MACHINE COMPONENT

Abstract

Machine component that includes: a core made of a steel used in mechanical engineering; as well as a medium carbon layer and a high carbon layer formed from steel for mechanical engineering, wherein the medium carbon layer covers the core, the high carbon layer covers the medium carbon layer and has a carbon concentration of 0.8 - 1.5%; wherein the steel for mechanical engineering contains in mass percent 0.13 - 0.30% C, 0.15 - 0.80% Si, 0.20 - 0.90% Mn, 0.90 - 2.00% Cr, 0.020 - 0.050% Al and 0.002 - 0.025% N, and also contains as impurities 0.030% or less P and 0.030% or less S, furthermore optionally selectively as a first group of optional components one or more elements selected from 0.10 - 2.00% Ni, 0.05 - 0.50% Mo, 0.01 - 0.10% Nb and 0.01 - 0.20% V, and optionally selectively as a second group of optional components in addition to or instead of the first group of optional components Contains 0.01 - 0.05% Ti and 0.0010 - 0.0050% B, the remainder consisting of Fe and unavoidable impurities, and the high-carbon layer consists of a martensite structure with dispersed carbides and a retained austenite structure, incorporated carbides with an aspect ratio of 1.5 or less constitute 90% or more of the total number of carbides, and the number of incorporated carbides at former austenite grain boundaries is 40% or less of the total number of carbides.

Description

Technical field

[0001] The present invention relates to a machine component which has excellent toughness and a surface layer hardened by carburizing, and which is used for a component which is to be subjected to high surface pressure.

[0002] The present application claims priority on the basis of Japanese patent application No. 2018-115349 filed on June 18, 2018, and the entire contents of that Japanese patent application are incorporated herein by reference. Technical background

[0003] Machine components, such as gears and shafts, which are subjected to high surface pressure, are manufactured by shaping a steel material through hot forging, cold forging, cutting, and similar processes. The resulting material is then subjected to carburizing operations, such as gas carburizing or vacuum carburizing, before use. If necessary, the material can also undergo grinding, shot peening, etc. Carburizing is a process in which carbon is introduced into a steel component from the surface. This occurs after heating the steel to a high temperature (not below the austenitizing temperature) to achieve a high solubility limit of carbon in the solid state.

[0004] Generally, carburizing introduces 0.7–0.8% carbon into the surface of the steel component. The component is then quenched. Quenching can be performed directly from the carburizing temperature, or it can be carried out after the component has cooled from the carburizing temperature to a standard quenching temperature. Alternatively, the component can be cooled and then reheated after carburizing before being quenched. This process is called tempering.

[0005] Due to the current reduction in size and weight of transmissions in motor vehicle powertrain units to reduce fuel consumption, gears, shafts, and the like are generally subjected to increasingly higher loads. In particular, the service life of gears can be shortened due to pitting corrosion on the tooth surface or tooth breakage. Patent document 1, however, proposes a steel with high hardness and excellent toughness, containing a large amount of carbon, with a carbon content of 0.55–1.10% by mass. The microstructure of the steel after quenching is a two-phase structure consisting of martensite and incorporated carbide, with the proportion of globular cementite in the total cementite and the proportion of cementite at the former austenite grain boundaries being controlled.With this type of steel, the carbon concentration in the steel component is kept high towards the inside, so the required toughness may not be achieved.

[0006] EP 2 530 178 A1 discloses a case-hardening steel and a carburized steel material produced therefrom by carburizing for mechanical components. The document teaches an alloy composition including C (0.10 - 0.35 wt%), Cr (0.5 - 2.5 wt%), Al (0.04 - 0.10 wt%), B, and Nb, with a significantly limited Ti content. For the surface layer (up to 0.4 mm), C contents ≥0.70 wt% or, in the case of "super-carburization," ≥0.85 wt%, as well as carbide sizes of max. 10 µm and on average ≤4 µm, are targeted in order to avoid coarse carbides and increase pitting fatigue strength. List of cited patent documents Patent document 1: JP 2017 - 57 479 A Patent document 2: EP 2 530 178 A1 Summary of the invention: Technical problem

[0007] One object of the present invention is to create a machine component that is surface-hardened and yet has improved toughness compared to conventional methods. Solution to the problem

[0008] To fulfill the above-mentioned task, the present invention provides a machine component as described below.

[0009] A machine component contains a core made of a steel for mechanical engineering, which has a component composition containing, by mass, 0.13–0.30% C, 0.15–0.80% Si, 0.20–0.90% Mn, 0.90–2.00% Cr, 0.020–0.050% Al, and 0.002–0.025% N, and also containing, as impurities, 0.030% or less P and 0.030% or less S, and optionally, as a first group of optional components, one or more elements selected from 0.10–2.00% Ni, 0.05–0.50% Mo, 0.01–0.10% Nb, and 0.01–0.20% V, and optionally, selectively as a second group of optional components in addition to or instead of the first group of optional components containing 0.01 - 0.05% Ti and 0.0010 - 0.0050% B, the remainder consisting of Fe and unavoidable impurities;as well as a medium-carbon layer and a high-carbon layer formed from the steel used in engineering, the medium-carbon layer covering the core, the high-carbon layer covering the medium-carbon layer, and the high-carbon layer having a carbon concentration of 0.8–1.5%. The high-carbon layer consists of a martensitic microstructure with carbides dispersed therein and a retained austenite microstructure. In the high-carbon layer, incorporated carbides with an aspect ratio of 1.5 or less constitute 90% or more of the total number of carbides. In the high-carbon layer, the number of incorporated carbides at former austenite grain boundaries constitutes 40% or less of the total number of carbides.

[0010] Of the carbides incorporated at the former austenite grain boundaries, 90% or more may have a particle size of 1 µm or less.

[0011] At the former austenite grain boundaries, a grain size of 15 µm or less may occur.

[0012] The layer with a high carbon content can extend from at least the surface down to a depth of 0.3 mm of the machine component. Effects of the invention

[0013] The machine component according to the solutions listed above, which has a core consisting of machine-grade steel with the component composition according to the solution listed above, and a surface layer enclosing a high-carbon layer made of machine-grade steel with a carbon concentration of 0.8 - 1.5%, exhibits excellent pitting resistance and toughness, making it suitable for the production of a machine component that is to be subjected to high surface pressure. Brief description of the drawings Fig. Figure 1 shows a cross-section through a machine component of an embodiment; Fig. Figure 2 shows an enlarged view of a cross-section through a section of the machine component of the embodiment; Fig. Figure 3 shows a microstructure of a layer with a high carbon content of the machine component of the embodiment; Fig. Figure 4 shows a form of a pitting corrosion roller test specimen; and Fig. Figure 5 represents a principle of a roller pitting test. Description of an embodiment

[0014] A gear serves as an example of a machine component. Fig. 1 and Fig.Figure 2 shows cross-sections through the gear. A machine component 1 according to one embodiment of the present invention includes a core 4 made of a steel for mechanical engineering, a medium-carbon layer 2 configured to cover the core, and a high-carbon layer 3 configured to cover the medium-carbon layer 2. A material having the shape of the machine component formed with the steel for mechanical engineering can be subjected to carburizing such that the medium-carbon layer 2 and the high-carbon layer 3 are produced in the surface layer of the material.Before describing the embodiment for implementing the invention, the reasons for limiting the component composition of the steel material forming the core 4 in the present invention, as well as the reasons for limiting the microstructure of the high carbon layer, will be explained. C: 0.13 - 0.30%

[0015] Carbon (C) is an element that influences the hardenability, forgeability, and machinability of the core of a steel component. If the C content is less than 0.13%, sufficient core hardness cannot be achieved, resulting in reduced strength. Therefore, C must be added in an amount of 0.13% or more, and an addition of 0.16% or more is desirable. Conversely, C, when present in large quantities, increases the hardness of the material and impairs machinability, such as cutting and forging. If the C content is too high, the core of the material becomes excessively hard, leading to reduced toughness. The C content must therefore be 0.30% or less, and is preferably 0.28% or less. Accordingly, the C content is specified as 0.13–0.30%, and preferably 0.16–0.28%. Si: 0.15 - 0.80%

[0016] Silicon (Si) is an element required for deoxidation. Si increases the resistance of steel components to temper softening and also improves pitting corrosion resistance. When Si is added in an amount of 0.15% or more, the depth of intergranular oxidation decreases, so to improve pitting corrosion resistance, the Si content must be 0.15% or more, and preferably 0.20% or more. On the other hand, Si, when present in large quantities, increases the hardness of the material and impairs machinability, such as cutting and forging, and inhibits carburization, thereby reducing pitting corrosion resistance. Therefore, the Si content must be specified at 0.80% or less, and preferably 0.70% or less. Accordingly, the Si content is specified at 0.15–0.80%, and preferably more than 0.30% but not more than 0.70%. Mn: 0.20 - 0.90%

[0017] Manganese (Mn) is an element required to ensure hardenability. Mn also causes intergranular oxidation or, during carburization, becomes concentrated in alloy oxides, forming a soft-quenched layer. To form a sufficiently soft-quenched layer, the Mn content must be at least 0.20% or more, and preferably 0.25% or more. On the other hand, Mn, when present in large quantities, increases the hardness of the material, impairs machinability (e.g., cutting and forging), and reduces toughness. Therefore, the Mn content must be 0.90% or less, and preferably 0.85% or less. Accordingly, the Mn content is specified as 0.20–0.90%, and preferably 0.25–0.85%. P: 0.030% or less

[0018] Phosphorus (P) is an impurity element that is unavoidably present in steel. P segregates at the grain boundary and causes reduced toughness. Therefore, the P content is limited to more than 0.000% and not more than 0.030%. S: 0.030% or less

[0019] S is an impurity element that is unavoidably present in steel. S binds to Mn to form MnS, which reduces toughness. Therefore, the S content is specified as being greater than 0.000% and not greater than 0.030%. The total amount of unavoidable impurities is preferably limited to less than 1.0%. Cr: 0.90 - 2.00%

[0020] Cr is an element that improves hardenability and also facilitates the incorporation of carbides by soft annealing. To achieve these effects, the Cr content must be 0.90% or more, and is preferably 1.00% or more. On the other hand, Cr is an element that, if added in excess, embrittles cementite and causes reduced toughness. Furthermore, Cr is an element that, when present in large quantities, inhibits carburization, leads to lower material hardness, and, if carburized, also forms coarse carbides, resulting in reduced pitting resistance. The Cr content must therefore be 2.00% or less, and is preferably 1.90% or less. Accordingly, the Cr content is specified as 0.90–2.00%, and preferably more than 1.50% but not more than 1.90%. Al: 0.020 - 0.050 %

[0021] Aluminum is an element that causes deoxidation in steelmaking and also suppresses grain coarsening because it binds to nitrogen to form Al₂O₃. To achieve this effect of suppressing grain coarsening, the Al content must be 0.020% or higher. However, if Al is added in large quantities, the formation of Al₂O₃ oxides in the steel increases and becomes a source of cracking, so the content is limited to 0.050% or less. Accordingly, the Al content is set at 0.020–0.050%. N: 0.002 - 0.025%

[0022] Nitrogen (N) is an element that is finely precipitated in steel as aluminum nitride, boron nitride, or other nitrides. It suppresses grain coarsening, which would otherwise reduce the strength, such as toughness, of the steel component. To achieve these effects, the N content must be 0.002% or higher. Conversely, if the N content exceeds 0.025%, large nitrides increase, reducing the steel's strength and workability. Therefore, the N content is typically set between 0.002% and 0.025%. Ni: 0.10 - 2.00%

[0023] Nickel (Ni) is an element that improves the hardenability and toughness of steel. However, nickel is an expensive element, so the cost increases when it is present in large quantities. Therefore, the nickel content is typically set at 0.10–2.00%. Mon: 0.05 - 0.50%

[0024] Mo is an element that improves the hardenability and toughness of steel. On the other hand, mo is an expensive element, so the cost increases when it is present in large quantities. Accordingly, the mo content is set at 0.05–0.50%. Note: 0.01 - 0.10%

[0025] Nitrogen (Nb) is an element that forms carbides or carbonitrides during carburization and promotes grain refinement. Furthermore, grains refined by Nb reduce the depth of intergranular oxidation, and even if cracking occurs due to intergranular oxidation, the crack length is shortened. However, if the Nb content is less than 0.01%, this crack length reduction effect cannot be achieved. Conversely, if the Nb content exceeds 0.10%, the grain refinement effect becomes saturated, and costs increase. Additionally, if the Nb content exceeds 0.10%, carbonitrides can form in large quantities, leading to poorer processing properties. Therefore, the Nb content is typically set between 0.01% and 0.10%. V: 0.01 - 0.20%

[0026] V is an element that forms carbides or carbonitrides during carburization and promotes grain refinement. Furthermore, the grains refined by V reduce the depth of intergranular oxidation, and even if cracking occurs due to intergranular oxidation, the crack length is shortened. However, if the V content is less than 0.01%, this crack length reduction effect cannot be achieved. If the V content exceeds 0.20%, the grain refinement effect becomes saturated, and costs increase. Additionally, if the V content exceeds 0.20%, carbonitrides can form in large quantities, leading to poorer processing properties. Therefore, the V content is typically set at 0.01–0.20%. Ti: 0.01 - 0.05%

[0027] Titanium (Ti) is an element that, when combined with nitrogen (B), enables B to exert an effect that improves hardenability. To improve hardenability, the combination of nitrogen and Ti to form titanium nitride is necessary. Therefore, Ti is added in an amount of 0.01% or more. It should be noted that the amount of Ti added is desirable to be 3.4 times or more than the amount of nitrogen added. On the other hand, if the added amount exceeds 0.05%, Ti forms fine carbides in large quantities, resulting in poorer processing properties. Accordingly, the Ti content is set at 0.01–0.05%. B: 0.0010 - 0.0050 %

[0028] Oxide (B) is an element that, when present in very small amounts, significantly improves the hardenability of steel. However, if the B content is less than 0.0010%, the effect is minimal. Conversely, B is an element that, when present in large amounts, reduces strength. Therefore, B is present in amounts of 0.0050% or less. Accordingly, the B content is specified as 0.0010–0.0050%.

[0029] A steel material used for a machine component 1 according to an embodiment of the present invention is, for example, the steel for mechanical engineering described below. The composition described below is the composition of a core 4 of the machine component 1. (a) A steel for mechanical engineering which contains, in mass percent, 0.13–0.30% C, 0.15–0.80% Si, 0.20–0.90% Mn, 0.030% or less P, 0.030% or less S, 0.90–2.00% Cr, 0.020–0.050% Al and 0.002–0.025% N, the remainder being Fe and unavoidable impurities; or (b) a steel for mechanical engineering which contains, in mass %, 0.13 - 0.30 % C, 0.15 - 0.80 % Si, 0.20 - 0.90 % Mn, 0.030 % or less P, 0.030 % or less S, 0.90 - 2.00 % Cr, 0.020 - 0.050 % Al and 0.002 - 0.025 % N, and furthermore contains one or more elements selected from 0.10 - 2.00% Ni, 0.05 - 0.50% Mo, 0.01 - 0.10% Nb and 0.01 - 0.20% V, the remainder consisting of Fe and unavoidable impurities; or (c) a steel for mechanical engineering which contains, in mass %, 0.13 - 0.30 % C, 0.15 - 0.80 % Si, 0.20 - 0.90 % Mn, 0.030 % or less P, 0.030 % or less S, 0.90 - 2.00 % Cr, 0.020 - 0.050 % Al and 0.002 - 0.025 % N, and furthermore contains 0.01 - 0.05% Ti and 0.0010 - 0.0050% B, the remainder consisting of Fe and unavoidable impurities; or (d) a steel for mechanical engineering which contains, in mass %, 0.13 - 0.30 % C, 0.15 - 0.80 % Si, 0.20 - 0.90 % Mn, 0.030 % or less P, 0.030 % or less S, 0.90 - 2.00 % Cr, 0.020 - 0.050 % Al and 0.002 - 0.025 % N, furthermore contains one or more elements selected from 0.10 - 2.00% Ni, 0.05 - 0.50% Mo, 0.01 - 0.10% Nb and 0.01 - 0.20% V, and furthermore contains 0.01 - 0.05% Ti and 0.0010 - 0.0050% B, the remainder consisting of Fe and unavoidable impurities.

[0030] With regard to the machine component of the present invention, in which the steel material with the component composition described above is used, the reasons for the determination of its properties are described in detail below. These properties are mainly attributable to the microstructure of a high-carbon layer 3 on the outermost surface of the machine component 1. A description of the requirements for the microstructure of the high-carbon layer 3 follows. The carbides in the high-carbon layer are mainly cementite (Fe3C), so in the following description, the carbides are considered to be cementite. In addition to cementite, the carbides can also be of type M. 23 Include C6, (FeCr)3C and the like. Fig. Figure 3 shows a structure of layer 3 with a high carbon content. (A) The high carbon layer consists of a martensite structure 7 containing dispersed embedded cementite 5 and a retained austenite structure 7, wherein the embedded cementite particles 5, having an aspect ratio of 1.5 or less, constitute 90% or more of the total cementite.

[0031] The aspect ratio, which defines the ratio of the major axis to the minor axis of the embedded cementite 5, is an index of spheroidization. A cementite particle with a large aspect ratio, for example, one with a plate-like or nearly columnar shape, becomes a source of stress concentration upon deformation due to its shape and, moreover, becomes an origin of cracking, thus reducing toughness. Therefore, from the perspective of improved toughness, the cementite particle should ideally approach a spherical shape. If the aspect ratio is 1.5 or less, the potential damage from becoming an origin of cracking can be reduced. Therefore, a larger proportion of embedded cementite particles with an aspect ratio of 1.5 or less is preferable.

[0032] This is the reason why the incorporated cementite particles with an aspect ratio of 1.5 or less constitute 90% or more, and desirablely 95-100%, of the total number of cementite particles. (B) As regards the cementite particles at the former austenite grain boundaries 6, the proportion of the number of cementite particles incorporated 5 at the former austenite grain boundaries 6 to the total number of cementite particles is 40% or less.

[0033] The microstructure of layer 3, with its high carbon content, falls within the range of hypereutectoid steel in terms of carbon concentration. In hypereutectoid steel, the type of brittle fracture that degrades impact strength is primarily grain boundary fracture along the former austenite grain boundaries 6. This is caused by cementite at the former austenite grain boundaries 6 or, in particular, by network-like carbides along the grain boundaries. Cementite particles precipitated at and located at the grain boundaries are more likely to become a source of cracking and are more detrimental compared to cementite particles within the grains. Accordingly, such cementite particles are preferably not present at the grain boundaries.Therefore, the proportion of the number of incorporated cementite particles 5 at the former austenite grain boundaries to the total cementite is set to 40% or less, preferably to 20% or less, and furthermore preferably to 5% or less down to 0%. (C) 90% or more of the incorporated cementite particles 5 at the former austenite grain boundaries 6 have a particle size of 1 µm or less.

[0034] Preferably, no cementite is present at the former austenite grain boundaries 6. In particular, network-like cementite particles or similarly coarse cementite particles along the grain boundaries pose an increased risk of becoming a source of grain boundary fracture. Therefore, adjustments are made such that 90% or more, and preferably 95–100%, of the incorporated cementite particles 5 have a particle size of 1 µm or less, which is considered harmful.

[0035] It should be noted that the percentage used here refers to the proportion in the case where the total number of carbides that can be viewed with a scanning electron microscope at a magnification of approximately 5000x is set at 100%. Very fine carbides that cannot be viewed at this magnification are not taken into account, as they have little effect on toughness. (D) At the former austenite grain boundaries 6, a grain size of 15 µm or less results.

[0036] By reducing the grain size A according to the length across the former austenite grain boundary 6, the size of the fracture surface of grain boundary fractures or cleavage fractures can be reduced, and the energy required for fracture can be increased, leading to improved toughness. Reducing the grain size is therefore a very effective means of improving toughness without reducing hardness.

[0037] In the manufacturing process according to the present invention, final quenching is carried out in the state in which the fine cementite particles have precipitated, and the quenching is carried out at a relatively low temperature. This is advantageous insofar as the original austenite grain size can be kept fine.

[0038] However, if the grain size defined by the former austenite grain boundaries 6 exceeds 15 µm, the toughness improvement is reduced. In particular, if carburizing is carried out at a heating temperature of 1050 °C or above, the former austenite grain size will remain large even after final quenching. Therefore, it is recommended that the grain size at the former austenite grain boundaries 6 be 15 µm or less.

[0039] The microstructure shown above contains precipitated fine carbides, which are generally difficult to produce using conventional carburizing processes. While patent document 1 describes a steel material with a carbon content of 0.55–1.10% in which carbides have precipitated, the precipitation of fine carbides in a low-carbon steel, such as a steel with a carbon content of 0.13–0.30% as in the example above, was previously inconceivable.

[0040] Layer 2, with medium carbon content, is located between core 4 and the high-carbon layer 3. The carbon content of layer 2 is at a moderate level, higher than that of core 4 and lower than that of high-carbon layer 3. The microstructure of layer 2 is essentially martensitic. Layer 2 contains fine, low-density carbides that have precipitated in its region near the high-carbon layer 3.

[0041] The embodiment for implementing the invention is described below with reference to examples. It should be noted that the percentage used for the component composition is given in mass percent.

[0042] Steels with the component compositions listed in Table 1 were produced in a 100 kg vacuum melting furnace, the remainder consisting of iron and unavoidable impurities. The resulting steels were drawn at 1250 °C to obtain bars with a diameter of 32 mm, which were then normalized for one hour at 925 °C.

[0043] Of the test specimens shown in Table 1, specimens 1 to 10 have component compositions that fall within the scope of protection of the present invention. Specimens 11 to 18 have component compositions that do not fall within the scope of protection of the present invention. The underlined values ​​are not within the scope of protection of the present invention. Ni in an amount of 0.09% or less and Mo in an amount of 0.04% or less are impurities.

[0044] Each test specimen was roughly divided into Fig.The four pitting roller test specimens (small roller) shown were formed (roughly machined) 8. During this rough machining, finishing work was carried out on the part 9 to be tested. An additional layer with a thickness of 0.2 mm was applied exclusively to a holding section 10 in preparation for final grinding after the subsequent heat treatment. Each test specimen was also roughly formed into a 10R Charpy circular impact test specimen. During this rough machining, an additional layer with a thickness of 2 mm was applied to sections other than the notch surface in preparation for machining to remove the carburizing layer after the subsequent heat treatment. Table 1 (Unit: mass %) Nr. C Si Mn P S Cr Ni Mon v Note Al N Ti B test specimens 1 0,13 0,80 0,35 0,015 0,016 1,61 0,06 0,03 - - 0,024 0,016 - - 2 0,18 0,24 0,85 0,011 0,004 0,90 - 0,02 0,01 - 0,030 0,018 - - 3 0,23 0,54 0,26 0,014 0,009 1,82 0,06 0,02 - 0,04 0,028 0,018 - - 4 0,25 0,31 0,80 0,014 0,013 1,19 0,10 0,15 - - 0,030 0,015 - - 5 0,16 0,70 0,20 0,009 0,004 1,50 0,03 0,03 - 0,07 0,050 0,025 - - 6 0,20 0,15 0,40 0,007 0,013 1,00 0,02 - - - 0,028 0,015 0,05 0,005 7 0,28 0,26 0,25 0,022 0,019 2,00 0,08 0,02 - 0,10 0,035 0,014 - - 8 0,30 0,20 0,43 0,016 0,012 1,25 0,07 0,50 0,20 0,01 0,020 0,017 - - 9 0,22 0,44 0,90 0,015 0,021 1,90 2,00 0,05 - - 0,028 0,002 - - 10 0,18 0,25 0,80 0,015 0,012 1,02 0,07 0,15 - - 0,032 0,016 - - 11 0,12 0,55 0,55 0,015 0,011 2,10 0,16 0,07 - - 0,027 0,017 - - 12 0,16 0,13 0,29 0,010 0,016 1,05 0,06 0,03 - - 0,024 0,014 - - 13 0,20 0,33 0,85 0,018 0,008 0,85 0,07 0,02 - - 0,033 0,018 - - 14 0,32 0,41 0,33 0,007 0,012 1,88 0,08 0,04 - - 0,028 0,016 - - 15 0,27 0,48 0,91 0,024 0,033 1,85 0,11 0,09 - - 0,027 0,015 - - 16 0,22 0,25 0,77 0,033 0,011 1,03 0,09 0,03 - - 0,030 0,019 - - 17 0,20 0,85 0,81 0,011 0,009 1,09 0,13 0,15 - - 0,025 0,022 - - 18 0,19 0,30 0,19 0,017 0,002 1,16 1,60 0,20 - - 0,032 0,020 - -

[0045] Table 2 lists the conditions for heat treatment, etc., of components using the test specimens Nos. 1 to 18 listed in Table 1. The component compositions of the steel components Nos. 1 to 10 according to the invention and the comparison steel components Nos. 11 to 18 in Table 2 correspond to those of the test specimens Nos. 1 to 18 shown in Table 1.

[0046] First, these components were subjected to gas carburizing under the heating conditions shown in Table 2 to achieve the carbon concentration on the surface of the test specimen shown in Table 2. The components were then cooled to 200 °C or below at the cooling rate shown in Table 2. During gas carburizing, a carburized layer is formed on the component surface. Starting from this carburized layer, subsequent machining processes create a high-carbon layer and a medium-carbon layer.

[0047] The components were each subjected to soft annealing, during which they were held at the reheating temperature shown in Table 2. In the present invention, the carbides must be grown to a suitable size and distributed in a suitable area ratio. For this purpose, the soft annealing must be carried out at a heating temperature that does not exceed the A cm -point (°C). The annealing temperatures in the present examples are all below the A cm -point (°C). The components were each held at the reheating temperature shown in Table 2 and then quenched. Afterwards, they were tempered by holding them at 180 °C for 1.5 hours and then air-cooled. The resulting components were then machined to produce the small-roll pitting roller test specimen 8 and the Charpy circular impact test specimen.

[0048] In the present embodiment, the components were cooled to room temperature once during each step of the process, from gas carburizing to soft annealing and quenching. Alternatively, the process can proceed to the next step when the temperature has fallen below the A1 point.

[0049] Reheating temperature refers to the temperature during soft annealing and the temperature during final quenching.

[0050] Subsequently, the in Fig. 4 shown pitting corrosion roller test specimens (small roller) 8, which was produced as explained above, and one in Fig. The test specimen 11 shown, in the form of a large roller, which is brought into contact with the small roller via an oil film in such a way that lubrication is ensured, is used to test the in Fig.The roller pitting test shown in Figure 5 is to be performed under the conditions listed in Table 3. Under the listed conditions, a slip ratio of -40% means that the circumferential speed of the large roller 11 is 40% lower than the circumferential speed of the small roller 8. The lubricant ATF (automatic transmission fluid) is a lubricating oil used for automatic transmissions in vehicles. A crown value of 150R means that the outer circumference of the roller has an arc shape with a radius of 150 mm in the direction of rotation. Table 3 slip ratio -40 % Surface print 3.3 GPa rotation speed of small roller 2000 rpm Test specimen in the form of a large roller (counterpart) SCM420 carburized and polished element Balliness value of large roller 150R lubricant ATF Oil temperature 80°C

[0051] The roller pitting test was performed using a vibrometer to detect excessive vibrations due to peeling or excessive deformation and to terminate the test upon detection of such vibrations. The number of cycles until the test was terminated was considered the specimen lifetime. Additionally, the Charpy test was performed at room temperature to assess toughness.

[0052] To investigate the grain size, the pitting corrosion roller test specimen (small roller) 8, which had undergone the tempering process described above, was cut into a sample piece and embedded in resin to allow examination of the cross-section from the surface layer to the interior. The area to be tested was then subjected to high-gloss polishing and grain boundary corrosion. An image of an average field of view was then generated using an optical microscope in the area from the outermost surface to 0.3 mm below the surface to determine an average grain size (diameter).

[0053] To examine carbides, the sample was embedded in resin as described above. The area to be tested was highly polished and then corroded with Nital. Using a scanning electron microscope, an image of an average field of view was generated in the area from the outermost surface to 0.3 mm below the surface to obtain a Fig. The aim was to obtain the microstructure shown in Figure 3, in which the identified carbides are depicted. Image analysis was performed on the identified carbides to examine the percentage of cementite particles with an aspect ratio of 1.5 or less, the percentage of cementite particles at the former austenite grain boundaries, the percentage of cementite particles with a particle size greater than 1 µm at the former austenite grain boundaries, and the former austenite grain size (µm).

[0054] It should be noted that for the test specimens which, after tempering, were subjected to surface treatment in the form of cutting, grinding, polishing, sandblasting, shot peening, hard shot peening and fine grain shot peening, similar considerations were carried out as those described above, with the treated surface being regarded as the surface layer.

[0055] The test results are presented in Table 4. The Charpy impact value and the pitting resistance are shown relative to those of the reference steel component 13, which was produced using test specimen no. 13 in Table 1, a steel according to JIS SCr420. The Charpy impact value of the steel of each of the numbered steel components according to the invention and the numbered reference steel components in Table 4 is given in Table 4 relative to the Charpy impact value of reference steel component no. 13. It was found that the toughness was good when the ratio of the Charpy impact value was 1.5 or greater. The pitting resistance of each of the numbered steel components according to the invention and the numbered reference steel components in Table 4 is given in Table 4 as a ratio for the case where the number of cycles until the occurrence of pitting in reference steel component no. 13 was set to 1.It was found that pitting corrosion resistance was good when the ratio of the number of cycles until pitting corrosion occurred was 2.0 or higher.

[0056] In the steel components Nos. 1 to 10 according to the invention, as shown in Tables 1 and 2, which were produced using test specimens Nos. 1 to 10 with the component compositions in Table 1 under the conditions listed in Table 2, cementite particles with an aspect ratio of 1.5 or less initially constituted 90–98% or 90% or more in the steel components Nos. 1 to 10 according to the invention, as shown in Table 4. That is to say, even if a cementite particle with a large aspect ratio were to become a source of stress concentration upon deformation due to its shape, and thus an origin of cracking and a reduction in toughness, the proportion of such cementite particles is small, so that the toughness is improved and not reduced.

[0057] Furthermore, in the steel components Nos. 1 to 10 according to the invention, the proportion of the number of embedded cementite particles at the former austenite grain boundaries to the total number of cementite particles was 11–40% or 40% or less. Furthermore, in the steel components Nos. 1 to 10 according to the invention, the embedded cementite particles at the former austenite grain boundaries with a particle size greater than 1 µm constituted 3–7%. That is, 90% or more of the embedded cementite particles 5 at the former austenite grain boundaries 6 had a particle size of 1 µm or less.Although cementite particles precipitated at and located at the former austenite grain boundaries (especially network-like carbides along the grain boundaries) are more likely to become a source of fracture and are more damaging compared to cementite particles within the grains, in the present invention the cementite particles at the grain boundaries have been reduced to 40% or less, and 90% or more of them have a size of 1 µm or less, which is less damaging. Furthermore, in the steel components Nos. 1 to 10 according to the invention, the former austenite grain size was 4–8 µm, or 8 µm or less overall. By reducing the former austenite grain size, the size of the fracture surface of grain boundary or cleavage fracture can be reduced, and the energy required for fracture can be increased, thereby improving toughness.Therefore, the machine component according to the present invention exhibits improved toughness.

[0058] For the steel components Nos. 1 to 10 according to the invention, the ratio of the Charpy impact value relative to 1.0 of the comparison steel component No. 13 was 1.6 to 2.9 or 1.5 or more, which indicates high toughness.

[0059] Likewise, for the steel components Nos. 1 to 10 according to the invention, the ratio of the number of cycles until the occurrence of pitting corrosion, relative to 1.0 of the comparison steel component No. 13, was 2.2 to 2.9, which indicates good pitting corrosion resistance.

[0060] It is evident from the above explanations that the machine components of the present invention all exhibit excellent resistance to pitting corrosion and excellent toughness.

[0061] It should be clear that the embodiment and examples disclosed herein are in every respect illustrative and not limiting. The scope of protection of the present invention is defined by the specifications of the claims and not by the description above, and is intended to include any modifications within the scope of protection and meaning equivalent to the specifications of the claims. Description of the reference symbols

[0062] 1: Gear (machine component); 2: Medium carbon layer; 3: High carbon layer; 4: Core; 5: Embedded cementite (embedded carbide); 6: Former austenite boundary; 7: Martensite or retained austenite structure; 8: Pitting roller specimen (small roller); 9: Part to be tested; 10: Holding section; 11: Large roller specimen; and A: Grain size.

Claims

[1] Machine component comprising: a core made of a steel used in mechanical engineering; as well as a medium carbon layer and a high carbon layer formed from steel for mechanical engineering, wherein the medium carbon layer covers the core, the high carbon layer covers the medium carbon layer and has a carbon concentration of 0.8 - 1.5%; wherein the steel for mechanical engineering contains in mass percent 0.13 - 0.30% C, 0.15 - 0.80% Si, 0.20 - 0.90% Mn, 0.90 - 2.00% Cr, 0.020 - 0.050% Al and 0.002 - 0.025% N, and also contains as impurities 0.030% or less P and 0.030% or less S, furthermore optionally selectively as a first group of optional components one or more elements selected from 0.10 - 2.00% Ni, 0.05 - 0.50% Mo, 0.01 - 0.10% Nb and 0.01 - 0.20% V, and optionally selectively as a second group of optional components in addition to or instead of the first group of optional components Contains 0.01 - 0.05% Ti and 0.0010 - 0.0050% B, the remainder consisting of Fe and unavoidable impurities, and the high-carbon layer consists of a martensite structure with dispersed carbides and a retained austenite structure, incorporated carbides with an aspect ratio of 1.5 or less constitute 90% or more of the total number of carbides, and the number of incorporated carbides at former austenite grain boundaries is 40% or less of the total number of carbides. [2] Machine component according to claim 1, wherein 90% or more of the molded carbides at the former austenite grain boundaries have a particle size of 1 µm or less. [3] Machine component according to claim 1 or 2, wherein the former austenite grain boundaries result in a grain size of 15 µm or less. [4] Machine component according to claim 1, wherein the layer with high carbon content is formed at least from a surface to a depth of 0.3 mm of the machine component.

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