Abstract:
Steel for mechanical components, wherein the composition thereof is, in percentages by weight: 0.19%≦C≦0.25%; 1.1%≦Mn≦1.5%; 0.8%≦Si≦1.2%; 0.01%≦S≦0.09%; trace levels≦P≦0.025%; trace levels≦Ni≦0.25%; 1%≦Cr≦1.4%; 0.10%≦Mo≦0.25%; trace levels≦Cu≦0.30%; 0.010%≦Al≦0.045%; 0.010%≦Nb≦0.045%; 0.0130%≦N≦0.0300%; optionally trace levels≦Bi≦0.10% and/or trace levels≦Pb≦0.12% and/or trace levels≦Te≦0.015% and/or trace levels≦Se≦0.030% and/or trace levels≦Ca≦0.0050%; 
 
the balance being iron and impurities resulting from the production operation, the chemical composition being adjusted so that the mean values J 3m , J 11m , J 15m  and J 25m  for five Jominy tests are such that: 
 
α=| J   11m   −J   3m ×14/22− J   25m ×8/22|≦2.5 HRC; and 
 
β= J   3m   −J   15m ≦9 HRC. 
Method for producing a mechanical component using this steel and a mechanical component produced in this manner.

Description:
TECHNICAL FIELD  
       [0001]     The invention relates to the field of steel metallurgy and, more particularly, steels for mechanical components, such as pinions.  
       BACKGROUND TO THE INVENTION  
       [0002]     Steels for gear manufacturing must have a high level of resistance to contact fatigue. Most of the time, components produced from these steels are subjected to a carburising or carbonitriding treatment which is intended to provide them with sufficient surface hardness and mechanical strength whilst maintaining a high level of core strength owing, in particular, to a carbon content in the order of from only 0.10 to 0.30%. The carbon content of the carburised layer may be up to approximately 1%.  
         [0003]     Various documents describe gear manufacturing steels which are intended to be carburised. These include U.S. Pat. No. 5,518,685, in which the contents of Si and Mn are maintained within relatively low limits (from 0.45 to 1% and from 0.40 to 0.70%, respectively) in order to prevent intergranular oxidation during the carburising operation. JP-A-4-21757 describes steels for gear manufacturing which are intended to be carburised using plasma or at reduced pressure then shot blasted, and which can have contents of Si and Mn higher than the previous steels. They have a high level of resistance to the surface pressure acting on the pinion whose life span is thus increased.  
         [0004]     WO-A-03 012 156 proposes a steel for mechanical components, such as pinions, the composition of which is: 0.12%≦C≦0.30%; 0.8%≦Si≦1.5%; 1.0%≦Mn≦1.6%; 0.4%≦Cr≦1.6%; Mo≦0.30%; Ni≦0.6%; Al≦0.06%; Cu≦0.30%; S≦0.10%; P≦0.03%; Nb≦0.050%. This steel has the advantage of minimising the operational plastic deformations of the component as a whole owing, in particular, to a judicious balance of the contents of silicon and manganese. The carburising or the carbonitriding must preferably take place under non-oxidising conditions, for example, at reduced pressure, so that the relatively high contents of silicon and manganese do not lead to problems of intergranular oxidation.  
         [0005]     Generally, the carburising or carbonitriding takes place at a temperature in the order of from 850 to 930° C. However, the current tendency is to attempt to carry out this operation at higher temperatures (high-temperature carburising or carbonitriding) in the order of from 950 to 1050° C. This increase in the processing temperature either allows the length of the processing time to be reduced with the same carburised depth or allows the carburised depth to be increased with the same length of processing time. The manufacturer thus has the choice of being able to increase the productivity of the installation or increase the effectiveness of the products obtained.  
         [0006]     However, the application of a high-temperature carburising or cabronitriding operation to the known steels which have been described presents a number of problems. Firstly, the high temperature may lead to an increase in poorly controlled grains, which is detrimental to the mechanical properties of the component. Secondly, the carburising or carbonitriding is followed by quenching during which the component is subjected to deformations. These may require the component to be remachined or, in the most extreme cases, cause it to be rejected. These problems are accentuated when the quenching is carried out on a component which has just been subjected to a carburising or carbonitriding operation at high temperature and not at a more normal temperature.  
         [0007]     The object of the invention is to provide, for metallurgists carrying out high-temperature carburising or carbonitriding of mechanical components, in particular pinions, a steel which overcomes the problems mentioned above whilst maintaining the required mechanical properties, and which is also compatible with the carburising and carbonitriding operations carried out at more normal temperatures.  
       SUMMARY OF THE INVENTION  
       [0008]     To this end, the subject-matter of the invention is a steel for mechanical components, characterised in that the composition thereof is, in percentages by weight: 
        0.19%≦C≦0.25%;     1.1%≦Mn≦1.5%;     0.8%≦Si≦1.2%;     0.01%≦S≦0.09%;     trace levels≦P≦0.025%;     trace levels≦Ni≦0.25%;     1%≦Cr≦1.4%;     0.10%≦Mo≦0.25%;     trace levels≦Cu≦0.30%;     0.010%≦Al≦0.045%;     0.010%≦Nb≦0.045%;     0.0130%≦N≦0.0300%;     optionally trace levels≦Bi≦0.10% and/or trace levels≦Pb≦0.12% 
 
 and/or trace levels≦Te≦0.015% and/or trace levels≦Se≦0.030% and/or trace levels≦Ca≦0.0050%; 
 
 the balance being iron and impurities resulting from the production operation, the chemical composition being adjusted so that the mean values J 3m , J 11m , J 15m  and J 25m  for five Jominy tests are such that: 
 
α=| J   11m   −J   3m ×14/22− J   25m ×8/22|≦2.5 HRC; and 
 
β= J   3m   −J   15m ≦9 HRC. 
       
 
         [0022]     Preferably, the composition thereof is adjusted so that 
 
β= J   3m   −J   15m ≦8 HRC. 
 
         [0023]     Preferably, the composition thereof is: 
        0.19%≦C≦0.25%;     1.2%≦Mn≦1.5%;     0.85%≦Si≦1.2%;     0.01%≦S≦0.09%;     trace levels≦P≦0.025%;     0.08%≦Ni≦0.25%;     1.1%≦Cr≦1.4%;     0.10%≦Mo≦0.25%;     0.06%≦Cu≦0.30%;     0.010%≦Al≦0.045%;     0.015%≦Nb≦0.045%;     0.0130%≦N≦0.0300%;        
 
         [0036]     optionally trace levels≦Bi≦0.07% and/or trace levels≦Pb≦0.12% and/or trace levels≦Te≦0.010% and/or trace levels≦Se≦0.020% and/or trace levels≦Ca≦0.045%, the balance being iron and impurities resulting from the production operation.  
         [0037]     Optimally, the composition thereof is: 
        0.20%≦C≦0.25%;     1.21%≦Mn≦1.45%;     0.85%≦Si≦1.10%;     0.01%≦S≦0.08%;     trace levels≦P≦0.020%;     0.08%≦Ni≦0.20%;     1.10%≦Cr≦1.40%;     0.11%≦Mo≦0.25%;     0.08%≦Cu≦0.30%;     0.010%≦Al≦0.035%;     0.025%≦Nb≦0.040%;     0.0130%≦N≦0.0220%;        
 
         [0050]     optionally trace levels≦Bi≦0.07% and/or trace levels≦Pb≦0.12% and/or trace levels≦Te≦0.010% and/or trace levels≦Se≦0.020% and/or trace levels≦Ca≦0.045%, the balance being iron and impurities resulting from the production operation.  
         [0051]     The subject-matter of the invention is also a method for producing a mechanical component from carburised or carbonitrided steel, characterised in that a steel of the above-mentioned type is used for this purpose, on which a machining operation, carburising operation or carbonitriding operation is carried out, then a quenching operation.  
         [0052]     The carburising or carbonitriding is preferably carried out at a temperature of from 950 to 1050° C.  
         [0053]     The subject-matter of the invention is also a steel mechanical component, such as a gear component, characterised in that it is produced using the above method.  
         [0054]     As will be appreciated, the invention is based on a precise adjustment of the content ranges of the main alloy elements, as well as the simultaneous presence, in well-defined contents, of aluminium, niobium and nitrogen.  
         [0055]     The desired effects are substantially of two types.  
         [0056]     Firstly, the selection of the contents of the main alloy elements is intended to achieve a Jominy curve with no significantly marked inflection point. This condition allows minimal deformations to be achieved during the quenching operation. In this respect, the carburising or the carbonitriding which is carried out at high temperature is, as has been mentioned, particularly demanding.  
         [0057]     It should be noted that the Jominy curve of a steel which is produced using a conventional standardised test characterises the quenchability of the steel. It is produced by measuring the hardness of a cylindrical test piece which has been quenched using a jet of water which sprays one of the ends thereof, along the length of one of the generating lines thereof. The hardness is measured at several distances x (in mm) from the sprayed end, and the corresponding value is designated J x . J xm  refers to the mean value obtained during five tests for measuring the hardness at distance x.  
         [0058]     As disclosed in document EP-A-0 890 653, to which the reader may refer for further details, the applicant demonstrated that a composition of the steel which produces a Jominy curve with no inflection point was advantageous for producing greatly reduced deformations during the quenching operation following a carburising or carbonitriding operation. This Jominy curve with no inflection point is produced when the values J 11m , J 3m , J 25m  and J 15m  satisfy the following relationships: 
 
α=| J   11m   −J   3m ×14/22− J   25m ×8/22|≦2.5 HRC; 
 
β= J   3m   −J   15m ≦9 HRC, or preferably ≦8 HRC. 
 
         [0059]     The composition of the steel according to the present invention is therefore adjusted so that this relationship is also produced in this instance.  
         [0060]     The composition is also adjusted, in particular owing to the combined presence of aluminium, niobium and nitrogen in defined contents, so that the size of the grains remains controlled, even when the carburising or the carbonitriding is carried out at high temperature.  
         [0061]     Finally, the composition of the steel must of course provide the desired mechanical properties for the use of the component. The criteria to be monitored more particularly include the carburised depth (conventionally defined as the depth at which the measured hardness is 550 HV), the deviation of hardness between the surface and the core of the carburised component, which must be as low as possible in order to minimise the deformations during quenching, and the core hardness which must be high so that the component responds effectively to stresses during operation, and therefore has a high level of strength in terms of endurance and fatigue.  
       BRIEF DESCRIPTION OF THE DRAWING  
       [0062]     The invention will be better understood from a reading of the following description, given with reference to the appended drawing which shows the Jominy curves of four reference steels and three steels according to the invention. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0063]     The steel according to the invention is intended primarily for producing mechanical components which are subjected to high levels of stress, such as gear elements, and which are intended to be carburised or carbonitrided (preferably at low pressure or in a non-oxidising atmosphere to prevent oxidation of the most oxidisable elements), both at normal temperatures of approximately from 850 to 930° C. and at high temperatures in the order of from 950 to 1050° C. These components must have a high level of fatigue endurance, high strength, and must be only slightly deformed during thermal treatments, such as the quenching operation following the carburising or carbonitriding operation. It has the following composition (all the percentages are percentages by weight).  
         [0064]     The carbon content thereof is between 0.19 and 0.25%. These contents are normal for gear manufacturing steels. Furthermore, this range allows the contents of the other elements to be adjusted, which allows the desired shape to be produced for the Jominy curve. The minimum content of 0.19% is further justified by the core hardness which can be achieved thereby after quenching. At more than 0.25%, there is a risk that the hardness will be too high to preserve the desirable machinability for the steel. The preferred range is from 0.20 to 0.25%.  
         [0065]     The manganese content thereof is between 1.1 and 1.5%. The minimum value is justified by the production of the desired Jominy curve in conjunction with the contents of the other elements. At more than 1.5%, there is the risk of the appearance of segregations and also banding during the annealing operations. Furthermore, such a high content would bring about excessive corrosion of the heat-resistant coating of the steel ladle during the production operation. It would not be desirable to further restrict this range of contents since producing the precise grade desired in the steelworks could be excessively difficult. The preferred range is from 1.2 to 1.5%, preferably from 1.21 to 1.45%.  
         [0066]     The silicon content thereof is between 0.8 and 1.2%. In this range, the desired shape of the Jominy curve can be produced in conjunction with the contents of the other elements. The minimum value of 0.8% is justified by the production of the desired core hardness, as well as by the limitation of the deviation of hardness between the surface and the core after carburising or carbonitriding. At more than 1.2%, there is a risk that excessive segregations will appear since silicon, though it segregates itself only slightly, tends to accentuate the segregation of other elements. There would also be an increased risk of oxidation during carburising or carbonitriding. The preferred range is from 0.85 to 1.20%, preferably from 0.85 to 1.10%.  
         [0067]     The sulphur content thereof is between 0.01 and 0.09%, the minimum value is justified by the production of correct machinability. At more than 0.09%, there is a risk of an excessively substantial reduction in the hot forgeability. The preferred range is from 0.01 to 0.08%.  
         [0068]     The phosphorus content is between trace levels and 0.025%. Generally, the standards in force tend to require a maximum phosphorus content of this order. Furthermore, beyond this value, there is a risk of interaction with niobium which renders the steel brittle during the hot moulding and/or the continuous casting of the steel in the form of blooms or billets. The phosphorus content is preferably 0.020% at the most.  
         [0069]     The nickel content thereof is between trace levels and 0.25%. This element, which is introduced intentionally at higher contents, would increase the cost of the metal unnecessarily. In practice, the nickel content which results naturally from the fusion of the raw materials of the casting could be sufficient, with no deliberate addition. The preferred range is from 0.08 to 0.20%.  
         [0070]     The chromium content thereof is between 1.00 and 1.40%. In this range, in conjunction with the contents of the other elements, the desired shape of the Jominy curve can be produced. Furthermore, the minimum content of 1.00% allows a high level of core hardness to be produced. At more than 1.40%, the cost of the production operation would be increased unnecessarily. The preferred range is from 1.10 to 1.40%.  
         [0071]     The molybdenum content thereof is between 0.10 and 0.25%. In this range, in conjunction with the contents of the other elements, the desired Jominy curve shape and core hardness are produced. The preferred range is from 0.11 to 0.25%.  
         [0072]     The copper content thereof is between trace levels and 0.30%. Again in this instance, as for nickel, the content produced after fusion of the raw materials is generally maintained purely and simply. At more than 0.30%, the ductility and the core strength of the component would be impaired. The preferred range is from 0.06 to 0.30%, preferably from 0.08 to 0.30% so as to optimise the shape of the Jominy curve and the hardness after quenching.  
         [0073]     The aluminium, niobium and nitrogen contents thereof must be controlled within precise limits. These are elements which, when interacting, bring about control of the fineness of the metal grain. This fineness is desirable in order to produce a high level of strength in the carburised or carbonitrided layer, a high level of fatigue strength and a reduction of the dispersion of the deformation during quenching. Furthermore, it is also important for producing the desired shape for the Jominy curve. Controlling the grain size is, in the context of the invention, all the more important since the steel must be capable of being subjected to a carburising or carbonitriding operation at high temperature without an excessive increase in the grain size occurring.  
         [0074]     This control of the grain is carried out substantially by means of precipitation of aluminium and/or niobium nitrides and carbonitrides. In order to produce this control, a significant presence of these two elements is therefore required, as well as nitrogen at a content which is substantially higher than that which is generally produced following a production operation carried out under normal conditions.  
         [0075]     The aluminium content must be between 0.010 and 0.045%. In addition to its grain control function mentioned above, this element controls the deoxidisation of the steel and the purity thereof in terms of inclusions of oxides. At less than 0.010%, the effects thereof, from the above perspectives, would be insufficient. At more than 0.045%, the purity in terms of inclusions of oxides risks being insufficient for the applications which are primarily intended. The preferred range is from 0.010 to 0.035%.  
         [0076]     The niobium content must be between 0.010 and 0.045%. At less than 0.010%, the effect of grain control would not be sufficient, in particular for the lowest contents of aluminium. At more than 0.045%, there is a risk of cracks appearing during the continuous casting of the steel, in particular if interaction with the phosphorus can occur, as has been indicated above. The preferred range is from 0.015 to 0.045%, preferably from 0.015 to 0.040%.  
         [0077]     In conjunction with the contents of aluminium and niobium as mentioned, the nitrogen content must be between 0.0130 and 0.0300% (130 to 300 ppm) so that the desired adjustment of the grain size and shape of the Jominy curve are produced. The preferred range is from 0.0130 to 0.0220%.  
         [0078]     If it appears desirable, one or more of the elements which are conventionally known may be added to the steel in order to improve the machinability thereof: lead, tellurium, selenium, calcium, bismuth in particular. The maximum contents are 0.10%, preferably 0.07% for Bi, 0.12% for Pb, 0.015%, preferably 0.010% for Te, 0.030%, preferably 0.020% for Se and 0.0050%, preferably 0.0045% for Ca.  
         [0079]     The other elements are those which are generally present in steel as impurities resulting from the production operation and are not added intentionally. It must be ensured in particular that the titanium content does not exceed 0.005%. Since the steel according to the invention is very rich in nitrogen, beyond this content there would be a risk of formation of coarse titanium nitrides and/or carbonitrides which can be seen by micrography, which would reduce the fatigue strength and impair the machinability. Furthermore, the titanium would thus capture the nitrogen which would no longer be available for controlling the grain.  
         [0080]     The invention will now be illustrated by means of examples. The appended Figure illustrates the Jominy curves of four steels whose compositions are set out in Table 1. The steels A, B, C and D are reference steels. The steels E, F and G are in accordance with the invention.  
                                                                         TABLE 1                           Compositions of the samples            Steel   C %   Mn %   Si %   S %   P %   Ni %   Cr %   Mo %   Cu %   Al %   Ti %   Nb %   N %               A   0.236   0.888   0.224   0.015   0.011   0.011   1.194   0.014   0.010   0.021   traces   traces   0.0124       (ref.)       B   0.195   1.188   0.069   0.023   0.012   0.208   1.228   0.096   0.162   0.021   traces   0.030   0.0179       (ref.)       C   0.192   1.205   0.845   0.029   0.014   0.080   0.995   0.099   0.110   0.025   traces   0.011   0.0110       (ref.)       D   0.245   1.215   0.840   0.035   0.012   0.085   0.980   0.103   0.098   0.035   traces   0.012   0.0090       (ref.)       E   0.230   1.287   0.920   0.018   0.017   0.201   1.269   0.200   0.211   0.032   traces   0.025   0.0174       (inv.)       F   0.201   1.453   1.191   0.041   0.014   0.139   1.381   0.246   0.122   0.031   0.002   0.038   0.0243       (inv.)       G   0.241   1.254   0.852   0.015   0.010   0.189   1.121   0.111   0.109   0.012   traces   0.016   0.0141       (inv.)                  
 
         [0081]     With sample A, the size α as defined above is equal to 8.7 and the size β as defined above is equal to 19.1. They are therefore far above the maximum required by the invention. It can be seen that the Jominy curve has a very marked point of inflection.  
         [0082]     With sample B, α is equal to 2.38 and β is equal to 11.1. β does not therefore comply with the requirements of the invention and the Jominy curve also has a significant point of inflection although this steel contains niobium and nitrogen within the prescribed limits. The basic reason for this is that the silicon content thereof is insufficient.  
         [0083]     With sample C, α is equal to 3.38 and β is equal to 10.7. Neither α nor β are within the prescribed limits and the Jominy curve has a marked point of inflection. Cr and Mo are just below the minimum values required and in particular the content of nitrogen is insufficient.  
         [0084]     With sample D, α is equal to 2.845 and β is equal to 9.5, which is again outside the prescribed limits. The Jominy curve has a marked point of inflection owing to the insufficient contents of Cr and nitrogen.  
         [0085]     However, for sample E according to the invention, α is equal to 0.41 and β is equal to 2.7. The required conditions are satisfied and it can be seen that the Jominy curve is almost rectilinear and has no point of inflection.  
         [0086]     In the same manner, for sample F according to the invention, α is equal to 0.23 and β is equal to 3.7. In this instance too, the Jominy curve thereof is almost rectilinear and has no point of inflection.  
         [0087]     In the same manner, for sample G according to the invention, α is equal to 0.83 and β is equal to 6.6. The Jominy curve thereof is almost rectilinear and has no marked point of inflection.  
         [0088]     The behaviour of steels A, B and E in Table 1 during carburising was also studied under normal temperature conditions and at high temperature.  
         [0089]     Carburising operations at normal temperature (930° C.) were carried out at low pressure under similar conditions using cylindrical samples in order to confer a carbon content of 0.75% on the carburised surface. These carburising operations were followed by quenching operations in a gaseous medium (in this instance in nitrogen, but a nitrogen/hydrogen mixture having 10% of hydrogen could, for example, have been used) under two different pressure conditions: 5 bar and 20 bar. It was thus intended to obtain a surface hardness of from 700 to 800 HV and a carburised depth (that is to say, the depth at which the hardness is 550 HV) of 0.50 mm. The results are given in Table 2 (tests at 5 bar) and in Table 3 (tests at 20 bar).  
                                 TABLE 2                           Behaviour during carburising with quenching       in gaseous media at 5 bar                Surface   Carburised   Core hardness HV outside       Steel   hardness HV   depth (mm)   the carburised zone               A   760   0.35   263       (ref.)       B   760   0.50   408       (ref.)       E   780   0.48   426       (inv.)                  
 
         [0090]    
       
         
               
             
               
               
               
               
             
           
               
                 TABLE 3 
               
             
             
               
                   
               
               
                   
               
               
                 Behaviour during carburising with quenching 
               
               
                 in gaseous media at 20 bar 
               
             
          
           
               
                   
                 Surface 
                 Carburised 
                 Core hardness HV outside 
               
               
                 Steel 
                 hardness HV 
                 depth (mm) 
                 the carburised zone 
               
               
                   
               
               
                 A 
                 780 
                 0.45 
                 318 
               
               
                 (ref.) 
               
               
                 B 
                 720 
                 0.55 
                 423 
               
               
                 (ref.) 
               
               
                 C 
                 738 
                 0.53 
                 408 
               
               
                 (ref.) 
               
               
                 E 
                 750 
                 0.55 
                 524 
               
               
                 (inv.) 
               
               
                   
               
             
          
         
       
     
         [0091]     These tests illustrate that the reference steel A does not allow the desired carburised depth to be readily achieved. This is due to its lack of quenchability.  
         [0092]     All three of the reference steels B and C and the steel E according to the invention allow the intended carburised depth to be produced under normal temperature conditions for carburising.  
         [0093]     The deviation ΔHV between the surface hardness and the core hardness is very comparable, for a quenching medium at 5 bar, for the reference steel B and the steel E according to the invention (ΔHV=352 and 354, respectively) and much lower than it is for the reference steel A (ΔHV=497). However, for a quenching medium at 20 bar, ΔHV is substantially less favourable for the reference steels B and C than for the steel E of the invention (ΔHV=297, 330 and 226, respectively). The result is that the residual stresses generated by these deviations of hardness, which are the cause of the deformations when the carburised components are quenched under strict conditions, may be minimised by the use of steels according to the invention.  
         [0094]     Finally, the highest levels of core hardness are produced with steel E according to the invention. Therefore, for gear components which are subjected to high levels of stress during operation and for which high levels of mechanical properties are sought (in particular high levels of hardness under the carburised layer and at the core) which are greater than the stresses to which the component is subjected during operation, in order to ensure a high level of fatigue endurance during operation, the steel according to the invention is that which, under given conditions for carburising, is most suitable for a high level of fatigue endurance during operation.  
         [0095]     Carburising tests were also carried out at high temperature (980° C.) on cylindrical samples of reference steels A and D and steel E according to the invention which have been described above. In this instance, the carburised surface again had a carbon content of 0.75%. In the two instances, a surface hardness of from 700 to 800 HV was sought and a carburised depth, at a hardness of 550 HV, of 0.50 mm. The quenching in a gaseous medium (nitrogen) which followed the carburising was carried out at a pressure of 20 bar for steels A and D and only 1.5 bar for steel E. The results are set out in Table 4. Evaluations of the grain size are also presented in accordance with the ASTM standard.  
                                         TABLE 4                           Behaviour during carburising with quenching in gaseous       media at 20 bar (steels A and C) and 1.5 bar (steel E)                        Core       Grain size                   hardness   Grain size   ASTM                   HV outside   ASTM in   outside           Surface       the   the   the           hardness   Carburised   carburised   carburised   carburised       Steel   HV   depth (mm)   zone   layer   layer               A   740   0.50   312   7/9   8/9       (ref.)       D   735   0.59   461   7/8   8/9       (ref.)       E   740   0.70   500   8/9    9/10       (inv.)                  
 
         [0096]     As with the carburising operation at the normal temperature of 930° C., the two steels allow the intended surface hardness to be achieved.  
         [0097]     The invention allows a carburised depth to be produced which is substantially greater than with reference A, although reference A has been quenched under much more strict conditions which are known to increase the carburised depth, all other things being equal.  
         [0098]     The deviation in hardness between the surface and the core is substantially less with the invention than with references A and D (ΔHV=240 for E, 428 for A and 274 for D, respectively). The advantages mentioned above with regard to deformations during quenching following carburising at normal temperature are also even further accentuated in this instance.  
         [0099]     The core hardness is higher with the invention than with the reference, in spite of a much lower pressure for the quenching medium. The consequences in terms of the improvement of the fatigue endurance during operation mentioned above for quenching at normal temperature are also found again in this instance.  
         [0100]     Finally, both in the carburised zone and outside the carburised zone, the steel according to the invention has an ASTM grain size which is finer than the reference steels A and D. Owing to this, it is less susceptible to the risks of increase in grain size during carburising at high temperature. This is a very significant advantage since the increase in size of grain on the carburised components has an extremely detrimental effect on the fatigue strength at the base of the tooth and on the strength of the carburised components. The steels according to the invention are therefore completely suitable for use in the production of gear components (or for any other components for which comparable properties are required) which are carburised or carbonitrided at high temperature, with all the economic advantages that this brings, without in any way sacrificing the effectiveness of the components.  
         [0101]     Other carburising tests were also carried out at low pressure on the reference steel A and on the steel E according to the invention.  
         [0102]     For a low-pressure carburising operation carried out at 930° C. on steel A followed by gas quenching at 20 bar, 72 mins. of carburising are required in order to produce the intended carburising depth of 0.50 mm for HV=550. Using steel E according to the invention, with low-pressure carburising at 930° C. followed by gas quenching (same gas as for steel A) at 1.5 bar, 30 mins. of carburising are sufficient to produce the same carburised depth of 0.50 mm for HV=550.  
         [0103]     For low-pressure carburising at a high temperature of 980° C., carried out on the reference steel A, 30 mins. of carburising are required and gas quenching at 20 bar in order to produce the intended carburising depth of 0.50 mm for HV=550. A carburising time of 20 mins. at low pressure at 980° C. is sufficient to produce the same carburising depth of 0.5 mm for HV=550 for steel E according to the invention and with gas quenching at a pressure of only 1.5 bar. The quenching gas used for steels A and E is, of course, the same.  
         [0104]     This shows that steel E according to the invention allows the carburising times to be reduced, both at the normal carburising temperature (930° C.) and at high temperature (980° C.), which allows the carburising costs to be reduced (quantity of carburising gas, carburising time, . . . ) and allows the productivity to be increased for the production of the carburised components.  
         [0105]     The steel according to the invention, owing to the controlled quenchability thereof, also allows the pressure of the quenching gases to be reduced in order to produce an identical carburising depth, which allows deformations in the carburised components to be further reduced or eliminated and allows savings and simplifications to be achieved in terms of technology for gas quenching components in the chambers of gas quenching furnaces.  
         [0106]     Carburising has also been carried out at low pressure on impact strength samples which have not been notched (dimensions: L=55 mm, cross-section 10×10 mm) at high temperature (980° C.), on the one hand on reference steel A before gas quenching at a pressure of 20 bar and, on the other hand, on steel E according to the invention but in this instance before gas quenching at a pressure of only 1.5 bar. The desired carburised depths were identical, as was the type of quenching gas. The samples which were carburised and quenched in this manner were then broken by means of impact at ambient temperature. The energy at break results obtained in this manner were, respectively: 
        19 Joules for the reference steel A,     29 Joules for the steel E according to the invention.        
 
         [0109]     At the same time, impact strength samples of reference steel A were carburised at low pressure at normal temperature (930° C.) in order to obtain the same carburised depth as above. They were then quenched with the same gas at a pressure of 20 bar. These samples were then broken as above at ambient temperature and the energy at break obtained in this manner was 17 Joules, that is, very substantially less than for steel E according to the invention which was carburised at high temperature.  
         [0110]     This indicates that, in spite of the sample of reference steel A having a lower core hardness (312 HV) than steel E according to the invention (500 HV), the toughness of steel E carburised at high temperature is higher than that of reference steel A carburised at high temperature or at normal temperature for the same final carburised depth. That is to say, using a steel according to the invention in order to carry out a high temperature carburisation operation which is intended to produce a specific carburised depth, does not impair, in fact the reverse, the toughness of carburised components which are produced from this steel compared with using a reference steel which has also been carburised at high temperature or at normal carburisation temperature in order to obtain the same carburised depth. The discrepancy in terms of core hardness between the two steels is not disadvantageous in this respect. This also indicates that the steels according to the invention are particularly suitable for carburisation at high temperature, both for reducing carburisation times, increasing productivity and reducing the carburisation costs compared with known steels which are carburised at normal temperature or at high temperature. The properties for use obtained with these components, such as the toughness, are not impaired compared with the reference steels.  
         [0111]     Under the conditions mentioned above, flexural fatigue samples of steel E according to the invention were also carburised at low pressure at high temperature (980° C.) comprising, at the centre thereof, a widened U-shaped notch. Carburisation was followed by gas quenching at a pressure of only 1.5 bar, both the intended carburised depths and the type of quenching gas being the same as in the tests on impact strength samples. In the same manner, a gas carburisation was carried out at the normal carburising temperature of 930° C. on steel A according to the prior art, in order to achieve the same carburised depth as above, on flexural fatigue samples identical to those of steel E. After carburisation, they were subjected to an oil quenching operation in order to increase the hardness and strength in terms of flexural fatigue of steel A. The endurance limits of the two batches of samples of steel E and A which were carburised in this manner were then compared in terms of flexural fatigue at 4 points, the widened U-shaped notch of these samples being centred in the region of the load applied in terms of flexural fatigue. The flexural fatigue tests were carried out for each steel A and E carburised and quenched under the above conditions for up to 10 million cycles.  
         [0112]     Under these conditions, the endurance limit at 10 million cycles for steel E according to the invention was 1405 MPa, and that of steel A only 1165 MPa.  
         [0113]     This indicates that using a steel according to the invention in order to carry out a high temperature carburisation operation which is intended to obtain a specific carburised depth does not impair the strength in terms of flexural fatigue but is instead very advantageous compared with a conventional carburisation operation which is carried out at normal carburisation temperature on a steel according to the prior art which has been carburised for the same depth and even quenched in oil in order to increase the strength thereof in terms of flexural fatigue.  
         [0114]     It should be added at this point that these tests of flexural fatigue are intended to simulate the fatigue strength of a gear tooth base, gear mechanism or gear component used in a gearbox of a motor vehicle. This again indicates that the steels according to the invention are particularly suitable for high temperature carburisation, both for reducing the carburisation times, increasing productivity, reducing carburisation costs compared with known steels which have been carburised at normal temperature, without impairing the properties for use obtained with components, such as the strength in terms of flexural fatigue of the carburised base of a gear tooth of a pinion or gear mechanism.