EP0779375A1 - Steel for the manufacture of divisible mechanical parts and parts made from this steel - Google Patents

Abstract

A steel alloy comprises by weight: 0.25-0.75% carbon, 0.2-1.5% silicon, 0.1-2% manganese, 0-1% each of nickel, chromium, molybdenum and copper, 0-0.2% vanadium, 0.02-0.35% sulphur, 0.04-0.2% phosphorous, 0-0.005% and, 0.005-0.02% nitrogen. The residue is iron plus possible impurities, with possibly<0.1% of at least one of lead, tellurium and selenium. The steel may have been treated with calcium. Also claimed is the use of such steel to make a mechanical component comprising at least two parts obtained by snapping a blank, and a component so made.

Description

The present invention relates to a steel for the manufacture of a breakable mechanical part, and in particular for the manufacture of a connecting rod for an internal combustion engine.

Certain mechanical parts such as, for example, the connecting rods of an internal combustion engine consist of at least two separable elements assembled by fixing means such as screws. These parts can be made of cast iron, sintered and forged metal powder, or forged steel. The invention relates to parts, and in particular connecting rods, in forged steel.

The steel constituting the forged steel connecting rods must be forgeable, easily machinable and have mechanical characteristics that ensure good service life of the connecting rods. The mechanical characteristics generally required are a hardness between 210 HB and 360 HB and a breaking strength between 650 MPa and 1200 MPa. to obtain sufficient resistance to fatigue, and an elastic limit between 300 MPa and 800 MPa in order to avoid deformation by exceeding the elastic limit.

The precise choice of the characteristics required for a particular connecting rod intended for a particular engine depends on the design of the connecting rod and on the nature of the engine in which it is incorporated. The steel which constitutes it is chosen as a function of these mechanical characteristics and of the manufacturing process which comprises, after forging, controlled cooling intended to obtain a ferritic-pearlitic structure which has the required mechanical characteristics and satisfactory machinability. The steels used are, in general, carbon steels of the XC42 type or low alloy steels of the 45M5, 30MSV6, 38MVS5 type (according to French standard). The carbon content is chosen mainly according to the level of hardness required, and the alloying elements are added either to increase the hardenability of the steel in order to increase the proportion of perlite, which is favorable for machinability, either to harden the ferrite and improve the yield strength to breaking strength ratio. With these steels, the separation of the different parts of the connecting rod can only be done by machining which requires a complex and costly range of machining.

Certain rods in cast iron or obtained by powder metallurgy can be separated into two elements by a fragile rupture operation according to a predetermined plane. This technique, called breakable parts, has several advantages and, in particular, that of simplifying considerably the manufacturing range by eliminating machining operations, it does however have drawbacks resulting from the nature of the materials that can be used.

In order to take advantage of the advantages of the breakable connecting rod technique to apply it to steel connecting rods, it has been proposed, in US patent 5,135,587, to use a steel whose chemical composition comprises, by weight: 0.6% 0.75% carbon, 0.2% 0.5% manganese, 0.04% 0.12% sulfur with Mn / S> 3, the remainder being iron and impurities; the content of impurities being less than 1.2%; the structure being practically 100% pearlitic and the grain size between 3 and 8 ASTM. The impurities taken from P, Si, Ni, V, Cu, Cr and Mo having individual contents preferably such as: Ni ≤ 0.2%, Mo ≤ 0.02%, Cr ≤ 0.1%, Cu ≤ 0 , 15% V ≤ 0.035% 0.15% ≤ If ≤ 0.35%, and P ≤ 0.03%. But this steel, which is of the XC70 type (according to the French standard), has the drawback of having an irregular behavior during the fragile rupture operation, in particular because, it is practically impossible to control industrially the proportion of phase proeutectoid, this can vary from 0% to 15% depending on the exact chemical analysis of the steel and the manufacturing means used., which makes it difficult to use industrially, moreover, it does not make it possible to obtain that the characteristics specific to the XC70 which limits its use to the parts for which these characteristics are adapted.

The object of the present invention is to remedy these drawbacks by proposing a steel which makes it possible to obtain the mechanical characteristics required for a wide range of applications, in particular in the field of connecting rods, and good machinability, while making it possible to produce the fragile rupture operation under satisfactory industrial conditions.

To this end, the subject of the invention is a steel for the manufacture of a breakable mechanical part, the chemical composition of which comprises, by weight: 0.25% ≤ C ≤ 0.75%, 0.2% ≤ Si ≤ 1.5%, 0.1% ≤ Mn ≤ 2%, 0% ≤ Ni ≤ 1%, 0% ≤ Cr ≤ 1%, 0% ≤ Mo ≤ 1%, 0% ≤ Cu ≤ 1%, 0% ≤ V ≤ 0.2%, 0.02% ≤ S ≤ 0.35%, 0.04% ≤ P ≤ 0.2%, 0% ≤ Al ≤ 0.005%, 0.005% ≤ N ≤ 0.02%, possibly at least one element taken from lead, tellurium and selenium in contents of less than 0.1%, the rest being iron and impurities resulting from the production, the steel possibly being treated with calcium.

Preferably, the chemical composition of the steel satisfies at least one of the following relationships: 0.06% ≤ P ≤ 0.12%, 0.8% ≤ Si ≤ 1.2%, 0.05% ≤ V ≤ 0 , 15%.

The chemical composition of the steel can be such that: 0.65% ≤ C ≤ 0.75%, 0.2% ≤ Si, 0.25% ≤ Mn ≤ 1%, Ni ≤ 0.15%, Cr ≤ 0.15%, Mo ≤ 0.05%, Cu ≤ 0.35%

The chemical composition of the steel can also be such that: 0.25% ≤ C ≤ 0.5%, 0.2% ≤ Si, Ni ≤ 0.15%, Cr ≤ 0.15%, Mo ≤ 0, 05%, Cu ≤ 0.35%, with, preferably, 0.25% ≤ Mn ≤ 1.3%.

The invention also relates to the use of a steel according to the invention for the manufacture of a mechanical part comprising at least two elements obtained by fragile rupture of a blank of said part, as well as said part. This part can be, in particular, a connecting rod for an internal combustion engine consisting for example of a steel whose hardness is between 210 HB and 360 HB, the breaking strength is between 650 MPa and 1200 MPa, with a majority relatively large grains, whose ASTM size index of austenitic grains is less than 5, preferably having a structure comprising at least 70% perlite.

The invention will now be described in a more precise but non-limiting manner.

• plus de 0,25% de carbone pour permettre d'obtenir une structure ferrito-perlitique ou perlitique de dureté supérieure à 210 HB, mais moins de 0,75% de façon à éviter la formation de carbures de fer défavorables à l'usinabilité;
• de 0,04% à 0,2% de phosphore, et de préférence, de 0,06% à 0,12% afin de fragiliser la structure, et en particulier la ferrite, obtenue après forgeage et traitement thermique; notamment lorsque la structure est essentiellement perlitique, cette teneur en phosphore permet d'obtenir une bonne reproductibilité de la rupture fragile d'ébauches de pièces de mécanique; de préférence, la teneur en phosphore doit être telle que: P ≥ 0,18 - 0,2 x C . On obtient ainsi une résilience Kcv inférieure à environ 7 Joules à la température ambiante, nécessaire pour obtenir une bonne aptitude à la rupture 100% fragile avec une déformation latérale inférieure ou égale à 120 µm;
• moins de 0,005% et de préférence moins de 0,003% d'aluminium afin d'éviter la présence d'inclusions d'alumine défavorables à l'usinabilité, et également pour éviter la formation de nitrures d'aluminium qui empêchent le grain de grossir pendant le réchauffage avant forgeage, ce qui est défavorable à l'aptitude à la rupture fragile;
• de 0,2% à 1,5% de silicium; le silicium est un élément désoxydant qui doit être ajouté en des teneurs supérieures à 0,2% pour assurer une bonne désoxydation, mais, en plus fortes teneurs cet élément durcit et fragilise la ferrite ce qui est favorable à une bonne usinabilité; pour obtenir cet effet favorable, sa teneur peut être fixée entre 0,8% et 1,2%;
• de 0% à 0,2%, et, de préférence, de 0,05% à 0,15% de vanadium pour durcir la ferrite et améliorer le rapport limite d'élasticité sur résistance à la rupture;
• de 0,02% à 0,35% , et de préférence, de 0,05% à 0,12% de soufre pour améliorer l'usinabilité;
• éventuellement, au moins un élément pris parmi le plomb, le tellure et le sélénium en des teneurs inférieures à 0,1% de façon à améliorer l'usinabilité;
• de 0,1% à 2% de manganèse et de préférence plus de 0,25% afin de fixer le soufre sous forme de sulfures de manganèse, et dans ce cas, la teneur en soufre peut être limitée à 1%; mais, le manganèse peut également être ajouté pour augmenter la trempabilité afin d'abaisser la température de début de transformation ferrito-perlitique et ainsi de limiter la teneur en ferrite, ce qui est favorable à l'usinabilité;
• éventuellement un ou plusieurs éléments pris parmi le nickel, le chrome, le molybdène et le cuivre, en des teneurs comprises entre 0% et 1% afin d'ajuster la trempabilité; lorsque ces éléments ne sont pas ajoutés volontairement, ils peuvent néanmoins exister à titre de résiduels apportés par les matières premières lors de l'élaboration, dans ce cas, les teneurs en nickel et en chrome sont inférieures à 0,15%, la teneur en molybdène est inférieure à 0,05% et la teneur en cuivre est inférieure à 0,35%.

The steel according to the invention is a carbon or low-alloy mechanical steel, the chemical composition of which comprises, by weight:

• more than 0.25% of carbon in order to obtain a ferrito-perlitic or perlitic structure with a hardness greater than 210 HB, but less than 0.75% so as to avoid the formation of iron carbides unfavorable to machinability;
• from 0.04% to 0.2% of phosphorus, and preferably from 0.06% to 0.12% in order to weaken the structure, and in particular the ferrite, obtained after forging and heat treatment; in particular when the structure is essentially pearlitic, this phosphorus content makes it possible to obtain good reproducibility of the fragile rupture of blanks of mechanical parts; preferably, the phosphorus content should be such that: P ≥ 0.18 - 0.2 x C. One thus obtains a resilience Kcv of less than approximately 7 Joules at ambient temperature, necessary to obtain a good breaking capacity 100% fragile with a lateral deformation less than or equal to 120 μm;
• less than 0.005% and preferably less than 0.003% aluminum in order to avoid the presence of inclusions of alumina unfavorable to the machinability, and also to avoid the formation of aluminum nitrides which prevent the grain from growing during reheating before forging, which is unfavorable to the brittle breaking capacity;
• from 0.2% to 1.5% silicon; silicon is a deoxidizing element which must be added in contents greater than 0.2% to ensure good deoxidation, but, at higher contents, this element hardens and weakens the ferrite, which is favorable for good machinability; to obtain this favorable effect, its content can be set between 0.8% and 1.2%;
• from 0% to 0.2%, and preferably from 0.05% to 0.15% of vanadium to harden the ferrite and improve the yield strength to breaking strength ratio;
• from 0.02% to 0.35%, and preferably from 0.05% to 0.12% of sulfur to improve the machinability;
• optionally, at least one element taken from lead, tellurium and selenium in contents of less than 0.1% so as to improve the machinability;
• from 0.1% to 2% of manganese and preferably more than 0.25% in order to fix the sulfur in the form of manganese sulphides, and in this case, the sulfur content can be limited to 1%; but, manganese can also be added to increase the hardenability in order to lower the temperature at the start of ferrito-pearlitic transformation and thus limit the ferrite content, which is favorable for machinability;
• optionally one or more elements taken from nickel, chromium, molybdenum and copper, in contents of between 0% and 1% in order to adjust the quenchability; when these elements are not added voluntarily, they may nevertheless exist as residuals provided by the raw materials during the preparation, in this case, the nickel and chromium contents are less than 0.15%, the content of molybdenum is less than 0.05% and the copper content is less than 0.35%.

In this family of steels, one can choose according to the use concerned, for example, a steel close to the eutectoid comprising from 0.65% to 0.75% of carbon, less than 1% of silicon, from 0, 25% to 1% manganese, less than 0.15% nickel, less than 0.15% chromium, less than 0.05% molybdenum, less than 0.35% copper and less than 0.005% of aluminum.

One can also use a steel less loaded with carbon whose chemical composition notably comprises 0.25% ≤ C ≤ 0.5%, Ni ≤ 0.15%, Cr ≤ 0.15%, Mo ≤ 0.05%, Cu ≤ 0.35%. This steel may be carbon steel, in which case it contains less than 0.5% of manganese. But it can also be a steel weakly alloyed with manganese, silicon, or possibly with vanadium. It can then contain between 1% and 2% of manganese, and / or between 0.5% and 1.5% of silicon, and / or between 0.5% and 0.2% of vanadium.

To manufacture a breakable part, a piece of steel according to the invention is supplied, it is heated to a temperature between 1100 ° C. and 1300 ° C. so. on the one hand to austenitize it. on the other hand to make the arain fat and, finally, to give it the ductility necessary for forging, then it is forged to give it the desired shape; forging ending at a temperature above 850 ° C. Directly after forging, it is cooled in a controlled manner to ambient temperature, for example in a cooling tunnel, at an average cooling rate between the end of forging temperature and 200 ° C. being between 0.5 ° C. / s and 15 ° C / s. By doing this, a ferrito-pearlitic structure is obtained with a majority of relatively large grains, the ASTM size index of the austenitic grain of which is less than 5, containing less than 30% ferrite, having the characteristics of hardness and traction. required, and a resilience of less than 7 Joules at room temperature. The blank of the part thus obtained is then machined, then divided into two elements by fragile rupture caused by an impact.

• C = 0,71 %
• Si = 0,250 %
• Mn = 0,8 %
• Ni = 0,08 %
• Cr = 0,05 %
• Mo = 0,01%
• Cu = 0,3 %
• S = 0,07 %
• P = 0,045 %
• Al = 0,002 %
• N = 0,012 %

le reste étant du fer et des impuretés résultant de l'élaboration.As a first example, connecting rods were manufactured using XC70 type steel, the chemical composition of which included, by weight:

• C = 0.71%
• If = 0.250%
• Mn = 0.8%
• Ni = 0.08%
• Cr = 0.05%
• Mo = 0.01%
• Cu = 0.3%
• S = 0.07%
• P = 0.045%
• Al = 0.002%
• N = 0.012%

the remainder being iron and impurities resulting from processing.

• structure: perlitique avec 0% à 15% de ferrite,
• HB compris entre 270 et 310
• Rm compris entre 900 MPa et 1050 MPa,
• Re compris entre 500 MPa et 600 MPa,
• Kcv inférieur à 7 Joules à la température ambiante.

Before forging, the pieces of steel were heated to 1250 ° C; the end of forging temperature was 1000 ° C. After forging, the blank was cooled by passing through a controlled cooling tunnel at average cooling rates of between 1 ° C / s and 3 ° C / s in order to simulate the effect of dispersions specific to industrial production. The characteristics obtained were:

• structure: pearlitic with 0% to 15% ferrite,
• HB between 270 and 310
• Rm between 900 MPa and 1050 MPa,
• Re between 500 MPa and 600 MPa,
• Kcv lower than 7 Joules at room temperature.

The blanks were then machined and then all separated into two elements by fragile rupture. This separation by brittle fracture was carried out without difficulty whatever the ferrite content.

• C = 0,505 %
• Si = 0,240 %
• Mn = 1,3 %
• Ni = 0,11 %
• Cr = 0,08 %
• Mo = 0,01%
• Cu = 0,32 %
• S = 0,085 %
• P = 0,075 %
• Al = 0,003 %
• N = 0,011 %

le reste étant du fer et des impuretés résultant de l'élaboration.As a second example, connecting rods were made using 50M5 type steel, the chemical composition of which included, by weight:

• C = 0.505%
• If = 0.240%
• Mn = 1.3%
• Ni = 0.11%
• Cr = 0.08%
• Mo = 0.01%
• Cu = 0.32%
• S = 0.085%
• P = 0.075%
• Al = 0.003%
• N = 0.011%

the remainder being iron and impurities resulting from processing.

• structure: perlitique avec 0% à 20% de ferrite,
• HB compris entre .260 et 300
• Rm compris entre 860 MPa et 1000 MPa,
• Re compris entre 400 MPa et 650MPa,
• Kcv inférieur à 6 Joules à la température ambiante.

Before forging, the piece of steel was heated to 1250 ° C; the end of forging temperature was 1000 ° C. After forging, the blank was cooled by passing through a controlled cooling tunnel at average cooling rates of between 1 ° C / s and 6 ° C / s in order to simulate the effect of dispersions specific to industrial manufacturing. The characteristics obtained were:

• structure: pearlitic with 0% to 20% ferrite,
• HB between .260 and 300
• Rm between 860 MPa and 1000 MPa,
• Re between 400 MPa and 650MPa,
• Kcv below 6 Joules at room temperature.

The blanks were then machined and then all separated into two elements by fragile rupture. This separation by brittle fracture was carried out without difficulty whatever the ferrite content.

• C = 0,39 %
• Si = 0,75 %
• Mn = 1,24 %
• Ni = 0,13 %
• Cr = 0,15 %
• Mo = 0,005%
• Cu = 0,2 %
• V = 0,105 %
• S=0,11 %
• P = 0,103 %
• Al = 0,004 %
• N = 0,009 %

le reste étant du fer et des impuretés résultant de l'élaboration.As a third example, connecting rods were made using 38MSV5 type steel, the chemical composition of which included, by weight:

• C = 0.39%
• If = 0.75%
• Mn = 1.24%
• Ni = 0.13%
• Cr = 0.15%
• Mo = 0.005%
• Cu = 0.2%
• V = 0.105%
• S = 0.11%
• P = 0.103%
• Al = 0.004%
• N = 0.009%

the remainder being iron and impurities resulting from processing.

• structure: perlitique avec 0% à 25% de ferrite,
• HB compris entre 260 et 310
• Rm compris entre 880 MPa et 1050 MPa,
• Re compris entre 500 MPa et 700 MPa,
• Kcv inférieur à 6,5 Joules.

Before forging, the piece of steel was heated to 1260 ° C; the end of forging temperature was 1030 ° C. After forging, the blank was cooled by passing through a controlled cooling tunnel at average cooling rates of between 1 ° C / s and 6 ° C / s in order to simulate the effect of dispersions specific to industrial manufacturing. The characteristics obtained were:

• structure: pearlitic with 0% to 25% ferrite,
• HB between 260 and 310
• Rm between 880 MPa and 1050 MPa,
• Re between 500 MPa and 700 MPa,
• Kcv less than 6.5 Joules.

The blanks were then machined and then all separated into two elements by fragile rupture. This separation by brittle fracture was carried out without difficulty whatever the ferrite content.

These examples show that with the steels according to the invention it is possible to reliably manufacture breakable connecting rods and more generally breakable parts having structures of the ferrito-pearlitic type easy to machine at low speed and at high cutting speed.

Claims (12)

Steel for the manufacture of a breakable mechanical part, characterized in that its chemical composition comprises, by weight: 0.25% ≤ C ≤ 0.75% 0.2% ≤ If ≤ 1.5% 0.1% ≤ Mn ≤ 2% 0% ≤ Ni ≤ 1% 0% ≤ Cr ≤ 1% 0% ≤ Mo ≤ 1% 0% ≤ Cu ≤ 1% 0% ≤ V ≤ 0.2% 0.02% ≤ S ≤ 0.35% 0.04% ≤ P ≤ 0.2% 0% ≤ Al ≤ 0.005% 0.005% ≤ N ≤ 0.02% - optionally at least one element taken from lead, tellurium and selenium in contents of less than 0.1%, the rest being iron and impurities resulting from the production, the steel possibly being treated with calcium. Steel according to claim 1 characterized in that its chemical composition is such that:
0.06% ≤ P ≤ 0.12% Steel according to claim 1 or claim 2 characterized in that its chemical composition is such that:
0.8% ≤ If ≤ 1.2% Steel according to any one of Claims 1 to 3, characterized in that its chemical composition is such that:
0.05% ≤ V ≤ 0.15% Steel according to any one of Claims 1 to 4, characterized in that its chemical composition is such that: 0.65% ≤ C ≤ 0.75% 0.25% ≤ Mn ≤ 1% Ni ≤ 0.15% Cr ≤ 0.15% Mo ≤ 0.05% Cu ≤ 0.35% Al ≤ 0.005% Steel according to any one of Claims 1 to 4, characterized in that its chemical composition is such that: 0.25% ≤ C ≤ 0.5% Ni ≤ 0.15% Cr ≤ 0.15% Mo ≤ 0.05% Cu ≤ 0.35% Steel according to claim 6 characterized in that its chemical composition is such that:
0.25% ≤ Mn ≤ 1% Use of a steel according to any one of claims 1 to 7 for the manufacture of a mechanical part comprising at least two elements obtained by fragile rupture of a blank. Use of a steel according to claim 8 characterized in that the part has a ferrito-pearlitic structure. Mechanical part comprising at least two elements obtained by fragile rupture of a blank, and in particular connecting rod for example for internal combustion engine, characterized in that it consists of a steel according to any one of claims 1 to 7. Piece according to claim 10 characterized in that the steel which constitutes it has a hardness between 210 HB and 360 HB, a breaking strength between 650 MPa and 1200 MPa, and a majority of grains whose size index ASTM of austenitic grain is less than 5. Piece according to claim 10 or claim 11 characterized in that the steel which constitutes it has a structure comprising at least 70% of perlite.