Shaft bearing steel

Abstract

To provide a bearing steel capable of suppressing lowering of hardness after quenching and tempering, while securing workability necessary for producing a component such as a rolling bearing and so on.SOLUTION: A bearing steel has a component composition made of in mass %, C: 0.85% or more and 1.20% or less, Si: 0.10% or more and 1.10% or less, Mn: 0.20% or more and 1.30% or less, Cr: 0.80% or more and 1.80% or less, Al: 0.050% or less, O: 0.0030% or less and Sb: 0.0005% or more and 0.0050% or less, and the remainder having a component composition composed of Fe and unavoidable impurities, wherein an area rate of the pro-eutectoid cementite is less than 1.3% and the remaining part has a microstructure that is a pearlite structure.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to bearing steel. [Background technology]

[0002] High-carbon chromium bearing steel (JIS standard: SUJ2) is the most commonly used steel for rolling bearings in automobiles, industrial machinery, etc. SUJ2 contains 0.95 to 1.10 mass% C (hereafter simply referred to as %), making it extremely hard. Therefore, the steel is first spheroidized and annealed to improve workability, then formed and processed, and then quenched and tempered to ensure the hardness required for rolling bearings. If the hardness decreases after quenching and tempering, this leads to a decrease in the rolling fatigue life, which is important for rolling bearings.

[0003] To address this problem, Patent Document 1 proposes a bearing steel to which an appropriate amount of Sb is added in order to prevent a decrease in hardness after quenching and tempering due to the generation of a decarburized layer during heat treatment. Patent Document 2 proposes a method for manufacturing a rolling bearing in which rolled coil material is subjected to warm precision rolling to ensure hardness after quenching and tempering. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 5-271866 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-191796 Summary of the Invention [Problem to be solved by the invention]

[0005] In rolling bearings, the decarburized layer is usually removed by cutting or grinding to improve vibration and acoustic characteristics. The bearing steel described in Patent Document 1 is effective in preventing a decrease in surface hardness after quenching and tempering when the decarburized layer remains, but there is a problem in that it is not possible to prevent a decrease in surface hardness after the decarburized layer has been removed by cutting or grinding. Furthermore, the method for manufacturing a rolling bearing described in Patent Document 2 requires warm precision rolling equipment, and therefore has the problem that it cannot be implemented using general rolling bearing manufacturing equipment.

[0006] The present invention advantageously solves the above problems, and aims to provide a bearing steel that can suppress a decrease in hardness after quenching and tempering while ensuring the workability required for manufacturing parts such as rolling bearings. [Means for solving the problem]

[0007] In order to solve the above problems, the present inventors conducted extensive research into the effects of the chemical composition, structure, and hot rolling conditions of steel on hardness. As a result, the present inventors discovered that the decrease in hardness after quenching and tempering is mainly caused by variations in the heating temperature, heating time, and cooling rate during quenching and tempering, but that hardness can decrease even when these variations are sufficiently suppressed and the conditions are essentially the same. The present inventors then conducted an investigation into the cause of hardness decrease, focusing on the microstructure of the material before quenching and tempering, and discovered the following: 1) When pro-eutectoid cementite increases in the microstructure of rolled steel, the hardness after quenching and tempering decreases. 2) When pro-eutectoid cementite increases in the microstructure of the steel after rolling, coarse spherical carbides increase in the microstructure after subsequent spheroidizing annealing. 3) If the number of coarse spheroidal carbides increases in the microstructure of steel after spheroidizing annealing, the dissolution of C from the spheroidal carbides into the austenite matrix during heating in the subsequent quenching and tempering treatment will be insufficient, resulting in a decrease in hardness.

[0008] As a result of further investigations, the present inventors have found that by controlling the chemical composition of the steel material and the hot rolling conditions, a bearing steel in which the area ratio of pro-eutectoid cementite is less than 1.3% in the microstructure after hot rolling, and that this steel is annealed to spheroidize so that the average grain size of spherical carbides is less than 0.50 μm and the maximum grain size of spherical carbides is 2.50 μm or less, can be prevented from losing hardness after quenching and tempering, leading to the completion of the present invention.

[0009] That is, the gist and configuration of the present invention are as follows. [1] In mass%, C: 0.85% or more and 1.20% or less, Si: 0.10% or more and 1.10% or less, Mn: 0.20% or more and 1.30% or less, Cr: 0.80% or more and 1.80% or less, Al: 0.050% or less, O: 0.0030% or less and Sb: 0.0005% or more and 0.0050% or less; The balance has a composition consisting of Fe and unavoidable impurities, A bearing steel having a microstructure in which the area ratio of pro-eutectoid cementite is less than 1.3%, with the remainder being pearlite. [2]Furthermore, in mass %, Cu: 0.05% or more and 0.20% or less, Ni: 0.05% or more and 0.20% or less [1] A bearing steel containing at least one selected from the group consisting of Mo: 0.03% or more and 0.10% or less. [3] In mass%, C: 0.85% or more and 1.20% or less, Si: 0.10% or more and 1.10% or less, Mn: 0.20% or more and 1.30% or less, Cr: 0.80% or more and 1.80% or less, Al: 0.050% or less, O: 0.0030% or less and Sb: 0.0005% or more and 0.0050% or less; The balance has a composition consisting of Fe and unavoidable impurities, The area ratio of the spheroidized particles is 12% or more and 20% or less, and the remainder is ferrite, The bearing steel has a microstructure in which the average grain size of the spherical carbides is less than 0.50 μm and the maximum grain size is 2.50 μm or less. [4]Furthermore, in mass %, Cu: 0.05% or more and 0.20% or less, Ni: 0.05% or more and 0.20% or less [3] Bearing steel containing at least one selected from the group consisting of Mo: 0.03% or more and 0.10% or less. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a bearing steel that has excellent hardness after quenching and tempering, and is extremely useful for application to rolling bearings. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a chart showing a typical manufacturing process for manufacturing a product such as a bearing part. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention will be specifically described below.

[0013] <Component composition> First, the chemical composition of the bearing steel of the present invention will be described. Note that "%" representing the chemical composition below means "mass %" unless otherwise specified.

[0014] C: 0.85% or more and 1.20% or less C is an essential element for ensuring the strength required for bearing steel, and should be contained in an amount of 0.85% or more. However, if it exceeds 1.20%, the amount of pro-eutectoid cementite increases after hot rolling, reducing the hardness after quenching and tempering. Therefore, the C content should be 1.20% or less. The C content is preferably 0.95% or more and 1.10% or less.

[0015] Si: 0.10% or more and 1.10% or less Silicon is an element effective in ensuring hardness after deoxidation and quenching / tempering, and should be contained at 0.10% or more. However, if it exceeds 1.10%, solid solution hardening of the ferrite phase leads to an increase in pro-eutectoid cementite and also deteriorates forgeability and machinability, so the Si content is set to 1.10% or less. The Si content is preferably 0.15% or more and 0.80% or less.

[0016] Mn: 0.20% or more and 1.30% or less Mn is an element necessary for deoxidation and is also effective in ensuring hardness after quenching and tempering, so its content should be 0.20% or more. However, if it exceeds 1.30%, it will increase pro-eutectoid cementite and deteriorate forgeability and machinability, so the Mn content should be 1.30% or less. The Mn content is preferably 0.30% or more and 1.20% or less.

[0017] Cr: 0.80% or more and 1.80% or less Cr is effective in ensuring hardness after quenching and tempering, significantly promotes carbide formation, and refines pearlite lamellar spacing and pearlite nodules. Ultimately, it is an element necessary for improving forgeability by spheroidizing carbides, and should be contained in an amount of 0.80% or more. However, a content exceeding 1.80% does not increase the effect and instead increases pro-eutectoid cementite, adversely affecting mechanical properties such as forgeability and fatigue strength. Therefore, the Cr content is set to 1.80% or less. The Cr content is preferably 0.90% or more and 1.60% or less.

[0018] Al: 0.050% or less Since Al is a deoxidizing material and its effect saturates even if it is contained in a large amount, leading to an increase in costs, the Al content is set to 0.050% or less, preferably 0.040% or less. The lower limit of the Al content is not particularly limited, but can be set to 0.005% or more from the viewpoint of deoxidation.

[0019] O: 0.0030% or less O combines with Al to form hard oxide-based nonmetallic inclusions, which reduce forgeability and rolling fatigue life, so it is desirable to keep the O content low. Therefore, the O content is set to 0.0030% or less, and preferably 0.0015% or less. There is no particular lower limit for the O content, and it may be 0%, but reducing it too much leads to increased costs, so it can be set to, for example, 0.0004% or more.

[0020] Sb: 0.0005% or more and 0.0050% or less Sb has the effect of destabilizing pro-eutectoid cementite and reducing its production, making it an important element in the present invention. Furthermore, Sb segregates at grain boundaries and inhibits oxygen penetration from the grain boundaries, thereby inhibiting decarburization during heating of steel, and is useful for inhibiting a decrease in hardness of the surface layer due to decarburization. From these points of view, the Sb content is set to 0.0005% or more. However, even if a large amount is added, the effect saturates and costs increase, so the Sb content is set to 0.0050% or less. The Sb content is preferably 0.0003% or more and 0.0035% or less.

[0021] In addition to the above elements, one or more optional components selected from the group consisting of Cu: 0.05% or more and 0.20% or less, Ni: 0.05% or more and 0.20% or less, and Mo: 0.03% or more and 0.10% or less may be contained.

[0022] Cu: 0.05% or more and 0.20% or less, Ni: 0.05% or more and 0.20% or less, Mo: 0.03% or more and 0.10% or less Cu, Ni, and Mo are all elements effective in improving hardenability and improving the rolling fatigue life of steel, so when these elements are added, the Cu content can be 0.05% or more, the Ni content can be 0.05% or more, and the Mo content can be 0.03% or more. However, if Cu or Mo is added in excess, pro-eutectoid cementite increases and forgeability decreases, and if Ni is added in excess, a large amount of retained austenite is formed, reducing the hardness of the steel and the rolling fatigue life. Therefore, when these elements are added, the Cu content can be 0.20% or less, the Ni content can be 0.20% or less, and the Mo content can be 0.10% or less.

[0023] The balance of the composition is Fe and unavoidable impurities.

[0024] <Microstructure> Next, the microstructure of the bearing steel of the present invention will be described.

[0025] The bearing steel of the present invention includes hot-rolled materials obtained by hot-rolling steel materials and spheroidized annealed materials obtained by subjecting hot-rolled materials to spheroidizing annealing.

[0026] Microstructure of hot-rolled material The microstructure after hot rolling has a pro-eutectoid cementite area ratio of less than 1.3%, with the remainder being a pearlite structure. If the pro-eutectoid cementite increases, the hardness after quenching and tempering decreases, leading to a decrease in the rolling fatigue life of rolling bearings. For this reason, the pro-eutectoid cementite area ratio in the microstructure of the hot-rolled material is set to less than 1.3%. The pro-eutectoid cementite area ratio is preferably 1.2% or less. There is no particular lower limit for the pro-eutectoid cementite, and it may be 0%.

[0027] Microstructure of spheroidized annealed material The spherical carbides in the microstructure after spheroidizing annealing of the hot-rolled material have an average grain size of less than 0.50 μm and a maximum grain size of 2.50 μm or less. If the number of coarse spherical carbides increases in the microstructure after spheroidizing annealing, the dissolution of C from the spherical carbides into the austenite matrix during heating in quenching and tempering treatments will be insufficient, resulting in a decrease in hardness. To prevent a decrease in hardness, the spherical carbides should have an average grain size of less than 0.50 μm and a maximum grain size of 2.50 μm or less. The average grain size of the spherical carbides is preferably 0.45 μm or less, and the maximum grain size is preferably 2.10 μm or less. The average grain size of the spherical carbides can be 0.05 μm or more.

[0028] The microstructure of the spheroidized annealed material can have an area ratio of 12% to 20% of spherical carbides. The area ratio of the spherical carbides is preferably 13% to 18% in order to ensure hardness after quenching and tempering. The remainder of the microcomposition is ferrite.

[0029] The area ratio of pro-eutectoid cementite is determined as follows. A test piece is taken from the hot-rolled material, and a vertical cross section (L cross section) parallel to the rolling direction is polished and etched with nital. Using a scanning electron microscope (SEM), the cross-sectional structure is observed in 20 fields of view (each field is 92 μm × 121 μm) at a magnification of 10,000 times. The area fraction of pro-eutectoid cementite observed in each field of view is determined by image analysis, and the arithmetic average of the 20 fields of view is taken as the area fraction of pro-eutectoid cementite.

[0030] The average particle size of the spherical carbides is determined as follows. A test piece is taken from the spheroidized annealed material, and a vertical cross section (L cross section) parallel to the rolling direction is polished and etched with picral. Using a scanning electron microscope (SEM), the cross-sectional structure is observed in 20 fields (each field is 92 μm × 121 μm) at a magnification of 10,000 times. The number average of the circle-equivalent diameters of the spheroidal carbides observed in each field is determined by image analysis, and the arithmetic average of the 20 fields is taken as the average particle size of the spheroidal carbides. The spheroidal carbides observed in each field must have an equivalent circle diameter (particle size) of 0.05 μm or more.

[0031] The maximum particle size of the spherical carbides is determined as follows. A test piece is taken from the spheroidized annealed material, and a vertical cross section (L cross section) parallel to the rolling direction is polished and etched with picral. Using a scanning electron microscope (SEM), the cross-sectional structure is observed in 20 fields (each field is 92 μm × 121 μm) at a magnification of 10,000 times. The maximum value of the circle-equivalent diameter of the spheroidal carbides observed in each field is determined by image analysis, and the maximum value of the 20 fields is taken as the maximum particle size of the spheroidal carbides. The spheroidal carbides observed in each field must have an equivalent circle diameter of 0.05 μm or more.

[0032] The area ratio is calculated as follows. A test piece is taken from the spheroidized annealed material, and a vertical cross section (L cross section) parallel to the rolling direction is polished and etched with picral. Using a scanning electron microscope (SEM), the cross-sectional structure is observed in 20 fields of view (92 μm × 121 μm per field of view) at a magnification of 10,000 times. The total area of ​​the spheroidal carbides observed in each field of view is determined by image analysis, and this is divided by the field area to determine the area ratio. The arithmetic average of the 20 fields of view is the area ratio of spheroidal carbides. The spheroidal carbides observed in each field of view must have an equivalent circle diameter (grain size) of 0.05 μm or more.

[0033] <Method of manufacturing bearing steel> Next, a method for producing the bearing steel of the present invention will be described. Figure 1 shows a typical manufacturing process for producing products such as bearing parts. Here, S1 is the wire rod manufacturing process, S2 is the transport process, and S3 is the product (bearing part) finishing process. In the wire rod manufacturing process (S1), steel ingots are hot-rolled to form wire rod, which is then softened by spheroidizing annealing before being inspected for quality and shipped. After the transport process (S2), in the product (bearing part) finishing process (S3), the wire rod is cut to the specified dimensions, cold forged, and if necessary processed by cutting, grinding, polishing, etc. to form the desired shape (for example, steel balls or rollers), and then quenched and tempered to form the product.

[0034] In one embodiment, the bearing steel of the present invention is a hot-rolled material, which is a wire rod obtained by hot-rolling a steel ingot having a predetermined chemical composition.

[0035] The chemical composition of the steel ingot is the same as the chemical composition of the bearing steel of the present invention, In mass%, C: 0.85% or more and 1.20% or less, Si: 0.10% or more and 1.10% or less, Mn: 0.20% or more and 1.30% or less, Cr: 0.80% or more and 1.80% or less, Al: 0.050% or less, O: 0.0030% or less and Sb: 0.0005% or more and 0.0050% or less; The balance has a composition consisting of Fe and unavoidable impurities.

[0036] The composition of the steel ingot is further expressed in mass percent as follows: Cu: 0.05% or more and 0.20% or less, Ni: 0.05% or more and 0.20% or less Mo: At least one selected from the group consisting of 0.03% to 0.10% may be contained.

[0037] In the method for producing bearing steel of the present invention, by satisfying predetermined conditions in the hot rolling process, it is possible to control the microstructure in the hot rolled material and the average grain size and maximum grain size of the spherical carbides after the hot rolled material is subjected to spheroidizing annealing within predetermined ranges.

[0038] Hot rolling process In the hot rolling process, a steel ingot with a predetermined chemical composition is prepared, and the steel ingot is heated to 950°C to 1250°C. This ingot is then hot rolled to a finishing rolling temperature of 800°C or higher, and cooled at a cooling rate of more than 0.4°C / s in the temperature range of 800 to 550°C. By satisfying these conditions, a hot-rolled material can be obtained that has a pro-eutectoid cementite area fraction of less than 1.3% and a microstructure with the remainder being pearlite, and this material can be used as bearing steel.

[0039] Rolling heating temperature: 950℃ to 1250℃ To prevent the precipitation of coarse cementite in the hot-rolled material, the steel ingot is heated to a temperature between 950°C and 1250°C and rolled to dissolve the carbides remaining from the melting and casting process.

[0040] If the rolling heating temperature is less than 950°C, the carbides remaining from the melting and casting process will not be sufficiently dissolved. On the other hand, if the temperature exceeds 1250°C, excessive surface scale will be generated, making surface defects more likely to occur, and the crystal grains will become coarse, resulting in poor forgeability. For this reason, the rolling heating temperature is set to 950°C or higher and 1250°C or lower. The rolling heating temperature is preferably 980°C or higher and 1200°C or lower.

[0041] Rolling finish temperature: 800°C or higher If the rolling finish temperature is less than 800°C, the hot deformation resistance increases and the rolling load becomes high. Therefore, the rolling finish temperature is set to 800°C or higher. The upper limit of the rolling finish temperature is preferably set to about 1100°C.

[0042] Cooling rate after rolling: over 0.4℃ / s In the cooling after rolling, the cooling rate in the temperature range of 800 to 550°C is set to more than 0.4°C / s. By setting the cooling rate in the pearlite transformation temperature range of 800 to 550°C to more than 0.4°C / s, the pearlite lamellar spacing after rolling can be refined and the spherical carbides after spheroidizing annealing can be finely controlled. The upper limit of the cooling rate is preferably about 30°C / s.

[0043] The hot-rolled material obtained in the hot rolling process can be subjected to spheroidizing annealing to obtain a spheroidized annealed material having a microstructure in which the average grain size of spherical carbides is less than 0.50 μm and the maximum grain size is 2.50 μm or less, and this material can be used as bearing steel.

[0044] Bearing steel, which is a spheroidized annealed material, can be cut to the specified dimensions, cold forged, and then processed as needed by cutting, grinding, polishing, etc. to form the desired shape (for example, steel balls or rollers), after which it can be hardened and tempered to produce products such as bearing parts. [Example]

[0045] Examples of the present invention will be specifically described below.

[0046] 150 kg of steels (steel types 1 to 10) having the compositions shown in Table 1 were melted in a vacuum melting furnace and hot rolled under the rolling conditions shown in Table 2 to form wire rods with a diameter of 15 mm. The obtained hot-rolled material was subjected to microstructural observation, in which the area ratio of pro-eutectoid cementite was determined by the method described above.

[0047] After the above-mentioned hot rolling, the steels (steel types 1 to 10) were subjected to spheroidizing annealing treatment to obtain spheroidized annealed materials. The spheroidizing annealing treatment was performed by heating the hot-rolled materials to 780°C and holding them at that temperature for 6.5 hours. The obtained spheroidized annealed material was subjected to microstructural observation. In the microstructural observation, the average grain size and maximum grain size of the spheroidal carbides, as well as the area ratio of the spheroidal carbides, were determined by the method described above. The remainder was ferrite.

[0048] Furthermore, a cold forgeability test was conducted on the spheroidized annealed material. For the cold forgeability test, cylindrical test pieces measuring 15 mm diameter x 22.5 mm were prepared from the spheroidized annealed material and compression tests were conducted using a 300 ton press at various compression ratios. The presence or absence of cracks on the side of the test piece was visually confirmed, and the compression ratio at which cracks occurred was evaluated as the critical compression ratio for cracking.

[0049] The spheroidized annealed material was then quenched and tempered to obtain quenched and tempered material. The quenching and tempering treatment was carried out by heating the material to 840°C, holding it there for 60 minutes, then quenching it in oil at 50°C, and then tempering it at 170°C for 1.5 hours. The hardness of the resulting quenched and tempered material was measured. The surface hardness of the quenched and tempered material was measured at a position 0.05 mm from the surface. Surface hardness was measured using a Vickers hardness tester in accordance with JIS Z 2244, with the hardness of the surface measured at six points under a test load of 2.94 N (300 gf), and the average value was taken as the surface hardness HV.

[0050] Table 2 shows the test results.

[0051] [Table 1]

[0052] [Table 2]

[0053] As is clear from Table 2, all of the invention examples Nos. 1 to 4 are superior to the comparative examples Nos. 5 to 13 in terms of surface hardness after quenching and tempering. The rolling conditions or chemical compositions of Comparative Examples Nos. 5 to 13 were outside the range of the present invention, and the resulting steel structures were outside the range of the present invention, resulting in poor hardness after quenching and tempering and / or poor cold forgeability. In No. 5, the rolling heating temperature and rolling finish temperature were below the appropriate ranges, so the steel structure within the range of the present invention was not obtained, and the hardness after quenching and tempering was low. In No. 6, the cooling rate after rolling was below the range of the present invention, so the area ratio of pro-eutectoid cementite was high, the average grain size and maximum grain size of carbide after spheroidizing annealing were larger than the range of the present invention, and the hardness after quenching and tempering was reduced. No. 7 had a carbon content below the appropriate range, resulting in low hardness after quenching and tempering. In No. 8, the C content exceeded the appropriate range, the pro-eutectoid cementite area ratio was high, the average carbide grain size and the maximum carbide grain size were also large, and the hardness after quenching and tempering was reduced. In No. 9, the Mo content, Si content, and Mn content exceeded the appropriate range, and the pro-eutectoid cementite area ratio, average carbide grain size, and maximum carbide grain size exceeded the ranges of the present invention. In this example, the hardness after quenching and tempering decreased, and the cold forgeability also decreased. In No. 10, the Cu content and Ni content exceeded the appropriate range, and the pro-eutectoid cementite area ratio, average carbide grain size, and maximum carbide grain size exceeded the ranges of the present invention. In this example, the hardness after quenching and tempering decreased. In No. 11, the Cr content and Mo content exceeded the appropriate range, the Sb content was lower than the lower limit of the range of the present invention, and the pro-eutectoid cementite area ratio, the average carbide grain size, and the maximum carbide grain size exceeded the range of the present invention. In this example, the hardness after quenching and tempering was reduced, and the cold forgeability was also reduced. In No. 12, the O content exceeds the appropriate range, resulting in reduced cold forgeability. In No. 13, the Si, Mn, and Cr contents were lower than the lower limit of the appropriate range, resulting in reduced hardness after quenching and tempering. Also, the low Cr content resulted in reduced cold forgeability.

Claims

1. In mass%, C: 0.85% or more and 1.20% or less, Si: 0.10% or more and 1.10% or less, Mn: 0.20% or more and 1.30% or less, Cr: 0.80% or more and 1.80% or less, Al: 0.050% or less, O: 0.0030% or less and Sb: 0.0005% or more and 0.0050% or less; The balance has a composition consisting of Fe and unavoidable impurities, A steel for rolling bearings having a microstructure in which the area ratio of pro-eutectoid cementite is less than 1.3%, with the remainder being pearlite.

2. Furthermore, in mass%, Cu: 0.05% or more and 0.20% or less, Ni: 0.05% or more and 0.20% or less 2. The rolling bearing steel according to claim 1, further comprising at least one element selected from the group consisting of Mo: 0.03% to 0.10%.

3. In mass%, C: 0.85% or more and 1.20% or less, Si: 0.10% or more and 1.10% or less, Mn: 0.20% or more and 1.30% or less, Cr: 0.80% or more and 1.80% or less, Al: 0.050% or less, O: 0.0030% or less and Sb: 0.0005% or more and 0.0050% or less; The balance has a composition consisting of Fe and unavoidable impurities, The area ratio of the spheroidal carbides is 12% or more and 20% or less, and the remainder is ferrite, The rolling bearing steel has a microstructure in which the average grain size of the spherical carbides is less than 0.50 μm and the maximum grain size is 2.50 μm or less.

4. Furthermore, in mass%, Cu: 0.05% or more and 0.20% or less, Ni: 0.05% or more and 0.20% or less 4. A steel for rolling bearings according to claim 3, containing at least one element selected from the group consisting of Mo: 0.03% to 0.10%.

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