EP1558769B1 - Method for making hardenable steel plates by firing, resulting steel plates - Google Patents

Method for making hardenable steel plates by firing, resulting steel plates Download PDF

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Publication number
EP1558769B1
EP1558769B1 EP03775481A EP03775481A EP1558769B1 EP 1558769 B1 EP1558769 B1 EP 1558769B1 EP 03775481 A EP03775481 A EP 03775481A EP 03775481 A EP03775481 A EP 03775481A EP 1558769 B1 EP1558769 B1 EP 1558769B1
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Prior art keywords
steel
weight
less
process according
temperature
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EP03775481A
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German (de)
French (fr)
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EP1558769A1 (en
Inventor
Joël MARSAL
Fernande Kirch
Dominique Mescolini
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ArcelorMittal France SA
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ArcelorMittal France SA
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Classifications

    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0226—Hot rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
    • C21D8/0236—Cold rolling
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
    • C21D8/0273—Final recrystallisation annealing
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/52—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length
    • 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
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • 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
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • 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
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium

Definitions

  • the present invention relates to a method for manufacturing baking hardening steel sheet, said "bake hardening", and the sheets and steel parts obtained by the implementation of this method.
  • These sheets and these pieces of steel may include an anticorrosion coating, such as that obtained by hot dip galvanizing or electrogalvanizing.
  • the steel sheets are more particularly intended for the manufacture of automotive appearance parts, such as hoods for example, while the parts of greater thickness than the sheets, are more particularly intended for producing pieces of steel. structure for automobile, too.
  • the appearance parts for the automobile must be made of a material easy to implement by stamping, having at the end of this implementation a good resistance to indentation, and the lightest possible to to reduce the consumption of the vehicle.
  • bake hardening also called steels BH
  • steels BH have the particularity to have a low yield strength before shaping, which makes them easily embossissables.
  • bake hardening also called steels BH
  • the parts or the plates of steels with BH have a limit of elasticity which has increased significantly, giving them good resistance to indentation.
  • this hardening property during the baking of the coating is in particular used to reduce the thickness, and therefore the weight, of these parts.
  • Bake-hardenable steels are known whose composition comprises manganese and silicon and a substantial amount of phosphorus, at around 0.1% by weight. These steels have good mechanical properties and a gain in yield strength after baking (BH) of the order of 45 MPa, but have a significant natural aging.
  • BH yield strength after baking
  • the present invention therefore aims to provide baking hardenable steels having good mechanical characteristics, a gain in yield strength after baking (BH) of at least 60 MPa and which are less sensitive to natural aging than the steels of the prior art.
  • BH gain in yield strength after baking
  • the first cooling comprises a first slow portion carried out at a speed of less than 10 ° C / s, then a fast second portion carried out at a rate of between 20 and 50 ° C / s.
  • the carbon content of the composition according to the invention is between 0.03 and 0.06% by weight, since this element substantially lowers the ductility. It is however necessary to have a minimum of 0.03% by weight to avoid any problem of aging.
  • the manganese content of the composition according to the invention must be between 0.50 and 1.10% by weight. Manganese improves the yield strength of steel while greatly reducing its ductility. Below 0.50% by weight, there are problems of aging, while beyond 1.10% by weight, it is too much to ductility.
  • the silicon content of the composition according to the invention must be between 0.08 and 0.20% by weight. It greatly improves the yield strength of steel while reducing its ductility slightly, but increases significantly its aging tendency. If its content is less than 0.08% by weight, the steel does not have good mechanical characteristics, whereas if it exceeds 0.20% by weight, it faces problems of surface appearance on which appear tigers.
  • the ratio of the manganese content with respect to the silicon content is between 4 and 15 in order to avoid any problem of solder brittleness. Indeed, if one places oneself outside these values, one observes the formation of embrittling oxides during this welding operation.
  • boron The main function of boron is to fix nitrogen by early precipitation of boron nitrides. It must therefore be present in sufficient quantity to prevent a large amount of nitrogen remains free, without however exceeding the stoichiometric amount because the free residual amount could cause metallurgical problems as well as a coloration of the coil banks. As an indication, it should be mentioned that the strict stoichiometry is reached for a B / N ratio of 0.77.
  • the aluminum content of the composition according to the invention is between 0.015 and 0.070% by weight, without it being of critical importance.
  • Aluminum is present in the grade according to the invention because of the casting process in which this element is added to deoxidize the steel. However, it is important not to exceed 0.070% by weight because then there would be problems of inclusions of aluminum oxides, harmful to the mechanical properties of steel.
  • Phosphorus is limited in the steel according to the invention to a content of less than 0.035% by weight, preferably less than 0.015% by weight. It makes it possible to increase the yield strength of the grade, but it also increases its aging tendency in heat treatments, which explains its limitation. It is also bad for ductility.
  • the titanium content of the composition must be less than 0.005% by weight, that of sulfur must be less than 0.015% by weight, that of nickel must be less than 0.040% by weight, that of copper must be less than 0.040% by weight. weight and that in molybdenum must be less than 0.010% by weight.
  • These different elements actually constitute the residual elements from the elaboration of the nuance that we meet most often. Their contents are limited because they are likely to form inclusions which diminish the mechanical characteristics of the shade.
  • these residual elements may also be niobium, which is not added to the composition, but which may be present in the form of traces, that is to say at a content of less than 0.004%, preferably less than 0.001%, and particularly preferably 0.
  • a second object of the invention which is defined in claim 8 is constituted by a baking curable sheet obtainable by the method according to the invention and which has a yield strength of between 260 and 360 MPa, a resistance to the tension between 320 and 460 MPa, a value of BH2 greater than 60 MPa and a plateau of elastic limit less than or equal to 0.2%.
  • the rest of the composition of the castings 1 to 5 is of course made of iron and possibly impurities resulting from the preparation.
  • a specimen is then subjected to a uniaxial tension of 2% and then a heat treatment of 170 ° C. for 20 minutes.
  • Slabs are made from castings 1 to 4 and then hot-rolled at a temperature above Ar 3. For these flows, the end of rolling temperature is between 854 and 880 ° C. The sheets thus obtained are reeled at a winding temperature between 580 and 620 ° C. for these pours, then they are cold rolled with a reduction rate which varies from 70 to 76%.
  • Specimens are then cut into these sheets and their yield strengths Re0 are measured. These specimens are then subjected to a 2% uniaxial tension and their yield strengths Re2% and their other mechanical characteristics are measured. Then, they are subjected to a conventional heat treatment at 170 ° C for 20 minutes and measured their new yield strengths ReTT. Their BH 2 are then calculated.
  • the castings 1 and 3 according to the invention have good mechanical characteristics, a good value of BH2 and have no or little yield point stop.
  • New specimens are then cut from the continuously annealed sheets and subjected to heat treatment at 75 ° C for 10 hours. This heat treatment is equivalent to a natural aging of 6 months at room temperature.
  • the following results are obtained: Specimen Re (MPa) rm (MPa) not P% AT% Casting 1 (fresh state) 296 384 0.208 0 36.6 Casting 1 (aged condition) 290 394 0,165 0.1 31.1 Casting 2 (fresh state) 305 422 0.189 0 33.1 Casting 2 (aged condition) 299 431 0,160 0 31.0 Casting 3 (fresh state) 284 379 0.194 0.2 35.3 Casting 3 (aged condition) 286 393 0.157 0.2 30.4
  • Slabs were made from castings 1 to 5 and then hot-rolled, the end-of-rolling temperature being 850/880 ° C.
  • the sheets thus obtained are reeled at a winding temperature of 580/620 ° C., then they are cold rolled with a reduction rate of 70/76% for these flows.
  • Specimens are then cut into these sheets and their yield strengths Re0 are measured. These specimens are then subjected to a 2% uniaxial tension and their yield strengths Re2% and their other mechanical characteristics are measured. Then, they are subjected to a conventional heat treatment at 170 ° C for 20 minutes and measured their new yield strengths ReTT. Their BH 2 are then calculated.
  • the castings 1 to 3 according to the invention have good mechanical characteristics, a very good value of BH 2 and show no or little yield strength stop, unlike casting which has 1.2% of bearing.
  • the castings 1 to 3 according to the invention do not exhibit a Z-type barrier (less than or equal to 0.2%), unlike the casting 5 which has a level of 1.8%.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Heat Treatment Of Sheet Steel (AREA)
  • Laminated Bodies (AREA)
  • Metal Rolling (AREA)
  • Heat Treatment Of Steel (AREA)
  • Meat, Egg Or Seafood Products (AREA)

Abstract

The fabrication of bake hardened steel strip comprises: (a) production of a steel with a given composition; (b) casting a slab of this steel, then hot rolling to produce a strip, the end rolling temperature being greater than the Ar3 point; (c) reeling of the strip at a temperature between 500 and 700 degrees C; (d) cold rolling the strip with a reduction rate of 50 to 80 %; (e) continuous annealing heat treatment with a duration of less than 15 minutes; (f) cold drawing with a reduction rate of between 1.2 and 2.5 %. Independent claims are also included for: (i) a bake hardened steel strip produced by this method; (ii) a component able to be cut from such steel strip, then painted and heated to at least 200 degrees C.

Description

La présente invention concerne un procédé de fabrication de tôles d'acier durcissables par cuisson, dites à "bake hardening", ainsi que les tôles et les pièces d'acier obtenues par la mise en oeuvre de ce procédé.The present invention relates to a method for manufacturing baking hardening steel sheet, said "bake hardening", and the sheets and steel parts obtained by the implementation of this method.

Ces tôles et ces pièces d'acier peuvent comporter un revêtement anticorrosion, tel que celui obtenu par galvanisation au trempé à chaud ou par électrozingage. Les tôles d'acier sont plus particulièrement destinées à la fabrication de pièces d'aspect pour l'automobile, comme des capots par exemple, tandis que les pièces de plus grande épaisseur que les tôles, sont plus particulièrement destinées à la réalisation de pièces de structure pour automobile, également.These sheets and these pieces of steel may include an anticorrosion coating, such as that obtained by hot dip galvanizing or electrogalvanizing. The steel sheets are more particularly intended for the manufacture of automotive appearance parts, such as hoods for example, while the parts of greater thickness than the sheets, are more particularly intended for producing pieces of steel. structure for automobile, too.

En effet, les pièces d'aspect pour l'automobile doivent être réalisées dans un matériau facile à mettre en oeuvre par emboutissage, présentant à l'issue de cette mise en oeuvre une bonne résistance à l'indentation, et le plus léger possible afin de diminuer la consommation du véhicule.Indeed, the appearance parts for the automobile must be made of a material easy to implement by stamping, having at the end of this implementation a good resistance to indentation, and the lightest possible to to reduce the consumption of the vehicle.

Or, ces différentes caractéristiques sont contradictoires : un matériau présente une bonne emboutissabilité lorsque sa limite d'élasticité est faible, mais une bonne résistance à l'indentation nécessite que sa limite d'élasticité soit élevée et son épaisseur importante.However, these different characteristics are contradictory: a material has good drawability when its elastic limit is low, but a good resistance to indentation requires that its yield strength is high and its thickness significant.

On a donc développé des aciers dits à "bake hardening" (encore appelés aciers à BH) présentant la particularité d'avoir une faible limite d'élasticité avant mise en forme, ce qui les rend facilement emboutissables. Mais, une fois emboutis, puis revêtus de peinture et soumis à un traitement thermique de cuisson (170°C pendant 20 minutes, par exemple), on constate que les pièces ou les tôles d'aciers à BH ont une limite d'élasticité qui a augmenté de façon considérable, ce qui leur confère une bonne résistance à l'indentation.So we have developed steels called "bake hardening" (also called steels BH) having the particularity to have a low yield strength before shaping, which makes them easily embossissables. But, once stamped, then coated with paint and subjected to a heat treatment of baking (170 ° C for 20 minutes, for example), one finds that the parts or the plates of steels with BH have a limit of elasticity which has increased significantly, giving them good resistance to indentation.

Dans le cas des pièces de structure, cette propriété de durcissement lors de la cuisson du revêtement est en particulier mise à profit pour réduire l'épaisseur, et donc le poids, de ces pièces.In the case of structural parts, this hardening property during the baking of the coating is in particular used to reduce the thickness, and therefore the weight, of these parts.

D'un point de vue métallurgique, ces modifications de caractéristiques s'expliquent par l'évolution du carbone en solution solide dans l'acier. Ce carbone a naturellement tendance à se fixer sur les dislocations de l'acier jusqu'à saturation de celles-ci, ce qui durcit l'acier. En contrôlant la quantité de carbone en solution solide et la densité de dislocations présentes dans l'acier au cours du procédé, on peut donc faire en sorte de durcir l'acier lorsqu'on le souhaite, en créant de nouvelles dislocations, que l'on sature avec le carbone restant en solution solide, et qui migre sous l'effet d'une activation thermique. Il convient cependant d'éviter la présence d'une trop grande quantité de carbone en solution solide, car il pourrait alors entraîner un vieillissement de l'acier sous forme d'un durcissement intempestif avant emboutissage qui irait à l'encontre du but visé.From a metallurgical point of view, these characteristic changes can be explained by the evolution of carbon in solid solution in steel. This carbon has a natural tendency to attach to the dislocations of the steel until saturation of these, which hardens the steel. By controlling the amount of carbon in solid solution and the density of dislocations present in the steel during the process, it is therefore possible to make the steel harden when desired, creating new dislocations, that the it saturates with the carbon remaining in solid solution, and which migrates under the effect of a thermal activation. However, it is advisable to avoid the presence of too much carbon in solid solution, because it could then cause an aging of the steel in the form of an untimely hardening before stamping which would go against the goal.

On connaît des aciers durcissables par cuisson dont la composition comprend du manganèse et du silicium et une quantité notable de phosphore, aux alentours de 0,1% en poids. Ces aciers ont de bonnes caractéristiques mécaniques et un gain en limite d'élasticité après cuisson (BH) de l'ordre de 45 MPa, mais présentent un vieillissement naturel important.Bake-hardenable steels are known whose composition comprises manganese and silicon and a substantial amount of phosphorus, at around 0.1% by weight. These steels have good mechanical properties and a gain in yield strength after baking (BH) of the order of 45 MPa, but have a significant natural aging.

La présente invention a donc pour but de mettre à disposition des aciers durcissables par cuisson présentant de bonnes caractéristiques mécaniques, un gain en limite d'élasticité après cuisson (BH) d'au moins 60 MPa et qui sont moins sensibles au vieillissement naturel que les aciers de l'art antérieur.The present invention therefore aims to provide baking hardenable steels having good mechanical characteristics, a gain in yield strength after baking (BH) of at least 60 MPa and which are less sensitive to natural aging than the steels of the prior art.

A cet effet, un premier objet de la présente invention qui est défini à la revendication 1 est constitué par un procédé de fabrication de tôles d'acier durcissables par cuisson comprenant :

  • l'élaboration d'un acier dont la composition comprend, exprimées en % en poids :
    • 0,03 ≤ C ≤ 0,06
    • 0,50 ≤ Mn ≤ 1,10
    • 0,08 ≤ Si ≤ 0,20
    • 0,015. ≤ Al ≤ 0,070
      N ≤ 0,007
      Ni ≤ 0,040
      Cu ≤ 0,040
      P ≤ 0,035
      S ≤ 0,015
      Mo ≤ 0,010
      Ti ≤ 0,005
    étant entendu qu'elle comprend également du bore en une quantité telle que : 0 , 64 ≤ B N ≤ 1 , 60
    Figure imgb0001
    le reste de la composition étant constitué de fer et d'impuretés résultant de l'élaboration,
  • la coulée d'une brame de cet acier, puis un laminage à chaud de cette brame pour obtenir une tôle, la température de fin de laminage étant supérieure à celle du point Ar3,
  • un bobinage de ladite tôle à une température comprise entre 500 et 700°C, puis
  • un laminage à froid de ladite tôle avec un taux de réduction de 50 à 80%,
  • un traitement thermique de recuit en continu d'une durée inférieure à 15 minutes, comprenant:
    • + un réchauffement de l'acier jusqu'à lui faire atteindre une température comprise entre 750 et 850°C,
    • + un maintien isotherme,
    • + un premier refroidissement jusqu'à une température comprise entre 380 et 500°C, et + un maintien isotherme, puis + un deuxième refroidissement jusqu'à température ambiante,
  • un écrouissage réalisé avec un taux de réduction compris entre 1,2 et 2,5%.
For this purpose, a first object of the present invention which is defined in claim 1 is constituted by a method of manufacturing bake-hardenable steel sheets comprising:
  • the preparation of a steel whose composition comprises, expressed in% by weight:
    • 0.03 ≤ C ≤ 0.06
    • 0.50 ≤ Mn ≤ 1.10
    • 0.08 ≤ If ≤ 0.20
    • 0.015. ≤ Al ≤ 0.070
      N ≤ 0.007
      Ni ≤ 0.040
      Cu ≤ 0.040
      P ≤ 0.035
      S ≤ 0.015
      Mo ≤ 0.010
      Ti ≤ 0.005
    it being understood that it also comprises boron in an amount such that: 0 , 64 ≤ B NOT ≤ 1 , 60
    Figure imgb0001
    the remainder of the composition consisting of iron and impurities resulting from the preparation,
  • casting a slab of this steel, and then hot rolling the slab to obtain a sheet, the end-of-rolling temperature being greater than that of point Ar3,
  • a winding of said sheet at a temperature between 500 and 700 ° C, then
  • a cold rolling of said sheet with a reduction rate of 50 to 80%,
  • a continuous annealing heat treatment lasting less than 15 minutes, comprising:
    • + a heating of the steel until it reaches a temperature of between 750 and 850 ° C,
    • + an isothermal hold,
    • + a first cooling to a temperature between 380 and 500 ° C, and + an isothermal holding, then + a second cooling to room temperature,
  • hardening achieved with a reduction ratio of between 1.2 and 2.5%.

Dans un mode de réalisation préféré, le premier refroidissement comprend une première partie lente effectuée à une vitesse inférieure à 10 °C/s, puis une seconde partie rapide effectuée à une vitesse comprise entre 20 et 50 °C/s.In a preferred embodiment, the first cooling comprises a first slow portion carried out at a speed of less than 10 ° C / s, then a fast second portion carried out at a rate of between 20 and 50 ° C / s.

Le procédé peut également comprendre les variantes suivantes, prises isolément ou en combinaison :

  • la teneur en manganèse et la teneur en silicium de l'acier sont telles que : 4 ≤ % Mn % Si ≤ 15
    Figure imgb0002
  • la teneur en manganèse de l'acier est comprise entre 0,55 et 0,65% en poids et la teneur en silicium de l'acier est comprise entre 0,08 et 0,12% en poids,
  • la teneur en manganèse de l'acier est comprise entre 0,95 et 1,05% en poids et la teneur en silicium de l'acier est comprise entre 0,16 et 0,20% en poids,
  • la teneur en azote de l'acier est inférieure à 0,005% en poids,
  • la teneur en phosphore de l'acier est inférieure à 0,015% en poids.
The method may also include the following variants, taken alone or in combination:
  • the manganese content and the silicon content of the steel are such that: 4 ≤ % mn % Yes ≤ 15
    Figure imgb0002
  • the manganese content of the steel is between 0.55 and 0.65% by weight and the silicon content of the steel is between 0.08 and 0.12% by weight,
  • the manganese content of the steel is between 0.95 and 1.05% by weight and the silicon content of the steel is between 0.16 and 0.20% by weight,
  • the nitrogen content of the steel is less than 0.005% by weight,
  • the phosphorus content of the steel is less than 0.015% by weight.

La teneur en carbone de la composition selon l'invention est comprise entre 0,03 et 0,06% en poids, car cet élément abaisse sensiblement la ductilité. Il est cependant nécessaire d'en avoir un minimum de 0,03% en poids pour éviter tout problème de vieillissement.The carbon content of the composition according to the invention is between 0.03 and 0.06% by weight, since this element substantially lowers the ductility. It is however necessary to have a minimum of 0.03% by weight to avoid any problem of aging.

La teneur en manganèse de la composition selon l'invention doit être comprise entre 0,50 et 1,10% en poids. Le manganèse améliore la limite d'élasticité de l'acier tout en réduisant fortement sa ductilité. En dessous de 0,50% en poids, on observe des problèmes de vieillissement, tandis que au-delà de 1,10% en poids, il nuit trop à la ductilité.The manganese content of the composition according to the invention must be between 0.50 and 1.10% by weight. Manganese improves the yield strength of steel while greatly reducing its ductility. Below 0.50% by weight, there are problems of aging, while beyond 1.10% by weight, it is too much to ductility.

La teneur en silicium de la composition selon l'invention doit être comprise entre 0,08 et 0,20 % en poids. Il améliore fortement la limite d'élasticité de l'acier tout en réduisant faiblement sa ductilité, mais augmente sensiblement sa tendance au vieillissement. Si sa teneur est inférieure à 0,08% en poids, l'acier ne présente pas de bonnes caractéristiques mécaniques, tandis que si elle dépasse 0,20% en poids, on se heurte à des problèmes d'aspect de surfaces sur lesquelles apparaissent des tigrages.The silicon content of the composition according to the invention must be between 0.08 and 0.20% by weight. It greatly improves the yield strength of steel while reducing its ductility slightly, but increases significantly its aging tendency. If its content is less than 0.08% by weight, the steel does not have good mechanical characteristics, whereas if it exceeds 0.20% by weight, it faces problems of surface appearance on which appear tigers.

Dans un mode de réalisation préféré de l'invention, le rapport de la teneur en manganèse par rapport à la teneur en silicium est compris entre 4 et 15 afin d'éviter tout problème de fragilité de soudure par étincelage. En effet, si on se place en dehors de ces valeurs, on observe la formation d'oxydes fragilisants lors de cette opération de soudage.In a preferred embodiment of the invention, the ratio of the manganese content with respect to the silicon content is between 4 and 15 in order to avoid any problem of solder brittleness. Indeed, if one places oneself outside these values, one observes the formation of embrittling oxides during this welding operation.

Le bore a pour fonction principale de fixer l'azote par précipitation précoce de nitrures de bore. Il doit donc être présent en quantité suffisante pour éviter qu'une trop grande quantité d'azote demeure libre, sans toutefois dépasser trop la quantité stoechiométrique car la quantité résiduelle libre pourrait poser des problèmes métallurgiques ainsi qu'une coloration des rives de bobine. A titre indicatif, on mentionnera que la stoechiométrie stricte est atteinte pour un rapport B/N de 0,77.The main function of boron is to fix nitrogen by early precipitation of boron nitrides. It must therefore be present in sufficient quantity to prevent a large amount of nitrogen remains free, without however exceeding the stoichiometric amount because the free residual amount could cause metallurgical problems as well as a coloration of the coil banks. As an indication, it should be mentioned that the strict stoichiometry is reached for a B / N ratio of 0.77.

La teneur en aluminium de la composition selon l'invention est comprise entre 0,015 et 0,070 % en poids, sans qu'elle présente une importance critique. L'aluminium est présent dans la nuance selon l'invention du fait du procédé de coulée au cours duquel on ajoute cet élément pour désoxyder l'acier. Il importe cependant de ne pas dépasser 0,070% en poids car on rencontrerait alors de problèmes d'inclusions d'oxydes d'aluminium, néfastes pour les caractéristiques mécaniques de l'acier.The aluminum content of the composition according to the invention is between 0.015 and 0.070% by weight, without it being of critical importance. Aluminum is present in the grade according to the invention because of the casting process in which this element is added to deoxidize the steel. However, it is important not to exceed 0.070% by weight because then there would be problems of inclusions of aluminum oxides, harmful to the mechanical properties of steel.

Le phosphore est limité dans l'acier selon l'invention à une teneur inférieure à 0,035 % en poids, de préférence inférieure à 0,015 % en poids. Il permet d'augmenter la limite d'élasticité de la nuance, mais il augmente parallèlement sa tendance au vieillissement dans les traitements thermiques, ce qui explique sa limitation. Il est également néfaste pour la ductilité.Phosphorus is limited in the steel according to the invention to a content of less than 0.035% by weight, preferably less than 0.015% by weight. It makes it possible to increase the yield strength of the grade, but it also increases its aging tendency in heat treatments, which explains its limitation. It is also bad for ductility.

La teneur en titane de la composition doit être inférieure à 0,005% en poids, celle en soufre doit être inférieure à 0,015 % en poids, celle en nickel doit être inférieure à 0,040% en poids, celle en cuivre doit être inférieure à 0,040% en poids et celle en molybdène doit être inférieure à 0,010% en poids. Ces différents éléments constituent en réalité les éléments résiduels issus de l'élaboration de la nuance que l'on rencontre le plus souvent. On limite leurs teneurs car ils sont susceptibles de former des inclusions qui diminuent les caractéristiques mécaniques de la nuance. Parmi ces éléments résiduels peut également se trouver du niobium, que l'on n'ajoute pas à la composition, mais qui peut être présent à l'état de traces, c'est à dire à une teneur inférieure à 0,004%, de préférence inférieure à 0,001%, et de façon particulièrement préférée égale à 0.The titanium content of the composition must be less than 0.005% by weight, that of sulfur must be less than 0.015% by weight, that of nickel must be less than 0.040% by weight, that of copper must be less than 0.040% by weight. weight and that in molybdenum must be less than 0.010% by weight. These different elements actually constitute the residual elements from the elaboration of the nuance that we meet most often. Their contents are limited because they are likely to form inclusions which diminish the mechanical characteristics of the shade. Among these residual elements may also be niobium, which is not added to the composition, but which may be present in the form of traces, that is to say at a content of less than 0.004%, preferably less than 0.001%, and particularly preferably 0.

Un second objet de l'invention qui est défini à la revendication 8 est constitué par une tôle durcissable par cuisson pouvant être obtenue par le procédé selon l'invention et qui présente une limite d'élasticité comprise entre 260 et 360 MPa, une résistance à la traction comprise entre 320 et 460 MPa, une valeur de BH2 supérieur à 60 MPa et un palier de limite d'élasticité inférieur ou égal à 0,2%.A second object of the invention which is defined in claim 8 is constituted by a baking curable sheet obtainable by the method according to the invention and which has a yield strength of between 260 and 360 MPa, a resistance to the tension between 320 and 460 MPa, a value of BH2 greater than 60 MPa and a plateau of elastic limit less than or equal to 0.2%.

La présente invention va être illustrée à partir des exemples qui suivent, le tableau ci-dessous donnant la composition des différents aciers testés en % en poids, parmi lesquels, les coulées 1 à 4 sont conformes à la présente invention tandis que la coulée 5 est utilisée à titre de comparaison : Coulée 1 Coulée 2 Coulée 3 Coulée 4 Coulée 5 C 0,044 0,045 0,038 0,043 0,066 Mn 0,546 0,989 0,598 1,000 0,625 Si 0,089 0,167 0,088 0,179 0,091 N 0,0033 0,0042 0,0032 0,0045 0,0039 B 0,0025 0,0029 0,0051 0,0029 - Al 0,047 0,031 0,038 0,029 0,058 P 0,006 0,0065 0,007 0,009 0,078 S 0,010 0,0056 0,01 0,008 0,0076 Cu 0,020 0,025 0,012 0,017 0,029 Ni 0,019 0,022 0,019 0,016 0,023 Ti 0,001 0,001 0,001 0,001 0,002 Mo 0,002 0,003 0,008 0,002 0,002 The present invention will be illustrated from the following examples, the table below giving the composition of the various steels tested in% by weight, among which, the castings 1 to 4 are in accordance with the present invention while the casting 5 is used for comparison: Casting 1 Casting 2 Casting 3 Casting 4 Casting 5 VS 0.044 0,045 0,038 0.043 0.066 mn 0.546 0.989 0.598 1,000 0.625 Yes 0.089 0,167 0.088 0.179 0.091 NOT 0.0033 0.0042 0.0032 0.0045 0.0039 B 0.0025 0.0029 0.0051 0.0029 - al 0,047 0.031 0,038 0,029 0.058 P 0.006 0.0065 0,007 0,009 0.078 S 0,010 0.0056 0.01 0,008 0.0076 Cu 0,020 0,025 0.012 0,017 0,029 Or 0,019 0,022 0,019 0.016 0,023 Ti 0,001 0,001 0,001 0,001 0,002 MB 0,002 0,003 0,008 0,002 0,002

Le reste de la composition des coulées 1 à 5 est bien entendu constitué de fer et éventuellement d'impuretés résultant de l'élaboration.The rest of the composition of the castings 1 to 5 is of course made of iron and possibly impurities resulting from the preparation.

Mesure du gain en limite d'élasticité après cuissonMeasurement of gain in elasticity limit after cooking

Afin de quantifier le gain possible en limite d'élasticité de l'acier, après cuisson, on procède à des essais conventionnels simulant une mise en oeuvre réelle au cours de laquelle on emboutit une tôle, puis on la cuit.In order to quantify the possible gain in yield strength of the steel, after firing, conventional tests are carried out simulating a real implementation during which a sheet is stamped and then fired.

On fait donc subir à une éprouvette une traction uniaxiale de 2%, puis un traitement thermique de 170°C pendant 20 minutes.A specimen is then subjected to a uniaxial tension of 2% and then a heat treatment of 170 ° C. for 20 minutes.

Au cours de ce processus , on mesure successivement :

  • la limite d'élasticité Re0 de l'éprouvette découpée dans la tôle d'acier venant de subir le recuit continu, puis
  • la limite d'élasticité Re2% de l'éprouvette ayant subie une traction uniaxiale de 2%, puis
  • la limite d'élasticité ReTT après traitement thermique de 170°C pendant 20 minutes.
During this process, we successively measure:
  • the elasticity limit Re0 of the specimen cut from the steel sheet which has undergone continuous annealing, and then
  • the elastic limit Re2% of the specimen having undergone a uniaxial tension of 2%, then
  • the yield strength ReTT after heat treatment of 170 ° C for 20 minutes.

La différence entre Re0 et Re2% permet de calculer le durcissement du à la mise en oeuvre (work hardening ou WH), tandis que la différence entre Re2% et ReTT conduit au durcissement du à la cuisson que l'on désigne, pour cet essai conventionnel, par BH2.The difference between Re0 and Re2% makes it possible to calculate the hardening of the implementation (work hardening or WH), while the difference between Re2% and ReTT leads to the hardening of the cooking that is designated, for this test conventional, by BH2.

Abréviations employéesAbbreviations used

A:AT:
allongement à la rupture en %elongation at break in%
Re:Re:
limite d'élasticité en MPayield strength in MPa
Rm:Rm:
résistance à la traction en MPatensile strength in MPa
n:not:
coefficient d'écrouissagecoefficient of hardening
P:P:
palier de limite d'élasticité en %level of yield strength in%
Exemple 1Example 1

On fabrique des brames à partir des coulées 1 à 4, puis on les lamine à chaud à une température supérieure à Ar3. Pour ces coulées, la température de fin de laminage est comprise entre 854 et 880°C. On bobine les tôles ainsi obtenues, à une température de bobinage entre 580 et 620°C pour ces coulées, puis on les lamine à froid avec un taux de réduction qui varie de 70 à 76%.Slabs are made from castings 1 to 4 and then hot-rolled at a temperature above Ar 3. For these flows, the end of rolling temperature is between 854 and 880 ° C. The sheets thus obtained are reeled at a winding temperature between 580 and 620 ° C. for these pours, then they are cold rolled with a reduction rate which varies from 70 to 76%.

Les tôles sont ensuite soumises à un recuit continu qui présente les étapes suivantes :

  • réchauffement de la tôle jusqu'à atteindre une température de 750°C, à une vitesse de réchauffage de 6°C/s, puis
  • maintien à cette température pendant 50 secondes,
  • refroidissement lent jusqu'à 650°C, à une vitesse de refroidissement de 4°C/s, puis
  • refroidissement rapide jusqu'à 400°C, à une vitesse de refroidissement de 28°C/s,
  • maintien à cette température pendant 170 secondes, puis
  • refroidissement à température ambiante, à une vitesse de refroidissement de 5°C/s.
The sheets are then subjected to continuous annealing which has the following steps:
  • sheet heating up to a temperature of 750 ° C, at a heating rate of 6 ° C / s, then
  • hold at this temperature for 50 seconds,
  • slow cooling down to 650 ° C, at a cooling rate of 4 ° C / s, then
  • rapid cooling up to 400 ° C, at a cooling rate of 28 ° C / s,
  • hold at this temperature for 170 seconds and then
  • cooling to room temperature at a cooling rate of 5 ° C / sec.

On découpe ensuite des éprouvettes dans ces tôles, et on mesure leurs limites d'élasticité Re0. Puis, on soumet ces éprouvettes à une traction uniaxiale de 2% et on mesure leurs limites d'élasticité Re2% ainsi que leurs autres caractéristiques mécaniques. Ensuite, on leur fait subir un traitement thermique conventionnel à 170°C pendant 20 minutes et on mesure leurs nouvelles limites d'élasticité ReTT. On calcule ensuite leurs BH2.Specimens are then cut into these sheets and their yield strengths Re0 are measured. These specimens are then subjected to a 2% uniaxial tension and their yield strengths Re2% and their other mechanical characteristics are measured. Then, they are subjected to a conventional heat treatment at 170 ° C for 20 minutes and measured their new yield strengths ReTT. Their BH 2 are then calculated.

Les résultats obtenus sont rassemblés dans le tableau suivant : Eprouvette Re
(MPa) Rm
(MPa) P
(%) BH2
(MPa) Coulée 1 296 384 0 67 Coulée 2 305 422 0 44* Coulée 3 284 379 0,2 64 * exemple hors de l'invention The results obtained are summarized in the following table: Specimen Re
(MPa) rm
(MPa) P
(%) BH2
(MPa) Casting 1 296 384 0 67 Casting 2 305 422 0 * 44 Casting 3 284 379 0.2 64 * example out of the invention

On constate que les coulées 1 et 3 selon l'invention présentent de bonnes caractéristiques mécaniques, une bonne valeur de BH2 et ne présentent pas ou peu de palier de limite d'élasticité.It is found that the castings 1 and 3 according to the invention have good mechanical characteristics, a good value of BH2 and have no or little yield point stop.

On découpe ensuite de nouvelles éprouvettes dans les tôles ayant subi le recuit continu, et on les soumet à un traitement thermique à 75°C pendant 10 heures. Ce traitement thermique est équivalent à un vieillissement naturel de 6 mois à température ambiante. On obtient les résultats suivants : Eprouvette Re
(MPa) Rm
(MPa) n P% A% Coulée 1
(état frais) 296 384 0,208 0 36,6 Coulée 1
(état vieilli) 290 394 0,165 0,1 31,1 Coulée 2
(état frais) 305 422 0,189 0 33,1 Coulée 2
(état vieilli) 299 431 0,160 0 31,0 Coulée 3
(état frais) 284 379 0,194 0,2 35,3 Coulée 3
(état vieilli) 286 393 0,157 0,2 30,4 New specimens are then cut from the continuously annealed sheets and subjected to heat treatment at 75 ° C for 10 hours. This heat treatment is equivalent to a natural aging of 6 months at room temperature. The following results are obtained: Specimen Re
(MPa) rm
(MPa) not P% AT% Casting 1
(fresh state) 296 384 0.208 0 36.6 Casting 1
(aged condition) 290 394 0,165 0.1 31.1 Casting 2
(fresh state) 305 422 0.189 0 33.1 Casting 2
(aged condition) 299 431 0,160 0 31.0 Casting 3
(fresh state) 284 379 0.194 0.2 35.3 Casting 3
(aged condition) 286 393 0.157 0.2 30.4

On constate après simulation d'un vieillissement naturel de 6 mois que les coulées 1 et 3 selon l'invention ne présentent pas de reprise de palier rédhibitoire à l'aspect Z (inférieur ou égal à 0.2%).After simulating a natural aging of 6 months, it can be seen that the streams 1 and 3 according to the invention do not show any Z-type barrier recovery (less than or equal to 0.2%).

Exemple 2Example 2

On fabrique des brames à partir des coulées 1 à 5, puis on les lamine à chaud, la température de fin de laminage étant de 850/880°C. On bobine les tôles ainsi obtenues, à une température de bobinage de 580/620°C, puis on les lamine à froid avec un taux de réduction variant de 70/76% pour ces coulées.Slabs were made from castings 1 to 5 and then hot-rolled, the end-of-rolling temperature being 850/880 ° C. The sheets thus obtained are reeled at a winding temperature of 580/620 ° C., then they are cold rolled with a reduction rate of 70/76% for these flows.

Les tôles sont ensuite soumises à un recuit continu qui présente les étapes suivantes :

  • réchauffement de la tôle jusqu'à atteindre une température de 820°C, à une vitesse de réchauffage de 7°C/s, puis
  • maintien à cette température pendant 30 secondes,
  • refroidissement lent jusqu'à 650°C, à une vitesse de refroidissement de 6°C/s, puis
  • refroidissement rapide jusqu'à 470°C, à une vitesse de refroidissement de 45°C/s,
  • maintien à cette température pendant 20 secondes, puis
  • refroidissement à température ambiante, à une vitesse de refroidissement de 11°C/s.
The sheets are then subjected to continuous annealing which has the following steps:
  • sheet heating up to a temperature of 820 ° C, at a heating rate of 7 ° C / s, then
  • hold at this temperature for 30 seconds,
  • slow cooling down to 650 ° C, at a cooling rate of 6 ° C / s, then
  • rapid cooling to 470 ° C, at a cooling rate of 45 ° C / s,
  • hold at this temperature for 20 seconds and then
  • cooling to room temperature at a cooling rate of 11 ° C / sec.

On découpe ensuite des éprouvettes dans ces tôles, et on mesure leurs limites d'élasticité Re0. Puis, on soumet ces éprouvettes à une traction uniaxiale de 2% et on mesure leurs limites d'élasticité Re2% ainsi que leurs autres caractéristiques mécaniques. Ensuite, on leur fait subir un traitement thermique conventionnel à 170°C pendant 20 minutes et on mesure leurs nouvelles limites d'élasticité ReTT. On calcule ensuite leurs BH2.Specimens are then cut into these sheets and their yield strengths Re0 are measured. These specimens are then subjected to a 2% uniaxial tension and their yield strengths Re2% and their other mechanical characteristics are measured. Then, they are subjected to a conventional heat treatment at 170 ° C for 20 minutes and measured their new yield strengths ReTT. Their BH 2 are then calculated.

Les résultats obtenus sont rassemblés dans le tableau suivant : Eprouvette Re
(MPa) Rm
(MPa) P
(%) BH2
(MPa) Coulée 1 290 389 0 74 Coulée 2 315 424 0 64 Coulée 3 282 377 0 82 Coulée 4 310 413 0,2 59* Coulée 5 333 436 1,2 40 * exemple hors de l'invention The results obtained are summarized in the following table: Specimen Re
(MPa) rm
(MPa) P
(%) BH2
(MPa) Casting 1 290 389 0 74 Casting 2 315 424 0 64 Casting 3 282 377 0 82 Casting 4 310 413 0.2 59 * Casting 5 333 436 1.2 40 * example out of the invention

On constate que les coulées 1 à 3 selon l'invention présentent de bonnes caractéristiques mécaniques, une très bonne valeur de BH2 et ne présentent pas ou peu de palier de limite d'élasticité, contrairement à la coulée 5 qui présente 1,2% de palier.It can be seen that the castings 1 to 3 according to the invention have good mechanical characteristics, a very good value of BH 2 and show no or little yield strength stop, unlike casting which has 1.2% of bearing.

On découpe ensuite de nouvelles éprouvettes dans les tôles ayant subi le recuit continu, et on les soumet à un traitement thermique à 75°C pendant 10 heures. Ce traitement thermique est équivalent à un vieillissement naturel de 6 mois à température ambiante. On obtient les résultats suivants : Eprouvette Re
(MPa) Rm
(MPa) n P% A % Coulée 1
(état frais) 290 389 0,197 0 32,6 Coulée 1
(état vieilli) 294 412 0,160 0,2 27,4 Coulée 2
(état frais) 315 424 0,180 0 32,8 Coulée 2
(état vieilli) 325 447 0,147 0 27,3 Coulée 3
(état frais) 282 377 0,185 0 20,4 Coulée 3
(état vieilli) 295 415 0,148 0 26,2 Coulée 4
(état frais) 310 413 0,187 0,2 31,7 Coulée 4
(état vieilli) 311 425 0,163 0,1 29,5 Coulée 5
(état frais) 333 436 0,186 1,2 31,6 Coulée 5
(état vieilli) 335 446 0,167 1,8 29,4 New specimens are then cut from the continuously annealed sheets and subjected to heat treatment at 75 ° C for 10 hours. This heat treatment is equivalent to a natural aging of 6 months at room temperature. The following results are obtained: Specimen Re
(MPa) rm
(MPa) not P% AT % Casting 1
(fresh state) 290 389 0.197 0 32.6 Casting 1
(aged condition) 294 412 0,160 0.2 27.4 Casting 2
(fresh state) 315 424 0,180 0 32.8 Casting 2
(aged condition) 325 447 0,147 0 27.3 Casting 3
(fresh state) 282 377 0.185 0 20.4 Casting 3
(aged condition) 295 415 0,148 0 26.2 Casting 4
(fresh state) 310 413 0.187 0.2 31.7 Casting 4
(aged condition) 311 425 0.163 0.1 29.5 Casting 5
(fresh state) 333 436 0,186 1.2 31.6 Casting 5
(aged condition) 335 446 0,167 1.8 29.4

On constate après simulation d'un vieillissement naturel de 6 mois que les coulées 1 à 3 selon l'invention ne présentent pas de palier rédhibitoire à l'aspect Z (inférieur ou égal à 0,2%), contrairement à la coulée 5 qui présente un palier de 1,8%.After simulating a natural aging of 6 months, it can be seen that the castings 1 to 3 according to the invention do not exhibit a Z-type barrier (less than or equal to 0.2%), unlike the casting 5 which has a level of 1.8%.

Claims (8)

  1. Process for manufacturing bake hardening steel sheet comprising:
    - the smelting of a steel, the composition of which comprises, expressed in % by weight:
    0.03 ≤ C ≤ 0.06
    0.50 ≤ Mn ≤ 1.10
    0.08 ≤ Si ≤ 0.20
    0.015 ≤ Al ≤ 0.070
    N ≤ 0.007
    Ni ≤ 0.040
    Cu ≤ 0.040
    P ≤ 0.035
    S ≤ 0.015
    Mo ≤ 0.010
    Ti ≤ 0.005
    it being understood that the steel also contains boron in an amount such that: 0.64 ≤ B N ≤ 1.60
    Figure imgb0005
    the balance of the composition consisting of iron and impurities resulting from the smelting;
    - the casting of a slab of this steel, this slab then being hot rolled in order to obtain a sheet, the end-of-rolling temperature being above that of the Ar3 point;
    - the coiling of said sheet at a temperature of between 500 and 700°C; then
    - the cold rolling of said sheet with a reduction ratio of 50 to 80%;
    - a continuous annealing heat treatment which is carried out for a time of less than 15 minutes, comprising:
    ■ a reheat of the steel until it reaches a temperature of between 750 and 850°C;
    ■ an isothermal soak;
    ■ a first cooling operation down to a temperature of between 380 and 500°C; and
    ■ an isothermal soak; and then
    ■ a second cooling operation down to the ambient temperature;
    - a skin pass which is carried out with a reduction ratio of between 1.2 and 2.5%.
  2. Process according to Claim 1, characterized in that said first cooling operation comprises a slow first part carried out at a rate of less than 10°C/s, followed by a rapid second part carried out at a rate of between 20 and 50°C/s.
  3. Process according to either of Claims 1 and 2, characterized in that, in addition, the manganese content and the silicon content of the steel are such that: 4 ≤ % Mn % Si ≤ 15.
    Figure imgb0006
  4. Process according to any one of Claims 1 to 3, characterized in that, in addition, the manganese content of the steel is between 0.55 and 0.65% by weight and the silicon content of the steel is between 0.08 and 0.12% by weight.
  5. Process according to any one of Claims 1 to 3, characterized in that, in addition, the manganese content of the steel is between 0.95 and 1.05% by weight and the silicon content of the steel is between 0.16 and 0.20% by weight.
  6. Process according to any one of Claims 1 to 5, characterized in that, in addition, the nitrogen content of the steel is less than 0.005% by weight.
  7. Process according to any one of Claims 1 to 6, characterized in that, in addition, the phosphorus content of the steel is less than 0.015% by weight.
  8. Bake hardening sheet that can be obtained by the process according to any one of Claims 1 to 7, characterized in that it has a yield strength of between 260 and 360 MPa, a tensile strength of between 320 and 460 MPa, a BH2 value of greater than 60 MPa and a yield plateau of less than or equal to 0.2%.
EP03775481A 2002-10-14 2003-10-10 Method for making hardenable steel plates by firing, resulting steel plates Expired - Lifetime EP1558769B1 (en)

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JP5376927B2 (en) * 2008-12-11 2013-12-25 日新製鋼株式会社 Manufacturing method of high proportional limit steel plate with excellent bending workability
WO2012127125A1 (en) * 2011-03-24 2012-09-27 Arcelormittal Investigatión Y Desarrollo Sl Hot-rolled steel sheet and associated production method
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BR0315255A (en) 2005-08-23
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CN1705757A (en) 2005-12-07
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DE60332951D1 (en) 2010-07-22
US20060157166A1 (en) 2006-07-20
KR20050055006A (en) 2005-06-10
CN100366760C (en) 2008-02-06
WO2004035838A1 (en) 2004-04-29
UA80448C2 (en) 2007-09-25
FR2845694B1 (en) 2005-12-30
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ZA200502882B (en) 2005-12-28
KR101044741B1 (en) 2011-06-28
FR2845694A1 (en) 2004-04-16
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ATE470729T1 (en) 2010-06-15
CA2502079A1 (en) 2004-04-29
EP1558769A1 (en) 2005-08-03
CA2502079C (en) 2011-08-09
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AU2003283507A1 (en) 2004-05-04
JP2006503183A (en) 2006-01-26
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RU2005114614A (en) 2005-10-10
BR0315255B1 (en) 2011-03-09

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