EP0567365B1 - Austenitic stainless steel with high machine ability and improved cold work ability - Google Patents

Austenitic stainless steel with high machine ability and improved cold work ability Download PDF

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EP0567365B1
EP0567365B1 EP93400903A EP93400903A EP0567365B1 EP 0567365 B1 EP0567365 B1 EP 0567365B1 EP 93400903 A EP93400903 A EP 93400903A EP 93400903 A EP93400903 A EP 93400903A EP 0567365 B1 EP0567365 B1 EP 0567365B1
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steel
austenitic stainless
stainless steel
steel according
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EP0567365A1 (en
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Olivier Bletton
Xavier Cholin
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Ugitech SA
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Ugine Savoie SA
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N

Definitions

  • the present invention relates to a weldable austenitic stainless steel, with high machinability and having good cold deformation characteristics.
  • JP-A-160 785 discloses a machinable steel, cold deformable and having in its weight composition, in particular a sulfur content of less than 0.03%, calcium and oxygen contents of between 10 and 300 ppm and between 30 and 300 ppm, a copper content between 0.8 and 5% and a lead content between 0.01 and 0.25%.
  • the improvement in machinability is generated by the introduction into the composition of a variable amount of lead.
  • austenitic stainless steels are difficult to machine, on the one hand, because of their low thermal conductivity and, on the other hand, because of their high work hardening locally inducing zones of high hardness. Poor flow of the heat produced at the tip of a cutting tool leads to rapid deterioration of the tool.
  • This element has the disadvantages of being difficult to dissolve homogeneously in a molten bath and, because of its high density, of having a tendency to accumulate in the bottom of metallurgical vessels.
  • boron can be introduced into the composition of austenitic stainless steel according to JP-A-160 785, as an element capable of counteracting the harmful effects of lead in the field of hot rolling, but the boron increases still the welding difficulties.
  • the introduction of the boron element also brings at least one other drawback, that of reducing the temperature range suitable for hot rolling, which imposes a more restrictive hot rolling process.
  • Austenitic stainless steel according to JP-A-160 785 may also contain titanium.
  • titanium which is generally introduced into stainless steels in order to improve the resistance to intragranular corrosion, disrupts the formation of calcium oxide inclusions and reduces their number.
  • titanium leads to the formation of hard inclusions which reduce the workability by promoting premature wear of the cutting tools.
  • Also known from FR-A-2 542 761 is a process for manufacturing a steel with high machinability.
  • a cause of the difficulty of machining stainless steels lies in the fact that they contain inclusions of hard oxides such as for example alumina or chromite which deteriorate the tools of chopped off.
  • One way of reducing the harmfulness of hard oxide inclusions is to introduce one or more alkaline earth compounds into the steel, in order to replace a good proportion of hard inclusions with calcium-based oxide inclusions, for example. It is also specified, on the one hand, that a certain quantity of sulfur combined with hard inclusions reduces its harmfulness, the sulfur content generally being less than 0.5 ⁇ 10 -4 % and, on the other hand, that another way to reduce the harmfulness of inclusions is to reduce their quantity thanks to good deoxidation and good settling of the bath during the production of steel.
  • the aim sought is the formation, with manganese and in a smaller proportion with chromium, of a manganese sulfide and chromium (Mn, Cr) S which generates in the form of specific inclusions a solid lubrication of the cutting tool during machining operations.
  • sulfur has an unfavorable effect on the resistance to corrosion and that despite this a chosen orientation is the introduction into a resulfurized steel of oxides of lime silicoaluminate and preferably of anorthite.
  • the present invention relates to a weldable austenitic stainless steel, with improved machinability and having good cold characteristics, in which the simultaneous presence in suitable proportion, of copper and of malleable oxides, chosen in a ternary diagram Al 2 O 3 , SiO 2 , CaO, very significantly improves a set of properties, some of which are contradictory.
  • the weldable austenitic stainless steel contains malleable oxides whose compositions are located in the ternary diagram AL 2 O 3 , SiO 2 and CaO, in the zone of the triple point anorthite, gehlenite, pseudo-wollastonite.
  • the sulfur ensures the formation of micro-precipitates of manganese sulphide and the copper ensures a reduction of the work hardness allowing to obtain a less hard surface and, during machining, to form chips that are also less hard at the machining temperature, resulting in a significant increase in the life of the tools.
  • the nitrogen concentration is between 0.03 and 0.05%.
  • the carbon concentration is less than 0.03%
  • This steel contains malleable oxides whose compositions are located on the ternary diagram AL 2 O 3 -SiO 2 -CaO, in the zone of the triple point anorthite, gehlenite and pseudo-wollastonite.
  • the nitrogen element introduced into the composition of austenitic stainless steels in a determined and limited proportion between 0.03 and 0.05% stabilizes the austenite at the expense of the formation of hardening martensite which tends to reduce the magnetic permeability .
  • the carbon concentration is less than 0.03%.
  • the copper element replacing sulfur and lead used alone and preferably with nitrogen, contributes to the stabilization of the austenite at the expense of the formation of the martensite for work hardening which results by a consequent reduction in magnetic permeability.
  • the austenitic stainless steel according to the invention has a set of characteristics and sometimes surprising physical properties, sometimes contradictory, such as machining, welding, cold deformation, non-magnetism and resistance to corrosion.
  • This reference steel is a referenced austenitic steel, the general composition of which is as follows: C less than 0.08%, Cr between 18 and 20%, Ni between 8 and 10.5%, S less than 0.03 %, Mo less than or equal to 0.5%, Nn less than or equal to 2% and N less than or equal to 0.1%.
  • the steel according to the invention has also been tested in "Gun Drilling" technology, that is to say in drilling with a guide gun and under high oil. pressure.
  • the choice of cutting conditions for good chip evacuation being assured, the machining of steel parts according to the invention is four times faster than with a reference steel and the number of parts machined before resharpening the tool is 6 times higher.
  • the steel according to the invention has a magnetic permeability of 1.4 (Curve G, Fig. 3) and only 1.9% of martensite is formed during wire drawing ( Curve E, Fig. 4).
  • the reference steel forms three times more martensite (curve F - Fig. 4) and therefore has a magnetic permeability of 2.4 (curve H - Fig. 3).
  • the passivation pH measured in a sodium chloride solution and defined by the pH above which the critical passivation current is less than 10 ⁇ A / cm 2 , is substantially equal to 3 with the steel according to the invention (curve J) and significantly lower than that of the reference steel which is 3.5 (curve K).
  • This area was compared with a non-heat affected area and found that this non-heat affected area includes the known sulfide inclusions of such resulfurized steel.
  • the steel according to the invention is suitable for machining at high speed, cold deformability comprising remarkable magnetic properties and a high resistance to cavernous corrosion as well as a capacity for welding.

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Abstract

A weldable austenitic stainless steel with high machinability and improved cold workability, characterised in that its weight composition is the following: C lower than 0.1 %, Si lower than 2 %, Mn lower than 2 %, S lower than 0.03 %, Ni between 8 and 10 %, Cr between 15 and 25 %, P lower than or equal to 0.04 %, Mo lower than 0.5 %, Cu between 1 and 5 %, N between 0.02 and 0.07 %, Ca higher than 30x10<-4> %, O higher than 70x10<-4> %, Al lower than 50x10<-4> %, the Ca/O ratio being between 0.3 and 0.6.

Description

La présente invention concerne un acier inoxydable austénitique soudable, à haute usinabilité et comportant de bonnes caractéristiques de déformation à froid.The present invention relates to a weldable austenitic stainless steel, with high machinability and having good cold deformation characteristics.

On connaît dans le JP-A-160 785 un acier usinable, déformable à froid et ayant en sa composition pondérale, notamment une teneur en soufre inférieure à 0,03%, des teneurs en calcium et oxygène respectivement comprises entre 10 et 300 ppm et entre 30 et 300 ppm, une teneur en cuivre comprise entre 0,8 et 5% et une teneur en plomb comprise entre 0,01 et 0,25%.JP-A-160 785 discloses a machinable steel, cold deformable and having in its weight composition, in particular a sulfur content of less than 0.03%, calcium and oxygen contents of between 10 and 300 ppm and between 30 and 300 ppm, a copper content between 0.8 and 5% and a lead content between 0.01 and 0.25%.

Dans cet acier inoxydable austénitique, on introduit de l'oxygène et du calcium ce qui permet de transformer des inclusions dures en inclusions à base d'oxyde de calcium.In this austenitic stainless steel, oxygen and calcium are introduced which makes it possible to transform hard inclusions into inclusions based on calcium oxide.

L'amélioration de l'usinabilité est générée par l'introduction, dans la composition, d'une quantité variable de plomb.The improvement in machinability is generated by the introduction into the composition of a variable amount of lead.

Il est bien connu que les aciers inoxydables austénitiques sont difficiles à usiner, d'une part, à cause de leur faible conductibilité thermique et, d'autre part, à cause de leur forte écrouissabilité induisant localement des zones de dureté élevée. Un mauvais écoulement de la chaleur produite à la pointe d'un outil de coupe entraîne une détérioration rapide de l'outil.It is well known that austenitic stainless steels are difficult to machine, on the one hand, because of their low thermal conductivity and, on the other hand, because of their high work hardening locally inducing zones of high hardness. Poor flow of the heat produced at the tip of a cutting tool leads to rapid deterioration of the tool.

Un moyen pour améliorer l'usinabilité de l'acier inoxydable austénitique, selon le JP-A-160 785, consiste à introduire l'élément plomb dans une proportion comprise entre 0,01 et 0,25%.One way to improve the machinability of austenitic stainless steel, according to JP-A-160 785, is to introduce the lead element in a proportion of between 0.01 and 0.25%.

Cet élément présente les inconvénients d'être difficile à dissoudre de façon homogène en bain fondu et, en raison de sa densité élevée, d'avoir tendance à s'accumuler dans le fond des récipients métallurgiques.This element has the disadvantages of being difficult to dissolve homogeneously in a molten bath and, because of its high density, of having a tendency to accumulate in the bottom of metallurgical vessels.

De plus, il forme des phases à bas point de fusion qui détériorent la déformabilité à chaud par formation de défauts lors du laminage et qui, dans le domaine de la soudabilité, génèrent dans la zone de soudure ce même défaut limitant la tenue mécanique de la soudure.In addition, it forms phases with a low melting point which deteriorate the hot deformability by formation of defects during rolling and which, in the field of weldability, generate in the weld zone this same defect limiting the mechanical strength of the welding.

D'autre part, du bore peut être introduit dans la composition de l'acier inoxydable austénitique selon le JP-A-160 785, comme élément susceptible de contrecarrer les effets nocifs du plomb dans le domaine du laminage à chaud, mais le bore augmente encore les difficultés de soudage.On the other hand, boron can be introduced into the composition of austenitic stainless steel according to JP-A-160 785, as an element capable of counteracting the harmful effects of lead in the field of hot rolling, but the boron increases still the welding difficulties.

L'introduction de l'élément bore apporte par ailleurs au moins un autre inconvénient, celui de réduire l'intervalle de la température apte au laminage à chaud, ce qui impose un procédé de laminage à chaud plus contraignant.The introduction of the boron element also brings at least one other drawback, that of reducing the temperature range suitable for hot rolling, which imposes a more restrictive hot rolling process.

L'acier inoxydable austénitique selon le JP-A-160 785 peut également contenir du titane.Austenitic stainless steel according to JP-A-160 785 may also contain titanium.

Or, le titane qui est généralement introduit dans les aciers inoxydables dans le but d'améliorer la résistance à la corrosion intragranulaire perturbe la formation des inclusions d'oxyde de calcium et réduit leur nombre.However, titanium, which is generally introduced into stainless steels in order to improve the resistance to intragranular corrosion, disrupts the formation of calcium oxide inclusions and reduces their number.

De plus, le titane conduit à la formation d'inclusions dures qui réduisent l'aptitude à l'usinage en favorisant l'usure prématurée des outils de coupe.In addition, titanium leads to the formation of hard inclusions which reduce the workability by promoting premature wear of the cutting tools.

On connaît également dans le FR-A-2 542 761 un procédé de fabrication d'un acier à haute usinabilité.Also known from FR-A-2 542 761 is a process for manufacturing a steel with high machinability.

Dans ce document, il est précisé qu'une cause de la difficulté d'usiner des aciers inoxydables réside dans le fait qu'ils contiennent des inclusions d'oxydes durs comme par exemple de l'alumine ou de la chromite qui détériorent les outils de coupe.In this document, it is specified that a cause of the difficulty of machining stainless steels lies in the fact that they contain inclusions of hard oxides such as for example alumina or chromite which deteriorate the tools of chopped off.

Un moyen de réduire la nocivité des inclusions d'oxydes durs est d'introduire dans l'acier un ou plusieurs composés alcalinoterreux, afin de remplacer dans une bonne proportion les inclusions dures par des inclusions d'oxyde à base de calcium, par exemple. Il est également précisé, d'une part, qu'une certaine quantité de soufre combinée avec des inclusions dures en réduit la nocivité, la teneur en soufre étant généralement inférieure à 0,5.10-4% et, d'autre part, qu'un autre moyen de réduire la nocivité des inclusions est de réduire leur quantité grâce à une bonne désoxydation et bonne décantation du bain lors de l'élaboration de l'acier.One way of reducing the harmfulness of hard oxide inclusions is to introduce one or more alkaline earth compounds into the steel, in order to replace a good proportion of hard inclusions with calcium-based oxide inclusions, for example. It is also specified, on the one hand, that a certain quantity of sulfur combined with hard inclusions reduces its harmfulness, the sulfur content generally being less than 0.5 × 10 -4 % and, on the other hand, that another way to reduce the harmfulness of inclusions is to reduce their quantity thanks to good deoxidation and good settling of the bath during the production of steel.

Dans le FR-A-2 648 477, il est proposé pour améliorer l'usinabilité des aciers inoxydables austénitiques d'introduire dans la composition une quantité de soufre, dans une proportion comprise entre 0,1 et 0,4% tout en assurant une proportion de calcium et d'oxygène respectivement supérieures à 30.10-4% et 70.10-4% et satisfaisant à la relation Ca/O comprise entre 0,2 et 0,6.In FR-A-2 648 477, it is proposed to improve the machinability of austenitic stainless steels to introduce into the composition an amount of sulfur, in a proportion of between 0.1 and 0.4% while ensuring a proportion of calcium and oxygen respectively greater than 30.10 -4 % and 70.10 -4 % and satisfying the Ca / O relationship between 0.2 and 0.6.

Dans ce document, le but recherché est la formation, avec le manganèse et dans une plus faible proportion avec le chrome d'un sulfure de manganèse et chrome (Mn, Cr) S qui génère sous forme d'inclusions spécifiques une lubrification solide de l'outil de coupe pendant les opérations d'usinage.In this document, the aim sought is the formation, with manganese and in a smaller proportion with chromium, of a manganese sulfide and chromium (Mn, Cr) S which generates in the form of specific inclusions a solid lubrication of the cutting tool during machining operations.

Il est également précisé que le soufre a un effet défavorable sur la résistance à la corrosion et que malgré cela une orientation choisie est l'introduction dans un acier resulfuré d'oxydes de silicoaluminate de chaux et préférentiellement d'anorthite.It is also specified that sulfur has an unfavorable effect on the resistance to corrosion and that despite this a chosen orientation is the introduction into a resulfurized steel of oxides of lime silicoaluminate and preferably of anorthite.

Or, la demanderesse a remarqué qu'un tel acier inoxydable austénitique bien qu'il ait de bonnes propriétés en usinabilité, comporte un autre inconvénient. En effet, le soufre réduit de façon conséquente les propriétés des aciers du point de vue soudabilité et du point de vue de la déformation à froid avec apparition de tapures par exemple en tréfilage.However, the Applicant has noticed that such an austenitic stainless steel, although it has good machinability properties, has another drawback. In fact, sulfur considerably reduces the properties of steels from the point of view of weldability and from the point of view of cold deformation with the appearance of tapures, for example in wire drawing.

Dans les documents précédemment mentionnés, l'amélioration de l'usinabilité de l'acier inoxydable austénitique est donc réalisée :

  • par introduction du plomb en tant que lubrifiant,
  • par introduction d'oxygène ou de calcium pour remplacer les inclusions dures par des inclusions à base de composés alcalinoterreux,
  • par réduction du nombre d'inclusions dures par désoxydation du bain fondu lors de l'élaboration,
  • par introduction d'une quantité de soufre par une technique de resulfurisation.
In the above-mentioned documents, the machinability of austenitic stainless steel is therefore improved:
  • by introducing lead as a lubricant,
  • by introducing oxygen or calcium to replace hard inclusions with inclusions based on alkaline earth compounds,
  • by reducing the number of hard inclusions by deoxidation of the molten bath during production,
  • by introducing an amount of sulfur by a resulfurization technique.

La présente invention a pour objet un acier inoxydable austénitique soudable, à usinabilité améliorée et comportant de bonnes caractéristiques à froid, dans lequel la présence simultanée en proportion convenable, de cuivre et d'oxydes malléables, choisis dans un diagramme ternaire Al2O3, SiO2, CaO, améliore de façon très sensible un ensemble de propriétés dont certaines sont antinomiques.The present invention relates to a weldable austenitic stainless steel, with improved machinability and having good cold characteristics, in which the simultaneous presence in suitable proportion, of copper and of malleable oxides, chosen in a ternary diagram Al 2 O 3 , SiO 2 , CaO, very significantly improves a set of properties, some of which are contradictory.

L'acier inoxydable austénitique selon la présente invention est caractérisé en ce que sa composition pondérale est la suivante :

  • C inférieur à 0,1%
  • Si inférieur à 2%
  • Mn inférieur à 2%
  • S inférieur à 0,03%
  • Ni compris entre 8 et 10%
  • Cr compris entre 15 et 25%
  • P inférieur ou égal à 0,04%
  • Mo inférieur à 0,5%
  • Cu compris entre 1 et 5%
  • N compris entre 0,02 et 0,07%
  • Ca supérieur à 30.10-4%
  • O supérieur à 70.10-4%
  • Al inférieur à 50.10-4%,
   le rapport Ca/O étant compris entre 0,3 et 0,6, le complément étant du fer et des impuretés inévitables.The austenitic stainless steel according to the present invention is characterized in that its weight composition is as follows:
  • C less than 0.1%
  • If less than 2%
  • Mn less than 2%
  • S less than 0.03%
  • Nor between 8 and 10%
  • Cr between 15 and 25%
  • P less than or equal to 0.04%
  • Mo less than 0.5%
  • Cu between 1 and 5%
  • N between 0.02 and 0.07%
  • Ca greater than 30.10 -4 %
  • O greater than 70.10 -4 %
  • Al less than 50.10 -4 %,
the Ca / O ratio being between 0.3 and 0.6, the balance being iron and unavoidable impurities.

L'acier inoxydable austénitique selon la présente invention est caractérisé en ce que sa composition pondérale est de préférence la suivante.

  • C inférieur ou égal à 0,08%
  • Si compris entre 0,2 et 0,75%
  • Mn compris entre 0,5 et 1,5%
  • S inférieur à 0,03%
  • Ni compris entre 8 et 10%
  • Cr compris entre 17 et 19%
  • Mo inférieur à 0,5%
  • Cu compris entre 3 et 4%
  • N compris entre 0,02 et 0,07%
  • Ca compris entre 0,003 et 0,01%
  • O compris entre 0,007 et 0,02%
  • Al inférieur à 0,005%
  • P inférieur ou égal à 0,04%,
   le rapport Ca/O étant compris entre 0,3 et 0,6, le complément étant du fer et des impuretés inévitables.The austenitic stainless steel according to the present invention is characterized in that its weight composition is preferably the following.
  • C less than or equal to 0.08%
  • If between 0.2 and 0.75%
  • Mn between 0.5 and 1.5%
  • S less than 0.03%
  • Nor between 8 and 10%
  • Cr between 17 and 19%
  • Mo less than 0.5%
  • Cu between 3 and 4%
  • N between 0.02 and 0.07%
  • Ca between 0.003 and 0.01%
  • O between 0.007 and 0.02%
  • Al less than 0.005%
  • P less than or equal to 0.04%,
the Ca / O ratio being between 0.3 and 0.6, the balance being iron and unavoidable impurities.

L'acier inoxydable austénitique soudable, à haute usinabilité et à bonne déformabilité à froid, contient des oxydes malléables dont les compositions se situent dans le diagramme ternaire AL2O3, SiO2 et CaO, dans la zone du point triple anorthite, gehlenite, pseudo-wollastonite.The weldable austenitic stainless steel, with high machinability and good cold deformability, contains malleable oxides whose compositions are located in the ternary diagram AL 2 O 3 , SiO 2 and CaO, in the zone of the triple point anorthite, gehlenite, pseudo-wollastonite.

Le soufre assure la formation de micro-précipités de sulfure de manganèse et le cuivre assure une réduction de l'écrouissabilité permettant d'obtenir une surface moins dure et, lors de l'usinage, de former des copeaux également moins durs à la température d'usinage d'où une augmentation importante de la durée de vie des outils.The sulfur ensures the formation of micro-precipitates of manganese sulphide and the copper ensures a reduction of the work hardness allowing to obtain a less hard surface and, during machining, to form chips that are also less hard at the machining temperature, resulting in a significant increase in the life of the tools.

Dans une forme préférentielle de l'invention, la concentration en azote est comprise entre 0,03 et 0,05%.In a preferred form of the invention, the nitrogen concentration is between 0.03 and 0.05%.

Dans une autre forme préférentielle de l'invention, la concentration en carbone est inférieure à 0,03%In another preferred form of the invention, the carbon concentration is less than 0.03%

Les caractéristiques et avantages apparaîtront au cours de la description qui va suivre, donnée uniquement à titre d'exemple, faite en se référant aux dessins annexés, sur lesquels :

  • la Fig. 1 représente deux courbes de l'évolution de l'usure d'un outil en fonction du temps pour un acier selon l'invention et un acier de référence,
  • la Fig. 2 représente deux courbes montrant l'évolution du coefficient de striction après tréfilage pour un acier selon l'invention et un acier de référence,
  • les Figs. 3 et 4 représentent, d'une part, une évolution comparée de la perméabilité magnétique en fonction du taux de déformation d'un acier selon l'invention et d'un acier de référence, et, d'autre part, l'évolution de la concentration en martensite en fonction du taux de déformation pour l'acier selon l'invention et le même acier de référence,
  • la Fig. 5 représente deux courbes montrant la mesure du courant critique en fonction du pH dans un test de corrosion en milieu chloruré pour l'acier selon l'invention comparé à un acier de référence,
The characteristics and advantages will become apparent during the description which follows, given solely by way of example, made with reference to the appended drawings, in which:
  • Fig. 1 represents two curves of the evolution of the wear of a tool as a function of time for a steel according to the invention and a reference steel,
  • Fig. 2 represents two curves showing the evolution of the necking coefficient after drawing for a steel according to the invention and a reference steel,
  • Figs. 3 and 4 represent, on the one hand, a comparative evolution of the magnetic permeability as a function of the strain rate of a steel according to the invention and of a reference steel, and, on the other hand, the evolution of the martensite concentration as a function of the deformation rate for the steel according to the invention and the same reference steel,
  • Fig. 5 represents two curves showing the measurement of the critical current as a function of the pH in a corrosion test in a chlorinated medium for the steel according to the invention compared with a reference steel,

La composition pondérale de l'acier inoxydable austénitique selon la présente invention est la suivante:

  • C inférieur à 0,1%
  • Si inférieur à 2%
  • Mn inférieur à 2%
  • S inférieur à 0,03%
  • Ni compris entre 8 et 10%
  • Cr compris entre 15 et 25%
  • P inférieur ou égal à 0,04%
  • Mo inférieur à 0,5%
  • Cu compris entre 1 et 5%
  • N compris entre 0,02 et 0,07%
  • Ca supérieur à 30.10-4%
  • O supérieur à 70.10-4%
  • Al inférieur à 50.10-4%,
   le rapport Ca/O étant compris entre 0,3 et 0,6.The weight composition of the austenitic stainless steel according to the present invention is as follows:
  • C less than 0.1%
  • If less than 2%
  • Mn less than 2%
  • S less than 0.03%
  • Nor between 8 and 10%
  • Cr between 15 and 25%
  • P less than or equal to 0.04%
  • Mo less than 0.5%
  • Cu between 1 and 5%
  • N between 0.02 and 0.07%
  • Ca greater than 30.10 -4 %
  • O greater than 70.10 -4 %
  • Al less than 50.10 -4 %,
the Ca / O ratio being between 0.3 and 0.6.

De préférence, la composition pondérale de l'acier inoxydable austénitique selon la présente invention est la suivante :

  • C inférieur ou égal à 0,08%
  • Si compris entre 0,2 et 0,75%
  • Mn compris entre 0,5 et 1,5%
  • S inférieur à 0,03%
  • Ni compris entre 8 et 10%
  • Cr compris entre 17 et 19%
  • Mo inférieur à 0,5%
  • Cu compris entre 3 et 4%
  • N compris entre 0,02% et 0,07%
  • Ca compris entre 0,003 et 0,01%
  • O compris entre 0,007 et 0,02%
  • Al inférieur à 0,005%
  • P inférieur ou égal à 0,04%,
   le rapport Ca/O étant compris entre 0,3 et 0,6.Preferably, the weight composition of the austenitic stainless steel according to the present invention is as follows:
  • C less than or equal to 0.08%
  • If between 0.2 and 0.75%
  • Mn between 0.5 and 1.5%
  • S less than 0.03%
  • Nor between 8 and 10%
  • Cr between 17 and 19%
  • Mo less than 0.5%
  • Cu between 3 and 4%
  • N between 0.02% and 0.07%
  • Ca between 0.003 and 0.01%
  • O between 0.007 and 0.02%
  • Al less than 0.005%
  • P less than or equal to 0.04%,
the Ca / O ratio being between 0.3 and 0.6.

Cet acier contient des oxydes malléables dont les compositions se situent sur le diagramme ternaire AL2O3-SiO2-CaO, dans la zone du point triple anorthite, gehlenite et pseudo-wollastonite.This steel contains malleable oxides whose compositions are located on the ternary diagram AL 2 O 3 -SiO 2 -CaO, in the zone of the triple point anorthite, gehlenite and pseudo-wollastonite.

Jusqu'à présent, l'amélioration de l'usinabilité des aciers inoxydables austénitiques est réalisée en introduisant dans la composition des aciers soit du plomb, soit du soufre, soit une quantité contrôlée de calcium et d'oxygène.Up to now, the improvement in the machinability of austenitic stainless steels has been achieved by introducing into the composition of the steels either lead or sulfur, or a controlled amount of calcium and oxygen.

Or, la demanderesse a remarqué d'une façon inattendue que la combinaison des élements cuivre, oxygène et calcium associée à une faible quantité de soufre apporte à l'acier inoxydable austénitique une usinabilité remarquable.However, the Applicant has noticed in an unexpected way that the combination of the elements copper, oxygen and calcium associated with a small amount of sulfur brings to austenitic stainless steel remarkable machinability.

L'élément cuivre a pour effet :

  • de réduire la dureté des copaux à la température d'usinage,
  • dans le domaine de la déformation à froid, d'assurer une réduction de l'écrouissabilité de l'acier par augmentation de l'énergie de défauts d'empilement,
  • et d'assurer une résistance à la corrosion en milieu acide par la formation d'un film d'oxyde métallique protecteur.
The copper element has the effect:
  • reduce the hardness of the chips to the machining temperature,
  • in the field of cold deformation, to ensure a reduction in the work hardening of the steel by increasing the energy of stacking defects,
  • and to provide corrosion resistance in an acid medium by the formation of a protective metal oxide film.

L'élément azote introduit dans la composition des aciers inoxydables austénitiques dans une proportion déterminée et limitée entre 0,03 et 0,05% stabilise l'austénite au dépend de la formation de la martensite d'écrouissage ce qui tend à réduire la perméabilité magnétique.The nitrogen element introduced into the composition of austenitic stainless steels in a determined and limited proportion between 0.03 and 0.05% stabilizes the austenite at the expense of the formation of hardening martensite which tends to reduce the magnetic permeability .

De préférence, la concentration en carbone est inférieure à 0,03%.Preferably, the carbon concentration is less than 0.03%.

La demanderesse a également constaté que l'élément cuivre en remplacement du soufre et du plomb utilisé seul et de préférence avec l'azote, contribue à la stabilisation de l'austénite au dépend de la formation de la martensite d'écrouissage ce qui se traduit par une réduction conséquente de la perméabilité magnétique.The Applicant has also found that the copper element, replacing sulfur and lead used alone and preferably with nitrogen, contributes to the stabilization of the austenite at the expense of the formation of the martensite for work hardening which results by a consequent reduction in magnetic permeability.

L'acier inoxydable austénitique selon l'invention présente un ensemble de caractéristiques et de propriétés physiques parfois surprenantes, parfois antinomiques, telles que usinage, soudage, déformation à froid, amagnétisme et résistance à la corrosion.The austenitic stainless steel according to the invention has a set of characteristics and sometimes surprising physical properties, sometimes contradictory, such as machining, welding, cold deformation, non-magnetism and resistance to corrosion.

Parmi, ces caractéristiques et propriétés physiques, on décrira dans ce qui suit successivement une série de tests comparatifs tel qu'un test d'usure d'outil en carbure en fonction du temps, un test comparatif de déformabilité à froid par tréfilage, un test comparatif de propriétés physiques de malléabilité attachées à la formation de la martensite dans l'acier, un test comparatif de résistance en corrosion caverneuse et un test de soudabilité.Among these physical characteristics and properties, a series of comparative tests will be described in the following, such as a carbide tool wear test as a function of time, a comparative cold deformability test by drawing, a test comparison of physical malleability properties associated with the formation of martensite in steel, a comparative test of resistance to cavernous corrosion and a weldability test.

Dans le domaine des aciers inoxydables austénitiques à usinabilité améliorée, les différentes orientations techniques et propriétés d'emploi obtenues sont généralement comparées à l'acier de référence internationale AlSi 304.In the field of austenitic stainless steels with improved machinability, the various technical orientations and properties of use obtained are generally compared with the steel of international reference AlSi 304.

Cet acier de référence est un acier austénitique référencé, dont la composition générale est la suivante : C inférieur à 0,08%, Cr compris entre 18 et 20%, Ni compris entre 8 et 10,5%, S inférieur à 0,03%, Mo inférieur ou égal à 0,5%, Nn inférieur ou égale à 2% et N inférieur ou égal à 0,1%.This reference steel is a referenced austenitic steel, the general composition of which is as follows: C less than 0.08%, Cr between 18 and 20%, Ni between 8 and 10.5%, S less than 0.03 %, Mo less than or equal to 0.5%, Nn less than or equal to 2% and N less than or equal to 0.1%.

Dans le domaine de l'usinage des aciers inoxydables austénitiques, des tests ont été réalisés avec des outils en carbure.In the field of machining of austenitic stainless steels, tests have been carried out with carbide tools.

On remarque sur la Fig. 1, après 30 minutes d'usinage, avec une vitesse de coupe de 220 m/mn, que l'usure en dépouille Vb de l'outil ayant usiné l'acier selon l'invention (Courbe A) est réduite d'environ 30% par rapport à l'usure en dépouille Vb obtenue après l'usinage de l'acier de référence (courbe B).We notice in Fig. 1, after 30 minutes of machining, with a cutting speed of 220 m / min, that the wear and tear Vb of the tool having machined the steel according to the invention (Curve A) is reduced by approximately 30 % relative to the wear in clearance Vb obtained after the machining of the reference steel (curve B).

L'acier selon l'invention a également été testé en technologie "Gun Drilling", c'est à dire en perçage avec un canon de guidage et sous huile à haute pression. Le choix des conditions de coupe pour une bonne évacuation des copeaux étant assuré, l'usinage des pièces en acier selon l'invention est quatre fois plus rapide qu'avec un acier de référence et le nombre de pièces usinées avant réaffutage de l'outil est 6 fois plus élevé.The steel according to the invention has also been tested in "Gun Drilling" technology, that is to say in drilling with a guide gun and under high oil. pressure. The choice of cutting conditions for good chip evacuation being assured, the machining of steel parts according to the invention is four times faster than with a reference steel and the number of parts machined before resharpening the tool is 6 times higher.

Du point de vue de la déformabilité à froid, les tests matérialisés sur la Fig. 2 montrent qu'après une déformation de 300% par tréfilage, l'acier selon l'invention (courbe C) conserve un coefficient de striction supérieur à 60%, alors qu'il atteind une valeur inférieure à 50% avec un acier de référence (courbe D).From the point of view of cold deformability, the tests materialized in FIG. 2 show that after a deformation of 300% by drawing, the steel according to the invention (curve C) retains a coefficient of necking greater than 60%, while it reaches a value less than 50% with a reference steel (curve D).

Une des caractéristiques physiques originale est liée au comportement magnétique de l'acier selon l'invention, comme matérialisé sur un test comparatif représenté sur les Fig. 3 et 4.One of the original physical characteristics is linked to the magnetic behavior of the steel according to the invention, as shown on a comparative test shown in FIGS. 3 and 4.

Lors d'une déformation de 100% par tréfilage l'acier selon l'invention a une perméabilité magnétique de 1,4 (Courbe G, Fig. 3) et il se forme, lors du tréfilage, seulement 1,9% de martensite (Courbe E, Fig. 4).During a deformation of 100% by wire drawing, the steel according to the invention has a magnetic permeability of 1.4 (Curve G, Fig. 3) and only 1.9% of martensite is formed during wire drawing ( Curve E, Fig. 4).

Dans les mêmes conditions, l'acier de référence forme trois fois plus de martensite (courbe F - Fig. 4) et a, de ce fait, une perméabilité magnétique de 2,4 (courbe H - Fig. 3).Under the same conditions, the reference steel forms three times more martensite (curve F - Fig. 4) and therefore has a magnetic permeability of 2.4 (curve H - Fig. 3).

Des tests de résistance à la corrosion ont également été effectués et sont représentés sur la Fig. 5.Corrosion resistance tests have also been carried out and are shown in FIG. 5.

Le pH de dépassivation mesuré dans une solution de chlorure de sodium et défini par le pH à partir duquel le courant critique de passivation est inférieur à 10µA/cm2, est sensiblement égal à 3 avec l'acier selon l'invention (courbe J) et nettement inférieur à celui de l'acier de référence qui est de 3,5 (courbe K).The passivation pH measured in a sodium chloride solution and defined by the pH above which the critical passivation current is less than 10 μA / cm 2 , is substantially equal to 3 with the steel according to the invention (curve J) and significantly lower than that of the reference steel which is 3.5 (curve K).

Du point de vue de la soudabilité, un test a été effectué en comparent deux soudures à l'arc sans métal d'apport, l'une étant réalisée entre l'acier de référence et un acier resulfuré, l'autre entre ce même acier de référence et un acier selon l'invention.From the point of view of weldability, a test was carried out by comparing two arc welds without filler metal, one being carried out between the reference steel and a resulfurized steel, the other between this same steel of reference and a steel according to the invention.

On a remarqué dans une zone affectée thermiquement de l'acier resulfuré, une forte précipitation de soufre dans les joints de grains.In a thermally affected area of resulfurized steel, a strong precipitation of sulfur has been observed in the grain boundaries.

On a comparé cette zone avec une zone non affectée thermiquement et a constaté que cette zone non affectée thermiquement comporte les inclusions de sulfure connues d'un tel acier resulfuré.This area was compared with a non-heat affected area and found that this non-heat affected area includes the known sulfide inclusions of such resulfurized steel.

En examinant la soudure réalisée entre l'acier de référence et l'acier selon l'invention, on a constaté que l'acier selon l'invention ne présente pas de modification de structure due au soudage.By examining the weld produced between the reference steel and the steel according to the invention, it was found that the steel according to the invention does not exhibit any structural modification due to welding.

Les différentes caractéristiques mentionnées ci-dessus assurent à l'acier selon l'invention une aptitude à un usinage à grande vitesse, une déformabilité à froid comportant des propriétés magnétiques remarquables et une résistance à la corrosion caverneuse élevée ainsi qu'une aptitude au soudage.The various characteristics mentioned above ensure that the steel according to the invention is suitable for machining at high speed, cold deformability comprising remarkable magnetic properties and a high resistance to cavernous corrosion as well as a capacity for welding.

La combinaison de l'élément cuivre associé à une faible quantité de soufre et une proportion déterminée de calcium et d'oxygène apporte à l'acier selon l'invention des caractéristiques remarquables sans induire les inconvénients inhérents aux éléments à bas points de fusion.The combination of the copper element associated with a small amount of sulfur and a determined proportion of calcium and oxygen provides the steel according to the invention with remarkable characteristics without inducing the drawbacks inherent in elements with low melting points.

Claims (5)

  1. Weldable austenitic stainless steel with high machinability and improved cold workability, characterised in that its composition by weight is as follows:
    C less than 0.1%
    Si less than 2%
    Mn less than 2%
    S less than 0.03%
    Ni between 8 and 10%
    Cr between 15 and 25%
    P less than or equal to 0.04%
    Mo less than 0.5%
    Cu between 1 and 5%
    N between 0.02 and 0.07%
    Ca greater than 30.10-4%
    O greater than 70.10-4%
    Al less than 50.10-4%,
       the Ca/O ratio being between 0.3 and 0.6, the remainder being iron and unavoidable impurities.
  2. Steel according to Claim 1, characterised in that its composition by weight is, preferably, as follows:
    C less than or equal to 0.08%
    Si between 0.2 and 0.75%
    Mn between 0.5 and 1.5%
    S less than 0.03%
    Ni between 8 and 10%
    Cr between 17 and 19%
    Mo less than 0.5%
    Cu between 3 and 4%
    N between 0.02 and 0.07%
    Ca between 0.003 and 0.01%
    O between 0.007 and 0.02%
    Al less than 0.005%
    P less than or equal to 0.04%
       the Ca/O ratio being between 0.3 and 0.6, the remainder being iron and avoidable impurities.
  3. Steel according to Claims 1 and 2, characterised in that it contains malleable oxides whereof the compositions are situated on the ternary diagram Al2O3-SiO2-CaO, in the area of the triple point anorthite, gehlenite and pseudo-wollastonite.
  4. Steel according to one of Claims 1 to 3, characterised in that the concentration of nitrogen is preferably between 0.03 and 0.05%.
  5. Steel according to one of Claims 1 to 3, characterised in that the concentration of carbon is preferably less than 0.03%.
EP93400903A 1992-04-17 1993-04-06 Austenitic stainless steel with high machine ability and improved cold work ability Expired - Lifetime EP0567365B1 (en)

Applications Claiming Priority (2)

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FR9204810A FR2690169B1 (en) 1992-04-17 1992-04-17 Austenitic stainless steel with high machinability and improved cold deformation.
FR9204810 1992-04-17

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FR2706489B1 (en) * 1993-06-14 1995-09-01 Ugine Savoie Sa Martensitic stainless steel with improved machinability.
FR2720410B1 (en) * 1994-05-31 1996-06-28 Ugine Savoie Sa Ferritic stainless steel with improved machinability.
FR2732694B1 (en) * 1995-04-07 1997-04-30 Ugine Savoie Sa AUSTENITIC RESULFUR STAINLESS STEEL WITH IMPROVED MACHINABILITY, ESPECIALLY USED IN THE FIELD OF MACHINING AT VERY HIGH CUTTING SPEEDS AND THE AREA OF DECOLLETING
FR2733252B1 (en) * 1995-04-21 1997-05-23 Ugine Savoie Sa AUSTENITIC STAINLESS STEEL FOR THE PREPARATION OF YARN IN PARTICULAR
US5512238A (en) * 1995-06-07 1996-04-30 Crs Holdings, Inc. Free-machining austenitic stainless steel
FR2757878B1 (en) * 1996-12-31 1999-02-05 Sprint Metal Sa STAINLESS STEEL STEEL WIRE AND MANUFACTURING METHOD
FR2759709B1 (en) * 1997-02-18 1999-03-19 Ugine Savoie Sa STAINLESS STEEL FOR THE PREPARATION OF TREWNED WIRE, ESPECIALLY OF PNEUMATIC REINFORCEMENT WIRE AND PROCESS FOR MAKING THE SAID WIRE
FR2776306B1 (en) * 1998-03-18 2000-05-19 Ugine Savoie Sa AUSTENITIC STAINLESS STEEL FOR THE PREPARATION OF YARN IN PARTICULAR
FR2780735B1 (en) * 1998-07-02 2001-06-22 Usinor AUSTENITIC STAINLESS STEEL WITH LOW NICKEL CONTENT AND CORROSION RESISTANT
FR2805829B1 (en) * 2000-03-03 2002-07-19 Ugine Savoie Imphy AUSTENITIC STAINLESS STEEL WITH HIGH MACHINABILITY, RESULFURIZING, AND COMPRISING IMPROVED CORROSION RESISTANCE
PT1412549E (en) * 2001-07-20 2011-12-22 Bekaert Sa Nv Bundle drawn stainless steel fibers
JP6589536B2 (en) * 2014-11-06 2019-10-16 日本製鉄株式会社 Manufacturing method of welded joint
CN113265585B (en) * 2021-05-14 2023-02-24 山西太钢不锈钢股份有限公司 Stainless steel for automobile safety airbag and production method and application thereof

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FR2648477B1 (en) * 1989-06-16 1993-04-30 Ugine Savoie Sa AUSTENITIC RESULFUR STAINLESS STEEL WITH IMPROVED MACHINABILITY

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IL105338A (en) 1997-02-18
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EP0567365A1 (en) 1993-10-27
FR2690169A1 (en) 1993-10-22
TR28589A (en) 1996-11-04
DE69311839T2 (en) 1997-11-13
DK0567365T3 (en) 1998-02-16
FI103416B (en) 1999-06-30
DE69311839D1 (en) 1997-08-07
ATE154953T1 (en) 1997-07-15
NO931384L (en) 1993-10-18
CA2094200C (en) 2002-08-13
US5362439A (en) 1994-11-08

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