JP2010018843A - Pearlite-based rail having excellent wear resistance and ductility - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the service life of a rail used for a heavy-load railroad by controlling the components of a rail steel, further controlling an Li oxide (LiO<SB>2</SB>), so as to improve the wear resistance and ductility of a pearlite texture, and further controlling its hardness. <P>SOLUTION: The pearlite-based rail having excellent wear resistance and ductility is characterized in that, in a steel rail comprising, by mass, 0.65 to 1.20% C, 0.05 to 2.00% Si, 0.05 to 2.00% Mn and 0.0005 to 0.10% Li, and the balance Fe with inevitable impurities, at least a part of the head surface part of the steel rail has a pearlite texture. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a pearlite rail intended to simultaneously improve wear resistance and ductility of a head in a rail used in heavy-duty railways.

  In overseas heavy-duty railroads, we are increasing the load of freight in order to increase the efficiency of rail transport. Especially for sharply curved rails, sufficient wear resistance is ensured in the GC section and the head side. However, the deterioration of the service life of the rail due to wear has become a problem. Against this background, there has been a demand for the development of a rail having wear resistance higher than that of a high-strength rail containing current eutectoid carbon steel. In order to solve these problems, the following rails have been developed. The main features of these rails are to increase the carbon content of the steel, to increase the volume ratio of the cemetite phase in the pearlite lamella, and to control the hardness (for example, to improve wear resistance) (See Patent Documents 1 and 2).

  In the disclosed technique of Patent Document 1, a hypereutectoid steel (C: more than 0.85 to 1.20%) is used to increase the cementite volume ratio in the lamellae in the pearlite structure, and the rail has excellent wear resistance. Can be provided.

  Moreover, in the open technology of patent document 2, using hypereutectoid steel (C: more than 0.85 to 1.20%), the cementite volume ratio in the lamella in the pearlite structure is increased, and at the same time, the hardness is increased. The rail can be controlled and has excellent wear resistance.

  However, in the disclosed technologies of Patent Documents 1 and 2, a certain level of wear resistance can be improved by increasing the volume ratio of the cemetite phase in the pearlite structure, that is, by increasing the carbon content of the steel. However, there is a problem that the ductility and toughness of the pearlite structure itself are remarkably lowered, and the rail breakage is liable to occur. Furthermore, there is a problem that a pro-eutectoid cementite structure which is harmful to the ductility and toughness of the rail is likely to be generated, and the rail breakage is liable to occur.

  Generally, to improve the ductility and toughness of pearlite steel, it is effective to refine the pearlite structure (pearlite block size), specifically, to refine the austenite structure before pearlite transformation and refine the pearlite structure. It is said. In order to achieve the fine graining of the austenite structure, reduction of the rolling temperature during hot rolling, increase of the reduction amount, and heat treatment by low temperature reheating after rail rolling are performed. In order to refine the pearlite structure, pearlite transformation is promoted from the austenite grains using transformation nuclei.

  However, in the production of rails, from the viewpoint of securing formability during hot rolling, there are limits to the reduction in rolling temperature and the increase in rolling reduction, and sufficient austenite grain refinement cannot be achieved. In addition, pearlite transformation from austenite grains using transformation nuclei has problems such as difficulty in controlling the amount of transformation nuclei and instability of pearlite transformation from within grains, and sufficient pearlite structure refinement. Could not be achieved.

  In order to drastically improve the ductility and toughness of a pearlite structure rail due to these problems, a method of refining the pearlite structure by performing low-temperature reheating after rail rolling and then performing pearlite transformation by accelerated cooling is a method. Has been used. However, in recent years, the carbon of rails has been increased to improve wear resistance, and when the above-mentioned low-temperature reheating heat treatment is performed, coarse carbides remain undissolved in the austenite grains, and the ductility and toughness of the pearlite structure after accelerated cooling are reduced. There is a problem that it falls. Moreover, since it is reheating, there are also economical problems such as high manufacturing cost and low productivity.

  Therefore, development of a method for producing a high carbon steel rail that secures formability during rolling and refines the pearlite structure after rolling has been demanded. In order to solve this problem, a method for producing a high carbon steel rail as described below has been developed. The main feature of these rails is the continuous rolling under small reduction by making use of the fact that the austenite grains of high carbon steel are relatively low temperature and easy to recrystallize even with a small reduction amount in order to refine the pearlite structure. Thus, finely sized particles are obtained, and the ductility and toughness of pearlite steel are improved (for example, see Patent Documents 3, 4, and 5).

  In the disclosed technology of Patent Document 3, a high ductility rail can be provided by rolling three or more consecutive passes in a predetermined time between passes in finish rolling of a steel rail containing high carbon steel.

  Moreover, in the disclosed technique of Patent Document 4, in the finish rolling of a steel rail containing high carbon steel, rolling is performed continuously for two or more passes at a predetermined time between passes, and further, after performing continuous rolling, accelerated cooling is performed after rolling. By performing the above, a high wear resistance and high toughness rail can be provided.

  Furthermore, in the disclosed technology of Patent Document 5, in the finish rolling of a steel rail containing high carbon steel, cooling is performed between passes, and further, continuous rolling is performed, and then accelerated cooling is performed after rolling. High toughness rails can be provided.

  However, in the disclosed technologies of Patent Documents 3 to 5, depending on the combination of the carbon content of steel, the temperature during continuous hot rolling, the number of rolling passes and the time between passes, the austenite structure cannot be refined and the pearlite structure is coarse. There is a problem that ductility and toughness are not improved.

JP-A-8-144016 JP-A-8-246100 Japanese Unexamined Patent Publication No. 7-173530 JP 2001-234238 A JP 2002-226915 A

  Against this background, it has been desired to provide a pearlite rail that has improved wear resistance of the pearlite structure and at the same time has improved wear resistance and ductility.

  Therefore, the present invention has been devised in view of the above-mentioned problems, and its object is to simultaneously improve the head wear resistance and ductility required for heavy-duty railroad rails. It is aimed at.

(1) By mass%, C: 0.65-1.20%, Si: 0.05-2.00%, Mn: 0.05-2.00%, Li: 0.0005-0.10% A pearlite-based rail excellent in wear resistance and ductility, characterized in that, in a steel rail that contains Fe and the balance of Fe and inevitable impurities, at least a part of the head surface of the steel rail has a pearlite structure.

(2) In the above (1), the total number of Li oxides having a diameter of 1 to 100 nm is present in 50 to 1000 in 1 μm ² in an arbitrary cross section in the pearlite structure. A pearlite rail with excellent wear resistance and ductility.

(3) Moreover, the following (1)-[10] component can be further selectively contained by the mass in the rail of said (1) or (2).
[1] Co: 0.05% to 1.00%,
[2] One or two of Cr: 0.05 to 2.00%, Mo: 0.01 to 0.50%,
[3] V: 0.005 to 0.50%, Nb: 0.002 to 0.050%, 1 type or 2 types,
[4] B: One kind of 0.0001 to 0.0050%,
[5] Cu: 0.05% to 1.00%,
[6] Ni: 0.01% to 1.00%,
[7] One or more of Ti: 0.0050 to 0.0500%, Mg: 0.0005 to 0.0200%, Ca: 0.0005 to 0.0150%,
[8] Al: one of 0.0100 to 1.00%,
[9] Zr: one of 0.0001 to 0.2000%,
[10] N: one of 0.0060 to 0.0200%,
And the balance consists of Fe and inevitable impurities.

(4) In any one of the above (1) to (3), at least a range of 20 mm in depth is a pearlite structure starting from the head corner portion and the top surface of the steel rail, and the hardness thereof is A pearlite rail excellent in wear resistance and ductility, characterized by being in the range of Hv 320 to 500.

According to the present invention, the wear resistance and ductility of the pearlite structure are improved by controlling the components of the rail steel, and further Li oxide (LiO ₂ ), and further the structure and hardness are controlled. This makes it possible to improve the service life of rails used in heavy-duty railways.

  Hereinafter, a pearlite rail excellent in wear resistance and ductility will be described in detail as an embodiment of the present invention. Hereinafter, the mass in the composition is simply described as%.

  First, the present inventors examined a method for suppressing grain growth of austenite grains by applying precipitates. Based on rail steel with 0.65 to 1.20% carbon added, materials with various oxides added are melted, laboratory rolling experiments simulating rolling conditions equivalent to rails are conducted, and ductility is determined by tensile tests. Evaluated. The hardness of the material was adjusted to the Hv400 level by controlling the heat treatment conditions.

As a result, it was found that by adding a certain amount of Li, Li oxide (LiO ₂ ) was generated, and austenite grains were refined by pinning, resulting in refined pearlite structure and improved ductility. FIG. 1 shows the relationship between the carbon content of steel and the total elongation value. Fig. 1 shows the result of a tensile test conducted by conducting a laboratory rolling experiment simulating rolling conditions equivalent to rails using steel having a carbon content of 0.65 to 1.20%, and further steel added with Li. It is shown by the relationship between the amount of carbon and the total elongation value. Here, the amount of Li oxide is shown by the value indicated by the amount of Li added in the steel. The amount of Li added is 0.02% in any steel. In any carbon content, it was confirmed that by adding Li, γ grains were pinned and the total elongation value was improved by refinement of the pearlite structure.

  Furthermore, the present inventors examined the optimal range of said Li oxide amount. Based on rail steel with a carbon content of 1.00%, in order to change the amount of Li oxide, a material in which the amount of Li addition was changed was melted, and a laboratory rolling experiment simulating rolling conditions equivalent to the rail was conducted, and a tensile test was performed. The effect of the amount of Li oxide on the ductility was investigated. The hardness of the material was adjusted to the Hv400 level by controlling the heat treatment conditions.

2 and 3 show the relationship between the number of Li oxides and the total elongation value. Figure 2 shows a lab rolling experiment that simulated the rolling conditions equivalent to the rail by melting the material with the Li addition amount changed to change the Li oxide amount based on the rail steel with 1.00% carbon content. The results of the tensile test are shown in relation to the number of Li oxides and the total elongation value. FIG. 3 is an enlarged view of a portion where the number of Li oxides is 0 to 100 in FIG. Note that the number of Li oxides was evaluated only for Li oxides having a diameter in the range of 1 to 100 nm, and the number in a cross section of 1 μm ² was counted. When the number of Li oxides is small, no significant change is observed in the total elongation value. When the number of Li oxides exceeds a certain amount, it is confirmed that the total elongation value increases and the ductility is improved. Furthermore, it was confirmed that when the number of Li oxides exceeds a certain amount, the total elongation value decreases and ductility decreases.

  From the results of these material tests, it was confirmed that the addition of Li is effective for improving the ductility in a steel rail containing high carbon. Furthermore, in order to improve ductility reliably and to secure wear resistance, it has been newly found that there is an optimum range for the amount of Li oxide.

  That is, the present invention aims to improve the service life of a rail by adding Li in a high-carbon steel rail, further controlling the amount of Li oxide, improving the wear resistance and ductility of the pearlite structure. It is related to the pearlite rail.

  Next, the reason for limitation of the present invention will be described in detail. Hereinafter, the mass in the composition is simply described as%.

(1) Reason for limitation of chemical component In claim 1, the reason why the chemical component of the rail steel is limited to the above-mentioned claims will be described in detail.

  C is an effective element that promotes pearlite transformation and ensures wear resistance. If the amount of C is less than 0.65%, the minimum strength and wear resistance required for the rail cannot be maintained. On the other hand, if the C content exceeds 1.20%, a large amount of coarse pro-eutectoid cementite structure is generated, and the wear resistance and ductility are lowered. For this reason, C addition amount was limited to 0.65-1.20%.

  When the C content is 0.90% or more, the wear resistance is further improved, and the service life of the rail is further improved. For this reason, in order to further improve the wear resistance of the rail and increase the life in a track under severe usage conditions, the C addition amount is 0.90 to 1.20% as shown in claim 3. It is desirable to limit to.

  Si is an essential component as a deoxidizing material. Moreover, it is an element which raises the hardness (strength) of a rail head by the solid solution strengthening to the ferrite phase in a pearlite structure | tissue. Furthermore, in hypereutectoid steel, it is an element that suppresses the formation of proeutectoid cementite structure and suppresses the decrease in ductility. However, when the Si content is less than 0.05%, these effects cannot be sufficiently expected. On the other hand, if the Si content exceeds 2.00%, many surface defects are generated during hot rolling, and weldability is deteriorated due to generation of oxides. Furthermore, the hardenability is remarkably increased, and a martensite structure is generated which is harmful to the wear resistance and ductility of the rail. For this reason, Si addition amount was limited to 0.05 to 2.00%.

  Mn is an element that increases the hardenability and refines the pearlite lamella spacing to ensure the hardness of the pearlite structure and improve the wear resistance. However, if the amount of Mn is less than 0.05%, the effect is small, and it is difficult to ensure the wear resistance required for the rail. Moreover, when the amount of Mn exceeds 2.00%, hardenability will increase remarkably and it will become easy to produce | generate the martensitic structure harmful to abrasion resistance and ductility. For this reason, Mn addition amount was limited to 0.05 to 2.00%.

Li is an element that improves the ductility by forming an oxide of Li (LiO ₂ ), the oxide is formed in the austenite structure, the austenite grains are refined by pinning, and the pearlite structure is refined as a result. is there. However, if the amount of Li is less than 0.0005%, the effect is weak, and further, the number of fine Li oxides having a diameter of 1 to 100 nm cannot be secured, and the ductility of the rail cannot be improved. When the Li content exceeds 0.10%, a coarse Li oxide with a diameter of more than 100 nm is rapidly generated and increased, the number of fine Li oxides cannot be secured, and the ductility of the rail decreases. In addition, since surface damage such as polling damage occurs in the rail head surface, the Li addition amount is limited to 0.0005 to 0.10%.

  In addition, the rail manufactured with the above component composition improves the hardness (strengthening) of the pearlite structure and pro-eutectoid ferrite structure, improves the ductility, prevents softening of the weld heat affected zone, and the cross-sectional hardness distribution inside the rail head. For the purpose of control, elements of Co, Cr, Mo, V, Nb, B, Cu, Ni, Ti, Mg, Ca, Al, Zr, and N are added as necessary.

Here, Co refines the lamellar structure and ferrite grain size of the wear surface and enhances the wear resistance of the pearlite structure. Cr and Mo increase the equilibrium transformation point of pearlite and ensure the hardness of the pearlite structure mainly by refining the pearlite lamella spacing. V, Nb suppresses the growth of austenite grains by carbides and nitrides generated in the hot rolling and subsequent cooling process, and further precipitates and hardens in the ferrite structure and pearlite structure. Improve hardness. In addition, carbides and nitrides are stably generated, and the weld joint heat-affected zone is prevented from being softened. B reduces the cooling rate dependence of the pearlite transformation temperature and makes the hardness distribution of the rail head uniform. Cu dissolves in the ferrite in the ferrite structure or pearlite structure, and increases the hardness of the pearlite structure. Ni improves the toughness and hardness of the ferrite structure and pearlite structure, and at the same time, prevents softening of the heat-affected zone of the weld joint. Ti refines the structure of the heat-affected zone and prevents embrittlement of the weld joint. Mg and Ca reduce the austenite grains during rail rolling, and at the same time, promote ferrite and pearlite transformation and improve toughness. Al moves the eutectoid transformation temperature to the high temperature side and increases the hardness of the pearlite structure. Zr suppresses the formation of segregation zone at the center of the slab and prevents the deterioration of the ductility of the rail by increasing the equiaxed crystallization rate of the solidification structure by the inclusion of ZrO ₂ inclusions as the solidification nucleus of the high carbon rail steel. . N is mainly intended to improve ductility by promoting pearlite transformation from the austenite grain boundary and making the pearlite structure fine.

  The reasons for limiting these components will be described in detail below.

  Co is an element that dissolves in the ferrite phase in the pearlite structure, further refines the fine ferrite structure formed by contact with the wheel on the wear surface of the rail head, and improves the wear resistance. If the Co content is less than 0.05%, the ferrite structure cannot be refined, and the effect of improving the wear resistance cannot be expected. Further, even if the Co amount is added to 1.00% or more, the above effect is saturated, and the ferrite structure corresponding to the addition amount cannot be refined. In addition, the economic efficiency decreases due to the increase in the alloy addition cost. For this reason, Co addition amount was limited to 0.05 to 1.00%.

  Cr increases the equilibrium transformation temperature, and as a result, refines the ferrite structure and pearlite structure and contributes to high hardness (strength), and at the same time, strengthens the cementite phase and improves the hardness (strength) of the pearlite structure However, when the Cr content is less than 0.05%, the effect is small, and the effect of improving the hardness of the rail steel is not seen at all. In addition, when excessive addition exceeding Cr amount of 2.00% is performed, hardenability increases, martensite structure is generated, and sprig damage starting from the martensite structure occurs at the head corner or the top of the head. , Surface damage resistance is reduced. Therefore, the Cr addition amount is limited to 0.05 to 2.00%.

  Mo, like Cr, is an element that increases the equilibrium transformation temperature and, as a result, contributes to higher hardness (strength) by making the ferrite structure and pearlite structure finer, and improves hardness (strength). If the amount is less than 0.01%, the effect is small, and the effect of improving the hardness of the rail steel is not seen at all. In addition, when the Mo amount exceeds 0.50%, the transformation rate is remarkably reduced, and sprig damage starting from the martensite structure occurs at the head corner and the top of the head, resulting in surface damage resistance. Decreases. For this reason, Mo addition amount was limited to 0.01 to 0.50%.

  V is a V carbide or V nitride produced by a cooling process after hot rolling by refining austenite grains due to the pinning effect of V carbide or V nitride when heat treatment is performed at a high temperature. It is an element effective for improving the ductility by increasing the hardness (strength) of ferrite structure and pearlite structure by precipitation hardening. In addition, it is an element effective in preventing V softening of the weld joint heat affected zone by generating V carbide and V nitride in a relatively high temperature range in the heat affected zone reheated to a temperature range below Ac1 point. is there. However, if the amount of V is less than 0.005%, the effect cannot be sufficiently expected, and no improvement in the hardness or ductility of the ferrite structure or pearlite structure is observed. If the V content exceeds 0.50%, the precipitation and hardening of V carbides and nitrides will be excessive, the ductility of the ferrite structure and pearlite structure will be reduced, and sprig damage will occur at the head corner and the top of the head. , Surface damage resistance is reduced. For this reason, V addition amount was limited to 0.005-0.50%.

  Nb, like V, is refined by the pinning effect of Nb carbide or Nb nitride when heat treatment is performed at a high temperature, and further Nb produced in the cooling process after hot rolling. It is an element effective for improving the ductility as well as increasing the hardness (strength) of ferrite structure and pearlite structure by precipitation hardening with carbide and Nb nitride. Moreover, in the heat affected zone reheated to a temperature range below the Ac1 point, Nb carbide and Nb nitride are stably generated from the low temperature range to the high temperature range, and the weld joint heat affected zone is prevented from being softened. It is an effective element. However, when the Nb content is less than 0.002%, the effect cannot be expected, and no improvement in hardness or toughness of the ferrite structure or pearlite structure is observed. If the Nb content exceeds 0.050%, the precipitation hardening of Nb carbides and nitrides becomes excessive, the ductility of the ferrite structure and pearlite structure decreases, and the sprig damage occurs at the head corner and the top of the head. , Surface damage resistance is reduced. For this reason, the amount of Nb added is limited to 0.002 to 0.050%.

  B forms iron boride (Fe23 (CB) 6) at the austenite grain boundary, reduces the cooling rate dependence of the pearlite transformation temperature due to the accelerating effect of the pearlite transformation, and has a more uniform hardness from the head surface to the inside. An element that imparts a distribution to the rail and prolongs the life of the rail. However, if the amount of B is less than 0.0001%, the effect is not sufficient, and the hardness distribution of the rail head is not improved. On the other hand, if the amount of B exceeds 0.0050%, a coarse borohydride is formed, resulting in a decrease in ductility and toughness. For this reason, B addition amount was limited to 0.0001 to 0.0050%.

  Cu is an element which dissolves in the ferrite phase in the ferrite structure and pearlite structure and improves the hardness (strength) of the pearlite structure by solid solution strengthening, but the effect is expected when the Cu content is less than 0.01%. Can not. Further, if the Cu content exceeds 1.00%, martensite structure harmful to ductility is generated due to remarkable hardenability improvement, and sprig damage occurs in the head corner portion and the top of the head, and the surface damage resistance decreases. . For this reason, the amount of Cu was limited to 0.01 to 1.00%.

  Ni is an element that improves the toughness of the ferrite structure and the pearlite structure, and at the same time increases the hardness (strength) by solid solution strengthening. Furthermore, in the weld heat affected zone, an intermetallic compound of Ni3Ti that is compounded with Ti is finely precipitated and is an element that suppresses softening by precipitation strengthening. However, when the Ni content is less than 0.01%, the effect is remarkable. If it is small and the Ni content exceeds 1.00%, the ductility of the ferrite structure and the pearlite structure is remarkably reduced, and the sprig damage is generated at the head corner and the top, and the surface damage resistance is lowered. For this reason, Ni addition amount was limited to 0.01 to 1.00%.

  By utilizing the fact that Ti carbide and Ti nitride precipitated during reheating during welding are not dissolved, the structure of the heat-affected zone heated to the austenite region is refined and brittleness of the welded joint is achieved. It is an effective ingredient for preventing oxidization. However, if the Ti amount is less than 0.0050%, the effect is small, and if the Ti amount exceeds 0.0500%, coarse Ti carbides and Ti nitrides are generated, and at the same time, the toughness of the rail is lowered. Fatigue damage occurs from coarse precipitates. For this reason, Ti addition amount was limited to 0.0050 to 0.050%.

  Mg combines with O, S, Al, or the like to form a fine oxide, suppresses grain growth during reheating during rail rolling, refines austenite grains, and produces a ferrite structure It is an element effective for improving the ductility of pearlite structure. Furthermore, MgO, MgS finely disperses MnS, forms a thin Mn band around MnS, contributes to the generation of ferrite and pearlite transformation, and as a result, mainly by reducing the pearlite block size, It is an element effective for improving the ductility of the pearlite structure. However, if the amount of Mg is less than 0.0005%, the effect is weak. If the amount of Mg exceeds 0.0200%, a coarse oxide of Mg is generated, and the toughness of the rail is lowered, and at the same time, fatigue is caused from coarse precipitates. Damage will occur. For this reason, Mg addition amount was limited to 0.0005 to 0.0200%.

Ca has a strong binding force with S and forms a sulfide as CaS. Further, CaS finely disperses MnS, forming a Mn dilute band around MnS, contributing to the generation of ferrite and pearlite transformation. As a result, the element is effective in improving the ductility of the pearlite structure mainly by reducing the pearlite block size. However, when the Ca content is less than 0.0005%, the effect is weak. When the Ca content exceeds 0.0150%, a coarse oxide of Ca is generated and the toughness of the rail is reduced. It was limited to 0005 to 0.0150%.
Al is an essential component as a deoxidizing material. Moreover, although it is an element which moves a eutectoid transformation temperature to the high temperature side and contributes to high hardness (strength) of a pearlite structure | tissue, the effect is weak if Al amount is less than 0.0100%. Also, if the Al content exceeds 1.00%, it becomes difficult to make a solid solution in the steel, coarse alumina inclusions are generated, the toughness of the rail is lowered, and at the same time fatigue damage is caused from the coarse precipitates. appear. Furthermore, since oxides are generated during welding and weldability is remarkably reduced, the amount of Al added is limited to 0.0100 to 1.00%.

Zr has a good lattice matching with γ-Fe because ZrO ₂ inclusions have a good lattice matching with γ-Fe, so that γ-Fe becomes a solidification nucleus of high-carbon rail steel that is a solidification primary crystal and increases the equiaxed crystallization rate of the solidification structure An element that suppresses the formation of a segregation band at the center of the slab and improves the characteristics of the segregation. However, when the amount of Zr is 0.0001% or less, the number of ZrO ₂ inclusions is small, and a sufficient action as a solidification nucleus is not exhibited. On the other hand, when the amount of Zr exceeds 0.2000%, a large amount of coarse Zr-based inclusions are generated, and the toughness is lowered, and at the same time, fatigue damage occurs from the coarse precipitates. For this reason, the amount of Zr was limited to 0.0001 to 0.2000%.

  N is an element effective for improving ductility by mainly segregating at the austenite grain boundary to promote ferrite and pearlite transformation from the austenite grain boundary and mainly by reducing the pearlite block size. However, if the N content is less than 0.0060%, the effect is weak. When the N content exceeds 0.0200%, it becomes difficult to make a solid solution in the steel, and bubbles that become the starting point of fatigue damage are generated, and fatigue damage occurs inside the rail head. For this reason, N addition amount was limited to 0.0060-0.0200%.

  Rail steel composed of the above components is melted in a commonly used melting furnace such as a converter, electric furnace, etc., and this molten steel is ingot-bundled, continuously cast, or hot. It is manufactured as a rail after rolling.

In order to uniformly disperse the fine oxide (LiO ₂ ), it is desirable to add Li using a high-temperature converter or the like. Furthermore, in order to prevent agglomeration of oxides in the remaining casting stage, the molten steel being solidified is stirred by electromagnetic force, and the shape of the casting nozzle is optimized to control the flow of the molten steel during casting. Is desirable.

(2) as Li oxide reasons for limiting the austenite grains fine precipitates (LiO _2), the reason for focusing on the Li oxide (LiO ₂₎ will be described in detail.

The inventors of the present invention examined precipitates that hardly precipitate in the transformed pearlite structure and do not adversely affect the wear resistance and ductility of the pearlite structure. As a result, the size of the precipitate itself is very fine. Further, it is difficult to coarsen in the austenite structure, and an oxide (LiO ₂ ) based on Li having strong oxidizing power is optimal as a finely dispersed precipitate. It was confirmed by experiment.

(3) Reason for limiting the size and number of Li oxide (LiO ₂ ) First, when limiting the number of Li oxides (LiO ₂ ) in 1 μm ^{2 of} pearlite structure, the size of Li oxide (LiO ₂ ) The reason for the limitation will be described in detail.

If the diameter of the Li oxide (LiO ₂ ) in the pearlite structure is in the range of 1 to 100 nm, when produced in the austenite structure, it exhibits a sufficient pinning effect at the grain boundary, resulting in a refined pearlite structure, Make sure to improve ductility. Therefore, the diameter of Li oxide (LiO ₂ ) to be evaluated was limited to a range of 1 to 100 nm.

Next, the reason why the total number of Li oxides (LiO ₂ ) in the pearlite structure 1 μm ² is limited will be described in detail.

When the total number of Li oxides (LiO ₂ ) is less than 50, the pinning effect in the austenite structure is not sufficiently exhibited, and the ductility of the rail is not improved. In addition, when the total number of Li oxides (LiO ₂ ) is 1000 or more, the pearlite structure itself becomes brittle, ductility is reduced, and furthermore, surface damage such as spalling damage occurs in the rail head surface portion. The total number of Li oxides (LiO ₂ ) was limited to 50 to 1000 in 1 μm ² .

In addition, as a method of limiting the size and number of the Li oxide (LiO ₂ ) to the above range, first, the manufacturing stage of the steel slab to be rolled as a rail at the same time that the Li addition amount is within the limited range. Adjustment in is necessary. Specifically, it is necessary to control the Li addition method in the converter when manufacturing the molten steel of the rail.

First, in order to efficiently add Li oxide (LiO ₂ ), it is desirable to add Li oxide after the refining of impurities such as P is almost completed. This makes it possible to add Li oxide to the molten steel without being adsorbed by the slag at the top of the molten steel.

Next, in order to control the size and number of Li oxide (LiO ₂ ), it is desirable to stir the molten steel with oxygen or an inert gas in the converter after the addition. This promotes the oxidation of Li. Furthermore, the agitation suppresses the coarsening of the Li oxide in the molten steel, and it becomes a fine oxide without agglomeration and can be dispersed in the molten steel.

By performing these controls, the size and number of Li oxides (LiO ₂ ) can be controlled within the above range.

  As for the precipitate, a thin film was collected from an arbitrary cross section, and observed using a transmission electron microscope, and observed at a magnification of 50,000 to 500,000. As the particle size of the precipitate, the area of each precipitate was obtained by observation, and the diameter of a circle corresponding to the area was used. The precipitates were observed in 20 fields of view, the number of precipitates corresponding to a predetermined diameter was counted, and this was converted into a number corresponding to a predetermined field area. The representative value of each rail steel was the average value of these 20 fields of view.

(4) Hardness of pearlite structure of rail head and reason for limitation of range Next, a pearlite structure having a depth of at least 20 mm and a hardness of Hv320 to 500 starting from the head corner and the top surface. The reason for the limitation will be described.

First, the reason why the hardness of the pearlite structure is limited to the range of Hv 320 to 500 will be described.
In this component system, when the hardness of the pearlite structure is less than Hv320, it is difficult to ensure the wear resistance of the rail head, and the service life of the rail is reduced. Further, flaking damage caused by plastic deformation occurs on the rolling surface, and the surface damage resistance of the rail head is greatly reduced. Further, when the hardness of the pearlite structure exceeds Hv500, the ductility of the pearlite structure is remarkably lowered, the spalling damage of the rolling surface is generated, and the surface damage resistance of the rail head is lowered. For this reason, the hardness of the pearlite structure | tissue was limited to the range of Hv320-500.

  Next, the reason why the necessary range of the pearlite structure having a hardness of Hv 320 to 500 is limited to a range of at least a depth of 20 mm starting from the head corner and the top surface will be described.

  If the range where the pearlite structure having a hardness of Hv 320 to 500 is less than 20 mm from the head surface at the head corner and the top of the head is less than 20 mm in depth, in order to maintain wear resistance on the rails of heavy-duty railways It is too small to improve the service life of the rail. Moreover, if the range where the pearlite structure | tissue of hardness Hv300-500 exists is 30 mm or more in depth starting from this head surface of a head corner part and a head part, a rail service life will improve further and it is more desirable.

  Here, FIG. 4 shows the designation of the pearlite rail excellent in wear resistance and ductility according to the present invention at the head cross-sectional surface position, and the region where the pearlite structure having a hardness of Hv 320 to 500 is required. In the rail head portion, 1 is a top portion, 2 is a head corner portion, and one of the head corner portions 2 is a gauge corner (GC) portion that mainly contacts a wheel.

  The rail head surface portion indicates a range up to a depth of 5 mm starting from the head corner portion and the top surface, and if the pearlite structure having the above component range is disposed at least in this portion, the rail has wear resistance. Can be improved.

  In addition, the pearlite structure having a hardness of Hv320 to 500 has a wear resistance of the rail as long as the pearlite structure is arranged in a range up to a depth of 20 mm starting from the head corner portion and the top surface, that is, at least within the oblique line in the figure. Is further secured, and the service life of the rail can be improved.

  Therefore, the pearlite structure having a hardness of Hv 320 to 500 is desirably arranged in the vicinity of the rail head surface where the wheel and the rail mainly contact each other, and the other part may be a metal structure other than the pearlite structure.

  As a method for obtaining a pearlite structure having a hardness of Hv 320 to 500 in the rail head, it is desirable to perform accelerated cooling on a high-temperature rail head having an austenite region after rolling or after reheating. As a method of accelerated cooling, a predetermined structure and hardness can be obtained by performing heat treatment by a method as described in JP-A-8-246100, JP-A-9-111352 and the like.

  The head metal structure of the rail of the present invention is desirably a pearlite structure as described above. However, depending on the component system of the rail and the heat treatment production method, a trace amount of pro-eutectoid ferrite structure, pro-eutectoid cementite structure, bainite structure and martensite structure of 5% or less may be mixed in the pearlite structure. However, even if these structures are mixed, there is no significant adverse effect on the wear resistance of the rail head. Therefore, as a pearlitic rail structure with excellent wear resistance, a very small amount of pro-eutectoid ferrite of 5% or less. A mixture of a structure, a proeutectoid cementite structure, a bainite structure, and a martensite structure may also be included. In other words, 95% or more of the head metal structure of the rail of the present invention may be a pearlite structure. In order to sufficiently secure wear resistance and ductility, 98% or more of the head metal structure is a pearlite structure. It is desirable to do. In Tables 1 and 2, the amount of trace in the column of microstructure refers to an amount of 5% or less, and the amount other than the pearlite structure not described as trace is an amount exceeding 5%. means.

Next, examples of the present invention will be described.
Table 1 shows the chemical composition of the test rail steel, the number of Li oxides (LiO ₂ ) having a diameter of 1 to 100 nm (in 1 μm ² ), the microstructure of the rail head, and the hardness. The microstructure and hardness are data at a position 2 mm below the head surface. The pearlite in the microstructure means that the area ratio is 95% or more of the pearlite structure, and the trace amount of the tissue other than the pearlite structure is the area ratio. Is 5% or less. Furthermore, the test piece was sampled from the position shown in FIG. 5, the result of the abrasion test conducted by the method shown in FIG. 6, and the result of the tensile test conducted by collecting the test specimen from the position shown in FIG. FIG. 5 illustrates the specimen collection positions in the wear tests shown in Tables 1 and 2, and FIG. 6 shows the outline of the wear test shown in Tables 1 and 2. FIG. 3 rail test piece, 4 mating material, 5 cooling nozzle. FIG. 7 illustrates test specimen collection positions in the tensile tests shown in Tables 1 and 2.

Table 2 shows the chemical composition of the comparative rail steel, the number of Li oxides (LiO ₂ ) having a diameter of 1 to 100 nm (in 1 μm ² ), the microstructure of the rail head, and the hardness. Note that the microstructure and hardness are data at a position 2 mm below the head surface, and the structure other than the pearlite structure in the microstructure means that the area ratio is more than 5%, but the trace amount of the tissue other than the pearlite structure is the area. It means that the rate is 5% or less. Furthermore, the test piece was sampled from the position shown in FIG. 5, the result of the abrasion test conducted by the method shown in FIG. 6, and the result of the tensile test conducted by collecting the test specimen from the position shown in FIG.

Various test conditions are as follows.
[1] Head tensile tester: Universal small tensile tester Test piece shape: JIS No. 4 similar parallel part length: 30 mm, parallel part diameter: 6 mm, distance between elongation measurement scores: 25 mm
Test piece sampling position: 6mm below the rail head surface (see Fig. 7)
Tensile speed: 10 mm / min, test temperature: normal temperature (20 ° C.)

[2] Head wear tester: Nishihara type wear tester (see Fig. 6)
Test piece shape: disk-shaped test piece (outer diameter: 30 mm, thickness: 8 mm)
Test piece sampling position: 2mm below the rail head surface (see Fig. 5)
Test load: 686 N (contact surface pressure 640 MPa)
Slip rate: 20%
Opposite material: Pearlite steel (Hv380)
Atmosphere: Air cooling: Forced cooling with compressed air (flow rate: 100 Nl / min)
Repeat count: 700,000 times

(1) Invention rail (32) Reference numerals 1 to 32
Abrasion resistance within the above-mentioned limited component range, and the number of fine Li oxides (LiO ₂ ) having a diameter of 1 to 100 nm (in 1 μm ² ), the microstructure of the rail head, and hardness within a specified range. Perlite rail with excellent ductility.

(2) Comparison rail (25) Reference numerals 33 to 57
Steel: 33-40: Rail whose component range is outside the scope of the present invention.
Steel: 41 to 51: Rail whose component range is within the scope of the present invention, but the number of fine Li oxides (LiO ₂ ) (in 1 μm ² ) is outside the scope of the present invention.
Steel: 52-54: Rail whose component range is within the scope of the present invention, but whose head microstructure is outside the above limits.
Steel: 55-57: Rail whose component range is within the scope of the present invention, but whose head hardness is not limited to the above.

  As shown in Tables 1 and 2, the rail steel of the present invention (steel: 1 to 32) is compared with the comparative rail steel (steel: 33 to 38), by keeping the chemical composition of the steel within a limited range. It is possible to stably obtain a pearlite structure within a certain hardness range without generating a structure such as a pro-eutectoid cementite structure, a martensite structure, and a bainite structure that adversely affects ductility and wear resistance.

  As shown in Tables 1 and 2, the rail steel of the present invention (steel: 1 to 32) has an added amount of Li within a certain range as compared with the comparative rail steel (steel: 39 to 40). Thereby, the ductility of the rail of a pearlite structure | tissue can be improved significantly.

  Furthermore, as shown in FIG. 8, the rail steel of the present invention (steel: 1 to 32) is compared with the comparative rail steel (steel: 41 to 51), and Li is added in the converter when manufacturing the molten steel of the rail. By controlling the method and the like and keeping the size and number of Li oxides within a certain range, the ductility of the rail of the pearlite structure can be greatly improved. FIG. 8 shows the relationship between the carbon amount and the total elongation value of the results of the head tensile test of the present rail steel shown in Table 1 and the comparative rail steel shown in Table 2.

  As shown in Tables 1 and 2, the rail steel of the present invention (steel: 1 to 32) has a wear resistance by making the microstructure a pearlite structure compared to the comparative rail steel (steel: 52 to 54). And ductility can be secured.

  As shown in Tables 1 and 2, the rail steel of the present invention (steel: 1 to 32) has a pearlite structure hardness within a certain range as compared with the comparative rail steel (steel: 55 to 57). Thus, wear resistance and ductility can be ensured.

  Furthermore, as shown in Table 1, Table 2, and FIG. 9, the rail steel of the present invention (steel: 1 to 32) dramatically improves the wear resistance by setting the carbon content to 0.90% or more. . FIG. 9 shows the relationship between the amount of carbon and the amount of wear in the results of the head wear test of the rail steel of the present invention shown in Table 1.

Using steel with a carbon content of 0.65 to 1.20%, and further steel with addition of Li, a laboratory rolling experiment simulating rolling conditions equivalent to rails was conducted, and the result of a tensile test was calculated as the carbon content of the steel. The figure shown in relation to the total elongation value. Based on rail steel with a carbon content of 1.00%, in order to change the amount of Li oxide, a material in which the amount of Li addition was changed was melted, and a laboratory rolling experiment simulating rolling conditions equivalent to the rail was conducted, and a tensile test was performed. The figure which showed the result of having performed by the relationship between the number of Li oxides, and total elongation value. The figure which expanded the part whose quantity of Li oxide shown in FIG. 2 is 100 or less. The figure which showed the name in the head cross-section surface position of the rail of this invention. The figure which showed the test piece collection position in the abrasion test shown in Table 1 and Table 2. FIG. The figure which showed the outline | summary of the abrasion test shown in Table 1 and Table 2. FIG. The figure which showed specimen collection in the tension test shown in Table 1 and Table 2. FIG. The figure which showed the relationship between carbon amount and the total elongation value as a result of the head tensile test of the present invention rail steel shown in Table 1 and the comparative rail steel shown in Table 2. The figure which showed the result of the head abrasion test of this invention rail steel shown in Table 1 with the relationship between carbon amount and abrasion amount.

Explanation of symbols

1: the top of the head,
2: Head corner,
3: Rail test piece,
4: Opponent material,
5: Cooling nozzle

Claims (14)

  In mass%, C: 0.65-1.20%, Si: 0.05-2.00%, Mn: 0.05-2.00%, Li: 0.0005-0.10% A pearlite rail excellent in wear resistance and ductility, characterized in that in the steel rail, the balance being Fe and inevitable impurities, at least part of the head surface of the steel rail has a pearlite structure. The wear resistance and ductility according to claim 1, wherein the total number of Li oxides having a diameter of 1 to 100 nm is present in 1 μm ² in an arbitrary cross section in the pearlite structure. Excellent perlite rail.   3. A pearlite rail excellent in wear resistance and ductility according to claim 1, wherein C is in the range of 0.90 to 1.20%.   The pearlitic rail excellent in wear resistance and ductility according to any one of claims 1 to 3, further comprising Co: 0.05 to 1.00% by mass.   In any 1 item | term of Claims 1-4, it is the mass% and contains further 1 type or 2 types of Cr: 0.05-2.00%, Mo: 0.01-0.50%. A pearlite rail with excellent wear resistance and ductility.   In any 1 item | term of Claims 1-5, it contains 1 type or 2 types of V: 0.005-0.50% and Nb: 0.002-0.050% further by the mass%. A pearlite rail with excellent wear resistance and ductility.   The pearlite rail excellent in wear resistance and ductility according to any one of claims 1 to 6, further comprising B: 0.0001 to 0.0050% in mass%.   The pearlite rail excellent in wear resistance and ductility according to any one of claims 1 to 7, further comprising Cu: 0.05 to 1.00% by mass%.   The pearlite rail excellent in wear resistance and ductility according to any one of claims 1 to 8, further comprising Ni: 0.01 to 1.00% by mass%.   In any 1 item | term of Claims 1-9, in mass%, Ti: 0.0050-0.0500%, Mg: 0.0005-0.0200%, Ca: 0.0005-0.0150% A pearlite rail excellent in wear resistance and ductility, characterized by containing one or more of the above.   The pearlite rail excellent in wear resistance and ductility according to any one of claims 1 to 10, further comprising Al: 0.0100 to 1.00% by mass.   The pearlite rail excellent in abrasion resistance and ductility according to any one of claims 1 to 11, further comprising Zr: 0.0001 to 0.2000% by mass.   The pearlite rail excellent in wear resistance and ductility according to any one of claims 1 to 12, further comprising N: 0.0060 to 0.0200% in mass%.   In any 1 item | term of Claims 1-13, the range of at least 20 mm in depth is a pearlite structure | tissue from the head corner part in the said steel rail, and the head top surface, and the hardness is Hv320-500 A pearlite rail with excellent wear resistance and ductility characterized by being in a range.

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