Metal treatment – Stock – Ferrous
Patent
1996-07-15
1998-06-09
Yee, Deborah
Metal treatment
Stock
Ferrous
148581, C11D 800, C11D 904
Patent
active
057627231
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a pearlitic steel rail which improves the wear resistance and breakage resistance that are required for rails at curved zones of heavy load railways, and drastically improves the service life of the rails, and a method of producing such rails.
BACKGROUND ART
Attempts have been made to improve a train speed and loading as one of the means for accomplishing higher efficiency of railway transportation. Such an improvement in efficiency of railway transportation means severe use of the rails, and a further improvement in the rail materials has become necessary. More concretely, wear drastically increases in the rails laid down in a curved zone of a heavy load railway and produces a problem from the aspect of longer service life of the rails.
However, high strength (high hardness) rails using eutectoid carbon steels and exhibiting a fine pearlite structure have been developed due to the recent improvements in high-strength rail heat-treatment technology as described below, and rail life in the curved zones in the heavy load railway has been remarkably improved.
(1) Heat-treated rails for ultra-heavy load having a sorbite structure of a fine pearlite structure at the head portion thereof (Japanese Examined Patent Publication (Kokoku) No. 54-25490);
(2) Production method for low alloy heat-treated rails which improves not only the wear resistance but also the drop of hardness at a weld portion by adding an alloy such as Cr, Nb, etc. (Japanese Examined Patent Publication (Kokoku) No. 59-19173); and
(3) Production method for a high strength rail of at least 130 kgf/mm.sup.2 produced by conducting accelerated cooling between 850.degree. C. to 500.degree. C. at a rate of 1.degree. to 4.degree. C./sec after rolling is completed or from a re-heated austenite zone temperature.
The characterizing feature of these rails is that they are high strength (high hardness) rails exhibiting the fine pearlite structure of eutectoid carbon-containing steel, and the rails are directed to improve the wear resistance.
In recent heavy load railways, however, an improvement in an axial load of cargos (the increase of train loading) has been strongly promoted so as to further improve railway transportation efficiency. In the case where the rails are sharply curved, the wear resistance cannot be secured even when the rails developed as described above are used, and the drop of rail life due to the wear has become a serious problem. With such a background, the development of rails having a higher wear resistance than that of the existing eutectoid carbon steels has been required.
The contact state between the wheel and the rail is complicated. Particularly, the contact state of the wheels is very different at the inner track rail compared to the outer track rail of the curved zone. On the outer track rail of the sharply curved zone of the heavy load railway, for example, the wheel flange is strongly pushed to the gage corner portion by the centrifugal force and receives sliding contact. On the head top portion of the inner track rail of the curved zone, on the other hand, the rail receives great slipping contact having large contact surface pressure from the wheel. As a result, in the case of the high strength wear-resistant rails according to the prior art wherein the head surface hardness is uniform inside the cross-section of the rail head portion, wear is promoted far more at the gage corner portion which receives the sliding contact of the outer track rail than the head top portion which receives the slipping contact of the inner track rail. On the other hand, the progress of the wear is always slower at the head top portion of the inner track rail than at the gage corner portion, and the contact surface pressure from the wheel is always maximal. Therefore, fatigue damage builds up on the head top surface before it is worn out.
The contact state with the wheels tends to the state described above in the high strength wear-resistant rails having uniform wear characteristics at the rail head
REFERENCES:
patent: 4886558 (1989-12-01), Teramoto et al.
Babazono Koji
Kageyama Hideaki
Kutaragi Ken
Uchino Kouichi
Ueda Masaharu
Nippon Steel Corporation
Yee Deborah
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