EP2155915B2 - 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 steel having a strength greater than 1200 MPa and an elongation at break greater than 8%. The automotive sector and general industry are in particular fields of application for these steel sheets.

In the automotive industry in particular, there is a continuous need to make vehicles lighter and to increase safety. Different families of steels have successively been proposed to meet this need for increased resistance: first of all, 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, a very hard constituent, within a softer ferritic matrix, makes it possible to obtain a resistance greater than 450MPa associated with a good suitability for cold forming.

In order to further increase resistance, steels have been developed which exhibit "TRIP" (Transformation Induced Plasticity) behavior with very advantageous combinations of properties (resistance-ability to deformation): these properties are linked to the structure of these steels consisting of a ferritic matrix comprising bainite and residual austenite. The presence of this last constituent confers a high ductility to an undeformed sheet. Under the effect of a subsequent deformation, for example during a uniaxial stress, the residual austenite of a TRIP steel part is gradually transformed into martensite, which results in significant consolidation and delays the appearance of a 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 sheets have been proposed, with a maximum resistance level of around 1000MPa. Obtaining significantly higher resistance levels, for example 1200-1400MPa, comes up against various difficulties:

• The increase in mechanical resistance requires a chemical analysis that is much more loaded with 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 earlier damage, as shown by the drop in elongation.
• An increase in the fraction of hardening constituents within the ferritic matrix is also observed: in this case, the islands, initially isolated and of small size when the resistance is low, gradually become connected and form large constituents which promote yet another early damage.

The possibilities of simultaneously obtaining very high levels of resistance and certain other usage properties by means of TRIP or Dual Phase microstructure steels thus seem limited. To achieve even higher strength, ie a level above 800-1000 MPa, so-called “multiphase” steels with a predominantly bainitic structure have been developed. In the automotive industry or in general industry, medium-thick multiphase steel sheets are used with profit for structural parts such as bumper crosspieces, uprights, various reinforcements.

Especially, in the field of multi-phase cold-rolled steel sheets over 980MPa, the patent EP1559798 describes 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, polygonal ferrite being less than 20%. The embodiments presented in this document show that the resistance does not exceed 1200 MPa.

The patent EP 1589126 also describes the manufacture of cold-rolled thin sheets, the product of which (strength x elongation) is greater than 20,000 MPa%. The composition of 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. Here again, the examples presented show that the resistance is still less than 1200 MPa. JP10280090 describes a steel sheet and the method of manufacturing very high strength cold rolled steel sheet, 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 possibly 0.01-0.15% Mo and 0.0005-0.0020% B, the remainder being iron and unavoidable residual impurities. The microstructure denotes steel comprising bainite and martensite. The microstructure can be achieved by checking hot rolling, coiling, pickling, cold rolling, heat treatment; the steel sheet will be of 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 at break greater than 8% and good cold formability. The invention also aims to provide a steel that is not very sensitive to damage during cutting by a mechanical process.

Furthermore, the invention aims to provide a process for the manufacture of thin sheets in which small variations in the parameters do not lead to significant modifications of the microstructure or of the mechanical properties. The invention also aims to provide a steel sheet that can be easily manufactured by cold rolling, that is to say the hardness of which 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 sheet of steel suitable for the possible deposition of a metal coating according to the usual methods.

It also aims to have a steel sheet that is not very sensitive to damage by cutting and capable of hole expansion.

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

For this purpose, the subject of the invention is a sheet according to one of claims 1 to 3.

According to a particular mode, 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 mode, 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≤0.003%.

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

The invention also relates to a method for manufacturing a cold-rolled steel sheet with a strength greater than 1200 MPa, an elongation at break greater than 10%, according to which a steel of composition: 0.10 % ≤ C ≤ 0.25%, 1%≤ Mn ≤ 3%, Al ≥ 0.010%, Si≤2.990%, it being understood 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 that Ti≤0.040%. A semi-finished product is cast from this steel, then the semi-finished product is brought to a temperature above 1150° C. and the semi-finished product is hot rolled to obtain a hot rolled sheet. The sheet is coiled and pickled, then it is cold rolled with a reduction rate 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 Ac3 and Ac3+20°C, for a time t ₁ of between 50 and 150s, then cools the sheet at a rate V _R1 greater than 40°C/s and less than 100°C/s down to a temperature T ₂ of 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 s then cooling is carried out at a speed V _R2 of less than 30° C./s down to ambient temperature. The invention also relates to a method for manufacturing a cold-rolled steel sheet with a strength greater than 1200 MPa, an elongation at break greater than 8%, according to which a steel of composition: 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 ≤ 0.25%, Cr ≤ 1.65%, it being understood 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 semi-finished product is cast from this steel, the semi-finished product is brought to a temperature above 1150° C., then the semi-finished product is hot rolled to obtain a hot rolled sheet. The sheet is coiled, it is pickled, then the sheet is cold rolled with a reduction rate 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 Ac3 and Ac3+20°C, for a time t ₁ of between 50 and 150 s then it is cooled at a speed V _R1 greater than 25°C/s and less than 100°C/s to a temperature T ₂ of between B _s and (M _s - 20° C.) The sheet is maintained at the temperature T ₂ for a time t ₂ comprised between 150 and 350 s then cooling is carried out at a speed V _R2 of less than 30° C./s down to ambient temperature.

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

The invention also relates to the use of a steel sheet cold rolled and annealed according to one of the above modes, or manufactured by a process according to one of the above modes, for the manufacture structural parts or reinforcing 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 characteristics and advantages of the invention will appear during the description below, given by way of example and made with reference to the appended figures attached hereto:

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

The inventors have demonstrated that the above problems are solved when the cold-rolled and annealed thin steel sheet exhibits a bainitic microstructure, with, in addition, islands of martensite and residual austenite, or "M-A" islands. For steels with the highest resistance, greater than 1600 MPa, the microstructure contains a greater quantity 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 connection with other elements of the composition (Cr, Mo, Mn) and with the annealing heat treatment after cold rolling, it increases the hardenability and makes it possible to obtain a bainitic transformation. The carbon contents according to the invention also lead to the formation of islands of residual martensite and austenite, the quantity, morphology and composition of which make it possible to obtain the properties referred to above.

Carbon also delays the formation of 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, the hardness is higher. The presence of proeutectoid ferrite resulting from the annealing must therefore be avoided to obtain high levels of mechanical resistance.

According to the invention, the carbon content is between 0.10 and 0.25% by weight: Below 0.10%, sufficient strength cannot be obtained and the stability of the residual austenite is not not satisfactory. Above 0.25%, weldability is reduced due to the formation of quench microstructures in the Heat Affected Zone.

According to a preferred embodiment, the carbon content is between 0.19 and 0.23%: within this range, the weldability is very satisfactory, and the quantity, stability and morphology of the M-A islands are particularly suitable for obtaining a favorable pair of mechanical properties (strength-elongation)

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

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

Silicon retards the precipitation of cementite on cooling from austenite after annealing. An addition of silicon according to the invention therefore contributes to stabilizing a sufficient quantity of residual austenite in the form of islands which subsequently and gradually transform into martensite under the effect of 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 austenite stabilization are similar; when the silicon and aluminum contents are such that: 1%≤Si+Al≤3%, satisfactory stabilization of the austenite is obtained, which makes it possible to form the desired microstructures while retaining satisfactory usage properties. Considering 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 quantity of residual austenite and to avoid intergranular oxidation during the hot coiling 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 an equivalent content, the effects of aluminum are in fact similar to those described above for silicon, but the risk of appearance of surface defects is however less.

The steels according to the invention optionally comprise molybdenum and/or chromium: the molybdenum increases the hardenability, prevents 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 the formation of bainite.

Chromium also contributes to avoid the formation of pro-eutectoid ferrite and to the refinement of the bainitic microstructure. Beyond 1.65%, the risk of obtaining a predominantly martensitic structure is high. Compared to molybdenum, however, its effect is 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 hardenability, 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 economical method, the steel can comprise very low or zero molybdenum and chromium contents, that is to say contents of less than 0.005% by weight for these two elements, and 0% boron.

To obtain a resistance greater than 1400MPa, the addition of chromium and/or molybdenum is required, in the quantities mentioned above. When the sulfur content is more than 0.015%, the formability is reduced due to the excessive presence of manganese sulfides.

The phosphorus content is limited to 0.1% in order to maintain sufficient hot ductility.

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

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 jointly between 0.004 and 0.008%, the precipitation of vanadium can occur during annealing after cold rolling in the form of fine carbonitrides which confer 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 less than or equal to 0.005%. According to a preferred embodiment, the steel preferentially contains between 0.0005 and 0.003% 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 the quantity mentioned above makes it possible to obtain a resistance greater than 1400 MPa.

The steel may optionally comprise titanium in an amount such that 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 an 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% bainite, these contents referring to surface percentages, the balance consists of islands of residual martensite and austenite (islands of M-A compounds)

This predominantly bainitic structure, not comprising proeutectoid ferrite of low hardness, has an elongation capacity at break 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 presents an example of the microstructure of a steel sheet according to the invention. The morphology of the MA islands was revealed by means of appropriate chemical reagents: after attack, the MA islands appear in white on a more or less dark bainitic matrix. Some small islands are located between the bainitic ferrite slats. The islets are observed at magnifications ranging from approximately 500 to 1500x on a statistically representative surface and the mean size of the islets and the mean distance between these islets are measured using image analysis software. In the case of the figure 1 , the areal percentage of the islands is 12% and the average size of the MA islands 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 shown that a specific morphology of the MA islands was to be particularly sought: when the average size of the islands is less than 1 micrometer and when the average distance between these islands is less than 6 micrometers, the following effects are simultaneously obtained:

• limited damage due to the absence of rupture initiation on large MA islands
• 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 with a strength greater than 1400 MPa and an elongation at break greater than 8%, the microstructure is composed of 45 to 65% 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 strength greater than 1600 MPa and an elongation at break greater than 8%, the microstructure is composed of 15 to 45% 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 process for manufacturing a cold-rolled and annealed thin sheet according to the invention is as follows:

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

The cast semi-finished products are first of all brought to a temperature above 1150°C in order to reach at all points a temperature favorable to the high deformations that the steel will undergo during rolling. Naturally, in the case of direct casting of thin slabs or thin strips between counter-rotating rolls, the hot rolling step of these semi-finished products starting at more than 1150°C can be done directly after casting so that an intermediate reheating 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 little dependent on the end-of-rolling temperature and of the cooling following the hot-rolling.

The hot sheet is then coiled. The coiling temperature is preferably below 550° C. to limit the hardness of the hot-rolled sheet and the intergranular oxidation on the surface. Excessive hardness of the hot-rolled sheet leads to excessive stresses during the subsequent cold rolling as well as possibly to edge defects.

The hot-rolled sheet is then pickled according to a process known per se so as to give it a surface condition suitable for cold rolling. The latter is carried out by reducing the thickness of the hot-rolled sheet by 30-80%.

• 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 hardened 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 particularly suited 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 transformation into 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 makes it possible to limit the subsequent formation of proeutectoid ferrite. It is important that the temperature T ₁ be lower than A _c3 +20°C in order to avoid an exaggerated magnification of the austenitic grain. Within this range (A _c3 -A _c3 +20°C), the characteristics of the final product are not very sensitive to a variation in temperature 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 austenitic grain size is more homogeneous and finer, which subsequently leads to the formation of a final microstructure also having these characteristics.

• Holding at temperature T ₁ for a time t ₁ of between 50s and 150s. This step leads to 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 contains practically no chromium, molybdenum and boron, i.e. 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 up to a temperature T ₂ of between M _s -30°C and M _s +30°C. For these conditions of cooling rate, the diffusion of carbon in austenite is limited. This effect is saturated above 100°C/s. Maintaining is carried out at this temperature T ₂ for a time t ₂ of between 150 and 350 s. 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 make it possible to avoid the formation of proeutectoid ferrite during cooling. We also obtain under these conditions a bainitic transformation of the greatest part of the austenite. The remaining fraction is transformed into martensite or is optionally stabilized in the form of residual austenite.
• When the steel has a chromium and molybdenum content such as 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 up to a temperature T ₂ between (B _s and M _s -20°C) Maintenance is carried out at this temperature T ₂ for a time t ₂ included between 150 and 350s. B _s designates 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 guarantee 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 following stage of the process is identical, whether or not the product comprises chromium and/or molybdenum: cooling is carried out at a rate V _R2 of less than 30° C./s down to ambient temperature. In particular, when the temperature T ₂ is low within the ranges according to the invention, the cooling at a rate V _R2 of less than 30° C. /s causes the islands of newly formed martensite to temper, which is favorable in terms of of use properties.

Example :

Steels have been produced, the composition of which is given in the table below, expressed as a percentage by weight. In addition to the steels I-1 to I-5 having been used for the manufacture of sheets according to the invention, the composition of steels R-1 to R-5 having been used for the manufacture of reference sheets has been indicated by way of comparison. . Table 1 Compositions of steels (% weight). I= According to the invention. R=reference Underlined values: Not in accordance with the invention. Steel VS (%) Min (%) Yes (%) Al (%) Si+Al (%) MB (%) 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

The composition of steel I-1 does not comply with claims 1 to 3.

Semi-finished products corresponding to the above compositions were reheated 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, ie a reduction rate of 70%. From the same composition, certain steels have been subjected to different manufacturing conditions. The references I1-a, I1-b and I1-c, I1-d denote, for example, four steel sheets manufactured under different conditions from the steel composition I1. Table 2 indicates the manufacturing conditions for 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 have also been given in Table 2.

The various microstructural constituents measured by quantitative microscopy have also been indicated: surface fraction of bainite, martensite and residual austenite.

The MA islands were highlighted by LePera's 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 in accordance with the invention. Galvanised steel T1 ₍ °C) Ac3 (°C) t1( _s ) V _R1 (°C/s) T2 ₍ °C) _Bsec (°C) _Msec (°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 mechanical tensile properties obtained (yield strength Re, strength Rm, uniform elongation Au, elongation at break At) have been entered in Table 3 below. The Re/Rm ratio was also indicated.

In some cases, the fracture energy at -40°C has been determined from impact specimens of the Charpy V type with a thickness reduced to 1.4 mm. We also evaluated the damage linked to a cut (shearing or punching for example) which could possibly reduce the subsequent deformation capacities of a cut part. For this purpose, specimens of dimension 20×80 mm ² were cut by shearing. A portion of these specimens was then subjected to edge polishing. The specimens were coated with photodeposited grids and then subjected to uniaxial tension until fracture. The values of the main deformations ε ₁ parallel to the direction of the stress were measured as close as possible to the initiation of the fracture from the deformed grids. This measurement was carried out on the specimens with mechanically cut edges and on the specimens with polished edges. Sensitivity to cutting is evaluated by the damage factor: Δ=ε ₁ (cut edges)-ε ₁ (polished edges)/ε ₁ (polished edges).

For certain plates, one also evaluated the damage in the vicinity of cut edges starting from samples of 105×105mm ² comprising a hole with an initial diameter of 10mm. The relative increase in the diameter of the hole is measured after introduction of 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 Bainite fraction (%) Fraction (MA) (%) Island size (MA)<1 micron and mean 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 Nope 490 1020 15 19 I1-c 63 17 Yes 500 1040 14 17 36 I1-d 83 17 Nope 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 n/a n/a n/a 700 1200 7 8.5 I3-a 88 12 Yes 750 1200 9.5 12.7 40 I3-b n/a n/a n/a 900 1300 9 8 I4 60 40 Yes 690 1420 8 11.2 -22.5 I5 45 55 n/a 800 1600 7.5 10 R1 n/a n/a n/a 800 950 4 6 R2 Ferrite 6 n/a 400 520 10 16 R3 Ferrite 5 n/a 300 450 16 21 R4 60 40 n/a 650 950 n/a 4 R5 Ferrite 17 Yes 404 856 12.4 16 -43

The sheets 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 mechanical resistance greater 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 fracture energy V at -40° C. greater than 40 Joules/cm ² . This allows the manufacture of parts resistant to the sudden propagation of a defect, in particular in the event of dynamic stresses. The microstructures of steels with a minimum strength of 1200 MPa and a minimum elongation at break of 10% according to the invention comprise a bainite content of between 65 and 90%, the balance consisting of MA islands. The figure 1 thus presents the microstructure of the I3a steel sheet comprising 88% bainite and 12% MA islands, revealed by an attack with the LePera reagent. 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 elongation at break of 8%, the steels according to the invention have a bainite content of between 45 and 65%, the balance being islands MA. In the case of steels having a minimum strength of 1600 MPa and a minimum elongation at break 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 MA of less than 1 micrometer, the inter-island distance being less than 6 micrometers.

The steels according to the invention also have good resistance to damage in the event of cutting since the damage factor Δ is limited to −23%. A steel sheet that does not have these characteristics (R5) can have a damage factor of 43%. The laminations according to the invention have good aptitude for hole expansion.

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

The steel sheets I1-b and I1-c were annealed at too low a temperature T ₁ , the austenitic transformation is not complete. Consequently, the microstructure contains proeutectoid ferrite (40% for I1b, 20% for I1-c) and an excessive content of MA islands. The mechanical resistance 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 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 below 10%.

For sheet I-3b, the holding temperature T ₂ is lower than Ms-20°C: consequently, cooling V _R1 causes the appearance of 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 lower than Ms-20°C. Due to the insufficient content of (Si+Al), the amount of MA islands formed is insufficient to obtain a strength greater than or equal to 1200MPa.

R2 and R3 steels have insufficient carbon, manganese, silicon+aluminum contents. The amount of MA compounds formed is less than 10%. Furthermore, the annealing temperature T ₁ lower than A _c3 leads to an excessive content of proeutectoid ferrite and of cementite, and to 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 the austenite with carbon on cooling is then insufficient to allow the formation of martensite and to obtain the strength and elongation properties targeted by the invention.

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

Starting from the manufacturing process of the steel sheet I2-a, a steel sheet I2-d was manufactured according to a process having identical characteristics, with the exception of the temperature T ₁ equal to 830°C, i.e. the temperature A _c3 . In the case where T ₁ is equal to A _c3 , the capacity for conical hole expansion is 25%. When the temperature T ₁ is equal to 850°C (A _c3 +20°C), the ability to expand is increased by up to 31%.

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

Claims (13)

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:

   0.10% ≤ C ≤ 0.25%

   1% ≤ Mn ≤ 3%

   Al ≥ 0.010%

   1.2% ≤ Si ≤ 1.8%

   S ≤ 0.015%

   P ≤ 0.1%

   N ≤ 0.008%,

   it being understood that

   1.2% ≤ Si+Al ≤ 3%,

   Mo ≤ 0.25%

   Cr ≤ 1.65%

   it being understood that

   Cr+(3 × Mo) ≥ 0.3%

   B = 0%,

   wherein the composition possibly comprises:

   0.05% ≤ V ≤ 0.15%

   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 65 to 90% bainite, the balance being formed from islands of martensite and residual austenite.
2. Cold-rolled and annealed sheet steel with a strength higher than 1400 MPa and elongation at break higher than 8%, the composition of which is as follows, the contents being expressed in weight:

   0.10% ≤ C ≤ 0.25%

   1% ≤ Mn ≤ 3%

   Al ≥ 0.010%

   1.2% ≤ Si ≤ 1.8%

   S ≤ 0.015%

   P ≤ 0.1%

   N ≤ 0.008%,

   it being understood that

   1.2% ≤ Si+Al ≤ 3%,

   Mo ≤ 0.25%

   Cr ≤ 1.65%

   it being understood that

   Cr+(3 × Mo) ≥ 0.3%

   wherein the composition possibly comprises:

   0.05% ≤ V ≤ 0.15%

   B ≤ 0.005%,

   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 comprises 45 to 65% bainite, the balance being formed from islands of martensite and residual austenite.
3. Cold-rolled and annealed sheet steel with a strength higher than 1600 MPa and elongation at break higher than 8%, the composition of which is as follows, the contents being expressed in weight:

   0.10% ≤ C ≤ 0.25%

   1% ≤ Mn ≤ 3%

   Al ≥ 0.010%

   1.2% ≤ Si ≤ 1.8%

   S ≤ 0.015%

   P ≤ 0.1%

   N ≤ 0.008%,

   it being understood that

   1.2% ≤ Si+Al ≤ 3%,

   Mo ≤ 0.25%

   Cr ≤ 1.65%

   it being understood that

   Cr+(3 × Mo) ≥ 0.3%

   0.0005 ≤ B ≤0.003 %,

   wherein the composition possibly comprises:

   0.05% ≤ V ≤ 0.15%

   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 comprises 15 to 45% bainite, the balance being formed from martensite and residual austenite.
4. Sheet steel according to any one of claims 1 to 3, characterised in that the composition of said steel contains, the content being expressed in weight:
   0.19% ≤ C ≤ 0.23%.
5. Sheet steel according to any one of claims 1 to 4, characterised in that the composition of said steel contains, the content being expressed in weight:
   1.5% ≤ Mn ≤ 2.5%.
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:
   1.2% ≤ Al ≤ 1.8%.
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:

   0.05% ≤ V ≤ 0.15%

   0.004 ≤ N ≤ 0.008%.
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:
   0.12% ≤ V ≤ 0.15%.
9. Sheet steel according to any one of claims 1 to 8, 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.
10. 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, the composition of said steel is as follows, the contents being expressed in weight:

    0.10% ≤ C ≤ 0.25%

    1% ≤ Mn ≤ 3%

    Al ≥ 0.010%

    1.2% ≤ Si ≤ 1.8%

    S ≤ 0.015%

    P ≤ 0.1%

    N ≤ 0.008%,

    it being understood that

    1.2% ≤ Si+Al ≤ 3%,

    Mo < 0.005%

    Cr < 0.005%

    B = 0%,

    wherein the composition possibly comprises:

    0.05% ≤ V ≤ 0.15%

    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
    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 for 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.
11. 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, the composition of said steel being according to any one of claims 1 to 3, wherein the contents of Mo and Cr are such that Mo ≤ 0.25%, Cr ≤ 1.65%, it being understood that Cr+(3 × Mo) ≥ 0.3%, 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 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.
12. Production process according to claim 10, characterised in that the temperature T₁ is preferably in the range of between Ac3+10°C and Ac3+20°C.
13. Use of a sheet steel cold-rolled and annealed according to any one of claims 1 to 9 or produced by a process according to any one of claims 10 to 12 for the production of structural parts or reinforcing elements in the automotive field.

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