TRACK PART AND METHOD FOR PRODUCING A TRACK PART

Abstract

In a track part, in particular a low-alloy steel rail for rail vehicles, the steel comprises, in the rail head of the track part, a ferrite portion of 5-15 vol %, an austenite portion of 5-20 vol %, a martensite portion of 5-20 vol %, and a portion of carbide-free bainite of 55-75 vol %.

Claims

1. A track part, in particular a low-alloy steel rail for rail vehicles, characterized in that the steel comprises, in the rail head of the track part, a ferrite portion of 5-15 vol %, an austenite portion of 5-20 vol %, a martensite portion of 5-20 vol %, and a portion of carbide-free bainite of 55-75 vol %.

2. The track part according to claim 1, characterized in that the portion of the carbide-free bainite is 60-70 vol %.

3. The track part according to claim 1, characterized in that the ferrite portion is 8-13 vol %.

4. The track part according to claim 1, characterized in that the bainite forms a matrix in which austenite, martensite and ferrite are preferably homogenously distributed.

5. The track part according to claim 1, characterized in that the austenite portion and the martensite portion are at least partially present in island form.

6. The track part according to claim 1, characterized in that the low-alloy steel comprises as alloying components carbon, silicon, manganese, chromium, molybdenum and optionally vanadium, phosphorus, sulfur, boron, titanium, aluminum and/or nitrogen, and the balance iron.

7. The track part according to claim 6, characterized in that no alloying component is present in an amount larger than 1.8 wt %.

8. The track part according to claim 6, characterized in that silicon is present in an amount smaller than 1.2 wt %.

9. The track part according to claim 6, characterized in that carbon is present in an amount smaller than 0.6 wt %, preferably smaller than 0.35 wt %.

10. The track part according to claim 6, characterized in that a low-alloy steel having the following reference analysis is used: 0.2-0.6 wt-% C 0.9-1.2 wt-% Si 1.2-1.8 wt-% Mn 0.15-0.8 wt-% Cr 0.01-0.15 wt-% Mo, and optionally 0-0.25 wt-% V, in particular 0.01-0.25 wt-% V 0-0.016 wt-% P, in particular 0.01-0.016 wt-% P 0-0.016 wt-% S, in particular 0.01-0.016 wt-% S balance: iron

11. The track part according to claim 6, characterized in that a low-alloy steel having the following reference analysis is used: 0.28-0.32 wt-% C 0.98-1.03 wt-% Si 1.7-1.8 wt-% Mn 0.28-0.32 wt-% Cr 0.08-0.13 wt-% Mo, and optionally 0-0.25 wt-% V, in particular 0.01-0.25 wt-% V 0-0.016 wt-% P, in particular 0.01-0.016 wt-% P 0-0.016 wt-% S, in particular 0.01-0.016 wt-% S balance: iron

12. The track part according to claim 6, characterized in that a low-alloy steel having the following reference analysis is used: 0.44-0.52 wt-% C 1.05-1.17 wt-% Si 1.4-1.7 wt-% Mn 0.36-0.80 wt-% Cr 0.01-0.08 wt-% Mo, and 0-0.25 wt-% V, in particular 0.01-0.25 wt-% V 0-0.016 wt-% P, in particular 0.01-0.016 wt-% P 0-0.016 wt-% S, in particular 0.01-0.016 wt-% S balance: iron

13. The track part according to claim 1, characterized in that the track part has a tensile strength R.sub.m of 1050-1400 N/mm.sup.2 in the head region.

14. The track part according to claim 1, characterized in that the track part has a hardness of 320-400 HB in the head region.

15. A method for producing a track part according to claim 1 from a hot-rolled section, characterized in that the rail head of the rolled section, immediately after having left the rolling stand, is subjected at rolling heat to controlled cooling, said controlled cooling comprising in a first step cooling at ambient air until reaching a first temperature of 780-830 C., in a second step accelerated cooling to a second temperature of 450-520 C., in a third step holding the second temperature, in a fourth step further accelerated cooling until reaching a third temperature of 420-470 C., in a fifth step holding the third temperature, and in a sixth step cooling to room temperature at ambient air.

16. The method according to claim 15, characterized in that said accelerated cooling in the second step is performed at a cooling rate of 2-5 C./s.

17. The method according to claim 15, characterized in that the third step extends over a period of 10-300 s.

18. The method according to claim 15, characterized in that said accelerated cooling in the fourth step is performed at a cooling rate of 2-5 C./s.

19. The method according to claim 15, characterized in that the fifth step extends over a period of 50-600 s, preferably 100-270 s.

20. The method according to claim 15, characterized in that reheating takes place during the third and/or the fifth steps.

21. The method according to claim 15, characterized in that the temperature is detected at a plurality of measuring points distributed over the length of the track part and a mean value of the temperature is formed, which is used for controlling said controlled cooling.

22. The method according to claim 15, characterized in that said controlled cooling is performed by immersing at least the rail head into a liquid coolant.

23. The method according to claim 15, characterized in that cooling during the second or fourth step is controlled such that the coolant initially forms a vapour film on the surface of the rail head and subsequently boils on the surface.

24. The method according to claim 23, characterized in that during the second and/or fourth step a film-breaking, gaseous pressure medium such as nitrogen is supplied to the rail head along the entire length of the track part to break the vapor film along the entire length of the track part and initiate the boiling phase.

25. The method according to claim 24, characterized in that the condition of the coolant is monitored during the second and/or fourth steps along the entire length of the track part and the film-breaking, gaseous pressure medium is supplied to the rail head as soon as the first occurrence of the boiling phase has been detected in a partial region of the track part length.

26. The method according to claim 24, characterized in that the film-breaking, gaseous pressure medium is supplied to the rail head about 20-100 s after the beginning of the second and/or fourth steps.

27. The method according to claim 15, characterized in that the track part is completely immersed into the coolant during the second step.

28. The method according to claim 15, characterized in that the track part is held in a position removed from the coolant during the third and/or fifth steps.

29. The method according to claim 15, characterized in that, during the fourth step, the track part is immersed into the coolant only with the rail head.

Description

EXAMPLE 1

[0074] In a first exemplary embodiment, a low-alloy steel having the following reference analysis was formed by hot-rolling to a running rail with a standard rail section: [0075] 0.3 wt % C [0076] 1.0 wt % Si [0077] 1.74 wt % Mn [0078] 0.31 wt % Cr [0079] 0.1 wt % Mo [0080] 0.014 wt % S [0081] 0.014 wt % P [0082] 20 ppm Al [0083] 70 ppm N

[0084] Boron and titanium were not alloyed. Balance: iron and inadvertent accompanying elements.

[0085] Immediately upon leaving the rolling stand, the rail was subjected at rolling heat to controlled cooling. Said controlled cooling is explained in more detail below with reference to the time-temperature transformation diagram depicted in FIG. 1, wherein the line denoted by 1 represents the cooling course. In a first step, the rail is cooled to a temperature of 810 C. at ambient air. In a second step, the rail is immersed into the liquid coolant over its entire length and by its entire cross section, and a cooling rate of 4 C./s was adjusted. After about 85 s, the rail was removed from the cooling bath, and an initial surface temperature of the rail head of 470 C. was measured, point 2 having been reached. During a period of about 45 s, the rail was held in a position removed from the coolant. Reheating to a temperature of 500 C. occurred within the first 5 seconds. When reaching point 3, the rail was again immersed into the cooling bath and cooled to 440 C. (point 4) at a cooling rate of 4 C./s. This temperature was held for 100 seconds. When reaching point 5, the rail was cooled to room temperature at ambient air.

[0086] The above-described controlled cooling resulted in a rail head having the following microstructure: [0087] 60-70 vol % carbide-free bainite, [0088] 8-13 vol % ferrite, [0089] 11-18 vol % austenite, [0090] 5-15 vol % martensite.

[0091] The microstructure is illustrated in FIG. 2. The following material properties were measured:

0.2% yield stress: 750 MPa10 MPa
Tensile strength: 1130 MPa10 MPa
Ultimate elongation: 17%1%
Surface hardness: 330 HB5 HB
Fracture toughness K.sub.Ic on standard sample at room temperature: 58 MPam3 MPam

EXAMPLE 2

[0092] In a second exemplary embodiment, a low-alloy steel having the following reference analysis was formed by hot-rolling to a running rail with a standard rail section: [0093] 0.5 wt % C [0094] 1.1 wt % Si [0095] 1.5 wt % Mn [0096] 0.7 wt % Cr [0097] 0.01 wt % Mo [0098] 0.20 wt % V [0099] 0.014 wt % S [0100] 0.014 wt % P [0101] 20 ppm Al [0102] 70 ppm N

[0103] Balance: Fe and inadvertent accompanying elements.

[0104] The heat treatment was performed as in Example 1.

[0105] In order to raise the wear resistance relative to that of Example 1 (0.3 wt % C), yet, at the same time, maintain the break resistance, a material having a significantly higher carbon content (0.5 wt %) was used in Example 2.

[0106] The advantage of a higher carbon content resides in enabling an enhanced enrichment both in the austenite and in the martensite, thus strengthening these two microstructural components, which has a very positive effect on the wear resistance. The heat treatment (accelerated cooling), due to the higher carbon content, reduces the increased inclination to perlite formationi.e. the region where perlite formation takes place is passed through very quickly such that no significant amounts of perlite can precipitate on the rail head surface (as far as to a depth of 10 mm). This means that the microstructure continues to comprise the previously indicated microstructural components.

[0107] The following material properties were measured:

0.2% yield stress: 900 MPa10 MPa
Tensile strength: 1320 MPa10 MPa
Ultimate elongation: 13%1%
Surface hardness: 380 HB5 HB
Fracture toughness K.sub.Ic on standard sample at room temperature: 53 MPam3 MPam