EP3445878B1 - A process for manufacturing a martensitic stainless steel part from a sheet - Google Patents

Description

The invention relates to the hot forming of stainless steels from a sheet to give them a complex shape and remarkable mechanical properties, these steels being intended, for example, for the automotive industry.

In order to lighten vehicles to limit their fuel consumption and therefore limit their CO ₂ emissions, manufacturers today use sheets of carbon steel or very high strength stainless steel, allowing thickness reductions sheets compared to steels more conventionally used in the past.

Martensitic steels (or, more generally, with martensitic structure for more than 50%) have such mechanical characteristics, but their cold forming capacity is limited. We therefore have to either form them cold in the ferritic state, then heat treat the part to obtain the martensitic structure, or put them into hot form in the austenitic state by terminating the treatment by quenching. in order to obtain the martensitic structure.

The production of parts of complex geometry by this second process with known steels (carbon steels containing boron, etc.) is however made difficult by the constraints of their limited quenchability, or the existence of metallurgical transformations at high temperature. which make it difficult to have a good grasp of the shaping and tempering process. There is a strong risk of obtaining a complex part which is not predominantly martensitic, therefore whose mechanical characteristics do not correspond to those targeted, or having to limit itself to obtaining a martensitic part of simple geometry, whose shape will be corrected, for example by means '' laser cutting.

We could think of carrying out several stages of hot shaping on transfer presses / tools to be followed, starting from steels conventionally known for these uses, in order to make shaping progressive and to limit the risks of appearance of defects. However, the part obtained will be made up of less than 80% martensite by volume and its mechanical properties and its resilience will be degraded: at least one of the target tensile strength Rm, elastic limit Rp0.2, elongation at break A, ease of folding or resilience will not be achieved. The time necessary to pass above the temperature Mf at the end of martensitic transformation to carry out at least two shaping steps, two transfer steps and a quenching step is too long, and the austenite is then partially transforms into ferrite / carbides / perlite.

Obtaining a structure composed of 80% by volume of minimum martensite is possible with known steels, but the cooling rate during quenching must be greater than 30 ° C / s per second on average. A multi-pass process using a press with tools to follow or so-called transfer, will not be able to achieve after austenitization more than a transfer step, followed by a shaping or hot cutting step, before quenching in the tool to guarantee a minimum of 30 ° C / s for the cooling rate.

The prior art represented by EP 0 170 598 A1 or FR 2 920 784 A1 does not seem to begin their treatments with a sheet composed of ferrite and / or martensite returned rich (0.5 to 5% vol) in carbides. Normally, martensitic steels (more than 50% M) have appropriate mechanical characteristics, but their cold forming capacity is limited. What is known is to treat the parts or the sheets in cold form in the ferritic state and then to obtain the martensitic structure, or to form them in the hot state in the austenitic state by terminating the treatment by quenching martensitic with or without income. The manufacturing processes according to the documents cited above are not an exception. The object of the invention is to propose a method for producing a hot-transformed martensitic steel part, making it possible to manufacture parts of complex shape from a sheet, this final part also having high mechanical properties. making it suitable, in particular, for use in the automotive industry.

• on prépare une tôle d'acier inoxydable de composition, en pourcentages pondéraux :
  • * 0,005% ≤ C ≤ 0,3% ;
  • * 0,2% ≤ Mn ≤ 2,0% ;
  • * traces ≤ Si ≤ 1,0% ;
  • * traces ≤ S ≤ 0,01% ;
  • * traces ≤ P ≤ 0,04% ;
  • * 10,5% ≤Cr ≤ 17,0% ; de préférence 10,5% ≤ Cr ≤ 14,0% ;
  • * traces ≤ Ni ≤ 4,0% ;
  • * traces ≤ Mo ≤ 2,0% ;
  • * Mo + 2 x W ≤ 2,0% ;
  • * traces ≤ Cu ≤ 3% ; de préférence traces ≤ Cu ≤ 0,5% ;
  • * traces ≤ Ti ≤ 0,5% ;
  • * traces ≤ Al ≤ 0,2% ;
  • * traces ≤ O ≤ 0,04% ;
  • * 0,05% ≤ Nb ≤ 1,0% ;
  • * 0,05% ≤ Nb + Ta ≤ 1,0% ;
  • * 0,25% ≤ (Nb + Ta)/(C + N) ≤ 8 ;
  • * traces ≤ V ≤ 0,3% ;
  • * traces ≤ Co ≤ 0,5% ;
  • * traces ≤ Cu +Ni + Co ≤ 5,0% ;
  • * traces ≤ Sn ≤ 0,05% ;
  • * traces ≤ B ≤ 0,1% ;
  • * traces ≤ Zr ≤ 0,5% ;
  • * Ti + V + Zr ≤ 0,5% ;
  • * traces ≤ H ≤ 5 ppm, de préférence traces ≤ H ≤ 1 ppm ;
  • * traces ≤ N ≤ 0,2% ;
  • * (Mn + Ni) ≥ (Cr -10,3 - 80 x [(C + N)²]) ;
  • * traces ≤ Ca ≤ 0,002% ;
  • * traces ≤ terres rares et/ou Y ≤ 0,06% ;
  • * le reste étant du fer et des impuretés résultant de l'élaboration ;
• la température de début de transformation martensitique (Ms) de la tôle étant ≥ 200°C ;
• la température de fin de transformation martensitique (Mf) de la tôle étant ≥-50°C ;
• la microstructure de la tôle étant composée de ferrite et/ou de martensite revenue et de 0,5% à 5% en volume de carbures ;
• la taille des grains ferritiques de la tôle étant de 1 à 80 µm, de préférence de 5 à 40 µm ;
• on procède éventuellement à une ou des transformations à chaud et/ou à froid de ladite tôle ;
• on réalise une austénitisation de la tôle en la maintenant à une température supérieure à Ac1, de manière à lui conférer une microstructure contenant au maximum 0,5% de carbures en fraction volumique et au maximum 20% de ferrite résiduelle en fraction volumique ;
• on transfère la tôle austénitisée sur un premier outil de mise en forme ou un outil de découpe, ledit transfert ayant une durée t0, pendant laquelle la tôle reste à une température supérieure à Ms et conserve au maximum 0,5% en volume de carbures et au maximum 20% en volume de ferrite résiduelle, la tôle se trouvant à une température TP0 à l'issue de ce transfert ;
• on réalise une première étape de mise en forme ou de découpe de la tôle, pendant une durée t1, et pendant laquelle la tôle reste à une température supérieure à Ms et conserve au maximum 0,5% en volume de carbures et au maximum 20% en volume de ferrite résiduelle ;
• on réalise un transfert de la tôle mise en forme ou découpée sur un deuxième outil de mise en forme ou de découpe, ou on modifie la configuration du premier outil de mise en forme ou de découpe, pendant une durée t2, pendant laquelle la tôle reste à une température supérieure à Ms et conserve au maximum 0,5% en volume de carbures et au maximum 20% en volume de ferrite résiduelle ;
• on réalise une deuxième étape de mise en forme ou de découpe de la tôle, pendant une durée t3, et pendant laquelle la tôle reste à une température supérieure à Ms et conserve au maximum 0,5% en volume de carbures et au maximum 20% en volume de ferrite résiduelle ;
• optionnellement, on réalise d'autres étapes de transfert de la tôle découpée ou mise en forme sur d'autres outils de découpe ou de mise en forme, ou de modification de la configuration de l'outil de mise en forme ou de découpe utilisé dans l'étape précédente, chacune étant suivie d'une étape de mise en forme ou de découpe, la tôle restant à une température supérieure à Ms et conservant au maximum 0,5% en volume de carbures et au maximum 20% en volume de ferrite résiduelle pendant chacune desdites étapes de transfert de la tôle ou de modification de la configuration de l'outil et chacune des opérations de mise en forme ou de découpe ;
• si on désigne par TPn la température atteinte par la tôle mise en forme ou découpée à l'issue de la dernière étape de découpe ou de mise en forme et par Σti la somme des durées des étapes de transfert et/ou de changement de configuration de l'outil et des étapes de mise en forme ou découpe, la grandeur (TP0-TPn)/Σti est d'au moins 0,5°C/s ;
• optionnellement on effectue une étape supplémentaire de mise en forme ou de découpe à une température comprise entre Ms et Mf, dans un domaine où la microstructure est constituée de martensite, d'au moins 5% d'austénite et d'au plus 20% de ferrite.
• et on laisse la tôle se refroidir jusqu'à la température ambiante pour obtenir la pièce finale, ladite pièce finale ayant une microstructure contenant au maximum 0,5% de carbures en fraction volumique et au maximum 20% de ferrite résiduelle en fraction volumique.

To this end, the subject of the invention is a method of manufacturing a piece of martensitic stainless steel from a sheet, by hot forming, characterized in that:

• a stainless steel sheet of composition is prepared, in weight percentages:
  • * 0.005% ≤ C ≤ 0.3%;
  • * 0.2% ≤ Mn ≤ 2.0%;
  • * traces ≤ Si ≤ 1.0%;
  • * traces ≤ S ≤ 0.01%;
  • * traces ≤ P ≤ 0.04%;
  • * 10.5% ≤Cr ≤ 17.0%; preferably 10.5% ≤ Cr ≤ 14.0%;
  • * traces ≤ Ni ≤ 4.0%;
  • * traces ≤ Mo ≤ 2.0%;
  • * Mo + 2 x W ≤ 2.0%;
  • * traces ≤ Cu ≤ 3%; preferably traces ≤ Cu ≤ 0.5%;
  • * traces ≤ Ti ≤ 0.5%;
  • * traces ≤ Al ≤ 0.2%;
  • * traces ≤ O ≤ 0.04%;
  • * 0.05% ≤ Nb ≤ 1.0%;
  • * 0.05% ≤ Nb + Ta ≤ 1.0%;
  • * 0.25% ≤ (Nb + Ta) / (C + N) ≤ 8;
  • * traces ≤ V ≤ 0.3%;
  • * traces ≤ Co ≤ 0.5%;
  • * traces ≤ Cu + Ni + Co ≤ 5.0%;
  • * traces ≤ Sn ≤ 0.05%;
  • * traces ≤ B ≤ 0.1%;
  • * traces ≤ Zr ≤ 0.5%;
  • * Ti + V + Zr ≤ 0.5%;
  • * traces ≤ H ≤ 5 ppm, preferably traces ≤ H ≤ 1 ppm;
  • * traces ≤ N ≤ 0.2%;
  • * (Mn + Ni) ≥ (Cr -10.3 - 80 x [(C + N) ² ]);
  • * traces ≤ Ca ≤ 0.002%;
  • * traces ≤ rare earths and / or Y ≤ 0.06%;
  • * the rest being iron and impurities resulting from processing;
• the temperature at the start of martensitic transformation (Ms) of the sheet being ≥ 200 ° C;
• the temperature at the end of martensitic transformation (Mf) of the sheet being ≥-50 ° C;
• the microstructure of the sheet being composed of ferrite and / or martensite returned and from 0.5% to 5% by volume of carbides;
• the size of the ferritic grains of the sheet being from 1 to 80 μm, preferably from 5 to 40 μm;
• optionally one or more hot and / or cold transformations of said sheet;
• an austenitization of the sheet is carried out by maintaining it at a temperature higher than Ac1, so as to give it a microstructure containing at most 0.5% of carbides in volume fraction and at most 20% of residual ferrite in volume fraction;
• the austenitized sheet is transferred to a first forming tool or a cutting tool, said transfer having a duration t0, during which the sheet remains at a temperature above Ms and retains a maximum of 0.5% by volume of carbides and maximum 20% by volume of residual ferrite, the sheet being at a temperature TP0 at the end of this transfer;
• a first step of shaping or cutting the sheet is carried out, for a duration t1, and during which the sheet remains at a temperature above Ms and retains a maximum of 0.5% by volume of carbides and a maximum of 20% by volume of residual ferrite;
• transferring the shaped or cut sheet to a second shaping or cutting tool, or modifying the configuration of the first shaping or cutting tool, for a period t2, during which the sheet remains to one temperature higher than Ms and retains a maximum of 0.5% by volume of carbides and a maximum of 20% by volume of residual ferrite;
• a second step of shaping or cutting the sheet is carried out, for a period t3, and during which the sheet remains at a temperature above Ms and keeps a maximum of 0.5% by volume of carbides and a maximum of 20% by volume of residual ferrite;
• optionally, other steps are carried out for transferring the cut or shaped sheet onto other cutting or shaping tools, or for modifying the configuration of the shaping or cutting tool used in the previous step, each being followed by a shaping or cutting step, the sheet remaining at a temperature above Ms and retaining at most 0.5% by volume of carbides and at most 20% by volume of ferrite residual during each of said steps of transferring the sheet or modifying the configuration of the tool and each of the shaping or cutting operations;
• if we designate by TPn the temperature reached by the sheet formed or cut at the end of the last cutting or shaping step and by Σti the sum of the durations of the steps of transfer and / or change of configuration of the tool and steps of shaping or cutting, the quantity (TP0-TPn) / Σti is at least 0.5 ° C / s;
• optionally, an additional shaping or cutting step is carried out at a temperature between Ms and Mf, in a field where the microstructure consists of martensite, at least 5% austenite and at most 20% ferrite.
• and the sheet is allowed to cool to room temperature to obtain the final part, said final part having a microstructure containing at most 0.5% of carbides in volume fraction and at most 20% of residual ferrite in volume fraction.

Said sheet may have a martensitic transformation start temperature (Ms) ≤ 400 ° C.

The martensitic transformation start temperature (Ms) of the sheet can be between 390 and 220 ° C.

The thickness of the sheet can be between 0.1 and 10 mm.

The austenitization temperature can be at least 850 ° C.

The austenitization temperature can be between 925 and 1200 ° C.

The sheet can be reheated, during at least one of the steps of transferring and / or changing the configuration of the tool or the steps of shaping or cutting the sheet.

A surface treatment can be carried out on the final part, intended to increase its roughness or its fatigue properties.

The final part can be left between 90 and 500 ° C for 10 s to 1 h, then let it cool naturally in air.

• Du choix d'une composition d'acier inoxydable martensitique ;
• Et de l'application d'un procédé de mise en forme à chaud particulier à une tôle présentant cette composition, ainsi que des caractéristiques structurelles initiales précises qui rendent possible l'utilisation dudit procédé pour l'obtention de la pièce finale, ou d'une pièce intermédiaire qui va ensuite subir des opérations visant à l'optimisation fine de certaines de ses propriétés mécaniques et/ou superficielles.

As will be understood, the invention is based on the combination:

• From the choice of a martensitic stainless steel composition;
• And of the application of a particular hot forming process to a sheet having this composition, as well as precise initial structural characteristics which make possible the use of said process for obtaining the final part, or an intermediate part which will then undergo operations aimed at fine-tuning some of its mechanical and / or surface properties.

This process begins with an austenitization of the sheet, that is to say by an elevation of its temperature above the temperature Ac1 of the steel so as to form austenite in place of ferrite and carbides constituting the starting microstructure, and under conditions which limit as much as possible the surface decarburization and oxidation of the sheet.

Then, successively, several steps of shaping the sheet (at least two) are carried out under temperature and duration conditions such that the ferrite + carbide structure obtained after the austenitization is preserved throughout the shaping. If necessary, it is possible to reheat or maintain the temperature between the shapings, or during these by means of heating tools, so that the temperature of the sheet during shaping and between layouts (during transfers of the sheet from one tool to another, where if the sheet remains on the same tool, during changes to the configuration of the tool) does not drop below Ms ( martensitic transformation start temperature).

It should be understood that the term “shaping step” includes operations as diverse as deformation or removal of material as, in particular, deep drawing, hot stamping, stamping, cutting , drilling, these steps can take place in any order chosen by the manufacturer.

After shaping, the part obtained is cooled without any particular constraints on cooling. This cooling can be preceded by a cutting or final shaping step carried out between Ms and Mf (end temperature of martensitic transformation), under conditions where the microstructure consists of at least 10% austenite, at most 20% ferrite, the rest being martensite.

• La figure 1 qui montre un schéma de fabrication d'une pièce faisant usage du procédé selon l'invention, utilisant un four à rouleaux classique, ainsi que l'évolution de la température de l'acier durant la fabrication ;
• La figure 2 qui montre un schéma de fabrication d'une pièce faisant usage du procédé selon l'invention, utilisant un four à induction, ainsi que l'évolution de la température de l'acier durant la fabrication.

The invention will be better understood on reading the description which follows, given with reference to the following appended figures:

• The figure 1 which shows a diagram of manufacturing a part making use of the method according to the invention, using a conventional roller oven, as well as the evolution of the temperature of the steel during manufacturing;
• The figure 2 which shows a diagram of manufacturing a part making use of the method according to the invention, using an induction furnace, as well as the evolution of the temperature of the steel during manufacturing.

The composition of the martensitic stainless steel used in the process according to the invention is as follows. All percentages are weight percentages.

Its C content is between 0.005% and 0.3%.

The minimum content of 0.005% is justified by the need to obtain an austenitization of the microstructure during the first stage of the hot forming process, so that the final mechanical properties targeted are obtained. Above 0.3%, the weldability and, above all, the resilience of the sheet become insufficient, in particular for an application in the automotive industry.

Its Mn content is between 0.2 and 2.0%.

A minimum of 0.2% is required to obtain austenitization. Above 2.0% of the oxidation problems are to be feared during heat treatments if these are not carried out in neutral or reducing atmospheres, and moreover obtaining the desired mechanical properties would no longer be guaranteed.

Its Si content is between traces (that is to say simple impurities resulting from the production, without Si having been added) and 1.0%.

Si can be used as a deoxidizer during production, just like Al, to which it can be added or substituted. Above 1.0%, it is considered that it excessively promotes the formation of ferrite and therefore makes austenitization more difficult, and that it weakens the sheet too much for the shaping of a complex part to be assured. perform satisfactorily.

Its S content is between traces and 0.01% (100 ppm), in order to guarantee suitable weldability and resilience to the final product.

Its P content is between traces and 0.04%, in order to guarantee that the final product will not be excessively brittle. P is also harmful to the weldability.

Its Cr content is between 10.5 and 17.0%, preferably between 10.5% and 14.0% to have faster dissolution of the carbides during austenitization.

The minimum content of 10.5% is justified to ensure the oxidability of the sheet. A content higher than 17% would make austenitization difficult and unnecessarily increase the cost of steel.

Its Ni content is between traces and 4.0%.

An addition of Ni is not essential to the invention. The presence of Ni within the prescribed limit of 4.0% maximum may, however, be advantageous for promoting austenitization. Exceeding the 4.0% limit would however lead to an excessive presence of residual austenite and an insufficient presence of martensite in the microstructure after cooling.

Its Mo content is between traces and 2.0%.

The presence of Mo is not essential. However, it is favorable to good resistance to corrosion. Above 2.0%, austenitization would be hindered and the cost of steel unnecessarily increased.

A presence of W is likewise possible, but since W is a very hardening element, this presence must be limited and put in relation to the Mo content. We consider that the sum Mo + 2 x W must be between traces and 2.0%.

Contrary to what is most usual when we consider in a steel shade the cumulation of the influences of Mo and W, we take into account not the relation Mo + W / 2 but the relation Mo + 2 x W. The relation Mo + W / 2 must be taken into account when we want to control the influence of these two elements on the formation of precipitates, for which W is twice as effective as Mo with equal added amount. However, in the case of the invention, the respective influences of Mo and W are favored on the hardness of the steel. And since W is a more hardening element than Mo, with equal added quantities, it is the relation Mo + 2 x W which must be taken into account according to the invention. This sum Mo + 2 x W must be between traces and 2.0%. Beyond this, the hardness becomes excessive and, all other things being equal, the mechanical properties to be favored in the context of the invention are reduced, in particular the bending angle capacity and the resilience.

Its Cu content is between traces and 3.0%, preferably between traces and 0.5%.

These Cu requirements are classic for this type of steel. In practice, this means that adding Cu is not useful and that the presence of this element is only due to the raw materials used. A content greater than 0.5%, which would correspond to a voluntary addition, is not desired for automotive applications, because it would degrade the weldability. Cu can however aid in austenitization, and if the steel of the invention is applied to a field which does not require welding, the Cu content can range up to 3.0%.

Its Ti content is between traces and 0.5%.

Ti is a deoxidizer, like Al and Si, but its cost and its lower efficiency than that of Al makes its use in general not very interesting from this point of view. It may be advantageous in that the formation of nitrides and carbonitrides of Ti can limit the growth of the grains and favorably influence certain mechanical properties and the weldability. However, this formation can be a drawback in the case of the process according to the invention, since Ti tends to hinder austenitization due to the formation of carbides, and TiNs degrade the resilience. A maximum content of 0.5% should therefore not be exceeded.

V and Zr are also elements capable of forming nitrides degrading the resilience, and in general, the sum Ti + V + Zr must not exceed 0.5%.

Its Al content is between traces and 0.2%.

Al is used as a deoxidizer during production. After deoxidation, there should not remain in the steel an amount exceeding 0.2%, because there would be a risk of forming an excessive amount of AlN degrading the mechanical properties, and also of having difficulties in obtain the martensitic microstructure.

Its O content is between traces and 0.04% (400 µm).

The requirements on the O content are those which are conventional on martensitic stainless steels, depending on the ability to shape them without cracks starting from the inclusions and the quality of the mechanical properties sought after. the final part, and which the excessive presence of oxidized inclusions is likely to alter. Conversely, if a minimum machinability of the sheet is sought, it may be advantageous to have oxidized inclusions in significant number, if their composition makes them sufficiently malleable so that they serve as a lubricant for the cutting tool. This technique for controlling the number and composition of oxidized inclusions is classic in the steel industry. Control of the composition of the oxides can be advantageously obtained by controlled addition of Ca and / or adjustment of the composition of the slag with which the liquid steel is in contact and in chemical equilibrium during the production.

It is essentially the addition of deoxidizers Al, Si, Ti, Zr during the elaboration, the possible addition of Ca, the care then taken in decanting the oxidized inclusions within the liquid steel and the subsistence of these deoxidizers in solidified steel which determine the final O content. If each of these elements, taken in isolation, can be absent or only very slightly present, it is nevertheless necessary that at least one of them (most often Al and / or Si) be present in a sufficient quantity to guarantee that the O content of the final sheet will not be too high for uneven shaping of the part, and for future applications of the part. These mechanisms governing the deoxidation of steels and the control of the composition and the quantity of their oxidized inclusions are well known to those skilled in the art, and apply in the context of the invention in a perfectly conventional manner.

Its Nb content is between 0.05% and 1.0%

Its total Nb + Ta content is between 0.05% and 1.0%.

Nb and Ta are important elements for obtaining good resilience, and Ta improves the resistance to pitting corrosion. But as they can interfere with austenitization, they must not be present in quantities exceeding what has just been prescribed. Also, Nb and Ta capture C and N by forming carbonitrides which prevent too strong growth of the austenite grains during austenitization. This is favorable for obtaining a very good resilience when cold, between -100 ° C. and 0 ° C. On the other hand, if the content of Nb and / or Ta is too high, C and N will be entirely trapped in the carbonitrides and there will no longer be enough in dissolved form for the targeted mechanical properties to be achieved, in particular the resilience and the mechanical resistance.

0.25 ≤ (Nb + Ta) / (C + N) ≤ 8 is therefore required to obtain a resilience of the order of 50 J / cm ² at 20 ° C or more.

Its V content is between traces and 0.3%.

Like Ti, V an embrittling element which is capable of forming nitrides, and must not be present in too large an amount. As said before, Ti + V + Zr must not exceed 0.5%.

Its Co content is between traces and 0.5%. This element is, like Cu, capable of helping austenitization. But it is useless to put more than 0.5%, because the austenitization can be assisted by less expensive means.

The total contents of Cu, Ni and Co must be between traces and 5.0%, so as not to leave too much residual austenite after the martensitic transformation and not to degrade the weldability in the applications which require it.

Its Sn content is between traces and 0.05% (500 ppm). This element is not desired because it is detrimental to the weldability and the ability of the steel to be hot processed. The 0.05% limit is a tolerance.

Its B content is between traces and 0.1%.

B is not obligatory, but its presence is advantageous for the hardenability and the forgeability of the austenite. It therefore facilitates shaping. Its addition above 0.1% (1000 ppm) does not provide any significant additional improvement.

Its Zr content is between traces and 0.5%, because it reduces the resilience and hinders austenitization. It is also recalled that the total content of Ti + V + Zr must not exceed 0.5%.

Its H content is between traces and 5 ppm, preferably not more than 1 ppm. Excessive H content tends to weaken martensite. It will therefore be necessary to choose a method of production of steel in the liquid state which can ensure this low presence of H. Typically, treatments ensuring a thorough degassing of the liquid steel (by massive injection of argon in the liquid steel, well-known process known as “AOD”, or by a vacuum passage during which the steel is decarbonized by release of CO, process known as “VOD”) are indicated.

Its N content is between traces and 0.2% (2000 ppm). N is an impurity of which the same treatments which make it possible to reduce the H content help to limit the presence, or even to reduce it substantially. It is not always necessary to have a particularly low N content, but for the reasons that have been said it is necessary that its content, considered jointly with those of elements with which it can combine to form nitrides or carbonitrides obeys the relation 8 ≥ (Nb + Ta) / (C + N) ≥ 0.25.

Also, good austenitization of the steel during the initial stage of the thermomechanical treatment is favored if the relation (Mn + Ni) ≥ (Cr -10.3 - 80 x [(C + N) ² ]) is respected. Sufficient resilience is obtained if this condition is met in addition to the others that have been defined. A sufficient level of gammagene elements is necessary to counterbalance the alphagene effect of Cr and to ensure correct austenitization, namely at least 80%, and from this point of view the effectiveness of the sum C + N is not linear.

Its Ca content is ≤ 0.002% (20 ppm).

Its total rare earth and Y content is between traces and 0.06% (600 ppm). These elements can improve the resistance to oxidation during austenitizations at very high temperatures.

The rest of the steel is made up of iron and impurities resulting from the production.

Other requirements on the composition of the steel relate to the temperatures at the start of martensitic transformation Ms and at the end of martensitic transformation Mf.

Ms should preferably be at most 400 ° C. If Ms is higher, there is a risk that the different transfer and shaping operations for the part will not follow each other quickly enough and that there will not be time to carry out all of the shaping at a temperature higher than Ms. However, this risk can be limited or avoided by providing that the part undergoes reheating or temperature maintenance between the shapings, and / or during these if using heating tools of known types integrating , for example, electrical resistors. This condition Ms ≤ 400 ° C is therefore not always imperative, but only recommended for economical and easy application of the process according to the invention under industrial conditions.

Ms must be greater than or equal to 200 ° C. to avoid subsistence in the final part of too high a content of residual austenite, which, in particular, would degrade Rp0.2 by bringing it below 800 MPa.

Preferably, Ms is between 390 and 320 ° C.

Mf must be greater than or equal to -50 ° C to guarantee that there will not be too much residual austenite in the final part.

Ms and Mf are preferably determined experimentally, for example by dilatometric measurements as is well known, see for example the article "Uncertainties in dilatometric determination of martensite start temperature", Yang and Badeshia, Materials Science and Technology, 2007/5, pp 556-560 .

Approximate formulas also make it possible to evaluate them from the composition of the steel, but an experimental determination is more certain.

It should be understood that the thermomechanical treatments which will be described can be carried out either on a bare sheet which may possibly be coated subsequently, or on a sheet already coated, for example with an Al-based alloy and / or Zn. This coating, typically 1 to 200 μm thick and present on one or two faces of the sheet, may have been deposited by any technique conventionally used for this purpose, it is simply necessary that, if it has been deposited before austenitization, it does not evaporate during the stay of the sheet at austenitization and deformation temperatures, and is not deteriorated during deformations.

The choice and optimization of the characteristics of the coating and of its mode of deposition so that these conditions are met do not go beyond what a person skilled in the art knows how to do, when he is required to form in a way classic stainless steel sheets already coated. If the coating takes place before austenitization, it may, however, be preferable to coatings based on Al over those based on Zn, as Al is less likely than Zn to evaporate at austenitization temperatures. .

The process according to the invention is as follows, applied to the manufacture and shaping of a sheet.

Firstly, an initial stainless steel sheet, bare or coated, having the composition just described and a thickness which is typically from 0.1 to 10 mm, is conventionally prepared. This preparation can include operations of hot and / or cold transformation and cutting of the semi-finished product resulting from the casting and the solidification of the liquid steel. This initial sheet must have a microstructure consisting of ferrite and / or returned martensite and from 0.5% to 5% by volume of carbides. The size of the ferritic grains, measured according to standard NF EN ISO 643, is between 1 and 80 μm, preferably between 5 and 40 μm. A ferritic grain size of 40 µm at most is recommended to promote the austenitization which will follow and thus obtain at least the 80% of austenite desired. A ferritic grain size of at least 5 µm is recommended to obtain good cold forming capacity.

The sheet is first austenitized by passing through an oven which brings it to a range of temperatures greater than Ac1 (temperature at the start of the appearance of austenite), therefore typically greater than approximately 850 ° C. for the compositions concerned). It should be understood that this austenitization temperature must relate to the entire volume of the sheet, and that the treatment must be long enough so that, taking into account the thickness of the sheet and the kinetics of the transformation, the austenitization is complete throughout this volume.

The maximum temperature of this austenitization is not a characteristic specific to the invention. It must simply be such that the sheet remains in an entirely solid state (the temperature must therefore be lower, in any case, than the solidus temperature of the steel) and is not too soft to support without transfer the transfer between the oven and the shaping tool which will follow the austenitization. Also, the temperature should not be raised to the point of causing significant surface oxidation and / or decarburization of the sheet in the heating atmosphere. A surface oxidation would lead to the need to descaling the sheet mechanically or chemically before it is shaped to avoid encrustation of scale in the surface of the sheet, and would cause a loss of material. Excessive decarburization (thickness of the decarburized surface ≥ 100 µm) would reduce the hardness and the tensile strength of the sheet. The risks of observing significant oxidation and / or decarburization depend, in a known manner, not only on the temperature and the austenitization time, but also on the treatment atmosphere of the furnace. A non-oxidizing, therefore neutral or reducing atmosphere (typically: argon, CO and their mixtures, etc.), preferably in air, allows the temperature of the treatment, which ensures complete austenitization in a minimum of time. If pure nitrogen or a highly hydrogenated atmosphere is used in an oven requiring a long residence time for austenitization, there is a risk of surface nitriding or of hydrogen being taken up by the steel. This should therefore be taken into account in the choice of the treatment atmosphere, and atmospheres of pure nitrogen or containing a relatively high hydrogen content will sometimes be avoided.

Typically, the austenitization takes place at a temperature between 925 and 1200 ° C. for a duration tm of 10 s to 1 h (this duration being that which the sheet passes over Ac1), preferably between 2 min and 10 min for heating in a conventional oven and between 30 s and 1 min for an induction oven. An induction furnace has the advantage, known in itself, of providing rapid reheating to the nominal austenitization temperature. It therefore allows a shorter treatment than a conventional oven to achieve the desired result. These temperatures and durations make it possible to ensure that the continuation of the treatments will lead to a sufficient formation of martensite, and this for a reasonable duration allowing good productivity of the process.

The purpose of this austenitization is to pass the metal from the initial ferrite + carbides structure to an austenitic structure containing at most 0.5% carbides by volume fraction, and at most 20% residual ferrite by volume fraction. An object of this austenitization is, in particular, to lead to a dissolution of at least the majority of the carbides initially present, so as to release C atoms to form the austenitic structure then the martensitic structure during the following stages of the process. The maximum residual ferrite content of 20%, which must remain until the final product, is justified by the resilience and the conventional elastic limit which it is desired to obtain.

The austenitized sheet is then transferred to a suitable forming tool (such as a stamping or stamping tool) or a cutting tool. This transfer has a duration t0 as short as possible, and during this transfer the sheet must remain at a temperature higher than Ms and keep an austenitic microstructure at 0.5% maximum of carbides and 20% maximum of residual ferrite. After this transfer, the sheet is at a temperature TP0, which is as close as possible to the nominal austenitization temperature for obvious reasons of energy saving.

A first shaping or cutting step is then carried out, of duration t1 typically between 0.1 and 10 s. The precise duration of this stage (like that of the other stages) is not in itself a fundamental characteristic of the invention. It must be sufficiently brief so that the temperature of the sheet does not drop below Ms, that there is no decarburization and / or significant oxidation of the sheet surface, and that an austenitic microstructure with 0.5% maximum of carbides and 20% maximum of residual ferrite is always present at the end of the operation. If necessary, one can use a shaping tool provided with sheet heating means so that these temperature and microstructure conditions are met, since the contact of a non-heating shaping tool with the sheet causes sheet cooling which is often greater than 100 ° C / s.

The absence of significant surface decarburization and oxidation can be obtained by adjusting the composition of the steel if necessary in the light of experience and, if possible, by maintaining a neutral or reducing around the sheet during its shaping

All these conditions relating to the shaping temperature and its evolution, and to the atmosphere surrounding the sheet during its shaping, are also valid for the following shaping steps.

The sheet thus formed is then transferred to another tool for a second shaping step in the broad sense of the term. As a variant, the same tool is used in the two stages but by modifying its configuration in the meantime (for example by replacing the punch in the case where a drawing is made in each of the two stages). The duration t2 of this transfer is typically from 1 to 10 s, the aim being that it is fast enough for the temperature of the sheet to remain above Ms during the transfer and for the microstructure to remain austenitic, at 0.5% maximum. of carbides and 20% maximum of residual ferrite.

The second shaping step is then carried out, of duration t3 typically between 0.1 and 10 s. The temperature of the sheet remains above Ms and the microstructure remains austenitic, at 0.5% maximum of carbides and 20% maximum of residual ferrite.

Other shaping steps (in the broad sense defined above), and their corresponding transfers, can follow this second shaping step.

The main thing is that during the execution of these transfers and these shaping / cutting, the temperature of the steel does not drop below Ms either, and that an austenitic microstructure, at 0.5% maximum of carbides and maximum 20% of residual ferrite is kept until the end of the last step n, of final temperature TPn. If necessary, as said, heated shaping tools can be used, as well as means for reheating the sheet between the shapings.

The average cooling rate between TP0 and TPn, defined by the quantity (TP0-TPn) / Σti, Σti constituting the sum of the durations of transfers and shaping, must be at least 0.5 ° C / s.

The consequence of this cooling rate between the start and the end of the shaping operations which have just been described, combined with the composition of the steel and with the operating method used during shaping, is that during the cooling, the steel does not enter the "nose" of the TRC diagram which corresponds to the bainitic transformation, but remains in the austenitic domain before passing directly into the domain where the martensitic transformation can take place. The composition of the steel is precisely chosen so that, compared to the carbon steels which it is the most common to use in the automotive industry for the production of sheets capable of being welded, this nose is shifted towards the durations higher, thus making possible on usual shaping tools the avoidance of the bainitic domain, a fortiori of the ferritic and perlitic domains, and therefore a execution as complete as possible of the transformation of austenite into martensite. However, it should be remembered that, as has been said, each step taken individually must make it possible to maintain an austenitic microstructure with a maximum of 0.5% of carbides and a maximum of 20% of residual ferrite. The duration / cooling rate of each stage must therefore be chosen accordingly, and, if necessary, reheating of the sheet between and / or during shaping or cutting is carried out so that this microstructure can be maintained during all the steps.

It is optionally possible to carry out at least one additional shaping step in the broad sense at a temperature between Ms and Mf, in a field where the microstructure comprises at least 5% by volume of austenite. If this additional step is a cut, the final shape of the part can be reached with less wear of the tools, and if this additional step is a deformation, at least 5% of austenite will provide sufficient ductility for this deformation to be still possible despite the sometimes already significant presence of martensite.

Finally, the sheet is allowed to cool, for example in the open air, to room temperature, thereby obtaining the final part according to the method of the invention. It is not necessary to impose a minimum speed during this cooling, because the composition of the steel ensures that the sheet will remain anyway in the area where the martensitic transformation can also take place during this cooling down to the ambient, at least if no means are used which significantly slow down cooling compared to natural cooling in the open air, such as a covering of the sheet. Of course, it is not excluded to accelerate this cooling, by means of forced air or a spray of water or another fluid.

The possibility of using at least two stages to obtain the final shape of the part gives access, thanks to the use of a steel having the specified and treated composition according to the invention, to complex shapes for the final part that the known methods making use of only one shaping of the initial sheet do not allow to reach, in any case not with sufficient quality.

Optionally, a surface treatment can be applied to the final part such as shot blasting or sandblasting, with the aim of increasing the roughness of its surface to improve the adhesion of a coating which would be subsequently applied, such as a paint, or to create residual stresses improving the fatigue resistance of the sheet. This type of operation is known in itself.

Also, a final heat treatment can be carried out on the final part, therefore after cooling to the ambient, to improve its elongation at break and bring it to a value of more than 8% according to ISO standards, which corresponds substantially more than 10% according to JIS standards. This treatment consists in making the final part stay between 90 and 500 ° C for 10 s to 1 h, then in performing a natural air cooling.

The part thus obtained by the process according to the invention has high mechanical properties at room temperature, in particular because of its high martensite content of at least 80%. Typically, Rm is at least 1000 MPa, Re is at least 800 MPa, the elongation at break A measured according to ISO 6892 standard is at least 8%, and the bending angle capacity for a 1.5 mm thickness is at least 60 °, measured according to VDA 238-100.

• Chauffage dans un four à rouleaux 1 classique pendant 2 min d'une tôle 2 d'épaisseur 1,5 mm, entre la température ambiante et une température TPi égale à 950°C ;
• Maintien dans le four 1 de la tôle 2 à ladite température TPi pendant une durée tm de 3 min ;
• Transfert de la tôle 2 entre le four 1 et un outil d'emboutissage à chaud 3, pendant une durée t0 de 1 s ; la température de l'acier diminue jusqu'à TP0 = 941°C ;
• Première étape de mise en forme (déformation), exécutée dans l'outil d'emboutissage à chaud 3 pendant une durée t1 de 0,5 s pour obtenir une tôle mise en forme 4 ; la température de l'acier diminue jusqu'à TP1 = 808°C ;
• Transfert de la tôle mise en forme 4 entre l'outil d'emboutissage à chaud 3 et un outil de perçage 5, pendant une durée t2 de 0,5 s ; la température de l'acier diminue jusqu'à TP2 = 799°C ;
• Deuxième étape de mise en forme, consistant en un perçage effectué dans l'outil de perçage 5 pendant une durée t3 de 1 s pour obtenir une tôle mise en forme et percée 6 ; la température de l'acier diminue jusqu'à TP3 = 667°C ;
• Transfert de la tôle 6 mise en forme et percée vers un outil de découpe 7 pour exécuter une découpe des bords de la tôle 6 afin de leur conférer leurs dimensions définitives pour obtenir un produit 8 ;
• Exécution d'un grenaillage du produit 8 dans une grenailleuse 9 pour optimiser sa tenue en fatigue ou l'adhérence d'une éventuelle couche de revêtement future.

The figure 1 schematically represents an example of an operating diagram for a method according to the invention, executed on a steel of composition in accordance with that of Example 2 of Table 1 which follows, of which Ms is 380 ° C and Mf is 200 ° C, and comprising the following stages:

• Heating in a conventional roller oven 1 for 2 min of a sheet 2 of thickness 1.5 mm, between ambient temperature and a temperature TPi equal to 950 ° C;
• Maintenance in the oven 1 of the sheet 2 at said temperature TPi for a period tm of 3 min;
• Transfer of the sheet 2 between the oven 1 and a hot stamping tool 3, for a time t0 of 1 s; the temperature of the steel decreases up to TP0 = 941 ° C;
• First shaping step (deformation), performed in the hot stamping tool 3 for a time t1 of 0.5 s to obtain a shaped sheet 4; the temperature of the steel decreases up to TP1 = 808 ° C;
• Transfer of the shaped sheet 4 between the hot stamping tool 3 and a drilling tool 5, for a period t2 of 0.5 s; the temperature of the steel decreases up to TP2 = 799 ° C;
• Second shaping step, consisting of drilling in the drilling tool 5 for a period t3 of 1 s to obtain a shaped and drilled sheet 6; the temperature of the steel decreases up to TP3 = 667 ° C;
• Transfer of the sheet 6 shaped and drilled to a cutting tool 7 to cut the edges of the sheet 6 to give them their final dimensions to obtain a product 8;
• Execution of a shot peening of the product 8 in a shot peening machine 9 to optimize its resistance to fatigue or the adhesion of a possible future coating layer.

• Chauffage dans un four à induction 10 classique pendant 20 s d'une tôle 2 d'épaisseur 1,5 mm, entre la température ambiante et une température TPi = 950°C ;
• Maintien dans le four à induction 10 de la tôle 2 à ladite température TPi pendant une durée tm de 30 s ;
• Transfert de la tôle 2 entre le four à induction 10 et un outil d'emboutissage à chaud 3, pendant une durée t0 de 1 s ; la température de l'acier diminue jusqu'à TP0 = 941°C ;
• Première étape de mise en forme (déformation), exécutée dans l'outil d'emboutissage à chaud 3 pendant une durée t1 de 0,5 s pour obtenir une tôle mise en forme 4 ; la température de l'acier diminue jusqu'à TP1 = 808°C ;
• Transfert de la tôle mise en forme 4 entre l'outil d'emboutissage à chaud 3 et un outil de perçage 5, pendant une durée t2 de 1 s ; la température de l'acier diminue jusqu'à TP2 = 799°C ;
• Deuxième étape de mise en forme, consistant en un perçage effectué dans l'outil de perçage 5 pendant une durée t3 de 0,5 s pour obtenir une tôle mise en forme et percée 6 ; la température de l'acier diminue jusqu'à TP3 = 667°C ;
• Transfert de la tôle 6 mise en forme et percée vers un outil de découpe 7 pendant une durée t4 de 1 s, pour exécuter une découpe des bords de la tôle 6 ; la température de la tôle diminue jusqu'à TP4 = 658°C ;
• Troisième étape de mise en forme consistant en un découpage des bords de la pièce 6 afin de lui conférer ses dimensions définitives et obtenir un produit 8, pendant une durée t5 de 0,5 s ; la température de la pièce diminue jusqu'à TP5 = 525°C ;
• Exécution d'un grenaillage 9 du produit 8 pour optimiser sa tenue en fatigue ou l'adhérence d'une éventuelle couche de revêtement future.

The figure 2 schematically represents another example of operating diagram for a process according to the invention, carried out on a sheet 2 of a steel of composition in accordance with that of Example 7 of Table 1 which follows, of which Ms is 380 ° C and Ms of 200 ° C, and comprising the following steps:

• Heating in a conventional induction furnace 10 for 20 s of a sheet 2 of thickness 1.5 mm, between room temperature and a temperature TPi = 950 ° C;
• Maintenance in the induction furnace 10 of the sheet 2 at said temperature TPi for a period tm of 30 s;
• Transfer of the sheet 2 between the induction furnace 10 and a hot stamping tool 3, for a time t0 of 1 s; the temperature of the steel decreases up to TP0 = 941 ° C;
• First shaping step (deformation), performed in the hot stamping tool 3 for a time t1 of 0.5 s to obtain a shaped sheet 4; the temperature of the steel decreases up to TP1 = 808 ° C;
• Transfer of the shaped sheet 4 between the hot stamping tool 3 and a drilling tool 5, for a period t2 of 1 s; the temperature of the steel decreases up to TP2 = 799 ° C;
• Second shaping step, consisting of drilling in the drilling tool 5 for a period t3 of 0.5 s to obtain a shaped and drilled sheet 6; the temperature of the steel decreases up to TP3 = 667 ° C;
• Transfer of the sheet 6 shaped and drilled to a cutting tool 7 for a period t4 of 1 s, to execute a cutting of the edges of the sheet 6; the sheet temperature decreases up to TP4 = 658 ° C;
• Third shaping step consisting in cutting the edges of the part 6 in order to give it its final dimensions and to obtain a product 8, for a duration t5 of 0.5 s; the room temperature decreases to TP5 = 525 ° C;
• Execution of shot peening 9 of product 8 to optimize its resistance to fatigue or the adhesion of a possible future coating layer.

The processes of the figure 1 and some figure 2 therefore do not differ fundamentally. The only difference is that the induction oven 10 allows faster heating and at a more regular speed than the conventional roller oven 1. The heating time and the holding time tm are therefore shortened, which is advantageous for productivity of the installation. Induction heating also more assuredly guarantees homogeneity of the temperature of the sheet throughout its volume, which is advantageous for the success of the stages of shaping and obtaining the desired final properties.
Table 1 below shows the compositions of example steels to which the process according to the invention as described above and shown in the figure 1 has been applied. Items not mentioned are only present in traces resulting from processing.

Table 2 shows the intermediate metallurgical structures (during the processing stages where the steel temperature is above Ms) and final structures of these same examples, with the mechanical properties of the final part: tensile strength Rm, elastic limit Rp0,2, elongation A, resilience KCU, bending angle capacity. In the columns relating to the intermediate structure, MC designates the proportion of carbides.

It can be seen from this table that the examples according to the invention are the only ones which make it possible to achieve all the objectives targeted in terms of mechanical properties.

Of course, if a preferred application of the invention is the shaping of sheets intended for the automobile industry, this is not exclusive, and the sheets thus formed can have any other application for which they are used. '' would prove to be advantageous, in particular all parts with a structural function relating to the aeronautical, building, railway fields.

The invention also includes the cases where a sheet having the composition required by the invention is joined to a sheet having another composition, and where the assembly thus obtained is deformed by the process which has just been described. Of course, the structures and properties according to the invention will normally only be obtained on the part of the assembly having the composition of the invention.

Claims (9)

1. Method of manufacturing a martensitic stainless steel part from a sheet by hot forming, characterized in that:

   - a stainless steel sheet with the following composition in percentages by weight is prepared:

   * 0.005% ≤ C ≤ 0.3%;

   * 0.2% ≤ Mn ≤ 2.0%;

   * traces ≤ Si ≤ 1.0%;

   * traces ≤ S ≤ 0.01%;

   * traces ≤ P ≤ 0.04%;

   * 10.5% ≤ Cr ≤ 17.0%; preferably 10.5% ≤ Cr ≤ 14.0%;

   * traces ≤ Ni ≤ 4.0%;

   * traces ≤ Mo ≤ 2.0%;

   * Mo + 2 x W ≤ 2.0%;

   * traces ≤ Cu ≤ 3%; preferably traces ≤ Cu ≤ 0.5%;

   * traces ≤ Ti ≤ 0.5%;

   * traces ≤ Al ≤ 0.2%;

   * traces ≤ O ≤ 0.04%;

   * 0.05% ≤ Nb ≤ 1.0%;

   * 0.05% ≤ Nb + Ta ≤ 1.0%;

   * 0.25% ≤ (Nb + Ta)/(C + N) ≤ 8;

   * traces ≤ V ≤ 0.3%;

   * traces ≤ Co ≤ 0.5%;

   * traces ≤ Cu + Ni + Co ≤ 5.0%;

   * traces ≤ Sn ≤ 0.05%;

   * traces ≤ B ≤ 0.1%;

   * traces ≤ Zr ≤ 0.5%;

   * Ti + V + Zr ≤ 0.5%;

   * traces ≤ H ≤ 5 ppm, preferably traces ≤ H ≤ 1 ppm;

   * traces ≤ N ≤ 0.2%;

   * (Mn + Ni) ≥ (Cr -10.3 - 80 x [(C + N) ²]);

   * traces ≤ Ca ≤ 0.002%;

   * traces ≤ rare earths and/or Y ≤ 0.06%;

   * the rest being iron and impurities resulting from the steelmaking;

   - the martensitic transformation start temperature (Ms) of the sheet is ≥ 200°C;

   - the martensitic transformation end temperature (Mf) of the sheet is ≥ -50°C;

   - the microstructure of the sheet is composed of ferrite and/or tempered martensite and 0.5% to 5% by volume of carbides;

   - the size of the ferritic grains of the sheet is from 1 to 80 µm, preferably from 5 to 40 µm;

   - one or more hot and/or cold transformations of the sheet may optionally be carried out;

   - the sheet is austenitized by maintaining it at a temperature greater than Ac1, in order to give it a microstructure containing at most 0.5% of carbides in volume fraction and at most 20% of residual ferrite in volume fraction;

   - the austenitized sheet is transferred to a first shaping tool or a cutting tool, wherein the transfer has a duration t0, during which the sheet remains at a temperature greater than Ms and retains at most 0.5% by volume of carbides and at most 20% by volume of residual ferrite, wherein the sheet is at a temperature TP0 at the end of this transfer;

   - a first shaping or cutting step of the sheet is carried out for a period t1, during which period the sheet remains at a temperature greater than Ms and retains at most 0.5% by volume of carbides and at most 20% by volume of residual ferrite;

   - transfer of the shaped or cut sheet metal is carried out on a second shaping or cutting tool, or the configuration of the first shaping or cutting tool is modified for a period t2 during which period the sheet metal is cut while remaining at a temperature greater than Ms and retaining at most 0.5% by volume of carbides and at most 20% by volume of residual ferrite;

   - a second shaping or cutting step of the sheet is carried out for a period t3, during which period the sheet remains at a temperature greater than Ms and retains at most 0.5% by volume of carbides and at most 20% by volume of residual ferrite;

   - optionally, other steps may be performed for transferring the cut or shaped sheet metal to other cutting or shaping tools, or to modifying the configuration of the shaping or cutting tool used in the preceding step, wherein each operation is followed by a shaping or cutting step, and wherein the sheet remains at a temperature greater than Ms and retains at most 0.5% by volume of carbides and at most 20% by volume of residual ferrite during each of the steps of transferring the sheet or modifying the configuration of the tool and each of the shaping or cutting operations;

   - if TPn denotes the temperature reached by the shaped or cut sheet at the end of the last cutting or shaping step and Σti the sum of the periods of the transfer and/or tool configuration change steps and the shaping or cutting steps, the magnitude (TP0-TPn)/Σti is at least 0.5°C/s;

   - optionally, an additional shaping or cutting step is carried out at a temperature between Ms and Mf, in a domain where the microstructure consists of martensite, at least 5% of austenite and at most 20% of ferrite,

   - and the sheet is allowed to cool to ambient temperature in order to obtain the final part, wherein the final part has a microstructure containing at most 0.5% of carbides in volume fraction and at most 20% of residual ferrite in volume fraction.
2. Method according to claim 1, characterized in that the sheet has a martensitic transformation start temperature (Ms) ≤ 400°C.
3. Method according to claim 2, characterized in that the martensitic transformation start temperature (Ms) of the sheet is between 390 and 220°C.
4. Method according to one of the claims 1 to 3, characterized in that the thickness of the sheet is between 0.1 and 10 mm.
5. Method according to one of the claims 1 to 4, characterized in that the austenitization temperature is at least 850°C.
6. Method according to claim 5, characterized in that the austenitization temperature is between 925 and 1200°C.
7. Method according to one of the claims 1 to 6, characterized in that reheating of the sheet is effected during at least one of the transfer and/or tool configuration change steps or shaping or cutting steps of the sheet.
8. Method according to one of the claims 1 to 7, characterized in that a surface treatment is performed on the final part that is intended to increase its roughness or its fatigue properties.
9. Method according to one of the claims 1 to 8, characterized in that the final part is kept between 90 and 500°C for 10 s to 1 h, and then allowed to cool naturally in air.

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