Method for production and forming of cast pieces of spheroidal graphite with improved mechanical properties

Abstract

The invention relates to a method of preparing and forming parts of spheroidal graphite cast iron having high-grade mechanical characteristics. The method comprises the following steps: a) preparing a mixture in the liquid state comprising by weight: 3% to 4% C; 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to 1.5% Cu; 0 to 0.5% Mo, 0.025% to 0.080% Mg, the balance being iron and impurities resulting from preparation; b) casting the mixture in the liquid state at a temperature of from 1350° C. to 1550° C. into a mold c) extracting a blank of the part from the mold at a temperature between solidus and AR3; d) forming the blank of the part by hot plastic deformation; the forming taking place at a temperature of from 1050° C. and AR3; e) quenching said part directly in the heat of forming at a temperature in the bainitic range, and maintaining said temperature; and f) cooling said part to ambient temperature.

Description

The present invention relates to a method of preparing and forming castings of spheroidal graphite cast iron with high-grade mechanical characteristics, and to the cast iron as obtained by implementing the method.

More precisely, the invention relates to a method of preparing and forming castings of spheroidal graphite cast iron having high toughness, i.e. high mechanical strength. The term “high toughness” is used typically to designate cast irons presenting traction strength in the range 1000 megapascals (MPa) to 1700 MPa, a ratio of elastic limit over traction strength Rp0.2/Rm≧0.68, and breaking elongation lying in the range about 4% to about 14%.

Advances in certain techniques make it necessary for cast irons, or more generally materials, to be available that have high-grade characteristics, capable of withstanding higher and higher mechanical stresses. This applies in particular in the automotive industry because of the increasing performance of motor vehicle engines, itself leading to an increase in the levels of stress applied to various mechanical members and components.

At present, numerous grades of molded cast iron are available for such mechanical applications. They represent a very wide variety of ranges of characteristics adapted to various kinds of stress and they often constitute materials that are competitive over other materials, and in particular over high performance forged steels, for manufacturing members that are subjected to very high stresses.

Nevertheless, in certain applications, economic constraints due to competition are more and more severe and it is therefore particularly advantageous to be able to extend the range of construction materials capable of satisfying very severe conditions of use and meeting the expectations of manufacturers for making various parts.

To achieve this object, the invention provides a method of preparing and forming parts of spheroidal graphite cast iron having high-grade mechanical characteristics, the method being characterized in that it comprises the following steps:

• • a) preparing a mixture in the liquid state having the following composition by weight: 3% to 4% C: 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to 1.5% Cu; 0 to 0.5% Mo, with a residual Mg content adapted to the thickness of the parts lying in the range 0.025% to 0.080%, the balance being iron and impurities resulting from preparation; the impurities being in particular S at a content of less than 0.015%, and P at a content of less than 0.10%;
  • b) casting the mixture in the liquid state at a temperature lying in the range 1350° C. to 1550° C. into a mold enabling a shape approaching that of the workpiece, which is referred to as the blank of the part;
  • c) extracting said blank of the part from the mold at a temperature Ts lying between the solidus and AR3, the solidus and AR3 representing the temperatures defining the austenitic region of said composition;
  • d) forming the blank of the part by hot plastic deformation directly in the heat of casting, in order to obtain the part with its final shape and dimensions, the forming taking place at a temperature Tf lying in the range 1050° C. and AR3;
  • e) quenching said part directly in the heat of forming at a temperature Tb lying in the bainitic range, and maintaining it at said temperature Tb for a duration tb; and
  • f) cooling said part to ambient temperature.

The term “quenching” or “bainitic staged quenching” is used here and throughout the text below in the broad sense, i.e. it relates to cooling that is fast enough to obtain a structure with practically no ferrito-pearlite and with practically no martensite either.

Quenching or bainitic staged quenching is intended to confer an essentially bainitic structure to the workpiece, i.e. constituted by more than 50% bainite.

For the grades of cast iron implemented using said composition, the rate of cooling corresponding to bainitic staged quenching lies in the range 15° C./sec to 150° C./sec.

In this first implementation of the method of the invention, and also in the following implementation when Ts is greater than Tf, all of the successive steps needed for making the cast iron workpiece from the step of casting to the step of cooling the workpiece to ambient temperature are performed in the heat of casting, i.e. without intermediate reheating between said steps. These two implementations of the method of the invention without intermediate reheating between the successive steps needed for making the cast iron workpiece provide particularly high performance in terms of energy consumption compared with conventional heat treatment and forming methods. They are therefore particularly inexpensive for preparing spheroidal graphite cast iron with high-grade mechanical characteristics.

In another implementation of the method of the invention, between step c) and step d), i.e. between the blank being extracted from the mold and the blank being formed by plastic deformation while hot, an additional step is performed in which the blank is maintained at a temperature Tm that is 200C to 500C higher than the forging temperature Tf for a duration lying in the range 10 min to 60 min in order to ensure that temperature within the blank is uniform, when the temperature Ts at which the blank is extracted from the mold is greater than the temperature Tf that is desired for forging; or when the temperature Ts at which the blank is extracted from the mold is lower than the temperature Tf that is desired for forging, in which case the blank is heated and maintained at the temperature Tm that is 20° C. to 50° C. higher than the forging temperature Tf for a duration lying in the range 10 min to 60 min.

In another implementation of the method of the invention, step e) for implementing direct bainitic staged quenching is replaced by one of the two following treatments:

• • 1) after hot plastic deformation, the workpiece is cooled into the intercritical region at a temperature Tir lying in the range AR1+20° C. and AR3, it is maintained at this temperature Tir for a duration lying in the range 15 min to 60 min to allow its structure to homogenize, and bainitic staged quenching is performed at a temperature Tb situated in the bainitic region, and the workpiece is maintained at a temperature Tb for a duration tb;
  • 2) after hot plastic deformation, the workpiece is allowed to cool to below the temperature AR1, and then the workpiece is reheated and maintained at a temperature Tic lying in the range AC1+20° C. to AC3, i.e. in the intercritical region for the composition of the cast iron for a duration lying in the range 30 min to 180 min in order to allow the structure to homogenize, and bainitic staged quenching is performed at a temperature Tb situated in the bainitic region, and the workpiece is maintained at the temperature Tb for a duration tb.

AR1, AR3, AC1, and AC3 represent the limits of the critical interval, also known as the intercritical region, of said composition as measured respectively during cooling (index R) and heating (index C); the temperature Tic or Tir that is maintained in the intercritical region or the critical interval lies in the range 740° C. to 850° C., depending on the composition of the cast iron in question.

Treatment 2) is more expensive in energy terms than treatment 1), but is nevertheless preferable since it makes it easier to adjust the structure of the cast iron in terms of ferrite content and austenite content prior to bainitic staged quenching.

This implementation of the invention with treatment in the intercritical region prior to bainitic staged quenching serves to reduce the hardness obtained on the workpieces, and thus makes them easier to machine subsequently, where appropriate; it also encourages very high toughness of the material by the presence of a larger proportion of ferrite in the structure obtained for the cast iron after bainitic staged quenching. In fact, the structure is constituted under these circumstances by two varieties of ferrite, a ferrite I resulting from the treatment in the intercritical region, and a ferrite II resulting from the treatment in the bainitic region, together with austenite; the ferrite II and the austenite represent the bainite.

In another implementation of the method of the invention, step e) consisting in performing bainitic staged quenching is preceded by treatment of maintaining a temperature lying in the range 950° C. to 900° C. for a duration lying in the range 15 min to 60 min for the purpose of making the temperatures of the various portions of the workpiece more homogeneous and thus making its chemical composition more homogeneous prior to the following bainitic staged quenching treatment.

Finally, in another implementation of the method of the invention, an additional step is added of cold calibration of the workpiece after step f) of cooling the workpiece to ambient temperature, which calibration is performed between at least two matrices having the shape of the finished part in order to improve its dimensional precision and increase the mechanical characteristics and the fatigue strength of the material of the workpiece by work-hardening its surface; prior to this operation of cold calibration, shot-blasting is performed, which serves to remove scale and to generate surface compression stresses serving to further reinforce the effect of the work-hardening due to calibration.

The various implementations of the method of the invention for preparing and forming parts made of spheroidal graphite cast iron enable cast iron to be obtained of structure that can be essentially bainitic or that can present a structure constituted by two varieties of ferrite: ferrite I, and ferrite II with austenite, the ferrite I being that which results from remaining in the critical interval at the temperature Tir or Tic, and the ferrite II being that which results from treatment in the bainitic region at the temperature Tb, and having mechanical characteristics at 20° C. that are typically traction strength Rm lying in the range 1000 MPa to 1700 MPa, a ratio of elastic limit over traction strength: Rp0.2/Rm greater than 0.68, and a breaking elongation coefficient lying in the range 4% to 14%. The temperature Tic or Tir maintained in the intercritical region or in the critical interval lies in the range 740° C. to 850° C., depending on the composition of the cast iron in question.

The term “hot plastic deformation” as used above and below designates deformation at a ratio lying overall in the range 2% to 60%.

The best results are obtained for a good strength-to-toughness compromise corresponding to hot plastic deformation ratios lying in the range 20% to 50% depending on the forging temperatures used in the range 1050° C. to AR3 of the cast iron, since at above 50%, large deformation is observed of the graphite nodules that is harmful for the mechanical characteristics. The deformation ratios given herein are defined as being the difference between the initial thickness of the section of the workpiece subjected to forging by flattening and the final thickness thereof after forging, relative to the initial thickness of said section of the workpiece and multiplied by 100 in order to express the result as a percentage.

The term “hot plastic deformation” or the term “forging” means above and below mainly an operation of stamping at a temperature greater than AC3 or AR3 of said cast iron composition, but also designates other forms of hot plastic deformation implemented at a temperature higher than AC3 or AR3 of said cast iron composition, such as free forging, stamping, rolling, hydroforming, etc.

In order to perform this hot plastic deformation in the context of the invention, it is necessary for the blank of the molded workpiece, which blank is approximately in the shape of the part, to possess at least one of its dimensions that is greater than that of the part proper so as to make said hot plastic deformation possible.

Preferably, the mold used for casting the workpiece blank is a permanent mold constituted by at least two metal half-portions coated in a release agent, however the mold may also be a semi-permanent sand mold constituted by at least two sand shell mold portions placed in a metal mold or it could also be a non-permanent mold of chemical sand or of green silica-clay sand.

When using sand molds, the workpiece blank must be hot-brushed or hot-sanded to remove grains of sand adhering thereto, prior to performing the forging operation.

The bainitic staged quenching temperature Tb lies in the range 260° C. to 420° C. This temperature Tb is preferably selected to lie in the range 260° C. to 300° C. when it is desired to have a high Rp0.2/Rm ratio together with high traction strength Rm; conversely, when it is desired to have traction strength Rm close to 1000 MPa or 1100 MPa, Tb should be selected to be greater than 300° C.

In order to obtain a properly-formed structure containing bainite constituted by ferrite and austenite saturated in carbon, the duration tb at which the bainitic staged quenching temperature Tb is maintained should preferably lie in the range 60 min to 180 min.

The invention also provides spheroidal graphite cast iron having high-grade mechanical characteristics prepared and formed using the method of the invention, and having a structure that is essentially bainitic, with the following mechanical characteristics at 20° C.:

• • traction strength Rm: 1000 MPa≦Rm≦1700 MPa;
  • ratio of elastic limit Rp0.2 over traction strength Rm: Rp0.2/Rm≧0.68; and
  • breaking elongation A: 4%≦A≦14%.

Finally, the invention provides a spheroidal graphite cast iron with high-grade mechanical characteristics prepared and formed by implementing the method of the invention, of structure made up of two varieties of ferrite: ferrite I and ferrite II together with austenite, the ferrite I being the result of spending time in the critical interval at the temperature Tir or Tic, and the ferrite II being the result of treatment in the bainitic region at the temperature Tb, the cast iron having the following mechanical characteristics at 20° C.:

• • traction strength Rm: 1000 MPa≦Rm≦1700 MPa;
  • ratio of elastic limit Rp0.2 over traction strength Rm: Rp0.2/Rm≧0.68; and
  • breaking elongation A: 4%≦A≦14%.

The present invention is particularly adapted, but not restricted, to fabricating automotive engine parts such as connecting rods for example or other moving parts of the engine. Lighter in weight and having mechanical characteristics that are equal to or better than those of the forged steel presently used in engines, connecting rods made of spheroidal graphite cast iron with high-grade mechanical characteristics prepared and formed by the method of the invention can enable the engine to have improved energy efficiency and thus achieve savings in its fuel consumption.

Other characteristics and advantages of the present invention appear better on reading the following description of two implementations of the invention given by way of non-limiting example.

Tests have been performed on various testpieces of spheroidal graphite cast iron having the following compositions:

Ref.	C %	Si %	Mn %	Ni %	Cu %	Mo %	Mg %
1	3.54	2.61	0.17	traces	1.02	traces	0.040
2	3.6	2.40	0.12	0.7	0.9	0.15	0.042
3	3.72	2.42	0.24	traces	0.6	traces	0.045
4	3.7	2.70	0.13	0.3	0.5	0.28	0.045

In accordance with the invention, these cast irons referenced 1 to 4 were prepared in an electric induction furnace, treated with ferro-silico-magnesium, and they were cast at a temperature lying in the range 1460° C. to 1500° C. into a metal mold of temperature regulated at 280° C. and coated in a protective release agent.

Five testpiece blanks were cast using each cast iron composition, the testpieces being in the form of rectangular bars having the following dimensions:

	Section in
Reference of blank	millimeters (mm)	Length in mm
A	20 × 10	150
B	20 × 12	150
C	20 × 15	150
D	20 × 18	150
E	20 × 24	150

For each mold casting, a full set of testpiece blanks was made. Four castings were undertaken per composition.

In an implementation of the invention, the testpiece blanks were extracted from the mold at a temperature that was found throughout testing to lie in the range 1000° C. to 980° C., and they were placed immediately in a bath comprising a fluidized bed of silica sand at a temperature regulated at 980° C. so as to ensure the same forging temperature for each blank.

Ten minutes after being placed in the fluidized bed bath at a regulated temperature of 980° C., the blanks were withdrawn one after another at intervals of about 10 seconds (sec) and they were formed by hot plastic deformation by being stamped between two matrices having the shape of the testpiece hollowed out therein. Each blank was thus plastically deformed by stamping at a temperature lying in the range 960° C. to 940° C. in order to bring the final section of each testpiece to 20 mm×10 mm.

Under such conditions, the deformation ratio to which each testpiece blank was subjected was as follows:

	Initial	Final
Blank	section	section	Deformation ratio
reference	in mm	in mm	in %
A	20 × 10	20 × 10	0%
B	20 × 12	20 × 10	(12 − 10)/12 × 100 = 16.7%
C	20 × 15	20 × 10	(15 − 10)/15 × 100 = 33.3%
D	20 × 18	20 × 10	(18 − 10)/18 × 100 = 44.4%
E	20 × 24	20 × 10	(24 − 10)/24 × 100 = 58.3%

Immediately after stamping, each testpiece was subjected to a deburring operation in a press and was immediately placed in a fluidized bed bath of zircon sand at a regulated temperature of 300° C. and of sufficiently large volume to guarantee temperature variation in the fluidized bed of less than 5° C. when the various testpieces were immersed therein at intervals of about 10 sec. Each testpiece was thus subjected to bainitic staged quenching at the temperature of 300° C. and was maintained at said temperature in the fluidized bed for a duration of 110 min, after which each testpiece was withdrawn from the fluidized bed bath and allowed to cool in air down to ambient temperature in accordance with the invention.

The table of FIG. 1 shows the results obtained. This table shows that the expected mechanical characteristics were indeed obtained, i.e.:

• • traction strength Rm: 1000 MPa≦Rm≦1700 MPa;
  • ratio of elastic limit Rp0.2 over traction strength Rm: Rp0.2/Rm≧0.68; and
  • breaking elongation A: 4%≦A≦14%; with an essentially bainitic structure constituted of ferrite and austenite.

Another series of tests was undertaken in accordance with another implementation of the invention using the cast iron composition referenced 2: in this other test, the conditions under which the testpiece blanks were prepared and the testpieces themselves were the same as in the above test except that an additional step of maintaining the testpieces at a temperature in the intercritical region was performed between the stamping and deburring operation and the operation of bainitic staged quenching. After forging and deburring in the same manner as in the above test, the testpieces were placed immediately in a fluidized bed at a regulated temperature Tic of 810° C., situated in the intercritical region and lying in the range AC1+20° C. to AC3 for a duration of 60 min, after which the testpieces were quenched in the bainitic region in the fluidized bed at a regulated temperature Tb of 300° C. and they were maintained at that temperature for a duration of 110 min, as in the above test, and under exactly the same conditions. At the end of that bainitic staged quenching treatment, the testpieces were withdrawn from the fluidized bed bath and allowed to cool in air down to ambient temperature, in accordance with the invention.

The table of FIG. 2 shows the results obtained. This table shows that the expected mechanical characteristics were indeed obtained, i.e.:

• • traction strength Rm: 1000 MPa≦Rm≦1700 MPa;
  • ratio of elastic limit Rp0.2 over traction strength Rm: Rp0.2/Rm≧0.68; and
  • breaking elongation A: 4%≦A≦14%;
    with a structure made up of two varieties of ferrite: a ferrite I resulting from being maintained in the intercritical region at the temperature Tic or Tir, and a ferrite II with austenite, the ferrite II coming from the treatment in the bainitic region performed at the temperature Tb.

Claims

1-20. (canceled)

21. A method of preparing and forming parts of spheroidal graphite cast iron having high-grade mechanical characteristics, the method comprising the following steps:

a) preparing a mixture in the liquid state having the following composition by weight: 3% to 4% C; 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to 1.5% Cu; 0 to 0.5% Mo, with a residual Mg content adapted to the thickness of the parts lying in the range 0.025% to 0.080%, the balance being iron and impurities resulting from preparation; the impurities including S at a content of less than 0.015%, and P at a content of less than 0.10%;

b) casting the mixture in the liquid state at a temperature of from 1350° C. to 1550° C. into a mold enabling a shape approaching that of the workpiece, which is referred to as the blank of the part;

c) extracting said blank of the part from the mold at a temperature Ts ranging from the solidus to AR3, wherein the solidus and AR3 representing the temperatures defining the austenitic region of said composition;

d) forming a blank of the part by hot plastic deformation directly in the heat of casting, in order to obtain the part with its final shape and dimensions, the forming taking place at a temperature Tf of from 1050° C. to AR3;

e) quenching said part directly in the heat of forming at a temperature Tb lying in the bainitic range, and maintaining the part at said temperature Tb for a duration tb; and

f) cooling said part to ambient temperature.

22. A method of preparing and forming parts of spheroidal graphite cast iron having high-grade mechanical characteristics, the method comprising the following steps:

a) preparing a mixture in the liquid state having the following composition by weight: 3% to 4% C; 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to 1.5% Cu; 0 to 0.5% Mo, with a residual Mg content adapted to the thickness of the parts lying in the range 0.025% to 0.080%, the balance being iron and impurities resulting from preparation; the impurities including S at a content of less than 0.015%, and P at a content of less than 0.10%;

b) casting the mixture in the liquid state at a temperature of from 1350° C. to 1550° C. into a mold enabling a shape approaching that of the workpiece, which is referred to as the blank of the part;

c) extracting said blank of the part from the mold at a temperature Ts ranging from the solidus and AR3, the solidus and AR3 representing the temperatures defining the austenitic region of said composition;

c′) maintaining the blank at a temperature Tm that is 20° C. to 50° C. higher than a forging temperature, for a duration of from 10 min to 60 min, so as to ensure that the temperature inside the blank is homogeneous;

d) forming a blank of the part by hot plastic deformation, in order to obtain the part with its final shape and dimensions, the forming taking place at a temperature Tfoffrom 1050° C. to AR3;

e) quenching said part directly in the heat of forming at a temperature Tb lying in the bainitic range, and maintaining the part at said temperature Tb for a duration tb; and

f) cooling said part to ambient temperature.

23. A method according to claim 22, wherein when the temperature Ts at which the blank is extracted from the mold is lower than the temperature Tf desired for forging, the blank is heated and maintained at the temperature Tm that is 20° C. to 50° C. greater than the forging temperature Tf for a duration of from 10 min to 60 min during step c′).

24. A method of preparing and forming parts of spheroidal graphite cast iron having high-grade mechanical characteristics, the method comprising the following steps:

a) preparing a mixture in the liquid state having the following composition by weight: 3% to 4% C; 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to 1.5% Cu; 0 to 0.5% Mo, with a residual Mg content adapted to the thickness of the parts lying in the range 0.025% to 0.080%, the balance being iron and impurities resulting from preparation; the impurities including S at a content of less than 0.015%, and P at a content of less than 0.10%;

b) casting the mixture in the liquid state at a temperature of from 1350° C. to 1550° C. into a mold enabling a shape approaching that of the workpiece, which is referred to as the blank of the part;

c) extracting said blank of the part from the mold at a temperature Ts ranging from the solidus and AR3, the solidus and AR3 representing the temperatures defining the austenitic region of said composition;

d) forming a blank of the part by hot plastic deformation directly in the heat of casting, in order to obtain the part with its final shape and dimensions, the forming taking place at a temperature Tf of from 1050° C. to AR3;

e) cooling the part in the intercritical region at a temperature Tir of from AR1+20° C. to AR3, the part being maintained at said temperature Tir for a duration of from 15 min to 60 min to allow its structure to become homogeneous, and performing bainitic staged quenching at a temperature Tb situated in the bainitic region, and maintaining the part at the temperature Tb for a duration tb, AR1, AR3, AC1, and AC3 representing the limits of the critical interval or of the intercritical region of said composition as measured respectively on cooling (index R) and on heating (index C); and

f) cooling said part to ambient temperature.

25. A method according to claim 24, wherein after the part has been formed by hot plastic deformation in step d), said part being allowed to cool to below the temperature AR1, and then heating the part and maintaining the part at a temperature Tic of from AC1+20° C. to AC3, the intercritical region of the composition of the cast iron, for a duration of from 30 min to 180 min in order to allow the part's structure to become homogeneous, and bainitic staged quenching is performed at a temperature Tb situated in the bainitic region and the part is maintained at the temperature Tb for a duration tb, AR1, AR3, AC1, and AC3 representing the limits of the critical interval of said composition as measured respectively on cooling (index R) and on heating (index C); and

said part is cooled to ambient temperature.

26. A method according to claim 21, wherein the step corresponding to bainitic staged quenching of the part at a temperature Tb is preceded by a treatment step comprising maintaining the part at a temperature of from 950° C. to 900° C. for a duration of from 15 min to 60 min for the purpose of making the temperatures of the various portions of the part homogeneous and making the part's chemical composition homogeneous prior to the bainitic staged quenching treatment.

27. A method according to claim 21, wherein an additional step of cold calibration of the part is added after the step of cooling the part to ambient temperature, the additional step being performed between at least two matrices having the shape of the finished part in order to improve its dimensional precision and to increase the mechanical characteristics and the fatigue strength of the material of the part by work-hardening its surface.

28. A method according to claim 27, wherein prior to the operation of cold calibration, a shot-blasting operation is performed for the purpose of removing scale and delivering compression stresses to the surface enabling the work-hardening effect due to calibration to be reinforced.

29. A method according to claim 21, wherein the mold used for casting the blank of the part is a permanent mold constituted by at least two metal half-portions coated in a release agent.

30. A method according to claim 21, wherein the rate of cooling corresponding to bainitic staged quenching lies in the range 15° C./sec to 150° C./sec.

31. A method according to claim 21, wherein the temperature Tic or Tir at which the part is maintained in the intercritical region or critical interval lies in the range 740° C. to 850° C.

32. A method according to claim 21, wherein the hot plastic deformation performed at a temperature greater than AC3 or AR3 for said cast iron composition comprises a stamping operation, an operation of forging, rolling, or hydroforming.

33. A method according to claim 21, wherein the hot plastic deformation is deformation at a ratio lying overall in the range 2% to 60%.

34. A method according to claim 21, wherein the cast blank of the part is of a shape approaching that of the part and possessing at least one dimension that is greater than that of the part so as to make said hot plastic deformation possible.

35. A method according to claim 21, wherein the temperature Tb of the bainitic staged quenching lies in the range 260° C. to 420° C.

36. A method according to claim 21, wherein the duration tb for which the temperature Tb of bainitic staged quenching is maintained is from 60 min to 180 min.

37. Spheroidal graphite cast iron having high-grade mechanical characteristics, and having the following composition by weight: 3% to 4% C; 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to 1.5% Cu; 0 to 0.5% Mo, with a residual Mg content adapted to the thickness of the parts lying in the range 0.025% to 0.080%, the balance being iron and impurities resulting from preparation; the impurities including S at a content of less than 0.015%, and P at a content of less than 0.10%, characterized in that its structure is essentially bainitic.

38. Spheroidal graphite cast iron having high-grade mechanical characteristics, and having the following composition by weight: 3% to 4% C; 1.7% to 3% Si; 0.1% to 0.7% Mn; 0 to 4% Ni; 0 to 1.5% Cu; 0 to 0.5% Mo, with a residual Mg content adapted to the thickness of the parts lying in the range 0.025% to 0.080%, the balance being iron and impurities resulting from preparation; the impurities including S at a content of less than 0.015%, and P at a content of less than 0.10%, characterized in that its structure is made up of two varieties of ferrite and austenite.

39. A cast iron part prepared from the spheroidal graphite cast iron according to claim 37.

40. A method according to claim 21, wherein all of the successive steps needed for making the cast iron part from the casting step to the step of cooling the part to ambient temperature are performed without intermediate heating between said steps.

41. A method according to claim 22, wherein an additional step of cold calibration of the part is added after the step of cooling the part to ambient temperature, the additional step being performed between at least two matrices having the shape of the finished part in order to improve its dimensional precision and to increase the mechanical characteristics and the fatigue strength of the material of the part by work-hardening its surface.

42. A method according to claim 41, wherein prior to the operation of cold calibration, a shot-blasting operation is performed for the purpose of removing scale and delivering compression stresses to the surface enabling the work-hardening effect due to calibration to be reinforced.

43. A method according to claim 22, wherein the mold used for casting the blank of the part is a permanent mold constituted by at least two metal half-portions coated in a release agent.

44. A method according to claim 22, wherein the rate of cooling corresponding to bainitic staged quenching lies in the range 15° C./sec to 150° C./sec.

45. A method according to claim 22, wherein the temperature Tic or Tir at which the part is maintained in the intercritical region or critical interval lies in the range 740° C. to 850° C.

46. A method according to claim 22, wherein the hot plastic deformation performed at a temperature greater than AC3 or AR3 for said cast iron composition comprises a stamping operation, an operation of forging, rolling, or hydroforming.

47. A method according to claim 22, wherein the hot plastic deformation is deformation at a ratio lying overall in the range 2% to 60%.

48. A method according to claim 22, wherein the cast blank of the part is of a shape approaching that of the part and possessing at least one dimension that is greater than that of the part so as to make said hot plastic deformation possible.

49. A method according to claim 22, wherein the temperature Tb of the bainitic staged quenching lies in the range 260° C. to 420° C.

50. A method according to claim 22, wherein the duration tb for which the temperature Tb of bainitic staged quenching is maintained is from 60 min to 180 min.

51. A method according to claim 24, wherein an additional step of cold calibration of the part is added after the step of cooling the part to ambient temperature, the additional step being performed between at least two matrices having the shape of the finished part in order to improve its dimensional precision and to increase the mechanical characteristics and the fatigue strength of the material of the part by work-hardening its surface.

52. A method according to claim 51, wherein prior to the operation of cold calibration, a shot-blasting operation is performed for the purpose of removing scale and delivering compression stresses to the surface enabling the work-hardening effect due to calibration to be reinforced.

53. A method according to claim 24, wherein the mold used for casting the blank of the part is a permanent mold constituted by at least two metal half-portions coated in a release agent.

54. A method according to claim 24, wherein the rate of cooling corresponding to bainitic staged quenching lies in the range 15° C./sec to 150° C./sec.

55. A method according to claim 24, wherein the temperature Tic or Tir at which the part is maintained in the intercritical region or critical interval lies in the range 740° C. to 850° C.

56. A method according to claim 24, wherein the hot plastic deformation performed at a temperature greater than AC3 or AR3 for said cast iron composition comprises a stamping operation, an operation of forging, rolling, or hydroforming.

57. A method according to claim 24, wherein the hot plastic deformation is deformation at a ratio lying overall in the range 2% to 60%.

58. A method according to claim 24, wherein the cast blank of the part is of a shape approaching that of the part and possessing at least one dimension that is greater than that of the part so as to make said hot plastic deformation possible.

59. A method according to claim 24, wherein the temperature Tb of the bainitic staged quenching lies in the range 260° C. to 420° C.

60. A method according to claim 24, wherein the duration tb for which the temperature Tb of bainitic staged quenching is maintained is from 60 min to 180 min.

61. A method according to claim 22, wherein the step corresponding to bainitic staged quenching of the part at a temperature Tb is preceded by a treatment step comprising maintaining the part at a temperature of from 950° C. to 900° C. for a duration of from 15 min to 60 min for the purpose of making the temperatures of the various portions of the part homogeneous and making the part's chemical composition homogeneous prior to the bainitic staged quenching treatment.

62. A method according to claim 23, wherein the step corresponding to bainitic staged quenching of the part at a temperature Tb is preceded by a treatment step comprising maintaining the part at a temperature of from 950° C. to 900° C. for a duration of from 15 min to 60 min for the purpose of making the temperatures of the various portions of the part homogeneous and making the part's chemical composition homogeneous prior to the bainitic staged quenching treatment.

63. A method according to claim 24, wherein the step corresponding to bainitic staged quenching of the part at a temperature Tb is preceded by a treatment step comprising maintaining the part at a temperature of from 950° C. to 900° C. for a duration of from 15 min to 60 min for the purpose of making the temperatures of the various portions of the part homogeneous and making the part's chemical composition homogeneous prior to the bainitic staged quenching treatment.

64. A method according to claim 25, wherein the step corresponding to bainitic staged quenching of the part at a temperature Tb is preceded by a treatment step comprising maintaining the part at a temperature of from 950° C. to 900° C. for a duration of from 15 min to 60 min for the purpose of making the temperatures of the various portions of the part homogeneous and making the part's chemical composition homogeneous prior to the bainitic staged quenching treatment.

65. A cast iron part prepared from the spheroidal graphite cast iron according to claim 38.

66. A method according to claim 22, wherein all of the successive steps needed for making the cast iron part from the casting step to the step of cooling the part to ambient temperature are performed without intermediate heating between said steps.