EP1443200A1 - A method for producing a piston for an internal combustion engine and a piston produced by the method - Google Patents

Abstract

The fabrication of a monolithic piston for an internal combustion engine involves reheating of a blank to an intermediate temperature between the solidus and liquidus temperatures and forming by thixotropic forging. An independent claim is also included for a monolithic piston fabricated by this method.

Description

The invention relates to the field of internal combustion engine pistons, especially for motor vehicles, trucks, agricultural machinery, machinery public works, boats.

In recent years, high performance internal combustion engines have been developed, in particular with high specific powers to comply with the new and future anti-pollution standards on CO ₂ emissions. This is particularly true in the case of diesel engines. This increase in specific powers leads to a very significant increase in the thermal and mechanical stresses to which the engine parts are subjected, and in particular the pistons. These, therefore, are becoming increasingly complex in design.

Usually the pistons are made of a single piece of alloy cast or forged aluminum. But the increased solicitation conditions which we just mentioned make the classic pistons unsuitable. Consequently, we have imagined various solutions to make the aluminum pistons compatible with high performance motors: insertion of alumina fibers in the alloy to strengthen it, addition of steel inserts to reduce expansion, deposit of graphite on the skirt to reduce friction, or machining of cooling channels to circulate air or oil through them maintain the piston at acceptable operating temperatures. But all of these solutions are expensive.

A possible solution could be the replacement of the alloy aluminum by a steel which, with comparable geometry, would have a better resistance to mechanical and thermal stresses and to fatigue and better resistance to temperature. In fact, steel was once used for make pistons but the use of steel to make pistons high efficiency motors is in fact not economically feasible at at first sight, due to the high density of this material. If we wanted to confer the piston a mass low enough to obtain high engine performance, very small wall thickness should be achieved after forging the piston. Such thickness is inaccessible by processes conventional forging if, for cost reasons, we want to continue making pistons in one piece.

The object of the invention is to make possible the manufacture, in economically advantageous conditions of pistons for internal combustion engines high performance, in particular by making it possible to use a steel for this purpose, or another dense alloy with high mechanical properties, instead of an alloy specially treated and / or shaped aluminum.

To this end, the invention relates to a process for manufacturing a piston for an internal combustion engine, said piston being formed of a metal part monobloc, characterized in that a piece of heating is carried out for bring it to an intermediate temperature between its solidus temperature and its liquidus temperature, and in that it is shaped by Thixoforging.

The invention also relates to an internal combustion engine piston, composed of a single piece of metal, characterized in that it has been manufactured by heating a piece to bring it to an intermediate temperature between its solidus temperature and its liquidus temperature, followed by setting form by thixoforgeage.

In an exemplary embodiment, its legs are constituted by calipers formed on the bottom of the interior cavity of the piston, provided with an orifice for the passage of the axis securing the piston and the connecting rod, and it has on its skirt of the recesses giving access to the holes of the stirrups.

The shape of the piston bottom wall can match that of the surface of the piston bottom on its side intended to be turned towards the chamber combustion.

The piston may include reinforcing ribs.

The piston can be made of carbon steel.

• 0,35% ≤ C ≤ 1,2%
• 0,10% ≤ Mn ≤ 2,0%
• 0,10% ≤ Si ≤ 1,0%
• traces ≤ Cr ≤ 4,5%
• traces ≤ Mo ≤ 2,0%
• traces < Ni ≤ 4,5%
• traces ≤ V ≤ 0,5%
• traces ≤ Cu ≤ 3,5%
• traces ≤ Al ≤ 0,060%
• traces ≤ Ca ≤ 0,050%
• traces ≤ B ≤ 100ppm
• traces ≤ Ti ≤ 0,050%
• traces ≤ Nb ≤ 0,050%

les autres éléments étant du fer et des impuretés classiques résultant de l'élaboration.Its composition can then be in weight percentages:

• 0.35% ≤ C ≤ 1.2%
• 0.10% ≤ Mn ≤ 2.0%
• 0.10% ≤ If ≤ 1.0%
• traces ≤ Cr ≤ 4.5%
• traces ≤ Mo ≤ 2.0%
• traces <Ni ≤ 4.5%
• traces ≤ V ≤ 0.5%
• traces ≤ Cu ≤ 3.5%
• traces ≤ Al ≤ 0.060%
• traces ≤ Ca ≤ 0.050%
• traces ≤ B ≤ 100ppm
• traces ≤ Ti ≤ 0.050%
• traces ≤ Nb ≤ 0.050%

the other elements being iron and conventional impurities resulting from the production.

It can also contain up to 0.180% of S and at least one of the elements chosen from up to 0.080% of Bi, up to 0.020% of Te, up to 0.040% Se, up to 0.070% Pb.

The piston can be made of hot tool steel.

The piston can be made of high speed steel.

The piston can be made of stainless steel.

The piston can be made of cast iron.

The piston can be made of an Fe-Ni based alloy.

The piston can be made of an alloy based on Ni-Co.

As will be understood, the invention is based on the use of shaping process called "thixoforgeage", known in itself but which had never been applied to the manufacture of pistons.

Thixoforgging is a process which consists in carrying out the form of a metal part by forging a piece after having brought it to a intermediate temperature between its solidus temperature and its temperature liquidus, so as to coexist within the plot of solid matter and intimately mixed liquid matter. This allows, compared to the processes conventional hot forging, to produce parts of complex geometry can have thin walls, and this with very low setting efforts in shape. Indeed, under the action of external forces, metals undergoing a thixoforgging operation behave like viscous fluids.

Thixoforging can be used for many kinds of alloys. In the rest of the description of the invention, we will focus on thixoforgging carbon steels, it being understood that other alloys may lend themselves to manufacture of pistons by thixoforgging.

The success of a steel thixoforging operation depends on first place of the primary structure obtained at an intermediate temperature between the solidus and the liquidus during the heating cycle of the plot before it is put in form by thixoforgeage. Experience shows that before the setting operation form, the piece must have a globular primary structure rather than dendritic. In the latter case, during heating, the segregation of various elements of alloy between the dendrites and the inter-dendritic spaces leads to a preferential fusion of metal in interdendritic spaces enriched with alloying elements. The resulting liquid tends to be expelled at the start of the forming operation, which results in an increase in efforts to apply (these being exerted on a metal more solid than expected) and the appearance faults within the room: segregation and internal health problems. When the thixoforgging shaping operation is carried out on a primary globular structure obtained by suitable heating, a homogeneous product, which can deform at high speed. The primary structure plot dendritic can be optimized to facilitate obtaining, when heating before thixoforgeage, of a homogeneous globular primary structure. This can be obtained by playing in particular on the intensity of the brewing electromagnetic during solidification of the continuously cast product which allows to fragment the dendrites, and on the intensity of the cooling of this product which conditions the growth of dendrites and the diffusion of segregating elements, all this for a given product size.

If we operate on a piece of wood from a rolled bar coming from a continuous casting bloom or ingot, this facilitates obtaining a structure globular during the heating preceding thixoforging, without being necessary to resort to a separate operation of globalization of the structure primary. Indeed, the multiple reheating and significant deformations undergone by the steel then led to a very nested and diffuse structure where a primary structure is practically impossible to reveal.

Heating the plot to reach the temperature of the thixoforgging is generally carried out by induction, to obtain a excellent temperature uniformity over the entire section of the plot and excellent reproducibility of the operation from one plot to another.

• la figure 1 qui représente en perspective et en coupe longitudinale un exemple de piston de l'art antérieur, réalisé classiquement en alliage d'aluminium forgé ;
• la figure 2 qui représente de la même façon un exemple de piston selon l'invention, pouvant se substituer au précédent, réalisé en acier au carbone thixoforgé.

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

• Figure 1 which shows in perspective and in longitudinal section an example of a piston of the prior art, conventionally made of forged aluminum alloy;
• Figure 2 which similarly shows an example of a piston according to the invention, which can replace the previous one, made of thixoforged carbon steel.

• une cavité intérieure 2, où on peut loger la bielle qui entraínera le piston 1 ;
• une jupe 3 constituant la paroi latérale du piston 1, destinée à venir au contact de la chemise du cylindre, notamment par l'intermédiaire de segments (non représentés) disposés dans des logements 4, 5, 6 ménagés sur la périphérie de la jupe 3, au niveau du fond 7 du piston 1 ;
• une surface 8 de fond de piston, constituant la partie du piston 1 faisant face à la chambre de combustion lorsque le piston 1 est placé dans le cylindre, et dont la conformation, représentée seulement à titre d'exemple, est classiquement conçue pour favoriser la combustion du carburant ;
• une patte 9, comportant un orifice 10 à parois renforcées vers l'intérieur du piston 1, ménagé dans la jupe 3 pour permettre le passage à travers l'orifice 10 de l'axe destiné à solidariser le piston 1 et la bielle ; une patte similaire est disposée symétriquement à l'opposé de la patte 9 sur la moitié du piston 1 non représentée.

The piston 1 of the prior art shown in section and in perspective in Figure 1, for reference, is designed to be used in a 1900 cm ³ diesel engine with high pressure direct injection displacement. It is made by forging an AS12UNG aluminum alloy reinforced with alumina fibers. Its outside diameter is 80 mm. Conventionally, its different portions consist of:

• an inner cavity 2, where we can accommodate the rod that will drive the piston 1;
• a skirt 3 constituting the side wall of the piston 1, intended to come into contact with the cylinder liner, in particular by means of segments (not shown) arranged in housings 4, 5, 6 formed on the periphery of the skirt 3 , at the bottom 7 of the piston 1;
• a surface 8 of the piston bottom, constituting the part of the piston 1 facing the combustion chamber when the piston 1 is placed in the cylinder, and the conformation of which, shown only by way of example, is conventionally designed to favor the fuel combustion;
• a lug 9, comprising an orifice 10 with reinforced walls towards the inside of the piston 1, formed in the skirt 3 to allow passage through the orifice 10 of the axis intended to secure the piston 1 and the connecting rod; a similar tab is arranged symmetrically opposite the tab 9 on half of the piston 1 not shown.

• la jupe 3 présente une épaisseur relativement élevée, de 6mm ;
• le fond de piston 7 est lui aussi épais, avec une distance maximale entre sa surface 8 et le fond 11 de la cavité intérieure 2 de 29mm,
• la distance entre le segment coup de feu (celui qui est placé dans le logement 6 le plus proche de la surface 8) et la surface 8 du fond de piston 7 est de 11mm ;
• la hauteur de compression, c'est à dire la distance entre le centre de l'orifice 10 de la patte 9 et la surface 8 du fond de piston 7, est de 51mm ;
• le diamètre de l'orifice 10 de la patte 9 est de 28mm ;
• la hauteur totale du piston 1 est de 68mm ;
• le poids du piston 1 est de 525g après usinage.

We can notice that:

• the skirt 3 has a relatively high thickness, of 6mm;
• the piston bottom 7 is also thick, with a maximum distance between its surface 8 and the bottom 11 of the interior cavity 2 of 29mm,
• the distance between the gunshot segment (the one placed in the housing 6 closest to the surface 8) and the surface 8 of the piston bottom 7 is 11mm;
• the compression height, ie the distance between the center of the orifice 10 of the tab 9 and the surface 8 of the piston bottom 7, is 51mm;
• the diameter of the orifice 10 of the tab 9 is 28mm;
• the total height of the piston 1 is 68mm;
• the weight of piston 1 is 525g after machining.

The piston 12 according to the invention shown in Figure 2 is called replace the piston 1 of the prior art which has just been described. It is produced by thixoforging of a carbon steel of composition (in percentages by weight): C = 0.962%; Mn = 0.341%; If = 0.237%; Cr = 1,500%; Ni = 0.089%; Mo = 0.017%; Cu = 0.161%; Al = 0.037%; S = 0.010%; P = 0.009%; V = 0.004%; Ti = 0.002%; Sn = 0.002%; N = 0.0041%. The elements functionally equivalent to those of piston 1 of the prior art are designated by the same references.

• la jupe 3 est beaucoup plus mince : son épaisseur n'est que de 1,5mm ;
• l'épaisseur du fond de piston 7 est très faible, de 3mm environ, et la forme de sa paroi épouse celle de sa surface 8 sur son côté destiné à être tourné vers la chambre de combustion ; le résultat est que la cavité intérieure 2 du piston 12 présente un grand volume, ce qui procure une grande économie de matière allégeant considérablement le piston 12 ;
• la distance entre le segment coup de feu placé dans le logement 6 et la surface 8 du fond de piston 7 est de 5mm ;
• la patte n'est plus intégrée à la jupe 3, mais est constituée par un étrier 13 triangulaire, ménagé au fond de la cavité 2 et perforé par l'orifice 10 ; un étrier similaire se trouve symétriquement à l'étrier 13 dans la moitié du piston 12 non représentée ; pour donner accès à l'étrier 13 et à l'orifice 10, la jupe 3 présente un large évidement 14, ce qui là encore permet d'alléger le piston 12, et aussi de réduire la surface de contact entre la jupe 3 et la chemise du cylindre, donc les frottements subis par le piston 12 en cours d'utilisation ;
• la hauteur de compression est de 32mm seulement ;
• le diamètre de l'orifice 10 de l'étrier 13 est de 20mm seulement, ce qui permet de réduire le diamètre de l'axe solidarisant le piston 12 et la bielle ;
• la hauteur totale du piston 12 est de 75mm (mais on pourrait la ramener à une valeur identique à celle du piston 1 de l'art antérieur) ;
• le poids du piston 12 est de 500g après usinage.

It is noted that, with respect to the piston 1 of the prior art:

• the skirt 3 is much thinner: its thickness is only 1.5mm;
• the thickness of the piston bottom 7 is very small, about 3 mm, and the shape of its wall matches that of its surface 8 on its side intended to be turned towards the combustion chamber; the result is that the internal cavity 2 of the piston 12 has a large volume, which provides a great saving of material considerably reducing the piston 12;
• the distance between the gunshot segment placed in the housing 6 and the surface 8 of the piston bottom 7 is 5mm;
• the tab is no longer integrated into the skirt 3, but is constituted by a triangular stirrup 13, formed at the bottom of the cavity 2 and perforated by the orifice 10; a similar caliper is symmetrically with the caliper 13 in the half of the piston 12 not shown; to give access to the stirrup 13 and to the orifice 10, the skirt 3 has a large recess 14, which again makes it possible to lighten the piston 12, and also to reduce the contact surface between the skirt 3 and the cylinder liner, therefore the friction undergone by the piston 12 during use;
• the compression height is only 32mm;
• the diameter of the orifice 10 of the stirrup 13 is only 20mm, which makes it possible to reduce the diameter of the axis joining the piston 12 and the connecting rod;
• the total height of the piston 12 is 75mm (but it could be reduced to a value identical to that of the piston 1 of the prior art);
• the weight of the piston 12 is 500g after machining.

This complex geometry cannot be obtained on a part carbon steel monobloc only through the use of the process of Thixoforging. It alone gives access, in particular, to the thinness of the skirt 3 which was cited.

Note that the weight gain provided by this configuration is feel not only on the piston itself, but on the entire crew mobile piston-piston-rod axis. As we have seen, the weight gain on the piston is 25g. Reducing the diameter of the piston pin from 28 to 20mm and its shortening from 80 to 50mm (the piston pin is in both cases a tube 6mm thick) saves 156g on this piece. The weight of the connecting rod can also be reduced by a few grams.

The dimensional modifications which have been indicated between the piston 1 of the prior art made of aluminum alloy and the piston 12 according to the invention made of thixoforged steel having the above-mentioned composition are made possible by the better mechanical and thermal characteristics of the steel. , highlighted in Table 1. All the characteristics were measured at 350 ° C. This temperature is an average temperature that the piston in operation reaches in extreme cases, but which can be greatly exceeded locally in the vicinity of the combustion chamber of the cylinder. compared characteristics of the aluminum alloy A512UNG reinforced and the steel of the previous example at 450 ° C. AS12UNG reinforced Steel Density 2.71 7.83 Young's modulus (MPa) 55000 190000 Poisson coefficient 0.3 0.27 Breaking strength (MPa) 100 1,100 Fatigue resistance (MPa) 50 400 Coefficient of expansion (10 ^-6 / K) 20 12 Coefficient of thermal conductivity (W / mK) 100 20

We see that the best mechanical characteristics of steel allow the use of a lesser amount of material to obtain a part equal resistance to stress, which compensates for the density superior of steel and get a piece that is even lighter than its equivalent in aluminum.

Furthermore, the mechanical characteristics of steel are more stable in temperature than those of aluminum.

Due to the lower thermal conductivity of steel, we can allow, as we have seen, to significantly shorten the distance between the gunshot segment and the bottom 8 of the piston. Spaces between segments may likewise be reduced. All this contributes to the decrease in the quantity of material used. On the other hand, the heat given off in the combustion thus remains concentrated on the bottom of the piston. Skirt 3 undergoes thus less temperature variations, which reduces the problems of expansion, as does the lower coefficient of expansion of steel by compared to other metal alloys such as aluminum. Skirt 3 and the cylinder liner expand in much the same way, allowing reduce operating clearance and dissipate heat more quickly shirt.

For the same reason, the heat of the combustion chamber is less evacuated by the steel piston than by the aluminum piston, which increases engine efficiency.

Reducing the compression height reduces the cylinder height, therefore improves engine compactness. This is another factor of decrease in engine weight.

If it turns out that the bottom 7 of the piston reaches temperatures excessive, one can foresee its cooling by a jet of oil directed in the cavity 2. This solution is, in any case, less complex than it is the use of cooling channels inside the piston, which is often necessary with aluminum pistons.

The geometry of the piston 12 which has just been described is only an example for implementing the invention, whether for the general appearance of the piston or for the precise dimensions of its different parts. So thixoforgeage offers the possibility of providing thin reinforcement ribs in different areas of the piston.

• 0,35% ≤ C ≤ 1,2%
• 0,10% ≤ Mn ≤ 2,0%
• 0,10% ≤ Si ≤ 1,0%
• traces ≤ Cr ≤ 4,5%
• traces ≤ Mo ≤ 2,0%
• traces < Ni ≤ 4,5%
• traces ≤ V ≤ 0,5%
• traces ≤ Cu ≤ 3,5%

A non-limiting example of steel which can be used to manufacture a piston by thixoforging is constituted by the following general range (in percentages by weight):

• 0.35% ≤ C ≤ 1.2%
• 0.10% ≤ Mn ≤ 2.0%
• 0.10% ≤ If ≤ 1.0%
• traces ≤ Cr ≤ 4.5%
• traces ≤ Mo ≤ 2.0%
• traces <Ni ≤ 4.5%
• traces ≤ V ≤ 0.5%
• traces ≤ Cu ≤ 3.5%

The other elements are iron and conventional impurities resulting development: P, Sn, N, As ...

• des éléments de désoxydation : Al (jusqu'à 0,060%) et/ou Ca (jusqu'à 0,050%) ;
• des éléments améliorant la trempabilité, tels que B (jusqu'à 100ppm) ;
• des éléments améliorant l'usinabilité : S (jusqu'à 0,180%), Bi (jusqu'à 0,080%), Te (jusqu'à 0,020%), Se (jusqu'à 0,040%) , Pb (jusqu'à 0,070%) ;
• des éléments bloquant le grossissement du grain tels que Ti (jusqu'à 0,050%) et Nb (jusqu'à 0,050%).

Optionally, we can add:

• deoxidation elements: Al (up to 0.060%) and / or Ca (up to 0.050%);
• elements which improve the hardenability, such as B (up to 100 ppm);
• elements improving machinability: S (up to 0.180%), Bi (up to 0.080%), Te (up to 0.020%), Se (up to 0.040%), Pb (up to 0.070%) );
• elements blocking the magnification of the grain such as Ti (up to 0.050%) and Nb (up to 0.050%).

• Exemple 1 : C = 0,377% ; Mn = 0,825% ; Si = 0,190% ; Cr = 0,167% ; Ni = 0,113% ; Cu = 0,143% ; Al = 0,022% ; S = 0,01% ; P = 0,007% ; Sn = 0,01 % ; N = 75ppm ; Ca = 6ppm.

Two examples of such steels can in particular be cited:

• Example 1: C = 0.377%; Mn = 0.825%; If = 0.190%; Cr = 0.167%; Ni = 0.113%; Cu = 0.143%; Al = 0.022%; S = 0.01%; P = 0.007%; Sn = 0.01%; N = 75ppm; Ca = 6ppm.

• Exemple 2 (celui utilisé pour réaliser le piston de la figure 2) : C = 0,962% ; Mn = 0,341% ; Si = 0,237% ; Cr = 1,500% ; Ni = 0,089% ; Mo = 0,017% ; Cu = 0,161 % ; Al = 0,037% ; S = 0,01% ; P = 0,009% ; V = 0,004% ; Ti = 0,002% ; Sn = 0,002% ; N = 41ppm.

The measured solidus temperature of this steel is 1430 ° C and the measured liquidus temperature is 1538 ° C. Thixoforging preferably takes place at 1480 ° C.

• Example 2 (the one used to make the piston in Figure 2): C = 0.962%; Mn = 0.341%; If = 0.237%; Cr = 1,500%; Ni = 0.089%; Mo = 0.017%; Cu = 0.161%; Al = 0.037%; S = 0.01%; P = 0.009%; V = 0.004%; Ti = 0.002%; Sn = 0.002%; N = 41ppm.

• Exemple 3 : C = 0,825% ; Mn = 0,649% ; Si = 0,213% ; Cr = 0,100% ; Ni = 0,062% ; Cu = 0,107% ; Al = 0,035% ; S =0,007% ; P = 0,007% ; N = 55ppm.

The measured solidus temperature of this steel is 1315 ° C and the measured liquidus temperature is 1487 ° C. Thixoforging preferably takes place at 1405 ° C.

• Example 3: C = 0.825%; Mn = 0.649%; If = 0.213%; Cr = 0.100%; Ni = 0.062%; Cu = 0.107%; Al = 0.035%; S = 0.007%; P = 0.007%; N = 55ppm.

The measured solidus temperature of this steel is 1360 ° C and the liquidus temperature measured is 1490 ° C. Thixoforgging takes place preferably at 1429 ° C.

It should be noted that the liquidus and solidus temperatures measured to which we have just alluded can differ significantly from liquidus and solidus temperatures calculated according to the composition of steel by the formulas conventionally available in the literature. In fact, these formulas are valid in the case where the steel sees its temperature drop by a few degrees per minute during solidification followed by cooling. To determine the optimum thixoforging temperature, the solidus and liquidus temperatures must be measured under the conditions to which the plots will be subjected, namely reheating from the ambient temperature, carried out by induction at a speed of the order of several tens of degrees per minute. But this determination may be carried out by a person skilled in the art using standard tests not showing of particular difficulties.

For the materials that have just been described, thixoforging must, from preferably take place with a liquid fraction representing 10 to 40% of the steel. Below 10% there is a risk that the metal will not flow properly and solidifies too quickly on contact with tools. At more than 40%, there are risks of subsidence and flow of the metal during the heating operation: the piece becomes difficult to correctly transfer to the formatting tools.

The steels whose composition has just been exposed are steels of construction or heat treatment used in forging and mechanics. They are likely to be suitable for the manufacture of pistons usable in the majority of motor vehicles, trucks, public works machinery, agricultural machinery, boats, etc.

For particularly demanding applications in terms of in particular, temperatures reached at the piston head, it is conceivable to use steels allowing hot work such as tool steels hot 38CrMoV5, 45CrMoV6, 55NiCrMoV7, conventional high speed steels or overburdened, and also cast iron or alloys based on iron-nickel or cobalt-nickel. We can also consider the use of stainless steels, for the case where the piston would be called upon to work in contact with fuels containing particularly corrosive additives, for example stainless steels martensitics Z40Cr13 to Z200Cr13. All these materials, as well as steels carbon of the type which can be used in the invention, have for characteristic a content of high carbon (0.35% at least), even very high. It is a very element favorable for thixoforgging operation because it lowers the solidus temperature and widens the solidification interval: this gives easier access to the interval optimal liquid fraction in the metal.

It can be seen that the invention can be applied to a wide variety alloys, the main thing being that their mechanical and thermal characteristics lend themselves well to their use to form pistons, and that they have a good thixoforging ability.

Claims (14)

Method for manufacturing a piston for an internal combustion engine, said piston being formed from a single piece of metal, characterized in that a piece of heating is carried out to bring it to a temperature intermediate between its solidus temperature and its liquidus temperature, and in that it is formed by thixoforgging. Piston (12) of an internal combustion engine, composed of a single piece of metal, characterized in that it was manufactured by heating a piece to bring it to an intermediate temperature between its solidus temperature and its liquidus temperature , followed by shaping by thixoforgging. Piston (12) according to claim 2, characterized in that its legs consist of stirrups (13) formed on the bottom of the interior cavity (2) of the piston (12), provided with an orifice (10) for the passage of the axis joining the piston (12) and the connecting rod, and in that it has on its skirt (3) recesses (14) giving access to the orifices (10) of the stirrups (13). Piston (12) according to claim 2 or 3, characterized in that the shape of the wall of the piston bottom (7) matches that of the surface (8) of the piston bottom (7) on its side intended to be turned towards the combustion chamber. Piston according to one of claims 2 to 4, characterized in that it has reinforcing ribs. Piston (12) according to one of claims 2 to 5, characterized in that it is made of carbon steel. Piston (12) according to claim 6, characterized in that its composition is, in weight percentages: 0.35% ≤ C ≤ 1.2% 0.10% ≤ Mn ≤ 2.0% 0.10% ≤ If ≤ 1.0% traces ≤ Cr ≤ 4.5% traces ≤ Mo ≤ 2.0% traces <Ni ≤ 4.5% traces ≤ V ≤ 0.5% traces ≤ Cu ≤ 3.5% traces ≤ Al ≤ 0.060% traces ≤ Ca ≤ 0.050% traces ≤ B ≤ 100ppm traces ≤ Ti ≤ 0.050% traces ≤ Nb ≤ 0.050% the other elements being iron and conventional impurities resulting from the production. Piston (12) according to claim 7, characterized in that it also comprises up to 0.180% of S and at least one of the elements chosen from up to 0.080% of Bi, up to 0.020% of Te, up to 0.040% Se, up to 0.070% Pb. Piston (12) according to one of claims 2 to 5, characterized in that it is made of hot tool steel. Piston (12) according to one of claims 2 to 5, characterized in that it is made of high-speed steel. Piston (12) according to one of claims 2 to 5, characterized in that it is made of stainless steel. Piston (12) according to one of claims 2 to 5, characterized in that it is made of cast iron. Piston (12) according to one of claims 2 to 5, characterized in that it is made of an alloy based on Fe-Ni. Piston (12) according to one of claims 2 to 5, characterized in that it is made of an alloy based on Ni-Co.

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