EP2155915B1 - Process for manufacturing cold-rolled and annealed steel sheets with very high strength, and sheets thus produced - Google Patents

Description

The invention relates to the manufacture of cold-rolled and annealed thin sheets of steels having a strength greater than 1200 MPa and an elongation at break greater than 8%. The automotive sector and the general industry are notably fields of application for these steel sheets.

In particular, there is a continuing need in the automotive industry for lighter vehicles and increased safety. Different families of steels have been successively proposed to meet this need for increased strength: first, steels comprising micro-alloy elements have been proposed. Their hardening is due to the precipitation of these elements and the refinement of the grain size. We then witnessed the development of "Dual-Phase" steels where the presence of martensite, constituting a great hardness, in a softer ferritic matrix, allows to obtain a resistance greater than 450MPa associated with good cold forming ability.

In order to further increase the resistance, steels having a "TRIP" (Transformation Induced Plasticity) behavior with very advantageous combinations of properties (resistance-ability to deformation) have been developed: these properties are related to the structure of these structures. steels consisting of a ferritic matrix comprising bainite and residual austenite. The presence of the latter component gives a high ductility to a non-deformed sheet. Under the effect of a subsequent deformation, for example during a uniaxial loading, the residual austenite of a TRIP steel part gradually changes to martensite, which results in a significant consolidation and delays the appearance of 'localized deformation.

• L'accroissement de résistance mécanique nécessite une analyse chimique nettement plus chargée en éléments d'alliage, au détriment de l'aptitude au soudage de ces aciers.
• On observe un accroissement de la différence de dureté entre la matrice ferritique et les constituants durcissants : ceci a pour conséquence une concentration locale des contraintes et des déformations et un endommagement plus précoce, comme en témoigne la baisse de l'allongement.
• On observe également un accroissement de la fraction des constituants durcissants au sein de la matrice ferritique : dans ce cas, les îlots, initialement isolés et de petite taille lorsque la résistance est faible, deviennent progressivement connexes et forment des constituants de grande taille qui favorisent là encore un endommagement précoce.

Dual Phase or TRIP steel plates have been proposed, with a maximum resistance level of the order of 1000 MPa. Obtaining significantly higher resistance levels, for example 1200-1400MPa, faces various difficulties:

• The increase in mechanical strength requires a much more heavily loaded chemical analysis of alloying elements, to the detriment of the weldability of these steels.
• An increase in the difference in hardness between the ferritic matrix and the hardening constituents is observed: this results in a local concentration of stresses and deformations and an earlier damage, as evidenced by the decrease in elongation.
• There is also an increase in the fraction of hardening components within the ferritic matrix: in this case, the islets, initially isolated and small when the resistance is low, become progressively connected and form large constituents which favor there still early damage.

The possibilities of simultaneously obtaining very high levels of resistance and certain other properties of use using TRIP or Dual Phase microstructure steels thus seem limited. To achieve an even higher resistance, ie a level greater than 800-1000 MPa, so-called "multiphase" steels have a predominantly bainitic structure. In the automotive industry or in the general industry, multi-phase steel sheets of medium thickness are used with advantage for structural parts such as bumper crosspieces, uprights, various reinforcements.

In particular, in the field of cold-rolled multiphase steel sheets over 980 MPa, the patent EP1559798 discloses the manufacture of steels of composition: 0.10-0.25% C, 1.0-2.0% Si, 1.5-3% Mn, the microstructure consisting of at least 60% bainitic ferrite and at least 5% residual austenite, the polygonal ferrite being less than 20%. The exemplary embodiments presented in this document show that the resistance does not exceed 1200 MPa.

The patent EP 1589126 also discloses the manufacture of cold-rolled thin sheet, the product of which (resistance x elongation) is greater than 20000 MPa%. The composition of the steels contains: 0.10-0.28% C, 1.0-2.0% Si, 1-3% Mn, less than 0.10% Nb. The structure consists of more than 50% bainitic ferrite, 5 to 20% residual austenite, and less than 30% polygonal ferrite. Again, the examples presented show that the resistance is still below 1200 MPa. JP10280090 discloses a steel sheet and the production process of cold-rolled steel sheet with very high strength, the sheet comprising by weight between 0.13-0.20% C, ≤0.6% Si, 1.8-2.8% Mn, ≤0.02% P, ≤0.015% S, 0.005-0.1% Al, ≤0.0060% N, and optionally 0.01-0.15% Mo and 0.0005-0.0020% B, the remainder being iron and unavoidable residual impurities. The microstructure dedicates steel comprising bainite and martensite. The microstructure can be achieved by hot rolling control, winding, pickling, cold rolling, heat treatment; the steel sheet will be tensile strength of about 780-1470 MPa. The present invention aims to solve the problems mentioned above. It aims to provide a cold rolled and annealed thin steel sheet having a mechanical strength greater than 1200 MPa together with an elongation greater than 8% rupture and good cold forming ability. The invention also aims at providing a steel that is not very sensitive to damage during cutting by a mechanical method.

Furthermore, the invention aims to provide a method of manufacturing thin sheets, small variations of the parameters do not lead to significant changes in the microstructure or mechanical properties. The invention also aims to provide a sheet of steel easily fabricated by cold rolling, that is to say whose hardness after the hot rolling step is limited so that the rolling forces remain moderate during of the cold rolling step.

It also aims to have a thin steel sheet suitable for the possible deposit of a metal coating according to the usual methods.

It also aims to have a steel sheet insensitive to damage by cutting and able to expand the hole.

It also aims to have a steel having good weldability by means of conventional assembly methods such as spot resistance welding.

For this purpose, the subject of the invention is a cold-rolled and annealed steel sheet with a resistance greater than 1200 MPa, the composition of which comprises the contents being expressed by weight: 0.10% ≤ C ≤ 0.25% , 1% ≤ Mn ≤ 3%, Al ≥ 0.010%, Si≤2.990%, S ≤ 0.015%, P ≤ 0.1%, N ≤0.008%, with the proviso that 1% ≤Si + Al ≤3%, the composition optionally comprising: 0.05% ≤ V ≤ 0.15%, B≤0.005%, Mo ≤ 0.25%, Cr ≤ 1.65%, it being understood that Cr + (3 × Mo) ≥0.3%, Ti in an amount such that Ti / N≥4 and that Ti 00.040%, the rest of the composition consisting of iron and unavoidable impurities resulting from the elaboration, the microstructure of said steel comprising 15 to 90% of bainite, the balance consisting of martensite and residual austenite.

The subject of the invention is also a sheet of steel with the above composition, with an elongation at break greater than 10%, characterized in that Mo <0.005%, Cr <0.005%, B = 0, the microstructure of the steel comprising 65 to 90% bainite, the balance consisting of islands of martensite and residual austenite

The invention also relates to a steel sheet of the above composition, characterized in that it contains: Mo ≤ 0.25%, Cr ≤ 1.65%, it being understood that Cr + (3 × Mo) ≥ 0.3%, B = 0, the microstructure of the steel comprising 65 to 90% of bainite, the remainder being islands of martensite and residual austenite

The subject of the invention is also a steel sheet of the above composition, with a resistance greater than 1400 MPa, with an elongation at break greater than 8%, characterized in that it contains: Mo ≤ 0.25%, Cr ≤ 1.65%, it being understood that Cr + (3 x Mo) ≥0.3%, the microstructure of the steel comprising 45 to 65% of bainite, the remainder being islands of martensite and residual austenite L Another subject of the invention is a steel sheet of the above composition, with a resistance greater than 1600 MPa, with an elongation at break greater than 8%, characterized in that it contains: Mo ≤ 0.25%, Cr ≤ 1.65%, it being understood that: Cr + (3 x Mo) ≥0.3%, the microstructure of the steel comprising 15 to 45% of bainite, the remainder consisting of martensite and residual austenite.

According to one particular embodiment, the composition comprises: 0.19% ≤ C ≤ 0.23% According to a preferred mode, the composition comprises: 1.5% ≤ Mn ≤ 2.5% Preferably, the composition comprises: 1.2% ≤Si ≤ 1.8% Preferably, the composition comprises: 1.2% ≤Al ≤ 1.8% According to a particular embodiment, the composition comprises: 0.05% ≤ V ≤ 0.15% 0.004 ≤ N ≤ 0.008%.

Preferably, the composition comprises: 0.12% ≤ V ≤ 0.15% According to a preferred mode, the composition comprises: 0.0005 B B 0,00 0.003%.

Preferably, the average size of the islands of martensite and residual austenite is less than 1 micrometer, the average distance between the islands being less than 6 microns.

The subject of the invention is also a process for manufacturing a cold-rolled steel sheet with a resistance greater than 1200 MPa, with an elongation at break greater than 10%, according to which a composition steel is supplied: 0.10 % ≤ C ≤ 0.25%, 1% ≤ Mn ≤ 3%, Al ≥ 0.010%, Si≤2.990%, provided that: 1% ≤Si + Al ≤3%, S ≤ 0.015%, P≤ 0, 1%, N≤0.008%, Mo <0.005%, Cr <0.005%, B = 0, the composition optionally comprising: 0.05% ≤V ≤ 0.15%, Ti in an amount such that Ti / N≥4 and than Ti≤0.040%. A semi-finished product is cast from this steel, then the semi-finished product is heated to a temperature above 1150 ° C. and the semi-finished product is hot-rolled to obtain a hot-rolled sheet. The sheet is reeled and stripped and then cold rolled with a reduction ratio of between 30 and 80% so as to obtain a cold-rolled sheet. The cold-rolled sheet is heated at a speed V _c of between 5 and 15 ° C./s up to a temperature T ₁ of between Ac 3 and Ac 3 + 20 ° C., for a time t ₁ of between 50 and 150 seconds. cools the sheet at a speed V _R1 greater than 40 ° C / s and less than 100 ° C / s to a temperature T ₂ between (M _s -30 ° C and M _s + 30 ° C). The sheet is maintained at said temperature T ₂ for a time t ₂ of between 150 and 350 seconds and then cooling is carried out at a speed V _{R2 of} less than 30 ° C./s up to room temperature. The subject of the invention is also a process for manufacturing a cold-rolled steel sheet with a resistance greater than 1200 MPa and an elongation at break greater than 8%, according to which a steel with a composition of 0.10 is supplied. % ≤ C ≤ 0.25%, 1% ≤ Mn ≤ 3%, Al ≥ 0.010%, Si≤2.990%, provided that 1% ≤Si + Al ≤3%, S ≤ 0.015%, P≤ 0.1 %, N≤0.008%, Mo ≤ 0.25%, Cr ≤ 1.65%, with the proviso that Cr + (3 x Mo) ≥0.3%, optionally 0.05% ≤ V ≤ 0.15%, B ≤0.005%, Ti in an amount such that Ti / N≥4 and Ti≤0.040%. A semifinished product is cast from this steel, the semi-finished product is heated to a temperature above 1150 ° C., and then the semi-finished product is hot-rolled to obtain a hot-rolled sheet. We reel the sheet, it is scoured, and then cold rolled sheet with a reduction rate of between 30 and 80% to obtain a cold rolled sheet. The cold-rolled sheet is heated at a speed V _c of between 5 and 15 ° C./s up to a temperature T ₁ of between Ac 3 and Ac 3 + 20 ° C., for a time t ₁ of between 50 and 150 seconds. it cools at a speed V _R1 greater than 25 ° C / s and less than 100 ° C / s to a temperature T ₂ between B _s and (M _s - 20 ° C) The sheet is maintained at temperature T ₂ for a time t ₂ of between 150 and 350 seconds, then cooling is carried out at a speed V _{R2 of} less than 30 ° C./s up to room temperature.

The temperature T ₁ is preferably between Ac3 + 10 ° C to Ac3 + 20 ° C.

The invention also relates to the use of a cold rolled steel sheet annealed in one of the above modes, or manufactured by a method according to one of the above modes, for the manufacture structural parts or reinforcement elements, in the automotive field.

• La figure 1 présente un exemple de structure d'une tôle d'acier selon l'invention, la structure étant révélée par réactif LePera.
• La figure 2 présente un exemple de structure d'une tôle d'acier selon l'invention, la structure étant révélée par réactif Nital.

Other features and advantages of the invention will become apparent from the description below, given by way of example and with reference to the appended figures attached hereto:

• The figure 1 shows an example of structure of a steel sheet according to the invention, the structure being revealed by LePera reagent.
• The figure 2 shows an example of structure of a steel sheet according to the invention, the structure being revealed by Nital reagent.

The inventors have demonstrated that the above problems were solved when the annealed cold-rolled thin steel sheet exhibited a bainitic microstructure, with in addition islands of martensite and residual austenite, or "M-A" islands. For steels with the highest strength greater than 1600 MPa, the microstructure contains a greater amount of martensite and residual austenite.

With regard to the chemical composition of steel, carbon plays a very important role in the formation of the microstructure and in the mechanical properties: in combination with other elements of the composition (Cr, Mo, Mn) and with the annealing heat treatment after cold rolling, it increases the hardenability and allows to obtain a bainitic transformation. The carbon contents according to the invention also lead to the formation of islands of martensite and residual austenite whose quantity, morphology, composition make it possible to obtain the properties referred to above.

The carbon also retards the formation of the pro-eutectoid ferrite after annealing heat treatment after cold rolling: otherwise, the presence of this phase of low hardness would cause excessive local damage at the interface with the matrix. Hardness is higher. The presence of proeutectoid ferrite resulting from the annealing must therefore be avoided in order to obtain high levels of mechanical strength.

According to the invention, the carbon content is between 0.10 and 0.25% by weight: Below 0.10%, sufficient strength can not be obtained and the stability of the residual austenite is not not satisfactory. Beyond 0.25%, the weldability is reduced due to the formation of quenching microstructures in the heat-affected zone.

According to a preferred mode, the carbon content is between 0.19 and 0.23%: within this range, the weldability is very satisfactory, and the quantity, the stability and the morphology of the islets MA are particularly adapted to obtain a favorable pair of mechanical properties (resistance-elongation)

In an amount of between 1 and 3% by weight, an addition of manganese, a gamma-type element, makes it possible to avoid the formation of proeutectoid ferrite during cooling after annealing after cold rolling. Manganese also helps to deoxidize steel during liquid phase processing. The addition of manganese also contributes to effective solid solution hardening and increased strength. Preferably, the manganese is between 1.5 and 2.5% so that these effects are obtained, and without risk of formation of harmful band structure.

Silicon and aluminum play an important role together according to the invention.

Silicon delays the precipitation of cementite during cooling from the austenite after annealing. An addition of silicon according to the invention Thus, it helps to stabilize a sufficient quantity of residual austenite in the form of islets which subsequently transform and progressively become martensite under the effect of a deformation. Another part of the austenite is transformed directly into martensite during cooling after annealing. Aluminum is a very effective element for the deoxidation of steel. As such, its content is greater than or equal to 0.010%. Like silicon, it stabilizes residual austenite.

The effects of aluminum and silicon on the stabilization of austenite are similar; when the silicon and aluminum contents are such that: 1% ≤Si + Al≤3%, a satisfactory stabilization of the austenite is obtained, which makes it possible to form the desired microstructures while retaining satisfactory use properties. Given that the minimum aluminum content is 0.010%, the silicon content is less than or equal to 2.990%.

The silicon content is preferably between 1.2 and 1.8% to stabilize a sufficient amount of residual austenite and to avoid intergranular oxidation during the hot winding step preceding the cold rolling. This also avoids the formation of strongly adherent oxides and the possible appearance of surface defects leading in particular to a lack of wettability in dip galvanizing operations.

These effects are also obtained when the aluminum content is preferably between 1.2 and 1.8%. At equivalent content, the effects of aluminum are indeed similar to those described above for silicon, but the risk of occurrence of superficial defects is however less.

The steels according to the invention optionally comprise molybdenum and / or chromium: molybdenum increases quenchability, avoids the formation of pro-eutectoid ferrite and effectively refines the bainitic microstructure. However, a content greater than 0.25% by weight increases the risk of forming a predominantly martensitic microstructure to the detriment of bainite formation.

Chromium also helps to prevent the formation of pro-eutectoid ferrite and the refinement of the bainitic microstructure. Beyond 1.65%, the risk of obtaining a predominantly martensitic structure is important. Compared to molybdenum, its effect is however less marked; according to the invention, the chromium and molybdenum contents are such that: Cr + (3 × Mo) ≥0.3%. The coefficients of chromium and molybdenum in this relationship reflect their influence on the quenchability, in particular the respective ability of these elements to avoid the formation of pro-eutectoid ferrite under the particular cooling conditions of the invention.

According to an economic mode of the invention, the steel may comprise very low or zero molybdenum and chromium contents, ie contents of less than 0.005% by weight for these two elements, and 0% boron.

To obtain a strength greater than 1400 MPa, the addition of chromium and / or molybdenum is required, in amounts mentioned above. When the sulfur content is greater than 0.015%, the shaping ability is reduced due to the excessive presence of manganese sulphides.

The phosphorus content is limited to 0.1% so as to maintain sufficient hot ductility.

The nitrogen content is limited to 0.008% to avoid possible aging.

The steel according to the invention optionally contains vanadium in an amount of between 0.05 and 0.15%. In particular, when the nitrogen content is between 0.004 and 0.008%, the precipitation of vanadium can occur during annealing after cold rolling in the form of fine carbonitrides which give additional hardening.

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

The steel may optionally comprise boron in an amount of less than or equal to 0.005%. In a preferred embodiment, the steel preferentially contains between 0.0005 and 0.003% of boron, which contributes to the suppression of pro-eutectoid ferrite in the presence of chromium and / or molybdenum. In addition to the other elements of addition, the addition of boron in quantity mentioned above makes it possible to obtain a resistance greater than 1400 MPa.

The steel may optionally comprise titanium in an amount such as Ti / N≥4 and Ti≤0.040%, which allows the formation of titanium carbonitrides and increases the hardening.

The rest of the composition consists of unavoidable impurities resulting from the elaboration. The contents of these impurities, such as Sn, Sb, As, are less than 0.005%.

According to one embodiment of the invention intended for the manufacture of steel sheets with a resistance greater than 1200 MPa, the microstructure of the steel is composed of 65 to 90% of bainite, these contents referring to surface percentages, the balance consists of islands of martensite and residual austenite (islets of MA compounds)

This bainitic structure, which does not contain proeutectoid ferrite of low hardness, has an elongation capacity greater than 10%.

According to the invention, the M-A islands regularly dispersed in the matrix have an average size of less than 1 micrometer.

The figure 1 shows an example of microstructure of a steel sheet according to the invention. The morphology of the islets MA was revealed by means of appropriate chemical reagents: after attack, the islets MA appear in white on a bainitic matrix more or less dark. Some small islands are located between the slats of bainite ferrite. The islands are observed at magnitudes ranging from 500 to 1500x on a statistically representative surface and the average size of the islands as well as the average distance between these islets is measured using an image analysis software. In the case of figure 1 , the surface percentage of the islets is 12% and the average size of the islets MA is less than 1 micrometer.

• un endommagement limité en raison de l'absence d'amorçage de la rupture sur des îlots M-A de grande taille
• un durcissement significatif en raison de la proximité de nombreux constituants M-A de faible taille

It has been demonstrated that a specific morphology of the MA islets is to be particularly sought: when the average size of the islets is less than 1 micrometer and when the average distance between these islets is less than 6 micrometers, the following effects are simultaneously obtained:

• limited damage due to lack of breakout initiation on large MA islands
• a significant hardening due to the proximity of many small MA constituents

According to another embodiment of the invention intended for the manufacture of steel sheets having a strength of greater than 1400 MPa and an elongation at break greater than 8%, the microstructure is composed of 45 to 65% of bainite, the balance being consisting of islands of martensite and residual austenite.

According to another embodiment of the invention intended for the manufacture of steel sheets with a resistance greater than 1600 MPa and an elongation at break greater than 8%, the microstructure is composed of 15 to 45% of bainite, the balance being consisting of martensite and residual austenite.

• 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 implementation of the method for manufacturing a cold rolled and annealed thin sheet according to the invention is 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. The casting can also be carried out in the form of thin 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 higher than 1150 ° C. to reach at any point a temperature favorable to the high deformations which the steel will undergo during rolling. Naturally, in the case of a direct casting of thin slabs or thin strips between contra-rotating rolls, the hot rolling step of these semi-finished products starting at more than 1150 ° C. can be done directly after casting. that an intermediate heating step is not necessary in this case.

The semi-finished product is hot-rolled. An advantage of the invention is that the final characteristics and the microstructure of the cold-rolled and annealed sheet are relatively independent of the end-of-rolling temperature and the cooling after hot rolling.

The sheet is then reeled hot. The winding temperature is preferably less than 550 ° C to limit the hardness of the hot-rolled sheet and the intergranular oxidation at the surface. Too much hardness of the hot-rolled sheet leads to excessive forces during subsequent cold rolling and possibly to edge defects.

The hot-rolled sheet is then etched according to a method known per se in order to give the latter a surface state suitable for cold rolling. This is done by reducing the thickness of the hot-rolled sheet by 30 to 80%.

• Une phase de chauffage avec une vitesse V_c comprise entre 5 et 15°C/s. jusqu'à une température T₁. Lorsque V_c est supérieure à 15°C/s, la recristallisation de la tôle écrouie par le laminage à froid peut ne pas être totale. Une valeur minimale de 5°C/s est requise pour la productivité. Une vitesse V_c comprise entre 5 et 15°C/s permet d'obtenir une taille de grain d'austénite particulièrement adaptée à la microstructure finale désirée.

An annealing heat treatment is then carried out, preferably by continuous annealing, which comprises the following phases:

• A heating phase with a speed V _c of between 5 and 15 ° C / s. up to a temperature T ₁ . When V _c is greater than 15 ° C / s, the recrystallization of the cold-worked sheet by cold rolling may not be complete. A minimum value of 5 ° C / s is required for productivity. A speed V _c of between 5 and 15 ° C./s makes it possible to obtain an austenite grain size that is particularly adapted to the desired final microstructure.

The temperature T ₁ is between A _c3 and A _c3 + 20 ° C, the temperature A _c3 corresponding to the total conversion to austenite during heating. A _c3 depends on the composition of the steel and the heating rate and can be determined for example by dilatometry. Total austenitization limits the subsequent formation of proeutectoid ferrite. It is important that the temperature T ₁ be less than A _c3 + 20 ° C in order to avoid exaggerated magnification of the austenitic grain. Within this range (A _c3 -A _c3 + 20 ° C), the characteristics of the final product are insensitive to a temperature variation T ₁ .

• Un maintien à la température T₁ pendant un temps t₁ compris entre 50s et 150s. Cette étape conduit à une homogénéisation de l'austénite.

Very preferably, the temperature T ₁ is between A _c3 + 10 ° C and A _c3 + 20 ° C. Under these conditions, the inventors have demonstrated that the austenite grain size is more homogeneous and finer, which subsequently leads to the formation of a final microstructure also having these characteristics.

• A maintenance temperature T ₁ for a time t ₁ between 50s and 150s. This step leads to a homogenization of the austenite.

• Lorsque l'acier ne comporte pratiquement pas de chrome, de molybdène et de bore, c'est à dire lorsque Cr<0,005%, Mo<0,005%, B=0%, on effectue un refroidissement avec une vitesse V_R1 supérieure à 40°C/s et inférieure à 100°C/s jusqu'à une température T₂ comprise entre M_s-30°C et M_s+30°C. Pour ces conditions de vitesse de refroidissement, la diffusion du carbone dans l'austénite est limitée. Cet effet est saturé au delà de 100°C/s. Un maintien est réalisé à cette température T₂ pendant un temps t₂ compris entre 150 et 350s. M_S désigne la température de début de transformation martensitique. Cette température dépend de la composition de l'acier mis en oeuvre et peut être déterminée par exemple par dilatométrie. Ces conditions permettent d'éviter la formation de ferrite proeutectoïde lors du refroidissement. On obtient également dans ces conditions une transformation bainitique de la plus grande partie de l'austénite. La fraction restante est transformée en martensite ou est éventuellement stabilisée sous forme d'austénite résiduelle.
• Lorsque l'acier comporte une teneur en chrome et en molybdène telles que Mo ≤ 0,25%, Cr ≤ 1,65%, et Cr+(3 x Mo) ≥0,3%, on effectue un refroidissement avec une vitesse V_R1 supérieure à 25°C/s et inférieure à 100°C/s jusqu'à une température T₂ comprise entre (B_s et M_s-20°C) Un maintien est réalisé à cette température T₂ pendant un temps t₂ compris entre 150 et 350s. B_s désigne la température de début de transformation bainitique. Ces conditions permettent d'obtenir les mêmes caractéristiques microstructurales que ci-dessus. L'addition de chrome et/ou de molybdène permet en particulier de garantir que la formation de ferrite proeutectoïde n'intervient pas. Dans les limites de vitesse de refroidissement V_R1 selon l'invention, les caractéristiques finales du produit sont relativement peu sensibles à une variation de cette vitesse V_R1.
• L'étape suivante du procédé est identique, que le produit comporte ou non du chrome et/ou du molybdène : on effectue un refroidissement à une vitesse V_R2 inférieure à 30°C /s jusqu'à la température ambiante. En particulier, lorsque la température T₂ est peu élevée au sein des plages selon l'invention, le refroidissement à une vitesse V_R2 inférieure à 30°C /s provoque un revenu des îlots de martensite nouvellement formée, ce qui est favorable en termes de propriétés d'usage.

The next step in the process depends on the chromium and molybdenum content of the steel:

• When the steel has practically no chromium, molybdenum and boron, that is to say when Cr <0.005%, Mo <0.005%, B = 0%, cooling is carried out with a speed V _R1 greater than 40 ° C / s and less than 100 ° C / s to a temperature T ₂ between M _s -30 ° C and M _s + 30 ° C. For these cooling rate conditions, carbon diffusion in the austenite is limited. This effect is saturated beyond 100 ° C / s. Hold is performed at this temperature T ₂ for a time t ₂ between 150 and 350s. M _S denotes the martensitic transformation start temperature. This temperature depends on the composition of the steel used and can be determined for example by dilatometry. These conditions prevent the formation of proeutectoid ferrite during cooling. In these conditions, a bainitic transformation of most of the austenite is also obtained. The remaining fraction is converted to martensite or is optionally stabilized as a residual austenite.
• When the steel has a content of chromium and molybdenum such that Mo ≤ 0.25%, Cr ≤ 1.65%, and Cr + (3 x Mo) ≥0.3%, cooling is carried out with a speed V _R1 greater than 25 ° C / s and less than 100 ° C / s to a temperature T ₂ between (B _s and M _s -20 ° C) Hold is performed at this temperature T ₂ for a time t ₂ included between 150 and 350s. B _s denotes the bainitic transformation start temperature. These conditions make it possible to obtain the same microstructural characteristics as above. The addition of chromium and / or molybdenum makes it possible in particular to ensure that the formation of proeutectoid ferrite does not occur. Within the cooling speed limits V _R1 according to the invention, the final characteristics of the product are relatively insensitive to a variation of this speed V _R1 .
• The next step in the process is the same, whether or not the product contains chromium and / or molybdenum: a speed V _{R2 of} less than 30 ° C / s to ambient temperature. In particular, when the temperature T ₂ is low within the ranges according to the invention, cooling at a speed V _{R2 of} less than 30 ° C./s causes a return of the islands of newly formed martensite, which is favorable in terms of of use properties.

Example:

Steels have been developed, the composition of which is given in the table below, expressed in percentage by weight. In addition to the steels I-1 to I-5 used in the manufacture of sheets according to the invention, the composition of steels R-1 to R-5 used for the manufacture of reference sheets has been indicated for comparison purposes. . Table 1 Compositions of steel (% by weight). I = according to the invention. R = reference Underlined values: Not in accordance with the invention. Steel VS (%) Mn (%) Yes (%) Al (%) Si + Al (%) Mo (%) Cr (%) Cr + (3xMo) (%) S (%) P (%) V (%) Ti (%) B (%) NOT (%) I-1 0.19 2 1.5 0,040 1.54 - - - 0,003 0,015 - - - 0,004 I-2 0.2 2 1.5 0,040 1.54 0.25 - 0.75 0,003 0,015 - - - 0,004 I-3 0.19 2 1.5 0,040 1.54 0.14 0.34 0.76 0,003 0,015 - - - 0,004 I-4 0.2 2 1.5 0,040 1.54 0.25 - 0.75 0,003 0,015 - 0,020 0.0038 0,004 I-5 0.2 2 1.5 0,040 1.54 0.25 - 0.75 0,003 0,015 0.15 0,020 0.0038 0,004 R-1 0.110 2.2 0.347 0.031 0,378 0.13 0.4 0.79 0,003 0,015 - 0,027 - 0,004 R-2 0,038 0.212 0,036 0.053 0.089 1.1 0.21 3.51 0,003 0,015 - 0,002 - 0,004 R-3 0,035 0.21 0,035 0,054 0.089 0.5 0,034 1,534 0,003 0,015 - 0,002 - 0,004 R-4 0.19 1.3 0.25 0,040 0.29 - 0.18 0.18 0,003 0,015 - 0,003 0.006 R-5 0,148 1,925 0.214 0,024 0.238 - 0.19 0.19 0,002 0.012 - 0,024 - 0.005

Semi-products corresponding to the above compositions were heated to 1200 ° C, hot rolled to a thickness of 3 mm and coiled at a temperature below 550 ° C. The sheets were then cold rolled to a thickness of 0.9 mm or a reduction rate of 70%. From the same composition, some steels have been subject to different manufacturing conditions. References I1-a, I1-b and I1-c, I1-d designate for example four steel sheets manufactured under different conditions from the steel composition I1. Table 2 shows the manufacturing conditions of the annealed sheets after cold rolling. The heating rate V _c is 10 ° C / s in all cases.

The transformation temperatures A _c3 , B _s and M _s were also reported in Table 2.

The different microstructural constituents measured by quantitative microscopy were also reported: surface fraction of bainite, martensite and residual austenite.

The MA islets have been highlighted by LePera reagent. Their morphology was examined using Scion® image analysis software. Table 2: Manufacturing conditions and microstructure of the hot-rolled sheets obtained. I = according to the invention. R = reference Underlined values: Not according to the invention. Galvanised steel T ₁ (° C) Ac3 (° C) t ₁ (s) V _R1 (° C / s) T ₂ (° C) B _s (° C) M _s (° C) t ₂ (s) V _R2 (C ° / s) I1-a 850 830 100 54 350 600 380 200 15 I1-b 800 830 100 54 400 600 380 200 15 I1-c 825 830 100 54 400 600 380 200 15 I1-d 850 830 100 54 450 600 380 200 15 I2-a 850 830 100 54 400 575 375 200 15 I2-b 850 830 120 54 400 575 375 240 15 I2-c 850 830 95 22 400 575 375 200 5 I3-a 850 830 100 54 400 565 395 200 15 I3-b 850 830 100 65 350 565 395 200 15 I4 850 830 100 54 400 575 375 200 15 I5 850 830 100 54 400 575 375 200 15 R1 850 845 100 54 400 520 425 200 15 R2 800 930 60 20 460 695 510 20 15 R3 800 915 60 20 460 760 520 20 15 R4 850 845 300 20 460 650 425 20 15 R5 800 900 60 20 460 605 425 60 20

The tensile mechanical properties obtained (yield strength Re, resistance Rm, uniform elongation Au, elongation at break At) were given in Table 3 below. The Re / Rm ratio was also indicated.

In some cases, the breaking energy at -40 ° C was determined from Charpy V type resilience specimens reduced to 1.4 mm thickness. Damage related to a cut (for example, shearing or punching) has also been evaluated which could possibly reduce the capacity for subsequent deformation of a cut piece. For this purpose, 20 × 80 mm ² specimens were cut by shearing. Some of these specimens were then polished at the edges. The specimens were coated with photodeposited grids and then subjected to uniaxial traction until rupture. The values of the principal deformations ε ₁ parallel to the direction of the stress have been measured as close as possible to the initiation of the rupture from the deformed grids. This measurement was carried out on the specimens with mechanically cut edges and on the specimens with polished edges. The sensitivity to cutting is evaluated by the damage factor: Δ = ε ₁ (cut edges) -ε ₁ (polished edges) / ε ₁ (polished edges).

For some sheets, damage was also evaluated in the vicinity of edges cut from samples of 105x105mm ² having a hole with an initial diameter of 10mm. The relative increase in the diameter of the hole is measured after introducing a conical punch until a crack appears. Table 3: Mechanical properties of cold-rolled and annealed sheets. Underlined Values: Not in accordance with the invention. Nd: not determined Galvanised steel Bainitic fraction (%) Fraction (MA) (%) Island size (MA) <1 micron and average distance <6 micrometer Re (MPa) Rm (MPa) At (%) At (%) KCV (-40 ° C) J / cm ² Damage Δ cut edges (%) Expansion (%) I1-a 89 11 Yes 718 1200 7.5 11.2 63 35 I1-b 43 17 No 490 1020 15 19 I1-c 63 17 Yes 500 1040 14 17 36 I1-d 83 17 No 550 1100 9 12 I2-a 88 12 Yes 800 1250 8.8 12.7 -14 I2-b 90 10 Yes 790 1260 8.2 12 I2-c nd nd nd 700 1200 7 8.5 I3-a 88 12 Yes 750 1200 9.5 12.7 40 I3-b nd nd nd 900 1300 9 8 I4 60 40 Yes 690 1420 8 11.2 -22.5 I5 45 55 nd 800 1600 7.5 10 R1 nd nd nd 800 950 4 6 R2 Ferrite 6 nd 400 520 10 16 R3 Ferrite 5 nd 300 450 16 21 R4 60 40 nd 650 950 nd 4 R5 Ferrite 17 Yes 404 856 12.4 16 -43

The sheets of composition according to the invention and manufactured according to the conditions of the invention (I1-a, I2-ab, 13-a, 14, 15) have a particularly advantageous combination of mechanical properties: on the one hand a resistance mechanical higher than 1200 MPa, on the other hand an elongation at break always greater than or equal to 10%. The steels according to the invention also have a Charpy V fracture energy at -40 ° C. of greater than 40 Joules / cm ² . This allows the manufacture of parts resistant to the sudden propagation of a fault especially in case of dynamic stresses. The microstructures of the steels with a minimum strength of 1200 MPa and a minimum breaking elongation of 10% according to the invention comprise a bainite content between 65 and 90%, the balance consisting of islets MA. The figure 1 thus shows the microstructure of I3a steel sheet with 88% bainite and 12% islet MA, revealed by a LePera reagent attack. The figure 2 presents this microstructure revealed by a Nital attack. In the case of steels having a minimum strength of 1400 MPa and a minimum breaking elongation of 8%, the steels according to the invention have a bainite content of between 45 and 65%, the balance being MA islands. In the case of steels having a minimum strength of 1600 MPa and a minimum breaking elongation of 8%, the steels according to the invention have a bainite content of between 15 and 35%, the balance being martensite and residual austenite. The steel sheets according to the invention have an island size of less than 1 micrometer MA, the inter-island distance being less than 6 micrometers.

The steels according to the invention also have good resistance to damage in case of cutting since the damage factor Δ is limited to -23%. A steel sheet that does not have these characteristics (R5) may have a 43% damage factor. The sheets according to the invention have good hole expansion capability.

The steels according to the invention also have good weldability: for welding parameters adapted to the thicknesses mentioned above, the welded joints are free of cold or hot cracks.

The steel sheets I1-b and I1-c were annealed at a temperature T ₁ too low, the austenitic transformation is not complete. As a result, the microstructure comprises proeutectoid ferrite (40% for I1b, 20% for I1-c) and an excessive content of MA islands. The mechanical strength is then reduced by the presence of proeutectoid ferrite.

For the steel sheet I1-d, the holding temperature T ₂ is greater than Ms + 30 ° C: the bainitic transformation which occurs at a higher temperature gives rise to a coarser structure and leads to insufficient mechanical strength.

For the steel sheet I-2c, the cooling rate V _R1 after annealing is not sufficient, the microstructure formed is more heterogeneous and the elongation at break is reduced to less than 10%.

For the sheet I-3b, the holding temperature T ₂ is less than Ms-20 ° C: consequently, the cooling V _R1 causes the appearance of a bainite formed at low temperature and martensite, associated with insufficient elongation .

The steel R1 has an insufficient (silicon + aluminum) content, the holding temperature T ₂ is less than Ms-20 ° C. Due to the insufficient content of (Si + Al), the amount of islets MA formed is insufficient to obtain a resistance greater than or equal to 1200 MPa.

R2 and R3 steels have insufficient carbon, manganese, silicon + aluminum contents. The amount of MA compounds formed is less than 10%. In addition, the annealing temperature T ₁ lower than A _c3 leads to an excessive content of proeutectoid ferrite and cementite, and insufficient strength.

The steel R4 has an insufficient content of (Si + Al) The cooling rate V _R1 is in particular too low. The enrichment of carbon austenite during cooling is then insufficient to allow the formation of martensite and to obtain the strength and elongation properties of the invention.

Steel R5 also has an insufficient content of (Si + Al) The insufficiently fast cooling rate after annealing leads to excessive proeutectoid ferrite content and insufficient mechanical strength.

Starting from the manufacturing process of the steel sheet I2-a, an I2-d steel sheet has been manufactured according to a process having identical characteristics, with the exception of the temperature T ₁ equal to 830 ° C. temperature A _c3 . In the case where T ₁ is equal to A _c3 , the cone hole expansion ability is 25%. When the temperature T ₁ is 850 ° C (Δ _c3 + 20 ° C), the expandability is increased to 31%.

Thus, the invention allows the manufacture of steel sheets combining a very high strength and high ductility. The steel sheets according to the invention are used profitably for the manufacture of structural parts or reinforcement elements in the automotive field and general industry.

Claims (15)

1. Cold-rolled and annealed sheet steel with a strength higher than 1200 MPa and elongation at break higher than 8%, the composition of which is as follows, the contents being expressed in weight: $$\mathrm{0.10}\%\le \mathrm{C}\le \mathrm{0.25}\%$$

   

   $$1\%\le \mathrm{Mn}\le 3\%$$

   

   $$\mathrm{Al}\ge 0.010\%$$

   

   $$\mathrm{1.2}\%\le \mathrm{Si}\le \mathrm{1.8}\%$$

   

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

   

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

   

   $$\mathrm{N}\le \mathrm{0.008}\%,$$

   

   it being understood that $$1.2\%\le \mathrm{Si}+\mathrm{Al}\le 3\%,$$

   

   wherein the composition possibly comprises: $$\mathrm{0.05}\%\le \mathrm{V}\le \mathrm{0.15}\%$$

   

   $$\mathrm{B}\le \mathrm{0.005}\%$$

   

   $$\mathrm{Mo}\le 0.25\%$$

   

   $$\mathrm{Cr}\le 1.65\%,$$

   

   it being understood that $$\mathrm{Cr}+\left(3\times \mathrm{Mo}\right)\ge 0.3\%$$

   

   Ti in a quantity such as Ti/N ≥ 4 and Ti ≤ 0.040%,
   the remainder of the composition being formed from iron and inevitable impurities resulting from smelting, wherein the microstructure of said steel amounts to 15 to 90% bainite, the balance being formed from martensite and residual austenite.
2. Sheet steel according to claim 1 with an elongation at break higher than 10%, characterised in that $$\mathrm{Mo}<0.005\%$$

   

   $$\mathrm{Cr}<0.005\%$$

   

   $$\mathrm{B}=\mathrm{0}\%,$$

   

   wherein the microstructure of said steel comprises 65 to 90% bainite, the balance being formed from islands of martensite and residual austenite.
3. Sheet steel according to claim 1, characterised in that it contains $$\mathrm{Mo}\le 0.25\%$$

   

   $$\mathrm{Cr}\le 1.65\%,$$

   

   it being understood that $$\mathrm{Cr}+\left(3\times \mathrm{Mo}\right)\ge 0.3\%$$

   

   $$\mathrm{B}=\mathrm{0}\%,$$

   

   wherein the microstructure of said steel comprises 65 to 90% bainite, the balance being formed from islands of martensite and residual austenite.
4. Sheet steel according to claim 1 with a strength higher than 1400 MPa, an elongation at break higher than 8%, characterised in that it contains $$\mathrm{Mo}\le 0.25\%$$

   

   $$\mathrm{Cr}\le 1.65\%,$$

   

   it being understood that $$\mathrm{Cr}+\left(3\times \mathrm{Mo}\right)\ge 0.3\%,$$

   

   wherein the microstructure of said steel comprises 45 to 65% bainite, the balance being formed from islands of martensite and residual austenite.
5. Sheet steel according to claim 1 with a strength higher than 1600 MPa, an elongation at break higher than 8%, characterised in that it contains $$\mathrm{Mo}\le 0.25\%$$

   

   $$\mathrm{Cr}\le 1.65\%$$

   

   $$\mathrm{0.0005}\le \mathrm{B}\le \mathrm{0.003}\%,$$

   

   it being understood that $$\mathrm{Cr}+\left(3\times \mathrm{Mo}\right)\ge 0.3\%,$$

   

   wherein the microstructure of said steel comprises 15 to 45% bainite, the balance being formed from martensite and residual austenite.
6. Sheet steel according to any one of claims 1 to 5, characterised in that the composition of said steel contains, the content being expressed in weight: $$\mathrm{0.19}\%\le \mathrm{C}\le \mathrm{0.23}\%\mathrm{.}$$

   
7. Sheet steel according to any one of claims 1 to 6, characterised in that the composition of said steel contains, the content being expressed in weight: $$0.5\%\le \mathrm{Mn}\le 2.5\%\mathrm{.}$$

   
8. Sheet steel according to any one of claims 1 to 7, characterised in that the composition of said steel contains, the content being expressed in weight: $$\mathrm{1.2}\%\le \mathrm{Al}\le \mathrm{1.8}\%\mathrm{.}$$

   
9. Sheet steel according to any one of claims 1 to 8, characterised in that the composition of said steel contains, the content being expressed in weight: $$\mathrm{0.05}\%\le \mathrm{V}\le \mathrm{0.15}\%$$

   

   $$\mathrm{0.004}\le \mathrm{N}\le \mathrm{0.008}\%\mathrm{.}$$

   
10. Sheet steel according to any one of claims 1 to 9, characterised in that the composition of said steel contains, the content being expressed in weight: $$\mathrm{0.12}\%\le \mathrm{V}\le \mathrm{0.15}\%\mathrm{.}$$

    
11. Sheet steel according to any one of claims 1 to 10, characterised in that the average size of said islands of martensite and residual austenite is less than 1 micrometre, and the average distance between said islands is less than 6 micrometres.
12. Process for the production of a cold-rolled sheet steel with a strength higher than 1200 MPa, an elongation at break higher than 10%, according to which

    • a steel is supplied with the composition according to claim 2, then

    • semi-finished product is cast from this steel, then

    • said semi-finished product is brought to a temperature higher than 1150°C, then

    • said semi-finished product is hot rolled to obtain a hot-rolled sheet, then

    • said sheet is wound, then

    • said hot-rolled sheet is pickled, then

    • said sheet is cold rolled at a reduction rate in the range of between 30 and 80% in order to obtain a cold-rolled sheet, then

    • said cold-rolled sheet is reheated at a rate V_c in the range of between 5 and 15°C/s to a temperature T₁ in the range of between Ac3 and Ac3+20°C during a time t₁ in the range of between 50 and 150s, then said sheet is cooled at a rate V_R1 higher than 40°C/s and lower than 100°C/s to a temperature T₂ in the range of between (M_s-30°C and M_s+30°C), said sheet is maintained at said temperature T₂ for a time t₂ in the range of between 150 and 350s, then a cooling is conducted at a rate V_R2 lower than 30°C/s to ambient temperature.
13. Process for the production of a cold-rolled sheet steel with a strength higher than 1200 MPa, an elongation at break higher than 8%, according to which

    • a steel is supplied with the composition according to any one of claims 1 or 3 to 5, wherein the contents of Mo and Cr are such that Mo ≤ 0.25%, Cr ≤ 1.65%, it being understood that Cr+(3 x Mo) ≥ 0.3%, then

    • a cold-rolled sheet is produced using a process comprising the steps of casting, reheating, hot rolling, winding, pickling, cold rolling according to claim 12, then

    • said cold-rolled sheet is reheated at a rate V_c in the range of between 5 and 15°C/s to a temperature T₁ in the range of between Ac3 and Ac3+20°C for a time t₁ in the range of between 50 and 150s, then said sheet is cooled at a rate V_R1 higher than 25°C/s and lower than 100°C/s to a temperature T₂ in the range of between B_s and (M_s-20°C), said sheet is maintained at said temperature T₂ for a time t₂ in the range of between 150 and 350s, then a cooling is conducted at a rate V_R2 lower than 30°C/s to ambient temperature.
14. Production process according to claim 12 or 13, characterised in that the temperature T₁ is preferably in the range of between Ac3+10°C and Ac3+20°C.
15. Use of a sheet steel cold-rolled and annealed according to any one of claims 1 to 11 or produced by a process according to any one of claims 12 to 14 for the production of structural parts or reinforcing elements in the automotive field.

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