Method for manufacturing steel product, steel product, and electric motor

Abstract

A method for manufacturing a steel product according to one embodiment of the present disclosure includes: a first heat treatment step for heating, at a first temperature, an object that is formed of a steel material and has an uneven part; a second heat treatment step for heating, after the first heat treatment step, the object at a second temperature that is lower than the first temperature; and a laser irradiation step for irradiating, after the second heat treatment step, at least a part of the uneven part of the object with laser light. The steel material contains, in mass%, 0.40-1.00% of C, 0.10-2.00% of Si, 0.10-1.00% of Mn, 0.030% or less of P, 0.030% or less of S, 1.10-3.20% of Cr, 0.010-0.10% of Al, and 0.15-0.50% of V, and additionally contains at least one of 2.50% or less of Ni and 1.00% or less of Mo, while having a (C + V) content of 0.60% or more in terms of mass%, with the balance being made up of Fe and unavoidable impurities.

Description

Method for manufacturing a steel product, a steel product, and an electric motor

[0001] The present disclosure relates to a method for manufacturing a steel product, a steel product, and an electric motor.

[0002] Conventionally, laser quenching is known for improving the hardness of a steel product including an object by irradiating the object formed of a steel material with a laser beam. For example, Non-Patent Document 1 discloses laser quenching of the root of a tooth of a gear formed of a steel material such as S45C.

[0003] Mitsuhiro Goto, Fuji High Frequency Industry Co., Ltd. Technical Report, "Things That Cannot Be Done by Laser Quenching and Troubleshooting Examples" [online], April 13, 2022, Fuji High Frequency Industry Co., Ltd. Homepage Internet <URL: https: / / www.fuji-koushuha.co.jp / technical-report / lh005>

[0004] In the above-described conventional technology, there is room for further improvement in terms of improving the durability of the steel product.

[0005] The present disclosure provides a technique capable of improving the durability of a steel product.

[0006] A method for manufacturing a steel product according to one aspect of the present disclosure includes a first heat treatment step of heating an object formed of a steel material and having uneven portions at a first temperature, a second heat treatment step of heating the object at a second temperature lower than the first temperature after the first heat treatment step, and a laser irradiation step of irradiating at least a part of the uneven portions of the object with a laser beam after the second heat treatment step. The steel material contains, in mass %, C: from 0.40% to 1.00%, Si: from 0.10% to 2.00%, Mn: from 0.10% to 1.00%, P: 0.030% or less, S: 0.030% or less, Cr: from 1.10% to 3.20%, Al: from 0.010% to 0.10%, V: from 0.15% to 0.50%, further contains at least one of Ni: 2.50% or less and Mo: 1.00% or less, and the amount of (C + V) is 0.60% or more in mass %, and the balance is steel composed of Fe and inevitable impurities.

[0007] According to the present disclosure, the durability of the steel product can be improved.

[0008] Figure 1 illustrates an example of a method for manufacturing a steel product according to an exemplary embodiment. Figure 2 is a diagram illustrating an example of an object according to an exemplary embodiment. Figure 3 is a diagram illustrating an example of a laser irradiation process according to an exemplary embodiment. Figure 4 is a diagram illustrating an example of the structure of a steel product according to an exemplary embodiment. Figure 5 is a diagram illustrating another example of the laser irradiation process according to an exemplary embodiment. Figure 6 is a diagram illustrating another example of the structure of a steel product according to an exemplary embodiment. Figure 7A is a diagram illustrating an example of an application of a steel product which is a gear according to an exemplary embodiment. Figure 7B is a diagram illustrating trocolloidal interference caused by a gear according to an exemplary embodiment. Figure 8 is a diagram illustrating an example of an electric motor according to an exemplary embodiment. Figure 9 is a diagram showing a cross-section of a gear according to Example 1. Figure 10A is a diagram showing the relationship between Vickers hardness and distance from the surface of the tooth tip in the gear according to Example 1. Figure 10B is a diagram showing the relationship between Vickers hardness and distance from the surface of the tooth in the gear according to Example 1. Figure 10C is a diagram showing the relationship between Vickers hardness and distance from the surface of the tooth root in the gear according to Example 1. Figure 10D is a diagram showing the relationship between Vickers hardness and distance from the surface of the tooth root in the gear according to Example 1. Figure 11 shows the results of fatigue tests on the gear according to Example 1, the gear according to Example 2, and the gear according to the comparative example.

[0009] Embodiments of the present disclosure will be described in detail below with reference to the drawings. In the following embodiments, the same parts are denoted by the same reference numerals, and redundant descriptions will be omitted.

[0010] <<1. Method for Manufacturing Steel Products>> First, a method for manufacturing steel products according to an exemplary embodiment will be described. Figure 1 is a diagram illustrating an example of a method for manufacturing steel products according to an exemplary embodiment.

[0011] An exemplary embodiment of the method for manufacturing steel products is a method for manufacturing steel products from an object.

[0012] The object is made of a steel material. Here, the steel material contains, by mass%, C: 0.40 to 1.00%, Si: 0.10 to 2.00%, Mn: 0.10 to 1.00%, P: 0.030% or less, S: 0.030% or less, Cr: 1.10 to 3.20%, Al: 0.010 to 0.10%, V: 0.15 to 0.50%, and further contains at least one of Ni: 2.50% or less and Mo: 1.00% or less, with a (C + V) amount of 0.60% or more by mass%, and the remainder being Fe and unavoidable impurities. For example, as the steel material, the steel disclosed in International Application No. 2019 / 035401 can be used.

[0013] An object has at least one protrusion or indentation. Each protrusion or indentation includes both a convex portion and a concave portion. A convex portion is a part that projects outward from a certain surface of the object. A concave portion is a part that is recessed inward from a certain surface of the object. The shape of the object is not particularly limited as long as it has protrusions and indentations. The shape of the object may be identical to, for example, the shape of a steel product. The object is obtained by processing a steel material into a predetermined shape having protrusions and indentations. A method for processing a steel material may be, for example, a method of cutting the steel material to have a predetermined shape using a cutting tool.

[0014] As shown in Figure 1, a method for manufacturing a steel product according to an exemplary embodiment includes a first heat treatment step S101, a second heat treatment step S102, and a laser irradiation step S103. The first heat treatment step S101 is sometimes called quenching. The second heat treatment step S102 is sometimes called tempering.

[0015] In the first heat treatment step S101, the object is heated to a first temperature. For example, a heating furnace can be used as the device for heating the object to the first temperature. The first temperature is measured, for example, by a thermocouple installed in the heating furnace. This allows the structure of the steel material constituting the object to be changed to martensite. After heating the object to the first temperature, the object is cooled.

[0016] In the second heat treatment step S102, the object is heated to a second temperature lower than the first temperature after the first heat treatment step S101. That is, in the second heat treatment step S102, the object, which has been heated to the first temperature, is heated to a second temperature lower than the first temperature. As the apparatus for heating the object to the second temperature, for example, a heating furnace can be used. The second temperature is measured by, for example, a thermocouple installed in the heating furnace. This makes it possible to change the structure of the steel material constituting the object to tempered martensite. The hardness of tempered martensite is less than the hardness of martensite. After heating the object to the second temperature, the object is cooled.

[0017] In the laser irradiation step S103, laser light is irradiated onto at least a portion of the uneven surface of the object after the second heat treatment step S102. That is, in the laser irradiation step S103, laser light is irradiated onto at least a portion of the uneven surface of the object after it has been heated to the second temperature. The device used to irradiate at least a portion of the uneven surface of the object with laser light is a laser device capable of heating at least a portion of the uneven surface of the object. For example, a semiconductor laser device can be used as the device used to irradiate at least a portion of the uneven surface of the object with laser light. In this way, steel products are manufactured from the object through the laser irradiation step S103.

[0018] In the laser irradiation step S103, at least a portion of the uneven surface of the object can be heated by irradiating it with laser light. In the laser-irradiated portion of the uneven surface of the object, the structure of the steel material constituting the object can be changed to martensite. On the other hand, in the portion of the uneven surface of the object that is not irradiated with laser light, the structure of the steel material constituting the object remains in the tempered martensite structure.

[0019] In other words, the uneven portion of the object after the laser irradiation step S103 includes a base having a structure containing tempered martensite and a coating layer having a structure containing martensite. Thus, the uneven portion of the object includes a base having a hardness less than that of the coating layer and a coating layer having a hardness greater than that of the base. In the uneven portion of the object, the coating layer covers at least a portion of the base.

[0020] Thus, the uneven surfaces of the object after the laser irradiation step S103, i.e., the uneven surfaces of the steel product, include a base having a lower hardness and a coating layer having a higher hardness. Because the coating layer of the uneven surfaces of the steel product has a higher hardness, the strength of the steel product can be improved. Because the base of the uneven surfaces of the steel product has a lower hardness, the toughness of the steel product can be improved. Therefore, according to the manufacturing method of the steel product according to the embodiment, a steel product with improved strength and toughness can be manufactured. Thus, according to the manufacturing method of the steel product according to the embodiment, a steel product with improved durability can be manufactured.

[0021] Objects having uneven surfaces are formed from the steel material described above. Therefore, when laser light is irradiated onto at least a portion of the uneven surface of the object in the laser irradiation step S103, the structure of the steel material constituting the object can be more efficiently transformed into martensite. In other words, by using the steel material described above, the quenching of at least a portion of the uneven surface of the object can be performed more efficiently.

[0022] This allows, for example, the microstructure of steel material in at least a portion of the uneven surface of an object to be more efficiently transformed into martensite by using only a single laser device. In other words, according to the method for steel products according to this embodiment, steel products with improved strength and toughness can be manufactured more easily.

[0023] If the object has multiple uneven surfaces, in the laser irradiation step S103, laser light may be continuously irradiated onto at least a portion of adjacent uneven surfaces. In this case, after irradiating at least a portion of the first uneven surface, laser light is irradiated onto at least a portion of the second uneven surface adjacent to the first uneven surface. Here, the object having multiple uneven surfaces is made of the steel material described above. Therefore, the change of the structure of the steel material in at least a portion of the first uneven surface adjacent to the second uneven surface to tempered martensite can be reduced by the laser light irradiated onto at least a portion of the second uneven surface.

[0024] In other words, by using the steel material described above, the tempering of at least a portion of each of the adjacent first and second uneven portions can be reduced. Accordingly, the hardening of at least a portion of each of the adjacent first and second uneven portions can be performed more independently. As a result, the hardening of at least a portion of each of the multiple uneven portions in the object can be performed more uniformly. Therefore, according to the manufacturing method of steel products according to the embodiment, more uniform steel products can be manufactured.

[0025] In the first heat treatment step S101, for example, the object is heated at a first temperature until its Vickers hardness exceeds 650 HV and falls below 950 HV. That is, the object is heated at a first temperature for a period of time until its Vickers hardness exceeds 650 HV and falls below 950 HV. This makes it easier to change the structure of the steel material constituting the object into martensite.

[0026] In the second heat treatment step S102, for example, the object is heated at a second temperature until its Vickers hardness is between 350 HV and 650 HV. That is, the object is heated at the second temperature for the duration required for its Vickers hardness to reach between 350 HV and 650 HV. This makes it easier to change the structure of the steel material constituting the object to tempered martensite. As a result, steel products with improved toughness can be manufactured more easily.

[0027] As described above, for example, the object is heated at a first temperature until its Vickers hardness exceeds 650 HV and is 950 HV or less, and then heated at a second temperature until its Vickers hardness is between 350 HV and 650 HV. In this case, steel products with improved toughness can be manufactured more easily.

[0028] In the second heat treatment step S102, for example, the object may be heated at a second temperature until its Vickers hardness is between 350 HV and 450 HV. That is, the object may be heated at a second temperature for a period of time until its Vickers hardness is between 350 HV and 450 HV. In this case, the hardness of the matrix contained in the uneven parts of the steel product can be reduced more appropriately. As a result, steel products with improved toughness can be manufactured. Consequently, steel products with improved durability can be manufactured.

[0029] The Vickers hardness of an object is measured according to the Japanese Industrial Standard "JIS Z 2244:2009 Vickers hardness test - Test method". A micro-Vickers hardness tester is used to measure the Vickers hardness of an object. The test force used in measuring Vickers hardness is 0.3 kgf.

[0030] The first temperature is, for example, between 930°C and 950°C. This makes it easier to change the structure of the steel material constituting the object into a martensitic structure.

[0031] The second temperature is, for example, between 550°C and 650°C. This allows the structure of the steel material constituting the object to be more easily transformed into tempered martensite. Therefore, steel products with improved toughness can be manufactured more easily.

[0032] As mentioned above, for example, the first temperature is between 930°C and 950°C, and the second temperature is between 550°C and 650°C. In this case, steel products with improved toughness can be manufactured more easily.

[0033] In the laser irradiation step S103, for example, laser light is irradiated onto at least a portion of the uneven surface of the object until the Vickers hardness of at least a portion of that portion exceeds 650 HV and falls below 950 HV. That is, laser light is irradiated onto at least a portion of the uneven surface of the object for the duration of time until the Vickers hardness of at least a portion of that portion exceeds 650 HV and falls below 950 HV. This makes it easier to change the structure of the steel material constituting at least a portion of the uneven surface of the object to martensite. Therefore, steel products with improved strength can be manufactured more easily.

[0034] The object is, for example, a gear having gear teeth as protrusions and recesses. Here, the gear can be considered an object having a disc portion and multiple protrusions and recesses as multiple gear teeth. The protrusions and recesses have concave and convex portions. The concave portion is a part that is recessed inward in the radial direction from the outer surface of the disc. The convex portion is a part that protrudes radially outward from the outer surface of the disc portion.

[0035] As described above, if the object is a gear having gear teeth as protrusions and recesses, it is possible to manufacture steel products that are gears with improved strength and toughness. In other words, it is possible to manufacture steel products that are gears with improved durability.

[0036] Figure 2 shows an example of an object according to an exemplary embodiment. As shown in Figure 2, the object is, for example, a gear 10. The gear 10 has a plurality of protrusions 11. The protrusions 11 include gear teeth 12 and grooves 13. The gear teeth 12 are an example of protrusions. The grooves 13 are an example of recesses. Thus, the gear 10 has a plurality of gear teeth 12. The gear 10 has a plurality of grooves 13 between the plurality of gear teeth 12.

[0037] In the laser irradiation step S103, for example, laser light is irradiated perpendicularly to the surface of a protrusion or recess in the uneven surface of an object.

[0038] The surface of a convex portion is the surface onto which the laser beam is irradiated. The surface of a convex portion may also be the outermost surface of the convex portion of the object. The surface of a concave portion is the surface onto which the laser beam is irradiated. The surface of a concave portion may also be the innermost surface of the concave portion of the object.

[0039] Irradiating a surface perpendicular to it is not limited to irradiating it strictly perpendicular to it, but also includes irradiating it substantially perpendicular to it. Irradiating a surface substantially perpendicular to it depends on the shape of the convex or concave part, but for example, it may be irradiating the surface with laser light at an angle of 0 degrees or more and 1 degree or less with respect to the normal to the surface.

[0040] As described above, when laser light is irradiated perpendicularly to the surface of a protrusion or recess in the uneven surface of an object, steel products with improved strength and toughness can be manufactured more easily.

[0041] In the laser irradiation step S103, for example, laser light is irradiated to the upper surface of the protrusions and the bottom surface of the recesses in the uneven portion, and to the side surfaces connecting the upper surface of the protrusions and the bottom surface of the recesses in the uneven portion. That is, laser light is irradiated to most or all of the surface of the uneven portion of the object. For example, a laser beam may be used with a spot shape and size that allows the laser light to be irradiated to all of the bottom surface of the recesses, all of the side surfaces connecting the upper surface of the protrusions and the bottom surface of the recesses, and at least a part of the upper surface of the protrusions.

[0042] In this case, the structure of the steel material constituting most or all of the surface of the uneven parts of the object can be changed to martensite. Therefore, it is possible to manufacture steel products with improved strength on most or all of the surface of the uneven parts. In this way, it is possible to manufacture steel products with improved durability.

[0043] For example, when the object is a gear having gear teeth as concavo-convex portions, in the laser irradiation step S103, laser light is irradiated onto the surface from the tooth root to the tooth tip of the gear teeth. Here, the tooth root of the gear teeth means a portion near the bottom surface of the concave portion in the concavo-convex portion as the gear teeth. The tooth tip of the gear teeth means a portion near the upper surface of the convex portion in the concavo-convex portion as the gear teeth. Further, as will be described later, the mid-tooth portion of the gear teeth means a portion near the side surface connecting the upper surface of the convex portion and the bottom surface of the concave portion in the concavo-convex portion as the gear teeth. As will be described later, the tooth bottom of the gear teeth means a portion near the portion where the side surface connecting the upper surface of the convex portion and the bottom surface of the concave portion in the concavo-convex portion as the gear teeth is connected to the bottom surface of the concave portion.

[0044] In this case, laser light is irradiated onto most or all of the surface of the gear teeth. Thereby, the structure of the steel material constituting most or all of the surface of the gear teeth can be changed to martensite. Therefore, a steel product that is a gear with improved strength over most or all of the surface of the gear teeth can be manufactured. Thus, a steel product that is a gear with improved durability can be manufactured.

[0045] In the laser irradiation step S103, for example, laser light is irradiated onto the bottom surface of the concave portion in the concavo-convex portion and the portion of the side surface connecting the upper surface of the convex portion and the bottom surface of the concave portion that is connected to the bottom surface of the concave portion. That is, laser light is selectively irradiated onto the bottom surface of the concave portion in the concavo-convex portion of the object and the portion near the bottom surface of the side surface connecting the upper surface of the convex portion and the bottom surface of the concave portion. For example, laser light having a spot shape and size capable of irradiating all of the bottom surface of the concave portion and the portion near the bottom surface of the side surface connecting the upper surface of the convex portion and the bottom surface of the concave portion may be used.

[0046] In this case, the structure of the steel material constituting the bottom surface of the recess in the concavo-convex portion of the object, and the portion near the bottom surface of the recess among the upper surface of the convex portion and the side surface connecting the bottom surface of the recess can be changed to martensite. Therefore, it is possible to manufacture a steel product with improved strength in the bottom surface of the recess, and the portion near the bottom surface of the recess among the upper surface of the convex portion and the side surface connecting the bottom surface of the recess. On the other hand, the structure of the steel material constituting the portions other than the bottom surface of the recess in the concavo-convex portion of the object and the portion near the bottom surface of the recess among the upper surface of the convex portion and the side surface connecting the bottom surface of the recess remains tempered martensite. Therefore, it is possible to obtain a steel product with improved toughness in the portions other than the portion near the bottom surface of the recess among the bottom surface of the recess, the upper surface of the convex portion, and the side surface connecting the bottom surface of the recess in the concavo-convex portion of the object. Thus, it is possible to manufacture a steel product with further improved durability.

[0047] For example, when the object is a gear having gear teeth as concavo-convex portions, in the laser irradiation step S103, laser light may be irradiated onto the surface from the tooth root to a part of the tooth middle in the gear teeth. For example, in the laser irradiation step S103, laser light may be irradiated onto the surface from the tooth root to the pitch point of the gear in the gear teeth. Here, the pitch point of the gear means the point where the tooth tip of the mating gear contacts the target gear when the target gear meshes with the mating gear.

[0048] In this case, the microstructure of the steel material constituting the surface of the gear tooth from the tooth root to a portion of the tooth can be changed to martensite. For example, the microstructure of the steel material constituting the surface of the gear tooth from the tooth root to the pitch point can be changed to martensite. Therefore, it is possible to manufacture steel products with improved strength on the surface of the gear tooth from the tooth root to the pitch point. On the other hand, the microstructure of the steel material constituting the surface of the gear tooth from a portion of the tooth to the tooth tip remains tempered martensite. For example, the microstructure of the steel material constituting the surface of the gear tooth from the pitch point to the tooth tip remains tempered martensite. Therefore, it is possible to obtain a steel product gear in which toughness is improved on the surface of the gear tooth from a portion of the tooth to the tooth tip. For example, it is possible to obtain a steel product gear in which toughness is improved on the surface of the gear tooth from the pitch point to the tooth tip. In this way, it is possible to manufacture steel products with further improved durability. For example, when a steel gear meshes with another gear, it is possible to reduce damage to the gear teeth of the mating gear caused by contact between the gear teeth of the steel gear and the gear teeth of the mating gear.

[0049] In the laser irradiation process S103, for example, the shape and size of the laser beam spot can be adjusted by using a beam shaper, composed of optical elements such as lenses, in the optical system of the device that irradiates the laser light.

[0050] Figure 3 illustrates an example of a laser irradiation process according to an exemplary embodiment. As shown in Figure 3, in the laser irradiation process S103 (see Figure 1), laser light LL is irradiated onto the uneven portion 11 (see Figure 2) of a gear 10 (see Figure 2) having a plurality of gear teeth 12 and a plurality of groove portions 13.

[0051] As shown in Figure 3, in the laser irradiation step S103, for example, laser light LL is irradiated perpendicularly to the surface 13a of the groove portion 13 in the uneven portion 11 of the gear 10. The laser light LL is irradiated onto the surface 13a of the groove portion 13 from, for example, a single semiconductor laser device. The surface 13a of the groove portion 13 is the innermost surface of the groove portion 13 of the gear 10. The laser light LL is irradiated onto the surface 13a of the groove portion 13 such that the optical axis OA of the laser light LL is perpendicular to the surface 13a of the groove portion 13. The laser light LL is irradiated not only onto the surface 13a of the groove portion 13, but also onto the side surface 14 connecting the groove portion 13 and the gear teeth 12, and a part of the surface 12a of the gear teeth 12.

[0052] The laser beam LL heats the surface 13a of the groove 13, the side surface 14 connecting the groove 13 and the gear teeth 12, and a portion of the surface 12a of the gear teeth 12. This changes the structure of the steel material in the parts of the groove 13 and gear teeth 12 irradiated with the laser beam LL to martensite. The structure of the steel material in the parts of the groove 13 and gear teeth 12 not irradiated with the laser beam LL is tempered martensite. Thus, the parts of the groove 13 and gear teeth 12 irradiated with the laser beam LL have a structure in which martensite covers the tempered martensite.

[0053] Similarly, in the laser irradiation step S103, laser light LL is irradiated perpendicularly to the surface 12a of the gear teeth 12 in the uneven portion 11 of the gear 10. In this way, laser light LL is irradiated to all of the surface 13a of the groove portion 13, the side surface 14 connecting the groove portion 13 and the gear teeth 12, and the surface 12a of the gear teeth 12. In other words, laser light LL is irradiated to the surface of the gear teeth 12 from the tooth root to the tooth tip. Next, in the laser irradiation step S103, for multiple groove portions 13 and multiple gear teeth 12, the process of irradiating the surface 13a of the groove portion 13 perpendicularly with laser light LL and the surface 12a of the gear teeth 12 perpendicularly is repeated. In this way, a gear 10 with improved strength and toughness can be manufactured as a steel product. That is, a gear 10 with improved durability can be manufactured as a steel product.

[0054] <<2. Steel Products>> Next, a steel product according to an exemplary embodiment will be described.

[0055] An exemplary embodiment of a steel product includes an object formed from a steel material and having an uneven surface. The steel material contains, by mass%, C: 0.40 to 1.00%, Si: 0.10 to 2.00%, Mn: 0.10 to 1.00%, P: 0.030% or less, S: 0.030% or less, Cr: 1.10 to 3.20%, Al: 0.010 to 0.10%, V: 0.15 to 0.50%, and further contains at least one of Ni: 2.50% or less and Mo: 1.00% or less, with a (C + V) amount of 0.60% or more by mass%, and the remainder being Fe and unavoidable impurities.

[0056] The uneven portion includes a base and a coating layer that covers at least a portion of the base. The coating layer is a layer on the surface side of the uneven portion. The base consists of the portion inside the coating layer in the uneven portion and the portion of the uneven portion not covered by the coating layer. The structure of the base includes tempered martensite. The structure of the coating layer includes martensite. The hardness of tempered martensite is less than the hardness of martensite. Thus, the uneven portion includes a base having a hardness less than the hardness of the coating layer and a coating layer having a hardness greater than the hardness of the base.

[0057] The substrate and coating layers contained within the uneven areas can be identified, for example, by taking cross-sectional images of the uneven areas using an industrial microscope. The cross-section of the uneven area is a plane perpendicular to the surface of the convex or concave areas, and includes both the convex and concave areas.

[0058] An exemplary steel product is manufactured, for example, by a method for manufacturing a steel product according to the exemplary embodiment described above. The base is obtained by a first heat treatment step of heating an object made of the steel material described above and having an uneven surface to a first temperature, and a second heat treatment step of heating the object to a second temperature lower than the first temperature after the first heat treatment step. The coating layer is obtained by a laser irradiation step of irradiating at least a portion of the uneven surface of the object with laser light after the second heat treatment step.

[0059] Thus, the uneven portion includes a base having lower hardness and a coating layer having higher hardness. Because the coating layer of the uneven portion has higher hardness, the strength of the steel product can be improved. Because the base of the uneven portion has lower hardness, the toughness of the steel product can be improved. Therefore, according to the embodiment, a steel product with improved strength and toughness can be provided. Thus, according to the embodiment, a steel product with improved durability can be provided.

[0060] The Vickers hardness of the base material is, for example, between 350 HV and 650 HV. In this case, base materials with lower hardness can be provided more easily. Therefore, steel products with improved toughness can be provided more easily.

[0061] The Vickers hardness of the coating layer is, for example, greater than 650 HV and less than or equal to 950 HV. In this case, a coating layer with greater hardness can be provided more easily. Therefore, steel products with improved strength can be provided more easily.

[0062] As mentioned above, for example, the Vickers hardness of the base is between 350 HV and 650 HV, while the Vickers hardness of the coating layer is greater than 650 HV and less than or equal to 950 HV. In this case, steel products with improved strength and toughness can be provided more easily. As a result, steel products with improved durability can be provided more easily.

[0063] The Vickers hardness of the base material may be, for example, 350 HV to 450 HV. In this case, it is possible to provide a steel product having a surface with irregularities including a base material with a more appropriately reduced hardness. Therefore, it is possible to provide a steel product with improved toughness. As a result, it is possible to provide a steel product with improved durability.

[0064] The Vickers hardness of the substrate or coating layer is measured according to the Japanese Industrial Standard "JIS Z 2244:2009 Vickers hardness test - Test method". A micro-Vickers hardness tester is used to measure the Vickers hardness of the substrate or coating layer. The test force used in measuring Vickers hardness is 0.3 kgf.

[0065] For example, the coating layer covers the entire base. In this case, the uneven portion includes the base and the coating layer that covers the entire base. That is, in the uneven portion, the coating layer, which has a hardness greater than that of the base, covers the entire base, which has a hardness less than that of the coating layer. Here, the base and the coating layer can be considered as the inner layer and outer layer of the uneven portion, respectively. Thus, it is possible to provide steel products with improved strength. In this way, it is possible to manufacture steel products with improved durability.

[0066] When the coating layer covers the entire base, for example, in the cross-section of an uneven surface, the maximum thickness of the coating layer on the surface of the protrusions in the uneven surface is greater than the maximum thickness of the coating layer on the surface of the recesses in the uneven surface. Furthermore, the minimum thickness of the coating layer on the sides connecting the protrusions and recesses in the uneven surface is less than the maximum thickness of the coating layer on the surface of the recesses in the uneven surface.

[0067] As described above, the cross-section of the uneven portion is a surface perpendicular to the surface of the convex portion or the concave portion, and includes both the convex and concave portions. The thickness of the coating layer on the surface of the convex portion is the thickness of the coating layer along the direction normal to the surface of the convex portion. The thickness of the coating layer on the surface of the convex portion may vary depending on the position on the surface of the convex portion. The thickness of the coating layer on the surface of the concave portion is the thickness of the coating layer along the direction normal to the surface of the concave portion. The thickness of the coating layer on the surface of the concave portion may vary depending on the position on the surface of the concave portion. The thickness of the coating layer on the side surface connecting the convex and concave portions is the thickness of the coating layer along the direction normal to the side surface connecting the convex and concave portions. The thickness of the coating layer on the side surface connecting the convex and concave portions may vary depending on the position on the side surface connecting the convex and concave portions.

[0068] The maximum thickness of the coating layer on the surface of the convex portion, the maximum thickness of the coating layer on the surface of the concave portion, and the minimum thickness of the coating layer on the side surface connecting the convex and concave portions in the uneven area are calculated by analyzing the image of the coating layer in a photograph of the cross-section of the uneven area.

[0069] When steel products have the above-described relationship of coating layer thickness, they can be manufactured, for example, by irradiating the uneven surfaces of an object with laser light. When irradiating the uneven surfaces of an object with laser light, the illuminance of the laser light on the surface of the convex parts tends to be greater than that on the surface of the concave parts. Accordingly, the thickness of the coating layer on the surface of the convex parts tends to be greater than that on the surface of the concave parts. Similarly, the illuminance of the laser light on the side surfaces connecting the convex and concave parts tends to be less than that on the surface of the concave parts. Accordingly, the thickness of the coating layer on the side surfaces connecting the convex and concave parts tends to be less than that on the surface of the concave parts.

[0070] Thus, when steel products have the aforementioned relationship of coating layer thickness, a coating layer containing martensite can be more easily provided by irradiating the uneven parts of the object with laser light. Therefore, steel products with improved strength and toughness can be more easily provided. As a result, steel products with improved durability can be more easily provided.

[0071] For example, the coating layer covers the base on the bottom surface of the recesses in the uneven portion, and on the portion of the side surface connecting the top surface of the convex portion and the bottom surface of the recesses that connects to the bottom surface of the recesses. In this case, the uneven portion includes the base and a coating layer that partially covers the base on the bottom surface of the recesses, and on the portion of the side surface connecting the top surface of the convex portion and the bottom surface of the recesses that connects to the bottom surface of the recesses. The base is partially exposed to the outside of the uneven portion on the top surface of the convex portion, and on the portion of the side surface connecting the top surface of the convex portion and the bottom surface of the recesses that connects to the top surface of the convex portion.

[0072] In other words, in the uneven areas, a coating layer having a hardness greater than that of the base partially covers the base, which has a hardness less than that of the coating layer. As a result, it is possible to provide a steel product in which the strength is improved in the parts of the base that are covered by the coating layer. On the other hand, it is possible to provide a steel product in which the toughness is improved in the parts of the base that are not covered by the coating layer. In this way, it is possible to provide a steel product with further improved durability.

[0073] The object is, for example, a gear having gear teeth as its protruding parts.

[0074] As described above, if the object is a gear having gear teeth as protrusions and recesses, it is possible to provide a steel product that is a gear with improved strength and toughness. In this way, it is possible to provide a steel product that is a gear with improved durability.

[0075] For example, if the object is a gear having gear teeth as a protruding part, the coating layer may be applied to cover the base material over the entire area of ​​the gear teeth. In this case, it is possible to provide a steel product that is a gear with improved strength over the entire area of ​​the gear teeth. In this way, it is possible to provide a steel product that is a gear with improved durability.

[0076] For example, if the object is a gear having gear teeth as a recessed portion, the coating layer may cover the base material in the region from the tooth root to a part of the tooth. For example, the coating layer may cover the base material in the region from the tooth root to the pitch point of the gear. In this case, it is possible to provide a steel product in which the strength of the gear is improved in the region from the tooth root to a part of the tooth. For example, it is possible to provide a steel product in which the strength of the gear is improved in the region from the tooth root to the pitch point of the gear. Furthermore, it is possible to provide a steel product in which the toughness of the gear is improved in the region from a part of the tooth to the tooth tip. For example, it is possible to provide a steel product in which the toughness of the gear is improved in the region from the pitch point to the tooth tip of the gear. In this way, it is possible to provide a steel product in which the durability of the gear is improved. For example, when a steel gear meshes with another gear, it is possible to reduce damage to the gear teeth of the mating gear caused by contact between the gear teeth of the steel gear and the gear teeth of the mating gear.

[0077] Figure 4 is a diagram illustrating an example of the structure of a steel product according to an exemplary embodiment.

[0078] As shown in Figure 4, an exemplary embodiment of a steel product includes an object formed from the steel material described above and having a plurality of protrusions 11. The object is a gear 10 (see Figure 2) having a plurality of gear teeth 12 and a plurality of grooves 13 as the plurality of protrusions 11. The plurality of protrusions 11 include a base 15 and a coating layer 16. The coating layer 16 covers the entire base 15. In other words, the coating layer 16 covers the base 15 over the entire area of ​​the gear teeth 12. The structure of the base 15 includes tempered martensite. The structure of the coating layer 16 includes martensite. The Vickers hardness of the base 15 is, for example, 350 HV to 650 HV. The Vickers hardness of the coating layer 16 is, for example, greater than 650 HV and 950 HV or less. In this case, the protrusions 11 include a base 15 having a lower hardness and a coating layer 16 having a higher hardness. Therefore, it is possible to provide steel products with improved strength and toughness. In this way, it is possible to provide steel products with improved durability.

[0079] As shown in Figure 4, in the cross-section of the uneven portion 11, the maximum thickness T1 of the coating layer 16 on the surface of the gear teeth 12 is greater than the maximum thickness T2 of the coating layer 16 on the surface of the groove portion 13. The minimum thickness T3 of the coating layer 16 on the side surface connecting the gear teeth 12 and the groove portion 13 is less than the maximum thickness T2 of the coating layer 16 on the surface of the groove portion 13. In this case, steel products can be manufactured by irradiating the multiple uneven portions 11 of the gear 10 with laser light. Therefore, steel products with improved strength and toughness can be provided more easily.

[0080] <<3. Another Example of a Method for Manufacturing Steel Products>> Figure 5 illustrates another example of a laser irradiation step according to an exemplary embodiment. As shown in Figure 5, in the laser irradiation step S103 (see Figure 1), laser light LL is irradiated onto the uneven portion 11 (see Figure 2) of a gear 10 (see Figure 2) having a plurality of gear teeth 12 and a plurality of groove portions 13.

[0081] As shown in Figure 5, in the laser irradiation step S103, for example, laser light LL is irradiated perpendicularly to the surface 13a of the groove portion 13 in the uneven portion 11 of the gear 10. The laser light LL is irradiated onto the surface 13a of the groove portion 13 from, for example, a single semiconductor laser device. The surface 13a of the groove portion 13 is the innermost surface of the groove portion 13 of the gear 10. The laser light LL is irradiated onto the surface 13a of the groove portion 13 such that the optical axis OA of the laser light LL is perpendicular to the surface 13a of the groove portion 13. Here, the side surface 14 connecting the groove portion 13 and the gear teeth 12 is divided into a portion that connects to the surface 13a of the groove portion 13 and a portion that connects to the surface 12a of the gear teeth 12. The laser light LL is irradiated not only to the surface 13a of the groove portion 13, but also to the portion of the side surface 14 connecting the groove portion 13 and the gear teeth 12 that connects to the surface 13a of the groove portion 13. However, the laser beam LL does not irradiate either the surface 12a of the gear teeth 12, nor the portion of the side surface 14 connecting the groove 13 and the gear teeth 12 that is connected to the surface 12a of the gear teeth 12.

[0082] The laser beam LL heats the surface 13a of the groove 13 and the portion of the side surface 14 connecting the groove 13 and the gear teeth 12 that connects to the surface 13a of the groove 13. This changes the structure of the steel material in the parts of the groove 13 and gear teeth 12 that are irradiated with the laser beam LL to martensite. The structure of the steel material in the parts of the groove 13 and gear teeth 12 that are not irradiated with the laser beam LL is tempered martensite. Thus, the parts of the groove 13 and gear teeth 12 that are irradiated with the laser beam LL have a structure in which martensite covers the tempered martensite.

[0083] In this way, the laser beam LL is irradiated onto the surface 13a of the groove 13 and the portion of the side surface 14 connecting the groove 13 and the gear teeth 12 that connects to the surface 13a of the groove 13. In other words, the laser beam LL is irradiated onto a portion of the surface of the gear teeth 12, from the tooth root to the middle of the tooth. For example, the laser beam may be irradiated onto the surface of the gear teeth 12 from the tooth root to the pitch point of the gear 10. Next, in the laser irradiation step S103, the laser beam LL is repeatedly irradiated perpendicularly to the surface 13a of the groove 13 as described above for the multiple grooves 13 and multiple gear teeth 12. In this way, a gear 10 with improved strength and toughness can be manufactured as a steel product. That is, a gear 10 with improved durability can be manufactured as a steel product.

[0084] <<4. Another Example of a Steel Product>> Figure 6 illustrates another example of the structure of a steel product according to an exemplary embodiment.

[0085] As shown in Figure 6, an exemplary embodiment of a steel product includes an object formed from the steel material described above and having a plurality of protrusions 11. The object is a gear 10 (see Figure 2) having a plurality of gear teeth 12 and a plurality of grooves 13 as the plurality of protrusions 11. The plurality of protrusions 11 include a base 15 and a coating layer 16. The coating layer 16 covers the base 15 on the surface of the grooves 13 and on the portion of the side surface connecting the upper surface of the gear teeth 12 and the surface of the grooves 13 that connects to the surface of the grooves 13. In other words, the coating layer 16 covers the base 15 in the region from the tooth root to a portion of the tooth in the gear teeth 12. For example, the coating layer 16 may cover the base 15 in the region from the tooth root to the pitch point of the gear 10 in the gear teeth 12.

[0086] The structure of the base 15 includes tempered martensite. The structure of the coating layer 16 includes martensite. The Vickers hardness of the base 15 is, for example, 350 HV to 650 HV. The Vickers hardness of the coating layer 16 is, for example, greater than 650 HV and 950 HV or less. In this case, the uneven portion 11 includes the base 15 having a lower hardness and the coating layer 16 having a higher hardness. Thus, it is possible to provide steel products with improved strength and toughness. In this way, it is possible to provide steel products with improved durability.

[0087] Figure 7A illustrates an example of an application for a steel product, which is a gear according to an exemplary embodiment. Figure 7B illustrates trocolloid interference caused by the gear according to an exemplary embodiment.

[0088] A steel product, which is a gear according to an exemplary embodiment, is used in a system in which a drive gear 10A meshes with a driven gear 10B, as shown in Figure 7A. Here, the drive gear 10A is a gear that rotates by a driving force supplied from a drive device (not shown). The driven gear 10B is a gear that rotates in conjunction with the rotation of the drive gear 10A. The driven gear 10B rotates in the opposite direction to the rotation of the drive gear 10A. For example, as shown in Figure 7A, when the drive gear 10A rotates clockwise, the driven gear 10B rotates counterclockwise.

[0089] The drive gear 10A has a plurality of gear teeth 12A and a plurality of grooves 13A. The driven gear 10B has a plurality of gear teeth 12B and a plurality of grooves 13B. When the drive gear 10A meshes with the driven gear 10B, the gear teeth 12A of the drive gear 10A contact the gear teeth 12B of the driven gear 10B at the pitch point PP of the drive gear 10A. Similarly, the gear teeth 12B of the driven gear 10B contact the gear teeth 12A of the drive gear 10A at the pitch point PP of the driven gear 10B. The pitch point PP of the driven gear 10B is the same as the pitch point PP of the drive gear 10A.

[0090] When the drive gear 10A meshes with the driven gear 10B, as the drive gear 10A rotates, the tips of the gear teeth 12B of the driven gear 10B come into contact with and move against the middle and bottom teeth of the gear teeth 12A of the drive gear 10A. This movement, in which the tips of the gear teeth 12B of the driven gear 10B come into contact with and move against the middle and bottom teeth of the gear teeth 12A of the drive gear 10A as the drive gear 10A rotates, is called trochoroid interference. For example, as the drive gear 10A rotates, the tips of the gear teeth 12B of the driven gear 10B move along a curve that passes through the pitch point PP of the drive gear 10A and cuts against the middle and bottom teeth of the gear teeth 12A of the drive gear 10A, as shown by the dotted arrow in Figure 7B. In this case, trochoroidal interference caused by the tooth tips of the gear teeth 12B of the driven gear 10B may cause damage to the gear teeth 12A of the drive gear 10A.

[0091] Here, a steel product that is a gear according to an exemplary embodiment can be used as the driven gear 10B. Specifically, a steel product according to an exemplary embodiment as shown in Figure 6 can be used, wherein the coating layer 16 (see Figure 6) covers the base 15 (see Figure 6) in the region from the tooth root to the pitch point of the gear on the gear teeth 12 (see Figure 6). In such a steel product, the gear teeth 12 include a base 15 having less hardness in the region from the tooth tip to the pitch point of the gear and a coating layer 16 having more hardness in the region from the tooth root to the pitch point of the gear. That is, a driven gear 10B can be provided having gear teeth 12 including a base 15 having less hardness in the region from the tooth tip to the pitch point PP of the driven gear 10B and a coating layer 16 having more hardness in the region from the tooth root to the pitch point PP of the driven gear 10B.

[0092] In this case, since the tooth tips of the gear teeth 12B of the driven gear 10B are made of a base 15 with lower hardness, it becomes possible to reduce damage to the gear teeth 12A of the drive gear 10A that may occur due to trocolloidal interference of the tooth tips of the gear teeth 12B of the driven gear 10B. In this way, by using a steel product which is a gear according to an exemplary embodiment as the driven gear 10B, the durability of the drive gear 10A can be improved.

[0093] Furthermore, a steel product, which is a gear according to an exemplary embodiment, may be used as the drive gear 10A. In this case, the durability of the drive gear 10A can be further improved.

[0094] <<5. Electric Motor>> Next, an electric motor according to an exemplary embodiment will be described.

[0095] An exemplary embodiment of the electric motor includes the steel product described above, where the object is a gear. In this case, an electric motor can be provided that includes a gear with improved strength and toughness. Thus, an electric motor can be provided that includes a gear with improved durability.

[0096] Figure 8 shows an example of an electric motor according to an exemplary embodiment.

[0097] As shown in Figure 8, the electric motor 20 is, for example, a geared motor including a motor and a reduction gear. The electric motor 20 includes, for example, a motor 21, a shaft 22, a sun gear 23, a plurality of planetary gears 24, an internal gear 25, and a carrier 26. The shaft 22 of the motor 21 is connected to the sun gear 23. The sun gear 23 meshes with the plurality of planetary gears 24. The plurality of planetary gears 24 mesh with the internal gear 25. The internal gear 25 is fixed to a housing (not shown). The carrier 26 is attached to the plurality of planetary gears 24.

[0098] The sun gear 23 is driven by the motor 21 through the shaft 22. When the sun gear 23 is rotated by the motor 21 through the shaft 22, the multiple planetary gears 24 that mesh with the sun gear 23 are also rotated inside the internal gear 25. The multiple planetary gears 24 rotate so as to revolve around the sun gear 23. When the multiple planetary gears 24 are rotated, the carrier 26 attached to the multiple planetary gears 24 is also rotated. The carrier 26 functions as an output shaft that transmits power from the motor 21.

[0099] Here, at least one of the sun gear 23, the multiple planetary gears 24, and the internal gear 25 can be a steel product of an exemplary embodiment in which the object is a gear.

[0100] Figure 8 shows an example where the electric motor 20 is a geared motor including a planetary gear reducer as a reduction gear. However, the electric motor in the exemplary embodiment is not limited to a geared motor including a planetary gear reducer as a reduction gear. For example, the electric motor in the exemplary embodiment may be a geared motor including a parallel shaft gear reducer as a reduction gear.

[0101] <<6. Examples>> Examples of the present disclosure will be described below in detail. The present disclosure is not limited to the examples shown below.

[0102] (Example 1) As a steel material, a steel was prepared containing, by mass%, C: 0.62%, Si: 0.99%, Mn: 0.39%, P: 0.011%, S: 0.005%, Cr: 1.98%, Al: 0.015%, V: 0.30%, Ni: 0.07%, and Mo: 0.30%, with a (C + V) amount of 0.92% by mass%, and the remainder consisting of Fe and unavoidable impurities. A gear was obtained formed from the above steel material and having a plurality of gear teeth and a plurality of grooves.

[0103] Next, the obtained gear was heated to 950°C using a heating furnace, and then oil-cooled to room temperature. The Vickers hardness of the obtained gear was 900 HV.

[0104] Next, the obtained gear was heated to 550°C using a heating furnace, and then air-cooled to room temperature. The Vickers hardness of the obtained gear was 500 HV.

[0105] The Vickers hardness of the gear was measured according to the Japanese Industrial Standard "JIS Z 2244:2009 Vickers hardness test - Test method". A micro-Vickers hardness tester was used to measure the Vickers hardness of the gear. The test force used in the Vickers hardness measurement was 0.3 kgf.

[0106] Next, laser light was irradiated onto multiple gear teeth and multiple grooves of the obtained gear. Using a single semiconductor laser device (LASERLINE LDFblue-4.0kW), laser light with a wavelength of 445 nm was irradiated perpendicularly to the surface of each of the multiple gear teeth and the surface of each of the multiple grooves of the gear. The output power of the semiconductor laser device was 320 W. Here, the shape and size of the laser spot were a 4 mm x 4 mm square. In this way, the gear according to Example 1 was manufactured.

[0107] Next, a photograph of the cross-section of the manufactured gear was taken using an industrial microscope (Olympus Digital Microscope DSX110). Figure 9 shows a cross-section of the gear according to Example 1. As shown in Figure 9, it was confirmed that the uneven portion of the manufactured gear, including the gear teeth 12 and grooves 13, includes a base 15 and a coating layer 16 covering the base 15.

[0108] As shown in Figure 9, it was confirmed that the maximum thickness of the coating layer 16 on the surface of the gear tooth 12 near the tooth tip is greater than the maximum thickness of the coating layer 16 on the surface of the groove 13 near the tooth root. As shown in Figure 9, it was confirmed that the minimum thickness of the coating layer 16 on the side surface connecting the gear tooth 12 and the groove 13 between the tooth and the tooth root is smaller than the maximum thickness of the coating layer 16 on the surface of the groove 13 near the tooth root.

[0109] Next, for each of the tooth tips, middle teeth, root teeth, and tooth roots shown in Figure 9, the Vickers hardness of the uneven surface was measured at a predetermined distance from the surface toward the inside of the uneven surface. The Vickers hardness of the uneven surface was measured according to the Japanese Industrial Standard "JIS Z 2244:2009 Vickers hardness test - Test method". A micro-Vickers hardness tester was used to measure the Vickers hardness of the uneven surface. The test force used in the measurement of Vickers hardness was 0.3 kgf.

[0110] Figure 10A shows the relationship between Vickers hardness and distance from the tooth tip surface in the gear according to Example 1. As shown in Figure 10A, the measured Vickers hardness of the uneven surface when the distance from the tooth tip surface was 1 mm or less was between 820 HV and 880 HV. On the other hand, the measured Vickers hardness of the uneven surface when the distance from the tooth tip surface was greater than 1 mm was between 474 HV and 513 HV. This confirmed that the layer at a distance of 1 mm or less from the tooth tip surface is a coating layer having a structure containing martensite. The layer at a distance greater than 1 mm from the tooth tip surface is confirmed to be a matrix having a structure containing tempered martensite.

[0111] Figure 10B shows the relationship between Vickers hardness and distance from the surface of the teeth in the gear according to Example 1. As shown in Figure 10B, the measured Vickers hardness of the uneven portion when the distance from the surface of the teeth was 0.6 mm or less was between 672 HV and 876 HV. On the other hand, the measured Vickers hardness of the uneven portion when the distance from the surface of the teeth was greater than 0.6 mm was between 479 HV and 527 HV. This confirmed that the layer at a distance of 0.6 mm or less from the surface of the teeth is a coating layer having a structure containing martensite. The layer at a distance greater than 0.6 mm from the surface of the teeth is confirmed to be a matrix having a structure containing tempered martensite.

[0112] Figure 10C shows the relationship between Vickers hardness and distance from the tooth root surface in the gear according to Example 1. As shown in Figure 10C, the measured Vickers hardness of the uneven portion when the distance from the tooth root surface was 0.4 mm or less was between 817 HV and 895 HV. On the other hand, the measured Vickers hardness of the uneven portion when the distance from the tooth root surface was greater than 0.4 mm was between 510 HV and 599 HV. This confirmed that the layer at a distance of 0.4 mm or less from the tooth root surface is a coating layer having a structure containing martensite. The layer at a distance greater than 0.4 mm from the tooth root surface is confirmed to be a matrix having a structure containing tempered martensite.

[0113] Figure 10D shows the relationship between Vickers hardness and distance from the tooth root surface in the gear according to Example 1. As shown in Figure 10D, the measured Vickers hardness of the uneven surface when the distance from the tooth root surface was 0.5 mm or less was between 862 V and 916 HV. On the other hand, the measured Vickers hardness of the uneven surface when the distance from the tooth root surface was greater than 0.5 mm was between 500 HV and 589 HV. This confirmed that the layer at a distance of 0.5 mm or less from the tooth root surface is a coating layer having a structure containing martensite. The layer at a distance greater than 0.5 mm from the tooth root surface is confirmed to be a matrix having a structure containing tempered martensite.

[0114] (Example 2) As a steel material, a steel was prepared containing, by mass%, C: 0.62%, Si: 0.99%, Mn: 0.39%, P: 0.011%, S: 0.005%, Cr: 1.98%, Al: 0.015%, V: 0.30%, Ni: 0.07%, and Mo: 0.30%, with a (C + V) amount of 0.92% by mass%, and the remainder being Fe and unavoidable impurities. A gear was obtained formed from the above steel material and having a plurality of gear teeth and a plurality of grooves.

[0115] Next, the obtained gear was heated to 930°C using a heating furnace, and then oil-cooled to room temperature. The Vickers hardness of the obtained gear was 900 HV.

[0116] Next, the obtained gear was heated to 650°C using a heating furnace, and then air-cooled to room temperature. The Vickers hardness of the obtained gear was 400 HV.

[0117] Next, laser light was irradiated onto multiple gear teeth and grooves of the obtained gear. Using a single semiconductor laser device (LASERLINE LDFblue-4.0kW), laser light with a wavelength of 445 nm was irradiated perpendicularly to the surfaces of each of the gear teeth and grooves of the gear. The output power of the semiconductor laser device was 320 W. Here, the shape and size of the laser spot were a 4 mm x 4 mm square. The Vickers hardness of the obtained gear was 700 HV near the surface of the uneven parts of the gear. In this way, the gear according to Example 2 was manufactured.

[0118] The Vickers hardness of the gear was measured according to the Japanese Industrial Standard "JIS Z 2244:2009 Vickers hardness test - Test method". A micro-Vickers hardness tester was used to measure the Vickers hardness of the gear. The test force used in the Vickers hardness measurement was 0.3 kgf.

[0119] (Comparative Example) As a steel material, a steel was prepared containing, by mass%, C: 0.62%, Si: 0.99%, Mn: 0.39%, P: 0.011%, S: 0.005%, Cr: 1.98%, Al: 0.015%, V: 0.30%, Ni: 0.07%, and Mo: 0.30%, with a (C + V) amount of 0.92% by mass%, and the remainder consisting of Fe and unavoidable impurities. A gear was obtained formed from the above steel material and having multiple gear teeth and multiple grooves.

[0120] Next, the obtained gear was heated to 950°C using a heating furnace, and then oil-cooled to room temperature. The Vickers hardness of the obtained gear was 900 HV.

[0121] Next, the obtained gear was heated to 160°C using a heating furnace, and then air-cooled to room temperature. The Vickers hardness of the obtained gear was 730 HV.

[0122] In this way, the gear relating to the comparative example was manufactured.

[0123] The Vickers hardness of the gear was measured according to the Japanese Industrial Standard "JIS Z 2244:2009 Vickers hardness test - Test method". A micro-Vickers hardness tester was used to measure the Vickers hardness of the gear. The test force used in the Vickers hardness measurement was 0.3 kgf.

[0124] Next, fatigue tests were conducted on the gear according to Example 1, the gear according to Example 2, and the gear according to the comparative example. In the fatigue tests, a sinusoidal load of 2 to 11 kN was repeatedly applied to the gear at room temperature and in air, and the number of load repetitions until the gear fractured was measured. The load ratio in the fatigue tests was 0.1. Here, the load ratio is the ratio of the minimum load applied to the gear to the maximum load applied to the gear.

[0125] Figure 11 shows the fatigue test results for the gear according to Example 1, the gear according to Example 2, and the gear according to the comparative example. In Figure 11, the vertical axis and horizontal axis represent load (kg) and number of cycles to break (N), respectively. ｆ (Number of cycles) is shown. Here, load refers to the weight of the load repeatedly applied to the gear. The number of cycles to break refers to the number of times the load is applied before the gear breaks. In Figure 11, the arrows indicate 1.00 × 10 ７ This means that the gear did not break when a load was applied.

[0126] As shown in Figure 11, it was confirmed that, under a common load, the number of cycles to break the gear in Example 1 and the number of cycles to break the gear in Example 2 were generally greater than the number of cycles to break the gear in the comparative example. In other words, it was confirmed that the durability of the gear in Example 1 and the gear in Example 2 is higher than the durability of the gear in the comparative example.

[0127] As shown in Figure 11, it was confirmed that the number of cycles to break the gear according to Example 2 was greater than the number of cycles to break the gear according to Example 1, under a common load. In other words, it was confirmed that the durability of the gear according to Example 2 is higher than that of the gear according to Example 1.

[0128] Although the embodiments of this disclosure have been described above, the technical scope of this disclosure is not limited to the embodiments described above, and various modifications are possible without departing from the gist of this disclosure. Furthermore, components from different embodiments and modifications may be combined as appropriate.

[0129] Furthermore, the effects described in each embodiment of this specification are merely illustrative and not limiting, and other effects may also occur.

[0130] Furthermore, this technology can be configured as follows: (1) A first heat treatment step of heating an object made of steel material and having an uneven surface to a first temperature; a second heat treatment step of heating the object to a second temperature lower than the first temperature after the first heat treatment step; and a laser irradiation step of irradiating at least a part of the uneven surface of the object with laser light after the second heat treatment step. A method for manufacturing steel products, wherein the steel material contains, by mass%, C: 0.40 to 1.00%, Si: 0.10 to 2.00%, Mn: 0.10 to 1.00%, P: 0.030% or less, S: 0.030% or less, Cr: 1.10 to 3.20%, Al: 0.010 to 0.10%, V: 0.15 to 0.50%, and further contains at least one of Ni: 2.50% or less and Mo: 1.00% or less, and the (C + V) amount is 0.60% or more by mass%, with the remainder being Fe and unavoidable impurities. (2) The method for manufacturing a steel product according to (1), wherein the first heat treatment step is to heat the object at the first temperature until the Vickers hardness of the object exceeds 650 HV and is 950 HV or less, and the second heat treatment step is to heat the object at the second temperature until the Vickers hardness of the object is 350 HV or more and 650 HV or less. (3) The method for manufacturing a steel product according to (2), wherein the second heat treatment step is to heat the object at the second temperature until the Vickers hardness of the object is 350 HV or more and 450 HV or less. (4) The method for manufacturing a steel product according to any one of (1) to (3), wherein the first temperature is 930°C or more and 950°C or less, and the second temperature is 550°C or more and 650°C or less. (5) The method for manufacturing a steel product according to any one of (1) to (4), wherein the laser irradiation step involves irradiating at least a portion of the uneven portion of the object with laser light until the Vickers hardness of at least a portion of the uneven portion of the object exceeds 650 HV and is 950 HV or less. (6) The method for manufacturing a steel product according to any one of (1) to (5), wherein the laser irradiation step involves irradiating the surface of a convex or concave portion of the uneven portion of the object with laser light perpendicular to the surface of the convex or concave portion.(7) The method for manufacturing a steel product according to any one of (1) to (6), wherein the laser irradiation step involves irradiating the upper surface of the protrusions and the bottom surface of the recesses in the uneven portion with laser light, and the side surface connecting the upper surface of the protrusions and the bottom surface of the recesses in the uneven portion with laser light. (8) The method for manufacturing a steel product according to any one of (1) to (6), wherein the laser irradiation step involves irradiating the bottom surface of the recesses in the uneven portion with laser light, and the portion of the side surface connecting the upper surface of the protrusions and the bottom surface of the recesses in the uneven portion that is connected to the bottom surface of the recesses. (9) The method for manufacturing a steel product according to any one of (1) to (8), wherein the object is a gear having gear teeth as the uneven portion. (10) The method for manufacturing a steel product according to (9), wherein the laser irradiation step involves irradiating the surface of the gear teeth from the tooth root to the tooth tip with laser light. (11) The method for manufacturing a steel product according to (9), wherein the laser irradiation step involves irradiating the surface of the gear teeth from the tooth root to the pitch point of the gear with laser light. (12) A steel product comprising an object formed of a steel material and having an uneven portion, wherein the steel material contains, by mass%, C: 0.40 to 1.00%, Si: 0.10 to 2.00%, Mn: 0.10 to 1.00%, P: 0.030% or less, S: 0.030% or less, Cr: 1.10 to 3.20%, Al: 0.010 to 0.10%, V: 0.15 to 0.50%, and further contains at least one of Ni: 2.50% or less and Mo: 1.00% or less, with a (C+V) amount of 0.60% or more by mass%, and the remainder being Fe and unavoidable impurities, wherein the uneven portion comprises a matrix and a coating layer covering at least a part of the matrix, the structure of the matrix comprises tempered martensite, and the structure of the coating layer comprises martensite. (13) The steel product according to (12), wherein the Vickers hardness of the base is 350 HV or more and 650 HV or less, and the Vickers hardness of the coating layer is greater than 650 HV and 950 HV or less. (14) The steel product according to (13), wherein the Vickers hardness of the base is 350 HV or more and 450 HV or less.(15) The steel product according to any one of (12) to (14), wherein the coating layer covers the entire base. (16) The steel product according to (15), wherein in the cross-section of the uneven portion, the maximum thickness of the coating layer on the surface of the protrusions in the uneven portion is greater than the maximum thickness of the coating layer on the surface of the recesses in the uneven portion, and the minimum thickness of the coating layer on the side surface connecting the upper surface of the protrusions and the bottom surface of the recesses in the uneven portion is less than the maximum thickness of the coating layer on the surface of the recesses in the uneven portion. (17) The steel product according to any one of (12) to (14), wherein the coating layer covers the base on the bottom surface of the recesses in the uneven portion, and on the portion of the side surface connecting the upper surface of the protrusions and the bottom surface of the recesses that connects the base to the bottom surface of the recesses. (18) The steel product according to any one of (12) to (17), wherein the object is a gear having gear teeth as the protrusions and recesses. (19) The steel product according to (18), wherein the coating layer covers the base over the entire area of ​​the gear teeth. (20) The steel product according to (18), wherein the coating layer covers the base in the area from the tooth root of the gear tooth to the pitch point of the gear. (21) An electric motor including the steel product according to any one of (18) to (20).

[0131] 10 Gear 10A Drive gear 10B Driven gear 11 Rough parts 12, 12A, 12B Gear teeth 12a Surface of gear teeth 13, 13A, 13B Groove 13a Surface of groove 14 Side surface 15 Base 16 Coating layer 20 Electric motor 21 Motor 22 Shaft 23 Sun gear 24 Planetary gear 25 Internal gear 26 Carrier OA Optical axis PP Pitch points T1, T2, T3 Thickness of coating layer LL Laser beam

Claims

1. A heat treatment step comprising: a first heat treatment step of heating an object made of steel material and having an uneven surface to a first temperature; a second heat treatment step of heating the object to a second temperature lower than the first temperature after the first heat treatment step; and a laser irradiation step of irradiating at least a portion of the uneven surface of the object with laser light after the second heat treatment step. A method for manufacturing steel products, wherein the steel material contains, by mass%, C: 0.40 to 1.00%, Si: 0.10 to 2.00%, Mn: 0.10 to 1.00%, P: 0.030% or less, S: 0.030% or less, Cr: 1.10 to 3.20%, Al: 0.010 to 0.10%, V: 0.15 to 0.50%, and further contains at least one of Ni: 2.50% or less and Mo: 1.00% or less, and the (C + V) amount is 0.60% or more by mass%, with the remainder being Fe and unavoidable impurities.

2. The method for manufacturing a steel product according to claim 1, wherein the first heat treatment step involves heating the object at the first temperature until the Vickers hardness of the object exceeds 650 HV and is 950 HV or less, and the second heat treatment step involves heating the object at the second temperature until the Vickers hardness of the object is 350 HV or more and 650 HV or less.

3. The method for manufacturing a steel product according to claim 2, wherein the second heat treatment step is to heat the object at the second temperature until the Vickers hardness of the object is 350 HV or more and 450 HV or less.

4. The method for manufacturing a steel product according to claim 1, wherein the first temperature is 930°C or higher and 950°C or lower, and the second temperature is 550°C or higher and 650°C or lower.

5. The method for manufacturing a steel product according to claim 1, wherein the laser irradiation step involves irradiating at least a portion of the uneven portion of the object with laser light until the Vickers hardness of at least a portion of the uneven portion of the object exceeds 650 HV and is 950 HV or less.

6. The method for manufacturing a steel product according to claim 1, wherein the laser irradiation step involves irradiating the surface of a protrusion or recess in the uneven portion of the object perpendicular to the surface of the protrusion or recess.

7. The method for manufacturing a steel product according to claim 1, wherein the laser irradiation step involves irradiating laser light onto the upper surface of the protrusions and the bottom surface of the recesses in the uneven portion, and onto the side surface connecting the upper surface of the protrusions and the bottom surface of the recesses in the uneven portion.

8. The method for manufacturing a steel product according to claim 1, wherein the laser irradiation step involves irradiating the bottom surface of a recess in the uneven portion with laser light, and the portion of the side surface connecting the top surface of a protrusion and the bottom surface of a recess in the uneven portion that is connected to the bottom surface of a recess.

9. The method for manufacturing a steel product according to claim 1, wherein the object is a gear having gear teeth as the protrusions and recesses.

10. The method for manufacturing a steel product according to claim 9, wherein the laser irradiation step involves irradiating the surface of the gear tooth from the tooth root to the tooth tip with laser light.

11. The method for manufacturing a steel product according to claim 9, wherein the laser irradiation step involves irradiating the surface of the gear tooth from the tooth root to the pitch point of the gear with laser light.

12. An object made of a steel material and having an uneven surface, wherein the steel material contains, by mass%, C: 0.40 to 1.00%, Si: 0.10 to 2.00%, Mn: 0.10 to 1.00%, P: 0.030% or less, S: 0.030% or less, Cr: 1.10 to 3.20%, Al: 0.010 to 0.10%, V: 0.15 to 0.50%, and further contains at least one of Ni: 2.50% or less and Mo: 1.00% or less, with a (C+V) amount of 0.60% or more by mass%, and the remainder being Fe and unavoidable impurities, the uneven surface includes a matrix and a coating layer covering at least a part of the matrix, the structure of the matrix includes tempered martensite, and the structure of the coating layer includes martensite. The coating layer is obtained by irradiating the uneven portion with laser light, in a steel product.

13. The steel product according to claim 12, wherein the Vickers hardness of the base is 350 HV or more and 650 HV or less, and the Vickers hardness of the coating layer is greater than 650 HV and 950 HV or less.

14. The steel product according to claim 13, wherein the Vickers hardness of the base is 350 HV or more and 450 HV or less.

15. The steel product according to claim 12, wherein the coating layer covers the entire base.

16. In the cross-section of the uneven portion, the maximum thickness of the coating layer on the surface of the protrusions in the uneven portion is greater than the maximum thickness of the coating layer on the surface of the recesses in the uneven portion, and the minimum thickness of the coating layer on the side surface connecting the upper surface of the protrusions and the bottom surface of the recesses in the uneven portion is less than the maximum thickness of the coating layer on the surface of the recesses in the uneven portion, as described in claim 15.

17. The steel product according to claim 12, wherein the coating layer covers the base on the bottom surface of the recesses in the uneven portion, and on the portion of the side surface connecting the top surface of the protrusions and the bottom surface of the recesses in the uneven portion that connects to the bottom surface of the recesses.

18. The steel product according to claim 12, wherein the object is a gear having gear teeth as the protrusions and recesses.

19. The steel product according to claim 18, wherein the coating layer covers the base over the entire area of ​​the gear teeth.

20. The steel product according to claim 18, wherein the coating layer covers the base in the region from the tooth root of the gear tooth to the pitch point of the gear.

21. An electric motor comprising the steel product described in claim 18.

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