JP7522984B1 - Rail and manufacturing method thereof - Google Patents

Abstract

Provided is a rail with excellent resistance to breakage of the rail web portion. The rail has a chemical composition containing 0.70-1.20 mass% C, 0.10-1.20 mass% Si, 0.10-1.80 mass% Mn, 0.035 mass% or less P, 0.020 mass% or less S, and 0.05-1.80 mass% Cr, with the balance being Fe and unavoidable impurities, and when the Vickers hardness at a position 0.5 mm deep from the surface of the rail web portion is measured within a range of ±17.5 mm above and below the center position of the rail height, the average Vickers hardness is Hv280 or more with a standard deviation of 5 or less.

Description

The present invention relates to a rail and a method for manufacturing the same.

In high-axle-load railways, which are primarily used for transporting ore, the load on the axles of freight cars is much higher than that of passenger cars, and the environment in which the rails are used is also harsh. For this reason, steel with a pearlite structure has traditionally been used for the rails, with an emphasis on wear resistance.

In recent years, efforts have been made to further increase the load capacity of freight cars in order to improve the efficiency of railway transport. In addition, the number of wheels passing over the rails is also increasing due to increased transport capacity.

When wheels pass over the rails, repeated bending stress is applied to the web of the outer gauge rail laid in the curved section. As a result, the frequency of rail replacement due to web breakage is increasing year by year. For this reason, there is a demand for rail steel with improved resistance to breakage of the rail web.

In light of the above-mentioned background, various studies have been conducted focusing on the rail web portion. For example, Patent Document 1 discloses a method of rapidly cooling the web portion at a cooling rate of 15°C/sec or more, then stopping the cooling at a temperature of 250-450°C, and when bainite transformation reaches 30% or more, cooling to below the Ms point to obtain a martensite structure, thereby forming a highly tough tempered martensite structure in the web portion.

Patent Document 2 discloses a method for manufacturing rails in which compressive residual stress is imparted by cooling from the top to the upper neck or bottom with high-pressure gas or hydrated gas, thereby improving the fracture resistance of the rail fastening parts.

Patent Document 3 discloses a rail with excellent fatigue damage resistance in the web portion, which has a specified chemical composition, in which 90% or more by area of the metal structure of the cross section of the rail web portion is pearlite structure, the minimum hardness value of the cross section of the rail web portion is Hv300 or more, and the difference between the maximum and minimum hardness values of the cross section of the rail web portion is Hv40 or less.

Japanese Patent Publication No. 62-99438 Japanese Patent Publication No. 59-47326 International Publication No. 2020/189232

However, the above conventional technologies have the following problems that need to be solved. The technology disclosed in Patent Document 1 has low production efficiency because it is necessary to maintain the temperature until the bainite transformation starts. In addition, the technology disclosed in Patent Document 2 places the highest priority on obtaining the wear resistance/fatigue damage resistance of the head, so the desired crack propagation inhibition ability of the abdomen is not necessarily obtained, and depending on the manufacturing conditions, there is a possibility of generating a martensite structure with high crack sensitivity. Furthermore, with the technology described in Patent Document 3, depending on the combination of ingredients and manufacturing conditions, there may be variations in surface hardness, and it is difficult to say that the fracture resistance of the abdomen is sufficient.

The present invention has been made to advantageously solve the above-mentioned problems, and aims to provide a rail with excellent resistance to breakage in the rail web portion, together with a manufacturing method thereof.

To solve the above problems, the inventors produced rails with different C, Si, Mn and Cr contents and thoroughly investigated the hardness and three-point bending characteristics of the rail web. As a result, they found that excellent breakage resistance can be obtained by ensuring a certain level of hardness on the web surface and by strictly controlling the variation in hardness at the above-mentioned position.

The present invention is based on the above findings, and has the following gist and configuration.
[1] C: 0.70 to 1.20% by mass,
Si: 0.10 to 1.20 mass%,
Mn: 0.10 to 1.50 mass%,
P: 0.035% by mass or less,
S: 0.020% by mass or less, and
Cr: 0.05 to 1.80 mass%;
The balance is Fe and unavoidable impurities.
When the Vickers hardness at a depth of 0.5 mm from the surface of the rail web is measured within a range of ±17.5 mm above and below the center of the rail height, the average Vickers hardness is Hv280 or more with a standard deviation of 5 or less.
rail.
[2] The composition further comprises:
V: 0.30% by mass or less,
Cu: 1.0 mass% or less,
Ni: 1.0 mass% or less,
Nb: 0.05% by mass or less,
Mo: 2.0% by mass or less,
Al: 0.07% by mass or less,
W: 1.0% by mass or less,
Co: 1.0 mass% or less,
B: 0.005% by mass or less,
Ti: 0.05% by mass or less,
Sb: 0.05% by mass or less,
Mg: 0.01% by mass or less,
Ca: 0.02% by mass or less, and
The rail of [1], containing at least one selected from the group consisting of Sn: 0.05% by mass or less.
[3] A method for manufacturing a rail according to [1] or [2],
In manufacturing a rail by hot rolling a steel material having the chemical composition described in [1] or [2], cooling after hot rolling is performed so that the average cooling rate from a cooling start temperature of 750°C or higher to a cooling stop temperature of 450 to 600°C at each of the following positions in the rail web portion: the center position of the rail height, a position 20 mm above the center position of the rail height, and a position 20 mm below the center position of the rail height is 0.4 to 5.0°C/sec, and the difference in the average cooling rate at each position is within 0.5°C/sec.
A method for manufacturing rails.

According to the present invention, it is possible to provide a rail with excellent resistance to fatigue breakage of the rail web portion, together with a manufacturing method thereof. The rail of the present invention contributes to extending the service life of rails for high axle load railways and preventing railway accidents, and is industrially useful. The rail manufacturing method of the present invention can stably manufacture the rail of the present invention, and is industrially useful.

FIG. FIG. 1 is a diagram showing positions for taking test pieces for measuring Vickers hardness of a rail web portion. FIG. 1 is a schematic diagram showing a method for cooling the rail web portion. FIG. 1 is a diagram showing positions for taking test pieces for a three-point bending test of a rail web portion. FIG. 1 is a diagram illustrating the shape of a test piece for a three-point bending test of a rail web portion.

<Rail parts>
First, the designations of each part of a rail according to the present invention will be explained with reference to the rail cross-sectional view of Fig. 1. In the rail 1 shown in Fig. 1, 11 denotes a rail head portion, 12 denotes a rail web portion, and 13 denotes a rail foot portion. In the following, the rail head portion, rail web portion, and rail foot portion may also be referred to as the head portion, web portion, and foot portion, respectively.

<Rail composition>
Next, the chemical composition of the steel for the rail of the present invention will be described. In the following description, "%" refers to "mass %" unless otherwise specified.

C: 0.70-1.20%
C is an essential element for ensuring the strength of the pearlite structure, i.e., breakage resistance. If the C content is less than 0.70%, it is difficult to obtain excellent breakage resistance. If the C content exceeds 1.20%, a large amount of proeutectoid cementite is generated at the austenite grain boundaries during cooling after hot rolling, resulting in a decrease in breakage resistance. Although proeutectoid cementite is present even when the C content is 1.20% or less, the amount generated is so small that the effect on breakage resistance is minor. From these points of view, the C content is set to the range of 0.70 to 1.20%, preferably 0.70 to 0.89%, and more preferably 0.70 to 0.85%.

Si: 0.10-1.20%
In addition to its effect as a deoxidizer, Si is an element that increases the pearlite equilibrium transformation temperature and refines the lamellar spacing, thereby strengthening the pearlite structure, i.e., improving breakage resistance. From this point of view, the Si content must be 0.10% or more, but if it exceeds 1.20%, bainite and martensite structures are likely to occur in the surface layer, which promotes hardness variation and results in a decrease in breakage resistance. Furthermore, the amount of Si oxide increases, which also deteriorates weldability. From these points of view, the Si content is set to a range of 0.10 to 1.20%, preferably 0.15 to 1.10%, and more preferably 0.20 to 1.00%.

Mn: 0.10-1.50%
Mn is an element that contributes to strengthening the pearlite structure, i.e., improving breakage resistance, by lowering the pearlite transformation temperature and narrowing the lamellar spacing. If the Mn content is less than 0.10%, sufficient effects cannot be obtained. On the other hand, if the Mn content exceeds 1.50%, bainite or martensite structures tend to form in the surface layer, which promotes hardness variation and, as a result, reduces breakage resistance. Furthermore, since Mn has the effect of shifting the eutectoid point to the low C side, excessive addition promotes the formation of pro-eutectoid cementite, resulting in a decrease in breakage resistance. From these points of view, the Mn content is set to a range of 0.10 to 1.50%, preferably 0.20 to 1.40%, and more preferably 0.30 to 1.30%.

P: 0.035% or less
If P is contained in an amount exceeding 0.035%, it deteriorates the breakage resistance and ductility. Therefore, the P content is set to 0.035% or less, and preferably 0.020% or less. The lower limit of the P content is not particularly limited and may be 0%, but in industry, it is usually more than 0%, and excessively reducing the P content leads to an increase in refining costs. From the viewpoint of economic efficiency, it is preferable that the P content is set to 0.001% or more.

S: 0.020% or less
S is an element that exists in steel mainly in the form of A-type inclusions, but if the S content exceeds 0.020%, the amount of inclusions increases significantly and coarse inclusions are generated, which leads to deterioration of breakage resistance and ductility. Therefore, the S content is set to 0.020% or less, preferably 0.015% or less, and more preferably 0.010% or less. The lower limit of the S content is not particularly limited and may be 0%, but it is usually more than 0% industrially, and excessive reduction of the S content leads to an increase in refining costs. From the viewpoint of economic efficiency, it is preferable to set the S content to 0.0005% or more.

Cr: 0.05-1.80%
Cr is an element that increases the pearlite equilibrium transformation temperature and refines the lamellar spacing, thereby strengthening the pearlite structure, i.e., improving breakage resistance. If the Cr content is less than 0.05%, sufficient effects cannot be obtained. On the other hand, if the Cr content exceeds 1.80%, the hardenability of the steel increases and bainite or martensite structures tend to form in the surface layer, which promotes hardness variation and results in a decrease in breakage resistance. From these points of view, the Cr content is set to the range of 0.05 to 1.80%, preferably 0.10 to 1.60%, and more preferably 0.15 to 1.40%.

The component composition of the rail used in the present invention may contain, in addition to the above essential components, at least one selected from the following:
V: 0.30 mass% or less, Cu: 1.0 mass% or less, Ni: 1.0 mass% or less, Nb: 0.05 mass% or less, Mo: 2.0 mass% or less, Al: 0.07 mass% or less, W: 1.0 mass% or less, Co: 1.0 mass% or less, B: 0.005 mass% or less, Ti: 0.05 mass%, Sb: 0.05 mass% or less, Mg: 0.01 mass% or less, Ca: 0 .02 mass% or less, and Sn: 0.05 mass% or less.
The reasons for the above optional elements will be explained below.

V: 0.30% or less
V is an element that forms carbonitrides in steel and disperses and precipitates in the matrix, improving breakage resistance. If the V content exceeds 0.30%, not only does breakage resistance and ductility deteriorate, but the alloy cost, i.e., the rail manufacturing cost, also increases. From these points of view, when the composition contains V, it is preferable that the V content be set at an upper limit of 0.30%. From the viewpoint of exerting the effect of improving breakage resistance, the V content is preferably 0.001% or more. A more preferable range of the V content is 0.001 to 0.15%.

Cu: 1.0% or less
Like Cr, Cu is an element that can further increase the strength of steel through solid solution strengthening. If the Cu content exceeds 1.0%, Cu cracking is likely to occur, so if the composition contains Cu, the Cu content is preferably 1.0% or less. The more preferable range of Cu content is 0.001 to 0.5%.

Ni: 1.0% or less
Ni is an element that can increase the strength of steel without deteriorating ductility. In addition, when added in combination with Cu, it is possible to suppress Cu cracking, so it is desirable to also include Ni when the composition contains Cu. However, if the Ni content exceeds 1.0%, the hardenability of the steel increases, the amount of martensite and bainite formed increases, and the fracture resistance decreases. From these points of view, when the composition contains Ni, it is preferable that the Ni content is 1.0% or less. A more preferable range for the Ni content is 0.001 to 0.5%.

Nb: 0.05% or less
Nb combines with C in the steel and precipitates as carbides during and after hot rolling to form rails, effectively reducing the size of pearlite colonies, and as a result, greatly improves breakage resistance, wear resistance, fatigue damage resistance, and ductility, and is an element that greatly contributes to extending the life of rails. However, even if the Nb content exceeds 0.05%, the improvement effect of each property is saturated and no effect commensurate with the increase in content can be obtained. From these points, when the composition contains Nb, the upper limit of the Nb content is preferably 0.05%. From the viewpoint of obtaining a sufficient effect on extending the life of rails, the Nb content is preferably 0.001% or more. A more preferable range of the Nb content is 0.001 to 0.03%.

Mo: 2.0% or less
Mo is an element that can further increase the strength of steel through solid solution strengthening. Mo also has the effect of shifting the eutectoid point to the high C side, and therefore also has the effect of suppressing the formation of pro-eutectoid cementite. However, if the Mo content exceeds 2.0%, the amount of bainite generated in the steel increases, and the breakage resistance decreases. From these points of view, when the composition contains Mo, the Mo content is preferably 2.0% or less. From the viewpoint of increasing strength, the Mo content is preferably 0.001% or more. The more preferable range of the Mo content is 0.001 to 1.0%.

Al: 0.07% or less
Al is an element that can be added as a deoxidizer. If the Al content exceeds 0.07%, a large amount of oxide-based inclusions are generated in the steel due to the high bonding strength of Al with oxygen, resulting in a decrease in the breakage resistance and ductility of the steel. Therefore, when the composition contains Al, the Al content is preferably 0.07% or less. There is no particular lower limit for the Al content, but it is preferably 0.001% or more for deoxidation. The more preferable range of the Al content is 0.001 to 0.03%.

W: 1.0% or less
W is an element that precipitates as carbides during and after hot rolling to form the rail shape, improving the strength and breakage resistance of the rail through precipitation strengthening. If the W content exceeds 1.0%, martensite is formed in the steel, resulting in a decrease in breakage resistance. From these points of view, when the composition contains W, the W content is preferably 1.0% or less. There is no particular lower limit for the W content, but in order to exert the effect of improving the strength and breakage resistance described above, it is preferably 0.001% or more. A more preferable range for the W content is 0.001 to 0.5%.

Cobalt: 1.0% or less
Co is an element that can increase the pearlite equilibrium transformation temperature and refine the lamellar spacing, thereby further increasing the strength of the steel. Co also has the effect of suppressing the precipitation of proeutectoid cementite. If the Co content exceeds 1.0%, martensite is formed in the steel, resulting in a decrease in breakage resistance. From these points of view, when the composition contains Co, the Co content is preferably 1.0% or less. There is no particular limit to the lower limit of the Co content, but it is preferably 0.001% or more in order to increase the strength. A more preferable range of the Co content is 0.001 to 0.5%.

B: 0.005% or less
B is an element that precipitates as a nitride in steel during and after hot rolling to form a rail shape, improving the strength and breakage resistance of the steel through precipitation strengthening. If the B content exceeds 0.005%, martensite is formed, resulting in a decrease in breakage resistance. From these points of view, when the composition contains B, the B content is preferably 0.005% or less. There is no particular limit to the lower limit of the B content, but in order to exert the effect of improving the above-mentioned strength and breakage resistance, it is preferably 0.001% or more. The B content is more preferably 0.001 to 0.003%.

Ti: 0.05% or less
Ti is an element that precipitates in steel as carbides, nitrides, or carbonitrides during and after hot rolling to form a rail shape, improving the strength and breakage resistance of the steel through precipitation strengthening. If the Ti content exceeds 0.05%, coarse carbides, nitrides, or carbonitrides are formed, resulting in a decrease in breakage resistance. From these points of view, when the composition contains Ti, the Ti content is preferably 0.05% or less. There is no particular limit to the lower limit of the Ti content, but in order to exhibit the effect of improving the above-mentioned strength and breakage resistance, it is preferably 0.001% or more. A more preferred range of the Ti content is 0.001 to 0.03%.

Sb: 0.05% or less
Sb is an element that has a remarkable effect of preventing decarburization of steel during reheating of rail steel material in a heating furnace before hot rolling. If the Sb content exceeds 0.05%, it has a negative effect on breakage resistance and toughness, so if the composition contains Sb, the Sb content is preferably 0.05% or less. There is no particular lower limit for the Sb content, but in order to exert the effect of reducing the decarburized layer, it is preferably 0.001% or more. A more preferable range for the Sb content is 0.001 to 0.03%.

Mg: 0.01% or less
Mg is an element that combines with oxygen to precipitate MgO, thereby further increasing strength. If the Mg content exceeds 0.01%, the increase in MgO adversely affects the breakage resistance and toughness of the steel, so if the composition contains Mg, the Mg content is preferably 0.01% or less. There is no particular lower limit for the Mg content, but in order to exert the above-mentioned strength-improving effect, it is preferably 0.001% or more. A more preferred range for the Mg content is 0.001 to 0.005%.

Ca: 0.02% or less
Ca is an element that combines with oxygen to precipitate CaO, thereby further increasing strength. If the Ca content exceeds 0.02%, the increase in CaO adversely affects the breakage resistance and toughness of the steel, so if the composition contains Ca, the Ca content is preferably 0.02% or less. There is no particular lower limit for the Ca content, but in order to exert the above-mentioned strength-improving effect, it is preferably 0.001% or more. A more preferred range for the Ca content is 0.001 to 0.01%.

Sn: 0.05% or less
Sn is an element that has a remarkable effect of preventing decarburization of steel during reheating of rail steel material in a heating furnace before hot rolling. If the Sn content exceeds 0.05%, it has a negative effect on the ductility and toughness of the steel, so if the composition contains Sn, the Sn content is preferably 0.05% or less. There is no particular lower limit for the Sn content, but in order to exert the effect of reducing the decarburized layer, it is preferably 0.001% or more. A more preferable range of the Sn content is 0.001 to 0.01%.

In the composition of the steel for the rail of the present invention, the balance of the above essential and optional components consists of Fe and unavoidable impurities. Here, examples of unavoidable impurities include N and O, with N being permitted up to 0.008% and O being permitted up to 0.004%. Note that, depending on the conditions of raw materials, materials, manufacturing equipment, etc., impurities other than N and O may be inevitably mixed into the steel. Examples of raw materials include iron ore, reduced iron, and scrap. The above impurities are permitted as long as they do not impede the objectives of the present invention. Examples of impurities other than N and O include Pb, Zr, Bi, Zn, Se, As, Te, Tl, Cd, Hf, Ag, Hg, Ga, Ge, and REM.

<Rail microstructure>
The rail of the present invention is a pearlitic rail, and the microstructure of the rail is pearlite in an area ratio of 95% or more. The remaining structure other than pearlite is permissible so long as the total area ratio is 5% or less since it does not significantly affect the fatigue crack propagation resistance. Examples of the remaining structure include ferrite, pro-eutectoid cementite, bainite, and martensite.

<Vickers hardness>
In the present invention, it is not enough for the composition of the components to simply satisfy the above ranges, and in order to obtain excellent breakage resistance in the web portion, it is important to control the Vickers hardness Hv at a position 0.5 mm deep from the surface of the rail web portion within a range of 17.5 mm above and below the center position of the rail height (a total range of 35 mm) within a specified range. In other words, the average Vickers hardness of the above locations is set to 280 or more, with its standard deviation not more than 5. In order to stably obtain the effect of improving breakage resistance, it is preferable that the average Vickers hardness of the above locations is set to Hv300 or more, with its standard deviation not more than 4.

In Figure 2, the rail height is represented by the length A from the bottom surface of the foot to the top, and the central position of the rail height is the central position of the rail height A (position A/2). A range of 17.5 mm above and below the central position of the rail height (a range of 35 mm in total) is within the shaded area in Figure 2. The Vickers hardness of the present invention can be measured, for example, with a pressing load of 98 N, at a position 0.5 mm deep from the surface of the location, over a range of 17.5 mm above and below the central position of the rail height (a range of 35 mm in total), at 1 mm intervals from the top to the bottom of the rail.

The reason why the hardness is set within the specified range of 17.5 mm above and below the center position of the rail height (a total range of 35 mm) is as follows.

As mentioned above, repeated "bending stress" is applied to the web of an outer gauge rail laid in a curved section. Meanwhile, roll marks and engravings are applied within an area of 17.5 mm vertically from the center of the rail height on the web of the rail. The inventors have discovered that when "bending stress" is applied, a portion of this roll mark or engraving becomes a source of stress concentration, and breakage occurs from this area. The present invention is based on this knowledge, and controls the hardness of the rail web within a predetermined range within an area of 17.5 mm vertically from the center of the rail height.

In this way, by strengthening the pearlite structure in the rail web portion and strictly controlling the variation in hardness in the surface layer of the rail web portion, it becomes possible to suppress stress concentration in locally softened areas, significantly improving the breakage resistance of the rail web portion. Excellent breakage resistance of the rail web portion is a characteristic generally required of rails, and is effective in extending the service life and preventing railway accidents for rails used in straight sections and rails that do not have roll marks or engravings on the rail web portion.

<Rail shape>
The shape of the rail in the present invention is not particularly limited, and may be any shape described in JIS E 1101:2001, BS EN13674-1:2011, American Railway Engineering and Maintenance-of-Way Association (AREMA), or the like.

<Rail manufacturing method>
The method for manufacturing a rail of the present invention will now be described. The rail of the present invention can be manufactured by sequentially carrying out the following treatments on a steel material having the above-mentioned chemical composition.

The steel material used as the rail material has the above-mentioned rail composition and can be manufactured by any method. In general, it is preferable to manufacture the steel material by casting, particularly continuous casting.

(1) Hot Rolling After the steel material is heated, it can be hot rolled into the shape of a rail.

(Heating temperature)
In the heating of the steel material prior to hot rolling, the heating temperature is preferably 1350° C. or less. If the heating temperature exceeds 1350° C., the steel material may partially melt due to an excessive temperature rise, which may cause defects inside the rail. On the other hand, although there is no particular lower limit for the heating temperature, it is preferably 1150° C. or more in order to reduce the deformation resistance during rolling.

(rolling finish temperature)
In the hot rolling, the rolling finish temperature is preferably 850°C or higher. If the rolling finish temperature is lower than 850°C, the rolling is performed in the low austenite temperature range, and processing strain is introduced into the austenite crystal grains, which makes it easier for the hardness of the pearlite structure generated by accelerated cooling to vary. Therefore, the rolling finish temperature is preferably 850°C or higher. There is no particular upper limit to the rolling finish temperature, but if the prior austenite grain size becomes extremely coarse, the breakage resistance and toughness will decrease, so it is preferably 1050°C or lower. Here, the rolling finish temperature is the surface temperature of the rail web part at the entrance side of the final rolling mill, and can be measured with a radiation thermometer.

Other hot rolling conditions are not particularly limited.

(2) Accelerated cooling Rails can be obtained by cooling after hot rolling. Accelerated cooling is performed on the rail web portion. When this is done, assuming that the rail height is A, the average cooling rate from the cooling start temperature of 750°C or higher to the cooling stop temperature of 450 to 650°C at each of the following positions is controlled to be 0.4 to 5.0°C/sec, from the center position of the rail height (A/2 position), a position 20 mm above the center position of the rail height (upper 20 mm position), and a position 20 mm below the center position of the rail height (lower 20 mm position), and the difference in the average cooling rate at these three positions is controlled to be within 0.5°C/sec.

The accelerated cooling method is not particularly limited, and can be performed, for example, by cooling using online heat treatment equipment. The cooling medium is not particularly limited, and one or more selected from air, spray water, mist, etc. can be used, but it is preferable to use air.

In this accelerated cooling, if the average cooling rate at the center position of the rail height (position A/2) or at positions 20 mm above and below the center position of the rail height (the upper 20 mm position and the lower 20 mm position) is less than 0.4°C/sec, the lamellar spacing of the abdominal surface layer will become coarse, and pro-eutectoid cementite will be more likely to form, resulting in a decrease in breakage resistance. In addition, the cooling time in the low temperature range will increase, which may decrease productivity and increase the manufacturing cost of the rail. Therefore, the average cooling rate at each of the above three positions is set to 0.4°C/sec or more, preferably 1.0°C/sec or more. On the other hand, if any of the average cooling rates at the above three positions exceeds 5.0°C/sec, bainite or martensite structures will form and breakage resistance will decrease. Therefore, the average cooling rate in the above range is set to 5.0°C/sec or less, preferably 4.0°C/sec or less.

Furthermore, if the difference in the average cooling rates at the above three positions exceeds 0.5°C/sec, the variation in hardness of the rail web surface layer will increase, making it easier for stress to concentrate in locally softened areas when bending stress is applied, and reducing breakage resistance. Therefore, the difference in the average cooling rates at the above three positions should be within 0.5°C/sec, and preferably within 0.3°C/sec. The difference in the average cooling rates at the three positions is the difference between the largest and smallest average cooling rates among the average cooling rates at the three positions.

Here, we will explain how to control the average cooling rate at the center position of the rail height (position A/2), the position 20 mm above, and the position 20 mm below. With regard to heat inflow, the areas above and below the A/2 position are susceptible to the effects of radiant heat and thermal conduction from the rail head and foot, respectively. If the average cooling rate at the three positions of A/2, the position 20 mm above, and the position 20 mm below are controlled within an appropriate range (0.4 to 5.0°C/sec), and the difference in the average cooling rate at the three positions is controlled within a certain range (within 0.5°C/sec), it is estimated that the average cooling rate within a range of 20 mm above and below the A/2 position (a total range of 40 mm) will be appropriate and uniform.

By appropriately strengthening the accelerated cooling on the upper side of the A/2 position and the lower side of the A/2 position, the difference in average cooling rate at the above three positions can be stably kept within 0.5℃/sec.

For example, the nozzles can be installed in three tiers in the height direction so that they directly hit the A/2 position, the upper side of the A/2 position, and the lower side, respectively, and the injection amount and type of coolant at the three positions can be changed according to the temperature and shape of the rail. For example, as shown in Figure 3, the air nozzles can be installed in three tiers, with the injection amount at the top 20 mm position set to the highest flow rate, the injection amount at the bottom 20 mm position set to the second highest flow rate, and the injection amount at the A/2 position set to the lowest flow rate.

Representative temperatures for determining the average cooling rate in accelerated cooling are the surface temperatures at position A/2, the position 20 mm above, and the position 20 mm below. These can be measured with a radiation thermometer. Here, the cooling start temperature is the surface temperature of the rail web part at the start of accelerated cooling, measured with a radiation thermometer, and the cooling stop temperature is the surface temperature of the rail web part after accelerated cooling has stopped (before reheating), measured with a radiation thermometer.

In the manufacturing method of the present invention, it is important to cool the rail web portion after hot rolling so that the surface temperature satisfies the above-mentioned condition. As long as this condition is satisfied, the cooling method for other parts of the rail (rail head, rail foot, etc.) is not particularly limited. The rail head and rail foot may be naturally cooled or may be subjected to accelerated cooling.

(3) Others After cooling, the rail material may be subjected to known treatments, for example, cold roller straightening.

The present invention will be explained in more detail below with reference to examples. However, the present invention is not limited to the examples, and appropriate modifications can be made within the scope of the spirit of the present invention, and all of these are included in the technical scope of the present invention.

A 60 kg rail material conforming to JIS E1101 was manufactured by heating and hot rolling a steel material having the chemical composition shown in Table 1 under the conditions shown in Table 2, and then accelerated cooling after hot rolling. The average cooling rate (°C/sec) was calculated by converting the temperature change from the start of cooling to the end of cooling per unit time (seconds). After cooling was stopped, the material was allowed to cool naturally.
In the examples, accelerated cooling of the rail web portion was performed using the air nozzles shown in Fig. 3, and when the cooling rate was to be changed, the amount of injected air was appropriately changed. In test No. 34, which is a comparative example, accelerated cooling was performed using only one set of air nozzles installed at the A/2 height among the air nozzles shown in Fig. 3.
At the start and end of the accelerated cooling, a two-dimensional radiation thermometer capable of measuring the temperature distribution in the height direction of the web of the rail was used to measure the surface temperatures at position A/2, the position 20 mm above, and the position 20 mm below, and the average cooling rate at these three positions was calculated.

The obtained rails were pearlitic rails. The Vickers hardness and three-point bending properties of the web portion of each rail were evaluated. The test results are shown in Table 2. Each evaluation item is explained in detail below.

<Vickers hardness>
After the accelerated cooling was completed, the rail tip was cut, and a section was taken from the rail web portion within a range of 20 mm above and below the center position of the rail height (A/2 position) shown in Figure 2 (within a total range of 40 mm), including the rail web surface, to prepare a test piece for Vickers hardness measurement. The test piece was embedded in resin and mirror polished, and the Vickers hardness was measured at a depth of 0.5 mm from the rail web surface, at 36 points with a 1 mm pitch from the top to the bottom of the rail, within a range of 17.5 mm above and below the center position of the rail height (A/2 position) (a total range of 35 mm). The average value and standard deviation were then calculated from each Vickers hardness obtained.

<Three-point bending characteristics>
As shown in Fig. 4, test pieces were taken from the rail web portion at two positions, 17.5 mm above and 17.5 mm below the central position of the rail height (position A/2), so as to include the rail web surface. The shape of the test piece is as shown in Fig. 5, with length L = 100 mm, width H = 10 mm, and thickness B = 10 mm. A notch was formed in the surface of the test piece that corresponds to the rail web surface at the central L/2 part of the length L. The notch depth N was 0.5 mm, the notch width C was 1.0 mm, and the notch bottom had a radius of curvature R = 0.5 mm.
The test piece was placed with the notch facing down on supports with a distance of 50 mm and a radius of curvature of 17 mm, and a bending strain was applied from the opposite side of the notch with an indenter with a radius of curvature of 16 mm at a pressing speed of 0.1 mm/sec. Tests were conducted on the test pieces taken from the above two locations, and if neither broke at a displacement of 1.0 mm, the rail web was evaluated as having excellent breakage resistance.

As shown in Table 2, the test results of the rail materials of the examples of the invention (Test Nos. 1 to 39 in Table 2) showed little variation in hardness of the surface layer of the rail web, and all of them showed good three-point bending characteristics. On the other hand, the comparative examples (Test Nos. 40 to 52 in Table 2) in which the component composition of the rail material did not satisfy the conditions of the present invention or a manufacturing method outside the scope of the present invention was not applied broke before reaching the specified displacement in the three-point bending test.

According to the present invention, it is possible to provide a rail with excellent resistance to breakage of the rail web portion, together with a manufacturing method thereof. The rail of the present invention contributes to extending the service life of rails for high axle load railways and preventing railway accidents, and is industrially useful. The rail manufacturing method of the present invention can stably manufacture the rail of the present invention, and is industrially useful.

1 Rail 11 Rail head (head)
12 Rail bottom (bottom)
13 Rail foot (foot)

Claims (3)

C: 0.70 to 1.20 mass%
Si: 0.10 to 1.20 mass%,
Mn: 0.10 to 1.50 mass%,
P: 0.035% by mass or less,
S: 0.020% by mass or less, and
Cr: 0.05 to 1.80 mass%;
The balance is Fe and unavoidable impurities.
When the Vickers hardness at a depth of 0.5 mm from the surface of the rail web is measured within a range of ±17.5 mm above and below the center of the rail height, the average Vickers hardness is Hv280 or more with a standard deviation of 5 or less.
rail. The composition further comprises:
V: 0.30% by mass or less,
Cu: 1.0 mass% or less,
Ni: 1.0 mass% or less,
Nb: 0.05% by mass or less,
Mo: 2.0% by mass or less,
Al: 0.07% by mass or less,
W: 1.0% by mass or less,
Co: 1.0 mass% or less,
B: 0.005% by mass or less,
Ti: 0.05% by mass or less,
Sb: 0.05% by mass or less,
Mg: 0.01% by mass or less,
Ca: 0.02% by mass or less, and
2. The rail according to claim 1, further comprising at least one selected from the group consisting of Sn: 0.05% by mass or less. A method for manufacturing a rail according to claim 1 or 2,
In manufacturing a rail by hot rolling a steel material having the chemical composition according to claim 1 or 2, cooling after hot rolling is carried out so that the average cooling rate from a cooling start temperature of 750°C or higher to a cooling stop temperature of 450 to 650°C at each of the following positions in the rail web portion: the center position of the rail height, a position 20 mm above the center position of the rail height, and a position 20 mm below the center position of the rail height is 0.4 to 5.0°C/sec, and the difference in the average cooling rate at each position is within 0.5°C/sec.
A method for manufacturing rails.

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