EP2245203B1 - Austenitic stainless steel sheet and method for obtaining this sheet - Google Patents

Description

The invention relates to stainless steel sheets having high mechanical properties and good resistance to corrosion in order to be intended in particular for the manufacture of automotive parts, such as structural parts or engine head gaskets.

It is stated, first of all, that the stainless steels considered here are in the sense given to this expression by the ISO 6929 standard, ie steels containing at least 10.5% by weight of chromium and not more than 1 , 2% by weight of carbon.

The growing demand to improve vehicle safety, coupled with the need to reduce carbon dioxide emissions, is prompting automakers to look for materials with ever-increasing mechanical characteristics. Among the desired qualities for the "steel" material, mention will in particular be made of the mechanical strength, the resistance to corrosion, to fatigue, the properties of deformability, of weldability. It is according to the purpose of use of the steel, that is to say of the functional part in which it will be transformed in the end, that some of these mechanical characteristics will be, more than others, favored by the steelmaker. The aim of the latter is now to adapt the steel it produces both to the mode of solicitation to which the final piece in service will be subjected, as the constraints related to the very manufacture of this piece by transformation of a blank resulting from the solidification of the metal.

With regard to the thermal engine parts to which the invention relates more particularly, it is generally used for their manufacture of austenitic steels. These are alloy steels containing chromium, nickel, manganese, nitrogen, carbon and optionally copper and molybdenum, in order to produce an austenitic microstructure, which has the advantage of presenting a large crystalline mesh for the iron (cubic face-centered), which increases the solubility of the various elements of alloys in iron, including carbon.

It turns out that conventional austenitic stainless steels are characterized by relatively modest mechanical properties in the annealed state. Indeed, unlike martensitic steels that take quenching, they do not harden significantly by heat treatment. To achieve sufficient mechanical strength for their use in the automotive industry, the austenitic stainless steels can be cold rolled, due to a martensitic transformation induced by the deformation. Depending on the reduction in thickness achieved, different levels of mechanical strength can be reached up to very high values (Rm = 1500MPa). However, the use of these hardened products poses several problems, on the one hand the cost associated with the additional rolling operation compared to an annealed product, on the other hand the low elongation capacities and the planar anisotropy. This is why solutions in the annealed state are sought.

The usual process for producing austenitic stainless steels is as follows: after hot rolling of a strip followed by annealing, a cold rolling is carried out whose rate depends on the final characteristics concerned. The steel then has good mechanical strength, but its ductility is too low, especially for its subsequent shaping. To overcome this, it is subjected to a final recrystallization treatment in the form of an annealing furnace, that is to say a heating with temperature maintenance the time required for complete recrystallization before controlled cooling.

The main purpose of annealing is to put the metal in a structural state close to the state of equilibrium. In short, the internal energy accumulated during the cold-working is evacuated. In fact, a recrystallization annealing will use this internal energy differential to promote the germination of new metal grains and their growth. It is understood that the greater the internal energy increase due to hardening is important, the more likely there will be new seeds during the annealing, and thus a small final grain size. Also, is it advantageous to perform a strong work hardening prior to annealing.

The recrystallization temperature is also an important parameter for controlling the final grain size since the grain boundary mobility increases with temperature. It is therefore recommended to lower the annealing temperature to obtain a fine grained structure.

However, the heating conventionally used during the recrystallization annealing is also a quenching, ie it must exceed the solvus of the chromium carbides to put in solution all the carbon in the austenite. The objective of this step is to avoid any risk of localized corrosion caused by decrepit areas around chromium carbides. The solution temperature of the chromium carbides thus constitutes a limit to the decrease of the annealing temperature in order to refine the microstructure. This limit depends on the chemical composition and mainly the carbon content.

An equilibrium has been found in the prior art using steels having low carbon contents, which makes it possible to lower the solvus of chromium carbides and to delay the kinetics of precipitation. As can be seen on the figure 1 with a heating rate of about 20 ° C / s, represented by the curve Vc, and a carbon content of less than 0.05%, the total recrystallization temperature Tc is reached without entering the precipitation area A ₁ chromium carbides relative to those steels with a C content of less than 0.05% C.

The mechanical strength of the steel can be further improved by hardening after this heat treatment. However, in order to better meet the demands of the automotive industry, it should nowadays be possible to further improve the mechanical strength of such steels beyond the limits imposed by the conventionally used routes. This is the reason why he was tempted to increase the carbon content. But to date, to the knowledge of the Applicant, all tests of grain size refinement have failed, resulting in a high precipitation of chromium carbides caused by the lowering of the annealing temperature.

EP-A-1739200 discloses an austenitic stainless steel strip having an elastic limit Rp0.2 greater than or equal to 600 MPa, a tensile strength Rm greater than or equal to 800 MPa, an elongation A80 of greater than or equal to 40%, the composition of which comprises % by weight: 0.025 ≤ C ≤ 0.15%; 0.20 ≤ If ≤ 1.0%; 0.50 ≤ Mn ≤ 2.0%; 6.0 ≤ Ni ≤ 12.0%; 16.0 ≤ Cr ≤ 20.0%; Mo ≤ 3.0%; 0.030 ≤ N ≤ 0.160%; Cu ≤ 0.50%; P ≤ 0.50%; S ≤ 0.015%; possibly 0.10 ≤ V ≤ 0.50%, and 0.03 ≤ Nb ≤ 0.50% with 0.10% ≤ Nb + V ≤ 0.50%, the balance being iron and any impurities resulting from the preparation, the average size of the austenite grains is less than or equal to 4 m, and the surface has a gloss greater than 50. This steel is only partially recrystallized, at a rate of 60-75%. Its cooling after the partial recrystallization treatment is carried out continuously. The presence of Cr carbides at the grain boundaries is considered to be avoided.

GB-A-473,331 describes treatments carried out on austenitic stainless steels in order to limit their sensitivity to intergranular corrosion. For this purpose, the precipitation of Cr carbides is carried out intragranularly, in particular by reheating at a temperature below the recrystallization temperature, and for a period of time sufficient to precipitate all excess C and redistribute the Cr homogeneously without causing significant recrystallization.

FR-A-2,864,108 discloses an austenitic stainless steel of composition 0.025% ≤ C ≤ 0.05%; 0.3% ≤ If ≤ 1%; 1% ≤ Mn ≤ 2%; 15% ≤ Cr ≤ 18.5%; 5% ≤ Ni ≤ 10%; Mo ≤ 3%; 0.1% ≤ N ≤ 0.16%; Cu ≤ 0.5%; P ≤ 0.5%; S ≤ 0.015%; optionally 0.1% ≤ V ≤ 0.5% and 0.1% ≤ Nb ≤ 0.5%, with 0.1% ≤ Nb + Ti + V ≤ 0.5%, the balance being iron and possible impurities resulting from the preparation, and whose microstructure is an essentially austenitic microstructure comprising from 0.1 to 0.2% by volume of martensitic islands distributed homogeneously. The targeted mechanical properties are achieved thanks to only partial recrystallization. Cooling after recrystallization is carried out continuously

The object of the invention is to provide an answer to this problem which has not yet been solved by means of a steel with a very fine austenitic microstructure, whose carbon content significantly increased compared with the practice of the prior art, makes it possible to obtain increased mechanical strength together with very good corrosion resistance.

For this purpose, the invention relates to a stainless steel sheet whose composition comprises, the contents being expressed by weight: 0.05% ≤ C ≤ 0.30%, 0.3% ≤ Si ≤ 1%, 0 , 5% ≤ Mn ≤ 3%, 4% ≤ Ni ≤ 10%, 15% ≤ Cr ≤ 20%, N ≤ 0.2%, P ≤ 0.05%, S ≤ 0.015%, optionally 0.1 ≤ V ≤ 0.5%, optionally Mo ≤ 3%, optionally Cu ≤ 0.5%, the rest of the composition being constituted of iron and unavoidable impurities resulting from the preparation, the microstructure of the steel being essentially austenitic completely recrystallized, the average size of the austenite grains being less than 2 micrometers, the sheet containing chromium carbides precipitated at the joints of austenitic grains for over 90% of them.

The composition preferably comprises, the contents being expressed by weight: 0.09% ≤ C ≤ 0.30%.

Preferentially, the composition comprises, the contents being expressed by weight: 16% ≤ Cr ≤ 18%.

• on approvisionne un acier de composition selon l'une quelconque des compositions ci-dessus, puis
• on coule l'acier sous forme de brame, puis
• on lamine à chaud la brame pour obtenir une tôle laminée à chaud, puis
• on recuit la tôle laminée à chaud à une température supérieure à 1000°C, puis
• on décape la tôle laminée à chaud, puis
• on lamine à froid la tôle laminée à chaud, à un taux de réduction supérieur à 40 %, puis
• on effectue un traitement thermique de recristallisation totale sur la tôle laminée à froid, le traitement thermique comprenant une phase de chauffage rapide, à une vitesse Vc comprise entre 50 et 800 °C/s jusqu'à une température comprise entre Tc et Tc+50°C, Tc désignant la température de recristallisation totale, de façon à obtenir une tôle chauffée et totalement recristallisée, puis
• on refroidit la tôle chauffée et totalement recristallisée, à une vitesse supérieure à 50°C/s jusqu'à une température Tm d'environ 750 °C, puis
• on maintient la tôle à la température Tm durant une durée comprise entre 1 et 100s afin d'obtenir une précipitation de carbures de chrome, puis
• on refroidit la tôle jusqu'à la température ambiante.

The invention also relates to a method for manufacturing a stainless steel sheet, according to which:

• a composition steel is supplied according to any one of the above compositions, and then
• the steel is cast as a slab, then
• the slab is hot rolled to obtain a hot rolled sheet, then
• the hot-rolled sheet is annealed at a temperature above 1000 ° C., and then
• the hot-rolled sheet is scoured, then
• the hot-rolled sheet is cold-rolled at a reduction rate greater than 40%, and then
• a total recrystallization heat treatment is carried out on the cold-rolled sheet, the heat treatment comprising a rapid heating phase, at a speed Vc of between 50 and 800 ° C./s up to a temperature of between Tc and Tc + 50 ° C, Tc denoting the total recrystallization temperature, so as to obtain a heated sheet and completely recrystallized, then
• the heated and completely recrystallized sheet is cooled at a speed greater than 50 ° C./s up to a temperature Tm of approximately 750 ° C., and then
• the sheet is maintained at the temperature Tm for a period of between 1 and 100 seconds in order to obtain a precipitation of chromium carbides, then
• the sheet is cooled to room temperature.

Preferably, the rapid heating is up to a temperature greater than 800 ° C and less than or equal to 900 ° C.

According to a preferred embodiment of the invention, once the heat treatment is complete, the cooled sheet is subjected to a cold deformation operation capable of generating the appearance of martensite within the steel structure.

The rapid heating is preferably carried out by electromagnetic induction.

Depending on the composition, especially the carbon content, the resistance may vary between about 1000 and 1600 MPa.

The invention also relates to a stainless steel sheet manufactured by the manufacturing method above.

The invention also relates to a mechanical stainless steel part obtained from a sheet made by the manufacturing process above. The invention also relates to the use of a sheet obtained by the above manufacturing process for the manufacture of structural parts for automobiles.

The invention also relates to the use of a sheet obtained by the above manufacturing method for the manufacture of motor cylinder head gaskets.

• La figure 1 est un diagramme représentant le chauffage d'un acier austénitique à teneur en carbone inférieure à 0,05% (dont le domaine A₁ de précipitation des carbures de chrome a été représenté) ou à teneur en carbone plus élevée (domaine de précipitation A₂) lors d'un recuit de recristallisation avec une vitesse de chauffage V_C selon l'art antérieur.
• La figure 2 est un diagramme similaire illustrant un mode de réalisation selon l'invention avec un recuit de recristallisation totale suivi d'une déstabilisation de la structure. Le domaine A de précipitation des carbures de chrome a été également figuré ainsi que la température de recristallisation totale Tc .

The invention will be better understood and other aspects and advantages will emerge more clearly on reading the following detailed description of an exemplary embodiment given with reference to the appended figures, in which:

• The figure 1 is a diagram showing the heating of a carbon steel austenitic less than 0.05% (which has a chromium carbide precipitating region A ₁ ) or a higher carbon content (A ₂ precipitation domain) ) during a recrystallization annealing with a heating rate V _C according to the prior art.
• The figure 2 is a similar diagram illustrating an embodiment according to the invention with a total recrystallization annealing followed by a destabilization of the structure. Domain A precipitation of chromium carbides was also included as well as the total recrystallization temperature Tc.

As will have already been understood, the invention essentially consists of a new sheet of austenitic stainless steel with very fine grains, having a significant carbon content, greater than 0.05 or 0.09%, and in a new process for obtaining a sheet from this steel which offsets the undesired effects of this increase in the carbon content by a very rapid heating annealing to quickly reach the recrystallization temperature.

As we have seen above, the main problem raised by the recrystallization annealing of austenitic stainless steel is that it can proceed to recrystallization without the precipitation of chromium carbides. On the one hand, these carbides are detrimental to the corrosion resistance of the steel, but they also prevent the recrystallization from starting. Now, as we can see on the figure 1 when the carbon content is increased, the nose of the precipitation zone of these carbides will shift to the left: the domain A ₁ is relative to steels with steels less than 0.05% C, the domain A ₂ to steels with higher carbon content. Carbides will form more easily and therefore faster. One solution would be to heat the steel at temperatures beyond that zone and hold it there until the carbides recoat. Unfortunately, the temperatures to achieve to achieve this are such that the time elapsed and the mobility of the grain boundaries do not allow then to obtain a fine grain.

Thus, when he increases the carbon content of steel to increase the mechanical strength, the skilled person is therefore to have to choose between good corrosion resistance or good fatigue resistance, through a fine grain and a high mechanical strength, while he legitimately wants to get both.

Surprisingly, the present inventors have discovered that it is possible to obtain a homogeneous and complete recrystallization or quenching of the steel before the chromium carbides precipitate, and this for carbon contents of up to at 0.3%, or even a little beyond. This could be achieved by increasing the heating rate above 50 ° C / s, although the total recrystallization temperature Tc increases with heating, which increases the risk of reaching the carbide precipitation zone.

For the sake of clarity, with conventional furnace heating rates of the order of 20 ° C / s, the maximum allowable carbon contents to obtain recrystallization and avoid precipitation of the carbides would be around 0.07 to 0, 08% on average. A maximum of 0,15% C even could have been reached sometimes by certain nuances.

• du carbone à une teneur comprise entre 0,05 et 0,30 % en poids. Si la teneur en C est inférieure à 0,05%, la résistance mécanique est insuffisante. Une teneur en carbone supérieure ou égale à 0,09% se prête particulièrement bien au procédé décrit selon la figure 2. En revanche si la teneur est supérieure à 0,30 %, les efforts de laminage à froid sont considérablement augmentés ce qui réduit la gamme dimensionnelle accessible.
• du silicium à une teneur comprise entre 0,3 et 1 % en poids. Le silicium est utilisé à titre de désoxydant de l'acier liquide. En outre, il participe au durcissement en solution solide et diminue l'énergie de faute d'empilement qui contrôle en partie la transformation martensitique induite par la déformation. On limite sa teneur à 1 % en poids car il a tendance à perturber le procédé de fabrication de la tôle d'acier en posant des problèmes de ségrégation pendant la coulée en brame d'acier;
• du manganèse à une teneur comprise entre 0,5 et 3%. Le manganèse favorise la formation d'austénite et augmente la solubilité de l'azote dans l'austénite. Pour une teneur inférieure à 0,5%, le manganèse ne peut plus piéger le soufre sous forme de MnS et la forgeabilité à chaud se dégrade, causant des défauts de surface sur les bandes laminées à chaud. Au delà de 3%, ces effets sont saturés.
• du chrome à une teneur comprise entre 15 et 20 %. Le chrome favorise la formation de martensite de déformation, et est un élément essentiel pour conférer à l'acier une bonne résistance à la corrosion. Si la teneur en chrome est inférieure à 15 %, la résistance à la corrosion sera insuffisante; si la teneur en chrome dépasse 20%, la fraction de ferrite pendant le laminage à chaud devient trop importante et peut conduire à la formation de criques de rives. Ces différents effets sont obtenus de façon stable dans une gamme préférentielle de 16 à 18% de chrome.
• du nickel à une teneur comprise entre 4 et 10 %. Le nickel stabilise l'austénite et favorise la re-passivation de l'acier. Il s'agit de la formation à la surface de l'acier d'un film protecteur très mince et de faible perméabilité ionique. Si la teneur en nickel est inférieure à 5 %, la résistance à la corrosion de l'acier est insuffisante. Si la teneur en nickel est supérieure à 10 %, l'austénite se surstabilise. On ne forme alors plus suffisamment de martensite de déformation et les caractéristiques de l'acier sont insuffisantes;
• de l'azote à une teneur inférieure ou égale à 0,2 %. En plus de son action en faveur de la formation d'austénite, l'azote retarde la précipitation des carbures de chrome. Au delà de 0,2 %, il risque de détériorer la ductilité à chaud de l'acier;
• du phosphore à une teneur inférieure ou égale à 0,05 %. Le phosphore est un élément ségrégeant aisément. Il favorise le durcissement en solution solide de l'acier, cependant sa teneur doit être limitée à 0,05 % car il augmente la fragilité de l'acier et diminue son aptitude au soudage;
• du soufre à une teneur inférieure ou égale à 0,015 %. Le soufre est également un élément qui ségrége, dont la teneur doit être limitée afin d'éviter les fissures lors du laminage à chaud.

To obtain a steel sheet according to the invention, it is first necessary to develop and then flow in the form of a slab, a stainless steel of composition as defined below, which comprises:

• carbon at a content of between 0.05 and 0.30% by weight. If the C content is less than 0.05%, the mechanical strength is insufficient. A carbon content greater than or equal to 0.09% is particularly suitable for the process described according to figure 2 . On the other hand, if the content is greater than 0.30%, the cold rolling forces are considerably increased, which reduces the accessible dimensional range.
• silicon at a content of between 0.3 and 1% by weight. Silicon is used as a deoxidizer for liquid steel. In addition, it participates in hardening in solid solution and reduces the stacking fault energy which partly controls the martensitic transformation induced by the deformation. Its content is limited to 1% by weight because it has a tendency to disturb the manufacturing process of the steel sheet by posing problems of segregation during the casting of steel slab;
• manganese at a content between 0.5 and 3%. Manganese promotes the formation of austenite and increases the solubility of nitrogen in austenite. At less than 0.5%, manganese can no longer trap sulfur as MnS and hot forgeability degrades, causing surface defects on hot-rolled strip. Beyond 3%, these effects are saturated.
• chromium at a content of between 15 and 20%. Chromium promotes the formation of deformation martensite, and is an essential element in giving steel good corrosion resistance. If the chromium content is less than 15%, the corrosion resistance will be insufficient; if the content chromium exceeds 20%, the fraction of ferrite during hot rolling becomes too large and can lead to the formation of bank cracks. These different effects are stably obtained in a preferred range of 16 to 18% chromium.
• nickel at a content of between 4 and 10%. Nickel stabilizes the austenite and promotes the re-passivation of the steel. This is the formation on the steel surface of a very thin protective film with low ionic permeability. If the nickel content is less than 5%, the corrosion resistance of the steel is insufficient. If the nickel content is greater than 10%, the austenite is over-stabilized. The formation of deformation martensite is then no longer sufficient and the characteristics of the steel are insufficient;
• nitrogen at a level not exceeding 0,2%. In addition to its action in favor of the formation of austenite, nitrogen delays the precipitation of chromium carbides. Beyond 0.2%, it may deteriorate the hot ductility of the steel;
• phosphorus at a level not exceeding 0,05%. Phosphorus is an easily segregating element. It promotes the solid solution hardening of steel, however its content must be limited to 0.05% because it increases the fragility of steel and decreases its weldability;
• sulfur at a level of less than or equal to 0.015%. Sulfur is also a segregating element whose content must be limited in order to avoid cracks during hot rolling.

• du vanadium à une teneur comprise entre 0,1 et 0,5 %. Le vanadium favorise la soudabilité de l'acier et freine la croissance des grains d'austénite dans la zone affectée par la chaleur. Au delà de 0,5 %, le vanadium ne contribue pas à l'amélioration de la soudabilité, et en dessous de 0,1 %, la soudabilité de l'acier n'est pas améliorée.
• du cuivre à une teneur inférieure ou égale à 0,5 %. Le cuivre favorise la formation d'austénite et contribue à la résistance contre la corrosion. Cependant, au delà d'une teneur de 0,5 %, l'austénite devient trop stable à température ambiante et la transformation martensitique par déformation est inhibée.
• du molybdène à une teneur inférieure ou égale à 3 %. Le molybdène favorise la formation de martensite de déformation et augmente la résistance à la corrosion, surtout s'il est combiné avec l'azote. Au delà de 3 %, la résistance à la corrosion de l'acier n'est plus améliorée et le durcissement à haute température rend le laminage à chaud trop difficile.

In addition, the composition may optionally include:

• vanadium at a content of between 0.1 and 0.5%. Vanadium promotes the weldability of steel and inhibits the growth of austenite grains in the heat affected zone. Beyond 0.5%, vanadium does not contribute to the improvement of the weldability, and below 0.1%, the weldability of the steel is not improved.
• copper at a level not exceeding 0.5%. Copper promotes the formation of austenite and contributes to resistance against corrosion. However, beyond a content of 0.5%, the austenite becomes too stable at room temperature and martensitic transformation by deformation is inhibited.
• molybdenum at a content of not more than 3%. Molybdenum promotes the formation of deformation martensite and increases corrosion resistance, especially when combined with nitrogen. Beyond 3%, the corrosion resistance of the steel is no longer improved and hardening at high temperature makes hot rolling too difficult.

The balance of the composition consists of iron and other elements usually expected to be found as impurities resulting from the processing of stainless steel, in proportions that do not affect the properties sought.

Once the slab is cast, it is hot rolled in a strip train to form a hot rolled sheet. This is annealed at a temperature above 1000 ° C in order to allow subsequent cold rolling. The sheet is then etched by a method known per se.

The hot rolled sheet is then cold rolled at room temperature at a reduction rate of greater than 40%.

This rolling will generate many dislocations within the steel. It will even form martensite (called martensite deformation) which is in the form of slats. These microstructural evolutions will increase the internal energy of steel. The increase in temperature during the heat treatment that will follow, will allow to bring the metal back to thermodynamic equilibrium.

When the work-hardening is sufficient, the force of return to equilibrium will allow the germination of new grains and their growth. Thus, the earlier the hardening has been important, the more fine grain will be obtained. This is why a reduction rate of less than 40% is insufficient to give the stainless steel according to the invention the required characteristics.

Finally, the cold-rolled sheet undergoes heat treatment so as to give the stainless steel a totally recrystallized structure.

The heat treatment according to the invention consists in subjecting the cold-rolled steel sheet to a total recrystallization annealing comprising, in a first step, a rapid heating phase at a speed of between 50 and 800 ° C./s. to reach a temperature between Tc and Tc + 50 ° C. We preferentially perform rapid heating at a temperature greater than 800 ° C and less than or equal to 900 ° C.

This temperature must be reached before the onset of precipitation of chromium carbides. After cooling under the conditions according to the invention, an ultra-fine austenitic grain having an average size of less than 2 microns is obtained.

Indeed, obtaining a fine grain does not only depend on the rate of preliminary hardening, but also the annealing conditions (temperature and hold time). It should be noted that the higher the carbon content of the steel, the higher the heating rate must be. Thus, for a carbon content of 0.05% can be satisfied with a heating rate of the order of 50 ° C / s, but it is necessary to reach 200 ° C / s when the carbon content is around 0.2%. For a carbon content of the order of 0.09-0.1%, the heating rate should reach about 100 ° C / s.

According to the invention, such a heating rate is achieved by the use of an electromagnetic induction heating device. Proper implementation of such a device, in particular by the choice of the frequency of the electric excitation current, makes it possible to obtain temperatures so high that it is no longer even necessary to provide a maintenance phase of homogenization as can be seen on the figure 2 .

Since the recrystallization temperature is reached faster than before, an advantage of the process according to the invention is that there is less loss of internal energy during the heating phase. It therefore becomes possible to obtain the same fineness of grain for a lower work hardening rate than in the past.

Although the increase in carbon content already allows itself to obtain high strength characteristics, it is still possible to improve them.

After the total recrystallization heat treatment, the sheet is then cooled in stages, as shown in FIG. figure 2 a first cooling is carried out at a speed greater than 50 ° C./s so as to be placed in the vicinity of the precipitation nose under isothermal conditions. This first cooling is carried out for example, up to a temperature Tm of about 750 ° C., that is to say between 700 and 800 ° C, which is maintained for a period of between 1 and 100 seconds. Then, the sheet is cooled to room temperature. In this way, the chromium carbides will predominantly precipitate, that is to say for more than 90% of them, at the austenitic grain boundaries. This precipitation after austenitization will destabilize the structure and increase the final mechanical characteristics of the steel. Indeed, chromium carbides predominantly precipitating at austenitic grain boundaries, and the latter being very thin (their average size is less than 2 micrometers), there is less risk at this level to deteriorate the resistance to intergranular corrosion.

Finally, it is also possible to subject the sheet to additional cold deformation, in particular by rolling, after the recrystallization treatment. This final plastic deformation will make it possible to transform part of the austenite into martensite deformation and to further increase the mechanical strength.

The invention will be particularly useful for the manufacture of motor cylinder head gaskets, which require a high yield strength and good resistance to fatigue and corrosion.

It goes without saying that the invention can not be limited to the examples explained herein, but that it extends to multiple variants or equivalents to the extent that its definition given in the appended claims is respected.

Claims (11)

1. Stainless steel sheet, the composition of which comprises, the contents being by weight: $$0.05\%\le \mathrm{C}\le 0.30\%$$

   

   $$0.3\%\le \mathrm{Si}\le 1\%$$

   

   $$0.5\%\le \mathrm{Mn}\le 3\%$$

   

   $$4\%\le \mathrm{Ni}\le 10\%$$

   

   $$15\%\le \mathrm{Cr}\le 20\%$$

   

   $$\mathrm{N}\le 0.2\%$$

   

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

   

   $$\mathrm{S}\le 0.015\%$$

   

   optionally 0.1 ≤ V ≤ 0.5%

   optionally Mo ≤ 3%

   optionally Cu ≤ 0.5%,

   the remainder of the composition being composed of iron and unavoidable impurities resulting from the production process, the microstructure of said steel being substantially totally recrystallised austenitic, the average size of the austenite grains being less than 2 micrometres, said sheet containing chromium carbides of which more than 90% are precipitated at the boundaries of said austenitic grains.
2. Steel sheet according to claim 1, characterised in that its composition comprises, the contents being by weight $$0.09\%\le \mathrm{C}\le 0.30\%.$$

   
3. Steel sheet according to claim 1 or 2, characterised in that its composition comprises, the contents being by weight $$16\%\le \mathrm{Cr}\le 18\%.$$

   
4. Process for manufacturing a stainless steel sheet, in which:

   - a steel having a composition according to any one of claims 1 to 3 is provided, then

   - the steel is cast in the form of a slab, then

   - said slab is hot-rolled to obtain a hot-rolled sheet, then

   - said hot-rolled sheet is annealed at a temperature greater than 1000°C, then

   - said hot-rolled sheet is pickled, then

   - said hot-rolled sheet is cold-rolled, with a rate of reduction greater than 40%, then

   - a heat treatment for total recrystallisation is carried out on said cold-rolled sheet, said heat treatment comprising a phase of rapid heating, at a rate Vc of between 50 and 800°C/s to a temperature of between Tc and Tc+50°C, Tc denoting the total recrystallisation temperature, so as to obtain a heated and totally recrystallised sheet, then

   - said heated and totally recrystallised sheet is cooled at a rate greater than 50°C/s to a temperature Tm of approximately 750°C, then

   - said sheet is maintained at said temperature Tm for a period of between 1 and 100 seconds in order to obtain chromium carbide precipitation, then

   - said sheet is cooled to ambient temperature.
5. Process according to claim 4, characterised in that said rapid heating is carried out to a temperature greater than 800°C and less than or equal to 900°C.
6. Process according to either claim 4 or claim 5, characterised in that, once said heat treatment is complete, said cooled sheet is subjected to a cold deformation operation capable of generating the appearance of martensite within the structure of the steel.
7. Process according to any one of claims 4 to 6, characterised in that said rapid heating is carried out by electromagnetic induction.
8. Stainless steel sheet obtained by the manufacturing process according to any one of claims 4 to 7.
9. Mechanical part made of stainless steel obtained from a sheet according to claim 8.
10. Use of a sheet according to claim 8 for the manufacture of structural parts for motor vehicles.
11. Use of a sheet according to claim 8 for the manufacture of engine cylinder head gaskets.

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