EP1378577A1 - Process for heat treating cold rolled formable steel strip and steel strip thus obtained - Google Patents
Process for heat treating cold rolled formable steel strip and steel strip thus obtained Download PDFInfo
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- EP1378577A1 EP1378577A1 EP03447154A EP03447154A EP1378577A1 EP 1378577 A1 EP1378577 A1 EP 1378577A1 EP 03447154 A EP03447154 A EP 03447154A EP 03447154 A EP03447154 A EP 03447154A EP 1378577 A1 EP1378577 A1 EP 1378577A1
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- annealing temperature
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- 229910000831 Steel Inorganic materials 0.000 title claims abstract description 100
- 239000010959 steel Substances 0.000 title claims abstract description 100
- 238000000034 method Methods 0.000 title claims abstract description 53
- 230000008569 process Effects 0.000 title claims abstract description 37
- 238000000137 annealing Methods 0.000 claims abstract description 56
- 238000010438 heat treatment Methods 0.000 claims abstract description 42
- 230000006641 stabilisation Effects 0.000 claims abstract description 30
- 238000001816 cooling Methods 0.000 claims abstract description 28
- 238000011105 stabilization Methods 0.000 claims abstract description 27
- 230000009466 transformation Effects 0.000 claims abstract description 23
- 229910000734 martensite Inorganic materials 0.000 claims abstract description 19
- 239000010960 cold rolled steel Substances 0.000 claims abstract description 7
- 238000004519 manufacturing process Methods 0.000 claims abstract description 6
- 238000010791 quenching Methods 0.000 claims description 26
- 230000000171 quenching effect Effects 0.000 claims description 24
- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 claims description 11
- 229910052799 carbon Inorganic materials 0.000 claims description 11
- 238000005097 cold rolling Methods 0.000 claims description 6
- PWHULOQIROXLJO-UHFFFAOYSA-N Manganese Chemical compound [Mn] PWHULOQIROXLJO-UHFFFAOYSA-N 0.000 claims description 5
- 238000005098 hot rolling Methods 0.000 claims description 5
- 229910052748 manganese Inorganic materials 0.000 claims description 5
- 239000011572 manganese Substances 0.000 claims description 5
- 238000005246 galvanizing Methods 0.000 claims description 2
- 230000009467 reduction Effects 0.000 claims description 2
- 238000004804 winding Methods 0.000 claims description 2
- 230000000087 stabilizing effect Effects 0.000 claims 2
- 238000005496 tempering Methods 0.000 abstract 3
- 229910001566 austenite Inorganic materials 0.000 description 23
- 229910000859 α-Fe Inorganic materials 0.000 description 15
- 238000011282 treatment Methods 0.000 description 11
- 238000010586 diagram Methods 0.000 description 10
- 238000012360 testing method Methods 0.000 description 7
- 239000000203 mixture Substances 0.000 description 6
- 230000002051 biphasic effect Effects 0.000 description 4
- 235000019362 perlite Nutrition 0.000 description 4
- 239000010451 perlite Substances 0.000 description 4
- 230000015572 biosynthetic process Effects 0.000 description 3
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- XLYOFNOQVPJJNP-UHFFFAOYSA-N water Substances O XLYOFNOQVPJJNP-UHFFFAOYSA-N 0.000 description 3
- 229910000851 Alloy steel Inorganic materials 0.000 description 2
- 229910001209 Low-carbon steel Inorganic materials 0.000 description 2
- 239000007789 gas Substances 0.000 description 2
- 230000007774 longterm Effects 0.000 description 2
- VNWKTOKETHGBQD-UHFFFAOYSA-N methane Chemical compound C VNWKTOKETHGBQD-UHFFFAOYSA-N 0.000 description 2
- 229910001339 C alloy Inorganic materials 0.000 description 1
- HCHKCACWOHOZIP-UHFFFAOYSA-N Zinc Chemical compound [Zn] HCHKCACWOHOZIP-UHFFFAOYSA-N 0.000 description 1
- 230000032683 aging Effects 0.000 description 1
- 229910045601 alloy Inorganic materials 0.000 description 1
- 239000000956 alloy Substances 0.000 description 1
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- QMQXDJATSGGYDR-UHFFFAOYSA-N methylidyneiron Chemical compound [C].[Fe] QMQXDJATSGGYDR-UHFFFAOYSA-N 0.000 description 1
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- 239000003345 natural gas Substances 0.000 description 1
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Images
Classifications
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- 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
- C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
- C21D9/48—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals deep-drawing sheets
-
- 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
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- C—CHEMISTRY; METALLURGY
- C21—METALLURGY OF IRON
- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
- C21D8/0447—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the heat treatment
- C21D8/0473—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
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
- C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
- C21D8/0426—Hot rolling
Definitions
- the present invention relates to the manufacture of steel strips by cold rolling, particularly steel belts for forming treatments, for example stamping deep.
- the invention relates more particularly to a process for the heat treatment of a steel strip cold rolled, in order to give it a high resistance and great formability.
- steel strips intended for forming operations are cold-rolled strips.
- the band has favorable properties, and particular good ductility, for the operations considered. In general, these properties are the result of heat treatments for annealing and well known cooling.
- the annealing is operated by heating the strip of steel up to an annealing temperature in the intercritical domain of the equilibrium diagram of steel, between transformation points A1 (temperature of the eutectoid) and A3 (minimum temperature at which the austenitic phase ⁇ is the only stable phase of steel).
- the steel strip is then maintained at this temperature annealing for a time sufficient to convert the initial ferritic structure of steel into a structure mixed ferrite and austenite.
- We then execute the quenching by suddenly cooling the steel strip up to a temperature below the Ms temperature of transformation of austenite into martensite.
- the annealing temperature located in the field intercritical between equilibrium points A1 and A3 of steel, is selected to provide training maximum of 20% (preferably 10 to 15%) by volume of austenite in the mixed structure of ferrite and of austenite, in order to give steel high without significantly affecting its ductility.
- temperatures from 725 to 825 ° C.
- the temperatures below 350 ° C for example temperatures from 120 to 340 ° C or room temperature.
- Heat treatment of steel strips according to the method described above is preferably performed in a continuous process, in an installation online.
- Reasons for investment and congestion at soil impose to limit the duration of the various stages of the process.
- the heating of the steel strip up to the annealing temperature is carried out at a speed at least 150 ° C / s, advantageously from 150 to 350 ° C / s, the maintaining the steel strip at this temperature does not exceed not 20s and the cooling rate up to the critical quenching temperature Ms is at least 300 ° C / sec (for example 300 to 1000 ° C / s).
- the invention aims to remedy this disadvantage of the known method described above, in providing a new process for heat treatment a cold-rolled steel strip, which gives it reproducible mechanical strength properties, especially a high ductility and a high resistance, in online industrial installations reduced size.
- the invention relates to a method for heat treatment of a cold rolled steel strip, comprising heating to an annealing temperature above the temperature A3, a stabilization at said annealing temperature and quenching up to a temperature below the critical temperature Ms of martensitic transformation, the method being characterized in that an annealing temperature is selected greater than the processing temperature A3 austenitic and in that between stabilization at the annealing temperature and quenching, we submit the tape from steel to cooling down to a temperature intermediate located in the intercritical domain between transformation points A1 and A3, and stabilize it at said intermediate temperature.
- temperatures A1 and A3 are well known in the middle steel.
- the temperature A1 corresponds to the temperature of the eutectoid of the equilibrium diagram of the iron-carbon alloy. It is around 725 ° C but can vary in function of alloy elements of steel and speed heating the steel strip up to the temperature of annealing.
- the temperature A3 is the minimum temperature at which phase ⁇ is the only stable phase of steel. It depends on the composition of the steel, particularly its carbon content, and the heating rate up to the annealing temperature.
- the critical transformation temperature Ms martensitic is also well known in the middle steel. It is the temperature limit under which, during a rapid cooling of the steel since the annealing temperature, the austenite is transformed into martensite without perlite formation. This temperature critical and the cooling rate required for the transformation into martensite depend on the composition of steel and annealing temperature.
- the annealing temperature is greater than the temperature A3 of the steel, so that the treated steel is then in the austenitic field of the phase equilibrium diagram of steel.
- the choice of the optimum temperature of the annealing will depend on the composition of steel, especially its carbon content. In the case of a low-alloy steel, including 0.08 at 0.15% by weight of carbon, the annealing temperature is generally above 830 ° C and advantageously between 850 and 880 ° C.
- Heating the steel strip to the annealing temperature is advantageously carried out at large speed, so as to reduce the bulk of the annealing installation and optimize the structure crystalline steel. Speeds above 100 ° C / sec are recommended, speeds of 100 to 300 ° C / s being preferred.
- Heating the steel to the temperature annealing can be done in an oven heated by any conventional heating means. This one can for example include natural gas heating, fuel oil or electric induction heating. Such ovens are well known in technique.
- the steel strip After heating up to the temperature annealing, the steel strip is subjected to a step of stabilization. During this period, the steel strip is kept at the annealing temperature for a while enough to turn the steel into austenite and form a homogeneous austenite. .
- the duration of this stage of stabilization depends on the temperature at which one the performs and the composition of the steel. This stage of stabilization being carried out in a thermostatically controlled oven, interest in making it as short as possible, for congestion and investment considerations. In practice, it is usually maintained under 1 minute, durations of 1 to 20 s generally suitable.
- the stabilization at Annealing temperature is followed by cooling up to an intermediate temperature located in the intercritical domain.
- the domain intercritical is the equilibrium domain of steel, located between the temperature A1 and the temperature A3. It corresponds the field of coexistence of a ferritic phase ⁇ and a austenitic phase ⁇ .
- the choice of the optimum value for the intermediate temperature will depend on the properties sought for the steel strip subjected to the treatment thermal. In general, it is chosen so that the equilibrium there corresponds to the maximum 20% (for example of 5 to 20% by weight of austenite, the values of 10 to 15% by weight austenite weight generally suitable.
- This optimum temperature depends on the steel temperature A3 concerned and, consequently, of the composition of the and can easily be determined from the diagram steel balance. In practice, temperatures of 600 to 750 ° C are suitable in most cases.
- Cooling to temperature intermediary is usually carried out using a gas cooling ("gas jet cooling"). We do it advantageously at a speed greater than 50 ° C./s, example of 100 to 150 ° C / s.
- Stabilization at intermediate temperature is to maintain the band of steel at the selected intermediate temperature during enough time to transform a fraction of ferrite austenite and achieve equilibrium thermodynamics of the two phases in the presence (ferritic and austenitic).
- the duration of stabilization at intermediate temperature is usually greater than 2 s. In practice, it is usually at least 5 s. We has no interest in exceeding 25s. The durations of 5 at 15s are generally well suited.
- the phase of stabilization at the intermediate temperature is performed preferably in a low inertia oven, also called a tunnel oven.
- the steel strip is subject to a quench. This is to cool the steel strip to a temperature below the temperature critical Ms, with sufficient cooling speed to turn the austenitic phase into martensite without perlite formation.
- the choice of optimum values for the quenching temperature and the cooling rate is going depend in particular on the composition of the steel and the intermediate temperature. These optimum values can be easily determined by laboratory tests. In practical, we choose a quenching start temperature between 600 and 750 ° C and a cooling rate greater than 100 ° C / s. Quenching can for example be performed by spraying a mist of water and air compressed or by immersion in a hot water bath (for example at a temperature of 70 to 90 ° C). For the cooling down to the quenching temperature, speeds of 300 to 1000 ° C / s are especially recommended.
- quenching is carried out in immersing the steel strip in a galvanizing bath, ideally a bath of molten zinc at a temperature of 460 to 500 ° C.
- the steel strip is subjected to an operation of overaging, intended to precipitate soluble carbon formed during the transformation of austenite into martensite.
- the method according to the invention makes it possible to confer to cold-rolled steel strips of properties optimum mechanical properties, which are reproducible in online industrial facilities, not requiring a Excessive congestion and investment.
- This result advantage of the process according to the invention in particular the reproducibility of the properties sought, is imputable, on the one hand, at the choice of an annealing temperature which places the point of thermodynamic equilibrium of steel in the austenitic zone and, on the other hand, to precede quenching a cooling down to a temperature intermediate that places the representative point of steel in the intercritical domain.
- the inventors found that this advantageous result of the process according to the invention originates in the speed of transformation and carbon diffusion in the steel during annealing and during the intermediate cooling that follows this one.
- the method according to the invention applies to all low carbon steel bands of the type biphasic (or "dual phase") defined above. It suits especially good to LC ('Low Carbon') steel strips cold rolled products of 0.3 to 2 mm thickness, intended for forming as applied in the manufacture of components used in the automotive industry. More particularly, the heat treatment process according to the invention applies especially well to steel strips low carbon and manganese contents, particularly steel strips containing from 0.06 to 0.2% by weight of carbon and from 0.6 to 2.0% by weight of manganese. Examples of LC steels for which the process according to the invention be well understood to include from 0.08 to 0.15% by weight of carbon and 0.6 to 1.5% by weight of manganese. Content silicon is preferably less than 0.5% by weight, for example, between 0.01 and 0.4% by weight.
- Cold-rolled steel strip in the heat treatment process according to the invention generally comes from a manufactured slab in a hot rolling mill.
- the invention also a method of manufacturing a steel strip adapted to forming, according to which a steel slab is subjected hot rolling at a temperature in the area austenitic steel, the sheet collected from the hot rolling at a winding at a temperature of 680 to 750 ° C, then to a cold rolling with a reduction rate from 50 to 80% and the sheet collected from the cold rolling to a heat treatment in accordance with the invention.
- the invention also relates to bands of steel suitable for forming, obtained by means of the manufacture according to the invention, described above, particularly steel belts for the industry automobile.
- Figure 1 is a diagram schematizing the different operating stages of two distinct processes for the heat treatment of a mild steel strip two-phase structure for forming.
- Figure 2 is a diagram showing the mechanism of formation of austenite with ordinate the volume fraction indicated by P, from a structure homogeneous ferritic (curve 11) or from a structure homogeneous austenitic (curve 12).
- the abscissa scale represents the time and the ordinate scale represents the temperatures.
- the line designated as a whole by the reference notation 1 represents the process of heat treatment of the state of the art.
- Line referred to as a whole by the reference notation 5 represents a heat treatment process according to the invention.
- the abscissa scale designates the time and the ordinate scale designates the content in austenite steel.
- Line 11 corresponds to the treatment according to the state of the art (shown schematically by line 1 in Figure 1) and line 12 corresponds to heat treatment according to the invention (schematized by line 5 in Figure 1).
- the steel initially in the equilibrium zone of the ferrite is heated to the annealing temperature T 1 located in the intercritical zone of the equilibrium diagram (between the temperatures A1 and A3) .
- T 1 located in the intercritical zone of the equilibrium diagram (between the temperatures A1 and A3) .
- a fraction of the ferrite turns into austenite and a substantial time t 1 is required to reach equilibrium.
- the steel is first heated to the annealing temperature T 2 located in the equilibrium zone of the austenite (higher than the temperature A3 of the equilibrium diagram). The time required to obtain a transformation of all the ferrite into austenite is very short and represented by t 2 . Then, the steel is cooled to the intermediate temperature T 3 located in the intercritical zone of the equilibrium diagram and maintained at this temperature the time required to obtain a transformation of a fraction of austenite to ferrite.
- each of the examples we implemented a low alloy steel strip (0.1% by weight of carbon and 1.25% by weight of manganese) 0.7 mm thick, from a cold rolling mill.
- the steel band was subjected to heat treatment, for the purpose of to confer a two-phase structure (or "dual phase") comprising martensite grains dispersed in a ferrite matrix.
- the steel strip has been subjected to a treatment according to the state of the art, shown schematically by line 1 in Figure 1.
- This treatment thermal featured a rapid heating 2 of some seconds up to an annealing temperature of 800 ° C, a stabilization 3 at this temperature for 3 minutes and quenching 4 to room temperature.
- the steel strip has been subjected to a treatment according to the state of the art, shown schematically by line 1 in Figure 1.
- This treatment thermal featured a quick heating of some seconds up to an annealing temperature of 800 ° C, a stabilization at this temperature for 3 minutes followed cooling in boiling water (2 seconds) up to a temperature of 641 ° C and quenching up to ambient temperature.
- the steel strip has been subjected to a treatment according to the state of the art, shown schematically by line 1 in Figure 1.
- This treatment thermal featured a quick heating of some seconds up to an annealing temperature of 800 ° C, a stabilization at this temperature for 3 minutes followed from an air cooling (14 seconds) to a temperature of 516 ° C and quenching to temperature room.
- the steel strip has been subjected to a treatment thermal device according to the invention, shown schematically by line 5 in Figure 1.
- This treatment thermal featured a rapid heating 6 of some seconds to an annealing temperature of 865 ° C, a stabilization 7 at this temperature for 40 seconds a rapid intermediate cooling 8 speed greater than 100 ° C / s up to a temperature of 650 ° C, a stabilization 9 at the temperature of 650 ° C for 10 seconds and quench to room temperature.
- a comparison of the results of Examples 1 to 3 shows that in the method of the prior art a structure stable biphasic ferrite and martensite requires a long-term stabilization at annealing temperature (3 minutes), followed by a slow preliminary cooling (14 seconds) before quenching. In the absence of this slow preliminary cooling, the resulting structure is not stable or reproducible or contains a phase of perlite untimely, detrimental to properties mechanical steel strip.
- the method of the invention allows to obtain the desired characteristics in term stability of the microstructure and mechanical properties, without requiring a long annealing period, nor a slow cooling before quenching.
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Abstract
Description
La présente invention a pour objet la fabrication de bandes d'acier par laminage à froid, particulièrement les bandes d'acier destinées à des traitements de formage, par exemple l'emboutissage profond.The present invention relates to the manufacture of steel strips by cold rolling, particularly steel belts for forming treatments, for example stamping deep.
L'invention concerne plus particulièrement un procédé pour le traitement thermique d'une bande d'acier laminée à froid, dans le but de lui conférer une haute résistance et une grande formabilité.The invention relates more particularly to a process for the heat treatment of a steel strip cold rolled, in order to give it a high resistance and great formability.
D'une manière générale, les bandes d'acier destinées à des opérations de formage, notamment de haute formabilité, sont des bandes laminées à froid. Ce type de bande présente en effet des propriétés favorables, et en particulier une bonne ductilité, pour les opérations envisagées. D'une manière générale, ces propriétés sont le résultat de traitements thermiques de recuit et de refroidissement bien connus.In general, steel strips intended for forming operations, particularly high formability, are cold-rolled strips. This kind of Indeed, the band has favorable properties, and particular good ductility, for the operations considered. In general, these properties are the result of heat treatments for annealing and well known cooling.
On sait par ailleurs qu'il est possible de produire des aciers présentant d'une part une haute résistance et d'autre part une ductilité élevée en leur conférant une structure biphasée (appelée aussi, en anglais, structure « dual phase »). Cette structure biphasée est essentiellement constituée par une matrice ferritique douce, dans laquelle sont dispersées de fines particules de martensite. En schématisant, on considère que la matrice ferritique assure la ductilité de l'acier, tandis que la résistance est donnée par la martensite, qui est une phase de dureté élevée, obtenue par une opération de trempe.We also know that it is possible to produce steels presenting on the one hand a high resistance and on the other hand a high ductility in them conferring a two-phase structure (also called, in English, "dual phase" structure). This structure biphasic is essentially constituted by a matrix ferritic soft, in which are dispersed fine martensite particles. By schematizing, we consider that the ferritic matrix ensures the ductility of the steel, while resistance is given by martensite, which is a phase of high hardness, obtained by an operation quenching.
D'un point de vue métallurgique, on forme la structure biphasée définie ci-dessus dans une bande d'acier en soumettant celle-ci à un traitement thermique adéquat en plusieurs étapes comportant un recuit et une trempe. Selon un procédé connu, le recuit est opéré en chauffant la bande d'acier jusqu'à une température de recuit située dans le domaine intercritique du diagramme d'équilibre de l'acier, entre les points de transformation A1 (température de l'eutectoïde) et A3 (température minimum à laquelle la phase austénitique γ est la seule phase stable de l'acier). La bande d'acier est ensuite maintenue à cette température de 'recuit pendant un temps suffisant pour convertir la structure ferritique initiale de l'acier en une structure mixte de ferrite et d'austénite. On exécute ensuite la trempe en refroidissant brutalement la bande d'acier jusqu'à une température inférieure à la température Ms de transformation de l'austénite en martensite.From a metallurgical point of view, we form the two-phase structure defined above in a steel strip by subjecting it to a suitable heat treatment in several stages including annealing and quenching. according to a known method, the annealing is operated by heating the strip of steel up to an annealing temperature in the intercritical domain of the equilibrium diagram of steel, between transformation points A1 (temperature of the eutectoid) and A3 (minimum temperature at which the austenitic phase γ is the only stable phase of steel). The steel strip is then maintained at this temperature annealing for a time sufficient to convert the initial ferritic structure of steel into a structure mixed ferrite and austenite. We then execute the quenching by suddenly cooling the steel strip up to a temperature below the Ms temperature of transformation of austenite into martensite.
Un procédé de ce type est notamment décrit et revendiqué dans le document WO-A-02/00947. Dans ce procédé connu, la température du recuit, située dans le domaine intercritique entre les points d'équilibre A1 et A3 de l'acier, est sélectionnée de façon à assurer la formation d'au maximum 20% (de préférence de 10 à 15%) en volume d'austénite dans la structure mixte de ferrite et d'austénite, afin de conférer à l'acier une résistance élevée sans affecter sensiblement sa ductilité. On cite les températures de 725 à 825°C. Pour la température de trempe, inférieure à la température critique Ms, on cite les températures inférieures à 350°C, par exemple les températures de 120 à 340°C ou la température ambiante.A process of this type is described in particular and claimed in WO-A-02/00947. In this process known, the annealing temperature, located in the field intercritical between equilibrium points A1 and A3 of steel, is selected to provide training maximum of 20% (preferably 10 to 15%) by volume of austenite in the mixed structure of ferrite and of austenite, in order to give steel high without significantly affecting its ductility. We quote temperatures from 725 to 825 ° C. For the quenching temperature, below the critical temperature Ms, we quote the temperatures below 350 ° C, for example temperatures from 120 to 340 ° C or room temperature.
Le traitement thermique des bandes d'acier selon le procédé décrit ci-dessus est de préférence effectué selon un processus continu, dans une installation en ligne. Des raisons d'investissement et d'encombrement au sol imposent de limiter la durée des différentes étapes du procédé. Ainsi, dans le procédé connu décrit dans la demande de brevet susdite, le chauffage de la bande d'acier jusqu'à la température de recuit est effectué à une vitesse d'au moins 150°C/s, avantageusement de 150 à 350°C/s, le maintien de la bande d'acier à cette température n'excède pas 20 s et la vitesse de refroidissement jusqu'à la température critique de trempe Ms est d'au moins 300°C/s (par exemple de 300 à 1000°C/s).Heat treatment of steel strips according to the method described above is preferably performed in a continuous process, in an installation online. Reasons for investment and congestion at soil impose to limit the duration of the various stages of the process. Thus, in the known process described in patent application aforesaid, the heating of the steel strip up to the annealing temperature is carried out at a speed at least 150 ° C / s, advantageously from 150 to 350 ° C / s, the maintaining the steel strip at this temperature does not exceed not 20s and the cooling rate up to the critical quenching temperature Ms is at least 300 ° C / sec (for example 300 to 1000 ° C / s).
Dans le procédé connu qui vient d'être décrit, il s'est révélé particulièrement difficile, voire impossible, d'atteindre l'équilibre thermodynamique de la structure mixte de ferrite et d'austénite à l'issue du recuit, ce qui rend le procédé instable. En pratique, il est en effet apparu que cet équilibre ne pouvait être atteint qu'à la suite d'une très longue durée de maintien de la bande d'acier à la température de recuit. Cette longue durée nécessite la construction d'une ligne très longue, représentant des investissements importants d'exploitation industrielle.In the known process that has just been described, it has proved particularly difficult, even impossible to reach the thermodynamic equilibrium of mixed structure of ferrite and austenite at the end of annealing, which makes the process unstable. In practice, he indeed it appeared that this balance could not be reached only after a very long period of maintenance of the steel strip at the annealing temperature. This long-term requires the construction of a very long, representing significant investments industrial exploitation.
L'invention vise à remédier à cet inconvénient du procédé connu décrit plus haut, en fournissant un procédé nouveau pour le traitement thermique d'une bande d'acier laminée à froid, qui confère à celle-ci des propriétés de résistance mécanique reproductibles, particulièrement une grande ductilité et une haute résistance, dans des installations industrielles en ligne d'encombrement réduit.The invention aims to remedy this disadvantage of the known method described above, in providing a new process for heat treatment a cold-rolled steel strip, which gives it reproducible mechanical strength properties, especially a high ductility and a high resistance, in online industrial installations reduced size.
L'invention concerne un procédé pour le traitement thermique d'une bande d'acier laminée à froid, comprenant un chauffage jusqu'à une température de recuit supérieure à la température A3, une stabilisation à ladite température de recuit et une trempe jusqu'à une température inférieure à la température critique Ms de transformation martensitique, le procédé étant caractérisé en ce qu'on sélectionne une température de recuit supérieure à la température A3 de transformation austénitique et en ce qu'entre la stabilisation à la température de recuit et la trempe, on soumet la bande d'acier à un refroidissement jusqu'à une température intermédiaire située dans le domaine intercritique entre les points de transformation A1 et A3, et on la stabilise à ladite température intermédiaire.The invention relates to a method for heat treatment of a cold rolled steel strip, comprising heating to an annealing temperature above the temperature A3, a stabilization at said annealing temperature and quenching up to a temperature below the critical temperature Ms of martensitic transformation, the method being characterized in that an annealing temperature is selected greater than the processing temperature A3 austenitic and in that between stabilization at the annealing temperature and quenching, we submit the tape from steel to cooling down to a temperature intermediate located in the intercritical domain between transformation points A1 and A3, and stabilize it at said intermediate temperature.
Dans le procédé selon l'invention, les températures A1 et A3 sont bien connues du milieu sidérurgique. La température A1 correspond à la température de l'eutectoïde du diagramme d'équilibre de l'alliage fer-carbone. Il est d'environ 725°C mais peut varier en fonction des éléments d'alliage de l'acier et de la vitesse de chauffage de la bande d'acier jusqu'à la température de recuit. La température A3 est la température minimum à laquelle la phase γ est la seule phase stable de l'acier. Elle dépend de la composition de l'acier, particulièrement de sa teneur en carbone, et de la vitesse de chauffage jusqu'à la température du recuit. In the process according to the invention, temperatures A1 and A3 are well known in the middle steel. The temperature A1 corresponds to the temperature of the eutectoid of the equilibrium diagram of the iron-carbon alloy. It is around 725 ° C but can vary in function of alloy elements of steel and speed heating the steel strip up to the temperature of annealing. The temperature A3 is the minimum temperature at which phase γ is the only stable phase of steel. It depends on the composition of the steel, particularly its carbon content, and the heating rate up to the annealing temperature.
La température critique Ms de transformation martensitique est également bien connue dans le milieu sidérurgique. Elle est la température limite sous laquelle, lors d'un refroidissement rapide de l'acier depuis la température du recuit, l'austénite est transformée en martensite sans formation de perlite. Cette température critique et la vitesse de refroidissement nécessaire à la transformation en martensite dépendent de la composition de l'acier et de la température de recuit.The critical transformation temperature Ms martensitic is also well known in the middle steel. It is the temperature limit under which, during a rapid cooling of the steel since the annealing temperature, the austenite is transformed into martensite without perlite formation. This temperature critical and the cooling rate required for the transformation into martensite depend on the composition of steel and annealing temperature.
Selon l'invention, la température du recuit est supérieure à la température A3 de l'acier, de sorte que l'acier traité se trouve alors dans le domaine austénitique du diagramme d'équilibre des phases de l'acier. Le choix de la température optimum du recuit va dépendre de la composition de l'acier, notamment de sa teneur en carbone. Dans le cas d'un acier faiblement allié, comprenant de 0,08 à 0,15% en poids de carbone, la température du recuit est généralement supérieure à 830°C et se situe avantageusement entre 850 et 880°C.According to the invention, the annealing temperature is greater than the temperature A3 of the steel, so that the treated steel is then in the austenitic field of the phase equilibrium diagram of steel. The choice of the optimum temperature of the annealing will depend on the composition of steel, especially its carbon content. In the case of a low-alloy steel, including 0.08 at 0.15% by weight of carbon, the annealing temperature is generally above 830 ° C and advantageously between 850 and 880 ° C.
Le chauffage de la bande d'acier jusqu'à la température de recuit est avantageusement effectué à grande vitesse, de manière à réduire l'encombrement de l'installation de recuit et à optimiser la structure cristalline de l'acier. Des vitesses supérieures à 100°C/s sont recommandées, les vitesses de 100 à 300°C/s étant préférées. Le chauffage de l'acier jusqu'à la température de recuit peut être effectué dans un four chauffé par tout moyen de chauffage classique. Celui-ci peut par exemple comprendre un chauffage au gaz naturel, un chauffage au mazout ou un chauffage électrique par induction. De tels fours sont bien connus en technique.Heating the steel strip to the annealing temperature is advantageously carried out at large speed, so as to reduce the bulk of the annealing installation and optimize the structure crystalline steel. Speeds above 100 ° C / sec are recommended, speeds of 100 to 300 ° C / s being preferred. Heating the steel to the temperature annealing can be done in an oven heated by any conventional heating means. This one can for example include natural gas heating, fuel oil or electric induction heating. Such ovens are well known in technique.
A l'issue du chauffage jusqu'à la température de recuit, la bande d'acier est soumise à une étape de stabilisation. Au cours de celle-ci, la bande d'acier est maintenue à la température du recuit pendant un temps suffisant pour transformer l'acier en austénite et former une austénite homogène. .La durée de cette étape de stabilisation dépend de la température à laquelle on l'effectue et de la composition de l'acier. Cette étape de stabilisation étant réalisée dans un four thermostatisé, on a intérêt à la rendre aussi courte que possible, pour des considérations d'encombrement et d'investissement. En pratique, on la maintient généralement sous 1 minute, les durées de 1 à 20 s convenant généralement bien.After heating up to the temperature annealing, the steel strip is subjected to a step of stabilization. During this period, the steel strip is kept at the annealing temperature for a while enough to turn the steel into austenite and form a homogeneous austenite. .The duration of this stage of stabilization depends on the temperature at which one the performs and the composition of the steel. This stage of stabilization being carried out in a thermostatically controlled oven, interest in making it as short as possible, for congestion and investment considerations. In practice, it is usually maintained under 1 minute, durations of 1 to 20 s generally suitable.
Selon l'invention, la stabilisation à la température de recuit est suivie d'un refroidissement jusqu'à une température intermédiaire située dans le domaine intercritique. Par définition, le domaine intercritique est le domaine d'équilibre de l'acier, situé entre la température A1 et la température A3. Il correspond au domaine de coexistence d'une phase ferritique α et d'une phase austénitique γ. Le choix de la valeur optimum pour la température intermédiaire va dépendre des propriétés recherchées pour la bande d'acier soumise au traitement thermique. En général, on la choisit de manière qu'à l'équilibre il y corresponde au maximum 20% (par exemple de 5 à 20%) en poids d'austénite, les valeurs de 10 à 15% en poids d'austénite convenant généralement bien. Cette température optimum dépend de la température A3 de l'acier concerné et, par conséquent, de la composition de celui-ci et peut aisément être déterminée au départ du diagramme d'équilibre des aciers. En pratique, des températures de 600 à 750°C conviennent bien dans la majorité des cas.According to the invention, the stabilization at Annealing temperature is followed by cooling up to an intermediate temperature located in the intercritical domain. By definition, the domain intercritical is the equilibrium domain of steel, located between the temperature A1 and the temperature A3. It corresponds the field of coexistence of a ferritic phase α and a austenitic phase γ. The choice of the optimum value for the intermediate temperature will depend on the properties sought for the steel strip subjected to the treatment thermal. In general, it is chosen so that the equilibrium there corresponds to the maximum 20% (for example of 5 to 20% by weight of austenite, the values of 10 to 15% by weight austenite weight generally suitable. This optimum temperature depends on the steel temperature A3 concerned and, consequently, of the composition of the and can easily be determined from the diagram steel balance. In practice, temperatures of 600 to 750 ° C are suitable in most cases.
Le refroidissement jusqu'à la température intermédiaire est généralement effectué à l'aide d'un refroidissement au gaz (« gas jet cooling »). On l'effectue avantageusement à une vitesse supérieure à 50°C/s, par exemple de 100 à 150°C/s.Cooling to temperature intermediary is usually carried out using a gas cooling ("gas jet cooling"). We do it advantageously at a speed greater than 50 ° C./s, example of 100 to 150 ° C / s.
Le refroidissement -de la bande d'acier jusqu'à la température intermédiaire est suivi d'une stabilisation à cette température. La stabilisation à la température intermédiaire consiste à maintenir la bande d'acier à la température intermédiaire sélectionnée pendant un temps suffisant pour transformer une fraction de l'austénite en ferrite et atteindre l'équilibre thermodynamique des deux phases en présence (ferritique et austénitique) . La durée de la stabilisation à la température intermédiaire est généralement supérieure à 2 s. En pratique, elle est habituellement d'au moins 5 s. On n'a pas intérêt à ce qu'elle dépasse 25 s. Les durées de 5 à 15 s conviennent généralement bien. La phase de stabilisation à la température intermédiaire est effectué de préférence dans un four à faible inertie, aussi nommé un four tunnel.Cooling of steel strip until the intermediate temperature is followed by a stabilization at this temperature. Stabilization at intermediate temperature is to maintain the band of steel at the selected intermediate temperature during enough time to transform a fraction of ferrite austenite and achieve equilibrium thermodynamics of the two phases in the presence (ferritic and austenitic). The duration of stabilization at intermediate temperature is usually greater than 2 s. In practice, it is usually at least 5 s. We has no interest in exceeding 25s. The durations of 5 at 15s are generally well suited. The phase of stabilization at the intermediate temperature is performed preferably in a low inertia oven, also called a tunnel oven.
A l'issue de la stabilisation à la température intermédiaire, la bande d'acier est soumise à une trempe. Celle-ci consiste à refroidir la bande d'acier jusqu'à une température inférieure à la température critique Ms, avec une vitesse de refroidissement suffisante pour transformer la phase austénitique en martensite sans formation de perlite. Le choix des valeurs optimum pour la température de trempe et la vitesse de refroidissement va dépendre notamment de la composition de l'acier et de la température intermédiaire. Ces valeurs optimum peuvent être déterminées aisément par des essais au laboratoire. En pratique, on choisit une température de début de trempe entre 600 et 750°C et une vitesse de refroidissement supérieure à 100°C/s. La trempe peut par exemple être effectuée par pulvérisation d'un brouillard d'eau et air comprimé ou par immersion dans un bain d'eau chaude (par exemple à une température de 70 à 90°C). Pour le refroidissement jusqu'à la température de trempe, des vitesses de 300 à 1000°C/s sont spécialement recommandées.At the end of the stabilization intermediate temperature, the steel strip is subject to a quench. This is to cool the steel strip to a temperature below the temperature critical Ms, with sufficient cooling speed to turn the austenitic phase into martensite without perlite formation. The choice of optimum values for the quenching temperature and the cooling rate is going depend in particular on the composition of the steel and the intermediate temperature. These optimum values can be easily determined by laboratory tests. In practical, we choose a quenching start temperature between 600 and 750 ° C and a cooling rate greater than 100 ° C / s. Quenching can for example be performed by spraying a mist of water and air compressed or by immersion in a hot water bath (for example at a temperature of 70 to 90 ° C). For the cooling down to the quenching temperature, speeds of 300 to 1000 ° C / s are especially recommended.
Dans une forme de réalisation particulière du procédé selon l'invention, on effectue la trempe en immergeant la bande d'acier dans un bain de galvanisation, idéalement un bain de zinc fondu à une température de 460 à 500°C.In a particular embodiment of the according to the invention, quenching is carried out in immersing the steel strip in a galvanizing bath, ideally a bath of molten zinc at a temperature of 460 to 500 ° C.
Dans une autre forme de réalisation particulière du procédé selon l'invention, à l'issue de la trempe, on soumet la bande d'acier à une opération de survieillissement, destinée à précipiter le carbone soluble formé lors de la transformation de l'austénite en martensite. Par cette opération, on réduit la sensibilité de l'acier au vieillissement et on améliore sa ductilité.In another embodiment of the process according to the invention, at the end of the quenching, the steel strip is subjected to an operation of overaging, intended to precipitate soluble carbon formed during the transformation of austenite into martensite. By this operation, we reduce the sensitivity aging steel and improves its ductility.
Par comparaison avec le procédé connu décrit plus haut, le procédé selon l'invention permet de conférer à des bandes d'acier laminées à froid des propriétés mécaniques optimum, qui sont reproductibles, dans des installations industrielles en ligne, ne nécessitant pas un encombrement et un investissement excessifs. Ce résultat avantageux du procédé selon l'invention, en particulier la reproductibilité des propriétés recherchées, est imputable, d'une part, au choix d'une température de recuit qui place le point d'équilibre thermodynamique de l'acier dans la zone austénitique et, d'autre part, à faire précéder la trempe d'un refroidissement jusqu'à une température intermédiaire qui place le point représentatif de l'acier dans le domaine intercritique. Les inventeurs ont trouvé que ce résultat avantageux du procédé selon l'invention trouve son origine dans la vitesse de transformation et de diffusion de carbone dans l'acier au cours du recuit et au cours du refroidissement intermédiaire qui suit celui-ci. Les inventeurs ont notamment trouvé que dans le procédé selon l'invention, la transformation directe d'une phase ferritique unitaire en une phase austénitique unitaire, par chauffage, suivie d'une transformation de ladite. phase austénitique unitaire en une phase mixte de ferrite et d'austénite par refroidissement requiert moins de temps qu'une transformation directe d'une phase ferritique unitaire en une phase mixte de ferrite et d'austénite par chauffage (procédé connu décrit dans le document WO-A-02/00947).Compared with the known method described above, the method according to the invention makes it possible to confer to cold-rolled steel strips of properties optimum mechanical properties, which are reproducible in online industrial facilities, not requiring a Excessive congestion and investment. This result advantage of the process according to the invention, in particular the reproducibility of the properties sought, is imputable, on the one hand, at the choice of an annealing temperature which places the point of thermodynamic equilibrium of steel in the austenitic zone and, on the other hand, to precede quenching a cooling down to a temperature intermediate that places the representative point of steel in the intercritical domain. The inventors found that this advantageous result of the process according to the invention originates in the speed of transformation and carbon diffusion in the steel during annealing and during the intermediate cooling that follows this one. In particular, the inventors found that in the process according to the invention, the direct transformation of a phase unitary ferritic in a unitary austenitic phase, by heating, followed by a transformation of said. phase austenitic unit in a mixed phase of ferrite and cooling austenite requires less time that a direct transformation of a ferritic phase unit in a mixed phase of ferrite and austenite by heating (known method described in document WO-A-02/00947).
Le procédé selon l'invention s'applique à toutes les bandes d'acier à basse teneur en carbone du type biphasé (ou « dual phase ») défini plus haut. Il convient spécialement bien aux bandes d'acier LC ('Low Carbon') laminées à froid de 0,3 à 2 mm d'épaisseur, destinées à un formage tel qu'appliqué dans la fabrication de composants utilisés dans l'industrie automobile. Plus particulièrement, le procédé de traitement thermique selon l'invention s'applique spécialement bien aux bandes d'acier à basses teneurs en carbone et en manganèse, notamment aux bandes d'acier contenant de 0,06 à 0,2% en poids de carbone et de 0,6 à 2,0% en poids de manganèse. Des exemples d'aciers LC pour lesquels le procédé selon l'invention convient bien comprennent de 0,08 à 0,15% en poids de carbone et de 0,6 à 1,5% en poids de manganèse. La teneur en silicium est de préférence inférieure à 0,5% en poids, par exemple située entre 0,01 et 0,4% en poids.The method according to the invention applies to all low carbon steel bands of the type biphasic (or "dual phase") defined above. It suits especially good to LC ('Low Carbon') steel strips cold rolled products of 0.3 to 2 mm thickness, intended for forming as applied in the manufacture of components used in the automotive industry. More particularly, the heat treatment process according to the invention applies especially well to steel strips low carbon and manganese contents, particularly steel strips containing from 0.06 to 0.2% by weight of carbon and from 0.6 to 2.0% by weight of manganese. Examples of LC steels for which the process according to the invention be well understood to include from 0.08 to 0.15% by weight of carbon and 0.6 to 1.5% by weight of manganese. Content silicon is preferably less than 0.5% by weight, for example, between 0.01 and 0.4% by weight.
La bande d'acier laminée à froid mise en oeuvre dans le procédé de traitement thermique selon l'invention provient généralement d'une brame fabriquée dans un laminoir à chaud.Cold-rolled steel strip in the heat treatment process according to the invention generally comes from a manufactured slab in a hot rolling mill.
En conséquence, l'invention concerne également un procédé de fabrication d'une bande d'acier adapté au formage, selon lequel on soumet une brame d'acier à un laminage à chaud à une température située dans la zone austénitique de l'acier, on soumet la tôle recueillie du laminage à chaud à un bobinage à une température de 680 à 750°C, puis à un laminage à froid avec un taux de réduction de 50 à 80% et on soumet ensuite la tôle recueillie du laminage à froid à un traitement thermique conforme à l'invention.Accordingly, the invention also a method of manufacturing a steel strip adapted to forming, according to which a steel slab is subjected hot rolling at a temperature in the area austenitic steel, the sheet collected from the hot rolling at a winding at a temperature of 680 to 750 ° C, then to a cold rolling with a reduction rate from 50 to 80% and the sheet collected from the cold rolling to a heat treatment in accordance with the invention.
L'invention a également pour objet des bandes d'acier adapté au formage, obtenues au moyen du procédé de fabrication selon l'invention, décrit ci-dessus, particulièrement des bandes d'acier destinées à l'industrie automobile.The invention also relates to bands of steel suitable for forming, obtained by means of the manufacture according to the invention, described above, particularly steel belts for the industry automobile.
La figure 1 est un diagramme schématisant les différentes étapes opératoires de deux procédés distincts pour le traitement thermique d'une bande d'acier doux à structure biphasée, destinée à un formage.Figure 1 is a diagram schematizing the different operating stages of two distinct processes for the heat treatment of a mild steel strip two-phase structure for forming.
La figure 2 est un schéma montrant le mécanisme de formation de l'austénite avec en ordonnée la fraction volumique indiquée par P, à partir d'une structure homogène ferritique (courbe 11) ou à partir d'une structure homogène austénitique (courbe 12).Figure 2 is a diagram showing the mechanism of formation of austenite with ordinate the volume fraction indicated by P, from a structure homogeneous ferritic (curve 11) or from a structure homogeneous austenitic (curve 12).
Dans ces figures, des mêmes notations de référence ont des significations identiques.In these figures, the same notations of reference have identical meanings.
A la figure 1, l'échelle des abscisses
représente le temps et l'échelle des ordonnées représente
les températures. La ligne désignée dans son ensemble par
la notation de référence 1 représente le procédé de
traitement thermique de l'état de la technique. La ligne
désignée dans son ensemble par la notation de référence 5
représente un procédé de traitement thermique conforme à
l'invention. In Figure 1, the abscissa scale
represents the time and the ordinate scale represents
the temperatures. The line designated as a whole by
the
Le procédé selon l'état de la technique, schématisé par la ligne 1 à la figure 1 comprend les étapes successives. suivantes
un chauffage 2 de la bande d'acier jusqu'à une température de recuit T1 située dans la zone intercritique (entre les températures A1 et A3) de l'acier traité;une stabilisation 3 de l'acier à la température de recuit T1;une trempe 4 jusqu'à la température ambiante, à une vitesse supérieure à la vitesse critique de transformation de l'austénite en martensite.
-
heating 2 of the steel strip to an annealing temperature T 1 located in the intercritical zone (between the temperatures A1 and A3) of the treated steel; - a
stabilization 3 of the steel at the annealing temperature T 1 ; - quenching 4 to room temperature, at a rate above the critical transformation rate of austenite to martensite.
Le procédé selon l'invention, schématisé par la ligne 5 à la figure 1, comprend les étapes opératoires suivantes :
un chauffage 6 de la bande d'acier jusqu'à une température de recuit T2 située dans la phase austénitique (au-dessus de la température A3) de l'acier traité ;une stabilisation 7 de l'acier à la température de recuit T2;un refroidissement 8 jusqu'à une température T3 située dans la zone intercritique (entre les températures A1 et A3) de l'acier traité;une stabilisation 9 de l'acier à la température T3;une trempe 10 jusqu'à la température ambiante, à une vitesse supérieure à la vitesse critique de transformation de l'austénite en martensite.
- heating the steel strip to an annealing temperature T 2 in the austenitic phase (above the temperature A3) of the treated steel;
- a
stabilization 7 of the steel at the annealing temperature T 2 ; - a
cooling 8 to a temperature T 3 located in the intercritical zone (between the temperatures A1 and A3) of the treated steel; - a
stabilization 9 of the steel at the temperature T 3 ; - quenching to room temperature, at a rate above the critical transformation rate of austenite to martensite.
A la figure 2, l'échelle des abscisses désigne le
temps et l'échelle des ordonnées désigne la teneur en
austénite de l'acier. La ligne 11 correspond au traitement
thermique selon l'état de la technique (schématisé par la
ligne 1 à la figure 1) et la ligne 12 correspond au
traitement thermique selon l'invention (schématisé par la
ligne 5 à la figure 1). In FIG. 2, the abscissa scale designates the
time and the ordinate scale designates the content in
austenite steel.
Dans le traitement thermique schématisé par la
ligne 11, l'acier initialement dans la zone d'équilibre de
la ferrite est chauffé à la température de recuit T1 située
dans la zone intercritique du diagramme d'équilibre (entre
les températures A1 et A3). Une fraction de la ferrite se
transforme en austénite et un temps substantiel t1 est
nécessaire pour atteindre l'équilibre.In the heat treatment schematized by
Dans le traitement thermique schématisé par la
ligne 12, l'acier est d'abord chauffé à la température de
recuit T2 située dans la zone d'équilibre de l'austénite
(supérieure à la température A3 du diagramme d'équilibre).
Le temps nécessaire pour obtenir une transformation de la
totalité de la ferrite en austénite est très court et
représenté par t2. Ensuite, l'acier est refroidi jusqu'à la
température intermédiaire T3 située dans la zone
intercritique du diagramme d'équilibre et maintenu à cette
température le temps nécessaire pour obtenir une
transformation d'une fraction d'austénite en ferrite.In the heat treatment schematized by
On observe sur le diagramme de la figure 2 qu'avec le procédé selon l'invention, le temps total t3 nécessaire pour obtenir la structure biphasique stable de ferrite et d'austénite est nettement inférieur au temps t1 qui est nécessaire dans le procédé de l'état de la technique.It can be observed in the diagram of FIG. 2 that with the process according to the invention, the total time t 3 necessary to obtain the stable biphasic structure of ferrite and austenite is clearly less than the time t 1 which is necessary in the process. of the state of the art.
Les exemples décrits ci-après vont faire apparaítre l'intérêt du procédé selon l'invention.The examples described below will do appear the interest of the method according to the invention.
Dans chacun des exemples on a mis en oeuvre une bande en acier faiblement allié (0,1% en poids de carbone et 1,25% en poids de manganèse) de 0,7 mm d'épaisseur, provenant d'un laminoir à froid. La bande d'acier a été soumise à un traitement thermique, dans le but de lui conférer une structure biphasée (ou « dual phase ») comprenant des grains de martensite dispersées dans une matrice de ferrite.In each of the examples we implemented a low alloy steel strip (0.1% by weight of carbon and 1.25% by weight of manganese) 0.7 mm thick, from a cold rolling mill. The steel band was subjected to heat treatment, for the purpose of to confer a two-phase structure (or "dual phase") comprising martensite grains dispersed in a ferrite matrix.
La bande d'acier a été soumise à un traitement
thermique conforme à l'état de la technique, représenté
schématiquement par la ligne 1 à la figure 1. Ce traitement
thermique a comporté un chauffage rapide 2 de quelques
secondes jusqu'à une température de recuit de 800°C, une
stabilisation 3 à cette température pendant 3 minutes et
une trempe 4 jusqu'à la température ambiante.The steel strip has been subjected to a treatment
according to the state of the art, shown
schematically by
A l'issue de l'essai, on a mesuré les propriétés mécaniques de la bande d'acier et on a analysé la structure cristalline de l'acier. Les résultats sont consignés dans le tableau 1 ci-dessous, dans lequel
- YS désigne la limite élastique exprimée en mégapascal (MPa) ;
- TS désigne la charge de rupture à la traction, exprimée en mégapascal (MPa) ;
- Eltot désigne l'allongement total à la rupture par traction, exprimé en % de la longueur initiale de l'éprouvette ;
- F désigne une phase de ferrite ;
- M désigne une phase de martensite ;
- P désigne une phase de perlite.
- YS is the elastic limit expressed in megapascal (MPa);
- TS is tensile tensile strength, expressed in megapascals (MPa);
- Eltot denotes the total tensile elongation, expressed as a% of the initial length of the specimen;
- F denotes a ferrite phase;
- M denotes a martensite phase;
- P denotes a phase of pearlite.
La bande d'acier a été soumise à un traitement
thermique conforme à l'état de la technique, représenté
schématiquement par la ligne 1 à la figure 1. Ce traitement
thermique a comporté un chauffage rapide de quelques
secondes jusqu'à une température de recuit de 800°C, une
stabilisation à cette température pendant 3 minutes suivie
d'un refroidissement dans l'eau bouillante (2 secondes)
jusqu'à une température de 641°C et une trempe jusqu'à la
température ambiante.The steel strip has been subjected to a treatment
according to the state of the art, shown
schematically by
Les propriétés mécaniques et la structure de l'acier à l'issue de l'essai sont consignés dans le tableau 1 ci-dessous.The mechanical properties and the structure of steel at the end of the test are recorded in the table 1 below.
La bande d'acier a été soumise à un traitement
thermique conforme à l'état de la technique, représenté
schématiquement par la ligne 1 à la figure 1. Ce traitement
thermique a comporté un chauffage rapide de quelques
secondes jusqu'à une température de recuit de 800°C, une
stabilisation à cette température pendant 3 minutes suivie
d'un refroidissement à l'air (14 secondes) jusqu'à une
température de 516°C et une trempe jusqu'à la température
ambiante.The steel strip has been subjected to a treatment
according to the state of the art, shown
schematically by
Les propriétés mécaniques et la structure de l'acier à l'issue de l'essai sont consignés dans le tableau 1 ci-dessous.The mechanical properties and the structure of steel at the end of the test are recorded in the table 1 below.
La bande d'acier a été soumise à un traitement
thermique conforme à l'invention, représenté
schématiquement par la ligne 5 à la figure 1. Ce traitement
thermique a comporté un chauffage rapide 6 de quelques
secondes jusqu'à une température de recuit de 865°C, une
stabilisation 7 à cette température pendant 40 secondes, un
refroidissement intermédiaire rapide 8 de vitesse
supérieure à 100°C/s jusqu'à une température de 650°C, une
stabilisation 9 à la température de 650°C pendant 10
secondes et une trempe 10 jusqu'à la température ambiante.The steel strip has been subjected to a treatment
thermal device according to the invention, shown
schematically by
Les propriétés mécaniques et la structure de
l'acier à l'issue de l'essai sont consignés dans le tableau
1 ci-dessous.
Une comparaison des résultats des exemples 1 à 3 montre que dans le procédé de l'art antérieur une structure biphasée stable de ferrite et de martensite nécessite une stabilisation de longue durée à la température de recuit (3 minutes), suivie d'un refroidissement préliminaire lent (14 secondes) avant la trempe. En l'absence de ce refroidissement préliminaire lent, la structure obtenue n'est pas stable ni reproductible ou contient une phase de perlite inopportune, préjudiciable aux propriétés mécaniques de la bande d'acier. Le procédé de l'invention permet d'obtenir les caractéristiques souhaitées en terme de stabilité de la microstructure et propriétés mécaniques, sans nécessiter une longue période de recuit, ni un refroidissement lent avant la trempe.A comparison of the results of Examples 1 to 3 shows that in the method of the prior art a structure stable biphasic ferrite and martensite requires a long-term stabilization at annealing temperature (3 minutes), followed by a slow preliminary cooling (14 seconds) before quenching. In the absence of this slow preliminary cooling, the resulting structure is not stable or reproducible or contains a phase of perlite untimely, detrimental to properties mechanical steel strip. The method of the invention allows to obtain the desired characteristics in term stability of the microstructure and mechanical properties, without requiring a long annealing period, nor a slow cooling before quenching.
Claims (16)
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| BE200200421 | 2002-07-02 | ||
| BE2002/0421A BE1015018A3 (en) | 2002-07-02 | 2002-07-02 | PROCESS FOR THE THERMAL TREATMENT OF A COLD ROLLED STEEL STRIP, PROCESS FOR MANUFACTURING A STEEL STRIP SUITABLE FOR CHEESE AND STEEL STRIP THUS OBTAINED. |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| EP1378577A1 true EP1378577A1 (en) | 2004-01-07 |
| EP1378577B1 EP1378577B1 (en) | 2008-12-03 |
Family
ID=29718746
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| EP03447154A Expired - Lifetime EP1378577B1 (en) | 2002-07-02 | 2003-06-17 | Process for heat treating cold rolled formable steel strip and steel strip thus obtained |
Country Status (5)
| Country | Link |
|---|---|
| EP (1) | EP1378577B1 (en) |
| AT (1) | ATE416265T1 (en) |
| BE (1) | BE1015018A3 (en) |
| DE (1) | DE60324989D1 (en) |
| ES (1) | ES2316713T3 (en) |
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2477664A (en) * | 2009-03-31 | 2011-08-10 | Kobe Steel Ltd | Cold rolled steel sheet with relationship between inclusion separation distance and yield strength |
| US8460800B2 (en) | 2009-03-31 | 2013-06-11 | Kobe Steel, Ltd. | High-strength cold-rolled steel sheet excellent in bending workability |
| EP2794936B1 (en) | 2011-12-22 | 2016-12-28 | ThyssenKrupp Rasselstein GmbH | Sheet steel for use as packaging steel and method for producing packaging steel |
| EP2794935B1 (en) | 2011-12-22 | 2017-01-11 | ThyssenKrupp Rasselstein GmbH | Easy-open lid for cans and method to produce an easy-open lid |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| CN113373388B (en) * | 2021-04-29 | 2022-08-05 | 宝鸡文理学院 | A method for improving the ductility and toughness of boron-containing eutectic alloys by using dual structure |
Citations (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2472021A1 (en) * | 1979-12-14 | 1981-06-26 | Nippon Kokan Kk | PROCESS FOR MANUFACTURING A COLD LAMINATED HIGH STRENGTH STEEL BAND HAVING EXCELLENT PRESS FORMABILITY |
| EP0053913A1 (en) * | 1980-12-04 | 1982-06-16 | Uss Engineers And Consultants, Inc. | Method for producing high-strength deep-drawable dual-phase steel sheets |
| JPS60100630A (en) | 1983-11-07 | 1985-06-04 | Kawasaki Steel Corp | Production of high-strength light-gage steel sheet having good ductility and bending workability |
| US5405463A (en) * | 1980-10-24 | 1995-04-11 | Nippon Kokan Kabushiki Kaisha | Continuous annealing process of producing cold rolled mild steel sheet excellent in deep drawability and aging resistibility |
| US5900082A (en) * | 1996-04-19 | 1999-05-04 | Naco, Inc. | Method of making a heat treated steel casting and a heat treated steel casting |
| US20010017172A1 (en) * | 1999-12-17 | 2001-08-30 | Maruta Kei-Ichi | Method for controlling structure of two-phase steel |
| WO2002000947A1 (en) * | 2000-06-29 | 2002-01-03 | Centre De Recherches Metallurgiques, Association Sans But Lucratif | Method for making a high resistance and high formability cole rolled steel strip |
-
2002
- 2002-07-02 BE BE2002/0421A patent/BE1015018A3/en not_active IP Right Cessation
-
2003
- 2003-06-17 EP EP03447154A patent/EP1378577B1/en not_active Expired - Lifetime
- 2003-06-17 DE DE60324989T patent/DE60324989D1/en not_active Expired - Lifetime
- 2003-06-17 AT AT03447154T patent/ATE416265T1/en not_active IP Right Cessation
- 2003-06-17 ES ES03447154T patent/ES2316713T3/en not_active Expired - Lifetime
Patent Citations (7)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR2472021A1 (en) * | 1979-12-14 | 1981-06-26 | Nippon Kokan Kk | PROCESS FOR MANUFACTURING A COLD LAMINATED HIGH STRENGTH STEEL BAND HAVING EXCELLENT PRESS FORMABILITY |
| US5405463A (en) * | 1980-10-24 | 1995-04-11 | Nippon Kokan Kabushiki Kaisha | Continuous annealing process of producing cold rolled mild steel sheet excellent in deep drawability and aging resistibility |
| EP0053913A1 (en) * | 1980-12-04 | 1982-06-16 | Uss Engineers And Consultants, Inc. | Method for producing high-strength deep-drawable dual-phase steel sheets |
| JPS60100630A (en) | 1983-11-07 | 1985-06-04 | Kawasaki Steel Corp | Production of high-strength light-gage steel sheet having good ductility and bending workability |
| US5900082A (en) * | 1996-04-19 | 1999-05-04 | Naco, Inc. | Method of making a heat treated steel casting and a heat treated steel casting |
| US20010017172A1 (en) * | 1999-12-17 | 2001-08-30 | Maruta Kei-Ichi | Method for controlling structure of two-phase steel |
| WO2002000947A1 (en) * | 2000-06-29 | 2002-01-03 | Centre De Recherches Metallurgiques, Association Sans But Lucratif | Method for making a high resistance and high formability cole rolled steel strip |
Non-Patent Citations (1)
| Title |
|---|
| PATENT ABSTRACTS OF JAPAN vol. 009, no. 241 (C - 306) 27 September 1985 (1985-09-27) * |
Cited By (6)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2477664A (en) * | 2009-03-31 | 2011-08-10 | Kobe Steel Ltd | Cold rolled steel sheet with relationship between inclusion separation distance and yield strength |
| GB2477664B (en) * | 2009-03-31 | 2012-01-18 | Kobe Steel Ltd | High-strength cold-rolled steel sheet excellent in bending workability |
| US8460800B2 (en) | 2009-03-31 | 2013-06-11 | Kobe Steel, Ltd. | High-strength cold-rolled steel sheet excellent in bending workability |
| EP2794936B1 (en) | 2011-12-22 | 2016-12-28 | ThyssenKrupp Rasselstein GmbH | Sheet steel for use as packaging steel and method for producing packaging steel |
| EP2794935B1 (en) | 2011-12-22 | 2017-01-11 | ThyssenKrupp Rasselstein GmbH | Easy-open lid for cans and method to produce an easy-open lid |
| EP2794936B2 (en) † | 2011-12-22 | 2019-10-02 | ThyssenKrupp Rasselstein GmbH | Sheet steel for use as packaging steel and method for producing packaging steel |
Also Published As
| Publication number | Publication date |
|---|---|
| ES2316713T3 (en) | 2009-04-16 |
| EP1378577B1 (en) | 2008-12-03 |
| ATE416265T1 (en) | 2008-12-15 |
| BE1015018A3 (en) | 2004-08-03 |
| DE60324989D1 (en) | 2009-01-15 |
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