Heat treatment method for increasing the depth of hardening layer in a steel rail and steel rail obtained with the method
The present invention relates to a heat treatment method for increasing the depth of hardening layer in a steel rail, and belongs to the field of steel rail production process. The technical problem to be solved in the present invention is to provide a heat treatment method for increasing the depth of hardening layer in a steel rail and a steel rail obtained with the method. The method comprises the following steps: cooling a finished rolling steel rail by natural cooling, till the temperature at the center of rail head surface is 660˜730° C.; cooling the steel rail by accelerated cooling at 1.5˜3.5° C./s cooling rate, till the temperature at the center of rail head surface is 500˜550° C.; increasing the cooling rate by 1.0˜2.0° C./s and further cooling down the steel rail, till the temperature at the center of rail head surface is 450° C. or lower; then, stopping the accelerated cooling, and cooling down the steel rail by air cooling to room temperature. With the heat treatment method disclosed in the present invention, a deep-hardening layer thicker than 25 mm can be obtained in the rail head part, the portion within 25 mm depth below the surface layer of rail head has hardness equivalent to the hardness of the surface layer of rail head, and the rail head is in a pearlite structure across its cross section. Thus, the service performance of the steel rail against the wearing incurred by the contact between the train wheels and the steel rails can be improved.
1. A heat treatment method for increasing the depth of hardening layer in a steel rail, the method comprising the following steps:
a. natural cooling: cooling a finished rolling steel rail by natural cooling, till the temperature at the center of rail head surface drops to 660˜730° C.;
b. first accelerated cooling stage: cooling the steel rail cooled down in step a by accelerated cooling at 1.5˜3.5° C./s cooling rate, till the temperature at the center of rail head surface is 500˜550° C.;
c. second accelerated cooling stage: cooling the steel rail cooled down in step b by accelerated cooling at a cooling rate that is 1.0˜2.0° C./s higher than the cooling rate in the first accelerated cooling stage, till the temperature at the center of rail head surface is 450° C. or lower;
d. air cooling: stopping the accelerated cooling, and cooling down the steel rail by air cooling to room temperature,
and wherein the carbon content in the steel rail is 0.75%˜0.90%, the silicon content in the steel rail is 0.54%˜0.96%, the manganese content in the steel rail is 0.69%˜1.35%, the phosphorus content in the steel rail is 0.009%˜0.018%, the sulphur content in the steel rail is 0.006%˜0.012%, the chromium content in the steel rail is 0.03%˜0.58%, the vanadium content in the steel rail is 0˜0.07%, and the remaining content is ferrum.
2. The heat treatment method for increasing the depth of hardening layer in a steel rail according to claim 1 , wherein the accelerated cooling in step b and step c is carried out by applying a cooling medium to the rail head surface and both sides of the steel rail.
3. The heat treatment method for increasing the depth of hardening layer in a steel rail according to claim 2 , wherein the cooling medium is at least one of compressed air, a water-vapor mixture, and a gas-oil mixture.
4. The heat treatment method for increasing the depth of hardening layer in a steel rail according to claim 1 , wherein the temperature of the finished rolling steel rail is 850˜1,000° C.
5. The heat treatment method for increasing the depth of hardening layer in a steel rail according to claim 2 , wherein the temperature of the finished rolling steel rail is 850˜1,000° C.
6. The heat treatment method for increasing the depth of hardening layer in a steel rail according to claim 3 , wherein the temperature of the finished rolling steel rail is 850˜1,000° C.