JP2010077481A - High internal hardness type pearlitic steel rail excellent in wear resistance and fatigue deterioration resistance, and method for manufacturing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high internal hardness type pearlitic steel rail which is excellent in both of wear resistance and fatigue deterioration resistance, by increasing hardness in a range of 25 mm deep from the surface of a railhead, and also to provide a preferable method for manufacturing the same. <P>SOLUTION: This pearlitic steel rail has a composition comprising, 0.73-0.85 mass% C, 0.5-0.75 mass% Si, 0.3-1.0 mass% Mn, 0.035 mass% or less P, 0.0005-0.012 mass% S, more than 0.5 mass% and equal to or less than 1.3 mass% Cr and the balance Fe with unavoidable impurities, while a value of [%Mn]/[%Cr] is controlled to 0.3 or more but less than 1.0, in which [%Mn] represents an Mn content and [%Cr] represents an Cr content. The railhead contains more than 0.20 mass% and equal to or less than 0.50 mass% precipitated Cr, and has an internal hardness of Hv 395 or more but less than Hv 480, which is defined by Vickers hardness in a range of at least 25 mm deep from the surface of the railhead. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an internal high-hardness pearlite steel rail excellent in wear resistance and fatigue damage resistance and a method for producing the same, and more particularly, in a severe mine railway such as an overseas mining railway having a heavy freight car and a sharp curve. The present invention relates to an internal high-hardness pearlite steel rail excellent in wear resistance and fatigue damage resistance that achieves a long life of a rail used under high axial load conditions, and a manufacturing method thereof.

  In high-axle heavy railways that mainly transport ores, the load on the axles of freight cars is far higher than that of passenger cars, and the use environment of the rails is also severe. Conventionally, steel having a pearlite structure has been used as a rail used in such an environment from the viewpoint of emphasizing wear resistance. However, in recent years, the load on the freight cars has been further increased in order to increase the efficiency of transportation by rail, and further improvements in wear resistance and fatigue damage resistance are required. The high-axle heavy railway is a railway having a large loading weight (a loading weight of, for example, about 150 tons or more) of one freight car such as a train or a freight car.

  In recent years, various studies have been conducted with the aim of further improving wear resistance. For example, in Patent Document 1 and Patent Document 2, the C amount is increased from 0.85% by mass to 1.20% by mass or less, and in Patent Document 3 and Patent Document 4, the C amount is increased from 0.85% by mass to 1.20% by mass and the rail head. Some devices have been devised such as increasing the amount of C and increasing the cementite fraction to improve the wear resistance, such as by heat-treating the part.

  On the other hand, the rails in the curved section of the high-axle heavy railway are subjected to rolling stress due to wheels and sliding force due to centrifugal force, so that wear of the rail becomes more severe and fatigue damage due to sliding occurs. As described above, when the C content is simply over 0.85% by mass and not more than 1.20% by mass, a pro-eutectoid cementite structure is formed depending on the heat treatment conditions, and the amount of the cementite layer in the brittle pearlite layered structure increases. Improvement is not expected. Therefore, Patent Document 5 proposes a technique for suppressing the formation of pro-eutectoid cementite by adding Al and Si and improving the fatigue damage resistance. However, it was difficult to satisfy both wear resistance and fatigue damage resistance in steel rails having a pearlite structure, such as the addition of Al, which generates oxides that cause fatigue damage.

In order to improve the service life of the rail, in Patent Document 6, the Vickers hardness in the range of at least 20mm in depth is set to Hv370 or more starting from the surface of the head corner and the top of the rail. The service life is improved. Further, in Patent Document 7, by controlling the pearlite block, the hardness of at least a depth of 20 mm starting from the surface of the head corner and the top of the rail is in the range of Hv300 to 500. The service life of the rail is improved.
Japanese Patent Laid-Open No. 8-109439 Japanese Patent Laid-Open No. 8-144016 JP-A-8-246100 JP-A-8-246101 Japanese Patent Laid-Open No. 2002-69585 Japanese Patent Laid-Open No. 10-195601 JP2003-293086

However, the usage environment of pearlite steel rails has become more severe, and in order to improve the service life of pearlite steel rails, it has been a challenge to further increase the hardness and extend the range of hardening depth. The present invention has been made to solve this problem. Compared with conventional hypoeutectoid, eutectoid and hypereutectoid pearlite steel rails, the amount of added Si, Mn, Cr and the amount of precipitated Cr are optimized. In addition, by optimizing the hardenability index (hereinafter referred to as DI) and the carbon equivalent (hereinafter referred to as C _eq ), the hardness within a range of 25 mm depth is increased at least from the rail top surface, The present invention provides an internal high hardness type pearlitic steel rail excellent in both wear and fatigue damage resistance properties together with a preferable manufacturing method thereof.

In order to solve the above-mentioned problems, the inventors manufactured a pearlite steel rail in which the contents of Si, Mn, and Cr were changed, and intensively investigated the structure, hardness, wear resistance, and fatigue damage resistance. As a result, the [% Mn] / [% Cr] value calculated from the Mn content [% Mn] and the Cr content [% Cr] is 0.3 or more and less than 1.0, and the precipitated Cr amount is 0.20 mass% or more and 0.50 mass% or less. As a result, the pearlite layer lamellar (hereinafter also referred to simply as “lamellar”) spacing becomes finer, and the internal hardness of the rail head defined by the hardness within a depth range of at least 25 mm from the surface of the rail head is Hv380. As described above, it has been found that the wear resistance and fatigue damage resistance are improved by being less than Hv480. Further, the hardenability index (ie DI value) is in the range of 5.6 to 8.6, the carbon equivalent (ie C _eq value) is in the range of 1.04 to 1.27, Mn content [% Mn], Cr content [% Cr], Wear resistance and fatigue damage resistance are improved by keeping the value of [% Si] + [% Mn] + [% Cr] calculated from the Si content [% Si] within the range of 1.55 to 2.50 mass%. It was found that the effect to be able to be maintained stably.

The present invention has been made based on these findings.
That is, the present invention is C: 0.73-0.85 mass%, Si: 0.5-0.75 mass%, Mn: 0.3-1.0 mass%, P: 0.035 mass% or less, S: 0.0005-0.012 mass%, Cr: more than 0.5 mass% 1.3% by mass or less, with the balance being Fe and inevitable impurities, Mn content [% Mn] and Cr content [% Cr] [% Mn] / [% Cr] value Is less than 0.3 and less than 1.0, the amount of Cr deposited on the rail head is more than 0.20% by mass and less than 0.50% by mass, and the rail head is defined by the Vickers hardness in the range of at least 25mm depth from the surface of the rail head The internal hardness of Hv395 is less than Hv480 and less than Hv480. This is a high-hardness pearlitic steel rail with excellent wear resistance and fatigue damage resistance.

In the internal high hardness type pearlite steel rail of the present invention, the C content of the above composition is [% C], the Si content is [% Si], the Mn content is [% Mn], and the P content is [%]. The P value, S content is [% S], Cr content is [% Cr], the DI value calculated by the following formula (1) satisfies the range of 5.6 to 8.6, and the following (2 It is preferable that the C _eq value calculated by the above formula satisfies the range of 1.04 to 1.27.
DI = (0.548 [% C] ^1/2 ) × (1 + 0.64 [% Si]) × (1 + 4.1 [% Mn])
× (1 + 2.83 [% P]) × (1−0.62 [% S]) × (1 + 2.23 [% Cr]) (1)
C _eq = [% C] + ([% Si] / 11) + ([% Mn] / 7) + ([% Cr] /5.8) (2)
Further, assuming that the Si content of the above composition is [% Si], the Mn content is [% Mn], and the Cr content is [% Cr], the [% Si] + [% Mn] + [% Cr] value is It is preferable to satisfy the range of 1.55 to 2.50% by mass. Further, in addition to the above composition, V: 0.001 to 0.30 mass%, Cu: 1.0 mass% or less, Ni: 1.0 mass% or less, Nb: 0.001 to 0.05 mass%, and Mo: 0.5 mass% or less are selected. It is preferable to contain 1 type (s) or 2 or more types.

Further, in the internal high hardness type pearlite steel rail of the present invention, the lamellar spacing of the pearlite layer in the range of a depth of at least 25 mm from the surface layer of the rail head is preferably 0.04 to 0.15 μm.
In the present invention, the steel material having the above composition is hot-rolled into a rail shape so that the rolling finishing temperature is 850 to 950 ° C., and then the rail head is 2.5 to 5 ° C. above the pearlite transformation start temperature. This is a method for producing an internal high-hardness pearlite steel rail with excellent wear resistance and fatigue damage resistance that is accelerated to 400-550 ° C at a cooling rate of / sec and then reheated at 20 ° C or higher.

  According to the present invention, it becomes possible to stably manufacture a pearlite steel rail having wear resistance and fatigue damage resistance far superior to those of conventional pearlite steel rails. This contributes to the prolongation of service life and the prevention of railway accidents, and has an industrially beneficial effect.

The reasons for limiting the requirements including the composition of the internal high hardness pearlitic steel rail of the present invention will be described.
C: 0.73-0.85 mass%
C forms cementite in the pearlite structure and is an essential element for ensuring the wear resistance, and the wear resistance improves as the content increases. However, if it is less than 0.73 mass%, it is difficult to obtain excellent wear resistance as compared with the conventional heat treated pearlite steel rail. On the other hand, if it exceeds 0.85% by mass, pro-eutectoid cementite is formed at the austenite grain boundaries during transformation after hot rolling, and the fatigue damage resistance is remarkably reduced. Therefore, the C content is 0.73 to 0.85 mass%. Preferably it is 0.75-0.85 mass%.

Si: 0.5-0.75 mass%
Si needs to be 0.5% by mass or more as a deoxidant and a strengthening element of the pearlite structure. However, if it exceeds 0.75% by mass, the weldability deteriorates due to the high bonding strength with Si. Furthermore, due to the high hardenability of Si, a martensite structure is easily formed on the surface layer of the internal high hardness type pearlitic steel rail. Accordingly, the Si content is 0.5 to 0.75 mass%. Preferably it is 0.5-0.70 mass%.

Mn: 0.3 to 1.0 mass%
Mn contributes to increasing the strength and ductility of internal hardened rails by lowering the pearlite transformation temperature and reducing the lamellar spacing, but excessive addition reduces the equilibrium transformation temperature of pearlite, and as a result In other words, it is an element that reduces the degree of supercooling and coarsens the lamellar spacing. If the amount is less than 0.3% by mass, a sufficient effect cannot be obtained. If the amount exceeds 1.0% by mass, a martensite structure is likely to be formed, and the material is liable to be hardened or embrittled during heat treatment and welding. Further, even if a pearlite structure is formed, the equilibrium transformation temperature is lowered, leading to coarse lamellar spacing. Therefore, the amount of Mn is 0.3 to 1.0% by mass. Preferably it is 0.3-0.8 mass%.

P: 0.035% by mass or less
The content of P exceeding 0.035% deteriorates ductility. Therefore, the P content is 0.035% by mass or less. Preferably it is 0.020 mass% or less.
S: 0.0005 to 0.012 mass%
S is present in the steel material mainly in the form of A-based inclusions, but when it exceeds 0.012% by mass, the amount of inclusions increases remarkably, and at the same time, coarse inclusions are produced, so that the cleanliness of the steel materials deteriorates. On the other hand, when the content is less than 0.0005% by mass, the steelmaking cost is increased. Therefore, the S amount is set to 0.0005 to 0.012% by mass. Preferably it is 0.0005-0.010 mass%. More preferably, it is 0.0005-0.008 mass%.

Cr: More than 0.5% by mass and less than 1.3% by mass
Cr is an element that raises the pearlite equilibrium transformation temperature and contributes to refinement of the lamellar spacing, and at the same time brings about further strengthening by solid solution strengthening. However, when the amount is 0.5% by mass or less, sufficient internal hardness cannot be obtained. On the other hand, when the amount exceeds 1.3% by mass, the hardenability increases and martensite is easily generated. Moreover, when it manufactures on the conditions which a martensite does not produce | generate, pro-eutectoid cementite produces | generates in a prior austenite grain boundary. Therefore, the wear resistance and fatigue damage resistance are reduced. Therefore, the Cr content is more than 0.5% by mass and 1.3% by mass or less. Preferably it is 0.6-1.2 mass%.

Precipitated Cr amount at rail head: Over 0.20% by mass and below 0.50% by mass
Cr not only strengthens by dissolving in cementite and ferrite in the pearlite lamellar but also precipitates and strengthens in the ferrite phase and cementite phase in the pearlite lamellar, thereby increasing the hardness of the rail. The inventors rolled pearly steel rails under the conditions shown in Table 2 for steel materials having the compositions shown in Table 1 to produce pearlite steel rails. Cooling was performed only on the rail head, and after cooling stopped, it was allowed to cool. The pearlite steel rail was examined for Vickers hardness and precipitated Cr content. The results are shown in Table 3. The rolling finishing temperature in Table 2 indicates the value obtained by measuring the temperature of the rail head side surface layer on the final rolling mill entry side with a radiation thermometer as the rolling finishing temperature. The cooling stop temperature indicates a value obtained by measuring the temperature of the rail head side surface layer on the cooling stop exit side with a radiation thermometer as the cooling stop temperature. The cooling rate was defined as the time change in temperature from the start of cooling to the stop of cooling. The recuperation temperature was determined by measuring the temperature of the surface layer on the side of the rail head at the entrance to the cooling floor with a radiation thermometer and calculating the difference between the measured value and the cooling stop temperature. As apparent from Table 3, when the amount of Cr deposited on the rail head is 0.20% by mass or less, the hardness in the range of at least 25 mm depth from the surface layer of the rail head does not satisfy the range of Hv395 to 480. On the other hand, if it exceeds 0.50% by mass, the amount of Cr added increases, so the hardenability of the pearlite steel rail increases and martensite is generated. Therefore, the amount of precipitated Cr in the rail head is set to be more than 0.20 mass% and 0.50 mass% or less. Preferably it is more than 0.20 mass% and 0.45 mass% or less.

[% Mn] / [% Cr]: 0.3 or more and less than 1.0
Mn and Cr are elements added to increase the hardness of the internal hardened pearlite steel rail. However, if the balance between the Mn content [% Mn] and the Cr content [% Cr] is not appropriate, martensite is generated on the surface layer of the internal high hardness type pearlite steel rail. The unit of [% Mn] and [% Cr] is mass%. If the value of [% Mn] / [% Cr] is less than 0.3, the amount of Cr added increases, and because of the high hardenability of Cr, martensite is generated on the surface layer of the internal hardened pearlite steel rail. It becomes easy. Moreover, when it manufactures on the conditions which a martensite does not produce | generate, pro-eutectoid cementite produces | generates in a prior austenite grain boundary. Therefore, fatigue damage resistance is reduced. On the other hand, when the value of [% Mn] / [% Cr] is 1.0 or more, the amount of Mn added increases and the hardenability of Mn increases. Becomes easier to generate. Moreover, since the amount of Cr added also decreases, it is not possible to expect Cr precipitation strengthening of cementite. The content of Mn and Cr is within the above ranges, and the value of [% Mn] / [% Cr] is set to 0.3 or more and less than 1.0, thereby preventing the formation of martensite on the surface layer and rails. The internal hardness of the head (hardness in a range of at least 25 mm depth from the head surface layer of the internal high-hardness pearlitic steel rail) can be controlled within a range described later. Therefore, the value of [% Mn] / [% Cr] is 0.3 or more and less than 1.0. Preferably it is 0.3 or more and 0.9 or less.

DI: 5.6 to 8.6
DI value is C content [% C], Si content [% Si], Mn content [% Mn], P content [% P], S content [% S], Cr It is a value calculated by the following formula (1) with the content as [% Cr]. The units of [% C], [% Si], [% Mn], [% P], [% S], and [% Cr] are all mass%.
DI = (0.548 [% C] ^1/2 ) × (1 + 0.64 [% Si]) × (1 + 4.1 [% Mn])
× (1 + 2.83 [% P]) × (1−0.62 [% S]) × (1 + 2.23 [% Cr]) (1)
This DI value represents hardenability and is used as an index for determining the quality of hardenability. In the present invention, while suppressing the formation of martensite on the surface layer of the internal high hardness type pearlite steel rail, It is preferably used as an index for achieving the target value of the internal hardness of the rail head and maintained within a suitable range. When the DI value is less than 5.6, the desired internal hardness can be obtained, but it is close to the lower limit of the target hardness range, and therefore further improvement in wear resistance and fatigue damage resistance cannot be expected. On the other hand, if the DI value exceeds 8.6, the hardenability of the internal high-hardness pearlite steel rail is increased, and martensite is easily generated on the surface layer of the rail head. Therefore, the DI value is preferably 5.6 to 8.6. More preferably, it is 5.6 to 8.2.

C _eq : 1.04-1.27
The C _eq value is calculated by the following formula (2), where C content is [% C], Si content is [% Si], Mn content is [% Mn], and Cr content is [% Cr]. Value. The units of [% C], [% Si], [% Mn], and [% Cr] are all mass%.
C _eq = [% C] + ([% Si] / 11) + ([% Mn] / 7) + ([% Cr] /5.8) (2)
This C _eq value is used to estimate the maximum hardness and weldability obtained from the alloy composition ratio, but in the present invention, martensite is suppressed from forming on the surface layer of the internal high-hardness pearlite steel rail. At the same time, it is preferably used as an index for achieving the target value of the internal hardness of the rail head and maintained within a suitable range. If the C _eq value is less than 1.04, the desired internal hardness can be obtained, but it is close to the lower limit of the target hardness range, so that further improvement in wear resistance and fatigue damage resistance cannot be expected. When the C _eq value exceeds 1.27, the hardenability of the internal high-hardness pearlite steel rail is increased, and martensite is easily generated on the surface layer of the rail head. Therefore, the C _eq value is preferably 1.04 to 1.27. More preferably, it is 1.04-1.20.

If the internal hardness of the rail head (hardness in the range of depth of at least 25 mm from the surface of the head of the internal hardened pearlitic steel rail is at least 25 mm deep) is less than Hv395 and less than Hv480, if the internal hardness of the rail head is less than Hv395, Abrasion is reduced and the service life of internal hardened pearlitic steel rails is reduced. On the other hand, when it becomes Hv480 or more, martensite is generated, and the fatigue damage resistance of the steel is lowered. Therefore, the internal hardness of the rail head is set to Hv395 or more and less than Hv480. Preferably it is Hv400 or more and less than Hv480. In addition, the definition area of the internal hardness of the rail head is at least 25 mm depth from the head surface layer of the internal high hardness type pearlite steel rail, and if this range includes less than 25 mm depth, This is because the wear resistance of the internal high-hardness pearlite steel rail decreases as it enters the inside from the surface layer of the rail head, and the service life is reduced.

[% Si] + [% Mn] + [% Cr]: 1.55 to 2.50 mass%
The sum of Si content [% Si], Mn content [% Mn] and Cr content [% Cr] (= [% Si] + [% Mn] + [% Cr]) is less than 1.55% by mass. If there is, it is difficult to satisfy the internal hardness of the rail head of Hv395 or more and less than Hv480. On the other hand, if it exceeds 2.50% by mass, a martensite structure is formed due to the high hardenability of Si, Mn, and Cr, and the ductility and toughness tend to decrease. Therefore, the [% Si] + [% Mn] + [% Cr] value is preferably 1.55 to 2.50 mass%. More preferably, it is 1.55-2.30 mass%.

The composition is further selected from V: 0.001 to 0.30 mass%, Cu: 1.0 mass% or less, Ni: 1.0 mass% or less, Nb: 0.001 to 0.05 mass%, and Mo: 0.5 mass% or less 1 A seed | species or 2 or more types may be added as needed.
V: 0.001 to 0.30 mass%
V forms a carbonitride, disperses and precipitates in the matrix, and improves the wear resistance. However, if it is less than 0.001% by mass, its effect is small, while if it exceeds 0.30% by mass, the workability deteriorates and it is manufactured. Cost increases. Moreover, since the alloy cost increases, the cost of the internal high hardness type pearlite steel rail increases. Therefore, when adding V, the amount of V is preferably 0.001 to 0.30 mass%. More preferably, it is 0.001-0.15 mass%.

Cu: 1.0% by mass or less
Cu, like Cr, is an element for further strengthening by solid solution strengthening. However, if it exceeds 1.0% by mass, Cu cracking tends to occur. Therefore, when adding Cu, it is preferable that the amount of Cu shall be 1.0 mass% or less. More preferably, it is 0.005-0.5 mass%.
Ni: 1.0% by mass or less
Ni is an element for increasing the strength without deteriorating the ductility. Moreover, in order to suppress Cu cracking by compounding with Cu, it is desirable to add Ni when Cu is added. However, addition exceeding 1.0% by mass increases the hardenability and produces martensite, which tends to decrease the wear resistance and fatigue damage resistance. Therefore, when adding Ni, the amount of Ni is preferably 1.0% by mass or less. More preferably, it is 0.005-0.5 mass%.

Nb: 0.001 to 0.05 mass%
Nb combines with C in the steel and precipitates as a carbide during and after rolling, and effectively acts to refine the pearlite colony size. As a result, the wear resistance, fatigue damage resistance, and ductility are greatly improved, which greatly contributes to the extension of the life of the internal hardened pearlite steel rail. However, if the Nb content is less than 0.001% by mass, it is difficult to obtain a sufficient effect. Moreover, even if it adds exceeding 0.05 mass%, the improvement effect of abrasion resistance and fatigue damage resistance will be saturated, and the effect corresponding to addition amount will not be acquired. Therefore, when Nb is added, the Nb amount is preferably 0.001 to 0.05% by mass. More preferably, it is 0.001-0.03 mass%.

Mo: 0.5% by mass or less
Mo is an element for further strengthening by solid solution strengthening. However, if it exceeds 0.5% by mass, a bainite structure is likely to occur, and the wear resistance tends to be lowered. Therefore, when adding Mo, the amount of Mo is preferably 0.5% by mass or less. More preferably, it is 0.005-0.3 mass%.

Lamella spacing of the pearlite layer in the range of at least 25mm depth from the top surface of the rail: 0.04-0.15μm
As for the lamellar spacing of the pearlite layer, the finer the inner hardened pearlite steel rail, the higher the hardness, which is advantageous from the viewpoint of improving wear resistance and fatigue damage resistance. Therefore, it is preferable that the thickness be 0.15 μm or less. In addition, if the lamellar spacing is to be less than 0.04 μm, a method of improving the hardenability and making it finer will be used. In this case, martensite is likely to be generated on the surface layer, and the fatigue damage resistance is adversely affected. Effect. Therefore, it is preferable that the thickness is 0.04 μm or more.

  In addition, a pearlite steel rail that contains other trace component elements within a range that does not substantially affect the effect of the present invention instead of a part of the remaining Fe in the composition according to the present invention is also included in the present invention. Belongs. Here, examples of the impurity include P, N, O and the like, and P can be allowed up to 0.035% by mass as described above. Further, N is allowable up to 0.006% by mass, and O is allowable up to 0.004% by mass. Furthermore, in this invention, Ti mixed as an impurity can be tolerated up to 0.0010% by mass. In particular, Ti forms an oxide and causes a reduction in fatigue damage resistance, which is a basic characteristic of the rail, and therefore it is preferable to control it to 0.0010% by mass or less.

  The internal high-hardness pearlitic steel rail of the present invention is obtained by hot rolling a steel material having the composition according to the present invention into a rail shape so that the rolling finishing temperature is 850 to 950 ° C., and subsequently at least the head of the rail-shaped body. Is preferably subjected to accelerated cooling from a temperature equal to or higher than the pearlite transformation start temperature to 400 to 550 ° C. at a cooling rate of 2.5 to 5 ° C./second, and then reheated at 20 ° C. or higher. The reason why the rolling finishing temperature is 850 to 950 ° C., the cooling rate of accelerated cooling is 2.5 to 5 ° C./second, the cooling stop temperature is 400 to 550 ° C., and the recuperation temperature is 20 ° C. or more is described below.

Rolling finishing temperature: 850-950 ° C
When the rolling finishing temperature is lower than 850 ° C., rolling is performed to a low temperature range of austenite, and not only processing strain is introduced into the austenite crystal grains, but also the degree of elongation of the austenite crystal grains becomes remarkable. The introduction of dislocations and an increase in the interfacial area of austenite grains increase the number of pearlite nucleation sites and the pearlite colony size becomes finer. Is coarsened and wear resistance is significantly reduced. On the other hand, when the rolling finishing temperature exceeds 950 ° C., the austenite crystal grains become coarse, so that the finally obtained pearlite colony size becomes coarse, and the fatigue damage resistance decreases. Therefore, the rolling finishing temperature is preferably 850 to 950 ° C.

Cooling rate from the temperature above the pearlite transformation start temperature: 2.5 to 5 ° C / sec When the cooling rate is less than 2.5 ° C / sec, the temperature difference between the surface layer of the rail head and the inside becomes small, and recuperation after cooling stops Therefore, the amount of Cr deposited decreases, and the internal hardness of the rail head decreases. On the other hand, when the cooling rate exceeds 5 ° C./second, a martensite structure is generated, and the service life of the internal high-hardness pearlite steel rail is reduced. Therefore, the cooling rate is preferably in the range of 2.5 to 5 ° C./second. Preferably it is 2.5-4.6 degrees C / sec. Although the pearlite transformation start temperature varies depending on the cooling rate, in the present invention, it means the equilibrium transformation temperature, and in the component range of the present invention, a cooling rate in this range from 720 ° C. or higher may be adopted.

Cooling stop temperature: 400 ~ 550 ℃
When the cooling stop temperature is less than 400 ° C, the cooling time in the low temperature region increases, so the productivity decreases and the cost of the internal high-hardness pearlite steel rail increases. On the other hand, if the temperature exceeds 550 ° C, the temperature inside the rail head stops cooling before the start of pearlite transformation or during the progress of pearlite transformation, so the amount of Cr dissolved in ferrite and cementite increases, and the target precipitation The amount is not obtained. Therefore, the cooling stop temperature is preferably 400 to 550 ° C. Preferably it is 450-550 degreeC.

Recuperating temperature: 20 ° C. or higher The inventors of the present invention manufactured a pearlite steel rail by rolling and cooling a steel material having the composition shown in Table 4 under the conditions shown in Table 5. Cooling was performed only on the rail head, and after cooling stopped, it was allowed to cool. The pearlite steel rail was investigated for recuperation and the amount of Cr deposited. The results are shown in Table 6. The rolling finishing temperature in Table 5 indicates a value obtained by measuring the temperature of the rail head side surface layer on the entry side of the final rolling mill with a radiation thermometer as the rolling finishing temperature. The cooling stop temperature indicates a value obtained by measuring the temperature of the rail head side surface layer on the cooling stop exit side with a radiation thermometer as the cooling stop temperature. The cooling rate was defined as the time change in temperature from the start of cooling to the stop of cooling. The recuperation temperature was determined by measuring the temperature of the surface layer on the side of the rail head at the entrance to the cooling floor with a radiation thermometer and calculating the difference between the measured value and the cooling stop temperature. As is apparent from Table 6, when the recuperation temperature is less than 20 ° C., the amount of Cr deposited decreases, and it is not possible to increase the strength to the inside of the rail head. In addition, in order to reduce the recuperation temperature to less than 20 ° C., it is necessary to stop the cooling at a high temperature, the waiting time in the cooling bed is increased, and the productivity of the internal high-hardness pearlite steel rail is lowered. Therefore, the recuperation temperature is 20 ° C. or higher. On the other hand, even when the recuperation temperature exceeded 100 ° C., the effect was saturated and no further increase in the amount of precipitated Cr was observed. Therefore, the recuperation temperature is preferably 20 to 100 ° C.

Next, a method for measuring or evaluating wear resistance, fatigue damage resistance, internal hardness of the rail head, lamellar spacing, and a method for analyzing precipitates will be described.
(Abrasion resistance)
As for wear resistance, it is most desirable to actually lay an internal high-hardness pearlite steel rail for evaluation, but this requires a long time for the test. Therefore, in the present invention, evaluation is performed by a comparative test that simulates the contact condition between an actual internal high-hardness pearlite steel rail and a wheel, using a Nishihara type wear tester capable of evaluating wear resistance in a short time. A Nishihara-type wear test piece 1 having an outer diameter of 30 mm is taken from the rail head and brought into contact with the tire test piece 2 and rotated for testing as shown in FIG. The arrows in FIG. 1 indicate the rotation directions of the Nishihara-type wear test piece 1 and the tire test piece 2, respectively. For the tire test piece, a round bar with a diameter of 32 mm was taken from the head of the normal rail described in JIS standard E1101, and it was heat-treated so that the Vickers hardness (load 98N) was Hv390 and the structure became a tempered martensite structure. After that, the shape shown in FIG. 1 was processed to obtain a tire test piece. The Nishihara-type wear test piece 1 is collected from two places on the rail head 3 as shown in FIG. A sample collected from the surface layer of the rail head 3 is a Nishihara type wear test piece 1a, and a sample collected from the inside is a Nishihara type wear test piece 1b. The center in the longitudinal direction of the Nishihara-type abrasion test piece 1b collected from the inside of the rail head 3 is located at a depth of 24 to 26 mm (average value 25 mm) from the upper surface of the rail head 3. Test environment conditions are dry, contact pressure: 1.4Gpa, slip rate: -10%, rotational speed: 675 times / minute (tire test piece is 750 times / minute), and the amount of wear after 100,000 rotations is measured. To do. A heat-treated pearlite steel rail is adopted as a reference steel material when comparing the amount of wear, and it is determined that the wear resistance is improved when the amount of wear is 10% or more less than this reference steel material. The allowance for improving wear resistance was calculated by {(amount of wear of reference material−amount of wear of test material) / (amount of wear of reference material)} × 100.

(Fatigue damage resistance)
For fatigue damage resistance, the contact surface is a curved surface with a radius of curvature of 15 mm, a 30 mm diameter Nishihara-type wear test piece 1 is taken from the rail head, and brought into contact with the tire test piece 2 and rotated as shown in FIG. Perform the test. The arrows in FIG. 3 indicate the rotation directions of the Nishihara type abrasion test piece 1 and the tire test piece 2, respectively. The Nishihara-type wear test piece 1 is collected from two places on the rail head 3 as shown in FIG. Since the position where the Nishihara type abrasion test piece 1 is collected and the position where the tire test piece is collected are the same as described above, the description thereof is omitted. The test environment is oil lubrication, contact pressure: 2.2Gpa, slip rate: -20%, rotation speed: 600 times / minute (tire test piece is 750 times / minute), the surface of the test piece every 25,000 times The fatigue damage life is determined by the number of rotations when a crack of 0.5 mm or more occurs. Uses heat-treated pearlitic steel rail as a reference steel when comparing the fatigue damage life, and determines that fatigue resistance is improved when the fatigue damage time is 10% or longer than this reference steel. . Fatigue damage resistance improvement allowance is calculated by {(number of rotations until the fatigue damage of the test material-number of rotations until the fatigue damage of the reference material) / (number of rotations until the fatigue damage of the reference material)} x 100 did.

(Internal hardness of rail head)
The range from the surface of the rail head to the depth of 25mm is measured at Hv98N, 1mm pitch. And the minimum value was made into the internal hardness of a rail head among all the hardness.
(Lamellar spacing)
Using a scanning electron microscope (SEM), observe the finest part of the lamellar spacing in the field of view at five magnifications of 20000 times at the surface of the rail head (approximately 1 mm deep) and at a position of 25 mm deep. And measure the lamellar spacing in the field of view. The lamellar interval is evaluated by an average value of the measured lamellar intervals of 5 fields of view.

(Analysis of Cr content)
As shown in FIG. 4, the amount of deposited Cr was measured using a test piece (10 mm × 10 mm × 100 mm) collected from the rail head. The amount of metal element deposited was 10% by mass acetylacetone-methanol electrolytic extraction, the residue was extracted from the test piece, and the value quantified by ICP emission analysis using the residue was taken as the amount of extracted metal.

  The steel material having the composition shown in Table 7 was rolled and cooled under the conditions shown in Table 8 to produce a pearlite steel rail. Cooling was performed only on the rail head, and after cooling stopped, it was allowed to cool. The pearlite steel rail was evaluated for Vickers hardness, lamellar spacing, wear resistance and fatigue damage resistance. The results are shown in Table 9. The rolling finishing temperature in Table 8 indicates a value obtained by measuring the temperature of the rail head side surface layer on the entry side of the final rolling mill with a radiation thermometer as the rolling finishing temperature. The cooling stop temperature indicates a value obtained by measuring the temperature of the rail head side surface layer on the cooling stop exit side with a radiation thermometer as the cooling stop temperature. The cooling rate was defined as the time change in temperature from the start of cooling to the stop of cooling.

  From these results, as in 3-B to 3-J and 3-U to 3-W, the addition amount of Si, Mn, and Cr is optimized, and the [% Mn] / [% Cr] value is set to 0.3. In addition, the amount of precipitated Cr exceeds 0.20% by mass and is less than 0.50% by mass, and the [% Si] + [% Mn] + [% Cr] value is controlled to 1.55 to 2.50% by mass. It can be seen that the wear resistance and fatigue damage resistance are improved by adding one or two or more components selected from Ni, Mo and Mo in an appropriate range.

In the invention examples, the DI value is in the range of 5.6 to 8.6, and the C _eq value is 1.04 to 1.27, such as 3-B to 3-E, 3-G, 3-J, 3-V, and 3-W. It can be seen that the wear resistance and fatigue damage resistance are improved as compared with 3-F, 3-H and 3-I.
It can be seen that when Ti is added as in 3-R of Comparative Example, fatigue damage resistance is reduced.

  Moreover, 3-S and 3-T of comparative examples are examples in which the cooling rate is slow. In 3-S, since the cooling rate is slow, the amount of Cr deposited is reduced, and high hardness cannot be achieved even inside the rail head. In 3-t, since the cooling rate was slow, martensite was generated in a part of the surface layer of the rail head, and the service life of the pearlite steel rail was reduced.

It is a figure which shows the Nishihara type abrasion test piece which evaluates abrasion resistance, (a) is a top view, (b) is a side view. It is sectional drawing of the rail head which shows the collection position of a Nishihara type abrasion test piece. It is a figure which shows the Nishihara type abrasion test piece which evaluates fatigue damage resistance, (a) is a top view, (b) is a side view. It is sectional drawing of the rail head which shows the collection position of the precipitation Cr analysis test piece.

Explanation of symbols

1 Nishihara-style wear specimens taken from pearlite steel rails
1a Nishihara type wear test specimen taken from the surface layer of the rail head
1b Nishihara-type wear specimens taken from inside the rail head 2 Tire specimen 3 Rail head 4 Precipitated Cr analysis specimen

Claims (6)

  C: 0.73 to 0.85 mass%, Si: 0.5 to 0.75 mass%, Mn: 0.3 to 1.0 mass%, P: 0.035 mass% or less, S: 0.0005 to 0.012 mass%, Cr: more than 0.5 mass% and 1.3 mass% or less And the balance is Fe and inevitable impurities, Mn content is [% Mn], Cr content is [% Cr], and [% Mn] / [% Cr] value is 0.3 or more and less than 1.0 The amount of Cr deposited at the rail head is between 0.20% and 0.50% by mass, and the internal hardness of the rail head defined by the Vickers hardness in the range of at least 25mm depth from the surface of the rail head is An internal high-hardness pearlite steel rail with excellent wear resistance and fatigue damage resistance, characterized by Hv395 or more and less than Hv480. C content of the composition is [% C], Si content is [% Si], Mn content is [% Mn], P content is [% P], S content is [% S], Cr content The amount is [% Cr], the DI value calculated by the following formula (1) is 5.6 to 8.6, and the C _eq value calculated by the following formula (2) is 1.04 to 1.27. The internal high hardness type pearlite steel rail excellent in wear resistance and fatigue damage resistance according to claim 1.
DI = (0.548 [% C] ^1/2 ) × (1 + 0.64 [% Si]) × (1 + 4.1 [% Mn])
× (1 + 2.83 [% P]) × (1−0.62 [% S]) × (1 + 2.23 [% Cr]) (1)
C _eq = [% C] + ([% Si] / 11) + ([% Mn] / 7) + ([% Cr] /5.8) (2)   When the Si content of the composition is [% Si], the Mn content is [% Mn], and the Cr content is [% Cr], the [% Si] + [% Mn] + [% Cr] value is 1.55 to 2.50. The internal high-hardness pearlitic steel rail having excellent wear resistance and fatigue damage resistance according to claim 1 or 2, wherein the internal high-hardness pearlitic steel rail is mass%.   In addition to the above composition, V: 0.001 to 0.30 mass%, Cu: 1.0 mass% or less, Ni: 1.0 mass% or less, Nb: 0.001 to 0.05 mass% and Mo: 0.5 mass% or less The internal high-hardness pearlite steel rail excellent in wear resistance and fatigue damage resistance according to any one of claims 1 to 3, comprising two or more kinds.   The wear resistance and fatigue resistance according to any one of claims 1 to 4, wherein a lamellar spacing of the pearlite layer in a range of at least 25 mm depth from the surface layer of the rail head is 0.04 to 0.15 µm. Internal hardened pearlite steel rail with excellent damage. The steel material having the composition described in any one of claims 1 to 4 is hot-rolled into a rail shape so that a rolling finishing temperature is 850 to 950 ° C, and subsequently the rail head surface portion is equal to or higher than a pearlite transformation start temperature. Internal high hardness type with excellent wear resistance and fatigue damage resistance characterized by accelerated cooling from temperature to 400-550 ° C at a cooling rate of 2.5-5 ° C / sec and then reheating at 20 ° C or higher Manufacturing method of pearlite steel rail.

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