée jusqu'à une température au moins égale à la température d'homogénéisation. Ainsi, pendant cette remontée en température, on tend vers une structure austénitique homogène, ceci afin d'homogénéiser la teneur en hydrogène, et on tend vers un gradient croissant de température du centre de la pièce vers la surface. La température au centre du lingot reste donc plus faible que la température de peau du lingot pendant toute la remontée en température. On permet ainsi un dégazage global et plus efficace du lingot. Furthermore, the inventors have found that satisfactory results are obtained when the ingot at the outlet of the ESR crucible is placed in an oven whose initial temperature T 0 is greater than the end of pearlitic transformation temperature in cooling Arl of this steel. and when the skin temperature of this ingot remains higher than the martensitic transformation temperature Ms of this steel. In the case where the initial temperature T 0 of the oven is lower than the homogenization temperature T, the temperature of the oven is, after the ingot has been placed in this oven, increased to a temperature at least equal to the temperature homogenization. Thus, during this rise in temperature, it tends to a homogeneous austenitic structure, in order to homogenize the hydrogen content, and tends to a rising temperature gradient from the center of the part to the surface. The temperature in the center of the ingot therefore remains lower than the skin temperature of the ingot throughout the rise in temperature. This allows a global degassing and more effective ingot.
Alternativement, la température initiale T0 du four peut être supérieure à la température d'homogénéisation, auquel cas la température du four est simplement maintenue au dessus de cette température d'homogénéisation. Alternatively, the initial temperature T 0 of the oven may be greater than the homogenization temperature, in which case the oven temperature is simply maintained above this homogenization temperature.
Les inventeurs ont constaté que le traitement d'homogénéisation était spécialement nécessaire lorsque :  The inventors have found that the homogenization treatment is especially necessary when:
- La dimension maximale du lingot est inférieure à environ 910 mm, et la teneur en H du lingot avant refusion sous laitier est supérieure à 10 ppm, et  The maximum dimension of the ingot is less than approximately 910 mm, and the H content of the ingot before slag remelting is greater than 10 ppm, and
- La dimension maximale du lingot est supérieure à environ 910 mm et la dimension minimale du lingot est inférieure à environ 1500 mm, et la teneur en H du lingot avant refusion sous laitier est supérieure à 3 ppm, et The maximum dimension of the ingot is greater than about 910 mm and the minimum dimension of the ingot is less than about 1500 mm, and the H content of the ingot before slag remelting is greater than 3 ppm, and
- La dimension minimale du lingot est supérieure à 1500 mm et la teneur en H du lingot avant refusion sous laitier est supérieure à 10 - The minimum dimension of the ingot is greater than 1500 mm and the H content of the ingot before slag remelting is greater than 10
PPm. Ppm.
La dimension maximale du lingot est celle des mesures dans sa partie la plus massive, et la dimension minimale du lingot est celle des mesures dans sa partie la moins massive ;  The maximum dimension of the ingot is that of the measurements in its most massive part, and the minimum dimension of the ingot is that of the measures in its least massive part;
· immédiatement après refusion sous laitier lorsque le lingot ne subit pas de mise en forme à chaud avant son refroidissement ultérieur.  · Immediately after slag remelting when the ingot is not hot-formed before it is subsequently cooled.
• Lorsque le lingot subit une mise en forme à chaud après refusion sous laitier, juste avant son refroidissement ultérieur. Comme indiqué ci-dessus, les inventeurs ont constaté que les concentrations en éléments légers peuvent être plus importantes (supérieures à 10 ppm) quand la dimension minimale du lingot ou du lingot déformé est supérieure à un seuil important de dimension (en l'espèce 1500 mm). L'explication de l'existence d'un seuil élevé (1500 mm) pour la dimension minimale du lingot est la suivante : lorsque la dimension minimale du lingot est supérieure à ce seuil, on se rapproche du cas du refroidissement lent (courbe C3) dans lequel il n'y a quasiment plus de différence structurale entre les dendrites et les régions interdendritiques au cours du refroidissement. De plus, la vitesse de refroidissement est suffisamment faible pour que la température soit sensiblement homogène entre le cœur la peau du lingot, donc pour que la diffusion des éléments légers vers la surface soit facilitée, ce qui permet un dégazage plus important. En revanche, lorsque la dimension minimale du lingot est inférieure à ce seuil, le cœur du lingot est, pendant le refroidissement, nettement plus chaud que sa surface, ce qui privilégie une diffusion des éléments légers vers le cœur et freine le dégazage. • When the ingot undergoes heat shaping after slag remelting, just before its subsequent cooling. As indicated above, the inventors have found that the concentrations of light elements may be greater (greater than 10 ppm) when the minimum dimension of the ingot or of the deformed ingot is greater than a large size threshold (in this case 1500 mm). The explanation of the existence of a high threshold (1500 mm) for the minimum dimension of the ingot is as follows: when the minimum dimension of the ingot is greater than this threshold, we approach the case of slow cooling (curve C3) in which there is almost no structural difference between dendrites and interdendritic regions during cooling. In addition, the cooling rate is sufficiently low so that the temperature is substantially homogeneous between the core of the skin of the ingot, so that the diffusion of light elements to the surface is facilitated, which allows a greater degassing. On the other hand, when the minimum dimension of the ingot is below this threshold, the core of the ingot is, during the cooling, much hotter than its surface, which favors a diffusion of the light elements towards the core and slows down the degassing.
Par ailleurs, il est préférable que le laitier soit préalablement déshydraté avant son utilisation dans le creuset d'ESR, car on minimise ainsi la quantité d'hydrogène présente dans le laitier, et donc on minimise la quantité d'hydrogène qui pourrait passer du laitier au lingot durant le procédé d'ESR.  Furthermore, it is preferable that the slag is previously dehydrated before use in the ESR crucible, because it minimizes the amount of hydrogen present in the slag, and thus minimizes the amount of hydrogen that could pass the slag ingot during the ESR process.
Les inventeurs ont réalisés des essais sur des aciers Z12CNDV12 élaborés avec le procédé selon l'invention, c'est-à-dire avec une homogénéisation effectuée immédiatement après la sortie du lingot du creuset d'ESR selon les paramètres suivants :  The inventors have carried out tests on Z12CNDV12 steels produced with the process according to the invention, that is to say with a homogenization carried out immediately after the ingress of the ingot of the ESR crucible according to the following parameters:
Essai n°l : Température en peau du lingot à 250°C, mise au four à 400°C, montée du four à la température d'homogénéisation de 1250°C, maintien métallurgique (dès que la température la plus froide du lingot atteint la température d'homogénéisation) de 75h, refroidissement jusqu'à la température ambiante.  Test No. 1: skin temperature of the ingot at 250 ° C., put in the oven at 400 ° C., rise of the oven at the homogenization temperature of 1250 ° C., metallurgical maintenance (as soon as the coldest temperature of the ingot reaches the homogenization temperature) of 75h, cooling to room temperature.
Essai n°2 : Température en peau du lingot à 600°C, mise au four à 450°C, montée du four à la température d'homogénéisation de 1000% maintien métallurgique (dès que la température la plus froide du lingot atteint la température d'homogénéisation) de 120h, refroidissement jusqu'à la température ambiante. Les résultats de ces essais sont présentés ci-dessous, Test No. 2: Skin temperature of the ingot at 600 ° C., put in the oven at 450 ° C., rise of the furnace at the homogenization temperature of 1000% metallurgical maintenance (as soon as the coldest temperature of the ingot reaches the temperature homogenization) of 120h, cooling to room temperature. The results of these tests are presented below,
La composition des aciers Z12CNDV12 est la suivante (norme DMD0242-20 indice E) :  The composition of the Z12CNDV12 steels is the following (standard DMD0242-20 index 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 + ll.V - 30.N) < 9C (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 satisfying the criterion 4.5≤ (Cr - 40.C - 2.Mn - 4.Ni + 6.Si + 4.Mo + ll.V - 30.N) <9
La température de transformation martensitique Ms mesurée est 220°C. The martensitic transformation temperature Ms measured is 220 ° C.
La quantité d'Hydrogène mesurée sur les lingots avant refusion sous laitier varie de 3,5 à 8,5ppm.  The amount of Hydrogen measured on the ingots before slag remelting varies from 3.5 to 8.5 ppm.
La figure 1 montre qualitativement les améliorations apportées par le procédé selon l'invention. On obtient expérimentalement la valeur du nombre N de cycles à rupture nécessaire pour rompre une éprouvette en acier soumise à une sollicitation cyclique en traction en fonction de la pseudo contrainte alternée C (il s'agit de la contrainte subie par l'éprouvette sous déformation imposée, selon la norme DMC0401 de Snecma utilisée pour ces essais).  Figure 1 qualitatively shows the improvements made by the method according to the invention. The value of the number N of rupture cycles necessary to break a steel specimen subjected to a cyclic stress in tension as a function of the pseudo-alternating stress C is obtained experimentally (this is the stress experienced by the test specimen under imposed deformation. , according to Sncma DMC0401 standard used for these tests).
Une telle sollicitation cyclique est représentée schématiquement en figure 2. La période T représente un cycle. La contrainte évolue entre une valeur maximale Cmax et une valeur minimale Cmin. Such a cyclic bias is shown schematically in FIG. 2. The period T represents a cycle. The constraint evolves between a maximum value C ma x and a minimum value C min .
En testant en fatigue un nombre statistiquement suffisant d'éprouvettes, les inventeurs ont obtenu des points N=f(C) à partir desquels ils ont tracé une courbe statistique moyenne C-N (contrainte C en fonction du nombre N de cycles de fatigue). Les écarts types sur les contraintes sont ensuite calculés pour un nombre de cycle donné.  By fatigue testing a statistically sufficient number of test pieces, the inventors obtained points N = f (C) from which they drew a mean statistical curve C-N (stress C as a function of the number N of fatigue cycles). The standard deviations on the stresses are then calculated for a given number of cycles.
Sur la figure 1, la première courbe 15 (en trait fin) est (schématiquement) la courbe moyenne obtenue pour un acier élaboré selon l'art antérieur. Cette première courbe moyenne C-N est entourée par deux courbes 16 et 14 en trait fin pointillé. Ces courbes 16 et 14 sont situées respectivement à une distance de +3 <s{ et -3 at de la première courbe 15, σ* étant l'écart-type de la distribution des points expérimentaux obtenus lors de ces essais en fatigue, et ±3σι correspond en statistique à un intervalle de confiance de 99,7%. La distance entre ces deux courbes 14 et 16 en trait pointillé est donc une mesure de la dispersion des résultats. La courbe 14 est le facteur limitant pour le dimensionnement d'une pièce. In FIG. 1, the first curve 15 (in fine lines) is (schematically) the average curve obtained for a steel produced according to the prior art. This first average curve CN is surrounded by two curves 16 and 14 in dashed fine lines. These curves 16 and 14 are situated respectively at a distance of +3 <s { and -3 at t from the first curve 15, where σ * is the standard deviation of the distribution of the experimental points obtained during these fatigue tests. and ± 3σι statistically corresponds to a confidence interval of 99.7%. The distance between these two curves 14 and 16 in dashed line is therefore a measure of the dispersion of results. Curve 14 is the limiting factor for dimensioning a part.
Sur la figure 1, la deuxième courbe 25 (en trait épais) est (schématiquement) la courbe moyenne obtenue à partir des résultats d'essais en fatigue effectués sur un acier élaboré selon l'invention sous une sollicitation selon la figure 2. Cette deuxième courbe moyenne C-N est entourée par deux courbes 26 et 24 en trait épais pointillé, situées respectivement à une distance de +3 σ2 et -3 σ2 de la deuxième courbe 25, σ2 étant l'écart-type de la distribution des points expérimentaux obtenus lors de ces essais en fatigue. La courbe 24 est le facteur limitant pour le dimensionnement d'une pièce. In FIG. 1, the second curve 25 (in thick line) is (schematically) the average curve obtained from the results of fatigue tests carried out on a steel produced according to the invention under a load according to FIG. CN average curve is surrounded by two curves 26 and 24 dashed thick line, respectively located at a distance of +3 σ 2 and -3 σ 2 of the second curve 25, σ 2 being the standard deviation of the distribution of points experimental results obtained during these fatigue tests. Curve 24 is the limiting factor for dimensioning a part.
On note que la deuxième courbe 25 est située au dessus de la première courbe 15, ce qui signifie que sous une sollicitation en fatigue à un niveau de contrainte C, les éprouvettes en acier élaboré selon l'invention se rompent en moyenne à un nombre N de cycles plus élevé que celui où les éprouvettes en acier selon l'art antérieur se rompent.  It is noted that the second curve 25 is located above the first curve 15, which means that under fatigue stress at a stress level C, the steel test pieces produced according to the invention break on average to a number N of cycles higher than that where the steel test pieces according to the prior art are broken.
De plus, la distance entre les deux courbes 26 et 24 en trait épais pointillé est plus faible que la distance entre les deux courbes 16 et 14 en trait fin pointillé, ce qui signifie que la dispersion en tenue à la fatigue de l'acier élaboré selon l'invention est plus faible que celle d'un acier selon l'art antérieur.  In addition, the distance between the two curves 26 and 24 in thick dashed line is smaller than the distance between the two curves 16 and 14 in dashed fine lines, which means that the dispersion in fatigue resistance of the developed steel according to the invention is lower than that of a steel according to the prior art.
La figure 1 illustre les résultats expérimentaux résumés dans le tableau 1 ci-dessous.  Figure 1 illustrates the experimental results summarized in Table 1 below.
Le tableau 1 donne les résultats pour une sollicitation en fatigue oligocyclique selon la figure 2 avec une contrainte Cmln nulle, à une température de 250°C, à N = 20 000 cycles, et N = 50 000 cycles. Une fatigue oligocyclique signifie que la fréquence de sollicitation est de l'ordre de 1 Hz (la fréquence étant définie comme le nombre de périodes T par seconde). Table 1 gives the results for a fatigue load of oligocyclic fatigue according to Figure 2 with a stress C mln zero, at a temperature of 250 ° C, at N = 20 000 cycles, and N = 50 000 cycles. Oligocyclic fatigue means that the bias frequency is of the order of 1 Hz (the frequency being defined as the number of periods T per second).
Tableau 1 Table 1
Condil :ions d'essai en  Condil: test ions in
Acier selon 1 art antérieur Acier élaboré st îlon l'invention fatigu e oligocyclique  Steel according to 1 prior art Steel developed stil the invention tired oligocyclic
N Température Qnin Dispersion Qnin Dispersion  N Temperature Qnin Dispersion Qnin Dispersion
2- 105 200°C 100%= M 120% M 130% M 44% M 5-104 400°C 100%=M 143% M 130% M 90% M 2-10 5 200 ° C 100% = M 120% M 130% M 44% M 5-10 4 400 ° C 100% = M 143% M 130% M 90% M
On note que pour une valeur donnée du nombre N de cycles, la valeur minimale de contrainte en fatigue nécessaire pour rompre un acier selon l'invention est supérieure à la valeur minimale M de contrainte en fatigue (fixée à 100%) nécessaire pour rompre un acier selon l'art antérieur. La dispersion (=6 σ) des résultats à ce nombre N de cycles pour un acier selon l'invention est inférieure à la dispersion des résultats pour un acier selon l'art antérieur (dispersions exprimées en pourcentage de la valeur minimale M). It should be noted that for a given value of the number N of cycles, the minimum value of fatigue stress required to break a steel according to the invention is greater than the minimum fatigue stress value M (set at 100%) necessary to break a steel according to the prior art. The dispersion (= 6 σ) of the results at this number N of cycles for a steel according to the invention is less than the dispersion of the results for a steel according to the prior art (dispersions expressed as a percentage of the minimum value M).
Avantageusement, la teneur en carbone de l'acier martensitique inoxydable est inférieure à la teneur en carbone en dessous de laquelle l'acier est hypoeutectoïde, par exemple une teneur de 0,49%. En effet, une telle teneur faible en carbone permet une meilleure diffusion des éléments d'alliage et un abaissement des températures de remise en solution des carbures primaires ou nobles, ce qui entraine une meilleure homogénéisation.  Advantageously, the carbon content of the stainless martensitic steel is lower than the carbon content below which the steel is hypoeutectoid, for example a content of 0.49%. Indeed, such a low carbon content allows a better diffusion of the alloying elements and a lowering of the temperatures of solution of the primary or noble carbides, which leads to a better homogenization.
Par exemple, l'acier martensitique a, avant sa refusion sous laitier, été élaboré à l'air.  For example, martensitic steel, before its slag remelting, was made in the air.

Claims

REVENDICATIONS
1. Procédé de fabrication d'un acier martensitique inoxydable comportant une étape de refusion sous laitier d'un lingot dudit acier puis une étape de refroidissement dudit lingot, caractérisé en ce que le lingot issu de la refusion sous laitier est, avant que la température de peau dudit lingot soit inférieure à la température de transformation martensitique Ms dudit acier, placé dans un four dont la température initiale T0 est alors supérieure à la température de fin de transformation perlitique en refroidissement Arl dudit acier, ledit lingot étant soumis dans ce four à un traitement d'homogénéisation pendant au moins un temps de maintien t après que la température du point le plus froid dudit lingot a atteint une température d'homogénéisation T, ce temps de maintien t étant égal à au moins une heure, et la température d'homogénéisation T variant entre environ 900°C et la température de brûlure dudit acier. 1. A method for manufacturing a stainless martensitic steel comprising a slag remelting step of an ingot of said steel and a cooling step of said ingot, characterized in that the ingot from the slag remelting is, before the temperature of skin of said ingot is less than the martensitic transformation temperature Ms of said steel, placed in a furnace whose initial temperature T 0 is then greater than the end of pearlitic transformation temperature in cooling Arl of said steel, said ingot being subjected in this furnace at a homogenization treatment for at least one holding time t after the temperature of the coldest point of said ingot has reached a homogenization temperature T, this holding time t being equal to at least one hour, and the temperature homogenization T varying between about 900 ° C and the burn temperature of said steel.
2. Procédé de fabrication d'un acier martensitique inoxydable selon la revendication 1, caractérisé en ce que ladite température initiale T0 du four est inférieure à ladite température d'homogénéisation T, la température du four étant augmentée depuis sa température initiale TD jusqu'à une température au moins égale à la température d'homogénéisation T. 2. Process for manufacturing a stainless martensitic steel according to claim 1, characterized in that said initial temperature T 0 of the furnace is lower than said homogenization temperature T, the temperature of the furnace being increased from its initial temperature T D to at a temperature at least equal to the homogenization temperature T.
3. Procédé de fabrication d'un acier martensitique inoxydable selon la revendication 1, caractérisé en ce que la température d'homogénéisation T appartient à une plage choisie dans le groupe comprenant les plages suivantes : 950°C à 1270°C, 980°C à 1250°C, 1000°C à 1200°C.  3. A method of manufacturing a stainless martensitic steel according to claim 1, characterized in that the homogenization temperature T is within a range selected from the group consisting of the following ranges: 950 ° C to 1270 ° C, 980 ° C at 1250 ° C, 1000 ° C to 1200 ° C.
4. Procédé de fabrication d'un acier martensitique inoxydable selon la revendication 1 ou 2, caractérisé en ce que temps de maintien t minimal appartient à une plage choisie dans le groupe comprenant les plages suivantes : 1 heure à 70 heures, 10 heures à 30 heures, 30 heures à 150 heures.  4. A method of manufacturing a stainless martensitic steel according to claim 1 or 2, characterized in that minimum holding time t belongs to a range selected from the group consisting of the following ranges: 1 hour to 70 hours, 10 hours to 30 hours. hours, 30 hours to 150 hours.
5. Procédé de fabrication d'un acier martensitique inoxydable selon la revendication 1, caractérisé en ce que le laitier utilisé dans ladite étape de refusion a été préalablement déshydraté.  5. A method of manufacturing a stainless martensitic steel according to claim 1, characterized in that the slag used in said remelting step has been previously dehydrated.
6. Procédé de fabrication d'un acier martensitique inoxydable selon la revendication 1 ou 2, caractérisé en ce que ledit temps de maintien t varie inversement à la variation de ladite température d'homogénéisation T. 6. A method of manufacturing a stainless martensitic steel according to claim 1 or 2, characterized in that said time of maintenance t varies inversely with the variation of said homogenization temperature T.
7. Procédé de fabrication d'un acier martensitique inoxydable selon l'une quelconque des revendications 1 à 6, caractérisé en ce qu'il est effectué sur ledit acier dans l'un des cas suivants ;  7. A method of manufacturing a stainless martensitic steel according to any one of claims 1 to 6, characterized in that it is carried out on said steel in one of the following cases;
- La dimension maximale dudit lingot avant refroidissement est inférieure à environ 910 mm, et la teneur en H du lingot avant refusion sous laitier est supérieure à 10 ppm,  The maximum dimension of said ingot before cooling is less than approximately 910 mm, and the H content of the ingot before slag remelting is greater than 10 ppm,
- La dimension maximale dudit lingot avant refroidissement est supérieure à environ 910 mm et sa dimension minimale inférieure à environ 1500 mm, et la teneur en H du lingot avant refusion sous laitier est supérieure à 3 ppm.  - The maximum dimension of said ingot before cooling is greater than about 910 mm and its minimum dimension less than about 1500 mm, and the H content of the ingot before slag remelting is greater than 3 ppm.
- La dimension minimale du lingot est supérieure à 1500 mm et la teneur en H du lingot avant refusion sous laitier est supérieure à 10 ppm.  - The minimum dimension of the ingot is greater than 1500 mm and the H content of the ingot before slag remelting is greater than 10 ppm.
8. Procédé de fabrication d'un acier martensitique inoxydable selon l'une quelconque des revendications 1 à 7, caractérisé en ce que la teneur en carbone dudit acier est inférieure à la teneur en carbone en dessous de laquelle l'acier est hypoeutectoïde.  8. A method of manufacturing a stainless martensitic steel according to any one of claims 1 to 7, characterized in that the carbon content of said steel is less than the carbon content below which the steel is hypoeutectoid.
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