Metal treatment – Stock – Ferrous
Patent
1995-08-15
1997-08-19
Yee, Deborah
Metal treatment
Stock
Ferrous
148584, 148902, 148334, C21D 800, C21D 904, C22C 3802
Patent
active
056584008
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to rails with high toughness of high-carbon pearlitic steels having high strength and wear resistance intended for railroad rails and industrial machines and their manufacturing processes.
DESCRIPTION OF THE PRIOR ART
Because of high strength and wear resistance, high-carbon steels with pearlitic structures are used in structural applications, for railroad rails required to withstand heavier axial loads due to increases in the weight of railroad cars and intended for faster transportation.
Many technologies for manufacturing high-performance rails have been known. Japanese Provisional Patent Publication No. 55-2768 (1980) discloses a process of manufacturing hard rails by cooling heated steel having a special composition that is liable to produce a pearlitic structure from above the Ac.sub.3 point to between 450.degree. and 600.degree. C., thereby producing a fine pearlitic structure through isothermal transformation. Japanese Provisional Patent Publication No. 58-221229 (1983) discloses a process of heat treatment for producing rails with improved wear resistance that produces fine pearlite by quenching a heated rail containing 0.65 to 0.85% carbon and 0.5 to 2.5% manganese, thereby producing fine pearlite in the rail or the head thereof. Japanese Provisional Patent Publication No. 59-133322 (1984) discloses a process of heat treatment for producing rails with a fine pearlitic structure having a hardness of Hv>350 and extending to a depth of approximately 10 mm from the surface of the rail head by immersing a rolled rail having a special composition that forms a stable pearlitic structure and heated to a temperature above the Ar.sub.3 point in a bath of molten salt of a certain Specific temperature.
Although pearlitic steel rails of desired strength and wear resistance can be readily produced by adding appropriate alloying elements, their toughness is much lower than that of steels consisting essentially of ferritic structures. In tests made on V notch Charpy test specimens No. 3 according to JIS at normal temperatures, for example, rails of eutectoid carbon steels with a pearlitic structure exhibit a toughness of approximately 10 to 20 J/cm.sup.2 and those of steels containing carbon above the eutectoid point exhibit a toughness of approximately 10 J/cm.sup.2. Tensile specimens No. 4 according to JIS exhibit an elongation of less than 10%. When steels having such low toughness are used in structural applications subject to repeated loading and vibration, fine initial defects and fatigue cracks can lead to brittle fractures at low stresses.
Generally, toughness of steel is improved by grain refinement of the metal structure or, more specifically, by refinement of austenite grains or transgranular transformation. Refinement of austenite grains is accomplished by application of low-temperature heating during or after rolling, or a combination of controlled rolling and heating treatment as disclosed in Japanese Provisional Patent Publication No. 63-277721 (1988). In the manufacture of rails, however, low-temperature heating during rolling, controlled rolling at low temperatures and heavy-draft rolling are not applicable because of formability limitations. Even today, therefore, toughness is improved by conventional heating treatment at low temperatures. Still, this process involves several problems, such as costliness and lower productivity, requiring prompt solutions to make itself as efficient as the latest technologies that provide greater energy and labor savings and higher productivity.
The object of this invention is to solve the problem described above. More specifically, the object of this invention is to provide rails with improved wear resistance, ductility and toughness and processes for manufacturing such rails by eliminating the problems in the conventional controlled rolling processes dependent upon low temperatures and heavy drafts, and applying a new controlled rolling process to control the grain size of the pearlite in eutectoid steels or
REFERENCES:
patent: 3726724 (1973-04-01), Davies et al.
patent: 4486248 (1984-12-01), Ackert et al.
patent: 4714500 (1987-12-01), Heller et al.
patent: 4767475 (1988-08-01), Fukuda et al.
Kuroki Toshiya
Uchino Kouichi
Ueda Masaharu
Nippon Steel Corporation
Yee Deborah
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