FR2827875A1 - Steel used in fabrication of mechanical components comprises specified amounts of carbon, silicon, manganese, chromium, molybdenum, nickel, aluminum, copper, sulfur, phosphorus, niobium and the rest is iron and impurities - Google Patents

Abstract

Steel comprises specified amounts of carbon, silicon, manganese, chromium, molybdenum, nickel, aluminum, copper, sulfur, phosphorus, niobium and the rest is iron and impurities. A steel for mechanical components has the following composition (by wt. %): (a) 0.12 <= carbon (C) <= 0.30%; (b) 0.8 <= silicon (Si) <= 1.5%; (c) 1.0 <= manganese (Mn) <= 1.6%; (d) 0.4 <= chromium (Cr) <= 1.6%; (e) 0 <= molybdenum (Mo) <= 0.30%; (f) 0 <= nickel (Ni) <= 0.6%; (g) 0 <= aluminum (Al) <= 0.06%; (h) 0 <= copper (Cu) <= 0.30%; (i) 0 <= sulfur (S) <= 0.10%; (j) phosphorus (P) <= 0.03%; (k) 0 <= niobium (Nb) <= 0.050%; and (l) the rest is iron and production impurities. An Independent claim is also included for a mechanical component made from this steel and subjected to a cementation or carbonitriding treatment.

Description

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 The invention relates to the field of iron and steel, and more specifically, that of steels for mechanical parts such as sprockets.

 Steels for gearing must have a high resistance to contact fatigue. Most of the time, they undergo a carburizing treatment or carbonitriding to provide them with sufficient surface hardness and mechanical strength, while retaining a good tenacity to heart thanks, in particular, to a carbon content of about 0 , 10 to 0.30% only. In the case-hardened layer, this carbon content can be up to about 1%.

 US-A-5,518,685 discloses steels for gearing to be case hardened. They contain mainly, in percentages by weight, 0.18 to 0.25% C, 0.45 to 1% Si, 0.40 to 0.70% Mn, 0.30 to 0.70% Ni, 1.0 to 1.5% Cr, 0.30 to 0.70% Mo, up to 0.50% Cu, 0.015 to 0.030% AI, 0.03 to 0.30% of V , 0.010 to 0.030% of Nb, up to 15 ppm of O, from 100 to 200 ppm of N. They undergo, after cementation, quench-tempering treatment avoiding the formation of core ferrite. The contents of Si and Mn are here kept within relatively low limits, to avoid intergranular oxidation during the cementation treatment.

 JP-A-4-21757 discloses steels for gearing intended to be cemented by plasma or under reduced pressure, then shot blasted.

 Their composition is, in percentages by weight, 0.10 to 0.30% of C, 0.25 to 1.50% of Si, 0.2 to 2% of Mn, up to 0.015% of P, up to 0.020% S, up to 2% Cr, 0.2 to 1% Mo, with Si + Mo between 0.6 and 2%, 0.010 to 0.060% AI, 50 to 250 ppm N and up to 15 ppm O. These steels have a high resistance to the surface pressure experienced by the pinion, which has a long service life.

 However, users of sprockets, for example for vehicle gearboxes, face the following problem. In the long run, there is the appearance of play between the various parts constituting the mechanical system in which the gables are integrated. These games degrade the functionalities of the parts, by increasing their cyclic stresses, the vibrations, the noise nuisances, and prematurely damage the parts.

These games are related to the dimensional modifications of the parts, which occur either during their thermal and / or thermochemical treatments, or during their use due to plastic deformations in service.

To avoid or limit the plastic deformations in service, according to the part and the stresses, it is sometimes enough to act on the properties of the layer

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 superficial corresponding to the area where the stresses exerted are at the highest level, especially in gear. But the properties of the substrate (the steel which has been carburized) with respect to the permanent deformations during cycling also influence the plastic deformations in service of the part.

 The object of the invention is to provide users of steel gearing materials with a low deformation in service, so as to minimize the appearance of games, by keeping the dimensions and geometry of the parts concerned.

caractérisé en ce que sa composition est, en pourcentages pondéraux : - 0,12 # C # 0,30% - 0,8 # Si # 1,5% - 1,0 # Mn # 1,6% - 0,4 # Cr # 1,6% - 0 # Mo # 0,30% - 0 # Ni # 0,6%

- 0 Al AI s 0 06% - OCu < 0, 30% - 0 s S s 0, 10% - 0 s P s 0, 03% - s b s, 5000 le reste étant du fer et des impuretés résultant de l'élaboration. For this purpose, the subject of the invention is a steel for mechanical parts,

characterized in that its composition is, in percentages by weight: - 0.12 # C # 0.30% - 0.8 # Si # 1.5% - 1.0 # Mn # 1.6% - 0.4 # Cr # 1.6% - 0 # MB # 0.30% - 0 # Ni # 0.6%

- 0 Al AI s 0 06% - OCu <0, 30% - 0 s S s 0, 10% - 0 s P s 0, 03% - sbs, 5000 the rest being iron and impurities resulting from the elaboration .

 It may also contain at least one of the following elements up to 0.02% of Te, up to 0.04% of Se, up to 0.07% of Pb, up to 0.05% of Ca up to 0.08% Bi.

 The invention also relates to a mechanical part, characterized in that it is made of a steel of the preceding type which has undergone a carburizing treatment or carbonitriding.

 Preferably, this piece is made of a steel of the above type which has undergone a low pressure carburizing treatment.

 The invention consists in adjusting the composition of the steel, in particular its Si and Mn contents, in order to obtain a cyclic plastic deformation in service of the entire part as small as possible.

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 The plastic deformation in use depends both on the forces exerted on the part and the material used. It is linked on the one hand to the intrinsic mechanical characteristics of the material, in particular to the evolution of the elastic limit during cycling, ie the yield strength, and on the other hand the structural stability in use, in particular the thermal or mechanical stability of the residual austenite often present in the materials used. This is likely to turn into martensite during a warm-up of the room. The inventors have determined the chemical composition conditions of a steel for carburized or carbonitrided parts making it possible to minimize the plastic deformation produced during each duty cycle. They were established by carrying out a first series of compression tests and stability measurements of austenite on steels whose composition reproduced that of the surface layer obtained after a cementation of steels according to the invention. These results were then supplemented by tests carried out on samples in all points in accordance with the invention, which showed that these steels were capable of constituting cemented parts having, at heart, the desired mechanical properties, in particular a low residual deformation. during cyclic solicitations.

 The invention will be better understood on reading the description which follows, given with reference to the following figures: FIG. 1 which shows the rate of remanent deformation undergone during fatigue-compression tests by two samples of steels simulating the invention and a reference steel, depending on the silicon content of the steel and the number of deformation cycles; - Figure 2 shows the same rate of remanent deformation after 106 cycles for different steels simulating the invention and reference steels, depending on their silicon and manganese content.

 FIG. 3 which shows the rate of remanent deformation experienced during fatigue-compression tests by a steel sample according to the invention and a reference sample with a low silicon content, as a function of the number of deformation cycles.

 According to the invention, the steels must have the following composition.

All percentages are percentages by weight.

 Their carbon content must be between 0.12 and 0.3%, which corresponds substantially to the levels usually encountered on steels for gearing. The carburizing or carbonitriding must carry, as it

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 is conventional, this content to more than 0.5%, usually between 0.7 and 1% in their surface layer. The cementation treatment is carried out in a conventional manner, preferably in a non-oxidizing atmosphere to avoid problems of oxidation of Si and Mn. Low pressure carburizing is a particularly indicated process.

 It is necessary to have a sufficiently high Mn content between 1 and 1.6% to give the steel a hardenability allowing less deformation to heat and / or thermochemical treatments. By associating it with a Si content of between 0.8 and 1.5%, the plastic deformations that can be caused by the cyclic stresses in service are also acted upon.

 The experimental results presented below will make it possible to specify the advantages of this range of contents of Si and Mn. It can already be said that a too low Si content results in a too large residual deformation during cyclic stresses. Too high a content is likely to cause in the carburizing layer the formation of ferrite islands detrimental to the required mechanical properties. It would also make it more difficult to shape the workpiece hot or cold. An excessively low Mn content is also detrimental to the deformation during cyclic stresses, since the small amount of residual austenity provides insufficient structural stability during deformation. An excessively high Mn content causes a too high residual austenite content, resulting in too low ductility mechanical characteristics and excessively high remanent deformations.

 The chromium content must be between 0.4 and 1.6%, so as to provide good steel hardenability and core mechanical properties sufficient in terms of hardness and strength. A content higher than 1.6% is no longer necessary from this point of view, and makes, in addition, the development of steel more difficult.

 The optionally available molybdenum makes it possible to adjust the hardenability of the steel. Above 0.30%, the addition becomes too expensive and superfluous to obtain the quenchability adjustment.

 The nickel content must be between 0% (or the content naturally resulting from elaboration without addition of Ni) and 0.6%. The addition of Ni makes it possible to obtain a better shock resistance which can be important when mounting the mechanical assembly to which the part is integrated. Above 0.6%, no additional effect is obtained and the cost of steel is unnecessarily increased.

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 The aluminum content must be between traces resulting from the preparation and 0.06%. This deoxidizing element is not essential, the deoxidation that obtain the silicon and manganese being sufficient. In addition, if the development and casting are not sufficiently treated, there is a risk of an excessive presence of alumina inclusions constituting sites of initiation for the fatigue cracks, when quantities of AI significant are added. However, if the conditions of development and casting are well controlled, it may be interesting to add the AI to prevent excessive grain growth during carburizing, which is favorable to a lower propagation of cracks.

 The copper content, resulting from the production of steel, must not exceed 0.30% in order not to degrade the ductility and the tenacity of the core material.

 The sulfur content can be between simple traces and 0.10%. This element can be added if one wishes to improve the machinability of the steel.

 The phosphorus content should not exceed 0.03%, so as not to cause excessive segregation at the grain boundaries during the income, which would weaken the steel.

 The niobium content may be between simple traces resulting from the elaboration and 0.050%. An addition of niobium makes it possible to obtain a more uniform grain size which favors the homogeneity of the plastic deformation in service and further minimizes this deformation. Beyond 0.050%, the effect of niobium no longer increases, and an addition at higher levels would unnecessarily increase the cost of steel.

Moreover, it is conceivable to add to the steel one or more elements making it possible to improve its machinability, namely tellurium (up to 0.02%), selenium (up to 0.04%), lead (up to 0.07%), calcium (up to 0.05%), or bismuth (up to 0.08%)
Tests were carried out on solid steel samples whose compositions are reported in Table 1.

 Samples 11 to 16 are a first series of steel samples which do not conform to the invention in that they have a carbon content higher than the required limit. But, as has been said, their composition simulates that of the cemented layer of steels which would be, in heart, in accordance with the invention. They allow to easily evaluate if this composition would be adapted to the resolution of the problem posed, what similar experiments realized

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 on steel samples according to the cemented or carbonitrided invention would not achieve with the same evidence. Table 1 also gives the composition of various reference samples, not in accordance with the invention and not simulating it, but making it possible to assess the aptitude for plastic deformations in service (destabilization of the residual austenite under cyclic stresses) in accordance with the invention. function of the elements Mn and Si, for cemented layers of analyzes close to those obtained on the steel of the invention. For all the samples in Table 1, the Ni content was less than 0.25%, the AI content was less than 0.050%, the Cu content was less than 0.2% and the N content was less than 150 ppm, knowing that this nitrogen content is not imperative and could be significantly higher without departing from the spirit of the invention. Indeed a high nitrogen content is not unacceptable for the types of steel involved.

--------------------------------------------------------------------------------------------------------------------l l No échantillon c Mn Si Cr Mo S P 1 0, 98 0, 30 0, 20 1, 50-0, 008 0, 010 2 0, 985 0, 345 1, 04 1, 49 0, 017 0, 009 0, 007 CI) 3 0, 95 0, 263 1, 51 1, 57 0, 020 0, 006 0, 012 4 0, 97 0, 297 2, 06 1, 52 0, 019 0, 006 0, 008 : 5 0, 95 1, 02 0, 50 1, 61 0, 021 0, 004 0, 011 (/) 6 0, 95 1, 02 2, 50 1, 57 0, 021 0, 007 0, 011 g 7 0, 94 2, 11 0, 45 1, 60 0, 020 0, 008 0, 011 8 0, 97 2, 05 1, 51 1, 54 0, 019 0, 008 0, 011 w W 9 0, 96 2, 07 2, 51 1, 61 0, 021 0, 007 0, 011 10 0, 77 1, 26 0, 25 1, 06 0, 107 0, 029 0, 018 11 0, 78 1, 27 0, 85 1, 06 0, 108 0, 031 0, 019 ru 12 0, 77 1, 25 1, 45 1, 05 0, 106 0, 031 0, 013 S J3 13 0, 97 1, 10 1. 08 1, 52 0, 023 0, 010 0, 008 M C r~ (D ≈14 0, 83 1, 02 0, 98 1, 39 0, 003 0, 009 0, 008 0 > 15 0, 88 1, 08 0, 88 1, 49 0, 003 0, 010 0, 007 us 16 0, 86 1, 06 1, 07 1, 53 0, 002 0, 008 0, 007 w Table 1: Compositions of the samples of the first series of tests (in% by weight)

-------------------------------------------------- -------------------------------------------------- ---------------- ll No sample c Mn If Cr Mo SP 1 0, 98 0, 30 0, 20 1, 50-0, 008 0, 010 2 0, 985 0 , 345 1, 04 1, 49 0, 017 0, 009 0, 007 CI) 3 0, 95 0, 263 1, 51 1, 57 0, 020 0, 006 0, 012 40, 97 0, 297 2, 06 1, 52 0, 019 0, 006 0, 008: 0, 95 1, 02 0, 50 1, 61 0, 021 0, 004 0, 011 (/) 6 0, 95 1, 02 2, 50 1 , 57 0, 021 0, 007 0, 011 g 7 0, 94 2, 11 0, 45 1, 60 0, 020 0, 008 0, 011 8 0, 97 2, 05 1, 51 1, 54 0, 019 0, 008 0, 011 w W 9 0, 96 2, 07 2, 51 1, 61 0, 021 0, 007 0, 011 10 0, 77 1, 26 0, 25 1, 06 0, 107 0, 029 0 , 018 11 0, 78 1, 27 0, 85 1, 06 0, 108 0, 031 0, 019 ru 12 0, 77 1, 25 1, 45 1, 05 0, 106 0, 031 0, 013 S J3 13 0, 97 1, 10 1. 08 1, 52 0, 023 0, 010 0, 008 MC r ~ (D ≈14 0, 83 1, 02 0, 98 1, 39 0, 003 0, 009 0, 008 0 15 0, 88 1, 08 0, 88 1, 49 0, 003 0, 010 0, 007 us 16 0, 86 1, 06 1, 07 1, 53 0, 002 0, 008 0, 007 w

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Une première expérience a consisté à évaluer, pour ces divers échantillons, la stabilité de l'austénite résiduelle. D'une part, on a mesuré la température à laquelle 50% du volume d'austénite résiduelle a été déstabilisée en martensite. D'autre part, on a mesuré le pourcentage d'austénite résiduelle déstabilisée à 350 C, qui est une température de toute façon supérieure à celle qu'atteignent les aciers de l'invention dans leurs utilisations privilégiées envlsagees.

A first experiment consisted in evaluating, for these various samples, the stability of the residual austenite. On the one hand, the temperature at which 50% of the residual austenite volume was destabilized to martensite was measured. On the other hand, the percentage of residual austenite destabilized at 350 ° C., which is a temperature in any case higher than that attained by the steels of the invention in their preferred uses, has been measured.

Table 2: Stability measures of residual austenite considered.

N'échantillon Température de % d'austénite résiduelle déstabilisation (OC) déstabilisée à 350 C 1 200 > 95 2 325 30 t. : I 350 35 4 325 25 L- 5 200 > 90 Q) (/) 6 400 < 20 g 7 275 > 60 8 400 < 15 LU 9 425 < 15 10 220 100 l l 11 350 50 ru 12 400 15 E c : 13 375 < 20 ë 0 c : 14 440 25 g > 15 425 35 16 375 < 45

On notera que d'une manière générale, les échantillons présentant la meilleure stabilité de l'austénite résiduelle sont ceux qui contiennent le plus de Si, la teneur en Mn exerçant également une influence en second rang. Les échantillons de référence 6, 8 et 9, qui ont au moins une des teneurs en Mn et Si au dessus de ce qu'exige l'invention, présentent de très bonnes Table 2: Stability measures of residual austenite

Sample% Residual austenite temperature destabilization (OC) destabilized at 350 C 1,200> 95 2,325 30 t. : I 350 35 435 25 L-5 200> 90 Q) (/) 6 400 <20 g 7 275> 60 8 400 <15 LU 9 425 <15 10 220 100 ll 11 350 50 ru 12 400 15 E c: 13,375 <20 ° C: 14,440 25 g> 15,425 35 16,375 <45

It should be noted that, in general, the samples having the best stability of the residual austenite are those containing the most Si, the Mn content also exerting a second-order influence. The reference samples 6, 8 and 9, which have at least one of the Mn and Si contents above that required by the invention, exhibit very good

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 characteristics of structural stability, but with the disadvantages noted above. Samples 12,13 and 14 simulating the invention have a very good residual austenite stability. Those of samples 11 and 16 are worse. But the sample 11, because of its relatively low silicon content relative to the previous ones, has a low amount of residual austenite initially. This relative lack of structural stability therefore does not compromise the achievement of the desired properties for the steels of the invention. Concerning sample 16, it is mainly its relatively high content of chromium that makes its initial quantity of residual austenite is low, and that the same comments can be made on it as for sample 11.

 FIG. 1 shows the remanent strain experienced during a fatigue-compression test by a steel sample placed in the form of a cylindrical stud with a diameter of 7 mm and a height of 12 mm as a function of the number of cycles, for a stress of 2000 MPa, at room temperature, and for different contents of Si. This test concerned the samples 10, 11 and 12 of Table 1, for which the Mn content was about 1.25%, and the If respectively 0.25%, 0.85% and 1.45%.

S 106 déformation rémanente va de 0, 42% après 5000 cycles à 0, 75% après 106 cycles. En revanche, cette déformation rémanente est nettement plus faible pour les aciers simulant les aciers de l'invention. Pour l'échantillon 11 à 0,85%

de Si, la déformation rémanente va de 0, 21% après 5000 cycles à 0, 32% après 6 1 106 cycles. Pour l'échantillon 12 à 1, 45% de Si, la déformation rémanente va de 0,18% après 5000 cycles à 0,24% après 106 cycles. It can be seen that for the reference sample 10 to 0.25% Si, the

S 106 remanent deformation ranges from 0.42% after 5000 cycles to 0.75% after 106 cycles. On the other hand, this remanent deformation is markedly lower for the steels simulating the steels of the invention. For sample 11 at 0.85%

of Si, the remanent strain ranges from 0.21% after 5000 cycles to 0.32% after 61 106 cycles. For sample 12 to 1, 45% Si, the remanent strain ranges from 0.18% after 5000 cycles to 0.24% after 106 cycles.

 FIG. 2 illustrates the remanent deformation undergone by various samples, in the form previously mentioned, after 106 cycles, as a function of the torque (Si%, Mn%). The numbers of the samples concerned, which can be divided into three curves, corresponding to Mn contents of the order of 0.3%, 1% and 2%, are shown in the figure. The best results are obtained with the reference samples, 2,3, 5,6 and with the samples 13,14, 15 and 16 simulating the invention, namely remanent deformations less than 0.8%. However, it has been seen that samples 2,3 and 5 also have insufficient residual austenite stability, whereas sample 6 has an excessive Si content, which is liable to lead to the formation of ferrite islands. and making the formatting of the piece difficult. The

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 Samples 13, 14, 15.16 at 1% Mn and about 1% Si give good results in all respects.

 FIG. 3 illustrates the residual deformation undergone by two samples of a second series of tests, put into the form previously cited, for a stress of 1000 MPa, at ambient temperature. These samples had the compositions reported in Table 3.

Table 3: Compositions of the samples of the second series of tests (in% by weight)

<Tb>
<tb> C <SEP> Mn <SEP> If <SEP> Cr <SEP> MB <SEP> S <SEP> P
<tb> sample <SEP> of <SEP> reference <SEP> 0.23 <SEP> 1.27 <SEP> 0.22 <SEP> 1.09 <SEP> 0.10 <SEP> 0.030 <SEP> 0.016
<tb> sample <SEP> according to <SEP> the invention <SEP> 0.23 <SEP> 1.32 <SEP> 0.95 <SEP> 1.11 <SEP> 0.10 <SEP> 0.032 <SEP > 0.016
<Tb>

The 0.95% silicon sample was therefore in all respects in accordance with the invention. The reference sample differed only in its lower silicon content. It can be seen in FIG. 3 that the elevation of the silicon content to a value in accordance with the invention provides the desired low residual deformation for the core of the cemented parts made from the steels of the invention.

 These tests show that the best compromise between the different properties required, for the mechanized mechanical parts to be resistant to contact fatigue and to deform only very little during the thermochemical treatments and during their use, is obtained for steels simulating the layer cemented steels according to the invention. This justifies the range of compositions imposed on the steels according to the invention, which undergo carburizing or carbonitriding when they are used to manufacture mechanical parts.

Claims (4)

1. Steel for mechanical parts, characterized in that its composition is, in percentages by weight:

 -------------------- - 0, 12 s C s 0, 30% - 0, 8 s If s 1, 5% - 1, 0 s Mn s 1 , 6% - 0, 4 s Crs 1, 6% - 0 s Mo <0, 30% zu Ni s 0, 6% - 0 # Al # 0.06% - 0 # Cu # 0.30% - 0 # S # 0.10%

 - 0 # P # 0.03% - 0 s Nb s 0, 050% the rest being iron and impurities resulting from the elaboration.  2. Steel for mechanical parts according to claim 1, characterized in that it contains at least one element selected from up to 0.02% of Te, up to 0.04% of Se, up to 0.07 % Pb, up to 0.05% Ca, up to 0.08% Bi.  3. Mechanical part, characterized in that it is made of a steel of the type according to one of claims 1 to 2 which has undergone a carburizing treatment or carbonitriding.  4. Mechanical part according to claim 3, characterized in that it has undergone a low pressure carburizing treatment.