EP3445878A1

EP3445878A1 - A process for manufacturing a martensitic stainless steel part from a sheet

Info

Publication number: EP3445878A1
Application number: EP17713465.7A
Authority: EP
Inventors: Pierre-Olivier Santacreu; Christophe Cazes; Guillaume BADINIER; Jean-Benoit MOREAU
Original assignee: Aperam SA
Current assignee: Aperam SA
Priority date: 2016-04-22
Filing date: 2017-03-21
Publication date: 2019-02-27
Anticipated expiration: 2037-03-21
Also published as: US11001916B2; RU2018136969A3; AU2017252037A1; RU2018136969A; BR112018071587B1; MX2018012841A; BR112018071587A2; KR102395730B1; SI3445878T1; HUE051293T2; KR20180136455A; CN109415776B; CA3022115A1; JP2019518609A; WO2017182896A1; US20190127829A1; RU2724767C2; JP6840771B2; EP3445878B1; ES2805067T3

Abstract

Process for manufacturing a martensitic stainless steel part, according to which a stainless steel sheet is prepared having the composition: 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 ≤ 7.0%; traces ≤ Ni ≤ 4.0%; traces ≤ Mo ≤ 2.0%; Mo + 2 x W ≤ 2.0%; traces ≤ Cu ≤ 3%; 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; traces ≤ N ≤ 0.2%; (Mn + Ni) ≥ (Cr - 0.3 – 80 x [(C + N)²]); traces ≤ Ca ≤ 0.002%; traces ≤ rare earth elements and/or Y ≤ 0.06%; the remainder being iron and impurities; the temperature Ms being ≥ 200°C; the temperature Mf being ≥ -50°C; the microstructure being composed of ferrite and/or tempered martensite and from 0.5% to 5% by volume of carbides; the size of the ferritic grains being from 1 to 80 μm; an austenization is carried out, in order to obtain a microstructure containing at most 0.5% of carbides and at most 20% of residual ferrite; the sheet is transferred to a first shaping tool, the sheet remaining at a temperature above Ms and retaining at most 0.5% of carbides and at most 20% of residual ferrite; a first shaping or cutting step is carried out, the sheet remaining at a temperature above Ms and retaining at most 0.5% of carbides and at most 20% of residual ferrite; the sheet is transferred to a second shaping tool; a second shaping step is carried out during which the sheet remains at a temperature above Ms and retains at most 0.5% of carbides and at most 20% of residual ferrite; - if TPn is the temperature reached by the sheet at the end of the last shaping step and Σti is the sum of the durations of the transfer and shaping steps, (TP0-TPn)/Σti is at least 0.5°C/s; - and the sheet is left to cool into a final part having a microstructure containing at most 0.5% of carbides and at most 20% of residual ferrite.

Description

Process for manufacturing a martensitic stainless steel part from a sheet

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 the vehicles to limit their fuel consumption and thus limit their C0 ₂ emissions, the manufacturers today use carbon steel plates or stainless steels with very high strengths, allowing reductions in thickness. sheet metal 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 ability to cold form is limited. It is therefore necessary either to cold-form them in the ferritic state, then to heat-treat the part to obtain the martensitic structure, or to heat-form them in the austenitic state by finishing the treatment with a quenching. in order to obtain the martensitic structure.

The realization of pieces of complex geometry by this second method 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 temperatures. which make difficult a good control of the course of the shaping and tempering. There is a strong risk of obtaining a complex part that is not predominantly martensitic, and therefore whose mechanical characteristics do not correspond to those aimed at, either of having to be limited to obtaining a martensitic piece of simple geometry, whose shape will be corrected, for example by means of a laser cut.

One could think of carrying out several hot forming steps on press transfers / tools to follow starting from the steels conventionally known for these uses, in order to make the shaping progressive and to limit the risks of occurrence of defects. But the piece obtained will consist of less than 80% of martensite by volume and its mechanical properties and resilience will be degraded: at least one of the tensile strength targets Rm, elastic limit Rp0.2, elongation at break A, ease of folding or resilience will not be achieved. The time it is necessary to pass above the end of martensitic transformation temperature Mf to achieve at least two shaping steps, two transfer steps and a quenching step is too long, and the austenite is then transforms partially 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 method using a press with tools to follow or said transfer, can not allow to achieve after austenitization more than a transfer step, followed by a shaping step or hot cutting, before quenching in the tool to guarantee a minimum of 30 ° C / s for the cooling rate.

The object of the invention is to propose a method for producing a martensitic steel piece that has been transformed while making it possible to manufacture pieces of complex shape from a sheet, this final piece having, moreover, high mechanical properties. making it suitable, in particular, for use in the automotive industry.

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

a stainless steel sheet of composition is prepared, 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% ≤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 earth and / or Y ≤ 0.06%;

the rest being iron and impurities resulting from the elaboration;

the martensitic transformation start temperature (Ms) of the sheet being≥

200 ° C;

the martensitic transformation end temperature (Mf) of the sheet being≥

50 ° C;

- The microstructure of the sheet being composed of ferrite and / or martensite back and 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 μηι;

- It is optionally carried out one or more transformations hot and / or cold of said sheet;

austenitization of the sheet is carried out by maintaining it at a temperature greater 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 greater than Ms and retains at most 0.5% by volume of carbides and at most 20% by volume of residual ferrite, the sheet being at a temperature ΤΡ0 at the end of this transfer;

a first stage of shaping or cutting of the sheet is carried out for a period t1 and 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;

a 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 t 2 during which the sheet metal is cut. 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;

a second stage of shaping or cutting of the sheet is carried out for a time t3, 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;

optionally, other steps are performed for transferring the cut or shaped sheet metal to other cutting or forming tools, or to modifying the configuration of the forming or cutting tool used. in the preceding step, each being followed by a shaping or cutting step, the sheet 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 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 TPn denotes the temperature reached by the sheet shaped or cut at the end of the last cutting or shaping step and by the sum of the durations of the transfer and / or change of steps; configuration of the tool and the shaping or cutting steps, the magnitude (TP0-TPn) / Σti is at least 0.5 ° C / s;

optionally an additional step of shaping or cutting is carried out at a temperature between Ms and Mf, in a field 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 room temperature in order 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 may be between 390 and 220 ° C.

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

The austenitization temperature may be at least 850 ° C.

The austenitization temperature may be between 925 and 1200 ° C. The sheet may 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.

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

The final piece can be kept between 90 and 500 ° C for 10 s to 1 h, then allowed to cool naturally in air.

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

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

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

Then successively executes several stages of shaping the sheet metal

(At least two) under conditions of temperature and duration such that the ferrite + carbide structure obtained after austenitization is retained throughout the shaping. If necessary, it is possible to carry out reheating or temperature maintenance between the shaping operations, or during these by means of heating tools, so that the temperature of the sheet being shaped and between formatting (during sheet metal transfers from one tool to another, or if the sheet remains on the same tool, during tool configuration changes) do not fall below Ms ( martensitic transformation start temperature).

It should be understood that the term "shaping step" includes such diverse operations of deformation or removal of material as, in particular, deep drawing, hot stamping, stamping, cutting. , holes, these steps can take place in any order at the choice of the manufacturer.

After shaping, the part obtained is cooled without particular constraints on the cooling. This cooling may be preceded by a cutting or final shaping step performed 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.

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

FIG. 1, which shows a manufacturing diagram of a part making use of the process according to the invention, using a conventional roller oven, as well as the evolution of the temperature of the steel during manufacture;

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

The composition of the martensitic stainless steel used in the process according to the invention is as follows. All percentages are percentages 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 austenitization of the microstructure during the first step of the hot forming process, so that the final mechanical properties are obtained. Above 0.3%, the weldability and, above all, the resilience of the sheet becomes 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 to achieve austenitization. Above

2.0% of oxidation problems are to be feared during thermal treatments if they are not carried out in neutral or reducing atmospheres, and moreover the obtaining of the desired mechanical properties would no longer be guaranteed.

Its Si content is between traces (i.e., simple impurities resulting from the elaboration without Si being added) and 1.0%.

If can be used as a deoxidizer during the elaboration, 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 thus makes it more difficult to austenitize, and that it weakens the sheet too much so that the shaping of a complex piece can certainly proceed satisfactorily.

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

Its P content is between traces and 0.04%, to ensure that the final product will not be excessively fragile. P is also bad for solderability. Its Cr content is between 10.5 and 17.0%, preferably between 10.5% and 14.0% to have faster carbide dissolution during austenitization.

The minimum content of 10.5% is justified to ensure the stainless steel sheet. A content greater 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, however, would 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. But it is favorable to a good resistance to corrosion. Above 2.0%, austenitization would be hampered 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 related to the Mo content. It is considered that the sum Mo + 2 x W must be between traces and 2.0%.

Contrary to what is most usual when one considers in a steel shade the cumulation of the influences of Mo and W, one takes into account not the relation Mo + W / 2 but the relation Mo + 2 x W. The relation Mo + W / 2 is to be taken into account when one wants 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 quantity. But in the case of the invention, the respective influences of Mo and W on the hardness of the steel are favored. And since W is a more hardening element than Mo, with equal added quantities, it is the relation Mo + 2 × W that must be taken into account according to the invention. This sum Mo + 2 x W must be between traces and 2.0%. Beyond, the hardness becomes excessive and, all things being equal, the mechanical properties to be preferred in the context of the invention are reduced, in particular the folding angle capacity and resilience.

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

These requirements on Cu are conventional for this type of steel. In practice, this means that an addition of 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 weldability. Cu may, however, assist in austenitization, and if the steel of the invention is applied to a non-weldable range, the Cu content may be 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 efficiency less than that of Al makes his job in general unattractive from this point of view. It may be of interest that the formation of Ti nitrides and carbonitrides can limit grain growth and favorably influence certain mechanical properties and weldability. However, this formation may be a disadvantage in the case of the process according to the invention, since Ti tends to hinder the austenitization due to the formation of carbides, and the TiN degrade the resilience. A maximum content of 0.5% is therefore not to be exceeded.

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

Its Al content is between traces and 0.2%.

Al is used as a deoxidizer during processing. It is not necessary that after the deoxidation there remain in the steel an amount exceeding 0,2%, because there would be a risk of forming an excessive amount of AIN degrading the mechanical properties, and also to have 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, as a function of the ability to form them without cracks starting from the inclusions and the quality of the mechanical properties sought on the final piece, and that 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 to the cutting tool. This technique for controlling the number and composition of oxidized inclusions is conventional in the iron and steel industry. The control of the composition of the oxides can be advantageously obtained by a controlled addition of Ca and / or an adjustment of the composition of the slag with which the liquid steel is in contact and in chemical equilibrium during the preparation.

It is essentially the addition of deoxidizers Al, Si, Ti, Zr during the elaboration, the possible addition of Ca, the care then brought to the decantation of the oxidized inclusions within the liquid steel and the subsistence of these deoxidants in the solidified steel which determine the final O content. If each of these elements, taken separately, can be absent or only very weakly present, it is nevertheless necessary that at least one of them (most often Al and / or Si) be present in a quantity sufficient to guarantee that the O content of the final sheet will not be too high for trouble-free formatting of the part, and for future applications of the part. These mechanisms governing the deoxidation of steels and the control of the composition and amount 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 resistance to pitting corrosion. But since 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 forming carbonitrides which prevent too much growth of the austenite grains during austenitization. This is favorable for obtaining a very good cold resilience, between -100 ° C and 0 ° C. On the other hand, if the Nb and / or Ta content is too high, C and N will be completely trapped in the carbonitrides and it will not remain sufficiently dissolved in order to reach the desired mechanical properties, in particular the resilience and mechanical resistance.

Therefore 0.25≤ (Nb + Ta) / (C + N) ≤ 8 is 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%.

As Ti, V a weakening element that is likely to form nitrides, and should not be present in too large a quantity. As said above, Ti + V + Zr must not exceed 0.5%.

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

The total contents of Cu, Ni and Co must be between trace amounts 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 that 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 limit of 0.05% is a tolerance.

Its B content is between traces and 0.1%. B is not obligatory, but its presence is advantageous for hardenability and forgeability of austenite. It therefore facilitates formatting. 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 for producing the 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 into the liquid steel, well-known process known as "AOD", or by a passage under vacuum in which the steel is decarburized by CO evolution, process known as "VOD") are indicated.

Its N content is between traces and 0.2% (2000 ppm). N is an impurity whose same treatments that make it possible to reduce the H content contribute to limiting the presence, or even substantially reducing it. 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, taken together 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, a good austenitization of the steel during the initial stage of the thermomechanical treatment is favored if one respects the relation (Mn + Ni) ≥ (Cr -10.3 - 80 x [(C + N) ² ]). Sufficient resilience is achieved if this condition is met in addition to the others that have been defined. A sufficient level of gammagenic elements is needed to counterbalance the alphagene effect of Cr and to ensure a correct austenitization of at least 80%, and from this point of view the efficiency of the sum C + N is not not linear.

Its Ca content is ≤ 0.002% (20 ppm).

Its total content of rare earths and Y 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 consists of iron and impurities resulting from the elaboration.

Other requirements on the composition of the steel relate to the martensitic transformation start temperature Ms and the end of martensitic transformation Mf. Ms should preferably be at most 400 ° C. If Ms is higher, there is a risk that the various operations of transfer and formatting of the part do not succeed each other quickly enough and that one does not have time to realize all the formatting to a temperature higher than Ms. However, this risk can be limited or avoided by providing that the room undergoes reheating or temperature maintenance between the shaping operations, and / or during the shaping if heated tools of known types incorporating for example, electrical resistors. This condition Ms≤ 400 ° C is not always imperative, but only recommended for an 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 the subsistence in the final part of a too high residual austenite content, which in particular would degrade Rp0,2 by raising it below 800 MPa.

Preferably, Ms is from 390 to 320 ° C.

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

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 that will be described can be performed either on a bare sheet which may possibly be subsequently coated, or on an already coated sheet, for example by an alloy based on AI and / or or Zn. This coating, typically of thickness 1 to 200 μηι and present on one or both sides of the sheet, may have been deposited by any technique conventionally used for this purpose, it is simply necessary that, if it was deposited before the austenitization, it does not evaporate during the stay of the sheet at the austenitization and deformation temperatures, and is not deteriorated during deformation.

The choice and the optimization of the characteristics of the coating and its mode of deposit so that these conditions are met do not go beyond what the skilled person knows when he is brought to shape in such a way. classic stainless steel sheets already coated. If the coating takes place prior to austenitization, however, AI-based coatings may be preferred over Zn-based coatings, since AI is less likely than Zn to evaporate at austenitization temperatures. . The method according to the invention is the following, applied to the manufacture and forming of a sheet.

In a first step, it is conventionally prepared an initial stainless steel sheet, bare or coated, having the composition which has just been described and a thickness which is typically from 0.1 to 10 mm. This preparation may include hot and / or cold processing operations and cutting of the semi-product resulting from the casting and solidification of the liquid steel. It is necessary that this initial sheet has a microstructure consisting of ferrite and / or martensite back and 0.5% to 5% by volume of carbides. The size of ferritic grains, measured according to standard NF EN ISO 643, is between 1 and 80 μηι, preferably between 5 and 40 μηι. A ferritic grain size of 40 μηι at most is recommended to promote the austenitization that will follow and thus obtain the desired 80% at least austenite. A ferritic grain size of at least 5 μηι is recommended to obtain a good capacity for cold forming.

The sheet is first austenitized by passing through a furnace that carries it in a temperature range greater than Ac1 (the onset temperature of the appearance of austenite), and therefore typically greater than about 850.degree. the compositions concerned). It should be understood that this austenitization temperature must concern the entire volume of the sheet, and that the treatment must be sufficiently long so that, given 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 specific feature of the invention. It must simply be such that the sheet remains in a completely solid state (the temperature must therefore be lower, in any case, at the solidus temperature of the steel) and is not too soft to withstand without damage the transfer between the oven and the shaping tool that will follow the austenitization. Also, the temperature should not be so high as to cause significant surface oxidation and / or decarburization of the sheet in the heating atmosphere. Superficial oxidation would lead to the necessity of descaling the sheet mechanically or chemically prior to shaping it to prevent encrustation of scale in the surface of the sheet, and would result in loss of material. Excessive decarburization (thickness of the decarburized surface ≥ 100 μηι) would reduce the hardness and 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 duration of austenitization, but also on the furnace treatment atmosphere. A non-oxidizing, therefore neutral or reducing, atmosphere (typically: argon, CO and their mixtures, etc.), preferably in air, makes it possible to increase 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 a furnace requiring a high residence time for austenitization, there is a risk of surface nitriding or recovery of hydrogen by the steel. This must be taken into account in the choice of 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 of between 925 and 1200 ° C. for a duration of 10 s to 1 h (this duration being that the sheet passes over Ac 1), preferably between 2 min and 10 min. min for heating in a conventional oven and between 30 s and 1 min for an induction furnace. An induction furnace has the advantage, known in itself, of providing rapid heating up to the nominal austenitization temperature. It therefore allows a treatment shorter than a conventional oven to achieve the desired result. These temperatures and times make it possible to ensure that the rest of the treatments will lead to a sufficient formation of martensite, and this for a reasonable duration allowing a good productivity of the process.

The purpose of this austenitization is to pass the metal of the initial ferrite + carbide structure to an austenitic structure containing at most 0.5% of carbides in volume fraction, and at most 20% of residual ferrite in volume fraction. One aim 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 and then the martensitic structure in the subsequent steps of the process. The maximum residual ferrite content of 20%, which must remain up to the final product, is justified by the resilience and the conventional yield strength that is desired.

The austenitized sheet is then transferred to a suitable shaping tool

(such as a stamping or embossing 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 greater than Ms and maintain an austenitic microstructure at a maximum of 0.5% of carbides and a maximum of 20% of residual ferrite. After this transfer, the sheet is at a temperature ΤΡ0, which is as close as possible to the nominal austenitization temperature for obvious reasons of energy saving.

A first step of shaping or cutting is then performed, of duration t1, typically between 0.1 and 10 s. The precise duration of this step (like those of the other steps) is not in itself a fundamental characteristic of the invention. It must be sufficiently short so that the temperature of the sheet does not fall below Ms, that one does not attend a decarburization and / or a significant oxidation of the surface of the sheet, and that an austenitic microstructure, at maximum 0.5% of carbides and 20% maximum of residual ferrite is always present at the end of the operation. If necessary, it is possible to use a shaping tool equipped with sheet heating means so that these temperature and microstructure conditions are fulfilled, since the contact of a non-heating shaping tool with the sheet causes a cooling of the sheet which is often greater than 100 ° C / s.

The absence of significant surface decarburization and oxidation can be achieved by adjusting the composition of the steel if necessary in the light of experience and, if possible, by maintaining a neutral or neutral atmosphere. reducer 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 shaped 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 both steps but by modifying its configuration in the interval (for example by replacing the punch in the case where a stamping is carried out in each of the two steps). The duration t 2 of this transfer is typically from 1 to 10 s, the aim being that it is fast enough for the sheet temperature to remain higher than Ms during the transfer and that the microstructure remains austenitic, at a maximum of 0.5%. of carbides and 20% maximum of residual ferrite.

The second formatting step, of duration t3, typically takes place between 0.1 and 10 s. The temperature of the sheet remains higher than Ms and the microstructure remains austenitic, with a maximum of 0.5% of carbides and a maximum of 20% of residual ferrite.

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

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

The average cooling rate between ΤΡ0 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 beginning and the end of the shaping operations which have just been described, combined with the composition of the steel and the procedure used during shaping, is that during In 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 going 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 it is 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 it possible to avoid the bainitic domain, especially the ferritic and pearlitic domains, on the usual shaping tools, and thus as complete as possible the transformation of austenite into martensite. But it must be remembered that, as has been said, each step taken individually must allow to retain an austenitic microstructure at maximum 0.5% of carbides and 20% maximum of residual ferrite. The duration / cooling rate pair of each step must therefore be chosen accordingly, and, if necessary, reheating of the sheet between and / or during shaping or cutting is performed so that this microstructure can be maintained during all steps.

Optionally, at least one further step of shaping can be carried out 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 presence sometimes already important of martensite.

Finally, the sheet is allowed to cool, for example in the open air, to room temperature, thus obtaining the final part according to the process 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 process. cooling down to ambient temperature, at least if no means are used which substantially slow down 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 accelerate this cooling, by means of pulsed air or a projection of water or other fluid.

The possibility of using at least two steps 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 using only a single shaping of the initial sheet does not achieve, in any case not with sufficient quality.

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

Also, a final heat treatment can be performed on the final part, therefore after cooling to 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, and then cooling naturally in the air.

The part thus obtained by the process according to the invention has high mechanical properties at ambient temperature, in particular because of 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 A measured according to ISO 6892 is at least 8%, and the bend angle capability for a thickness of 1.5 mm is at least 60 °, measured according to the VDA 238-100 standard.

FIG. 1 diagrammatically represents an exemplary 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 200 ° C, and comprising the following steps:

- Heating in a conventional roller oven 1 for 2 min of a sheet 2 of thickness 1, 5 mm, between the ambient temperature and a TPi temperature equal to 950 ° C;

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

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

First forming step (deformation), performed in the hot stamping tool 3 for a duration t1 of 0.5 s to obtain a shaped sheet metal 4; the temperature of the steel decreases to TP1 = 808 ° C; Transfer of the shaped sheet 4 between the hot stamping tool 3 and a drilling tool 5, for a duration t2 of 0.5 s; the temperature of the steel decreases to TP2 = 799 ° C;

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

Transfer of the shaped sheet 6 and pierced to a cutting tool 7 to perform a cutting of the edges of the sheet 6 to give them their final dimensions to obtain a product 8;

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

FIG. 2 diagrammatically represents another example of an operating diagram for a method according to the invention, executed 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 for 20 s of a sheet 2 of thickness 1.5 mm, between room temperature and a temperature TPi = 950 ° C .;

- Hold in the induction furnace 10 of the sheet 2 at said TPi temperature for a time 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 to ΤΡ0 = 941 ° C;

- First forming 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 to TP1 = 808 ° C;

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

Second forming step, consisting of a drilling performed in the drilling tool 5 for a duration t3 of 0.5 s to obtain a shaped and pierced sheet metal 6; the temperature of the steel decreases to TP3 = 667 ° C;

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

of piece 6 to give it its final dimensions and get a product

8, for a duration t5 of 0.5 s; the room temperature decreases until

TP5 = 525 ° C;

Execution of a blast 9 of the product 8 to optimize its resistance to fatigue or the adhesion of a possible future coating layer.

The processes of Figure 1 and Figure 2 therefore do not differ fundamentally. The only difference is that the induction furnace 10 allows faster heating and at a more steady speed than the conventional roller oven 1. The heating time and the holding time tm are thus shortened, which is advantageous for the productivity of the installation. Induction heating also more assuredly ensures homogeneity of the temperature of the sheet throughout its volume, which is advantageous for the success of the shaping steps and obtaining the final properties aimed.

Table 1 which follows shows the compositions of examples of steels to which the method according to the invention as described above and shown in Figure 1 has been applied.

The elements not mentioned are present only in the state of traces resulting from the elaboration.

Ex. C Mn P Si Si Al Ni Cr Cu Mo W Sn Nb Ta V Ti

%%% ppm%%%%%%% ppm%%%%

1 0.102 0.35 0.022 6 0.40 0.002 0.093 12.0 0.044 0.01 0.007 2 0.11 traces 0.10 0.007

2 0.06 0.51 0.019 40 0.38 0.002 0.41 11, 2 trace traces 1 trace 0.10 trace traces

3 0.06 0.51 0.019 40 0.38 0.002 0.41 11, 2 0.600 trace amounts 1 0.10 traces Traces traces

4 0.055 0.40 0.020 40 0.42 0.002 0.35 11, 3 0.030 0.20 0, 10 1 0.05 0.10 0.10 0.005

5 0.055 0.40 0.020 40 0.42 0.002 0.35 11, 3 0.030 0.20 0.50 1 0.10 traces 0.10 0.005

6 0.006 1, 14 0.016 9 0.38 0.030 0.88 12.0 0.01 1 0.001 traces 10 0.11 traces 0.027 0.077

7 0.14 0.45 0.020 5 0.35 0.001 0.40 11, 1 0.091 0.027 traces 320 0.14 trace amounts 0.004

8 0.10 0.30 0.020 10 0.40 0.002 3.0 12.0 0.020 1, 90 traces 50 0.10 traces 0.08 0.004

9 0.10 0.30 0.020 10 0.40 0.002 2.5 12.0 0.020 1, 65 traces 50 0.10 traces 0.08 0.004

10 0.10 0.30 0.020 10 0.40 0.002 2.5 12.0 0.020 1, 65 traces 50 0.10 0.50 0.08 0.004

1 1 0.10 0.30 0.020 10 0.40 0.002 2.0 12.0 0.020 1, 35 traces 50 0.10 traces 0.08 0.004

12 0.10 0.30 0.020 10 0.40 0.002 2.0 15.0 0.020 1, 35 traces 50 0.10 traces 0.08 0.004

13 0.10 0.30 0.020 10 0.40 0.002 1, 5 12.0 0.020 1, 05 traces 50 0.10 traces 0.08 0.004

14 0.10 0.30 0.020 10 0.40 0.002 1, 5 12.0 0.020 1, 05 0.60 50 0.10 traces 0.08 0.004

0.102 0.35 0.022 6 0.40 0.002 0.09 12.0 0.044 0.01 0.007 2 0.11 traces 0.10 0.007

16 0.055 0.40 0.020 40 0.42 0.002 0.35 11, 3 0.030 0.20 0.007 1 0.02 0.0002 0.10 0.50

17 0.395 0.34 0.027 18 0.28 0.002 0.19 14.1 0.044 0.63 traces 2 0.001 0.001 0.001 0.001

18 0.41 0.20 0.027 18 0.28 0.002 0.30 13.9 0.050 0.70 traces 2 0.001 0.001 0.001 0.001

19 0.125 0.41 0.033 10 0.36 0.001 0.29 12.32 0.1 10 0.02 trace amounts 0.006 traces 0.08 0.009

20 0.23 1, 16 0.017 6 0.28 0.057 0.025 0.17 0.025 0.006 traces 1 0.001 traces 0.004 0.04

21 0.23 0.37 0.024 10 0.51 traces 0.15 13.2 0.080 0.014 trace amounts 0.005 traces 0.077 0.004 22 0.461 0.33 0.024 19 0.33 traces 0.094 13.7 0.080 0.011 trace amounts 0.005 traces 0.077 0.004

23 0.345 0.377 0.024 15 0.29 traces 0.52 16.2 0.091 0.027 traces traces 0.005 traces 0.077 0.004

24 0.107 0.59 0.020 20 0.56 0.001 0.38 11, 4 0.091 0.027 traces 90 0.006 traces 0.077 0.001

25 0.009 0.20 0.030 100 0.55 0.030 0.20 11, 1 0.300 0.30 traces 500 0.001 trace traces 0.175

26 0.033 0.30 0.018 17 0.27 traces 0.12 16.1 0.023 0.004 traces 40 0.48 traces 0.08 0.007

27 0.009 0.25 0.018 17 0.58 traces 0.12 14.8 0.023 0.004 traces 40 0.48 traces 0.08 0.007

28 0.021 0.41 0.021 30 0.16 0.002 1.4 16.8 0.050 0.15 traces 20 0.003 traces 0.12 0.005

29 0.015 0.313 0.029 11 0.42 0.004 0.20 16.5 0.120 0.024 traces 67 0.15 traces 0.1 1 0.106

0.05 0.35 0.021 80 0.30 0.0006 0.12 12.6 0.031 0.01 traces 50 0.003 traces 0.097 0.007

31 0.0463 0.914 0.019 14 0.37 0.012 1, 61 13.9 0.073 0.027 traces 54 0.006 traces 0.108 0.362

32 0.10 0.30 0.020 10 0.40 0.002 3.1 12.0 1, 50 1, 90 traces 50 0.10 traces 0.08 0.004

33 0.10 0.30 0.020 10 0.40 0.002 2.5 12.0 0.020 1, 65 traces 50 0.50 0.70 0.03 0.004

Ex. Zr Co B N H 0 Ca Y (Mn + Ni) - Cu + Ni + 2xW (Nb + T Nb + Ta Ti + Zr + V

%% ppm ppm ppm ppm ppm ppm [Cr-10.3- + Co% a) / (C +%%

80 (C + N) ² ]% N)

%

1 0.005 0.019 4 250 0.2 10 5 0.1 0.03 0.156 0.024 0.835 0.106 0.112

2 traces 0.036 traces 100 <0.1 10 5 0.1 0.41 0.446 traces 1, 4289 0.100 traces

3 traces 0.036 traces 100 <0.1 10 5 0.1 0.41 1, 046 traces 1, 429 0.100 traces

4 0.002 0.036 traces 110 <0.1 10 5 0.1 0.10 0.416 0.400 2.273 0.150 0.107

5 0.002 0.036 traces 110 <0.1 10 5 0.1 0.10 0.416 1, 200 1, 515 0.100 0.107

6 traces 0.021 traces 110 0.2 10 5 0.1 0.38 0.912 0.001 6.471 0.110 0.104 c: 7 traces 0.037 traces 500 0.2 10 5 0.1 2.93 0.528 0.027 0.737 0.140 0.004

_c 8 traces 0.050 traces 1000 0.2 10 5 0.1 4.80 3.070 1, 900 0.500 0.100 0.084

9 traces 0.050 traces 1000 0.2 10 5 0.1 4.30 2.570 1, 650 0.500 0.100 0.084

10 traces 0.050 traces 1000 0.2 10 5 0.1 4.30 2.570 1, 650 3,000 0.600 0.084

11 traces 0.050 traces 1000 0.2 10 5 0.1 3.30 2.070 1, 350 0.500 0.100 0.084

12 traces 0.050 traces 1000 0.2 10 5 0.1 0.80 2.070 1, 350 0.500 0.100 0.084

13 traces 0.050 traces 1000 0.2 10 5 0.1 3.30 1, 570 1, 050 0.500 0.100 0.084

14 traces 0.050 traces 1000 0.2 10 5 0.1 3.30 1, 570 2.250 0.500 0.100 0.084

0.005 0.019 4 250 10 10 5 0.1 0.03 0.156 0.024 0.835 0.106 0.112

16 0.05 0.036 traces 110 <0.1 10 5 0.1 0.10 0.416 0.400 0.306 0.020 0.650

17 0.001 traces 4 900 0.2 10 5 0.1 15.56 0.234 0.644 0.004 0.002 0.003

18 0.001 traces 4 900 0.2 10 5 0.1 16.90 0.350 0.714 0.004 0.002 0.003

19 traces 0.020 traces 270 0.2 18 5 0.1 0.53 0.420 0.020 0.039 0.006 0.089

20 traces 0.008 2 37 0.2 10 5 0.1 15.68 0.058 0.006 0.004 0.001 0.044 c:

aj 21 traces 0.020 traces 200 0.2 10 5 0.1 2.62 0.250 0.014 0.020 0.005 0.081

•at)

• a) 22 traces 0.020 traces 298 0.2 10 5 0.1 16.27 0.194 0.011 0.010 0.005 0.081 rr

23 traces 0.020 traces 1600 0.2 10 5 0.1 15.40 0.634 0.027 0.010 0.005 0.081

24 traces 0.020 3 140 0.2 10 5 0.1 -0.10 0.491 0.027 0.193 0.006 0.079

25 traces 0.020 traces 180 0.2 10 5 0.1 -0.34 0.520 0.300 0.037 0.001 0.175

26 traces 0.018 traces 400 0.2 10 5 0.1 -5.00 0.161 0.004 6.575 0.480 0.087

27 traces 0.018 traces 186 0.2 10 5 0.1 -4.09 0.161 0.004 17.391 0.480 0.087

28 traces 0.046 traces 270 0.2 10 5 0.1 -4.49 1, 496 0.150 0.062 0.003 0.125 traces 0.026 5 244 0.2 10 5 0.1 -5.68 0.346 0.024 6.254 0.153 0.216 traces 0.016 2 200 0.2 10 5 0.1 -1, 47 0.167 0.010 0.043 0.003 0.104 traces 0.052 2 99 0.2 10 0.1 -0.87 1, 732 0.274 0.114 0.006 0.470 traces 0.50 traces 1000 0.2 10 5 0.1 4.90 5.100 1, 900 0.500 0.100 0.084 traces 0.050 traces 1000 0.2 10 5 0, 1 4.30 2.570 1, 650 6,000 1, 200 0.084

Table 1: Compositions of test samples

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

Ex. Metallurgical Structure Structure Final Piece

metallurgical intermediate Rm (MPa) / KCU

final Rp0.2 (MPa) (J / cm ² ) / angle of

/ A (%) bending (°)

1 MC = 0.1%; ferrite = 17% Martensite = 82.90% 1170/810 / 8.2 65/80

2 MC = 0.02%; ferrite = 3% Martensite = 96.98% 1130/850 / 9.4 75/100

3 MC = 0.02%; ferrite = 1% Martensite = 98.98% 1150/870 / 9.3 80/110

4 MC = 0.02%; ferrite = 5% Martensite = 94.98% 1145/840 / 9.1 75/110

MC = 0.02%; ferrite = 6% Martensite = 93.98% 1180/870 / 9.0 65/90

6 MC = 0.12%; ferrite = 19% Martensite = 80.88% 1175/810 / 9.7 70/80 o

^" c 7 MC = 0.05% ferrite = 1% Martensite = 98.95% 1550/1150 / 8.4 55/70

o

>

MC = 0.05%; ferrite = 0% Martensite = 88% 1570/1100 / 9.1 55/65

MC = 0.05%; ferrite = 0% Martensite = 95% 1560/11 10 / 9.0 55/70

MC = 0.05%; ferrite = 0% Martensite = 93% 1550/1130 / 9.3 60/80

11 MC = 0.05%; ferrite = 0% Martensite = 97% 1500/1050 / 9.5 70/80

MC = 0.1%; ferrite = 0% Martensite = 84% 1600/1200 / 8.5 60/70

MC = 0.05%; ferrite = 0% Martensite = 95% 1510/11 10 / 9.2 65/75

14 MC = 0.05%; ferrite = 0% Martensite = 98% 1200/810 / 8.1 30/55

MC = 0.05%; ferrite = 15% Martensite = 84.95% 1210/820 / 8.3 10/50

MC = 0.01%; ferrite = 80% Martensite = 19.99% 1010/700 / 10.5 60/70

MC = 0.35%; ferrite = 0% Martensite = 75% 1600/1000/4 30/10

MC = 0.40%; ferrite = 0% Martensite = 72% 1620/1010 / 4.2 30/15 w

o o 19 MC = 0.05%; ferrite = 15% Martensite = 84.95% 1230/900 / 9.0 25/70

ω

MC = 0%; ferrite = 0% Martensite = 100% 1500/1050 / 4.5 70/45

^■ S

this

MC = 0.1%; ferrite = 5% Martensite = 94.90% 1800/1100 / 7.5 30/50

22 MC = 1%; ferrite = 0% Martensite = 70% 1 00/1000 / 3.2 20/15

C = 1%; ferrite = 0% Martensite = 73% 1750/1050/3 25/10

MC = 0.01%; ferrite = 15% Martensite = 84.99% 950/720/6 90/130

MC = 0.01%; ferrite = 95% Martensite = 4.99% 460/320/27 80/130 MC = 0.01%; ferrite = 80% Martensite = 19.99% 750/520/12 50/110

MC = 0.01%; ferrite = 99.99% Martensite = 0% 540/350/29 80/130

MC = 0.005%; ferrite = 99.995% Martensite = 0% 800/700/13 60/120

MC = 0.003%; ferrite = 99.997% Martensite = 0% 500/350/20 65/120

MC = 0.01%; ferrite = 18% Martensite = 81, 99% 1000/780 / 8.5 20/105

31 MC = 0.01%; ferrite = 45% Martensite = 54.99% 850/650/5 40/90

32 MC = 0.05%; ferrite = 25% Martensite = 74.95% 1600/1050 / 8.5 35/65

33 MC = 0.05%; ferrite = 65% Martensite = 34.95% 900/700/12 60/110

Table 2: Intermediate and Final Metallurgical Structures and Final Mechanical Properties of the Examples in Table 1 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 desired objectives in terms of mechanical properties.

Of course, if a preferred application of the invention is the shaping of sheets for the automotive industry, it is not exclusive, and the sheets thus shaped can have any other application for which they are 'would prove advantageous, especially any structural function parts belonging to the fields aeronautics, building, railway.

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

Claims

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

a stainless steel sheet of composition is prepared, 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% ≤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 earth and / or Y ≤ 0.06%;

the rest being iron and impurities resulting from the elaboration; the martensitic transformation start temperature (Ms) of the sheet being≥

200 ° C;

the martensitic transformation end temperature (Mf) of the sheet being≥

50 ° C;

40 μηι;

a 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 t 2 during which the sheet metal is cut. 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;

a second stage of shaping or cutting of the sheet is carried out for a time t3, 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; optionally, other steps are performed for transferring the cut or shaped sheet metal to other cutting or forming tools, or to modifying the configuration of the forming or cutting tool used. in the preceding step, each being followed by a shaping or cutting step, the sheet 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 during each of said steps of transferring the sheet or modifying the configuration of the tool and each of the shaping or cutting operations;

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

3. - Process 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 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 claims 1 to 4, characterized in that the austenitization temperature is at least 850 ° C.

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

7.- Method according to one of claims 1 to 6, characterized in that performs a reheating of the sheet during at least one of the transfer steps and / or configuration change of the tool or steps of forming or cutting the sheet.

8. - Method according to one of claims 1 to 7, characterized in that performs a surface treatment on the final part, intended to increase its roughness or fatigue properties.

9. - Method according to one of claims 1 to 8, characterized in that the final room is held between 90 and 500 ° C for 10 s to 1 h, and then allowed to cool naturally in air.