EP1485511A2 - High-resistant, low-density hot laminated sheet steel and method for the production thereof - Google Patents

High-resistant, low-density hot laminated sheet steel and method for the production thereof

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Publication number
EP1485511A2
EP1485511A2 EP03725278A EP03725278A EP1485511A2 EP 1485511 A2 EP1485511 A2 EP 1485511A2 EP 03725278 A EP03725278 A EP 03725278A EP 03725278 A EP03725278 A EP 03725278A EP 1485511 A2 EP1485511 A2 EP 1485511A2
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EP
European Patent Office
Prior art keywords
phase
silicon
sheet according
aluminum
sheet
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Granted
Application number
EP03725278A
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German (de)
French (fr)
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EP1485511B1 (en
Inventor
Xavier Bano
Christian Giraud
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ArcelorMittal France SA
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USINOR SA
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Classifications

    • CCHEMISTRY; METALLURGY
    • C21METALLURGY OF IRON
    • C21DMODIFYING 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
    • C21D8/00Modifying the physical properties by deformation combined with, or followed by, heat treatment
    • C21D8/02Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
    • C21D8/04Modifying 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
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/06Ferrous alloys, e.g. steel alloys containing aluminium
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/02Ferrous alloys, e.g. steel alloys containing silicon
    • CCHEMISTRY; METALLURGY
    • C22METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22CALLOYS
    • C22C38/00Ferrous alloys, e.g. steel alloys
    • C22C38/04Ferrous alloys, e.g. steel alloys containing manganese

Definitions

  • the invention relates to a hot-rolled steel sheet, with very high strength and low density, from a strip rolling train, as well as its manufacturing process.
  • Reducing motor vehicles becomes a necessity due to the imposition of reducing C02 emissions to 140g / Km in 2008. This reduction can only be achieved by increasing the level of mechanical strength of steels to compensate for the decrease in sheet thickness. It is therefore necessary to increase the mechanical characteristics while reducing the thicknesses of the sheets with which the parts used are made. This approach finds its limits with the reduction in the rigidity of the parts and the appearance of unacceptable vibrations and noises in targeted applications in the automotive field where noise is an element of discomfort.
  • the object of the invention is to provide users of hot-rolled steel sheets with a sheet of low density and having resistance levels comparable to steel sheets with high mechanical resistance currently used or even of a higher level, and this, in order to combine the two advantages of low density and high mechanical strength.
  • the first object of the invention is a hot-rolled steel sheet of very high strength and low density, characterized in that its composition in% by weight comprises: 0.04% ⁇ carbon ⁇ 0.5%
  • the steel comprises in its composition, in% by weight:
  • the sheet according to the invention is such that the silicon content is between 3 and 6%, and that the aluminum content is between 1 and 2%.
  • the sheet according to the invention is such that the silicon content is between 2 and 3%, and that the aluminum content is between 7 and 10%.
  • the silicon and aluminum contents of the sheet according to the invention are such that:
  • the sheet according to the invention can also have the following characteristics, alone or in combination:
  • the sheet has a microstructure consisting of a primary ferrite phase and a secondary ferrite phase, the average grain size of said primary ferrite being greater than the average grain size of said secondary ferrite, said microstructure also containing carburetted phases,
  • the sheet has a primary ferrite phase obtained during the heating of the steel carried out prior to hot rolling, and a secondary ferrite phase obtained after hot rolling, as well as carburetted phases, - the sheet comprises a primary ferrite phase whose average grain size greater than 5 ⁇ m, and a secondary ferrite phase are the average grain size less than 2 ⁇ m.
  • the second object of the invention is a method of manufacturing a hot-rolled sheet which comprises the steps consisting in:
  • the temperature of end of hot rolling being higher than the temperature AR3 of the austenitic phase formed during reheating, in order to carry out a rolling under austenitic conditions, thus transforming the austenitic phase into a phase secondary ferrite and fuel phases.
  • FIG. 1 a curve showing the evolution of the density of a steel as a function of the silicon, aluminum and / or silicon plus aluminum content
  • FIG. 2 the microstructure of a steel according to the invention comprising 0.04% of carbon (casting I)
  • FIG. 3 the microstructure of a steel according to the invention comprising 0.160% of carbon (casting J)
  • FIG. 4 the microstructure of a steel according to the invention comprising 0.268% of carbon (casting K)
  • FIG. 5 the microstructure of a steel comprising 0.505% of carbon, shown for comparison (casting L ).
  • the hot rolled steel on a strip train according to the invention has a high mechanical strength and a low density.
  • molybdenum ⁇ 2% taken alone or in combination, the rest being iron and elements inherent in the preparation, comprises:
  • the carbon content of the sheet according to the invention is between 0.04 and 0.5% by weight, preferably between 0.04 and 0.3% by weight.
  • the evolution of the structure of the steel as a function of the carbon content is presented in FIGS. 2 to 5 and shows that the structure of the steel according to the invention (FIGS. 2 to 4) consists of primary ferrite with coarse grains, and a mixture of carburetted phase and fine secondary ferrite with smaller grains. If the carbon content falls below 0.04%, the microstructure does not include carburetted phases and loses in mechanical characteristics. On the other hand, if the carbon content exceeds 0.5% in weight, the structure becomes very fragile, and we observe that the microstructure no longer includes primary ferrite (see Figure 5).
  • the steel according to the invention can reach levels of mechanical resistance ranging from 620 MPa to more than 1000 MPa and densities of the order of 7.55 and down to 7 g / cm3 depending on the contents.
  • silicon and aluminum and additive elements as shown in Figure 1.
  • the mechanical characteristics can be reinforced by the addition of micro alloy elements such as niobium, titanium, vanadium, the latter two, which are less dense than iron.
  • the sheet according to the invention can be manufactured by any suitable process.
  • This process first of all comprises reheating the slab at high temperature (preferably above 900 ° C.), prior to hot rolling.
  • high temperature preferably above 900 ° C.
  • the present inventors have discovered that during this reheating step, the slab has a microstructure composed of a so-called primary ferritic phase which forms at high temperature, and which co-exists with an austenitic phase.
  • the austenitic phase is then completely transformed into a mixture of carburetted phase and secondary ferrite, the average grain size of which is smaller than that of the primary ferritic phase which remains.
  • a carbon-manganese couple will advantageously be chosen in order to have an AR3 transformation temperature such that it is possible to guarantee rolling under austenitic conditions.
  • Table 1 below presenting different analyzes according to the invention, shows the influence of the different elements on the characteristics of the steels.
  • the flows A, C, F, H and L are given for comparison, while the flows B, D, E, G, I, J and K are according to the invention.
  • the rolling temperature is 895 ° C, and the winding temperature of 600 ° C, with a cooling rate of 49 ° C / s, which gives the steel a mechanical resistance of 750 MPa.
  • Lowering the winding temperature increases the level of mechanical resistance.
  • the winding temperature of which is 20 ° C. with a cooling rate of 5 ° C./s, which makes it possible to reach a level of mechanical resistance of 902 MPa.
  • the level of mechanical strength can also be adjusted by carbon and manganese contents and / or other addition elements as presented above.
  • Certain operations such as, for example, re-rolling or a heat treatment such as annealing, can be used to modify or adjust the level of mechanical properties.
  • the steel proposed meets two contradictory needs in the field of hot-rolled steels on the one hand, high mechanical properties and on the other hand, low density.
  • the existing solutions for making steels with very high levels of mechanical strength are based on the use of addition elements which do not allow a large variation in density, and the existing solutions for making steels with low density are based on the use of addition elements which do not allow a high level of mechanical strength to be achieved.
  • the steel of the invention combines these two properties, namely, a high level of mechanical resistance and a very low density for lightening of a part usable in the automobile.

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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)
  • Heat Treatment Of Steel (AREA)
  • Laminated Bodies (AREA)
  • Pressure Welding/Diffusion-Bonding (AREA)
  • Heat Treatment Of Strip Materials And Filament Materials (AREA)

Abstract

The invention relates to a high-resistant, low-density hot laminated sheet steel comprising the following elements expressed in weight per cent: 0.04%:9 carbon<=0.5%; 0.05%<=manganese<=3%, being able to contain hardening elements: 0.01%<=niobium<=0.1 0.01%<=titanium<=0.2% 0.01<=vanadium<=0.2%, either individually or combined, and/or elements acting on the transformation temperatures, 0.0005%<=boron<=0.005%; 0.05%<=nickel<=2%; 0.05%<=chrome<=2%; 0.05%<=molybdenum<=2%, either individually or combined, the remainder being iron and elements which are inherent to production, characterized in that it comprises: 2%<=silicon<=10%; 1%<=aluminium<=10%. The invention also relates to the production thereof.

Description

Tôle d'acier laminé à chaud à très haute résistance et de faible densité, et procédé de fabrication Very high strength, low density hot rolled steel sheet and manufacturing process
L'invention concerne une tôle d'acier laminé à chaud, à très haute résistance et de faible densité, issu d'un train de laminage à bande, ainsi que son procédé de fabrication.The invention relates to a hot-rolled steel sheet, with very high strength and low density, from a strip rolling train, as well as its manufacturing process.
L'allégement des véhicules automobiles devient une nécessité de par l'imposition de diminuer les émissions de C02 à 140g /Km en 2008. Cet allégement ne peut être réalisé qu'en augmentant le niveau de résistance mécanique des aciers pour compenser la diminution d'épaisseur des tôles. Il est donc nécessaire d'augmenter les caractéristiques mécaniques tout en réduisant les épaisseurs des tôles avec lesquelles sont réalisées les pièces utilisées. Cette démarche trouve ses limites avec la diminution de la rigidité des pièces et l'apparition de vibrations et de bruits rédhibitoires dans les applications visées du domaine de l'automobile où le bruit est un élément d'inconfort.Reducing motor vehicles becomes a necessity due to the imposition of reducing C02 emissions to 140g / Km in 2008. This reduction can only be achieved by increasing the level of mechanical strength of steels to compensate for the decrease in sheet thickness. It is therefore necessary to increase the mechanical characteristics while reducing the thicknesses of the sheets with which the parts used are made. This approach finds its limits with the reduction in the rigidity of the parts and the appearance of unacceptable vibrations and noises in targeted applications in the automotive field where noise is an element of discomfort.
Dans le domaine des aciers plats laminés à chaud, dont les caractéristiques mécaniques sont obtenues par laminage contrôlé sur train à large bande, le niveau de résistance le plus élevé est obtenu avec les aciers THR de structure bainitique, qui permettent d'atteindre un niveau de résistance mécanique compris entre 800 MPa et 1000 MPa, mais leur densité est celle d'un acier courant, c'est-à-dire une densité de 7,8 g/cm3.In the field of hot-rolled flat steels, the mechanical properties of which are obtained by controlled rolling on a wide-band train, the highest level of resistance is obtained with THR steels of bainitic structure, which make it possible to reach a level of mechanical strength between 800 MPa and 1000 MPa, but their density is that of a common steel, that is to say a density of 7.8 g / cm 3 .
Il est par ailleurs possible d'obtenir un acier de plus faible densité en utilisant un élément d'addition tel que l'aluminium, acier dans lequel un ajout de 8,5% d'aluminium permet d'abaisser la densité jusqu'à 7g/cm3. Cette solution ne permet pas d'atteindre des niveaux de résistance mécanique supérieure à 480 MPa. L'ajout d'autres éléments d'addition comme le chrome, le vanadium et le niobium, à des teneurs allant respectivement jusqu'à 1%, 0,1%, 0,4%, ne permet pas de dépasser en résistance mécanique un niveau de 580 MPa. Dans cette démarche, le gain en densité est annihilé par la faiblesse des caractéristiques en résistance mécanique obtenues.It is also possible to obtain a lower density steel by using an addition element such as aluminum, steel in which an addition of 8.5% of aluminum makes it possible to lower the density up to 7 g / cm3. This solution does not allow mechanical strength levels greater than 480 MPa to be achieved. The addition of other addition elements such as chromium, vanadium and niobium, at contents ranging respectively to 1%, 0.1%, 0.4%, does not make it possible to exceed in mechanical strength a 580 MPa level. In this approach, the gain in density is annihilated by the weakness of the mechanical strength characteristics obtained.
Le but de l'invention est de proposer aux utilisateurs de tôles d'acier laminées à chaud une tôle de faible densité et comportant des niveaux de résistance comparables aux tôles d'acier à haute résistance mécanique actuellement utilisé voire d'un niveau supérieur, et cela, afin de cumuler les deux avantages de faible densité et de résistance mécanique élevée. L'invention a pour premier objet une tôle d'acier laminé à chaud à très haute résistance et de faible densité, caractérisé en ce que sa composition en % en poids comprend : 0,04% < carbone < 0,5%The object of the invention is to provide users of hot-rolled steel sheets with a sheet of low density and having resistance levels comparable to steel sheets with high mechanical resistance currently used or even of a higher level, and this, in order to combine the two advantages of low density and high mechanical strength. The first object of the invention is a hot-rolled steel sheet of very high strength and low density, characterized in that its composition in% by weight comprises: 0.04% <carbon <0.5%
0,05% < manganèse < 3% pouvant contenir les éléments de durcissement :0.05% <manganese <3% which may contain the hardening elements:
0,01 %< niobium < 0,1% 0,01 % < titane < 0,2% 0,01 < vanadium < 0,2%, pris seul ou en combinaison, et/ou les éléments agissant sur les températures de transformation,0.01% <niobium <0.1% 0.01% <titanium <0.2% 0.01 <vanadium <0.2%, taken alone or in combination, and / or the elements acting on processing temperatures ,
0,0005% < bore < 0,005% 0,05% < nickel < 2% 0,05% < chrome < 2% 0,05% < molybdène < 2% , pris seul ou en combinaison, le reste étant du fer et des éléments inhérents à l'élaboration, caractérisé en ce qu'il comporte :0.0005% <boron <0.005% 0.05% <nickel <2% 0.05% <chromium <2% 0.05% <molybdenum <2%, taken alone or in combination, the rest being iron and elements inherent in the preparation, characterized in that it comprises:
2% < silicium < 10%, 1% < aluminium < 10%.2% <silicon <10%, 1% <aluminum <10%.
Dans un mode de réalisation préféré de l'invention, l'acier comporte dans sa composition, en % en poids :In a preferred embodiment of the invention, the steel comprises in its composition, in% by weight:
0,04% carbone < 0,3% r /0.04% carbon <0.3% r /
0,08% < manganèse < 3% 2% < silicium < 6%,0.08% <manganese <3% 2% <silicon <6%,
1 % < aluminium < 10%.1% <aluminum <10%.
Dans un autre mode de réalisation préféré, la tôle selon l'invention est telle que la teneur en silicium est comprise entre 3 et 6%, et que la teneur en aluminium est comprise entre 1 et 2 %.In another preferred embodiment, the sheet according to the invention is such that the silicon content is between 3 and 6%, and that the aluminum content is between 1 and 2%.
Dans un autre mode de réalisation préféré, la tôle selon l'invention est telle que la teneur en silicium est comprise entre 2 et 3 %, et que la teneur en aluminium est comprise entre 7 et 10 %. Dans un autre mode de réalisation préféré, les teneurs en silicium et en aluminium de la tôle selon l'invention sont telles que :In another preferred embodiment, the sheet according to the invention is such that the silicon content is between 2 and 3%, and that the aluminum content is between 7 and 10%. In another preferred embodiment, the silicon and aluminum contents of the sheet according to the invention are such that:
%Si + %AI > 9% Si +% AI> 9
La tôle selon l'invention peut également présenter les caractéristiques suivantes, seules ou en combinaison :The sheet according to the invention can also have the following characteristics, alone or in combination:
- la tôle présente une microstructure constituée d'une phase de ferrite primaire, et d'une phase de ferrite secondaire, la taille moyenne de grains de ladite ferrite primaire étant supérieure à la taille moyenne de grains de ladite ferrite secondaire, ladite microstructure contenant également des phases carburées,the sheet has a microstructure consisting of a primary ferrite phase and a secondary ferrite phase, the average grain size of said primary ferrite being greater than the average grain size of said secondary ferrite, said microstructure also containing carburetted phases,
- la tôle présente une phase de ferrite primaire obtenue lors du réchauffage de l'acier effectué préalablement au laminage à chaud, et une phase de ferrite secondaire obtenue à l'issue du laminage à chaud, ainsi que des phases carburées, - la tôle comprend une phase de ferrite primaire dont la taille moyenne de grains supérieure à 5 μm, et une phase de ferrite secondaire sont la une taille moyenne de grains inférieure à 2 μm.- the sheet has a primary ferrite phase obtained during the heating of the steel carried out prior to hot rolling, and a secondary ferrite phase obtained after hot rolling, as well as carburetted phases, - the sheet comprises a primary ferrite phase whose average grain size greater than 5 μm, and a secondary ferrite phase are the average grain size less than 2 μm.
L'invention a pour second objet un procédé de fabrication d'une tôle laminée à chaud qui comprend les étapes consistant à :The second object of the invention is a method of manufacturing a hot-rolled sheet which comprises the steps consisting in:
- réchauffer une brame dont la composition est conforme à l'invention, formant ainsi une brame dont la microstructure comprend une phase de ferrite primaire et une phase d'austénite,reheating a slab whose composition is in accordance with the invention, thus forming a slab whose microstructure comprises a primary ferrite phase and an austenite phase,
- puis à laminer έj chaud ladite brame, la température de fin de laminage à chaud étant supérieure à la température AR3 de la phase austénitique formée lors du réchauffage, afin de réaliser un laminage dans des conditions austénitiques, transformant ainsi la phase austénitique en une phase de ferrite secondaire et des phases carburées.- Then to laminate έj hot said slab, the temperature of end of hot rolling being higher than the temperature AR3 of the austenitic phase formed during reheating, in order to carry out a rolling under austenitic conditions, thus transforming the austenitic phase into a phase secondary ferrite and fuel phases.
La description qui suit fera bien comprendre l'invention en référence aux figures annexées, qui représentent :The following description will make the invention easier to understand with reference to the appended figures, which represent:
- en figure 1 : une courbe présentant l'évolution de la densité d'un acier en fonction de la teneur en silicium, en aluminium et/ou silicium plus aluminium,in FIG. 1: a curve showing the evolution of the density of a steel as a function of the silicon, aluminum and / or silicon plus aluminum content,
- en figure 2 : la microstructure d'un acier selon l'invention comprenant 0,04% de carbone (coulée I), - en figure 3 : la microstructure d'un acier selon l'invention comprenant 0,160% de carbone (coulée J),in FIG. 2: the microstructure of a steel according to the invention comprising 0.04% of carbon (casting I), in FIG. 3: the microstructure of a steel according to the invention comprising 0.160% of carbon (casting J),
- en figure 4 : la microstructure d'un acier selon l'invention comprenant 0,268% de carbone (coulée K), - en figure 5 : la microstructure d'un acier comprenant 0,505% de carbone, montré à titre de comparaison (coulée L).- in FIG. 4: the microstructure of a steel according to the invention comprising 0.268% of carbon (casting K), - in FIG. 5: the microstructure of a steel comprising 0.505% of carbon, shown for comparison (casting L ).
L'acier laminé à chaud sur train à bande selon l'invention présente une résistance mécanique élevée et une faible densité. L'acier à la composition générale pondérale suivante :The hot rolled steel on a strip train according to the invention has a high mechanical strength and a low density. Steel with the following general weight composition:
0,04% < carbone < 0,5% 0,05% < manganèse < 3% pouvant contenir les éléments de durcissement :0.04% <carbon <0.5% 0.05% <manganese <3% which may contain the hardening elements:
0,01 %< niobium < 0,1 % 0,01% < titane < 0,2%0.01% <niobium <0.1% 0.01% <titanium <0.2%
0,01 < vanadium < 0,2%, pris seul ou en combinaison, et/ou les éléments agissant sur les températures de transformation,0.01 <vanadium <0.2%, taken alone or in combination, and / or the elements acting on the processing temperatures,
0,0005% < bore < 0,005% 0,05% < nickel < 2% 0,05% < chrome < 2%0.0005% <boron <0.005% 0.05% <nickel <2% 0.05% <chromium <2%
0,05% molybdène < 2% , pris seul ou en combinaison, le reste étant du fer et des éléments inhérents à l'élaboration, comporte :0.05% molybdenum <2%, taken alone or in combination, the rest being iron and elements inherent in the preparation, comprises:
2% < silicium < 10%, 1% < aluminium < 10%.2% <silicon <10%, 1% <aluminum <10%.
La teneur en carbone de la tôle selon l'invention est comprise entre 0,04 et 0,5% en poids, de préférence entre 0,04 et 0,3% en poids. L'évolution de la structure de l'acier en fonction de la teneur en carbone est présentée dans les figures 2 à 5 et montre que la structure de l'acier selon l'invention (figures 2 à 4) est constituée de ferrite primaire à gros grains, et d'un mélange de phase carburées et de ferrite secondaire fine à plus petits grains. Si la teneur en carbone tombe en dessous de 0,04%, la microstructure ne comprend pas de phases carburées et perd en caractéristiques mécaniques. Par contre, si la teneur en carbone dépasse 0,5% en poids, la structure devient très fragile, et on observe que la microstructure ne comprend plus de ferrite primaire (cf. Figure 5).The carbon content of the sheet according to the invention is between 0.04 and 0.5% by weight, preferably between 0.04 and 0.3% by weight. The evolution of the structure of the steel as a function of the carbon content is presented in FIGS. 2 to 5 and shows that the structure of the steel according to the invention (FIGS. 2 to 4) consists of primary ferrite with coarse grains, and a mixture of carburetted phase and fine secondary ferrite with smaller grains. If the carbon content falls below 0.04%, the microstructure does not include carburetted phases and loses in mechanical characteristics. On the other hand, if the carbon content exceeds 0.5% in weight, the structure becomes very fragile, and we observe that the microstructure no longer includes primary ferrite (see Figure 5).
Sans vouloir être lié par une théorie, on pense que l'obtention de cette microstructure originale est due à la combinaison des teneurs en carbone, silicium et aluminium. Elle permet d'atteindre d'excellentes caractéristiques mécaniques. En effet, l'acier selon l'invention peut atteindre des niveaux de résistance mécanique allant de 620 MPa à plus de 1000 MPa et des densités de l'ordre de 7,55 et descendant jusqu'à 7 g/cm3 suivant les teneurs en silicium et en aluminium et en éléments d'addition, comme le montre la figure 1.Without wishing to be bound by a theory, it is believed that obtaining this original microstructure is due to the combination of the contents of carbon, silicon and aluminum. It achieves excellent mechanical characteristics. In fact, the steel according to the invention can reach levels of mechanical resistance ranging from 620 MPa to more than 1000 MPa and densities of the order of 7.55 and down to 7 g / cm3 depending on the contents. silicon and aluminum and additive elements, as shown in Figure 1.
Les caractéristiques mécaniques peuvent être renforcées par addition d'élément de micro alliage comme le niobium, le titane, le vanadium, les deux derniers, moins denses que le fer.The mechanical characteristics can be reinforced by the addition of micro alloy elements such as niobium, titanium, vanadium, the latter two, which are less dense than iron.
La tôle selon l'invention pourra être fabriquée par tout procédé adapté.The sheet according to the invention can be manufactured by any suitable process.
Néanmoins, on préfère employer le procédé selon l'invention. Ce procédé comprend tout d'abord un réchauffage de la brame à haute température (de préférence supérieure à 900°C), préalablement au laminage à chaud. Les présents inventeurs ont découvert que lors de cette étape de réchauffage, la brame présente une microstructure composée d'une phase ferritique dite primaire qui se forme à haute température, et qui co-existe avec une phase austénitique.However, it is preferred to use the method according to the invention. This process first of all comprises reheating the slab at high temperature (preferably above 900 ° C.), prior to hot rolling. The present inventors have discovered that during this reheating step, the slab has a microstructure composed of a so-called primary ferritic phase which forms at high temperature, and which co-exists with an austenitic phase.
En laminant à chaud de telle sorte que la température de fin de laminage reste supérieure à la valeur.de AR3 calculée pour la phase austénitique seule, on réalise un laminage dans des conditions austénitiques.By hot rolling so that the end of rolling temperature remains higher than the value of AR3 calculated for the austenitic phase alone, rolling is carried out under austenitic conditions.
On observe que la phase austénitique se transforme alors complètement en un mélange de phase carburées et de ferrite secondaire, dont la taille de grains moyenne est inférieure à celle de la phase ferritique primaire qui subsiste.It is observed that the austenitic phase is then completely transformed into a mixture of carburetted phase and secondary ferrite, the average grain size of which is smaller than that of the primary ferritic phase which remains.
On choisira avantageusement un couple carbone-manganèse afin d'avoir une température de transformation AR3 telle qu'on puisse garantir un laminage dans des conditions austénitiques. Le tableau 1 suivant, présentant différentes analyses selon l'invention, montre l'influence des différents éléments sur les caractéristiques des aciers.A carbon-manganese couple will advantageously be chosen in order to have an AR3 transformation temperature such that it is possible to guarantee rolling under austenitic conditions. Table 1 below, presenting different analyzes according to the invention, shows the influence of the different elements on the characteristics of the steels.
TABLEAU 1TABLE 1
Les coulées A, C, F, H et L sont données à titre de comparaison, tandis que les coulées B, D, E, G, I, J et K sont selon l'invention. The flows A, C, F, H and L are given for comparison, while the flows B, D, E, G, I, J and K are according to the invention.
Les données, présentées que le tableau 1, montrent que l'aluminium seul ne permet pas d'obtenir à la fois une faible densité de l'acier et un haut niveau de résistance dudit acier.The data, presented as Table 1, show that aluminum alone does not make it possible to obtain both a low density of steel and a high level of resistance of said steel.
Dans l'exemple de l'acier référencé E, la température de laminage est de 895°C, et la température de bobinage de 600°C, avec une vitesse de refroidissement de 49°C/s, ce qui donne à l'acier une résistance mécanique de 750 MPa. L'abaissement de la température de bobinage permet d'augmenter le niveau de résistance mécanique. C'est le cas de l'exemple de l'acier référencé B, dont la température de bobinage est de 20°C avec une vitesse de refroidissement de 5°C/s, ce qui permet d'atteindre un niveau de résistance mécanique de 902 MPa.In the example of the steel referenced E, the rolling temperature is 895 ° C, and the winding temperature of 600 ° C, with a cooling rate of 49 ° C / s, which gives the steel a mechanical resistance of 750 MPa. Lowering the winding temperature increases the level of mechanical resistance. This is the case of the example of steel referenced B, the winding temperature of which is 20 ° C. with a cooling rate of 5 ° C./s, which makes it possible to reach a level of mechanical resistance of 902 MPa.
Si on augmente la vitesse de refroidissement pour un acier référencé C réalisé avec un laminage à une température de 870°C, un bobinage à une température de 120°C et une vitesse de refroidissement de 130°C/s, on obtient un acier avec une résistance mécanique de 1296 MPa.If the cooling rate is increased for a steel referenced C produced with rolling at a temperature of 870 ° C, a winding at a temperature of 120 ° C and a cooling rate of 130 ° C / s, a steel is obtained with a mechanical resistance of 1296 MPa.
Le niveau de résistance mécanique peut aussi être ajusté par des teneurs en carbone et manganèse et/ou d'autres éléments d'addition comme présentés ci-dessus. Certaines opérations, comme par exemple un re-laminage ou un traitement thermique tel q'un recuit, peuvent être utilisées pour modifier ou ajuster le niveau des propriétés mécaniques.The level of mechanical strength can also be adjusted by carbon and manganese contents and / or other addition elements as presented above. Certain operations, such as, for example, re-rolling or a heat treatment such as annealing, can be used to modify or adjust the level of mechanical properties.
Selon l'invention, l'acier proposé répond à deux besoins contradictoires du domaine des aciers laminés à chaud d'une part, des propriétés mécaniques élevées et d'autre part, une faible densité. Les solutions existantes pour réaliser des aciers de très hauts niveaux de résistance mécanique sont basées sur l'utilisation d'éléments d'addition ne permettant pas une forte variation de la densité, et les solutions existantes pour réaliser des aciers de faible densité sont basées sur l'utilisation d'éléments d'addition ne permettant pas d'atteindre de haut niveau de résistance mécanique. L'acier de l'invention combine ces deux propriétés, à savoir, un haut niveau de résistance mécanique et une très faible densité pour un allégement de pièce utilisable dans l'automobile. According to the invention, the steel proposed meets two contradictory needs in the field of hot-rolled steels on the one hand, high mechanical properties and on the other hand, low density. The existing solutions for making steels with very high levels of mechanical strength are based on the use of addition elements which do not allow a large variation in density, and the existing solutions for making steels with low density are based on the use of addition elements which do not allow a high level of mechanical strength to be achieved. The steel of the invention combines these two properties, namely, a high level of mechanical resistance and a very low density for lightening of a part usable in the automobile.

Claims

REVENDICATIONS.CLAIMS.
Tôle d'acier laminé à chaud à très haute résistance et de faible densité, caractérisé en ce que sa composition en % en poids comprend : 0,04% < carbone < 0,5%Very high strength, low density hot rolled steel sheet, characterized in that its composition in% by weight comprises: 0.04% <carbon <0.5%
0,05% < manganèse < 3% et éventuellement les éléments de durcissement suivants :0.05% <manganese <3% and possibly the following hardening elements:
0,01 %< niobium < 0,1%0.01% <niobium <0.1%
0,01% < titane < 0,2% 0,01 < vanadium < 0,2%, pris seul ou en combinaison, et éventuellement les éléments agissant sur les températures de transformation :0.01% <titanium <0.2% 0.01 <vanadium <0.2%, taken alone or in combination, and possibly the elements acting on the processing temperatures:
0,0005% < bore < 0,005%0.0005% <boron <0.005%
0,05% < nickel < 2%0.05% <nickel <2%
0,05% < chrome < 2% 0,05% < molybdène < 2%, pris seul ou en combinaison, le reste étant du fer et des éléments inhérents à l'élaboration, caractérisé en ce qu'il comporte :0.05% <chromium <2% 0.05% <molybdenum <2%, taken alone or in combination, the rest being iron and elements inherent in the preparation, characterized in that it comprises:
2% < silicium < 10%,2% <silicon <10%,
1% < aluminium < 10%.1% <aluminum <10%.
Tôle selon la revendication 1 , caractérisé en outre en ce que ladite composition comprend :The sheet of claim 1, further characterized in that said composition comprises:
0,04% < carbone < 0,3% θ|θ8% < manganèse < 3% ' 2% < silicium < 6%,0.04% <carbon <0.3% θ | θ8% <manganese <3% '2% <silicon <6%,
1% < aluminium < 10%.1% <aluminum <10%.
Tôle selon l'une ou l'autre des revendications 1 et 2, caractérisé en outre en ce que la teneur en silicium est comprise entre 3 et 6%, et en ce que la teneur en.aluminium est comprise entre 1 et 2 %.Sheet according to either of Claims 1 and 2, further characterized in that the silicon content is between 3 and 6%, and in that the aluminum content is between 1 and 2%.
Tôle selon l'une ou l'autre des revendications 1 et 2, caractérisé en outre en ce que la teneur en silicium est comprise entre 2 et 3 %, et en ce que la teneur en aluminium est comprise entre 7 et 10 %. Tôle selon l'une quelconque des revendications 1 , 2 ou 4, caractérisé en outre en ce que les teneurs en silicium et en aluminium sont telles que :Sheet according to either of Claims 1 and 2, further characterized in that the silicon content is between 2 and 3%, and in that the aluminum content is between 7 and 10%. Sheet according to any of claims 1, 2 or 4, further characterized in that the silicon and aluminum contents are such that:
%Si + %AI > 9% Si +% AI> 9
Tôle selon l'une quelconque des revendications 1 à 5, caractérisée en ce qu'elle présente en outre une microstructure constituée d'une phase de ferrite primaire, et d'une phase de ferrite secondaire, la taille moyenne de grains de ladite ferrite primaire étant supérieure à la taille moyenne de grains de ladite ferrite secondaire, ladite microstructure contenant également des phases carburées.Sheet according to any one of Claims 1 to 5, characterized in that it also has a microstructure consisting of a primary ferrite phase and a secondary ferrite phase, the average grain size of said primary ferrite being greater than the average grain size of said secondary ferrite, said microstructure also containing carburetted phases.
Tôle selon la revendication 6, caractérisé en outre en ce que ladite phase de ferrite primaire est obtenue lors du réchauffage de l'acier effectué préalablement au laminage à chaud, et ladite phase de ferrite secondaire est obtenue à l'issue du laminage à chaud.The sheet of claim 6, further characterized in that said primary ferrite phase is obtained upon reheating the steel prior to hot rolling, and said secondary ferrite phase is obtained upon completion of hot rolling.
Tôle selon l'une ou l'autre des revendications 6 ou 7, caractérisée en ce que ladite phase de ferrite primaire présente une taille moyenne de grains supérieure à 5 μm, et en ce que ladite phase de ferrite secondaire présente une taille moyenne de grains inférieure à 2 μm.Sheet according to either of claims 6 or 7, characterized in that said primary ferrite phase has an average grain size greater than 5 μm, and in that said secondary ferrite phase has an average grain size less than 2 μm.
Procédé de fabrication d'une tôle laminée à chaud selon l'une quelconque des revendications 6 à 8, caractérisé en ce qu'il comprend les étapes consistant à :Method for manufacturing a hot-rolled sheet according to any one of Claims 6 to 8, characterized in that it comprises the steps consisting in:
- réchauffer une. brame dont la composition est conforme à celles de l'une quelconque des revendications 1 à 5, formant ainsi une brame dont la microstructure comprend une phase de ferrite primaire et une phase d'austénite,- reheat one. slab, the composition of which conforms to those of any one of claims 1 to 5, thus forming a slab, the microstructure of which comprises a primary ferrite phase and an austenite phase,
- puis à laminer à chaud ladite brame, la température de fin de laminage à chaud étant supérieure à ia température AR3 de ia phase austénitique formée iors du réchauffage, afin de réaliser un laminage dans des conditions austénitiques, transformant ainsi la phase austénitique en une phase de ferrite secondaire et des phases carburées. - Then hot rolling said slab, the end of hot rolling temperature being higher than the temperature AR3 of the austenitic phase formed during reheating, in order to carry out rolling under austenitic conditions, thus transforming the austenitic phase into a phase secondary ferrite and fuel phases.
EP03725278A 2002-03-11 2003-03-10 High-resistant, low-density hot laminated steel sheet and method for the production thereof Expired - Lifetime EP1485511B1 (en)

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