The invention relates to the hot-shaping of stainless steels from a sheet to impart a complicated shape and excellent mechanical properties, these steels being intended, for example, in the automotive industry.

To lighten vehicles to reduce their fuel consumption and limit emissions of C0 2 , automakers today use carbon steel sheet or stainless steels with very high resistance, allowing reductions in thickness sheet metal over the more conventionally used steels in the past.

Martensitic steels (or, more generally, martensitic structure to more than 50%) have such mechanical characteristics, but their cold shaping capacity is limited. It is therefore necessary either to format cold ferritic state and then heat treating the part to obtain the martensite structure or hot shaping them to the austenitic state by terminating the treatment by quenching to obtain the martensite structure.

The realization of complex geometry parts by the second method with the known steels (carbon steels containing boron, ...) is however hampered by the constraints is their limited hardenability, or the existence of high-temperature metallurgical transformations that hinder a good command of the progress of the shaping and tempering. a complex part not predominantly martensitic, so the mechanical characteristics do not correspond to those mentioned, or having to be limited to obtain a martensitic piece of simple geometry we are likely to get, which is the correct form, for example through a laser cutting.

One might think to perform several steps of hot-forming press on transfers / tools to follow starting steels conventionally known for such uses, to make the gradual shaping and limiting the risk of occurrence of defects. But the resulting part will be composed of less than 80% martensite by volume and its mechanical properties and resilience will be degraded: at least one of target resistance strength Rm, yield strength Rp0.2, elongation at break A, ease of folding or resilience will not be reached. The time it is necessary to pass over the end temperature Mf of martensitic transformation to provide at least two shaping stages, two stages of transfer and quenching step is too long, and

Obtaining a structure composed of 80% by volume of martensite minimum is possible with the 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 progressive dies or said transfer, will not allow to realize after austenitization more than a transfer step, followed by a shaping step or hot cutting, before hardening within the tool to ensure a minimum of 30 ° C / s cooling rate.

The object of the invention is to provide a method for producing a piece of martensitic steel transformed to warm making possible the manufacture of complex shapes from a sheet, this endpiece having also high mechanical properties the making it suitable in particular for use in the automotive industry.

To this end, the invention relates to a method of manufacturing a part made of martensitic stainless steel from a metal sheet, by hot shaping, characterized in that:

- preparing a stainless steel sheet composition, in percentages by weight:

* 0,005%≤ C≤ 0,3% ;

* 0,2%≤ Mn≤ 2,0% ;

* traces≤ Si≤ 1 ,0% ;

* traces≤ S≤ 0,01 % ;

* traces≤ P≤ 0,04% ;

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

* traces≤ Ni≤ 4,0% ;

* traces≤ Mo≤ 2,0% ;

* I + 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≤ Co≤ Cu + Ni + 5.0%;

* traces≤ Sn≤ 0,05% ;

* traces≤ B≤ 0,1 % ;

* traces≤ Zr≤ 0,5% ;

* Ti + V + Zr≤ 0,5%;

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

* 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 balance being iron and impurities resulting from preparation;

- the starting temperature of martensitic transformation (Ms) of the sheet étant≥

200°C ;

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

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

- the size of ferritic grains of the sheet being from 1 to 80 μηι, preferably from 5 to

40 microns;

- optionally carrying out one or more transformations hot and / or cold from said sheet;

- austenitization of the sheet is carried out by maintaining it at a higher than Ac1 temperature so as to impart a microstructure containing at most 0.5% by volume fraction carbides and maximum 20% residual ferrite volume fraction;

- transferring the austenitized sheet on a first shaping tool or a cutting tool, said transfer tO having a duration during which the sheet remains at a temperature above Ms and retains at most 0.5% by volume of carbides and at most 20% by volume of residual ferrite, the sheet is at a temperature ΤΡ0 after the transfer;

- one carries out a first shaping step or cutting of the sheet, during a time t1, and for which the sheet remains at a temperature above Ms and retains at most 0.5% by volume of carbides and a maximum of 20 % by volume of residual ferrite;

- there is provided a transfer sheet shaping or cut on a second shaping tool or cutting, or modifying the configuration of the first shaping tool or cutting, for a time t2, during which the sheet remains at a temperature above Ms and retains at most 0.5% by volume of carbides and at most 20% by volume of residual ferrite;

- one carries out a second shaping step or cutting of the sheet, for a duration t3, and during which the sheet remains at a temperature above Ms and retains at most 0.5% by volume of carbides and a maximum of 20 % by volume of residual ferrite;

- optionally, it performs other of the sheet transfer steps cut or shaped on other cutting tools or shaping or modifying the configuration of the shaping tool or cutting used in the previous step, each being followed by a shaping step or cutting, the sheet remaining at a higher temperature Ms and retaining at most 0.5% by volume of carbides and at most 20% by volume ferrite residual for each of said steps of transfer of the sheet or changing the configuration of the tool and each of the shaping or cutting operations;

- if we denote by TPn the temperature reached by the sheet shaping or cut at the end of the final stage of cutting or shaping and parΣti the sum of the durations of transfer steps and / or change configuration of the tool and shaping steps or cutting, the variable (TP0-TPn) / Σti is at least 0.5 ° C / s;

- optionally performing a further step of shaping or cutting at a temperature between Ms and Mf, in an area where the microstructure consists of martensite, at least 5% austenite and not more than 20% ferrite.

- and the sheet is allowed to cool to room temperature to obtain the final piece, said end piece having a microstructure containing at most 0.5% by volume fraction carbides and at most 20% residual volume fraction of ferrite.

Said sheet can have a starting temperature of martensitic transformation (Ms) ≤ 400 ° C

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

The sheet thickness may be between 0.1 and 10 mm.

The austenitizing temperature may be at least 850 ° C.

The austenitizing temperature may be between 925 and 1200 ° C.

Can effect reheating of the sheet for at least one of the steps of transfer and / or change of configuration of the tool or shaping steps or sheet cutting.

a surface treatment can be performed on the final part, for increasing its roughness or fatigue properties.

the final part it may lie between 90 and 500 ° C for 10 s to 1 h, then allowed to cool naturally in air.

As will have been understood, the invention relies on the combination:

Choosing a stainless steel composition martensitic; - And the application of a shaping method of hot particular a sheet having this composition, as well as precise initial structural characteristics that make possible the use of said method for obtaining the final part, or an intermediate piece will then undergo operations to fine tuning some of its mechanical properties and / or superficial.

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

Then successively performs several shaping steps of the plate

(At least two) under temperature and time conditions such that the structure ferrite + carbides obtained after austenitization is maintained throughout the entire shaping. If necessary, we can proceed to reheating or maintain temperature between layouts, or for them by means of heating tools, so that the temperature of the shaping of current sheet and between layouts (for the transfer of the sheet from one tool to another, or if the sheet remains on the same tool in the tool configuration changes) does not drop below Ms ( start temperature of martensitic transformation).

It should be understood that the term "forming step", it also includes various operations of deformation or material removal, in particular, the deep drawing, hot stampings, stampings, cutouts , holes, these steps can take place in any order chosen by the manufacturer.

After shaping, the component obtained is cooled without particular constraints on cooling. This cooling may be preceded by a step of cutting or ultimate shaping performed between Ms and Mf (end temperature

martensitic transformation), under conditions where the microstructure is constituted by at least 10% austenite, more than 20% ferrite, the remainder being martensite.

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

- Figure 1 shows a flow diagram of a part making use of the method according to the invention, using an oven at conventional rollers, and the evolution of the temperature of the steel during the manufacture;

Figure 2 shows a flow diagram of a part making use of the method according to the invention, using an induction furnace, and changes of the temperature of the steel during the manufacture.

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

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 hot-shaping process, so that the final mechanical properties described are obtained. Above 0.3%, the weldability and, particularly, the resilience of the sheet become insufficient, especially for an application in the automotive industry.

Its Mn content is between 0.2 and 2.0%.

A minimum of 0.2% is required for austenitization. Above

2.0% of oxidation problems are to be expected during heat treatment if they are not carried out in neutral or reducing atmospheres, and most obtain the desired mechanical properties is no longer guaranteed.

Its Si content is between traces (that is to say, single impurities resulting from the smelting, without Si was added) and 1, 0%.

If can be used as a deoxidizer during the development, as Al, which it can supplement or replace. Beyond 1, 0%, it is considered that overly favors the formation of ferrite and therefore makes it more difficult austenitizing and that too brittle sheet for the shaping of a complex part can certainly will perform satisfactorily.

Its S content is between traces and 0.01% (100 ppm) in order to ensure a suitable weldability and resiliency to the final product.

Sa teneur en P est comprise entre des traces et 0,04%, afin de garantir que le produit final ne sera pas excessivement fragile. P est également néfaste pour la soudabilité.

Sa teneur en Cr est comprise entre 10,5 et 17,0%, de préférence entre 10,5% et 14,0% pour avoir une dissolution plus rapide des carbures pendant l'austénitisation.

La teneur minimale de 10,5% se justifie pour assurer l'inoxydabilité de la tôle. Une teneur supéreure à 17% rendrait difficile l'austénitisation et augmenterait inutilement le coût de l'acier.

Sa teneur en Ni est comprise entre des traces et 4,0%.

Un ajout de Ni n'est pas indispensable à l'invention. La présence de Ni dans la limite prescrite de 4,0% au maximum peut, cependant être avantageuse pour favoriser l'austénitisation. Un dépassement de la limite de 4,0% conduirait cependant à une présence excessive d'austénite résiduelle et à une présence insuffisante de martensite dans la microstructure après le refroidissement.

Sa teneur en Mo est comprise entre des traces et 2,0%.

La présence de Mo n'est pas indispensable. Mais elle est favorable à une bonne tenue à la corrosion. Au-dessus de 2,0%, l'austénitisation serait gênée et le coût de l'acier inutilement augmenté.

Une présence de W est, de même, possible, mais comme W est un élément très durcissant, cette présence doit être limitée et mise en relation avec la teneur en Mo . On considère qu'il faut que la somme Mo + 2 x W soit comprise entre des traces et 2,0%.

Contrairement à ce qui est le plus habituel lorsqu'on considère dans une nuance d'acier le cumul des influences de Mo et W, on prend en compte non la relation Mo + W/2 mais la relation Mo + 2 x W. La relation Mo + W/2 est à prendre en compte lorsqu'on veut maîtriser l'influence de ces deux éléments sur la formation de précipités, pour laquelle W est deux fois plus efficace que Mo à quantité ajoutée égale. Mais dans le cas de l'invention, on privilégie les influences respectives de Mo et W sur la dureté de l'acier. Et comme W est un élément plus durcissant que Mo, à quantités ajoutées égales, c'est la relation Mo + 2 x W qui doit être à prendre en compte selon l'invention. Cette somme Mo + 2 x W doit être comprise entre des traces et 2,0%. Au-delà, la dureté devient excessive et, toutes choses étant égales par ailleurs, les propriétés mécaniques à privilégier dans le cadre de l'invention sont diminuées, en particulier la capacité d'angle de pliage et la résilience.

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

These requirements on Cu are typical for this type of steel. In practice, this means that a Cu addition is not useful and that the presence of this element is due only raw materials used. A higher content of 0.5%, which would correspond to a voluntary addition is not required for automotive applications because it would degrade the weldability. Cu may however help austenitizing, and if the steel is applied to the invention to a field that does not require welding, the Cu content is up to 3.0%.

Its Ti content is between traces and 0.5%.

Ti is a deoxidizer such as Al and Si, but its cost and lower efficiency than that of Al makes his job generally unattractive from this point of view. It may have an interest in that the Ti nitrides and carbonitrides training may limit grain growth and positively influence certain mechanical properties and weldability. However, this training can be a disadvantage in the case of the method according to the invention, since Ti tends to hinder the austenitizing due to the formation of carbides, and TiN degrade resilience. A maximum of 0.5% is therefore not exceeded.

V and Zr are also elements capable of forming nitrides degrading resiliency, and generally, it is necessary that the sum Ti + Zr + V not exceeding 0.5%.

The Al content is between traces and 0.2%.

Al is used as a deoxidizer during the development. Do not until after the deoxidation it remains in the steel an amount exceeding 0.2%, as there would be a risk of forming an excessive amount of AlN degrading the mechanical properties, and also to have difficulties get the martensitic microstructure.

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

The requirements on the content of O are those on conventional martensitic stainless steels, depending on the ability to their shaping without cracks will initiate from inclusions and quality of the desired mechanical properties on the final part, and that the excessive presence of oxide inclusions is likely to alter. Conversely, if a minimum workability of the sheet is desired, it may be advantageous to have oxidized inclusions in significant numbers, if their composition makes them sufficiently malleable so that they serve as a lubricant to the cutting tool. This technique number control and the composition of the oxide inclusions is conventional steel.

This is essentially the addition of oxidants Al, Si, Ti, Zr when developing the possible addition of Ca, then the care given to the settling of oxide inclusions in the liquid steel and the livelihoods of these oxidants in the solidified steel that determine the final content of O. While each of these elements taken in isolation, may be absent or only very weakly present, it is nevertheless necessary that at least one of them (usually Al and / or Si) is present in an amount sufficient to ensure that the O content of the final sheet will not be too high for shaping uneventful of the workpiece, and for future applications of the room.These mechanisms governing the deoxidation of steel and controlling the composition and quantity of their oxide inclusions are well known in the art, and apply under the perfectly conventional way of invention.

Its Nb content is between 0.05% and 1 0%

Its total content of Nb + Ta is 0.05% and 1, 0%.

Nb and Ta are important to obtain good resilience, and Ta improves resistance to pitting corrosion. But since they can interfere with the austenitizing, they should not be present in quantities exceeding what is just prescribed. Also, Nb and Ta capture C and N to form carbonitrides which prevent excessive grain growth of austenite during austenitization. This is favorable for obtaining a very good cold resilience, between -100 ° C and 0 ° C. However, if the content of Nb and / or Ta is too high, C and N are completely trapped in carbonitrides and none left enough dissolved form so that the mechanical properties referred to are achieved, including resilience and mechanical resistance.

therefore requires 0,25≤ (Nb + Ta) / (C + N) ≤ 8

to obtain a resilience of about 50 J / cm 2 at 20 ° C or more.

The V content is between traces and 0.3%.

As Ti, V a weakening element is likely to form nitrides, and should not be present in too much. As said earlier, it is necessary that Ti + Zr + V not exceeding 0.5%.

Its Co content is between traces and 0.5%. This entry, like Cu, which may assist in the austenitization. But it is useless to put more than 0.5%, as the austenitizing may be assisted by less expensive means.

Total Cu content, Ni and Co must be between traces and 5.0%, for not to leave too much residual austenite after the martensitic transformation and not to degrade the weldability in applications that require it.

The Sn content is between traces and 0.05% (500 ppm). This element is not desired because it is detrimental to weldability and steel capacity to be converted to heat. The limit of 0.05% is a tolerance.

Its B content is between traces and 0.1%.

B is optional, but its presence is beneficial to the hardenability and the forgeability of the austenite. It thus facilitates the formatting. Its addition to the above 0.1% (1000 ppm) provides no significant additional improvement.

The Zr content is between traces and 0.5% as it decreases resilience and discomfort austenitizing. It also recalls that the total content of Ti + V + Zr should not exceed 0.5%.

Its H content is between traces and 5 ppm, preferably not more than 1 ppm. Excessive content of H tends to weaken martensite. He'll have to choose a method of steelmaking in the liquid state that can ensure this low presence of H. Typically, treatments ensuring a high degassing liquid steel (by massive injection of argon in the liquid steel, well known process called "AOD", or by a vacuum passage in which the steel is decarburized by release of CO, said method "VOD") are indicated.

Its N content is between traces and 0.2% (2000 ppm). N is an impurity with the same treatments that reduce the H content contribute to limiting the presence or to reduce significantly. It is not always necessary to have a particularly low N content, but for the reasons that we say it is necessary that its content, considered in conjunction with those elements with which it can combine to form nitrides or carbonitrides 8≥ obey the relation (Nb + Ta) / (C + N) ≥ 0,25.

Also, good austenitization of the steel during the initial stage of the thermomechanical treatment is favored if it meets the relationship (Mn + Ni) ≥ (Cr -10.3 to 80 x [(C + N) 2 ]). Sufficient resilience is achieved if this condition is satisfied in addition to the other that have been defined. Must be a sufficient level of gammagenic elements to offset the effect of Cr and alphagenic ensure proper austenitizing, namely at least 80%, and from this point of view the effectiveness of the sum C + N is not not linear.

Its Ca content est≤ 0.002% (20 ppm).

Its total content of rare earths and Y is between traces and 0.06% (600 ppm). These elements may improve the oxidation resistance at very high temperatures austénitisations.

The rest of the steel is constituted by iron and impurities resulting from the smelting.

Other requirements on the composition of the steel concern the beginning of martensitic transformation temperatures Ms and end of martensitic transformation Mf.

Ms should preferably be at most 400 ° C. If MS is higher, there is a risk that the various transfer operations, and the room layout does not succeed quickly enough and we did not have time to do all the formatting to a temperature higher than Ms. however, it can limit or avoid this risk by providing that the play suffers from reheating or maintain temperature between layouts, and / or during the latter when using heated tools of known types including for example, electrical resistors. This condition Ms≤ 400 ° C is not always mandatory but only recommended for an economical and easy application of the method according to the invention under industrial conditions.

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

Preferably, Ms is between 390 and 320 ° C.

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

Ms and Mf are determined, preferably, 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 help evaluate from the composition of the steel, but an experimental determination is safer.

It should be understood that the thermomechanical treatments that will be described can be performed either on a bare metal which will eventually be covered later, or on a sheet already coated, for example by an AI-based alloy and / or Zn. This coating, typically thickness of 1 to 200 μηι and present on one or both sides of the sheet, may have been deposited by any conventionally used for this technique, it is simply that, if it was filed before the austenitizing, it does not evaporate during the stay of the sheet to the austenitization temperature and deformation, and is not damaged during the deformation.

The selection and optimization of characteristics of the coating and its method of deposition so that these conditions are met does not go beyond what is known to the skilled person, when brought to shape so classic stainless steel sheets already coated. If the coating occurs before austenitizing, we can, however, focus on coatings based AI compared to those based on Zn, as Ai is less likely to evaporate Zn temperatures austenitizing .

The method of the invention is the following, applied to the manufacture and shaping a metal sheet.

Initially, one conventional manner of preparing a sheet original stainless steel, bare or coated, with the composition which has just been described and a thickness which is typically from 0.1 to 10 mm. This preparation can include hot transformation operations and / or cold and cutting the semi-product from the casting and solidification of the liquid steel. It is necessary that this initial plate has a microstructure consisting of ferrite and / or martensite and from 0.5% to 5% by volume of carbides. The size of the ferritic grain, measured according to standard NF EN ISO 643, is between 1 and 80 μηι, preferably between 5 and 40 μηι. A ferrite grain size of 40 μηι at most is recommended to promote the austenitization that follows and thus obtain at least 80% of desired austenite.

The first step is to austenitization of the sheet by passage through an oven which carries it in a range of greater than Ac1 temperature (onset temperature of the onset of austenite), thus typically above about 850 ° C for relevant compositions). It should be understood that the austenitizing temperature should concern the entire volume of the metal, and that treatment should be long enough so that, given the thickness of the sheet and the kinetics of the transformation, austenitizing is complete throughout this volume.

The maximum temperature of this austenitization is not a specific characteristic of the invention. It must simply be such that the metal remains in a fully solid state (the temperature should be lower, at least in the steel solidus temperature) and not too softened to withstand without damage the transfer between oven and the forming tool that follows the austenitization. Also, the temperature must not be so high as to cause oxidation and / or a significant surface decarburizing the sheet in the heating atmosphere. Surface oxidation lead to the need to descale the sheet mechanically or chemically prior to shaping to prevent incrustation of calamine in the sheet surface, and result in a loss of material. Excessive decarburization (thickness of the surface décarburée≥ 100 μηι) would decrease the hardness and tensile strength of the sheet. The risks observed oxidation and / or a significant decarbonization depend, as is known, not only on the temperature and duration austenitizing, but also the baking atmosphere. A non-oxidizing, therefore neutral or reducing atmosphere (typically argon, CO and mixtures thereof ...), preferably in air, increases without damage the temperature not only the temperature and duration austenitizing, but also the baking atmosphere. A non-oxidizing, therefore neutral or reducing atmosphere (typically argon, CO and mixtures thereof ...), preferably in air, increases without damage the temperature not only the temperature and duration austenitizing, but also the baking atmosphere. A non-oxidizing, therefore neutral or reducing atmosphere (typically argon, CO and mixtures thereof ...), preferably in air, increases without damage the temperature

treatment, which ensures a complete austenitizing in a short time. When using pure nitrogen or a strongly hydrogen-containing atmosphere in a furnace requiring a high residence time for austenitizing, there is a risk of surface nitriding or hydrogen taken up by the steel. It will therefore be reflected in the choice of the treatment atmosphere, and pure nitrogen atmosphere or containing a relatively high hydrogen content will sometimes be avoided.

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

The purpose of this austenitization is to pass the metal structure of ferrite + carbides initial austenitic structure containing a maximum of 0.5% carbides volume fraction and maximum 20% residual ferrite volume fraction. An object of this austenitization is, in particular, lead to dissolution of at least a majority of carbide initially present so as to release atoms of C to form austenite structure and the martensite structure during subsequent process steps. The maximum content of residual ferrite 20%, to be preserved until the final product, is justified by the resilience and conventional elastic limit that is to be obtained.

The austenitized sheet is then transferred to a suitable shaping tool

(Such as a stamping or punching tool) or a cutting tool. This transfer tO a duration as short as possible, and during this transfer the plate must remain at a temperature above Ms and maintain an austenitic microstructure at 0.5% maximum carbides and 20% maximum residual ferrite. After this transfer, the sheet is a ΤΡ0 temperature which is as close as possible to the austenitizing temperature rating for obvious reasons of energy saving.

Then performs a first shaping step or cutting, lasted t1 typically between 0.1 and 10 s. The precise duration of this step (like those of other steps) is not in itself a fundamental characteristic of the invention. It must be short enough to give the temperature of the metal does not drop below Ms, we do not attend a decarbonization and / or

significant oxidation of the surface of the sheet, and an austenitic microstructure, 0.5% maximum carbides and 20% maximum residual ferrite is still present at the end of the operation. If needed, one can use a tool shaping provided with means for heating the sheet so that these conditions of temperature and microstructure are satisfied, since the contact of a shaping tool with non-heating of the sheet causes a cooling sheet which is often greater than 100 ° C / s.

The absence of significant decarbonization and surface 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 atmosphere or reductive around the sheet during its shaping

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

The sheet thus formed is then transferred to another tool for a second shaping step in the broad sense of the term. Alternatively, using the same tool in both stages, but alter its configuration in the interval (for example, replacement of the punch in the case where a drawing is performed in each of the two steps). T2 duration of this transfer is typically from 1 to 10 s, in order to be sufficiently fast so that the temperature of the sheet remains above Ms during the transfer and that the microstructure remains austenitic, 0.5% maximum carbides and 20% or less of residual ferrite.

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

Other shaping steps (broadly defined), and their corresponding transfers, can follow the second shaping step.

The key is that during the execution of these transfers and these layouts / cuts, the temperature of the steel does not descend below Ms, and an austenitic microstructure at 0.5% maximum carbides and 20% maximum residual ferrite is retained until the end of the last stage n, of TPn final temperature. If necessary, as we have said, heated shaping tools can be used, as the means of heating the sheet metal between the layouts.

The average cooling rate between ΤΡ0 and TPn, defined by the magnitude (TP0-TPn) / Σti, Σti constituting the sum of the durations of transfers and edited, must be at least 0.5 ° C / s.

The consequence of this cooling rate between the beginning and end of forming operations just described, combined with the steel composition and the procedure used during the shaping is that when cooling the steel does not enter the "nose" of the CCT diagram that corresponds to the bainite transformation, but remains in the austenitic range before moving directly into the field which can operate the martensitic transformation. The steel composition is precisely selected so that, compared to carbon steels it is most common to use in the automotive industry for the production of sheet suitable for being welded, the nose is shifted to the durations higher, making it possible on the usual shaped layout tools avoidance bainitic domain, let alone of pearlite and ferrite areas, and therefore as complete as possible execution of the transformation of austenite to martensite. But it should be recalled that, as mentioned, each individual step taken must allow to keep a maximum of 0.5% austenitic microstructure carbides and 20% maximum residual ferrite. The couple duration / each stage cooling rate should be selected accordingly and, if necessary, reheating of the sheet between and / or during the formatting or cuts are executed for this microstructure can be maintained for all steps. and therefore as complete as possible execution of the transformation of austenite to martensite. But it should be recalled that, as mentioned, each individual step taken must allow to keep a maximum of 0.5% austenitic microstructure carbides and 20% maximum residual ferrite. The couple duration / each stage cooling rate should be selected accordingly and, if necessary, reheating of the sheet between and / or during the formatting or cuts are executed for this microstructure can be maintained for all steps. and therefore as complete as possible execution of the transformation of austenite to martensite. But it should be recalled that, as mentioned, each individual step taken must allow to keep a maximum of 0.5% austenitic microstructure carbides and 20% maximum residual ferrite. The couple duration / each stage cooling rate should be selected accordingly and, if necessary, reheating of the sheet between and / or during the formatting or cuts are executed for this microstructure can be maintained for all steps. Maximum 5% carbides and 20% maximum residual ferrite. The couple duration / each stage cooling rate should be selected accordingly and, if necessary, reheating of the sheet between and / or during the formatting or cuts are executed for this microstructure can be maintained for all steps. Maximum 5% carbides and 20% maximum residual ferrite. The couple duration / each stage cooling rate should be selected accordingly and, if necessary, reheating of the sheet between and / or during the formatting or cuts are executed for this microstructure can be maintained for all steps.

Can, optionally, performing at least one additional step shaping wider at a temperature between Ms and Mf, in an area where the microstructure comprises at least 5% by volume of austenite. If this additional step is cutting, the final shape of the workpiece can be achieved with less tool wear, and if this additional step is a deformation, at least 5% austenite will provide sufficient ductility so that this deformation is still possible despite the sometimes significant presence of martensite.

Finally the sheet was 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, the fact that the composition of the steel ensures that the sheet will remain anyway in the area which can operate martensitic transformation also during this cooling to ambient temperature, at least if not using means substantially slowing the cooling compared to a natural cooling in the open air, such as a rollover of the sheet. Of course, it is not excluded to expedite this cooling, forced air means or spraying water or another fluid.

The possibility of using at least two steps to obtain the final shape of the room gives access, through the use of a steel having the specified composition and treated according to the invention, complex shapes for the final part that known processes making use of only one form of the initial sheet layout does not achieve, at least not with sufficient quality.

Optionally, a surface treatment can be applied to the final piece, such as a shot blasting or sand blasting, in order to increase the roughness of its surface to improve the adhesion of a coating which may subsequently be applied, such as a painting, or to create residual stresses enhancing the fatigue strength of the sheet. This type of operation is known in itself.

Also, a final heat treatment may be performed on the final part, so after cooling to room temperature, to improve its elongation at break and bring it to a value of more than 8% according to ISO standards, which corresponds substantially more of 10% according to JIS. This treatment is to stay the final part between 90 and 500 ° C for 10 s to 1 h, and then to perform natural cooling in air.

The piece thus obtained by the method according to the invention have high mechanical properties at ambient temperature, particularly due to its high martensite content of at least 80%. Typically, Rm is at least 1000 MPa, Re of at least 800 MPa, the elongation at break measured according to ISO 6892 is at least 8%, and the bending angle of capacity for thickness of 1, 5 mm is at least 60 °, measured according to VDA 238-100.

1 schematically shows an exemplary process scheme for a method according to the invention running on a steel of composition according to that of Example 2 of Table 1 which follows, in which Ms is 380 ° C and 200 Mf ° C, and comprising the steps of:

- Heating in a roller hearth furnace 1 vector for 2 min a sheet 2 of thickness 1, 5 mm, between room temperature and TPi temperature equal to 950 ° C;

Maintaining in the furnace 1 of the sheet 2 at said temperature for a time TPi tm 3 min;

- Transfer of the sheet 2 between the furnace 1 and a hot embossing tool 3, for a to 1 s; the steel temperature decreases to ΤΡ0 = 941 ° C;

First shaping step (deformation), executed in the hot stamping tool 3 during a t1 period of 0.5 s to obtain a sheet shaped 4; the steel temperature decreases to TP1 = 808 ° C;

Transfer of the sheet shaping 4 between the hot stamping tool 3 and a drilling tool 5, during a t2 period of 0.5 s; the steel temperature decreases to TP2 = 799 ° C;

Second shaping step consisting of a bore in the drilling tool 5 during a t3 period of 1 s to obtain a sheet shaped and pierced 6; the steel temperature decreases to TP3 = 667 ° C;

Transfer of the sheet 6 shaping and pierced to a cutting tool 7 for performing a cutting of the edges of the sheet 6 in order to give them their final dimensions to obtain a product 8;

- Execution of a blasting of the product 8 in a shot blasting 9 to optimize its fatigue strength or adhesion of a possible layer of coating future.

2 schematically shows another example of process flow diagram for a method according to the invention performed on a sheet 2 of a steel of composition according to that of Example 7 in Table 1 which follows, in which Ms is 380 Ms ° C and 200 ° C, and comprising the steps of:

Heating in an oven at 10 for 20 sec induction vector 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 TPi to said temperature for a period tm of 30 s;

Transfer of the sheet 2 between the induction furnace 10 and an embossing tool in hot 3 tO for a duration of 1 s; the steel temperature decreases to ΤΡ0 = 941 ° C;

- First shaping step (deformation), executed in the hot stamping tool 3 during a t1 period of 0.5 s to obtain a sheet shaped 4; the steel temperature decreases to TP1 = 808 ° C;

Transfer of the sheet shaping 4 between the hot stamping tool 3 and a drilling tool 5, during a t2 period of 1 s; the steel temperature decreases to TP2 = 799 ° C;

Second shaping step consisting of a bore in the drilling tool 5 during a t3 period of 0.5 s to obtain a sheet shaped and pierced 6; the steel temperature decreases to TP3 = 667 ° C;

Transfer of the sheet 6 shaping and pierced to a cutting tool 7 during a t4 period of 1 s, for performing a cutting of the edges of the sheet 6; the temperature of the sheet decreases to TP4 = 658 ° C;

CLAIMS

1 .- A method for manufacturing a component martensitic stainless steel from a metal sheet, by hot shaping, characterized in that:

- preparing a stainless steel sheet composition, in percentages by weight:

* 0,005%≤ C≤ 0,3% ;

* 0,2%≤ Mn≤ 2,0% ;

* traces≤ Si≤ 1 ,0% ;

* traces≤ S≤ 0,01 % ;

* traces≤ P≤ 0,04% ;

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

* traces≤ Ni≤ 4,0% ;

* traces≤ Mo≤ 2,0% ;

* I + 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≤ Co≤ Cu + Ni + 5.0%;

* traces≤ Sn≤ 0,05% ;

* traces≤ B≤ 0,1 % ;

* traces≤ Zr≤ 0,5% ;

* Ti + V + Zr≤ 0,5%;

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

* 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 balance being iron and impurities resulting from preparation;

- the starting temperature of martensitic transformation (Ms) of the sheet étant≥

200°C ;

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

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

- the size of ferritic grains of the sheet being from 1 to 80 μηι, preferably from 5 to

40 microns;

- optionally carrying out one or more transformations hot and / or cold from said sheet;

- austenitization of the sheet is carried out by maintaining it at a higher than Ac1 temperature so as to impart a microstructure containing at most 0.5% by volume fraction carbides and maximum 20% residual ferrite volume fraction;

- transferring the austenitized sheet on a first shaping tool or a cutting tool, said transfer tO having a duration during which the sheet remains at a temperature above Ms and retains at most 0.5% by volume of carbides and at most 20% by volume of residual ferrite, the sheet is at a temperature ΤΡ0 after the transfer;

- one carries out a first shaping step or cutting of the sheet, during a time t1, and for which the sheet remains at a temperature above Ms and retains at most 0.5% by volume of carbides and a maximum of 20 % by volume of residual ferrite;

- there is provided a transfer sheet shaping or cut on a second shaping tool or cutting, or modifying the configuration of the first shaping tool or cutting, for a time t2, during which the sheet remains at a temperature above Ms and retains at most 0.5% by volume of carbides and at most 20% by volume of residual ferrite;

- one carries out a second shaping step or cutting of the sheet, for a duration t3, and during which the sheet remains at a temperature above Ms and retains at most 0.5% by volume of carbides and a maximum of 20 % by volume of residual ferrite;

- optionally, it performs other of the sheet transfer steps cut or shaped on other cutting tools or shaping or modifying the configuration of the shaping tool or cutting used in the previous step, each being followed by a shaping step or cutting, the sheet remaining at a higher temperature Ms and retaining at most 0.5% by volume of carbides and at most 20% by volume ferrite residual for each of said steps of transfer of the sheet or changing the configuration of the tool and each of the shaping or cutting operations;

- if we denote by TPn the temperature reached by the sheet shaping or cut at the end of the final stage of cutting or shaping and parΣti the sum of the durations of transfer steps and / or change configuration of the tool and shaping steps or cutting, the variable (TP0-TPn) / Σti is at least 0.5 ° C / s;

- optionally performing a further step of shaping or cutting at a temperature between Ms and Mf, in an area where the microstructure consists of martensite, at least 5% austenite and not more than 20% ferrite.

- and the sheet is allowed to cool to room temperature to obtain the final piece, said end piece having a microstructure containing at most 0.5% by volume fraction carbides and at most 20% residual volume fraction of ferrite.

2. - Method according to claim 1, characterized in that said sheet has a starting temperature of martensitic transformation (Ms) ≤ 400 ° C

3. - Method according to claim 2, characterized in that the starting temperature of martensitic transformation (Ms) of the sheet is between 390 and 220 ° C.

4. - Method according to one of claims 1 to 3, characterized in that the sheet thickness is between 0.1 and 10 mm.

5. - Method according to one of claims 1 to 4, characterized in that the austenitizing temperature is at least 850 ° C.

6. A process according to claim 5, characterized in that the austenitizing temperature is between 925 and 1200 ° C.

7. A process according to one of claims 1 to 6, characterized by effecting a heating of the sheet for at least one of the steps of transfer and / or the tool configuration change or steps of shaping the sheet or blank.

8. - Process according to one of claims 1 to 7, characterized by carrying out a surface treatment on the final part, for increasing its roughness or fatigue properties.

9. - A method according to one of claims 1 to 8, characterized in that one stay the final part between 90 and 500 ° C for 10 s to 1 h, then allowed to cool naturally in air.