EP1534869A2 - Very high mechanical strength steel and method for making a sheet thereof coated with zinc or zinc alloy - Google Patents

Very high mechanical strength steel and method for making a sheet thereof coated with zinc or zinc alloy

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Publication number
EP1534869A2
EP1534869A2 EP03769565A EP03769565A EP1534869A2 EP 1534869 A2 EP1534869 A2 EP 1534869A2 EP 03769565 A EP03769565 A EP 03769565A EP 03769565 A EP03769565 A EP 03769565A EP 1534869 A2 EP1534869 A2 EP 1534869A2
Authority
EP
European Patent Office
Prior art keywords
sheet
zinc
steel
high mechanical
zinc alloy
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Granted
Application number
EP03769565A
Other languages
German (de)
French (fr)
Other versions
EP1534869B1 (en
Inventor
Antoine Moulin
Jean-Luc Lapointe
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
ArcelorMittal France SA
Original Assignee
USINOR SA
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Filing date
Publication date
Application filed by USINOR SA filed Critical USINOR SA
Publication of EP1534869A2 publication Critical patent/EP1534869A2/en
Application granted granted Critical
Publication of EP1534869B1 publication Critical patent/EP1534869B1/en
Anticipated expiration legal-status Critical
Expired - Lifetime legal-status Critical Current

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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
    • 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
    • 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
    • C21D8/0278—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips involving a particular surface treatment
    • 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
    • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C23C2/022—Pretreatment of the material to be coated, e.g. for coating on selected surface areas by heating
    • C23C2/0224—Two or more thermal pretreatments
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/005—Ferrite
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/008—Martensite
    • 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
    • C21D8/0247—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
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
    • Y10T428/00—Stock material or miscellaneous articles
    • Y10T428/12—All metal or with adjacent metals
    • Y10T428/12493—Composite; i.e., plural, adjacent, spatially distinct metal components [e.g., layers, joint, etc.]
    • Y10T428/12771—Transition metal-base component
    • Y10T428/12785—Group IIB metal-base component
    • Y10T428/12792—Zn-base component
    • Y10T428/12799—Next to Fe-base component [e.g., galvanized]

Definitions

  • the present invention relates to a steel with very high mechanical strength, as well as to a process for manufacturing a sheet of this steel coated with zinc or zinc alloy.
  • dual phase steels have a microstructure composed of ferrite and martensite, which allows them to reach tensile strengths ranging from 400 MPa to more than 1200MPa.
  • these grades are fairly heavily loaded with elements such as chromium, silicon, manganese, aluminum or phosphorus.
  • these grades are problematic when it is desired to coat them with a protective coating against corrosion, by hot-dip galvanizing, for example. Indeed, it is observed that the surface of the sheets has very poor wettability with respect to zinc or zinc alloys. The sheets then have uncoated parts, which constitute privileged areas for the initiation of corrosion.
  • the present invention therefore aims to provide a steel composition which does not have the drawbacks of the compositions of the prior art, and which in particular has good suitability for coating with zinc or zinc alloys, while retaining high mechanical characteristics.
  • a first object of the invention consists of a steel with very high mechanical strength, the chemical composition of which comprises, in% by weight:
  • the steel comprises: 0.080% ⁇ C ⁇ 0.120%
  • the steel comprises:
  • This embodiment makes it possible to obtain a steel sheet having a tensile strength of the order of 500 MPa.
  • the steel comprises:
  • This embodiment makes it possible to obtain a steel sheet having a tensile strength of the order of 600 MPa.
  • the steel has a microstructure consisting of ferrite and martensite.
  • a second object of the invention consists of a steel sheet with very high mechanical strength in accordance with the invention, and coated with zinc or zinc alloy.
  • a third object of the invention consists of a process for manufacturing a steel sheet according to the invention coated with zinc or a zinc alloy, and which comprises the steps consisting in:
  • the sheet is held at the holding temperature for 10 to 1000 seconds.
  • the bath containing zinc or a molten zinc alloy is maintained at a temperature between 450 and 480 ° C, and the immersion time of the sheet is between 2 and 400 seconds.
  • the bath mainly contains zinc.
  • a fourth object of the invention consists of the use of a sheet of very high mechanical strength of steel coated with zinc or zinc alloy, for the manufacture of automobile parts.
  • the present invention is based on the new observation that by limiting the contents of manganese, silicon and chromium to the maximum values claimed, it is possible to obtain excellent coatability of the grades thus produced. Depending on the level of mechanical characteristics sought, the contents of quenching elements such as carbon and molybdenum will be adjusted, which it has been observed that they do not harm this coating.
  • C ⁇ represents the carbon content of the austenite before cooling.
  • the steel composition according to the invention contains between 0.060 and 0.250% by weight of carbon, since it has been observed that for a carbon content of less than 0.060%, the grade was no longer hardenable, and no longer made it possible to obtain the high mechanical characteristics sought.
  • the composition also contains between 0.400 and 0.950% by weight of manganese.
  • the lower limit is required to obtain a grade of hardenable steel, while the upper limit must be respected in order to ensure good coating of the grade.
  • the composition also contains up to 0.300% by weight of silicon.
  • the upper limit must be respected in order to ensure good coating of the shade.
  • composition also contains up to 0.300% by weight of chromium.
  • the upper limit must be respected in order to ensure good coating of the shade.
  • composition according to the invention must contain between 0.100 and
  • composition can also optionally contain up to
  • the steel composition can also contain various inevitable residual elements, among which there may be mentioned N, Nb, Cu, Ni, W, V.
  • the steel according to the invention finds in particular applications in the field of the manufacture of parts for the automobile, and more particularly for the manufacture of visible parts such as bodywork elements, which will have good appearance after painting, unlike those manufactured up to now with the steels of the prior art.
  • the sheets are then subjected to galvanization by dipping in a zinc bath, with a residence time in the bath depending on the line speed chosen (between 80 and 150 m / min), then cooled at a speed of 5 ° C. / s up to room temperature.
  • Test 1 Influence of the molybdenum content and the presence of boron
  • the martensite content is increased, which makes it possible to increase the tensile strength and to lower the elastic limit.
  • the sheets of grades A, B, C and F are hot dip galvanized and the dew point is adjusted to -40 ° C.
  • the sheets produced in shades A and B have gaps in their coatings, unlike shades C and F which have continuous coatings.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Organic Chemistry (AREA)
  • Materials Engineering (AREA)
  • Mechanical Engineering (AREA)
  • Metallurgy (AREA)
  • Thermal Sciences (AREA)
  • Physics & Mathematics (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Heat Treatment Of Sheet Steel (AREA)
  • Coating With Molten Metal (AREA)
  • Electroplating Methods And Accessories (AREA)
  • Laminated Bodies (AREA)
  • Heat Treatment Of Steel (AREA)

Abstract

The invention concerns a very high mechanical strength steel, whereof the chemical composition comprises in wt. %: 0.060%=C=0.250%; 0.400%=Mn=0.950%; Si=0.300%; Cr=0.300%; 0.100%=Mo=0.500%; 0.020%=AI=0.100%; P=0.100%; B=0.010%; Ti=0.050%, the rest being iron and impurities resulting from preparation. The invention also concerns a method for making a sheet of said steel coated with zinc or zinc alloy.

Description

ACIER A TRES HAUTE RESISTANCE MECANIQUE ET PROCEDE DE FABRICATION D'UNE FEUILLE DE CET ACIER REVETUE DE ZINC OU D'ALLIAGE DE ZINC VERY HIGH MECHANICAL STRENGTH STEEL AND METHOD FOR MANUFACTURING A SHEET OF SUCH A ZINC COATED STEEL OR A ZINC ALLOY
La présente invention concerne un acier à très haute résistance mécanique, ainsi qu'un procédé de fabrication d'une feuille de cet acier revêtue de zinc ou d'alliage de zinc.The present invention relates to a steel with very high mechanical strength, as well as to a process for manufacturing a sheet of this steel coated with zinc or zinc alloy.
Il existe plusieurs familles d'aciers à très haute résistance mécanique qui diffèrent par leurs compositions et par leurs microstructures. Ainsi, les aciers dits dual phase ont une microstructure composée de ferrite et de martensite, qui leur permet d'atteindre des résistances à la traction allant de 400 MPa à plus de 1200MPa.There are several families of very high mechanical strength steels which differ in their compositions and in their microstructures. Thus, so-called dual phase steels have a microstructure composed of ferrite and martensite, which allows them to reach tensile strengths ranging from 400 MPa to more than 1200MPa.
Afin d'obtenir les microstructures qui permettront d'atteindre des caractéristiques mécaniques élevées, ces nuances sont assez fortement chargées en des éléments tels que le chrome, le silicium, le manganèse, l'aluminium ou le phosphore. Ces nuances posent cependant problème lorsque l'on souhaite les revêtir d'un revêtement protecteur contre la corrosion, par galvanisation au trempé à chaud, par exemple. En effet, on observe que la surface des tôles présente une très mauvaise mouillabilité vis-à-vis du zinc ou des alliages de zinc. Les tôles comportent alors des parties non revêtues, qui constituent des zones privilégiées pour l'amorce d'une corrosion.In order to obtain the microstructures which will make it possible to achieve high mechanical characteristics, these grades are fairly heavily loaded with elements such as chromium, silicon, manganese, aluminum or phosphorus. However, these grades are problematic when it is desired to coat them with a protective coating against corrosion, by hot-dip galvanizing, for example. Indeed, it is observed that the surface of the sheets has very poor wettability with respect to zinc or zinc alloys. The sheets then have uncoated parts, which constitute privileged areas for the initiation of corrosion.
Pour pallier ce problème, différentes approches ont été proposées. Ainsi, on connaît des procédés consistant à effectuer un pré-revêtement d'un métal permettant de fournir une meilleure base d'accrochage pour le zinc. On a proposé à cet effet de déposer du fer, de l'aluminium, du cuivre et d'autres éléments, en général par électrodéposition. Ces procédés présentent l'inconvénient d'ajouter une étape supplémentaire avant la galvanisation proprement dite.To overcome this problem, different approaches have been proposed. Thus, methods are known which consist in pre-coating a metal which makes it possible to provide a better bonding base for zinc. To this end, it has been proposed to deposit iron, aluminum, copper and other elements, generally by electrodeposition. These methods have the disadvantage of adding an additional step before the actual galvanization.
Il a également été proposé de faire passer les tôles dans des fours de recuit présentant, notamment, des atmosphères particulières, permettant d'oxyder sélectivement le fer, afin de former une couche d'oxyde de fer sur laquelle le zinc se dépose bien. Un tel procédé est cependant d'un réglage très délicat et nécessite un contrôle très strict des conditions d'oxydation.It has also been proposed to pass the sheets through annealing ovens having, in particular, specific atmospheres, allowing to selectively oxidize the iron, in order to form a layer of iron oxide on which the zinc is deposited well. Such a process is however very delicate to set up and requires very strict control of the oxidation conditions.
La présente invention a donc pour but de mettre à disposition une composition d'acier ne présentant pas les inconvénients des compositions de l'art antérieur, et présentant en particulier une bonne aptitude au revêtement par du zinc ou des alliages de zinc, tout en conservant des caractéristiques mécaniques élevées.The present invention therefore aims to provide a steel composition which does not have the drawbacks of the compositions of the prior art, and which in particular has good suitability for coating with zinc or zinc alloys, while retaining high mechanical characteristics.
A cet effet, un premier objet de l'invention est constitué par un acier à très haute résistance mécanique, dont la composition chimique comprend, en % en poids :To this end, a first object of the invention consists of a steel with very high mechanical strength, the chemical composition of which comprises, in% by weight:
0,060% < C < 0,250% 0,400% < Mn < 0,950% Si < 0,300% Cr < 0,300%0.060% <C <0.250% 0.400% <Mn <0.950% If <0.300% Cr <0.300%
0,100% < Mo < 0,500% 0,020% < Al < 0,100% P < 0,100% B < 0,010% Ti < 0,050% le reste étant du fer et des impuretés résultant de l'élaboration. Dans un mode de réalisation préféré, l'acier comprend : 0,080% < C < 0,120%0.100% <Mo <0.500% 0.020% <Al <0.100% P <0.100% B <0.010% Ti <0.050% the remainder being iron and impurities resulting from the production. In a preferred embodiment, the steel comprises: 0.080% <C <0.120%
0,800% < Mn < 0,950% Si < 0,300% Cr < 0,300% 0,100% < Mo < 0,300% 0,020% < Al < 0,100%0.800% <Mn <0.950% If <0.300% Cr <0.300% 0.100% <Mo <0.300% 0.020% <Al <0.100%
P < 0,100% B < 0,010% Ti < 0,050% le reste étant du fer et des impuretés résultant de l'élaboration. Ce mode de réalisation permet d'obtenir une feuille d'acier ayant une résistance à la traction de l'ordre de 450MPa.P <0.100% B <0.010% Ti <0.050% the rest being iron and impurities resulting from the production. This embodiment makes it possible to obtain a steel sheet having a tensile strength of the order of 450 MPa.
Dans un autre mode de réalisation préféré, l'acier comprend :In another preferred embodiment, the steel comprises:
0,080% < C < 0,120% 0,800% < Mn < 0,950%0.080% <C <0.120% 0.800% <Mn <0.950%
Si < 0,300% Cr < 0,300% 0,150% < Mo < 0,350% 0,020% < Al < 0,100% P < 0,100% B < 0,010% Ti < 0,050% le reste étant du fer et des impuretés résultant de l'élaboration.If <0.300% Cr <0.300% 0.150% <Mo <0.350% 0.020% <Al <0.100% P <0.100% B <0.010% Ti <0.050% the remainder being iron and impurities resulting from the production.
Ce mode de réalisation permet d'obtenir une feuille d'acier ayant une résistance à la traction de l'ordre de 500MPa.This embodiment makes it possible to obtain a steel sheet having a tensile strength of the order of 500 MPa.
Dans un autre mode de réalisation préféré, l'acier comprend :In another preferred embodiment, the steel comprises:
0,100% < C < 0,140% 0,800% < Mn < 0,950%0.100% <C <0.140% 0.800% <Mn <0.950%
Si < 0,300% Cr < 0,300% 0,200% < Mo < 0,400% 0,020% < Al < 0,100% P < 0,100% B < 0,010% Ti < 0,050% le reste étant du fer et des impuretés résultant de l'élaboration.If <0.300% Cr <0.300% 0.200% <Mo <0.400% 0.020% <Al <0.100% P <0.100% B <0.010% Ti <0.050% the rest being iron and impurities resulting from the production.
Ce mode de réalisation permet d'obtenir une feuille d'acier ayant une résistance à la traction de l'ordre de 600MPa.This embodiment makes it possible to obtain a steel sheet having a tensile strength of the order of 600 MPa.
Dans un autre mode de réalisation préféré, l'acier présente une microstructure constituée de ferrite et de martensite. Un deuxième objet de l'invention est constitué par une feuille d'acier à très haute résistance mécanique conforme à l'invention, et revêtue de zinc ou d'alliage de zinc. Un troisième objet de l'invention est constitué par un procédé de fabrication d'une feuille d'acier selon l'invention revêtue de zinc ou d'alliage de zinc, et qui comprend les étapes consistant à :In another preferred embodiment, the steel has a microstructure consisting of ferrite and martensite. A second object of the invention consists of a steel sheet with very high mechanical strength in accordance with the invention, and coated with zinc or zinc alloy. A third object of the invention consists of a process for manufacturing a steel sheet according to the invention coated with zinc or a zinc alloy, and which comprises the steps consisting in:
- élaborer une brame dont la composition est conforme à l'invention, laminer à chaud, puis à froid ladite brame pour obtenir une feuille,- develop a slab whose composition is in accordance with the invention, hot roll, then cold roll said slab to obtain a sheet,
- chauffer ladite feuille à une vitesse comprise entre 2 et 100°C/s jusqu'à atteindre une température de maintien comprise entre 700 et 900°C,- heating said sheet at a speed of between 2 and 100 ° C / s until a holding temperature of between 700 and 900 ° C is reached,
- refroidir ladite feuille à une vitesse comprise entre 2 et 100°C/s jusqu'à atteindre une température proche de celle d'un bain contenant du zinc ou un alliage de zinc fondu, puis- cooling said sheet at a speed between 2 and 100 ° C / s until reaching a temperature close to that of a bath containing zinc or a molten zinc alloy, then
- revêtir ladite feuille de zinc ou d'un alliage de zinc par immersion dans ledit bain et la refroidir jusqu'à température ambiante à une vitesse de refroidissement comprise entre 2 et 100°C/s.- coating said sheet of zinc or a zinc alloy by immersion in said bath and cooling it to ambient temperature at a cooling rate of between 2 and 100 ° C / s.
Dans un autre mode de réalisation préféré, la feuille est maintenue à la température de maintien pendant 10 à 1000 secondes.In another preferred embodiment, the sheet is held at the holding temperature for 10 to 1000 seconds.
Dans un autre mode de réalisation préféré, le bain contenant du zinc ou un alliage de zinc fondu est maintenu à une température comprise entre 450 et 480°C, et le temps d'immersion de la feuille est compris entre 2 et 400 secondes. Dans un autre mode de réalisation préféré, le bain contient principalement du zinc.In another preferred embodiment, the bath containing zinc or a molten zinc alloy is maintained at a temperature between 450 and 480 ° C, and the immersion time of the sheet is between 2 and 400 seconds. In another preferred embodiment, the bath mainly contains zinc.
Un quatrième objet de l'invention est constitué par l'utilisation d'une feuille à très haute résistance mécanique d'acier revêtue de zinc ou d'alliage de zinc, pour la fabrication de pièces d'automobiles. La présente invention est basée sur le constat nouveau qu'en limitant les teneurs en manganèse, silicium et chrome aux valeurs maximum revendiquées, on peut obtenir une excellente revêtabilité des nuances ainsi produites. En fonction du niveau de caractéristiques mécaniques recherché, on ajustera les teneurs en éléments trempants tels que le carbone et le molybdène, dont on a pu constater qu'ils ne nuisent pas à cette revêtabilité.A fourth object of the invention consists of the use of a sheet of very high mechanical strength of steel coated with zinc or zinc alloy, for the manufacture of automobile parts. The present invention is based on the new observation that by limiting the contents of manganese, silicon and chromium to the maximum values claimed, it is possible to obtain excellent coatability of the grades thus produced. Depending on the level of mechanical characteristics sought, the contents of quenching elements such as carbon and molybdenum will be adjusted, which it has been observed that they do not harm this coating.
A cet effet, on pourra par exemple utiliser la formule classique donnant le logarithme décimal de la vitesse critique de trempe V (en°C/s): Log(V) = 4,5 - 2,7%Cγ - 0,95%Mn - 0,18%Si - 0,38%Cr - 1 ,17%Mo - 1 ,29(%C x %Cr) - 0,33(%Cr x %Mo)For this purpose, we could for example use the classic formula giving the decimal logarithm of the critical quenching speed V (in ° C / s): Log (V) = 4.5 - 2.7% Cγ - 0.95% Mn - 0.18% Si - 0.38% Cr - 1.17% Mo - 1.29 (% C x% Cr) - 0.33 (% Cr x% Mo)
où Cγ représente la teneur en carbone de l'austénite avant le refroidissement.where Cγ represents the carbon content of the austenite before cooling.
La composition d'acier selon l'invention contient entre 0,060 et 0,250% en poids de carbone, car on a observé que pour une teneur en carbone inférieure à 0,060 %, la nuance n'était plus trempable, et ne permettait plus d'obtenir les caractéristiques mécaniques élevées recherchées. Au-delà deThe steel composition according to the invention contains between 0.060 and 0.250% by weight of carbon, since it has been observed that for a carbon content of less than 0.060%, the grade was no longer hardenable, and no longer made it possible to obtain the high mechanical characteristics sought. Beyond
0,250% en poids, le carbone détériore fortement la soudabilité de la nuance.0.250% by weight, carbon greatly deteriorates the weldability of the grade.
La composition contient également entre 0,400 et 0,950% en poids de manganèse. De même que pour le carbone, la limite inférieure est requise pour obtenir une nuance d'acier trempable, tandis que la limite supérieure doit être respectée afin d'assurer une bonne revêtabilité de la nuance.The composition also contains between 0.400 and 0.950% by weight of manganese. As with carbon, the lower limit is required to obtain a grade of hardenable steel, while the upper limit must be respected in order to ensure good coating of the grade.
La composition contient aussi jusqu'à 0,300% en poids de silicium. La limite supérieure doit être respectée afin d'assurer une bonne revêtabilité de la nuance.The composition also contains up to 0.300% by weight of silicon. The upper limit must be respected in order to ensure good coating of the shade.
La composition contient en outre jusqu'à 0,300% en poids de chrome. La limite supérieure doit être respectée afin d'assurer une bonne revêtabilité de la nuance.The composition also contains up to 0.300% by weight of chromium. The upper limit must be respected in order to ensure good coating of the shade.
Enfin, la composition selon l'invention doit contenir entre 0,100 etFinally, the composition according to the invention must contain between 0.100 and
0,500% en poids de molybdène car on a observé que pour une teneur inférieure à 0,100%, la nuance ne permettait plus d'obtenir les caractéristiques mécaniques élevées recherchées. Au-delà de 0,500% en poids, le molybdène détériore fortement la soudabilité de la nuance.0.500% by weight of molybdenum because it has been observed that for a content of less than 0.100%, the grade no longer makes it possible to obtain the desired high mechanical characteristics. Above 0.500% by weight, molybdenum greatly deteriorates the weldability of the grade.
La composition peut également contenir, à titre optionnel, jusqu'àThe composition can also optionally contain up to
0,010% en poids de bore que l'on protégera alors si nécessaire par une teneur de 0,050% en poids au maximum de titane. Ce dernier élément présentant une affinité pour l'azote plus importante que le bore, le piège par formation de nitrures de titane. La composition d'acier peut également contenir différents éléments résiduels inévitables, parmi lesquels on peut citer N, Nb, Cu, Ni, W, V.0.010% by weight of boron which will then be protected if necessary by a content of 0.050% by weight at most of titanium. This last element having a greater affinity for nitrogen than boron, traps it by the formation of titanium nitrides. The steel composition can also contain various inevitable residual elements, among which there may be mentioned N, Nb, Cu, Ni, W, V.
On préfère en particulier limiter la teneur en azote qui peut rendre l'acier sensible au vieillissement.It is particularly preferred to limit the nitrogen content which can make the steel sensitive to aging.
Grâce à sa galvanisabilité améliorée, l'acier selon l'invention trouve notamment des applications dans le domaine de la fabrication de pièces pour l'automobile, et plus particulièrement pour la fabrication de pièces visibles telles que des éléments de carrosserie, qui présenteront un bon aspect après peinture, contrairement à ceux fabriqués jusqu'à présent avec les aciers de l'art antérieur.Thanks to its improved galvanizability, the steel according to the invention finds in particular applications in the field of the manufacture of parts for the automobile, and more particularly for the manufacture of visible parts such as bodywork elements, which will have good appearance after painting, unlike those manufactured up to now with the steels of the prior art.
La présente invention va à présent être illustrée à partir des observations et des exemples suivants, donnés à titre d'exemples non limitatifs, le tableau 1 donnant la composition chimique des aciers testés, en 10"3% en poids.The present invention will now be illustrated from the following observations and examples, given by way of nonlimiting examples, Table 1 giving the chemical composition of the steels tested, in 10 "3 % by weight.
Tableau 1Table 1
selon l'invention according to the invention
Ces différentes compositions ont été élaborées sous forme de lingots de 15 kg. Les lingots ont été ensuite réchauffés à 1250°C pendant 45 minutes, puis laminés à chaud en 7 passes, la température de fin de laminage étant de 900°C. Les tôles ainsi obtenues ont été refroidies par trempe à l'eau avec ralentisseur à une vitesse de refroidissement de l'ordre de 25°C/s, puis bobinées à 550°C avant d'être refroidies.These different compositions have been produced in the form of 15 kg ingots. The ingots were then reheated at 1250 ° C for 45 minutes, then hot rolled in 7 passes, the temperature at the end of rolling being 900 ° C. The sheets thus obtained were cooled by water quenching with a retarder at a cooling rate of the order of 25 ° C / s, then wound at 550 ° C before being cooled.
Elles ont ensuite été laminées à froid avec un taux de réduction de 70% avant de subir le cycle thermique suivant :They were then cold rolled with a reduction rate of 70% before undergoing the following thermal cycle:
- chauffage à une vitesse de l'ordre de 30°C/s jusqu'à atteindre une température de maintien variant entre 770 et 810°C pendant un temps variant entre 50 et 80 secondes, pour simuler des vitesses de ligne allant de 80 à 150 m/min, - refroidissement de la feuille à une vitesse de l'ordre de 10°C/s jusqu'à atteindre 470°C.- heating at a speed of the order of 30 ° C / s until reaching a holding temperature varying between 770 and 810 ° C for a time varying between 50 and 80 seconds, to simulate line speeds ranging from 80 to 150 m / min, - cooling of the sheet at a speed of the order of 10 ° C / s until reaching 470 ° C.
Les feuilles sont ensuite soumises à une galvanisation au trempé dans un bain de zinc, avec un temps de séjour dans le bain dépendant de la vitesse de ligne choisie (entre 80 et 150 m/min), puis refroidies à une vitesse de 5°C/s jusqu'à température ambiante.The sheets are then subjected to galvanization by dipping in a zinc bath, with a residence time in the bath depending on the line speed chosen (between 80 and 150 m / min), then cooled at a speed of 5 ° C. / s up to room temperature.
Pour chaque feuille, on mesure ensuite les caractéristiques mécaniques suivantes :For each sheet, the following mechanical characteristics are then measured:
- Rm : résistance à la traction en MPa- Rm: tensile strength in MPa
- Rel : limite d'élasticité en MPa, - A : allongement à la rupture en %- Rel: elastic limit in MPa, - A: elongation at break in%
- Ag : allongement réparti en %.- Ag: elongation distributed in%.
- P : palier en %, ainsi que la proportion de martensite des feuilles (%M). Essai 1 : Influence de la teneur en molybdène et de la présence de bore- P: plateau in%, as well as the proportion of leaf martensite (% M). Test 1: Influence of the molybdenum content and the presence of boron
Cette influence a été étudiée pour les nuances A à F, pour une température de maintien de 790°C et une vitesse de ligne de 120 m/min.This influence has been studied for grades A to F, for a holding temperature of 790 ° C and a line speed of 120 m / min.
*selon l'invention * according to the invention
Pour les nuances selon l'invention, on constate qu'en augmentant la teneur en molybdène, on augmente la teneur en martensite, ce qui permet d'augmenter la résistance à la traction et d'abaisser la limite d'élasticité.For the grades according to the invention, it is found that by increasing the molybdenum content, the martensite content is increased, which makes it possible to increase the tensile strength and to lower the elastic limit.
Par contre, l'addition de bore n'entraîne pas d'augmentation du pourcentage de martensite, mais conduit plutôt à un affinement de la martensite et des phases carburées.On the other hand, the addition of boron does not lead to an increase in the percentage of martensite, but rather leads to a refinement of the martensite and of the carburetted phases.
Essai 2 : Influence du traitement thermiqueTest 2: Influence of heat treatment
Cette influence a été étudiée pour la nuance D pour trois vitesses de ligne et pour trois températures de maintien (en m/min):This influence has been studied for grade D for three line speeds and for three holding temperatures (in m / min):
On constate que la température de maintien et la vitesse de ligne ont une faible influence sur les caractéristiques mécaniques obtenues. Ceci présente un grand intérêt pour une application industrielle qui en doit pas être sensible à ce type de variations. Cette influence a ensuite été étudiée pour la nuance FIt can be seen that the holding temperature and the line speed have a small influence on the mechanical characteristics obtained. This is of great interest for an industrial application which must not be sensitive to this type of variation. This influence was then studied for the grade F
On constate que l'ajout de bore à la nuance selon l'invention stabilise de façon remarquable la proportion de martensite formée qui ne varie absolument pas, quels que soient les paramètres du traitement thermique.It is found that the addition of boron to the grade according to the invention remarkably stabilizes the proportion of martensite formed which does not vary absolutely, whatever the parameters of the heat treatment.
Essai 3 : GalvanisabilitéTest 3: Galvanizability
On galvanise au trempé à chaud des feuilles des nuances A, B, C et F et en réglant le point de rosée à -40°C. Les feuilles réalisées dans les nuances A et B présentent des manques dans leurs revêtements, au contraire des nuances C et F qui présentent des revêtements continus. The sheets of grades A, B, C and F are hot dip galvanized and the dew point is adjusted to -40 ° C. The sheets produced in shades A and B have gaps in their coatings, unlike shades C and F which have continuous coatings.

Claims

REVENDICATIONS
1. Acier à très haute résistance mécanique, caractérisé en ce que sa composition chimique comprend, en % en poids :1. Steel with very high mechanical resistance, characterized in that its chemical composition comprises, in% by weight:
0,060% < C < 0,250% 0,400% < Mn < 0,950% Si < 0,300%0.060% <C <0.250% 0.400% <Mn <0.950% If <0.300%
Cr < 0,300% 0,100% < Mo < 0,500% 0,020% < Al < 0,100% P < 0,100% B < 0,010% Ti < 0,050% le reste étant du fer et des impuretés résultant de l'élaboration. Cr <0.300% 0.100% <Mo <0.500% 0.020% <Al <0.100% P <0.100% B <0.010% Ti <0.050% the remainder being iron and impurities resulting from the production.
2. Acier selon la revendication 1 , caractérisé en ce qu'il comprend en outre2. Steel according to claim 1, characterized in that it further comprises
0,080% < C < 0,120% 0,800% < Mn < 0,950% Si < 0,300% Cr < 0,300% 0,100% < Mo < 0,300%0.080% <C <0.120% 0.800% <Mn <0.950% Si <0.300% Cr <0.300% 0.100% <Mo <0.300%
0,020% < Al < 0,100%0.020% <Al <0.100%
P < 0,100%P <0.100%
B < 0,010%B <0.010%
Ti < 0,050% le reste étant du fer et des impuretés résultant de l'élaboration.Ti <0.050% the remainder being iron and impurities resulting from the production.
3. Acier selon la revendication 1 , caractérisé en ce qu'il comprend en outre :3. Steel according to claim 1, characterized in that it further comprises:
0,080% < C < 0,120% 0,800% < Mn < 0,950%0.080% <C <0.120% 0.800% <Mn <0.950%
Si < 0,300% Cr < 0,300% 0,150% < Mo < 0,350% 0,020% < Al < 0,100% P < 0,100% B < 0,010% Ti < 0,050% le reste étant du fer et des impuretés résultant de l'élaboration. If <0.300% Cr <0.300% 0.150% <Mo <0.350% 0.020% <Al <0.100% P <0.100% B <0.010% Ti <0.050% the rest being iron and impurities resulting from the production.
4. Acier selon la revendication 1 , caractérisé en ce qu'il comprend en outre :4. Steel according to claim 1, characterized in that it further comprises:
0,100% < C < 0,140% 0,800% < Mn < 0,950% Si < 0,300%0.100% <C <0.140% 0.800% <Mn <0.950% If <0.300%
Cr < 0,300% 0,200% < Mo < 0,400% 0,020% < Al < 0,100% P < 0,100% B < 0,010% Ti < 0,050% le reste étant du fer et des impuretés résultant de l'élaboration. Cr <0.300% 0.200% <Mo <0.400% 0.020% <Al <0.100% P <0.100% B <0.010% Ti <0.050% the remainder being iron and impurities resulting from the production.
5. Acier selon l'une quelconque des revendications 1 à 4, caractérisé en ce que sa microstructure est constituée de ferrite et de martensite. 5. Steel according to any one of claims 1 to 4, characterized in that its microstructure consists of ferrite and martensite.
6. Feuille à très haute résistance mécanique d'acier selon l'une quelconque des revendications 1 à 5, caractérisée en ce qu'elle est revêtue de zinc ou d'alliage de zinc. 6. Sheet with very high mechanical strength of steel according to any one of claims 1 to 5, characterized in that it is coated with zinc or zinc alloy.
7. Procédé de fabrication d'une feuille d'acier selon la revendication 6, caractérisé en ce qu'il comprend les étapes consistant à :7. A method of manufacturing a steel sheet according to claim 6, characterized in that it comprises the steps consisting in:
- élaborer une brame dont la composition est conforme à l'une quelconque des revendications 1 à 4, laminer à chaud, puis à froid ladite brame pour obtenir une feuille, - chauffer ladite feuille à une vitesse comprise entre 2 et 100°C/s jusqu'à atteindre une température de maintien comprise entre 700 et 900°C,- Developing a slab, the composition of which conforms to any one of Claims 1 to 4, hot rolling, then cold laminating said slab to obtain a sheet, - heating said sheet at a speed of between 2 and 100 ° C / s until reaching a holding temperature between 700 and 900 ° C,
- refroidir ladite feuille à une vitesse comprise entre 2 et 100°C/s jusqu'à atteindre une température proche de celle d'un bain contenant du zinc ou un alliage de zinc fondu, puis - revêtir ladite feuille de zinc ou d'un alliage de zinc par immersion dans ledit bain et la refroidir jusqu'à température ambiante, à une vitesse de refroidissement comprise entre 2 et 100°C/s. - cooling said sheet at a speed between 2 and 100 ° C / s until reaching a temperature close to that of a bath containing zinc or a molten zinc alloy, then - coating said sheet of zinc or a zinc alloy by immersion in said bath and cooling it to ambient temperature, at a cooling rate of between 2 and 100 ° C / s.
8. Procédé selon la revendication 7, caractérisé en ce que la feuille est maintenue à ladite température de maintien pendant 10 à 1000 secondes.8. Method according to claim 7, characterized in that the sheet is maintained at said holding temperature for 10 to 1000 seconds.
9. Procédé selon l'une ou l'autre des revendications 7 ou 8, caractérisé en ce que ledit bain contenant du zinc ou un alliage de zinc fondu est maintenu à une température comprise entre 450 et 480°C, et en ce que le temps d'immersion de ladite feuille est compris entre 2 et 400 secondes.9. Method according to either of claims 7 or 8, characterized in that said bath containing zinc or a molten zinc alloy is maintained at a temperature between 450 and 480 ° C, and in that the immersion time of said sheet is between 2 and 400 seconds.
10. Procédé selon l'une quelconque des revendications 7 à 9, caractérisé en ce que ledit bain contient principalement du zinc.10. Method according to any one of claims 7 to 9, characterized in that said bath contains mainly zinc.
11. Utilisation d'une feuille à très haute résistance mécanique d'acier revêtue de zinc ou d'alliage de zinc, selon la revendication 6, pour la fabrication de pièces d'automobiles. 11. Use of a sheet of very high mechanical strength steel coated with zinc or zinc alloy, according to claim 6, for the manufacture of automobile parts.
EP03769565A 2002-09-06 2003-09-04 Very high mechanical strength steel and method for making a sheet thereof coated with zinc or zinc alloy Expired - Lifetime EP1534869B1 (en)

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FR0211040A FR2844281B1 (en) 2002-09-06 2002-09-06 HIGH MECHANICAL STRENGTH STEEL AND METHOD OF MANUFACTURING SHEET OF ZINC-COATED STEEL OR ZINC ALLOY STEEL
FR0211040 2002-09-06
PCT/FR2003/002641 WO2004022793A2 (en) 2002-09-06 2003-09-04 Very high mechanical strength steel and method for making a sheet thereof coated with zinc or zinc alloy

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