EP0725156B1 - High ductility steel, manufacturing process and utilization - Google Patents

High ductility steel, manufacturing process and utilization Download PDF

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Publication number
EP0725156B1
EP0725156B1 EP96400061A EP96400061A EP0725156B1 EP 0725156 B1 EP0725156 B1 EP 0725156B1 EP 96400061 A EP96400061 A EP 96400061A EP 96400061 A EP96400061 A EP 96400061A EP 0725156 B1 EP0725156 B1 EP 0725156B1
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Prior art keywords
steel
component
temperature
minutes
room temperature
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French (fr)
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EP0725156A1 (en
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Jean Beguinot
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Industeel France SAS
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Industeel France SAS
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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
    • C22C38/54—Ferrous alloys, e.g. steel alloys containing chromium with nickel with boron
    • 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
    • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten

Definitions

  • the present invention relates to a weldable steel having a high tensile strength and good ductility.
  • Staged quenching heat treatment comprising a cooling at a cooling rate greater than or equal to 50 ° C / s up to a holding temperature, then an isothermal holding at this temperature, and finally, cooling down to the temperature ambient, is well suited for thin sheets or small parts of mechanical, but it is completely unsuitable for thick sheets, especially when they are large. Cool a sheet metal to a cooling speed greater than 50 ° C / s is all the more difficult that the sheet is thick, and, by the mere fact of the laws that govern transfers heat, it even becomes impossible when the thickness of the sheet exceeds 15 mm.
  • Inter-critical treatments are also unsuitable for manufacture of sheets with very high yield strength. Indeed, these treatments consist in bringing the steel to an intermediate temperature between the austenitization start temperature and the temperature complete austenitization, so that such treatment followed by quenching leads to mixed structures made up of a mixture of structures hardened and very softened ferrite. The presence of very softened ferrite significantly reduces the level of breaking strength it is possible to get.
  • the object of the present invention is to remedy these disadvantages by proposing a weldable steel which makes it possible to manufacture industrially welded sheets greater than 8 mm thick, having a tensile strength greater than 1200 MPa, and having a very good ductility, i.e. a uniform elongation rate greater than 5%.
  • the chemical analysis is adjusted so that: 0.005% ⁇ Ti ⁇ 0.1% 0.01% ⁇ Al ⁇ 0.5% 0.003 ⁇ N ⁇ 0.02% and, when the steel is in the solid state, the number of precipitates of titanium nitrides of size greater than 0.1 ⁇ m counted over an area of 1 mm 2 of a micrographic section, is preferably less than 4 times the total content of titanium precipitated in the form of nitrides, expressed in thousandths of% by weight.
  • the steel contains from 0.5% to 3% of chromium, less than 2% manganese, and the molydene content plus half the tungsten content is between 0.1% and 2%.
  • the sum of the silicon contents and aluminum is between 1.5% and 2.5%, and it is preferable that the carbon content is between 0.2% and 0.3%.
  • the room can be allowed to cool in air.
  • the invention finally relates to a steel part, and in particular a sheet of thickness greater than 8 mm, obtained by the process according to the invention, the tensile strength of which is greater than 1200 MPa and the ductility measured by uniform elongation is greater than 5%.
  • the structure of the part comprises from 5% to 30% and preferably from 10% to 20% residual austenite.
  • steel contains titanium, its structure preferably comprises more than 30% bainite.
  • This part is particularly suitable for manufacturing equipment for mines or quarries which must resist abrasion or manufacture of metal construction parts or parts boilermaking.
  • the steel according to the invention is a structural steel weakly or moderately alloyed which makes it possible to obtain, by a treatment suitable thermal, a mixed structure consisting of bainite and / or martensite, and from 5% to 30%, and preferably from 10% to 20%, of strongly austenite loaded with carbon.
  • the inventors found that such a structure had the advantage of combining a very high tensile strength and a very good ductility, even for low carbon contents, which allows good weldability, but only if the steel contains sufficient alloying elements increasing the hardenability.
  • the increase in ductility results from the instability of the austenite which transforms into martensite when the steel undergoes plastic deformation.
  • austenite content of the structure must be greater than 5% and preferably greater than 10%; however this content must remain below 30% and preferably 20% to avoid reducing the elastic limit too much.
  • the steel must contain more than 0.15% carbon, and preferably more than 0.2%.
  • the content carbon should remain less than or equal to 0.303%, and preferably, less than 0.3%.
  • the optimal content of carbon is between 0.2% and 0.24%.
  • steel To promote carbon enrichment of austenite during heat treatment, steel must contain at least one element taken from silicon and aluminum.
  • the sum of the silicon contents and aluminum should be greater than 1%, and preferably, greater than 1.5%. However, in order to avoid development difficulties, this sum must remain below 3%, and preferably below 2.5%. So the aluminum and silicon contents are each between 0% and 3%.
  • Manganese which greatly increases the hardenability, is also necessary in contents higher than 0.1% to obtain a good hot ductility, but its content must remain below 4.5%, and preferably less than 2%, so as not to stabilize the austenite too much. Of preferably, the manganese content must be between 1.2% and 1.7%.
  • Nickel which is not essential, increases the hardenability and has a favorable effect on the weldability and on the toughness at low temperature. But this item is expensive. In addition, too strong contents, it stabilizes the austenite too much. Also its content must remain below 9%. Preferably, the nickel content must be between 1.5% and 2.5%.
  • Chromium, molybdenum and tungsten are also not essential, but these elements increase the hardenability and above all can form very hardening carbides.
  • chromium content must be greater than 0.5%, and, from preferably also less than 3%, and more preferably less than 1.5%.
  • Tungsten in Any Content Has Effects equivalent to that of molybdenum in half content. Also for these two elements we consider the sum of the molybdenum content and the half the tungsten content. Beyond 3% the effect is no longer significant for the steels concerned, and this value is a maximum. Well that these two elements are not essential, it is desirable that the sum of the molybdenum content and half the content of tungsten is greater than 0.1%. Preferably, the sum of the content molybdenum and half the tungsten content should be less at 2%, and preferably less than 0.5%.
  • steel contains less than 0.02% nitrogen, however it may be desirable to increase the content of this element up to 0.3% to provide additional hardening without harming weldability.
  • the structure of the steel contains more than 30% of bainite, its toughness can be increased by adding between 0.005% and 0.1% of titanium.
  • the steel must then contain between 0.01% and 0.5% aluminum and between 0.003% and 0.02% nitrogen, in addition, the titanium must be added to the steel in a very gradual way to limit the precipitation of large titanium nitrides in liquid steel.
  • a steel is thus obtained which, in the solid state, is such that the number of precipitates of titanium nitrides of size greater than 0.1 ⁇ m counted over an area of 1 mm 2 of a micrographic section, is less than 4 times the total content of precipitated titanium in the form of titanium nitrides, expressed in thousandths of% by weight.
  • titanium is in this form in steel, it considerably refines the structure and the bainitic sub-structure. This has the effect of lowering the resilience transition temperature by at least 30 ° C and significantly increasing the resilience at room temperature, when the steel structure comprises at least 30% bainite.
  • the rest of the chemical composition of steel is made up by iron and by impurities resulting from the production.
  • the steel contains 0.2% 0.24% carbon, 1.5% to 2.5% silicon plus aluminum, 1.2% 1.7% manganese, 1.5% 2.5% nickel, 0.5% 1.5% chromium, from 0.1% to 0.5% molybdenum, possibly from 0.0005% to 0.005% boron, possibly from 0.005% to 0.1% of titanium introduced as shown above.
  • This heat treatment makes it possible to obtain a structure consisting of martensite and / or lower bainite which are not very softened and from 5% to 30% of residual austenite highly enriched in carbon.
  • the slow passage in the vicinity of M s allows the carbon enrichment of the austenite. It must therefore be long enough, but not too long so as not to soften the structure too much.
  • Heat treatment can be done either in the warm of shaped by hot plastic deformation, either after this surgery.
  • the semi-finished product When the heat treatment is done in the hot forming by hot plastic deformation, the semi-finished product must be reheated before plastic deformation to a temperature higher than Ac 3 and lower than 1300 ° C to avoid an excessive magnification of the austenitic grain and the plastic deformation (rolling for example) must preferably end above Ab 3 to prevent the ferrito-pearlitic transformation from starting.
  • the cooling to a temperature close to M s carried out at a cooling rate greater than 0.3 ° C / s, can be carried out, for example, by controlled watering with water.
  • the slow passage in the vicinity of M s can then be obtained by air cooling which can also serve as cooling to room temperature.
  • cooling to room temperature which follows the slow passage in the vicinity of M s , can advantageously be achieved by cooling with water in order to limit as much as possible the self-income of the structure obtained.
  • cooling to the vicinity of M s , the slow passage in the vicinity of M s and cooling to the ambient can be carried out directly by air cooling. This is particularly the case, when the product is a sheet of thickness at least equal to 30 mm. It is also possible to treat, by air cooling, sheets of thickness less than 30 mm, by stacking several sheets so as to form a packet of thickness greater than 30 mm.
  • the product When the heat treatment is carried out after shaping by hot plastic deformation and return to ambient temperature of the product, the product must be austenitized by reheating above Ac 3 in order to obtain a complete austenitization, then it can be cooled either in the same way as when the heat treatment is carried out in the hot forming, or by any suitable means to carry out the recommended thermal cycle.
  • the titanium of steel C was introduced in accordance with the invention.
  • the steels according to the invention have elongations uniforms at least 2.5 times higher than those of steels according to the art prior.
  • These parts are in particular sheets of thickness greater than 8 mm.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Heat Treatment Of Steel (AREA)
  • Heat Treatment Of Sheet Steel (AREA)

Description

La présente invention concerne un acier soudable, ayant une résistance à la traction élevée et une bonne ductilité.The present invention relates to a weldable steel having a high tensile strength and good ductility.

Pour fabriquer des équipements destinés, par exemple, soit à résister à l'abrasion, soit à résister à des chocs concentrés et très énergétiques, on utilise des tôles d'épaisseur supérieure à 8 mm en acier à haute résistance mécanique (résistance à la traction supérieure à 1200 MPa) faiblement allié trempé revenu dont la structure est martensitique ou martensito-bainitique. Les équipements ainsi fabriqués, ont une tenue en service d'autant meilleure que la résistance à la traction de l'acier est élevée, mais aussi que l'énergie de rupture est grande. L'énergie de rupture est d'autant plus grande que la ductilité de l'acier est grande. Cette ductilité est mesurée par le taux d'allongement juste avant striction lors d'un essai de traction (allongement uniforme). Les tôles étant, en général, soudées, l'acier utilisé doit également être soudable. Les aciers faiblement alliés trempés revenus dont la structure est martensitique ou martensito-bainitique permettent de combiner une résistance à la traction élevée et une soudabilité satisfaisante, mais ils ont l'inconvénient de présenter une ductilité très médiocre : l'allongement uniforme devient inférieur à 5% dès que la résistance à la traction dépasse 1200 MPa.To manufacture equipment intended, for example, either for resist abrasion, that is to resist concentrated shocks and very energetic, we use steel sheets thicker than 8 mm high mechanical strength (tensile strength greater than 1200 MPa) low alloy tempered tempered with a martensitic structure or martensito-bainitic. The equipment thus produced has a resistance in service the better the tensile strength of steel high, but also that the breaking energy is large. The energy of the greater the rupture, the greater the ductility of the steel. This ductility is measured by the rate of elongation just before necking during a tensile test (uniform elongation). The sheets being, in general, welded, the steel used must also be weldable. Steels weakly tempered alloys with a martensitic structure or martensito-bainitique allow to combine a tensile strength high and satisfactory weldability, but they have the disadvantage of have very poor ductility: uniform elongation becomes less than 5% as soon as the tensile strength exceeds 1200 MPa.

Pour concilier une résistance à la traction élevée et une bonne ductilité, on a proposé d'utiliser des aciers contenant notamment entre 0,5% et 3% de silicium et soumis à un traitement de trempe étagée après soit une austénitisation complète, soit un traitement inter-critique. Cependant, ces aciers et ces traitements thermiques présentent des inconvénients.To reconcile high tensile strength and good ductility, it has been proposed to use steels containing in particular between 0.5% and 3% silicon and subjected to a stepped quenching treatment after either a complete austenitization, or an inter-critical treatment. However, these steels and these heat treatments have disadvantages.

Les aciers considérés, soit ne sont pas soudables, soit ne permettent pas d'obtenir une résistance à la traction suffisante, soit enfin ne permettent d'obtenir l'ensemble des caractéristiques souhaitées que sur des tôles minces d'épaisseur sensiblement inférieure à 8 mm.The steels considered either are not weldable or do not not provide sufficient tensile strength, or finally allow to obtain all the desired characteristics only on thin sheets of thickness substantially less than 8 mm.

Le traitement thermique de trempe étagée, comprenant un refroidissement à une vitesse de refroidissement supérieure ou égale à 50°C/s jusqu'à une température de maintien, puis un maintien isotherme à cette température, et enfin, un refroidissement jusqu'à la température ambiante, est bien adapté à des tôles minces ou à des petites pièces de mécanique, mais il est tout à fait inadapté à des tôles épaisses, en particulier lorsqu'elles sont de grande dimension. Refroidir une tôle à une vitesse de refroidissement supérieure à 50°C/s est d'autant plus difficile que la tôle est épaisse, et, du seul fait des lois qui régissent les transferts de chaleur, cela devient même impossible lorsque l'épaisseur de la tôle dépasse 15 mm. De plus, compléter un refroidissement rapide par un maintien isotherme est une opération courante pour de petites pièces de mécanique, par exemple en utilisant un bain de sel, ou pour une bande mince bobinée en sortie de laminoir à chaud, mais c'est une opération très malcommode et donc très coûteuse lorsque elle doit être effectuée sur une tôle épaisse de grande dimension.Staged quenching heat treatment, comprising a cooling at a cooling rate greater than or equal to 50 ° C / s up to a holding temperature, then an isothermal holding at this temperature, and finally, cooling down to the temperature ambient, is well suited for thin sheets or small parts of mechanical, but it is completely unsuitable for thick sheets, especially when they are large. Cool a sheet metal to a cooling speed greater than 50 ° C / s is all the more difficult that the sheet is thick, and, by the mere fact of the laws that govern transfers heat, it even becomes impossible when the thickness of the sheet exceeds 15 mm. In addition, supplement rapid cooling with a insulated holding is a common operation for small parts of mechanical, for example using a salt bath, or for a band thin wirewound at the exit of the hot rolling mill, but it is a very inconvenient and therefore very expensive when it must be performed on a large thick sheet.

Les traitements inter-critiques sont également inadaptés à la fabrication de tôles à très haute limite d'élasticité. En effet, ces traitements consistent à porter l'acier à une température intermédiaire entre la température de début d'austénitisation et la température d'austénitisation complète, si bien qu'un tel traitement suivi d'une trempe conduit à des structures mixtes constituées d'un mélange de structures trempées et de ferrite très adoucie. La présence de ferrite très adoucie réduit significativement le niveau de résistance à la rupture qu'il est possible d'obtenir. Inter-critical treatments are also unsuitable for manufacture of sheets with very high yield strength. Indeed, these treatments consist in bringing the steel to an intermediate temperature between the austenitization start temperature and the temperature complete austenitization, so that such treatment followed by quenching leads to mixed structures made up of a mixture of structures hardened and very softened ferrite. The presence of very softened ferrite significantly reduces the level of breaking strength it is possible to get.

Dans le brevet US 2,791,500, on a proposé, pour fabriquer des pièces de structure à hautes caractéristiques pour l'aéronautique, un acier contenant essentiellement de 1,5% à 3,5% de nickel, de 0,7% à 1,5% de chrome, de 0,1% à 0,5% de molybdène, de 0,35% à 0,45% de carbone, de 1,3% à 2% de silicium, de 0,5% à 1 % de manganèse, au moins 0,02%, et de préférence pas plus de 1%, d'aluminium, le reste étant du fer. Cet acier trempé et revenu permet d'obtenir une résistance à la traction supérieure à 1930 MPa. In US Patent 2,791,500, it has been proposed to manufacture structural parts with high characteristics for aeronautics, steel containing essentially from 1.5% to 3.5% of nickel, from 0.7% to 1.5% of chromium, from 0.1% to 0.5% of molybdenum, from 0.35% to 0.45% of carbon, 1.3% to 2% silicon, 0.5% to 1% manganese, at least 0.02%, and preferably not more than 1%, of aluminum, the remainder being iron. This quenched and tempered steel provides tensile strength greater than 1930 MPa.

Le but de la présente invention est de remédier à ces inconvénients en proposant un acier soudable qui permette de fabriquer de façon industrielle des tôles d'épaisseur supérieure à 8 mm, soudables, ayant une résistance à la traction supérieure à 1200 MPa, et ayant une très bonne ductilité, c'est à dire un taux d'allongement uniforme supérieur à 5%.The object of the present invention is to remedy these disadvantages by proposing a weldable steel which makes it possible to manufacture industrially welded sheets greater than 8 mm thick, having a tensile strength greater than 1200 MPa, and having a very good ductility, i.e. a uniform elongation rate greater than 5%.

A cet effet, l'invention a pour objet un acier dont la composition chimique, en poids, comprend : 0,15% ≤ C ≤ 0,303% 0% ≤ Si ≤ 3% 0% ≤ Al ≤ 3% 0,1% ≤ Mn ≤ 4,5% 0% ≤ Ni ≤ 9% 0% ≤ Cr ≤ 6% 0% ≤ Mo + W/2 ≤ 3% 0% ≤ V ≤ 0,5% 0% ≤ Nb ≤ 0,5% 0% ≤ Zr ≤ 0,5% N ≤ 0,3%

  • éventuellement de 0,0005% à 0,005% de bore,
  • éventuellement de 0,005% à 0,1% de titane,
  • éventuellement au mois un élément pris parmi Ca, Se, Te, Bi et Pb en des teneurs inférieures à 0,2%,
le reste étant du fer et des impuretés résultant de l'élaboration,
la composition chimique satisfaisant en outre aux relations : 1% ≤ Si + Al ≤ 3% et, 4,6x(%C)+1,05x(%Mn) + 0,54x(%Ni) + 0,66x(%Mo + %W/2) + 0,5x(%Cr) +K ≥ 3,8 avec
  • K = 0,5 lorsque l'acier contient du Bore,
  • K = 0 lorsque l'acier ne contient pas de Bore.
    •  To this end, the subject of the invention is a steel whose chemical composition, by weight, comprises: 0.15% ≤ C ≤ 0.303% 0% ≤ If ≤ 3% 0% ≤ Al ≤ 3% 0.1% ≤ Mn ≤ 4.5% 0% ≤ Ni ≤ 9% 0% ≤ Cr ≤ 6% 0% ≤ Mo + W / 2 ≤ 3% 0% ≤ V ≤ 0.5% 0% ≤ Nb ≤ 0.5% 0% ≤ Zr ≤ 0.5% N ≤ 0.3%
    • optionally from 0.0005% to 0.005% boron,
    • optionally from 0.005% to 0.1% of titanium,
    • optionally at least one element taken from Ca, Se, Te, Bi and Pb in contents of less than 0.2%,
    the rest being iron and impurities resulting from the production,
    the chemical composition also satisfying the relationships: 1% ≤ Si + Al ≤ 3% and, 4.6x (% C) + 1.05x (% Mn) + 0.54x (% Ni) + 0.66x (% Mo +% W / 2) + 0.5x (% Cr) + K ≥ 3.8 with
  • K = 0.5 when the steel contains Boron,
  • K = 0 when the steel does not contain Boron.

    • Dans un mode particulier de réalisation, l'analyse chimique est ajustée pour que: 0,005% ≤ Ti ≤ 0,1% 0,01% ≤ Al ≤ 0,5% 0,003 ≤ N ≤ 0,02% et, lorsque l'acier est à l'état solide, le nombre de précipités de nitrures de titane de taille supérieure à 0,1 µm comptés sur une aire de 1 mm2 d'une coupe micrographique, est de préférence inférieure à 4 fois la teneur totale en titane précipité sous forme de nitrures, exprimée en millièmes de % en poids.In a particular embodiment, the chemical analysis is adjusted so that: 0.005% ≤ Ti ≤ 0.1% 0.01% ≤ Al ≤ 0.5% 0.003 ≤ N ≤ 0.02% and, when the steel is in the solid state, the number of precipitates of titanium nitrides of size greater than 0.1 μm counted over an area of 1 mm 2 of a micrographic section, is preferably less than 4 times the total content of titanium precipitated in the form of nitrides, expressed in thousandths of% by weight.

    De préférence l'acier contient de 0,5% à 3% de chrome, moins de 2% de manganèse, et la teneur en molydène plus la moitié de la teneur en tungstène est comprise entre 0,1% et 2%.Preferably the steel contains from 0.5% to 3% of chromium, less than 2% manganese, and the molydene content plus half the tungsten content is between 0.1% and 2%.

    Il est souhaitable que la somme des teneurs en silicium et aluminium soit comprise entre 1,5% et 2,5%, et il est préférable que la teneur en carbone soit comprise entre 0,2% et 0,3%.It is desirable that the sum of the silicon contents and aluminum is between 1.5% and 2.5%, and it is preferable that the carbon content is between 0.2% and 0.3%.

    De préférence, la composition chimique de l'acier comprend, en poids : 0,20% ≤ C ≤ 0,24% 0% ≤ Si ≤ 2,5% 0% ≤ Al ≤ 2,5% 1,2% ≤ Mn ≤ 1,7% 1,5% ≤ Ni ≤ 2,5% 0,5% ≤ Cr ≤ 1,5% 0,1% ≤ Mo + W/2 ≤ 0,5% la composition chimique satisfaisant en outre aux relations : 1,5% ≤ Si + Al ≤ 2,5% et, 4,6x(%C)+1,05x(%Mn) + 0,54x(%Ni) + 0,66x(%Mo + %W/2) + 0,5x(%Cr) +K ≥ 3,8 avec,

  • K = 0,5 lorsque l'acier contient du Bore,
  • K = 0 lorsque l'acier ne contient pas de Bore.
    • Preferably, the chemical composition of the steel comprises, by weight: 0.20% ≤ C ≤ 0.24% 0% ≤ If ≤ 2.5% 0% ≤ Al ≤ 2.5% 1.2% ≤ Mn ≤ 1.7% 1.5% ≤ Ni ≤ 2.5% 0.5% ≤ Cr ≤ 1.5% 0.1% ≤ Mo + W / 2 ≤ 0.5% the chemical composition also satisfying the relationships: 1.5% ≤ Si + Al ≤ 2.5% and, 4.6x (% C) + 1.05x (% Mn) + 0.54x (% Ni) + 0.66x (% Mo +% W / 2) + 0.5x (% Cr) + K ≥ 3.8 with,
  • K = 0.5 when the steel contains Boron,
  • K = 0 when the steel does not contain Boron.

    • L'invention concerne également un procédé pour la fabrication d'une pièce en acier à haute résistance et haute ductilité selon lequel :

    • on élabore un acier conforme à l'invention,
    • on coule l'acier et on le fait solidifier sous forme d'un demi-produit,
    • on forme par déformation plastique à chaud le demi-produit pour obtenir une pièce en acier,
    • on austénitise la pièce par chauffage au dessus de Ac3, puis on la refroidit jusqu'à la température ambiante de telle sorte que la vitesse de refroidissement entre la température d'austénitisation et Ms + 150°C soit supérieure à 0,3°C/s, que le temps de séjour entre Ms + 150°C et Ms - 50°C soit compris entre 5 minutes et 90 minutes, et que la vitesse de refroidissement en dessous de Ms - 50°C soit supérieure à 0,02°C/s.
    The invention also relates to a method for manufacturing a high-strength, high-ductility steel part, according to which:
    • a steel is produced in accordance with the invention,
    • we pour the steel and solidify it in the form of a semi-finished product,
    • the semi-finished product is formed by hot plastic deformation to obtain a steel part,
    • the part is austenitized by heating above Ac 3 , then cooled to room temperature so that the cooling rate between the austenitization temperature and M s + 150 ° C is greater than 0.3 ° C / s, that the residence time between M s + 150 ° C and M s - 50 ° C is between 5 minutes and 90 minutes, and that the cooling rate below M s - 50 ° C is greater than 0.02 ° C / s.

    Dans un autre mode de réalisation du procédé:

    • on élabore un acier conformément à l'invention,
    • on coule l'acier et on le fait solidifier sous forme d'un demi-produit,
    • on réchauffe le demi-produit à une température inférieure à 1300°C et on le met en forme par déformation plastique à chaud de telle façon que la température de fin de mise en forme par déformation plastique à chaud soit supérieure à Ac3, pour obtenir une pièce en acier,
    • on refroidit la pièce en acier jusqu'à la température ambiante de telle sorte que la vitesse de refroidissement entre la température d'austénitisation et Ms + 150°C soit supérieure à 0,3°C/s, que le temps de séjour entre Ms + 150°C et Ms - 50°C soit compris entre 5 minutes et 90 minutes, et que la vitesse de refroidissement en dessous de Ms - 50°C soit supérieure à 0,02°C/s.
    In another embodiment of the method:
    • a steel is produced in accordance with the invention,
    • we pour the steel and solidify it in the form of a semi-finished product,
    • the semi-finished product is heated to a temperature below 1300 ° C. and it is shaped by hot plastic deformation so that the temperature at the end of shaping by hot plastic deformation is greater than Ac 3 , to obtain a steel part,
    • the steel part is cooled to ambient temperature so that the cooling rate between the austenitization temperature and M s + 150 ° C is greater than 0.3 ° C / s, that the residence time between M s + 150 ° C and M s - 50 ° C is between 5 minutes and 90 minutes, and that the cooling rate below M s - 50 ° C is greater than 0.02 ° C / s.

    Dans les deux cas, pour refroidir la pièce jusqu'à la température ambiante, on peut laisser la pièce refroidir à l'air.In both cases, to cool the room down to the at room temperature, the room can be allowed to cool in air.

    L'invention concerne enfin une pièce en acier, et notamment une tôle d'épaisseur supérieure à 8 mm, obtenue par le procédé selon l'invention, dont la résistance à la traction est supérieure à 1200 MPa et la ductilité mesurée par l'allongement uniforme est supérieure à 5%. La structure de la pièce comporte de 5% à 30% et de préférence de 10% à 20% d'austénite résiduelle. Lorsque l'acier contient du titane, sa structure comporte, de préférence, plus de 30% de bainite.The invention finally relates to a steel part, and in particular a sheet of thickness greater than 8 mm, obtained by the process according to the invention, the tensile strength of which is greater than 1200 MPa and the ductility measured by uniform elongation is greater than 5%. The structure of the part comprises from 5% to 30% and preferably from 10% to 20% residual austenite. When steel contains titanium, its structure preferably comprises more than 30% bainite.

    Cette pièce est particulièrement adaptée à la fabrication d'équipements pour mines ou carrières devant résister à l'abrasion, ou à la fabrication de pièces de construction métallique ou de pièces chaudronnées.This part is particularly suitable for manufacturing equipment for mines or quarries which must resist abrasion or manufacture of metal construction parts or parts boilermaking.

    L'invention va maintenant être décrite plus en détails mais de façon non limitative.The invention will now be described in more detail, but from without limitation.

    L'acier selon l'invention est un acier de construction faiblement ou moyennement allié qui permet d'obtenir, par un traitement thermique adapté, une structure mixte constituée de bainite et/ou de martensite, et de 5% à 30%, et de préférence de 10% à 20%, d'austénite fortement chargée en carbone. Les inventeurs on constaté qu'une telle structure avait l'avantage de combiner une très grande résistance à la traction et une très bonne ductilité, même pour des teneurs en carbone faibles, ce qui permet d'obtenir une bonne soudabilité, mais à condition que l'acier contienne suffisamment d'éléments d'alliage augmentant la trempabilité. L'augmentation de ductilité résulte de l'instabilité de l'austénite qui se transforme en martensite lorsque l'acier subit une déformation plastique. La transformation de l'austénite en martensite, induite par la déformation plastique, a un effet sur le coefficient d'écrouissage qui favorise l'augmentation du taux d'allongement uniforme mesuré lors d'un essai de traction. Pour que cet effet soit significatif, la teneur en austénite de la structure doit être supérieure à 5% et de préférence supérieure à 10% ; cependant cette teneur doit rester inférieure à 30% et de préférence 20% pour éviter de trop réduire la limite d'élasticité.The steel according to the invention is a structural steel weakly or moderately alloyed which makes it possible to obtain, by a treatment suitable thermal, a mixed structure consisting of bainite and / or martensite, and from 5% to 30%, and preferably from 10% to 20%, of strongly austenite loaded with carbon. The inventors found that such a structure had the advantage of combining a very high tensile strength and a very good ductility, even for low carbon contents, which allows good weldability, but only if the steel contains sufficient alloying elements increasing the hardenability. The increase in ductility results from the instability of the austenite which transforms into martensite when the steel undergoes plastic deformation. The transformation of austenite into martensite, induced by deformation plastic, has an effect on the work hardening coefficient which favors the increase in the uniform elongation rate measured during a traction. For this effect to be significant, the austenite content of the structure must be greater than 5% and preferably greater than 10%; however this content must remain below 30% and preferably 20% to avoid reducing the elastic limit too much.

    Pour permettre d'obtenir une résistance à la traction supérieure à 1200 MPa, l'acier doit contenir plus de 0,15% de carbone, et de préférence, plus de 0,2%. Pour éviter de détériorer la soudabilité, la teneur en carbone doit rester inférieure ou égale à 0,303%, et de préférence, inférieure à 0,3%. Pour les applications envisagées, la teneur optimale en carbone est comprise entre 0,2% et 0,24%.To provide superior tensile strength at 1200 MPa, the steel must contain more than 0.15% carbon, and preferably more than 0.2%. To avoid deteriorating the weldability, the content carbon should remain less than or equal to 0.303%, and preferably, less than 0.3%. For the intended applications, the optimal content of carbon is between 0.2% and 0.24%.

    Pour favoriser l'enrichissement en carbone de l'austénite pendant le traitement thermique, l'acier doit contenir au moins un élément pris parmi le silicium et l'aluminium. La somme des teneurs en silicium et aluminium doit être supérieure à 1%, et de préférence, supérieure à 1,5%. Cependant, afin d'éviter des difficultés d'élaboration, cette somme doit rester inférieure à 3%, et de préférence, inférieure à 2,5%. Ainsi, les teneurs en aluminium et en silicium sont chacune comprises entre 0% et 3%.To promote carbon enrichment of austenite during heat treatment, steel must contain at least one element taken from silicon and aluminum. The sum of the silicon contents and aluminum should be greater than 1%, and preferably, greater than 1.5%. However, in order to avoid development difficulties, this sum must remain below 3%, and preferably below 2.5%. So the aluminum and silicon contents are each between 0% and 3%.

    Pour obtenir les propriétés recherchées, et notamment pour permettre de fabriquer dans des conditions satisfaisantes des tôles d'épaisseur supérieure à 8 mm ayant les caractéristiques requises, l'acier doit être suffisamment trempant pour qu'un traitement thermique adapté permette d'obtenir une structure constituée d'austénite et de bainite inférieure ou de martensite, et qui ne contienne ni de ferrite granulaire ni de ferrite-perlite. Pour cela, l'acier doit contenir au moins un élément pris parmi le manganèse, le nickel, le chrome, le molybdène, le tungstène ou le bore, et sa composition chimique doit satisfaire la relation: 4,6x(%C)+1,05x(%Mn) + 0,54x(%Ni) + 0,66x(%Mo + %W/2) + 0,5x(%Cr) +K ≥ 3,8 avec

  • K = 0,5 lorsque l'acier contient du Bore,
  • K = 0 lorsque l'acier ne contient pas de Bore.
    • To obtain the desired properties, and in particular to make it possible to manufacture, under satisfactory conditions, sheets of thickness greater than 8 mm having the required characteristics, the steel must be sufficiently quenching so that a suitable heat treatment makes it possible to obtain a structure consisting of austenite and lower bainite or martensite, and which contains neither granular ferrite nor ferrite-perlite. For this, the steel must contain at least one element taken from manganese, nickel, chromium, molybdenum, tungsten or boron, and its chemical composition must satisfy the relationship: 4.6x (% C) + 1.05x (% Mn) + 0.54x (% Ni) + 0.66x (% Mo +% W / 2) + 0.5x (% Cr) + K ≥ 3.8 with
  • K = 0.5 when the steel contains Boron,
  • K = 0 when the steel does not contain Boron.

    • Le manganèse, qui augmente fortement la trempabilité, est également nécessaire en des teneurs supérieures à 0,1% pour obtenir une bonne ductilité à chaud, mais sa teneur doit rester inférieure à 4,5%, et de préférence, inférieure à 2%, pour ne pas trop stabiliser l'austénite. De préférence, la teneur en manganèse doit être comprise entre 1,2% et 1,7%.Manganese, which greatly increases the hardenability, is also necessary in contents higher than 0.1% to obtain a good hot ductility, but its content must remain below 4.5%, and preferably less than 2%, so as not to stabilize the austenite too much. Of preferably, the manganese content must be between 1.2% and 1.7%.

    Le nickel, qui n'est pas indispensable, augmente la trempabilité et a un effet favorable sur la soudabilité et sur la ténacité à basse température. Mais cet élément est coûteux. De plus, en trop fortes teneurs, il stabilise trop l'austénite. Aussi sa teneur doit rester inférieure à 9%. De préférence, la teneur en nickel doit être comprise entre 1,5% et 2,5%. Nickel, which is not essential, increases the hardenability and has a favorable effect on the weldability and on the toughness at low temperature. But this item is expensive. In addition, too strong contents, it stabilizes the austenite too much. Also its content must remain below 9%. Preferably, the nickel content must be between 1.5% and 2.5%.

    Le chrome, le molybdène et le tungstène ne sont pas non plus indispensables, mais ces éléments augmentent la trempabilité et surtout peuvent former des carbures très durcissants.Chromium, molybdenum and tungsten are also not essential, but these elements increase the hardenability and above all can form very hardening carbides.

    Au delà de 6% le chrome n'a plus d'effet significatif pour les aciers considérés, aussi sa teneur maximale est limitée à cette valeur. De préférence, la teneur en chrome doit être supérieure à 0,5%, et, de préférence également, inférieure à 3%, et mieux encore, inférieure à 1,5%.Above 6% chromium no longer has a significant effect on steels considered, also its maximum content is limited to this value. Of preferably, the chromium content must be greater than 0.5%, and, from preferably also less than 3%, and more preferably less than 1.5%.

    Le tungstène en une teneur quelconque a des effets équivalents à ceux du molybdène en une teneur moitié. Aussi pour ces deux éléments on considère la somme de la teneur en molybdène et de la moitié de la teneur en tungstène. Au delà de 3% l'effet n'est plus significatif pour les aciers concernés, et cette valeur est un maximum. Bien que ces deux éléments ne soient pas indispensables, il est souhaitable que la somme de la teneur en molybdène et de la moitié de la teneur en tungstène soit supérieure à 0,1%. De préférence, la somme de la teneur en molybdène et de la moitié de la teneur en tungstène doit être inférieure à 2%, et, de préférence, inférieure à 0,5%.Tungsten in Any Content Has Effects equivalent to that of molybdenum in half content. Also for these two elements we consider the sum of the molybdenum content and the half the tungsten content. Beyond 3% the effect is no longer significant for the steels concerned, and this value is a maximum. Well that these two elements are not essential, it is desirable that the sum of the molybdenum content and half the content of tungsten is greater than 0.1%. Preferably, the sum of the content molybdenum and half the tungsten content should be less at 2%, and preferably less than 0.5%.

    Pour augmenter la trempabilité sans modifier les autres propriétés de l'acier, on peut sans que ce soit obligatoire, ajouter entre 0,0005% et 0,005% de bore.To increase the hardenability without modifying the others properties of steel, we can without it being compulsory, add between 0.0005% and 0.005% boron.

    Afin d'augmenter un peu la dureté, on peut ajouter au moins un élément pris parmi le vanadium, le niobium ou le zirconium, en des teneurs comprises entre 0% et 0,5% pour chacun de ces éléments.In order to slightly increase the hardness, we can add at least an element taken from vanadium, niobium or zirconium, in contents between 0% and 0.5% for each of these elements.

    Habituellement, l'acier contient moins de 0,02% d'azote, cependant il peut être souhaitable d'augmenter la teneur en cet élément jusqu'à 0,3% pour apporter un durcissement complémentaire sans nuire à la soudabilité. Usually, steel contains less than 0.02% nitrogen, however it may be desirable to increase the content of this element up to 0.3% to provide additional hardening without harming weldability.

    Lorsque la structure de l'acier contient plus de 30% de bainite, on peut augmenter sa ténacité en ajoutant entre 0,005% et 0,1% de titane. Pour que cette addition soit efficace, l'acier doit alors contenir entre 0,01% et 0,5% d'aluminium et entre 0,003% et 0,02% d'azote, de plus, le titane doit être ajouté dans l'acier de façon très progressive pour limiter la précipitation de gros nitrures de titane dans l'acier liquide. Pour cela, on peut, par exemple, recouvrir l'acier liquide non désoxydé par un laitier, ajouter du titane dans le laitier, puis ajouter l'aluminium dans l'acier liquide, et enfin, brasser avec un gaz neutre. On obtient ainsi un acier qui, à l'état solide, est tel que le nombre de précipités de nitrures de titane de taille supérieure à 0,1 µm comptés sur une aire de 1 mm2 d'une coupe micrographique, est inférieure à 4 fois la teneur totale en titane précipité sous forme de nitrures de titane, exprimée en millièmes de % en poids. Lorsque le titane est sous cette forme dans l'acier, il affine considérablement la structure et la sous structure bainitique. Ceci a pour effet d'abaisser d'au moins 30°C la température de transition en résilience et d'augmenter significativement la résilience à la température ambiante, lorsque la structure de l'acier comporte au moins 30% de bainite.When the structure of the steel contains more than 30% of bainite, its toughness can be increased by adding between 0.005% and 0.1% of titanium. For this addition to be effective, the steel must then contain between 0.01% and 0.5% aluminum and between 0.003% and 0.02% nitrogen, in addition, the titanium must be added to the steel in a very gradual way to limit the precipitation of large titanium nitrides in liquid steel. For this, one can, for example, cover the non-deoxidized liquid steel with a slag, add titanium to the slag, then add the aluminum to the liquid steel, and finally, stir with a neutral gas. A steel is thus obtained which, in the solid state, is such that the number of precipitates of titanium nitrides of size greater than 0.1 μm counted over an area of 1 mm 2 of a micrographic section, is less than 4 times the total content of precipitated titanium in the form of titanium nitrides, expressed in thousandths of% by weight. When titanium is in this form in steel, it considerably refines the structure and the bainitic sub-structure. This has the effect of lowering the resilience transition temperature by at least 30 ° C and significantly increasing the resilience at room temperature, when the steel structure comprises at least 30% bainite.

    Enfin, pour améliorer la ténacité ou pour améliorer l'usinabilité, on peut ajouter au moins un élément pris parmi le calcium, le sélénium, le tellure, le bismuth ou le plomb, en des teneurs inférieures à 0,2%.Finally, to improve toughness or to improve machinability, at least one element taken from calcium, selenium, tellurium, bismuth or lead, in contents lower than 0.2%.

    Le reste de la composition chimique de l'acier est constitué par du fer et par des impuretés résultant de l'élaboration.The rest of the chemical composition of steel is made up by iron and by impurities resulting from the production.

    Dans un mode préféré de réalisation, l'acier contient de 0,2% à 0,24% de carbone, de 1,5% à2,5% de silicium plus aluminium, de 1,2% à 1,7% de manganèse, de 1,5% à 2,5% de nickel, de 0,5% à 1,5% de chrome, de 0,1% à 0,5% de molybdène, éventuellement de 0,0005% à 0,005% de bore, éventuellement de 0,005% à 0,1% de titane introduit comme indiqué ci dessus.In a preferred embodiment, the steel contains 0.2% 0.24% carbon, 1.5% to 2.5% silicon plus aluminum, 1.2% 1.7% manganese, 1.5% 2.5% nickel, 0.5% 1.5% chromium, from 0.1% to 0.5% molybdenum, possibly from 0.0005% to 0.005% boron, possibly from 0.005% to 0.1% of titanium introduced as shown above.

    Avec l'acier ainsi défini on peut fabriquer des pièces en acier, et notamment des tôles d'épaisseur supérieure à 8 mm, dont la résistance à la traction est supérieure à 1200 MPa et dont l'allongement uniforme est supérieur à 5%. Pour cela, on élabore un acier liquide conforme à l'invention, on le coule et on le fait se solidifier sous forme d'un demi produit qu'on met en forme par déformation plastique à chaud, par exemple par laminage ou par forgeage, et qu'on le soumet à un traitement thermique constitué par :

    • une austénitisation à une température supérieure à la température Ac3 de complète austénitisation de l'acier;
    • suivi d'un refroidissement jusqu'à la température ambiante dans des conditions telles que la vitesse de refroidissement entre la température d'austénitisation et la température égale à Ms+150°C, et de préférence Ms+100°C, (Ms est la température de début de transformation martensitique ) soit supérieure à 0,3°C/s, et telles que le temps de passage entre Ms+150°C, et de préférence Ms+100°C, et Ms-50 C, et de préférence Ms, soit compris entre 5 minutes et 90 minutes, et de préférence, entre 15 minutes et 50 minutes. Le refroidissement jusqu'à la température ambiante doit se faire à une vitesse de refroidissement supérieure à 0,02°C/s pour éviter un adoucissement excessif de la martensite.
    With the steel thus defined, it is possible to manufacture steel parts, and in particular sheets of thickness greater than 8 mm, whose tensile strength is greater than 1200 MPa and whose uniform elongation is greater than 5%. For this, a liquid steel is produced in accordance with the invention, it is poured and it is made to solidify in the form of a semi-product which is shaped by hot plastic deformation, for example by rolling or forging, and that it is subjected to a heat treatment consisting of:
    • austenitization at a temperature above the temperature Ac 3 of complete austenitization of the steel;
    • followed by cooling to room temperature under conditions such as the cooling rate between the austenitization temperature and the temperature equal to M s + 150 ° C, and preferably M s + 100 ° C, (M s is the martensitic transformation start temperature) is greater than 0.3 ° C / s, and such that the passage time between M s + 150 ° C, and preferably M s + 100 ° C, and M s - 50 C, and preferably M s , is between 5 minutes and 90 minutes, and preferably between 15 minutes and 50 minutes. Cooling to room temperature should be done at a cooling rate greater than 0.02 ° C / s to avoid excessive softening of the martensite.

    Ce traitement thermique permet d'obtenir une structure constituée de martensite et/ou de bainite inférieure peu adoucies et de 5% à 30% d'austénite résiduelle fortement enrichie en carbone. En particulier, le passage lent au voisinage de Ms permet l'enrichissement en carbone de l'austénite. Il doit donc être suffisamment long, mais pas trop pour ne pas trop adoucir la structure.This heat treatment makes it possible to obtain a structure consisting of martensite and / or lower bainite which are not very softened and from 5% to 30% of residual austenite highly enriched in carbon. In particular, the slow passage in the vicinity of M s allows the carbon enrichment of the austenite. It must therefore be long enough, but not too long so as not to soften the structure too much.

    Le traitement thermique peut être fait soit dans la chaude de mise en forme par déformation plastique à chaud, soit après cette opération.Heat treatment can be done either in the warm of shaped by hot plastic deformation, either after this surgery.

    Lorsque le traitement thermique est fait dans la chaude de mise en forme par déformation plastique à chaud, le demi produit doit être réchauffé avant déformation plastique à une température supérieure à Ac3 et inférieure à 1300°C pour éviter un grossissement excessif du grain austénitique, et la déformation plastique (le laminage par exemple) doit se terminer préférentiellement au dessus de Ac3 pour éviter que la transformation ferrito-perlitique ne commence.When the heat treatment is done in the hot forming by hot plastic deformation, the semi-finished product must be reheated before plastic deformation to a temperature higher than Ac 3 and lower than 1300 ° C to avoid an excessive magnification of the austenitic grain and the plastic deformation (rolling for example) must preferably end above Ab 3 to prevent the ferrito-pearlitic transformation from starting.

    Dans tous les cas, le refroidissement jusqu'à une température voisine de Ms, réalisé à une vitesse de refroidissement supérieure 0,3°C/s, peut être effectué, par exemple, par un arrosage contrôlé avec de l'eau. Le passage lent au voisinage de Ms peut alors être obtenu par un refroidissement à l'air qui peut servir également de refroidissement jusqu'à la température ambiante. Cependant, le refroidissement jusqu'à la température ambiante, qui suit le passage lent au voisinage de Ms, peut être réalisé avantageusement par refroidissement à l'eau afin de limiter le plus possible l'auto revenu de la structure obtenue.In all cases, the cooling to a temperature close to M s , carried out at a cooling rate greater than 0.3 ° C / s, can be carried out, for example, by controlled watering with water. The slow passage in the vicinity of M s can then be obtained by air cooling which can also serve as cooling to room temperature. However, cooling to room temperature, which follows the slow passage in the vicinity of M s , can advantageously be achieved by cooling with water in order to limit as much as possible the self-income of the structure obtained.

    Lorsque la massivité du produit s'y prête, le refroidissement jusqu'au voisinage de Ms, le passage lent au voisinage de Ms et le refroidissement jusqu'à l'ambiante peuvent être réalisés directement par un refroidissement à l'air. C'est le cas notamment, lorsque le produit est une tôle d'épaisseur au moins égale à 30 mm. On peut également traiter par refroidissement à l'air des tôles d'épaisseur inférieure à 30 mm, en empilant plusieurs tôles de façon à former un paquet d'épaisseur supérieure à 30 mm.When the bulk of the product is suitable, cooling to the vicinity of M s , the slow passage in the vicinity of M s and cooling to the ambient can be carried out directly by air cooling. This is particularly the case, when the product is a sheet of thickness at least equal to 30 mm. It is also possible to treat, by air cooling, sheets of thickness less than 30 mm, by stacking several sheets so as to form a packet of thickness greater than 30 mm.

    Lorsque le traitement thermique est effectué après la mise en forme par déformation plastique à chaud et retour à la température ambiante du produit, le produit doit être austénitisé par réchauffage au dessus de Ac3 afin d'obtenir une austénitisation complète, puis il peut être refroidi soit de la même façon que lorsque le traitement thermique est effectué dans la chaude de mise en forme, soit par tout moyen adapté pour réaliser le cycle thermique recommandé.When the heat treatment is carried out after shaping by hot plastic deformation and return to ambient temperature of the product, the product must be austenitized by reheating above Ac 3 in order to obtain a complete austenitization, then it can be cooled either in the same way as when the heat treatment is carried out in the hot forming, or by any suitable means to carry out the recommended thermal cycle.

    A titre d'exemple, on a réalisé des tôles de 20 mm d'épaisseur avec les aciers A et C selon l'invention, et à titre de comparaison, avec l'acier B selon l'art antérieur.For example, we made sheets of 20 mm thick with steels A and C according to the invention, and as comparison, with steel B according to the prior art.

    Les compositions chimiques de ces aciers étaient, en millièmes de % en poids: C Si Al Mn Ni Cr Mo B Ti A 215 2050 65 1430 2044 1020 210 2,7 0 B 252 395 67 1570 660 1615 207 2,9 0 C 219 1994 27 1447 2020 1008 203 2,6 23 The chemical compositions of these steels were, in thousandths of% by weight: VS Yes Al Mn Or Cr Mo B Ti AT 215 2050 65 1430 2044 1020 210 2.7 0 B 252 395 67 1570 660 1615 207 2.9 0 VS 219 1994 27 1447 2020 1008 203 2.6 23

    Le titane de l'acier C a été introduit conformément à l'invention.The titanium of steel C was introduced in accordance with the invention.

    Les traitements thermiques auxquels ont été soumis les tôles ont tous comporté une austénitisation de 30 minutes à 900°C suivie de :

    • acier A, premier exemple conforme à l'invention : refroidissement à l'air de deux tôles empilées (épaisseur du bloc 40 mm),
    • acier A, deuxième exemple conforme à l'invention : refroidissement à l'air d'une tôle avec palier de 20 minutes à 338°C ( Ms +20°C ), refroidissement à l'air jusqu'à l'ambiante,
    • acier C, exemple conforme à l'invention : refroidissement à l'air de deux tôles empilées (épaisseur du bloc 40 mm),
    • acier B, selon l'art antérieur, refroidissement à l'air d'une tôle.
    The heat treatments to which the sheets were subjected all included an austenitization of 30 minutes at 900 ° C. followed by:
    • steel A, first example according to the invention: air cooling of two stacked sheets (block thickness 40 mm),
    • steel A, second example according to the invention: air cooling of a sheet with 20-minute plateau at 338 ° C (M s + 20 ° C), air cooling to room temperature,
    • steel C, example according to the invention: air cooling of two stacked sheets (block thickness 40 mm),
    • steel B, according to the prior art, air cooling of a sheet.

    Les caractéristiques mécaniques obtenues ont été les suivantes: Rm Re allongement Kcv austénite MPa MPa uniforme total J/cm2 résiduelle A 1er 1487 769 8,7% 16,5% 45 12% A 2ème 1442 743 9,5% 17,7% 49 13% B art ant 1492 1045 3,2% 9,9% 61 3,5% C 1483 775 8,9% 16,5% 74 12% The mechanical characteristics obtained were as follows: Rm Re elongation Kcv austenite MPa MPa uniform total J / cm 2 residual 1st 1487 769 8.7% 16.5% 45 12% A 2nd 1442 743 9.5% 17.7% 49 13% B art ant 1492 1045 3.2% 9.9% 61 3.5% VS 1483 775 8.9% 16.5% 74 12%

    Toujours à titre d'exemple, on a réalisé des tôles de 20 mm d'épaisseur avec les aciers D et F selon l'invention, et à titre de comparaison avec les aciers E et G selon l'art antérieur.Still by way of example, we made 20 mm sheets thick with steels D and F according to the invention, and as comparison with steels E and G according to the prior art.

    Les compositions chimiques de ces aciers étaient, en millièmes de pour-cent en poids : C Si Al Mn Ni Cr Mo B D 303 880 1050 195 4110 559 175 0 E 357 380 27 1450 1546 685 223 0 F 152 928 954 1475 2536 1047 215 2,8 G 182 351 23 1492 254 1717 176 0 The chemical compositions of these steels were, in thousandths of a percent by weight: VS Yes Al Mn Or Cr Mo B D 303 880 1050 195 4110 559 175 0 E 357 380 27 1450 1546 685 223 0 F 152 928 954 1475 2536 1047 215 2.8 G 182 351 23 1492 254 1717 176 0

    Les tôles réalisées avec les aciers D, E et G ont été austénitisées à 900°C pendant 30 minutes, puis,

    • pour l'acier D, deux tôles de 20 mm d'épaisseur, empilées, ont été refroidies à l'air,
    • pour E et G, une tôle de 20 mm d'épaisseur à été refroidie à l'air.
    The sheets produced with steels D, E and G were austenitized at 900 ° C. for 30 minutes, then,
    • for steel D, two 20 mm thick sheets, stacked, were air-cooled,
    • for E and G, a 20 mm thick sheet has been air-cooled.

    Avec l'acier F dans lequel le titane a été introduit conformément à l'invention, on a réalisé une tôle de 40 mm d'épaisseur traitée dans la chaude de laminage. Un lingot a été chauffé à 1200°C puis laminé, la température de fin de laminage a été supérieure à 950 C ; après laminage la tôle à été refroidie à l'air.With the steel F in which the titanium was introduced in accordance with the invention, a 40 mm thick sheet was made processed in the hot rolling. An ingot was heated to 1200 ° C then laminated, the end of rolling temperature was greater than 950 C; after rolling the sheet was cooled in air.

    Les caractéristiques mécaniques obtenues ont été : Rm Re allongement Mpa Mpa uniforme total D invention 1945 997 5,8% 12,1 % E art antérieur 1930 1490 1,8% 7,4% F invention 1259 645 10,1% 18,1% G art antérieur 1262 951 4,1% 11,9% The mechanical characteristics obtained were: Rm Re elongation Mpa Mpa uniform total D invention 1945 997 5.8% 12.1% E prior art 1930 1490 1.8% 7.4% F invention 1259 645 10.1% 18.1% G prior art 1262 951 4.1% 11.9%

    Ces exemples font apparaítre l'augmentation de ductilité apportée par l'invention ainsi que l'effet favorable du titane sur la résilience (exemple C).These examples show the increase in ductility provided by the invention as well as the favorable effect of titanium on the resilience (example C).

    On constate sur tous ces exemples que, à résistance à la traction comparable, les aciers selon l'invention ont des allongements uniformes au moins 2,5 fois plus élevés que ceux des aciers selon l'art antérieur.It can be seen from all of these examples that, with resistance to comparable tensile strength, the steels according to the invention have elongations uniforms at least 2.5 times higher than those of steels according to the art prior.

    Sur la tôle réalisée avec l'acier A on a, également, fait un essai de déformation dynamique en compression à la vitesse de 104s-1 et on a constaté une consolidation comparable à celle d'une tôle selon l'art antérieur dont la dureté statique est de 500 Hb alors que la dureté statique de la tôle selon l'invention n'est que de 400 HB.On the sheet produced with steel A, a dynamic compression deformation test was also carried out at the speed of 10 4 s -1 and a consolidation comparable to that of a sheet according to the prior art was found, the static hardness is 500 Hb while the static hardness of the sheet according to the invention is only 400 HB.

    Du fait de sa très bonne ductilité associée à une très haute résistance mécanique, l'acier selon l'invention est particulièrement bien adapté à la fabrication

    • de pièces résistant à l'usure abrasive pour équipements utilisés notamment dans l'industrie minérale (en particulier, mines, carrières, cimenteries) ou dans les travaux publics telles que dents, tôles, lames, racloirs, cribles, marteaux de dispositifs d'abattage, de concassage, de broyage, de criblage, de pelletage, de nivelage ou de transport ;
    • de tôles soumises à des chocs intenses ou à des impacts concentrés et très énergétiques;
    • de pièces pour les constructions métalliques ou chaudronnées soumises à un formage à froid important et/ou exigeant une sécurité élevée en service favorisée par une valeur réduite du rapport Re/Rm et une déformabilité avant striction importante ; par exemple : réservoirs sous pression, charpentes métalliques, flèches de grue, et plus généralement, pièces résistantes soumises à un emboutissage ou à un étirement à froid ou à température modérée.
    Due to its very good ductility associated with very high mechanical strength, the steel according to the invention is particularly well suited to manufacturing
    • parts resistant to abrasive wear for equipment used in particular in the mineral industry (in particular, mines, quarries, cement works) or in public works such as teeth, sheets, blades, scrapers, screens, hammers of felling devices , crushing, grinding, screening, shoveling, leveling or transport;
    • sheets subjected to intense shocks or concentrated and very energetic impacts;
    • parts for metal or sheet metal constructions subjected to significant cold forming and / or requiring high safety in service favored by a reduced value of the Re / Rm ratio and a significant deformability before necking; for example: pressure tanks, metal frames, crane jibs, and more generally, resistant parts subjected to deep drawing or cold or moderate temperature stretching.

    Ces pièces sont notamment des tôles d'épaisseur supérieure à 8 mm.These parts are in particular sheets of thickness greater than 8 mm.

    Claims (13)

    1. Steel characterized in that its chemical composition comprises, by weight: 0.15% ≤ C ≤ 0.303% 0% ≤ Si ≤ 3% 0% ≤ Al ≤ 3% 0.1% ≤ Mn ≤ 4.5% 0% ≤ Ni ≤ 9% 0% ≤ Cr ≤ 6% 0% ≤ Mo + W/2 ≤ 3% 0% ≤ V ≤ 0.5% 0% ≤ Nb ≤ 0.5% 0% ≤ Zr ≤ 0.5% N ≤ 0.3%
      optionally from 0.0005% to 0.005% of boron,
      optionally from 0.005% to 0.1% of titanium,
      optionally at least one element taken from Ca, Se, Te, Bi and Pb in amounts less than 0.2%,
      the balance being iron and impurities resulting from smelting,
      the chemical composition furthermore satisfying the relationships: 1% ≤ Si + Al ≤ 3% and, 4.6×(%C) + 1.05×(%Mn) + 0.54×(%Ni) + 0.66×(%Mo + %W/2) + 0.5x(%Cr) + K ≥ 3.8 where
      K = 0.5 when the steel contains boron,
      K = 0 when the steel does not contain boron.
    2. Steel according to Claim 1, characterized in that: 0.005% ≤ Ti ≤ 0.1% 0.01% ≤ Al ≤ 0.5% 0.003 ≤ N ≤ 0.02% and in that, in the solid state, the number of titanium nitride precipitates of size greater than 0.1 µm, counted over an area of 1 mm2 of a micrograph section, is less than 4 times the total content of titanium precipitated in the form of nitrides, this content being expressed in thousandths of a % by weight.
    3. Steel according to Claim 1 or Claim 2, characterized in that: 0.5% ≤ Cr ≤ 3% 0.1% ≤ Mo + W/2 ≤ 2% Mn ≤ 2%.
    4. Steel according to Claim 1, Claim 2 or Claim 3, characterized in that: 1.5% ≤ Si + Al ≤ 2.5%.
    5. Steel according to any one of Claims 1 to 4, characterized in that, 0.2% ≤ C ≤ 0.3%.
    6. Steel according to any one of Claims 1 to 5, characterized in that its chemical composition by weight comprises: 0.20% ≤ C < 0.24% 0% ≤ Si ≤ 2.5% 0% ≤ Al ≤ 2.5% 1.2% ≤ Mn ≤ 1.7% 1.5% ≤ Ni ≤ 2.5% 0.5% ≤ Cr ≤ 1.5% 0.1% ≤ Mo + W/2 ≤ 0.5% the chemical composition furthermore satisfying the relationships: 1.5% ≤ Si + Al ≤ 2.5% and 4.6%×(%C) + 1.05×(%Mn) + 0.54×(%Ni) + 0.66×(%Mo + %W/2) + 0.5×(%Cr) + K ≥ 3.8 where
      K = 0.5 when the steel contains boron,
      K = 0 when the steel does not contain boron.
    7. Process for the manufacture of a component made of high-strength high-ductility steel, characterized in that:
      a steel according to any one of Claims 1 to 6 is smelted;
      the steel is cast and solidified in the form of a semi-finished product;
      the semi-finished product is formed by hot plastic deformation in order to obtain a steel component;
      the component is austenized by heating above Ac3°C and then cooled down to room temperature in such a way that the rate of cooling between the austenization temperature and Ms + 150°C is greater than 0.3°C/s, such that the residence time between Ms + 150°C and Ms - 50°C is between 5 minutes and 90 minutes and such that the cooling down to room temperature is faster than 0.02°/s.
    8. Process for the manufacture of a component made of high-strength high-ductility steel, characterized in that:
      a steel according to any one of Claims 1 to 6 is smelted;
      the steel is cast and solidified in the form of a semi-finished product;
      the semi-finished product is heated to a temperature of less than 1300°C and shaped by hot plastic deformation in order to obtain a steel component, in such a way that the temperature at the end of hot plastic deformation is greater than Ac3;
      the steel component is cooled down to room temperature in such a way that the rate of cooling between the austenization temperature and Ms + 150°C is greater than 0.3°C/s, such that the residence time between Ms + 150°C and Ms - 50°C is between 5 minutes and 90 minutes and such that the cooling down to room temperature is faster than 0.02°/s.
    9. Process according to Claim 7 or Claim 8, characterized in that, in order to cool the component from the austenization temperature down to room temperature, the component is left to cool in air.
    10. Steel component according to any one of Claims 1 to 6, characterized in that its microstructure consists of bainite and/or of martensite and of 5 to 30% of austenite, its tensile strength is greater than 1200 MPa and its ductility measured by uniform elongation is greater than 5%.
    11. Steel component according to Claim 10, characterized in that its structure contains at least 30% of bainite.
    12. Component according to Claim 10 or Claim 11, characterized in that it is a sheet having a thickness of greater than 8 mm.
    13. Use of a steel according to any one of Claims 1 to 6 for the manufacture of equipment components for the mineral industry or for civil engineering, these components having to be abrasion-resistant, or for the manufacture of metal construction components or components fabricated from sheet-metal.
    EP96400061A 1995-01-31 1996-01-11 High ductility steel, manufacturing process and utilization Expired - Lifetime EP0725156B1 (en)

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    FR9501079A FR2729974B1 (en) 1995-01-31 1995-01-31 HIGH DUCTILITY STEEL, MANUFACTURING PROCESS AND USE
    FR9501079 1995-01-31

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    JPH08239738A (en) 1996-09-17
    US5695576A (en) 1997-12-09
    FR2729974B1 (en) 1997-02-28
    EP0725156A1 (en) 1996-08-07
    DE69613868T2 (en) 2001-11-29
    DE69613868D1 (en) 2001-08-23
    FR2729974A1 (en) 1996-08-02

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