JP2000054075A - Bainitie-base rail excellent in electric conductivity and rolling fatigue damage resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide electric conductivity and rolling fatigue damage resistance, both required of a rapid transit section, by specifying respective contents of C, Si, Mn, Cr, Mo, B, Nb, P, and S in a rail steel to control electric resistance value and forming the structure of railhead into bainitic structure. SOLUTION: A rail steel is provided with a composition consisting of, by weight, 0.15-0.45% C, 0.10-0.50% Si, 0.20-1.80% Mn, 0.20-1.80% Cr, 0.01-0.60% Mo, 0.0001-0.0020% B, 0.005-<0.01% Nb, <=0.030% P, <=0.030% S, and the balance Fe, and an electric resistance value represented by an expression is regulated to <=26.5 μΩ.cm. Further, by regulating heat treatment conditions, the structure in the area within a depth of at least 15 mm, with railhead corner surface and railhead surface as the starting point, is formed into bainitic structure of >=Hv 240 hardness. If necessary, one or >=2 kinds among 0.01-0.30%, by weight, V, 0.05-0.50% Cu, 0.05-1.00% Ni, and 0.005-0.050% Ti are incorporated into the above steel composition.

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 particularly in a high-speed operation section of a passenger railway and has both electric conductivity and anti-rolling fatigue damage.

[0002]

2. Description of the Related Art In recent years, the speed of trains has been increased in passenger railways in order to improve transportation efficiency. Along with this, rails in straight sections where high-speed operation is mainly performed, due to repeated contact between the rails and wheels, such as crack damage on the rail head surface called dark spot damage,
Rolling fatigue damage is likely to occur. The dark spot damage is likely to occur on a rail in a high-altitude operation section of a passenger railway in which a rail having a conventional pearlite structure is used.

The present inventors have studied the relationship between the formation of a fatigue layer (fatigue damage layer, texture) generated by repeated contact between a rail and a wheel and the metallographic structure. 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). A high-strength bainite rail that adds high-temperature heat after hot rolling, or accelerates and cools the head of a rail that has been heated to a high temperature by adding alloy elements such as Mn, Cr, and Mo as low-carbon components. Manufacturing method (JP-A-6-248347)
No.).

[0004] 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, and 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.

[0005]

However, when the alloying amount of the rail steel is increased in order to obtain a bainite structure like the above-mentioned inventive rail, the electric resistance value of the rail generally increases. As a result, track currents, which are the power sources of electric trains and electric locomotives, are reduced, and in railway sections where trains are densely operated, operational problems such as a decrease in train running speed due to a decrease in current are expected. As a countermeasure, track electrification systems such as shortening the interval between substations and increasing track voltage are considered to compensate for current loss and secure track current.
The remodeling of these track electrical equipment requires a lot of expense and is uneconomical. Therefore, as a method of reducing the electric resistance of a rail having a bainite structure, the following products have been developed. A rail exhibiting a bainite structure in which an alloy element of Mn, Cr, and Mo is appropriately held down and the electric resistance value is 23.7 μΩ · cm or less (Japanese Patent Application Laid-Open No. 7-305144). A rail exhibiting a bainite structure in which the amount of Mn effective for forming a bainite structure is increased, the amount of an alloy of Cr and Mo is optimized, and the electric resistance is reduced, and a method of manufacturing the same (Japanese Patent Application Laid-Open No. -13144 publication).

The feature of these rails is to provide a bainite-structured rail having a certain level of strength by suppressing the amount of alloy to suppress the electric resistance value.

However, in such a rail having a bainite structure with a small amount of added alloy, the bainite structure is not stable depending on the combination of alloying elements such as Mn and Cr and the difference in cooling rate after rail rolling, and the austenite grain boundary is not obtained. In addition, a ferrite structure harmful to abrasion resistance and a pearlite structure harmful to rolling fatigue damage were liable to be mixed, and it was difficult to stably produce a bainite structure rail.

Accordingly, the present inventors have studied additional elements that stabilize the bainite structure without reducing the electrical conductivity of steel. As a result of the experiment, it was confirmed that the formation of ferrite structure from austenite grain boundaries was suppressed,
At the same time, Nb, which promotes bainite transformation by improving hardenability, is added in a complex manner,
Compared with steels containing only B and Nb, quenching properties are synergistically improved, and the formation of ferrite structure harmful to wear resistance and pearlite structure harmful to rolling fatigue damage generated at austenite grain boundaries is stable. Confirmed that it can be prevented.

Further, by adding B and Nb having a small electric resistance in combination, the hardenability of the steel is increased and the bainite transformation is promoted. Therefore, it is added to stabilize the bainite transformation and secure the hardness. M with high electric resistance
It has been confirmed that the addition amount of alloying elements such as n, Cr, and Mo can be reduced, and the electric resistance can be further reduced as compared with the inventive rail steel described in the above.
That is, an object of the present invention is to provide a rail exhibiting a bainite structure having electrical conductivity and anti-rolling fatigue damage required for a high-speed operation section of a passenger railway.

[0010]

The present invention achieves the above object, and its gist is that the weight percent
C: 0.15 to 0.45%, Si: 0.10 to 0.50%, Mn: 0.20 to 1.80%, Cr: 0.20 to 1.80%, Mo: 0. B: 0.0001% to 0.0020% Nb: 0.005% to less than 0.01%, P: ≦ 0.030%, S: ≦ 0.030%, Further, if necessary, V: 0.01 to 0.30%, Cu: 0.05 to 0.50%, Ni: 0.05 to 1.00%, Ti: 0.005 to 0.050% It contains one or more kinds, and the balance consists of iron and inevitable impurities.
Excellent in electrical conductivity and rolling fatigue damage resistance, characterized in that the bainite structure has a hardness of Hv 240 or more at least in a range of a depth of at least 15 mm starting from the surface of the head corner and the top of the crown. Low resistance bainite rail. * Electrical resistance calculation formula (μΩ · cm) Calculation formula = 10.1 + 6.1 [C] +13.8 [Si] +6.3 [Mn] +5.2 [Cr] +17.2 [P] +11.2 [S] +2.9 [Ti] + 2.5 [Ni] + 6.O [Cu] +5.5 [V] +3.3 [Mo] Formula 1 where [element] is% by weight.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. (1) Chemical Components First, the reasons for determining the chemical components of the rail as described above will be described. C is an essential element for ensuring the strength and wear resistance of the bainite structure, but if less than 0.15%, a ferrite structure is formed in the bainite structure, and the strength and wear resistance required for the bainite rail. Is difficult to secure. On the other hand, if the content exceeds 0.45%, a large amount of pearlite structure harmful to the occurrence of rolling fatigue in the bainite structure is likely to be formed, and the bainite transformation rate is significantly reduced, and a martensite structure harmful to rail toughness. Is easily generated, so the C amount is 0.15 to
Limited to 0.45%.

[0012] Si is an element that improves the strength by forming a solid solution with the base ferrite in the bainite structure. However, if the content is less than 0.10%, the strength cannot be expected to be improved. Further, Si is the element having the highest contribution to the electric resistance. However, if added in excess of 0.50%, the adverse effect of increasing the electric resistance as compared with the contribution to the strength becomes very large. The amount of Si was limited to 0.10 to 0.50%.

Mn is an element indispensable for enhancing the hardenability of steel and for stably forming a bainite structure similarly to C. In this component system, the Mn content is 0.20%
If it is less than 3, the effect is weak, and depending on the combination of the added elements, a ferrite structure harmful to wear resistance and a pearlite structure harmful to surface damage resistance tend to be formed in the bainite structure. Further, if it exceeds 1.80%, it becomes very difficult to keep the electric resistance within the range of the original rail, so the Mn content is limited to 0.20 to 1.80%.

[0014] Cr is an important element for finely dispersing carbides in the bainite structure and ensuring strength.
If it is less than 0.20%, the effect is weak, and it is difficult to stably obtain a bainite structure depending on the combination of the additional elements. On the other hand, when the content exceeds 1.80%, a martensitic structure is easily formed on the surface of the rail head, and the toughness and wear resistance of the rail are reduced, and the electric resistance cannot be kept within the current range of rail like Mn. From
The amount of Cr was limited to 0.20 to 1.80%.

Mo, like Mn or Cr, stably forms a bainite structure.
r is a promising element that can improve the strength without extremely increasing the electric resistance.
%, The contribution to the strength is small, and if added over 0.60%, the adverse effect of increasing the electric resistance becomes very large compared to the contribution to the strength. Further, Mo is easily segregated and easily forms a martensite structure harmful to the toughness of the rail in the segregated portion.
Limited to 0%.

B is an element that suppresses the formation of a pro-eutectoid ferrite structure formed from the prior austenite grain boundaries and a pearlite structure that is transformed with the structure, and stably forms a bainite structure by increasing hardenability. But,
If it is less than 0.0001%, the effect is weak, and 0.0020%
%, A large amount of nitrides and carbides of B are formed at the austenite grain boundaries, conversely reducing the hardenability,
In order to easily generate a proeutectoid ferrite structure or a pearlite structure, the B content was limited to 0.0001 to 0.0020%.

Nb, like B, is an element that suppresses the formation of a pro-eutectoid ferrite structure or a pearlite structure formed from a prior-austenite grain boundary and stably forms a bainite structure by increasing hardenability. The effect is 0.00
If it is less than 5%, it cannot be expected, and even if 0.01% or more is added, the hardenability is not significantly improved, so the Nb content is limited to 0.005% to less than 0.01%.

The reason why B and Nb are added in a complex manner is that the hardenability is remarkably increased as compared with the steel to which B and Nb are added alone, and the ferrite structure and the pearlite structure from the austenite grain boundary are increased. This is because the formation is prevented and the bainite structure is stably generated. In addition, since the hardenability is increased by the complex addition of B and Nb having low electric resistance, alloy elements such as Mn, Cr, and Mo having high electric resistance added to stabilize bainite transformation and secure hardness are added. This is because the addition amount can be reduced, and the electric resistance can be further reduced.

P and S are not elements to be added intentionally, but if each contains more than 0.030%, the electric resistance excessively increases, a segregation zone is formed, and the martensitic structure lowers the toughness. Etc. are easily generated, and each is limited to 0.030% or less. 0.025 each
% Is more preferable.

The rails manufactured with the above-mentioned composition have one or more of the following elements added as necessary for the purpose of preventing strength, ductility, toughness and deterioration of the material during welding. . V: 0.01 to 0.30%, Cu:
0.05-0.50%, Ni: 0.05-1.00%,
Ti: 0.005 to 0.050%

V increases the strength of the bainite structure by precipitation hardening by V carbide and V nitride generated in the cooling process during hot rolling, and at the same time, increases the crystal grain growth when heat treatment is performed at a high temperature. It is an effective component for refining austenite grains by suppressing action and improving the ductility and toughness of bainite structure. However, if it is less than 0.01%, its effect cannot be expected sufficiently, and it exceeds 0.30%. Since no further effect can be expected even if added, the V
〜0.30%.

Cu is an element which improves the strength without deteriorating the toughness of the steel, and its effect is greatest in the range of 0.05 to 0.50%. Cu content is 0.05 to 0.5
Limited to 0%.

Ni is an element that stabilizes austenite and has an effect of lowering the bainite transformation temperature, refining the bainite structure, and improving the toughness.
If it is less than 5%, the effect is remarkably small.
%, The amount of Ni is limited to 0.05 to 1.00% in order to greatly reduce the bainite transformation rate and to easily generate a martensite structure harmful to the toughness of the rail.

Ti is Ti carbide precipitated during melting and solidification. Utilizing the fact that Ti nitride does not dissolve even at the high temperature of rail rolling heating, it aims to refine austenite crystal grains during rail rolling heating and contributes to improvement of ductility and toughness of bainite structure. Further, it is an element that precipitates excessive nitrogen in steel as nitride (TiN), suppresses precipitation of nitride (BN) of B, and as a result, secures hardenability by solid solution B. If the content is less than 005%, these effects are not sufficiently exerted. If the content exceeds 0.050%, coarse nitride (TiN) or carbide (T
iC) is generated and the ductility and toughness of the rail deteriorate, and at the same time, it tends to be a starting point of fatigue damage during use of the rail.
The amount of Ti was limited to 0.005 to 0.050%.

The rail steel having the above-mentioned composition is melted in a commonly used melting furnace such as a converter or an electric furnace, and the molten steel is formed by ingot-bulking or continuous casting.
Further, it is manufactured as a rail through hot rolling. Further, if necessary, accelerated cooling is performed on the hot-rolled rail having high-temperature heat or the rail head heated to a high temperature for the purpose of heat treatment to adjust the hardness of the rail head.

(2) Range of bainite structure and its hardness First, the range having the bainite structure was determined to have a depth of 15 m starting from the head surface at the corner and top of the head.
The reason for limiting to the range of m will be described. As for the rail head, the wear life of the rail used in the straight section of the high-speed railway is 15 mm. Therefore, if the area having the bainite structure is less than 15 mm, the area for preventing the rolling fatigue damage required for the rail head is required. Surface damage such as dark spot damage and flaking damage occurs before the wear life is reached, and a sufficient life improvement effect cannot be expected. Therefore, the range having the bainite structure is limited to 15 mm or more.

Here, FIG. 1 shows the designation of the cross-sectional surface position of the head of the low electric resistance bainite type rail according to the present invention, which is excellent in weldability and anti-rolling fatigue damage. 1 at the rail head
Is a crown, 2 is a head corner, and one of the head corners 2 is a gauge corner (G.
C. ) Department.

Further, the hardness of the bainite structure is set to Hv24.
The reason for limiting the number to 0 or more will be described. Hardness is Hv2
If it is less than 40, it is difficult to secure the basic strength and wear resistance required for the rail. 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 bainite structure is limited to Hv 240 or more.

(3) Electric resistance value The electric resistance value (μΩ · cm) is calculated to be 26.5 μΩ · c.
The reason defined below is explained. Table 1 shows an example of the chemical composition of a typical railroad passenger rail, a calculated value of electrical resistance according to Equation 1, and an actually measured value. Here, elements that are not contained and elements that are not described in the formula are treated as zero.

[0030]

[Table 1]

* Electric resistance calculation formula (μΩ · cm) Calculation formula = 10.1 + 6.1 [C] +13.8 [Si] +6.3 [Mn] +5.2 [Cr] +17.2 [P] +11.2 [S] +2.9 [ Ti] +2.5 [Ni] + 6.O [Cu] +5.5 [V] +3.3 [Mo] (1) where [element] is% by weight.

As shown in Table 1, the measured value of the electric resistance of the rail has a very good correlation with the value calculated by the equation (1). Considering the manufacturing variation of rail chemical components,
When the calculated electrical resistance exceeds 26.5 μΩ · cm,
It is considered that there is a high possibility that the electric resistance value increases in actual measurement as compared with a typical railroad rail with a high carbon pearlite structure currently used. Therefore, when the calculated value of the electric resistance exceeds 26.5 μΩ · cm, the electric resistance also increases in the actual measurement as compared with the current rail, and as described above, the track current, which is the power source of the train, decreases, and the train operates. In an overcrowded line section, an operation problem such as a decrease in train running speed due to a decrease in current is expected to occur. Therefore, the calculated electric resistance value of the rail is 26.5.
It was limited to μΩ · cm or less.

The metal structure of the rail steel of the present invention is desirably a bainite structure. However, depending on the combination of the component systems, the cooling method of the rail, and the segregation state of the material, a trace amount of pearlite structure and martensite structure may be contained in the bainite structure. In some cases, a pro-eutectoid ferrite structure is formed. However, even if these structures are formed in trace amounts in the bainite structure, they do not significantly affect the rolling fatigue resistance, wear resistance and strength of the rail. Includes mixed organizations.

As described above, the bainite-based rail of the present invention has electrical conductivity and anti-rolling fatigue damage required for a high-speed operation section of a passenger railway.

[0035]

Next, an embodiment of the present invention will be described. Table 2 shows the chemical composition of the rail steel of the present invention, the microstructure, the hardness of the rail head, and the electrical resistance value calculated by the following equation 1. Further, the results of a water lubrication rolling fatigue test using a rolling fatigue damage tester shown in FIG. 2 and the results of actual measurement of rail electrical resistance shown in FIG. 3 are also shown.

[0036]

[Table 2]

Table 3 shows the chemical composition of the comparative rail steel,
The microstructure, the hardness of the base material and the heat-affected zone, and the electrical resistance value calculated by the following equation 1 are shown. Further, the results of a water lubrication rolling fatigue test using a rolling fatigue damage tester shown in FIG. 2 and the results of actual measurement of rail electrical resistance shown in FIG. 3 are also shown.

[0038]

[Table 3]

* Electric resistance calculation formula (μΩ · cm) Calculation formula = 10.1 + 6.1 [C] +13.8 [Si] +6.3 [Mn] +5.2 [Cr] +17.2 [P] +11.2 [S] +2.9 [ Ti] +2.5 [Ni] + 6.O [Cu] +5.5 [V] +3.3 [Mo] (1) where [element] is% by weight.

The configuration of the rail is as follows.・ Rail steel of the present invention (10 pieces) Code: A to J Within the above component range, the calculated electrical resistance is 26.5 μΩcm.
In the following steel rail, at least a depth of 1 mm from the head surface of the rail head corner portion and the crown portion as a starting point.
Electrical conductivity, characterized in that the metal structure in a portion within a range of 5 mm is a bainite structure having a hardness of Hv240 or more;
Bainite rail with excellent rolling fatigue resistance. -Comparative rail steel (10 pieces) Codes: K to T Codes K, L: Comparative rail steels of pearlite structure currently used in passenger railways. Symbols M to T: comparative rail steels outside the above component range.

The rolling fatigue test was performed under the following conditions. -Testing machine: Rolling fatigue tester-Specimen shape: disk-shaped specimen (Rail outer diameter: 200 mm, rail material cross section: 60K
(1/4 model of rail) (wheel outer diameter: 200 mm, cross section of wheel material: 1/4 model of circular tread wheel)-Test load: 1.0 ton (radial load)-Atmosphere: dry + water lubrication (60 / min)-Number of rotations: drying; 100 rpm, water lubrication; 300 rpm-Number of repetitions: dry state from 0 to 5000 times, then
Until damage occurs due to water lubrication. (If no damage occurs, stop the test at 2 million times)

The measurement of the electric resistance of the rail was carried out in the following manner. As a method for measuring the electric resistance of the rail shown in FIG. 3, a voltage drop method was used. As a measuring method, current terminals were attached to the pillars on both sides of the rail end, and the reference current was 25A.
And set the voltage terminal on the rail foot at a position about 1 m inside the current terminal.

From the current value and the voltage value thus measured, the electric resistance was obtained by the following equation. ρ (μΩ · cm) = (ρ ₂₀ × A) × 10 ⁻¹ / LA A: cross-sectional area of measurement rail (mm ² ), L: distance between voltage terminals (mm) where ρ ₂₀ is at 20 ° C. The following formula was used for the resistance value (μΩ) of the measurement material converted to the value. ρ ₂₀ = ρ _t / {1 + α (t−20)} ρ _t ; electric resistance measured at temperature t, α was obtained from a change in resistance value actually measured by actually changing the rail temperature with a temperature correction coefficient.

[0044]

As shown in Tables 2 and 3, the rail steel of the present invention has an electric resistance which is higher than that of the comparative rail steel by adjusting the appropriate selection and addition amount of alloying elements such as Mn, Cr and Mo. By having a low bainite structure, it is possible to prevent rolling fatigue damage which has been a problem with the current pearlite structure rail.

Further, as shown in Table 2, Table 3 and FIG. 4, the addition of B and Nb having low electric resistance in combination increases the hardenability of steel and promotes the bainite transformation. It is possible to reduce the amount of alloying elements such as Mn and Cr, which have high electric resistance added to secure the stability and hardness of the structure. The electric resistance can be further reduced.

That is, according to the present invention, it is possible to provide a rail having both electrical conductivity and anti-rolling fatigue damage required in a high-speed operation section of a passenger train.

[Brief description of the drawings]

FIG. 1 is a diagram showing names of rail head cross-sectional surface positions.

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

FIG. 3 is a schematic diagram showing a method for measuring electric resistance.

FIG. 4 is a diagram showing the relationship between the hardness of the head of the rail steel of the present invention and the comparative rail steel and the measured electrical resistance.

[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

Claims (2)

[Claims] C .: 0.15 to 0.45%, Si: 0.10 to 0.50%, Mn: 0.20 to 1.80%, Cr: 0.20 to 1. 80%, Mo: 0.01 to 0.60%, B: 0.0001 to 0.0020%, Nb: 0.005% to less than 0.01%, P: 0.030%, S: 0 0.030%, the balance being iron and unavoidable impurities, having an electrical resistance value of 26.5 μΩ · cm or less according to the following formula 1, and having a depth of at least 15 mm starting from the head corner and top surface. A low-resistance bainite rail excellent in electrical conductivity and anti-rolling fatigue damage, characterized by having a bainite structure having a hardness of Hv240 or more. * Electrical resistance calculation formula (μΩ · cm) Calculation formula = 10.1 + 6.1 [C] +13.8 [Si] +6.3 [Mn] +5.2 [Cr] +17.2 [P] +11.2 [S] +2.9 [Ti] + 2.5 [Ni] + 6.O [Cu] +5.5 [V] +3.3 [Mo] Formula 1 where [element] is% by weight. 2. In% by weight, C: 0.15 to 0.45%, Si: 0.10 to 0.50%, Mn: 0.20 to 1.80%, Cr: 0.20 to 1. 80%, Mo: 0.01 to 0.60%, B: 0.0001 to 0.0020%, Nb: 0.005% to less than 0.01%, P: 0.030%, S: 0 0.030%, V: 0.01 to 0.30%, Cu: 0.05 to 0.50%, Ni: 0.05 to 1.00%, Ti: 0.005 to 0. 050% of one or more kinds, the balance being iron and unavoidable impurities, and having an electrical resistance of 26.
Excellent in electrical conductivity and rolling fatigue damage resistance, characterized in that the bainite structure has a hardness of Hv 240 or more at least in a range of a depth of at least 15 mm starting from the surface of the head corner and the top of the crown. Low resistance bainite rail. * Electrical resistance calculation formula (μΩ · cm) Calculation formula = 10.1 + 6.1 [C] +13.8 [Si] +6.3 [Mn] +5.2 [Cr] +17.2 [P] +11.2 [S] +2.9 [Ti] + 2.5 [Ni] + 6.O [Cu] +5.5 [V] +3.3 [Mo] Formula 1 where [element] is% by weight.