Steel parts

Abstract

To provide a steel member that is used as a bearing, a gear or the like and has high surface fatigue strength.SOLUTION: A steel comprising, in mass%, C: 0.10-0.30%, Si: 1.60-3.00%, Mn: 0.20-2.00%, Cr: 0.10-4.00%, Al: 0.005-0.100%, N: 0.0010-0.0250%, P: 0.030% or less, S: 0.005-0.025%, and Mo: 0-1.00% is subjected to carburizing and cold working, providing a steel member that has a C level of 0.60-1.00 mass% at the surface of a portion to come into contact with other parts due to a slide or the like, and also has a Vickers hardness of 800 HV or more at the portion to come into contact with other parts when tempered at 300°C.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a steel member, and more particularly to a steel member having high surface fatigue strength and suitable for use in power transmission parts of automobiles, etc., such as gears, continuously variable transmissions, and constant velocity joints. [Background technology]

[0002] For example, high surface fatigue strength is required for steel parts such as power transmission components like gears in automatic transmissions, sheaves in continuously variable transmissions, and constant velocity joints. These parts are generally made from case-hardened steel with a carbon content of around 0.2%, such as JIS SCr420 or SCM420, which is carburized, quenched, and tempered to form a hardened layer of martensitic structure with a carbon content of around 0.8% on the surface of the part, thereby increasing surface fatigue strength. Because hardness is prioritized over toughness for these parts, the tempering temperature is usually below 200°C.

[0003] To improve automobile fuel efficiency, there is a demand for smaller and lighter mechanical structural parts such as gears, which require higher surface fatigue strength than conventional products. The sliding surfaces of mechanical structural parts such as gears can reach temperatures of around 300°C due to frictional heat. When the temperature of steel rises to around 300°C and then falls, the hardness of the steel decreases, similar to that of tempering, resulting in a decrease in surface fatigue strength. It is known that there is a good correlation between surface fatigue strength and the hardness of steel members tempered at 300°C (hereinafter referred to as "300°C tempered hardness"). Therefore, in order to achieve high 300°C tempered hardness, various steels and steel members have been proposed that contain alloying elements such as Si, Cr, and Mo to improve temper softening resistance.

[0004] Patent Document 1 discloses a high-strength case-hardened steel in which the impact strength is improved by adding B, fine TiC is dispersed in the steel by adding Ti, and the depth of the grain boundary oxide layer formed during carburization is shallow and temper softening resistance is excellent by adding Si.

[0005] Patent Document 2 discloses bearing steel parts characterized by a 300°C temper softening resistance of 130 HV or less, which is defined as "surface hardness after carburizing, quenching, or carbonitriding, quenching, and tempering treatment" minus "surface hardness after tempering treatment at 300°C." [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2004-300550 [Patent Document 2] Japanese Patent Application Laid-Open No. 2006-097096 Summary of the Invention [Problem to be solved by the invention]

[0007] However, no matter how much tempering softening resistance is improved, tempering at 300°C does not improve hardness. Since the hardness after carburizing, quenching, and tempering is usually around 700 to 800 HV, it has been difficult to achieve a 300°C tempered hardness of 800 HV or more. Furthermore, by performing cold working such as shot peening after carburizing, quenching, and tempering, it is possible to achieve a surface hardness of 1000 HV or more. However, because the hardness also decreases significantly when tempered at 300°C, it has been difficult to achieve a 300°C tempered hardness of 800 HV or more, even with cold working.

[0008] The present invention has been made in view of the above circumstances, and has an object to provide a steel member to be used as a bearing, gear, etc., which has high surface fatigue strength, in which the hardness of the part that comes into contact with other parts by sliding, etc., when tempered at 300°C is 800 HV or more. [Means for solving the problem]

[0009] The present inventors have conducted extensive research into methods for obtaining steel members with high surface fatigue strength, in other words, steel members with high hardness when tempered at 300°C. As a result, they have found that the hardness before tempering at 300°C can be improved by tempering and cold working the steel members after carburizing and quenching, and that the addition of a large amount of Si can significantly suppress the decrease in hardness during tempering at 300°C, thereby achieving a hardness of 800 HV or more when tempered at 300°C.

[0010] The present invention has been made based on the above findings, and the gist of the present invention is as follows.

[0011] [1] A steel member made of steel containing, by mass%, C: 0.10-0.30%, Si: 1.60-3.00%, Mn: 0.20-2.00%, Cr: 0.10-4.00%, Al: 0.005-0.100%, N: 0.0010-0.0250%, P: 0.03% or less, S: 0.005-0.025%, and Mo: 0-1.00%, with the balance being Fe and impurities, wherein the C concentration on the surface of a portion that comes into contact with other parts by sliding or the like is 0.6-1.0 mass%, and when the steel member is tempered at 300°C, the Vickers hardness of the portion that comes into contact with other parts is 800 HV or more. [Effects of the Invention]

[0012] According to the present invention, a steel member having high surface fatigue strength, in which the surface hardness after tempering at 300° C. is 800 HV or more, can be obtained in the portion that comes into contact with other parts due to sliding or the like. DETAILED DESCRIPTION OF THE INVENTION

[0013] First, the chemical composition of the steel constituting the steel member of the present invention will be described. Hereinafter, "%" in the chemical composition means "mass %."

[0014] [C: 0.10~0.30%] C is an important element that greatly affects the strength of steel parts, and 0.10% or more is necessary to ensure sufficient internal hardness after carburizing, quenching, and tempering. If the C content exceeds 0.30%, workability decreases, so the content should be 0.30% or less.

[0015] [Si: 1.60~3.00%] Si is a useful element that improves temper softening resistance and suppresses softening caused by temperature rise. In particular, adding a large amount of Si can significantly suppress the softening caused by tempering of the hardness increased by cold working after quenching. This effect is unique to Si and is not found in Cr or Mo, which also improve temper softening resistance. To achieve this effect, the Si content is set to 1.60% or more. To ensure that this effect is exerted, the Si content is preferably 2.00% or more. If the Si content is too high, not only will workability decrease, but the above effect will saturate and no effect commensurate with the content can be expected. Therefore, the Si content is set to 3.00% or less. To ensure that the above effect is not saturated, the Si content is preferably 2.50% or less.

[0016] [Mn: 0.20~2.00%] Mn is a useful element that improves the hardenability of steel while suppressing red shortness and improving hot ductility. To achieve this effect, the Mn content must be 0.20% or more. However, if the Mn content exceeds 2.00%, workability decreases, so the Mn content is set to 2.00% or less.

[0017] [Cr: 0.10~4.00%] Cr is a useful element for improving the hardenability and temper softening resistance of steel, and for this reason, 0.10% or more is contained. To more reliably improve the hardenability and temper softening resistance of steel, the Cr content is preferably 1.00% or more. If the Cr content exceeds 4.00%, the workability of the steel decreases, so the Cr content is set to 4.00% or less. To more reliably prevent the decrease in workability, the Cr content is preferably 3.00% or less.

[0018] [Al: 0.005 to 0.100%] Al has a deoxidizing effect and also prevents coarsening of austenite grains by combining with N to form AlN during heat treatment, thereby increasing toughness. To obtain this effect, the Al content must be 0.005% or more. If the Al content exceeds 0.100%, the cleanliness of the steel decreases and the above effect saturates, so the Al content should be 0.100% or less.

[0019] [N:0.0010~0.0250%] N combines with Al to form AlN, which prevents austenite grains from coarsening and increases toughness. To obtain this effect, the N content must be 0.0010% or more. If the N content exceeds 0.0250%, the above effect saturates, so the N content should be 0.0250% or less.

[0020] [P:0.030% or less] P is an element contained as an impurity. Since P segregates at grain boundaries and reduces grain boundary strength, it is better to keep the P content as low as possible. Therefore, the P content is set to 0.030% or less.

[0021] [S:0.005~0.025%] S is added at a content of 0.005% or more to improve machinability. However, if the S content is too high, S that is not fixed by Mn forms as FeS at grain boundaries, reducing hot ductility. Furthermore, the large amount of MnS formed reduces wear resistance and cold ductility. Therefore, the S content is set to 0.025% or less.

[0022] [Mo: 0-1.00%] Mo is a useful element that improves the hardenability and temper softening resistance of steel materials. Therefore, the steel according to this embodiment may contain a predetermined amount of Mo in place of part of the remaining Fe. When Mo is contained, the above-mentioned effects can be obtained even with a small content, but to ensure the effects, the content is preferably 0.1% or more. However, if the content exceeds 1.00%, workability decreases, so the Mo content is set to 1.00% or less.

[0023] The balance of the above chemical components is iron (Fe) and impurities. Here, the term "impurities" refers to components that are mixed in from ores or scraps used as raw materials for steel, or from the manufacturing process environment, etc., and are not intentionally contained in the steel material.

[0024] Next, carburizing and quenching, tempering treatment and cold working will be described.

[0025] The carburizing, quenching, and tempering conditions must be selected so that the surface carbon concentration in the area that comes into contact with other components through sliding or other processes is 0.60 to 1.00%. To achieve a 300°C tempered hardness of 800 HV or more, the carbon concentration must be 0.60% or more. To achieve an even higher 300°C tempered hardness, the carbon concentration is preferably 0.70% or more. Conversely, if the carbon concentration exceeds 1.00%, not only will the formation of lenticular martensite, which has poor toughness, be promoted, but a large amount of untransformed austenite will remain even after cold working, making it impossible to achieve a 300°C tempered hardness of 800 HV or more. Therefore, the carbon concentration must be 1.00% or less.

[0026] Carburizing conditions that result in a surface carbon concentration of 0.60 to 1.00% can be achieved, for example, by vacuum carburizing using acetylene gas at 1000°C with a carburizing period of 2 minutes and a diffusion period of 7 minutes. Quenching after vacuum carburizing can be performed, for example, by oil quenching. Tempering can be performed, for example, by holding at 160°C for 1 hour.

[0027] The carbon concentration on the surface is measured as follows: First, the steel member is cut on a plane perpendicular to the surface of the portion that comes into contact with other parts due to sliding or the like, and the cut surface is mirror-polished. Then, using an electron probe microanalyzer (EPMA) with an accelerating voltage of 15 kV, a probe current of 30 nA, and an electron beam diameter of 1 μm, the carbon concentration is measured at 1,000 points at a depth of 50 μm from the surface at 1 μm intervals, and the average value is taken as the surface carbon concentration.

[0028] Shot peening is suitable for cold working after carburizing, quenching, and tempering, but other methods such as roller burnishing and rolling can also be used. The surface hardness after cold working is preferably 900 HV or more, and more preferably 1000 HV or more.

[0029] The order of tempering and cold working may be reversed.

[0030] Finally, we will explain the method for measuring the 300°C tempered hardness. 300°C tempering involves holding the steel member at 300°C for one hour and then allowing it to cool. Because a large amount of Si is added, the decrease in hardness due to 300°C tempering is significantly suppressed. The steel member is then cut on a plane perpendicular to the surface of the part that comes into contact with other parts by sliding, etc., and the cut surface is polished. Furthermore, Vickers hardness is measured at a depth of 50 μm from the surface under a load of 0.3 kgf according to the method specified in JIS Z 2244:2009. Vickers hardness is measured at five points using the above procedure, and the average value is taken as the 300°C tempered hardness.

[0031] As described above, a steel member having high surface fatigue strength can be obtained by carburizing, quenching, tempering, and cold working a steel having the above-mentioned chemical composition. Surface fatigue strength can be evaluated by Vickers hardness after tempering at 300°C, and the steel member of the present invention has a high 300°C tempered hardness of 800 HV or more. [Example]

[0032] Next, the steel member according to the embodiment of the present invention will be specifically described with reference to examples and comparative examples. Note that the examples shown below are merely examples of the steel member according to the embodiment of the present invention, and the steel member according to the embodiment of the present invention is not limited to the examples shown below.

[0033] Steel having the chemical composition listed in Table 1 was vacuum melted and then cast using a mold to produce a 10 kg steel ingot. The resulting steel ingot was heated to 1200°C and held there for 1 hour, after which it was hot forged into a round bar with an outer diameter of 30 mm. Round bar test specimens with a diameter of 15 mm and height of 20 mm were machined from these round bars. They were then vacuum carburized under various conditions to achieve the target surface carbon concentration shown in Table 2, oil quenched at 120°C, and tempered at 160°C for 1 hour. Furthermore, with the exception of some test specimens, both end surfaces were cold worked by shot peening. Shot peening was performed using a 0.8 mm diameter, 700 HV steel ball at a projection pressure of 0.4 MPa and a coverage of 300%.

[0034] Next, the round bar test specimens were cut along a plane passing through the central axis, and the cut surfaces were mirror-polished. Using an EPMA with an acceleration voltage of 15 kV, a probe current of 30 nA, and an electron beam diameter of 1 μm, the carbon concentration at a depth of 50 μm from the end face was measured at 1,000 points at 1 μm intervals, and the average value was taken as the actual surface carbon concentration. Furthermore, in accordance with the method specified in JIS Z 2244:2009, the Vickers hardness at a depth of 50 μm from the end face was measured at five points at 0.5 mm intervals with a load of 0.3 kgf, and the average value was taken as the hardness before tempering at 300°C. The specimens were then tempered at 300°C for 1 hour, and the Vickers hardness at a depth of 50 μm from the end face was measured at five points at 0.5 mm intervals with a load of 0.3 kgf, in accordance with the method specified in JIS Z 2244:2009, and the average value was taken as the hardness after tempering at 300°C.

[0035] [Table 1]

[0036] Table 2 shows the measurement results of the hardness after tempering at 300°C.

[0037] [Table 2]

[0038] Nos. 1 to 6 in Table 2 are examples, and the others (Nos. 7 to 12) are comparative examples.

[0039] Comparative Examples No. 7 and No. 8 are examples in which the Si content was low, resulting in a large decrease in hardness when tempered at 300°C, and sufficient hardness was not obtained after tempering at 300°C. Comparative Example No. 9 is an example in which the C concentration in the surface was low, resulting in sufficient hardness after tempering at 300°C. Comparative Example No. 10 is an example in which the C concentration in the surface was too high, resulting in sufficient hardness after tempering at 300°C. Comparative Examples No. 11 and No. 12 are examples in which the hardness before tempering at 300°C was low because shot peening was not performed, and sufficient hardness was not obtained after tempering at 300°C.

[0040] On the other hand, for No. 1 to No. 6, which correspond to examples of the present invention, the hardness before tempering was high, and furthermore, the decrease in hardness when tempered at 300°C was small. As a result, it was confirmed that the hardness when tempered at 300°C was high, i.e., the surface fatigue strength was high.

[0041] Although the preferred embodiments of the present invention have been described in detail above, the present invention is not limited to these examples. It is clear that a person skilled in the art to which the present invention pertains can conceive of various modifications and alterations within the scope of the technical ideas set forth in the claims, and it is understood that these also naturally fall within the technical scope of the present invention. [Industrial Applicability]

[0042] According to the present invention, a steel member having excellent surface fatigue strength can be obtained, and is of great industrial utility.

Claims

1. In mass%, C: 0.10-0.30%, Si: 1.60-3.00%, Mn: 0.20-2.00%, Cr: 1.00-4.00%, Al: 0.005-0.100%, N: 0.0010-0.0250%, P: 0.030% or less, S: 0.005 to 0.025%, and Mo: 0-1.00% A steel member made of steel containing a balance of Fe and impurities, The carbon concentration on the surface of the portion that comes into contact with other components due to sliding or the like is 0.60 to 1.00 mass %, When the steel member is tempered at 300°C, the Vickers hardness of the portion that comes into contact with the other part is 800 HV or more. A steel member characterized by:

2. In mass %, C: 0.10-0.30%, Si: 1.60-3.00%, Mn: 0.20-2.00%, Cr: 0.10-4.00%, Al: 0.005-0.100%, N: 0.0010-0.0250%, P: 0.030% or less, S: 0.005 to 0.025%, and Mo: 0-1.00% A steel member made of steel containing a balance of Fe and impurities, The carbon concentration on the surface of the portion that comes into contact with other components due to sliding or the like is 0.60 to 1.00 mass %, When the steel member is tempered at 300°C, the Vickers hardness of the portion that comes into contact with the other part is 842 HV or more. A steel member characterized by:

3. A steel member as described in claim 2, characterized in that the Cr content is 1.00 to 4.00 mass%.

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