EP2707513B1 - Method for the production of very-high-strength martensitic steel and sheet or part thus obtained - Google Patents

Description

The invention relates to a method for manufacturing sheet or steel parts with a martensitic structure, with a mechanical strength greater than that which could be obtained by austenitization and then simple fast cooling treatment with martensitic quenching, and properties of mechanical strength and durability. elongation allowing their application to the manufacture of energy absorbing parts in motor vehicles. In some applications, it is sought to produce steel parts combining high mechanical strength, high impact resistance and good corrosion resistance. This type of combination is particularly desirable in the automotive industry where significant vehicle lightening is sought. This can be achieved particularly through the use of steel parts with very high mechanical properties whose microstructure is martensitic or bainito-martensitic. Anti-intrusion parts, structure or participating in the safety of motor vehicles such as: bumper cross members, door or center pusher reinforcements, wheel arms, require for example the qualities mentioned above. Their thickness is preferably less than 3 millimeters.

The patent EP0971044 discloses the manufacture of a steel sheet coated with aluminum or an aluminum alloy, the composition of which comprises in weight content: 0.15-0.5% C, 0.5-3% Mn , 0.1-0.5% Si, 0.011% Cr, Ti <0.2%, Al <0.1%, P <0.1%, S <0.05%, 0.0005% <B < 0.08%, the rest being iron and impurities inherent in the elaboration. This sheet is heated so as to obtain an austenitic transformation and hot stamped so as to produce a part, which is then cooled rapidly to obtain a martensitic or martensito-bainitic structure. In this way, it is possible to obtain, for example, a mechanical strength greater than 1500 MPa. However, we seek to obtain parts with even greater mechanical strength. We search still, at a given level of mechanical strength, to reduce the carbon content of the steel so as to improve its weldability.

There is also known a manufacturing process called "ausforming" in which a steel is totally austenitized and then rapidly cooled to an intermediate temperature, generally around 700-400 ° C, in which range the austenite is metastable. This austenite is hot deformed and then rapidly cooled so as to obtain a totally martensitic structure. The patent GB1,080,304 thus describes the composition of a steel sheet for such a process, which comprises 0.15-1% C, 0.25-3% Mn, 1-2.5% Si, 0.5-3% Mo , 1-3% Cu, 0.2-1% V.

Similarly, the patent GB 1,166,042 discloses a steel composition suitable for this ausforming process, which comprises 0.1-0.6% C, 0.25-5% Mn, 0.5-2% Al, 0.5-3% Mo, 0.01-2% Si, 0.01-1% V.

These steels include significant additions of molybdenum, manganese, aluminum, silicon and / or copper. These are intended to create a larger metastability domain for austenite, ie to delay the onset of the transformation from austenite to ferrite, bainite or perlite, at the temperature at which performs hot deformation. Most studies on ausforming have been conducted on steels with a carbon content greater than 0.3%. Thus, these compositions adapted to the ausforming have the disadvantage of requiring special precautions for welding, and also have particular difficulties in the case where it is desired to perform a metal coating quenching. In addition, these compositions have expensive addition elements.

Moreover, the publication of S. Morito et al .: "Influence of austenite grain size on the morphology and crystallography of martensite in Low C steels". ISIJ International, vol. 45, 2005 . This publication illustrates the relationships between austenitic grain size, and the sizes of martensite packs, blocks, and slats obtained after quenching with water. The results presented, however, are only relative to C-Mn and C-Mn-V steels and to a simple austenitization and quenching process, without hot deformation.

In addition, this document does not contain any teaching on the slenderness factor of martensite slats.

We also know the publication of Tsuji and Maki: "Enhanced structural refinement by deformation and plastic transformation" Scripta Materiala, 2009 . This presents the general principles for combining hot deformation and transformation to obtain ultrafine structures. The process of ausforming is mentioned for the manufacture of martensitic structures. However the publication reports that the size of the martensitic slats is not modified by the ausforming. We also know the publication of S. Morito et al .: "Effect of the block size on the strength of the martensite in low C steels" Material Science and Engineering A, 2006 . This publication aims to clarify the relationship between the yield strength and the size of the blocks or martensitic packages of two grades of steel. These grades are Fe-C or Fe-C-Mn steels, without any other significant addition of alloying elements. In addition, this publication only discloses test results after austenitization and quenching without hot deformation.

It seeks to have a method of manufacturing sheets or steel parts does not have the above disadvantages, with a higher tensile strength of more than 50 MPa that could be obtained through austenitization followed by a simple martensitic quenching of the steel in question. The inventors have demonstrated that, for carbon contents ranging from 0.15 to 0.40% by weight, the tensile tensile strength Rm of steels manufactured by total austenitization followed by a simple martensitic quench, do not practically depended only on the carbon content and was connected to it with very good precision, according to the expression (1): Rm (megapascals) = 3220 (C) + 908.

In this expression, (C) denotes the carbon content of the steel, expressed as a percentage by weight. At a carbon content C given for a steel, a method of manufacture is thus sought which makes it possible to obtain an ultimate tensile strength of 50 MPa for expression (1), ie a strength greater than 3220 (C ) + 958 MPa for this steel. It is sought to have a method for producing sheet metal with a very high yield strength, say greater than 1300 MPa. It is also sought to have a method for the manufacture of sheets or parts usable directly, that is to say without the need for a tempering treatment after quenching. It is also sought to have a manufacturing process for the manufacture of a sheet or a readily coated part by dipping in a metal bath.

These sheets or these parts must be weldable by the usual methods and not have expensive additions of alloying elements.

The present invention aims to solve the problems mentioned above. It aims in particular to provide sheets with a yield strength greater than 1300 MPa, a mechanical strength expressed in megapascals greater than (3220 (C) +958) MPa, and preferably a total elongation greater than 3%.

à limite d'élasticité supérieure à 1300 MPa, à résistance mécanique supérieure à (3220(C)+958) mégapascals, étant entendu que (C) désigne la teneur en carbone en pourcentage pondéral de l'acier, comprenant les étapes successives et dans cet ordre selon lesquelles :

• on approvisionne un demi-produit d'acier dont la composition comprend, les teneurs étant exprimées en poids, 0,15% ≤ C ≤ 0,40%, 1,5%≤ Mon ≤ 3%, 0,005% ≤ Si ≤ 2%, 0,005%≤ Al ≤ 0,1%, 1,8% ≤ Cr≤ 4%, 0%≤ Mo ≤2%, étant entendu que 2,7%≤0,5 (Mn)+(Cr)+3(Mo)≤5,7%, S ≤ 0,05%, P≤ 0,1%, et optionnellement: 0%≤ Nb≤0,050%, 0,01%≤ Ti≤0,1%, 0,0005% ≤ B ≤ 0,005%, 0,0005% ≤ Ca ≤ 0,005%, le reste de la composition étant constitué de fer et d'impuretés inévitables résultant de l'élaboration,
• on réchauffe le demi-produit à une température T₁ comprise entre 1050°C et 1250°C, puis
• on effectue un laminage de dégrossissage du demi-produit réchauffé, à une température T₂ comprise entre 1000 et 880°C, avec un taux de réduction ε_a cumulé supérieur à 30% de façon à obtenir une tôle avec une structure austénitique complètement recristallisée de taille moyenne de grain inférieure à 40 micromètres et préférentiellement à 5 micromètres, le taux de réduction cumulé ε_a étant défini par: $\mathrm{Ln}\frac{{\mathit{e}}_{\mathit{ia}}}{{\mathit{e}}_{{\mathit{f}}_{\mathit{a}}}},\mathrm{.}$
  
  e_ia désignant l'épaisseur du demi-produit avant le laminage à chaud de dégrossissage et e_fa l'épaisseur de la tôle après le laminage de dégrossissage, puis
• on refroidit non complètement la tôle jusqu'à une température T₃ comprise entre 600°C et 400°C dans le domaine austénitique métastable, à une vitesse V_R1 supérieure à 2°C/s, puis
• on effectue un laminage à chaud de finition à la température T₃, de la tôle non complètement refroidie, avec un taux de réduction cumulé ε_b supérieur à 30% de façon à obtenir une tôle, le taux de réduction cumulé ε_b étant défini par: $\mathrm{Ln}\frac{{\mathit{e}}_{\mathit{ib}}}{{\mathit{e}}_{{\mathit{f}}_b}},$
  
  e_ib désignant l'épaisseur de la tôle avant le laminage à chaud de finition et e_fa l'épaisseur de la tôle après le laminage de finition, puis
• on refroidit la tôle à une vitesse V_R2 supérieure à la vitesse critique de trempe martensitique.

For this purpose, the subject of the invention is a method for manufacturing a sheet of steel with a totally martensitic structure having an average slat size of less than 1 micrometer, the average elongation factor of the slats being between 2 and 5 , it being understood that the elongation factor of a slat of maximum dimension I _max and minimum I _min is defined by $\frac{\mathit{l}\max }{\mathit{l}\min },$

with a yield strength greater than 1300 MPa, with a mechanical strength greater than (3220 (C) +958) megapascals, it being understood that (C) denotes the carbon content by weight percentage of the steel, comprising the successive stages and in this order according to which:

• supplying a semi-finished steel product whose composition comprises, the contents being expressed by weight, 0.15% ≤ C ≤ 0.40%, 1.5% ≤ Mon ≤ 3%, 0.005% ≤ Si ≤ 2% , 0.005% ≤ Al ≤ 0.1%, 1.8% ≤ Cr ≤ 4%, 0% ≤ Mo ≤2%, with 2.7% ≤ 0.5 (Mn) + (Cr) + 3 ( Mo) ≤ 5.7%, S ≤ 0.05%, P ≤ 0.1%, and optionally: 0% ≤ Nb 0 0.050%, 0.01% ≤ Ti 0 0.1%, 0.0005% ≤ B ≤ 0.005%, 0.0005% ≤ Ca ≤ 0.005%, the remainder of the composition consisting of iron and unavoidable impurities resulting from the preparation,
• the semi-finished product is heated to a temperature T ₁ of between 1050 ° C. and 1250 ° C., and then
• the semi-product heated at a temperature T ₂ of between 1000 and 880 ° C. is rough-rolled with a cumulative reduction ratio ε _a greater than 30% so as to obtain a sheet with a completely recrystallized austenitic structure of average grain size less than 40 micrometers and preferably 5 micrometers, the cumulative reduction rate ε _a being defined by: $\mathrm{Ln}\frac{{\mathit{e}}_{\mathit{ia}}}{{\mathit{e}}_{{\mathit{f}}_{\mathit{at}}}},\mathrm{.}$
  
  e _ia designating the thickness of the semi-finished product before the hot rolling of roughing and e _fa the thickness of the sheet after the rough rolling, and then
• the sheet is cooled to a temperature T ₃ of between 600 ° C. and 400 ° C. in the austenitic metastable domain, at a speed V _R1 greater than 2 ° C./s, and then
• finishing the hot rolling at the temperature T ₃ of the non-completely cooled sheet with a cumulative reduction ratio ε _b greater than 30% so as to obtain a sheet, the cumulative reduction ratio ε _b being defined by : $\mathrm{Ln}\frac{{\mathit{e}}_{\mathit{ib}}}{{\mathit{e}}_{{\mathit{f}}_b}},$
  
  e _ib denoting the thickness of the sheet before finishing hot rolling and e _fa the thickness of the sheet after finishing rolling, and then
• the sheet is cooled to a speed V _R2 greater than the critical speed of martensitic quenching.

• on approvisionne un flan d'acier dont la composition comprend, les teneurs étant exprimées en poids, 0,15% ≤ C ≤ 0,40%, 1,5%≤ Mn ≤ 3%, 0,005% ≤ Si ≤ 2%, 0,005%≤ Al ≤ 0,1%, 1,8% ≤ Cr≤ 4%, 0%≤ Mo ≤2%, étant entendu que 2,7%≤0,5 (Mn)+(Cr)+3(Mo)≤5,7%, S ≤ 0,05%, P≤ 0,1%, optionnellement: 0%≤ Nb≤0,050%, 0,01%≤ Ti≤0,1%, 0,0005% ≤ B ≤ 0,005%, 0,0005% ≤ Ca ≤ 0,005%, le reste de la composition étant constitué de fer et d'impuretés inévitables résultant de l'élaboration,

• on chauffe le flan à une température T₁ comprise entre A_C3 et A_C3+250°C de telle sorte que la taille moyenne de grain austénitique soit inférieure à 40 micromètres, et préférentiellement à 5 micromètres, puis
• on transfère le flan chauffé au sein d'une presse d'emboutissage à chaud ou d'un dispositif de mise en forme à chaud, puis
• on refroidit le flan jusqu'à une température T₃ comprise entre 600°C et 400°C, à une vitesse V_R1 supérieure à 2°C/s de façon à éviter une transformation de l'austénite,
• l'ordre des deux dernières étapes pouvant être interverti, puis,
• on emboutit ou on met en forme à chaud à la température T₃ le flan refroidi, d'une quantité ε_c supérieure à 30% dans au moins une zone, pour obtenir une pièces, ε_c étant défini par $\overline{{\epsilon }_c}=\frac{2}{\sqrt{3}}\sqrt{\left({\epsilon }_1^2+{\epsilon }_1{\epsilon }_2+{\epsilon }_2^2\right)},$
  
  où ε₁ et ε₂ sont les déformations principales cumulées sur l'ensemble des étapes de déformation à la température T₃, puis,
• on refroidit la pièce à une vitesse V_R2 supérieure à la vitesse critique de trempe martensitique.

The subject of the invention is also a method for manufacturing a piece of steel with a totally martensitic structure having an average slat size of less than 1 micrometer, the average elongation factor of the slats being between 2 and 5, comprising the successive stages and in this order according to which:

• a steel blank whose composition comprises, the contents being expressed by weight, is supplied with 0.15% ≤ C ≤ 0.40%, 1.5% ≤ Mn ≤ 3%, 0.005% ≤ Si ≤ 2%, 0.005 % ≤ Al ≤ 0.1%, 1.8% ≤ Cr≤4%, 0% ≤ Mo ≤2%, with 2.7% ≤0.5 (Mn) + (Cr) +3 (Mo) ≤ 5.7%, S ≤ 0.05%, P ≤ 0.1%, optionally: 0% ≤ Nb ≤ 0.050%, 0.01% ≤ Ti ≤ 0.1%, 0.0005% ≤ B ≤ 0.005 %, 0.0005% ≤ Ca ≤ 0.005%, the remainder of the composition being iron and impurities inevitable resulting from the elaboration,

• the blank is heated to a temperature T ₁ between A _C3 and A _C3 + 250 ° C so that the average austenitic grain size is less than 40 microns, and preferably 5 microns, and then
• the heated blank is transferred into a hot stamping press or a hot forming device, and then
• the blank is cooled to a temperature T ₃ of between 600 ° C and 400 ° C, at a speed V _R1 greater than 2 ° C / s so as to avoid transformation of the austenite,
• the order of the last two steps can be inverted, then,
• the cooled blank is stamped or shaped at a temperature of T ₃ ε _c greater than 30% in at least one area, to obtain parts, ε _c being defined by $\widetilde{{\epsilon }_{\mathrm{vs}}}=\frac{2}{\sqrt{3}}\sqrt{\left({\epsilon }_1^2+{\epsilon }_1{\epsilon }_2+{\epsilon }_2^2\right)},$
  
  where ε ₁ and ε ₂ are the main deformations accumulated over all the deformation steps at the temperature T ₃ , then,
• the workpiece is cooled to a speed V _R2 greater than the critical speed of martensitic quenching.

In a preferred embodiment, the blank is hot-stamped so as to obtain a workpiece, then the workpiece is held in the stamping tool so as to cool it at a speed V _R2 greater than the critical speed of martensitic quenching. .

In a preferred embodiment, the blank is pre-coated with aluminum or an aluminum-based alloy.

According to another preferred embodiment, the blank is pre-coated with zinc or a zinc-based alloy.

Preferably, the sheet or piece of steel obtained by any one of the above manufacturing processes is subjected to a subsequent heat treatment of tempering at a temperature T ₄ of between 150 and 600 ° C. for a period of time between 5 and 30 minutes.

The subject of the invention is also an unreturned steel sheet having a yield strength greater than 1300 MPa, with a mechanical strength greater than (3220 (C) +958) megapascals, with the proviso that (C) denotes the weight percent carbon content of the steel, obtained by any of the above manufacturing processes, having a totally martensitic structure, having a medium size of slats less than 1 micrometer, the average elongation factor of slats being between 2 and 5

The invention also relates to a piece of unreturned steel obtained by any one of the above part manufacturing processes, the part comprising at least one zone of totally martensitic structure having an average slat size of less than 1 micrometer, the average elongation factor of the slats being between 2 and 5, the yield strength in said zone being greater than 1300 MPa and the mechanical strength being greater than (3220 (C) +958) megapascals, it being understood that (C) refers to the percentage carbon content of the steel. The subject of the invention is also a sheet or a piece of steel obtained by the process with the above treatment of income, the steel having a totally martensitic structure, having in at least one zone an average slat size of less than 1 , 2 micrometer, the average elongation factor of the slats being between 2 and 5.

The inventors have demonstrated that the problems described above were solved by means of a specific ausforming process implemented on a particular range of steel compositions. Contrary to previous studies which showed that ausforming required the addition of expensive alloying elements, the inventors have surprisingly demonstrated that this effect can be obtained thanks to substantially less charged compositions of alloying elements.

• La figure 1 présente un exemple de microstructure de tôle d'acier fabriquée par le procédé selon l'invention.
• La figure 2 présente un exemple de microstructure du même acier fabriqué par un procédé de référence, par chauffage dans le domaine austénitique puis simple trempe martensitique.
• La figure 3 présente un exemple de microstructure de pièce d'acier fabriquée par le procédé selon l'invention.

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

• The figure 1 shows an example of microstructure of steel sheet manufactured by the method according to the invention.
• The figure 2 shows an example of microstructure of the same steel manufactured by a reference method, by heating in the austenitic domain and then simple martensitic quenching.
• The figure 3 shows an example of a steel part microstructure manufactured by the method according to the invention.

The composition of the steels used in the process according to the invention will now be detailed.

When the carbon content of the steel is less than 0.15% by weight, the quenchability of the steel is insufficient given the process used and it is not possible to obtain a totally martensitic structure. When this content is greater than 0.40%, welded joints made from these sheets or these parts have insufficient toughness. The optimum carbon content for the implementation of the invention is between 0.16 and 0.28%.

Manganese lowers the initial formation temperature of martensite and slows the decomposition of austenite. In order to obtain sufficient effects to allow the implementation of the ausforming, the manganese content must not be less than 1.5%. Moreover, when the manganese content exceeds 3%, segregated zones are present in excessive quantity which is detrimental to the implementation of the invention. A preferred range for the implementation of the invention is 1.8 to 2.5% Mn.

The silicon content must be greater than 0.005% so as to contribute to the deoxidation of the steel in the liquid phase. The silicon should not exceed 2% by weight due to the formation of surface oxides which significantly reduce the processability in processes involving a continuous passage of the steel sheet in a coating metal bath.

Chromium and molybdenum are very effective in retarding the transformation of austenite and in separating the ferrito- pearlitic and bainitic transformation domains, ferrito- pearlitic transformation occurring at higher temperatures than bainitic transformation. These transformation domains are in the form of two distinct "noses" in an isothermal transformation diagram TTT (Transformation-Temperature-Time) from the austenite, which allows the implementation of the method according to the invention.

The chromium content of the steel must be between 1.8% and 4% by weight in order for its delay effect on the transformation of the austenite to be sufficient. The chromium content of the steel takes into account the content of other elements that increase the quenchability such as manganese and molybdenum: in fact, given the respective effects of manganese, chromium and molybdenum on the transformations from the austenite, a combined addition of these elements must be carried out respecting the following condition, the respectively noted quantities (Mn) (Cr) (Mo) being expressed in percentage by weight: 2.7% ≤0.5 (Mn) + (Cr) 3 (MB) ≤5,7%. However, the molybdenum content must not exceed 2% because of its excessive cost.

The aluminum content of the steel according to the invention is not less than 0.005% so as to obtain sufficient deoxidation of the steel in the liquid state. When the aluminum content is greater than 0.1% by weight, casting problems can occur. It is also possible to form inclusions of alumina in too large a quantity or size which play a detrimental role on the tenacity:

The sulfur and phosphorus contents of the steel are respectively limited to 0.05 and 0.1% in order to avoid a reduction in the ductility or toughness of the parts or sheets produced according to the invention.

The steel may optionally contain niobium and / or titanium, which makes it possible to refine further refinement of the grain. Due to the heat curing these additions confer, they must however be limited to 0.050% for niobium and between 0.01 and 0.1% for titanium so as not to increase the forces during hot rolling. .

As an option, the steel can also contain boron: indeed, the significant deformation of the austenite can accelerate the conversion to ferrite on cooling, a phenomenon that should be avoided. Addition of boron in an amount of between 0.0005 and 0.005% by weight makes it possible to guard against early ferritic transformation.

Optionally, the steel may also contain calcium in an amount between 0.0005 and 0.005%: by combining with oxygen and sulfur, calcium prevents the formation of large inclusions, harmful to the ductility of the sheets or parts thus manufactured.

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

Afin d'être statistiquement représentative, cette observation porte sur au moins 1000 lattes martensitiques. Le facteur d'allongement moyen $\frac{\overline{\mathit{l}\max }}{\mathit{l}\min }$

est ensuite déterminé pour l'ensemble de ces lattes observées.The sheets or steel parts manufactured according to the invention are characterized by a totally slab martensite structure of great fineness: due to the specific thermomechanical cycle and composition, the average size of the martensitic slats is less than 1 micrometer and their average elongation factor is between 2 and 5. These microstructural characteristics are determined for example by observing the microstructure by scanning electron microscopy using a field effect gun ("MEB-FEG" technique) at a magnification higher than 1200x, coupled to an EBSD detector ("Electron Backscatter Diffraction"). It is defined that two contiguous slats are distinct when their disorientation is greater than 5 degrees. The average slat size is defined by the intercepts method known per se: the mean size of the intercepted slats is evaluated by randomly defined lines with respect to the microstructure. The measurement is performed on at least 1000 martensitic slats in order to obtain a representative average value. The morphology of the individual slats is determined by image analysis using known software in themselves: the maximum I _max and minimum I _min dimension of each martensitic slat and its elongation factor are determined. $\frac{\mathit{l}\max }{\mathit{l}\min }\mathrm{.}$

In order to be statistically representative, this observation concerns at least 1000 martensitic slats. The average elongation factor $\frac{\widetilde{\mathit{l}\max }}{\mathit{l}\min }$

is then determined for all of these slats observed.

The method according to the invention makes it possible to manufacture either rolled sheets or hot-stamped or heat-formed parts. These two modes will be successively exposed.

• On approvisionne tout d'abord un demi-produit d'acier dont la composition a été exposée ci-dessus. Ce demi-produit peut se présenter par exemple sous forme de brame issue de coulée continue, de brame mince ou de lingot. A titre d'exemple indicatif, une brame de coulée continue a une épaisseur de l'ordre de 200mm, une brame mince une épaisseur de l'ordre de 50-80mm. On réchauffe ce demi-produit à une température T₁ comprise entre 1050°C et 1250°C. La température T₁ est supérieure à A_c3, température de transformation totale en austénite au chauffage. Ce réchauffage permet donc d'obtenir une austénitisation complète de l'acier ainsi que la dissolution d'éventuels carbonitrures de niobium existant dans le demi-produit. Cette étape de réchauffage permet également de réaliser les différentes opérations ultérieures de laminage à chaud qui vont être présentées : on effectue un laminage, dit de dégrossissage, du demi-produit à une température T₂ comprise entre 1000 et 880°C.

The process for manufacturing hot-rolled sheets according to the invention comprises the following steps:

• First, a semi-finished steel product is supplied whose composition has been exposed above. This semi-finished product may for example be in the form of slab from continuous casting, thin slab or ingot. As an indicative example, a continuous casting slab has a thickness of about 200 mm, a thin slab a thickness of about 50-80 mm. This half-product is heated to a temperature T ₁ of between 1050 ° C. and 1250 ° C. The temperature T ₁ is greater than A _c3 , the total conversion temperature to austenite during heating. This reheating thus makes it possible to obtain a complete austenitization of the steel as well as the dissolution of any possible niobium carbonitrides in the semi-finished product. This reheating step also makes it possible to carry out the various subsequent hot rolling operations that will be presented: a roughing operation is carried out on the semi-finished product at a temperature T ₂ of between 1000 and 880 ° C.

Selon l'invention, le taux de réduction cumulé ε_a lors du laminage de dégrossissage doit être supérieur à 30%. Dans ces conditions, l'austénite obtenue est totalement recristallisée avec une taille moyenne de grain inférieure à 40 micromètres, voire à 5 micromètres lorsque la déformation ε_a est supérieure à 200% et lorsque la température T₂ est comprise entre 950 et 880°C. On refroidit ensuite non complètement la tôle, c'est à dire jusqu'à une température intermédiaire T₃, de façon à éviter une transformation de l'austénite, à une vitesse V_R1 supérieure à 2°C/s jusqu'à une température T₃ comprise entre 600°C et 400°C, domaine de température dans lequel l'austénite est métastable, c'est à dire dans un domaine où elle ne devrait pas être présente dans des conditions d'équilibre thermodynamique. On effectue alors un laminage à chaud de finition à la température T₃, le taux de réduction cumulé ε_b étant supérieur à 30%. Dans ces conditions, on obtient une structure austénitique déformée plastiquement dans laquelle n'intervient pas la recristallisation. On refroidit ensuite la tôle à une vitesse V_R2 supérieure à la vitesse de trempe critique martensitique.The cumulative reduction rate of the various stages of rolling at roughing is noted ε _a . If e _ia denotes the thickness of the semi-finished product before the hot rolling of roughing and e _fa the thickness of the sheet after this rolling, the reduction rate cumulated by ${\mathrm{\epsilon }}_{\mathrm{at}}=\mathrm{Ln}\frac{e_{\mathit{ia}}}{e_{f_{\mathrm{at}}}\mathrm{.}}$

According to the invention, the cumulative reduction ratio ε _a during the rough rolling must be greater than 30%. Under these conditions, the austenite obtained is completely recrystallized with an average grain size of less than 40 micrometers or even 5 microns when the strain ε _a is greater than 200% and when the temperature T ₂ is between 950 and 880 ° C. . The sheet is then not completely cooled, that is to say up to an intermediate temperature T ₃ , so as to avoid transformation of the austenite, at a speed V _R1 greater than 2 ° C./s up to a temperature T ₃ between 600 ° C and 400 ° C, temperature range in which the austenite is metastable, that is to say in a field where it should not be present under conditions of thermodynamic equilibrium. A finishing hot rolling is then carried out at the temperature T ₃ , the cumulative reduction ratio ε _b being greater than 30%. Under these conditions, a plastically deformed austenitic structure is obtained in which no action is taken. recrystallization. The sheet is then cooled at a speed V _R2 greater than the critical martensitic quenching speed.

Although the above method describes the manufacture of flat products (sheets) from slabs in particular, the invention is not limited to this geometry and to this type of products, and can be used for the manufacture of long products, bars, profiles, by successive stages of hot deformation.

• On approvisionne tout d'abord un flan en acier dont la composition contient en poids : 0,15% ≤ C ≤ 0,40%, 1,5%≤ Mn ≤ 3%, 0,005% ≤ Si ≤ 2%, 0,005%≤ Al ≤ 0,1%, 1,8% ≤ Cr≤ 4%, 0%≤ Mo ≤2%, étant entendu que 2,7%≤0,5 (Mn)+(Cr)+3(Mo)≤5,7%, S ≤ 0,05%, P≤ 0,1%, et optionnellement : 0%≤ Nb≤0,050%, 0,01%≤ Ti≤0,1%, 0,0005% ≤ B ≤ 0,005%, 0,0005% ≤ Ca ≤ 0,005%.

The process for manufacturing stamped or hot-formed parts is as follows:

• Firstly, a steel blank whose composition contains by weight: 0.15% ≤ C ≤ 0.40%, 1.5% ≤ Mn ≤ 3%, 0.005% ≤ Si ≤ 2%, 0.005% ≤ Al ≤ 0.1%, 1.8% ≤ Cr≤4%, 0% ≤ Mo ≤2%, with 2.7% ≤0.5 (Mn) + (Cr) +3 (Mo) ≤5 , 7%, S ≤ 0.05%, P ≤ 0.1%, and optionally: 0% ≤ Nb≤0.050%, 0.01% ≤ Ti ≤ 0.1%, 0.0005% ≤ B ≤ 0.005% , 0.0005% ≤ Ca ≤ 0.005%.

This flat blank is obtained by cutting a sheet or a coil in a form related to the final geometry of the target part. This blank may be uncoated or optionally pre-coated. The pre-coating may be aluminum or an aluminum-based alloy. In the latter case, the sheet may advantageously be obtained by continuously dipping in a bath of aluminum-silicon alloy comprising by weight 5-11% of silicon, 2 to 4% of iron, optionally between 15 and 30 ppm of calcium, the rest being aluminum and unavoidable impurities resulting from the elaboration.

The blank may also be pre-coated with zinc or a zinc-based alloy. The pre-coating may be in particular of the type galvanized with continuous dipping ("GI") or galvanized-alloyed ("GA")

The blank is heated to a temperature T ₁ between A _c3 and A _c3 + 250 ° C. In the case where the blank is pre-coated, the heating is preferably carried out in an oven under ordinary atmosphere; during this step, an alloying between the steel and the precoat is observed. The alloyed coating protects the underlying steel from oxidation and decarburization and is suitable for subsequent hot deformation. We keep the blank the temperature T ₁ to ensure the homogeneity of the temperature within it. Depending on the thickness of the blank, for example 0.5 to 3 mm, the holding time at the temperature T ₁ varies from 30 seconds to 5 minutes.

• on transfère le flan ainsi chauffé au sein d'une presse d'emboutissage à chaud ou bien au sein d'un dispositif de mise en forme à chaud : ce dernier peut être par exemple un dispositif de « roll-forming » dans lequel le flan est déformé progressivement par profilage à chaud dans une série de rouleaux jusqu'à atteindre la géométrie finale de la pièce désirée. Le transfert du flan jusqu'à la presse ou jusqu'au dispositif de mise en forme doit s'effectuer suffisamment rapidement pour ne pas provoquer de transformation de l'austénite.
• on refroidit ensuite le flan à une vitesse V_R1 supérieure à 2°C/s de façon à éviter la transformation de l'austénite, jusqu'à une température T₃ comprise entre 600°C et 400°C, domaine de température dans lequel l'austénite est métastable.

Under these conditions, the steel structure of the blank is completely austenitic. The limitation of the temperature to A _c3 + 250 ° C has the effect of restricting the magnification of the austenitic grain to an average size of less than 40 micrometers. When the temperature is between Ac3 and Ac3 + 50 ° C, the average grain size is preferably less than 5 micrometers.

• the blank thus heated is transferred into a hot stamping press or into a hot forming device: the latter may for example be a "roll-forming" device in which the blank is progressively deformed by hot forming in a series of rollers until reaching the final geometry of the desired part. The transfer of the blank to the press or to the shaping device must be carried out quickly enough not to cause transformation of the austenite.
• the blank is then cooled at a speed V _R1 greater than 2 ° C / s so as to avoid transformation of the austenite, to a temperature T ₃ of between 600 ° C and 400 ° C, temperature range in which the austenite is metastable.

According to one variant, it is also possible to reverse the order of these last two steps, ie to cool the blank first with a speed V _R1 greater than 2 ° C./s, and then to transfer this blank in the stamping press or hot forming device, so that it can be stamped or shaped hot as follows.

où ε_c et ε₂ sont les déformations principales cumulées sur l'ensemble des étapes de déformation à la température T₃. Dans une première variante, le mode de formage à chaud est choisi de telle sorte que la condition ε_c >30% soit satisfaite en tout endroit de la pièce formée.The blank is stamped or shaped at a temperature T ₃ of between 400 and 600 ° C., this hot deformation can be carried out in a single step or in several successive steps, as in the case of the mentioned roll-forming. above. Starting from an initial flat blank, the stamping makes it possible to obtain a part whose shape is not developable. Regardless of the hot layout mode, the cumulative deformation ε _c must be greater than 30% so as to obtain a non-recrystallized deformed austenite. As the deformation modes may vary from one place to another due to the geometry of the part and the local mode of stress (expansion, shrinkage, traction or uniaxial compression), the term ε _c the equivalent deformation defined in each point of the piece by $\widetilde{{\epsilon }_{\mathrm{vs}}}=\frac{2}{\sqrt{3}}\sqrt{\left({\epsilon }_1^2+{\epsilon }_1{\epsilon }_2+{\epsilon }_2^2\right)},$

where ε _c and ε ₂ are the main deformations accumulated over all the deformation steps at the temperature T ₃ . In a first variant, the hot forming mode is chosen so that the condition ε _c > 30% is satisfied anywhere in the formed part.

Optionally, it is also possible to implement a hot forming process where this condition is fulfilled only in certain particular places, corresponding to the most stressed areas of the rooms where it is desired to obtain particularly high mechanical characteristics. In these conditions, a part is obtained whose mechanical properties are variable, which may result in some places with a simple martensitic quench (in the case of possible zones not locally deformed during hot forming) and result in other zones of the zone. method according to the invention which leads to a martensitic structure with an extremely reduced slat size and increased mechanical properties.

After hot deformation, the workpiece is cooled at a speed V _R2 greater than the critical speed of martensitic quenching so as to obtain a totally martensitic structure. In the case of hot stamping, this cooling can be achieved by maintaining the piece in the tool with close contact therewith. This cooling by thermal conduction can be accelerated by cooling the stamping tool, for example through channels machined in the tool for the circulation of a refrigerant.

In addition to the steel composition used, the hot stamping process of the invention differs from the usual process of starting hot stamping as soon as the blank has been positioned in the press. According to this conventional method, the flow limit of the steel is the lowest at high temperature and the forces required by the press are the lowest. By comparison, the method according to the invention consists in observing a waiting time so that the blank reaches a temperature range suitable for the ausforming, then hot stamping the blank at a significantly lower temperature than in the usual process. For a given blank thickness, the stamping force required by the press is slightly higher but the final structure obtained thinner than in the usual process leads to greater mechanical properties of yield strength, strength and stability. ductility. To meet a specification corresponding to a given level of stress, it is therefore possible to reduce the thickness of the blanks and thereby reduce the stamping force of the parts according to the invention.

In addition, according to the conventional hot stamping method, the hot deformation immediately after stamping must be limited, this high temperature deformation tending to favor the formation of ferrite in the most deformed areas, which is sought to avoid. The method according to the invention does not include this limitation.

Whatever the variant of the process according to the invention, the sheets or the steel parts may be used as such or subjected to a heat treatment of tempering, carried out at a temperature T ₄ of between 150 and 600 ° C. for a period of time. between 5 and 30 minutes. This treatment of income has the effect of increasing the ductility at the price of a decrease in yield strength and strength. The inventors have, however, demonstrated that the process according to the invention, which gives a tensile strength Rm of at least 50 MPa higher than that obtained after conventional quenching, retained this advantage, even after treatment of tempering with temperatures. ranging from 150 to 600 ° C. The fineness characteristics of the microstructure are preserved by this treatment of income, the average size of slats being less than 1.2 micrometer, the average elongation factor of slats being between 2 and 5.

As a non-limitative example, the following results will show the advantageous characteristics conferred by the invention.

Example 1

Steel semi-finished products have been supplied whose compositions, expressed in contents by weight (%), are as follows: Steel VS mn Yes Cr MB al S P Nb Ti B 0.5Mn + Cr + 3MB AT 0.195 1,945 0.01 1,909 0.05 003 0,003 0.02 0.01 0.012 0.0014 3.03 B 0.24 1.99 0.01 1.86 0,008 0027 0,003 0.02 0,008 - - 2.88

31 mm thick semi-finished products were heated and held for 30 minutes at a temperature T ₁ of 1050 ° C. and then subjected to a 5-pass roughing rolling at a temperature T ₂ of 910 ° C. up to a thickness of 6 mm, a cumulative reduction ratio ε _is 164%. At this stage, the structure is completely austenitic and completely recrystallized with an average grain size of 30 microns. The sheets thus obtained were then cooled at a rate of 25 ° C./s up to the temperature T ₃ of 550 ° C. where they were rolled in 5 passes with a cumulative reduction rate ε _b of 60% and then cooled down. up to room temperature with a speed of 80 ° C / sec so as to obtain a completely martensitic microstructure. In comparison, steel sheets of the above composition were heated and held for 30 minutes at 1250 ° C. and then cooled by quenching with water so as to obtain a completely martensitic microstructure (reference treatment).

By means of tensile tests, the yield strength Re, the tensile strength Rm, and the total elongation A were determined for sheets obtained by these different methods of manufacture. The estimated value of the resistance after single martensitic hardening (3220% (C) +908) (MPa) and the difference ΔRm between this estimated value and the resistance actually measured are also shown.

The microstructure of the Scanning Electron Microscopy plates was also observed using a field effect gun (MEB-FEG technique) and EBSD detector and quantified the average size. slats of the martensitic structure and their average elongation factor $\frac{\widetilde{\mathit{l}\max }}{\mathit{l}\min }\mathrm{.}$

Invention A1 550 1588 1889 5,9 1536 353 0,9 3 B1 550 1572 1986 6,5 1681 306 0,8 4 Référence A2 Sans 1223 1576 6,9 1536 40 2 7 Valeurs soulignées : non conformes à l'invention The results of these different characterizations are presented below. Tests A1 and A2 designate tests carried out on the composition of steel A under two different conditions, the test B1 was made from the composition of steel B. Test conditions and mechanical results obtained Trial Temperature T ₃ (° C) Re (MPa) Rm (MPa) AT (%) 3220% C + 908 (MPa) ΔRm (MPa) Average size of slats (μm)

Invention A1 550 1588 1889 5.9 1536 353 0.9 3 B1 550 1572 1986 6.5 1681 306 0.8 4 Reference A2 Without 1223 1576 6.9 1536 40 2 7 Underlined values: not in accordance with the invention

The figure 1 shows the microstructure obtained in the case of test A1. By comparison, figure 2 shows the microstructure of the same steel simply heated to 1250 ° C., held for 30 minutes at this temperature and then quenched with water (test A2). The process according to the invention makes it possible to obtain a martensite with a significantly greater average slat size. thin and less elongated than in the reference structure.

In the case of the A2 (simple martensitic quenching) test, it is observed that the value of the estimated resistance (1536 MPa) from expression (1) is close to that determined experimentally (1576 MPa)

In tests A1 and B1 according to the invention, the values of ΔRm are 353 and 306 MPa respectively. The method according to the invention therefore makes it possible to obtain mechanical strength values that are clearly higher than those which would be obtained by a simple martensitic quenching. This increase in strength (353 or 306 MPa) is equivalent to that which would be obtained from equation (1) by simple martensitic quenching applied to steels in which an additional addition of 0.11% or 0.09 about% would have been achieved. Such an increase in the content However, the carbon-based process would have adverse consequences with respect to weldability and toughness, whereas the process according to the invention makes it possible to achieve very high values of mechanical strength without these disadvantages.

The sheets manufactured according to the invention because of their lower carbon content, have good weldability by conventional methods, in particular spot resistance welding.

Heat treatments were then carried out under different conditions of temperature and duration on the steel in condition B1 above: for a temperature up to 600 ° C and a duration of up to 30 minutes, the Average size of martensitic slats remains less than 1.2 micrometers.

Example 2

Steel blanks of thickness 3 mm of the following composition, expressed in terms of weight (%), were supplied: Steel VS mn Yes Cr MB al S P Nb 0.5Mn + Cr + 3Mo B 0.24 1.99 0.01 1.86 0,008 0,027 0,003 0.02 0,008 2.88

• soit refroidis à 50°C/s jusqu'à la température T₃ de 525°C puis emboutis à cette température avec une déformation équivalente ε_c supérieure à 50%, et enfin refroidis à une vitesse supérieure à la vitesse critique de trempe martensitique (essai B2)
• soit refroidis à 50°C/s jusqu'à la température de 525°C, puis refroidis à une vitesse supérieure à la vitesse critique de trempe martensitique (essai B3)

The blanks were subjected to heating at 1000 ° C (ie Ac3 + 210 ° C) for 5 minutes. These were then:

• be cooled to 50 ° C / s up to the temperature T ₃ of 525 ° C and then stamped at this temperature with equivalent deformation ε _c greater than 50%, and finally cooled to above the critical martensitic quenching speed (test B2)
• cooled to 50 ° C / s to 525 ° C, then cooled to above the critical martensitic quenching rate (test B3)

Invention B2 525 1531 1912 1681 299 0,9 3 Référence B3 - 1320 1652 1681 29 1,8 5 Valeurs soulignées : non conformes à l'invention The table below shows the mechanical properties obtained: Test conditions and mechanical results obtained Trial Temperature T ₃ (° C) Re (MPa) Rm (MPa) 3220% C + 908 (MPa) | ΔRm | (MPa) Average size of slats (μm)

Invention B2 525 1531 1912 1681 299 0.9 3 Reference B3 - 1320 1652 1681 29 1.8 5 Underlined values: not in accordance with the invention

The figure 3 shows the microstructure obtained in condition B3 according to the invention, characterized by a very thin slat size (0.9 micrometer) and a low elongation factor.

Thus, the invention allows the manufacture of sheets, or bare or coated parts, with very high mechanical characteristics, under very satisfactory economic conditions.

These sheets or parts will be used profitably for the manufacture of safety parts, including anti-intrusion parts or underbody, reinforcement bars, middle feet, for the construction of motor vehicles.

Claims (9)

1. Method for manufacturing a steel sheet with a completely martensitic structure having a mean lath size of less than 1 micrometre, the mean elongation factor of said laths being between 2 and 5, it being understood that the elongation factor of a lath with a maximum dimension l_max and minimum dimension l_min is defined by $\frac{l_{\mathit{max}}}{l_{\mathit{min}}},$

   

   with an elastic limit greater than 1300 MPa and a mechanical strength greater than (3220(C)+958) megapascals, it being understood that (C) designates the carbon content as a percentage by weight of said steel, comprising the successive steps in this order, according to which:

   - a semi-finished product is procured, the composition of which comprises, the proportions being expressed by weight, $$0.15\%\le \mathrm{C}\le 0.40\%$$

   

   $$1.5\%\le \mathrm{Mn}\le 3\%$$

   

   $$0.005\%\le \mathrm{Si}\le 2\%$$

   

   $$0.005\%\le \mathrm{Al}\le 0.1\%$$

   

   $$1.8\%\le \mathrm{Cr}\le 4\%$$

   

   $$0\%\le \mathrm{Mo}\le 2\%$$

   

   it being understood that $$2.7\%\le 0.5\left(\mathrm{Mn}\right)+\left(\mathrm{Cr}\right)+3\left(\mathrm{Mo}\right)\le 5.7\%$$

   

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

   

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

   

   and optionally: $$0\%\le \mathrm{Nb}\le 0.050\%$$

   

   $$0.01\%\le \mathrm{Ti}\le 0.1\%$$

   

   $$0.0005\%\le \mathrm{B}\le 0.005\%$$

   

   $$0.0005\%\le \mathrm{Ca}\le 0.005\%,$$

   

   the rest of the composition consisting of iron and unavoidable impurities resulting from the production,

   - the semi-finished product is heated to a temperature T₁ of between 1050°C and 1250°C, and then

   - a roughing-down rolling of said heated semi-finished product is carried out, at a temperature T₂ between 1000 and 880°C, with a total degree of reduction ε_a greater than 30% so as to obtain a sheet with a completely recrystallised austenitic structure with a mean grain size of less than 40 micrometres and preferably less than 5 micrometres, it being understood that the total reduction rate ε_a is defined by: $\mathit{Ln}\frac{{\mathit{e}}_{\mathit{ia}}}{{\mathit{e}}_{{\mathit{f}}_{\mathit{a}}}},$

   

   e_ia designating the thickness of the semi-finished product before said roughing-down hot rolling and e_fa the thickness of the sheet after said roughing-down rolling, and then

   - the sheet is cooled not completely to a temperature T₃ of between 600°C and 400°C in the metastable austenitic range, at a rate V_R1 greater than 2°C/s, then

   - a finishing hot rolling is carried out, at said temperature T₃, of the not completely cooled sheet, with a total degree of reduction ε_b greater than 30% so as to obtain a sheet, it being understood that said total degree of reduction ε_b is defined by: $\mathit{Ln}\frac{{\mathit{e}}_{\mathit{ib}}}{{\mathit{e}}_{{\mathit{f}}_b}},$

   

   e_ib designating the thickness of the sheet before said finished hot rolling and e_fa the thickness of the sheet after said finishing rolling, then

   - said sheet is cooled at a rate V_R2 greater than the critical martensitic hardening rate.
2. Method for manufacturing a steel piece with a completely martensitic structure having a mean lath size of less than 1 micrometre, the mean elongation factor of said laths being between 2 and 5, it being understood that the elongation factor of a lath with a maximum dimension l_max and minimum dimension l_min is defined by $\frac{l_{\mathit{max}}}{l_{\mathit{min}}},$

   

   comprising the successive steps in this order according to which:

   - a steel blank is procured, the composition of which comprises, the proportions being expressed by weight, $$0.15\%\le \mathrm{C}\le 0.40\%$$

   

   $$1.5\%\le \mathrm{Mn}\le 3\%$$

   

   $$0.005\%\le \mathrm{Si}\le 2\%$$

   

   $$0.005\%\le \mathrm{Al}\le 0.1\%$$

   

   $$1.8\%\le \mathrm{Cr}\le 4\%$$

   

   $$0\%\le \mathrm{Mo}\le 2\%$$

   

   it being understood that $$2.7\%\le 0.5\left(\mathrm{Mn}\right)+\left(\mathrm{Cr}\right)+3\left(\mathrm{Mo}\right)\le 5.7\%$$

   

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

   

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

   

   and optionally: $$0\%\le \mathrm{Nb}\le 0.050\%$$

   

   $$0.01\%\le \mathrm{Ti}\le 0.1\%$$

   

   $$0.0005\%\le \mathrm{B}\le 0.005\%$$

   

   $$0.0005\%\le \mathrm{Ca}\le 0.005\%,$$

   

   the rest of the composition consisting of iron and unavoidable impurities resulting from the production,

   - said blank is heated to a temperature T₁ of between A_c3 and Ac₃+250°C so that the mean austenitic grain size is less than 40 micrometres, and preferably less than 5 micrometres, then

   - said heated blank is transferred into a hot drawing press or a hot forming device, then

   - said blank is cooled to a temperature T₃ between 600°C and 400°C, at a rate V_R1 greater than 2°C/s so as to prevent transformation of the austenite,

   - the order of the last two sections being able to be inverted, then

   - said cooled blank is pressed or formed hot at said temperature T₃, by a quantity ε_c greater than 30% in at least one region, in order to obtain a piece, it being understood that said quantity ε_c is defined by ε_c = $\frac{2}{\sqrt{3}}\sqrt{\left({\epsilon }_1^2+{\epsilon }_1{\epsilon }_2+{\epsilon }_2^2\right)},$

   

   where ε₁ and ε₂ are the total main deformations over all the deformation steps at the temperature T₃, then

   - said piece is cooled at a rate V_R2 greater than the critical martensitic hardening rate.
3. Method for manufacturing a piece according to claim 2, characterised in that said blank is hot drawn so as to obtain a piece, then said piece is kept in the drawing tool so as to cool said piece at a speed V_R2 greater than the critical martensitic hardening speed.
4. Method for manufacturing a steel piece according to either one of claims 2 or 3, characterised in that said blank is precoated with aluminium or an aluminium-based alloy.
5. Method for manufacturing a steel piece according to any one of claims 2 to 4, characterised in that said blank is precoated with zinc or a zinc-based alloy.
6. Method for manufacturing a steel sheet or piece according to any one of claims 1 to 5, characterised in that said sheet or piece is subjected to subsequent tempering heat treatment at a temperature T₄ between 150°C and 600°C for a period of between 5 and 30 minutes.
7. Steel sheet with an elastic limit greater than 1300 MPa, and a mechanical strength greater than (3220(C)+958) megapascals, it being understood that (C) designates the carbon content as a percentage by weight of said steel, obtained by a method according to claim 1, with a totally martensitic structure, having a mean lath size of less than 1 micrometre, the mean elongation factor of said laths being between 2 and 5.
8. Steel piece obtained by a method according to any one of claims 2 to 5, comprising at least one region with a totally martensitic structure having a mean lath size of less than 1 micrometre, the mean elongation factor of said laths being between 2 and 5, the elastic limit in said at least one region being greater than 1300 MPa, and the mechanical strength being greater than (3220(C)+958) megapascals, it being understood that (C) designates the carbon content as a percentage by weight of said steel.
9. Steel sheet or piece obtained by a method according to claim 6, with a totally martensitic structure, having, in at least one region, a mean lath size of less than 1.2 micrometres, the mean elongation factor of said laths being between 2 and 5.

Images