JP7780959B2 - steel parts - Google Patents

Description

The present invention relates to steel members, and in particular to steel members with high surface fatigue strength suitable for use in power transmission parts such as gears, continuously variable transmissions, and constant velocity joints in automobiles.

For example, high surface fatigue strength is required for steel parts such as power transmission parts like automatic transmission gears, continuously variable transmission sheaves, and constant velocity joints. These parts are generally made from case-hardened steel with a C 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 C content of around 0.8% on the surface of the part, thereby increasing the surface fatigue strength. Because hardness is prioritized over toughness for these parts, the tempering temperature is usually below 200°C.

To improve automobile fuel efficiency, there is a demand for smaller and lighter mechanical structural parts such as gears, which requires higher surface fatigue strength than conventional products. The sliding surfaces of mechanical structural parts such as gears can experience 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 when it is tempered, and therefore the surface fatigue strength decreases. It is known that there is a good correlation between surface fatigue strength and the hardness of steel members when 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, which improve temper softening resistance.

Patent Document 1 discloses a high-strength case-hardened steel that improves impact strength by adding B, disperses fine TiC in the steel by adding Ti, and further reduces the depth of the grain boundary oxide layer formed during carburization and provides excellent temper softening resistance by adding Si.

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."

Japanese Patent Application Laid-Open No. 2004-300550 Japanese Patent Application Laid-Open No. 2006-097096

However, no matter how much tempering softening resistance is increased, tempering at 300°C does not improve hardness. Since the hardness after carburizing, quenching, and tempering is typically 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 tempering at 300°C, it has been difficult to achieve a 300°C tempered hardness of 800 HV or more, even with cold working.

The present invention has been developed in light of the above circumstances, and aims to provide a steel member for use as a bearing, gear, etc., which has high surface fatigue strength and a 300°C tempered hardness of 800 HV or more in the part that comes into contact with other parts due to sliding, etc.

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

The present invention was made based on the above findings, and its gist is as follows:

[1] A steel member comprising, 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; characterized in that the C concentration on the surface of a portion that comes into contact with another part 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 the other part is 800 HV or higher.

The present invention makes it possible to obtain steel members with high surface fatigue strength, with a 300°C tempered hardness of 800 HV or more on the surface of parts that come into contact with other parts due to sliding, etc.

First, we will explain the chemical composition of the steel that constitutes the steel member of the present invention. Hereinafter, "%" in chemical composition refers to "mass %."

[C: 0.10-0.30%]
C is an important element that greatly affects the strength of steel members, 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 is set to 0.30% or less.

[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 more reliably exert this effect, the Si content is preferably 2.00% or more. If the Si content is too high, not only will the workability decrease, but the 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 more reliably suppress saturation of the effect, the Si content is preferably 2.50% or less.

[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.

[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. In order to more reliably improve the hardenability and temper softening resistance of steel, the Cr content is preferably 1.00% or more. If the content exceeds 4.00%, the workability of the steel decreases, so the Cr content is set to 4.00% or less. In order to more reliably suppress the decrease in workability, the Cr content is preferably 3.00% or less.

[Al: 0.005-0.100%]
Al has a deoxidizing effect and also prevents coarsening of austenite grains by combining with N to form AlN during heat treatment, thereby improving 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 is set to 0.100% or less.

[N:0.0010-0.0250%]
N combines with Al to form AlN, thereby preventing coarsening of austenite grains and increasing 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 is set to 0.0250% or less.

[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 for the P content to be as low as possible. Therefore, the P content is set to 0.030% or less.

[S: 0.005-0.025%]
S is contained in an amount of 0.005% or more to improve machinability. However, if the S content is too high, S that is not fixed by Mn is generated as FeS at grain boundaries, resulting in a decrease in hot ductility. Furthermore, the large amount of MnS generated reduces wear resistance and cold ductility. Therefore, the S content is set to 0.025% or less.

[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, since a content exceeding 1.00% reduces workability, the Mo content is set to 1.00% or less.

The balance of the above chemical components is iron (Fe) and impurities. Here, "impurities" refers to components that are mixed in from the ore or scrap used as raw materials for steel, or from the manufacturing process environment, etc., and are not intentionally included in the steel material.

Next, we will explain carburizing, quenching, tempering, and cold working.

The conditions for carburizing, quenching, and tempering must be selected so that the surface carbon concentration in areas that come into contact with other parts due to sliding, etc., 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 should preferably be 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.

Carburizing conditions that result in a surface C 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 carried out, for example, by oil quenching. Tempering can be carried out, for example, by holding the material at 160°C for 1 hour.

The surface carbon concentration 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, etc., and the cut surface is mirror-polished. Then, using an electron probe microanalyzer (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 is measured at 1,000 points at a depth of 50 μm from the surface, spaced at 1 μm intervals, and the average value is taken as the surface carbon concentration.

Shot peening is the preferred cold working method 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 higher, and more preferably 1000 HV or higher.

The order of tempering and cold working may be reversed.

Finally, we will explain how to measure 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 during this process, the decrease in hardness due to 300°C tempering is greatly suppressed. The steel member is then cut on a plane perpendicular to the surface of the part that will come into contact with other parts due to 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.

As a result, by carburizing, quenching, tempering, and cold working steel with the above-mentioned chemical composition, it is possible to obtain steel members with high surface fatigue strength. Surface fatigue strength can be evaluated by Vickers hardness after tempering at 300°C, and the steel members of the present invention have a high 300°C tempered hardness of 800 HV or more.

Next, we will specifically explain the steel member according to an embodiment of the present invention, showing examples and comparative examples. Note that the example shown below is merely an example of the steel member according to an embodiment of the present invention, and the steel member according to an embodiment of the present invention is not limited to the example shown below.

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 measuring 15 mm in diameter and 20 mm in height 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%.

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 was measured at 1,000 points at a depth of 50 μm from the end face at 1 μm intervals, and the average value was used as the actual surface carbon concentration. Furthermore, in accordance with the method specified in JIS Z 2244:2009, the Vickers hardness was measured at 5 points at a depth of 50 μm from the end face at 0.5 mm intervals under a load of 0.3 kgf, and the average value was used as the hardness before tempering at 300°C. The specimen was then tempered at 300°C for 1 hour, and the Vickers hardness was measured at 5 points 50 μm deep from the end face at 0.5 mm intervals under a load of 0.3 kgf in accordance with the method specified in JIS Z 2244:2009, with the average value being taken as the 300°C tempered hardness.

The measurement results for hardness after tempering at 300°C are shown in Table 2.

No. 1 to No. 6 in Table 2 are examples, and the others (No. 7 to No. 12) are comparative examples.

Comparative Examples No. 7 and No. 8 are examples in which the low Si content resulted in a significant decrease in hardness when tempered at 300°C, and sufficient hardness was not achieved after tempering at 300°C. Comparative Example No. 9 is an example in which the low C concentration on the surface resulted in sufficient hardness after tempering at 300°C. Comparative Example No. 10 is an example in which the excessively high C concentration on the surface resulted in sufficient hardness after tempering at 300°C. Comparative Examples No. 11 and No. 12 are examples in which the lack of shot peening resulted in low hardness before tempering at 300°C, and sufficient hardness was not achieved after tempering at 300°C.

On the other hand, Nos. 1 to 6, which correspond to examples of the present invention, had high hardness before tempering, and furthermore, the decrease in hardness upon tempering at 300°C was small. As a result, it was confirmed that they had high hardness upon tempering at 300°C, i.e., high surface fatigue strength.

While 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 with ordinary skill in the technical field to which the present invention pertains could conceive of various modifications or 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.

This invention makes it possible to obtain steel members with excellent surface fatigue strength, making them highly useful in industry.

Claims (3)

In mass%,
C: 0.10-0.30%,
Si: 1.60-3.00%,
Mn: 0.20-2.00%,
Cr: 1.00 to 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 to 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 %,
A steel member characterized in that, when the steel member is tempered at 300°C, the Vickers hardness of the portion that comes into contact with the other component is 800 HV or more. 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: The steel member according to claim 2, characterized in that the Cr content is 1.00 to 4.00 mass %.