EP2188402B1 - Martensitic stainless steel, method for making parts from said steel and parts thus made - Google Patents

Martensitic stainless steel, method for making parts from said steel and parts thus made Download PDF

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EP2188402B1
EP2188402B1 EP08830336A EP08830336A EP2188402B1 EP 2188402 B1 EP2188402 B1 EP 2188402B1 EP 08830336 A EP08830336 A EP 08830336A EP 08830336 A EP08830336 A EP 08830336A EP 2188402 B1 EP2188402 B1 EP 2188402B1
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Prior art keywords
steel
trace levels
traces
temperature
hardness
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German (de)
French (fr)
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EP2188402A1 (en
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André GRELLIER
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Aubert and Duval SA
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Aubert and Duval SA
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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
    • 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
    • C21D6/002—Heat treatment of ferrous alloys containing Cr
    • 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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • 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
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22—Ferrous alloys, e.g. steel alloys containing chromium 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
    • C22C38/24—Ferrous alloys, e.g. steel alloys containing chromium 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 present invention relates to iron and steel, more specifically, martensitic stainless steels, intended for example for the manufacture of molds for the production of plastics by injection.
  • the industry uses stainless steels of the AISI 420 family with a chromium content of 12 to 15% (in percentages by weight, like all the contents indicated in the rest of the text). , a silicon content of less than 1%, a manganese content of less than 1%, a carbon content of 0.16 to 0.45%, and a nitrogen content which is the result naturally of the production and generally up to 0.03%.
  • the vanadium content does not exceed 0.1% and results from the simple melting of the raw materials.
  • the molybdenum content results from the melting of the raw materials and does not exceed 0.2%, unless 0.2 to 1.0% is added to improve the corrosion resistance.
  • the X40Cr14 nomenclature steel capable of exceeding a hardness of 50 HRC by virtue of its carbon content of 0.36-0.45%, offers an appreciable abrasion resistance.
  • JP10110248 relates to martensitic stainless steels, for example for the manufacture of molds for the production of plastics by injection and comprises, in percentages by weight, C: 0.15 - 0.04%, N 0.02 -0.15%, Cr: 11 - 15%, V, 0 - 0.25%, Mo + 1 ⁇ 2 W: 0 - 3%, Ni: 0.01 - 0.2%, Mn: 0 - 2%, Si: 0 - 2%, Cu 0.1 - 1%, Ti: 0 - 0.25%, Nb: 0 - 0.25%, Al: 0 - 0, 1%, S: 0 - 0.005%, B: 0 - 0.1%, Ca : 0 - 0.1%, Mg: 0 - 0.1%, Fe remain and a method of manufacturing a piece of this martensitic stainless steel.
  • the choice of the low temperatures allows only a partial release of the stresses, and if the composition of the steel and the quenching cycle left residual austenite, the income not decomposing it, the hardness target is not reached.
  • High temperature incomes break down austenite and relax residual stresses, but decrease toughness and corrosion resistance.
  • traces ⁇ Nb ⁇ 0.010%.
  • Said incomes may each be carried out at a temperature of 200 to 400 ° C., preferably 300 to 380 ° C. for a minimum of 2 hours, while maintaining the nominal temperature at least 1 hour at the core, so as to obtain a hardness from 49 to 55 HRC.
  • Said incomes can each be carried out at a temperature of 530 to 540 ° C for a minimum of 2 hours while maintaining a core temperature of the nominal temperature of at least 1 hour, so as to obtain a hardness between 42 and 50 HRC.
  • the invention also relates to a martensitic stainless steel part, characterized in that the element manufactured by the process is manufactured according to the preceding method.
  • It may be a mold element for the manufacture of plastic articles.
  • the invention is based on a steel composition whose carbon and chromium contents are simultaneously at the bottom of the ranges usually required, and sometimes even below for the chromium content, with the imposition of conditions. other elements present or to be limited or avoided.
  • a manufacturing method is associated with this composition.
  • the approach of the inventors has focused on the real consideration of the properties of the steel resulting from the manufacture, and in particular of the industrial treatment as described above, and not according to laboratory conditions.
  • the research was carried out with the aim of optimizing the action of the alloying elements to limit the quantity introduced.
  • the toughness mediocre for this family of steels, is, for a given hardness, the lower the chromium content is high. It could be improved by balancing the composition, especially with additions of nickel and manganese to maintain a residue of austenite quenching. This solution, which also has no effect if the revenues are achieved above 500 ° C, however, proves unstable and handicaps the achievement of hardness. It was not retained, especially since it was not compatible with the desired lowering of the content of alloying elements.
  • Table 1 groups the compositions of the samples studied.
  • the "Reference" sample corresponds to a standard X40Cr14 type steel.
  • the samples Exp.1 to Exp.7 are not in accordance with the invention but make it possible to identify the disadvantages of not respecting all the conditions required by the invention.
  • Samples Inv.1 and Inv.2 are in accordance with the invention.
  • the object of the invention is therefore to design an optimized steel intended to be treated according to the range of industrial quenching speeds, preferably with a subsequent double low temperature ( ⁇ 400 ° C.) for a hardness of 52 HRC with tenacity and a corrosion resistance equal to or greater than that of the reference steel AISI 420 or X40Cr14 in its usual implementation.
  • the invention has the objective of minimizing the addition of alloying elements, in particular metal elements, in order to reduce the cost of production, to prevent the presence of residual austenite after quenching, and to reduce the amplitude. interdendritic segregation is detrimental to the tenacity and quality of polish.
  • the nitrogen content must be between 0.05% and 0.15% and preferably between 0.08% and 0.12%. This element is therefore systematically present at a high content, because it is essential to form type V carbonings (C, N) capable of preventing grain growth after austenization once the dissolved chromium carbides. An excessive content would however be detrimental by exceeding the solubility limit in the solid state and would be a source of metallurgical defects. Nitrogen combines with carbon to impart hardness and contributes to corrosion resistance. The nitrogen content can be adjusted by blowing nitrogen gas during the preparation of the liquid steel.
  • Chrome gives steel its resistance to corrosion. Given the industrial quenching speeds practiced, and the income range chosen, and according to the mechanisms mentioned above, its content must be between 10 and 12.4% and preferably between 11.0 and 12.4% .
  • the vanadium must be present at a content of between 0.10% and 0.40% and preferably between 0.15% and 0.35%. Its presence is essential to form with carbon and nitrogen a sufficient density of micro- and nano-precipitates able to prevent the growth of the grain. Too high a content would be detrimental by the excessive fixation of the carbon which would be lacking for the hardening, and by the formation, during the solidification, of carbides isolated or in clusters unfavorable to the tenacity and the quality of the polish.
  • Molybdenum completes the action of chromium for corrosion resistance; it is present, by recycling or by voluntary addition, at percentages between 0.10 and 1.0%. A higher content would be detrimental by increasing the amplitude of interdendritic segregation, and by the risk of forming delta ferrite.
  • Nickel may be present at levels less than 1.0%, mainly because of the input by the raw materials. No favorable action of an addition within this limit for toughness was noted. On the other hand, a higher content would be likely to maintain residual austenite in the treated state.
  • Silicon is naturally present for the elaboration and deoxidation of steel. Its content must be limited to 1.0% and preferably 0.5%, since it acts on the solidification process and the delta-gamma transformation and can therefore cause the presence of delta ferrite or local segregations consecutive to the presence of this phase at the end of solidification before wrought.
  • Tungsten may be present at levels less than 1.0% without having a favorable or detrimental effect on the product. Nevertheless, by its individual action or by synergy with molybdenum, it can promote the presence of delta ferrite in the state of use, or local precipitation or segregation originating from the presence of delta ferrite at any stage of the process. thermomechanical. It will be preferred to meet the condition 0.10% ⁇ Mo + W / 2 ⁇ 1.20%.
  • Cobalt and copper have no identified beneficial effect but may be present at levels less than or equal to 1.0%; higher grades may favor the presence of residual austenite.
  • the total contents of Mn, Cu and Co is ⁇ 1.8%, so as to limit the risks of presence of residual austenite.
  • Titanium and niobium are very reactive elements that form very hard precipitates harmful to the quality of polishing. Their content must be kept as low as possible: at most 0.010%, preferably at most 0.003% for Ti, and at most 0.050%, preferably at most 0.010% for Nb.
  • the added aluminum for the deoxidation of the steel can remain present in oxide inclusions very harmful for the polishing.
  • the level of addition must be adapted to the processing methods used. A maximum level of 0.050% is tolerable, provided that it does not lead to the presence of inclusions of alumina or silico-aluminates in large quantities which would lead to an exceedance of the acceptable O content (0.0040% better, 0.0015%).
  • the sulfur is preferentially limited to a content of less than 0.003% to prevent the formation of sulphide inclusions.
  • it may be chosen to make a voluntary addition in the range 0.003 to 0.020% preferably associated with another element (SE up to 0.010%, Ca up to 0.020%, La up to 0.040%). %, Ce up to 0.040%) promoting the formation of globular sulphides to improve machinability, to the detriment to a certain extent of the quality of the polish.
  • the maximum oxygen content is 0.0040%, preferably 0.0015%.
  • the phosphorus content is limited to 0.03% which is a common content in this class of steels. There was no adverse effect of P in this range.
  • Boron may be added to improve quenchability at a level not exceeding 0.0050%.
  • the products shall be manufactured in accordance with the state of the art for high-grade special steels for plastics molding applications, with the objective of limiting the inclusion content and segregation for obtaining a quality polish.
  • the preparation must include, after melting, a phase of deoxidation and elimination of inclusions in a metallurgical reactor.
  • a phase of deoxidation and elimination of inclusions in a metallurgical reactor.
  • it will be practiced remelting by consumable electrode slag to improve the inclusion cleanliness and distribute the alloy elements, and above all the nitrogen, homogeneously throughout the mass.
  • thermomechanical transformation by forging or rolling completed by annealing must follow to complete the homogeneity and compactness of the microstructure.
  • the products After machining the workpiece to the final shape and before putting into operation, the products must, according to the preferred procedure, undergo a heat treatment comprising austenization at about 1020 ° C (from 990 to 1040 ° C, preferably 1000- 1030 ° C), a controlled quenching, for example under a neutral gas pressure, at a speed of between 10 and 40 ° C / min adapted to the size of the workpiece, then two returns at a temperature of 200 to 400 ° C, preferably between 300 and 380 ° C to obtain a hardness close to 52 HRC ⁇ 2 HRC, and generally between 49 and 55 HRC.
  • a heat treatment comprising austenization at about 1020 ° C (from 990 to 1040 ° C, preferably 1000- 1030 ° C)
  • a controlled quenching for example under a neutral gas pressure
  • the steel defined by the invention may be treated with a double feed from 530 ° C. to 560 ° C. for hardnesses less than or equal to 50 HRC and greater than or equal to at 42 HRC, conditions in which corrosion resistance is sufficient.
  • the chromium carbides (M 23 C 6 ) existing in the delivery state are redissolved during the austenization that precedes quenching, and the temperature of the maintenance is limited to 1020/1030 ° C to prevent grain growth.
  • the temperature of the maintenance is limited to 1020/1030 ° C to prevent grain growth.
  • this dissolution temperature there remains a significant amount of carbides distributed heterogeneously.
  • the effective density of micrometric carbides observed on industrial products and illustrated on the Figure 1 effectively decreases significantly between the reference composition and the compositions of the invention, which constitutes a favorable factor for the quality of the polished state.
  • the corrosion resistance capacity is, theoretically according to the basic knowledge, primarily related to the chromium content available in the matrix; thermodynamic calculations show that carbons undissolved in austenization set about 0.9% chromium. This amount of chromium unavailable for corrosion resistance becomes less than 0.1% for vanadium and nitrogen alloyed experimental grades. According to the following formula:
  • the electrochemical method performed according to the ASTM G 108 standard consists, in a 1% by weight aqueous H 2 SO 4 solution, of polarizing the sample for 15 minutes at a potential of -550mV / ECS and then carrying out a forward scan. and back to 60mV / min from -550mV to + 500mV.
  • the intensity-potential curves at the return can have two peaks, one (Pic1) due to the dissolution of the matrix, the second (Pic 2), at a higher potential, connected to the dissolution at the right of carbide precipitates of chromium. Steel is all the more sensitive to corrosion as the dissolution current is intense. Characteristic curves are presented in figures 2 and 3 .
  • the figure 2 demonstrates for the casting INV1 that the steel is strongly sensitized with respect to corrosion for incomes made in the hardening zone around 500 ° C. If corrosion resistance is a characteristic that should be favored for the applications envisaged, we will favor low-temperature incomes (200-380 ° C).
  • a quenching speed compatible with the know-how of the heat treatment and between 10 and 40 ° C./min in the temperature range 800 to 400 ° C. will be chosen.
  • compositions of the invention make it possible to obtain the hardness of 52 HRC or more after quenching under industrial conditions and double-tempered at 380 ° C., despite the softening experienced in this field for this family of steel from crude. quenching, as shown in figure 5 .

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Abstract

The invention relates to a martensitic stainless steel characterised in that it comprises in weight percent: - 0.22% < C < 0.32% - 0.05% < N < 0.15%, with 0.33% < C+N < 0.43% - 10% ≤ Cr ≤ 12.4% - 0.10% ≤ V ≤ 0.40% - 0.10% ≤ Mo ≤ 1.0% - traces < Ni ≤ 1.0% - traces < Mn < 1.0% - traces < Si < 1.0% - traces ≤ W ≤ 1.0% - traces < Co < 1.0% - traces ≤ Cu ≤ 1.0% - traces < Ti < 0.010% - traces ≤ Nb ≤ 0.050% - traces ≤ Al ≤ 0.050% - traces < S < 0.020% - traces < O < 0.0040% - traces < P < 0.03% - traces < B < 0.0050% - traces < Ca < 0.020% - traces < Se < 0.010% - traces < La < 0.040% - traces < Ce < 0.040%, the balance consisting of iron and impurities resulting from the production. The invention also relates to a method for producing a part made of such a steel and to the part thus obtained, as a mould element for the production of plastic articles.

Description

La présente invention concerne la sidérurgie, plus précisément, les aciers inoxydables martensitiques, destinés par exemple à la fabrication de moules de production de matières plastiques par injection.The present invention relates to iron and steel, more specifically, martensitic stainless steels, intended for example for the manufacture of molds for the production of plastics by injection.

Pour la réalisation de moules d'injection de matières plastiques, l'industrie utilise les aciers inoxydables de la famille AISI 420 présentant une teneur en Chrome de 12 à 15% (en pourcentages pondéraux, comme toutes les teneurs indiquées dans la suite du texte), une teneur en silicium de moins de 1%, une teneur en manganèse de moins de 1%, une teneur en carbone de 0,16 à 0,45%, et une teneur en azote qui est cette résultant naturellement de l'élaboration et allant généralement jusqu'à 0,03%. Généralement, la teneur en vanadium ne dépasse pas 0,1% est résulte de la simple fusion des matières premières. De même, la teneur en molybdène résulte de la fusion des matières premières et ne dépasse pas 0,2%, à moins qu'on n'en ajoute de 0,2 à 1,0% pour améliore la résistance à la corrosion. Plus spécifiquement, l'acier de nomenclature X40Cr14, capable grâce à sa teneur en carbone de 0,36-0,45% de dépasser une dureté de 50 HRC, offre une résistance à l'abrasion appréciable.For the production of plastic injection molds, the industry uses stainless steels of the AISI 420 family with a chromium content of 12 to 15% (in percentages by weight, like all the contents indicated in the rest of the text). , a silicon content of less than 1%, a manganese content of less than 1%, a carbon content of 0.16 to 0.45%, and a nitrogen content which is the result naturally of the production and generally up to 0.03%. Generally, the vanadium content does not exceed 0.1% and results from the simple melting of the raw materials. Similarly, the molybdenum content results from the melting of the raw materials and does not exceed 0.2%, unless 0.2 to 1.0% is added to improve the corrosion resistance. More specifically, the X40Cr14 nomenclature steel, capable of exceeding a hardness of 50 HRC by virtue of its carbon content of 0.36-0.45%, offers an appreciable abrasion resistance.

JP10110248 concerne des aciers inoxydables martensitiques, destinés par exemple à la fabrication de moules de production de matières plastiques par injection et il comprend, en pourcentages pondéraux, C : 0,15 - 0.04%, N 0,02 -0,15%, Cr: 11 - 15%, V, 0 - 0,25%, Mo + ½ W : 0 - 3%, Ni : 0,01 - 0,2%, Mn : 0 - 2%, Si : 0 - 2%, Cu : 0,1 - 1%, Ti : 0 - 0,25%, Nb : 0 - 0,25%, Al : 0 - 0, 1 %, S : 0 - 0,005%, B : 0 - 0.1%, Ca : 0 - 0,1%, Mg : 0 - 0,1 %, reste Fe et un procédé de fabrication d'une pièce en cet acier inoxydable martensitique. JP10110248 relates to martensitic stainless steels, for example for the manufacture of molds for the production of plastics by injection and comprises, in percentages by weight, C: 0.15 - 0.04%, N 0.02 -0.15%, Cr: 11 - 15%, V, 0 - 0.25%, Mo + ½ W: 0 - 3%, Ni: 0.01 - 0.2%, Mn: 0 - 2%, Si: 0 - 2%, Cu 0.1 - 1%, Ti: 0 - 0.25%, Nb: 0 - 0.25%, Al: 0 - 0, 1%, S: 0 - 0.005%, B: 0 - 0.1%, Ca : 0 - 0.1%, Mg: 0 - 0.1%, Fe remain and a method of manufacturing a piece of this martensitic stainless steel.

Compte tenu de l'application envisagée, la performance du matériau doit être évaluée par l'obtention d'un bon compromis entre les propriétés suivantes :

  • la résistance à l'usure recherchée pour pouvoir produire le maximum de pièces avec une régularité géométrique garantie, y compris avec des matières plastiques rendues abrasives par intégration de fibres ou autres additifs de renfort; cette résistance à l'usure est conférée par une dureté élevée;
  • une ténacité suffisante pour éviter des ruptures pendant le traitement thermique, les opérations de montage-démontage ou le service; pour ces aciers notablement fragiles, cette propriété se révèle contradictoire avec la précédente, la ténacité diminuant lorsque la dureté augmente ;
  • une bonne polissabilité permettant d'obtenir sans difficultés un poli de qualité sur la surface du moule afin de produire des pièces en matières plasti-ques avec un aspect de surface lisse et uniforme; l'acier doit aussi pouvoir maintenir cet état poli aussi longtemps que possible,
  • une résistance à la corrosion suffisante pour éviter la piqûration, le ternissement, l'altération du poli pendant le stockage des moules et pendant le service, dans le contexte de la production de matières plastiques peu ou moyennement chimiquement agressives ; les substances plus actives par exemple par re-largage d'ions chlorures, demandent des aciers ou alliages d'autres familles.
Given the intended application, the performance of the material must be evaluated by obtaining a good compromise between the following properties:
  • the wear resistance sought to be able to produce the maximum number of parts with a guaranteed geometric regularity, including with plastics made abrasive by integration of fibers or other reinforcing additives; this resistance to wear is conferred by a high hardness;
  • sufficient toughness to prevent breakage during heat treatment, assembly-disassembly operations or service; for these notably fragile steels, this property is contradictory with the previous one, the tenacity decreasing when the hardness increases;
  • good polishability to obtain without difficulty a quality polish on the surface of the mold in order to produce plastic parts with a smooth and uniform surface appearance; the steel must also be able to maintain this polished state as long as possible,
  • sufficient corrosion resistance to prevent pitting, tarnishing, discolouration during mold storage and during service, in the context of the production of low or moderately chemically aggressive plastics; the more active substances, for example by re-release of chloride ions, require steels or alloys from other families.

Après usinage d'une ébauche à des cotes proches de la forme finale, les moules subissent dans un four à atmosphère contrôlée le traitement thermique suivant :

  • montée à la température de trempe dans l'intervalle 1000 à 1050°C, suivi d'un maintien de quelques dizaines de minutes dans ce domaine,
  • trempe sous pression de gaz jusqu'à une température de l'ordre de 80°C;
  • remontée en température pour deux cycles de revenus.
After machining a blank close to the final shape, the molds undergo in a controlled atmosphere furnace the following heat treatment:
  • rising to the quenching temperature in the range 1000 to 1050 ° C, followed by a maintenance of a few tens of minutes in this area,
  • quenching under pressure of gas up to a temperature of the order of 80 ° C;
  • temperature rise for two income cycles.

Il est habituellement proposé deux domaines de températures de revenu :

  • revenus à basse température : 150 à 250°C
  • revenus vers 490/530°C dans la zone de durcisseme nt secondaire de l'acier.
It is usually proposed two income temperature domains:
  • Low temperature incomes: 150 to 250 ° C
  • returned to 490/530 ° C in the secondary hardening zone of steel.

Normalement, les deux revenus successifs sont effectués tous deux dans le même domaine.Normally, the two successive incomes are both in the same field.

Des arbitrages doivent être décidés pour le choix précis des paramètres du traitement.Arbitrations must be decided for the precise choice of treatment parameters.

Pour la trempe, il est métallurgiquement recommandé de rechercher des vitesses de trempe élevées pour bénéficier d'une microstructure martensitique favorable. Cependant, des vitesses de trempe élevées favorisent les déformations et génèrent des contraintes résiduelles susceptibles de conduire à des ruptures. Pratiquement, les pressions de gaz sont limitées à des valeurs de 2 à 4 bars.For quenching, it is metallurgically recommended to seek high quench rates to benefit from a favorable martensitic microstructure. However, high quenching speeds favor the deformations and generate residual stresses liable to lead to breaks. Practically, the gas pressures are limited to values of 2 to 4 bars.

Lors de l'arrêt de trempe, avant d'enchaîner sur les revenus, des ruptures sont possibles si le refroidissement se déroule jusqu'à la température ambiante. Mais le choix habituel d'arrêter le refroidissement vers 80°C ouvre au risque de conserver de l'austénite résiduelle, notamment si les revenus ultérieurs sont fixés au-dessous de 500°C, et par voie de conséquence de ne pouvoir obtenir la dureté nominale recherchée.During the quenching stop, before going back on the incomes, breaks are possible if the cooling takes place until the ambient temperature. But the usual choice to stop cooling to 80 ° C opens the risk of retaining residual austenite, especially if the subsequent revenues are set below 500 ° C, and consequently can not get the hardness nominal sought.

Pour les revenus, le choix des basses températures ne permet qu'une libération partielle des contraintes, et si la composition de l'acier et le cycle de trempe ont laissé subsister de l'austénite résiduelle, le revenu ne la décomposant pas, la dureté visée n'est pas atteinte. Les revenus haute température décomposent l'austénite et relaxent les contraintes résiduelles, mais diminuent la ténacité et la résistance à la corrosion.For the incomes, the choice of the low temperatures allows only a partial release of the stresses, and if the composition of the steel and the quenching cycle left residual austenite, the income not decomposing it, the hardness target is not reached. High temperature incomes break down austenite and relax residual stresses, but decrease toughness and corrosion resistance.

Se pose également le problème du coût de ces aciers, de par les teneurs élevées en éléments d'alliage qu'ils requièrent, et qu'il faudrait pouvoir minimiser sans dégrader les propriétés recherchées.There is also the problem of the cost of these steels, due to the high levels of alloying elements they require, and that it should be possible to minimize without degrading the desired properties.

Le but de l'invention est la définition d'une composition économique d'acier pour les applications de moules pour la fabrication d'articles en matières plastiques présentant par rapport aux références AISI 420 et X40Cr14, les propriétés suivantes:

  • dureté équivalente préférentielle de 49 à 55 HRC à l'état traité pour résister à l'abrasion ;
  • résistance à la corrosion équivalente ;
  • ténacité améliorée à dureté égale ;
  • polissabilité améliorée ;
  • tout cela pour des conditions de traitement thermique industrielles comparables aux conditions habituelles.
The object of the invention is the definition of an economic composition of steel for mold applications for the manufacture of plastic articles having compared with the references AISI 420 and X40Cr14, the following properties:
  • preferred equivalent hardness of 49 to 55 HRC in the treated state to resist abrasion;
  • equivalent corrosion resistance;
  • improved toughness at equal hardness;
  • improved polishability;
  • all this for industrial heat treatment conditions comparable to the usual conditions.

A cet effet, l'invention a pour objet un acier inoxydable martensitique, caractérisé en ce qu'il comprend, en pourcentages pondéraux :

  • 0,22% ≤ C ≤ 0,32%
  • 0,05% ≤ N ≤ 0,15%, avec 0,33% ≤ C+N ≤ 0,43%
  • 10% ≤ Cr ≤ 12,4%
  • 0,10% ≤ V ≤ 0,40%
  • 0,10% ≤ Mo ≤ 1,0%
  • traces ≤ Ni ≤ 1,0%
  • traces ≤ Mn ≤ 1,0%
  • traces ≤ Si ≤ 1,0%
  • traces ≤ W ≤ 1.0%
  • traces ≤ Co ≤ 1,0%
  • traces ≤ Cu ≤ 1.0%
  • traces ≤ Ti ≤ 0,010%
  • traces ≤ Nb ≤ 0,050%
  • traces ≤Al ≤ 0,050%
  • traces ≤ S ≤ 0,020%
  • traces ≤ O ≤ 0,0040%
  • traces ≤ P ≤ 0,03%
  • traces ≤ B ≤ 0.0050%
  • traces ≤ Ca ≤ 0,020%
  • traces ≤ Se ≤ 0,010%
  • traces ≤ La ≤ 0.040%
  • traces ≤ Ce ≤ 0,040%
le reste étant du fer et des impuretés résultant de l'élaboration.For this purpose, the subject of the invention is a martensitic stainless steel, characterized in that it comprises, in weight percentages:
  • 0.22% ≤ C ≤ 0.32%
  • 0.05% ≤ N ≤ 0.15%, with 0.33% ≤ C + N ≤ 0.43%
  • 10% ≤ Cr ≤ 12.4%
  • 0.10% ≤ V ≤ 0.40%
  • 0.10% ≤ Mo ≤ 1.0%
  • traces ≤ Ni ≤ 1.0%
  • traces ≤ Mn ≤ 1.0%
  • traces ≤ If ≤ 1.0%
  • traces ≤ W ≤ 1.0%
  • traces ≤ Co ≤ 1.0%
  • traces ≤ Cu ≤ 1.0%
  • traces ≤ Ti ≤ 0.010%
  • traces ≤ Nb ≤ 0.050%
  • traces ≤Al ≤ 0.050%
  • traces ≤ S ≤ 0.020%
  • traces ≤ O ≤ 0.0040%
  • traces ≤ P ≤ 0.03%
  • traces ≤ B ≤ 0.0050%
  • traces ≤ Ca ≤ 0.020%
  • traces ≤ Se ≤ 0.010%
  • traces ≤ The ≤ 0.040%
  • traces ≤ Ce ≤ 0.040%
the rest being iron and impurities resulting from the elaboration.

De préférence 0,08% ≤ N ≤ 0,12%.Preferably 0.08% ≤ N ≤ 0.12%.

De préférence 11,0% ≤ Cr ≤ 12,4%.Preferably 11.0% ≤ Cr ≤ 12.4%.

De préférence 0,15% ≤ V ≤ 0,35%.Preferably 0.15% ≤ V ≤ 0.35%.

De préférence traces ≤ Si ≤ 0,50%.Preferably traces ≤ Si ≤ 0.50%.

De préférence 0,10% ≤ Mo + W/2 ≤ 1,20%.Preferably 0.10% ≤ Mo + W / 2 ≤ 1.20%.

De préférence traces ≤ Ti ≤ 0,003%.Preferably traces ≤ Ti ≤ 0.003%.

De préférence traces ≤ Nb ≤ 0,010%.Preferably traces ≤ Nb ≤ 0.010%.

De préférence traces ≤ O ≤ 0,0015%.Preferably traces ≤ 0 ≤ 0.0015%.

De préférence traces ≤ S ≤ 0,003%.Preferably traces ≤ S ≤ 0.003%.

De préférence traces ≤ Mn + Cu + Co ≤ 1,8%.Preferably traces ≤ Mn + Cu + Co ≤ 1.8%.

L'invention a également pour objet un procédé de fabrication d'une pièce en acier inoxydables martensitique, caractérisé en ce que :

  • on élabore, on coulpe, on forge ou lamine et on recuit un acier du type précèdent ;
  • on réalise un usinage dudit acier pour lui donner la forme de ladite pièce ;
  • on réalise une austénisation dudit acier usiné à une température de 990-1040°C, de préférence 1000-1030°C ;
  • on réalise une trempe de l'acier austénisé à une vitesse comprise entre 10 et 40°C/min dans l'intervalle de température 800 à 400°C ;
  • on réalise deux revenus de l'acier trempé, pour lui conférer sa dureté finale.
The invention also relates to a process for manufacturing a martensitic stainless steel part, characterized in that:
  • one makes, sinks, forges or rolls and anneals a steel of the preceding type;
  • machining said steel to give it the shape of said piece;
  • the machined steel is austenized at a temperature of 990-1040 ° C, preferably 1000-1030 ° C;
  • quenching the austenized steel at a rate of between 10 and 40 ° C / min in the temperature range 800 to 400 ° C;
  • two incomes of hardened steel are realized to give it its final hardness.

Lesdits revenus peuvent être chacun effectués à une température de 200 à 400°C, de préférence 300 à 380°C pendant un minimum de 2h en assurant un maintien à coeur de la température nominale d'au moins 1h, de manière à obtenir une dureté de 49 à 55 HRC.Said incomes may each be carried out at a temperature of 200 to 400 ° C., preferably 300 to 380 ° C. for a minimum of 2 hours, while maintaining the nominal temperature at least 1 hour at the core, so as to obtain a hardness from 49 to 55 HRC.

Lesdits revenus peuvent être chacun effectués à une température de 530 à 540°C pendant un minimum de 2h en assurant un maintien à coeur de la température nominale d'au moins 1h, de manière à obtenir une dureté comprise entre 42 et 50 HRC.Said incomes can each be carried out at a temperature of 530 to 540 ° C for a minimum of 2 hours while maintaining a core temperature of the nominal temperature of at least 1 hour, so as to obtain a hardness between 42 and 50 HRC.

L'invention a également pour objet une pièce en acier inoxydable martensitique, caractérisée en ce que l'élément fabriqué par le procédé est fabriqué selon le procédé précédent.The invention also relates to a martensitic stainless steel part, characterized in that the element manufactured by the process is manufactured according to the preceding method.

Il peut s'agir d'un élément de moule destiné à la fabrication d'articles en matières plastiques.It may be a mold element for the manufacture of plastic articles.

Comme on l'aura compris l'invention repose sur une composition d'acier dont les teneurs en carbone et chrome sont simultanément dans le bas des gammes habituellement requises, voire parfois en-dessous pour la teneur en chrome, avec l'imposition de conditions précises sur d'autres éléments présents ou devant être limités ou évités. Un procédé de fabrication est associé à cette composition.As will be understood, the invention is based on a steel composition whose carbon and chromium contents are simultaneously at the bottom of the ranges usually required, and sometimes even below for the chromium content, with the imposition of conditions. other elements present or to be limited or avoided. A manufacturing method is associated with this composition.

La démarche des inventeurs s'est concentrée sur la prise en considération réelle des propriétés de l'acier issu de la fabrication, et en particulier du traitement industriel tel que décrit plus haut, et non selon des conditions de laboratoire. La recherche a été réalisée avec la préoccupation d'optimiser l'action des éléments d'alliages pour en limiter la quantité introduite.The approach of the inventors has focused on the real consideration of the properties of the steel resulting from the manufacture, and in particular of the industrial treatment as described above, and not according to laboratory conditions. The research was carried out with the aim of optimizing the action of the alloying elements to limit the quantity introduced.

Les principales considérations ayant conduit à l'invention sont les suivantes.The main considerations leading to the invention are as follows.

La polissabilité et la qualité de surface de l'état poli de l'acier sont dégradées par :

  • la présence d'inclusions non métalliques d'oxydes non réfléchissantes de la lumière, et qui de plus s'effritent ou se déchaussent au contact de l'abrasif, et forment en s'évacuant des rayures ou "queues de comètes" à la surface du moule ;
  • les ségrégations interdendritiques se formant naturellement lors de la solidification du lingot et générant sur la surface des moules des zones ou lignes plus dures alternant avec des zones ou lignes plus douces, provoquant au polissage des vallonnements du fait que les zones douces se creusent plus vite que les zones dures ;
  • la présente de carbures de chrome micrométriques non dissous à la trempe.
Polishability and surface quality of the polished state of steel are degraded by:
  • the presence of nonmetallic inclusions of non-reflective oxides of the light, and which moreover crumble or come off in contact with the abrasive, and form by evacuating stripes or "tails of comets" on the surface mold;
  • the interdendritic segregations that form naturally during the solidification of the ingot and generate on the surface of the molds zones or harder lines alternating with softer zones or lines, causing the polishing of undulations because the soft zones are digging faster than hard zones;
  • the present micrometric chromium carbides undissolved in quenching.

D'une manière générale, la ténacité, médiocre pour cette famille d'aciers, est, pour une dureté donnée, d'autant plus faible que la teneur en chrome est élevée. Elle pourrait être améliorée par un équilibrage de la composition, notamment avec des additions de nickel et manganèse permettant de conserver un résidu d'austénite à la trempe. Cette solution, qui par ailleurs n'a plus d'effet si les revenus sont réalisés au-dessus de 500°C, se révèle cependant instable et handicape l'obtention de la dureté. Elle n'a pas été retenue, d'autant plus qu'elle n'était pas compatible avec l'abaissement recherché de la teneur en éléments d'alliage.In general, the toughness, mediocre for this family of steels, is, for a given hardness, the lower the chromium content is high. It could be improved by balancing the composition, especially with additions of nickel and manganese to maintain a residue of austenite quenching. This solution, which also has no effect if the revenues are achieved above 500 ° C, however, proves unstable and handicaps the achievement of hardness. It was not retained, especially since it was not compatible with the desired lowering of the content of alloying elements.

Pour atteindre tes objectifs fixés, il a été choisi :

  • d'une part de produire l'acier selon des procédés connus limitant la présence d'inclusions non métalliques oxydées, donc conférant à l'acier une basse teneur en O,
  • et d'autre part de réduire globalement les éléments d'alliages, d'introduire de l'azote, et d'optimiser les équilibres entre éléments pour relever la ténacité, réduire la ségrégation interdendritique et limiter la densité de précipités micrométriques.
To achieve your goals, it was chosen:
  • on the one hand to produce the steel according to known methods limiting the presence of oxidized non-metallic inclusions, thus conferring on the steel a low O content,
  • and on the other hand to reduce overall alloying elements, introduce nitrogen, and optimize the equilibrium between elements to increase toughness, reduce interdendritic segregation and limit the density of micrometric precipitates.

L'invention sera mieux comprise par la description qui suit, donnée en référence aux figures annexées suivantes :

  • la figure 1 qui montre des micrographies d'échantillons d'un acier de référence et de deux aciers selon l'invention, montrant la densité et la distribution des carbures micrométriques à l'état d'emploi de ces aciers ;
  • la figure 2 qui montre l'influence de la température des deux revenus sur la résistance à la corrosion d'un acier selon l'invention ;
  • la figure 3 qui montre l'influence de la température des revenus sur la résistance à la corrosion ;
  • la figure 4 qui montre les interactions de la teneur en Cr et de la vitesse de trempe sur la résistance à la corrosion ;
  • la figure 5 qui montre la dureté des aciers selon l'invention et d'un acier de référence en fonction de la température des revenus.
The invention will be better understood from the description which follows, given with reference to the following appended figures:
  • the figure 1 which shows micrographs of samples of a reference steel and two steels according to the invention, showing the density and the distribution of micrometer carbides at the state of use of these steels;
  • the figure 2 which shows the influence of the temperature of the two incomes on the corrosion resistance of a steel according to the invention;
  • the figure 3 which shows the influence of the temperature of the incomes on the resistance to corrosion;
  • the figure 4 which shows the interactions of Cr content and quenching rate on corrosion resistance;
  • the figure 5 which shows the hardness of the steels according to the invention and a reference steel as a function of the temperature of the incomes.

Le tableau 1 regroupe les compositions des échantillons étudiés. L'échantillon « Référence » correspond à un acier de type X40Cr14 classique. Les échantillons Exp.1 à Exp.7 sont non-conformes à l'invention mais permettent de dégager les inconvénients qu'il y a à ne pas respecter toutes les conditions requises par l'invention. Les échantillons Inv.1 et Inv.2 sont conformes à l'invention. Tableau 1: Composition chimique des aciers étudiés Identification C % N % Si % Mn % Ni % Cr % Mo %, V % Co % Cu % Al % W % Nb % S ppm P ppm Ti ppm O ppm Référence 0.41 0.02 0.38 0.50 0.27 14.56 0.29 0.06 0.04 0.08 0.023 0.04 0.01 5 210 34 12 Exp. 1 0.27 0.11 0.27 0.40 0.32 14.17 0.16 0.14 0.02 0.06 0.019 0.02 0.01 2 170 20 7 Exp. 2 0.27 0.12 0.28 0.37 0.32 10.71 0.16 0.13 0.93 0.03 0.015 0.07 0.02 3 150 12 11 Exp. 3 0.27 0.11 0.32 0.42 0.31 10.75 0.15 0.01 0.01 0.02 0.012 0.05 0.03 4 140 30 8 Exp. 4 0.26 0.12 0.30 0.40 0.31 10.80 0.15 0.36 0.01 0.06 0.019 0.07 0.02 2 150 28 8 Exp. 5 0.23 0.13 0.31, 0.42 0.32 12.75 0.15 0.14 0.04 0.07 0.021 0.02 0.02 2 190 18 7 Exp. 6 0.27 0.11 0.29 0.42 1.04 12.89 0.15 0.13 0.02 0.09 0.010 0.02 0.01 4 160 16 10 Exp. 7 0.28 0.11 0.27 0.42 0.35 0.89 0.89 0.13 0.01 0.02 0.013 0.02 0.01 3 170 12 11 Inv 1 0.28 0.10 0.38 0.46 0.32 12.38 0.29 0.18 0.03 0.03 0.014 0.03 0.01 1 170 13 9 Inv 2 0.27 0.12 0.37 0.43 0.32 11.04 0.32 0.22 0.04 0.01 0.013 0.01 0.02 1 200 16 7 Table 1 groups the compositions of the samples studied. The "Reference" sample corresponds to a standard X40Cr14 type steel. The samples Exp.1 to Exp.7 are not in accordance with the invention but make it possible to identify the disadvantages of not respecting all the conditions required by the invention. Samples Inv.1 and Inv.2 are in accordance with the invention. <b> Table 1: Chemical composition of the steels studied </ b> Identification VS % NOT % Yes % Mn% Neither% Cr% Mo%, V% Co% Cu% Al% W% Nb% S ppm P ppm Ti ppm O ppm Reference 0.41 0.02 0.38 0.50 0.27 14.56 0.29 0.06 0.04 0.08 0023 0.04 0.01 5 210 34 12 Exp. 1 0.27 0.11 0.27 0.40 0.32 14.17 0.16 0.14 0.02 0.06 0019 0.02 0.01 2 170 20 7 Exp. 2 0.27 0.12 0.28 0.37 0.32 10.71 0.16 0.13 0.93 0.03 0015 0.07 0.02 3 150 12 11 Exp. 3 0.27 0.11 0.32 0.42 0.31 10.75 0.15 0.01 0.01 0.02 0012 0.05 0.03 4 140 30 8 Exp. 4 0.26 0.12 0.30 0.40 0.31 10.80 0.15 0.36 0.01 0.06 0019 0.07 0.02 2 150 28 8 Exp. 5 0.23 0.13 0.31, 0.42 0.32 12.75 0.15 0.14 0.04 0.07 0021 0.02 0.02 2 190 18 7 Exp. 6 0.27 0.11 0.29 0.42 1.04 12.89 0.15 0.13 0.02 0.09 0010 0.02 0.01 4 160 16 10 Exp. 7 0.28 0.11 0.27 0.42 0.35 0.89 0.89 0.13 0.01 0.02 0013 0.02 0.01 3 170 12 11 Inv 1 0.28 0.10 0.38 0.46 0.32 12.38 0.29 0.18 0.03 0.03 0014 0.03 0.01 1 170 13 9 Inv 2 0.27 0.12 0.37 0.43 0.32 11.04 0.32 0.22 0.04 0.01 0013 0.01 0.02 1 200 16 7

L'invention a donc pour objet de concevoir un acier optimisé destiné à être traité selon la gamme des vitesses de trempe industrielles avec de préférence un double revenu ultérieur à basse température (< 400°C) pour une dureté de 52 HRC avec une ténacité et une résistance à la corrosion égales ou supérieures à celles de l'acier de référence AISI 420 ou X40Cr14 dans sa mise en oeuvre habituelle.The object of the invention is therefore to design an optimized steel intended to be treated according to the range of industrial quenching speeds, preferably with a subsequent double low temperature (<400 ° C.) for a hardness of 52 HRC with tenacity and a corrosion resistance equal to or greater than that of the reference steel AISI 420 or X40Cr14 in its usual implementation.

De plus, l'invention poursuit l'objectif de limiter au maximum les additions d'éléments d'alliage en particulier les éléments métalliques pour diminuer le coût de production, prévenir la présence d'austénite résiduelle après la trempe, et réduire l'amplitude de la ségrégation interdendritique néfaste à la ténacité et à la qualité du poli.In addition, the invention has the objective of minimizing the addition of alloying elements, in particular metal elements, in order to reduce the cost of production, to prevent the presence of residual austenite after quenching, and to reduce the amplitude. interdendritic segregation is detrimental to the tenacity and quality of polish.

A cet effet, les inventeurs sont parvenus aux résultats suivants sur la définition de la composition des aciers de l'invention.For this purpose, the inventors have reached the following results on the definition of the composition of the steels of the invention.

La teneur en azote doit être comprise entre 0,05% et 0,15% et de préférence entre 0,08% et 0,12%. Cet élément est donc présent systématiquement à une teneur élevée, car il est indispensable pour former des carbonitures de type V(C,N) aptes à éviter le grossissement du grain à l'austénisation une fois les carbures de chrome dissous. Une teneur excessive serait cependant préjudiciable par le dépassement de la limite de solubilité dans l'état solide et serait source de défauts métallurgiques. L'azote s'associe au carbone pour conférer la dureté et participe à la résistance à la corrosion. La teneur en azote peut être ajustée par insufflation d'azote gazeux lors de l'élaboration de l'acier liquide.The nitrogen content must be between 0.05% and 0.15% and preferably between 0.08% and 0.12%. This element is therefore systematically present at a high content, because it is essential to form type V carbonings (C, N) capable of preventing grain growth after austenization once the dissolved chromium carbides. An excessive content would however be detrimental by exceeding the solubility limit in the solid state and would be a source of metallurgical defects. Nitrogen combines with carbon to impart hardness and contributes to corrosion resistance. The nitrogen content can be adjusted by blowing nitrogen gas during the preparation of the liquid steel.

Le carbone contribue majoritairement à conférer la dureté requise, associé à l'azote. Compte tenu de la dureté recherchée après revenu à basse température, le pourcentage doit être compris entre 0,22% et 0,32%. De plus la somme C+N doit être comprise entre 0,33% et 0,43% pour permettre, après revenu, d'obtenir la dureté visée.Carbon contributes mainly to confer the required hardness, associated with nitrogen. Given the hardness sought after low temperature income, the percentage must be between 0.22% and 0.32%. Moreover the sum C + N must be between 0.33% and 0.43% to allow, after income, to obtain the target hardness.

Le chrome confère à l'acier sa résistance à la corrosion. Compte tenu des vitesses de trempe industrielles pratiqués, et de l'intervalle de revenu choisi, et selon les mécanismes évoqués plus haut, sa teneur doit être comprise entre 10 et 12,4% et de préférence entre 11,0 et 12,4%.Chrome gives steel its resistance to corrosion. Given the industrial quenching speeds practiced, and the income range chosen, and according to the mechanisms mentioned above, its content must be between 10 and 12.4% and preferably between 11.0 and 12.4% .

Le vanadium doit être présent à une teneur comprise entre 0,10% et 0.40% et de préférence entre 0,15% et 0,35%. Sa présence est indispensable pour former avec le carbone et l'azote une densité suffisante de micro- et nano-précipités aptes à éviter le grossissement du grain. Une teneur trop importante serait néfaste par la fixation excessive du carbone qui ferait défaut pour le durcissement, et parla formation, lors de la solidification, de carbures isolés ou en amas défavorables à la ténacité et à la qualité du poli.The vanadium must be present at a content of between 0.10% and 0.40% and preferably between 0.15% and 0.35%. Its presence is essential to form with carbon and nitrogen a sufficient density of micro- and nano-precipitates able to prevent the growth of the grain. Too high a content would be detrimental by the excessive fixation of the carbon which would be lacking for the hardening, and by the formation, during the solidification, of carbides isolated or in clusters unfavorable to the tenacity and the quality of the polish.

Le molybdène complète l'action du chrome pour la tenue à la corrosion; il est présent, par les recyclages ou par addition volontaire, à des pourcentages compris entre 0.10 et 1.0%. Une teneur supérieure serait néfaste par l'accroissement de l'amplitude de la ségrégation interdendritique, et par le risque de former de la ferrite delta.Molybdenum completes the action of chromium for corrosion resistance; it is present, by recycling or by voluntary addition, at percentages between 0.10 and 1.0%. A higher content would be detrimental by increasing the amplitude of interdendritic segregation, and by the risk of forming delta ferrite.

Le nickel peut être présent à des teneurs inférieures à 1,0%, notamment du fait de l'apport par les matières premières. Aucune action favorable d'une addition dans cette limite pour la ténacité n'a été relevée. En revanche une teneur supérieure serait susceptible de maintenir de l'austénite résiduelle à l'état traité.Nickel may be present at levels less than 1.0%, mainly because of the input by the raw materials. No favorable action of an addition within this limit for toughness was noted. On the other hand, a higher content would be likely to maintain residual austenite in the treated state.

Le manganèse est un élément naturellement présent dans cette famille d'acier du fait des procédés d'élaboration et des matières premières disponibles. Aucun effet bénéfique n'a été-identifié, et il s'avère nécessaire de limiter sa concentration à 1,0% pour éviter l'austénite résiduelle après traitement thermique.Manganese is a naturally occurring element in this family of steel because of the production processes and raw materials available. No beneficial effect has been identified, and it is necessary to limit its concentration to 1.0% to avoid residual austenite after heat treatment.

Le silicium est naturellement présent pour l'élaboration et la désoxydation de l'acier. Sa teneur doit être limitée à 1,0% et de préférence 0,5%, car il agit sur le processus de solidification et la transformation delta-gamma et de ce fait peut provoquer la présence de ferrite delta ou de ségrégations locales consécutives à la présence de cette phase en fin de solidification avant corroyage.Silicon is naturally present for the elaboration and deoxidation of steel. Its content must be limited to 1.0% and preferably 0.5%, since it acts on the solidification process and the delta-gamma transformation and can therefore cause the presence of delta ferrite or local segregations consecutive to the presence of this phase at the end of solidification before wrought.

Le tungstène peut être présent à des teneurs inférieures à 1,0% sans avoir d'effet favorable ou néfaste pour le produit. Néanmoins, par son action individuelle ou par synergie avec le molybdène, il peut favoriser la présence de ferrite delta à l'état d'utilisation, ou de précipitations ou ségrégations locales ayant pour origine la présence de ferrite delta à un stade quelconque du processus thermomécanique. Il sera préféré de respecter la condition 0,10% ≤ Mo + W/2 ≤ 1,20%.Tungsten may be present at levels less than 1.0% without having a favorable or detrimental effect on the product. Nevertheless, by its individual action or by synergy with molybdenum, it can promote the presence of delta ferrite in the state of use, or local precipitation or segregation originating from the presence of delta ferrite at any stage of the process. thermomechanical. It will be preferred to meet the condition 0.10% ≤ Mo + W / 2 ≤ 1.20%.

Le cobalt et le cuivre n'ont pas d'effet bénéfique identifié mais peuvent être présents à des teneurs inférieures ou égales à 1.0%; des teneurs supérieures pourraient favoriser la présence d'austénite résiduelle.Cobalt and copper have no identified beneficial effect but may be present at levels less than or equal to 1.0%; higher grades may favor the presence of residual austenite.

Il est préférable que le total des teneurs en Mn, Cu et Co soit ≤ 1,8%, de manière à limiter les risques de présence d'austénite résiduelle.It is preferable that the total contents of Mn, Cu and Co is ≤ 1.8%, so as to limit the risks of presence of residual austenite.

Le titane et le niobium sont des éléments très réactifs qui forment des précipités très durs néfastes pour la qualité du polissage. Leur teneur doit être maintenue aussi basse que possible : au plus 0,010%, de préférence au plus 0,003% pour Ti, et au plus 0,050%, de préférence au plus 0,010% pour Nb.Titanium and niobium are very reactive elements that form very hard precipitates harmful to the quality of polishing. Their content must be kept as low as possible: at most 0.010%, preferably at most 0.003% for Ti, and at most 0.050%, preferably at most 0.010% for Nb.

L'aluminium ajouté pour la désoxydation de l'acier peut rester présent dans des inclussions d'oxydes très néfastes pour le polissage. Le niveau d'addition doit être adapté aux procédés d'élaboration pratiqués. Une teneur maximale de 0,050% est tolérable, à condition qu'elle ne conduise pas à la présence d'inclusions d'alumine ou de silico-aluminates en grande quantité qui conduirait à un dépassement de la teneur en O acceptable (0,0040%, mieux 0,0015%).The added aluminum for the deoxidation of the steel can remain present in oxide inclusions very harmful for the polishing. The level of addition must be adapted to the processing methods used. A maximum level of 0.050% is tolerable, provided that it does not lead to the presence of inclusions of alumina or silico-aluminates in large quantities which would lead to an exceedance of the acceptable O content (0.0040% better, 0.0015%).

Le soufre est préférentiellement limité à une teneur inférieure à 0.003% pour éviter la formation d'inclusions de sulfure. Toutefois, optionnellement, il peut être choisi d'en pratiquer une addition volontaire dans l'intervalle 0.003 à 0,020% associé de préférence à un autre élément (Se jusqu'à 0.010%, Ca jusqu'à 0,020%, La jusqu'à 0,040%, Ce jusqu'à 0,040%) favorisant la formation de sulfures globulaires afin d'améliorer l'usinabilité, au détriment dans une certaine mesure de la qualité du poli.The sulfur is preferentially limited to a content of less than 0.003% to prevent the formation of sulphide inclusions. However, optionally, it may be chosen to make a voluntary addition in the range 0.003 to 0.020% preferably associated with another element (SE up to 0.010%, Ca up to 0.020%, La up to 0.040%). %, Ce up to 0.040%) promoting the formation of globular sulphides to improve machinability, to the detriment to a certain extent of the quality of the polish.

La teneur en oxygène maximale est de 0,0040%, de préférence 0,0015%. En effet, cet élément est un indice de la densité inclusionnaire, néfaste au poli de surface lorsqu'elle est trop élevée. Cette teneur doit être maintenue aussi basse que possible et le procédé d'élaboration de l'acier doit être choisi en conséquence. Dans la pratique, des procédés connus permettent de descendre jusqu'à O = 5ppm dans des conditions économiquement acceptables.The maximum oxygen content is 0.0040%, preferably 0.0015%. Indeed, this element is an index of the inclusion density, harmful to the surface polish when it is too high. This content should be kept as low as possible and the process of making the steel should be selected accordingly. In practice, known methods allow to go down to O = 5 ppm under economically acceptable conditions.

La teneur en phosphore est limitée à 0,03% qui est une teneur courante dans cette classe d'aciers. On n'a pas noté d'effet nocif de P dans cette gamme.The phosphorus content is limited to 0.03% which is a common content in this class of steels. There was no adverse effect of P in this range.

Du bore peut être ajouté pour améliorer la trempabilité, à une teneur ne dépassant pas 0,0050%.Boron may be added to improve quenchability at a level not exceeding 0.0050%.

Les teneurs préférentielles indiquées pour certains éléments peuvent être imposées seules, et pas forcément en combinaison avec les autres teneurs préférentielles indiquées.The preferred contents indicated for certain elements may be imposed alone, and not necessarily in combination with the other preferred levels indicated.

Les éléments non cités peuvent être présents à des teneurs du niveau d'impuretés résultant de l'élaboration, ne modifiant pas les propriétés que l'invention cherche à optimiser.The elements not mentioned can be present at levels of impurity level resulting from the development, not modifying the properties that the invention seeks to optimize.

Les produits doivent être fabriqués selon les règles de l'art en vigueur pour les aciers spéciaux de haute qualité destinés aux applications de moulage d'articles en matières plastiques, avec l'objectif de limiter la teneur en inclusions et la ségrégation pour l'obtention d'un poli de qualité. L'élaboration doit comprendre, après fusion, une phase de désoxydation et d'élimination des inclusions dans un réacteur métallurgique. Préférentiellement, notamment pour la fabrication de moules de forte taille et pour l'obtention des plus hautes quantités de poli, il sera pratiqué une refusion par électrode consommable sous laitier pour améliorer la propreté inclusionnaire et distribuer les éléments d'alliages, et avant tout l'azote, de manière homogène dans toute la masse.The products shall be manufactured in accordance with the state of the art for high-grade special steels for plastics molding applications, with the objective of limiting the inclusion content and segregation for obtaining a quality polish. The preparation must include, after melting, a phase of deoxidation and elimination of inclusions in a metallurgical reactor. Preferably, particularly for the manufacture of large molds and for obtaining the highest amounts of polish, it will be practiced remelting by consumable electrode slag to improve the inclusion cleanliness and distribute the alloy elements, and above all the nitrogen, homogeneously throughout the mass.

Une transformation thermomécanique par forgeage ou laminage terminée par un recuit doit suivre pour compléter l'homogénéité et la compacité de la microstructure.A thermomechanical transformation by forging or rolling completed by annealing must follow to complete the homogeneity and compactness of the microstructure.

Après usinage de la pièce à la forme finale et avant mise en service, les produits doivent, selon le mode opératoire préféré, subir un traitement thermique comprenant une austénisation aux environs de 1020°C (de 990 à 1040°C, de préférence 1000-1030°C), une trempe contrôlée, par exemple sous pression de gaz neutre, à une vitesse comprise entre 10 et 40°C/min adaptée à la taille de la pièce, puis deux revenues à une température de 200 à 400°C, de préférence entre 300 et 380°C pour obtenir une dure té voisine de 52 HRC ± 2 HRC, et de manière générale comprise entre 49 et 55 HRC .After machining the workpiece to the final shape and before putting into operation, the products must, according to the preferred procedure, undergo a heat treatment comprising austenization at about 1020 ° C (from 990 to 1040 ° C, preferably 1000- 1030 ° C), a controlled quenching, for example under a neutral gas pressure, at a speed of between 10 and 40 ° C / min adapted to the size of the workpiece, then two returns at a temperature of 200 to 400 ° C, preferably between 300 and 380 ° C to obtain a hardness close to 52 HRC ± 2 HRC, and generally between 49 and 55 HRC.

Optionnellement, pour des applications ne nécessitant pas une dureté supérieure à 50HRC, l'acier défini par l'invention pourra être traité par double revenu de 530°C à 560°C pour des duretés inférieure s ou égales à 50 HRC et supérieures ou égales à 42 HRC, conditions dans lesquelles la résistance à la corrosion s'avère suffisante.Optionally, for applications that do not require a hardness higher than 50HRC, the steel defined by the invention may be treated with a double feed from 530 ° C. to 560 ° C. for hardnesses less than or equal to 50 HRC and greater than or equal to at 42 HRC, conditions in which corrosion resistance is sufficient.

Pour l'acier de référence, les carbures de Chrome (M23C6) existants à l'état de livraison sont remis en solution lors de l'austénisation qui précède la trempe, et la température du maintien est limitée vers 1020/1030 °C pour éviter le grossissement du grain. Cependant, à cette température de mise en solution, il subsiste une quantité significative de carbures répartis de manière hétérogène. Par une substitution d'environ 0,10 à 0,15% de la teneur en carbone par de l'azote, un abaissement d'environ 2% de la teneur en chrome et une introduction simultanée de vanadium, il est observé à la température adéquate de trempe que le grain, fixé par des précipités nanométriques de carbonitrures de vanadium V(C,N) ne grossit pas alors que la plus grande partie des carbures de chrome s'est dissoute.For the reference steel, the chromium carbides (M 23 C 6 ) existing in the delivery state are redissolved during the austenization that precedes quenching, and the temperature of the maintenance is limited to 1020/1030 ° C to prevent grain growth. However, at this dissolution temperature, there remains a significant amount of carbides distributed heterogeneously. By substituting about 0.10 to 0.15% of the carbon content with nitrogen, lowering the chromium content by about 2%, and introducing vanadium simultaneously, it is observed at room temperature. Quenching the grain, fixed by nanoscale precipitates of vanadium carbonitrides V (C, N) does not grow while most of the chromium carbides has dissolved.

Pour trois des compositions étudiées, le calcul comparé des équilibres à 1030° par simulation thermodynamique avec le logiciel THERMOCALC (d'utilisation courante par les métallurgistes) illustre cette mutation (voir tableau 2). Tableau 2: Calcul des équilibres thermodynamiques à 1015°C pour trois compositions représentatives Identification Composition nominale % Molaire de M23C6 % Molaire de V(C,N) Composition de matrice P.R.E. C(%) N (%) Cr (%) Mo (%) V (%) Cr (%) Mo (%) N (%) Référence 0.41 0.02 14.56 0,29 0.06 2,84 0.00 13,45 0,26 0,02 15.0 Inv. 1 0.28 0.10 12.38 0,29 0.18 0.00 0.20 12,35 0,29 0,09 16.0 Inv. 2 0.27 0.12 11,04 0,32 0.22 0.00 0.26 11,00 0,32 0,10 15.1 For three of the compositions studied, the comparison of equilibria at 1030 ° by thermodynamic simulation with the THERMOCALC software (commonly used by metallurgists) illustrates this mutation (see Table 2). <b> Table 2: Calculation of thermodynamic equilibria at 1015 ° C for three representative compositions </ b> Identification Nominal composition % Molar of M23C6 % Molar of V (C, N) Matrix composition PRE VS(%) NOT (%) Cr (%) Mo (%) V (%) Cr (%) Mo (%) NOT (%) Reference 0.41 0.02 14.56 0.29 0.06 2.84 0.00 13.45 0.26 0.02 15.0 Inv. 1 0.28 0.10 12.38 0.29 0.18 0.00 0.20 12.35 0.29 0.09 16.0 Inv. 2 0.27 0.12 11,04 0.32 0.22 0.00 0.26 11,00 0.32 0.10 15.1

La densité effective des carbures micrométriques observée sur des produits industriels et illustrée sur la Figure 1 diminue effectivement de manière significative entre la composition de référence et les compositions de l'invention, ce qui constitue un facteur favorable pour la qualité de l'état poli.The effective density of micrometric carbides observed on industrial products and illustrated on the Figure 1 effectively decreases significantly between the reference composition and the compositions of the invention, which constitutes a favorable factor for the quality of the polished state.

Pour l'acier de référence, la capacité de résistance à la corrosion est, théoriquement selon les connaissances de base, avant tout reliée à la teneur en chrome disponible dans la matrice; les calculs thermodynamiques montrent que les carbures non dissous à l'austénisation fixent environ 0,9% de chrome. Cette quantité de chrome non disponible pour la résistance à la corrosion devient inférieure à 0,1% pour les nuances expérimentales alliées en vanadium et azote. Selon la formule suivante :For the reference steel, the corrosion resistance capacity is, theoretically according to the basic knowledge, primarily related to the chromium content available in the matrix; thermodynamic calculations show that carbons undissolved in austenization set about 0.9% chromium. This amount of chromium unavailable for corrosion resistance becomes less than 0.1% for vanadium and nitrogen alloyed experimental grades. According to the following formula:

P.R.E. (Pitting Resistance Equivalent) = %Cr + 3,3 x %Mo + 30 x %N permettant conventionnellement de classer les compositions selon leur résistance à la piqûration et appliquée sur la composition effective de la matrice, il se révèle selon le tableau 2 que les compositions expérimentales Inv 1 et 2 présentent un coefficient voisin de celui de la référence.PRE. (Pitting Resistance Equivalent) =% Cr + 3.3 x% Mo + 30 x% N conventionally allowing to classify the compositions according to their resistance to pitting and applied to the actual composition of the matrix, it is revealed according to Table 2 that the experimental compositions Inv 1 and 2 have a coefficient close to that of the reference.

Au-delà des considérations exprimées ci-dessus exprimant un potentiel à l'état brut de trempe, il convient de réaliser des mesures dans l'état effectif du métal au stade de l'usage. La méthode électrochimique pratiquée issue de la norme ASTM G 108, consiste, dans une solution aqueuse de H2SO4 à 1% en poids, à polariser 15 minutes l'échantillon à un potentiel de -550mV/ECS puis à réaliser un balayage aller et retour à 60mV/mn de -550mV à +500mV. Les courbes caractéristiques intensité-potentiel au retour peuvent présenter deux pics, l'un (Pic1) dû à la dissolution de la matrice, le second (Pic 2), à un potentiel plus élevé, relié à la dissolution au droit des précipités de carbures de chrome. L'acier est d'autant plus sensible à la corrosion que le courant de dissolution est intense. Des courbes caractéristiques sont présentées en figures 2 et 3.Beyond the considerations expressed above expressing a potential in the quenched state, measurements should be made in the effective state of the metal at the stage of use. The electrochemical method performed according to the ASTM G 108 standard consists, in a 1% by weight aqueous H 2 SO 4 solution, of polarizing the sample for 15 minutes at a potential of -550mV / ECS and then carrying out a forward scan. and back to 60mV / min from -550mV to + 500mV. The intensity-potential curves at the return can have two peaks, one (Pic1) due to the dissolution of the matrix, the second (Pic 2), at a higher potential, connected to the dissolution at the right of carbide precipitates of chromium. Steel is all the more sensitive to corrosion as the dissolution current is intense. Characteristic curves are presented in figures 2 and 3 .

Selon les pratiques habituelles pour les aciers de référence, tout en obtenant la dureté voisine de 52HRC recherchée, deux paramètres du traitement thermique s'avèrent influents pour la résistance à la corrosion: la température du revenu et la vitesse de trempe.According to the usual practices for the reference steels, while obtaining the hardness close to 52HRC sought, two parameters of the heat treatment are influential for the resistance to corrosion: the temperature of the income and the quenching speed.

Ces effets ont été précisés par des essais en laboratoire:These effects have been clarified by laboratory tests:

a) effet du revenu:a) income effect :

La figure 2 démontre pour la coulée INV1 que l'acier se sensibilise fortement vis-à-vis de la corrosion pour des revenus réalisés dans la zone de durcissement vers 500°C. Si la résistance à la corrosion est une caractéristique à privilégier impérativement pour les applications envisagées, on privilégiera donc les revenus à basse température (200-380°C).The figure 2 demonstrates for the casting INV1 that the steel is strongly sensitized with respect to corrosion for incomes made in the hardening zone around 500 ° C. If corrosion resistance is a characteristic that should be favored for the applications envisaged, we will favor low-temperature incomes (200-380 ° C).

Cette tendance se confirme pour toutes les compositions testées, comme le montre la figure 3. Celle-ci montre l'influence d'un double revenu de 2h à 380°C ou à une température avoisinant 500°C, sur la résistance à la corrosion, en considérant le courant de corrosion au pic 2 de la figure 2. La température exacte des doubles revenus à 500°C environ a été ajustée pour qu'elle permette d'obtenir la même dureté qu'après un double revenu à 380°C. On constate en particulier que l'échantillon selon l'invention a une résistance à la corrosion très comparable à celle de l'échantillon de référence pour un double revenu à 380°C.This trend is confirmed for all the compositions tested, as shown in figure 3 . This shows the influence of a double income of 2h at 380 ° C or at a temperature of around 500 ° C, on the corrosion resistance, considering the corrosion current at peak 2 of the figure 2 . The exact temperature of the double incomes at about 500 ° C. was adjusted to obtain the same hardness as after a double income at 380 ° C. It is found in particular that the sample according to the invention has a corrosion resistance very comparable to that of the reference sample for a double income at 380 ° C.

Pour les basses températures de revenu, il a par ailleurs été vérifié que la résistance à la corrosion décroît légèrement entre 200°C et 380°C pour se dégrader rapidement au-dessus de 400°C.For low tempering temperatures, it has also been verified that the corrosion resistance decreases slightly between 200 ° C and 380 ° C to rapidly degrade above 400 ° C.

Pour que les revenus aient l'effet escompté, il faut qu'ils durent au moins 2h, et que leur température nominale soit maintenue au coeur de la pièce pendant au moins 1h.For income to have the desired effect, they must last at least 2 hours, and their nominal temperature is maintained in the heart of the room for at least 1 hour.

b) effet de la vitesse de trempe :b) effect of quenching speed :

De manière inattendue, comme en témoigne la figure 4 qui compare deux coulées expérimentales se distinguant l'une de l'autre par la seule teneur en Chrome, l'augmentation de la teneur en cet élément n'améliore pas la résistance à la corrosion dans les conditions de la trempe industrielle avec une vitesse de refroidissement de l'ordre de 20°C/minute dans l'intervalle 900/400°C. La basse vitesse de refroidissement provoque le développement du Pic 2 révélateur de la précipitation de carbures ou nitrures, et dont l'amplitude est d'autant plus importante que la teneur en Chrome est élevée et amplifie le courant de corrosion de la matrice (Pic 2).Unexpectedly, as evidenced by the figure 4 which compares two experimental streams distinguished from each other by the single chromium content, the increase in the content of this element does not improve the corrosion resistance under the conditions of industrial quenching with a speed of cooling of the order of 20 ° C / minute in the range 900/400 ° C. The low cooling rate causes the development of peak 2 revealing the precipitation of carbides or nitrides, and whose amplitude is all the more important that the chromium content is high and amplifies the corrosion current of the matrix (Pic 2 ).

Ces résultats sont confirmés pour les diverses compositions étudiées.These results are confirmed for the various compositions studied.

Selon l'invention, on choisira une vitesse de trempe compatible avec le savoir-faire du traitement thermique et comprise entre 10 et 40°C/min dans l'intervalle de température 800 à 400°C.According to the invention, a quenching speed compatible with the know-how of the heat treatment and between 10 and 40 ° C./min in the temperature range 800 to 400 ° C. will be chosen.

En conclusion, dans le contexte d'une trempe industrielle, c'est avec les revenus à basse température que la meilleure tenue à la corrosion est obtenue, et dans cette configuration, la variation de la teneur en chrome dans l'intervalle 10,5 à 15% ne confirme pas l'effet bénéfique habituellement reconnu pour cet élément d'alliage.In conclusion, in the context of industrial quenching, it is with the low temperature incomes that the best corrosion resistance is obtained, and in this configuration, the variation of the chromium content in the interval 10.5 at 15% does not confirm the beneficial effect usually recognized for this alloying element.

Les mêmes effets défavorables de la diminution de la vitesse de trempe et de l'augmentation de la température du revenu sont constatés sur la ténacité. Cette propriété est communément simplement appréciée à partir des caractéristiques mécaniques conventionnelles d'allongement et de striction lors de l'essai de traction et d'énergie de flexion par choc sur barreaux non entaillées de dimensions 55x10x7 mm. Pour les essais concernés, on a réalisé sur tous les échantillons une trempe de 16°C/min puis un double revenu de 2h. Les résultats regroupés dans le Tableau 3 démontrent :

  • pour la composition Inv. 2 prise comme exemple, l'effet néfaste de l'abaissement de la vitesse de trempe ;
  • l'effet fragilisant du double revenu au voisinage de 500°C ;
  • pour le double revenu à basse température (380°C), la supériorité du compromis dureté/ténacité des deux aciers de l'invention par rapport à la référence.
Tableau 3: Caractéristiques mécaniques mesurées pour trois compositions sur prélèvements représentatifs de produits Industriels Composition Vitesse de Trempe : (°C/min) Double revenu de 2 heures à 380°C Double revenu de 2 heures à haute température Dureté A Z Flexion par choc Eprouvette non entaillée Revenu Dureté A Z Flexion par choc Eprouvette non entaillée HRC % % J °C HRC % % J Référence 16 51,3 4,5 9 110 500 51,3 3,0 3 32 Inv. 1 16 53,2 7,5 21 210 520 53,0 3,5 5 38 Inv. 2 16 53,9 6.5 17 130 520 52,3 6,0 17 35 Inv. 2 60 53,8 9.5 35 260 The same adverse effects of the decrease of the quenching rate and the increase of the temperature of the income are noted on the tenacity. This property is commonly appreciated simply from the conventional mechanical features of elongation and necking during tensile testing and impact bending energy on unscored bars of dimensions 55x10x7 mm. For the tests concerned, on all the samples, a quenching of 16 ° C / min and then a double income of 2h were carried out. The results summarized in Table 3 demonstrate:
  • for the composition Inv. 2 taken as an example, the detrimental effect of lowering the quenching rate;
  • the weakening effect of double income at around 500 ° C;
  • for the low-temperature double-temperature (380 ° C.), the superiority of the hardness / toughness compromise of the two steels of the invention compared to the reference.
<b> Table 3: Measured mechanical characteristics for three compositions on representative samples of industrial products </ b> Composition Quenching speed: (° C / min) Double income of 2 hours at 380 ° C Double income of 2 hours at high temperature Hardness AT Z Impact bending Untouched specimen Returned Hardness AT Z Impact bending Untouched specimen HRC % % J ° C HRC % % J Reference 16 51.3 4.5 9 110 500 51.3 3.0 3 32 Inv. 1 16 53.2 7.5 21 210 520 53.0 3.5 5 38 Inv. 2 16 53.9 6.5 17 130 520 52.3 6.0 17 35 Inv. 2 60 53.8 9.5 35 260

Les compositions de l'invention permettent l'obtention de la dureté de 52 HRC ou davantage après trempe dans les conditions industrielles et double revenu à 380°C, malgré l'adoucissement subi dans ce domaine pour cette famille d'acier à partir du brut de trempe, comme le montre la figure 5.The compositions of the invention make it possible to obtain the hardness of 52 HRC or more after quenching under industrial conditions and double-tempered at 380 ° C., despite the softening experienced in this field for this family of steel from crude. quenching, as shown in figure 5 .

Claims (16)

  1. Martensitic stainless steel, characterised in that it comprises, in percentages by weight
    - 0.22% ≤ C ≤ 0.32%
    - 0.05% ≤ N ≤ 0.15%, with 0.33% ≤ C+N ≤ 0.43%
    - 10% ≤ Cr ≤ 12.4%
    - 0.10% ≤ V ≤ 0.40%
    - 0.10% ≤ Mo ≤ 1.0%
    - trace levels ≤ Ni ≤ 1.0%
    - trace levels ≤ Mn ≤ 1.0%
    - trace levels Si ≤ 1.0%
    - trace levels ≤ W < 10%
    - trace levels ≤ Co ≤ 1.0%
    - trace levels ≤ Cu ≤ 1.0%
    - trace levels ≤ Ti ≤ 0.010%
    - trace levels ≤ Nb ≤ 0.050%
    - trace levels ≤ Al ≤ 0.050%
    - trace levels ≤ S ≤ 0.020%
    - trace levels ≤ O ≤ 0.0040%
    - trace levels ≤ P ≤ 0.03%
    - trace levels ≤ B ≤ 0.0050%
    - trace levels ≤ Ca ≤ 0.020%
    - trace levels ≤ Se ≤ 0.010%
    - trace levels ≤ La ≤ 0.040%
    - trace levels ≤ Ce ≤ 0.040%
    the remainder being iron and impurities resulting from the production operation.
  2. Steel according to claim 1, characterised in that 0.08% < N ≤ 0.12%.
  3. Steel according to claim or claim 2, characterised in that 11.0% ≤ Cr ≤12.4%.
  4. Steel according to any one of claims 1 to 3, characterised in that 0.15% ≤ V ≤0.35%.
  5. Steel according to any one of claims 1 to 4, characterised in that trace levels ≤ Si ≤ 0.5%.
  6. Steel according to any one of claims 1 to 5, characterised in that 0.10% ≤ Mo + W/2 ≤ 1.20%.
  7. Steel according to any one of claims 1 to 6, characterised in that trace levels ≤ Ti ≤ 0.003%.
  8. Steel according to any one of claims 1 to 7, characterised in that trace levels ≤ Nb ≤ 0.010%.
  9. according to any one of claims 1 to 8, characterised in that trace levels ≤ O ≤ 0.0015%.
  10. Steel according to any one of claims 1 to 9. characterised in that trace levels ≤ S ≤ 0.003%.
  11. Steel according to any one of claims 1 to 10, characterised in that trace levels ≤ Mn + Cu + Co ≤ 1.8%.
  12. Method for producing a component of martensitic stainless steel, characterised in that:
    - a steel according to any one of claims 1 to 11 is produced, cast, forged or rolled and annealed;
    - the steel is machined in order to confer thereon the shape of the component;
    - the machined steel is austenitised at a temperature of from 990-1040°C, preferably 1000-1030°C;
    - the austenitised steel is quenched at a rate of between 10 and 40°C/min in the temperature range of from 800 to 400°C;
    - two tempering operations are carried out on the quenched steel, in order to confer thereon its final hardness.
  13. Method according to claim 12, characterised in that the tempering operations are each carried out at a temperature of from 200 to 400°C, preferably from 300 to 380°C for a minimum of 2 hours whilst ensuring that the nominal temperature is maintained in the core for at least 1 hour, in order to obtain a hardness of from 49 to 55 HRC.
  14. Method according to claim 12, characterised in that the tempering operations are each carried out al a temperature of from 530 to 560°C for a minimum of 2 hours whilst ensuring that the nominal temperature is maintained in the core for at least 1 hour, in order to obtain a hardness of between 42 and 50 HRC.
  15. Component of martensitic stainless steel, characterised in that the element produced using the method is produced in accordance with any one of claims 12 to 14.
  16. Component according to claim 15, characterised in that it is a mould element which is intended for the production of articles of plastics materials.
EP08830336A 2007-09-10 2008-08-25 Martensitic stainless steel, method for making parts from said steel and parts thus made Revoked EP2188402B1 (en)

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FR2920784B1 (en) 2010-12-10
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EP2188402A1 (en) 2010-05-26
RU2010114173A (en) 2011-10-20
JP2010539325A (en) 2010-12-16
WO2009034282A1 (en) 2009-03-19
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US20100276038A1 (en) 2010-11-04
FR2920784A1 (en) 2009-03-13

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