JPH044372B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Application Field] The present invention relates to a rail heat treatment method, and particularly to a rail heat treatment method that can eliminate variations in hardness due to non-uniform cooling and downsize the air source equipment in the heat treatment equipment. [Conventional technology] Rail wear has become an urgent problem as railway vehicles become heavier, with higher axle loads and faster transportation, and there is a demand for the supply of high-strength rails with wear resistance. ing. As is well known, the wear parts of a rail are the top surface and inside surface of the rail. Therefore, it is necessary that at least the surface layer of the rail head has a fine pearlite structure. As shown in Figure 1, heat treatment methods for obtaining this fine pearlite structure include isothermal transformation heat treatment, which mainly controls the cooling stop temperature and maintains the transformation temperature, and continuous cooling transformation, which mainly controls the cooling rate to perform cooling. There is heat treatment. As a cooling medium, blast air, spray water, air-water mixtures,
Heat treatment methods using boiling water, steam, molten salt, etc. are disclosed in JP-A-54-148124 and JP-A-54-147124.
Publication No. 1985-85929, Japanese Patent Publication No. 1987-85929
It is disclosed in JP-A No. 133322, JP-A-61-149436, etc. [Problems to be Solved by the Invention] However, these heat treatment methods have the following problems. (1) Heat treatment using air blast: Uniform cooling can be performed using air blast cooling, but its cooling capacity is smaller than, for example, when using water spray. Therefore, in order to improve wear resistance and strength, it is necessary to add alloying elements, but this increases the manufacturing cost of the rail. Therefore,
There is a method of installing a blast cooling nozzle close to the rail surface and injecting a large amount of compressed air onto the rail surface to ensure the desired cooling capacity, but online heat treatment after rolling is not possible. Since this requires a long cooling area, the air source equipment becomes large, which is disadvantageous in terms of equipment. (2) Heat treatment by spraying water or air-water mixtures: The cooling capacity of these refrigerants is significantly superior to that of blast air. As an example of the cooling capacity of water, a steel billet with a water density of 200 to 1000/min・m ²
Figure 2 shows the relationship between the surface temperature of the steel slab and the heat transfer coefficient when the steel slab is cooled at Maximum at 350℃. This is due to nucleate boiling of the cooling water.
When the rail surface is cooled by spray water, the cooling water transitions to nucleate boiling, with scale generated on the rail surface during rolling and heat treatment as nuclei.
This local nucleate boiling causes the temperature in this part to drop rapidly, thereby generating a martensitic structure or a bainite structure, resulting in variations in the hardness of the rail head. The cooling capacity is adjusted by the amount of water sprayed, but as the amount of sprayed water decreases, it becomes difficult to maintain cooling uniformity. In the case of atomization of air-water mixtures, not only is the problem of non-uniform cooling, but also a considerable amount of air is required and problems similar to blast cooling. (3) Heat treatment by immersing the rail head in boiling water: A steam film is formed on the rail head and the desired cooling capacity is obtained through this steam film, but the steam film is not uniformly formed. Moreover, it is almost impossible to maintain it and is not a realistic method. (4) Heat treatment by steam injection: Although the cooling capacity is greater than blast cooling, it still requires a large amount of steam to obtain a fine pearlite structure, which is disadvantageous in terms of equipment. (5) Heat treatment by immersing the rail head in a molten salt bath: This poses no problem in terms of cooling rate control and cooling uniformity, but it requires a device to remove the molten salt that adheres to the rail surface after heat treatment. This requires a large amount of molten salt to adhere to the rail head. Therefore, it is disadvantageous in terms of heat treatment equipment and running costs. Among the heat treatment methods disclosed in the above-mentioned publications, for example, the heat treatment method disclosed in JP-A-54-147124 is a constant temperature transformation heat treatment method of the two heat treatment methods mentioned above. Because it is necessary to complete the transformation, it is held at a high temperature for a long time, so a softening phenomenon due to self-annealing tends to occur, which is undesirable. Therefore, an object of the present invention is to provide a method for heat treatment of rails, which can reduce the size of the air source equipment necessary for heat treatment, has no variation in hardness, and turns the head structure of the rail into a fine pearlite structure. be. [Means for Solving the Problems] The present invention provides continuous cooling transformation heat treatment to the rail head to transform the structure of the rail head into a fine pearlite structure, by spraying the rail head with spray water. cooling, then cooling the rail head with a hot water jet, then cooling the rail head with air, and switching from cooling with the spray water to cooling with the hot water jet; Temperature 530℃
The above, and the temperature at which the cooling by the hot water jet is switched to the cooling by the air is set to a temperature of 420° C. or more, which is lower than the temperature at which the cooling by the spray water is switched to the cooling by the hot water jet. It is something. What is a hydrothermal jet?100
This is a high-speed gas-liquid two-phase flow obtained by jetting high-temperature, high-pressure water at a temperature of ℃ or higher from a nozzle. In this invention, the heat treatment method is limited to the continuous cooling transformation heat treatment method shown in FIG. 1 because this heat treatment method allows the rail to be quickly cooled even after the transformation treatment. On the other hand, the isothermal transformation heat treatment method is not preferable because, as described above, a self-softening annealing phenomenon occurs after the transformation is completed. In this invention, the temperature for switching from water spray cooling to hot water jet cooling is 530°C or higher, and the temperature for switching from hot water jet cooling to blast cooling is 420°C below the temperature for switching from water spray cooling to hot water jet cooling.
The reason for the above period will be explained. Figure 3 shows C: 0.77%, Si: 0.25%, Mn: 0.85
%, P: 0.01%, S: 0.007% (weight %) is subjected to continuous cooling transformation heat treatment, and shows the relationship between the cooling time from Ac ₃ points and the metal structure and hardness. As is clear from FIG. 3, in order to form a pearlite structure, it is necessary to cool from above the austenitization temperature to below the transformation point temperature at a cooling rate of 11° C./sec or less. In addition, in order to prevent self-softening annealing after heat treatment, the maximum recuperation temperature must be 450℃ as shown in Figure 4.
It must be cooled to below. In addition, Fig. 4 shows C: 0.77%, Si: 0.25%, Mn: 0.86%,
A rail made of known steel containing P: 0.017% and S: 0.008% (or more by weight) was cooled at a temperature of 4.8%.
It is a graph showing the relationship between the recuperation temperature, the hardness calculated from the tensile strength, and the strength 5 mm below the rail head when cooling at a rate of °C/sec. Therefore, a test piece of 136 lb/yard rail with a length of 500 mm (C: 0.75%, Si: 0.24%, Mn: 0.90
%, P: 0.016%, S: 0.008% or more by weight), a thermocouple was attached at a position 5 mm from the upper surface of the head, and the test piece was heated to 900°C. After this, the test piece could be moved back and forth. When placed on a trolley, the rail temperature reaches 800℃.
The rail is left to cool in the atmosphere until
As shown in Figures A and B, the cooling zone (-
A trolley (not shown) on which the rail 1 is placed is set so that the cooling rate by spray water from the water cooling nozzle 2 provided above and on both sides of the rail 1 is 2, 5, 10°C/sec. ) to move back and forth to rail 1.
was cooled, and this cooling was stopped at various times to examine the subsequent recuperation temperature of the rail 1. The cooling conditions at this time are shown in Table 1.

[Table] Figures 6A, B, and C show the relationship between the cooling time and the maximum recuperation temperature on the rail surface after cooling has stopped. As is clear from FIGS. 6A, B, and C, it can be seen that the maximum recuperation temperature on the rail surface varies greatly from a certain temperature depending on the cooling rate. Next, according to the cooling conditions shown in Table 2, that is, in Table 1, the nozzle type is replaced with a hot water injection nozzle, and hot water (high temperature, high pressure water of 100°C or higher) is used as a cooling medium. The cooling rate was set to 2, 5, 10℃/sec, and the other conditions were the same as in the above test. I investigated the relationship. The results are shown in FIGS. 7A, B, and C.

[Table] As is clear from FIGS. 7A, B, and C, as in the case of FIG. 6, it can be seen that the maximum recuperation temperature on the rail surface varies greatly from a certain temperature depending on the cooling rate. Next, according to the test conditions described above, the relationship between the rail surface temperature at the time of cooling stop and the rail recuperation maximum temperature was determined using a computer. The results are shown in FIG. As can be seen from Figures 6, 7, and 8, the reason why the maximum recuperation temperature on the rail surface varies is that in the case of water spray cooling, the rail surface temperature is approximately
It can be seen that when the temperature reaches 530°C, the rail surface temperature reaches approximately 420°C in the case of hot water jet cooling. Therefore, in this invention, the temperature at which water spray cooling is switched to hot water jet cooling is set to 530°C or higher, and the temperature at which hot water jet cooling is switched to blast cooling is set to be lower than the temperature at which water spray cooling is switched to hot water jet cooling. The temperature was set at 420℃ or higher. Next, embodiments of the invention will be described. 136 lb/yard rail specimen with length 500 mm (C: 0.76%, Si: 0.25%, Mn: 0.91%, P:
Attach a thermocouple at a position 5 mm from the upper surface of the head of 0.017%, S: 0.007% or more), and
The specimen was heated to 800°C, and then placed on a reciprocating trolley and moved back and forth through the water spray cooling zone (between the middle and middle of the figure) as shown in Figure 9 A, B, C, and D. Then, the rail 1 was cooled by spray water from water cooling nozzles 2 provided above and on both sides until the rail surface temperature reached 550°C, and then,
By reciprocating the hot water jet cooling zone (between - in the figure), the rail surface temperature is raised to 420°C by the hot water jet from the hot water jet cooling nozzle 4 provided above and on both sides of the rail 1 head. Cool until
Subsequently, the air cooling zone (between the two in the figure) was moved back and forth, and the rail surface temperature was cooled by air from the air cooling nozzles 3 provided above and on both sides of the rail 1 until the rail surface temperature reached 200°C. . The maximum recuperation surface temperature at this time was 330°C. Table 3 shows the cooling conditions.

[Table] Then, a rail head was cut out from this test piece, and its macrostructure and Vickers hardness were examined. As a result, the macrostructure was a fine pearlite structure, and no abnormal structure was observed. Further, the results of the Vickers hardness distribution are shown in FIG. As is clear from FIG. 10, it can be seen that the Vickers hardness of the rail head has small variations and has sufficient wear resistance. Next, C: 0.78%, Si: 0.56%, Mn: 0.86%,
P: 0.002%, S: 0.007%, Cr: 0.447%, V:
A 136 pound/yard rail containing 0.054% (or more by weight) immediately after rolling was placed in a water cooling zone equipped with a nozzle shown in Figure 9D, a hot water jet cooling zone equipped with a nozzle shown in Figure 9C, and a hot water jet cooling zone equipped with a nozzle shown in Figure 9C. The air cooling zone equipped with the nozzle shown in Figure B is moved at a speed of 7.2 m/min, and the rail surface temperature is cooled to 550℃ in the water spray cooling zone, and then the rail surface temperature is lowered in the hot water jet cooling zone. It was cooled to 450°C, and then the rail surface temperature was cooled to 300°C in the air cooling zone. The cooling conditions at this time are shown in Table 4.

【table】

〔Effect of the invention〕

As explained above, according to the present invention, the rail head is uniformly cooled compared to the case where the rail head is heat-treated using only spray water, so the variation in hardness is small, and the rail head is exposed to blasts. Since the amount of air used is significantly smaller than when heat treatment is performed by cooling only, various useful effects such as the ability to downsize the air source equipment are brought about.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between time and temperature in isothermal transformation heat treatment and continuous cooling transformation heat treatment, and FIG. 2 is a graph showing the relationship between heat transfer coefficient and surface temperature when water volume density is used as a parameter. Figure 3 is a graph showing the relationship between the cooling rate of continuous cooling transformation temperature, metal structure and hardness, and Figure 4 is the hardness calculated from tensile strength and 5mm below the head.
FIG. 5A is a front view showing the cooling method of the rail test piece, FIG. 5B is a view taken along line AA in FIG. 5A, and FIGS. Graph showing the relationship between maximum recuperation temperature and cooling time during water cooling,
Figures 7A to C are graphs showing the relationship between maximum recuperation temperature and cooling time during hot water jet cooling, and Figure 8 is a graph showing the relationship between maximum recuperation temperature and cooling time when cooling rate is used as a parameter. FIG. 9A is a front view showing the cooling method of the present invention, FIG. 9A is a graph showing the relationship with the rail surface temperature, FIG. The BB view of A and the same figure D are Fig. 9A
Figure 10 is a graph showing the relationship between the Vickers hardness and the distance from the surface, and Figure 11 is a graph showing the relationship between the Vickers hardness 20 mm below the rail surface and the position in the longitudinal direction of the rail. be. In the drawings: 1...Rail, 2...Water cooling nozzle, 3...Air cooling nozzle, 4...Hot water jet cooling nozzle.

Claims (1)

[Claims] 1. Apply continuous cooling transformation heat treatment to the rail head,
In converting the structure of the rail head into a fine pearlite structure, the rail head is cooled by spray water, then the rail head is cooled by a hot water jet, and then the rail head is cooled by a hot water jet. cooling the part with air, and setting the switching temperature from cooling by the spray water to cooling by the hot water jet to 530 ° C. or higher,
The temperature at which the cooling by the hot water jet is switched to the cooling by the air is set to a temperature of 420° C. or more below the temperature at which the cooling by the spray water is switched to the cooling by the hot water jet.
Rail heat treatment method.