Rolling bearing

Details Rolling bearing Rolling bearing

1999-12-13

2001-10-23

King, Roy (Department: 1742)

Metal treatment

Stock

Carburized or nitrided

C148S233000, C148S225000

Reexamination Certificate

active

06306227

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a rolling bearing, and more particularly, it relates to a large-sized rolling bearing such as a bearing, which an average diameter of a rolling element is at least 20 mm, for a rolling mill in the steel industry.
2. Description of the Prior Art
In general, the material for a large-sized thick bearing employed in the steel industry or the like is prepared by carburized material such as SNCM815 having a large content of alloying element, in order to ensure required case depth and toughness. With SNCM 815 having a large content of Ni and requiring a high material cost, however, surface hardness is hard to attain in carburizing and a long time is required for carburizing, to disadvantageously result in a high cost for the rolling bearing and dispersion of the rolling contact fatigue life.
If the content of the alloying element is small, on the other hand, hardenability is so lowered that required hardness distribution (case depth) or core hardness cannot be obtained although the material cost is reduced.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a rolling bearing capable of reducing the material cost and increasing the life or reducing the carburizing time by ensuring surface hardness and optimizing the case depth.
The inventors have made deep study to find out that the material cost can be reduced by reducing the content of Ni (nickel) and increasing the content of C (carbon) for the composition of SNCM 815 and a material for a rolling bearing having a long life and fracture strength comparable to that of the prior art can be obtained by controlling surface hardness and core hardness to proper values through proper carburizing. It has also been proved that reduction of toughness resulting from the increase of the C content can be suppressed mainly by low-order regulation of specific unavoidable impurities.
1) Accordingly, the inventive rolling bearing has a rolling bearing ring and a rolling element, and the diameter of the rolling element is at least 20 mm while at least one of the rolling bearing ring and the rolling element consists of carburized or carbonitrided steel containing at least 0.2 percent by weight (wt %) and not more than 0.35 wt % of C and at least 2.2 wt % and not more than 3.6 wt % of Ni.
In the inventive rolling bearing, the Ni content is reduced as compared with SNCM 815 while the C content is increased, whereby the material cost can be reduced and the case depth can be increased as compared with SNCM 815 through the same carburizing. If the inventive rolling bearing has the same case depth as SNCM 815, the carburization time can be reduced as compared with SNCM 815.
The C content is set in the range of at least 0.2 wt % and not more than 0.35 wt % since required core hardness for ensuring core strength cannot be ensured if the C content is less than 0.2 wt % while forgeability and machinability are deteriorated and core toughness specific to carburized steel cannot be ensured if the C content exceeds 0.35 wt %. If the Ni content is less than 2.2 wt %, core toughness as well as hardenability are reduced. If the content of high-priced Ni exceeds 3.6 wt %, the cost for the rolling bearing cannot be reduced while heat treatment such as process annealing is required. Therefore, the upper limit for the Ni content is 3.6 wt %.
2) The aforementioned rolling bearing preferably contains at least 0.7 wt % and not more than 0.9 wt % of Cr (chromium), at least 0.2 wt % and not more than 0.25 wt % of Mo (molybdenum) and not more than 0.015 wt % of P (phosphorus).
The content of Cr, which is a high-priced chemical component, is preferably increased for forming a carbide and attaining heat resistance. The effects of forming a carbide and attaining heat resistance are reduced if the Cr content is small, and hence the lower limit for the Cr content is 0.7 wt %. If the Cr content is excessive, however, the cost for the rolling bearing is increased. Therefore, the upper limit for the Cr content is 0.9 wt %.
Mo is a carbide forming element by coexisting with Cr, improving temper resistance and increasing the life of the rolling bearing. The life of the rolling bearing is reduced if the Mo content is too small, and hence the lower limit for the Mo content is 0.2 wt %. The cost for the rolling bearing is excessively increased if the Mo content is excessive, and hence the upper limit for the Mo content is 0.25 wt %.
In order to suppress increase of core hardness or reduction of core toughness resulting from the increase of the C content and the reduction of the Ni content, the content of P exerting bad influence on toughness is controlled to not more than 0.015 wt %.
3) In the aforementioned rolling bearing, the steel preferably contains at least 0.25 wt % and not more than 0.35 wt % of C and at least 2.2 wt % and not more than 2.9 wt % of Ni.
Thus, the material cost can be reduced by reducing the Ni content, while required core hardness for ensuring core strength can be readily ensured by increasing the C content.
4) In the aforementioned rolling bearing, the steel preferably contains at least 0.25 wt % and not more than 0.30 wt % of C, at least 2.2 wt % and not more than 2.9 wt % of Ni, at least 0.7 wt % and not more than 0.9 wt % of Cr, at least 0.2 wt % and not more than 0.25 wt % of Mo, at least 0.15 wt % and not more than 0.4 wt % of Si (silicon), at least 0.3 wt % and not more than 2.0 wt % of Mn (manganese), at least 0.001 wt % and not more than 0.01 wt % of Ti (titanium), at least 0.001 wt % and not more than 0.005 wt % of N (nitrogen), not more than 0.015 wt % of P, not more than 0.05 wt % of Cu (copper), not more than 0.01 wt % of Nb (niobium) and not more than 0.01 wt % of V (vanadium).
When high core toughness and high rolling contact fatigue life are necessary, the chemical composition must be limited mainly by small amount regulation of specific impurity elements in particular. In other words, reduction of the contents of unavoidable impurity elements compensates for reduction of toughness resulting from the increase of the C content. Further, the rolling contact fatigue life is improved due to reduction of the contents of the elements leading to fatigue failure. The reasons for limiting the chemical composition are as follows:
Si: 0.15 to 0.4 wt %.
Si is necessary as a deoxidizing element when ingoting the steel. Therefore, Si must be added by at least 0.15 wt %, while Si reduces toughness or prompts boundary oxidation to readily form an origin point of fracture if excessively added and hence the upper limit for the Si content is set to 0.4 wt %.
Mn: 0.3 to 2.0 wt %.
Mn is necessary as a deoxidizing and desulfurizing element when ingoting the steel. Further, Mn is necessary for providing prescribed core hardness to a carburized/hardened part and increasing an effective case depth as an element improving hardenability and increasing the strength of the core part. Therefore, Mn must be added by at least 0.3 wt %. If Mn is excessively added, however, hardenability is excessively increased to reduce toughness and deteriorate machinability and cold workability, and hence the upper limit for the Mn content is set to 2.0 wt %.
P: not more than 0.015 wt %, Cu: not more than 0.05 wt %, Nb: not more than 0.01 wt %, V: not more than 0.01 wt %.
P, Cu, Nb and V are harmful elements reducing toughness and causing fatigue failure when employed as the materials for the rolling element. In relation to the present invention, it has been recognized effective to regulate these elements to small amount in particular. Excellent toughness and rolling contact fatigue life can be attained by setting the upper limits for the contents of P, Cu, Nb and V to 0.015 wt %, 0.05 wt %, 0.01 wt % and 0.01 wt % respectively.
Ti: 0.001 to 0.01 wt %.
Ti is an element suppressing austenite grain growth in carburizing. Ti must be added by at least 0.001 wt % in order to attain this effect, while toughness and rolling contact fatigue life are reduced if Ti is excessively added. Thus, the

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