EP0779375B1 - Steel for the manufacture of divisible mechanical parts and parts made from this steel - Google Patents

Description

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

Certain mechanical parts such as, for example, the connecting rods internal combustion engine consist of at least two elements separable assembled by fixing means such as screws. These parts can be 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 allowing to ensure good service life of the connecting rods. Characteristics mechanical properties generally required are a hardness of between 210 HB and 360 HB and a breaking strength of between 650 MPa and 1200 MPa. to obtain sufficient resistance to fatigue, and an elastic limit included between 300 MPa and 800 MPa in order to avoid deformation by exceeding the elastic limit.

The precise choice of characteristics required for a particular connecting rod intended for a particular engine depends on the design of the rod and the nature of the engine in which it is incorporated. The steel which constitutes it is chosen in function of these mechanical characteristics and the manufacturing process which includes, after forging, a controlled cooling intended to obtain a ferrito-pearlitic structure which has mechanical characteristics required and satisfactory machinability. The steels used are, in general carbon steels like XC42 or low alloy steels like 45M5, 30MSV6, 38MVS5 (according to French standard). The carbon content is chosen mainly according to the level of hardness required, and the elements of alloy are added either to increase the hardenability of the steel so increase the proportion of perlite, which is favorable for machinability, i.e. to harden ferrite and improve the yield strength to strength ratio rupture. With these steels, the separation of the different parts of the connecting rod does not can only be done by machining which requires a complex machining range and expensive.

Certain rods in cast iron or obtained by powder metallurgy can be separated into two parts by a fragile breaking operation according to a predetermined plan. This technique, called breakable pieces, has several advantages and, in particular, that of simplifying considerably the production range by eliminating operations on the other hand, it has drawbacks resulting from the nature of the usable materials.

In order to take advantage of the advantages of the breakable rod technique for apply it to steel connecting rods, it has been proposed in US patent 5,135,587, using a steel whose chemical composition comprises, by weight: from 0.6% to 0.75% carbon, 0.2% to 0.5% manganese, 0.04% to 0.12% sulfur with Mn / S> 3, the remainder being iron and impurities; content impurities being less than 1.2%; the structure being almost 100% perlitic and the grain size between 3 and 8 ASTM. Impurities taken among 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 disadvantage of having an irregular behavior during the fragile rupture operation, in particular because, it is practically impossible to control industrially the proportion of proeutectoid phase, this can vary from 0% to 15% depending on the exact chemical analysis of the steel and the means of manufacture used, which makes it difficult to use industrially, moreover, it only makes it possible to obtain the characteristics specific to the XC70 which limits its use to parts for which these characteristics are suitable.

The object of the present invention is to remedy these drawbacks by offering a steel which makes it possible to obtain the mechanical characteristics required for a wide range of applications, particularly in the field connecting rods, and good machinability, while allowing the operation to be carried out brittle failure under satisfactory industrial conditions.

To this end, the invention relates to a steel for the manufacture of a breakable mechanical part whose 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%, possibly at minus one element taken from lead, tellurium and selenium in contents less than 0.1%, the remainder being iron and impurities resulting from elaboration, 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% ≤ If ≤ 1.2%, 0.05% ≤ V ≤ 0.15%.

The chemical composition of the steel can be such that: 0.65% ≤ C ≤ 0.75%, 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%, Ni ≤ 0.15%, Cr ≤ 0.15%, Mo ≤ 0.05%, Cu ≤ 0.35%, preferably with 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 piece. This part can be, in particular, a connecting rod for a motor internal combustion 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 of relatively large grains, of which the ASTM size index of the austenitic grains is less than 5, having, preferably, a structure comprising at least 70% perlite.

The invention will now be described in more detail but not limiting.

• 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, in 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% 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 can 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, we 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% silicon, 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% aluminum.

One can also use a steel less loaded with carbon whose chemical composition includes 0.25% ≤ C ≤ 0.5%, Ni ≤ 0.15%, Cr ≤ 0.15%, Mo ≤ 0.05%, Cu ≤ 0.35%. This steel can be carbon steel, at which case it contains less than 0.5% manganese. But it can be also 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 make a breakable piece, we supply a piece of steel according to the invention, it is heated to a temperature between 1100 ° C. and 1300 ° C so on the one hand to austenitize. on the other hand to make the grain fat and finally give it the ductility necessary for forging, then we forge it give it the desired shape; forging ending at a temperature higher than 850 ° C. Directly after forging, it is cooled so controlled up to room temperature, for example in a tunnel cooling, 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 so, we obtain a ferrito-pearlitic structure with a majority of relatively large grains, including the ASTM grain size index austenitic is less than 5, containing less than 30% ferrite, having the hardness and tensile properties required, and a resilience lower than 7 Joules at room temperature. The part blank thus obtained is then machined, then divided into two elements by fragile rupture generated by a shock.

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 made 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 separated in half elements by fragile rupture. This separation by fragile rupture was made 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 separated in half elements by fragile rupture. This separation by fragile rupture was made 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 separated in half elements by fragile rupture. This separation by fragile rupture was made without difficulty whatever the ferrite content.

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

Claims (12)

1. Steel for the manufacture of a separable mechanical component, characterized in that its chemical composition 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%

   optionally at least one element taken from lead, tellurium and selenium in contents of less than 0.1%,

   the balance being iron and impurities resulting from the smelting, the steel being optionally treated with calcium.
2. Steel according to Claim 1, characterized in that its chemical composition is such that:
      0.06% ≤ P ≤ 0.12%.
3. Steel according to Claim 1 or Claim 2, characterized in that its chemical composition is such that:
      0.8% ≤ Si ≤ 1.2%.
4. Steel according to any one of Claims 1 to 3, characterized in that its chemical composition is such that:
      0.05% ≤ V ≤ 0.15%.
5. 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%.
6. 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%.
7. Steel according to Claim 6, characterized in that its chemical composition is such that:
      0.25% ≤ Mn ≤ 1.3%.
8. Use of a steel according to any one of Claims 1 to 7 for the manufacture of a mechanical component, comprising at least two elements, obtained by brittle fracture of a blank.
9. Use of a steel according to Claim 8, characterized in that the component has a ferritopearlitic structure.
10. Mechanical component, comprising at least two elements, obtained by brittle fracture of a blank, and especially a connecting rod, for example for an internal-combustion engine, characterized in that it is composed of a steel according to any one of Claims 1 to 7.
11. Component according to Claim 10, characterized in that the steel of which it is composed has a hardness of between 210 HB and 360 HB and a tensile strength of between 650 MPa and 1200 MPa, most of the grains having an austenitic grain ASTM size index of less than 5.
12. Component according to Claim 10 or Claim 11, characterized in that the steel of which it is composed has a structure consisting of at least 70% pearlite.