EP1913169B1 - Manufacture of steel sheets having high resistance and excellent ductility, products thereof - Google Patents

Description

The invention relates to the manufacture of steel sheets, more particularly "TRIP" ("Transformation Induced Plasticity") steels, ie having a plasticity induced by an allotropic transformation.

In the automotive industry, there is a continuing need for vehicle lightening which results in the search for steels with increased yield strength or resistance. Thus, high strength steels having micro-alloy elements have been proposed. The hardening is obtained simultaneously by precipitation and by refining the grain size. In order to achieve even higher strength levels, TRIP steels have been developed which have advantageous combinations of properties (strength-ability to deform). These properties are related to the structure of these steels, consisting of a ferritic matrix comprising phases of bainite and residual austenite. In hot rolled sheets, the residual austenite is stabilized by increasing the content of elements such as silicon or aluminum, these elements delaying the precipitation of carbides in the bainite. TRIP steel cold-rolled sheet is produced by reheating during annealing in a field where austenitization occurs partially, followed by rapid cooling to avoid the formation of perlite and subsequent maintenance. isothermal in the bainitic domain: a part of the austenite is transformed into bainite, another part is stabilized by the increase of the carbon content of the islands of residual austenite. Thus, the initial presence of ductile residual austenite is associated with high deformability. Under the effect of a subsequent deformation, for example during stamping, the residual austenite of a TRIP steel part gradually changes to martensite, which results in a significant hardening. A steel with a TRIP behavior thus makes it possible to guarantee an aptitude significant deformation and high mechanical strength, both properties being usually antagonistic. This combination provides a high energy absorption potential, a quality typically sought in the automotive industry for shock-resistant parts.

Carbon plays an important role in the manufacture of TRIP steels: on the one hand, its presence in sufficient quantities within the islands of residual austenite is necessary for the local martensitic transformation temperature to be lowered below ambient temperature. On the other hand, it is usually added to increase strength economically. However, this addition of carbon must remain limited to ensure that the weldability of the products remains satisfactory: otherwise, the ductility of the welded joints and the cold cracking resistance are reduced. A manufacturing process is therefore sought to increase the strength of the TRIP steel sheets, in particular above about 900-1100 MPa for a carbon content of the order of 0.2% by weight without the total elongation. be reduced below a value of 18%. An increase in resistance of more than 100 MPa from current levels is desirable.

It also seeks a method of manufacturing hot or cold rolled steel sheet which is insensitive to small variations in industrial manufacturing conditions, particularly to temperature variations. It is thus sought to obtain a product characterized by a microstructure and mechanical properties that are insensitive to small variations in these manufacturing parameters. It is also sought to obtain a high tenacity product with excellent breaking strength.

The present invention aims to solve the problems mentioned above.

For this purpose, the subject of the invention is a composition for the manufacture of steel having a TRIP behavior, comprising, the contents being expressed by weight: 0.08% ≤ C ≤ 0.23%, 1% ≤ Mn ≤ 2 %, 1≤ If ≤ 2%, Al ≤ 0.030%, 0.1% ≤ V ≤ 0.25%, Ti ≤ 0.010%, S ≤ 0.015%, P ≤ 0.1%, 0.004% ≤ N ≤ 0.012% , and optionally one or more selected elements among: Nb ≤ 0.1%, Mo ≤ 0.5%, Cr ≤ 0.3%, the remainder of the composition consisting of iron and unavoidable impurities resulting from the preparation. Preferably, the carbon content is such that: 0.08% ≤ C ≤ 0.13%. According to a preferred variant, the carbon content is such that: 0.13% <C ≤ 0.18%.

Preferentially, the carbon content is such that: 0.18% <C ≤ 0.23%.

Preferably, the manganese content is such that: 1.4% ≤ Mn ≤ 1.8%. Preferentially, the manganese content satisfies: 1.5% ≤ Mn s 1.7%.

Preferably, the silicon content is such that: 1.4% ≤ Si ≤ 1.7%. Preferably, the aluminum content satisfies: Al ≤ 0.015%.

According to a preferred mode, the vanadium content is such that: 0.12% ≤V ≤ 0.15%.

Preferentially, the titanium content is such that: Ti ≤ 0.005%. The invention also relates to a steel sheet of the above composition, the microstructure of which consists of ferrite, bainite, residual austenite, and possibly martensite.

According to a preferred embodiment, the microstructure of the steel comprises a residual austenite content of between 8 and 20%.

The microstructure of the steel preferably comprises a martensite content of less than 2%.

Preferably, the average size of the residual austenite islands is less than or equal to 2 microns.

The average size of the residual austenite islands is preferably less than or equal to 1 micrometer.

• on approvisionne un acier selon l'une quelconque des compositions ci-dessus,
• on procède à la coulée d'un demi-produit à partir de cet acier,
• on porte ledit demi-produit à une température supérieure à 1200°C,
• on lamine à chaud le demi-produit,
• on refroidit la tôle ainsi obtenue,
• on bobine la tôle, la température T_fl de fin du laminage à chaud, la vitesse V_r du refroidissement, la température de bobinage T_bob étant choisies de telle sorte que la microstructure de l'acier soit constituée de ferrite, de bainite, d'austénite résiduelle, et éventuellement de martensite.

The invention also relates to a method for manufacturing a hot-rolled sheet exhibiting a TRIP behavior, according to which:

• a steel is supplied according to any one of the above compositions,
• a semi-finished product is cast from this steel,
• said semi-finished product is heated to a temperature above 1200 ° C,
• the semi-finished product is hot-rolled,
• the sheet thus obtained is cooled,
• the sheet is wound, the end temperature T _fl of the hot rolling, the speed V _r of the cooling, the winding temperature T _bob being chosen so that the microstructure of the steel consists of ferrite, bainite, d residual austenite, and possibly martensite.

Preferably, the temperature T _fl at the end of the hot rolling, the speed V _r of the cooling, the temperature T _bob of the winding are chosen such that the microstructure of the steel comprises a residual austenite content of between 8 and 20%. .

Also preferably, the temperature T _fl of the hot rolling end, the cooling speed V _r , the winding temperature T _bob are chosen such that the microstructure of the steel comprises a martensite content of less than 2%.

Preferably, the temperature T _fl of the hot rolling end, the cooling speed V _r , the winding temperature T _bob are chosen such that the average size of the residual austenite islands is less than or equal to 2 microns. and very preferably less than 1 micrometer.

• on lamine à chaud le demi-produit jusqu'à une température de fin de laminage T_fl supérieure ou égale à 900°C,
• on refroidit la tôle ainsi obtenue avec une vitesse de refroidissement V_r supérieure ou égale à 20°C/s,
• on bobine la tôle à une température T_bob inférieure à 450°C. Préférentiellement, la température de bobinage T_bob est inférieure à 400°C. L'invention a également pour objet un procédé de fabrication d'une tôle laminée à froid présentant un comportement TRIP, selon lequel on approvisionne une tôle d'acier laminé à chaud fabriquée selon l'un quelconque des procédés décrits ci-dessus, on décape la tôle, on lamine à froid la tôle, on fait subir à la tôle un traitement thermique de recuit, le traitement thermique comprenant une phase de chauffage à une vitesse de chauffage V_cm, une phase de maintien à une température de maintien T_m pendant un temps de maintien t_m, suivie d'une phase de refroidissement à une vitesse de refroidissement V_rm lorsque la température est inférieure à Ar3, suivie d'une phase de maintien à une température de maintien T'_m pendant un temps de maintien t'_m, les paramètres V_cm, T_m, t_m, V_rm, T'_m, t'_m étant choisis de telle sorte que la microstructure dudit acier soit constituée de ferrite, de bainite, d'austénite résiduelle, et éventuellement de martensite.

The invention also relates to a method for manufacturing a hot-rolled sheet exhibiting a TRIP behavior, according to which:

• the semi-finished product is hot-rolled to a rolling end temperature T _fl greater than or equal to 900 ° C.,
• the sheet thus obtained is cooled with a cooling rate V _r greater than or equal to 20 ° C / s,
• the sheet is reeled at a temperature T _bob lower than 450 ° C. Preferably, the winding temperature T _bob is less than 400 ° C. The subject of the invention is also a process for manufacturing a cold-rolled sheet exhibiting a TRIP behavior, according to which a hot-rolled steel sheet manufactured according to any of the processes described above is supplied, the sheet, the sheet is cold-rolled, the sheet is subjected to an annealing heat treatment, the heat treatment comprising a heating phase at a heating rate V _cm , a holding phase at a holding temperature T _m during a holding time t _m , followed by a cooling phase at a cooling rate V _rm when the temperature is lower than Ar3, followed by a holding phase at a holding temperature T ' _m during a holding time t ' _m , the parameters V _cm , T _m , t _m , V _rm , T' _m , t ' _m being chosen such that the microstructure of said steel consists of ferrite, bainite, residual austenite, and possibly of martensite.

According to a preferred embodiment, the parameters V _cm , T _m , t _m , V _rm , T ' _m , t' _m are chosen such that the microstructure of the steel comprises a residual austenite content of between 8 and 20%. .

Preferentially, the parameters V _cm , T _m , tm, V _rm , T ' _m , t' _m are chosen such that the microstructure of the steel comprises less than 2% of martensite.

According to a preferred embodiment, the parameters V _cm , T _m , t _m , V _rm , T ' _m , t' _m are chosen such that the average size of the residual austenite islands is less than 2 micrometers, very preferably less than at 1 micrometer. The subject of the invention is also a process for manufacturing a cold-rolled sheet exhibiting a TRIP behavior, according to which the sheet is subjected to an annealing heat treatment, the heat treatment comprising a heating phase at a speed V _cm. greater than or equal to 2 ° C / s, a holding phase at a holding temperature T _m between Ac1 and Ac3 for a holding time t _m of between 10 and 200s, followed by a cooling phase at a speed of cooling V _rm greater than 15 ° C / s when the temperature is lower than Ar3, followed by a holding phase at a temperature T ' _m between 300 and 500 ° C for a holding time t' _m between 10 and 1000 s.

The holding temperature T _m is preferably between 770 and 815 ° C.

The invention also relates to the use of a steel sheet having a TRIP behavior, according to one of the variants described above, or manufactured by one of the processes described above, for the manufacture of parts of structure or reinforcement elements in the automotive field.

Other features and advantages of the invention will become apparent from the description below, given by way of example.

With regard to the chemical composition of steel, carbon plays a very important role in the formation of the microstructure and the mechanical properties: According to the invention, a bainitic transformation takes place from an austenitic structure formed at high temperature , and bainitic ferrite slats are formed. Given the much lower solubility of carbon in ferrite compared to austenite, the carbon of the austenite is rejected between the slats. Thanks to certain alloying elements of the steel composition according to the invention, in particular silicon and manganese, the precipitation of carbides, in particular of cementite, intervenes very little. Thus, the austenite interlatte is progressively enriched in carbon without the precipitation of carbides intervening. This enrichment is such that the austenite is stabilized, that is to say that the martensitic transformation of this austenite does not occur during cooling to room temperature. According to the invention, the carbon content is between 0.08 and 0.23% by weight. Preferably, the carbon content is in a first range of 0.08 to 0.13% by weight. In a second preferred range, the carbon content is greater than 0.13% and is less than or equal to 0.18% by weight. The carbon content is in a third preferred range, where it is greater than 0.18 and less than or equal to 0.23% by weight.

Since carbon is a particularly important element for curing, the minimum carbon content of each of the three preferred ranges makes it possible to obtain a minimum strength of 600 MPa, 800 MPa and 950 MPa on cold-rolled and annealed sheets, respectively at each of the three preferred ranges. beaches above. The maximum carbon content of each of three ranges makes it possible to guarantee satisfactory weldability, particularly in spot welding, if the level of resistance obtained in these three preferred ranges is taken into account.

In an amount of between 1 and 2% by weight, an addition of manganese, a gammagenic element, contributes to reducing the martensitic transformation start temperature Ms and stabilizing the austenite. This addition of manganese also contributes to an effective hardening in solid solution and thus to obtaining increased strength. Manganese is preferentially included between 1.4 and 1.8% by weight: in this way a satisfactory hardening and an increase in the stability of the austenite are combined without, however, excessively increasing the quenchability in the welded joints. Optimally, the manganese content is between 1.5 and 1.7% by weight. In this way, the effects sought above are obtained without risk of formation of a harmful band structure that would come from a possible segregation of manganese during solidification.

In an amount of between 1 and 2% by weight, silicon inhibits the precipitation of cementite during cooling from austenite by considerably retarding the growth of carbides: this is due to the fact that the solubility of silicon in cementite is very low. low and that this element increases the carbon activity in the austenite. In this way, a possible cementite seed forming will be surrounded by a silicon-rich austenitic zone which has been rejected at the precipitate-matrix interface. This silicon-enriched austenite is also richer in carbon and the growth of cementite is slowed down because of the small diffusion resulting from the reduced carbon gradient between the cementite and the surrounding austenitic zone. This addition of silicon thus contributes to stabilizing a sufficient amount of residual austenite to obtain a TRIP effect. In addition, this addition of silicon makes it possible to increase the resistance thanks to hardening in solid solution. However, an excessive addition of silicon causes the formation of strongly adherent oxides, which are difficult to eliminate during a stripping operation, and the possible appearance of surface defects due in particular to a lack of wettability in dip galvanizing operations. In order to obtain the stabilization of a sufficient amount of austenite while reducing the risk of surface defects, the silicon content is preferably between 1.4 and 1.7% by weight.

Aluminum is a very effective element for the deoxidation of steel. Like silicon, it is very slightly soluble in cementite and could be used as such to prevent the precipitation of cementite during maintenance at a bainitic transformation temperature and stabilize the austenite residual. However, according to the invention, the aluminum content is less than or equal to 0.030% by weight: in fact, as will be seen below, a very effective hardening is obtained by means of a precipitation of vanadium carbonitrides: when the aluminum content is greater than 0.030%, there is a risk of precipitation of aluminum nitride which reduces by the same amount of nitrogen capable of precipitating with vanadium. Preferably, when this amount is less than or equal to 0.015% by weight, any risk of precipitation of aluminum nitride is discarded and the full effect of hardening by the precipitation of vanadium carbonitrides is obtained.

For the same reason, the titanium content is less than or equal to 0.010% by weight in order not to precipitate a significant amount of nitrogen in the form of titanium nitrides or carbonitrides. Given the high affinity of titanium for nitrogen, the titanium content is preferably less than or equal to 0.005% by weight. Such a titanium content then makes it possible to avoid the precipitation of (Ti, V) N on hot-rolled sheets.

Vanadium and nitrogen are important elements of the invention: The inventors have demonstrated that, when these elements are present in defined amounts according to the invention, they precipitate in the form of very fine vanadium carbonitrides associated with hardening. important. When the vanadium content is less than 0.1% by weight or when the nitrogen content is less than 0.004% by weight, the precipitation of vanadium carbonitrides is limited and curing is insufficient. When the vanadium content is greater than 0.25% by weight or when the nitrogen content is greater than 0.012% by weight, the precipitation occurs at an early stage after hot rolling in the form of coarser precipitates. The size of these precipitates does not make it possible to take full advantage of the potential hardening of vanadium, especially when it is aimed at producing a cold-rolled and annealed steel sheet. In the latter case, the inventors have demonstrated that it is necessary to limit the precipitation of vanadium to the hot rolling step in order to make the most of a fine hardening precipitation during a subsequent annealing. In addition, limiting the precipitation of vanadium at this stage reduces efforts necessary during the subsequent cold rolling and therefore to make the best use of the performance of industrial installations.

When the vanadium content is between 0.12 and 0.15% by weight, the uniform or breaking elongation is particularly increased.

In an amount greater than 0.015% by weight, sulfur tends to precipitate excessively in the form of manganese sulfides which greatly reduce the formability.

Phosphorus is a known element to segregate at grain boundaries. Its content shall be limited to 0.1% by weight so as to maintain sufficient hot ductility and to promote breakage by peeling during tensile-shear tests carried out on spot-welded joints.

Optionally, elements such as chromium and molybdenum that delay bainitic transformation and promote hardening by solid solution, can be added in amounts of less than or equal to 0.3 or 0.5% by weight, respectively. Niobium may also optionally be added in an amount of less than or equal to 0.1% by weight so as to increase the resistance by additional precipitation of carbonitrides.

• On approvisionne un acier de composition selon l'invention
• On procède à la coulée d'un demi-produit à partir de cet acier. Cette coulée peut être réalisée en lingots ou en continu sous forme de brames d'épaisseur de l'ordre de 200mm. On peut également effectuer la coulée sous forme de brames minces de quelques dizaines de millimètres d'épaisseur ou de bandes minces entre cylindres d'acier contra-rotatifs.

The method of manufacturing a hot-rolled sheet according to the invention is implemented as follows:

• A composition steel is supplied according to the invention
• A semi-finished product is cast from this steel. This casting can be carried out in ingots or continuously in the form of slabs of the order of 200mm thickness. It is also possible to perform the casting in the form of slabs of a few tens of millimeters thick or thin strips between contra-rotating steel rolls.

The cast semifinished products are first brought to a temperature above 1200 ° C. in order to attain at all points a temperature favorable to the high deformations which the steel will undergo during rolling as well as to avoid at this stage of manufacture the presence of vanadium carbonitrides. Naturally, in the case of a direct casting of thin slabs or thin strips between contra-rotating rolls, the step of hot rolling of these semi-products starting at more than 1200 ° C. can be done directly after so that an intermediate reheat step is not then necessary. As will be seen, this minimum temperature of 1200 ° C. also makes it possible to perform hot rolling in a completely austenitic phase under satisfactory conditions on a continuous hot rolling mill.

The semi-finished product is hot-rolled up to a rolling end temperature T _f equal to or greater than 900 ° C. In this way, the rolling is entirely carried out in the austenitic phase where the solubility of the vanadium carbonitrides is greater and where the probability of precipitation of V (CN) is the lowest. For the same reason, the sheet thus obtained is then cooled with a cooling rate V _r greater than or equal to 20 ° C./s in order to avoid a precipitation of the vanadium carbonitrides in the ferrite. This cooling can be carried out for example by means of spraying water on the sheet.

If it is desired to manufacture a hot-rolled sheet according to the invention, the sheet obtained is then reeled at a temperature of less than or equal to 450 ° C. In this way, the quasi-isothermal maintenance associated with this winding leads to the formation of a microstructure consisting of bainite, ferrite, residual austenite, possibly a small amount of martensite, as well as a hardening precipitation of vanadium carbonitrides. When the winding temperature is less than or equal to 400 ° C, the total elongation and the distributed elongation are increased.

Are more particularly chosen T _fl end temperature of hot rolling, the cooling rate V _r and the winding _bob temperature T such that the microstructure comprises a residual austenite content of between 8 and 20%: When the amount residual austenite is less than 8%, a sufficient TRIP effect can not be demonstrated during mechanical tests: in particular, it is demonstrated during tensile tests that the coefficient of hardening n is less than 0 , 2 and decreases rapidly with the deformation ε. The criterion of Consider applies to these steels and the rupture occurs when n = ε _true , the elongation is thus strongly limited. In the case of TRIP behavior, residual austenite is progressively transforms into martensite during deformation, n is greater than 0.2 and necking appears for larger deformations. When the residual austenite content is greater than 20%, the residual austenite formed under these conditions has a relatively low carbon content and is destabilized too easily during a subsequent phase of deformation or cooling.

• La vitesse de refroidissement V_r sera choisie de façon à être la plus rapide possible pour éviter une transformation perlitique (ce qui irait à l'encontre de l'obtention d'une teneur en austénite résiduelle comprise entre 8 et 20%) tout en restant au sein des capacités de contrôle d'une ligne industrielle de façon à obtenir une homogénéité microstructurale dans le sens longitudinal et transversal de la tôle laminée à chaud.
• La température de bobinage sera choisie suffisamment basse de façon à éviter une transformation perlitique, ce qui se traduirait par une transformation bainitique incomplète et une teneur en austénite résiduelle inférieure à 8%.

Among the parameters T _fl , V _r , T _bob selected to obtain a residual austenite amount of between 8 and 20%, the parameters V _r , T _bob are the most important:

• The cooling rate V _r will be chosen so as to be as rapid as possible in order to avoid a pearlitic transformation (which would be contrary to obtaining a residual austenite content of between 8 and 20%) while remaining within the control capabilities of an industrial line so as to obtain a microstructural homogeneity in the longitudinal and transverse direction of the hot-rolled sheet.
• The winding temperature will be chosen sufficiently low so as to avoid a pearlitic transformation, which would result in an incomplete bainitic transformation and a residual austenite content of less than 8%.

• La vitesse de refroidissement V_r, qui doit être la plus rapide possible pour éviter une transformation perlitique, tout en évitant que ce refroidissement ne conduise à une température inférieure à M_s, cette dernière température désignant la température de début de transformation martensitique, caractéristique de la composition chimique de l'acier mis en oeuvre.
• Pour la même raison, on choisira une température de bobinage supérieure à M_s.
• On choisira aussi préférentiellement les paramètres T_fl, V_r, T_bob, de telle sorte que la taille moyenne des îlots d'austénite résiduelle de la microstructure soit inférieure ou égale à 2 micromètres. En effet, lorsque l'austénite se transforme en martensite sous l'influence de l'abaissement de la température ou d'une déformation, les îlots de martensite de taille moyenne supérieure à 2 micromètres jouent un rôle préférentiel pour l'endommagement par suite d'une décohésion avec la matrice.
• A titre préférentiel, on choisira encore plus particulièrement les paramètres T_fl, V_r, T_bob, de telle sorte que la taille moyenne des îlots d'austénite résiduelle de la microstructure soit inférieure ou égale à 1 micromètre afin d'augmenter leur stabilité, de limiter l'endommagement à l'interface matrice-îlots et de repousser la striction vers des valeurs de déformation plus élevées.

The parameters T _f , V _r , T _bob will preferably be chosen so that the microstructure of the hot-rolled steel sheet contains less than 2% of martensite. In the opposite case, the elongation is reduced as well as the absorption energy related to the area under the traction curve (σ-ε). The excessive presence of martensite leads to a mechanical behavior approaching that of a Dual-Phase steel with an initial value of the coefficient of hardening n high decreasing when the rate of deformation increases. Optimally, the microstructure does not contain martensite. Among the parameters T _fl , V _r , T _bob chosen in order to obtain a martensite content of less than 2%, the most important parameters are:

• The cooling rate V _r , which must be as fast as possible to avoid a pearlitic transformation, while avoiding that this cooling leads to a temperature below M _s , the latter temperature designating the start temperature of martensitic transformation, characteristic of the chemical composition of the steel used.
• For the same reason, we will choose a winding temperature greater than M _s .
• The parameters T _f , V _r , T _bob will also preferably be chosen so that the average size of the residual austenite islands of the microstructure is less than or equal to 2 micrometers. Indeed, when the austenite is transformed into martensite under the influence of the lowering of the temperature or of a deformation, the islands of martensite of average size superior to 2 micrometers play a preferential role for the damage as a result of decohesion with the matrix.
• As a preference, the parameters T _f , V _r , T _bob will be chosen even more particularly, so that the average size of the residual austenite islands of the microstructure is less than or equal to 1 micrometer in order to increase their stability. to limit the damage to the matrix-island interface and to push the necking towards higher deformation values.

• Une température T_fl de fin de laminage dans le domaine austénitique pas trop élevée de façon à obtenir une taille de grain austénitique relativement fine avant transformation allotropique.
• La vitesse de refroidissement V_r la plus rapide possible pour éviter une transformation perlitique.

In order to obtain a fine size of islands of residual austenite, one will choose:

• A rolling end temperature T _fl in the austenitic range not too high so as to obtain a relatively fine austenitic grain size before allotropic transformation.
• The cooling rate V _r as fast as possible to avoid a pearlitic transformation.

In order to manufacture a cold-rolled sheet according to the invention, a hot-rolled sheet is first produced according to one of the variants which have been explained above. Indeed, the inventors have found that the microstructures and the mechanical properties obtained by the cold rolling and annealing manufacturing process which will be exposed, depend relatively little on the manufacturing conditions within the limits of the process variants set out above. , in particular variations of the winding temperature T _bob . In this way, the sheet metal manufacturing process Cold rolled has the advantage of being insensitive to unforeseen variations in the manufacturing conditions of hot-rolled sheets.

As a preference, however, a winding temperature of less than or equal to 400 ° C. will be chosen so as to keep more vanadium in solid solution available for precipitation during the subsequent annealing of the cold-rolled sheet.

The hot-rolled sheet is scraped by a method known per se so as to give it a surface state suitable for cold rolling. The latter is carried out under customary conditions, for example by reducing the thickness of the hot-rolled sheet by 30 to 75%.

• Une phase de chauffage avec une vitesse V_cm supérieure ou égale à 2°C/s jusqu'à une température T_m située dans le domaine intercritique, c'est à dire une température située entre les températures de transformation A_c1 et A_c3 : Lors de cette phase, on observe une recristallisation de la structure écrouie, une dissolution de la cémentite et une croissance de l'austénite au delà de la température de transformation A_c1 ainsi qu'une précipitation de carbonitrures de vanadium dans la ferrite : ces précipités sont de très petite taille, de diamètre typiquement inférieur à 5 nanomètres à l'issue de cette phase de chauffage.

A clean annealing treatment is then carried out to recrystallize the work-hardened structure and to confer the particular microstructure according to the invention. This treatment, preferably carried out by continuous annealing, comprises the following successive phases:

• A heating phase with a velocity V _cm greater than or equal to 2 ° C / s to a temperature T _m located in the intercritical range, ie a temperature situated between the transformation temperatures A _c1 and A _c3 : During this phase, recrystallization of the hardened structure, dissolution of the cementite and growth of the austenite beyond the transformation temperature A _c1 and precipitation of vanadium carbonitrides in the ferrite are observed: these precipitates are of very small size, of diameter typically less than 5 nanometers at the end of this heating phase.

• Une phase de maintien à une température intercritique T_m comprise entre A_c1 et A_c3 pendant un temps t_m compris entre 10s et 200s. Dans ces conditions bien définies, les inventeurs ont mis en évidence que la précipitation de carbonitrures de vanadium se poursuivait dans la ferrite pratiquement sans aucune précipitation dans la phase austénitique nouvellement formée. La fraction volumique de précipités s'accroît parallèlement à une augmentation du diamètre moyen de ces précipités. De la sorte, on obtient un durcissement particulièrement efficace de la ferrite intercritique.

When the heating rate is less than 2 ° C / sec, the volume fraction of precipitated vanadium decreases. In addition, the productivity of manufacturing is reduced excessively.

• A maintenance phase at an intercritical temperature T _m between A _c1 and A _c3 for a time t _m between 10s and 200s. Under these well-defined conditions, the inventors have demonstrated that the precipitation of vanadium carbonitrides continued in the ferrite virtually without any precipitation in the newly formed austenitic phase. The volume fraction of precipitates increases in parallel with an increase in the average diameter of these precipitates. Of In this way, a particularly effective hardening of the intercritical ferrite is obtained.

Then a rapid cooling is carried out at a speed V _rm greater than 15 ° C / s when the temperature is lower than Ar3. Rapid cooling when the temperature is lower than Ar3 is important in order to limit the formation of ferrite before the bainitic transformation. This rapid cooling phase when the temperature is lower than Ar 3 may be preceded, if necessary, by a slower cooling phase from the temperature T _m .

During this cooling phase, the inventors have demonstrated that a complementary precipitation of vanadium carbonitrides in the ferritic phase was practically not involved.

It is then carried out at a temperature T ' _m of between 300 ° C. and 500 ° C. for a hold time t' _m of between 10 s and 1000 s: a bainitic transformation and a carbon enrichment of the islets are thus obtained. residual austenite in such an amount that this residual austenite is stable even after cooling to room temperature.

Preferably, the holding temperature T _m is between 770 and 815 ° C: below 770 ° C, the recrystallization may be insufficient. Beyond 815 ° C, the fraction of intercritical austenite formed is too large and the hardening of ferrite by the precipitation of vanadium carbonitrides is less effective: in fact, the intercritical ferrite content is less and the total amount of vanadium precipitates, vanadium being rather soluble in the austenite. On the other hand, vanadium carbonitride precipitates that form are more likely to grow and coalesce at high temperatures.

According to a preferred embodiment of the invention, after the cold rolling step, the sheet is subjected to a heat treatment annealing whose parameters V _cm , T _m , tm, V _rm , T ' _m , t' _m are chosen so that the microstructure of the steel obtained consists of ferrite, bainite and residual austenite, possibly martensite. Advantageously, parameters will be chosen such that the residual austenite content is between 8 and 20%. These Parameters will preferably be chosen such that the average size of the residual austenite islands is less than or equal to 2 micrometers, optimally less than or equal to 1 micrometer. These parameters will also be chosen so that the martensite content is less than 2%. Optimally, the microstructure does not include martensite.

• T_m, température située dans le domaine intercritique entre les températures de transformation A_c1 (début de transformation austénitique) et A_c3 (fin de transformation austénitique), doit être choisie de façon à obtenir au moins 8% d'austénite formée à haute température. Cette condition est nécessaire pour que la structure après refroidissement contienne au moins 8% d'austénite résiduelle. La température T_m ne doit cependant pas être trop proche de A_c3 pour éviter un grossissement du grain austénitique à haute température, entraînant par la suite une taille trop importante des îlots d'austénite résiduelle.
• Le temps t_m doit être choisi suffisamment long pour que la transformation partielle en austénite ait le temps d'intervenir.
• La vitesse de refroidissement V_rm doit être suffisamment rapide pour éviter la formation de perlite, celle-ci ne permettant pas d'obtenir les résultats visés ci-dessus.
• La température T'_m sera choisie de façon à ce que la transformation de l'austénite formée lors du maintien à la température T_m, soit une transformation bainitique, et conduise à un enrichissement en carbone suffisant pour que cette austénite formée à haute température soit stabilisée dans une quantité comprise entre 8 et 20%.

In order to obtain these results, the choice of the parameters T _m , t _m , V _rm , T ' _m is more particularly important:

• T _m , temperature in the intercritical range between transformation temperatures A _c1 (austenitic transformation start) and A _c3 (end of austenitic transformation), must be chosen to obtain at least 8% of austenite formed at high temperature . This condition is necessary for the structure after cooling to contain at least 8% residual austenite. However, the temperature T _m must not be too close to A _c3 to avoid a magnification of the austenitic grain at high temperature, which subsequently causes the islands of residual austenite to be too large.
• The time t _m must be chosen long enough for the partial transformation into austenite to have time to intervene.
• The cooling rate V _rm must be fast enough to prevent the formation of perlite, which does not make it possible to obtain the results referred to above.
• The temperature T ' _m will be chosen so that the transformation of the austenite formed during the maintenance at the temperature T _m , is a bainitic transformation, and lead to a sufficient carbon enrichment so that this austenite formed at high temperature is stabilized in an amount of between 8 and 20%.

By way of non-limiting example, the following results will show the advantageous characteristics conferred by the invention.

Example 1

Steels have been developed, the composition of which is given in the table below, expressed in percentage by weight. In addition to the steels I1 to I3 according to the invention, the composition of a reference steel R1 has been indicated for comparison: Table 1 Compositions of steel (% by weight). I = according to the invention. R = reference Steel VS mn Yes al V Ti S P NOT I1 0.223 1.58 1.59 <0.030 0,100 0,002 <0.005 <0.030 0,008 I2 0,225 1.58 1.60 <0.030 0.155 0,002 <0.005 <0.030 0,009 I3 0,225 1.58 1.60 <0.030 0.209 0,002 <0.005 <0.030 0,009 R1 0.221 1.60 1.59 <0.030 0.005 (*) 0,002 <0.005 <0.030 0.001 (*) (*): Not in accordance with the invention

Semi-products corresponding to the above compositions were heated to 1200 ° C and hot rolled so that the rolling temperature was above 900 ° C. The sheets of 3 mm thus obtained were cooled with a speed of 20 ° C / s by water spray, and then wound at a temperature of 400 ° C. The tensile mechanical properties obtained (yield strength Re, resistance Rm, uniform elongation Au, elongation at break At) were given in Table 2 below. The ductile-brittle transition temperature was also determined using Charpy V type specimens of reduced thickness (e = 3mm). The residual austenite content measured by X-ray diffraction was also reported. Table 2: Mechanical tensile properties, transition temperature and residual austenite content of hot-rolled sheets. Steel Re (MPa) Rm (MPa) At (%) At (%) Transition temperature (° C) Residual austenite content (%) I1 731 884 13 22 nd nd I2 724 891 26 38 -35 nd I3 755 916 24 36 nd 10.8 R1 615 793 14 28 0 <1% na: Not determined.

The sheets manufactured according to the invention have a very high strength, significantly higher than 800 MPa for a carbon content of about 0.22%. Their microstructure is composed of ferrite, bainite and residual austenite, as well as martensite in an amount of less than 2%. In the case of steel 13 (residual austenite content: 10.8%), the carbon concentration of the residual austenite islets is 1.36% by weight. This indicates that the austenite is sufficiently stable to obtain a TRIP effect as shown by the behavior observed during the tensile tests performed on these steel sheets.

The reference steel sheet R1 of bainito-pearlitic structure, with a very low residual austenite content, does not exhibit TRIP behavior. Its resistance is less than 800 MPa, a level significantly lower than that of the steels of the invention.

I2 steel according to the invention also has excellent toughness since its ductile-brittle transition temperature is significantly lower (-35 ° C) than that of a reference steel (0 ° C).

Example 2

3 mm thick hot-rolled sheets of steels of compositions I2 and R1 made according to example 1 were cold-rolled to a thickness of 0.9 mm. An annealing heat treatment was then carried out comprising a heating phase at a rate of 5 ° C./s, a holding phase at a holding temperature T _m of between 775 and 815 ° C. (temperatures in the Ac1- Ac3) for a holding time of 180s, followed by a first cooling phase at 6-8 ° C / s, followed by cooling at 20 ° C / s in a range where the temperature is lower than Ar3, d a hold phase at 400 ° C for 300s to form bainite, and a final cooling at 5 ° C / s.

The resulting microstructure was observed after Klemm reagent attack highlighting residual austenite islands and the average size of these islets was measured using image analysis software.

In the case of the reference steel R1, the average size of the islands is 1.1 microns. In the case of the steel according to the invention 12, the microstructure general is thinner with an average island size of 0.7 micron. In addition, these islets have a more even character. In the case of steel 12, these characteristics particularly reduce the stress concentrations at the matrix-island interface.

The mechanical properties after cold rolling and annealing are as follows: Table 3: Mechanical tensile characteristics of cold-rolled and annealed sheets. Steel Holding temperature T _m Re (MPa) Rm (MPa) At (%) I2 775 630 1000 25 795 658 980 28 815 650 938 26 R1 775 480 830 nd 795 480 820 30 815 470 820 30 na: Not determined.

The steel I2 manufactured according to the invention has a resistance greater than 900 MPa. At a maintenance temperature T _m comparable, its resistance is significantly increased compared to the reference steel.

The cold-rolled and annealed steels according to the invention have mechanical properties which are not very sensitive to small variations in certain manufacturing parameters such as the winding temperature or the annealing temperature T _m .

Thus, the invention allows the manufacture of steels with a TRIP behavior with increased mechanical strength. Parts made from steel sheets according to the invention are used with advantage for the manufacture of structural parts or reinforcement elements in the automotive field.

Claims (30)

1. Composition for the manufacture of steel exhibiting TRIP behaviour, comprising, the contents being expressed by weight: $$0.08\%\le \mathrm{C}\le 0.23\%$$

   

   $$1\%\le \mathrm{Mn}\le 2\%$$

   

   $$1\le \mathrm{Si}\le 2\%$$

   

   $$\mathrm{Al}\le 0.030\%$$

   

   $$0.1\%\le \mathrm{V}\le 0.25\%$$

   

   $$\mathrm{Ti}\le 0.010\%$$

   

   $$\mathrm{S}\le 0.015\%$$

   

   $$\mathrm{P}\le 0.1\%$$

   

   $$0.004\%\le \mathrm{N}\le 0.012\%,$$

   

   
   and, optionally, one or more elements chosen from: $$\mathrm{Nb}\le 0.1\%$$

   

   $$\mathrm{Mo}\le 0.5\%$$

   

   $$\mathrm{Cr}\le 0.3\%,$$

   

   
   the balance of the composition consisting of iron and inevitable impurities resulting from the smelting.
2. Composition according to claim 1, characterized in that it comprises the content being expressed by weight: $$0.08\%\le \mathrm{C}\le 0.13\%\mathrm{.}$$

   
3. Composition according to claim 1, characterized in that it comprises, the content being expressed by weight: $$0.13\%<\mathrm{C}\le 0.18\%\mathrm{.}$$

   
4. Composition according to claim 1, characterized in that it comprises, the content being expressed by weight: $$0.18\%<\mathrm{C}\le 0.23\%\mathrm{.}$$

   
5. Composition according to any one of claims 1 to 4, characterized in that it comprises, the content being expressed by weight: $$1.4\%\le \mathrm{Mn}\le 1.8\%\mathrm{.}$$

   
6. Composition according to any one of claims 1 to 5, characterized in that it comprises, the content being expressed by weight: $$1.5\%\le \mathrm{Mn}\le 1.7\%\mathrm{.}$$

   
7. Composition according to any one of claims 1 to 6, characterized in that it comprises, the content being expressed by weight: $$1.4\%\le \mathrm{Si}\le 1.7\%\mathrm{.}$$

   
8. Composition according to claim any one of claims 1 to 7, characterized in that it comprises, the content being expressed by weight: $$\mathrm{Al}\le 0.015\%\mathrm{.}$$

   
9. Composition according to claim any one of claims 1 to 8, characterized in that it comprises, the content being expressed by weight: $$0.12\%\le \mathrm{V}\le 0.15\%\mathrm{.}$$

   
10. Composition according to claim any one of claims 1 to 9, characterized in that it comprises, the content being expressed by weight: $$\mathrm{Ti}\le 0.005\%\mathrm{.}$$

    
11. Sheet of steel of composition according to any one of claims 1 to 10, characterized in that the microstructure of said steel consists of ferrite, bainite, residual austenite and, optionally, martensite.
12. Sheet of steel according to claim 11, characterized in that the microstructure of said steel has a residual austenite content of between 8 and 20%.
13. Sheet of steel according to any one of claims 11 or 12, characterized in that the microstructure of said steel has a martensite content of less than 2%.
14. Sheet of steel according to any one of claims 11 to 13, characterized in that the mean size of the residual austenite islands does not exceed 2 microns.
15. Sheet of steel according to any one of claims 11 to 14, characterized in that the mean size of the residual austenite islands does not exceed 1 micron.
16. Process for manufacturing a hot-rolled sheet exhibiting TRIP behaviour, in which:

    - a steel of composition according to any one of claims 1 to 10 is supplied;

    - a semi-finished product is cast from this steel;

    - said semi-finished product is raised to a temperature above 1200°C;

    - said semi-finished product is hot-rolled;

    - the sheet thus obtained is cooled;

    - said sheet is coiled,

    - characterized in that the temperature T_er of the end of said hot rolling, the rate V_c of said cooling and the temperature T_coil of said coiling are chosen in such a way that the microstructure of said steel consists of ferrite, bainite, residual austenite and, optionally, martensite.
17. Process according to claim 16, characterized in that the temperature T_er of the end of said hot rolling, the rate V_c of said cooling and the temperature T_coil of said coiling are chosen in such a way that the microstructure of said steel has a residual austenite content of between 8 and 20%.
18. Process according to claim 16 or 17, characterized in that the temperature T_er of the end of said hot rolling, the rate V_c of said cooling and the temperature T_coil of said coiling are chosen in such a way that the microstructure of said steel has a martensite content of less than 2%.
19. Process according to any one of claims 16 to 18, characterized in that the temperature T_er of the end of said hot rolling, the rate V_c of said cooling and the temperature T_coil of said coiling are chosen in such a way that the mean size of the residual austenite islands does not exceed 2 microns.
20. Process according to any one of claims 16 to 19, characterized in that the temperature T_er of the end of said hot rolling, the rate V_c of said cooling and the temperature T_coil of said coiling are chosen in such a way that the mean size of the residual austenite islands does not exceed 1 micron.
21. Process for manufacturing a hot-rolled sheet according to claim 16, characterized in that the temperature T_er of the end of said rolling is not less than 900°C,the rate V_c of said cooling is not less than 20°C/s and the temperature T_coil of said coiling is below 450°C
22. Process according to claim 21, characterized in that the coiling temperature T_coil is below 400 ° C.
23. Process for manufacturing a cold-rolled sheet exhibiting TRIP behaviour, in which:

    - a hot-rolled steel sheet manufactured according to any one of claims 16 to 22 is supplied;

    - said sheet is pickled;

    - said sheet is cold-rolled; and

    - said sheet is made to undergo an annealing heat treatment, said heat treatment comprising a heating phase at a heating rate V_hs, a soak phase at a soak temperature T_s for a soak time is followed by a cooling phase at a cooling rate V_cs when the temperature is below Ar3, followed by a soak phase at a soak temperature T'_s for a soak time t'_s, characterized in that the parameters V_hs, T_s, t_s, V_cs, T'_s and t'_s are chosen in such a way that the microstructure of said steel consists of ferrite, bainite, residual austenite and, optionally, martensite.
24. Process according to claim 23, characterized in that the parameters V_hs, T_s, t_s, V_cs, T'_s and t'_s, are chosen in such a way that the microstructure of said steel has a residual austenite content of between 8 and 20%.
25. Process according to any one of claims 23 or 24, characterized in that the parameters V_hs, T_s, t_s, V_cs, T'_s and t'_s, are chosen in such a way that the microstructure of said steel has a martensite content of less than 2%.
26. Process according to any one of claims 23 to 25, characterized in that the parameters V_hs, T_s, t_s, V_cs, T'_s and t'_s, are chosen in such a way that the mean size of the residual austenite islands is less than 2 microns.
27. Process according to any one of claims 23 to 26, characterized in that the parameters V_hs, T_s, t_s, V_cs, T'_s and t'_s are chosen in such a way that the mean size of the residual austenite islands is less than 1 micron.
28. Process for manufacturing a cold-rolled sheet exhibiting TRIP behaviour according to claim 23, characterized in that said sheet is made to undergo an annealing heat treatment, said heat treatment comprising a heating phase at a heating rate V_hs of 2°C/s or higher, a soak phase at a soak temperature T_s of between A_c1 and A_c3 for a soak time t_s of between 10 and 200 s, followed by a cooling phase at a cooling rate V_cs of greater than 15°C/s when the temperature is below Ar3, followed by a soak phase at a temperature T'_s of between 300 and 500 ° C for a soak time t_s of between 10 and 1000 s.
29. Process according to claim 28, characterized in that said soak temperature T_s is between 770 and 815° C.
30. Using of a sheet of steel as claimed in any one of claims 11 to 15, or manufactured by the process of any one of claims 16 to 29, for the manufacture of structural components or of reinforcing elements in the automobile field.