EP3146083B1 - Double-annealed steel sheet having high mechanical strength and ductility characteristics, method of manufacture and use of such sheets - Google Patents

Description

The present invention covers the manufacture of double-annealed steels with high strength, simultaneously having a mechanical strength and a deformation capacity for carrying out cold forming operations. The invention more specifically relates to steels having a mechanical strength greater than or equal to 980 MPa, having a yield strength greater than or equal to 650 MPa, a uniform elongation greater than or equal to 15%, an elongation at break greater than or equal to 20%.

Strong demand for reduced greenhouse gas emissions, coupled with growing automotive safety requirements and fuel prices, has led motor vehicle manufacturers to use more and more high strength steels improved in the bodywork to reduce the thickness of parts and thus the weight of vehicles while maintaining the mechanical strength performance of the structures. In this perspective, steels combining high strength with sufficient formability for shaping without the appearance of cracks are becoming increasingly important. It has thus been proposed, in time and successively, several families of steels offering various levels of mechanical strength. These families include DP steels for Dual Phase, TRIP steels for Transformation Induced Plasticity, multiphase steels or even low density steels (FeAl).

In order to meet this demand for increasingly light vehicles, it is therefore necessary to have steels increasingly resistant to compensate for the drop in thickness. However, it is known that in the field of carbon steels, an increase in mechanical strength is generally accompanied by a loss of ductility. In addition, motorized land vehicle manufacturers are defining increasingly complex parts that require steels with high levels of ductility.

We were able to read the patent EP1365037A1 showing a steel containing the following chemical components, in% by weight, C: 0.06 to 0.25% Si + Al: 0.5 to 3% Mn: 0.5 to 3% P: 0.15 or less, S: 0.02% or less, and optionally additionally containing at least one of the following components in% by weight: Mo: 1% or less, Ni: 0.5% or less, Cu: 0 , 5% or less, Cr: 1% or less, Ti: 0.1% or less, Nb: 0.1% or less, V: 0.1% or less, Ca: 0.003% or less, and / or REM: 0.003% or less associated with a microstructure composed mainly of returned martensite or bainite revenue representing 50% or more in surface proportion, or of returned martensite or bainite revenue which represents 15% or more with respect to a congestion factor with respect to the entire structure and further comprising ferrite, the returned martensite or the bainite returned and a second phase structure comprising the austenite back which represents from 3 to 30% in surface proportion and comprising in particular in addition to possibly bainite and / or martensite, residual austenite having a C (C gamma R) concentration of 0.8% or more. This patent application does not achieve sufficiently high levels of resistance and necessary to significantly reduce the thicknesses and therefore the weight of the sheets used in the automotive industry for example.

On the other hand, we know about the patent US20110198002A1 which has a high-strength, hot-coated steel with a strength greater than 1200 MPa, an elongation of more than 13% and a hole expansion of more than 50% and the method of manufacturing such steel from the chemical composition following: 0.05-0.5% carbon, 0.01-2.5% silicon, 0.5-3.5% manganese, 0.003-0.100% phosphorus, up to 0.02% sulfur, and 0.010-0.5% d aluminum, the rest being impurities. The microstructure of this steel comprises in terms of surface proportions 0-10% of ferrite, 0-10% of martensite, and 60-95% of martensite returned and containing, in proportions determined by X-ray diffraction: 5-20% d residual austenite. Nevertheless, the ductilities achieved by the steels according to this invention are low and it affects the shaping of the piece from the product obtained from the teachings of this application.

Finally, we also know the publication "fatigue strength of newly developed high-strength low alloy TRIP-steered steels with good hardenability" presenting the study of a steel with the following composition: 0.4% C, 1.5% Si , 1.5% Mn, 0-1.0% Cr, 0-0.2% Mo, 0.05% Nb, 0-18ppm B for automotive application This steel has a very good fatigue behavior, surpassing that of This behavior is all the more marked with additions of B, Cr and Mo. The microstructure of this steel has a TRIP effect with a high content of metastable residual austenite which suppresses the pre-cracks and their propagation because of plastic de-stressing and martensite formation during transformation from austenite This article discloses a method for the production of steels with excellent resistance-ductility trade-offs, but the chemical compositions disclosed as well as the methods of production are not only not compatible with industrial production but they will give rise to difficulties of coating.

The object of the present invention is to solve the problems mentioned above. It aims to provide a cold-rolled steel having a mechanical strength greater than or equal to 980 MPa, a yield strength greater than or equal to 650 MPa together with a uniform elongation greater than or equal to 15%, a higher breaking elongation or equal to 20% and its manufacturing process. The invention also aims to provide a steel with an ability to be produced stably.

For this purpose, the subject of the invention is a steel sheet whose composition comprises, the contents being expressed as a percentage of the weight, 0.20% ≤ C ≤ 0.40%, preferentially 0.22% ≤ C ≤ 0 , 32%, 0.8% ≤ Mn ≤ 1.4%, preferably 1.0% ≤ Mn ≤ 1.4%, 1.60% ≤ Si ≤ 3.00%, preferentially 1.8% ≤ Si ≤ 2 , 5%,
0.015 ≤ Nb ≤ 0.150%, preferably 0.020% ≤ Nb ≤ 0.13%, Al ≤ 0.1%, Cr ≤ 1.0%, preferentially Cr ≤ 0.5%, S ≤ 0.006%, P ≤ 0.030%, Ti ≤ 0.05%, V ≤ 0.05%, Mo <0.03%, B ≤0.003%, N ≤ 0.01%, the remainder of the composition consisting of iron and unavoidable impurities resulting from the elaboration, the microstructure being constituted, in surface proportions, of 10 to 30% residual austenite, 30 to 60% annealed martensite, 5 to 30% bainite, 10 to 30% fresh martensite and less than 10% ferrite.

Preferably, the steel sheet according to the invention comprises a coating of zinc or zinc alloy or a coating of Al or Al alloy. These coatings may or may not be alloyed with iron, it will be called galvanized sheet (GI / GA)

Ideally the sheets according to the invention have a mechanical behavior such that the mechanical strength is greater than or equal to 980 MPa, the elastic limit is greater than or equal to 650 MPa, the uniform elongation greater than or equal to 15% and the elongation at break greater than or equal to 20%.

• on approvisionne un acier de composition selon l'invention
• on coule ledit acier sous forme de demi-produit, puis
• on porte ledit demi-produit à une température T_rech comprise entre 1100°C et 1280°C pour obtenir un demi-produit réchauffé, puis
• on lamine à chaud ledit demi-produit réchauffé, la température de fin de laminage à chaud T_fl étant supérieure ou égale à 900°C pour obtenir une tôle laminée à chaud, puis,
• on bobine ladite tôle laminée à chaud à une température T_bob comprise entre 400 et 600°C pour obtenir une tôle laminée à chaud bobinée, puis,
• on refroidit ladite tôle laminée à chaud bobinée jusqu'à la température ambiante, puis,
• on débobine et on décape ladite tôle laminée à chaud bobinée, puis,
• on lamine à froid ladite tôle laminée à chaud avec un taux de réduction compris entre 30 et 80% de façon à obtenir une tôle laminée à froid, puis,
• on recuit une première fois ladite tôle laminée à froid en la réchauffant à une vitesse V_C1 comprise entre 2 et 50°C/s jusqu'à une température T_soaking1 comprise entre TS1= 910,7 - 431,4*C - 45,6*Mn + 54,4*Si - 13,5*Cr + 52,2*Nb, les teneurs étant exprimées en pourcentage du poids, et 950°C, pendant une durée t_soaking1 comprise entre 30 et 200 secondes, puis :
  • on refroidit ladite tôle en la soumettant à un refroidissement jsuqu'à la température ambiante à une vitesse supérieure ou égale à 30°C/s, puis,
  • on recuit une seconde fois ladite tôle en la réchauffant à une vitesse V_C2 comprise entre 2 et 50°C/s jusqu'à une température T_soaking2 comprise entre Ac1 et TS2=906,5 - 440,6*C - 44,5*Mn + 49,2*Si - 12,4*Cr + 55,9*Nb, pendant une durée t_soaking2 comprise entre 30 et 200 secondes, puis,
  • on refroidit ladite tôle en la soumettant à un refroidissement à une vitesse supérieure ou égale à 30°C/s jusqu'à la température de fin de refroidissement T_OA comprise entre 420°C et 480°C, puis,
  • on maintient ladite tôle dans la plage de température allant de 420 à 480°C pendant une durée t_OA comprise entre 5 et 120 secondes, puis,
  • optionnellement on dépose un revêtement sur ladite tôle avant de refroidir ladite tôle jusqu'à l'ambiante.

The subject of the invention is also a method for producing a cold-rolled, doubly annealed and optionally coated steel sheet comprising the following successive stages:

• supplying a composition steel according to the invention
• said steel is cast as a semi-finished product and
• said half-product is brought to a temperature T _rech between 1100 ° C and 1280 ° C to obtain a heated half-product, then
• said heated half-product is hot-rolled, the hot-rolling end temperature T _f being greater than or equal to 900 ° C to obtain a hot-rolled sheet, and then
• said hot-rolled sheet is reeled at a temperature T _bob of between 400 and 600 ° C to obtain a rolled hot-rolled sheet, then,
• said hot-rolled rolled sheet is cooled to room temperature, and then
• said wound hot-rolled sheet is uncoiled and stripped, then,
• said hot-rolled sheet is cold-rolled with a reduction ratio of between 30 and 80% so as to obtain a cold-rolled sheet, and then
• said cold-rolled sheet is first annealed by heating it at a speed V _C1 of between 2 and 50 ° C / s to a T _soaking1 temperature of between TS1 = 910.7 and 431.4 ° C-45, 6 * Mn + 54.4 * Si - 13.5 * Cr + 52.2 * Nb, the contents being expressed as a percentage of the weight, and 950 ° C, for a duration t _soaking1 of between 30 and 200 seconds, then:
  • said sheet is cooled by subjecting it to cooling to ambient temperature at a speed greater than or equal to 30 ° C / s, then,
  • said sheet is re-annealed a second time by heating it at a speed V _C2 of between 2 and 50 ° _C./s up to a temperature T _{soaking 2} between Ac1 and TS2 = 906.5 - 440.6 ° C - 44.5 * Mn + 49.2 * Si - 12.4 * Cr + 55.9 * Nb, for a duration t _soaking2 between 30 and 200 seconds, then,
  • said sheet is cooled by subjecting it to cooling at a speed greater than or equal to 30 ° C / s to the end-of-cooling temperature T _OA of between 420 ° C and 480 ° C, and then
  • said sheet is maintained in the temperature range of 420 to 480 ° C for a period t _OA of between 5 and 120 seconds, then,
  • optionally, a coating is deposited on said sheet before cooling said sheet to ambient.

In a preferred embodiment, a so-called base annealing of said wound hot-rolled sheet is carried out before cold rolling so that the sheet is heated and then maintained at a temperature of between 400 ° C. and 700 ° C. for a period of between and 24 hours,

Preferably, the sheet is maintained at the end of cooling temperature T _OA isothermally between 420 and 480 ° C between 5 and 120 seconds.

Preferably, the double annealed cold-rolled sheet is then cold-rolled with a cold rolling ratio of between 0.1 and 3% before depositing a coating.

In a preferred embodiment, the doubly annealed sheet is finally heated to a holding temperature T _base of between 150 ° C and 190 ° C for a hold time t _basis between 10h and 48h.

Preferably, after maintaining the T _OA the sheet is coated by dipping in a liquid bath of one of the following elements: Al, Zn, Al alloy or Zn alloy.

The sheet according to the invention, cold-rolled, doubly annealed and coated, or manufactured by a method according to the invention is used for the manufacture of parts for land motor vehicles.

Other features and advantages of the invention will become apparent from the description below

According to the invention, the carbon content, by weight, is between 0.20 and 0.40%. If the carbon content of the invention is below 0.20% by weight, the mechanical strength becomes insufficient and the residual austenite fraction is still insufficient and not stable enough to achieve a uniform elongation greater than 15%. Beyond 0.40%, the weldability becomes more and more reduced because low-tenacity microstructures are formed in the heat-affected zone (ZAT) or in the melted zone in the case of resistance welding. In a preferred embodiment, the carbon content is between 0.22 and 0.32%. Within this range, the weldability is satisfactory, the stabilization of the austenite is optimized and the fresh martensite fraction is within the target range of the invention.

Manganese is, according to the invention between 0.8 and 1.4%, it is a hardening element with a solid solution of substitution, it stabilizes the austenite and lowers the transformation temperature Ac3. Manganese therefore contributes to an increase in mechanical strength. According to the invention, a a minimum of 0.8% by weight is necessary to obtain the desired mechanical properties. However, beyond 1.4%, its gammagenic character leads to a slowing down of the bainitic transformation kinetics taking place during the maintenance at the end of cooling temperature T _OA and the bainite fraction is still insufficient to reach a resistance. elasticity greater than 650 MPa. Preferably, a range of manganese content of between 1.0% and 1.4% is chosen, thus combining a satisfactory mechanical strength without increasing the risk of reducing the bainite fraction and thus reducing the elastic resistance, neither increase quenchability in welded alloys, which would adversely affect the weldability of the sheet according to the invention.

The silicon must be between 1.6 and 3.0%. In this range, the stabilization of the residual austenite is made possible by the addition of silicon, which considerably slows the precipitation of the carbides during the annealing cycle and more particularly during the bainitic transformation. This is because the solubility of silicon in cementite is very low and this element increases the carbon activity in the austenite. Any formation of cementite will therefore be preceded by a step of rejection of Si at the interface. The enrichment of carbon austenite therefore leads to its stabilization at room temperature on the double-annealed and coated steel sheet. Subsequently, the application of an external constraint, shaping for example, will lead to the transformation of this austenite martensite. This transformation has the result of also improving the resistance to damage. Silicon is also a strongly hardening element with a solid solution and thus makes it possible to achieve the elastic and mechanical resistances targeted by the invention. In view of the properties targeted by the invention, an addition of silicon in an amount greater than 3.0% will substantially promote the ferrite and the target strength would not be reached, in addition strongly adherent oxides would form which would lead to defects surface and non-adherence of the Zinc or Zinc alloy coating. The minimum content must also be set at 1.6% by weight to obtain the stabilizing effect on the austenite. So preferentially, the silicon content will be between 1.8 and 2.5% in order to optimize the aforementioned effects.

The chromium content must be limited to 1.0%, this element makes it possible to control the formation of pro-eutectoid ferrite during cooling during annealing from said holding temperature T _soaking1 or T _soaking2 , because this ferrite, in a high quantity decreases the mechanical strength required for the sheet according to the invention. This element also makes it possible to harden and refine the bainitic microstructure. However, this element considerably slows down the kinetics of bainitic transformation. However, for contents above 1.0%, the bainite fraction is still insufficient to reach a yield strength greater than 650 MPa.

Nickel and copper have effects substantially similar to that of manganese. These two elements will be in residual contents namely 0.05% for each element but only because their costs are much higher than that of manganese.

The aluminum content is limited to 0.1% by weight, this element is a powerful alphagene favoring the formation of ferrite. A high aluminum content would increase the Ac3 point and thus make the industrial process expensive in terms of energetic annealing. In addition, it is considered that high levels of aluminum increase the erosion of refractories and the risk of clogging of the nozzles during the casting of the steel upstream of the rolling. In addition aluminum segregates negatively and, it can lead to macro-segregations. In excessive amounts, aluminum reduces hot ductility and increases the risk of defects in continuous casting. Without a strong control of the casting conditions, micro and macro segregation defects ultimately result in central segregation on the annealed steel sheet. This central band will be harder than its surrounding matrix and will damage the formability of the material.

The sulfur must be less than 0.006%, beyond which the ductility is reduced due to the excessive presence of sulphides such as MnS, so-called manganese sulphides, which reduce the ability to deform.

Phosphorus should be less than 0.030%, it is a hardening element in solid solution but significantly reduces spot weldability and hot ductility, particularly because of its ability to segregate at grain boundaries or its tendency to co-segregation with manganese. For these reasons, its content should be limited to 0.030% in order to obtain a good spot welding ability.

The niobium must be between 0.015 and 0.150%, it is a micro-alloy element which has the particularity of forming precipitates hardening with carbon and / or nitrogen. These precipitates, already present during the hot rolling operation, delay the recrystallization during annealing and thus refine the microstructure, which contributes to the hardening of the material. It also makes it possible to improve the elongation properties of the product, by allowing high temperature annealing without lowering the elongation performance by a refinement effect of the structures. The niobium content must nevertheless be limited to 0.150% to avoid excessively high hot rolling forces. In addition, above 0.150%, a saturating effect is expected on the positive effects of Niobium, particularly on the effect of hardening by refinement of the microstructure. On the other hand, the niobium content must be greater than or equal to 0.015%, which makes it possible to harden the ferrite when it is present and such hardening is sought and also a sufficiently important refinement for greater stabilizing the residual austenite and thus ensuring a uniform elongation in the scope of the invention, preferably the Nb content is between 0.020 and 0.13 to optimize the aforementioned effects.

The other micro-alloy elements such as titanium and vanadium are limited to a maximum content of 0.05% because these elements have the same advantages as niobium but they have the particularity of reducing the ductility of the product more strongly.

The nitrogen is limited to 0.01% in order to avoid phenomena of aging of the material and to minimize the precipitation of aluminum nitrides (AlN) during solidification and thus to weaken the semi-finished product.

Boron and molybdenum are at levels of impurities or, at levels individually lower than 0.003 for boron and 0.03 for Mo.

The rest of the composition consists of iron and unavoidable impurities resulting from the elaboration.

According to the invention, the microstructure of the steel after the first annealing must contain, in surface proportion, less than 10% of polygonal ferrite, the remainder of the microstructure being composed of fresh or returned martensite. If the polygonal ferrite content is greater than 10%, the strength and yield strength of the steel after the second annealing will be less than 980 MPa and 650 MPa respectively. In addition, a polygonal ferrite content greater than 10% after the first annealing will result in a polygonal ferrite content at the end of the second annealing greater than 10% which would lead to a yield strength and too much mechanical strength. low compared to the scope of the invention.

The microstructure of the steel after the second annealing must contain, in surface proportions, 10 to 30% residual austenite. If the residual austenite content is less than 10%, the uniform elongation will be less than 15% because the residual austenite will be too stable and can not be transformed into martensite during mechanical stresses bringing a significant gain on the work hardening. the steel actually delays the appearance of the necking which results in an increase of the uniform elongation. If the residual austenite content is greater than 30%, the residual austenite will be unstable because not sufficiently enriched in carbon during the second annealing and maintenance at the end of cooling temperature T _OA , and the ductility of the steel after the Second annealing will be reduced, which will lead to a uniform elongation of less than 15% and / or a total elongation of less than 20%.

In addition, the steel according to the invention after the second annealing must contain, in surface proportions, from 30 to 60% of annealed martensite, which is a martensite resulting from the first annealing, annealed during the second annealing and which is distinguished from a fresh martensite by a smaller quantity of crystallographic defects, and which is distinguished from a martensite returned by the absence of carbides within its laths. If the annealed martensite content is less than 30%, the ductility of the steel will be too low because the residual austenite content will be too low because not enough enriched in carbon and the fresh martensite content will be too much this leads to a uniform elongation of less than 15%. If the annealed martensite content is greater than 60%, the ductility of the steel will be too low because the residual austenite will be too stable and can not be transformed into martensite under the effect of mechanical stresses, which will have the effect of to reduce the ductility of the steel according to the invention, and will lead to a uniform elongation of less than 15% and / or a total elongation of less than 20%.

Still according to the invention, the microstructure of the steel after the second annealing must contain, in surface proportions, from 5 to 30% of bainite. The presence of bainite in the microstructure is justified by the role it plays in the carbon enrichment of the residual austenite. Indeed, during the bainitic transformation and thanks to the presence of silicon in significant amount, the carbon is redistributed from bainite to austenite which has the effect of stabilizing the latter at room temperature. If the bainite content is less than 5%, residual austenite will not be sufficiently enriched in carbon and the latter will not be stable enough, which will favor the presence of fresh martensite which will cause a significant decrease in ductility. The uniform elongation will then be less than 15%. If the bainite content is greater than 30%, this will lead to a residual austenite which is too stable and can not be converted into martensite under the effect of mechanical stresses, which will lead to a uniform elongation of less than 15%. and / or a total elongation of less than 20%.

Finally, the steel according to the invention and after the second annealing must contain, in surface proportions, 10 to 30% fresh martensite. If the fresh martensite content is less than 10%, the strength of the steel will be less than 980 MPa. If it is greater than 30%, the residual austenite content will be too low and the steel will not be sufficiently ductile, moreover, the uniform elongation will be less than 15%.

The sheet according to the invention may be manufactured by any suitable method.

First, a steel of composition according to the invention is supplied. Then, one proceeds to the casting of a half-product from this steel. This casting can be carried out in ingots or continuously in the form of slabs.

The reheating temperature should be between 1100 and 1280 ° C. The cast semifinished products must be heated to a temperature T _rech at 1100 ° C to obtain a semi-finished product warmed in order to reach at any point a temperature favorable for the large deformations that the steel will undergo during rolling. This temperature range also makes it possible to be in the austenitic range and to ensure complete dissolution of the precipitates resulting from the casting. However, if the temperature T _rech is greater than 1280 ° C, the austenitic grains grow undesirably and will lead to a coarser final structure and the risks of surface defects related to the presence of liquid oxide are increased. It is of course also possible to hot roll directly after casting without heating the slab.

The semi-finished product is then hot-rolled in a temperature range in which the structure of the steel is totally austenitic: if the end-of-rolling temperature T _f is less than 900 ° C., the rolling forces are very large and may cause significant energy consumption or even breakages of rolling mill. Preferably, a rolling end temperature greater than 950 ° C. will be observed in order to guarantee rolling in the austenitic range and thus to limit the rolling forces.

The hot-rolled product is then rolled at a temperature T _bob of between 400 and 600 ° C. This temperature range makes it possible to obtain ferritic, bainitic or pearlitic transformations during the quasi-isothermal maintenance associated with the winding followed by slow cooling to minimize the martensite fraction after cooling. A winding temperature above 600 ° C leads to the formation of unwanted surface oxides. When the winding temperature is too low, below 400 ° C, the hardness of the product after cooling is increased, which increases the efforts required during the subsequent cold rolling.

The hot-rolled product is then cleaned if necessary by a process known per se.

Optionally, an intermediate base annealing of the hot-rolled sheet wound between T _RB1 and T _{RB2 is carried out} with T _RB1 = 400 ° C and T _RB2 = 700 ° C for a period of between 5 and 24 hours. This heat treatment makes it possible to have a mechanical strength of less than 1000 MPa at any point of the hot-rolled sheet, the difference in hardness between the center of the sheet and the banks thus being minimized. This greatly facilitates the next step of cold rolling by softening the formed structure.

Cold rolling is then carried out with a reduction ratio preferably comprised between 30 and 80%.

The first annealing of the cold-rolled product is then carried out, preferably in a continuous annealing installation, with a mean heating rate V _C of between 2 and 50 ° C. per second. In relation to the annealing temperature T _soaking1 , this heating rate range makes it possible to obtain a recrystallization and an adequate refinement of the structure. Below 2 ° C per second, the risks of surface decarburization are considerably increased. Above 50 ° C per second, traces of non-recrystallization and insoluble carbides would appear during the maintenance which would have the effect of reducing the residual austenite fraction and thus adversely affect the ductility.
The heating is carried out up to a _soaking temperature of annealing T between temperature TS1 and 950 ° C where TS1 = 910.7 - 431.4 * C- 45.6 * Mn + 54.4 * Si - 13.5 * Cr + 52.2 * Nb with temperatures in ° C and chemical compositions in percent by mass, When T _soaking1 is lower than TS1, it promotes the presence of polygonal ferrite beyond 10% and therefore outside the scope of the invention. Conversely, if T _soaking1 is above 950 ° C, the austenitic grain sizes increase considerably which is detrimental to the refinement of the final microstructure and therefore to the yield strength levels which would be below 650 MPa .

A holding time t _soaking1 of between 30 and 200 seconds at the temperature T _soaking1 allows the dissolution of the previously formed carbides, and especially a sufficient transformation to austenite. Below 30 seconds the dissolution of the carbides would be insufficient. On the other hand, a holding time greater than 200 s is hardly compatible with productivity requirements. continuous annealing installations, in particular the speed of travel of the reel. In addition, the same risk of austenitic grain enlargement as in the case of T _soaking1 above 950 ° C appears, with the same risk of having a yield strength of less than 650 MPa. The duration of maintenance t _soaking1 is therefore between 30 and 200 s.

At the end of maintaining the first annealing, the sheet is cooled to room temperature, the cooling rate V _ref1 being fast enough to prevent the formation of ferrite. For this purpose, this cooling rate is greater than 30 ° C./s, which makes it possible to obtain a microstructure with less than 10% ferrite, the rest being martensite. Preferentially, a fully martensitic microstructure will be favored after the first annealing.

The second annealing of the cold-rolled product is then carried out and annealed a first time, preferably in a continuous galvanization annealing installation, with an average heating rate V _C greater than 2 ° C. per second in order to avoid the risks of surface decarburization. Preferably, the average heating rate must be less than 50 ° C per second to avoid the presence of insoluble carbides during maintenance which would have the effect of reducing the residual austenite fraction.
The heating is carried out up to a _{soaking 2} annealing temperature T between the temperature Ac1 = 728 - 23.3 * C - 40.5 * Mn + 26.9 * Si + 3.3 * Cr + 13.8 * Nb
and TS2 = 906.5 - 440.6 * C - 44.5 * Mn + 49.2 * Si - 12.4 * Cr + 55.9 * Nb with temperatures in ° C and chemical compositions in percent by weight. When T _soaking2 is less than Ac1, it is not possible to obtain the microstructure targeted by the invention since only the income from the martensite resulting from the first annealing would take place. When T _soaking2 is greater than TS2 the annealed martensite content will be less than 30% which will favor the presence of a large amount of fresh degrading martensite de facto strongly the ductility of the product.

A holding time t _{soaking 2} of between 30 and 200 seconds at the _soaking temperature T ₂ allows the dissolution of the previously formed carbides, and especially sufficient transformation into austenite. Below 30 seconds the dissolution of the carbides may be insufficient. On the other hand, a hold time greater than 200 s is hardly compatible with the productivity requirements of continuous annealing equipment, in particular the speed of travel of the reel. In addition, the same risk of austenitic grain enlargement as in the case of t _soaking1 beyond 200s appears, with the same risk of having a yield strength of less than 650 MPa. The soaking time t _soaking2 is therefore between 30 and 200 s.

At the end of the maintenance of the second annealing, the sheet is cooled until a temperature of end of cooling T _OA between T _OA1 = 420 ° C and T _OA2 = 480 ° C is _reached , the cooling rate V _ref2 being sufficiently fast to avoid the massive formation of ferrite ie a content greater than 10%. For this purpose, this cooling rate is greater than 20 ° C per second.

The end of cooling temperature must be between T _OA1 = 420 ° C and T _OA2 = 480 ° C. Below 420 ° C, the bainite formed will be hard and this may affect the ductility which could be less than 15% for the uniform elongation, moreover, this temperature is too low for the case where it is desired to enter in a bath of Zn which is generally at 460 ° C, the bath would therefore be continuously cooled. If the temperature T _OA is greater than 480 ° C, it is likely to precipitate carbonite, carbide phase which will reduce the available carbon to stabilize the austenite. In addition, in the case of dip galvanized coating, there is a risk of evaporating liquid Zn while losing control of the reaction between the bath and the steel if it is too high in temperature, ie above 480 ° C.

The holding time t _OA in the temperature range T _OA1 (° C) to T _OA2 (° C) must be between 5 and 120 seconds in order to allow the bainitic transformation and thus the stabilization of the austenite by carbon enrichment. of said austenite. It must also be greater than 5 s to ensure a bainite content according to the invention without which the yield strength would be less than 650 MPa. It must also be less than 120 seconds to limit the bainite content to 30% as referred to in the invention, otherwise the residual austenite content would be less than 10% and the ductility of the steel would be too low, which would be manifest by a uniform elongation of less than 15% and / or a total elongation of less than 20%.

At the end of this hold between T _OA1 (° C) and T _OA2 (° C), the doubly annealed sheet is coated with a deposit of Zinc or Zinc alloy (the Zn content in mass percentage being predominant) by hot dip coating before cooling to room temperature. Preferably, zinc or zinc alloy may also be coated by any electrolytic or physicochemical process known in itself bare annealed sheet. A coating based on aluminum or aluminum-based alloy (the Al content in weight percentage being the majority) can also be deposited by hot quenching.

A post annealing heat treatment is then preferably carried out on the cold-rolled and doubly annealed and coated sheet at a holding temperature T _base of between 150 ° C. and 190 ° C. for a hold time t _base of between 10h and 48h to improve the elasticity limit and the pliability. This treatment will be called: post annealing base.

The present invention will now be illustrated from the following nonlimiting examples.

Examples

Steels have been developed, the composition of which is given in the table below, expressed in percentage by weight. Table 1 shows the chemical composition of the steel used to manufacture the sheets of the examples. Table 1: Chemical compositions (% wt) and critical temperatures Ae1, TS1 and TS2 being in ° C. Steel VS mn Yes al Cr MB Cu Or V Nb S P B Ti NOT Ae1 TS1 TS2 AT 0.26 1.3 2.12 0,027 0,002 0,002 0.005 0.006 0,002 0.124 0.0027 0,019 0.0005 0,004 0,002 728 862 846 B 0.28 1.17 1.99 0.03 0,003 0,003 0,007 0,008 0,003 0,017 0.0036 0.014 0.00042 0,007 0.0014 727 844 829 VS 0.29 1.17 1.98 0,029 0,003 0,003 0,007 0,008 0,003 0.068 0.0036 0.014 0.0004 0.006 0.0016 728 845 830 D 0.21 1.25 3.04 0,023 0,004 0.005 0.005 0,004 0,002 0.00 0.0033 0,018 0.0006 0,004 0.0015 754 927 907 E 0.19 1.68 1.55 0.053 0,024 0.006 0,007 0,017 0,004 0,001 0,002 0,009 0.0007 0,003 0,004 697 836 824

The references D and E of Table 1 denote steels whose compositions do not conform to the invention. The contents not in accordance with the invention are underlined.

Note in particular that the references D and E are not in accordance with the invention because their compositions are free of Niobium, which will limit the yield strength and the mechanical strength of the final sheet by the absence of precipitation hardening.

It is also noted that the references D and E are not in accordance with the invention because their silicon contents are outside the target range. Beyond 3.00%, the silicon will promote a quantity of ferrite which is too great and the intended mechanical strength would not be reached. Below 1.60% by weight, the stabilization of residual austenite will not be large enough to achieve the desired ductility.

It is also noted that the reference E is not in accordance with the invention because the carbon content is lower than the target which will limit the final strength and ductility of the sheet. In addition, the Mn content is too high, which will limit the final amount of bainite in the sheet, which will have the effect of limiting the ductility of the sheet by an excessive presence of fresh martensite.

Plates corresponding to the above compositions were produced according to manufacturing conditions compiled in Table 2.

From these compositions, some steels have been subjected to different annealing conditions. The conditions before hot rolling are identical with a reheating of between 1200 ° C. and 1250 ° C., an end-of-rolling temperature of between 930 ° C. and 990 ° C. and a winding of between 540 ° C. and 560 ° C. The hot rolled products are then all stripped and then directly cold rolled with a reduction rate of between 50 and 70%.

• température de réchauffage : T_rech
• température de fin de laminage : T_fl
• température de bobinage : T_BOB
• taux de réduction au laminage à froid
• vitesse de chauffe au premier recuit : V_C1
• température de maintien au premier recuit : T_soaking1
• temps de maintien au premier recuit à T_soaking1 : t_soaking1
• vitesse de refroidissement au premier recuit : V_ref1
• vitesse de chauffe au deuxième recuit : V_C2
• température de maintien au deuxième recuit : T_soaking2
• temps de maintien au deuxième recuit à T_soaking1 : t_soaking2
• vitesse de refroidissement au deuxième recuit : V_ref2
• température de fin de refroidissement T_OA
• temps de maintien à la température T_OA : t_OA
• les températures calculées Ac1, TS1 et TS2 (en °C)

Tableau 2 :Condition de recuit des exemples et rérérences Acier ID T_rech (°C) T_fl(°C) T_BOB (°C) Taux de réduction (%) V_C1 (°C/s) T _{Soaking 1}(°C) t_Soaking1 (s) V_ref1 (°C/s) V_C2 (°C/s) T_{Soaking 2} (°C) t_Soaking2 (s) V_ref2 (°C/s) T_OA (°C) t_OA (s) Ac1 TS1 TS2 A A_1 1240 963 551 62 15 900 120 800 15 770 120 95 460 15 728 862 847 A A_2 1240 963 551 62 15 900 120 800 15 770 120 95 460 20 728 862 847 A A_3 1240 963 551 62 15 900 120 800 15 770 120 95 450 25 728 862 847 A A_4 1240 963 551 62 15 900 120 300 15 770 120 95 450 30 723 862 847 A A_5 1240 963 551 62 15 800 120 800 15 770 120 95 460 15 728 862 847 A A_6 1240 963 551 62 15 800 120 800 15 770 120 95 460 20 728 862 847 B B_1 1245 951 546 59 15 900 120 800 15 750 120 95 400 15 728 845 829 B B_2 1245 951 546 59 15 840 120 800 15 750 120 95 450 30 728 845 829 B B_3 1245 951 546 59 15 840 120 800 15 770 120 95 450 30 728 845 829 B B_4 1245 951 546 59 15 840 120 800 15 790 120 95 450 30 728 845 829 c C_1 1245 951 546 59 15 900 120 800 15 750 120 95 450 15 728 846 830 C C_2 1245 951 546 59 15 840 120 800 15 750 120 95 450 30 728 846 830 C C_3 1245 951 546 59 15 840 120 800 15 770 120 95 450 30 728 846 830 C C_4 1245 951 546 59 15 840 120 800 15 790 120 95 450 30 726 846 830 C C_5 1245 951 546 59 - - - - 15 770 120 95 450 30 728 846 830 D D_1 1243 965 553 61.5 15 850 120 800 15 800 120 95 460 30 754 927 907 D D_2 1243 965 553 61.5 15 850 120 800 15 800 120 95 460 30 754 927 907 E E_1 1210 952 541 52 15 870 120 800 5 820 87 36 450 25 697 837 825 E E_2 1210 952 541 52 15 870 120 800 5 840 87 36 450 25 697 837 825 E E_3 1210 952 541 52 15 870 120 800 5 850 87 36 450 25 697 837 825 E E_4 1210 952 541 52 15 870 120 800 5 860 87 36 450 25 697 837 825 E E_5 1210 952 541 52 15 870 120 800 3 800 110 23 450 38 697 837 825 E E_6 1210 952 541 52 15 870 120 800 3 820 110 24 450 38 697 837 825 Table 2 also indicates the manufacturing conditions for the cold-rolled sheet after the following denominations:

• reheating temperature: T _rech
• end of rolling temperature: T _fl
• winding temperature: T _BOB
• reduction rate at cold rolling
• heating rate at first annealing: V _C1
• holding temperature at first annealing: T _soaking1
• hold time at first annealing at T _soaking1 : t _soaking1
• cooling rate at first annealing: V _ref1
• heating rate at second annealing: V _C2
• holding temperature at second annealing: T _soaking2
• hold time at second annealing at T _soaking1 : t _soaking2
• cooling rate at the second annealing: V _ref2
• end of cooling temperature T _OA
• hold time at temperature T _OA : t _OA
• the calculated temperatures Ac1, TS1 and TS2 (in ° C)

Table 2: Annealing condition of examples and rerérences Steel ID T _rech (° C) T _fl (° C) T _BOB (° C) Reduction rate (%) V _C1 (° C / s) T _{Soaking 1} (° C) t _Soaking1 (s) V _ref1 (° C / s) V _C2 (° C / s) T _{Soaking 2} (° C) t _Soaking2 (s) V _ref2 (° C / s) T _OA (° C) t _OA (s) Ac1 TS1 TS2 AT A_1 1240 963 551 62 15 900 120 800 15 770 120 95 460 15 728 862 847 AT A_2 1240 963 551 62 15 900 120 800 15 770 120 95 460 20 728 862 847 AT A_3 1240 963 551 62 15 900 120 800 15 770 120 95 450 25 728 862 847 AT A_4 1240 963 551 62 15 900 120 300 15 770 120 95 450 30 723 862 847 AT AT 5 1240 963 551 62 15 800 120 800 15 770 120 95 460 15 728 862 847 AT A_6 1240 963 551 62 15 800 120 800 15 770 120 95 460 20 728 862 847 B B_1 1245 951 546 59 15 900 120 800 15 750 120 95 400 15 728 845 829 B B_2 1245 951 546 59 15 840 120 800 15 750 120 95 450 30 728 845 829 B B_3 1245 951 546 59 15 840 120 800 15 770 120 95 450 30 728 845 829 B B_4 1245 951 546 59 15 840 120 800 15 790 120 95 450 30 728 845 829 vs C_1 1245 951 546 59 15 900 120 800 15 750 120 95 450 15 728 846 830 VS C_2 1245 951 546 59 15 840 120 800 15 750 120 95 450 30 728 846 830 VS C_3 1245 951 546 59 15 840 120 800 15 770 120 95 450 30 728 846 830 VS C_4 1245 951 546 59 15 840 120 800 15 790 120 95 450 30 726 846 830 VS C_5 1245 951 546 59 - - - - 15 770 120 95 450 30 728 846 830 D D_1 1243 965 553 61.5 15 850 120 800 15 800 120 95 460 30 754 927 907 D D_2 1243 965 553 61.5 15 850 120 800 15 800 120 95 460 30 754 927 907 E E_1 1210 952 541 52 15 870 120 800 5 820 87 36 450 25 697 837 825 E E_2 1210 952 541 52 15 870 120 800 5 840 87 36 450 25 697 837 825 E E_3 1210 952 541 52 15 870 120 800 5 850 87 36 450 25 697 837 825 E E_4 1210 952 541 52 15 870 120 800 5 860 87 36 450 25 697 837 825 E E_5 1210 952 541 52 15 870 120 800 3 800 110 23 450 38 697 837 825 E E_6 1210 952 541 52 15 870 120 800 3 820 110 24 450 38 697 837 825

References A5 to A6, B1 to B4, C2 to C5, D1 and D2, E1 to E6 of Table 2 denote steel sheets manufactured under conditions not in accordance with the invention from steels whose compositions are given in Table 1. The parameters not in accordance with the invention are underlined.

It will be noted that the references A5, A6, B2 to B4, C2 to C4, D1 and D2 are not in accordance with the invention since the holding temperature at the first annealing T _soaking1 is lower than the calculated temperature TS1, which would favor a large quantity of ferrite at the first annealing thus limiting the mechanical strength of the sheet after the second annealing.

It is also noted that the references E2, E3 and E4 are not in accordance with the invention by their chemical composition and by the fact that the holding temperature at the second annealing T _soaking2 is greater than the calculated temperature TS2, which will have the effect to reduce the amount of annealed martensite after the second annealing, limiting the final ductility of the sheet due to too much fresh martensite.

Note also that the reference B1 is not in accordance with the invention because the temperature T _OA is outside the range 420 ° C - 480 ° C, which will limit the amount of residual austenite after the second annealing and therefore limit the ductility of the sheet.

Note also that the reference C5 is not in accordance with the invention because only a single annealing, in accordance with the invention and the claims of the second annealing, has been applied to the sheet. The absence of the first annealing leads to the absence of martensite annealed in the microstructure which greatly limits the yield strength and the ultimate strength of the sheet.

Note finally that the two references E5 and E6 are not in accordance with the invention the cooling rate at the second annealing V _Ref2 is less than 30 ° C / s which promotes the formation of ferrite cooling, which will have the effect to reduce the elastic limit and the mechanical strength of the sheet.

Examples A1 to A4, C1 are those according to the invention.

The mechanical properties are then measured using an ISO 12.5 × 50 specimen and the contents of each of the phases present in the microstructures developed by cross section of the material from the chemical compositions given in Table 1 following the methods described in Table 2. The uni-axial tractions to obtain these mechanical properties are made in the direction parallel to that of the rolling to cold.

• %M1 : fraction surfacique de martensite après le premier recuit
• %F1 : fraction surfacique de ferrite après le premier recuit
• %M2 : fraction surfacique de martensite après le deuxième recuit
• %F2 : fraction surfacique de ferrite après le deuxième recuit
• %RA : fraction surfacique d'austénite résiduelle après le deuxième recuit
• %AM : fraction surfacique de martensite recuite après le deuxième recuit
• %B : fraction surfacique de bainite après le deuxième recuit
• la limite d'élasticité : Re
• la résistance mécanique : Rm
• l'allongement uniforme : Al. Unif.
• Allongement total : Al. Total.

Tableau 3 : Fractions surfaciques de chacune des phases des microstructures et des propriétés mécaniques des références et de l'invention Acier ID %M1 %F1 %M2 %F2 %RA %AM %B Re (MPa) Rm (MPa) Al. Unif. (%) Al. Total. (%) Re/Rm A A_1 97 3 22 3 17 48 10 667 1000 20,6 24,1 0,67 A A_2 96 4 21 4 18 45 12 723 992 17,3 24,3 0,73 A A_3 97 3 17 3 19 46 15 671 984 22,3 28,3 0,68 A A_4 98 2 15 2 21 45 17 684 986 21,5 26,7 0,69 A A_5 59 41 22 41 17 11 9 496 1018 20,1 21,7 0,49 A A_6 60 40 20 40 19 10 11 511 1007 21,5 23,3 0,51 B B_1 98 2 6 2 14 56 22 634 881 16,8 20,5 0,72 B B_2 86 14 8 14 16 48 14 682 925 24,7 30,7 0,74 B B_3 85 15 13 15 19 41 12 662 926 23,8 29,4 0,71 B B_4 84 16 18 16 19 36 11 679 917 25,8 31,3 0,74 C C_1 97 3 14 3 18 53 12 694 981 23,2 29,0 0,71 C C_2 83 17 6 17 17 45 15 714 905 13,7 16,6 0,79 C C_3 81 19 10 19 19 38 14 703 928 24,0 29,4 0,76 C C_4 81 19 19 19 16 33 13 692 916 21,4 26,5 0,76 C C_5 - - 25 48 15 - 12 469 850 17,4 22,2 0,55 D D_1 64 36 17 36 15 26 6 488 999 16,6 22,0 0,49 D D_2 63 37 18 37 14 22 9 500 1039 17,3 19,9 0,48 E E_1 98 2 8 14 21 31 26 600 893 16 20,6 0,67 E E_2 97 3 17 16 18 15 34 550 899 18,8 23,5 0,61 E E_3 98 2 19 17 16 8 40 551 904 18,9 23,6 0,61 E E_4 96 4 15 19 15 3 48 483 872 19,7 25 0,55 E E_5 98 2 13 21 14 43 9 472 925 16,9 20,5 0,51 E E_6 99 1 19 19 16 32 14 545 897 16,3 20,1 0,61 The contents of each of the phases after each annealing and the tensile mechanical properties obtained are given in Table 3 below with the following abbreviations:

• % M1: surface fraction of martensite after first annealing
• % F1: surface fraction of ferrite after the first annealing
• % M2: surface fraction of martensite after the second annealing
• % F2: surface fraction of ferrite after the second annealing
• % RA: surface fraction of residual austenite after the second annealing
• % AM: surface fraction of martensite annealed after the second annealing
• % B: surface fraction of bainite after the second annealing
• the yield strength: Re
• mechanical resistance: Rm
• uniform elongation: Al. Unif.
• Total elongation: Al. Total.

Table 3: Fraction fractions of each of the microstructure phases and the mechanical properties of the references and the invention Steel ID M1% F1% M2% % F2 % RA % AM % B Re (MPa) Rm (MPa) Al. Unif. (%) Al. Total. (%) Re / Rm AT A_1 97 3 22 3 17 48 10 667 1000 20.6 24.1 0.67 AT A_2 96 4 21 4 18 45 12 723 992 17.3 24.3 0.73 AT A_3 97 3 17 3 19 46 15 671 984 22.3 28.3 0.68 AT A_4 98 2 15 2 21 45 17 684 986 21.5 26.7 0.69 AT AT 5 59 41 22 41 17 11 9 496 1018 20.1 21.7 0.49 AT A_6 60 40 20 40 19 10 11 511 1007 21.5 23.3 0.51 B B_1 98 2 6 2 14 56 22 634 881 16.8 20.5 0.72 B B_2 86 14 8 14 16 48 14 682 925 24.7 30.7 0.74 B B_3 85 15 13 15 19 41 12 662 926 23.8 29.4 0.71 B B_4 84 16 18 16 19 36 11 679 917 25.8 31.3 0.74 VS C_1 97 3 14 3 18 53 12 694 981 23.2 29.0 0.71 VS C_2 83 17 6 17 17 45 15 714 905 13.7 16.6 0.79 VS C_3 81 19 10 19 19 38 14 703 928 24.0 29.4 0.76 VS C_4 81 19 19 19 16 33 13 692 916 21.4 26.5 0.76 VS C_5 - - 25 48 15 - 12 469 850 17.4 22.2 0.55 D D_1 64 36 17 36 15 26 6 488 999 16.6 22.0 0.49 D D_2 63 37 18 37 14 22 9 500 1039 17.3 19.9 0.48 E E_1 98 2 8 14 21 31 26 600 893 16 20.6 0.67 E E_2 97 3 17 16 18 15 34 550 899 18.8 23.5 0.61 E E_3 98 2 19 17 16 8 40 551 904 18.9 23.6 0.61 E E_4 96 4 15 19 15 3 48 483 872 19.7 25 0.55 E E_5 98 2 13 21 14 43 9 472 925 16.9 20.5 0.51 E E_6 99 1 19 19 16 32 14 545 897 16.3 20.1 0.61

References A5 and A6, B1 to B4, C2 to C5, D1 and D2, E1 to E6 of Table 3 denote steel sheets manufactured under conditions described in Table 2 from steels whose compositions are given in the table. 1. Mechanical properties and phase fractions not in accordance with the invention are underlined.

Examples A1 to A4 and C1 are those according to the invention.

Note that the references A5, A6, D1 and D2 are not in accordance with the invention because the elastic limit is less than 650 MPa, which is explained by a large amount of ferrite at the end of the first annealing and a small fraction of martensite annealed at the end of the second annealing, which is due to a holding temperature T _soaking1 lower than the calculated temperature TS1.

It is also noted that the references B2 to B4 and C2 to C4 are not in accordance with the invention because the mechanical strength is less than 980 MPa, which is explained by an amount of ferrite greater than 10% after the first annealing, which will limit the fresh martensite fraction at the end of the second annealing, which is due to a holding temperature T _soaking1 lower than the calculated temperature TS1.

Note also that the B1 reference is not in accordance with the invention because the elastic limit is less than 650 MPa and the mechanical strength is less than 980 MPa, which is explained by a quantity of fresh martensite too low to the result of the second annealing, which is due to an end-of-cooling temperature T _{OA of} less than 420 ° C.

It is also noted that the references E1 to E6 are not in accordance with the invention since the elastic limit is less than 650 MPa and the mechanical strength is less than 980 MPa. The non-compliance of these examples reflects an unsuitable chemical composition, in particular the content of elements that harden too low (carbon, silicon) and the lack of precipitation hardening due to the absence of Niobium. This effect is all the more marked for the references E2 to E6, because the process with respect to the invention has not been respected and the quantities of phases obtained are outside the target ranges.

Note finally that the C5 reference is not in accordance with the invention because only a single annealing, corresponding to the method of the second annealing in view of the invention, has been applied, which reflects the absence of annealed martensite necessary for obtain the elastic limit and the mechanical strength referred to in the invention.

The invention also makes it possible to provide a steel sheet capable of depositing a coating of Zinc or Zn alloy, in particular by a hot-quenching process in a liquid Zn bath followed or not by a heat treatment of alliation.

Finally, it makes it possible to provide a steel having a good weldability by means of the usual assembly methods such as, for example and without limitation, spot resistance welding.

The steel sheets according to the invention will be used with advantage for the manufacture of structural parts, reinforcing elements, security, anti abrasive or transmission discs for applications in land motor vehicles.

Claims (16)

1. Steel sheet, the composition of which comprises, the contents being expressed in percentage by weight, $$0.20\%\le \mathrm{C}\le 0.40\%$$

   

   $$0.8\%\le \mathrm{Mn}\le 1.4\%$$

   

   $$1.60\%\le \mathrm{Si}\le 3.00\%$$

   

   $$0.015\le \mathrm{Nb}\le 0.150\%$$

   

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

   

   $$\mathrm{Cr}\le 1.0\%$$

   

   $$\mathrm{S}\le 0.006\%$$

   

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

   

   $$\mathrm{Ti}\le 0.05\%$$

   

   $$\mathrm{V}\le 0.05\%$$

   

   $$\mathrm{Mo}<0.03\%$$

   

   $$\mathrm{B}\le 0.003\%$$

   

   $$\mathrm{N}\le 0.01\%$$

   

   the remainder of the composition being formed from iron and inevitable impurities resulting from manufacture, the microstructure being formed, in surface proportions, from 10 to 30% of residual austenite, from 30 to 60% of annealed martensite, from 5 to 30% of bainite, from 10 to 30% of fresh martensite and from less than 10% of ferrite.
2. Steel sheet according to claim 1, the composition of which comprises, the content being expressed in weight $$0.22\%\le \mathrm{C}\le 0.32\%$$

   
3. Steel sheet according to claim 1 or 2, the composition of which comprises, the content being expressed in weight $$1.0\%\le \mathrm{Mn}\le 1.4\%$$

   
4. Steel sheet according to any of the claims 1 to 3, the composition of which comprises, the content being expressed in weight $$1.80\%\le \mathrm{Si}\le 2.5\%$$

   
5. Steel sheet according to any of the claims 1 to 4, the composition of which comprises, the content being expressed in weight $$\mathrm{Cr}\le 0.5\%$$

   
6. Steel sheet according to any of the claims 1 to 5, the composition of which comprises, the content being expressed in weight $$0.020\%\le \mathrm{Nb}\le 0.13\%$$

   
7. Steel sheet according to any of the claims 1 to 6, comprising a coating of zinc or of zinc alloy.
8. Steel sheet according to any of the claims 1 to 6, comprising a coating of aluminium or of aluminium alloy.
9. Steel sheet according to any of the claims 1 to 8, the mechanical strength of which is greater than or equal to 980 MPa, the limit of elasticity is greater than or equal to 650 MPa, the uniform elongation greater than or equal to 15% and the breaking elongation greater than or equal to 20%.
10. Method for manufacture of a twice-annealed, cold-rolled steel sheet, comprising the following successive steps:

    - making available a steel of composition according to any of the claims 1 to 6, then

    - casting said steel in the form of a semifinished product, then

    - bringing said semifinished product to a temperature T_rech between 1,100°C and 1,280°C in order to obtain a reheated semifinished product, then

    - hot-rolling said reheated semifinished product, the temperature at the end of the hot-rolling T_fl being greater than or equal to 900°C in order to obtain a hot-rolled metal sheet, then

    - coiling said hot-rolled sheet at a temperature T_bob between 400 and 600°C in order to obtain a coiled hot-rolled metal sheet, then

    - cooling said coiled hot-rolled metal sheet to ambient temperature, then

    - uncoiling and pickling said coiled hot-rolled metal sheet, then

    - cold-rolling said hot-rolled metal sheet with a reduction ratio between 30 and 80% in order to obtain a cold-rolled metal sheet, then

    - annealing, for a first time, said cold-rolled metal sheet by reheating it at a speed Vci between 2 and 50°C/s up to a temperature T_soaking1 between TS1 = 910.7 - 431.4*C - 45.6*Mn + 54.4*Si - 13.5*Cr + 52.2*Nb and 950°C, the contents being expressed in percentage by weight, for a duration t_soaking1 between 30 and 200 seconds, then:

    - cooling said metal sheet by subjecting it to cooling to ambient temperature at a speed greater than or equal to 30°C/s, then

    - annealing, for a second time, said metal sheet by reheating it at a speed V_C2 between 2 and 50°C/s up to a temperature T_soaking2 between Ac1 and TS2 = 906.5 - 440.6*C - 44.5*Mn + 49.2*Si - 12.4*Cr + 55.9*Nb, the contents being expressed in percentage by weight, for a duration t_soaking2 between 30 and 200 seconds, then

    - cooling said metal sheet by subjecting it to cooling at a speed greater than or equal to 30°C/s to the end cooling temperature T_OA between 420°C and 480°C, then

    - soaking said metal sheet within the temperature range from 420 to 480°C for a duration t_OA between 5 and 120 seconds, then

    - optionally depositing a coating on the cold-rolled and annealed metal sheet

    - cooling said metal sheet to ambient temperature.
11. Method for manufacture according to claim 10, in which furthermore, annealing, termed basic, of said coiled hot-rolled metal sheet is effected before cold-rolling such that the metal sheet is heated then soaked at a temperature between 400°C and 700°C for a duration between 5 and 24 hours.
12. Method for manufacture according to any of the claims 10 or 11, in which said metal sheet is soaked isothermally at the end cooling temperature T_OA between 420 and 480°C between 5 and 120 seconds.
13. Method for manufacture according to any of the claims 10 to 12, in which the twice-annealed, cold-rolled metal sheet is then cold-rolled with a cold-rolling ratio between 0.1 and 3% before deposition of a coating.
14. Method for manufacture according to any of the claims 10 to 13, in which the metal sheet is finally heated to a soaking temperature T_base between 150°C and 190°C for a soaking time t_base between 10 h and 48 h.
15. Method for manufacture according to any of the claims 10 to 12, in which, at the end of the soaking at T_OA, the metal sheet is dip-coated in a liquid bath of one of the following elements: aluminium, zinc, alloy of aluminium or alloy of zinc.
16. Use of a metal sheet according to any of the claims 1 to 9, or a metal sheet manufactured by a method according to any of the claims 10 to 15 for manufacture of parts for vehicles.