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EP0379061B1 - Exterior cover part for a watch, and method for manufacturing the same - Google Patents

Exterior cover part for a watch, and method for manufacturing the same Download PDF

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Publication number
EP0379061B1
EP0379061B1 EP19900100499 EP90100499A EP0379061B1 EP 0379061 B1 EP0379061 B1 EP 0379061B1 EP 19900100499 EP19900100499 EP 19900100499 EP 90100499 A EP90100499 A EP 90100499A EP 0379061 B1 EP0379061 B1 EP 0379061B1
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EP
European Patent Office
Prior art keywords
steel
less
exterior part
part according
chromium
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Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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EP19900100499
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German (de)
French (fr)
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EP0379061A1 (en
Inventor
Eric Loth
Pierre-Alain Walder
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ETA SA Manufacture Horlogere Suisse
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Eta SA Fabriques dEbauches
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    • GPHYSICS
    • G04HOROLOGY
    • G04BMECHANICALLY-DRIVEN CLOCKS OR WATCHES; MECHANICAL PARTS OF CLOCKS OR WATCHES IN GENERAL; TIME PIECES USING THE POSITION OF THE SUN, MOON OR STARS
    • G04B43/00Protecting clockworks by shields or other means against external influences, e.g. magnetic fields
    • G04B43/007Antimagnetic alloys
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/18Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/22Ferrous alloys, e.g. steel alloys containing chromium with molybdenum or tungsten
    • GPHYSICS
    • G04HOROLOGY
    • G04BMECHANICALLY-DRIVEN CLOCKS OR WATCHES; MECHANICAL PARTS OF CLOCKS OR WATCHES IN GENERAL; TIME PIECES USING THE POSITION OF THE SUN, MOON OR STARS
    • G04B37/00Cases
    • G04B37/22Materials or processes of manufacturing pocket watch or wrist watch cases

Definitions

  • the present invention relates to an outer piece of clothing for a watch made of stainless steel, such as for example a case or case, as well as a method of manufacturing such a piece.
  • the external steel parts for a watch such as the cases or middle parts, are made of austenitic stainless steel.
  • austenitic stainless steel One of the main reasons for this choice is that this type of steel can be easily stamped or stamped.
  • austenitic steels contain a significant proportion of nickel (around 10%), which constitutes a major drawback for a watch case.
  • nickel is a highly allergenic constituent so that to avoid any contamination by the wearer's skin, many countries have set up strict standards, thereby prohibiting the sale on their territory of timepieces that do not satisfy not to these standards. What is more, nickel is on the world markets subject to significant price increases, which increases the cost of timepieces accordingly.
  • this steel has a high magnetic permeability.
  • the box which advantageously forms a screen protects the movement against magnetic fields liable to disturb its operation
  • nickel is only present in this composition in a trace state so that, on the one hand, the allergenic character is thereby eliminated and, on the other hand, the cost of the box. and the timepiece fitted with it can be considerably reduced.
  • ferritic steel has so far never been generalized in the production of clothing for watches. This is because manufacturers and metallurgists in the watch industry have always rejected ferritic steels because of their strong propensity for grain enlargement during heat treatments.
  • the box is considered fragile and even small impacts (falls) can cause it to break.
  • the box has an appearance qualified as "orange peel", that is to say as hammered or bumpy.
  • the invention aims to overcome all these difficulties by providing a piece of clothing made of stainless steel forming a magnetic screen, at the lowest possible cost and having no allergenic character, but it also aims to provide a piece of clothing sufficiently solid and of an aspect allowing its commercial exploitation successfully.
  • the subject of the invention is also a method which is characterized in that, in order to manufacture a piece of clothing as defined above, the steel used is in the form of a rolled strip, from which blanks are cut by a stamping operation, then formed by cold stamping operations, with a recrystallization heat treatment between each of the operations, and in that the parts obtained are then finished with finishing operations.
  • Stainless steels are metallic alloys based on iron (Fe) and chromium (Cr), and also comprising other materials such as nickel (Ni), carbon (C), nitrogen (N) , manganese (Mn), molybdenum (Mo), silicon (Si) and niobium (Nb).
  • Ni, C, N, Mn nickel-C
  • Ni, C, N, Mn manganese
  • Mo molybdenum
  • Si silicon
  • Nb niobium
  • Ni, C, N, Mn tend to favor a crystalline structure of cubic type, centered faces, also called gamma type. They are called gammagens.
  • Ni * Ni% + 30 (C + N)% + 0.5 Mn%.
  • Martensitic steels are characterized by a low content of alphagenic and gammagenic compounds. They behave similar to carbon steels. They therefore allow themselves to be soaked and machined well. However, because of their low chromium content, they unfortunately have insufficient resistance to oxidation to be able to be used in the manufacture of clothing parts for watches. This type of steel will therefore not be examined further.
  • Austenitic steel has a face-centered cubic type structure, that is to say of gamma type. Its composition is therefore particularly rich in gammagenic compounds, in particular nickel, but also in alphagenic compounds.
  • Ferritic steel has a centered cubic type structure, that is to say an alpha type structure. Its composition is therefore particularly rich in alphagenic compounds.
  • certain alloys comprise both zones of two or even each of the three types of stainless steel.
  • the first values, (elastic limits Re) represented a first obstacle to the use of ferritic steels since, on the one hand, the tools for stamping have pressure limits in use and that, on the other hand, need of energy automatically leads to an increase in machining costs which must be passed on to the cost price of the box.
  • the ferritic stainless steel used has a composition which meets the following conditions: concentration in chromium equivalent of between 17.5 and 22%; nickel equivalent concentration less than or equal to 2%; and chromium concentration less than 18.5%.
  • the concentration of gamma compounds is substantially defined by the carbon and nitrogen content.
  • An increase in this content causes an increase in the sensitivity to oxidation and a decrease in the sensitivity to grain enlargement.
  • the proportion of chromium or chromium equivalent must be increased, which has the effect of reducing the deformability. Applicant's practical tests have shown that it is advantageous to choose a steel with a low carbon and nitrogen content, despite the increased sensitivity to grain enlargement, since the number of stamping operations can be greatly reduced.
  • the ferritic steel used must therefore have a chromium equivalent value of between 17.5% and 22%.
  • the steel oxidizes too easily, above, it becomes difficult to deform.
  • the increase in the chromium concentration contributes to reducing the deformability which is why this concentration should not exceed 18.5%.
  • the steels chosen will have a composition comprised within the following limits:
  • a piece of watch clothing for example a middle part
  • a steel strip 1802 laminated to a thickness close to that of the finished middle part.
  • the blanks are then cut by a stamping operation, then formed by cold stamping, using a die and a punch.
  • the latter together define a housing having the final shape of the middle part.
  • three stampings are enough to obtain the final shape.
  • the middle parts thus obtained are then finished with conventional finishing operations. It is very clear that after each stamping and work hardening operation, the material undergoes a recrystallization heat treatment.

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  • Engineering & Computer Science (AREA)
  • Metallurgy (AREA)
  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Chemical & Material Sciences (AREA)
  • Manufacturing & Machinery (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Organic Chemistry (AREA)
  • Heat Treatment Of Sheet Steel (AREA)
  • Heat Treatment Of Steel (AREA)
  • Heat Treatment Of Articles (AREA)

Description

La présente invention a pour objet une pièce extérieure d'habillement pour montre en acier inoxydable, telle que par exemple une boîte ou carrure, ainsi qu'un procédé de fabrication d'une telle pièce.The present invention relates to an outer piece of clothing for a watch made of stainless steel, such as for example a case or case, as well as a method of manufacturing such a piece.

De manière générale, les pièces extérieures en acier pour montre, telles que les boîtes ou carrures, sont réalisées en acier inoxydable austénitique. L'une des raisons principales de ce choix est que ce type d'acier peut être facilement embouti ou étampé.In general, the external steel parts for a watch, such as the cases or middle parts, are made of austenitic stainless steel. One of the main reasons for this choice is that this type of steel can be easily stamped or stamped.

Toutefois, les aciers austénitiques contiennent une proportion importante de nickel (environ 10 %), ce qui constitue pour une boîte de montre un inconvénient majeur. En effet, le nickel est un constituant fortement allergène si bien que pour éviter toute contamination par la peau du porteur, de nombreux pays ont mis sur pied des normes sévères, interdisant de ce fait la vente sur leur territoire de pièce d'horlogerie ne satisfaisant pas à ces normes. Qui plus est, le nickel est sur les marchés mondiaux sujet à des hausses de prix importantes ce qui augmente d'autant le coût des pièces d'horlogerie.However, austenitic steels contain a significant proportion of nickel (around 10%), which constitutes a major drawback for a watch case. In fact, nickel is a highly allergenic constituent so that to avoid any contamination by the wearer's skin, many countries have set up strict standards, thereby prohibiting the sale on their territory of timepieces that do not satisfy not to these standards. What is more, nickel is on the world markets subject to significant price increases, which increases the cost of timepieces accordingly.

Par ailleurs, il est également connu de la demande de brevet suisse no 8453/68 de réaliser des boîtes de montre en acier ferritique. La composition de cet acier est la suivante :
   Cr : 19,2 % ; C + N : 0,03% ; Mo : 2,0 % ; Mn : 0,2 %;
   Si : 0,6 % et Nb : 0,31 %.
Furthermore, it is also known from Swiss patent application No. 8453/68 to produce watch cases from ferritic steel. The composition of this steel is as follows:
Cr: 19.2%; C + N: 0.03%; Mo: 2.0%; Mn: 0.2%;
If: 0.6% and Nb: 0.31%.

La composition chimique de cet acier est choisie de manière qu'il présente une grande perméabilité magnétique. Ainsi, la boîte qui forme avantageusement écran protège le mouvement contre les champs magnétiques susceptibles de perturber son fonctionnementThe chemical composition of this steel is chosen so that it has a high magnetic permeability. Thus, the box which advantageously forms a screen protects the movement against magnetic fields liable to disturb its operation

De plus, le nickel n'est présent dans cette composition qu'à l'état de trace si bien que, d'une part, le caractère allergène s'en trouve supprimé et que, d'autre part, le coût de la boîte et de la pièce d'horlogerie qui en est équipée peut être considérablement diminué.In addition, nickel is only present in this composition in a trace state so that, on the one hand, the allergenic character is thereby eliminated and, on the other hand, the cost of the box. and the timepiece fitted with it can be considerably reduced.

Malgré tous ces avantages, l'utilisation de l'acier ferritique ne s'est jusqu'ici jamais généralisée dans la fabrication de pièce d'habillement pour montre. Ceci parce que les fabricants et les métallurgistes de l'industrie horlogère ont toujours écarté les aciers ferritiques à cause de leur forte propension au grossissement de grain lors des traitements thermiques.Despite all these advantages, the use of ferritic steel has so far never been generalized in the production of clothing for watches. This is because manufacturers and metallurgists in the watch industry have always rejected ferritic steels because of their strong propensity for grain enlargement during heat treatments.

En effet, lorsqu'un acier ferritique est embouti puis soumis à un traitement thermique de recristallisation, on constate que, dans les parties de la pièce faiblement écrouie, il n'y a pas recristallisation mais plutôt un grossissement du grain. Lorsque le grain devient trop grand, la matière tend à se fissurer, et ceci même pour de faibles déformations.In fact, when a ferritic steel is stamped and then subjected to a recrystallization heat treatment, it is found that, in the parts of the part which is slightly cold worked, there is no recrystallization but rather a magnification of the grain. When the grain becomes too large, the material tends to crack, even for small deformations.

C'est typiquement le cas pour les opérations d'emboutissage où les passes se succèdent et conduisent chacune à sensibiliser davantage la matière.This is typically the case for stamping operations where the passes follow one another and each lead to greater awareness of the material.

Dans une application horlogère, telle que la fabrication de boîtes ou carrures, ce grossissement de grain et les micro-fissurations qui en résultent sont inacceptables.In a watchmaking application, such as the manufacture of cases or squares, this magnification of grain and the micro-cracks which result therefrom are unacceptable.

En effet, ces fissurations à l'intérieur de la boîte se conduisent comme des défauts cristallographiques dont le comportement est, sur le plan mécanique, celui des amorces de ruptures.Indeed, these cracks inside the box behave like crystallographic defects whose behavior is, mechanically speaking, that of the initiations of ruptures.

Ainsi, la boîte est considérée comme fragile et même de faibles chocs (chutes) peuvent entraîner son brisement.Thus, the box is considered fragile and even small impacts (falls) can cause it to break.

De plus, la boîte présente un aspect qualifié de "peau d'orange", c'est-à-dire comme martelée ou bosselée.In addition, the box has an appearance qualified as "orange peel", that is to say as hammered or bumpy.

En surface, les grains, comme on le sait séparés par des joints, se comportent optiquement comme de nombreuses facettes. Même après polissage, cet effet subsiste puisque cette structure organisée en grains se retrouve dans toute la matière.On the surface, the grains, as we know separated by joints, behave optically like many facets. Even after polishing, this effect remains since this structure organized in grains is found in all the material.

L'invention a pour but de lever toutes ces difficultés en fournissant une pièce d'habillement réalisée en acier inoxydable formant écran magnétique, d'un coût le moins élevé possible et ne présentant aucun caractère allergène, mais elle a aussi pour but de fournir une pièce d'habillement suffisamment solide et d'un aspect permettant avec succès son exploitation commerciale.The invention aims to overcome all these difficulties by providing a piece of clothing made of stainless steel forming a magnetic screen, at the lowest possible cost and having no allergenic character, but it also aims to provide a piece of clothing sufficiently solid and of an aspect allowing its commercial exploitation successfully.

Ainsi, l'invention a pour objet une pièce d'habillement en acier inoxydable purement ferritique qui comporte des éléments gammagènes choisis parmi le nickel, le carbone, l'azote et le manganèse, dont la concentration globale est donnée en équivalent nickel Ni* exprimée par la formule :

        Ni* =Ni % + 30 (C + N) % + 0,5 Mn % ;


et des éléments alphagènes choisis parmi le chrome, le molybdène, le silicium et le niobium, dont la concentration est donnée en équivalent chrome Cr* exprimée par la formule :

        Cr* = Cr % + Mo % + 1,5 Si% + 0,5 Nb %


   est qui est caractérisée en ce que l'acier inoxydable ferritique a une composition nominale satisfaisant aux conditions suivantes : concentration en équivalent chrome comprise entre 17,5 et 22 %, concentration en équivalent nickel inférieure ou égale à 2 % et concentration en chrome plus petite ou égale à 18,5 %.
Thus, the subject of the invention is a piece of clothing made of purely ferritic stainless steel which includes gamma elements chosen from nickel, carbon, nitrogen and manganese, the overall concentration of which is given in nickel equivalent Ni * expressed by the formula:

Ni * = Ni% + 30 (C + N)% + 0.5 Mn%;


and alphagenic elements chosen from chromium, molybdenum, silicon and niobium, the concentration of which is given in chromium Cr * equivalent expressed by the formula:

Cr * = Cr% + Mo% + 1,5 Si% + 0,5 Nb%


is which is characterized in that the ferritic stainless steel has a nominal composition satisfying the following conditions: chromium equivalent concentration between 17.5 and 22%, nickel equivalent concentration less than or equal to 2% and smaller chromium concentration or equal to 18.5%.

L'invention a aussi pour objet un procédé qui se caractérisé en ce que, pour fabriquer une pièce d'habillement telle que définie ci-dessus, l'acier utilisé se présente sous forme d'une bande laminée, dans laquelle des ébauches sont découpées par une opération d'étampage, puis formées par des opérations d'emboutissage à froid, avec un traitement thermique de recristallisation entre chacune des opérations, et en ce que les pièces obtenues sont ensuite terminées par des opérations de finition.The subject of the invention is also a method which is characterized in that, in order to manufacture a piece of clothing as defined above, the steel used is in the form of a rolled strip, from which blanks are cut by a stamping operation, then formed by cold stamping operations, with a recrystallization heat treatment between each of the operations, and in that the parts obtained are then finished with finishing operations.

L'invention sera mieux comprise à la lecture de la description qui va suivre d'un mode de réalisation fait en référence au dessin dans lequel la figure unique montre deux courbes représentant la limite élastique (Re) et la résistance à la traction (Rm) en fonction du taux d'écrouissage, respectivement d'un acier austénitique classique et d'un acier ferritique tel qu'utilisé dans les pièces selon l'invention.The invention will be better understood on reading the following description of an embodiment made with reference to the drawing in which the single figure shows two curves representing the elastic limit (Re) and the tensile strength (Rm) depending on the rate of work hardening, respectively of a conventional austenitic steel and a ferritic steel as used in the parts according to the invention.

Pour bien comprendre les caractéristiques particulières d'une pièce selon l'invention, il faut au préalable rappeler dans les grandes lignes ce que sont les différents types d'aciers inoxydables.To fully understand the particular characteristics of a part according to the invention, it is first necessary to recall in broad outline what the different types of stainless steel are.

Les aciers inoxydables sont des alliages métalliques à base de fer (Fe) et de chrome (Cr), et comportant en outre d'autres matériaux tels que du nickel (Ni), du carbone (C), de l'azote (N), du manganèse (Mn), du molybdène (Mo), du silicium (Si) et du niobium (Nb). Certains de ces matériaux (Ni, C, N, Mn) tendent à favoriser une structure cristalline de type cubique faces centrées, appelée aussi de type gamma. Ils sont appelés gammagènes. L'effet global de ces matériaux peut être évalué en équivalent nickel (Ni*) par la formule suivante :

        Ni* = Ni % + 30 (C + N) % + 0,5 Mn %.

Stainless steels are metallic alloys based on iron (Fe) and chromium (Cr), and also comprising other materials such as nickel (Ni), carbon (C), nitrogen (N) , manganese (Mn), molybdenum (Mo), silicon (Si) and niobium (Nb). Some of these materials (Ni, C, N, Mn) tend to favor a crystalline structure of cubic type, centered faces, also called gamma type. They are called gammagens. The overall effect of these materials can be evaluated in nickel equivalent (Ni *) by the following formula:

Ni * = Ni% + 30 (C + N)% + 0.5 Mn%.

D'autres de ces matériaux (C, Mo, Si et Nb) tendent à favoriser une structure cristalline de type cubique centré, appelée aussi de type alpha. Ils sont appelés alphagènes. L'effet global de ces matériaux peut être évalué en équivalent chrome (Cr*) par la formule suivante :

        Cr* = Cr % + Mo % + 1,5 Si % + 0,5 Nb %.

Others of these materials (C, Mo, Si and Nb) tend to favor a crystal structure of centered cubic type, also called alpha type. They are called alphagenes. The overall effect of these materials can be evaluated in chromium equivalent (Cr *) by the following formula:

Cr * = Cr% + Mo% + 1.5 Si% + 0.5 Nb%.

La structure cristalline, et en conséquence les caractéristiques mécaniques de ces alliages, varient en fonction de leur teneur en Cr* et Ni*. Ceci est illustré dans le diagramme de Schaeffler - De Long. Il existe en effet trois familles d'aciers inoxydables :

  • les aciers martensitiques ;
  • les aciers austénitiques ; et
  • les aciers ferritiques .
The crystalline structure, and consequently the mechanical characteristics of these alloys, vary according to their Cr * and Ni * content. This is illustrated in the Schaeffler - De Long diagram. There are indeed three families of stainless steels:
  • martensitic steels;
  • austenitic steels; and
  • ferritic steels.

Les aciers martensitiques se caractérisent par une faible teneur en composés alphagènes et gammagènes. Ils ont un comportement voisin des aciers aux carbones. Ils se laissent donc tremper et bien usiner. Toutefois à cause de leur faible teneur en chrome, ils présentent malheureusement une résistance insuffisante à l'oxydation pour pouvoir être utilisé dans la fabrication de pièces d'habillement pour montre. Ce type d'acier ne sera donc pas examiné de manière plus approfondie.Martensitic steels are characterized by a low content of alphagenic and gammagenic compounds. They behave similar to carbon steels. They therefore allow themselves to be soaked and machined well. However, because of their low chromium content, they unfortunately have insufficient resistance to oxidation to be able to be used in the manufacture of clothing parts for watches. This type of steel will therefore not be examined further.

L'acier austénitique présente une structure de type cubique faces centrées, c'est-à-dire de type gamma. Sa composition est donc particulièrement riche en composés gammagènes, notamment en nickel, mais également en composés alphagènes.Austenitic steel has a face-centered cubic type structure, that is to say of gamma type. Its composition is therefore particularly rich in gammagenic compounds, in particular nickel, but also in alphagenic compounds.

L'acier ferritique présente une structure de type cubique centré, c'est-à-dire de type alpha. Sa composition est donc particulièrement riche en composés alphagènes.Ferritic steel has a centered cubic type structure, that is to say an alpha type structure. Its composition is therefore particularly rich in alphagenic compounds.

Par ailleurs, certains alliages comportent à la fois des zones de deux, voire de chacun des trois types d'aciers inoxydables.In addition, certain alloys comprise both zones of two or even each of the three types of stainless steel.

Ainsi que cela a été dit plus haut, les caractéristiques mécaniques des différents types d'aciers varient de manière sensible. Le tableau ci-après donne les valeurs de plusieurs paramètres pour un acier de type purement ferritique et plus particulièrement de la nuance telle que commercialisée par la maison Sandvik (Suède) sous la dénomination Sandvik 1802, et un acier purement austénitique de type 304L. Cet alliage austénitique est celui qui a été jusqu'à lors le plus utilisé pour la fabrication de pièces d'habillement.As has been said above, the mechanical characteristics of the different types of steel vary significantly. The table below gives the values of several parameters for a steel of purely ferritic type and more particularly of the grade as marketed by Sandvik (Sweden) under the name Sandvik 1802, and a purely austenitic steel of type 304L. This austenitic alloy is the one that has been used the most for the manufacture of clothing pieces until then.

Les valeurs données dans les tableaux ci-dessous sont celles fournies par les acièristes et, par voie de conséquence, celles qui avaient jusqu'ici toujours été prises en compte par les fabricants de pièces d'habillement dans le choix de leurs aciers.

Figure imgb0001
The values given in the tables below are those supplied by steel manufacturers and, consequently, those which had hitherto always been taken into account by manufacturers of clothing parts in the choice of their steels.
Figure imgb0001

Si l'on se réfère à ce tableau, on constate que la limite élastique de l'acier ferritique est sensiblement supérieure à celle de l'acier austénitique et de plus que l'allongement avant rupture (type normalisé A5) est de 30 % pour l'acier ferritique et de 63 % pour l'austénitique. Cela revient à dire que la force à appliquer pour obtenir une déformation de l'acier ferritique est plus élevée que pour l'acier austénitique et que, de plus, la déformation possible est plus faible avec le ferritique qu'avec l'austénitique.If one refers to this table, one notes that the elastic limit of ferritic steel is appreciably higher than that of austenitic steel and moreover that the elongation before rupture (standard type A5) is of 30% for ferritic steel and 63% for austenitic. This amounts to saying that the force to be applied to obtain a deformation of the ferritic steel is higher than for austenitic steel and that, moreover, the possible deformation is lower with ferritic than with austenitic.

En d'autres termes, les fabricants de pièces d'habillement ont toujours eu à l'esprit que la mise en oeuvre par emboutissage ou étampage (c'est-à-dire par déformation plastique) d'un acier ferritique demanderait plus d'énergie qu'avec un acier austénitique et, de plus, que la déformation obtenue serait beaucoup moins grande qu'avec ce dernier (au dessus d'un facteur 2).In other words, the manufacturers of clothing parts have always had in mind that the implementation by stamping or stamping (that is to say by plastic deformation) of a ferritic steel would require more energy than with an austenitic steel and, moreover, that the deformation obtained would be much less than with the latter (above a factor of 2).

Les premières valeurs, (limites élastiques Re) ont représenté un premier obstacle à l'utilisation des aciers ferritiques puisque, d'une part, les outillages pour l'emboutissage présentent des limites de pression en utilisation et que, d'autre part, le besoin d'énergie conduit automatiquement à une élévation des coûts d'usinage qu'il faut repercuter sur le prix de revient de la boîte.The first values, (elastic limits Re) represented a first obstacle to the use of ferritic steels since, on the one hand, the tools for stamping have pressure limits in use and that, on the other hand, need of energy automatically leads to an increase in machining costs which must be passed on to the cost price of the box.

Enfin, l'allongement (déformation plastique résultante du matériau) étant beaucoup plus faible pour l'acier ferritique, on a toujours supposé que le nombre de passes par opération d'emboutissage allait être beaucoup plus important qu'avec l'acier austénitique.Finally, the elongation (plastic deformation resulting from the material) being much lower for ferritic steel, it has always been assumed that the number of passes per stamping operation would be much greater than with austenitic steel.

Dès lors, on a toujours conclu qu'un grand nombre de recuits allait être nécessaire et, comme chaque traitement thermique est précisément la cause du grossissement de grain, on a considéré qu'il serait impossible d'obtenir une boîte ou carrure de montre de résistance suffisante et d'aspect commercialement acceptable.Therefore, it was always concluded that a large number of anneals would be necessary and, since each heat treatment is precisely the cause of the grain enlargement, it was considered that it would be impossible to obtain a watch case or case of sufficient strength and commercially acceptable appearance.

On comprend aisément pourquoi, au vu de toutes ces hypothèses, les fabricants de pièces d'habillement ont toujours renoncé à utiliser les aciers ferritiques.It is easy to understand why, in view of all these assumptions, manufacturers of clothing parts have always given up using ferritic steels.

Cependant, en faisant de nombreux essais et en étudiant de façon approfondie et judicieuse le comportement des aciers ferritiques dans leur application à la fabrication par emboutissage des pièces d'habillement, telles que les boîtes ou carrures de montre, la demanderesse a constaté que la situation était plus favorable qu'il n'y paraît de prime abord.However, by carrying out numerous tests and by studying in depth and judiciously the behavior of ferritic steels in their application to the production by stamping of clothing parts, such as watch cases or cases, the Applicant has found that the situation was more favorable than it first appears.

Un des résultats de cette étude est illustré sur la figure, qui représente schématiquement l'évolution de la limite élastique Re et de la limite de rupture Rm en fonction du taux d'écrouissage des aciers respectivement ferritique de type 1802 et austénitique de type 304L.One of the results of this study is illustrated in the figure, which schematically represents the evolution of the elastic limit Re and the breaking limit Rm as a function of the rate of work hardening of respectively ferritic steels of type 1802 and austenitic of type 304L.

Il apparaît sur cette figure que si la limite élastique de l'acier ferritique type 1802 est supérieure à celle de l'acier austénitique pour de faibles taux d'écrouissage, cette limite croît beaucoup moins rapidement lorsque ces taux augmentent. Plus précisément, on constate que pour déformer un acier ferritique type 1802, il faut appliquer une pression d'environ 370 N/mm² lorsqu'il n'est pas écroui et de 750 N/mm² lorsque le taux d'écrouissage est de 60 %. Pour un acier austénitique type 304L, la pression n'est que de 320 N/mm² lorsqu'il n'est pas écroui, mais avoisine 1′200 N/mm² avec un taux d'écrouissage de 60 %.It appears on this figure that if the elastic limit of ferritic steel type 1802 is higher than that of austenitic steel for low rates of work hardening, this limit increases much less quickly when these rates increase. More precisely, we note that to deform a ferritic steel type 1802, it is necessary to apply a pressure of approximately 370 N / mm² when it is not hardened and of 750 N / mm² when the rate of work hardening is 60% . For an austenitic steel type 304L, the pressure is only 320 N / mm² when it is not work hardened, but is around 1,200 N / mm² with a work hardening rate of 60%.

Or, lorsqu'une pièce telle qu'une boîte ou carrure de montre est formée par emboutissage, l'écrouissage peut être très important dans certaines régions. C'est pourquoi, d'après cette courbe, on a pu supposé que l'acier inoxydable ferritique se laisserait plus facilement former par emboutissage à froid que l'acier austénitique et ce malgré toutes les hypothèses. Encore fallait-il que la différence soit importante, car si le nombre de traitements thermiques de recristallisation de l'acier ferritique est trop élevé, le risque de grossissement du grain augmente et avec lui la susceptibilité à la fissuration.However, when a part such as a watch case or case is formed by stamping, work hardening can be very important in some regions. This is why, according to this curve, it has been assumed that ferritic stainless steel would be more easily formed by cold stamping than austenitic steel, despite all the assumptions. The difference still had to be significant, because if the number of heat treatments for recrystallization of ferritic steel is too high, the risk of grain enlargement increases and with it the susceptibility to cracking.

Des résultats satisfaisants ont été obtenus par la requérante lorsque l'acier inoxydable ferritique utilisé a une composition qui répond aux conditions suivantes : concentration en équivalent chrome compris entre 17,5 et 22 % ; concentration en équivalent nickel inférieure ou égale à 2 % ; et concentration en chrome inférieure à 18,5 %.Satisfactory results have been obtained by the applicant when the ferritic stainless steel used has a composition which meets the following conditions: concentration in chromium equivalent of between 17.5 and 22%; nickel equivalent concentration less than or equal to 2%; and chromium concentration less than 18.5%.

Dans cet alliage, la concentration en composés gammagènes est sensiblement définie par la teneur en carbone et en azote. Une augmentation de cette teneur provoque une augmentation de la sensibilité à l'oxydation et une diminution de la sensibilité au grossissement de grain. Pour compenser l'augmentation de la sensibilité à l'oxydation, il faut augmenter la proportion de chrome ou d'équivalent chrome , ce qui a pour effet de réduire la déformabilité. Les essais pratiques de la requérante ont montré qu'il est avantageux de choisir un acier à faible teneur en carbone et en azote, malgré l'augmentation de la sensibilité au grossissement de grain, car le nombre d'opérations d'emboutissage peut être fortement réduit. Par exemple, alors que la fabrication d'une carrure de montre avec quatre cornes nécessite une douzaine d'emboutissages successifs lorsqu'elle est faite en acier austénitique, on s'est rendu compte que trois suffisaient avec l'acier purement ferritique référence 1802 vendu par la maison Sandvik (Suède). La composition nominale de cet acier est la suivante :

Figure imgb0002
Figure imgb0003
In this alloy, the concentration of gamma compounds is substantially defined by the carbon and nitrogen content. An increase in this content causes an increase in the sensitivity to oxidation and a decrease in the sensitivity to grain enlargement. To compensate for the increase in sensitivity to oxidation, the proportion of chromium or chromium equivalent must be increased, which has the effect of reducing the deformability. Applicant's practical tests have shown that it is advantageous to choose a steel with a low carbon and nitrogen content, despite the increased sensitivity to grain enlargement, since the number of stamping operations can be greatly reduced. For example, while the manufacture of a watch case with four horns requires a dozen successive stampings when it is made of austenitic steel, we realized that three were enough with the purely ferritic steel reference 1802 sold by Sandvik (Sweden). The nominal composition of this steel is as follows:
Figure imgb0002
Figure imgb0003

En introduisant ces valeurs dans les formules de calcul des équivalent chrome et équivalent nickel, on obtient les valeurs suivantes :
   Cr* : 21,05 %
   Ni* : 0,9 %
By introducing these values into the formulas for calculating the chrome equivalent and the nickel equivalent, the following values are obtained:
Cr *: 21.05%
Ni *: 0.9%

Pour obtenir un résultat satisfaisant il faut donc que l'acier ferritique utilisé ait une valeur d'équivalent chrome comprise entre 17,5 % et 22 %. En dessous, l'acier s'oxyde trop facilement, en dessus, il devient difficilement déformable. L'augmentation de la concentration en chrome contribue à réduire la déformabilité c'est pourquoi cette concentration ne doit pas dépasser 18,5 %.To obtain a satisfactory result, the ferritic steel used must therefore have a chromium equivalent value of between 17.5% and 22%. Below, the steel oxidizes too easily, above, it becomes difficult to deform. The increase in the chromium concentration contributes to reducing the deformability which is why this concentration should not exceed 18.5%.

Avantageusement, les aciers choisis auront une composition comprise dans les limites suivantes :

Figure imgb0004
Advantageously, the steels chosen will have a composition comprised within the following limits:
Figure imgb0004

Par ailleurs, dans l'application qui nous intéresse, à savoir la fabrication de pièces d'habillement pour montres telles que des carrures, on a pu déterminer que des compositions nominales suivantes :
   soufre compris entre 0,3 et 0,5 %
   et plus particulièrement 0,3 % ;
   molybdène au moins 1 % et de
   préférence 2,3 % ;
   nobium entre 0,1 % et 0,4% ;
   et titane 0,6 %
offrent des caractéristiques avantageuses et notamment présentent d'excellents compromis oxydation/déformabilité.
Furthermore, in the application which interests us, namely the manufacture of clothing parts for watches such as casebands, it has been possible to determine that the following nominal compositions:
sulfur between 0.3 and 0.5%
and more particularly 0.3%;
molybdenum at least 1% and
preferably 2.3%;
nobium between 0.1% and 0.4%;
and titanium 0.6%
offer advantageous characteristics and in particular have excellent oxidation / deformability compromises.

Dans ces conditions, on comprendra que le nombre très faible de passes d'étampage et d'emboutissage (3) a permis de diminuer d'autant le nombre de recuits (recristallisation) et, par conséquent, de ne pas augmenter la sensibilité de l'acier au grossissement de grain.Under these conditions, it will be understood that the very low number of stamping and stamping passes (3) has made it possible to reduce all the more the number of anneals (recrystallization) and, consequently, not to increase the sensitivity of the steel to grain enlargement.

Pour fabriquer une pièce d'habillement de montre, par exemple une carrure, on part d'une bande d'acier 1802, laminée à une épaisseur voisine de celle qu'aura la carrure terminée. Les ébauches sont ensuite découpées par une opération d'étampage, puis formées par emboutissage à froid, au moyen d'une matrice et d'un poinçon. Ces derniers définissent ensemble un logement ayant la forme finale de la carrure. Pour une carrure habituelle, trois emboutissages suffisent pour obtenir la forme définitive. Les carrures ainsi obtenues sont ensuite terminées par les opérations classiques de finition. Il est bien clair qu'après chaque opération d'étampage et d'écrouissage, la matière subit un traitement thermique de recristallisation.To make a piece of watch clothing, for example a middle part, one starts with a steel strip 1802, laminated to a thickness close to that of the finished middle part. The blanks are then cut by a stamping operation, then formed by cold stamping, using a die and a punch. The latter together define a housing having the final shape of the middle part. For a usual build, three stampings are enough to obtain the final shape. The middle parts thus obtained are then finished with conventional finishing operations. It is very clear that after each stamping and work hardening operation, the material undergoes a recrystallization heat treatment.

Il est important que les ébauches soient découpées dans une bande laminée plutôt que sciées dans un tube. En effet, la pratique a montré que si la pression d'emboutissage est appliquée selon un axe parallèle à l'axe de déformation de la matière générée par son étirage, ce qui est le cas avec des ébauches sciées, la déformabilité avant fissuration est plus faible que lorsque l'axe de déformation de la matière est perpendiculaire à l'axe d'application de la pression.It is important that the blanks are cut from a laminated strip rather than sawn from a tube. Indeed, practice has shown that if the stamping pressure is applied along an axis parallel to the axis of deformation of the material generated by its drawing, which is the case with sawn blanks, the deformability before cracking is more weak only when the axis of deformation of the material is perpendicular to the axis of application of the pressure.

L'exemple décrit ci-dessus se rapporte à une carrure de boîte de montre. Il est bien évident que d'autres pièces de l'habillement pourraient également être fabriquées en acier purement ferritique comme défini dans les revendications. C'est le cas par exemple des maillons d'un bracelet, du fond de la boîte, etc.The example described above relates to a case case middle. It is obvious that other pieces of clothing could also be made of purely ferritic steel as defined in the claims. This is the case for example of the links of a bracelet, the bottom of the case, etc.

Claims (11)

  1. Exterior part for a watch formed of purely ferritic steel including gammagen elements chosen from among nickel (Ni), carbon (C), nitrogen (N) and manganese (Mn), the concentration of which is given in nickel equivalent Ni* expressed by the formula :

            Ni* = Ni% + 30(C + N)% + 0.5Mn%

    and alphagen elements chosen from among chromium (Cr), molybdenum (Mo), silicon (Si) and niobium (Nb), the concentration of which is given in chromium equivalent Cr* expressed by the formula :

            Cr* = Cr% + Mo% + 1.5 Si% + 0.5 Nb%

    characterized in that said steel has a nominal composition satisfying the following conditions :
       Cr* comprising between 17.5% and 22%;
       Ni* less than or equal to 2%; and
       Cr concentration less than or equal to 18.5%.
  2. Exterior part according to claim 1, characterized in that said steel exhibits the following nominal composition :
    carbon and nitrogen combined : less than 0.07%; silicon : less than 0.8%; chromium : between 17 and 18.5%; molybdenum : between 1 and 4%; niobium and titanium combined : less than 1%, manganese : less than 1.2%; sulphur : less than 0.5%; and nickel : less than 1%; the balance being iron and unavoidable impurities.
  3. Exterior part according to claim 1 or 2, characterized in that said steel includes a nominal sulphur content between 0.3 and 0.5%.
  4. Exterior part according to claim 1, 2 or 3, characterized in that said steel includes 0.3% sulphur.
  5. Exterior part according to claim 1, 3 or 4, characterized in that said steel includes at least 1% molybdenum.
  6. Exterior part according to claim 5, characterized in that said steel includes 2.3% molybdenum.
  7. Exterior part according to anyone of the preceding claims, characterized in that said steel has a nominal composition satisfying the following conditions :
          Cr* = 21.05%
          Ni* = 0.9%.
  8. Exterior part according to anyone of the preceding claims, characterized in that said steel includes between 0.1 and 0.4% niobium.
  9. Exterior part according to any one of the preceding claims, characterized in that the steel exhibits substantially the following nominal composition : carbon and nitrogen combined : less than 0.03%; silicon : 0.5%; manganese : 0.5%; sulphur : 0.3%; chromium : 18%; molybdenum : 2.3%; titanium : 0.6%; the balance being iron and unavoidable impurities.
  10. Method of manufacture for an exterior part according to one of the claims 1 to 9, characterized in that the steel employed is in the form of a rolled strip, in that a blank is cut from the strip by a stamping operation, then formed by cold pressing operations, with recrystallization heat treatment between each of said operations, and in that the part obtained from the blanks is then subjected to finishing operations.
  11. Method of manufacture according to claim 10, characterized in that the number of stamping and pressing operations is less than or equal to 5.
EP19900100499 1989-01-18 1990-01-11 Exterior cover part for a watch, and method for manufacturing the same Expired - Lifetime EP0379061B1 (en)

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CH14789 1989-01-18
CH147/89 1989-01-18

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EP0964071A1 (en) * 1998-06-12 1999-12-15 Asulab S.A. Ferritic stainless steel and exterior cover part for a watch made with such a steel
US20060130938A1 (en) * 2002-10-04 2006-06-22 Firth Ag Ferritic steel alloy
EP3176653B1 (en) * 2015-12-03 2018-09-26 Cartier International AG Clock component having a magnetic shielding function and comprising a steel alloy

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FR1140573A (en) * 1956-01-25 1957-07-29 Birmingham Small Arms Co Ltd Ferritic chromium steels
FR2055782B1 (en) * 1969-08-18 1973-11-16 Uebelhardt Roger
BE805253A (en) * 1973-09-25 1974-01-16 Nyby Bruk Ab Welded constructions - of corrosion resistant chrome steel sheets, eg vess-els, pipes heat-exchangers for hot water
CA1184402A (en) * 1980-04-11 1985-03-26 Sumitomo Metal Industries, Ltd. Ferritic stainless steel having good corrosion resistance

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