Metal treatment – Stock – Ferrous
Reexamination Certificate
2002-02-20
2003-12-23
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S906000, C384S625000, C384S912000
Reexamination Certificate
active
06666931
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to rolling parts and power transmission parts formed of carbon steel produced through induction hardening.
2. Description of the Background Art
Conventionally, rolling bearings, a typical rolling part, have often been formed for example of SUJ2 or other similar high-carbon chromium bearing steel, SCM420 or other similar, case-hardened steel carburized, or the like. They are sufficiently reliable steels for bearings. However, they are expensive as they contain Cr, Mo and other expensive elements, and using such elements also results in consumption of rare resources and thus desirably should be avoided. In particular, when SCM420 and other similar case-hardened steels are thermally processed they are required to be heated for a long period of time, or carburized, and this also consumes a large amount of thermal energy.
In contrast, in recent years automobile leg bearings and constant velocity joints (CVJ), ball screws, and any other similar rolling parts also sliding as they roll are formed of S53C or other similar medium carbon steel, with the rolling portion alone induction-hardened. Medium carbon steel has an alloy element content smaller than the afore-mentioned bearing steel and case-hardened steel and it is inexpensive and has satisfactory workability. It is disadvantageous, however, as it is inferior in important characteristics, i.e., it has a shorter rolling life.
To overcome the above disadvantage, conventionally members have been increased in size to alleviate load or surface pressure and they have thus been used without problems. In the future, however, energy conservation and miniaturization will result in high surface pressure acting on such members and a longer rolling fatigue life is thus demanded. Furthermore, CVJs, ball screws and the like can have a rolling portion also sliding as they roll, and they are thus also required to have long life accordingly. Furthermore, miniaturization requires that members be reduced in thickness and that a raw material itself corresponding to a non-hardened portion also have enhanced fatigue strength.
Furthermore as a result of the miniaturization of the part a large amount of heat is emitted and confined and the entirety of the part is thus exposed to higher temperature than conventional.
Thus, energy conservation, miniaturization and the like result in a rolling portion being used under severer conditions and a material providing a long rolling life is thus demanded. As has been described previously, CVJs, ball screws and the like can have a portion sliding as it rolls. As such they are required to have a long life not only as a simply rolling part but a rolling part which also slides. Thus the “rolling life” as aforementioned refers not only to that of a rolling part that simply rolls but that of a rolling part also sliding as it rolls
In addition to the above demand, miniaturization and associated reduction of members in thickness entail acceptance of relatively large load. As such, a raw material itself of a non-hardened portion is also required to have larger fatigue strength. In order to in crease the life of the exact raw material without increasing the cost thereof it is effective to increase the contents, as represented in percentage, of C, Si, Mn or any other similar, inexpensive alloy element in conventionally used medium carbon steel. In other words, increasing these inexpensive alloy elements in amount enhances the strength of the raw material and hence the fatigue strength the exact raw material.
If the raw material is excessively hardened, however, it would be inferior in workability. The present invention is directed to a rolling part having a complex shape, for example thread cutting. As such, turnability, forgeability, pierceability and other similar working characteristics are also important. Thus, high-carbon steel and high-alloy steel, such as bearing steel, are unsuitable. Case-hardened steel that is carburized is also unsuitable for the above rolling parts as it needs to have a treaded portion protected against carbonization and its complex shape facilitates over-carburization and if boundary oxidization occurs under mill scale the steel can be impaired in strength. To enhance workability, the raw material can have its hardness adjusted for example by a quenching and tempering process after it is cast and molded. To reduce the cost of the raw material, however, desirably the quenching and tempering process is excluded and the raw material that is not quenched or tempered is processed.
SUMMARY OF THE INVENTION
There can be obtained a rolling part and power transmission part excluding any expensive elements as its constituents and formed of inexpensive elements C, Si and Mn optimized to allow the same to have characteristics equivalent to those of rolling parts using bearing steel, and a power transmission part using the rolling part. The above characteristics are as follows: An application miniaturized and thus incapable of accepting temperature elevation, requires a rolling life allowing for its high temperature use.
(a) Rolling portion corresponding to an induction-hardened portion:
(a1) rolling life thereof as it simply rolls
(a2) rolling life thereof as it slides while rolling
(b) Non-hardened portion
(b1) limit of typical fatigue characteristics, or rotating bending fatigue
(b2) workability
The present invention in one aspect provides a rolling part formed of steel containing 0.5 to 0.8% by weight of C, 0.5 to 1.2% by weight of Si and 0.3 to 1.3% by weight of Mn and having a surface hardness of no less than HRC 59.
The steel contains 0.5 to 0.8% by weight of C to ensure that induction-hardening provides a surface hardness of no less than a predetermined value. It contains C with a lower limit of 0.5% by weight to ensure a long rolling life with a large load imposed while Si, Mn and the like are contained, as predetermined. Carbon forms carbide and to obtain steady hardness larger carbon contents, as represented in percentage, are preferable. Too high carbon contents, as represented in percentage, however, impair cold-workability, and a soaking process for prevention of component segregation, spheroidization of carbide, and other similar, particular heat treatments are required, which is costly. To ensure good cold-workability and dispense with soaking, C has an upper limit set to be 0.8% by weight.
The steel contains 0.5 to 1.2% by weight of Si because Si is an element increasing a rolling life and it also prevents the steel from softening when it is exposed to high temperature, and it acts to delay microstructural change, cracking, and the like attributable to large load applied repetitively. Medium carbon steel containing 0.5 to 1.2% by weight of C, as provided in the present invention, and containing less than 0.5% by weight of Si, cannot exhibit its effect and provides a rolling life increasing as no less than 0.5% by weight of Si increases. More than 1.2% by weight of Si, however, significantly impairs cold-workability and hot-workability and increases production cost. Si thus has an upper limit set to be 1.2% by weight.
0.3% by weight of Mn contained in the steel improves the steel in hardenability and it dissolves into solution in the steel to enhance the steel in toughness and also increases retained austenite beneficial in increasing a rolling life. Mn, however, as well as Si, reinforces a raw material and if its content as represented in percentage is too high it impairs workability and machinability. Mn thus has an upper limit set to be 1.3% by weight.
Desirably Al is low in level to ensure a long rolling life, although it is not necessarily required to be particularly low if it has approximately a normal level, and so is P.
As the above alloy components integrally act, the steel can be produced in the same process line as conventional carbon steel and provide a material providing a long rolling life. Of the above alloy elements, C, Si and Mn contribute to providing inexpensive medium carbon steel having a
Fujii Yukio
Maeda Kikuo
McDermott & Will & Emery
NTN Corporation
LandOfFree
Rolling part and power transmission part does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Rolling part and power transmission part, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Rolling part and power transmission part will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3180992