Method for producing bainitic rail steels, track element and installation for carrying out the method

Abstract

A track part, in particular a rail for rail vehicles, made from low-alloy steel, includes a rail head. The rail head in the track part includes a ferrite portion of 5-15% by volume and a multiphase bainite structure consisting of upper and lower bainite portions. A method for producing bainitic rail steels, track elements, such as the track part, and installation for carrying out the method(s) are also provided.

1 .- 34 . (canceled) 
     
     
         35 . A track part, in particular a rail for rail vehicles, made from low-alloy steel, wherein the steel in the rail head of the track part comprises a ferrite portion of 5-15% by volume and a multiphase bainite structure consisting of upper and lower bainite portions. 
     
     
         36 . A track part according to  claim 35 , wherein the portion of upper bainite is 5-75% by volume and the portion of lower bainite is 15-90% by volume. 
     
     
         37 . A track part according to  claim 35 , wherein the ferrite portion is 8-13% by volume. 
     
     
         38 . A track part according to  claim 35 , wherein the ferrite is acicular ferrite. 
     
     
         39 . A track part according to  claim 38 , wherein the multi-phase bainite is intercalated in the acicular ferrite structure. 
     
     
         40 . A track part according to  claim 35 , wherein the steel in the rail head of the track part comprises a residual martensite/austenite portion of <2% by volume. 
     
     
         41 . A track part according to  claim 35 , wherein the low-alloy steel comprises silicon, manganese and chromium as well as, optionally, at least one of vanadium, molybdenum, phosphorus, sulfur and nickel as an alloying component. 
     
     
         42 . A track part according to  claim 41 , wherein none of the alloying components is present in a portion higher than 1.5% by weight. 
     
     
         43 . A track part according to  claim 35 , wherein the low-alloy steel has the following reference analysis:
 0.4-0.55% by weight C   0.3-0.6% by weight Si   0.9-1.4% by weight Mn   0.3-0.6% by weight Cr   0.1-0.3% by weight V   0.05-0.20% by weight Mo   0-0.02% by weight P   0-0.02% by weight S   0-0.15% by weight Ni.   
     
     
         44 . A track part according to  claim 35 , wherein the track part has a tensile strength R m  higher than 1150 N/mm 2  in the head region. 
     
     
         45 . A track part according to  claim 35 , wherein the track part has a hardness of above 340 HB in the head region. 
     
     
         46 . A method for producing a track part according to  claim 35 , the method including hot rolling the low-alloy steel in a roll stand to form a rolled track part, and subjecting the rail head of the rolled track part to controlled cooling immediately upon leaving the roll stand at rolling heat, said controlled cooling comprising,
 in a first step, accelerated cooling at a cooling rate of 2-5° C./s, starting at a temperature of 740-850° C. until reaching a first temperature of 450-525° C. thereby allowing the formation of ferrite,   in a second step maintaining said first temperature to effect the formation of ferrite,   in a third step further cooling within a temperature range allowing the formation of multiphase bainite until a second temperature of 280-350° C., wherein the cooling extends for a period of 50-100 s, and   in a fourth step maintaining said second temperature.   
     
     
         47 . A method according to  claim 46 , wherein said method further includes detecting the temperature on a plurality of measuring points distributed over the length of the track part and forming a temperature mean value, and using said temperature mean value in controlling said controlled cooling. 
     
     
         48 . A method according to  claim 46 , wherein said controlled cooling includes immersing at least the rail head into a liquid coolant. 
     
     
         49 . A method according to  claim 48 , wherein the cooling during the third step is controlled so as to cause the coolant to initially form a vapor film on the surface of the rail head and then boil on said surface. 
     
     
         50 . A method according to  claim 49 , wherein during the third step, a film-breaking, gaseous pressure medium is supplied to the rail head along the entire length of the track part in order to break the vapor film along the entire length of the track part and trigger the boiling phase. 
     
     
         51 . A method according to  claim 50 , wherein the state of the coolant during the third step is monitored along the entire length of the track part and the film-breaking, gaseous pressure medium is supplied to the rail head upon detecting the first appearance of the boiling phase in a portion of the track part length. 
     
     
         52 . A method according to  claim 50 , wherein the film-breaking, gaseous pressure medium is supplied to the rail head about 20-100 after the beginning of the third step. 
     
     
         53 . A method according to  claim 46 , wherein the track part is completely immersed in the coolant during the first step. 
     
     
         54 . A method according to  claim 46 , wherein the track part is held in a position removed from the coolant during the second step. 
     
     
         55 . A method according to  claim 46 , wherein the track part is immersed in the coolant merely by the rail head during the third step. 
     
     
         56 . A method according to  claim 46 , wherein the track part is cyclically immersed into the coolant and removed from the coolant during the fourth step. 
     
     
         57 . A device for carrying out the method according to  claim 46 , wherein comprising a cooling tank corresponding to the length of the track part and capable of being filled with coolant, a lifting and lowering device for the track part to immerse the track part into the cooling tank and lift it out of the same, a temperature-measuring device for measuring the temperature of the track part, pressure medium generating means for injecting pressure medium into the coolant, means for controlling the temperature of the coolant, and a control device to which the measurements of the temperature measuring device are fed and which interacts with the lifting and lowering device for controlling the lifting and lowering operations, and with the means for controlling the temperature of the coolant as a function of the temperature measurements, and furthermore with the pressure-medium generating means. 
     
     
         58 . A device according to  claim 57 , wherein sensors for detecting coolant boiling on the surface of the rail head are provided, whose sensor measurements are fed to the control device in order to activate the pressure medium generating means as a function of the sensor measurements. 
     
     
         59 . A device according to  claim 58 , wherein a plurality of sensors are provided for detecting coolant boiling on the surface of the rail head, which sensors are distributed over the length of the cooling tank. 
     
     
         60 . A device according to  claim 57 , wherein the sensor measurements of the plurality of sensors are fed to the control device, said control device activating the pressure medium generating means as soon as at least one sensor has detected coolant boiling on the surface of the rail head. 
     
     
         61 . A device according to  claim 57 , wherein the control device is configured to perform controlled cooling comprising in a first step accelerated cooling until reaching a first temperature allowing the formation of ferrite, in a second step maintaining said first temperature to effect the formation of ferrite, in a third step further cooling within a temperature range allowing the formation of multiphase bainite until a second temperature, and in a fourth step maintaining said second temperature. 
     
     
         62 . A device according to  claim 61 , wherein the control device is configured to reduce the temperature of the rail head in the first step to a first temperature of 450-525° C. at a cooling rate of 2-5° C./s, to keep the temperature of the rail head in the second step at the first temperature, and to reduce the temperature of the rail head during the third step to a second temperature of 280-350° C., preferably for a period of 50-100 s. 
     
     
         63 . A device according to  claim 61 , wherein the control device is configured to activate the pressure medium generating means during the third step.