JPH044373B2 - - 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 temperature 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 rail heat treatment method that can reduce the size of the air source equipment necessary for heat treatment, eliminate variations in hardness, and turn the head structure of the rail into a fine pearlite structure. It is in. [Means for Solving the Problems] This invention provides continuous cooling transformation heat treatment to the rail head to transform the structure of the rail head into a fine pearlite structure. Then, the rail head is blast cooled to a temperature of 420° C. or higher at which cooling is switched from hot water jet cooling to blast cooling. Here, the hydrothermal jet is a high-speed gas-liquid two-phase flow obtained by jetting high-temperature, high-pressure water at 100°C 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 reason why the temperature at which rail head cooling is switched from hot water jet cooling to blast cooling is set to 420° C. or higher will be explained. Figure 3 shows C: 0.77%, Si: 0.25%, Mn: 0.85
%, P: 0.01%, S: 0.007% (hereinafter referred to as weight %) is subjected to continuous cooling transformation heat treatment, and the relationship between the cooling time from Ac ₃ points and the metal structure and hardness is shown. 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 cooling rate 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 (-
The rail 1 is cooled so that the cooling rate of the hot water from the hot water jet cooling nozzle 2 provided above and on both sides of the rail 1 is 2, 5, 10°C/sec.
The rail 1 was cooled by reciprocating a trolley (not shown) on which the rail 1 was mounted, and this cooling was stopped at various times to examine the recuperation temperature of the rail 1 thereafter. The cooling conditions at this time are shown in Table 1.

【table】

[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 above-mentioned test conditions, the relationship between the rail surface temperature at the time of cooling stop and the maximum rail recuperation temperature was determined using a computer. The results are shown in FIG. As can be seen from FIGS. 6 and 7, variations in the maximum recuperation temperature on the rail surface occur when the rail surface temperature reaches approximately 420°C. Therefore, in this invention, as a cooling medium for the rail head, a hot water jet in which hot water (high-temperature, high-pressure water with a temperature of 100°C or higher) is injected from a jet nozzle is used in an area where the surface temperature of the rail head is high. However, stable cooling can be achieved by stopping the use of hot water jets until the head surface temperature reaches at least 420°C, and then using air. Next, embodiments of the invention will be described. 500 mm long 136 lb/yd rail specimen (C: 0.76%, Si: 0.25%, Mn: 0.91%, P:
0.017%, S: 0.007% 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 800°C. After this, the test piece was placed on a trolley that could be moved back and forth. As shown in FIGS. 8A, B, and C, the hot water jet cooling nozzles 2 are installed above and on both sides of the head of the rail 1 by reciprocating the hot water jet cooling zone (between - in the figure). Cool the rail surface with hot water until the rail surface temperature reaches 420℃,
Subsequently, the blast cooling zone (between the two sides in the figure) is moved back and forth, and the air from the air cooling nozzles 3 provided above and on both sides of the rail 1 cools the rail surface until the temperature reaches 220°C. did. The maximum recuperation surface temperature at this time was 350°C. Table 2 shows the cooling conditions.

〔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 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 cooling rate, metal structure and hardness in continuous cooling transformation heat treatment;
The figure shows hardness converted from tensile strength and 5
Graph showing the relationship between the strength of mm and the recuperation temperature, 5th
Figure A is a front view showing the cooling method of the rail test piece;
Figure B is a view from A-A in Figure 5A, and Figures 6A to C.
7 is a graph showing the relationship between maximum recuperation temperature and cooling time, FIG. Figure A is a front view showing the cooling method of the present invention, and Figure B is A of Figure 8A.
-A view and C are B-B views of Figure 8A, Figure 9 is a graph showing the relationship between Vickers hardness and distance from the rail surface, and Figure 10 is 20 mm below the rail surface. 2 is a graph showing the relationship between the Vickers hardness of the rail and the position in the longitudinal direction of the rail. In the drawings, 1...Rail, 2...Hot water jet cooling nozzle, 3...Air cooling nozzle.

Claims (1)

[Claims] 1. Apply continuous cooling transformation heat treatment to the rail head,
When the structure of the rail head is made into a fine pearlite structure, the rail head is cooled by a hot water jet, and then the rail head is blast-cooled,
A rail heat treatment method characterized by setting the temperature at which cooling is switched from hot water jet cooling to blast cooling to 420°C or higher.