Metal treatment – Stock – Ferrous
Patent
1996-11-13
1998-11-03
Yee, Deborah
Metal treatment
Stock
Ferrous
148333, 148334, 148581, 148584, C21D 800, C21D 904, C22C 3802
Patent
active
058302867
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a steel rail having the improved wear resistance and internal fatigue breakage resistance required for heavy haul railways, and a method of producing the same.
BACKGROUND ART
Improvements in train speeds and loading have been made in the past as means for improving the efficiency of railway transportation. Such high efficiency of railway transportation means severe use of the rails, and further improvements in rail materials have been required. More concretely, the increase of wear is heavy in rails laid down in a curve zone of a heavy load railway, and the drop of service life of the rail has become remarkable. However, the service life of the rail has been drastically improved in recent years due to the improvements in heat-treating technologies for further strengthening the rails, and high strength rails using an eutectoid carbon steel and having a fine pearlite structure have been developed. For example, 1 heat-treated rails for heavy loads having a sorbite structure or a fine pearlite structure at the head portion thereof (Japanese Examined Patent Publication (Kokoku) No. 54-25490), 2 ( low alloy heat-treated rail improving not only the wear resistance but also the drop of hardness at a weld portion by the addition of alloys such as Cr, Nb, etc, (Japanese Examined Patent Publication (Kokoku) No. 59-19173), etc, have been developed.
The characterizing features of these rails are that they are high strength rails exhibiting a fine pearlite structure by a eutectoid carbon-containing steel, and are directed to improve the wear resistance.
To further accomplish higher railway transportation efficiency, however, a load of an axial direction of cargos has been strongly promoted in recent heavy load railways, and even when the rails described above are used, the wear resistance cannot be easily secured particularly in a sharply curved track, and the occurrence of the fatigue breakage inside the head portion of the rails might develop. With the background described above, rails having higher wear resistance and higher internal fatigue breakage resistance than the existing eutectoid carbon-containing high strength steel rail have been required.
DISCLOSURE OF THE INVENTION
To improve the wear resistance of the pearlite structure having the eutectoid carbon component used as the conventional rail steels and to further improve the internal fatigue breakage resistance of the rail head portion, possible means may be generally a method which improves the hardness of the pearlite structure and keeps this hardness inside the rail head portion, too.
However, the existing hardness has reached the upper limit in the high strength rails exhibiting the pearlite structure of the eutectoid carbon component. When a heat-treatment cooling rate and the addition amount of alloys are increased so as to improve the hardness and to keep the hardness inside the rail head portion, too, an abnormal hardened phase such as a martensite structure is formed in the pearlite structure, and ductility and fatigue breakage resistance of the rail are lowered.
Another means for solving the problems may be the utilization of a metallic structure having a higher wear resistance other than the pearlite structure, but no material which is more economical and has higher wear resistance than the fine pearlite structure has been found.
Therefore, inventing a rail steel which does not contain an abnormal hardened structure such as martensite, can improve the wear resistance while keeping the pearlite structure, and is effective for improving the internal fatigue breakage resistance of the rail head, and inventing a production method of such a rail steel, are the problems to be solved.
Under such circumstances, the inventors of the present invention have examined the wear mechanism of the pearlite structure, and have made the following observation. the rolling contact with a wheel, ferrite among lamellar ferrite and cementite constituting pearlite, which has a lower hardness, is squeezed out, and only cemen
REFERENCES:
patent: 5658400 (1997-08-01), Uchino et al.
Iron and Steel 65(4) (1979) Mashiro, Ueda, Kozo Fukuda, Hiroyuki Ichinose, p. S498.
Iron and Steel 66(4) (1980) Kozo Fukuda, Masahiro Ueda, Hiroyuiki Ichinose, p. S277.
Uchino Kouichi
Ueda Masaharu
Nippon Steel Corporation
Yee Deborah
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