EP2480693B1 - Stainless steel having local variations in mechanical resistance - Google Patents

Stainless steel having local variations in mechanical resistance Download PDF

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Publication number
EP2480693B1
EP2480693B1 EP09740179.8A EP09740179A EP2480693B1 EP 2480693 B1 EP2480693 B1 EP 2480693B1 EP 09740179 A EP09740179 A EP 09740179A EP 2480693 B1 EP2480693 B1 EP 2480693B1
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EP
European Patent Office
Prior art keywords
sheet
mechanical resistance
steel sheet
martensitic
local
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EP09740179.8A
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German (de)
French (fr)
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EP2480693A1 (en
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Pierre-Olivier Santacreu
Aurélien PIC
Fabrice Pinard
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Aperam SA
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Aperam 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
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/021—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular fabrication or treatment of ingot or slab
    • 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
    • C21D1/06—Surface hardening
    • C21D1/09—Surface hardening by direct application of electrical or wave energy; by particle radiation
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C—CHEMISTRY; METALLURGY
    • 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

Definitions

  • the present invention relates to the forming of stainless steel sheets and more particularly those having high mechanical strengths.
  • Stainless steel sheets are widely used in the automotive, construction and general industrial sectors because of their excellent resistance to corrosion.
  • these sheets are generally shaped to be used, for example, in the form of profiles, square tubes, bumper beams, stretchers, door frames. These shaping are most often performed by folding, profiling and stamping.
  • DE102006059885 discloses a carbon steel component for a motor vehicle, having a surface with a structural variation in some areas, laser-treated to form a three-dimensional resistance-improving structure.
  • FR2864108 discloses a stainless steel sheet having high mechanical strength and good elongation, and whose properties are homogeneous.
  • the patent US5,735,163 thus describes a blank shaping process in which a local portion of the blank is cured before shaping. This hardening is generated by a high density energy supply. The resulting rise in temperature results in the transformation of the local microstructure into martensite or bainite, which locally increases the mechanical strength.
  • the formation of hardened lines parallel to the direction of the deformation makes it possible to avoid the breaking of indefinable shades.
  • the structural transformation related to the formation of martensite or bainite on the outer side of the blank to be bent generates a local compressive stress. During folding, this constraint partially cancels the extension stress generated by folding, thus limiting the springback.
  • this process solves only one of the problems mentioned above.
  • this process can not be applied to steels having a high mechanical strength, already sufficiently difficult to implement.
  • this process assumes the use of steels capable of undergoing a martensitic or bainitic phase transformation during annealing followed by quenching, which in fact limits its use to carbon-manganese steels.
  • the present invention aims to facilitate the forming of stainless steel sheets having a high mechanical strength. It has been designed and realized to overcome the defects presented previously and to obtain other advantages.
  • the invention firstly relates to a stainless steel sheet containing a minimum of 10.5% by weight of Cr and a maximum of 1.2% by weight of C, the microstructure of which is martensitic or austenitic. martensitic and comprises at least 2% by volume of martensite.
  • This sheet is essentially characterized in that it comprises at least one local portion of lower mechanical strength, having a martensite rate at least 10% lower than that of the remainder of said sheet; said local portion being at least partially of a thickness equal to that of said sheet.
  • a third object of the invention consists of a steel part obtainable by deformation of a steel sheet according to the invention or of a sheet obtained by the method according to the invention, said deformation taking place in at least one of said local portions of lower mechanical strength.
  • the terms 2H, C700 to C1300 (called hardened state), 1E, 1D, 2B, 2D, 2R, 2E (so-called annealed condition) refer in particular to the standards which define the production ranges and the technical delivery conditions of the steels concerned. (NF EN 10088-1 and -2 for stainless steels). C1500 will designate a range of manufacture of a 2H hardened nut ensuring a mechanical strength higher than 1500MPa.
  • the stainless steel sheets considered by the present invention are characterized by their mechanical strength. This is controlled on the one hand by the addition elements, but also by the heat treatments and the mechanical treatments that the sheet can undergo.
  • This composition features an austenitic stainless steel that solidifies to primary ferrite and contains a non-zero amount of work hardening martensite after deformation. Although predominantly austenite, conventional austenitic grades contain traces of residual ferrite from the solidification as well as traces of martensite resulting from rolling operations.
  • Heat treatment and mechanical treatment allow, in turn, to modify this mechanical strength in a certain proportion.
  • a homogeneous work hardening (production range 2H: C700 to C1500) on the entire sheet results in a partial transformation of the austenite to martensite and possibly a hardening of the austenite by densification. of the dislocation network.
  • This hardening achieves mechanical strengths well above 780 MPa, maximum value achievable on a stainless steel annealed type 1D, 1E, 2B, 2D, 2E, 2R.
  • the steel thus worked is of austeno-martensitic structure that is to say formed at ordinary temperature of a mixture of austenite and martensite, the volume fraction of martensite being at least 2%.
  • a localized heat treatment to the zones to be deformed causes a partial reversion of the martensite to austenite and possibly a softening of the austenite by reducing the number of dislocations.
  • This heat treatment makes it possible to lower the mechanical resistance of the sheet locally. A portion of lower mechanical strength is thus obtained.
  • This mechanical resistance can be lowered up to 500 MPa, the minimum achievable on annealed austenitic stainless steel.
  • This heat treatment can be performed without this list being exhaustive, by laser, by induction, by electron beam or by welding with the wheel.
  • the thermal cycle includes in particular a temperature rise above the transformation start temperature of martensite to austenite, called reversion temperature of martensite.
  • This temperature is a function of the grade of steel considered but within the scope of the invention, and to cover all the austenitic grades, the reversion temperature is higher than 550 ° C.
  • the durations of the heat treatment, heating, maintenance and cooling are a function of the grade of the sheet, its thickness and the method used: they must be determined beforehand and must allow a minimum decrease of 10% of the volume fraction of martensite and possibly the dislocation density. This minimal decrease makes it possible to overcome the local variations inherent in the cold-working process. A partial melting of the steel on the surface of the sheet and on a thickness not exceeding 0,5e is admissible.
  • the heat treated area is quenched by self-cooling, the heat being transmitted to the surrounding areas. This phenomenon eliminates the control of quenching parameters for obtaining a sheet according to the invention.
  • a stainless steel sheet according to the invention can be shaped according to the usual techniques well known to those skilled in the art, among which may be mentioned as examples folding, profiling, stamping. During this shaping, the portion of lower mechanical strength, which encompasses the deformed zone undergoes hardening. A partial transformation of the austenite into martensite and possibly a hardening of the austenite by densification of the dislocation network make it possible to recover at least partially the initial microstructure of this portion of the sheet.
  • a steel piece, shaped at the level of at least one of the lower strength portions of a steel sheet according to the invention is characterized by the presence, in the vicinity of the neutral fiber, of a zone having a martensite rate lower than that of the sheet.
  • the detection of this zone can be made by measurement of residual stresses or by measurement of the martensite fraction.
  • neutral fiber is meant the set of points which, in case of application of a global deformation, do not undergo local deformation.
  • the local portions of lesser mechanical strength may not be shaped and serve as preferential zones of deformation during dynamic loading, typically at deformation rate of between 1 and 1000s -1 as the crash.
  • the measurement of the martensite ratio is performed by a local measurement of the magnetic induction - using a ferritescope. This measurement gives an average percentage by volume of martensite on the thickness of the sheet.
  • This indirect measurement assumes the use of a corrective factor depending on the grade of steel considered. In the case of 1.4318 (301LN) or 1.4310 (301) stainless steel, the corrective factor is 1.7.
  • a direct measurement by sigmametry (saturation magnetic induction) is also possible, although more restrictive to implement.
  • a sheet 1 of stainless steel according to the invention is treated locally so as to obtain four linear portions 3 of lower mechanical strength.
  • the sheet 1 described above is folded at the level of the portions 3 of least mechanical strength so as to obtain the profiled steel piece 2.
  • a sheet 11 of stainless steel according to the invention is treated locally so as to obtain linear portions 13 of lesser mechanical strength.
  • the sheet 11 described above is folded at four portions 13 of least mechanical strength so as to obtain the profiled steel piece 12.
  • the portions 13 of least strength unformatted have a provision guiding the deformation of the workpiece profiled steel 12 during a crash-type dynamic solicitation.
  • a sheet 21 of stainless steel according to the invention is treated locally so as to obtain a portion 23 of lower mechanical strength.
  • the sheet 21 described above is stamped at the portion 23 of least mechanical strength so as to obtain the piece of steel 22.
  • a stainless steel sheet 31 according to the invention treated locally so as to obtain portions 33 of lesser mechanical strength is profiled by means of a profiling line 34 so as to obtain a piece of profiled steel 32.
  • a steel coil 46 is unwound and undergoes a local heat treatment by means of a laser 45 so as to obtain a sheet 41 of stainless steel according to the invention having four linear portions 43 of lesser mechanical strength.
  • a stainless steel sheet 51 according to the invention undergoes a local heat treatment by means of a laser 55 so as to obtain four linear portions 53 of lesser mechanical strength.
  • a hardened stainless steel 1.4318 (301LN) is used such that its mechanical strength Rm (conventional stress maximum tensile strength) of at least 1000 MPa (state C1000 of the manufacturing range 2H according to EN 10088/2).
  • Rm conventional stress maximum tensile strength
  • the thickness of the sheet is 0.8 mm and the metal contains about 45% by volume of martensite and 55% by volume of austenite.
  • a localized heat treatment is carried out using a CO 2 type laser of 4kW.
  • the power in this case is 20%
  • the displacement of the source is 0.85m / min (1m / min also tested)
  • the focal point is located 25mm above the upper surface of the sheet.
  • the laser treatment makes it possible to obtain, along the treatment line, an annealed structure in which the percentage of martensite passes to a content of less than 10% and even 1.5% in the center, close to the annealed state of this metal, that is to say before hardening (state 2B).
  • the structure of the treated line comprises an austenitic melted zone limited in width L_zf at 2-4 times the thickness of the sheet and depth P_zf less than 50% of the thickness of the sheet as well as a heat affected zone of a width L_zat between 3 and 6 times the thickness of the sheet.
  • This area underwent an almost total reversion of martensite.
  • the set of two identified areas constitutes the portion of least mechanical strength.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Articles (AREA)
  • Heat Treatment Of Sheet Steel (AREA)
  • Forging (AREA)
  • Bending Of Plates, Rods, And Pipes (AREA)

Description

Domaine de l'inventionField of the invention

La présente invention est relative à la mise en forme de tôles d'aciers inoxydables et plus particulièrement de celles présentant des résistances mécaniques élevées.The present invention relates to the forming of stainless steel sheets and more particularly those having high mechanical strengths.

Contexte de l'inventionContext of the invention

Les tôles d'aciers inoxydables sont largement utilisées dans les secteurs de l'automobile, de la construction et de l'industrie en général, en raison de leur excellente résistance à la corrosion. Dans le cadre de ces applications, ces tôles sont le plus généralement mises en forme de façon à être utilisées, par exemple, sous forme de profilés, de tubes carrés, de poutres de pare-choc, de brancards, de cadres de portes. Ces mises en forme sont le plus souvent réalisées par pliage, profilage et emboutissage.Stainless steel sheets are widely used in the automotive, construction and general industrial sectors because of their excellent resistance to corrosion. In the context of these applications, these sheets are generally shaped to be used, for example, in the form of profiles, square tubes, bumper beams, stretchers, door frames. These shaping are most often performed by folding, profiling and stamping.

L'utilisation, dans le cadre de ces applications, de nuances d'acier inoxydables présentant une résistance mécanique élevée, supérieure à 780MPa, est rendue très difficile par un allongement à la rupture qui diminue rapidement avec l'augmentation de la résistance à la rupture. Ce phénomène est source de nombreux inconvénients :

  • les rayons minimum de pliage sont en général supérieurs à deux fois l'épaisseur de la tôle (et jusqu'à six fois) avec au mieux un angle de pliage ne dépassant pas 120°, ne permettant pas la fabrication de tubes à faibles rayons de courbure
  • Le retour élastique est très marqué et rend difficile un éventuel soudage des profilés
  • L'allongement résiduel limité dans les zones déformées est source de ruptures fragiles lors d'une sollicitation dynamique, typiquement à vitesse de déformation comprise entre 1 et 1000s-1 comme en crash.
The use in these applications of stainless steel grades with a high mechanical strength greater than 780 MPa is made very difficult by an elongation at break which decreases rapidly with the increase of the breaking strength. . This phenomenon causes many disadvantages:
  • the minimum bending radii are generally greater than twice the thickness of the sheet (and up to six times) with at best a bending angle not exceeding 120 °, not allowing the manufacture of tubes with small radii of curvature
  • The springback is very pronounced and makes it difficult to weld sections
  • The limited residual elongation in the deformed zones causes brittle fractures during dynamic loading, typically at a deformation rate of between 1 and 1000s -1, as in crashing.

DE102006059885 divulgue un composant en acier au carbone pour un véhicule à moteur, ayant une surface comportant une variation de structure dans certains secteurs, traitée par laser du façon à former une structure tridimensionnelle améliorant la résistance. DE102006059885 discloses a carbon steel component for a motor vehicle, having a surface with a structural variation in some areas, laser-treated to form a three-dimensional resistance-improving structure.

FR2864108 divulgue une tôle en acier inoxydable présentant une grande résistance mécanique et un bon allongement, et dont les propriétés sont homogènes. FR2864108 discloses a stainless steel sheet having high mechanical strength and good elongation, and whose properties are homogeneous.

Une solution consiste à traiter localement la zone à mettre en forme de façon à en faciliter la déformation. Le brevet US 5,735,163 décrit ainsi un procédé de mise en forme de flans dans lequel une portion locale du flan est durcie avant mise en forme. Ce durcissement est généré par un apport énergétique de forte densité. L'élévation de température qui en résulte entraine la transformation de la microstructure locale en martensite ou en bainite, ce qui augmente localement la résistance mécanique. Dans le cas d'un emboutissage, la formation de lignes durcies parallèlement à la direction de la déformation permet d'éviter la rupture de nuances peu emboutissables. Dans le cas d'un pliage, la transformation structurale liée à la formation de martensite ou de bainite sur le coté extérieur du flan à plier génère une contrainte compressive locale. Lors du pliage, cette contrainte annule partiellement la contrainte en extension générée par le pliage, limitant ainsi le retour élastique.One solution is to locally treat the area to be shaped to facilitate deformation. The patent US5,735,163 thus describes a blank shaping process in which a local portion of the blank is cured before shaping. This hardening is generated by a high density energy supply. The resulting rise in temperature results in the transformation of the local microstructure into martensite or bainite, which locally increases the mechanical strength. In the case of a stamping, the formation of hardened lines parallel to the direction of the deformation makes it possible to avoid the breaking of indefinable shades. In the case of a bend, the structural transformation related to the formation of martensite or bainite on the outer side of the blank to be bent generates a local compressive stress. During folding, this constraint partially cancels the extension stress generated by folding, thus limiting the springback.

De par la réduction du retour élastique, ce procédé ne résout qu'un seul des problèmes évoqués ci-dessus. De plus, de par le durcissement local qu'il génère, ce procédé ne peut s'appliquer aux aciers présentant une résistance mécanique élevée, déjà suffisamment difficiles à mettre en oeuvre. Enfin, ce procédé suppose l'utilisation d'aciers aptes à subir une transformation de phase martensitique ou bainitique lors d'un recuit suivi d'une trempe, ce qui, de fait, limite son utilisation à des aciers au carbone-manganèse.By reducing the springback, this process solves only one of the problems mentioned above. In addition, due to the local hardening that it generates, this process can not be applied to steels having a high mechanical strength, already sufficiently difficult to implement. Finally, this process assumes the use of steels capable of undergoing a martensitic or bainitic phase transformation during annealing followed by quenching, which in fact limits its use to carbon-manganese steels.

Résumé de l'inventionSummary of the invention

La présente invention a pour but de faciliter la mise en forme de tôles d'aciers inoxydables présentant une résistance mécanique élevée. Elle a été conçue et réalisée pour surmonter les défauts présentés précédemment et pour obtenir d'autres avantages.The present invention aims to facilitate the forming of stainless steel sheets having a high mechanical strength. It has been designed and realized to overcome the defects presented previously and to obtain other advantages.

A cet effet, l'invention a pour premier objet un tôle d'acier inoxydable contenant un minimum de 10,5% en poids de Cr et un maximum de 1,2% en poids de C, dont la microstructure est martensitique ou austéno-martensitique et comprend au minimum 2% en volume de martensite. Cette tôle est essentiellement caractérisée en ce qu'elle comprend au moins une portion locale de moindre résistance mécanique, présentant un taux de martensite inférieur d'au moins 10% à celui du reste de ladite tôle ; ladite portion locale étant au moins partiellement d'épaisseur égale à celle de ladite tôle.For this purpose, the invention firstly relates to a stainless steel sheet containing a minimum of 10.5% by weight of Cr and a maximum of 1.2% by weight of C, the microstructure of which is martensitic or austenitic. martensitic and comprises at least 2% by volume of martensite. This sheet is essentially characterized in that it comprises at least one local portion of lower mechanical strength, having a martensite rate at least 10% lower than that of the remainder of said sheet; said local portion being at least partially of a thickness equal to that of said sheet.

La tôle d'acier selon l'invention, d'épaisseur e, peut également comprendre les caractéristiques optionnelles suivantes, prises isolément ou en combinaison :

  • La portion locale de moindre résistance mécanique présente une largeur comprise entre e et 25e en surface de ladite tôle.
  • La résistance mécanique à la rupture de la tôle d'acier est supérieure ou égale à 850MPa en dehors de ladite portion locale.
  • La portion locale de moindre résistance mécanique est obtenue :
    • ∘ soit par traitement thermique local d'une tôle d'acier inoxydable martensitique ou austéno-martensitique de résistance mécanique homogène.
    • ∘ Soit par écrouissage différentiel d'une tôle d'acier inoxydable austénitique ou austéno-martensitique de résistance mécanique homogène.
  • La portion locale de moindre résistance mécanique présente un taux de martensite au moins deux fois inférieur à celui du reste de la tôle et préférentiellement au moins quatre fois inférieur à celui du reste de la tôle.
The steel sheet according to the invention, of thickness e, can also comprise the following optional characteristics, taken separately or in combination:
  • The local portion of least mechanical strength has a width between e and 25e at the surface of said sheet.
  • The mechanical strength at break of the steel sheet is greater than or equal to 850 MPa outside said local portion.
  • The local portion of least mechanical strength is obtained:
    • ∘ either by local heat treatment of a martensitic or austeno-martensitic stainless steel sheet of homogeneous mechanical strength.
    • ∘ By differential hardening of austenitic or austeno-martensitic stainless steel sheet of homogeneous mechanical strength.
  • The local portion of least mechanical strength has a martensite rate at least two times lower than the rest of the sheet and preferably at least four times lower than the rest of the sheet.

On comprendra donc que la solution au problème technique posé consiste à traiter localement des zones de la tôle de façon à en abaisser la résistance mécanique et d'ainsi en faciliter la déformation.It will therefore be understood that the solution to the technical problem is to locally treat areas of the sheet so as to lower the mechanical strength and thereby facilitate the deformation.

Un second objet de l'invention est constitué par un procédé de fabrication d'une tôle d'acier selon l'invention, comprenant essentiellement les étapes selon lesquelles :

  • On approvisionne une tôle d'acier austénitique, martensitique ou austéno-martensitique, ledit acier étant un acier inoxydable contenant un minimum de 10,5% en poids de Cr et un maximum de 1,2% en poids de C
  • On écrouit éventuellement tout ou partie de ladite tôle
  • On traite au moins une portion locale de ladite tôle de façon à obtenir une portion locale de moindre résistance mécanique, présentant un taux de martensite inférieur d'au moins 10% à celui du reste de ladite tôle ; ladite portion locale étant au moins partiellement d'épaisseur égale à celle de ladite tôle d'acier.
A second subject of the invention consists of a method of manufacturing a steel sheet according to the invention, essentially comprising the steps according to which:
  • Austenitic, martensitic or austeno-martensitic steel sheet is supplied, said steel being a stainless steel containing a minimum of 10.5% by weight of Cr and a maximum of 1.2% by weight of C
  • Any or all of said sheet
  • At least one local portion of said sheet is treated so as to obtain a local portion of least mechanical strength, having a martensite at least 10% lower than the remainder of said sheet; said local portion being at least partially of a thickness equal to that of said steel sheet.

Le procédé selon l'invention peut également comprendre la caractéristique optionnelle suivante :

  • la portion locale de moindre résistance mécanique est obtenue :
    • ∘ soit par traitement thermique local d'une tôle d'acier martensitique ou austéno-martensitique de résistance mécanique homogène, le traitement thermique résultant d'une élévation thermique par laser, par induction, par faisceau d'électrons ou par soudage à la molette.
    • ∘ Soit par écrouissage différentiel d'une tôle d'acier austénitique ou austéno-martensitique de résistance mécanique homogène.
The method according to the invention may also include the following optional feature:
  • the local portion of least mechanical strength is obtained:
    • ∘ either by local heat treatment of a martensitic or austeno-martensitic steel sheet of homogeneous mechanical strength, the heat treatment resulting from a thermal rise by laser, induction, electron beam or by knurled welding.
    • ∘ By differential work hardening of austenitic or austeno-martensitic steel sheet of homogeneous mechanical strength.

Un troisième objet de l'invention est constitué par une pièce d'acier pouvant être obtenue par déformation d'une tôle d'acier selon l'invention ou d'une tôle obtenue par le procédé selon l'invention, ladite déformation ayant lieu dans l'une au moins desdites portions locales de moindre résistance mécanique.A third object of the invention consists of a steel part obtainable by deformation of a steel sheet according to the invention or of a sheet obtained by the method according to the invention, said deformation taking place in at least one of said local portions of lower mechanical strength.

La pièce selon l'invention peut également comprendre les caractéristiques optionnelles suivantes :

  • Elle peut être obtenue par le pliage, le profilage ou l'emboutissage de l'une au moins desdites portions locales de moindre résistance mécanique.
  • Elle peut être obtenue par découpe d'une tôle d'acier selon l'invention ou d'une tôle obtenue par le procédé selon l'invention.
  • Elle peut être utilisée pour la fabrication de structures métalliques résistant à des sollicitations dynamiques.
The part according to the invention may also include the following optional features:
  • It can be obtained by folding, profiling or stamping at least one of said local portions of lower mechanical strength.
  • It can be obtained by cutting a steel sheet according to the invention or a sheet obtained by the method according to the invention.
  • It can be used for the manufacture of metal structures resistant to dynamic stresses.

D'autres caractéristiques et avantages de l'invention apparaitront à la lecture de la description qui va suivre.Other features and advantages of the invention will appear on reading the description which follows.

Les termes 2H, C700 à C1300 (dits état écroui), 1E, 1D, 2B, 2D, 2R, 2E (dits état recuit), se rapportent notamment aux normes qui définissent les gammes de fabrication et les conditions techniques de livraison des aciers concernés (NF EN 10088-1 et -2 pour les aciers inoxydables). C1500 désignera une gamme de fabrication d'un écroui 2H garantissant une résistance mécanique supérieure à 1500MPa.The terms 2H, C700 to C1300 (called hardened state), 1E, 1D, 2B, 2D, 2R, 2E (so-called annealed condition) refer in particular to the standards which define the production ranges and the technical delivery conditions of the steels concerned. (NF EN 10088-1 and -2 for stainless steels). C1500 will designate a range of manufacture of a 2H hardened nut ensuring a mechanical strength higher than 1500MPa.

Les tôles d'aciers inoxydables considérées par la présente invention sont caractérisées par leur résistance mécanique. Celle-ci est contrôlée d'une part par les éléments d'addition, mais également par les traitements thermiques et les traitements mécaniques que la tôle peut subir.The stainless steel sheets considered by the present invention are characterized by their mechanical strength. This is controlled on the one hand by the addition elements, but also by the heat treatments and the mechanical treatments that the sheet can undergo.

Les éléments d'addition définissent la nuance de base de la tôle considérée et donc sa résistance mécanique intrinsèque. Dans le cadre de la présente invention, on entend par acier inoxydable à structure austénitique une tôle comprenant en pourcentage pondéral :

  • 10,5 ≤ Cr ≤20
  • 0,005 ≤ C ≤ 1,2
  • 0,005 ≤ N ≤2
  • 0,6 ≤ Ni ≤ 15
  • 0,1 ≤ Mn ≤ 15
  • 0,1 ≤ Mo ≤ 5
  • 0,1 ≤ Cu ≤ 3
  • 0,05 ≤ Si ≤ 3
  • 0,0001 ≤ Ti ≤ 1
  • 0,0001 ≤ Nb ≤ 1
The addition elements define the basic shade of the sheet in question and therefore its intrinsic mechanical strength. In the context of the present invention, stainless steel with an austenitic structure is understood to mean a sheet comprising in weight percentage:
  • 10.5 ≤ Cr ≤20
  • 0.005 ≤ C ≤ 1.2
  • 0.005 ≤ N ≤2
  • 0.6 ≤ Ni ≤ 15
  • 0.1 ≤ Mn ≤ 15
  • 0.1 ≤ Mo ≤ 5
  • 0.1 ≤ Cu ≤ 3
  • 0.05 ≤ Si ≤ 3
  • 0.0001 ≤ Ti ≤ 1
  • 0.0001 ≤ Nb ≤ 1

Le reste de la composition étant constitué de fer et d'impuretés inévitables dues à l'élaboration.The rest of the composition being made of iron and unavoidable impurities due to the elaboration.

Etant entendu en outre que les teneurs respectent les relations suivantes :

  • 1,48<Créq/Niéq <2,2 avec: Cr éq = % Cr + 1,37 %Mo + 1,5 %Si + 2 %Nb + 3 %Ti
    Figure imgb0001
     Ni éq = % Ni + 0 , 31%Mn + 22%C + 14 , 2%N + %Cu
    Figure imgb0002
  • α'(30/20) >0, α' étant défini par la relation suivante:
    a'(30/20) = 374,05 - 3,73 %Cr - 23,03 %Ni - 503,11 %C - 161,70 %N - 21,55 %Mn
It is further understood that the contents respect the following relationships:
  • 1.48 <Cr eq / Ni eq <2.2 with: Cr eq = % Cr + 1.37 % Mo + 1.5 %Yes + 2 % Nb + 3 % Ti
    Figure imgb0001
     Or eq = % Or + 0 , 31% Mn + 22% C + 14 , 2% N + % Cu
    Figure imgb0002
  • α '(30/20)> 0, α' being defined by the following relation:
    a '(30/20) = 374.05 - 3.73% Cr - 23.03% Ni - 503.11% C - 161.70% N - 21.55% Mn

Cette composition caractérise un acier inoxydable austénitique qui solidifie en ferrite primaire et qui contient une quantité non nulle de martensite d'écrouissage après déformation. Bien que majoritairement constituées d'austénite, les nuances austénitiques conventionnelles contiennent des traces de ferrite résiduelle provenant de la solidification ainsi que des traces de martensite résultant des opérations de laminage.This composition features an austenitic stainless steel that solidifies to primary ferrite and contains a non-zero amount of work hardening martensite after deformation. Although predominantly austenite, conventional austenitic grades contain traces of residual ferrite from the solidification as well as traces of martensite resulting from rolling operations.

Le traitement thermique et le traitement mécanique, seuls ou combinés, permettent, quant à eux, de modifier cette résistance mécanique dans une certaine proportion.Heat treatment and mechanical treatment, alone or in combination, allow, in turn, to modify this mechanical strength in a certain proportion.

La présente invention considère notamment deux variantes possibles :

  • un traitement mécanique homogène sur l'intégralité de la tôle suivi d'un traitement thermique local
  • un traitement mécanique inhomogène sur l'intégralité de la tôle
The present invention considers in particular two possible variants:
  • a homogeneous mechanical treatment on the whole sheet followed by a local heat treatment
  • an inhomogeneous mechanical treatment on the entire sheet

Dans les deux cas, la modification des caractéristiques mécaniques est rendue possible par la capacité de la tôle considérée à subir d'une part des transformations de phase et d'autre part des variations de densité de dislocations.In both cases, the modification of the mechanical characteristics is made possible by the capacity of the sheet considered to undergo on the one hand phase transformations and on the other hand dislocation density variations.

Dans le cas de la première variante envisagée, un écrouissage homogène (gamme de fabrication 2H : C700 à C1500) sur l'intégralité de la tôle entraine une transformation partielle de l'austénite en martensite et éventuellement un durcissement de l'austénite par la densification du réseau de dislocations. Cet écrouissage permet d'atteindre des résistances mécaniques bien supérieures à 780MPa, valeur maximale atteignable sur un acier inoxydable recuit de type 1D, 1E, 2B, 2D, 2E, 2R. L'acier ainsi écroui est de structure austéno-martensitique c'est-à-dire constitué à température ordinaire d'un mélange d'austénite et de martensite, la fraction volumique de martensite étant au minimum de 2%. Dans une deuxième étape, un traitement thermique localisé aux zones à déformer entraine une réversion partielle de la martensite en austénite et éventuellement un adoucissement de l'austénite par la diminution du nombre de dislocations. Ce traitement thermique permet d'abaisser localement la résistance mécanique de la tôle. Une portion de moindre résistance mécanique est ainsi obtenue. Cette résistance mécanique peut être abaissée jusqu'à 500MPa, valeur minimale atteignable sur un acier inoxydable austénitique recuit. Ce traitement thermique peut être réalisé, sans que cette liste soit exhaustive, par laser, par induction, par faisceau d'électrons ou par soudage à la molette. Quelle que soit la technique utilisée, le cycle thermique comprend notamment une élévation de température au-dessus de la température de début de transformation de la martensite en austénite, appelée température de réversion de la martensite. Cette température est fonction de la nuance d'acier considérée mais dans le cadre de l'invention, et pour couvrir l'ensemble des nuances austénitiques, la température de réversion est prise supérieure à 550°C. Les durées du traitement thermique, chauffage, maintien et refroidissement sont fonction de la nuance de la tôle, de son épaisseur et du procédé utilisé: elles doivent être déterminées préalablement et doivent permettre une diminution minimale de 10% de la fraction volumique de martensite et éventuellement de la densité de dislocation. Cette diminution minimale permet de s'affranchir des variations locales inhérentes au procédé d'écrouissage. Une fusion partielle de l'acier à la surface de la tôle et sur une épaisseur n'excédant pas 0,5e est admissible. La zone traitée thermiquement est trempée par auto-refroidissement, la chaleur se transmettant aux zones avoisinantes. Ce phénomène supprime le contrôle des paramètres de trempe pour l'obtention d'une tôle selon l'invention.In the case of the first variant envisaged, a homogeneous work hardening (production range 2H: C700 to C1500) on the entire sheet results in a partial transformation of the austenite to martensite and possibly a hardening of the austenite by densification. of the dislocation network. This hardening achieves mechanical strengths well above 780 MPa, maximum value achievable on a stainless steel annealed type 1D, 1E, 2B, 2D, 2E, 2R. The steel thus worked is of austeno-martensitic structure that is to say formed at ordinary temperature of a mixture of austenite and martensite, the volume fraction of martensite being at least 2%. In a second step, a localized heat treatment to the zones to be deformed causes a partial reversion of the martensite to austenite and possibly a softening of the austenite by reducing the number of dislocations. This heat treatment makes it possible to lower the mechanical resistance of the sheet locally. A portion of lower mechanical strength is thus obtained. This mechanical resistance can be lowered up to 500 MPa, the minimum achievable on annealed austenitic stainless steel. This heat treatment can be performed without this list being exhaustive, by laser, by induction, by electron beam or by welding with the wheel. Whatever the technique used, the thermal cycle includes in particular a temperature rise above the transformation start temperature of martensite to austenite, called reversion temperature of martensite. This temperature is a function of the grade of steel considered but within the scope of the invention, and to cover all the austenitic grades, the reversion temperature is higher than 550 ° C. The durations of the heat treatment, heating, maintenance and cooling are a function of the grade of the sheet, its thickness and the method used: they must be determined beforehand and must allow a minimum decrease of 10% of the volume fraction of martensite and possibly the dislocation density. This minimal decrease makes it possible to overcome the local variations inherent in the cold-working process. A partial melting of the steel on the surface of the sheet and on a thickness not exceeding 0,5e is admissible. The heat treated area is quenched by self-cooling, the heat being transmitted to the surrounding areas. This phenomenon eliminates the control of quenching parameters for obtaining a sheet according to the invention.

Dans le cas de la deuxième variante envisagée (traitement mécanique inhomogène), un écrouissage est réalisé à l'aide de cylindres de laminage structurés. L'écrouissage des aciers inoxydables est habituellement réalisé à l'aide de rouleaux lisses. Dans le cas présent, ces cylindres sont gravés ou cannelés de telle sorte que des portions de la tôle écrouie soient épargnées par cet écrouissage et ainsi conservent leur structure austénitique moins écrouie. On désigne cet écrouissage spécifique sous le nom d'écrouissage différentiel. Des portions de moindre résistance mécanique sont ainsi obtenues.In the case of the second variant envisaged (inhomogeneous mechanical treatment), a cold-working is carried out using structured rolling rolls. The hardening of stainless steels is usually achieved using smooth rollers. In this case, these cylinders are etched or grooved so that portions of the work-hardened sheet are spared by this work hardening and thus retain their austenitic structure less work hardened. This specific hardening is referred to as differential hardening. Portions of lesser mechanical strength are thus obtained.

Quelque soit la variante utilisée, les conditions opératoires sont contrôlées de façon à respecter les conditions suivantes :

  • la portion de moindre résistance mécanique est au moins partiellement d'épaisseur égale à l'épaisseur e de la tôle,
  • la portion de moindre résistance mécanique inclut la zone qui pourrait être déformée lors d'une étape ultérieure de mise en forme. A cet effet, on cherchera à inclure les zones mises en forme pour lesquelles les rayons de pliage sont compris entre 2 et 6 fois l'épaisseur de la tôle (cas de la mise en forme des aciers inoxydables présentant les plus hautes résistances mécaniques, sans recours à la présente invention). Pour cette raison, la portion de moindre résistance mécanique est de préférence d'une largeur comprise entre e et 25e,
  • Cette portion peut présenter des formes variées, être linéaire, curviligne, avoir un contour fermé ou encore peut présenter des intersections avec d'autres portions de moindre résistance mécanique.
  • Cette portion présente un taux de martensite inférieur d'au moins 10% à celui du reste de la tôle.
Whatever the variant used, the operating conditions are controlled so as to comply with the following conditions:
  • the portion of least mechanical strength is at least partially of thickness equal to the thickness e of the sheet,
  • the portion of least mechanical strength includes the area that could be deformed in a subsequent shaping step. For this purpose, it will be sought to include the shaped areas for which the bending radii are between 2 and 6 times the thickness of the sheet (in the case of the shaping of stainless steels having the highest mechanical strengths, without use of the present invention). For this reason, the portion of least mechanical strength is preferably of a width between e and 25e,
  • This portion may have various shapes, be linear, curvilinear, have a closed contour or may have intersections with other portions of lower strength.
  • This portion has a martensite rate at least 10% lower than the rest of the sheet.

La présence sur la tôle d'acier inoxydable de portions de moindre résistance mécanique, obtenues par l'une ou l'autre des variantes décrites ci-dessus, permet :

  • un pliage sévère de cette tôle jusqu'à des angles de 180° et jusqu'à des rayons minimum de pliage valant 0,5 fois l'épaisseur de la tôle
  • une mise en forme facilitée puisque prédéterminée, évitant un glissement de la tôle ou une mauvaise localisation de la zone déformée
  • une forte diminution du retour élastique lors du profilage, ce retour étant équivalent à celui que l'on aurait avec un acier inoxydable recuit de type 2B, 2D, 2R, 2E, 1E, 1D
  • Une réduction de l'effort de pliage, cet effort étant équivalent à celui que l'on aurait avec un acier inoxydable recuit de type 2B, 2D, 2R, 2E, 1E, 1D, soit une réduction de 25 à 50% en fonction de la nuance d'acier inoxydable considérée.
The presence on the stainless steel sheet portions of lower mechanical strength, obtained by one or other of the variants described above, allows:
  • a severe folding of this sheet to angles of 180 ° and up to minimum bending radii of 0.5 times the thickness of the sheet
  • a facilitated formatting since predetermined, avoiding slippage of the sheet or a bad location of the deformed area
  • a strong decrease of the elastic return during the profiling, this return being equivalent to that which one would have with a stainless steel annealing of type 2B, 2D, 2R, 2E, 1E, 1D
  • A reduction of the bending force, this effort being equivalent to that which one would have with a stainless steel annealing of type 2B, 2D, 2R, 2E, 1E, 1D, a reduction of 25 to 50% according to the grade of stainless steel considered.

Dans le cas d'une tôle d'acier inoxydable selon l'invention ayant subi un traitement thermique local, on notera également l'avantage présenté par la légère coloration de la tôle générée par ce traitement thermique : elle permet de localiser sans difficulté la zone à déformer. Dans le cas d'une tôle d'acier inoxydable selon l'invention ayant subi un écrouissage différentiel, la localisation de la zone à déformer est rendue possible par un aspect moins brillant et une rugosité différente de la portion locale.In the case of a stainless steel sheet according to the invention having undergone a local heat treatment, it will also be noted the advantage presented by the slight coloration of the sheet generated by this heat treatment: it makes it possible to locate the zone without difficulty. to deform. In the case of a stainless steel sheet according to the invention having undergone differential hardening, the location of the zone to be deformed is made possible by a less glossy appearance and roughness different from the local portion.

Une tôle d'acier inoxydable selon - l'invention peut être mise en forme selon les techniques habituelles bien connues de l'homme du métier, parmi lesquelles on pourra citer à titre d'exemples le pliage, le profilage, l'emboutissage. Lors de cette mise en forme, la portion de moindre résistance mécanique, qui englobe la zone déformée, subit un écrouissage. Une transformation partielle de l'austénite en martensite et éventuellement un durcissement de l'austénite par la densification du réseau de dislocations permettent de retrouver au moins partiellement la microstructure initiale de cette portion de la tôle. Dans les cas de modes de déformation pour lesquels il existe une fibre neutre, une pièce d'acier, mise en forme au niveau de l'une au moins des portions de moindre résistance mécanique d'une tôle d'acier selon l'invention, se caractérise par la présence, au voisinage de la fibre neutre, d'une zone présentant un taux de martensite inférieur à celui de la tôle. La détection de cette zone peut être faite par mesure de contraintes résiduelles ou par mesure de la fraction de martensite. On entend par fibre neutre l'ensemble des points qui, en cas d'application d'une déformation globale, ne subissent pas de déformation locale.A stainless steel sheet according to the invention can be shaped according to the usual techniques well known to those skilled in the art, among which may be mentioned as examples folding, profiling, stamping. During this shaping, the portion of lower mechanical strength, which encompasses the deformed zone undergoes hardening. A partial transformation of the austenite into martensite and possibly a hardening of the austenite by densification of the dislocation network make it possible to recover at least partially the initial microstructure of this portion of the sheet. In the case of deformation modes for which there is a neutral fiber, a steel piece, shaped at the level of at least one of the lower strength portions of a steel sheet according to the invention, is characterized by the presence, in the vicinity of the neutral fiber, of a zone having a martensite rate lower than that of the sheet. The detection of this zone can be made by measurement of residual stresses or by measurement of the martensite fraction. By neutral fiber is meant the set of points which, in case of application of a global deformation, do not undergo local deformation.

Une pièce d'acier, mise en forme au niveau de l'une au moins des portions de moindre résistance mécanique d'une tôle d'acier selon l'invention, permet :

  • Une amélioration de la tenue mécanique statique ou dynamique, l'allongement résiduel plus important dans les zones mises en forme évitant des ruptures fragiles en comportement dynamique (crash).
  • Un soudage entre deux bords de la tôle facilité par la minimisation du retour élastique
A piece of steel, shaped at the level of at least one of the portions of least mechanical strength of a steel sheet according to the invention, allows:
  • An improvement of the static or dynamic mechanical resistance, the greater residual elongation in the shaped zones avoiding fragile fractures in dynamic behavior (crash).
  • Welding between two edges of the sheet facilitated by the minimization of the springback

Par ailleurs, les portions locales de moindre résistance mécanique peuvent ne pas être mises en forme et servir de zones préférentielles de déformation lors d'une sollicitation dynamique, typiquement à vitesse de déformation comprise entre 1 et 1000s-1 comme le crash.Furthermore, the local portions of lesser mechanical strength may not be shaped and serve as preferential zones of deformation during dynamic loading, typically at deformation rate of between 1 and 1000s -1 as the crash.

Afin d'illustrer l'invention, des essais ont été réalisés et vont être décrits à titre d'exemples non limitatifs, notamment en référence aux figures 1 à 7 qui représentent :

  • Figure 1A : Exemple de microstructure d'une tôle selon l'invention avant traitement thermique localisé. Coupe métallographique avec attaque électrolytique.
  • Figure 1B : Grossissement de la Figure 1A avec en sombre la martensite et en clair l'austénite.
  • Figure 1C : Exemple de microstructure d'une tôle selon l'invention après traitement thermique localisé. Coupe métallographique avec attaque électrolytique.
  • Figure 1D : Grossissement de la Figure 1C. Détail de la zone non traitée.
  • Figure 1E : Grossissement de la Figure 1C. Détail de la portion locale de moindre résistance mécanique.
  • Figure 2 : Variation, en moyenne dans l'épaisseur de la tôle, du taux de martensite au voisinage de la portion de moindre résistance mécanique (A) et structure de cette portion (B).
  • Figure 3A : Tôle selon l'invention présentant des zones de moindre résistance mécanique.
  • Figure 3B : pièce après pliage de la tôle présentée en figure 3A.
  • Figure 4A : Tôle selon l'invention présentant des zones de moindre résistance mécanique.
  • Figure 4B : pièce après pliage de la tôle présentée en Figure 4A.
  • Figure 5A : Tôle selon l'invention présentant des zones de moindre résistance mécanique.
  • Figure 5B : pièce après emboutissage de la tôle présentée en Figure 5A.
  • Figure 6 : exemple de profilage d'une tôle selon l'invention au moyen d'une ligne de profilage et pièce obtenue.
  • Figure 7A : premier mode de réalisation d'une tôle selon l'invention
  • Figure 7B : autre mode de réalisation d'une tôle selon l'invention
In order to illustrate the invention, tests have been carried out and will be described by way of non-limiting examples, in particular with reference to Figures 1 to 7 that represent :
  • Figure 1A : Example of microstructure of a sheet according to the invention before localized heat treatment. Metallographic section with electrolytic attack.
  • Figure 1B : Magnification of Figure 1A with dark martensite and clear austenite.
  • Figure 1C : Example of microstructure of a sheet according to the invention after localized heat treatment. Metallographic section with electrolytic attack.
  • Figure 1D : Magnification of Figure 1C . Detail of the untreated area.
  • Figure 1E : Magnification of Figure 1C . Detail of the local portion of least mechanical resistance.
  • Figure 2 : Variation, on average in the thickness of the sheet, of the martensite rate in the neighborhood of the portion of least mechanical strength (A) and structure of this portion (B).
  • Figure 3A : Sheet according to the invention having areas of lower mechanical strength.
  • Figure 3B : piece after folding of the sheet presented in figure 3A .
  • Figure 4A : Sheet according to the invention having areas of lower mechanical strength.
  • Figure 4B : piece after folding of the sheet presented in Figure 4A .
  • Figure 5A : Sheet according to the invention having areas of lower mechanical strength.
  • Figure 5B : piece after stamping the sheet presented in Figure 5A .
  • Figure 6 example of profiling a sheet according to the invention by means of a profiling line and part obtained.
  • Figure 7A first embodiment of a sheet according to the invention
  • Figure 7B another embodiment of a sheet according to the invention

La mesure du taux de martensite est effectuée par une mesure locale de l'induction magnétique - à l'aide d'un ferritescope. Cette mesure donne un pourcentage moyen en volume de martensite sur l'épaisseur de la tôle. Cette mesure indirecte suppose l'utilisation d'un facteur correctif fonction de la nuance d'acier considérée. Dans le cas d'un acier inoxydable 1.4318 (301LN) ou 1.4310(301), le facteur correctif est de 1,7. Une mesure directe par sigmamétrie (induction magnétique à saturation) est également envisageable, bien que plus contraignante à mettre en oeuvre.The measurement of the martensite ratio is performed by a local measurement of the magnetic induction - using a ferritescope. This measurement gives an average percentage by volume of martensite on the thickness of the sheet. This indirect measurement assumes the use of a corrective factor depending on the grade of steel considered. In the case of 1.4318 (301LN) or 1.4310 (301) stainless steel, the corrective factor is 1.7. A direct measurement by sigmametry (saturation magnetic induction) is also possible, although more restrictive to implement.

ExemplesExamples

En référence à la figure 3A, une tôle 1 d'acier inoxydable selon l'invention est traitée localement de façon à obtenir quatre portions linéaires 3 de moindre résistance mécanique. En référence à la figure 3B, la tôle 1 décrite précédemment est pliée au niveau des portions 3 de moindre résistance mécanique de façon à obtenir la pièce d'acier profilé 2.With reference to the figure 3A , a sheet 1 of stainless steel according to the invention is treated locally so as to obtain four linear portions 3 of lower mechanical strength. With reference to the figure 3B , the sheet 1 described above is folded at the level of the portions 3 of least mechanical strength so as to obtain the profiled steel piece 2.

En référence à la figure 4A, une tôle 11 d'acier inoxydable selon l'invention est traitée localement de façon à obtenir des portions linéaires 13 de moindre résistance mécanique. En référence à la figure 4B, la tôle 11 décrite précédemment est pliée au niveau de quatre portions 13 de moindre résistance mécanique de façon à obtenir la pièce d'acier profilé 12. Les portions 13 de moindre résistance mécanique non mises en forme présentent une disposition guidant la déformation de la pièce d'acier profilé 12 lors d'une sollicitation dynamique de type crash.With reference to the Figure 4A a sheet 11 of stainless steel according to the invention is treated locally so as to obtain linear portions 13 of lesser mechanical strength. With reference to the Figure 4B , the sheet 11 described above is folded at four portions 13 of least mechanical strength so as to obtain the profiled steel piece 12. The portions 13 of least strength unformatted have a provision guiding the deformation of the workpiece profiled steel 12 during a crash-type dynamic solicitation.

En référence à la figure 5A, une tôle 21 d'acier inoxydable selon l'invention est traitée localement de façon à obtenir une portion 23 de moindre résistance mécanique. En référence à la figure 5B, la tôle 21 décrite précédemment est emboutie au niveau de la portion 23 de moindre résistance mécanique de façon à obtenir la pièce d'acier 22.With reference to the Figure 5A a sheet 21 of stainless steel according to the invention is treated locally so as to obtain a portion 23 of lower mechanical strength. With reference to the Figure 5B , the sheet 21 described above is stamped at the portion 23 of least mechanical strength so as to obtain the piece of steel 22.

En référence à la figure 6, une tôle 31 d'acier inoxydable selon l'invention traitée localement de façon à obtenir des portions 33 de moindre résistance mécanique est profilée au moyen d'une ligne de profilage 34 de façon à obtenir une pièce d'acier profilé 32.With reference to the figure 6 , a stainless steel sheet 31 according to the invention treated locally so as to obtain portions 33 of lesser mechanical strength is profiled by means of a profiling line 34 so as to obtain a piece of profiled steel 32.

En référence à la figure 7A, une bobine d'acier 46 est déroulée et subit un traitement thermique local au moyen d'un laser 45 de façon à obtenir une tôle 41 d'acier inoxydable selon l'invention présentant quatre portions linéaires 43 de moindre résistance mécanique.With reference to the Figure 7A a steel coil 46 is unwound and undergoes a local heat treatment by means of a laser 45 so as to obtain a sheet 41 of stainless steel according to the invention having four linear portions 43 of lesser mechanical strength.

En référence à la figure 7B, une tôle 51 d'acier inoxydable selon l'invention subit un traitement thermique local au moyen d'un laser 55 de façon à obtenir quatre portions linéaires 53 de moindre résistance mécanique.With reference to the Figure 7B , a stainless steel sheet 51 according to the invention undergoes a local heat treatment by means of a laser 55 so as to obtain four linear portions 53 of lesser mechanical strength.

Selon un mode préféré de réalisation, on utilise un acier inoxydable 1.4318 (301LN) écroui tel que sa résistance mécanique Rm (contrainte conventionnelle maximale en traction) soit au minimum de 1000MPa (état C1000 de la gamme de fabrication 2H selon la norme EN 10088/2). Dans cet exemple, l'épaisseur de la tôle est de 0,8mm et le métal contient environ 45% en volume de martensite et 55% en volume d'austénite .According to a preferred embodiment, a hardened stainless steel 1.4318 (301LN) is used such that its mechanical strength Rm (conventional stress maximum tensile strength) of at least 1000 MPa (state C1000 of the manufacturing range 2H according to EN 10088/2). In this example, the thickness of the sheet is 0.8 mm and the metal contains about 45% by volume of martensite and 55% by volume of austenite.

Un traitement thermique localisé, selon une ligne, est réalisé à l'aide d'un laser de type CO2 de 4kW. La puissance dans le cas présent est de 20%, le déplacement de la source est de 0,85m/min (1m/min également testé) et le point focal est situé à 25 mm au-dessus de la surface supérieure de la tôle. En référence à la figure 2, le traitement laser permet d'obtenir le long de la ligne de traitement une structure recuite où le pourcentage de martensite passe à une teneur inférieure à 10% et même 1,5% au centre, proche de l'état recuit de ce métal, c'est-à-dire avant écrouissage (état 2B). La structure de la ligne traitée comprend une zone fondue austénitique limitée en largeur L_zf à 2-4 fois l'épaisseur de la tôle et de profondeur P_zf inférieure à 50% de l'épaisseur de la tôle ainsi qu'une zone affectée thermiquement d'une largeur L_zat comprise entre 3 et 6 fois l'épaisseur de la tôle. Cette zone a subi une réversion presque totale de la martensite. L'ensemble des deux zones identifiées constitue la portion de moindre résistance mécanique.A localized heat treatment, according to a line, is carried out using a CO 2 type laser of 4kW. The power in this case is 20%, the displacement of the source is 0.85m / min (1m / min also tested) and the focal point is located 25mm above the upper surface of the sheet. With reference to the figure 2 , the laser treatment makes it possible to obtain, along the treatment line, an annealed structure in which the percentage of martensite passes to a content of less than 10% and even 1.5% in the center, close to the annealed state of this metal, that is to say before hardening (state 2B). The structure of the treated line comprises an austenitic melted zone limited in width L_zf at 2-4 times the thickness of the sheet and depth P_zf less than 50% of the thickness of the sheet as well as a heat affected zone of a width L_zat between 3 and 6 times the thickness of the sheet. This area underwent an almost total reversion of martensite. The set of two identified areas constitutes the portion of least mechanical strength.

Des essais de pliage sont réalisés sur les tôles C1000 ainsi traitées selon l'invention et sur des tôles non traitées. On observe que le pliage de la tôle C1000 traitée selon l'invention est possible jusqu'à des angles de 180° sans difficulté, comme pour la tôle recuite 2B. Le pliage est en revanche difficile à 90° avec la tôle C1000 non traitée avec présence de petites fissures, et impossible à 180° avec rupture parfois complète de l'éprouvette (Tab.1). Tableau1 : Essais de pliage sur une nuance 1.4318 état 2B, écrouie C1000 et écrouie C1000 avec traitement thermique laser Rm (MPa) Angle de pliage Echantillon 90° 180° 2B 780 ○ ○ C1000 1000

Figure imgb0003
● C1000 selon l'invention ○ ○ ○ pliage correct,
Figure imgb0003
présence de fissures, ● rupture de l'échantillon Folding tests are carried out on the C1000 sheets thus treated according to the invention and on untreated sheets. It is observed that the folding of the C1000 sheet treated according to the invention is possible up to 180 ° angles without difficulty, as for the annealed sheet 2B. On the other hand, the bending is difficult at 90 ° with the unprocessed C1000 sheet with small cracks, and impossible at 180 ° with sometimes complete rupture of the test piece (Tab.1). Table 1: Bend tests on grade 1.4318 condition 2B, work hardened C1000 and work hardened C1000 with laser heat treatment Rm (MPa) Angle of folding Sample 90 180 ° 2B 780 ○ ○ C1000 1000
Figure imgb0003
 ● C1000 according to the invention ○ ○ ○ correct folding,
Figure imgb0003
presence of cracks, ● breakage of the sample

Claims (12)

  1. A stainless steel sheet containing a minimum of 10.5% by weight of Cr and a maximum of 1.2% by weight of C, the microstructure of which is martensitic or austeno-martensitic and comprises at least 2% by volume of martensite, characterizing in that it comprises at least one local portion of lesser mechanical resistance, having a martensite content at least 10% lower than that of the remainder of said sheet, said local portion being at least partly with a thickness equal to that of said sheet.
  2. The steel sheet according to claim 1, with a thickness e, the said local portion of which has a width comprised between e and 25e at the surface of said sheet.
  3. The steel sheet according to any of claims 1 or 2, the mechanical resistance at break of which is greater than or equal to 850 MPa outside said local portion.
  4. The steel sheet according to any of claims 1 to 3, said local portion of which with lesser mechanical resistance is obtained:
    - either by local heat treatment of a martensitic or austeno-martensitic stainless steel sheet of homogeneous mechanical resistance
    - or by differential work-hardening of an austenitic or austeno-martensitic stainless steel sheet of homogeneous mechanical resistance.
  5. The steel sheet, according to any of claims 1 to 4, said local portion of which with lesser mechanical resistance has a martensite content at least twice smaller than that of the remainder of the sheet.
  6. The steel sheet according to claim 4, said local portion of which with lesser mechanical resistance has a martensite level at least four times smaller than that of the remainder of the sheet.
  7. A method for manufacturing a steel sheet, according to any of claims 1 to 6, comprising the steps according to which:
    - an austenitic, martensitic or austeno-martensitic steel sheet is supplied, said steel being a stainless steel containing a minimum of 10.5% by weight of Cr and a maximum of 1.2% by weight of C.
    - optionally, all or part of said sheet is work-hardened so that the microstructure comprises at least 2% by volume of martensite.
    - said sheet is treated so as to obtain at least one local portion of lesser mechanical resistance, having a martensite content at least 10% lower than that of the remainder of said sheet; said local portion being at least partly with a thickness equal to that of said steel sheet.
  8. The method according to claim 7, wherein said local portion of lesser mechanical resistance is obtained:
    - either by local heat treatment of a martensitic or austeno-martensitic steel sheet of homogeneous mechanical resistance, the heat treatment resulting from a thermal rise in temperature by laser, by induction, by an electron beam or by seam welding
    - or by differential work-hardening of an austenitic or austeno-martensitic steel sheet of homogeneous mechanical resistance.
  9. A steel part which may be obtained by deformation of a steel sheet according to any of claims 1 to 6 or of a sheet obtained by the method according to any of claims 7 to 8, said deformation occurring in at least one of said local portions of lesser mechanical resistance.
  10. The steel part according to claim 9, obtained by bending, profiling or stamping of at least one of said local portions of lesser mechanical resistance.
  11. A steel part which may be obtained by cutting a steel sheet according to any of claims 1 to 6 or a sheet obtained by the method according to any of claims 7 to 8.
  12. The use of a part according to any of claims 9 to 11 for manufacturing metal structures withstanding dynamic stresses.
EP09740179.8A 2009-09-21 2009-09-21 Stainless steel having local variations in mechanical resistance Not-in-force EP2480693B1 (en)

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WO2011033180A1 (en) 2011-03-24
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