EP1592816A1 - Method of producing a cold-rolled band of dual-phase steel with a ferritic/martensitic structure and band thus obtained - Google Patents

Method of producing a cold-rolled band of dual-phase steel with a ferritic/martensitic structure and band thus obtained

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Publication number
EP1592816A1
EP1592816A1 EP04706710A EP04706710A EP1592816A1 EP 1592816 A1 EP1592816 A1 EP 1592816A1 EP 04706710 A EP04706710 A EP 04706710A EP 04706710 A EP04706710 A EP 04706710A EP 1592816 A1 EP1592816 A1 EP 1592816A1
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EP
European Patent Office
Prior art keywords
cooling
temperature
band
steel strip
rolled
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
EP04706710A
Other languages
German (de)
French (fr)
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EP1592816B1 (en
Inventor
Antoine Moulin
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 EP1592816A1 publication Critical patent/EP1592816A1/en
Application granted granted Critical
Publication of EP1592816B1 publication Critical patent/EP1592816B1/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
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C21D1/185—Hardening; Quenching with or without subsequent tempering from an intercritical temperature
    • 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

Definitions

  • the present invention relates to a method of manufacturing a dual-phase steel strip with a ferrito-martensitic structure, cold-rolled, and the strip obtainable by this method, which is more particularly intended for the manufacture of automobile parts. by deep drawing.
  • Very high strength steels have been developed in recent years, in particular to meet the specific needs of the automotive industry, which are in particular the reduction of weight and thus the thickness of parts, and the improvement of safety. which goes through the increase of the resistance to fatigue and the shock resistance of the parts. These improvements should not further deteriorate the fitness of the sheets used in the manufacture of parts.
  • the object of the present invention is to overcome the disadvantages of the steels of the prior art by providing a steel strip capable of deep drawing, and having both excellent mechanical characteristics and excellent characteristics of anisotropy .
  • the invention firstly relates to a method of manufacturing a dual-phase steel strip with a ferrite-martensitic structure, cold-rolled, characterized in that a slab is hot-rolled, the chemical composition of which comprises , in weight :
  • the cooling rate between 600 ° C and the ambient temperature being between 100 ° C / s and 1500 ° C / s,
  • the chemical composition further comprises, by weight:
  • the method according to the invention may also comprise the following characteristics, alone or in combination:
  • the strip is hot-rolled at a temperature above 850.degree. C., the hot strip is reeled at a temperature of between 550.degree.
  • the strip is cold-rolled with a reduction ratio of between 70 and 80%
  • the continuous annealing of the cold rolled strip comprises a temperature rise phase and then a holding phase at a predetermined temperature
  • the holding temperature is between Ad and 900 ° C.
  • the holding temperature is between 750 and 850 ° C
  • cooling to room temperature comprises a first slow cooling between the holding temperature and 600 ° C, during which the cooling rate is less than 50 ° C / s, then a second cooling at a higher speed and between 100 ° C / s and 1500 ° C / s, up to room temperature.
  • the second subject of the invention is a dual-phase steel strip with a cold-rolled ferrito-martensitic structure, the chemical composition of which comprises, by weight:
  • composition of the strip is as follows: 0.020% ⁇ C ⁇ 0.060%
  • the strip according to the invention may also comprise the following characteristics, alone or in combination: it has a tensile strength Rm greater than 450 MPa,
  • the invention has the third object the use of a steel strip according to the invention, for the manufacture of parts for automotive deep drawing.
  • the method according to the invention consists of hot rolling a slab of specific composition, then winding the hot strip obtained at a temperature between 550 and 850 ° C.
  • This winding at high temperature is indeed favorable to the development of what is called a texture, ie an anisotropic structure.
  • a texture ie an anisotropic structure.
  • Such a winding indeed makes it possible to coalesce the precipitation of Fe3C cementite, and to reduce the amount of carbon redissolved during annealing, which is harmful to the development of the recrystallization texture.
  • the process then involves cold rolling the web with a reduction ratio of 60 to 90%, and then continuously annealing the web in the intercritical range.
  • Intercritical annealing makes it possible to re-dissolve the majority of the carburized phases formed during the winding after the recrystallization.
  • the fact that the austenitization and dissolution of the carburized phases occurs after recrystallization makes it possible to retain the carbon trapped during the recrystallization and to release it once the texture of the recrystallized ferrite is developed. The texture will therefore not be affected by the carbon in solid solution, as in the case of a low temperature winding, but only altered by the isotropic nature of the martensite formed.
  • the method then consists in cooling the strip to room temperature, in one or more stages, the cooling rate between 600 ° C. and the ambient temperature being between 100 ° C./s and 1500 ° C./s, and optionally to incur an income at a temperature below 300 ° C.
  • the martensitic phase fraction formed can be reduced by lowering the holding temperature to lower values in the intercritical range, or by performing slow cooling before quenching.
  • the income optionally practiced according to the invention consists in precipitating a part of the solid solution carbon trapped in martensite, without decreasing the proportion of this martensite.
  • the maximum temperature of this income is 300 ° C, preferably 250 ° C, and more preferably 200 ° C.
  • composition according to the invention comprises carbon with a content of between 0.010% and 0.100%. This element is essential to obtain good mechanical properties, but should not be present in too large quantities, because it would generate the formation of a large proportion of martensitic phase.
  • the composition also comprises chromium at a content of between 0.010% and 1.0% which aids in the desired formation of martensite.
  • the composition also comprises silicon at a content of between 0.010% and 0.50%. It greatly improves the yield strength of steel while reducing its ductility slightly and deteriorating its coating.
  • the composition also comprises phosphorus at a content of between 0.001% and 0.20%, which hardens the microstructure without affecting its texture.
  • the composition also comprises aluminum at a content of between 0.010% and 0.10% which avoids aging by trapping nitrogen.
  • compositions consist of iron and unavoidable impurities resulting from the elaboration.
  • the hot-rolled strips were then cooled by quenching with water at a cooling rate of the order of 25 ° C./s until the winding temperature was reached.
  • Grade A was coiled at 720 ° C while one sample of Grade B was coiled at 550 ° C and the other at 720 ° C.
  • the different samples were then cold-rolled to a 75% reduction rate, then subjected to an annealing treatment at a holding temperature of 750 ° C for some samples, and 800 ° C for 'other. Cooling to room temperature is then carried out at a rate of about 25 ° C./s, by quenching with water.
  • the overall anisotropy of a steel is determined by the average normal anisotropy coefficient r: rT + rL + 2 (r45 °))
  • FIG. 1 shows the relationship between the average coefficient r and the martensite ratio formed% m for the grades A and B. It can be seen that the higher the martensite content, the more steel is isotropic.
  • FIG. 2 shows the microstructure obtained with the grade A, wound at 720 ° C., then annealed at 750 ° C. to finally obtain 12% of martensite.
  • ferrite and formed martensite There is a good distinction between ferrite and formed martensite.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Physics & Mathematics (AREA)
  • Thermal Sciences (AREA)
  • Crystallography & Structural Chemistry (AREA)
  • Mechanical Engineering (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Heat Treatment Of Sheet Steel (AREA)
  • Metal Rolling (AREA)

Abstract

The invention relates to a method of producing a cold-rolled band of dual-phase steel with a ferritic/martensitic structure. The inventive method consists in hot rolling a slab having a chemical composition which comprises, by weight, 0.01 % </= C </= 0.1 %, 0.05 % </= Mn </= 1 %, 0.01 % </= Cr </= 1 %, 0.01 % </= Si </= 0.5 %, 0.001 % </= P </= 0.2 %, 0.01 % </= Al </= 0.1 %, N </= 0.01 %, the remainder being iron and impurities resulting from the preparation thereof. The method also comprises the following subsequent steps consisting in: hot winding the band obtained at a temperature of between 550 and 850 DEG C; cold rolling the band with a reduction ratio of between 60 and 90 %; annealing the band continuously in the intercritical region; cooling said band to ambient temperature, in one or more steps, the rate of cooling between 600 DEG C and ambient temperature being between 100 DEG C/s and 1500 DEG C/s; and, optionally, tempering same at a temperature of less than 300 DEG C. The aforementioned annealing and cooling operations are performed such that the end band comprises between 1 and 15 % martensite. The invention also relates to the steel band thus formed.

Description

Procédé de fabrication d'une bande d'acier dual-phase à structure ferrito-martensitique, laminée à froid et bande obtenue Process for producing a cold-rolled, ferrite-martensitic, dual-phase steel strip and strip obtained
La présente invention concerne un procédé de fabrication d'une bande d'acier dual-phase à structure ferrito-martensitique, laminée à froid, et la bande pouvant être obtenue par ce procédé, qui est plus particulièrement destinée à la fabrication de pièces pour automobile par emboutissage profond.The present invention relates to a method of manufacturing a dual-phase steel strip with a ferrito-martensitic structure, cold-rolled, and the strip obtainable by this method, which is more particularly intended for the manufacture of automobile parts. by deep drawing.
Les aciers à très haute résistance ont été développés ces dernières années, notamment afin de répondre aux besoins spécifiques de l'industrie automobile, qui sont en particulier la réduction du poids et donc de l'épaisseur des pièces, et l'amélioration de la sécurité qui passe par l'augmentation de la résistance à la fatigue et de la tenue aux chocs des pièces. Ces améliorations ne doivent en outre pas détériorer l'aptitude à la mise en forme des tôles utilisées pour la fabrication des pièces.Very high strength steels have been developed in recent years, in particular to meet the specific needs of the automotive industry, which are in particular the reduction of weight and thus the thickness of parts, and the improvement of safety. which goes through the increase of the resistance to fatigue and the shock resistance of the parts. These improvements should not further deteriorate the fitness of the sheets used in the manufacture of parts.
Ainsi, on a développé des aciers dits dual-phase, dont la structure est ferrito-martensitique et qui permettent d'atteindre des résistances à la traction Rm de plus de 400 MPa, mais qui ne présentent pas de bonnes caractéristiques d'emboutissabilité, car leur coefficient d'anisotropie r moyen est proche de 1 . Par ailleurs, leur aptitude à la galvanisation est mauvaise, car ils contiennent de fortes quantités de silicium ou d'autres éléments néfastes au bon mouillage de la surface de la bande par le zinc en fusion. Par ailleurs, on connaît des aciers dont la structure est monophasée et qui présentent un coefficient moyen d'anisotropie r élevé, mais ont des caractéristiques mécaniques moyennes, avec une résistance à la traction Rm ne dépassant pas 400 MPa.Thus, so-called dual-phase steels, whose structure is ferritic-martensitic and which make it possible to reach tensile strengths Rm of more than 400 MPa, but which do not have good draw-off characteristics, have been developed because their mean anisotropy coefficient r is close to 1. In addition, their galvanizing ability is poor because they contain high amounts of silicon or other elements harmful to the good wetting of the surface of the strip by the molten zinc. In addition, steels are known whose structure is single-phase and have a high average coefficient of anisotropy r, but have average mechanical characteristics, with a tensile strength Rm of not more than 400 MPa.
On citera à titre d'exemple les aciers à bas interstitiels, ou les aciers calmés à l'aluminium et re-phosphorés. Les tentatives d'amplifier les mécanismes de durcissement classiques pour ces types d'acier ne permettent pas d'améliorer sensiblement leurs caractéristiques mécaniques. En outre, cet acier doit être apte à la galvanisation.By way of example, mention may be made of low-interstitial steels, or aluminum-killed and re-phosphorus-treated steels. Attempts to amplify conventional hardening mechanisms for these types of steel not significantly improve their mechanical characteristics. In addition, this steel must be suitable for galvanizing.
Le but de la présente invention est de remédier aux inconvénients des aciers de l'art antérieur en proposant une bande d'acier apte à l'emboutissage profond, et présentant à la fois d'excellentes caractéristiques mécaniques et d'excellentes caractéristiques d'anisotropie.The object of the present invention is to overcome the disadvantages of the steels of the prior art by providing a steel strip capable of deep drawing, and having both excellent mechanical characteristics and excellent characteristics of anisotropy .
A cet effet, l'invention a pour premier objet un procédé de fabrication d'une bande d'acier dual-phase à structure ferrito-martensitique, laminée à froid caractérisé en ce qu'on lamine à chaud une brame dont la composition chimique comprend, en poids :For this purpose, the invention firstly relates to a method of manufacturing a dual-phase steel strip with a ferrite-martensitic structure, cold-rolled, characterized in that a slab is hot-rolled, the chemical composition of which comprises , in weight :
0,010% < C < 0,100% 0,050% < Mn < 1 ,0% 0,010% < Cr < 1 ,0% 0,010% < Si < 0,50% 0,001 % < P < 0,20%0.010% <C <0.100% 0.050% <Mn <1.0% 0.010% <Cr <1.0% 0.010% <If <0.50% 0.001% <P <0.20%
0,010% < Al < 0,10% N < 0,010% le reste étant du fer et des impuretés résultant de l'élaboration, ledit procédé comprenant ensuite les étapes consistant à : - bobiner la bande à chaud obtenue à une température comprise entre0.010% <Al <0.10% N <0.010% the balance being iron and impurities resulting from the elaboration, said method then comprising the steps of: - winding the hot strip obtained at a temperature between
550 et 850°C, puis550 and 850 ° C, then
- à laminer à froid la bande avec un taux de réduction compris entre 60 et 90%, puis- cold rolling the strip with a reduction rate of between 60 and 90%, then
- à recuire la bande de façon continue dans le domaine intercritique, et - à la refroidir jusqu'à la température ambiante, en une ou plusieurs étapes, la vitesse de refroidissement entre 600°C et la température ambiante étant comprise entre 100°C/s et 1500°C/s,- continuously annealing the strip in the intercritical range, and - cooling it to room temperature, in one or more steps, the cooling rate between 600 ° C and the ambient temperature being between 100 ° C / s and 1500 ° C / s,
- et éventuellement à lui faire subir un revenu à une température inférieure à 300°C, les opérations de recuit et de refroidissement étant menées de telle sorte que la bande comprenne finalement de 1 à 15% de martensite. Dans un mode de réalisation préféré, la composition chimique comprend en outre, en poids :and possibly to make it undergo an income at a temperature below 300 ° C, the annealing and cooling operations being carried out so that the band finally comprises 1 to 15% of martensite. In a preferred embodiment, the chemical composition further comprises, by weight:
0,020% < C < 0,060% 0,300% < Mn < 0,500% 0,010% < Cr < 1 ,0%0.020% <C <0.060% 0.300% <Mn <0.500% 0.010% <Cr <1.0%
0,010% Si < 0,50% 0,010% < P < 0,100% 0,010% < Al < 0,10% N < 0,010% le reste étant du fer et des impuretés résultant de l'élaboration.0.010% If <0.50% 0.010% <P <0.100% 0.010% <Al <0.10% N <0.010% the balance being iron and impurities resulting from processing.
Le procédé selon l'invention peut également comprendre les caractéristiques suivantes, seules ou en combinaison :The method according to the invention may also comprise the following characteristics, alone or in combination:
- on lamine la bande à chaud à une température supérieure à 850°C, - on bobine la bande à chaud à une température comprise entre 550 etthe strip is hot-rolled at a temperature above 850.degree. C., the hot strip is reeled at a temperature of between 550.degree.
750°C,750 ° C,
- on lamine à froid la bande avec un taux de réduction compris entre 70 et 80%,the strip is cold-rolled with a reduction ratio of between 70 and 80%,
- le recuit continu de la bande laminée à froid comprend une phase de montée en température, puis une phase de maintien à une température prédéterminée,the continuous annealing of the cold rolled strip comprises a temperature rise phase and then a holding phase at a predetermined temperature,
- la température de maintien est comprise entre Ad et 900°C,the holding temperature is between Ad and 900 ° C.
- la température de maintien est comprise entre 750 et 850°C,the holding temperature is between 750 and 850 ° C,
- le refroidissement jusqu'à la température ambiante comprend un premier refroidissement lent entre la température de maintien et 600°C, au cours duquel la vitesse de refroidissement est inférieure à 50°C/s, puis un second refroidissement à une vitesse plus élevée et comprise entre 100°C/s et 1500°C/s, jusqu'à la température ambiante. L'invention a également pour deuxième objet une bande d'acier dual- phase à structure ferrito-martensitique, laminée à froid, dont la composition chimique comprend, en poids :cooling to room temperature comprises a first slow cooling between the holding temperature and 600 ° C, during which the cooling rate is less than 50 ° C / s, then a second cooling at a higher speed and between 100 ° C / s and 1500 ° C / s, up to room temperature. The second subject of the invention is a dual-phase steel strip with a cold-rolled ferrito-martensitic structure, the chemical composition of which comprises, by weight:
0,010% < C < 0,100% 0,050% < Mn < 1 ,0%0.010% <C <0.100% 0.050% <Mn <1.0%
0,010% < Cr < 1 ,0% 0,010% < Si < 0,50% 0,001 % < P < 0,20% 0,010% < Al < 0,10% N < 0,010% le reste étant du fer et des impuretés résultant de l'élaboration, la bande comportant en outre entre 1 % et 15% de martensite.0.010% <Cr <1.0% 0.010% <Si <0.50% 0.001% <P <0.20% 0.010% <Al <0.10% N <0.010% the remainder being iron and impurities resulting from the elaboration, the band further comprising between 1% and 15% of martensite.
Dans un mode de réalisation préféré, la composition de la bande est la suivante : 0,020% < C < 0,060%In a preferred embodiment, the composition of the strip is as follows: 0.020% <C <0.060%
0,300% < Mn < 0,500% 0,010% < Cr < 1 ,0% 0,010% < Si < 0,50% 0,010% < P'< 0,100% 0,010% < Al < 0,10%0.300% <Mn <0.500% 0.010% <Cr <1 0% 0.010% <Si <0.010% 0.50% <P <0.100% 0.010% <Al <0.10%
N < 0,010% le reste étant du fer et des impuretés résultant de l'élaboration.N <0.010% the balance being iron and impurities resulting from the elaboration.
La bande selon l'invention peut également comprendre les caractéristiques suivantes, seules ou en combinaison : - elle présente une résistance à la traction Rm supérieure à 450 MPa,The strip according to the invention may also comprise the following characteristics, alone or in combination: it has a tensile strength Rm greater than 450 MPa,
- elle présente une résistance à la traction Rm supérieure à 500 MPa,it has a tensile strength Rm greater than 500 MPa,
- elle présente une résistance à la traction Rm supérieure à 600 MPa,it has a tensile strength Rm greater than 600 MPa,
- elle présente un coefficient d'anisotropie moyen r supérieur à 1 ,1 ,it has an average coefficient of anisotropy r greater than 1, 1,
- elle présente un coefficient d'anisotropie moyen r supérieur à 1 ,3, - elle comporte en outre entre 1% et 10% de martensite,it has an average coefficient of anisotropy r greater than 1.3, it furthermore comprises between 1% and 10% of martensite,
- elle comporte en outre entre 5% et 8% de martensite. Enfin, l'invention a pour troisième objet l'utilisation d'une bande d'acier selon l'invention, pour la fabrication de pièces pour automobile par emboutissage profond.it also comprises between 5% and 8% of martensite. Finally, the invention has the third object the use of a steel strip according to the invention, for the manufacture of parts for automotive deep drawing.
Le procédé selon l'invention consiste à laminer à chaud une brame de composition spécifique, puis à bobiner la bande à chaud obtenue à une température comprise entre 550 et 850°C.The method according to the invention consists of hot rolling a slab of specific composition, then winding the hot strip obtained at a temperature between 550 and 850 ° C.
Ce bobinage à température élevée est en effet favorable au développement de ce que l'on appelle une texture, c'est à dire une structure anisotrope. Un tel bobinage permet en effet de faire coalescer la précipitation de cémentite Fe3C, et de réduire la quantité de carbone remise en solution lors du recuit, nocive au développement de la texture de recristallisation.This winding at high temperature is indeed favorable to the development of what is called a texture, ie an anisotropic structure. Such a winding indeed makes it possible to coalesce the precipitation of Fe3C cementite, and to reduce the amount of carbon redissolved during annealing, which is harmful to the development of the recrystallization texture.
Le procédé consiste ensuite à laminer à froid la bande avec un taux de réduction compris entre 60 et 90%, puis à la recuire la bande de façon continue dans le domaine intercritique. Le recuit intercritique permet de re-dissoudre la majorité des phases carburées formées lors du bobinage après la recristallisation. Le fait que l'austénitisation et la dissolution des phases carburées intervienne après la recristallisation permet de conserver le carbone piégé lors de la recristallisation et de le libérer une fois que la texture de la ferrite recristallisée est développée. La texture ne sera donc pas affectée par le carbone en solution solide, comme dans le cas d'un bobinage à basse température, mais uniquement altérée par le caractère isotrope de la martensite formée.The process then involves cold rolling the web with a reduction ratio of 60 to 90%, and then continuously annealing the web in the intercritical range. Intercritical annealing makes it possible to re-dissolve the majority of the carburized phases formed during the winding after the recrystallization. The fact that the austenitization and dissolution of the carburized phases occurs after recrystallization makes it possible to retain the carbon trapped during the recrystallization and to release it once the texture of the recrystallized ferrite is developed. The texture will therefore not be affected by the carbon in solid solution, as in the case of a low temperature winding, but only altered by the isotropic nature of the martensite formed.
Le procédé consiste ensuite à refroidir la bande jusqu'à la température ambiante, en une ou plusieurs étapes, la vitesse de refroidissement entre 600°C et la température ambiante étant comprise entre 100°C/s et 1500°C/s, et éventuellement à lui faire subir un revenu à une température inférieure à 300°C.The method then consists in cooling the strip to room temperature, in one or more stages, the cooling rate between 600 ° C. and the ambient temperature being between 100 ° C./s and 1500 ° C./s, and optionally to incur an income at a temperature below 300 ° C.
Cette phase de refroidissement rapide permet de former de la martensite dans la structure de l'acier, ce qui permet d'obtenir de très bonnes caractéristiques mécaniques. Cependant, on veille à ne pas former trop de martensite, car celle-ci est isotrope et réduit donc le coefficient moyen d'anisotropie r.This rapid cooling phase makes it possible to form martensite in the structure of the steel, which makes it possible to obtain very good mechanical characteristics. However, care is taken not to train too much martensite, because it is isotropic and therefore reduces the average coefficient of anisotropy r.
La trempe a l'eau permet de former des proportions importantes de phases carburées par rapport à l'analyse considérée. On peut réduire la fraction de phase martensitique formée en abaissant la température de maintien vers des valeurs plus basses dans le domaine intercritique, ou bien encore en pratiquant un refroidissement lent avant trempe.Quenching with water makes it possible to form significant proportions of carburized phases with respect to the analysis under consideration. The martensitic phase fraction formed can be reduced by lowering the holding temperature to lower values in the intercritical range, or by performing slow cooling before quenching.
On peut également réduire la différence de dureté entre la matrice ferritique et la phase martensitique, en refroidissant plus lentement la bande ou en pratiquant un court revenu, de l'ordre d'une minute, de la phase martensitique formée après trempe à l'eau.It is also possible to reduce the difference in hardness between the ferritic matrix and the martensitic phase, by cooling the strip more slowly or by practicing a short time, of the order of one minute, of the martensitic phase formed after quenching with water. .
Il est à noter que ce revenu n'est en aucun cas un traitement de survieillissement comme on en trouve dans l'art antérieur. En effet, ces traitements de survieillissement (ou overaging en anglais), qui sont généralement effectués entre 300 et 500°C, ont notamment pour effet de supprimer la martensite qui est un élément essentiel de la présente invention.It should be noted that this income is in no way a survival treatment as found in the prior art. Indeed, these overaging treatments (or overaging in English), which are generally carried out between 300 and 500 ° C, have the effect in particular of suppressing martensite which is an essential element of the present invention.
Le revenu éventuellement pratiqué selon l'invention, consiste à précipiter une partie du carbone en solution solide piégé dans la martensite, sans diminuer la proportion de cette martensite. La température maximale de ce revenu est de 300°C, de préférence de 250°C, et de façon plus particulièrement préférée, de 200°C.The income optionally practiced according to the invention consists in precipitating a part of the solid solution carbon trapped in martensite, without decreasing the proportion of this martensite. The maximum temperature of this income is 300 ° C, preferably 250 ° C, and more preferably 200 ° C.
La composition selon l'invention comprend du carbone à une teneur comprise entre 0,010% et 0,100%. Cet élément est essentiel à l'obtention de bonnes caractéristiques mécaniques, mais ne doit pas être présent en trop grande quantité, car il générerait la formation d'une trop grande proportion de phase martensitique.The composition according to the invention comprises carbon with a content of between 0.010% and 0.100%. This element is essential to obtain good mechanical properties, but should not be present in too large quantities, because it would generate the formation of a large proportion of martensitic phase.
Elle comprend également du manganèse à une teneur comprise entreIt also includes manganese with a content between
0,050% et 1 ,0%. Le manganèse améliore la limite d'élasticité de l'acier tout en réduisant fortement sa ductilité, ce pour quoi on limite sa teneur. La composition comprend également du chrome à une teneur comprise entre 0,010% et 1 ,0% qui aide à la formation recherchée de martensite. La composition comprend également du silicium à une teneur comprise entre 0,010% et 0,50%. Il améliore fortement la limite d'élasticité de l'acier tout en réduisant faiblement sa ductilité et en détériorant sa revêtabilité.0.050% and 1.0%. Manganese improves the yield strength of steel while greatly reducing its ductility, for which it limits its content. The composition also comprises chromium at a content of between 0.010% and 1.0% which aids in the desired formation of martensite. The composition also comprises silicon at a content of between 0.010% and 0.50%. It greatly improves the yield strength of steel while reducing its ductility slightly and deteriorating its coating.
La composition comprend également du phosphore à une teneur comprise entre 0,001 % et 0,20%, qui durcit la microstructure sans affecter sa texture.The composition also comprises phosphorus at a content of between 0.001% and 0.20%, which hardens the microstructure without affecting its texture.
La composition comprend également de l'aluminium à une teneur comprise entre 0,010% et 0,10% qui évite le vieillissement en piégeant l'azote.The composition also comprises aluminum at a content of between 0.010% and 0.10% which avoids aging by trapping nitrogen.
ExemplesExamples
A titre d'exemple non limitatif, et afin de mieux illustrer l'invention, deux nuances d'acier ont été élaborées. Leur composition, en millièmes de pourcent est donnée dans le tableau suivant :As a non-limiting example, and to better illustrate the invention, two steel grades have been developed. Their composition, in thousandths of a percent, is given in the following table:
Le reste des compositions est constitué de fer et d'impuretés inévitables résultant de l'élaboration.The rest of the compositions consist of iron and unavoidable impurities resulting from the elaboration.
Abréviations employéesAbbreviations used
Re : limite d'élasticité en MPa Rm : résistance à la traction en MPa r : coefficient d'anisotropieRe: yield strength in MPa Rm: tensile strength in MPa r: anisotropy coefficient
P : palier %m : proportion de martensite Après élaboration, les deux nuances ont été austénitisée à 1250°C pendant une heure, afin d'obtenir une mise en solution des nitrures d'aluminium. Les brames ont ensuite été laminées à chaud de telle sorte que la température de fin de laminage soit supérieure à 900°C, la valeur de AR3 étant de 870°C environ pour les deux nuances.P:% m level: proportion of martensite After elaboration, the two shades were austenitized at 1250 ° C. for one hour, in order to obtain dissolution of the aluminum nitrides. The slabs were then hot-rolled so that the end-of-rolling temperature was above 900 ° C, the value of AR3 being about 870 ° C for both grades.
Les bandes laminées à chaud ont ensuite été refroidies par trempe à l'eau, à une vitesse de refroidissement de l'ordre de 25°C/s, jusqu'à atteindre la température de bobinage. La nuance A a été bobinée à 720°C, tandis qu'un échantillon de la nuance B a été bobiné à 550°C et l'autre à 720°C.The hot-rolled strips were then cooled by quenching with water at a cooling rate of the order of 25 ° C./s until the winding temperature was reached. Grade A was coiled at 720 ° C while one sample of Grade B was coiled at 550 ° C and the other at 720 ° C.
Les différents échantillons ont ensuite été laminés à froid jusqu'à atteindre un taux de réduction de 75%, puis ont été soumis à un traitement de recuit à une température de maintien de 750°C pour certains échantillons, et de 800°C pour d'autres. Le refroidissement jusqu'à la température ambiante est alors effectué à une vitesse de l'ordre de 25°C/s, par trempe à l'eau.The different samples were then cold-rolled to a 75% reduction rate, then subjected to an annealing treatment at a holding temperature of 750 ° C for some samples, and 800 ° C for 'other. Cooling to room temperature is then carried out at a rate of about 25 ° C./s, by quenching with water.
On mesure ensuite les caractéristiques mécaniques et d'anisotropie des aciers obtenus.The mechanical and anisotropic characteristics of the steels obtained are then measured.
Les résultats sont rassemblés dans le tableau suivant : The results are summarized in the following table:
L'anisotropie globale d'un acier est déterminée par le coefficient d'anisotropie normale r moyen : rT+rL+2(r45°))The overall anisotropy of a steel is determined by the average normal anisotropy coefficient r: rT + rL + 2 (r45 °))
où rT désigne la valeur de r mesurée dans la direction transversale au sens de laminage de la bande, rL désigne la valeur de r mesurée dans la direction longitudinale au sens de laminage de la bande, r45° désigne la valeur de r mesurée à 45° par rapport au sens de laminage de la bande. Pour une température de bobinage de 720°C, on a représenté en figure 1 , la relation existant entre le coefficient r moyen et le taux de martensite formé %m pour les nuances A et B. On constate que plus le taux de martensite augmente, plus l'acier est isotrope.where rT is the value of r measured in the direction transverse to the rolling direction of the strip, rL denotes the value of r measured in the longitudinal direction in the rolling direction of the strip, r45 ° denotes the value of r measured at 45 ° with respect to the rolling direction of the strip. For a winding temperature of 720 ° C., FIG. 1 shows the relationship between the average coefficient r and the martensite ratio formed% m for the grades A and B. It can be seen that the higher the martensite content, the more steel is isotropic.
Par ailleurs, on constate que plus le taux de martensite est important, plus les caractéristiques mécaniques sont élevées.Furthermore, it is found that the higher the martensite rate, the higher the mechanical characteristics are.
A titre d'illustration, on a fait figurer en figure 2 la microstructure obtenue avec la nuance A, bobinée à 720°C, puis recuit à 750°C pour obtenir finalement 12% de martensite. On y distingue bien la ferrite et la martensite formée. By way of illustration, FIG. 2 shows the microstructure obtained with the grade A, wound at 720 ° C., then annealed at 750 ° C. to finally obtain 12% of martensite. There is a good distinction between ferrite and formed martensite.

Claims

REVENDICATIONS
1. Procédé de fabrication d'une bande d'acier dual-phase à structure ferrito- martensitique, laminée à froid caractérisé en ce qu'on lamine à chaud une brame dont la composition chimique comprend, en poids :1. A method for producing a cold-rolled, ferritomethylsitic steel, dual-phase steel strip characterized in that a slab is hot-rolled, the chemical composition of which comprises, by weight:
0,010% < C < 0,100% 0,050% < Mn < 1 ,0%0.010% <C <0.100% 0.050% <Mn <1.0%
0,010% < Cr < 1 ,0% 0,010% < Si < 0,50% 0,001 % < P < 0,20% 0,010% < Al < 0,10% N < 0,010%0.010% <Cr <1.0% 0.010% <If <0.50% 0.001% <P <0.20% 0.010% <Al <0.10% N <0.010%
le reste étant du fer et des impuretés résultant de l'élaboration, ledit procédé comprenant ensuite les étapes consistant à :the remainder being iron and impurities resulting from the preparation, said process then comprising the steps of:
- bobiner la bande à chaud obtenue à une température comprise entre 550 et 850°C, puiswinding the hot strip obtained at a temperature of between 550 and 850 ° C., and
- à laminer à froid la bande avec un taux de réduction compris entre 60 et 90%, puis- cold rolling the strip with a reduction rate of between 60 and 90%, then
- à recuire la bande de façon continue dans le domaine intercritique, etto anneal the strip continuously in the intercritical domain, and
- à la refroidir jusqu'à la température ambiante, en une ou plusieurs étapes, la vitesse de refroidissement entre 600°C et la température ambiante étant comprise entre 100°C/s et 1500°C/s,- cooling it to room temperature, in one or more steps, the cooling rate between 600 ° C and the ambient temperature being between 100 ° C / s and 1500 ° C / s,
- et éventuellement à lui faire subir un revenu à une température inférieure à 300°C, les opérations de recuit et de refroidissement étant menées de telle sorte que la bande comprenne finalement de 1 à 15% de martensite. and possibly to make it undergo an income at a temperature below 300 ° C, the annealing and cooling operations being carried out so that the band finally comprises 1 to 15% of martensite.
2. Procédé selon la revendication 1 , caractérisé en outre en ce que la composition chimique de l'acier comprend :The method of claim 1, further characterized in that the chemical composition of the steel comprises:
0,020% < C < 0,060% 0,300% < Mn < 0,500% 0,010% Cr < 1 ,0%0.020% <C <0.060% 0.300% <Mn <0.500% 0.010% Cr <1.0%
0,010% < Si < 0,50% 0,010% P < 0,100% 0,010% < Al < 0,10% N < 0,010% le reste étant du fer et des impuretés résultant de l'élaboration.0.010% <Si <0.50% 0.010% P <0.100% 0.010% <Al <0.10% N <0.010% the rest being iron and impurities resulting from the preparation.
3. Procédé selon l'une ou l'autre des revendications 1 ou 2, caractérisé en ce que l'on lamine la bande à chaud à une température supérieure à 850°C.3. Method according to either of claims 1 or 2, characterized in that the hot strip is rolled at a temperature above 850 ° C.
4. Procédé selon l'une quelconque des revendications 1 à 3, caractérisé en ce que l'on bobine la bande à chaud à une température comprise entre4. Process according to any one of Claims 1 to 3, characterized in that the hot strip is reeled at a temperature between
550 et 750°C.550 and 750 ° C.
5. Procédé selon l'une quelconque des revendications 1 à 4, caractérisé en ce que l'on lamine à froid la bande avec un taux de réduction compris entre 70 et 80%. 5. Process according to any one of Claims 1 to 4, characterized in that the strip is cold-rolled with a reduction ratio of between 70 and 80%.
6. Procédé selon l'une quelconque des revendications 1 à 5, caractérisé en ce que le recuit continu de la bande laminée à froid comprend une phase de montée en température, puis une phase de maintien à une température prédéterminée.6. Method according to any one of claims 1 to 5, characterized in that the continuous annealing of the cold rolled strip comprises a temperature rise phase, then a holding phase at a predetermined temperature.
7. Procédé selon la revendication 6, caractérisé en ce que la température de maintien est comprise entre Ad et 900°C.7. Method according to claim 6, characterized in that the holding temperature is between Ad and 900 ° C.
8. Procédé selon la revendication 7, caractérisé en outre en ce que la température de maintien est comprise entre 750 et 850°C.The method of claim 7, further characterized in that the holding temperature is between 750 and 850 ° C.
9. Procédé selon l'une quelconque des revendications 1 à 8, caractérisé en ce que le refroidissement jusqu'à la température ambiante comprend un premier refroidissement lent entre la température de maintien et 600°C, au cours duquel la vitesse de refroidissement est inférieure à 50°C/s, puis un second refroidissement à une vitesse plus élevée et comprise entre 100°C/s et 1500°C/s, jusqu'à la température ambiante. 9. Method according to any one of claims 1 to 8, characterized in that the cooling to room temperature comprises a first slow cooling between the holding temperature and 600 ° C, during which the cooling rate is lower at 50 ° C / s and then a second cooling at a higher speed and between 100 ° C / s and 1500 ° C / s, up to room temperature.
10. Procédé selon la revendication 9, caractérisé en ce que le second refroidissement est réalisé par trempe à l'eau. 10. The method of claim 9, characterized in that the second cooling is carried out by quenching with water.
11. Procédé selon l'une quelconque des revendications 1 à 8, caractérisé en ce que le refroidissement est effectuée en une seule opération, à une vitesse de refroidissement comprise entre 100°C/s et 1500°C/s. 11. Process according to any one of claims 1 to 8, characterized in that the cooling is carried out in a single operation, at a cooling rate of between 100 ° C / s and 1500 ° C / s.
12. Procédé selon la revendication 11 , caractérisé en ce que le refroidissement est réalisé par trempe à l'eau. 12. Method according to claim 11, characterized in that the cooling is carried out by quenching with water.
13. Bande d'acier dual-phase à structure ferrito-martensitique, laminée à froid, dont la composition chimique comprend, en poids :13. Cold-rolled, ferritic-martensitic, dual-phase steel strip, the chemical composition of which comprises, by weight:
0,010% < C 0,100% 0,050% < Mn < 1 ,0% 0,010% < Cr < 1 ,0% 0,010% < Si < 0,50%0.010% <C 0.100% 0.050% <Mn <1.0% 0.010% <Cr <1.0% 0.010% <If <0.50%
0,001% < P < 0,20% 0,010% < Al < 0,10% N < 0,010% le reste étant du fer et des impuretés résultant de l'élaboration, la bande comportant en outre entre 1% et 15% de martensite.0.001% <P <0.20% 0.010% <Al <0.10% N <0.010% the rest being iron and impurities resulting from the preparation, the band further comprising between 1% and 15% of martensite.
14. Bande d'acier selon la revendication 13, caractérisée en ce que sa composition chimique comprend en outre :Steel strip according to claim 13, characterized in that its chemical composition further comprises:
0,020% < C < 0,060% 0,300% < Mn < 0,500% 0,010% < Cr < 1 ,0%0.020% <C <0.060% 0.300% <Mn <0.500% 0.010% <Cr <1.0%
0,010% < Si < 0,50% 0,010% < P < 0,100% 0,010% < Al < 0,10% N < 0,010% le reste étant du fer et des impuretés résultant de l'élaboration. 0.010% <Si <0.50% 0.010% <P <0.100% 0.010% <Al <0.10% N <0.010% the rest being iron and impurities resulting from the preparation.
15. Bande d'acier selon l'une ou l'autre des revendications 13 ou 14, caractérisée en ce qu'elle présente une résistance à la traction Rm supérieure à 450 MPa.15. Steel strip according to any one of claims 13 or 14, characterized in that it has a tensile strength Rm greater than 450 MPa.
16. Bande d'acier selon la revendications 15, caractérisée en ce qu'elle présente une résistance à la traction Rm supérieure à 500 MPa.16. Steel strip according to claim 15, characterized in that it has a tensile strength Rm greater than 500 MPa.
17. Bande d'acier selon la revendication 16, caractérisée en outre en ce qu'elle présente une résistance à la traction Rm supérieure à 600 MPa.Steel strip according to claim 16, further characterized in that it has a tensile strength Rm greater than 600 MPa.
18. Bande d'acier selon l'une quelconque des revendications 13 à 17, caractérisée en ce qu'elle présente un coefficient d'anisotropie moyen r supérieur à 1 ,1.18. Steel strip according to any one of claims 13 to 17, characterized in that it has an average coefficient of anisotropy r greater than 1.1.
19. Bande d'acier selon la revendication 18, caractérisée en outre en ce qu'elle présente un coefficient d'anisotropie moyen r supérieur à 1 ,3.Steel strip according to claim 18, further characterized in that it has an average coefficient of anisotropy r greater than 1.3.
20. Bande d'acier selon l'une quelconque des revendications 13 à 19, caractérisée en ce qu'elle comporte en outre entre 1 % et 10% de martensite.20. Steel strip according to any one of claims 13 to 19, characterized in that it further comprises between 1% and 10% of martensite.
21. Bande d'acier selon la revendication 20, caractérisée en ce qu'elle comporte en outre entre 5% et 8% de martensite.21. Steel strip according to claim 20, characterized in that it further comprises between 5% and 8% of martensite.
22. Utilisation d'une bande d'acier selon l'une quelconque des revendications 13 à 21 , pour la fabrication de pièces pour automobile par emboutissage profond. 22. Use of a steel strip according to any one of claims 13 to 21 for the manufacture of automotive parts by deep drawing.
EP04706710.3A 2003-02-05 2004-01-30 Method of producing a cold-rolled band of dual-phase steel with a ferritic/martensitic structure and band thus obtained Expired - Lifetime EP1592816B1 (en)

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JP2006520431A (en) 2006-09-07
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UA87454C2 (en) 2009-07-27
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MXPA05008189A (en) 2005-10-05
CA2514736A1 (en) 2004-09-16
RU2341566C2 (en) 2008-12-20
JP4528769B2 (en) 2010-08-18
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HUE052206T2 (en) 2021-04-28
ES2831249T3 (en) 2021-06-08
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US20060144482A1 (en) 2006-07-06
FR2850671A1 (en) 2004-08-06

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