Metal treatment – Stock – Ferrous
Reexamination Certificate
2000-05-17
2002-04-23
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C420S127000, C420S110000, C420S104000, C420S119000
Reexamination Certificate
active
06375763
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a pearlitic steel railroad rail used, for example, in a mine railroad, in particular having excellent wear resistance.
2. Description of Related Art
Since freight cars used in a mine railroad sometimes make a sharp turn carrying mainly heavy minerals, the wheel axle of these cars is overloaded compared with that of passenger cars, and therefore the railroad rail for this use is exposed to severe conditions. From this viewpoint, the rail used in a mine railroad is fabricated from so-called pearlitic steel consisting essentially of pearlitic structure which is desirable for wear resistance.
With a recent increase in loading weight in freight cars due to an improvement of railroading efficiency, further excellent wear resistance is requested for the rail in a mine railroad. To answer the request, the Japanese Unexamined Patent Publications No. 8-109439 and No 8-144016 propose a railroad rail prepared from hypereutectoid steel containing more than 0.85% of C in which the ratio of cementitic structure, an important factor giving excellent wear resistance to pearlitic steel, is increased. The Japanese Unexamined Patent Publications No 8-246100 and No 8-246101 disclose a railroad rail fabricated from hypereutectoid steel in which the pearlitic structure is hardened in the head part of rail by heat treatment.
However, in the railroad rails described in these Patent Publications, there sometimes appear cracks due to the thermal dilation and contraction caused by a change of atmospheric temperature, for example, over 80° C. in summer and below 30° C. in winter, because the ductility of the rail fabricated from hypereutectoid steel is very poor. Furthermore, if the rail is applied to a long rail with a length of 200~1500 m, of which the demand is increasing in recent time, cracks tend to generate in the welded part because of its large thermal contraction compared with the case of the rail having a conventional length.
SUMMARY OF THE INVENTION
An object of the present invention is consequently to provide a pearlitic steel railroad rail having not only excellent wear resistance but also sufficient ductility to prevent the cracks from generating due to a temperature difference.
The above object can be achieved by the pearlitic steel railroad rail which comprises 0.75 to 0.84% C, 0.10 to 1.0% Si, 0.4 to 2.5% Mn, 0.035% or less P, 0.035% or less S and 0.05 to 0.5% Nb, by weight.
REFERENCES:
patent: 4714500 (1987-12-01), Heller et al.
patent: 4933024 (1990-06-01), Fukuda et al.
patent: 8-109439 (1996-04-01), None
patent: 8-144016 (1996-06-01), None
patent: 8-246100 (1996-09-01), None
patent: 8-246101 (1996-09-01), None
Mitao Shinji
Yokoyama Hiroyasu
NKK Corporation
Yee Deborah
LandOfFree
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