JP2002363698A - Rail having excellent rolling fatigue damage resistance and wear resistance, and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To simultaneously improve the rolling fatigue damage resistance and wear resistance in a passenger railroad and a freight railroad. SOLUTION: The steel rail having excellent rolling fatigue damage resistance and wear resistance has a mixed metallic structure of a bainitic structure and a pearlitic structure. In the optional cross section of the steel rail, the range of the area ratio of the pearlitic structure in the whole structure is 10 to 50%. The rail has components containing, by mass, 0.25 to 0.60% C, 0.05 to 2.00% Si, 0.10 to 3.00% Mn and 0.10 to 3.00% Cr, and the balance Fe with inevitable impurities.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a rail which is required for a passenger railroad or a freight railroad and has improved rolling fatigue damage resistance and abrasion resistance, and a method of manufacturing the same.

[0002]

2. Description of the Related Art In recent years, in passenger railways and freight railways, the speed of trains and the loading of freight cars have been increased in order to improve transportation efficiency. As a result, the rail use environment has become severer, mainly in straight section rails, and the occurrence of rolling fatigue damage on the rail head surface such as dark spot damage due to repeated contact between the rails and wheels has increased. Rolling fatigue damage such as dark spot damage has a characteristic that it is likely to occur on rails in straight sections of passenger railways and freight railways in which rails having a conventional pearlite structure are used.

[0003] The present inventors have considered the formation of a fatigue layer (fatigue damage layer, texture) on the rail head surface generated by repeated contact with wheels, which is considered to be the source of this rolling fatigue damage, and the formation of a metallic structure. Study the relationship. As a result, in a pearlite structure having a layer structure of a ferrite phase and a cementite phase, a fatigue damage layer (a layer in which the structure is crushed) easily accumulates, and further, a texture is easily developed, whereas It is clear that the bainite structure, in which granular hard carbides are dispersed in the ferrite structure, is unlikely to accumulate a fatigue damage layer, furthermore, it is difficult to develop a texture that triggers fatigue damage, and as a result, rolling fatigue damage is unlikely to occur. became.

Further, as a result of experiments, it has been confirmed that the bainite structure has a greater amount of wear than a pearlite structure having the same hardness, and as a result, the fatigue layer (fatigue damage layer, texture) itself on the head surface due to the promotion of wear. Was self-removed, and the occurrence of rolling fatigue damage was suppressed.
From these results, it was confirmed that the bainite structure was superior to the pearlite structure in rolling fatigue resistance.

Therefore, the following products and manufacturing methods have been developed as rails having a bainite structure. A high-strength rail which exhibits a bainite structure as it is rolled by adding a large amount of alloying elements such as Mn, Cr, and Mo in a low-carbon component (Japanese Patent Application Laid-Open No. 5-271871). Manufacture of high-strength bainite rails with low carbon content, alloy elements such as Mn, Cr, Mo, etc. added, and high temperature heat-treated rails after hot rolling, or accelerated cooling of rail heads heated to high temperatures Method (Japanese Patent Laid-Open No. 7-34132). The feature of these rails is that, in order to stably generate a bainite structure excellent in surface fatigue damage resistance, the amount of carbon is reduced as compared with conventional plain carbon steel rails,
The alloy element is added with a large amount of alloy elements such as n, Cr, and Mo, and is subjected to an appropriate heat treatment to further secure the strength.

[0006]

The above-mentioned bainite structure rail has better rolling fatigue resistance than the conventional rail having a pearlite structure, but the bainite structure has a low hardness. If the contact surface pressure and tangential force of rails / wheels are significantly higher than that of passenger railways, such as in the case of freight railways, darkness may occur due to the above-mentioned systematic effect (the formation of a fatigue layer on the rail head surface is small). Rolling fatigue damage such as spot damage can be prevented, but the bainite structure has a greater amount of wear than the pearlite structure of the same hardness. There was such a problem.

[0007] From such a background, it has been desired to develop a rail having a rolling fatigue resistance and an abrasion resistance in a steel rail having a bainite structure. That is, an object of the present invention is to simultaneously improve the rolling fatigue resistance and the wear resistance in a passenger railway or a freight railway.

[0008]

The present invention attains the above-mentioned object, and its gist is as follows. (1) In the steel rail, the metal structure is a mixed structure of a bainite structure and a pearlite structure, and the area ratio of the pearlite structure in the entire structure is 10 to 50% in an arbitrary cross section of the steel rail. A rail with excellent rolling fatigue resistance and wear resistance characterized by: (2) The rail is expressed in mass%, C: 0.25 to 0.60%, Si: 0.05 to 2.00%, Mn: 0.10 to 3.00%, Cr: 0.10 to 0%. 3.00%, and the balance can be Fe and inevitable impurities. (3) Further, in the above (2) rail, in mass%,
The following components can be selectively contained. Mo: 0.01 to 1.00% Ni: 0.05 to 2.00%, Cu: 0.05 to
1.00%, one or more of Co: 0.10 to 2.00%, V: 0.01 to 0.30%, Nb: 0.005 to
One or two of 0.050%, Ti: 0.005 to 0.050% Mg: 0.0001 to 0.0300%, Ca: 0.
One or two kinds of 0001 to 0.0150%, B: 0.0001 to 0.0050% (4) Also, the rails described in the above (1) to (3) have a head corner portion and a top surface. From the starting point, the hardness at least in the range of 20 mm in depth can be in the range of Hv 300 to 450.

(5) Any one of the above (2) to (4)
The steel slab containing the components described in the section was hot-rolled into a rail shape and then hot-rolled to a steel rail having a temperature of Ar1 point or higher, or heated to a temperature of Ac1 point + 30 ° C or higher for the purpose of heat treatment. In a steel rail, at least the rail head is 0.5 to 10 ° C./se from the austenitic zone temperature.
accelerated cooling at a cooling rate of c, stop accelerated cooling when the temperature of the head of the steel rail reaches 500 to 250 ° C.,
2. A method for producing a rail comprising the structure according to claim 1 and having excellent rolling fatigue damage resistance and wear resistance, wherein the rail is naturally cooled thereafter. (6) After hot rolling a steel slab containing the component according to any one of the above (2) to (4) into a rail shape, the steel rail having a temperature of at least the Ar1 point as hot rolled, or In a steel rail heated to a temperature of at least Ac1 point + 30 ° C. for the purpose of heat treatment, at least the rail head is accelerated and cooled at a cooling rate of 0.5 to 10 ° C./sec from the austenite region temperature, and the head of the steel rail Part temperature is 700-60
When the temperature reaches 0 ° C., the accelerated cooling is stopped and 15 to 600 s
After cooling at ec, accelerated cooling was performed at a cooling rate of 3 ° C./sec or more, and the temperature of the steel rail head was 500 to 25.
A method for producing a rail having excellent rolling fatigue resistance and wear resistance, wherein accelerated cooling is stopped when the temperature reaches 0 ° C., and then natural cooling is performed.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail. The present inventors have studied a method for improving wear resistance of a steel rail having a bainite structure. First, the present inventors investigated the reason why the wear resistance of bainite steel was lower than that of pearlite structure. As a result, it became clear that the bainite structure had a smaller amount of work hardening on the rolling surface than the pearlite structure, and the wear resistance was reduced. Next, the present inventors considered a method of increasing the amount of work hardening on the rolling surface in order to improve the wear resistance. As a result, it was confirmed that the amount of work hardening greatly depends on the metal structure, and it was confirmed that the pearlite structure had the highest amount of work hardening among the metal structures.

Accordingly, the present inventors have devised a method of improving the wear resistance of the bainite structure by mixing a pearlite structure which is harmful to the rolling fatigue damage resistance into the bainite structure. As a result, even if a certain amount of pearlite structure, which is detrimental to rolling fatigue damage resistance, is mixed in bainite structure, surface damage such as dark spot damage does not occur, and the rolling fatigue resistance of rail steel does not deteriorate. In addition, it has been clarified that the abrasion resistance is improved by increasing the work hardening amount of the rolling surface due to the pearlite structure.

The present inventors further studied the optimum area ratio of the pearlite structure in the entire structure of the rail. As a result, it has been found that the area ratio of the pearlite structure needs to be controlled within a certain range in order to simultaneously improve the rolling fatigue resistance and the wear resistance of the rail based on the bainite structure. In addition, the present inventors have studied the rail site and the hardness of the rail based on the bainite structure, in which the area ratio of the pearlite structure needs to be controlled within a certain range. As a result, at least a certain depth range starting from the head corner and the top surface,
It was found that if the hardness of the part is within a certain range, the rolling fatigue resistance and the wear resistance are improved at the same time, and the service life of the rail is most efficiently improved.

[0013] In addition to these inventions, the present inventors have studied the chemical composition of the rail for controlling the area ratio of the pearlite structure within a certain range and the heat treatment manufacturing method in the rail based on the bainite structure. As a result of the experiment, C,
By controlling the addition amounts of Si, Mn, and Cr within a certain range,
At the same time, accelerate the rail head with high temperature heat at a cooling rate within a certain range from the austenite temperature,
After that, it has been found that the area ratio of the pearlite structure in the entire structure of the rail can be controlled by natural cooling or by allowing it to cool for a certain period of time in a high temperature range and then further performing accelerated cooling.

As a result of the above examination in the laboratory, by optimizing the chemical composition range and at the same time, by optimizing the heat treatment manufacturing conditions of the rail head, the bainite structure effective for rolling fatigue resistance and the bainite structure are improved. In a mixed structure of an appropriate amount of pearlite structure effective for abrasion, a certain range of hardness can be imparted, and rolling fatigue damage resistance and abrasion resistance can be simultaneously improved at the rail head. That is, an object of the present invention is to simultaneously improve the rolling fatigue resistance and wear resistance required for a passenger railway or a freight railway.

Hereinafter, the reasons for limitation of the present invention will be described in detail. First, the area ratio of the pearlite structure in the whole structure,
The chemical composition of the rail steel, the necessary range and hardness of the mixed structure of the bainite structure and the pearlite structure, and the reason why the method for producing the rail heat treatment is limited to the above claims will be described in detail.

(1) Area ratio of pearlite structure in entire structure: First, the reason why the area ratio of pearlite structure in all structures is limited to a certain range will be described. When the area ratio of the pearlite structure is less than 10%, an increase in the amount of work hardening on the rolling surface of the rail steel mainly composed of bainite structure cannot be expected, and the wear resistance of the rail steel cannot be improved.
If the area ratio of the pearlite structure exceeds 50%, the accumulation of the fatigue layer (fatigue damage layer, texture) on the rolling surface is likely to progress due to the increase in the pearlite structure, causing dark spot damage and the like to cause the rail head. Significantly reduces the rolling fatigue resistance and the service life of the rail.
For these reasons, the area ratio of the pearlite structure in the entire structure was limited to the range of 10 to 50%. To maximize the rolling fatigue resistance and wear resistance of the rail steel, from the viewpoint of ensuring sufficient rolling fatigue resistance,
It is most desirable that the area ratio of the pearlite structure in the entire structure be in the range of 20 to 40%.

The metal structure of the rail steel of the present invention is desirably a mixed structure of a bainite structure and a pearlite structure. However, a minute amount of martensite structure or ferrite structure may be mixed depending on the manufacturing method. However, even if a small amount of martensite structure or ferrite structure is mixed in the above mixed structure, it does not significantly affect the rolling fatigue resistance and wear resistance of the rail steel. Also includes some mixture of the organizations described above.

(2) Chemical composition of rail steel: Next, the reason for limiting the chemical composition of the rail in the present invention as described above will be described. The unit is mass%. C is
It is an essential element for ensuring the strength and wear resistance of the bainite structure, and is an element that promotes the formation of a pearlite structure. Abrasion resistance cannot be improved. On the other hand, if it exceeds 0.60%, the amount of pearlite structure generated in the mixed structure becomes large, the accumulation of the fatigue layer on the rolling surface proceeds, and the rolling head fatigue resistance to rolling is significantly reduced. Was limited to 0.25 to 0.60%.

Si is an essential element for improving the strength by solid solution hardening of a bainite structure or a pearlite structure to a ferrite matrix and securing the strength of the rail steel. However, if the content is less than 0.05%, the effect is almost expected. It is not possible, and surface damage due to plastic deformation occurs on the rolling surface, and the rolling fatigue resistance is reduced. On the other hand, if the content exceeds 2.00%, surface flaws are likely to be generated during hot rolling of the rail, and the ductility of the pearlite structure is significantly reduced to cause surface damage such as spalling. ~
Limited to 2.00%.

Mn is an element that stably forms a bainite structure by promoting bainite transformation, and at the same time, lowers bainite transformation temperature and pearlite transformation temperature by improving hardenability, thereby securing the strength of rail steel. However, if the Mn content is less than 0.10%, the effect is weak, surface damage due to plastic deformation occurs on the rolling surface, and the rolling fatigue resistance is reduced. Further, depending on the combination of the additional elements, a large amount of ferrite structure is generated, and the wear resistance of the rail steel is reduced. If it exceeds 3.00%,
Since the transformation rate of the bainite structure or the pearlite structure is remarkably reduced and a large amount of martensite structure is generated and the wear resistance of the rail is greatly reduced, the Mn content is reduced to 0.10 to 0.10.
It was limited to 3.00%.

Cr is an important element for lowering the bainite transformation temperature by strengthening hardenability and strengthening the bainite structure. At the same time, it raises the equilibrium transformation point of pearlite and refines the pearlite structure. However, if it is less than 0.10%, the effect is weak, the reinforcement of the rail steel becomes insufficient, and surface damage due to plastic deformation occurs on the rolling surface, and the rolling contact fatigue damage Is reduced. On the other hand, if the content exceeds 3.00%, the transformation rate of the bainite structure or the pearlite structure is remarkably reduced like Mn, and a large amount of martensite structure is generated, and the wear resistance of the rail is greatly reduced.
The amount of Cr was limited to 0.10 to 3.00%.

Further, the rail manufactured with the above-mentioned composition has a stable bainite structure, a sufficient rolling fatigue resistance by strengthening a bainite structure and a pearlite structure, prevention of softening of a heat-affected zone in a welded portion, and a bainite structure. In order to improve the ductility and toughness of the pearlite structure and to control the microstructure generated in the mixed structure, Mo, Ni, Cu, Co, V,
Elements of Nb, Ti, Mg, Ca, and B are added as needed.

Here, Mo stabilizes the bainite transformation, and further improves the hardness of the bainite structure by lowering the bainite transformation temperature. Ni, Cu, and Co aim to strengthen the bainite structure and the pearlite structure by lowering the bainite transformation temperature and strengthening the solid solution of the bainite structure and the pearlite structure in the base ferrite. V and Nb are elements that increase the strength of a bainite structure or a pearlite structure by carbides and nitrides generated in a cooling process after hot rolling and, at the same time, a heat-affected zone reheated to a temperature range below the Ac1 point. The main purpose of addition is to form carbides in the steel and prevent a decrease in the hardness of the heat-affected zone of the weld joint.

[0024] Further, Ti seeks to refine the austenite crystal grains during rail rolling heating, and to improve the ductility and toughness of the bainite structure and the pearlite structure. Mg and Ca finely disperse MnS by generating fine oxides and sulfides, and improve the ductility and toughness of the bainite structure by promoting bainite transformation and pearlite transformation and refinement of the structure. The main purpose of B is to suppress the formation of a pro-eutectoid ferrite structure formed from the prior austenite grain boundaries and to stabilize the wear resistance of the rail steel.

Each of these components will be described in detail below. Mo, like Mn, is an element that stably forms a bainite structure by the effect of promoting bainite transformation, and at the same time, is an element that lowers the bainite transformation temperature by improving hardenability and improves the hardness of the bainite structure. At the same time, it is an element that strengthens the pearlite structure by increasing the equilibrium transformation point of the pearlite to refine the pearlite structure. If the Mo content is less than 0.01%, the effect is not sufficient, and stabilization and strengthening of the bainite structure and strengthening of the pearlite structure cannot be achieved. 1.0
If it exceeds 0%, the transformation rate of the bainite structure or the pearlite structure is reduced as in the case of Mn and Cr, and a large amount of martensite structure is formed, and the wear resistance of the rail is greatly reduced. It was limited to 01 to 1.00%.

Ni is an element that stabilizes austenite, lowers the transformation temperature of bainite, and improves the hardness of the bainite structure. At the same time, Ni is solid-solution strengthened in the ferrite matrix in the bainite structure or pearlite structure. The element strengthens the bainite structure and the pearlite structure. If the content is less than 0.05%, the effect is extremely small, and the bainite structure and the pearlite structure cannot be strengthened. Further, if it is added in excess of 2.00%, the transformation rate of the bainite structure or the pearlite structure is reduced, and a large amount of martensite structure is generated, and the wear resistance of the rail is greatly reduced. To 2.00%.

Cu, like Ni, is an element that stabilizes austenite, lowers the bainite transformation temperature, and improves the hardness of the bainite structure, and at the same time, reduces the ferrite base in the bainite structure and the pearlite structure. It is an element that strengthens the bainite structure and the pearlite structure by solid solution strengthening.
The largest amount is in the range of 00%, and if it exceeds 1.00%, red heat embrittlement is likely to occur.
To 1.00%.

Co is an element that forms a solid solution in the base ferrite having a bainite structure or a pearlite structure and improves wear resistance by solid solution strengthening. Further, Co is formed by increasing the transformation energy of pearlite to promote the pearlite structure. , Is an element that aims to produce a stable pearlite structure,
If it is less than 0.10%, the effect cannot be expected.
Even if excessive addition exceeding 0% is carried out, the effect corresponding to the addition amount is not improved, and the economical efficiency is significantly impaired due to the increase in the addition cost.
Limited to 00%.

V combines with C or N to form V carbide or V nitride in the cooling process after hot rolling.
It is an element that strengthens the bainite structure and pearlite structure by precipitating in the ferrite matrix in the bainite structure and pearlite structure. It is an effective component to prevent the decrease in the hardness of the heat-affected zone of the welded joint. However, if it is less than 0.01%, its effect cannot be expected sufficiently. The V content is limited to 0.01 to 0.30% because carbides of V form and the ductility and toughness of the rail decrease.

Like V, Nb forms carbides of Nb and nitrides of Nb in a cooling process after hot rolling and precipitates on a ferrite matrix in a bainite structure or a pearlite structure, thereby forming a bainite structure or a pearlite structure. At the same time as an element that strengthens the structure, Nb carbide is generated in the heat-affected zone reheated to a temperature range below the Ac1 point, and is an effective component to prevent a decrease in the hardness of the heat-affected zone at the weld joint. However, if the content is less than 0.005%, the effect cannot be expected sufficiently. If the addition is excessively more than 0.05%, an intermetallic compound of Nb or a coarse precipitate is formed, and the ductility of the rail and Since the toughness is reduced, the amount of Nb is set to 0.1.
005-0.05%.

Titanium is used for refining austenite crystal grains during rail rolling heating by utilizing the fact that Ti nitride precipitated during melting and solidification does not dissolve even at the high temperature of rail rolling heating. Although it is an element that contributes to the improvement of the ductility and toughness of the structure, if it is less than 0.005%, these effects are not sufficiently exhibited, and 0.050%
%, Coarse nitrides (TiN) are formed to reduce the ductility and toughness of the rail, and at the same time, become the starting point of fatigue damage during the use of the rail and shorten the service life of the rail. It was limited to 0.005 to 0.050%.

Mg combines with O or S or Al to form a fine oxide, suppresses the growth of crystal grains during reheating during rail rolling, refines austenite grains, and improves the bainite structure. And an element effective for improving the ductility and toughness of the pearlite structure. Furthermore, MgO, Mg
S finely disperses MnS, forms a dilute band of Mn around MnS, promotes bainite transformation and pearlite transformation, and as a result, refines bainite structure and pearlite structure, thereby improving ductility and toughness. It is an effective element to make it. However, if it is less than 0.0001%, the effect is weak, and if it exceeds 0.0300%, a coarse oxide of Mg is formed to deteriorate the ductility and toughness of the rail. Limited to 0300%.

Ca has a strong bonding force with S, forms a sulfide as CaS, and furthermore, CaS finely disperses MnS and forms a dilute band of Mn around MnS to form bainite transformation or pearlite transformation. It is an element effective for improving ductility and toughness by accelerating and, as a result, making the bainite structure and the pearlite structure finer. However, if less than 0.0001%, the effect is weak, and 0.0150%.
%, A coarse oxide of Ca is formed to deteriorate the ductility and toughness of the rail.
It was limited to 1 to 0.0150%.

B is an element that suppresses the formation of a pro-eutectoid ferrite structure formed from the prior austenite grain boundary, and at the same time, forms an iron carbide boride to promote the pearlite transformation. However, if it is less than 0.0001%, the effect is weak,
If added in excess of 0.0050%, coarse nitrides and carbides of B are formed at the austenite grain boundaries, and the ductility and toughness of the rail steel are greatly reduced.
It was limited to 1 to 0.0050%.

(3) Necessary range of mixed structure of bainite structure and pearlite structure: Next, the reason why the mixed structure of the mixed structure of bainite structure and pearlite structure is limited to a certain region of the rail head. Will be described.
In the rail head, the area where the mixed tissue exists,
If the head corners and the top of the head are less than 20 mm from the head surface as a starting point, the area where the rolling fatigue resistance and wear resistance are required in the rail head is small, and the rail service life is sufficiently improved. This is because the effect cannot be obtained. When the range of the mixed structure is 30 mm or more starting from the head surface of the head corner and the head, the effect of improving the life is further increased, which is more preferable.

Here, FIG. 1 shows the designation of the surface position of the cross section of the head of the rail of the present invention which is excellent in the rolling fatigue resistance and the wear resistance. In the rail head, 1 is a crown, 2 is a head corner, and one of the head corners 2 is a gauge corner (GC) part mainly in contact with wheels. Here, the mixed structure of bainite structure and pearlite structure is
At least in the shaded area in the figure, sufficient rail service life can be improved.

(4) Hardness of Mixed Structure of Bainite Structure and Pearlite Structure: Next, the reason why the hardness of the mixed structure of the rail head is limited to the range of Hv 300 to 450 will be described. When the hardness becomes less than Hv300, the G.V.
C. Since metal flow is generated in the part due to strong contact between the rail and the wheel, and surface damage such as creaking and flaking is likely to occur with this, the hardness of the rail head is limited to Hv300 or more. Further, when the hardness exceeds Hv450, the ductility of the mixed damage is significantly reduced, and surface damage such as spalling damage is easily generated on the rail head. Therefore, the hardness of the rail head is set in the range of Hv300 to 450. Limited.

(5) A method for producing a rail by heat treatment:
The reasons why the temperature conditions and the accelerated cooling conditions before the accelerated cooling during the manufacture of the rails are limited as described above at 0 and 11 will be described in detail. First, a temperature condition before the rail head is accelerated and cooled will be described. In order to stably obtain a mixed structure of a bainite structure and a pearlite structure, at least the entire rail head must be austenitized. The temperature of the rail head immediately after rolling is in the temperature range of Ar1 point or higher, and the temperature of the reheated rail head is in the temperature range of Ac1 point + 30 ° C. or higher. Although the upper limit of the temperature is not particularly defined, if the temperature is too high, the liquid phase appears and the austenite phase becomes unstable, so that the upper limit of the temperature is substantially 1450 ° C.

Here, the above-mentioned "rail head" refers to the one shown in FIG.
Are the hatched parts in the figure including the rail top part (reference number: 1) and the head corner part (reference number: 2). The accelerated cooling conditions described below correspond to the top of the rail shown in FIG.
If the temperature is measured in a range of 1 to 5 mm from the head surface of the head corner (code: 2) and at a depth of 2 to 5 mm, at least a range of a depth of the rail head of 20 mm can be represented. It is possible to control at least the metal structure and hardness of the hatched portion shown in FIG.

Next, the rail head is moved 0.5 degrees from the above temperature range.
In a method of performing accelerated cooling at a cooling rate of -10 ° C / sec, stopping accelerated cooling when the head of the steel rail reaches 500 to 250 ° C, and then naturally cooling, an accelerated cooling rate and an accelerated cooling stop temperature are provided. Will be described below. First, the reason why the accelerated cooling rate from the austenite region is limited to the range of 0.5 to 10 ° C./sec will be described.

When accelerated cooling at a rate of less than 0.5 ° C./sec in the above-mentioned component system, a large amount of pearlite structure is formed, dark spot damage is generated on the rail head, and rolling fatigue resistance of the rail head is remarkable. descend. Furthermore, many bainite structures with low hardness are generated in a high temperature range, and surface damage such as flaking damage due to plastic deformation is likely to occur. When accelerated cooling at more than 10 ° C./sec, a sufficient amount of pearlite structure is not generated depending on the component system.
Since a bainite structure is generated, it becomes difficult to improve the wear resistance of the rail steel. For these reasons, the accelerated cooling rate range was limited to the range of 0.5 to 10 ° C./sec.

Next, the reason why the accelerated cooling stop temperature from the austenite region temperature is limited to the range of 500 to 250 ° C. will be described. When cooling is stopped in this component system beyond 500 ° C, bainite transformation starts in the high temperature range immediately after accelerated cooling, bainite structure with low hardness is generated, and flaking damage due to plastic deformation occurs at the rail head. Etc. are likely to occur. Further, when the temperature is cooled to less than 250 ° C., a large amount of martensite structure is generated during accelerated cooling depending on the component system. Furthermore, even in the cooling zone after accelerated cooling, the bainite transformation does not proceed, and a large amount of the remaining austenite structure is transformed into a martensite structure, and the wear resistance of the rail steel is significantly reduced. For these reasons, the accelerated cooling stop temperature from the austenite region temperature is limited to the range of 500 to 250 ° C.

Next, the rail head is moved from the above temperature range by 0.5.
After accelerated cooling at a cooling rate of 〜１０10 ° C./sec, the accelerated cooling is stopped when the temperature of the head of the steel rail reaches 700 to 600 ° C., and after cooling for 15 to 600 seconds, the cooling rate is further increased to 3 ° C./sec. In the method of accelerated cooling in seconds or more, and stopping the accelerated cooling when the temperature of the head of the steel rail reaches 500 to 250 ° C., and then naturally cooling, the accelerated cooling rate and the accelerated cooling stop temperature are set as described above. The reason for the limitation is described.

In a component system having a small addition amount of C, which has a high effect of promoting the pearlite transformation, or a component system containing a large amount of an element stabilizing the bainite transformation such as Mn or Mo, the pearlite transformation region is shifted to a longer time side. During the accelerated cooling from the austenite region temperature, a sufficient amount of pearlite structure may not be obtained, and the wear resistance of the rail steel may not be improved. Therefore, the above heat treatment is performed for the purpose of stably obtaining a mixed structure of a bainite structure and a pearlite structure.

First, the reason why the accelerated cooling rate from the austenite region is limited to the range of 0.5 to 10 ° C./sec will be described. When accelerated cooling at less than 0.5 ° C / sec in the above component system, a large amount of pearlite structure is formed in the subsequent cooling zone, dark spot damage is generated on the rail head, and rolling fatigue damage of the rail head is caused. Properties are significantly reduced. Further, when accelerated cooling at more than 10 ° C./sec, depending on the component system, a pearlite structure is not sufficiently generated in the subsequent cooling region, and it becomes difficult to improve the wear resistance of the rail steel. For these reasons, the accelerated cooling rate range was limited to the range of 0.5 to 10 ° C./sec.

Next, the reason why the accelerated cooling stop temperature from the austenite region temperature is limited to the range of 700 to 600 ° C. will be described. When the cooling is stopped in this component system above 700 ° C, ferrite transformation starts in the subsequent cooling zone, and the formation of a ferrite structure with low hardness causes surface damage such as flaking damage due to plastic deformation at the rail head. Is more likely to occur. Further, when the temperature is cooled to less than 600 ° C., a sufficient pearlite structure is not generated in the subsequent cooling region depending on the component system, and the wear resistance of the rail steel cannot be improved. For these reasons, the accelerated cooling stop temperature from the austenite region temperature is limited to the range of 700 to 600 ° C.

Next, the reason for limiting the cooling time will be described. If the cooling time is less than 15 seconds, the generation time of the pearlite structure is short, and a sufficient amount of the pearlite structure cannot be obtained, and the wear resistance of the rail cannot be improved. On the other hand, if the cooling time exceeds 600 sec, depending on the component system, the generation amount of the pearlite structure becomes excessive, dark spot damage is generated on the rail head, and the rolling fatigue resistance of the rail head is significantly reduced. For these reasons, the cooling time after accelerated cooling is limited to the range of 15 to 600 sec.

Next, the reason why the accelerated cooling rate from 700 to 600 ° C. is limited to 3 ° C./sec or more will be described.
If the accelerated cooling rate from 700 to 600 ° C. is less than 3 ° C./sec, depending on the component system, many bainite structures with low hardness are generated in the high temperature range during cooling, and the surface such as flaking damage due to plastic deformation is generated. Damage is more likely to occur. For this reason, the accelerated cooling rate after cooling is limited to 3 ° C./sec or more.

Next, the reason why the accelerated cooling stop temperature from 700 to 600 ° C. is limited to the range of 500 to 250 ° C. will be described. When cooling is stopped in this component system beyond 500 ° C, bainite transformation starts in the high temperature range immediately after accelerated cooling, bainite structure with low hardness is generated, and flaking damage due to plastic deformation occurs at the rail head. Etc. are likely to occur. Further, when the temperature is cooled to less than 250 ° C., a large amount of martensite structure is generated during accelerated cooling depending on the component system. Furthermore, bainite transformation does not progress even in the cooling zone after accelerated cooling, and a large amount of remaining austenite structure transforms to martensite structure,
The wear resistance of the rail steel is significantly reduced. For these reasons, the accelerated cooling stop temperature from the austenite region temperature is 5
The range was limited to 00 to 250 ° C.

That is, in the present invention, at least the rail head of a steel rail having a temperature of not less than the Ar1 point as hot rolled or heated to a temperature of not less than the Ac1 point + 30 ° C. for the purpose of heat treatment. From each temperature range, accelerated cooling at a cooling rate of 0.5 to 10 ° C./sec, and the head of the steel rail is 500 to 250 ° C.
At the time when the temperature of the steel rail is reached, and then natural cooling, or accelerated cooling at a cooling rate of 0.5 to 10 ° C./sec from the respective temperature ranges, and the temperature of the head of the steel rail is 700.
When the temperature reaches ６００600 ° C., the accelerated cooling is stopped.
After allowing to cool for 00 seconds, the steel rail was further accelerated and cooled at a cooling rate of 3 ° C./sec or more, and the temperature of the steel rail head was 500 to
When the temperature reaches 250 ° C, accelerated cooling is stopped and then spontaneous cooling promotes the formation of pearlite structure, and at the same time suppresses the formation of bainite structure in high temperature range and stabilizes bainite transformation in low temperature range And a mixed structure of a bainite structure and a pearlite structure having a certain range of hardness can be obtained.

It is to be noted that the accelerated cooling is mainly performed on the rail head, which is required to have at least mainly the resistance to rolling fatigue damage and wear, and for which the amount of formation of the bainite structure and the pearlite structure must be accurately controlled. It is a premise. However, in order to control the bending of the rail during the heat treatment and the residual stress after the heat treatment, it is desirable to perform the same heat treatment on the rail pillars and the bottom in addition to the rail head. As a cooling medium at the time of accelerated cooling, to obtain a predetermined cooling rate, air, mist, a mixed refrigerant of water and air, or a combination thereof, and a rail head to oil, hot water, polymer + water, and a salt bath. It is desirable to use immersion or the like for part or the whole.

[0052]

Next, an embodiment of the present invention will be described. Table 1 shows the chemical composition of the rail steel of the present invention, the microstructure of the head, the area ratio thereof, the hardness of the head, and the heat treatment conditions for the parts. Table 1 also shows the results of a water lubricated rolling fatigue test performed by the rolling fatigue damage tester shown in FIG. 2 and the wear test results shown in FIG. Table 2 shows the chemical composition of the comparative rail steel, the microstructure of the head, the area ratio thereof, the hardness of the head, and the conditions of the partial heat treatment. Table 2 also shows the results of a water lubricated rolling fatigue test using the rolling fatigue damage tester shown in FIG. 2 and the results of the wear test shown in FIG. Further, FIG. 4 shows the area ratio and wear of the pearlite structure in the entire structure of the rail steel of the present invention (reference numerals G to J: 0.4 mass% C) and the comparative rail steel (reference numerals U and W: 0.4 mass% C). The relationship between the quantities is shown.

The configuration of the rail is as follows. -Rail steel of the present invention (12 in total) Code: A to L The chemical composition is within the above-mentioned limited range, the metal structure is a mixed structure of bainite structure and pearlite structure, and in any cross section, pearlite in the whole structure The range of the area ratio of the structure is 10 to 50%, and further, starting from the head surface of the head corner and the top of the steel rail,
A rail having excellent rolling fatigue damage resistance and fracture resistance, characterized in that its hardness is in the range of Hv 300 to 450 at least up to a depth of 20 mm. -Comparative rail steel (12 rails in total) Codes: M to X Codes M to O: Comparative rail steels (3) having a pearlite structure whose chemical composition is outside the above-mentioned limited range and currently used in passenger railways. Symbols P to R: comparative rail steels (three) whose chemical components are outside the above-mentioned limited range. Symbols S to X: Comparative rail steels (six pieces) whose chemical components are within the above-mentioned limited range but heat treatment conditions are outside the above-mentioned limited range.

The various test conditions are as follows. -Rolling fatigue test conditions Test machine: Rolling fatigue test machine (see Fig. 2) Specimen shape: Disc-shaped specimen (Rail outer diameter: 200 mm, rail material cross-sectional shape: 60)
(1/4 model of K rail) (wheel outer diameter: 200mm, cross section of wheel material: 1/4 model of circular tread wheel) Test load: 1.0 ton (radial load) Atmosphere: dry + water lubrication (60cc / min) ) Number of revolutions: dry; 100 rpm, water lubrication; 300 rpm
m Number of repetitions: Dry to 0 to 5000 times, then to damage and wear limit by water lubrication (if no damage occurs, stop the test after 2 million times). -Wear test conditions Test machine: Nishihara type wear test machine (see Fig. 3) Test specimen shape: Disc-shaped test specimen (outer diameter: 30mm, thickness: 8mm) Test specimen sampling position: 2mm below the rail head surface (Fig. 5) Test load: 686 N (640 MPa) Sliding rate: 20% Counterpart material: Pearlite steel (Hv380) Atmosphere: Air cooling: Forced cooling with compressed air (flow rate: 10)
0Nl / min) Number of repetitions: 700,000 times

[0055]

[Table 1]

[0056]

[Table 2]

[0057]

As shown in Tables 1 and 2, the rail steel of the present invention has a mixed structure of a bainite structure and a pearlite structure by appropriately selecting the added elements of C, Si, Mn, and Cr. 4 prevents rolling fatigue damage such as dark spot damage, which has been a problem with the pearlite microstructured rail steel (compared rail steel, code: M to O), and at the same time, has an appropriate amount of pearlite microstructure as shown in FIG. The wear resistance of the rail can be improved by the presence of, and the wear life of the rail can be improved without lowering the rolling fatigue resistance. Furthermore, by appropriately selecting the accelerated cooling rate during the heat treatment of the head, the accelerated cooling stop temperature, and the cooled state after the accelerated cooling, a mixed structure of a bainite structure and a pearlite structure can be obtained stably.

[Brief description of the drawings]

BRIEF DESCRIPTION OF DRAWINGS FIG. 1 is a diagram showing a designation of a rail head cross-sectional surface position and a required range of a mixed structure of bainite structure and pearlite structure in a rail head.

FIG. 2 is a schematic diagram of a rolling fatigue damage tester.

FIG. 3 is a schematic diagram of a Nishihara type abrasion tester.

FIG. 4 shows the rail steel of the present invention (reference numerals G to J: 0.4 mass%).
C) and comparison rail steel (reference U, W: 0.4 mass% C)
FIG. 4 is a diagram showing the relationship between the area ratio of the pearlite structure in the entire structure and the wear amount.

FIG. 5 is a diagram showing a wear test piece sampling position on a rail head.

[Explanation of symbols]

 1: Top of head 2: Head corner 3: Wheel test piece 4: Rail disc test piece 5: Motor (wheel side) 6: Motor (rail side) 7: Water lubrication device 8: Rail test piece 9: Counterpart material 10 ： Cooling nozzle

Claims (11)

[Claims] In a steel rail, a metal structure is a mixed structure of a bainite structure and a pearlite structure, and in an arbitrary cross section of the steel rail, a range of an area ratio of a pearlite structure in a whole structure is 10 to 50%. A rail with excellent rolling fatigue resistance and abrasion resistance. 2. Mass%: C: 0.25 to 0.60%, Si: 0.05 to 2.00%, Mn: 0.10 to 3.00%, Cr: 0.10 to 3.0%. And a balance of Fe and unavoidable impurities, the metal structure of which is a mixed structure of bainite structure and pearlite structure. A rail having excellent rolling fatigue resistance and wear resistance, wherein the area ratio of the pearlite structure is 10 to 50%. 3. The rail according to claim 2, wherein the rail further comprises Mo: 0.01 to 1.00% by mass%. 4. One or more of Ni: 0.05 to 2.00%, Cu: 0.05 to 1.00%, and Co: 0.10 to 2.00% by mass%. The rail according to claim 2 or 3, wherein the rail has excellent rolling fatigue damage resistance and wear resistance. 5. The composition according to claim 2, further comprising one or more of V: 0.01 to 0.30% and Nb: 0.005 to 0.050% by mass%.
5. The rail excellent in rolling fatigue damage resistance and abrasion resistance according to any one of items 4 to 4. 6. The method according to claim 2, further comprising: 0.005 to 0.050% by mass of Ti.
A rail with excellent rolling fatigue damage resistance and abrasion resistance as described in the item. 7. The composition according to claim 2, further comprising one or more of Mg: 0.0001 to 0.0300% and Ca: 0.0001 to 0.0150% by mass%.
7. The rail having excellent rolling fatigue damage resistance and abrasion resistance according to any one of items 6 to 6. 8. The method according to claim 2, further comprising: B: 0.0001 to 0.0050% by mass%.
A rail with excellent rolling fatigue damage resistance and abrasion resistance as described in the item. 9. The steel rail according to any one of claims 1 to 8, wherein a hardness of at least 20 mm in depth starting from the head corner portion and the top surface is Hv.
A rail having excellent rolling fatigue damage resistance and wear resistance, which is in the range of 300 to 450. 10. A hot-rolled steel slab containing the component according to any one of claims 2 to 9 and then hot-rolled as a steel rail at a temperature of Ar1 point or higher, or heat-treated. In a steel rail heated to a temperature of at least Ac1 point + 30 ° C. for the purpose of cooling, at least the rail head is accelerated and cooled at a cooling rate of 0.5 to 10 ° C./sec from the austenitic zone temperature, and the steel rail head is heated. Temperature of 500 ~ 2
The method according to claim 1, wherein the accelerated cooling is stopped when the temperature reaches 50 ° C, and then the natural cooling is performed. 11. A steel slab containing the component according to any one of claims 2 to 9 is hot-rolled into a rail shape, and then hot-rolled as a steel rail having a temperature of at least the Ar1 point or a heat treatment. In a steel rail heated to a temperature of at least Ac1 point + 30 ° C. for the purpose of cooling, at least the rail head is accelerated and cooled at a cooling rate of 0.5 to 10 ° C./sec from the austenitic zone temperature, and the steel rail head is heated. The temperature of 700-6
When the temperature reaches 00 ° C., the accelerated cooling is stopped, and 15 to 600
After allowing the steel rail to cool at a rate of 3 ° C./sec or more, the temperature at the head of the steel rail is 500 to 2
The method according to claim 1, wherein the accelerated cooling is stopped when the temperature reaches 50 ° C, and then the natural cooling is performed.