SI20334A - Steel and method for making cleavable mechanical parts - Google Patents

Abstract

The invention concerns steel for making a cleavable mechanical part characterised in that its chemical composition comprises (expressed in weight proportions): 0.2 % 2 C 2 0,4 %; 0,1 % 2 Si 1.5 %; 0.3 % 2 Mn 2 1.4 %; 0 % 2 Ni +Cr 2 0.45 %; 0 % 2 Mo 2 0.06 %; 0 % 2 Cu 2 0.5 %; 0.2 % 2 V 2 0.5 %; P 2 0.150 %; 0.005 % 2 N 2 0.02 %; optionally one or several elements selected among lead up to 0.1 %, tellurium up to 0.15 %, bismuth up to 0.15 %. selenium up to 0.02 %, sulphur up to 0.35 % and calcium up to 0.005 %; optionally at least one element selected among titanium up to 0.05 %, niobium up to 0.1 % and aluminium up to 0.07 %, the remainder being iron and impurities and residuals resulting from the preparation; the steel having an essentially ferrito-pearlitic structure, the ferritic fraction being less than 20 %, the tensile strength of the steel being less than 1000 MPa, the yield strength being more than 700 MPa, the ratio Re/Rm being more than 0.73 and the Kcv impact strength being less than 7 Joules/cm2. The invention also concerns a method for making a cleavable part and the resulting part.

Description

Steel and manufacturing process for detachable machine parts

The invention relates to steel for the manufacture of detachable machine parts, in particular for the manufacture of a connecting rod for an internal combustion engine.

Certain machine parts such as e.g. internal combustion engine connecting rods consist of at least two separable elements which are interconnected by means of fasteners such as screws. These parts can be made of cast iron, sintered and pressed metal powder, or forged steel. The invention relates to parts made of forged steel.

The steel from which the forged steel connecting rods are made must be well forged and machined, and at the same time have satisfactory mechanical properties that guarantee the required reliability of the connecting rod during operation. Typically, the required mechanical properties are hardness between 210 HB and 360 HB and a tensile ultimate tensile strength of 650 MPa to 1200 MPa, which also provides sufficient abrasion resistance, as well as a plasticity limit of 300 MPa to 800 MPa, thereby avoiding deformation due to exceeding the plasticity limit. For some sheaths, the plasticity limit Re should be higher than 700 MPa and the tensile strength should be less than 1100 MPa.

The connecting rods, which consist of two separate parts - the body and the head - can be made by forging workpieces. The forgings are then mechanically machined and brittle fractured into two parts in a predetermined plane. This technology, it is said, for the production of detachable parts has many advantages, in particular in that the contraction of the machining operations significantly simplifies the sequence of manufacturing steps. On the other hand, this requires the use of separable steel, which means that it allows the brittle fracture operation to be carried out properly.

In order to facilitate the manufacture of separable connecting rods, the Japanese patent application proposed the use of steel with a content of 0.4% to 0.6% carbon, 0.5% to 5% silicon, 0.1% to 0 , 8% manganese, from 0.1% to 0.5% chromium, from 0.1% to 0.5% vanadium and from 0.01% to 0.2%, preferably more than 0.05% phosphorus, at the remainder being iron with impurities and optionally small additives to improve mechanical workability. However, the disadvantage of such a steel is that it has almost entirely a pearlitic structure, which limits the ferrite fraction and therefore negates the effects of carbonitriding, while at the same time the uniformity of distribution of ferrite m is good. In addition, the tensile properties (Re and Rm) of this steel are very sensitive to cooling conditions, which reduces the possibility of mass production of parts.

The French patent application FR 2,742,448 also proposed the use of steel with a content of 0.25% to 0.5% carbon, 0.2% to 1.5% silicon, 0.1% to 2% manganese, less as 0.15% chromium, less than 0.15% nickel, less than 0.05% molybdenum, less than 0.35% copper, from 0% to 0.2% vanadium, from 0.04% to 0.2 % phosphorus and from 0.005% to 0.02% nitrogen, the remainder being iron with impurities and optionally small additives to improve mechanical workability. This steel can be counted on with a well-resolved ferrite-pearlite structure. On the other hand, it does not allow to guarantee both, namely the plasticity limit above 700

MPa and tensile strengths below 1100 MPa. In addition, resolution can only be ensured by sufficient phosphorus addition, which results in disadvantages in terms of increasing the risk of segregation and impaired mechanical workability.

The purpose of the present invention is to avoid these drawbacks by proposing means for making detachable forgings whose plasticity limit is greater than 700 MPa and tensile strength is less than 1100 MPa and should have good mechanical machining properties while making the operation possible brittle breaking in satisfactory industrial conditions.

To this end, the subject of the invention is steel for the manufacture of detachable machine parts, the chemical composition of which (by weight) is as follows:

0.2% <C <0.4%

0.1% <Si <1.5%

0.3% <Mn <1.4%

0.2% <V <0.5%

P <0,150%

0.005% <N <0.02%

- if necessary, it contains one or more elements selected from lead up to 0,1%, tellurium up to 0,15%, bismuth up to 0,15%, selenium up to 0,02%, sulfur up to 0,35% and calcium up to 0,005% ,

- if necessary, at least one element selected from titanium up to 0.05%, niobium up to 0.1% and aluminum up to 0.07%, the remainder constituting iron together with impurities and smelting residues, the steel being substantially ferritic, pearlitic structure, where the ferrite fractions are at least 20%, the tensile strength of the steel is less than 1100 MPa, the plasticity limit is higher than 700 MPa, the Re / Ra ratio is greater than 0.73 and the refractive energy Kcv is lower than 7 J / cm ² .

The phosphorus content is preferably less than 0.03%. It is also preferred that the residue, meaning nickel, chromium and molybdenum, be such that:

Ni + Cr <0.45%

Mo <0.06%.

The invention also relates to a part made of this type of steel, the structure of which is essentially ferrite-pearlite, the ferrite fraction is at least 20%, its tensile strength is less than 1100 MPa, its plasticity limit is higher than 700 MPa, the Re / Ra ratio is more than 0.73 and a refractive energy Kcv of less than 7 J / cm ² . That part, which can be a drawbar, is separable.

Finally, the invention relates to a method of manufacturing a steel part comprising at least two separate parts, wherein

- a steel blank having the chemical composition of steel according to the invention is heated to a temperature between Ac ₃ + 150 ° C and 1370 ° C;

- the blank is then forged, thus producing a forging;

- after forging, the forgings are cooled to room temperature at a rate between 0.4 ° C / s and 1.5 ° C / s, from 700 ° C to give a ferrite-pearlite structure, and the ferrite fraction is is at least 20%, the tensile strength of the steel is less than 1100 MPa, the plasticity limit is greater than 700 MPa, the Re / Rm ratio is greater than 0.73 and the refractive energy Kcv is less than 7 J / cm ² ;

- the forging is then subjected to one or more mechanical processing operations; and

- the resulting part is brittle into at least two parts.

The invention will now be described in more detail, but without any restriction.

The steel of the invention is a carbon or low alloy structural steel whose chemical composition is by weight as follows:

- more than 0,2% and preferably more than 0,21% carbon, in order to ensure sufficient mechanical strength; however, less than 0,4% of carbon, in order to avoid hypersensitivity to mechanical properties with regard to cooling conditions and also to avoid excessive hardness which would complicate mechanical treatment;

- from 0.1% to 1.5% of silicon, the silicon being a deoxidizing element which, in order to ensure proper deoxidation, must be added in quantities of more than 0.1% and in quantities of less than 1.5% and preferably between 0.6% and 1.2%, this element consolidates and distributes ferrite, which is conducive to improving the workability of the mean, while also facilitating brittle fracture;

- from 0.3% to 1.4% of manganese due to the retention of sulfur in the form of manganese sulphides and to the stabilization of hardness in order to provide a ferrite-pearlite structure having at least 30% ferrite content, preferably in terms of mechanical workability; at least 90% of the structure is said to be ferrite-pearlite;

- from 0,2% to 0,4% and preferably more than 0,21 vanadium for the purpose of hardening the ferrite and for providing a plasticity limit of over 700 MPa and a ratio of plasticity to tensile strength of at least 0,72, which is preferable for providing brittle brittleness ;

- up to 0.2%, preferably less than 0.03% phosphorus; While high phosphorus content is preferred in terms of providing brittle brittleness, it is advisable to limit the content to 0.03%, thereby limiting segregation which adversely affects mechanical workability;

- from 0.005% to 0.02% nitrogen due to the formation of vanadium nitride, which hardens ferrite;

- optionally one or more elements selected from lead up to 0.1%, tellurium up to 0.15%, bismuth up to 0.15%, selenium up to 0.02%, sulfur up to 0.35% and calcium up to 0.005%, thus also improving mechanical workability;

- optionally at least one element selected from titanium up to 0.05%, niobium up to 0.1% and aluminum up to 0.07% to limit grain growth during forging.

The remainder of the composition is iron, together with impurities and residues from smelting. The residues already included in the raw material are, in particular, nickel, chromium and vanadium. In order to provide a predominantly ferrite-pearlite structure containing more than 20% and preferably more than 30% ferrite, the content of these residues should be controlled so that the sum of Ni + Cr content remains below 0.45% and that the molybdenum content remains below 0.05%.

In addition, in the context of appropriate structure control, it is preferable that, when the manganese and vanadium content are so high, Mn + 10V <5.2%.

In order to produce the separable part, the steel blank of the composition according to the invention must be heated to a temperature between Ac ₃ + 150 ° C and 1370 ° C, in order to austenize and dissolve the vanadium, then hot forge and forge and forge at above 800 ° C. Immediately after forging - when the temperature is 700 ° C - the forgings are cooled in a controlled manner to room temperature, e.g. in air, at a cooling rate of between 0,5 ° C / s and 1, 5 ° C / s. Such treatment results in a ferrite-pearlite structure (ie at least 90% ferrite-pearlite), in the structure at least 20% and preferably more than 30% ferrite, with a plasticity limit Re higher than 700 MPa and a tensile strength lower than 1100 MPa and preferably higher than 850 MPa. In addition, at room temperature, the refractive energy Kcv is lower than 7 J / cm ² and the Re / Rm ratio is greater than 0.73 and preferably greater than 0.75. The latter conditions make it possible to achieve clear fractures without deformed edges, thus indicating good resolution. The forgings thus obtained are then mechanically machined and then separated into two elements by brittle fracturing.

In the first case, connecting rods were made using the steel according to the invention, the chemical composition by weight being as follows:

C = 0.319%

Si = 0.61%

Mn = 1.02%

V = 0,3%

Ni = 0,214%

Cr = 0.21%

Mo = 0.05%

Cu = 0.21%

S = 0.059%

P = 0.017%

Al = 0.02%

N = 0.0085% of the residue, however, represented the iron, together with impurities arising from the smelting.

Before forging, the steel blanks were heated to 1270 ° C and the temperature at the end of the forging was 1005 ° C.

After forging, the forgings were cooled in air at an average cooling rate of 0.9 ° C / s at 700 ° C. The properties obtained were as follows:

Structure: 98% ferrite-pearlite with 60% ferrite

Rm = 1050 MPa Re = 840 MPa A% = 14.5%

Kcv = 4 J / cm ² at room temperature.

The forgings were then mechanically machined and then all separated by brittle breaking into two elements each. This separation into two parts took place without any problems.

In the second case, the connecting rods were made of steel according to the invention, the chemical composition of which by weight was as follows:

C = 0,208%

Si = 0.605%

Mn = 1.02%

V = 0,3%

Ni = 0.212%

Cr = 0.211%

Mo = 0.05%

Cu = 0.205%

S = 0.059%

P = 0.016%

Al = 0,022%

N = 0.0077% of the remainder, however, represented the iron, together with the impurities arising from the smelting.

Prior to forging, the steel blank was heated to 1270 ° C and the temperature at the end of forging was 1025 ° C. After forging, the forgings were cooled in air at an average cooling rate of 0.9 ° C / s at 700 ° C. The properties obtained were as follows:

Structure: 98% ferrite-pearlite with 60% ferrite

Rm = 950 MPa Re - 740 MPa A% = 17%

Kcv = 5 J / cm ² at room temperature.

The forgings were then mechanically machined, and each was then divided into two elements with fragile fracturing. This fracture separation took place without any problems.

For:

ASCOMETAL

-iv A, i »'· ftrLNTAftVd ...

m BORŠTAR s.p

I. fl V E N t, J A (S: I

Claims (8)

PATENT APPLICATIONS 1. Steel for the manufacture of detachable machine parts, characterized in that its chemical composition by weight is as follows: 0.2% <C <0.4% 0.1% <Si <1.5% 0.3% <Mn <1.4% 0.2% <V <0.5% P <0,150% 0.005% <N <0.02% - if necessary, it contains one or more elements selected from lead up to 0,1%, tellurium up to 0,15%, bismuth up to 0,15%, selenium up to 0,02%, sulfur up to 0,35% and calcium up to 0,005 %, - if necessary, at least one element selected from titanium up to 0.05% is not up to 0.1% and aluminum is up to 0.07%, the remainder being iron together with impurities and smelting residues, whereby the steel is essentially ferritic -perlite structure, where the ferrite fractions are at least 20%, the tensile strength of the steel is less than 1100 MPa, the plasticity limit is higher than 700 MPa, the Re / Rm ratio is greater than 0.73 and the refractive energy Kcv is lower than 7 J / cm ² . Steel according to claim 1, characterized in that its chemical composition is such that P <0.03%. Steel according to claim 1, characterized in that the residual content such as nickel, chromium and molybdenum is such that Ni + Cr <0.45% Mo <0.06%. Steel according to one of Claims 1, 2 and 3, characterized in that the ferrite fractions are at least 30%. Steel according to one of Claims 1, 2, 3 and 4, characterized in that the Re / Rm ratio is greater than 0.75. Steel according to any one of claims 1 to 5, characterized in that the tensile strength is greater than or equal to 850 MPa. 7. A steel part according to any one of the preceding articles, characterized in that it is a connecting rod. 8. A method of manufacturing a steel part comprising at least two separable parts. characterized in that - steel blank consisting of steel, the chemical composition of which by weight is as follows: 0,2% <C <0,4% 0.1% <Si <1.5% 0.3% <Mn <1.4% 0.2% <V <0.5% P <0,150% 0.005% <N <0.02%, if necessary containing one or more elements selected from lead up to 0.1%, tellurium up to 0.15%, bismuth up to 0.15%, selenium up to 0.02%, sulfur up to 0, 35% and calcium up to 0.005% and, if necessary, at least one element selected from titanium up to 0.05%, niobium up to 0.1% and aluminum up to 0.07%, the remainder being iron together with impurities and smelting residues, heated to a temperature between Ac ₃ + 150 ° C and 1370 ° C; - that the workpiece will then forge and receive forgings, - after forging, the forgings are cooled to room temperature at a rate of between 0,4 ° C / s and 1, 5 ° C / s, from 700 ° C to a predominantly ferritic-perlite structure, and the ferrite fraction is is at least 20%, the tensile strength of the steel is less than 1100 MPa, the plasticity limit is greater than 700 MPa, the Re / Rm ratio is greater than 0.73 m. The refractive energy Kcv is less than 7 J / cm ² ; - the forging is then subjected to one or more mechanical processing operations; and then - the resulting part is brittle into at least two parts.