FR2958660A1 - STEEL FOR MECHANICAL PIECES WITH HIGH CHARACTERISTICS AND METHOD FOR MANUFACTURING THE SAME. - Google Patents

Abstract

Steel for mechanical parts with high characteristics, characterized in that its composition, in percentages by weight, is: - 0.05% ≤ C ≤ 0.25%; -1.2% ≤ Mn≤ 2%; - 1% ≤ Cr≤ 2.5%; - (830 - 270 C% - 90 Mn% - 70 Cr%) ≤ 560; - traces ≤ If ≤ 1.5%; - traces ≤ Ni ≤ 1%; - traces 5 Mo ≤ 0.5%; - traces ≤ Cu ≤ 1%; - traces ≤ V ≤ 0.3%; - traces ≤ Al ≤ 0.1%; - traces ≤ B≤ 0.005%; - traces ≤ Ti ≤ 0.03% - traces ≤ Nb ≤ 0.06%; - traces ≤ S ≤ 0.1%; - traces ≤ Ca ≤ 0.006%; - traces ≤ Te ≤ 0.03%; - traces ≤ Se ≤ 0.05%; - traces ≤ Bi ≤ 0.05%; - traces ≤ Pb ≤ 0.1%; the rest being iron and impurities resulting from the elaboration. Process for manufacturing a mechanical part having this composition

Description

Steel for mechanical parts with high characteristics and its manufacturing process

The invention relates to steels for mechanical parts with high characteristics, obtained by hot forging or bar machining. Certain grades of steel make it possible to obtain high mechanical characteristics on a forged part or a raw rolling bar, without the use of controlled cooling or a subsequent heat treatment. They are based on obtaining a bainitic homogeneous microstructure. Such grades are already proposed, such as those disclosed in EP-B1-0 787 812 or EP-A-1 426 453, which are used industrially for the production of forged parts for internal combustion engines. However, in order to obtain high mechanical characteristics on the parts described in these documents, it is necessary, unless it is limited to diameters of the order of 20 mm, to use carbon contents greater than or equal to 0, 25%. If it is now possible to guarantee a tensile strength of the order of 1200 MPa after natural cooling, thanks, in particular to shades such as those described in EP-A-1 426 453, the obtaining of these mechanical characteristics are often at the price of a resilience less than or equal to 30 J.cm-2. The object of the invention is to propose a new steel grade for mechanical parts which, combined with appropriate heat and thermomechanical treatments, makes it possible simultaneously to obtain mechanical properties (tensile strength Rm, yield strength Re, ratio Re / Rm, elongation at break A, necking Z) advantageous, and an improved KCU resilience compared to steels known for this same use. For this purpose, the subject of the invention is a steel for mechanical parts with high characteristics, characterized in that its composition, in percentages by weight, is: 0.05% <0.25%; -1.2% 5Mn <_2%; - 1% 5. Cr 5 2.5%; 30 - (830 to 270 C% to 90 Mn% to 70% Cr) 5,560; - traces 5 Si 5 1.5%; -traces sNi51%; - traces 5 Mo 5 0.5%; traces 5 Cu 5 1%; Traces <= 0.3%; traces 5 Al 5 0.1%; traces <B <0.005%; traces <_ Ti <0.03%; traces <_ Nb <0.06%; -traces 5S5.50.1%; - traces <_ Ca <0.006%; -traces <_Te50.03%; traces 5 Se 0.05%; - traces <_ Bi <0.05%; traces 5 Pb 5 0.10 / 0; the rest being iron and impurities resulting from the elaboration. Preferably, traces of <0.3%. Preferably, 0.8% to 1.5%. Preferably, Ni <0.5%. Preferably, 0.04% <0.5% Mo.

Preferably 0.5% to 0.3%. Preferably 0.005% <0.1%. Preferably 0.0005% <0.005%, and traces <0.0080% and Ti%? 3.5%. Preferably 0.005% Ti 0.03%. Preferably 0.005% 0.1%.

Preferably, the structure of the steel is bainitic and contains at most 20% in total of martensite and / or ferrite and / or perlite. The invention also relates to a method of manufacturing a steel part such as a mechanical part with high characteristics, characterized in that it comprises the following steps: - a slab or a steel bar is prepared whose composition is in accordance with what has been said previously; - Hot forming of the billet or the bar in the austenitic field, by forging or rolling; the hot-formed billet or bar is cooled at a rate giving it a bainitic structure containing at most 20% in total of martensite and / or pearlite and / or ferrite; and one or more machining operations are carried out to give the part its final dimensions and surface state. Before or after the machining or operations, one can proceed to an income made between 35 200 and 350 CC for 30 minutes to 4 hours. The billet or hot formed bar can be naturally cooled in still air.

The billet or hot formed bar can be cooled with air blown. As will be understood, the invention is based on a composition and on its association with a metallurgical structure, which can be obtained by simple means such as a cooling air calm or blown.

The shades of the present invention make it possible to obtain, on the basis of a so-called "low-middle" carbon, and by lowering the starting point of transformation mainly by the incorporation of chromium and manganese, tensile strengths of the order of 1200 MPa or more, for resilience of at least 40 J.cm -2, can even reach 70 J.cm-2.

However, these grades offer Re / Rm ratios in the forged heat or in the raw rolling state of the order of 0.6, and therefore yield strengths that are significantly lower than those obtained on hardened grades. -revenues of the same mechanical resistance. But, as will be demonstrated, it is also possible, according to the invention, by a subsequent low temperature income, to increase very significantly (of the order of 25%) the elastic limit, without increasing mechanical resistance. This type of income is to be distinguished from the incomes sometimes used on micro-alloyed steels, driven at 550-650 ° C, which allow the precipitation of alloy carbides. Indeed, while the latter are often accompanied by a significant loss of resilience, the low-temperature incomes produced in the context of the invention have a beneficial effect on resilience (up to about 30% increase ). We will now justify the choice of composition ranges for the various elements of the shade according to the invention. All grades are given in percentages by weight.

The content of C is between 0.05 and 0.25%. This interval, called "low-medium carbon" because its upper limit is located in the low zone of the levels considered as carbon means and its lower limit belongs to the field of low carbon, allows a very homogeneous microstructure and hardness even in the presence of segregations . In particular, for carbon contents of less than 0.2%, the hardness of martensitic microstructures that may be present in the segregated zones is only slightly greater than that of the bainitic microstructure. In addition, these carbon contents allow ductilities and resilience higher than those obtained at the same level of mechanical strength, for contents greater than 0.25%.

The Mn content is between 1.2 and 2%. Manganese is used, together with chromium, as the main element to lower the formation temperature of bainite (Bs) during continuous cooling. Since a relatively low carbon content is used, relatively high levels of Mn are required, which furthermore must contribute to satisfying the condition imposed on the C, Mn, Cr contents for the calculation of Bs (see below). . Manganese is limited to 2% to avoid segregation problems too pronounced. The Cr content is between 1.2 and 2.5%. In the present invention, Cr is used in the same way as Mn to lower the bainitic transformation start temperature Bs. The contents of C, Mn and Cr must also be such that 830 -270 C% -90 90 Mn% -70 Cr% <_- 560. Indeed, the bainitic transformation start temperature Bs can be classically estimated from the following formula: Bs = 830 -270 C% - 90 Mn% - 70 Cr% - 37 Ni% - 83 Mo% where the contents are expressed as percentages by weight (see, for example, Bhadeshia, Bainite in Steels, 10M 2001). In the context of the invention, given the relatively low levels of Ni and Mo steel, we can limit ourselves to consider only the contributions of C, Mn and Cr. In any case, if Ni and Mo are present at the levels located at the top of the forks which will be seen further, they will contribute to lowering Bs. It is thus ensured that in all cases we will obtain a Bs less than or equal to 20,560. 'C. If is between traces and 1.5%. Silicon can be used to prevent the formation of carbides that would deteriorate resilience during bainitic transformation. At carbon contents below 0.2%, however, this formation of carbides remains weak, and the addition of Si loses its interest from this point of view. On the other hand, by promoting the formation of residual austenite, Si improves the fatigue strength for certain applications. In some cases, however, its use can also be excluded by the need to avoid excessive decarburization on the surface. Two variants of the invention can therefore be envisaged. In a first variant, the Si content results simply from the production conditions, namely the raw materials used and the possible partial oxidation of the Si that they brought to the bath of liquid metal, and no significant voluntary addition of If not done. In this case, a Si content between traces and 0.3% is typically obtained. In a second variant, Si is voluntarily added to obtain a content of 0.8 to 1.5%. Ni is between traces and 1%, preferably between traces and 0.5%. It can be present only by its introduction by the raw materials as a residual element, or be added in small quantity to contribute to the decrease of the temperature Bs. But its content is limited to 1%, better 0.5% for reasons of cost, this element being expensive and likely to have its price fluctuate widely on the market. Mo is between traces and 0.5%, preferably between 0.04 and 0.5%. The role of molybdenum on quenchability is well established: it avoids the formation of ferrite and perlite but does not slow down the formation of bainite. It can therefore be added in variable quantity depending on the diameter of the part. A second benefit of molybdenum is to limit susceptibility to reversible brittleness (see Bhadeshia, Mater Sci Forum, High Performance Bainitic Steels, vol 500-501, 2005). Finally, molybdenum strengthens the austenite by passing it in solid solution. Since the mechanical strength of austenite is one of the main factors governing the fineness of the bainitic structure (Singh and Bhadeshia, Mater Sci Eng A, 1998, Vol 245, p72), the addition of Mo contributes indirectly to obtaining a finer structure. The upper limit is established primarily for economic reasons.

V is between traces and 0.3%, preferably between 0.05 and 0.3%. The addition of vanadium allows additional hardening; however, contrary to what happens with ferrito-pearlitic steels, this hardening does not seem to be done by precipitation; it is indeed demonstrated experimentally that after hot deformation (hot forging or rolling) and natural cooling, only a very small fraction of vanadium is in precipitated form. Like molybdenum, vanadium enhances austenite by precipitation and solid solution, and can therefore indirectly contribute to the fineness of the bainitic structure, hence its hardening effect. Its addition is limited to 0.3% for economic reasons. Cu is between traces and 1%. It may possibly be used to contribute to hardening, but would entail implementation difficulties for contents greater than 1%. Al is between traces and 0.1%, preferably between 0.005 and 0.1%. Al is optionally added to ensure the deoxidation of the steel and to prevent excessive growth of the austenitic grains during high temperature maintenance (eg a cementation treatment) which would be carried out on the part after the implementation of the process. process according to the invention. B is between traces and 0.005%, preferably between 0.0005 and 0.005%. This optional element can be used for large diameter parts, particularly if the Mo content is low, in order to guarantee the homogeneity of the structure (to limit the presence of ferrite). In this case, it will be preferable to couple the addition of B with addition of Ti which will capture the nitrogen to form nitrides and avoid the formation of boron nitrides. Thus all the boron will be available to play its role of homogenizer of the structure. In this case, traces of 0.0080% and 3.5% N are required. Ti is between traces and 0.03%, preferably between 0.005 and 0.03%. As we have just said, this optional element is to be used mainly for boron shades, with the relationship between Ti% and N% which has just been exposed. Nb is between traces and 0.06%. This optional element can be used to refine the austenitic structure after forging or hot rolling, with consequent decrease in bainite package sizes and acceleration of bainitic transformation (Bhadeshia, Royal Proc Soc., 2010, Vol 466 p.3).

S is between traces and 0.1%. As is well known, this element can, if necessary, be left at a relatively high level, or added voluntarily, to improve the machinability of the steel. It is then given a content of 0.005 to 0.1%. Preferably, this significant presence of S is then accompanied by an addition of Ca up to 0.006%, and / or Te up to 0.03%, and / or Se up to 0.05%, and / or or Bi up to 0.05% and / or Pb up to 0.1%. This improvement in machinability can be sought in particular for applications where the part is stressed in fatigue, or for applications where its mechanical properties are improved, at least locally, by a sufficient pre-stressing to prevent the propagation of cracks ( crankshaft burnishing, autofrettage of the high-pressure injection rails). The other elements contained in the steel according to the invention are iron and impurities resulting from the preparation, present at usual contents. The preferred ranges cited for various elements are independent of each other. A steel which would be in one or some of these preferential ranges and not in the others would therefore be considered as part of the invention. Industrially, the workpiece may be produced by hot forming a billet or bar having the composition described above, such as hot forging or hot rolling, or by machining a bar ready for use. employment.

In the first case, the industrial process involves a hot shaping step carried out in austenitic phase (typically 1100-1250t), followed by a natural cooling. One of the important points of the invention is the possibility of obtaining high mechanical characteristics without the use of heat treatments after forging or rolling, nor any particular very restrictive control of the rate of cooling which can be carried out naturally, in the air calm. Nevertheless, if the installations allow it, an adaptation of the cooling may in some cases be used, either because of the diameter of the parts (with large parts, too slow cooling can lead to an appearance of ferrite and / or perlite too much), or to obtain mechanical characteristics superior to those which would be obtained by a natural cooling. Air-blast cooling may be sufficient to achieve this objective. However, care should be taken that the cooling is not so rapid as to cause a massive appearance of martensite. In addition, a heat treatment of low temperature (200 to 350 ° C for periods of 30 minutes to 4 hours) ) makes it possible to obtain, on the shades according to the invention, a very significant increase in the elastic limit without increasing the hardness and without decreasing the resilience. According to the parts concerned, several machining operations can take place after the forging or rolling and before or after the income to obtain the precise dimensions and surface characteristics of the final part.

The mechanical characteristics being obtained by natural cooling, they are also likely to be reached starting from a hot rolled bar ready for use, if it already has the desired metallurgical structure (essentially bainitic) which will be described more far. The composition of the steels of the invention is such that the probability of obtaining the desired structure naturally after a simple air cooling of the hot-rolled bar under usual conditions is not negligible, if the dimensions of the bar lead to an adequate cooling rate. Subsequently, results obtained with steel compositions according to the invention and reference compositions are presented. These results are obtained on laboratory castings forged in 40 mm rounds, or on industrial castings forged in circles of equivalent diameter. In order to allow a meaningful comparison of the results, the mechanical characteristics are evaluated after 10001 austenitization followed by a natural cooling with calm air or a forced cooling with the blown air. In addition, for reference, two bainitic grades allowing to obtain high mechanical characteristics in the hot forge, and already used on crankshafts, rails and other forgings with high mechanical strength, are added: samples A (corresponding to EP -BO 787 812) and B (corresponding to EPA-1 426 453). The compositions of these samples are shown in Table 1, as well as their bainitic transformation start temperature Bs calculated as previously stated on the basis of the contents of C, Mn and Cr.35 E ch C% Mn% Cr% Si% Ni % Mo% V% Other Bs (t) A (Ref.) 0.25 1.52 0.84 0.80 0.17 0.10 0.19 Ti, B 567 B (Ref.) 0.29 1, 13 1.15 0.80 0.20 0.12 0.18 Nb, Ti, B 570 C (Inv.) 0.18 1.62 1.51 0.18 0.17 0.19 0.09 530 D (Inv.) 0.16 1.68 1.54 0.94 0.06 0.20 0.09 Ti, B 528 E (Inv.) 0.21 1.78 1.38 1.06 0.10 0 , 0.09 Ti, B 517 F (Inv.) 0.18 1.60 1.54 0.86 0.96 0.19 0.20 Ti, B 530 G (Inv.) 0.18 1.53 1.45 0.97 0.10 0.17 0.09 Ti, B 542 Table 1: Compositions and Bs of the tested samples The contents of Ti, Nb and B are typically 0.030%, 0.025% and 0.003% respectively when these elements are present.

Table 2 presents the mechanical characteristics measured on the products obtained from these samples. It should be emphasized here that the results obtained, in absolute terms, should be analyzed only in the precise context to which they refer. Indeed, it is common to observe differences in the mechanical properties obtained on forgings or rolled parts of the same composition but of different dimensions, generally in the direction of an increase in mechanical characteristics equivalent diameter. The hierarchy between the shades examined will nevertheless remain the same for samples having all the same dimensions, which would be different from those of the examples cited here. The word "AS" after the reference of the sample means that the cooling has, in his case, been led to the supply air. Ech. Structure Re Rm Re / Rm AZ (%) KCU (MPa) (MPa) (%) (J.cm-2) A bainite 666 1114 0.60 19 39 39 B bainite 739 1226 0.60 18 41 27 A-AS bainite 694 1119 0.62 14 30 32 C bainite 738 1185 0.62 15 53 50 D bainite 709 1173 0.60 14 44 44 D-AS bainite 759 1203 0.63 15 57 69 E bainite 796 1303 0.61 15 39 47 E-AS Bainite + 10% 989 1344 0.74 12 46 58 martensite F bainite 745 1213 0.61 17 49 44 F-AS bainite 774 1238 0.63 16 50 50 G bainite 769 1212 0.63 17 51 Table 2 Mechanical Characteristics of the Samples After Austenitization and Cooling The mechanical properties of the examples of steels according to the invention C to G thus show a significant increase in the mechanical strength with respect to the average carbon bainitic grades A and B, the carbon content of which is the medium-high carbon category. The yield strengths are 60 to 130 MPa higher and the mechanical strengths are 70 to 190 MPa, all things being equal.

They also allow an increase in the resilience of up to about 100% compared to medium-high carbon grades (C: 50 J / cm 2 against 39 J / cm 2 for A, 32 J / cm 2 for A-As and 27 J / cm 2). cm2 for B), always all things being equal. As shown in Table 3, the structure is bainitic in all cases, with the exception of E-AS casting cooled with air. This is demonstrated by the ratio Re / Rm which is established at a value of about 0.6, typical of a bainitic structure, except in the case of E-AS where martensite is present and where Re / Rm takes a value. higher. A presence of martensite is not in itself prohibitive, insofar as the mechanical characteristics remain very high (in particular if the resilience remains greater than 40 J / cm 2). On the other hand, insofar as the fraction of martensite formed is very sensitive to the exact conditions of cooling, it is to be expected that a considerable dispersion of the mechanical characteristics on parts made under industrial conditions for which the control of the cooling of the part can not always be optimal. It is therefore preferable to set a goal of limiting the total presence of martensite, ferrite and perlite to no more than 20%. However, it is necessary to emphasize the important role of the dimension of the parts in the analysis of the mechanical characteristics: thus, if the shade E cooled with the air blows presents martensite on a round of 40 mm of diameter, one found that it allows, conversely, to guarantee a homogeneous bainitic structure with diameters of 50 to 300 mm.

If a particularly high value of Re is desired, it is possible to apply to the part a low temperature income, before or after the final machining. As shown in Table 3, such an income makes it possible to obtain a higher elastic limit of up to 200 MPa than that obtained after normalization, while maintaining or increasing resilience (up to + 25% ) and without increasing the tensile strength. Machinability will not be affected. It is further noted that the results obtained vary little in a temperature range of 250-350 ° C for income. An industrial treatment can therefore be easily achieved without a very precise control of income conditions is necessary. Ech. Treatment Re Rm Re / Rm A (%) Z (%) KCU (MPa) (MPa) (J.cm "Z) F without income 745 1213 0.61 17 49 44 income 940 1184 0.79 12 59 59 300t, 2h income 937 1165 0.80 15 60 57 350t, 2h C without income 738 1185 0.62 15 53 50 income 897 1156 0.78 15 58 250t, 2h income 920 1144 0.80 14 60 300'C, 2h income 912 1138 0.80 14 56 350'C, 2h Table 3: Mechanical characteristics obtained after income

Claims (15)

REVENDICATIONS1. Steel for mechanical parts with high characteristics, characterized in that its composition, in percentages by weight, is: -0.05% 5C50.25%; -1.2% 5Mn <_2%; -1% 5 Cr 52.5%; (830 to 270 C% to 90 Mn% to 70%) to 560; -traces 5. If 51.5%; - traces 5 Ni 5 1%; - traces <_ Mo <_ 0.5%; -traces 5. Cu 51%; -traces <50.3%; traces 5 Al 5 0.1%; traces 5 B <0.005%; traces <_ Ti <0.03% - traces <_ Nb <0.06%; - traces _ S50,1%; - traces <_ Ca <0.006%; traces <0.03%; traces 5 Se 0.05%; - traces <_ Bi <0.05%; traces S Pb 0.1%; the rest being iron and impurities resulting from the elaboration. 2. Steel according to claim 1, characterized in that traces <_ Si 0.3%. 3. Steel according to claim 1, characterized in that 0.8 <_ Si <_ 1.5%. 4. Steel according to one of claims 1 to 3, characterized in that Ni <_ 0.5%. 5. Steel according to one of claims 1 to 4, characterized in that 0.04% 5 Mo 0.5%. Steel according to one of claims 1 to 5, characterized in that 0.05% <0.3%. 7. Steel according to one of claims 1 to 6, characterized in that 0.005% ≤ 0.1%. Steel according to one of claims 1 to 7, characterized in that 0.0005% B <0.005% and traces N <0.0080% and Ti%> 3.5 N%. 9. Steel according to one of claims 1 to 8, characterized in that 0.005% Ti <0.03%. Steel according to one of claims 1 to 9, characterized in that 0.005% <0.1%. 11. Steel according to one of claims 1 to 10, characterized in that its structure is bainitic and contains at most 20% in total of martensite and / or ferrite and / or pearlite. 12. A method of manufacturing a steel part such as a mechanical part with high characteristics, characterized in that it comprises the following steps: - a billet or a steel bar is prepared whose composition complies with one of claims 1 to 10; - Hot forming of the billet or the bar in the austenitic field, by forging or rolling; the billet or hot-formed bar is cooled at a speed giving it a bainitic structure containing at most 20% in total of martensite and / or pearlite and / or ferrite; and one or more machining operations are carried out to give the part its final dimensions and surface state. 13. The method of claim 12, characterized in that, before or after the machining or machining, one proceeds to an income made in a temperature range of 200 to 350t for 30 minutes to 4 hours. 14. The method of claim 12 or 13, characterized in that the slab or bar formed hot is cooled naturally in still air. 15. The method of claim 12 or 13, characterized in that the billet or the bar formed hot is cooled in the blown air.