Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
2000-04-19
2001-11-13
Wyszomierski, George (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S584000
Reexamination Certificate
active
06315844
ABSTRACT:
The invention relates to a process for manufacture of a high-strength track section of steel, in particular of a switch section such as a frog, tongue rail or wing rail. The invention further relates to a switch section made of steel.
Due to the increasing speed of trains the requirements placed on the track super-structure are also increasing. Here it is rails and switches in particular that must have a high resistance to wear, crushing and fatigue damage. In addition, resistance to fracture and suitability for welding must be assured. These requirements were the reason for the use of rails with minimum tensile strengths of 1100 N/mm
2
.
A track section and a process for manufacture thereof is known from EP 0 620 865 B1, which a vacuum-treated steel is used with 0.53 to 0.62% C, 0.1 to 0.25% Si, 0.65 to 1.1% Mn, 0.8 to 1.3% Cr, 0.05 to 0.11% Mo, 0.05 to 0.11% V, <0.02% P, optionally up to 0.025% Al, optionally up to 0.5% Nb, remainder iron and usual smelting-related impurities, with the ratio of Mn:Cr being around 0.80<Mn:Cr<0.85 and the ratio of Mo:V around 1, and with the track section in the form of a switch section being a rolled rail section as the starting material that has a martensitic structure at least in the rail head thanks to tempering and quenching. The result is strengths of over 1500 N/mm
2
in the rail head.
The literature source DE-Z: “Stahl und Eisen” magazine, 115 (1995), No. 2, pages 93-98, describes a process for manufacture of a natural-hardness bainitic rail with high tensile strength. A steel of as-rolled hardness is here subjected to a heat treatment such that firstly heating to 750° C. takes place, in order to then achieve a required bainitic structure by air cooling to below approx. 450° C. Then tempering over a period of one hour can take place at around 550° C. in order to then cool down the rail to room temperature. With the appropriate process, rail portions with a tensile strength of over 1400 N/mm
2
can be achieved.
The problem underlying the present invention is to develop a process for manufacture of a track section of the type mentioned at the outset or to develop a switch section itself such that a high tensile strength and yield strength are achieved and the useful life prolonged, so that in particular use in highly strained switches is possible. The problem is solved in accordance with the process in that steel with a chemical orientation analysis of 0.3 to 0.6% C, 0.8 to 1.5% Si, 0.7 to 1.0% Mn, 0.9 to 1.4% Cr, 0.6 to 1.0% Mo, remainder iron and usual smelting-related impurities, is after cooling from the rolling heat and formation of a bianitic basic structure with a tensile strength of approx. 1100 N/mm
2
, first subjected to a pretreatment stage in that the steel with the bainitic basic structure is heated to a temperature T
4
of 400° C.<T
4
<550° C. and then cooled in controlled form in such a way that the steel has after the pretreatment a tensile strength of at least 1200 N/mm
2
, then heated to a temperature T
1
with 750° C.<T
1
<920° C., then cooled in accelerated form to a temperature T
2
with 450° C.<T
2
<250° C. in a polymer/water mixture, in molten salt or in a powder, then heated up again to a temperature T
3
>T
2
with 400° C.<T
3
<560° C., kept at the temperature T
3
for a time t
1
of 60 mins.<t
1
<150 mins., and then cooled to room temperature.
Particularly good results can be achieved when the steel is, after reaching the temperature T
1
at its core, kept there over a period t
2
with 10 mins.<t
2
<30 mins., in particular t
2
in approx. 20 mins.
In particular, it is provided that the steel is, after a selective cooling from its rolling heat and formation of the bainitic basic structure with a tensile strength of approx. 1100 N/mm
2
, heated to a temperature T
1
of in particular around 860° C., in order to then be cooled in accelerated form to the temperature T
2
and subjected to a cooling medium such that the steel is cooled at its core to the temperature T
2
.
It is further provided that the steel with bianitic basic structure can be connected to standard rail steels such as 900 A and S 1100 by flash-butt welding, with the joined track sections then being able to be subjected to a joint heat treatment.
In accordance with the invention, a volume-tempering process of a steel with bainitic basic structure takes place, where selective heat treatment results without difficulty in tensile strengths of up to 1700 N/mm
2
, yield strengths (technical yield strengths with 0.2% permanent elongation) of up to 1400 N/mm
2
, breaking elongation A5 (%) of more than 10 and constrictions of more than 25%.
A switch section of steel, manufactured using previously described processes, is characterized in that the track section is made of steel with a chemical orientation analysis of 0.3 to 0.6% C, 0.8 to 1.5% Si, 0.7 to 1.0% Mn, 0.9 to 1.4% Cr, 0.6 to 1.0% Mo, remainder iron and usual smelting-related impurities, has a bianitic basic structure, a tensile strength of more than 1600 N/mm
2
, a technical yield strength of more than 1250 N/mm
2
, a breaking elongation>10% and a constriction>25%.
In particular, the steel has a chemical orientation analysis of 0.4 to 0.5% C, around 1% Si, around 0.8% Mn, around 1.0 Cr, and 0.6 to 1.0% Mo, the strength being up to 1700 N/mm
2
and the yield strength up to 1400 N/mm
2
. In this case, the steel is heat-treated such that the breaking elongation is >10% and the constriction >25%.
Further details, advantages and features of the invention are shown not only in the claims and in the features they contain—singly and/or in combination—but also in the following description of embodiments.
REFERENCES:
patent: 4933024 (1990-06-01), Fukuda et al.
patent: 5482576 (1996-01-01), Heller et al.
patent: 5879474 (1999-03-01), Bhadeshia et al.
patent: 1239110 (1965-10-01), None
Kunitz Walter
Ratz Gerhard
BWG Butzbacher Weichenbau Gesellschaft mbH & Co. KG
Combs-Morillo Janelle
Dennison, Scheiner Schultz & Wakeman
Wyszomierski George
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