JP2001152291A - Bainitic rail excellent in resistance to rolling fatigue damage and internal fatigue damage and welded joint properties, and its manufacturing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a bainitic rail in which rolling fatigue damage such as dark spot damage that has been a problem in the present rail steel of pearlitic structure is prevented and the occurrence of internal fatigue damage in a railhead is suppressed, and simultaneously, reduction of hardness in a welded-joint heat-affected zone and also a region thereof are reduced and local wear in a railhead surface part is suppressed. SOLUTION: The rail has a composition consisting of, by mass, 0.15-0.55% C, 0.10-2.00% Si, 0.20-3.00% Mn, 0.20-3.00% Cr, 0.01-0.30% V, >0.01-0.50% Nb and the balance Fe with inevitable impurities and satisfying V+10×Nb=0.14 to 0.50. Moreover, at least the region within 20 mm from the surface of railhead corners and top part of railhead shows a baintic structure, and further, handness is HV 330 to 500 and the difference in hardness is <= HV 50.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a rolling and fatigue resistance required for rails for passenger railways and freight railways.
The present invention relates to a bainite-based rail having improved internal fatigue damage resistance and weld joint characteristics.

[0002]

2. Description of the Related Art In recent years, passenger railways and freight railways have been increasing the speed of trains and the loading of freight wagons in order to improve transportation efficiency. Along with this, in rails mainly in straight sections, the operating environment of the rails has become severer, and the occurrence of rolling fatigue damage on the rail head surface called dark spot damage due to repeated contact between the rails and wheels has increased.
This dark spot damage has a characteristic that it is likely to occur on a rail in a straight section of a passenger railway or a freight railway where a rail having a conventional pearlite structure is used.

[0003] The inventors of the present invention have developed a fatigue layer (fatigue damage layer,
The relationship between the formation of texture and the microstructure was studied. As a result, in the pearlite structure having a layered structure of ferrite phase and cementite phase, the fatigue damage layer easily accumulates, and further, the texture is easily developed, whereas the granular hard carbide is formed on the soft ferrite structure ground. It has been found that the bainite structure in which is dispersed has a difficulty in accumulating a fatigue damage layer, and furthermore, a texture that triggers surface fatigue damage does not easily develop, and as a result, dark spot damage does not easily occur.

Therefore, the following products and manufacturing methods have been developed as rails having a bainite structure. A high-strength rail exhibiting a bainite structure as-rolled by adding a large amount of alloying elements such as Mn, Cr, and Mo with a low carbon component (Japanese Patent Application Laid-Open No. 5-271871). High-strength bainite rails that add high carbon elements such as Mn, Cr, and Mo as low-carbon components and retain high-temperature heat after hot rolling, or accelerate and cool the heads of rails heated to high temperatures. Manufacturing method (JP-A-7-34132).

[0005] The features of these rails are that, in order to stably generate a bainite structure excellent in rolling fatigue damage resistance, the amount of carbon is reduced as compared with conventional ordinary carbon steel rails, and at the same time, Mn, Cr, Mo are reduced. In addition, a large amount of alloying elements such as are added, and an appropriate heat treatment is performed to secure the strength.

[0006]

[Occurrence of Internal Fatigue Damage] The bainite structure used in these rails has a property that the transformation start temperature greatly depends on the cooling rate. Therefore, when the cross-sectional area is large like the rail head and the cooling speed between the head surface and the inside of the head is largely different, bainite transformation is performed on the rail head surface with a relatively fast cooling speed and inside the relatively slow cooling speed. A difference is inevitably generated in the starting temperature, and as a result, a large difference in hardness is likely to occur between the rail head surface portion and the inside.

FIG. 1 shows an example of a head section hardness distribution of a bainite rail. As described above, the bainite rail has a characteristic that the hardness rapidly decreases from the rail head surface to the inside. With such a hardness distribution, the change in material strength in the cross section of the rail head becomes significantly large,
Along with this, the strain (plastic deformation region) generated on the rail due to contact with the wheel tends to concentrate on the low hardness (strength) portion inside the rail head. As a result, in a freight railroad in which the rail usage environment is severe, fatigue cracks are likely to occur from the low strength portion inside the rail head, causing a problem that the service life of the rail is shortened.

As a countermeasure for this, it is conceivable to increase the amount of alloy such as Mn or Cr, or to increase the cooling rate during heat treatment. However, if the amount of alloy or the cooling rate during heat treatment is excessively increased, Although the internal hardness can be improved, there is a problem that an abnormal structure such as martensite is formed on the surface of the rail head and the wear resistance and toughness are greatly reduced.

There is also a method of improving the hardness of the inside of the head by improving the heat treatment cooling method to prevent the formation of an abnormal structure in the surface of the rail head and improving the characteristics of the inside of the head. At the same time, it was difficult to control the cooling rate applied, and it was difficult to obtain an appropriate hardness distribution in the cross section of the rail head.

[Problem of Decreasing Hardness of Weld Heat-Affected Zone] In addition to these problems, the bainite structure has a characteristic that the hardness is liable to decrease when subjected to heat as compared with the pearlite structure. As a result, when the bainite structure rail is joined by an existing welding method such as flash butt welding, gas pressure welding, thermite welding, or enclosed arc welding, Ac1
In the heat-affected zone reheated to a temperature range below the point, the hardness-reduced area expands compared to the current pearlite structure rail,
Further, the hardness decrease tends to increase.

FIG. 2 shows bainite steel (0.3% C, Hv
380) and current pearlite steel (0.7% C, Hv39)
FIG. 6 shows an example of the hardness distribution in the longitudinal direction of the head surface portion of the flash butt welding joint of the rail 0). As shown in FIG. 2, on the surface of the rail head of bainite steel, Ac1
The hardness-reduced region of the heat-affected zone reheated to a temperature range below the temperature is expanded, and the hardness-reduced amount increases.

Therefore, the bainite-structured rail is more susceptible to local wear in the heat-affected zone of the weld joint than in the pearlite-structured rail due to the expansion of the reduced hardness region and the increased amount of reduced hardness. As a result, there has been a problem that noise and vibration during train running increase.

[0013] The present invention solves the above-mentioned problems, and aims to improve the rolling fatigue resistance, the internal fatigue damage resistance required for freight railways and the like, and at the same time, to improve the welding joint characteristics. The method of manufacture is provided.

[0014]

The present invention attains the above object, and its gist is as follows: (1) In mass%, C: 0.15 to 0.55%, Si: 0 0.10 to 2.00%, Mn: 0.20 to 3.00%, Cr: 0.20 to 3.00%, V: 0.01 to 0.30%, Nb: more than 0.01 to 0. And the sum of V + 10 × Nb is 0.14 or more and 0.5
0 or less, and if necessary, Mo: 0.01 to 1.00%, Ti: 0.005 to 0.050%, Ni: 0.05 to 1.00%, Cu: 0.05 to 0.50%, Mg: 0.0010 to 0.0100%, Ca: 0.0010 to 0.0150%, B: One or more of 0.0001 to 0.0050%, the balance being A rail made of Fe and unavoidable impurities, wherein at least a range of a depth of 20 mm from a head corner portion and a top surface of the rail exhibits a bainite structure, and the bainite structure in the range has a hardness of Hv330 to 500. Range,
In addition, the bainite-based rail has a bainite structure having a hardness difference of Hv50 or less, and has excellent rolling fatigue damage resistance, internal fatigue damage resistance, and weld joint characteristics. Also

(2) The composition according to the above (1),
A slab consisting of Fe and unavoidable impurities is hot-rolled into a rail shape.
Accelerated cooling from a temperature of one or more points at a cooling rate of 1 to 20 ° C./sec.When the head of the steel rail reaches 500 to 300 ° C., the accelerated cooling is stopped. A method for producing a bainite-based rail having excellent rolling fatigue damage resistance, internal fatigue damage resistance, and weld joint characteristics characterized by cooling. (3) After the accelerated cooling stop described in the above (2), the temperature rise at the rail head surface due to the transformation heat generation and the reheating generated inside the rail is suppressed to 50 ° C. or less from the accelerated cooling stop temperature. A method for producing bainite rails with excellent rolling fatigue damage resistance, internal fatigue damage resistance, and weld joint characteristics. And (4) rolling stop fatigue resistance and internal fatigue damage resistance, characterized in that after the accelerated cooling stop described in the above (2), the rail head surface is controlled and cooled to a normal temperature range at a temperature of 1 to 40 ° C./min. This is a method for producing bainite-based rails having excellent welding joint characteristics.

[0016]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail. The present inventors prevent the formation of an abnormal structure such as martensite in the surface of the rail head, improve the hardness inside the rail head, and control the cooling by controlling the selection of an appropriate alloy and the amount of the alloy added. A stable bainite structure is generated stably from the top to the bottom of the rail at different speeds, and at the same time, the area where the hardness of the weld heat affected zone is reduced on the surface of the rail head is prevented from expanding. We studied ways to prevent this.

Here, FIG. 3 shows the names of the surface positions of the rail head sections of the present invention which are excellent in the resistance to rolling fatigue damage, the resistance to internal fatigue damage, and the characteristics of welding joints. 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, as shown in FIG. 3, the rail head surface part is 2 to 10 from the head surface of the rail head part 1 and the head corner part 2 which are required mainly for surface damage resistance.
mm area. Further, the inside of the rail head mainly refers to a region of 10 to 30 mm deeper than the above range in which internal fatigue damage resistance is required.

First, in order to improve the resistance to internal fatigue damage, the inventors of the present invention conducted laboratory studies on alloy elements for improving the hardness inside the rail head. The bainite structure has a structure in which granular hard carbides are dispersed in soft ferrite ground. Then, the element which finely precipitates in the ferrite ground of bainite structure and strengthens the precipitation strengthening of the bainite structure was examined by experiments. Actual rail (carbon content 0.15 to 0.5
As a result of conducting an accelerated cooling experiment using 5%), the rail to which a certain amount of V and further Nb was added in combination, during cooling, in the inside of the rail head whose cooling rate was slower than the rail head surface part during cooling. It has been found that carbides and nitrides of V and carbon / nitrides of Nb precipitate on ferrite grounds in the bainite structure, and the hardness inside the rail head is stably improved by the synergistic effect of both alloys.

Next, the present inventors prevented the expansion of the reduced hardness region of the weld heat affected zone on the surface of the rail head,
A method for preventing an increase in the amount of decrease in hardness was studied. First, it is considered that precipitation strengthening is effective in preventing a decrease in the hardness of the weld heat-affected zone, and in the weld heat-affected zone reheated to a temperature range of less than or equal to the Ac point, fine precipitation occurs on a ferrite ground having a bainite structure. The element which strengthens the precipitation of was studied by experiments. As a result, V and Nb do not precipitate as carbides or nitrides on the bainite-structured ferrite ground during cooling on the rail head surface where the cooling rate is relatively high as compared with the inside of the rail head, and solid-dissolve in the bainite structure. Is confirmed,
It was confirmed that during the reheating process due to the effect of welding heat, it was likely to precipitate on the ferrite ground with bainite structure and to strengthen the bainite structure by precipitation.

The present inventors further studied the bainite steel (carbon content 0.15 to 0.55) in which the added amount of V and Nb was changed.
%), Austenitizing, accelerated cooling, and a tempering experiment reproducing the effect of welding heat were performed to confirm the precipitation strengthening behavior of V and Nb. As a result, it was confirmed that Nb had an effect of stably depositing carbide from a low temperature range to a high temperature range, and V had an effect of depositing a large amount of carbide in a relatively high temperature range. Therefore, by combining Nb and V, in the heat-affected zone of the rail welding joint, the hardness decreases from a portion where the reheating temperature is low and the hardness decreases little to a portion where the reheating temperature is high and the hardness decreases greatly. It is possible to control the amount of precipitate according to the amount,
It has been found that it is possible to prevent an increase in the hardness reduction region and an increase in the hardness reduction amount.

Based on these findings, the present inventors further investigated the relationship between the added amounts of V and Nb and the increase in hardness in the weld heat affected zone on the rail head surface, and found the balance between the added amounts of V and Nb and The optimal component range was studied in detail. As a result, when compared with the same addition amount (wt%), it became clear that the hardness increase (precipitation strengthening) of Nb in the heat affected zone was about 10 times as effective as V. Is increased by V (wt%) + 10 × Nb (wt
%). Further, in order to stably improve the decrease in hardness of the weld heat affected zone, in addition to the respective additive component ranges of Nb and V, V
It has been found that a certain range exists in the sum of (wt%) + 10 × Nb (wt%).

Hereinafter, the reasons for limitation of the present invention will be described in detail. (1) Chemical Composition of Rail Steel First, the reason for limiting the chemical composition of the rail in the present invention as described above will be described. C is an essential element for ensuring the strength and wear resistance of the bainite structure,
If it is less than 0.15%, a ferrite structure is generated in the bainite structure, and it becomes difficult to secure the strength and wear resistance required for the bainite rail. On the other hand, if the content exceeds 0.55%, a large amount of pearlite structure harmful to the occurrence of rolling fatigue damage in the bainite structure is likely to be generated, and the bainite transformation rate is significantly reduced, and a martensite structure harmful to rail toughness is reduced. Because it is easy to generate,
C content was limited to 0.15 to 0.55%.

[0023] Si is an element which improves the strength by solid solution hardening of the ferrite matrix in the bainite structure, but its effect cannot be expected if it is less than 0.10%. On the other hand, if the content exceeds 2.00%, surface flaws are likely to be generated during hot rolling of the rail, and a martensite structure is generated in the bainite structure to reduce the wear resistance and toughness of the rail. It was limited to 0.10 to 2.00%.

Mn is an element indispensable for enhancing the hardenability of steel and stably forming a bainite structure, similarly to C. In this component system, the effect is weak when the Mn content is less than 0.20%, and depending on the combination of the added elements, the bainite structure has a ferrite structure harmful to wear resistance and a pearlite harmful to surface damage resistance. Tissues are easier to generate. On the other hand, if the content exceeds 3.00%, the transformation speed of the bainite structure is reduced, and a martensite structure harmful to the wear resistance and toughness of the rail is easily generated.
The amount of n was limited to 0.20 to 3.00%.

[0025] Cr is an important element for finely dispersing carbides in the bainite structure and ensuring strength, but if its content is less than 0.20%, its effect is weak, and depending on the combination of added elements, It becomes difficult to stably obtain a bainite structure. When the content exceeds 3.00%, a martesite structure is easily generated in the bainite structure, and the toughness and wear resistance of the rail are reduced. Therefore, the Cr content is limited to 0.20 to 3.00%.

V combines with C or N to form V carbides and V nitrides during cooling after rolling, and precipitates on ferrite ground in bainite structure, so that the rail head having a relatively slow cooling rate is formed. In the heat-affected zone reheated to a temperature range below the Ac1 point, V carbide is generated in a relatively high temperature range, and the bainite structure in the heat-affected zone of the weld joint is simultaneously an element that increases the hardness inside the weld zone. Is an effective component for preventing a decrease in hardness, but if it is less than 0.01%, its effect cannot be expected sufficiently, and if it exceeds 0.30%, no further effect can be expected. , V amount to 0.0
Limited to 1 to 0.30%.

Like V, Nb increases the strength of the bainite structure by precipitation hardening due to Nb carbon / nitride generated in the cooling process after hot rolling, and at the same time, heat re-heated to a temperature range of less than Ac1 point. In the affected zone, Nb carbide is a component effective for stably generating Nb carbide from a low temperature range to a high temperature range and preventing a decrease in hardness of a bainite structure in a heat-affected zone of a welding joint. The effect cannot be expected sufficiently at a content of 0.01% or less, and an excessive addition exceeding 0.05% generates an intermetallic compound of Nb or a coarse precipitate to lower the toughness of the rail. Therefore, the Nb amount exceeds 0.01 to
Limited to 0.05%.

The purpose of adding V and Nb in a complex manner is that V carbide is added to the ferrite ground in the bainite structure during cooling after rolling in the inside of the rail head where the cooling rate is slower than the surface of the rail head. And carbon and nitride of Nb and Nb precipitate, and the synergistic effect of the two alloys stably improves the hardness inside the rail head, and Nb is low in the bainite structure affected by welding heat. V has the effect of precipitating carbides stably from the temperature range to the high temperature range, and V has the effect of precipitating a large amount of carbides in the relatively high temperature range. By combining Nb and V, the thermal effect of the rail welding joint is This is because it is possible to control the amount of the precipitate in the part according to the hardness reduction amount.

In addition, rails manufactured with the above-described component compositions are made of Mo, Ti, Ni, Cu, Mg, Ca, B for the purpose of preventing strength, ductility, toughness, and material deterioration during welding.
One or more of the above elements are added as necessary in the following component ranges. Mo: 0.01 to 1.00%, Ti: 0.005 to 0.050%, Ni: 0.05 to 1.00%, Cu: 0.05 to 0.50%, Mg: 0.0010 0.0100%, Ca: 0.0010 to 0.0150%, B: 0.0001 to 0.0050% Here, Mo stabilizes bainite structure and improves strength.
i, Ni improve ductility and toughness, Cu improves strength, Mg,
Ca is added for the purpose of improving ductility and toughness, and B is added for the purpose of stabilizing the bainite structure.

Mo, like Mn or Cr, is an element that lowers the bainite transformation temperature and contributes to stabilization of the bainite transformation and strengthening of the bainite structure.
%, The effect is not sufficient, and if it exceeds 1.00%, the transformation rate of the bainite structure is remarkably reduced, and similarly to Mn and Cr, a martensite structure harmful to the wear resistance and toughness of the rail is likely to be generated. Therefore, the amount of Mo is 0.01 to
Limited to 1.00%.

Titanium is used to refine the austenite crystal grains during rail rolling heating by utilizing the fact that Ti carbides and Ti nitrides precipitated during melting and solidification do not dissolve even at the high temperature of rail rolling heating. Is an element that contributes to the improvement of ductility and toughness. However, if it is less than 0.005%, these effects are not sufficiently exhibited, and if it exceeds 0.050%, coarse nitride (TiN) or carbide is not added. (T
iC) is generated and the ductility and toughness of the rail are reduced, and at the same time, it is likely to become a starting point of fatigue damage during use of the rail.
The amount of Ti was limited to 0.005 to 0.050%.

Ni is an element that stabilizes austenite, has the effect of lowering the bainite transformation temperature, refining the bainite structure, and improving ductility and toughness.
If it is less than 0.05%, the effect is extremely small.
If it is added in excess of 00%, the bainite transformation rate is greatly reduced, and a martensite structure harmful to the wear resistance and toughness of the rail is easily formed.
Limited to 1.00%.

Cu is an element which improves the strength without impairing the toughness of the steel, and its effect is greatest in the range of 0.05 to 0.50%, and when it exceeds 0.50%, red embrittlement occurs. Cu content is 0.05 to 0.50
%.

Mg combines with O, S, Al, etc. to form a fine oxide, suppresses the growth of crystal grains during reheating during rail rolling, and refines austenite grains. It is an element effective for improving the ductility and toughness of the bainite structure. Further, MgO and MgS are converted to MnS
Is an element effective for improving ductility and toughness by dispersing finely and forming a thin band of Mn around MnS to promote bainite transformation and thereby refine the bainite structure. However, the effect is weak when it is less than 0.0010%, and when it exceeds 0.0100%, M
The amount of Mg was limited to 0.0010 to 0.0100% because g coarse oxides were generated to deteriorate rail ductility and toughness.

Ca has a strong bonding force with S, forms a sulfide as CaS, and CaS finely disperses MnS, forms a dilute band of Mn around MnS, and promotes bainite transformation. As a result, the element is effective for improving the ductility and toughness by making the bainite structure fine. However, if the content is less than 0.0010%, the effect is weak, and if it is added more than 0.0150%, a coarse oxide of Ca is formed to deteriorate rail ductility and toughness.
The amount was limited to 0.0010 to 0.0150%.

B is an element that suppresses the formation of a pro-eutectoid ferrite structure generated from a prior austenite grain boundary and a pearlite structure that is transformed with the structure, thereby stably forming a bainite structure. However, if the content is less than 0.0001%, the effect is weak. If the content is more than 0.0050%, nitrides and carbides of B are formed at austenite grain boundaries, and conversely, a proeutectoid ferrite structure or a pearlite structure which deteriorates hardenability. To make B easily, the amount of B is set to 0.0001 to 0.0.
050%.

(2) Equation of the sum of V and Nb and its range Next, in the present invention, as an index indicating the hardness of the welding heat affected zone of the rail, the equation of V (wt%) + 10 × Nb (wt%) and The reason for limiting the range will be described. Investigation was made on the relationship between the added amount of V and Nb and the increase in hardness in the weld heat affected zone on the rail head surface in the bainite-structured rail. It became clear that the increase in hardness (precipitation strengthening) in the portion was about 10 times as effective as V. Therefore, the above expression was limited as an index indicating the amount of hardness increase in the welding heat affected zone at the rail head surface.

Further, the reason why the range of the value of the above expression is limited will be described. When the value of the above expression is less than 0.14, the increase in hardness in the weld heat affected zone is extremely reduced, and it is difficult to prevent the decrease in the hardness of the weld heat affected zone and the increase in the decrease. Abrasion occurs, resulting in an increase in noise and vibration during train running. Further, when the value of the above formula is 0.50 or more, the hardness increase in the weld heat affected zone varies due to segregation of V and Nb, and as a result, the hardness distribution of the weld heat affected zone is not stabilized, Since noise and vibration during train running increase due to the occurrence of local wear, V (w
t%) + 10 × Nb (wt%) is 0.14 to 0.1%.
The range was limited to 50.

The values of V and Nb in the above formulas are all mass%. If the addition amount of either V or Nb does not satisfy the limited range of the above components, even if the value of the above expression satisfies the range, the hardness characteristics of the weld heat affected zone are not sufficiently improved. In the invention, it is out of the limited range.

(3) Hardness of bainite structure and its range Next, the reason why the hardness of the bainite structure is limited to the range of Hv 330 to 500 will be described. Hardness is Hv330
When the distance is less than G., in a curved section having a gentle radius of curvature according to the straight section, G.P. C. The metal flow is likely to be generated by the strong contact between the rail and the wheel in the part, which causes surface damage such as creaking and flaking. Further, in a freight railroad or the like having a severe use environment, fatigue cracks are easily generated from a low-strength portion inside the rail head, and the service life of the rail is shortened. When the hardness exceeds Hv500,
Since the wear is extremely suppressed and surface damage resistance is likely to occur, and the ductility and toughness are extremely reduced and the ductility required for the rail cannot be secured, the hardness of the bainite structure is reduced. Hv was limited to the range of 330 to 500.

Next, the reason why the range having the bainite structure is limited to a range of at least a depth of 20 mm starting from the surface of the head at the corner and top of the head will be described.
If it is less than 20 mm, the rolling fatigue resistance required for the rail head is small, and a sufficient life improving effect cannot be obtained. When the bainite structure is present at a depth of 30 mm or more starting from the head surface at the corner and top of the head, the life improvement effect is further increased, which is more desirable.

If the maximum value of the difference in hardness exceeds Hv50, there is a concern that internal fatigue damage as described above may occur. However, if the maximum value of the difference in hardness is within Hv50, the cracks that occur are small and do not pose a serious problem in practical use. Therefore, if the maximum value of the difference in hardness is within Hv50, it has sufficient characteristics as a practical rail. Further, when the maximum value of the difference in hardness is within Hv40, it is more preferable because the above-mentioned minor crack damage hardly occurs and the resin has sufficient internal fatigue damage resistance.

It is desirable that the metal structure of the rail of the present invention be a bainite structure. However, depending on the production method, a minute amount of martensite structure, ferrite structure, or pearlite structure may be mixed in the bainite structure. But,
Even if a small amount of martensite, ferrite, or pearlite microstructure is mixed into the bainite microstructure, the bainite microstructure does not significantly affect the rolling fatigue resistance, internal fatigue resistance, and weld joint characteristics of the rail. The organization of the system rail includes a slight mixture of the organizations described above.

(4) Heat-treating production method of rail The rail steel composed of the above-mentioned components is converted into a converter steel,
Melting is performed in a commonly used melting furnace such as an electric furnace, and the molten steel is made into a steel slab by an ingot-bulking method or a continuous casting method, and is further manufactured as a rail through hot rolling. Next, the head part of the hot-rolled rail is accelerated and cooled at a cooling rate of 1 to 20 ° C./sec from a temperature of Ar 1 point or higher,
When the top of the steel rail reaches 500 to 300 ° C., the accelerated cooling is stopped, and then the natural cooling is continued to the normal temperature range, or after the accelerated cooling is stopped, the heat generated by the transformation and the regenerated heat generated from the inside of the rail. Perform cooling to suppress temperature rise. Alternatively, control cooling is performed after acceleration cooling is stopped.

The rail manufactured in this manner suppresses bainite transformation in a high temperature region by accelerated cooling, and then stabilizes bainite transformation in a low temperature region by cooling to a room temperature region. Complete
It becomes possible to obtain a bainite structure having a certain range of hardness. Then, for the purpose of further stabilizing the hardness of the rail head and further increasing the hardness, at the rail head surface after accelerated cooling, the temperature rise due to transformation heat generation and reheating generated from inside the rail is suppressed. Cooling is performed, as well as a controlled cooling rate.

Next, the reason why the temperature condition before the accelerated cooling, the accelerated cooling condition, and the cooling condition after the accelerated cooling at the time of manufacturing the rail are limited as described above will be described in detail. First, temperature conditions before the rail head surface portion is accelerated and cooled will be described. In order to stably obtain a bainite structure, at least the entire rail head must be austenitized,
The accelerated cooling start temperature was defined as Ar1 point or higher. here,
The above “rail head” refers to the top of the head shown in FIG.
1) indicates the entire region of the head including the head corner (symbol: 2).

Normally, hot rolling is completed at a temperature higher than the Ar1 point, so that accelerated cooling may be performed subsequently. However, reheating may be performed if there is a reason such as an equipment configuration. In this case, it is preferable to heat the rail head to a temperature higher than the Ac1 point + 30 ° C. in order to ensure austenitization. The upper limit of the temperature is not particularly defined, but if the temperature is too high, a liquid phase appears and the austenite phase becomes unstable, so that the upper limit is substantially 1450 ° C.

Next, the reason why the accelerated cooling rate up to the accelerated cooling stop temperature is limited to the range of 1 to 20 ° C./sec will be described. When accelerated cooling at less than 1 ° C./sec in the above component system, a ferrite structure or a pearlite structure is generated depending on the component system, and surface damage such as dark spot damage occurs. Furthermore, many bainite structures with low hardness are generated in a high temperature range, and flaking damage or the like due to plastic deformation occurs due to a decrease in strength. Further, when accelerated cooling is performed at a rate exceeding 20 ° C./sec, large uncontrollable reheat is generated from the inside of the rail in natural cooling after accelerated cooling. For this reason, bainite transformation starts inside the rail head in a high temperature range during reheating, the hardness of the bainite structure decreases, the difference in hardness between the rail head surface and the inside increases, and internal fatigue damage occurs. If controlled cooling is continued after accelerated cooling, bainite transformation is not completed during this controlled cooling, a large amount of martensite structure is generated, and the wear resistance and toughness of the rail are greatly reduced. Speed range 1-2
The range was limited to 0 ° C / sec.

Next, the reason why the accelerated cooling stop temperature from the austenite region temperature is limited to the range of 500 to 300 ° C. will be described. When the cooling is stopped above 500 ° C in this component system, 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 due to plastic deformation due to reduced strength. appear. Further, when the temperature is cooled to less than 300 ° C., in the natural cooling after the accelerated cooling, the recuperation amount from the inside of the rail is not sufficient, and the bainite transformation does not completely end during the recuperation,
Many martensite structures are generated, and the toughness of the rail is significantly reduced. When controlled cooling is continued after accelerated cooling, bainite transformation is not completed during this controlled cooling, a large amount of martensite structure is generated, and the wear resistance and toughness of the rail are greatly reduced. The temperature was limited to the range of 500-300C.

Next, the reason why the temperature rise due to the transformation heat generation and the reheating generated inside the rail in the rail head after the accelerated cooling is limited to 50 ° C. or less will be described. When the top of the rail of this component is accelerated and cooled from the austenitic region temperature and accelerated cooling is stopped in the temperature range of 500 to 300 ° C, the temperature rises up to 150 ° C at the top of the rail depending on the selection of the accelerated cooling rate. Has been confirmed by experiments. With such a temperature rise, a bainite structure having a hardness in a certain range within the range defined in claim 1 is stably generated, and there is no major problem. However, since this temperature rise is greatly affected by the accelerated cooling speed and the accelerated cooling stop temperature, the amount of temperature rise varies depending on the location,
As a result, the bainite transformation temperature changes, and the hardness tends to slightly vary or decrease.

Therefore, for the purpose of further stabilizing the hardness of the rail head and further increasing the hardness, the heat generated by transformation of the surface of the rail head after stopping the accelerated cooling and the recuperation generated from inside the rail. The temperature rise was defined as 50 ° C. or less.
In addition, this cooling includes a constant temperature change in temperature and an irregular temperature change in a temperature range of 0 to 50 ° C.

Next, the reason why the controlled cooling rate of the rail head after accelerated cooling is limited to the range of 1 to 40 ° C./min will be described. When cooled at less than 1 ° C / min, depending on the accelerated cooling stop temperature, bainite transformation starts in the high temperature region immediately after accelerated cooling, a large amount of bainite structure with low hardness is generated, and flaking damage due to plastic deformation due to reduced strength is caused. Occurs. Also, when cooling over 40 ° C / min,
During the controlled cooling after the accelerated cooling, the bainite transformation is not completely terminated, a martensite structure is formed, and the wear resistance and toughness of the rail are significantly reduced. For this reason, the cooling rate is 1 to 40
C./min.

As a cooling medium at the time of accelerated cooling, air, mist, water / air mixed refrigerant,
Alternatively, it is desirable to use a combination of these, and immersion of the rail head or the whole in oil, hot water, polymer + water, or a salt bath. Further, it is desirable to use air or a mist mainly composed of air as a cooling medium at the time of cooling for suppressing the temperature rise due to the transformation heat generation after the accelerated cooling and the reheating generated inside the rail, and the control cooling.

[0054]

Next, an embodiment of the present invention will be described. Example about rail Table 1 shows the chemical composition, crown hardness and head microstructure of the rail steel of the present invention. Table 1 also shows the results of a water lubricated rolling fatigue test by the rolling fatigue damage tester shown in FIG.
The results of the rolling fatigue test shown in FIG. 5 are also shown. Table 2 shows the chemical composition, crown hardness and head microstructure of the comparative rail steel. Table 2 also shows the results of a water-lubricated rolling fatigue test using a rolling fatigue damage tester shown in FIG. 4 and the results of a rolling fatigue test shown in FIG. In FIG. 4, 3 is a wheel test piece, 4 is a rail disc test piece, 5 is a wheel side motor, and 6 is a rail side motor. In FIG. 5,
7 is a water lubrication device, 8 is a slider for moving a rail, 9 is a rail, 10 is a wheel, 11 is a motor, and 12 is a load load device.

FIGS. 6 and 7 show the rail steel of the present invention (symbol: F,
It is an example of a head section hardness distribution of I). 8 and 9
Is an example of a head section hardness distribution of comparative rail steel (symbols: R, S). FIG. 10 is an example of the longitudinal hardness distribution of the rail head surface portion of the flash butt welding joint portion of the rail steel of the present invention (symbol: F) and the comparative rail steel (symbol: U). FIG. 11 shows an example of the longitudinal hardness distribution in the rail head surface portion of the flash butt welding joint portion of the rail steel of the present invention (symbol: I) and the comparative rail steel (symbol: V).

Examples relating to a method for manufacturing a rail Table 3 shows the heat treatment conditions, head hardness and head microstructure of the rail steel of the present invention. Table 3 also shows the results of a water-lubricated rolling fatigue test using a rolling fatigue damage tester shown in FIG. 4, and the results of a rolling fatigue test shown in FIG. Table 4
The head heat treatment conditions, crown hardness and head microstructure of the comparative rail steel are shown in FIG. Table 4 also shows the results of a water-lubricated rolling fatigue test using a rolling fatigue damage tester shown in FIG. 4 and the results of a rolling fatigue test shown in FIG.

The configuration of the rail is as follows. Examples related to rails ・ Rail steel of the present invention (11 pieces) Table 1 symbols: A to K Chemical components are within the above-mentioned limited range, and at least depth 20 starting from the head corner and top surface of the rail.
The range of mm shows a bainite structure, and the hardness of the bainite structure in the above range is in the range of Hv330 to 500, and the difference in hardness is bainite structure having a hardness difference of not more than Hv40 or Hv50, and anti-rolling fatigue damage, A bainitic rail with excellent internal fatigue damage resistance and excellent weld joint characteristics.

· Comparative rail steel (11 pieces) Table 2 code: L
-V Symbols LM: Comparative rail steel of pearlite structure currently used in passenger railways. Symbols N to S: comparative rail steels whose chemical components are outside the above-mentioned limited range. Reference symbol T: a comparative rail steel having a chemical component within the above-mentioned limited range, but having a hardness outside the above-described limited range. Symbols U and V: Comparative rail steels in which the value of V (wt%) + 10 × Nb (wt%) is out of the above-mentioned limited range.

Examples relating to a method for manufacturing a rail ・ Rail steel of the present invention (7 pieces) Symbols: 1 to 7
At least a range of a depth of 20 mm from a head corner portion and a crown surface of the rail exhibits a bainite structure,
The hardness of the bainite structure in the above range is Hv330 to 500.
And the hardness difference is Hv40 or Hv40.
It is a bainite-based rail excellent in rolling fatigue damage resistance, internal fatigue damage resistance, and weld joint characteristics having a bainite structure of v50 or less.

Comparative Rail Steel (7 pieces) Code: 8 to 14 This is a comparative rail steel in which the chemical composition is within the above-mentioned limited range, but the head-surface heat treatment conditions are out of the above-mentioned limited range.

The rolling fatigue test was performed under the following conditions. -Testing machine: Rolling fatigue tester (see Fig. 4)-Specimen shape: disk-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) ・ Rotation speed: Drying: 100 rpm, water lubrication: 300 r
pm ・ Number of repetitions: 0 to 5000 times in a dry state.
Damage is caused by water lubrication and wear limit is reached (if no damage occurs, the test is stopped after 2 million cycles).

The conditions for the rolling fatigue test were as follows.・ Testing machine: Rolling fatigue tester (See Fig. 5) ・ Specimen shape Rail: 136 lbs rail x 2m Wheel: AAR type (diameter 920mm) ・ Radial load: 147000N ・ Thrust load: 9800N ・ Lubrication: Dry + oil (Intermittent lubrication) ・ Repeat times: Until damage occurs (if no damage occurs, the test is stopped after 10 million times).

The flash butt welding conditions were as follows.・ Welding machine: K-355 (Soviet Union) ・ Capacity: 150 KVA ・ Secondary current: 20,000 Amp. Maximum ・ Clamping force: 125 ton maximum ・ Remaining heat time: 90 sec ・ Flushing time: 150 sec ・ Upset amount: 15 mm

[0064]

[Table 1]

[0065]

[Table 2]

[0066]

[Table 3]

[0067]

[Table 4]

[0068]

As shown in Tables 1 and 2, the rail steel of the present invention has a bainite structure by appropriately selecting alloying elements such as Mn and Cr as compared with the comparative rail steel.
As shown in FIG. 9, the rail steel of the present invention adjusts the addition of V and Nb and adjusts the amount of addition to increase the hardness inside the rail head as compared with the comparative rail steel. Rolling fatigue damage such as dark spot damage, which has been a problem with rail steel, can be prevented, and at the same time, the occurrence of internal fatigue damage at the rail head can be suppressed.

By controlling the sum of V (wt%) + 10 × Nb (wt%) within the above-mentioned claims, as shown in FIG. 10 and FIG. In addition, the area thereof is reduced, and local wear of the rail head surface can be suppressed. According to the present invention, it has become possible to provide a rail which has the anti-rolling fatigue damage resistance, the anti-internal fatigue damage resistance, and the weld joint characteristics required for railroad rails.

[Brief description of the drawings]

FIG. 1 is a diagram showing an example of a bainite rail head section hardness distribution.

FIG. 2 is a diagram showing a head surface longitudinal hardness distribution of a flash butt welding joint of a rail having a bainite structure and a pearlite structure.

FIG. 3 is a diagram showing names of rail cross-sectional surface positions and a necessary range of a bainite structure.

FIG. 4 is a schematic view of a rolling fatigue damage tester.

FIG. 5 is a schematic diagram of a rolling fatigue tester.

FIG. 6 is a diagram showing a head section hardness distribution of the rail steel (symbol: F) of the present invention.

FIG. 7 is a diagram showing a head section hardness distribution of the rail steel (symbol: I) of the present invention.

FIG. 8 is a diagram showing a head section hardness distribution of a comparative rail steel (symbol: R).

FIG. 9 is a diagram showing a head section hardness distribution of comparative rail steel (symbol: S).

FIG. 10 is a view showing a longitudinal hardness distribution in a rail head surface portion of a flash butt welding joint portion of the rail steel of the present invention (symbol: F) and a comparative rail steel (symbol: U).

FIG. 11 is a diagram showing a longitudinal hardness distribution in a rail head surface portion of a flash butt welding joint portion of the rail steel of the present invention (symbol: I) and a comparative rail steel (symbol: V).

[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: Slider for rail movement, 9: Rail , 10: wheels, 11: motor, 12: load applying device

 ────────────────────────────────────────────────── ─── Continued on the front page F term (reference) 4K042 AA04 BA02 BA03 BA04 BA11 BA14 CA02 CA03 CA05 CA06 CA08 CA09 CA10 CA12 CA13 DA01 DC02 DE02 DE03 DE05 DE06

Claims (14)

[Claims] 1. Mass%, C: 0.15 to 0.55%, Si: 0.10 to 2.00%, Mn: 0.20 to 3.00%, Cr: 0.20 to 3. 00%, V: 0.01 to 0.30%, Nb: more than 0.01 to 0.05%, V + 10 × N
b is 0.14 or more and 0.50 or less, and the remainder is F
e and a rail made of unavoidable impurities, wherein at least a range of a depth of 20 mm from the head corner portion and the top surface of the rail has a bainite structure, and the bainite structure in the range has a hardness of Hv330 to 500. A bainite-based rail excellent in rolling fatigue damage resistance, internal fatigue damage resistance, and weld joint characteristics, characterized by having a bainite structure having a hardness difference of not more than Hv50 within a range. 2. The anti-rolling fatigue damage resistance, the internal fatigue damage resistance, and the weld joint characteristics according to claim 1, further comprising: Mo: 0.01 to 1.00% by mass%. Excellent bainite rail. 3. The composition according to claim 1, further comprising one or two types of Ti: 0.005 to 0.050% and Ni: 0.05 to 1.00% by mass%. 2. A bainitic rail having excellent rolling fatigue damage resistance, internal fatigue damage resistance, and weld joint characteristics described in 2. 4. The anti-rolling fatigue damage resistance and the internal fatigue resistance according to claim 1, further comprising Cu: 0.05 to 0.50% by mass. Bainite rail with excellent weldability and weldability. 5. The method according to claim 1, further comprising one or more of Mg: 0.0010 to 0.0100% and Ca: 0.0010 to 0.0150% by mass%. 4. Rolling fatigue damage resistance, internal fatigue damage resistance,
Bainite type rail with excellent weld joint characteristics. 6. The anti-rolling fatigue damage resistance and the internal fatigue damage according to claim 1, further comprising B: 0.0001 to 0.0050% by mass%. Bainite rail with excellent weldability and weldability. 7. In mass%, C: 0.15 to 0.55%, Si: 0.10 to 2.00%, Mn: 0.20 to 3.00%, Cr: 0.20 to 3%. 00%, V: 0.01 to 0.30%, Nb: more than 0.01 to 0.05%, V + 10 × N
b is 0.14 or more and 0.50 or less, and the remainder is F
e) and a steel slab consisting of unavoidable impurities are hot-rolled into a rail shape, and the head of the steel rail is accelerated and cooled from a temperature of 1 point or more at a cooling rate of 1 to 20 ° C./sec. When accelerated cooling is stopped when the head of the steel reaches 500 to 300 ° C., and then naturally cooled to room temperature, the rolling fatigue resistance, internal fatigue damage resistance, and weld joint characteristics An excellent bainite rail manufacturing method. 8. The method according to claim 7, wherein after the accelerated cooling is stopped, the temperature rise at the rail head surface due to the transformation heat generation and the reheating generated inside the rail is suppressed to 50 ° C. or less than the accelerated cooling stop temperature. A method for producing a bainite-based rail having excellent rolling fatigue damage resistance, internal fatigue damage resistance, and weld joint characteristics as described. 9. The anti-rolling fatigue damage resistance and internal resistance according to claim 7, wherein after stopping the accelerated cooling, the rail head surface portion is continuously cooled at a temperature of 1 to 40 ° C./min to a normal temperature range. A method for manufacturing bainite rails with excellent fatigue damage properties and weld joint characteristics. 10. The anti-rolling fatigue damage resistance and the internal fatigue damage resistance according to claim 7, further comprising: Mo: 0.01 to 1.00% by mass%. Of bainite rails with excellent weldability and weldability. 11. The composition according to claim 7, further comprising one or two types of Ti: 0.005 to 0.050% and Ni: 0.05 to 1.00% by mass%. 10. The method for producing a bainite-based rail excellent in rolling fatigue damage resistance, internal fatigue damage resistance, and welding joint characteristics according to any one of items 10. 12. The anti-rolling fatigue damage resistance and the internal fatigue resistance according to claim 1, further comprising Cu: 0.05 to 0.50% by mass%. Of bainite rails with excellent weldability and weldability. 13. The method according to claim 7, further comprising one or more of Mg: 0.0010 to 0.0100% and Ca: 0.0010 to 0.0150% by mass%. 13. The method for producing a bainite-based rail excellent in rolling fatigue damage resistance, internal fatigue damage resistance, and welding joint characteristics according to any one of 12. 14. The anti-rolling fatigue resistance and the internal fatigue resistance according to claim 7, further comprising B: 0.0001 to 0.0050% by mass%. Of bainite rails with excellent weldability and weldability.