EP2742165A1 - Steel for manufacturing carburized steel parts, carburized steel parts produced with said steel, and method for manufacturing same - Google Patents

Abstract

The invention relates to steel for manufacturing carburized steel parts, characterized in that the composition thereof is: 0.1 wt % ≤ C ≤ 0.15 wt %; 0.8 wt % ≤ Mn ≤ 2 wt %; 1 wt % ≤ Cr ≤ 2.5 wt %; 0.2 wt % ≤ Mo ≤ 0.6 wt %; trace elements ≤ Si ≤ 0.35 wt %; trace elements ≤ Ni ≤ 0.7 wt %; trace elements ≤ B ≤ 0.005 wt %; trace elements ≤ Ti ≤ 0.1 wt %; trace elements ≤ N ≤ 0.01 wt % if 0.0005 wt % (5ppm) ≤ B ≤ 0.005 wt %, and trace elements ≤ N ≤ 0.02 wt % if trace elements ≤ B ≤ 0.0005 wt % (5ppm); - traces ≤ Al ≤ 0.1 wt %; trace elements ≤ V ≤ 0.3 wt %; trace elements ≤ P ≤ 0.025 wt %; trace elements ≤ Cu ≤ 1 wt %, preferably < 0.6 wt %; and trace elements ≤ S ≤ 0.1 wt %, the remainder being iron and impurities resulting from production. The invention also relates to a carburized steel part produced with said steel, and to the method for manufacturing same.

Description

 Steel for the manufacture of case-hardened parts, case-hardened part made of this steel and its method of manufacture

 The invention relates to iron and steel, and more particularly to the grades of case hardening steels with a high degree of resilience.

 One of the main applications of such steels is the manufacture of large format mechanical parts and more particularly the manufacture of drill bits.

 Such a drill bit is a forged tool consisting of three rotating steel cones "entangled" in one another and making it possible to crush the geological formations during oil or gas exploration operations. These three cones are rotated, via one or more bearings, on three welded steel arms. The machining tracks machined on the arms and inside the cones are generally, in conventional production processes, treated superficially by atmospheric carburization to reach a conventional depth, where the Vickers hardness is 550 HV, included on average between 1 and 1.5 mm.

 The present invention relates to a new steel grade that can be used for making cones and arms. There are several kinds of such trephines. One of them is the bit with "inserted tooth", that is to say in which spikes, most often made of tungsten carbide obtained by powder metallurgy, are crimped into each of the cones. The steel subject of the present invention is not limited, in its use, to this type of drill bit, and could also be used for the production of "machined tooth" bits.

 Currently, the reference grades used for the manufacture of the cones and arms constituting the drill bits are steels strongly alloyed with nickel at levels of up to 3.5% by weight (especially the grade of 15NiCrMo13). This alloying element is usually considered necessary to give the product the level of ductility essential to withstand the severity of the mechanical stresses in service. This ductility must be associated with tensile characteristics and high quenchability. The properties typically sought for these shades are, in fact:

a hardenability such that the Jominy curve is horizontal, typically at least over a depth of 20 mm; as described in document FR-A-2 765 890, this characteristic can be evaluated by the coefficient β corresponding to the difference in hardness between Jominy points J ₅ and J ₃ . For quenchability to be sufficient, the Jominy curve should satisfy β <3.5 HRC and Ji> 40 HRC; it is recalled that the Jominy curve is a curve which reflects the hardenability of a steel; it is obtained at by means of a standardized test (in particular by the NF EN ISO 642 standard) by measuring the hardness along a generatrix of a cylindrical specimen soaked by a jet of water watering one of its ends; the hardness measured at a depth of x mm from said end is designated J _x ;

 a yield strength Re greater than 900 MPa;

 a tensile strength Rm greater than 1200 MPa;

 a resilience Kv measured at 20 ° C. of greater than 75 J;

 a good aptitude for atmospheric carburizing; it results, in particular by the possibility of finding conditions of atmospheric cementation making it possible to reach the following characteristics:

 • a carburising depth at 550 HV between 1 .2 and 1 .5 mm;

• superficial hardness greater than 670 HV;

 An austenitic grain size such that the grain index is greater than ASTM 5;

 A maximum superficial content in residual austenite (at 20 μηι under the skin of the metal) in the lower case-hardened layer at a content of about 40%, such as that which is typically sought after treatment of cementation type shades. 15NiCrMo13.

 However, the steels usually used in this context have, as has been said, a Ni content requiring a significant addition of this expensive element and whose purchase price is very variable over time. It has therefore been attempted to replace these steels with Ni-free carburizing steels having improved resilience.

 US-A-6 146 472 provides an example. The increase in resilience is obtained by controlling the magnification resistance of the austenitic grain via the use of an Nb-Al-N microalloy, combined with an optimized heat treatment. However, the resilience values indicated in this document are at best close to 60J, and the examples presented are castings which do not make it possible to check the quenchability criterion β <3.5 HRC.

 Similarly, US-A-2005/0081962 discloses a carburizing steel not using Ni, but whose resilience does not exceed 51 J, which is not sufficient.

 The object of the invention is to provide a carburizing steel used in particular for the manufacture of drill bits, not requiring addition of Ni and nevertheless meeting all the criteria of ductility, hardenability, Re, Rm and Kv cited above.

For this purpose, the subject of the invention is a steel for the manufacture of cemented parts, characterized in that its composition, in percentages by weight, is: - 0.1% ≤C≤ 0.15%;

 - 0.8% <Mn <2%;

 - 1% <Cr <2.5%;

 - 0.2% <Mo <0.6%;

 - traces <If <0.35%;

 traces <Ni <0.7%, preferably traces <Ni <0.3%;

 - traces <B <0.005%;

 traces <Ti <0.1%, preferably traces <Ti <0.04%;

 - traces <N <0.01% if 0.0005% <B <0.005%, and traces <N <0.02% if traces <B <0.0005%;

 - traces <Al <0.1%;

 - traces <V <0.3%;

 - traces <P <0.025%;

 - traces <S <0.1%;

 traces <Cu <1%, preferably <0.6%;

the rest being iron and impurities resulting from the elaboration.

 Preferably, traces <0 <30 ppm.

 The subject of the invention is also a case-hardened steel piece, characterized in that it is made of a steel having the preceding composition and in that it has undergone a cementation.

 It can be a drill bit.

 The invention also relates to a method for manufacturing a cemented part, characterized in that:

 a blank of said workpiece is made of a steel having the preceding composition. This shaping can be performed by any method (forging, machining, rolling ...);

 and a cementation is carried out on said blank.

 In the case of an atmospheric cementation process, the succession of steps may be as follows:

 - heating up to the temperature of the enrichment stage.

 enrichment stage at a temperature of 900 to 980 ° C. for 3 to 20 hours and at a carbon potential of between 0.8 and 1.2%;

 diffusion at a temperature of 820 to 880 ° C. at a carbon potential of between 0.6 and 0.8%, with a treatment time of 1 to 4 hours;

oil quenching at a temperature of less than or equal to 1 ° C - Income at a temperature between 150 and 250 ^ and for a period of between 1 and 4 hours.

 In the case where the cementation is carried out at low pressure, said pressure may be from 5 to 20 mbar, and the succession of steps of the cementation may be the following:

 - heating up to the temperature of the enrichment stage.

 enrichment stage at a temperature of 900 to 980 ° C. for 3 to 20 hours and at a carbon potential of between 0.8 and 1.2%;

 diffusion at a temperature of 890 to 950 ° C. at a carbon potential of between 0.6 and 0.8%, with a treatment time of 1 to 4 hours;

 quenching at a temperature of less than or equal to 100.degree.

 - Income at a temperature between 150 and 250 ° C and for a period of between 1 and 4 hours.

 As will be understood, the invention is based on a careful adjustment of the composition of the steel, to meet all the criteria mentioned above.

 In addition, the steel that is the subject of the present invention is also different from that described in US Pat. No. 6,146,472 in that the accessible resilience is significantly greater, and in that the improvement of the resilience is not generated at least mainly, by a control of the grain size. This has the advantage of not modifying the ability of the grade to thermomechanical treatment and limit the risk of abnormal magnification of the austenitic grain during cementation. The segregating effect of niobium which may lead to a heterogeneous austenitic grain size is, in particular, avoided. The level of resilience accessible by the present invention is also significantly higher.

 The type of carburizing used with the steel described in the present invention is not limited to the atmospheric carburization process which could be replaced by other surface hardening processes, for example low pressure carburizing.

The present invention is based on a steel whose composition is defined below. All grades are given in percentages by weight. By using a composition defined as described below, it is possible to develop, without the voluntary addition of nickel and without using large quantities of other expensive elements, a steel having a hardenability, mechanical characteristics after quenching followed by and a carburizing ability (carbon uptake, resilience to the core, carburizing depth, residual austenite content, etc.) similar to those of the reference grades of 3.5% Ni usually used for the manufacture of carbon fiber drill bits. drilling. The content of C is between 0.10% and 0.15%, a carbon content limited to a relatively narrow range, and which is low, compared with those generally encountered in case-hardening steels. This low carbon content makes it possible to achieve very high resilience in the core of the cemented parts. The loss of quenchability and the decrease in the hardness of the after-carburized products, which would normally result from this lowering of the C content, are compensated by an optimized adjustment of the concentration of the other alloying elements.

 The Mn content is between 0.8% and 2%. Manganese is used with chromium and molybdenum to compensate for the loss of hardenability associated with decreased carbon content. For its effect to be sufficient, a content greater than or equal to 0.8% is required. Since this alloying element can pose problems of segregation, it is preferable that its concentration does not exceed 2%.

 The Cr content is between 1% and 2.5%. Like manganese, chromium is used to ensure a sufficient degree of quenchability to the grade. The minimum content of 1% is chosen so that the effect of this alloying element on quenchability is sufficient. The maximum content of 2.5% is defined in order to avoid a detrimental effect on the use properties, in particular by formation of coarse chromium carbides.

 The Mo content is between 0.2% and 0.6%. Molybdenum is a third element used to adjust the quenchability of the grade. It is also an alloying element that can be used judiciously to increase resilience, especially at low temperatures. Molybdenum also exacerbates the effect of boron on hardenability, and can therefore be used for this purpose in the case of boron alloyed grades. For a content of less than 0.2% the increase in hardenability is too low and this value is therefore chosen as the minimum level. For high concentrations, molybdenum tends to decrease the ability of forging steels. In addition, because it is an expensive alloying element, its use at an excessive content would lead to a loss of economic benefit brought about by the non-use of nickel. For these reasons, a maximum content of 0.6% is preferred.

 The Si content is less than 0.35%. Like aluminum, silicon can be used as a deoxidizing element. For a steel which has been deoxidized by addition of silicon, the residual content of this element does not in any case generally exceed 0.35%. It should also not exceed a content of 0.35% in the steels of the invention, because silicon is an alloying element that can limit, by barrier effect, the carbon uptake during cementation.

The Ni content is less than or equal to 0.7%, preferably 0.3%. As has been said, one of the objects of the present invention is to make it possible to do without an addition voluntary of this element. However, it is still present in the residual state in the raw materials used to make steel, especially in scrap metal. The 0.3% content corresponds to the maximum level most commonly encountered when no voluntary addition of nickel is made during the elaboration.

 The B content is less than 0.005%. Boron is an optional element. It can be used to optimally adjust the quenchability of the grade if the levels of Mn, Cr and Mo are not quite sufficient for this purpose. But for this alloy element to actually act on the quenchability, it must be maintained in solid solution. For that, the precipitation of nitrides or oxides of boron must be avoided. This result can be obtained by adding a higher affinity alloy element with nitrogen, for example titanium, and controlling the production process to limit the dissolution of nitrogen and oxygen in the steel.

 The Ti content is less than 0.1% and preferably less than 0.04%. Titanium is optionally added to allow the boron to be maintained in solid solution by precipitation of titanium nitrides which reduce the amount of nitrogen that may be combined with boron. Its content should optimally be chosen according to the amount of nitrogen in the grade. To be completely effective, a stoichiometric amount of titanium must be added to ensure precipitation of all of the nitrogen contained in the steel as TiN, and thus maintain the boron in solid solution. This is verified if the Ti / N ratio is greater than 3.4. In the case of substoichiometric titanium addition, the effect of boron on quenchability can still be expressed but is less pronounced. Beyond the prescribed limit, there is a risk of formation of TiN too coarse during solidification, and moreover the addition of Ti becomes excessively expensive.

 The N content is less than 0.02%, preferably less than 0.01%. In the case of elaboration with addition of boron and titanium, it is necessary to control the nitrogen content of the steel to limit the risk of formation of coarse titanium nitride TiN, which can deteriorate the properties of use of the product. In the case of boron addition (from 5 to 50 ppm), a nitrogen content of less than 0.01% is therefore recommended. If the boron is not used (B <5ppm), it is not absolutely essential to strictly control the nitrogen content, which can then go up to 0.02% without detrimental effect on the properties of the elaborated steel. .

The Al content must be at most 0.1%: Aluminum is an optional element. It can be used as a deoxidizer for steel to replace silicon, and to optimize the behavior of the austenitic grain during cementation. The V content is at most 0.3%. Vanadium is an optional element. It can be used as a micro-alloy element for better control of grain size during carburizing, providing further improvement in resilience.

 The P content is at most 0.025%. This limit is recommended to avoid risk of weakening the steel. At an excessively high content, this element tends to segregate at the austenitic grain boundaries, which can lead to an increase in the ductile-brittle transition temperature and a decrease in the resilience at room temperature.

 The Cu content is at most 1%, preferably at most 0.6%. A maximum content of 1% is recommended because it is an expensive element that provides no benefit of quenchability or resilience. The preferred maximum value of 0.6% is a content usually recognized as being below which copper has no significant effect on the mechanical properties of steel. Nevertheless, use at a higher level is possible without modifying the ability of the grade to be used for the manufacture of drill bits.

 The negative effects of copper at higher levels are in particular the risk of appearance of surface defects during rolling (crazing). Additions of Ni and / or Si may make it possible to remedy this, but since the invention requires relatively low levels of these elements, we can not rely heavily on them to limit the harmful effects of copper, hence the limits of 1%, better 0.6%, for the copper content of the steels of the invention.

 The S content is not strictly imposed in the definition of steel according to the most general invention, but it must be controlled according to the intended application. A low content will be sought if it is desired to improve the inclusion cleanliness by not forming sulphides (typically <0.01%) and a higher content may be chosen (typically from 0.03% to 0.1%) if a gain in machinability is sought and provided that the inclusion cleanliness remains consistent with the requirements of the intended application for steel.

 The O content is most often at most 0.003% (30 ppm), so as to optimize the inclusion cleanliness. This limit may possibly be exceeded if the future application of the steel does not require a very good inclusion cleanliness, and in any case a specific O content does not constitute an intrinsic property of the steel according to the invention.

The control of the oxygen content is ensured by inerting systems during casting and by a control of the content of deoxidizing elements such as Si and Al. When these deoxidizing elements are present at low levels, it is nevertheless possible to ensure a low oxygen content in the liquid metal:

 or by agitating the liquid metal so as to place it in chemical equilibrium with the slag, the composition of which is controlled so that this chemical equilibrium leads to the establishment of a low dissolved oxygen content in the liquid metal, and then avoiding the reoxidation of the liquid metal until casting by an effective protection against atmospheric oxygen, for example the inerting of the surface of the metal with argon, and the confinement of the pouring jets in refractory tubes themselves filled with argon;

 or by driving the elaboration of the liquid metal at least partially under reduced pressure, so that the dissolved oxygen content is limited by the carbon present in the liquid steel, which causes the departure of the excess dissolved oxygen under form of CO; then, as in the previous case, the low dissolved oxygen content must be preserved until casting by an effective protection of the liquid steel against atmospheric reoxidation.

 For the other elements, they can be present in the state of simple impurities resulting from the elaboration.

 In the case of an atmospheric cementation process intended to obtain a surface carbon content typically of 0.5 to 0.8%, the succession of steps may be as follows:

 heating up to the temperature of the enrichment stage; a speed of the order of 10 mm for this heating is advisable to provide a good control of the evolution of the grain size;

 enrichment stage at a temperature of 900 to 980 ° C and a carbon potential of between 0.8 and 1.2 for a period of 3 to 20 hours; these conditions may vary depending on the exact composition of the steel being treated, and especially on the carburizing depth involved; typically for a temperature of 960 ° C a treatment of 3 to 6h makes it possible to temper the part to a depth of 1 to 1, 5 mm;

 diffusion at a temperature of 820 to 880 ° C. at a carbon potential of between 0.6 and 0.8%, with a treatment time of 1 to 4 hours, typically of the order of 2 hours; the criteria for choosing the diffusion temperature are mainly, and conventionally for the person skilled in the art, related to the situation of the phase transformation points of the treated grade, and to the fact that this temperature must not be too high to minimize the deformations of the part during the quenching which follows;

oil quenching at a temperature of less than or equal to 100%; - Income at a temperature between 150 and 250 ^ and for a period typically of the order of 2h, in any case between 1 and 4h.

 This type of carburizing is just one example, and other methods can be used. In particular, it is possible to use a low-pressure carburizing to avoid possible surface oxidation and / or intergranular problems during the treatment, and also to reach carburizing depths greater than the 1 to 2 mm usually accessible by atmospheric carburizing and / or reducing the cementation time due to the high temperature at which low pressure carburizing is practiced.

 In the case of a low pressure carburizing process conducted at a pressure of a few millibars (5 to 20 mbar), the succession of steps may be as follows, to aim at a surface C content typically of 0.5 to 0. , 8%:

 heating up to the temperature of the enrichment stage; a speed of the order of 10 mm for this heating is advisable to provide a good control of the evolution of the grain size;

 enrichment stage at a temperature of 900 to 980 ° C. and at a carbon potential of between 0.8 and 1.2 for a period of 3 to 20 hours; these conditions may vary depending on the exact composition of the steel being treated, and especially on the carburizing depth involved; typically for a temperature of 960 'Ό a treatment of 3 to 6h allows to temper the part to a depth of 1 to 1, 5 mm avoiding the surface oxidation problems that may be encountered during an air cementation.

 diffusion at a temperature of 890-950 ° at a carbon potential of between 0.6 and 0.8%, with a treatment time of 1 to 4 hours, typically of the order of 2 hours; the criteria for choosing the diffusion temperature are mainly, and conventionally for the person skilled in the art, related to the situation of the phase transformation points of the treated grade, and to the fact that this temperature must not be too high to minimize the deformations of the part during the quenching which follows;

 quenching, for example with oil or gas (quenching pressure of between 3 and 10 bar), at a temperature of less than or equal to 1 ° C.;

 - Income at a temperature between 150 and 250 ^ and for a period typically of the order of 2h, in any case between 1 and 4h.

Of course, the mechanical properties obtained on the final product depend not only on the composition of the steel, but also the thermal and thermomechanical treatments it undergoes until the product is obtained. It may however be noted that in the case where the final product must be cemented, the conditions of its hot shaping by forging, rolling or otherwise, are of little importance. Indeed, the Cementation is accompanied by a quenching and tempering operation which gives the product a new structure and erases the consequences of hot shaping. It is then this treatment which confers on the product the essential of its mechanical properties, if it is not itself followed by any other treatment which could modify them.

 The results of tests carried out on steels according to the invention and reference steels will now be described. All the results presented were obtained on laboratory castings forged at 1200 square-section bars of 40 mm side.

 Before forging, the steel is in the form of ingots of square section 100x100 mm and 200 mm high. After forging, the 40x40 mm section bars are cooled in still air and then normalized for 2 hours at a temperature of 875, 900 or 925 ^, chosen according to the transformation point Ac3 of the grade. This standardization is intended to homogenize the carbon content and initial microstructure throughout the product.

 The composition of the different shades tested is given in Table 1. Castings Nos. 1 to 4 are those whose composition is in accordance with the present invention. Castings Nos. 5 to 10 are those in which at least one of the alloying elements is outside the claimed ranges. All concentrations are given in% by weight, except nitrogen, oxygen and boron, which are given in ppm by weight. The table also shows the temperature of the transformation point Ac3 (in ° C) of each of the grades.

Ech C If Mn Ni Cr Mo S P O Al N Cu B Ti V Ac3

%%%%%%%% PP% PP% PP%% ° C m m m

 Inv. 1 0.1 0.2 1, 4 0.0 2.1 0.4 0.0 0.0 7 0.0 115 0.0 3 0.0 0.0 857

 07 74 84 67 07 43 08 09 22 7 09 1

Inv. 2 0.1 0.2 1, 5 0.0 2.0 0.4 0.0 0.0 21 0.0 120 0.0 3 0.0 0.0 845

 37 39 36 72 58 01 08 08 02 8 06 1

Inv. 3 0.1 0.2 0.8 0.0 1, 4 0.4 0.0 0.0 0.0 67 0.0 0.0 0.0 0.0 874

 14 35 93 59 28 06 05 07 30 6 23 1

Inv. 4 0.1 0.2 1, 0 0.0 1, 5 0.3 0.0 0.0 13 0.0 78 0.0 29 0.0 0.2 861

 46 43 17 56 19 76 06 07 18 5 19 1

Ref. 5 0.1 0.0 1, 2 0.1 1, 3 0.0 0.0 0.0 18 0.0 148 0.2 1 0.0 0.0 826

 60 40 40 80 80 90 31 8 20 1 02 0

Ref. 6 0.0 0.3 1, 2 0.2 0.9 0.0 0.0 0.0 0.0 0.0 0.2 0.2 0.0 0.0 0.0 881

70 80 50 60 40 80 04 12 30 1 22 1 Ref. 7 0.1 0.2 1, 3 0.0 1, 6 0.1 0.0 0.0 9 0.0 175 0.0 25 0.0 0.0 848

30 60 50 50 60 00 08 08 20 5 06 1

Ref. 8 0.1 0.9 1, 4 0.0 1, 4 0.2 0.0 0.0 10 0.0 141 0.0 2 0.0 0.0 882

 20 30 70 50 40 10 07 08 20 4 05 1

Ref. 9 0.1 0.2 1, 1 0.0 1, 9 0.1 0.0 0.0 14 0.0 155 0.0 10 0.0 0.0 851

 20 50 10 60 20 60 06 08 20 6 06 1

Ref. 10 0.1 0.8 0.5 0.0 1, 5 0.8 0.0 0.0 12 0.0 143 0.0 4 0.0 0.0 916

 30 80 30 50 10 60 08 07 20 4 06 1

Table 1: Ac3 compositions and temperatures of the tested samples

Due to the significant presence of AI at fairly comparable levels, the O contents of the various samples are all between 7 and 21 ppm and do not significantly affect the properties obtained.

 The quenchability of the different samples was evaluated by means of Jominy tests. The austenitization temperature was chosen, according to the transformation point of the steel in question, among the temperatures 875, 900 and 925 ° C.

 To evaluate the mechanical characteristics, pieces of square section of

20 mm of side were taken in each of the bars forged then treated by the following thermal cycle:

 heating up to the austenitization temperature;

 austenitization at 930 ° C for 30 minutes;

oil quenching at 30 ° C ^. ;

 - income at Ι ΘΟ 'Ό for 2 hours;

 - air cooling.

 This heat treatment cycle makes it possible to estimate the resilience expected at the core of the parts treated by cementation.

Traction and resilience test pieces (Charpy type with V notch) were then machined in the parts thus treated. Table 2 presents the results obtained and will be compared to the properties required for the production of drill bits. The underlined data indicate results that do not meet criterion Ji> 40, or quenchability criterion β <3.5 HRC, or whose elasticity, tensile and resilience characteristics are insufficient. Echo T Jominy (° C) Ji β Re (MPa) Rm (MPa) Kv 20 ° C (J)

Inv. 1 875 40.9 1, 9 995 1280 121

 Inv. 2,875 43.5 2.9 967 1291 108

 Inv. 3 900 40.2 3 907 1204 160

 Inv. 4,900 43.6 2,949 1212 89

 Ref. 5,875 44.4 10.1 1016 1319 46

 Ref. 6,900 35.4 10.7 746 850 -

Ref. 7,875 41 10,1 821 1,106,123

 Ref. 8 925 43 L 3 982 1298 12,

 Ref. 9,875 41, 1,887 1,184,109

 Ref. 10 925 41, 2 10.4 820 1 1 14 142

Table 2: Mechanical properties of the tested samples

It is noted that all grades whose composition is in accordance with the present invention are characterized by mechanical characteristics greater than the minimum required for the production of drill bits, ie Re greater than 900 MPa, Rm greater than 1200 MPa, Kv at 20 ° C higher at 75 J, and by a hardenability satisfying the criteria β <3.5 HRC and Ji> 40 HRC. Conversely, all the shades whose composition is outside the present invention have insufficient quenchability and / or mechanical characteristics too low. It is, in particular, the case of the sample 6 for which the mechanical characteristics Re and Rm are in any case frankly too low for the steel to be usable to manufacture drill bits, and for which it has not been considered useful to measure resilience.

 The ability to cement the steels according to the invention was also tested. Carburizing tests were carried out under the following conditions.

 These tests were carried out on cylinders of diameter 25 mm and length 120 mm placed in industrial loads of the order of 150 to 200 kg. After carburising, the cemented cylinders are characterized in the following manner:

 - determination, according to standard NF EN ISO 2639, of the conventional 550 HV HVAC and the surface hardness by microhardness tests under a load of 0.1 kg (called "depth at 550 HV0.1");

- determination of the size of austenitic grain layer and heart after attack Béchet-Beaujard; this evaluation was carried out in accordance with standard NF EN ISO 643; - X-ray diffractometry determination of the residual austenite content at 20 μηι below the surface of the parts.

 The cementation tests were carried out on grades No. 1 and No. 3 placed in an industrial cementation charge and treated at atmospheric pressure under the following conditions:

heating at 9 ^< C / min up to 950 ° C;

 - isothermal maintenance at 950 ° for 5h with a carbon potential (called "carbon enrichment potential") of 1 .2%; it is recalled that a gaseous carburizing atmosphere is characterized by its carbon potential, which is the carbon content of a sample of the equilibrium steel in the austenitic state with the carburizing atmosphere, at temperature and pressure. of use;

 cooling to 870 ° C. and keeping at this temperature for 2 hours with a carbon potential (called "diffusion carbon potential") of between 0.6 and 0.7%;

 oil quenching at 30 ° C .;

 - income at 190 ° C for 2 hours.

 These conditions are those of a standard carburization cycle used to treat the grades 1 3NiCrMo13 currently used for the manufacture of drill cones.

 A cylinder 1 1NiCrMo13 has therefore been placed in the carburization charge to serve as a reference and determine the reference characteristics that must achieve, for the sample format considered, shades developed in accordance with the present invention. The composition of the casting used as a reference is given in Table 3.

Table 3: Composition of the reference sample in 13NiCrMo1 steel 3

The carbon potential in the diffusion phase (diff carbon potential) was adapted to the treated grade so as to control the residual austenite surface content.

 The characterization results of the reference grade are given in

Table 4. Those of the two grades elaborated according to the present invention are given in Table 5. It is noted that the characteristics of the shades according to the present invention are identical or almost identical to those of the grade of reference, and thus respect in all points the minima required for the manufacture of drill bits namely:

 carburizing depth of between 1 and 1.5 mm;

 - superficial hardness greater than 670 HV;

 - residual austenite content of less than 40;

 - ASTM grain index greater than 5

Table 4: Cementation Test Results (Reference)

Table 5: Results of the cementation tests (Invention)

Claims

 1 .- Steel for the manufacture of cemented parts, characterized in that its composition, in percentages by weight is:

 - 0.1% ≤C≤ 0.15%;

 - 0.8% <Mn <2%;

 - 1% <Cr <2.5%;

 - 0.2% <Mo <0.6%;

 - traces <If <0.35%;

 - traces <Ni <0.7%;

 - traces <B <0.005%;

 - traces <Ti <0.1%;

 - traces <N <0.01% if 0.0005% <B <0.005%, and traces <N <0.02% if traces <B <0.0005%;

 - traces <Al <0.1%;

 - traces <V <0.3%;

 - traces <P <0.025%;

 traces <Cu <1%, preferably <0.6%;

 - traces <S <0.1%

the rest being iron and impurities resulting from the elaboration.

 2. Steel according to claim 1, characterized in that its O content is less than or equal to 30 ppm.

 3. - Hardened steel piece, characterized in that it is a steel having the composition according to claim 1 or 2, and in that it has undergone a cementation.

 4. - Cemented part according to claim 3, characterized in that it is a drill bit.

 5. - Process for manufacturing a cemented part, characterized in that:

 - Preparing a blank of said piece of a steel according to claim 1, and is shaped by forging;

 and a cementation is carried out on said blank.

 6. A process according to claim 5, characterized in that the carburizing is carried out at atmospheric pressure, and in that the succession of steps of the carburizing is the following:

 - heating up to the temperature of the enrichment stage.

enrichment stage at a temperature of 900 to 980 ° C. for 3 to 20 hours and at a carbon potential of between 0.8 and 1.2%; diffusion at a temperature of 820 to 880 ° C. at a carbon potential of between 0.6 and 0.8%, with a treatment time of 1 to 3 hours;

 - oil quenching at a temperature of less than or equal to

 - Income at a temperature between 150 and 250 ^ and for a period of between 1 and 4 hours.

 7.- Method according to claim 5, characterized in that the carburizing is carried out at low pressure, in that said pressure is 5 to 20 mbar, and in that the succession of steps of the cementation is as follows:

 - heating up to the temperature of the enrichment stage.

 enrichment stage at a temperature of 900 to 980 ° C. for 3 to 20 hours and at a carbon potential of between 0.8 and 1.2%;

 diffusion at a temperature of 890 to 950 ° C. at a carbon potential of between 0.6 and 0.8%, with a treatment time of 1 to 4 hours;

 quenching at a temperature of less than or equal to

 - Income at a temperature between 150 and 250 ^ and for a period of between 1 and 4 hours.