JP7405250B2 - Rail manufacturing method - Google Patents

Description

The present invention accelerates cooling of a high-temperature rail that has been hot-rolled above the austenite range temperature or heated above the austenite range temperature using a cooling medium (air, water, mist, etc.), thereby improving the durability of the head of the high-temperature rail. The present invention relates to a rail with improved wear resistance and a method for manufacturing the same.

A general method for manufacturing a conventional high-hardness rail in which the rail head is made of a fine pearlite structure to improve wear resistance will be described.

Rails that have been hot-rolled above the austenite range temperature or heated above the austenite range temperature are carried into the heat treatment equipment in an upright position (the top of the head is upward and the sole of the foot is downward). . In this case, it may be delivered to the heat treatment equipment as a rolled length of, for example, about 100 m, or it may be divided (hereinafter referred to as sawing) into pieces each having a length of, for example, about 25 m. It may also be brought in from If the principle is to cool the material after sawing, the heat treatment equipment may also be divided into zones of corresponding lengths.

In the heat treatment apparatus, the toes of the rail are restrained by, for example, clamps, and the top surface, side surface of the head, sole surface of the foot, and, if necessary, the abdominal surface are forcibly cooled with a cooling medium (air, water, mist, etc.). By controlling the temperature history, the entire head including the inside of the head has a fine pearlite structure.

After the heat treatment is completed, the product is transported to a cooling bed in an upright or inverted state, left to cool to about room temperature, and undergoes processes such as straightening and inspection to become a product.

In order to increase the hardness by accelerated cooling, it is effective to perform pearlite transformation at a low temperature, and by increasing the cooling rate, it is possible to lower the transformation temperature. In addition, in order to obtain high hardness to the inside of the head, cooling is performed by heat conduction from the head surface, which requires cooling for a relatively long time.

However, if the cooling rate is high and the amount of cooling is too large, a structure other than pearlite, such as a bainite structure or a martensitic structure, will be generated near the cooling surface. Compared to a pearlite structure, a bainite structure lowers wear resistance as its fraction increases, and a martensitic structure significantly lowers toughness, so it is not preferable.

Therefore, in order to generate a fine pearlite structure by accelerated cooling, precise temperature history control was required. Patent Document 1 describes a method of controlling temperature history while monitoring transformation behavior with a thermometer installed in a cooling device.

International Publication No. 2014/157198

FIG. 2 schematically shows the pressure schedule of the top air header and the history of the rail head surface temperature for implementing desired cooling in Patent Document 1. As shown in FIG. 2, cooling starts at the cooling start temperature in the austenite region, and temperature rise due to transformation heat generation starts at timing t1 in the figure. If the temperature rise is too large, transformation occurs at high temperatures and hardness decreases. To prevent this, it is necessary to increase the header pressure at approximately the same time as the temperature rise, or slightly before it, to increase the cooling capacity. This makes it possible to reduce temperature rise due to transformation heat generation and increase hardness. At this time, the interior transforms later than the surface, so in order to make the interior highly hard, it is necessary to cool the surface to a bainite formation temperature range and increase the cooling rate of the interior by heat conduction. However, by measuring the temperature during cooling from the rail surface, it is not possible to determine the completion of internal transformation. Therefore, it is common to increase the amount of cooling during accelerated cooling so that the surface temperature of the recuperated head after the accelerated cooling ends is equal to or lower than the pearlite transformation temperature.

However, when cooling the entire length of the rail and attempting to control cooling with the method described in Patent Document 1 for a rail in which temperature variations have occurred in the longitudinal direction before the cooling starts, the timing of the start of transformation varies depending on the temperature variations. It will change. For this reason, it is necessary to install thermometers at all positions where there are variations, which increases equipment installation costs. Further, even if a thermometer can be installed, desired control may not be possible depending on the conditions. For example, in the case of blast cooling, cooling air is usually supplied from one blower to a plurality of cooling headers. In this case, if the cooling rate needs to be changed significantly in order to cope with temperature variations in the longitudinal direction, the flow pressure regulating valve installed in each header cannot handle this, and cooling control according to the transformation behavior cannot be done. It disappears. As a result, pearlite transformation occurs at high temperatures and hardness decreases, or pearlite transformation is not completed during accelerated cooling and a large amount of bainite structure or martensite structure is generated.

Further, in order to correct temperature variations in the longitudinal direction of the rail before cooling starts, heating using IH or the like may be considered, but this increases the equipment installation cost. Furthermore, even when the rails are cooled by passing through the cooling device from the end of the rail instead of cooling the entire length all at once, if the cooling time is long, the length of the cooling equipment becomes long. As a result, equipment installation costs increase, such as the need for thermal rundown compensation by IH.

Furthermore, temperature variations include variations due to materials and variations within the cross section of the head. Due to variations in temperature and time during heating, rolling, and transportation to the accelerated cooling device, temperature variations occur depending on the material. Further, since the rail is transported from rolling to the accelerated cooling device in an overturned state, the heat dissipation state of the rail head is different, and a temperature difference occurs within the cross section of the head.

Therefore, the present invention has been made to solve the above-mentioned problems, and its purpose is to manufacture high-hardness, high-quality rails using a simple cooling method, regardless of temperature variations in the rails before cooling starts. This provides a method to do so. Here, "high hardness" means a surface hardness of HB430 or higher and an internal hardness of HB385 or higher. Furthermore, "high quality" means that the bainite production rate at a depth of 5 mm from the cooling surface of the rail head, which reduces wear resistance, is 15% or less.

As a result of intensive investigation, the authors found that when a bainite structure is formed, it is formed in large quantities during cooling after accelerated cooling. That is, it was found that austenite that was not transformed during accelerated cooling was transformed into pearlite or bainite, depending on the temperature history of the recuperation after the accelerated cooling and the subsequent cooling process.

As shown in FIG. 3, at the end of accelerated cooling, heat conduction causes recuperation so that the temperature distribution within the cross section that occurred during accelerated cooling almost disappears, and the temperature of the surface increases. If the pearlite transformation is not completed at the end of accelerated cooling and austenite remains, and the temperature is in the bainite transformation temperature range during the reheating process, the remaining austenite transforms into bainite. If pearlite transformation hardly occurs during accelerated cooling, a large amount of bainite will be produced. In addition, in the transformation rate graph of FIG. 3, 100-transformation rate (%) is the residual rate of austenite.

On the other hand, as shown in FIG. 1, even if the pearlite transformation is not completed at the end of accelerated cooling and austenite remains, and the temperature is in the bainite formation temperature range, the pearlite transformation temperature range is reached again in the reheating process. Most of the remaining austenite transforms into pearlite during the recuperation process and subsequent natural cooling. It was found that the desired pearlite structure could be obtained by this method.

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] A rail manufacturing method comprising the step of accelerating cooling of a rail having a temperature equal to or higher than the austenite range temperature, the rail manufacturing method comprising the step of accelerating cooling of a rail having a temperature equal to or higher than the austenite range temperature, the rail manufacturing method comprising: % or less is austenite, and the temperature of the rail head surface at the completion of recuperation that occurs after the completion of accelerated cooling is in the pearlite transformation temperature range.
[2] The method for producing a rail according to [1], wherein the maximum temperature of the rail head surface in the recuperation process that occurs after the accelerated cooling is higher than the lower limit of the pearlite transformation temperature range and lower limit of the pearlite transformation temperature range +75°C or lower. .
[3] In [1] or [2], the rail is allowed to cool after the accelerated cooling is completed, and after the temperature of the rail head surface becomes 200° C. or less, the rail is cooled at a rate of 1° C./s or more. Method of manufacturing the rails described.
[4] A rail in which the bainite production rate is 15% or less in a range up to a depth of 5 mm from the cooling surface of the rail head.

By using the manufacturing method of the present invention, a rail with high hardness and high quality can be manufactured regardless of temperature variations in the rail before cooling starts.

FIG. 1 is a schematic diagram showing the relationship between the temperature history of the rail head surface from the start of accelerated cooling and the transformation rate in a range up to a depth of 5 mm from the cooling surface of the rail head, according to the present invention. FIG. 2 is a schematic diagram of the pressure schedule and the history of the rail head surface temperature of the top air header for cooling the rail head surface according to Patent Document 1. Figure 3 shows the temperature history of the rail head surface and the transformation rate in the range from the cooling surface to a depth of 5 mm when the temperature at the completion of recuperation after the end of accelerated cooling is in the bainite transformation temperature range, according to a comparative example. FIG.

Hereinafter, the manufacturing method of the present invention will be explained with reference to the drawings.

As shown in Figure 1, when the pearlite transformation in a region 5 mm deep from the cooling surface of the rail head is not completed at the end of accelerated cooling, austenite remains, and the temperature of the rail head surface is in the bainite generation temperature range. think of. By reaching the pearlite transformation temperature range again in the subsequent reheating process, most of the remaining austenite transforms into pearlite during the reheating process and the subsequent natural cooling. Note that the remaining austenite is usually 70% or less. At this time, a fine pearlite structure can be obtained by controlling the accelerated cooling so that the temperature of the rail head surface at the completion of the recuperation that occurs after the accelerated cooling is in the pearlite transformation temperature range. In addition, the remaining amount of austenite can be confirmed by a conventional method.

Here, a trace amount of bainite may be generated in the bainite generation temperature range during accelerated cooling and reheating process. If the bainite production rate in the region 5 mm deep from the cooling surface of the rail head is 15% or less, the difference in wear resistance from full pearlite, that is, 100% pearlite, is negligible. It should be noted that confirmation that pearlite transformation has not been completed and austenite remains after accelerated cooling can be performed by measurement using a transformation rate meter, reproduction of temperature results measured during cooling using a THERMEC, etc. Alternatively, a comparison may be made with a rail in which the cooling time is intentionally extended and the temperature range after reheating is set to the bainite transformation temperature range. The bainite transformation temperature range here refers to the temperature range in which bainite is generated when the material is maintained in an isothermal state. By creating a isothermal transformation curve in advance using a test piece, it is possible to understand the transformation temperature range of bainite and pearlite.

Moreover, the temperature of the rail head surface is a value obtained by measuring the temperature of the corner of the rail head with a radiation thermometer. Note that when the rail is conveyed in an upright position after accelerated cooling, the temperature within the cross section of the rail head becomes almost the same during the recuperation process, so any part of the range of both lateral surfaces including the central surface of the head There is no problem in this position. Further, the region at a depth of 5 mm from the cooling surface of the rail head means the average value of the microstructure of the region at a depth of 5 mm from the central surface and both temporal corner surfaces of the rail head.

Further, it is preferable that the maximum temperature of the rail head during the recuperation process after the end of accelerated cooling is +75° C. or less from the lower limit of the pearlite transformation temperature range, since the pearlite produced will be finer and the hardness of the rail will be higher. More preferably, the temperature is +50° C. or lower from the lower limit of the pearlite transformation temperature.

In cooling to about room temperature on a cooling bed, it is preferable to let the rail head cool until the temperature of the rail head surface becomes 200° C. or less, and then cool at a rate of 1° C./s or more. If the temperature of the rail is 200° C. or lower, all the rails have completed transformation, so the characteristics are not affected and the cooling time can be shortened. Furthermore, cooling on the cooling bed does not affect the warpage of the rail. Furthermore, if the temperature is allowed to cool to 200° C. or lower, the temperature difference from room temperature becomes small, so it takes one hour or more to cool to about room temperature. By setting the cooling rate to 1° C./s or more, the processing time on the cooling bed can be significantly shortened.

The timing of cooling on the cooling bed may be such that the time when the temperature becomes 200° C. or lower is determined in advance, and cooling may be started after a predetermined time. Alternatively, cooling may be started after measuring the temperature of the surface of the rail head with a thermometer and confirming that it is below 200°C. Regarding the cooling method on the cooling bed, known methods such as water spray cooling from above can be used without any problem.

It is not necessary to always measure the temperature of the rail head during the recuperation process after the end of accelerated cooling and during the subsequent natural cooling, but it is sufficient to measure the temperature at a timing of 30 seconds or more and 150 seconds or less after the end of accelerated cooling. If it is shorter than 30 seconds, the recuperation has not yet been completed, and it is not possible to determine whether the temperature of the rail head surface is within the desired pearlite transformation temperature range, and the hardness of the rail may be reduced. . In addition, if it is longer than 150 seconds, the amount of temperature drop due to natural cooling after the recuperation process will become large, making it difficult to grasp the temperature during the recuperation process, and there is a risk that the hardness of the rail will decrease. . After accelerated cooling, the rails are generally transferred to a cooling bed and left to cool naturally to near room temperature. For this reason, it is preferable to measure the temperature during transportation to the cooling bed, which is 30 seconds or more and 150 seconds or less after the end of accelerated cooling. This makes it possible to measure the entire rail length with a single thermometer.

If the measurement results show that the desired temperature is not achieved, the amount of cooling may be adjusted for the next rail and subsequent rails. That is, if the temperature is high, in order to increase the amount of cooling, the flow rate of the refrigerant to be injected may be increased to increase the cooling capacity, or the cooling time may be extended. Furthermore, if the temperature is low, in order to reduce the amount of cooling, the flow rate of the refrigerant to be injected may be reduced to lower the cooling capacity, or the cooling time may be shortened.

Regarding the cooling rate during accelerated cooling, the rail head surface is cooled at 1°C/s or more and 7°C/s or less, and pearlite transformation is started in the vicinity of the surface, that is, a range up to 5 mm deep from the cooling surface of the rail head surface. It is preferable. More preferably, it is 4°C/s or more and 6°C/s or less. When cooling by blast cooling or the like, the temperature decreases and the cooling capacity also decreases, so it is preferable to increase the air volume as the temperature of the rail decreases.

When transformation near the surface begins, the temperature rises due to the heat generated by the transformation. It is preferable that the temperature rise due to transformation heat generation is 50° C. or less. More preferably it is 30°C or lower.

After the temperature rise due to transformation heat generation in the vicinity of the surface ends, cooling is preferably performed at a rate of 1° C./s or more and 5° C./s or less. More preferably, it is 1.5°C/s or more and 2.5°C/s or less. If it is higher than 5° C./s, a larger cooling device is required, which increases the equipment cost. Moreover, the variation in the amount of adjustment of the cooling amount becomes large, and more precise control of the cooling measure is required, which increases the equipment cost.

The bainite production rate must be 15% or less in the range up to a depth of 5 mm from the cooling surface of the rail head cooled by the above cooling method. It is preferable that the remaining structure has a pearlite production rate of 85% or more. This is because when the bainite production rate is greater than 15%, the wear resistance is inferior to that of full pearlite. In addition, the bainite production rate here is the area ratio of bainite that can be visually recognized by microstructure observation using a normal optical microscope. Note that similarly to the formation rate of structures other than bainite, the generation rate means the area ratio.

Note that the component composition of the rail may be within a conventionally known range, and in terms of mass%, for example, C content: 0.7 to 1.00%, Si content: 0.20 to 1.20%, Mn content: 0.20 or more and 1.50 or less%, P content: 0.035 or less%, S content: 0.012 or less%, Cr content: 0.20 or more and 1.50 or less %, and optionally at least one selected from Cu, Ni, Mo, V, Nb, Al, Ti, and Sb, respectively from 0.01 to 1.00%, B, Ca, Mg, and REM. It is preferable that at least one kind is contained in an amount of 0.001 to 0.10%, and the balance is iron and inevitable impurities. There is no particular limitation on the steel structure other than the region up to a depth of 5 mm from the cooling surface of the rail head of the present invention, and a conventional structure may be used.

A long rail having the composition shown in Table 1 and hot-rolled at 900°C is inserted into the cooling device almost simultaneously over its entire length, and the header is brought close to the rail from a state where the surface temperature of the rail head is 770°C. Cooled by air. The temperature at the corner of the rail head during cooling was measured using a radiation thermometer, and the cooling rate of the rail head surface was measured. From the start of accelerated cooling until the temperature rises due to transformation heat generation near the rail surface, cooling is performed at a rate of 5.5℃/s, and after the temperature rise due to transformation heat generation near the surface ends, cooling is performed at 1.5℃/s. Cooled. After the accelerated cooling was completed, the rail was taken out from the cooling device and transported to the cooling bed. At this time, the temperature of the surface of the head of the rail while it was being transported to the cooling bed was measured, and this was taken as the temperature of the surface of the head of the rail at the completion of recuperation. The cooling time after the temperature rise due to transformation heat generation in the vicinity of the surface was completed was adjusted so that this temperature became a predetermined value.

A sample was cut from the rail at room temperature according to JIS Z 2243, and the hardness was measured at the surface position at the center of the head and at a position 23 mm inside. We investigated the rate. The results are shown in Table 2. In addition, the measured surface temperature history was reproduced using THERMEC, and the transformation behavior during cooling was investigated. All structures other than pearlite were bainite. In addition, when creating a constant temperature transformation curve for the components shown in Table 1, the transformation temperature range of pearlite was 750 to 525°C. A surface hardness of HB430 or higher was judged to be good. Moreover, the internal hardness was judged to be good if the internal hardness was HB385 or higher. The range of the present invention is a pearlite production rate of 85% or more at room temperature, and it was determined that the higher the pearlite production rate at room temperature, the better the structure. Note that the value obtained by subtracting each pearlite production rate from 100% at the end of accelerated cooling is regarded as the remaining amount of austenite.

In Example 1, the temperature of the rail head surface at the completion of reheating was 610° C., so the hardness and texture were good. In Example 2, the temperature of the rail head surface upon completion of reheating was 550° C., so the hardness was further increased compared to Example 1. In both examples, the pearlite transformation rate on the rail surface immediately after the end of accelerated cooling was 35%, but pearlite transformation occurred during the subsequent reheating process.

On the other hand, in the comparative example, the pearlite transformation rate on the rail surface immediately after the end of accelerated cooling was 35%, and the temperature of the rail head surface at the end of recuperation was 450°C, so the recuperation process after the end of accelerated cooling was And perlite did not metamorphose. As a result, a large amount of bainite was generated near the surface, resulting in a significant decrease in surface hardness.

Claims (2)

The surface hardness of the rail head is HB430 or higher, the internal hardness at the 23mm position inside the rail is HB385 or higher, and the rail head has a temperature of 5mm deep from the cooling surface of the rail head. A method for manufacturing a rail, wherein the bainite production rate in the range of 15% or less, the remainder being pearlite,
70% or less of the area at a depth of 5 mm from the cooling surface of the rail head at the end of the accelerated cooling is austenite,
A method for producing a rail, wherein the temperature of the rail head surface at the completion of recuperation occurring at the end of accelerated cooling is greater than or equal to the lower limit of the pearlite transformation temperature range and less than or equal to the lower limit of the pearlite transformation temperature range +50°C. The rail manufacturing method according to claim 1, wherein the rail is allowed to cool after the accelerated cooling is completed, and after the temperature of the rail head surface becomes 200° C. or less, the rail is cooled at a rate of 1° C./s or more.

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