EP2171112B1 - Method for producing steel sheets having high resistance and ductility characteristics, and sheets thus obtained - Google Patents

Abstract

The invention relates to a hot-rolled steel sheet having a resistance higher than 800 MPa and an elongation at break higher than 10%, and having the following composition in weight: 0.050% ≤ C ≤ 0.090%, 1%< Mn ≤ 2%, 0.015% ≤ Al ≤ 0.050 %, 0.1 %≤Si ≤ 0.3%, 0.10% ≤ Mo ≤ 0.40%, S ≤ 0.010%, P≤ 0.025%, 0.003% ≤ N ≤ 0.009%, 0.12% ≤ V ≤ 0.22%, Ti≤ 0.005%, Nb ≤ 0.020% and optionally Cr ≤ 0.45%, the balance consisting of iron and unavoidable impurities resulting from the production, wherein the microstructure of the sheet or the part includes, in surface fraction, at least 80% of upper bainite, the optional balance consisting of lower bainite, martensite and residual austenite, the sum of the martensite and residual austensite contents being lower than 5%.

Description

The invention relates to the manufacture of sheets or hot-rolled parts of so-called "multiphase" steels, simultaneously having a very high strength and a deformation capacity for performing cold or warm shaping operations. The invention more specifically relates to predominantly bainitic microstructure steels having a strength greater than 800 MPa and an elongation rate greater than 10% rupture.

The automotive industry is in particular a preferred field of application for these hot-rolled steel sheets.

In particular, there is a continuing need in this industry for lighter vehicles and increased safety. Thus, different families of steels have been proposed to meet the growing needs:

Steels were first proposed comprising micro-alloy elements whose hardening is obtained simultaneously by precipitation and by grain size refinement. The development of these steels was followed by that of "Dual-Phase" steels where the presence of martensite within a ferritic matrix makes it possible to obtain a strength greater than 450 MPa combined with a good cold forming ability.

To obtain higher resistance levels, steels with a "Transform Induced Plasticity" (TRIP) behavior have been developed with advantageous combinations of properties (resistance-ability to deformation): these properties are related to the structure of these steels. consisting of a ferritic matrix comprising bainite and residual austenite. Under the effect of a deformation, the residual austenite of a TRIP steel part gradually changes to martensite, which results in a significant consolidation and delays the appearance of a necking.

To achieve a high yield strength / resistance ratio at the same time, an even greater resistance, ie a level greater than 800 MPa, multiphase steels with predominantly bainitic structure have been developed; in the automotive industry or in the general industry, these steels are used profitably for the manufacture of structural parts. The ability to shape these parts, however, simultaneously requires sufficient elongation. This requirement may also be required when the parts are welded and then shaped: in this case, the welded joints must have a sufficient fitness for shaping and not lead to premature fractures at the joints.

JP 2003 321739 A discloses a steel sheet with high characteristics of strength and ductility and the method of manufacturing this sheet. The sheet composition comprises, the contents being expressed by weight: 0.03-0.1% C, 0.5-1.7% Mn, 0-0.1% Al, 0-2% Si, 0.1-0.5% Mo, 0-0.01% S, 0 -0.06% P, 0-0.006% N, 0.01-0.15% V, 0.007-0.2% Ti, 0.005-0.02% Nb, remains Fe and unavoidable impurities resulting from the elaboration. The microstructure is composed of 5-70% bainite, the rest being essentially ferrite.

The present invention aims to solve the problems mentioned above. It aims to provide a hot-rolled steel sheet having a mechanical strength greater than 800 MPa together with an elongation rate greater than 10% fracture, both in long direction and in cross-direction relative to the rolling .

The invention also aims at providing a steel sheet that is not very sensitive to damage during cutting by a mechanical method.

It also aims to have a steel sheet having a good aptitude for forming welded assemblies made from this steel, in particular assemblies obtained by welding LASER.

The invention also aims to provide a method of manufacturing a steel sheet in the uncoated, electrogalvanized or galvanized, or aluminized state. This therefore requires that the mechanical characteristics of this steel are insensitive to the thermal cycles associated with continuous dipping zinc coating processes.

The invention also aims to have a sheet or piece of hot rolled steel available even in small thickness, that is to say for example between 1 and 5mm. The hot hardness of the steel must not be too high to facilitate rolling.

• 0,050% ≤ C ≤ 0,090%, 1% ≤ Mn ≤ 2%, 0,015% ≤ Al ≤ 0,050 %, 0,1%≤Si ≤ 0,3%, 0,10% ≤ Mo ≤ 0,40%, S ≤ 0,010%, P≤ 0,025%, 0,003%≤N≤0,009%, 0, 12% ≤ V ≤ 0,22%, Ti≤ 0,005%, Nb≤ 0,020% et à titre optionnel, Cr≤ 0,45%, le reste de la composition étant constitué de fer et d'impuretés inévitables résultant de l'élaboration, la microstructure de la tôle ou de la pièce d'acier comprenant, en fraction surfacique, au moins 80% de bainite supérieure, le complément éventuel étant constitué de bainite inférieure, de martensite et d'austénite résiduelle, la somme des teneurs en martensite et en austénite résiduelle étant inférieure à 5%.

For this purpose, the subject of the invention is a sheet or piece of hot-rolled steel with a resistance greater than 800 MPa, with an elongation at break greater than 10%, the composition of which comprises the contents being expressed by weight:

• 0.050% ≤ C ≤ 0.090%, 1% ≤ Mn ≤ 2%, 0.015% ≤ Al ≤ 0.050%, 0.1% ≤Si ≤ 0.3%, 0.10% ≤ Mo ≤ 0.40%, S ≤ 0.010%, P ≤ 0.025%, 0.003% ≤N ≤ 0.009%, 0.12% ≤ V ≤ 0.22%, Ti ≤ 0.005 %, Nb 0,0 0.020% and optionally Cr 0,4 0.45%, the remainder of the composition consisting of iron and unavoidable impurities resulting from the preparation, the microstructure of the sheet or the steel piece comprising, in surface fraction, at least 80% of higher bainite, the optional supplement consisting of lower bainite, martensite and residual austenite, the sum of the martensite and residual austenite contents being less than 5%.

The composition of the steel preferably comprises the content being expressed by weight: 0.050% ≤ C ≤ 0.070%

Preferably, the composition comprises, the content being expressed by weight: 0.070% <C ≤ 0.090%

In a preferred embodiment, the composition comprises: 1.4% ≤ Mn ≤ 1.8%.

Preferably, the composition comprises: 0.020% ≤ Al ≤ 0.040%.

The composition of the steel preferably comprises: 0.12% ≤ V ≤ 0.16%. In a preferred embodiment, the composition of the steel comprises 0.18% ≤ Mo ≤ 0.30%.

By way of preference, the composition comprises: Nb ≤ 0.005%

Preferably, the composition comprises: 0.20% ≤ Cr ≤ 0.45%

In a particular embodiment, the sheet or the part is coated with a coating based on zinc or aluminum-based.

The subject of the invention is also a piece of steel with a composition and a microstructure defined above, characterized in that it is obtained by heating at a temperature T of between 400 and 690 ° C. and then a warm stamping in a temperature range between 350 ° C and (T-20 ° C), then a subsequent cooling to room temperature.

The invention also relates to a beam welded assembly with high energy density made from a sheet or piece of steel in one of the modes above.

The invention also relates to a method of manufacturing a sheet or piece of hot-rolled steel with a resistance greater than 800 MPa, elongation at break greater than 10%, according to which a steel of the above composition is supplied, a semi-product is cast which is heated to a temperature above 1150 ° C. The semi-finished product is hot rolled to a temperature T _FL in a temperature range where the microstructure of the steel is entirely austenitic so as to obtain a sheet. This is then cooled to a cooling rate V _R of 75 and 200 ° C./s, then the sheet is reeled at a temperature T _bob of between 500 and 600 ° C. According to a preferred embodiment, the end of rolling temperature T _FL is between 870 and 930 ° C.

Preferably, the cooling rate V _R is between 80 and 150 ° C / s.

Preferably, the sheet is pickled, then optionally skin-passed, and then coated with zinc or zinc alloy.

According to a preferred embodiment, the coating is carried out continuously by dipping.

The subject of the invention is also a process for manufacturing a hot-stamped part, according to which a steel sheet is supplied according to one of the above characteristics, or manufactured by a method according to one of the above-mentioned characteristics. above, then cutting said sheet to obtain a blank. The blank is heated partially or completely to a temperature T of between 400 and 690 ° C., where a holding time of less than 15 minutes is carried out so as to obtain a heated blank, and then the blank heated to a temperature is pressed. between 350 and T-20 ° C to obtain a piece that is cooled to room temperature with a speed V ' _R

According to a particular mode, the speed V ' _R is between 25 and 100 ° C / s.

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

• La figure 1 illustre l'influence de la teneur en carbone sur l'allongement en sens long de soudures de raboutage réalisées par faisceau LASER
• La figure 2 illustre la microstructure d'une tôle ou pièce d'acier selon l'invention
• La figure 3 illustre la microstructure d'une pièce d'acier emboutie à tiède selon l'invention

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 accompanying figures in which:

• The figure 1 illustrates the influence of carbon content on long-term elongation of LASER beam splicing welds
• The figure 2 illustrates the microstructure of a sheet or piece of steel according to the invention
• The figure 3 illustrates the microstructure of a piece of hot-stamped steel according to the invention

With regard to the chemical composition of steel, carbon plays an important role in the formation of the microstructure and in the mechanical properties.

According to the invention, the carbon content is between 0.050 and 0.090% by weight: Below 0.050%, sufficient strength can not be obtained. Beyond 0.090%, the microstructure formed consists mainly of lower bainite, this structure being characterized by the presence of carbides precipitated within bainitic ferrite slats: the mechanical strength thus obtained is high but the elongation is then significantly reduced.

According to a particular embodiment of the invention, the carbon content is between 0.050 and 0.070%. The figure 1 illustrates the influence of carbon content on the long-term elongation of LASER beam splicing welds: a particularly high elongation at break of 17-23% is associated with a carbon content of 0.050 at 0.070%. These high elongation values make it possible to ensure that LASER-welded sheets can be stamped satisfactorily, even taking into account any local imperfections such as geometrical singularities of weld seams resulting in stress concentrations, or microporosities. in the molten metal. Compared with 0.12% C steels of the prior art, carbon reduction was expected to improve weldability. However, it has been demonstrated that a significant lowering of the carbon content not only makes it possible to obtain a high breaking elongation, but also simultaneously maintains the mechanical strength at a level greater than 800 MPa, which was not expected for contents as low as 0.050% C.

According to another preferred embodiment, the carbon content is greater than 0.070% and less than or equal to 0.090%: even if this range does not lead to such a high ductility, the elongation at break of the LASER welds is greater than 15% and remains comparable to that of the base steel sheet.

In an amount of between 1 and 2% by weight, manganese increases the quenchability and avoids the formation of ferrite cooling after 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. Preferentially, the manganese is between 1.4 and 1.8%: thus forming a completely bainitic structure without risk of appearance of harmful band structure.

In a range of contents between 0.015% and 0.050%, aluminum is an effective element for the deoxidation of steel. This efficiency is obtained in a particularly economical and stable manner when the aluminum content is between 0.020 and 0.040%.

In an amount greater than or equal to 0.1%, silicon contributes to liquid phase deoxidation and hardening in solid solution. An addition of silicon above 0.3%, however, causes the formation of strongly adherent oxides and the possible appearance of surface defects, due in particular to a lack of wettability in dip galvanizing operations.

In an amount greater than or equal to 0.10%, molybdenum retards bainitic transformation during cooling after rolling, contributes to hardening by solid solution and refines the size of bainitic slats. According to the invention, the molybdenum content is less than or equal to 0.40% to prevent the excessive formation of quenching structures. This limited molybdenum content also makes it possible to lower the manufacturing cost.

In a preferred embodiment, the molybdenum content is greater than or equal to 0.18% and less than or equal to 0.30%. In this way, the level is ideally adjusted to avoid the formation of ferrite or perlite in the steel sheet on the cooling table after hot rolling.

In excess of 0.010%, sulfur tends to precipitate excessively in the form of manganese sulphides which greatly reduce the shaping ability.

Phosphorus is a known element to segregate at grain boundaries. Its content must be limited to 0.025% in order to maintain sufficient hot ductility.

As an option, the composition may comprise chromium in an amount of less than or equal to 0.45%. Thanks to the other elements of the composition and to the process according to the invention, its presence is however not absolutely necessary, which has the advantage of avoiding expensive additions.

An addition of chromium between 0.20 and 0.45% can be carried out in addition to the other elements increasing the quenchability: below 0.20%, the effect on the quenchability is not sufficiently marked. Above 0.45%, the coating can be reduced.

According to the invention, the steel contains less than 0.005% Ti and less than 0.020% Nb. In the opposite case, these elements fix too much nitrogen in the form of nitrides or carbonitrides. There is not enough nitrogen available to precipitate with vanadium. In addition, excessive precipitation of niobium would increase the hot hardness and would not easily allow the realization of thin-rolled hot-rolled sheets.

In a particularly economical mode, the niobium content is less than 0.005%

Vanadium is an important element according to the invention: the steel contains a vanadium content of between 0.12 and 0.22%. Compared to a vanadium-free steel, the increase in strength due to a hardening precipitation of carbonitrides can be up to 300 MPa. Below 0.12%, there is no significant effect on the mechanical tensile characteristics. Beyond 0.22% of vanadium, under the manufacturing conditions according to the invention, there is a saturation of the effect on the mechanical characteristics. A content of less than 0.22% thus makes it possible to obtain high mechanical characteristics in a very economical manner with respect to steels which contain higher levels of vanadium.

For a vanadium content between 0.13 and 0.15%, microstructure refinement and structural hardening are particularly effective.

According to the invention, the nitrogen content is greater than or equal to 0.003% in order to obtain a precipitation of vanadium carbonitrides in a sufficient quantity. However, the nitrogen content is less than or equal to 0.009% to avoid the presence of solid solution nitrogen or the formation of larger carbonitrides, which would reduce ductility.

The rest of the composition consists of unavoidable impurities resulting from the preparation, such as for example Sb, Sn, As.

• d'au moins 80% de bainite supérieure, cette structure étant constituée de lattes de ferrite bainitique et de carbures situés entre ces lattes, la précipitation intervenant lors de la transformation bainitique. Cette matrice présente des propriétés de résistance élevées combinées à une ductilité importante. Très préférentiellement, la microstructure est constituée d'au moins 90% de bainite supérieure : la microstructure est alors très homogène et permet d'éviter une localisation des déformations.
• en complément éventuel, la structure contient :
• De la bainite inférieure, dont la précipitation de carbures intervient au sein des lattes ferritiques ; par rapport à la bainite supérieure, la bainite inférieure présente une résistance un peu plus importante mais une ductilité moins grande.
• Eventuellement de la martensite. Celle-ci est fréquemment associée à de l'austénite résiduelle sous forme de composés « M-A » (martensite-austénite résiduelle) La teneur totale en martensite et en austénite résiduelle doit être limitée à 5% pour ne pas diminuer la ductilité.

The microstructure of the sheet or piece of steel according to the invention consists of:

• at least 80% higher bainite, this structure consisting of bainitic ferrite slats and carbides located between these laths, the precipitation occurring during the bainitic transformation. This matrix has high strength properties combined with high ductility. Very preferably, the microstructure consists of at least 90% higher bainite: the microstructure is then very homogeneous and avoids a localization of the deformations.
• as a possible complement, the structure contains:
• Lower bainite, the precipitation of carbides intervenes within the ferritic slats; compared with the upper bainite, the lower bainite has a somewhat greater resistance but a smaller ductility.
• Possibly martensite. This is frequently associated with residual austenite in the form of "MA" compounds (residual martensite-austenite). The total content of martensite and residual austenite must be limited to 5% in order not to reduce the ductility.

The microstructural percentages above are surface fractions that can be measured on polished and etched sections.

The microstructure therefore does not include primary or proeutectoid ferrite: it then has a great homogeneity since the difference in mechanical properties between the matrix (upper bainite) and the other possible constituents (lower bainite and martensite) is small. During a mechanical stress, the deformations are distributed homogeneously. Accumulation of dislocations does not occur at the interfaces between the constituents and premature damage is avoided, as opposed to this can be noted in structures with a significant amount of primary ferrite, phase whose flow limit is very low, or martensite with a very high level of resistance. In this way, the steel sheet according to the invention has a particular aptitude for certain demanding deformation modes such as the expansion of holes, the mechanical stressing of cut edges, folding.

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

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

• A steel composition is supplied according to the invention, and then a semi-finished product is cast from this steel. This casting may be carried out in ingots, or continuously in the form of slabs of thickness of the order of 200 mm. 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 in a temperature range where the structure of the steel is totally austenitic up to an end-of-rolling temperature T _FL . The temperature T _FL is preferably between 870 and 930 ° C to obtain a grain size adapted to the bainitic transformation that follows.

Cooling is then carried out at a speed V _R of between 75 and 200 ° C./s: a minimum speed of 75 ° C./s makes it possible to avoid the formation of proeutectoid ferrite and of perlite, whereas a lower speed V _R or 200 ° C / s avoids excessive formation of martensite.

Optimally, the speed V _R is between 80 and 150 ° C / s: A minimum speed of 80 ° C / s leads to the formation of upper bainite with a very small slat size, combined with excellent mechanical properties. A speed of less than 150 ° C / s makes it possible to avoid, for the most part, the formation of martensite.

• Après cette phase de refroidissement rapide, la tôle laminée à chaud est bobinée à une température T_bob comprise entre 500 et 600°C. La transformation bainitique se produit pendant cette phase de bobinage ; de la sorte, on évite la formation de ferrite proeutectoïde ou de perlite causée par une température de bobinage trop élevée et on évite également la formation de constituants de trempe qui serait causée par une température de bobinage trop basse. De plus, la précipitation de carbonitrures intervenant dans cette gamme de température de bobinage permet d'obtenir un durcissement supplémentaire.

The cooling rate range according to the invention can be obtained by means of a water spray or an air-water mixture, depending on the thickness of the sheet, at the output of the finishing mill.

• After this rapid cooling phase, the hot-rolled sheet is wound at a T _bob temperature of between 500 and 600 ° C. The bainitic transformation occurs during this winding phase; in this way, the formation of proeutectoid ferrite or perlite caused by a too high winding temperature is avoided and the formation of quenching constituents which is caused by a too low winding temperature is also avoided. In addition, the precipitation of carbonitrides occurring in this winding temperature range makes it possible to obtain additional hardening.

The sheet can be used in the bare state or coated. In the latter case, the coating may be for example a coating based on zinc or aluminum. Depending on the use envisaged, the sheet is scoured after rolling according to a method known per se, so as to obtain a surface state suitable for promoting the implementation of the subsequent coating.

In order to erase the bearing observed during a mechanical tensile test, the sheet may be subjected to a slight cold deformation, usually less than 1% ("skin-pass"). The sheet is then coated with zinc or aluminum. a zinc-based alloy, for example by electrogalvanizing or continuous galvanizing dipping. In the latter case, it has been demonstrated that the particular microstructure of the steel, mainly composed of higher bainite, is not very sensitive to the thermal conditions of the subsequent galvanizing treatment, so that the mechanical characteristics of the sheets coated continuously with dipping have a great stability even in case of untimely fluctuation of these conditions. The sheet in the galvanized state therefore has mechanical characteristics very similar to those in the naked state.

The sheets are then cut by methods known in themselves from in order to obtain blanks suitable for shaping.

• On chauffe tout d'abord les flans définis ci-dessus à une température T comprise entre 400 et 690°C. La durée de maintien à cette température peut aller jusqu'à 15 minutes sans qu'il y ait de risque que la résistance Rm de la pièce finale ne diminue au dessous de 800MPa. La température de chauffage doit être supérieure à 400°C pour diminuer suffisamment la limite d'écoulement de l'acier et permettre l'emboutissage qui va suivre avec des efforts peu importants, et faire en sorte que le retour élastique de la pièce emboutie soit également minime ce qui permet la fabrication de pièce avec une bonne précision géométrique. Cette température est limitée à 690°C d'une part pour éviter une transformation partielle au chauffage en austénite, qui conduirait à la formation de constituants de trempe au refroidissement, d'autre part pour éviter un adoucissement de la matrice qui conduirait à une résistance inférieure à 800MPa sur la pièce emboutie.
• On effectue ensuite un emboutissage de ces flans chauffés dans une gamme de température allant de 350°C à (T-20°C) pour former une pièce que l'on refroidit jusqu'à température ambiante. On réalise de la sorte un emboutissage « à tiède » avec les effets suivants :
• On diminue la contrainte d'écoulement de l'acier. Ceci permet d'utiliser des presses d'emboutissage moins puissantes et/ou de fabriquer des pièces plus difficiles à réaliser que par emboutissage à froid.
• La gamme de température de l'emboutissage à tiède tient compte de la légère diminution de température lorsque le flan est extrait du four et transféré à la presse d'emboutissage : pour une température de chauffage de T°C, l'emboutissage peut débuter à une température de (T-20°C). La température d'emboutissage doit cependant être supérieure à 350°C afin de limiter le retour élastique et le niveau de contraintes résiduelles sur la pièce finale. Par rapport à un emboutissage à froid, cette diminution du retour élastique permet la fabrication de pièces avec une meilleure tolérance géométrique finale.
• De façon surprenante, on a découvert que la microstructure particulière des aciers selon l'invention présente une grande stabilité de propriétés mécaniques (résistance, allongement) lors de l'emboutissage à tiède : en effet, une variation de la température d'emboutissage ou de vitesse de refroidissement après emboutissage, ne conduisent pas à une modification importante de la microstructure et des précipités telles que les carbonitrures.
• Dans la limite des conditions de l'invention, une modification inopinée ou une fluctuation des paramètres de chauffage (température ou temps de maintien) ou de refroidissement (contact plus ou moins parfait de la pièce avec l'outillage) ne conduisent pas alors à un rejet des pièces ainsi produites.
• Lors du chauffage et de l'emboutissage à tiède, une modification des composés M-A éventuellement présents en faible quantité initiale ne se traduit pas par une dégradation des propriétés mécaniques. On ne note pas par exemple d'influence négative lié à une déstabilisation de l'austénite résiduelle.
• La microstructure après emboutissage à tiède est très proche de la microstructure avant emboutissage. De la sorte, si on chauffe et on emboutit à tiède non pas la totalité d'un flan, mais seulement une partie (la partie à emboutir ayant été chauffée localement par un moyen approprié, par exemple par induction) la microstructure et les propriétés de la pièce finale seront bien homogènes dans ses différentes parties.

The inventors have also demonstrated that it was possible to take advantage of the microstructure according to the invention to produce parts stamped particularly advantageously according to the following method:

• The blanks defined above are first heated to a temperature T of between 400 and 690 ° C. The holding time at this temperature can be up to 15 minutes without there being any risk that the resistance Rm of the final part decreases below 800 MPa. The heating temperature must be greater than 400 ° C to sufficiently decrease the flow limit of the steel and allow the stamping to follow with little effort, and ensure that the elastic return of the stamped part is also minimal which allows the manufacture of part with a good geometric precision. This temperature is limited to 690 ° C on the one hand to avoid a partial transformation to austenite heating, which would lead to the formation of cooling quenching constituents, on the other hand to avoid a softening of the matrix that would lead to a resistance less than 800MPa on the stamped part.
• These heated blanks are then stamped in a temperature range of 350 ° C to (-20 ° C) to form a part which is cooled to room temperature. In this way, a "warm" stamping is carried out with the following effects:
• The flow stress of the steel is reduced. This makes it possible to use less powerful stamping presses and / or to make parts that are more difficult to produce than by cold stamping.
• The temperature range of warm stamping takes into account the slight decrease in temperature when the blank is removed from the oven and transferred to the stamping press: for a heating temperature of T ° C, stamping can begin at a temperature of (T-20 ° C). The drawing temperature must however be greater than 350 ° C in order to limit the springback and the level of residual stresses on the final part. Compared to a cold stamping, this reduction of the springback allows the manufacture of parts with a better final geometric tolerance.
• Surprisingly, it has been found that the particular microstructure of the steels according to the invention has a high stability of mechanical properties (strength, elongation) during hot stamping: indeed, a variation of the stamping temperature or the cooling rate after stamping, do not lead to a significant modification of the microstructure and precipitates such as carbonitrides.
• Within the limits of the conditions of the invention, an unexpected modification or a fluctuation of the heating parameters (temperature or hold time) or cooling (more or less perfect contact of the workpiece with the tooling) do not then lead to a rejection of the parts thus produced.
• During heating and hot stamping, a modification of the MA compounds possibly present in a small initial quantity does not result in a degradation of the mechanical properties. For example, there is no negative influence related to a destabilization of the residual austenite.
• The microstructure after warm stamping is very close to the microstructure before stamping. In this way, if one warms and hot stamps not all of a blank, but only a part (the part to be stamped having been locally heated by a suitable means, for example by induction) the microstructure and the properties of the final piece will be homogeneous in its different parts.

Example 1

Steels have been developed, the composition of which is given in the table below, expressed in percentage by weight. In addition to the steel I-1 used for the production of sheets according to the invention, the composition of R-1 and R-2 steels used for the manufacture of reference sheets has been indicated for comparison. Table 1 Compositions of steel (% by weight). Steel VS (%) Mn (%) Yes (%) Al (%) S (%) P (%) Mo (%) Cr (%) NOT(%) V (%) Nb (%) I-1 0,070 1,604 0.218 0,028 0,002 0.014 0.313 0,400 0.006 0,150 - I2 0,072 1,592 0,204 0.031 0,003 0,024 0,200 0.414 0.006 0.211 0,017 R1 0,125 1,670 0,205 0,030 0,002 0,025 0.307 0.414 0,004 0.105 - R2 0,102 1,680 0,204 0,023 0,002 0,028 0,315 0,408 0,007 0,205 - I = according to the invention. R = reference
Underlined values: Not in accordance with the invention.

Semi-finished products corresponding to the above compositions were reheated to 1220 ° C and hot rolled to a thickness of 2.3 mm in a field where the structure is fully austenitic. The manufacturing conditions of these steels (end of rolling temperature T _FL , cooling rate V _R , winding temperature T _bob ) are indicated in table 2 Table 2 Manufacturing conditions. Steel T _FL (° C) V _R (° C / s) T _bob (° C) I1 910 80 520 I2 875 80 600 R1 880 80 520 R2 885 100 450 Underlined values: not in accordance with the invention

The tensile mechanical properties obtained (elastic limit Re, resistance Rm, elongation at break A) were given in Table 3 below. Table 3: Mechanical characteristics (long direction compared to rolling) Steel Re (MPa) Rm (MPa) Elongation at break A (%) I1 820 880 11 I2 767 831 16 R1 740 835 8 R2 870 927 7.5 Underlined values: not in accordance with the invention

The high values of the mechanical characteristics are obtained both in the long direction and in the transverse direction with respect to rolling for the steels according to the invention.

The microstructure of steel I1 illustrated in figure 2 comprises more than 80% higher bainite, the remainder being lower bainite and MA compounds. The total content of martensite and residual austenite is less than 5%. The size of the old austenitic grains and bainitic batten bundles is about 10 microns. The limitation of the size of the batten packets and the strong disorientation between the adjacent packets results in a high resistance to the propagation of any microcracks. Due to the small difference in hardness between the various constituents of the microstructure, the steel is not very sensitive to damage during cutting by a mechanical process.

The steel sheet R1, having a carbon content too high and a vanadium content too low, has an elongation insufficient rupture. The R2 steel has a carbon content and phosphorus too high, its winding temperature is also too low. As a result, its elongation at break is also significantly less than 10%.

LASER autogenous welded joints were made under the following conditions: power: 4.5kW, welding speed: 2.5m / min. The lengthwise elongation of the LASER welds of I-1 steel is 17%, whereas it is 10 and 13% respectively for the R-1 and R-2 steels. These values lead, particularly for steel R1, to difficulties in stamping welded joints.

Steel sheets I1 according to the invention were also galvanized under the following conditions: after heating at 680 ° C., the sheets were cooled to 455 ° C. and then quenched continuously in a Zn bath at this temperature and finally cooled to room temperature. The mechanical characteristics of the galvanized sheets are as follows: Re = 824 MPa, Rm = 879 MPa, A = 12%. These properties are virtually identical to those of the uncoated sheet, which indicates that the microstructure of the steels according to the invention is very stable vis-à-vis the thermal cycles of galvanization.

Example 2

A steel sheet I-1, manufactured using the parameters defined in Table 2 for this steel, was cut to obtain blanks. After heating at temperatures of 400 ° C. or 690 ° C., held at these temperatures for 7 or 10 minutes and hot-drawing at temperatures of 350 ° C. or 640 ° C. respectively, the parts obtained were cooled to a speed V _'R 25 ° C / sec 100 ° C / s to room temperature. The speed V ' _R denotes the average speed of cooling between the temperature T and the ambient temperature. The mechanical strength Rm of the parts thus obtained is indicated in Table 4: Table 4: Resistance Rm obtained after hot stamping under various conditions Cooling 25 ° C / s 100 ° C / s cooling Heating: 400 ° C- 7 minutes 880 MPa 875MPa Heating: 400 ° C- 10 minutes 875 MPa 885MPa Heating: 690 ° C-10 minutes 810MPa 810MPa

The stamped parts according to the conditions of the invention thus have a low sensitivity to a variation of the manufacturing conditions: after heating at 400 ° C., the final resistance varies little (10 MPa) when the duration of the heating and / or the speed of the cooling are modified.

Even for heating at 690 ° C, the resistance of the piece obtained is greater than 800 MPa.

Compared to the initial microstructure, there is a slight additional precipitation of carbides. The structure remains practically identical to that of the non-hot stamped sheet metal, as illustrated by figure 3 relating to a room heated at 400 ° C for 7 minutes and then swirled at 380 ° C.

Thus, the invention allows the manufacture of laminations or pieces of bainitic matrix steels without excessive addition of expensive elements. These combine 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 (18)

1. Hot-rolled steel sheet or part having a tensile strength greater than 800 MPa and an elongation at break greater than 10%, the composition of which comprises, the contents being expressed by weight:

   0.050% ≤ C ≤ 0.090%

   1% ≤ Mn ≤ 2%

   0.015% ≤ Al ≤ 0.050%

   0.1% ≤ Si ≤ 0.3%

   0.10% ≤ Mo ≤ 0.40%

   S ≤ 0.010%

   P ≤ 0.025%

   0.003% ≤ N ≤ 0.009%

   0.12% ≤ V ≤ 0.22%

   Ti ≤ 0.005%

   Nb ≤ 0.020%

   and, optionally,

   Cr ≤ 0.45%

   the balance of the composition consisting of iron and inevitable impurities resulting from the smelting, the microstructure of said sheet or said part comprising, as a surface fraction, at least 80% upper bainite, the possible complement consisting of lower bainite, martensite and residual austenite, the sum of the martensite and residual austenite contents being less than 5%.
2. Steel sheet or part according to Claim 1, characterized in that the composition of said steel comprises, the content being expressed by weight:

   0.050% ≤ C ≤ 0.070%.
3. Steel sheet or part according to Claim 1, characterized in that the composition of said steel comprises, the content being expressed by weight:

   0.070% < C ≤ 0.090%.
4. Steel sheet or part according to any one of Claims 1 to 3, characterized in that the composition of said steel comprises, the content being expressed by weight:

   1.4% ≤ Mn ≤ 1.8%.
5. Steel sheet or part according to any one of Claims 1 to 4, characterized in that the composition of said steel comprises, the content being expressed by weight:

   0.020% ≤ Al ≤ 0.040%.
6. Steel sheet or part according to any one of Claims 1 to 5, characterized in that the composition of said steel comprises, the content being expressed by weight:

   0.12% ≤ V ≤ 0.16%.
7. Steel sheet or part according to any one of Claims 1 to 6, characterized in that the composition of said steel comprises, the content being expressed by weight:

   0.18% ≤ Mo ≤ 0.30%.
8. Steel sheet or part according to any one of Claims 1 to 7, characterized in that the composition of said steel comprises, the content being expressed by weight:

   Nb ≤ 0.005%.
9. Steel sheet or part according to any one of Claims 1 to 8, characterized in that the composition of said steel comprises, the content being expressed by weight:

   0.20% ≤ Cr ≤ 0.45%.
10. Steel sheet or part according to any one of Claims 1 to 9, characterized in that said sheet or said part is coated with a zinc-based or aluminium-based coating.
11. Process for manufacturing a hot-rolled steel sheet having a tensile strength greater than 800 MPa and an elongation at break greater than 10%, in which:

    - a steel with the composition according to any one of Claims 1 to 9 is provided;

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

    - said semi-finished product is heated to a temperature above 1150°C;

    - said semi-finished product is hot-rolled to a temperature T_ER in a temperature range in which the microstructure of the steel is entirely austenitic so as to obtain a sheet;

    - said sheet is cooled in such a way that the cooling rate V_c is between 75 and 200°C/s; and then

    - said sheet is coiled at a temperature T_coil of between 500 and 600°C.
12. Process for manufacturing a hot-rolled steel sheet according to Claim 11, characterized in that the end-of-rolling temperature T_ER is between 870 and 930°C.
13. Process for manufacturing a hot-rolled steel sheet according to Claim 11 or 12, characterized in that the cooling rate V_c is between 80 and 150°C/s.
14. Manufacturing process in which a sheet manufactured according to any one of Claims 11 to 13 is pickled, then optionally skin-passed and then coated with zinc or a zinc alloy or else with aluminium or an aluminium alloy.
15. Process for manufacturing a steel sheet according to Claim 14, characterized in that said coating is carried out continuously by hot-dip coating.
16. Process for manufacturing a warm-drawn part, characterized in that:

    - a steel sheet according to any one of Claims 1 to 10, or manufactured by a process according to any one of Claims 11 to 15, is provided; then

    - said sheet is cut so as to obtain a blank; then

    - said blank is partly or completely heated to a temperature T of between 400 and 690°C, where it is maintained for a time of less than 15 minutes so as to obtain a heated blank; then

    - said heated blank is drawn at a temperature of between 350 and T-20°C in order to obtain a part; and then

    - said part is cooled down to ambient temperature at a rate V'_c.
17. Manufacturing process according to Claim 16, characterized in that the rate V'_c is between 25 and 100°C/s.
18. Use of a hot-rolled steel sheet according to any one of Claims 1 to 10, or manufactured by a process according to any one of Claims 11 to 17, for the manufacture of structural parts or reinforcing elements in the automotive field.

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