EP2812455A1

EP2812455A1 - Process for making a steel part, and steel part so obtained

Info

Publication number: EP2812455A1
Application number: EP12719049.4A
Authority: EP
Inventors: Fabien Perrard; Martin J. M. VERHOOFSTAD
Original assignee: Ascometal SA; Weweler BV
Current assignee: Asco Industries; Weweler BV
Priority date: 2012-02-10
Filing date: 2012-02-10
Publication date: 2014-12-17
Also published as: WO2013117953A1

Abstract

Process for making a steel part, characterized in that: - a half-product is prepared, having the following composition: * 0.30% < C ≤ 0.46%; * 1 % ≤ Mn ≤ 2%; * 1 % ≤ Cr ≤ 1.5%; * 2.3% ≤ Mn + Cr ≤ 3% * traces ≤ Si ≤ 1.5%; * traces ≤ Ni ≤ 2%, preferably traces < Ni ≤ 0.5%; * 0.04% ≤ Mo ≤ 0.50%, preferably 0.04% ≤ Mo ≤ 0.25%; * traces ≤ Cu ≤ 0.5%; * 0.005 ≤ Ti + Nb ≤ 0.06%; * traces ≤ V ≤ 0.3%; * 0.001 % ≤ Al ≤ 0.1 %; * traces ≤ B ≤ 0.01 %, preferably 0.0005% ≤ B ≤ 0.01 %; * traces ≤ S ≤ 0.1 %, preferably 0.002% ≤ S ≤ 0.1 %%; * traces ≤ N ≤ 200 ppm; * traces ≤ P ≤ 0.020%; * traces ≤ O ≤ 35 ppm; the remaining elements being Fe and impurities resulting from the steelmaking process; * with Ms = 561 - (474 x C%) - (33 x Mn%) - (17 x Cr%) - (21 x Mo%) - (17 x Ni%) °C ≤ 330 °C; - this half product is hot-formed at 1100-1300 °C by rolling or forging; - it undergoes an isothermal bainitic quench at a temperature Tb in the bainitic range, with a duration sufficient so as to give it a bainitic structure with less than 20% of martensite and/or pearlite and/or ferrite; Steel part made according to this process.

Description

Process for making a steel part, and steel part so obtained

The invention deals with the steel industry, and more particularly with the making of massive parts with high mechanical properties.

The making of massive steel parts (which can be defined as parts having an equivalent diameter of at least 1 5 mm and generally at most 1 50 mm) having high mechanical properties is, generally, performed by a succession of steps including, for example, a hot forming step of a half-product by hot rolling and/or forging, followed by a cooling in still or blown air, then by a quenching-tempering step which comprises a reheating of the half-product up to the austenite range, a quench in oil and a temper. After this thermal treatment, further steps, like machining, surface hardening and painting, may be performed.

Classically the "equivalent diameter" of a part is the diameter of a circle which would have the same area than the transverse cross-section of the part. If the cross-section varies along the part, this calculation is based on the maximal value of the cross-section.

This process allows to give the material homogeneous mechanical properties in the whole product section when the hardenability of the steel grade fits the cooling power of the quenching step, and so an advantageous resistance to fatigue.

Though, this process has also drawbacks. On the one hand, it is relatively complex to practice, since it is performed in several steps, including reheatings which are energy-consuming and an oil quench with its associated risks of fire. On the other hand, these reheatings tend to downgrade the surface condition of the half-product by oxidation and decarburization, and sometimes by the appearance of cracks during the oil quench.

Document WO-A-201 0/059037 has suggested another solution for the making of such massive steel parts. It implies the reheating of a slug, its hot forming, and its cooling in an appropriate medium for the the obtention of a mainly bainitic structure. Also, the cooling following the reheating may be performed in air and followed by an austenitizing and an isothermal bainitic quenching. But the steel compositions to which these processes would fit best are not precised.

Steels with bainitic structures used for making high performance mechanical parts were already described in EP-B1 -0 787 812 and EP-A- 1 426 453. These steels allow to obtain, after hot forming and continuous cooling, controlled or not, a homogeneous bainitic structure, with a tensile strength UTS of about 1200 MPa. These steels are, so, replacement solutions to quenched- tempered grades, and have the advantage of allowing simplified thermal treatments: no quenching-tempering is required, and also no isothermal bainitic quench. But the UTS values, they allow to obtain, are limited to about 1200 MPa, which is not sufficient for the most stringent uses of massive parts.

In order to obtain higher UTS values, typically 1400-2000 MPa, and so to be able to propose alternative solutions to quenched-tempered structures having these mechanical properties, it is necessary to replace the continuous cooling by an isothermal bainitic quench in an appropriate medium. Such a treatment allows to refine the bainitic structure, and so to obtain higher UTS values, even for massive parts. Such a thermal treatment combines a quick initial cooling (in order to avoid to penetrate into the ferrite-perlitic range) and an adequately lengthy stay at sufficiently low a temperature for the obtention of a high UTS value. Such steel grades were already described in, for example, EP-B1 -1 200 638, but they require high carbon contents and treatment durations of several tens of hours, so not compatible with many of the uses which could be considered. EP-A-0 461 652 disclosed grades with lower carbon contents, but adapted only for the treatment of small components (flat springs with an equivalent diameter equal to or less than 15 mm.

The purpose of the invention is to propose a steel grade and a thermal treatment which allow to obtain massive parts (that is with an equivalent diameter of 15 to 150 mm or more) having a bainitic structure, a very high tensile strength (1400 to 2000 MPa) and good resistance to fatigue in conditions compatible with industrial requirements for a mass production of such parts.

To this end, the invention is a process for making a steel part, characterized in that:

- a half-product is prepared, having the following composition, expressed in percentages in weight:

^* 0.30% < C < 0.46%;

^* 1 % < Mn < 2%;

^* 1 % < Cr < 1 .5%; ^* 2.3% < Mn + Cr < 3%

^* traces < Si < 1 .5%;

^* traces < Ni < 2%, preferably traces < Ni < 0.5%;

^* 0.04% < Mo < 0.50%, preferably 0.04% < Mo < 0.25 % ;

* traces < Cu < 0.5%;

^* 0.005 < Ti + Nb < 0.06%;

^* traces < V < 0.3% ;

^* 0.001 % < Al < 0.1 % ;

^* traces < B < 0.01 %, preferably 0.0005% < B < 0.01 %;

* traces < S < 0.1 %, preferably 0.002% < S < 0.1 %%;

^* traces < N < 200 ppm;

^* traces < P < 0.030%;

^* traces < O < 35 ppm;

^* optionally up to 0.006% of Ca, and/or up to 0.03% of Te and/or up to 0.05% of Se, and/or up to 0.05% of Pb, and/or up to 0.05% of Bi;

the remaining elements being Fe and impurities resulting from the steelmaking process ;

^* with Ms = 561 - (474 x C%) - (33 x Mn%) - (17 x Cr%) - (21 x Mo%) - (17 x Ni%) °C < 330°C;

- this half product is hot-formed at 1 100-1300 °C by rolling or forging;

- it undergoes an isothermal bainitic quench at a temperature Tb in the bainitic range, with a duration sufficient so as to give it a bainitic structure with less than 20% of martensite and/or pearlite and/or ferrite;

- and optionally it undergoes finishing operations which give the part its final dimensions and surface condition.

Preferably, Ti/N > 3, if B > 8 ppm.

The hot-formed half-product may be allowed to cool, and is then reheated up to the austenitic range before the isothermal bainitic quench.

The hot-formed half-product may undergoe the isothermal bainitic quench directly after hot-forming.

The isothermal bainitic quench duration may be 30 min to 2.5 h.

The finishing operations may be at least one of a machining, a grinding, a shot-peening. The equivalent diameter of the final product may be up to 150 mm.

Another object of the invention is a steel part, characterized in that is has been obtained by this process.

It may be a mechanical part.

It may be a flexible trailing arm for wheel axle suspension of a heavy vehicle, such as a lorry or a trailer.

As it has been understood, the first item of the invention is a new steel grade which particularly fits the requirements linked to the thermal process which constitutes the second item of the invention, and allows to obtain a metallurgical structure which is essentially bainitic and contains at most 20% of martensite or ferrite/pearlite. Such a structure is obtained by:

- hot forming a half-product, the composition of which corresponds to the requirements cited above;

- and quenching this hot formed half-product in an appropriate medium (generally a salt bath) for obtaining an isothermal bainitic transformation; this quenching is preferably performed just after the hot forming step, but may also be performed after the half-product has cooled down in air to the ambient temperature and has, then, be reheated to a convenient temperature before quenching.

The advantage of this process is that is easier to perform and less energy- consuming (particularly when the half-product is quenched just after its hot forming) than the prior art processes, and less prone to lead to surface cracks and decarburization. The energy consumption can be reduced by about 35% and the weight of the produced part may be reduced by about 15% due to the increase of the in-use properties of the steel, as compared to the prior art.

The invention will be better understood by reading the following description, which refers to appended figures:

- Figure 1 which shows the microstructure of a reference steel of the 51 CrV4 grade;

- Figure 2 which shows the microstructure of a steel according to the invention.

The requirements for the steel grade are justified as follows. All contents are percentages in weight. The C content must be between 0.3 and 0.46%. This relatively narrow range allows to guarantee a low sensitiveness to decarburization during the thermal treatments. Over 0.46%, it becomes difficult to obtain sufficiently quickly a bainitic transformation within the half-product, so that the isothermal quenching could be performed in 2.5 hours or less. Higher C contents would also downgrade the steel toughness. Lower C contents would not be able to lead to sufficiently high mechanical strengths for the main considered uses of the final products.

The Mn content must be between 1 and 2%. As well as Cr, Mn is used for controlling the start temperature of the martensitic transformation Ms, and also for delaying the onset of bainitic transformation in order to obtain a homogeneous transformation in the massive part. A Ms of at most 330 °C is required, so, in combination with the required C content, relatively high contents in Mn and Cr are needed, linked in the manner which shall be seen later on. Nevertheless, the Mn content must be limited to 2% in order to avoid serious segregation problems.

The Cr content must be between 1 and 1 .5%. In addition to its contribution to the control of Ms, the presence of Cr ensures sufficient hardenability for the obtention of a fine and homogeneous bainitic structure after the isothermal quench.

In addition, the Mn and Cr contents must respect the condition 2.3% < Mn + Cr < 3%. Experiments have shown that below 2.3%, it is not possible to obtain the required mechanical characteristics, due to the lacking hardenability of the steel. The incubation period is too short for ensuring a homogeneous structure: different kinds of bainite can be found in the structure, that are coarse upper bainite formed during the cooling and fine lower bainite formed during the isothermal transformation. In other words, too much of the bainite is formed during the cooling phase of the quench instead of during the isothermal period. And above 3%, the bainitic transformation would not be quick enough for the invention to be used in an industrial context.

Concerning Ms, it is mainly determined by the C, Mn, Cr, Mo and Ni. The classical formula by which it is calculated is:

Ms = 561 - (474 x C%) - (33 x Mn%) - (17 x Cr%) - (21 x Mo%) - (17 x Ni%) °C. It is this formula which must be taken into account for the definition of the invention. As said before, it must be of at most 330 °C. Such a relatively low Ms is necessary for obtaining the required mechanical characteristics, because it allows to extend the bainitic range down to the low temperatures, so as to easily isothermally keep the half-product in this bainitic range. That makes easier the obtention of a high UTS, between 1400 and 2000 MPa.

The Si content is between 0% (or unavoidable traces resulting from the steelmaking) and 1 .5%. Si is, so, not compulsory, but can be used in order to avoid the precipitation of carbides during the bainitic transformation, which would downgrade the resilience. It can also increase the yield strength. But 1 .5% should generally not be exceeded, in order to avoid too high a decarburization at the surface of the material.

The Ni content is between 0% (or unavoidable traces resulting from the steelmaking) and 2%. There is no absolute need to add Ni to the amount which would naturally result from the melting of the raw materials used for the steelmaking step, but some Ni can be added to lower Ms. Nevertheless, Ni should not exceed 2%, preferably 0.5%, in order not to uselessly increasing the cost of the steel.

The Mo content is between 0.04 and 0.50%, preferably between 0.04 and 0.25%. This element improves hardenability by avoiding the formation of ferrite and pearlite. Its added amount can depend on the equivalent diameter of the part. If this diameter is high, the highest amounts of Mo of the chosen range are preferred, in order to better ensure that the part will be structurally homogeneous. Also, Mo strengthens austenite when it is in solid solution. Since the mechanical strength of austenite is one of the factors which govern the small size of the bainite laths, an addition of Mo indirectly contributes to the obtention of a finer structure. On the other hand, it is known that Mo can slow down the formation of bainite for contents higher than 0.25%. That is the reason why the preferred upper limit is set at 0.25%.

The Cu content is set between 0% (or merely traces unavoidably resulting from the melting of the raw materials) and 1 %. Cu can be present either as a residual element, or be added in order to strengthen the bainitic structure by Cu precipitation. In the case where precipitation strengthening is sought, the Cu content may reach 1 %. If the Cu content is over 1 %, it can be detrimental during the rolling operation, due to the occurrence of surface defects.

The total of the contents in Ti and Nb must be between 0.005 and 0.06%. The invention necessitates to control the grain size during the hot forming step, in particular when the hot forming step is immediately followed by the isothermal quench. To this end, the use of Ti, or Nb, or a combination of both these elements, is necessary in order to form carbonitrides with sufficiently high dissolution temperatures for allowing them to avoid the grains to grow too much during the stay of the half-product at high temperatures. So, the bainitic structure will remain fine.

When Ti is used to protect boron from forming nitrides, which is useful when the content in B is at least 8 ppm, Ti content shall be no less than 3 times the nitrogen content in order to obtain the proper effect on hardenability,

The V content is between 0% or traces resulting from the steelmaking and 0.3%. This element is, so, only optional, but preferred since, from a content of 0.05%, it is able to strengthen the austenite by its precipitation and its presence in solid solution, and so indirectly contribute to the fineness of the bainitic structure. The upper limit of 0.3% is justified in that for higher V contents, the size of the precipitates would be too high and they would not have the desired effects.

The Al content is between 0.001 % and 0.1 %. This element can be added for deoxidation needs, and also for limiting the growth of the austenitic grains during the stays of the half-product at high temperatures. Above 0.1 %, the formed Al oxides would be too large and be detrimental to the required mechanical properties of the steel.

The B content is between 0% (or traces resulting from the steelmaking) and 0.01 %. This optional element can be used preferably between 0.0005 and 0.01 %, mainly for high equivalent diameter parts, It is particularly useful if the Mo content is low, in order to ensuring a homogeneous structure thanks to a limitation of the presence of ferrite. But to this end, B must remain in solution in the matrix and its combination with N should be avoided. So, when B is intended to be equal or higher than 8 ppm, it is advisable to add B in conjunction with Ti, with a Ti/N ratio of at least 3, as said above. The S content is between 0% (or traces resulting from steelmaking) and 0.1 %, preferably between 0.002 and 0.1 %. Similarly, Ca may be present up to 0.006%, Te up to 0.03%, Se up to 0.05%, Pb up to 0.05%, Bi up to 0.05%. At least one or all these elements may be present. They are not compulsory, but as it is well known, they may be added to improve the machinability of the part.

The N content is at most 200 ppm, and has no imperative lower limit. It is typically about 60-80ppm following vacuum degassing, as a natural result of the steelmaking operations if nothing is made for increasing the N content. However, higher amounts of N may be used in conjunction with nitride-forming elements, such as Al or V, in order to promote austenite grain size stability. Beyond 200 ppm of N, significant difficulties are expected during casting due to the possible precipitation of coarse nitrides within liquid steel, and there is a risk of embrittlement by ageing.

Also, as said above, when B > 8 ppm, the Ti/N ratio is preferably > 3, in order to avoid an excessive presence of BN.

The P content is at most 0.030% and has no imperative lower limit. The upper limit is justified in that beyond 0.030%, P would be detrimental for the mechanical properties, because it would induce an excessive brittleness of thegrain boundaries.

The O content is at most 35 ppm, so that the steel has sufficiently low an amount of oxide inclusions which could impair its mechanical properties. As it is well known by the steelmakers, such an O content can be obtained by using sufficiently high amounts of Al and /or Si for limiting the dissolved O content of the liquid steel, and protecting liquid steel from air during steelmaking and casting. A removal of O in liquid steel can also be at least partly performed by a deoxidation under vacuum by formation of CO. Generally speaking, the precise O content will depend on the customer's requirements, regarding the mechanical properties of the steel which can be downgraded if the O content is too high.

All elements which have not been cited may be present up to levels merely resulting from the steelmaking process.

The treatment process of the invention comprises the following steps. A half-product having the composition cited above is prepared by any steelmaking process known in the art which can allow its obtention (ingot casting, continuous casting and so on).

This half-product is heated in the austenitic range, typically between 1 100 and 1300°C, and hot-formed by rolling or forging. The hot-formed half-product then undergoes an isothermal bainitic quench. To this end, two processing routes can be used.

In a first possible processing route, the hot-formed half-product, still in the austenite stability temperature range, is cooled down to room temperature or to any temperature sufficiently low to ensure transformation of the austenite. It is then reheated in the austenitic range, and undergoes an isothermal quench down to a temperature Tb in the bainitic range (bainitization temperature), where it is kept for a duration of 0.5 to 2.5 h in order to obtain a bainitic microstructure containing no more than 20% of martensite and/or ferrite and/or pearlite. This is typically carried out by quenching in a medium set at a temperature Tb higher than ambient temperature.

It must be understood that in this regard, retained austenite, if present, must be assimilated to bainite.

The duration of the isothermal holding must, in all cases, be sufficient to generate the required microstructure as defined above. The duration is also preferably adapted to make the structure as homogeneous as possible within the part. Therefore its precise optimal value will depend on the shape and dimensions of the part;

In a second possible processing route, the hot-formed half product is directly quenched from the hot forming temperature to the bainitizing temperature Tb. The remainder of the treatment is as per first route, i.e. keeping the half- product in the bainitic range for a duration of 0.5 to 2.5 h, in order to generate a microstructure as previously described.

In both cases, the isothermally quenched product so obtained may undergo finishing steps like machining, grinding, shot-peening and so on, which give the product its definitive dimensions and/or surface condition.

For half-products having an equivalent diameter of up to 150 mm, these two variants of the process have quite similar effects on the mechanical properties of the product. This is, inter alia, due to the addition of microelements Ti and Nb which keep the grain size at a thin scale. The choice between the two variants will depend mainly on the industrial equipments which are available on the processing site. It has also to be noticed that the direct route, performed without any cooling, is the most energetically efficient.

An example of implementation of the invention will now be described, together with comparative examples.

These examples are applied to steels for mechanical parts with high mechanical characteristics, more particularly steels for vehicle suspension systems, like those used for trailing arms of heavy vehicles such as lorries or trailers. These parts are massive ones, their weight is about 20-40 kg, their thickness is several centimeters (corresponding to equivalent diameters of about 100 mm), and their shape is complex. They are usually obtained after a shaping process including several reheatings and one or several hot rollings followed by coolings in still air, an austenization during which the part is formed and which is followed by an oil quench, a tempering and various finishing steps which often include a prestressing shot-peening. The steel grade most generally used is the 51 CrV4, the composition of which is C = 0.47-0.55%, Si < 0.40%, Mn = 0.70- 1 .10%, P < 0.035%, S < 0.035%, V = 0.10-0.20% according to the ISO 10089 Standard. This grade is intended for making springs and is used in the quenched- tempered state. Its hardenability is convenient for parts having a large cross- section, so that sufficient a hardness is obtained for the core of the part, while the formation of surface cracks is normally avoided during cooling.

But 51 CrV4 is not satisfactory, because brittleness can appear after tempering if the quenching step is not well performed (quenching rate, temperature and holding time in oil bath), and also if tempering is not well performed (for instance if the holding time is too short or the holding temperature too low).

Drawbacks of the making process are mainly risks of fire, quenching cracks and energy consumption, and also its complexity.

The invention is intended to be an alternative and more convenient solution to the use of this grade and of its associated processing mode. Table 1 shows the compositions of the steels used for experiments which have shown the advantages of the invention. Steel 1 is a reference steel of the 51 CrV4 grade. Steel 2 is a reference steel which nearly corresponds to the present invention, except for the fact that its sum Mn + Cr is lower than 2.3. Steel 5 3 is a reference steel which nearly corresponds to the present invention, except for the fact that its sum Mn + Cr is higher than 3. Steels 4 to 7 correspond to the present invention.

Table 1 : compositions of the sampl

Table 2 shows the durations t (in hours) necessary for obtaining a complete bainitic transformation and the corresponding steel Vickers hardness Hv after an isothermal stay at different temperatures Tb for the steel of the invention

15 and for the reference steel 3. The UTS values are estimated from the hardness values according to the ISO 182625 standard, since the trials were performed on dilatometry samples, which were 3 mm in diameter and 12 mm in length, on which UTS cannot be directly measured. No results of trials performed on reference sample 1 are shown, since on such very small diameter samples they would not

20 give results which would be significantly very different from those obtained on the steels of the invention. That would not be the case for massive parts, for which the invention is primarily intended.

Sample Ms (°C) Mn + Cr( %) Tb (°C) t (h) Hv UTS (MPa)

3 (Ref.) 220 3.3 275 4.9 588 1976 3 (Ref.) 220 3.3 300 3.4 548 1826

2 (Ref) 282 2.1 325 0.4 507 1675

2 (Ref.) 282 2.1 350 0.3 464 1518

2 (Ref.) 282 2.1 375 0.2 422 1368

4 (Inv.) 288 2.7 300 1 .4 536 1782

4 (Inv.) 288 2.7 325 0.7 487 1602

4 (Inv.) 288 2.7 350 0.9 441 1436

4 (Inv.) 288 2.7 375 0.8 411 1330

5 (Inv.) 272 2.7 275 2.2 563 1882

5 (Inv.) 272 2.7 300 1 .3 531 1763

5 (Inv.) 272 2.7 325 1 486 1598

5 (Inv.) 272 2.7 350 1 .3 448 1461

6 (Inv.) 265 2.8 275 2 565 1890

6 (Inv.) 265 2.8 300 1 .5 534 1774

6 (Inv.) 265 2.8 325 1 .4 477 1565

6 (Inv.) 265 2.8 350 1 .2 440 1432

7 (Inv.) 287 2.6 290 0.9 550 1834

7 (Inv.) 287 2.6 310 0.8 529 1756

7 (Inv.) 287 2.6 330 0.7 495 1631

7 (Inv.) 287 2.6 350 0.5 449 1465

Table 2: Tranformation durations and corresponding hardnesses and estimated UTS for different isothermal quench temperatures The steels according to the invention have a transformation duration between 30 min and 2.2 h. Note that reference steel 2, the Mn + Cr content of which is only 2.1 %, has a shorter transformation duration, with the drawbacks on structural homogeneousness which have been cited above. On the contrary, reference steel 3 which has a Mn + Cr content of 3.3% has a transformation duration of up to 5 h, so by far higher that what happens in the case of the invention.

As can be seen from table 2, for obtaining high hardnesses, leading to tensile strengths UTS higher than 1400 MPa, it is necessary to have the isothermal quench performed at 350 °C or less. So, a Ms temperature lower than 330 °C is recommended for the steels of the invention.

In order to allow significant comparisons of the results, the mechanical characteristics of all samples are measured after an austenization treatment performed at 850 °C and a quench in a salt bath, on small size slugs (15 x 15 x100 mm, that is with an equivalent diameter of 17 mm) for following tables 3 and 4, and also on real parts for some of investigated samples.

Table 3 shows the mechanical characteristics (yield strength YS, tensile strength UTS and YS/UTS ratio) obtained after a bainitic isothermal quench on small size samples (15x15x100 mm) for different steels according to the invention and reference steels, after an isothermal quench which was sufficiently long so as to obtain a homogeneous bainitic structure. These mechanical properties were, so, obtained in ideal cases where the cooling speed was sufficiently high for the process being usable in industrial practice. They show the upper limits of these properties which may be obtained on massive parts.

Table 3; mechanical properties obtained after a quench in a salt bath on small size samples

The obtained results show that the thermal treatment conditions which allow to surely obtain UTS equal or higher than 1400 MPa are those for which the condition Mn + Cr > 2.3% is fulfilled. In the case of reference steel 2 which does not fulfill this condition, though the individual contents in the diverse elements are according to the invention, the obtained structure is bainitic, indeed. But, as said before, it is a mixture of coarse upper bainite formed during the continuous cooling and of fine lower bainite formed during the isothermal transformation. So, this steel has not sufficient a hardenability to surely obtain a homogeneous bainitic structure which can be obtained only if the majority of the bainite is formed during the isothermal step. Concerning reference steel 1 which is a 51 CrV4 steel, the same can be said concerning the structure: it is a coarse mixed bainitic structure. It allows, nevertheless, to obtain good mechanical properties on samples having said small size, but these properties cannot be obtained also on massive parts, as will be seen later on.

Table 4 shows the evolution of the structure and of the mechanical properties according to the duration of an isothermal quench at 290 °C for the steel of the invention of sample 7.

Table 4: evolution of the mechanical properties of sample 7 during isothermal quenching It was found that keeping the steel at 290 °C during at least 0.5 h is necessary for obtaining an homogeneous bainitic structure with less than 20% of martensite, and the high mechanical properties linked to this feature. A duration of 15 min only would lead to a martensite proportion of 25%.

In particular, this minimal duration of 0.5 h allows to obtain a YS/UTS ratio higher than 0.60. A duration of 1 h allows to reach a YS/UTS ratio of 0.74, which, afterwards, evolves in a relatively limited manner. These results show that a mixed martensite + bainite structure does not allow to reach optimal mechanical properties levels, especially concerning the YS/UTS ratio. In this example, a martensite content not higher than 1 1 % is already sufficient for significantly affecting mechanical properties. Generally speaking, a maximum of 20% martensite and/or pearlite and/or ferrite can be tolerated to obtain in-use properties typical of the invention

For higher durations, it is expected that the mechanical properties would evolve only very slowly, following the transformation of residual austenite into carbides. So, higher durations would not cause any significant downgrading of these properties.

Table 5 shows the mechanical properties obtained in industrial conditions on massive parts (trailing arms) having an equivalent diameter of 100 mm. Two types of thermal treatments at temperature T with a duration t are considered for each case, that are reference steel 1 and inventive steel 7:

- a treatment of quenching-tempering (reference treatment) ;

- an isothermal bainitic quench according to the present invention.

The average fatigue life of the trailing arms is also precised. It is measured under flexion with a R-ratio R = -0.5 and minimum and maximum loads of -57 kN and +28 kN respectively, which are typical testing conditions for this kind of products. The applied stresses are of the order of magnitude of those which the part undergoes during its use on a vehicle

Table 5: structures, mechanical properties and fatigue life of massive parts

The reference fatigue life is the one obtained with reference steel 1 of the 51 CrV4 grade in the quenched-tempered condition, with a martensitic structure. The use of the reference steel 1 in bainitic condition leads to a fatigue life which is only close to the reference value, because, in fact, this sample has a non homogeneous structure, which comprises a significant amount of martensite. Its microstructure can be seen on Figure 1 , where residual austenite is in bright white, and untempered martensite needles are in grey, the remaining being mixed coarse upper and lower bainite.

But the use of the steel 7, which has a composition according to the invention and a bainitic structure, allows to increase very significantly (by five) the fatigue life. In its case, the very fine bainitic structure which is obtained allows to increase the ability of the material to handle very high loads. Its microstructure can be seen on Figure 2, where a homogeneous bainitic structure can be seen, together with homogeneously distributed residual austenite islands.

Especially, it increases the effect of residual stresses caused by a shot- peening on the fatigue life. As said before, shot-peening is one of the finishing treatments which may be performed on the part at the end of its making process.

Moreover, the lower carbon content of the steel according to the invention, as compared to the reference steel 51 CrV4, leads to a lower sensitivity to decarburization during the thermal treatments, such a sensitivity being detrimental to fatigue life.

The process of the invention can be used, in particular, for making massive steel parts, that are, as said before, parts with an equivalent diameter up to 150 mm. It is particularly well adapted to the making, for example, of a mechanical part such as a flexible trailing arm for wheel axle suspension of a heavy vehicle, such as a lorry or a trailer, as described in WO-A-2010/059037.

Claims

1 .- Process for making a steel part, characterized in that:

^* 0.30% < C < 0.46%;

^* 1 % < Mn < 2%;

^* 1 % < Cr < 1 .5%;

^*

2.3% < Mn + Cr < 3%

* traces < Si < 1 .5%;

^* traces < Ni < 2%, preferably traces < Ni < 0.5%;

^* 0.04% < Mo < 0.50%, preferably 0.04% < Mo < 0.25%;

^* traces < Cu < 0.5%;

^* 0.005 < Ti + Nb < 0.06%;

* traces < V < 0.3% ;

^* 0.001 % < Al < 0.1 % ;

^* traces < B < 0.01 %, preferably 0.0005% < B < 0.01 %;

^* traces < S < 0.1 %, preferably 0.002% < S < 0.1 %%;

^* traces < N < 200 ppm;

* traces < P < 0.020%;

^* traces < O < 35 ppm;

- this half product is hot-formed at 1 100-1300 °C by rolling or forging;

3. - Process according to claim 1 or 2 , characterized in that Ti/N > 3 and B > 8 ppm.

4. - Process according to any one of claims 1 to 3, characterized in that the hot-formed half-product is allowed to cool, and is then reheated up to the austenitic range before the isothermal bainitic quench.

5. - Process according to any one of claims 1 to 3, characterized in that the hot-formed half-product undergoes the isothermal bainitic quench directly after hot-forming.

6. - Process according to any one of claims 1 to 5, characterized in that the isothermal bainitic quench duration is 30 min to 2.5 h.

7. - Process according to any one of claims 1 to 6, characterized in that the finishing operations are at least one of a machining, a grinding, a shot-peening.

8. - Process according to any one of claims 1 to 7, characterized in that the equivalent diameter of the final product is up to 150 mm.

9.- Steel part, characterized in that is has been obtained by the process according to any one of the claims 1 to 8.

10. - Steel part according to claim 9, characterized in that it is a mechanical part.

1 1 . - Steel part according to claim 10, characterized in that it is a flexible trailing arm for wheel axle suspension of a heavy vehicle, such as a lorry or a trailer.