JPH042647B2 - - Google Patents

Description

[Detailed description of the invention]

``Object of the Invention'' The present invention relates to a method for heat treatment of rails, and particularly to produce rails with less fatigue damage such as wear of the rail head on curved sections and shearing defects on the rail head during high-speed operation under heavy axle loads. Therefore, the aim is to produce the required fine pearlite structure stably and at low cost. Industrial application field A heat treatment method for rails that responds quickly to the heavier axle loads and faster speeds of rail transportation. Conventional technology In recent years, rail transport has become more oriented toward heavy axle loads and higher speeds, and the conditions for using rails have become more severe. It is known to exhibit the best properties in terms of wear resistance and damage resistance required for. However, the following two manufacturing methods are generally known as methods for obtaining such a metallurgical structure. The first method uses carbon steel rail components such as Si, Cr,
This is a method (hereinafter referred to as the alloy steel rail method) in which the fine pearlite structure is obtained under natural cooling conditions after rolling by adding a large amount of alloying elements such as Mo and V alone or in combination. The second method is to obtain the fine pearlite structure by accelerating a carbon steel rail from which pearlite is obtained stably after rolling or reheating at a predetermined cooling rate from the austenite region temperature (hereinafter referred to as the heat-treated rail method). ). However, this heat treatment rail method is further divided into the following methods. Method: After rolling, the material is cooled and then reheated offline to the austenite region using gas flame heating or high-frequency induction heating, followed by heat treatment. Method: After hot rolling, the austenite region is then acceleratedly cooled and heat treated at a predetermined cooling rate online. In addition, as the forced cooling method for the heat treatment, the following 3 methods are available:
Two methods are known. ○B Cooling by blast. ○B Cooling by water spray and water mist. ○C Cooling by immersion in a molten salt bath, etc. Problems to be Solved by the Invention However, each of the above conventional methods has its own problems. That is, even if the first alloy steel rail method can be manufactured as rolled, it requires a large amount of expensive alloying elements, resulting in high manufacturing costs. Furthermore, when welding long rails, the cooling rate after welding is around 2°C/sec, which is faster than the cooling rate after rolling, so a martensitic structure is generated in the welded joint. Therefore, after welding the joint, post-heat treatment is required, which inevitably reduces the efficiency of joint welding. The second heat treatment rail method is unavoidably disadvantageous because it increases the time and cost of reheating the material once it has been cooled. Although this heat treatment rail method does not have the disadvantages mentioned above, methods ○A to ○C adopted for forced cooling have the following disadvantages. B) Blast cooling has a small cooling capacity, and it is difficult to secure the required cooling rate determined by the components. In the case of off-line heat treatment, the required cooling capacity is reduced by making the temperature gradient from the surface of the rail head to the inside steeper during reheating, thereby utilizing heat removal into the rail. Furthermore, since the cooling capacity depends on the speed of collision with the material to be cooled, possible ways to improve the cooling performance include reducing the distance between the blast nozzle and the material to be cooled, and increasing the discharge pressure. In the former case, it is necessary to control the blast nozzle to follow the bending of the rail during the forced cooling process, and as the heat treatment speed increases, the equipment mechanism becomes complicated. In the latter case, the flow rate of the blast increases, leading to an increase in the size of the supply device, which is disadvantageous in terms of equipment costs and running costs. Note that there is a method of adding alloys such as Cr, Mo, etc. to increase hardenability and reduce the required cooling rate, but the addition of alloys increases manufacturing costs. ○B Cooling by water spray and water mist,
Compared to blast cooling, the cooling medium cost is low, the cooling capacity is large, and the cooling capacity can be adjusted by changing the flow density. As the cooling capacity increases, a water wetting phenomenon occurs that reaches its maximum at the point where film boiling shifts to nucleate boiling. In the case of water mist, the same phenomenon occurs although it differs slightly depending on the air-water ratio, and this phenomenon largely depends on the surface temperature and surface properties of the material to be cooled.
These cooling characteristics of water make cooling control difficult, resulting in uneven quality due to variations in cooling conditions.
Localized supercooling due to water wetting causes the formation of bainite and martensite structures that are harmful to rail properties. ○C In immersion cooling in a molten salt bath, etc., the rail position is restricted to the immersion position, and it is difficult to adjust the cooling capacity for each part or over time. Therefore, this is disadvantageous in terms of equipment design and control of heat treatment conditions. "Structure of the Invention" Means for Solving the Problems Forced cooling is performed from the austenite region to 750 to 500 degrees Celsius at a cooling rate of 1 to 30 degrees C/sec, and in order to make the rail head a fine pearlite structure, A method for heat treatment of rails, characterized in that the head of the rail is forcibly cooled by passing through a high temperature gas-liquid mixed spray zone obtained by spraying pressurized hot water of 100 to 170°C from arranged nozzles. Function The droplets of hot water ejected from the nozzle provide stable cooling with little change in cooling capacity. Cooling capacity can be adjusted in response to changes in hardenability due to rail components, and droplets that collide with the material to be cooled are instantaneously vaporized, resulting in appropriate drainage. EXAMPLES To further explain the present invention as described above, in the present invention, after repeated studies to solve the problems of the conventional methods described above, the rail head is made of a fine pearlite structure with excellent wear resistance and damage resistance. In order to obtain a stable, inexpensive and We succeeded in obtaining a cooling or cooling control method for highly efficient heat treatment. That is, in the present invention, generally in wt% (hereinafter simply referred to as %), C: 0.50 to 0.85%, Si: 0.10 to 1.00%,
Mn: 0.50-1.50%, P: 0.035% or less, S: 0.035
% or less, Al: 0.05% or less, the balance being Fe and unavoidable impurities, as necessary.
Cr: 0.05~1.50%, Mo: 0.05~0.20%, V: 0.03
~0.10%, Ni: 0.10~1.00%, Nb: 0.005~0.050
A rail having a chemical composition in which one or more of
To heat-treat the rail head to a fine pearlite structure by accelerating cooling at a rate of ℃/sec, pressurized hot water of 100 to 170℃ is injected through a nozzle to cover the entire temperature range of the forced cooling described above. In other words, the rail is passed through a row in which such hot water spray headers are arranged, and the head of the rail is forcibly cooled. In addition, this forced cooling is performed from the start of forced cooling to 500℃ using the hot water spray cooling header, and from 500℃ to the forced cooling stop temperature, the head is forced to pass through the above rail through the row of blast cooling headers. It provides cooling. In the present invention, the temperature of the rail head is accelerated from the austenite region to the temperature of the austenite region, and hot water spray or a combination of hot water spray and blast is used as the cooling medium when heat treating the rail head to a fine pearlite structure. This is due to the following reasons.
In other words, in order to obtain a fine pearlite structure in the rail head, the cooling rate specified by the rail component (for example, AREA
For system components, accelerated cooling is performed at a rate of 3 to 9℃/sec).
In addition, the stop temperature of forced cooling must be set at 450 to 270°C on the rail surface, taking into account heat recovery from the rail abdomen, etc., in order to prevent softening of the fine pearlite structure of the transformed head. The hot water spray of the present invention has high temperature and high pressure (100 to 170
℃, 0 to 7.0 Kgf/cm ² G) is injected into the atmosphere from a nozzle, and the droplets generated at this time are used to cool the air. A portion of the hot water injected from the nozzle vaporizes and rapidly expands to several hundred times its volume.
The hot water is sheared and accelerated by this steam and becomes fine droplets, forming a gas-liquid mixed spray. As shown in Figure 6, the cooling performance characteristics of hot water spray are that the cooling performance changes little as the surface temperature of the material to be cooled changes from a high temperature range to a low temperature range, which means that cooling control is easy and stable. shows. In the case of water spray, the cooling capacity changes depending on the surface temperature of the material to be cooled, and the cooling capacity rapidly increases due to the water wetting phenomenon. This phenomenon depends on the surface temperature and surface properties, so in the case of hot-rolled materials, it is necessary to take into account that water wetting may occur at a surface temperature of 500°C to 550°C. It has also been shown that the cooling ability of hot water spray is equivalent to or higher than that of water mist, and the explosively expanding steam atomizes the hot water, making it possible to obtain stable droplets over a wide range of flow rates with the same nozzle. Therefore, it is extremely advantageous for adjusting the cooling capacity in response to changes in hardenability due to rail components. With hot water spray, the droplets that collide with the material to be cooled instantly vaporize, so even hot water spray alone has extremely good water removal properties. By providing this, the water can be completely drained and the cooling conditions can be stabilized. Next, the reason why hot water spray cooling is used as the cooling medium from the start of cooling to 500°C, and blast cooling is used from 500°C until forced cooling is stopped, is as follows. That is, in order to obtain fine pearlite, the austenite-pearlite transformation requires forced cooling at a cooling rate of 1 to 30°C/sec between 700°C and 500°C, so the above-mentioned hot water spray cooling is used as the cooling medium. The forced cooling after the transformation is a recuperation-suppressing cooling to prevent the softening of the fine pearlite, and the cooling capacity may be less than half that of the transformation. In addition, in order to avoid the phenomenon of water wetting that is noticeable with water spray, etc., using blast cooling as the cooling medium in the latter half is industrially practical and effective in terms of quality stability. That is, by using the hot water spray cooling of the present invention as a cooling medium during the austenite to pearlite transformation, the required cooling rate can be sufficiently ensured,
In addition, since the cooling capacity is stable, a fine pearlite structure can be stably obtained at the rail head without generating bainite or martensite that is harmful to the rail. Note that the forced cooling method according to the present invention as described above can be applied to both cases where after hot rolling is completed, heat treatment is performed on-line following the hot rolling, and when heat treatment is performed after reheating off-line. To explain a specific embodiment of the present invention as shown in the attached drawings, FIG. The mixed spray forms and forcibly cools the head of the rail 1, which is the material to be cooled. The head and bottom portions of the rail 1 are provided with spray shielding means 3 and the like to prevent cooling by hot water spray. Furthermore, FIG. 2 shows an example of the arrangement of a blast cooling header used in combination with the hot water spray nozzle shown in FIG. The head of the rail 1 is forcibly cooled. Furthermore, FIG. 3 shows an example of the arrangement of the hot water spray nozzle and the purge nozzle. By arranging the purge nozzle 5, the injected hot water can be drained more completely, which is effective against local unevenness caused by water wetting. FIG. 4 is an example of an arrangement in which hot water spray cooling and blast cooling are combined, and the hot water spray cooling area 6 and the blast cooling area 7 are placed directly in the rail passing direction, and the rail 1 is forcedly cooled by passing through it. Next, Table 1 shows the results of the rails manufactured by the present invention by subsequent heat treatment after completion of hot rolling.As a comparison material, the rails manufactured by the conventional method were reheated off-line and then blast cooled. The rail that has been subjected to fracture quenching is also shown. Table 1 shows the chemical composition of the sample materials and the cooling conditions for heat treatment, where A and B are the materials of the present invention,
C and D are rails heat treated by the conventional method as comparison materials. B and D are rail steels in which a low alloy is added to the basic composition.

[Table] Table 2 below shows the mechanical properties of the central portion of the rail head after heat treatment. The materials of the present invention, both A and B, have a tensile strength of 130 Kgf/mm ² or more, which is equivalent to the conventional heat-treated rail.

[Table] Figure 5 shows the cross-sectional hardness distribution relationship depending on the depth (distance) from the surface at the center of each rail head obtained as described above. Slack quenched comparison material C,
Compared to D, present inventions A and B have a deeper effective hardening layer depth due to heat treatment, and high hardness is ensured to the inside. Furthermore, the microstructure showed good results, with no structures such as evaite or martensite, and it was confirmed that rails of stable quality could be manufactured at low cost by the cooling method of the present invention. "Effects of the Invention" According to the present invention as described above, the rail is capable of producing a rail with less fatigue damage such as abrasion of the rail head at curved sections or shearing scratches on the rail head during high-speed operation, especially under heavy axle loads. This invention can be produced stably and at low cost, and has great industrial results.

[Brief explanation of drawings]

The drawings show the technical contents of the present invention, and Fig. 1 is an explanatory diagram of the arrangement relationship in the rail cross-sectional direction of the hot water spray according to the present invention, and Fig. 2 is similar to Fig. 1 regarding the blast header. Figure 3 is an explanatory diagram of the arrangement relationship in the cross-sectional direction of the rail. Figure 3 is an explanatory diagram of the arrangement relationship of hot water spray and purge nozzles in the rail length direction. Figure 4 is the arrangement in the rail length direction by a combination of hot water spray and air blast. 5 is a chart showing cross-sectional hardness distribution measurement results for manufacturing examples of the present invention and comparative examples, and FIG. 6 is a diagram summarizing the cooling performance of hot water spray, water mist, and water spray. This is a diagram. However, in these drawings, 1 is the rail, 2
is a hot water spraying means, 3 is a spray shielding means, and 4 is a spray shielding means.
5 is a blast header, 5 is a purge nozzle, 6 is a hot water spray cooling area, 7 is a blast cooling area, and 5th
In the figure, A and B show the ones according to the present invention, and C and D show the ones according to the comparative example, respectively.

Claims (1)

[Claims] 1 Forced cooling is performed from the austenite region to 750 to 500 degrees Celsius at a cooling rate of 1 to 30 degrees Celsius/sec, and when the rail head is made into a fine pearlite structure, pressurized heat of 100 to 170 degrees Celsius is applied from nozzles arranged in series. A rail heat treatment method characterized by forcibly cooling the rail head by passing it through a high-temperature gas-liquid mixed spray zone obtained by jetting water.