JP6631403B2 - Rails with excellent wear resistance and toughness - Google Patents

Description

  The present invention relates to a rail having excellent wear resistance and toughness.

  With the economic development, new development of natural resources such as coal is being promoted. Specifically, mining of natural resources has been promoted in areas where the natural environment has been severe, which has not been developed yet. As a result, the track environment of overseas freight railways that transport resources has become extremely severe. Rails have been required to have higher wear resistance than ever. Against this background, the development of rails with improved wear resistance has been required.

  In order to improve the wear resistance of rail steel, high-strength rails as disclosed in Patent Documents 1 and 2 have been developed. The main feature of these rails is to reduce the pearlite lamella spacing by heat treatment to increase the hardness of the steel or increase the carbon content of the steel to improve the wear resistance, and to increase the cementite content in the pearlite lamella. The volume ratio of the phase is increased.

  In the technique disclosed in Patent Document 1, the rail head is accelerated and cooled immediately after the end of rolling or after cooling and reheating after the end of rolling at a cooling rate of 1 to 4 ° C./sec between 850 to 500 ° C. from the austenitic zone temperature. By doing so, a rail having excellent wear resistance can be provided.

  In the technology disclosed in Patent Document 2, a hypereutectoid steel (C: more than 0.85 to 1.20%) is used to increase the volume ratio of cementite in the lamella in the pearlite structure, and is excellent in wear resistance. Rails can be provided.

  In the techniques disclosed in Patent Documents 1 and 2, by increasing the volume ratio of cementite phase in the pearlite structure lamella by increasing the hardness by reducing the lamellar spacing in the pearlite structure, it is possible to maintain a constant The wear resistance in the range can be improved. However, the techniques disclosed in Patent Literatures 1 and 2 have a problem in that the toughness of the pearlite structure itself is significantly reduced, and rail breakage is likely to occur.

  From such a background, it has been desired to provide a pearlite-based rail having improved wear resistance of a pearlite structure and improved wear resistance and toughness at the same time.

  In general, in order to improve the toughness of pearlite steel, refinement of pearlite structure, specifically, refinement of austenite structure before pearlite transformation and refinement of pearlite block size are said to be effective. . In order to achieve the refinement of the austenite structure, a reduction in the rolling temperature and an increase in the rolling reduction during hot rolling, and a heat treatment by low-temperature reheating after the rail rolling are performed. Further, in order to achieve a finer pearlite structure, promotion of pearlite transformation from within austenite grains using transformation nuclei is performed.

  However, in the manufacture of rails, from the viewpoint of ensuring the formability during hot rolling, there is a limit to the reduction of the rolling temperature and the increase of the rolling reduction, and it was not possible to achieve sufficient austenite grain refinement. In addition, the pearlite transformation from inside austenite grains using transformation nuclei has problems such as difficulty in controlling the amount of transformation nuclei and instability of pearlite transformation from inside the grains. Could not be achieved.

  From these problems, in order to drastically improve the toughness of the pearlite structure rail, a method of performing low-temperature reheating after rail rolling, and then performing pearlite transformation by accelerated cooling to refine the pearlite structure is used. Have been. However, in recent years, high carbonization of rails has been promoted to improve wear resistance, and coarse carbides remain dissolved in austenite grains during the above-mentioned low-temperature reheating heat treatment, and ductility and toughness of pearlite structure after accelerated cooling decrease. There is a problem of doing. In addition, when reheating is essential, there are also problems of economy such as high production cost and low productivity.

  Therefore, it has been required to develop a method for manufacturing a high-carbon steel rail that secures formability during rolling and refines the pearlite structure after rolling. In order to solve this problem, a method for manufacturing a high carbon steel rail as shown in Patent Documents 3 to 5 has been developed. The main feature of these production methods is that in order to refine the pearlite structure, the austenitic grains of high carbon steel are used at a relatively low temperature, and are easily recrystallized even with a small rolling reduction. . Thereby, fine grains of uniform size are obtained by continuous rolling under small pressure, and the ductility and toughness of the pearlite steel are improved.

  In the technique disclosed in Patent Document 3, in the finish rolling of a steel rail containing high carbon steel, a high toughness rail can be provided by performing continuous rolling of three or more passes within a predetermined rolling pass time. .

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

  According to the technique disclosed in Patent Document 5, in finish rolling of a steel rail containing high carbon steel, cooling is performed between passes, continuous rolling is performed, and accelerated cooling is performed after rolling, thereby achieving high wear resistance and high toughness. Rails can be provided.

  However, even if the techniques disclosed in Patent Literatures 3 to 5 are applied, the toughness of the rail may be reduced. Therefore, there has been an issue of developing a technology for securing the wear resistance of the rail and at the same time stably improving the toughness of the rail.

Therefore, control of inclusions in steel, which is an element that makes toughness unstable, has been required. In order to solve this problem, a method for manufacturing a high carbon steel rail as shown in Patent Documents 6 and 7 has been developed. The main feature of these manufacturing methods is that in order to suppress the production of MnS and Al ₂ O ₃ , which are typical inclusions of the rail, the addition of a deoxidizing element or the application of a vacuum treatment is performed to reduce the amount of oxygen contained in the molten steel. It is a technique to reduce inclusions in molten steel by reducing as much as possible. Accordingly, the techniques of Patent Documents 6 and 7 improve the toughness of the rail.

  In the technique disclosed in Patent Document 6, a method for producing a high-carbon high-purity molten steel that reduces MnO inclusions and reduces MnS elongated inclusions precipitated from MnO as a starting point is proposed. In this technology, after melting in an atmospheric refining furnace, the steel is tapped in a non-deoxidized or weakly deoxidized state, the dissolved oxygen in the molten steel is reduced to 30 ppm or less by vacuum treatment at a degree of vacuum of 1 Torr or less, and then Al and Si are melted in And then add Mn to the molten steel to reduce the number of secondary deoxidation products that are crystallization nuclei of MnS crystallized in the final solidified portion and reduce the MnO concentration in the oxide. , MnS crystallization is suppressed.

  In the technology disclosed in Patent Document 7, a method for manufacturing a rail in which the amount of oxygen and the amount of Al in steel are reduced is proposed. According to this technique, a rail having excellent damage resistance can be manufactured by limiting the total oxygen amount based on the relationship between the total oxygen value of oxide-based inclusions and the damage property. Further, the technique of Patent Document 7 improves the damage resistance and toughness of the rail by limiting the amount of dissolved Al or the composition of inclusions to a preferable range.

  However, in recent years, rails have been required to have higher toughness, and even if the techniques disclosed in Patent Documents 6 and 7 are applied, the rails may have insufficient toughness. Therefore, there has been an issue of developing a technique for ensuring wear resistance and at the same time stably improving toughness.

  Therefore, control of impurities in steel has been required as an element that makes toughness unstable. In order to solve this problem, a high-strength pearlite rail and a manufacturing method as disclosed in Patent Document 8 have been developed. The main feature of this rail is a technique of fixing P in a solid solution state, which is an element that reduces toughness, as a P compound. Thus, the technique of Patent Document 8 improves the toughness of the rail.

  The technique disclosed in Patent Literature 8 is to add Zr, Nb, Ti, and Mo, and fix solid solution P, which is an element that reduces toughness, as a P compound of Zr, Nb, Ti, and Mo. , To reduce the amount of dissolved P and improve the toughness of the pearlite steel.

  However, even when the technology disclosed in Patent Document 8 is applied, there is a case where the toughness is insufficient particularly in rails for freight railways overseas.

JP-A-57-198216 JP-A-8-144016 JP-A-7-173530 JP 2001-234238 A JP 2002-226915 A JP-A-5-263121 JP-A-2001-220651 JP 2001-40453 A

  Although the cause of the above-mentioned decrease in toughness was unknown, it was presumed to be due to scrap used as an auxiliary material for rails in order to reduce manufacturing costs. In the production of rails using blast furnace molten iron as the main raw material, in order to suppress this decrease in toughness, the amount of scrap used in the production of molten steel must be restricted and the quality of the scrap must be thoroughly controlled to prevent contamination of impurities. Has been dealt with. However, no detailed studies have been made on the effects of various impurity elements contained in scrap on toughness and wear resistance of rails. Also, keeping the mechanical properties of the rail high while increasing the amount of scrap used has not been sufficiently studied. For this reason, the present inventors considered that the limitation on the amount of scrap used and the management of the grade of the scrap excessively increased the manufacturing cost of the rail.

  As described above, there has been an issue of technology development capable of manufacturing a rail at a low cost, which secures wear resistance and stably improves toughness at the same time.

  The present invention has been devised in view of the above-mentioned problems, and an object of the present invention is to provide a rail for freight railways having high wear resistance and toughness at low cost.

The gist of the present invention is a rail shown below.
(1) The rail with excellent wear resistance and toughness according to one embodiment of the present invention has, in unit mass%, C: 0.75 to 1.20%, Si: 0.10 to 2.00%, and Mn: 0.10-2.00%, P ≦ 0.0250%, S ≦ 0.0250%, Pb: 0.0005-0.0020%, with the balance being Fe and impurities, from the surface of the head shell 95% by area or more of the area up to the depth of 20 mm is a pearlite structure, and the hardness of the area from the outer surface of the head to the depth of 20 mm is in the range of Hv300 to 500.
(2) The rail having excellent wear resistance and toughness according to the above (1) is a Pb having a particle size of 1.0 to 5.0 μm in a cross section of 2 to 20 mm in depth from the outer surface of the head. The number density of the oxide-based inclusions may be 100 or less per 1000 μm ^{2 of the} area to be detected.
(3) The rail having excellent wear resistance and toughness described in the above (1) and (2) further has a unit mass% of:
Group a: one or two of Cr: 2.00% or less and Mo: 0.50% or less, Group b: Co: 1.00% or less, Group c: B: 0.0050% or less, Group d : One or two kinds of Cu: 1.00% or less and Ni: 1.00% or less, Group e: V: 0.50% or less, Nb: 0.0500% or less, and Ti: 0.0500% One or more of the following, f group: one or more of Mg: 0.0200% or less, Ca: 0.0200% or less, and REM: 0.0500% or less, g group: Zr: 0 0.0200% or less, h group: Al: 1.00% or less, and i group: N: 0.0200% or less.

  ADVANTAGE OF THE INVENTION According to this invention, the wear resistance of a rail, toughness, and service life are not spoiled by using scrap as an auxiliary raw material, and controlling the content of Pb contained in rail steel, and manufacturing of a rail Cost can be reduced.

It is the graph which showed the relationship between the Pb content of a rail, and the impact value of a rail. It is the figure which showed the structure of the rail, and the area | region which contains the pearlite structure | tissue of 95 area% or more. 4 is a graph showing the relationship between the number of Pb oxide-based inclusions having a particle size of 1.0 to 5.0 μm (pieces / 1000 μm ² ) and the impact value of a rail. It is a figure showing an outline of a wear test. It is a figure showing a wear test piece sampling position. It is the figure which showed the impact test piece sampling position. It is the graph which showed the relationship between the Pb content of a rail, and the impact value of a rail. It is the graph which showed the relationship between C content of a rail, and the impact value of a rail.

  The present inventors have studied a method of stably preventing a decrease in toughness of a rail for freight railroads and reducing a manufacturing cost by using scrap as an auxiliary raw material in the production of rails using blast furnace molten iron as a main raw material.

For this purpose, rails were manufactured using blast furnace hot metal and scrap, and the relationship between impurities contained therein and the toughness of the rails was investigated in more detail than before. In particular, even a trace amount of impurity element, which is considered to have no influence on the rail characteristics according to the prior art, was also subjected to detailed analysis. The main component ranges, manufacturing conditions and toughness evaluation test conditions of the rail used in this study are as shown below. Hereinafter, the unit “% by mass” of the content of the alloy element is simply described as “%”.
[Rail alloy components]
● Component range: 5 main elements (0.75 to 1.20% C-0.10 to 2.00% Si-0.10 to 2.00% Mn-0.0020 to 0.0250% P-0. 0020 to 0.0250% S) and impurities The components of the five main elements were controlled so as to be within the above ranges, but the other impurity elements were not controlled.
[Basic characteristics of rail]
● Metal structure of the rail head: Perlite (95% or more area ratio)
● Hardness of rail head: Hv300-500
[Toughness evaluation test]
● Test method: Impact test ● Specimen: JIS No. 3 2mm U notch ● Specimen sampling position: 2mm below outer surface of rail head (see Fig. 6, 4mm below notch)
● Test temperature: room temperature (+ 20 ℃)
As a result of the impact test, the impact value and toughness of the rail were varied. This variation was considered to be due to factors other than the main components. As a result of detailed analysis of the rail steel with reduced toughness, it is clear that the effect of the content of Pb as an element other than the above-mentioned main components (C, Si, Mn, P, and S) is very large. Was.

  The present inventors have specifically controlled the Pb content of the rail and investigated the toughness in detail. As a result, it was necessary to control the Pb content in order to keep the toughness of the rail high while using scrap as an auxiliary material. I confirmed that there is. In particular, the present inventors have found that, in the production of rails using blast furnace molten iron as a main raw material, deterioration of toughness can be suppressed by controlling the Pb content mixed from scrap introduced when producing molten steel.

On the other hand, the present inventors have also found that even when Pb is contained in an amount of 0.005% or more, deterioration of rail toughness is within an allowable range. The present inventors manufactured a plurality of rail steels with varied Pb contents and investigated the relationship between the Pb content and toughness in order to more specifically clarify the effect of the Pb content on toughness. did. The components of the rail, the manufacturing conditions, and the test conditions for evaluating toughness used in this investigation are as shown below.
[Alloy composition of rail steel]
● Base steel component (1) 0.75% C-0.60% Si-0.60% Mn-0.0150% P-0.0120% S
(2) 1.00% C-0.60% Si-0.60% Mn-0.0150% P-0.0120% S
(3) 1.20% C-0.60% Si-0.60% Mn-0.0150% P-0.0120% S
● Pb content: 0 to 0.0060%
[Rail steel manufacturing conditions]
● Hot rolling conditions Reheating temperature: 1250 ° C, final rolling temperature: 1000 ° C
● Heat treatment conditions after hot rolling Cooling rate: 3 to 10 ° C / sec, cooling start temperature: 800 ° C, cooling stop temperature: 580 ° C
[Basic properties of rail steel]
● Metal structure of the rail head: Perlite (95% or more area ratio)
● Hardness of rail head: Hv350-400
[Toughness evaluation test]
● Test method: Impact test ● Specimen: JIS No. 3 2mm U notch ● Specimen sampling position: 2mm below outer surface of rail head (see Fig. 6, 4mm below notch)
● Test temperature: room temperature (+ 20 ℃)

  FIG. 1 shows the results of the impact test. The horizontal axis is the Pb content, and the vertical axis is the impact value. As a result, it was confirmed that the impact value significantly changed by controlling the Pb content in the rail steels having any carbon contents. In particular, it has been clarified that by controlling the Pb content to 0.0020% or less, a significant decrease in the impact value can be suppressed and the toughness required for the rail steel can be secured.

  Pb with a content of more than 0.0003% (for example, about 0.0020%) is not usually included in rails manufactured using blast furnace hot metal as a main raw material. Since impurities derived from scrap may impair the mechanical properties of the rail, when manufacturing rails using blast furnace hot metal as the main raw material, use of scrap or restriction of the amount of scrap used and management of the grade of scrap are performed. . In the prior art, the relationship between Pb exceeding 0.0003% and toughness has not been studied in detail so far. According to the investigation by the present inventors, rails manufactured using blast furnace hot metal as a main raw material contained only a very small amount of Pb with a detection lower limit of about 0 to 3 ppm (0.0000 to 0.0003%). . The experimental results shown in FIG. 1 show that, contrary to such common technical knowledge, as long as the metal structure and hardness of the rail head surface are appropriately controlled, the toughness is increased even when the Pb content is increased to 0.0020%. This indicates that there is no significant deterioration.

  Further, it is clear from the experimental results shown in FIG. 1 that the impact value is further stabilized by controlling the Pb content to 0.0015% or less and the Pb content to 0.0010% or less. became.

The impact test is an evaluation test for confirming the required toughness of the rail steel. As a result of analyzing the impact value of pearlite steel (0.75 to 1.20% C) so far, if the impact value of this test is 10.0 J / cm ² or more, it is confirmed that the toughness of the rail is secured on the actual track. Have been. Here, it was determined whether toughness was ensured based on an impact value of 10.0 J / cm ² or more as an evaluation criterion.

  Next, the present inventors examined the minimum hardness required to secure the wear resistance of the rail mainly composed of the pearlite structure. By subjecting hypereutectoid steel (1.00% C-0.60% Si-0.60% Mn-0.0150% P-0.0120% S) to hot rolling and heat treatment under various conditions. A prototype of a rail having different rail head hardness was manufactured, and the relationship between the hardness of the rail head and wear resistance was investigated. The wear resistance was evaluated by performing a wear test on a test piece taken from the rail head. As a result, in order to ensure the wear resistance of the rail head containing 0.0005 to 0.0020% of Pb, it is necessary to use a metal structure mainly composed of a pearlite structure in a range from the outer surface of the head to a depth of 20 mm. It was confirmed that the hardness had to be controlled in the range of 300 to 500 Hv.

  In addition, as shown in FIG. 2, the rail head 3 refers to a portion above the constricted portion at the center in the height direction of the rail when the rail is viewed in cross section. The head outer surface is an area of the rail head 3 where the crown 1 and the head corner 2 are combined. The crown 1 is the top surface of the rail head 3 when the rail is erected. The head corner portion 2 is a surface composed of corners on both sides of the crown 1 and the upper half of the side surface of the rail head 3 when the rail is erected. One of the head corner portions 2 is a gauge corner (GC) portion that mainly contacts the wheel. A region from the outer surface of the head to a depth of 20 mm may be referred to as a front part (a front part 3a hatched in FIG. 2).

  The rail according to the present embodiment having excellent wear resistance and toughness obtained based on the above findings will be described in detail below. Hereinafter, the unit “% by mass” of the content of the alloy component is simply described as “%”.

  First, the reasons for limiting the chemical components of the rail of the present embodiment will be described in detail.

(1) Reasons for limiting the chemical composition of steel

  In the rail of the present embodiment, the reason for limiting the chemical composition of steel will be described in detail.

  C is an element effective for promoting the pearlite transformation and ensuring the wear resistance of the rail. If the C content is less than 0.75%, a proeutectoid ferrite structure is generated in the present component system, so that the minimum strength and wear resistance required for a rail mainly composed of a pearlite structure cannot be maintained. On the other hand, when the C content exceeds 1.20%, a proeutectoid cementite structure is easily generated, and the wear resistance of the pearlite structure is reduced. For this reason, the C content was limited to 0.75 to 1.20%. In order to stabilize the formation of the pearlite structure, the C content is desirably 0.80 to 1.10%.

  Si is an element that forms a solid solution with the ferrite phase having a pearlite structure, increases the hardness of the rail head, and improves the wear resistance of the rail. However, if the Si content is less than 0.10%, these effects cannot be sufficiently expected. On the other hand, if the Si content exceeds 2.00%, many rail surface flaws are generated during hot rolling of the rail. Further, when the Si content exceeds 2.00%, the hardenability of the rail is remarkably increased, a martensite structure is formed at the rail head, and the wear resistance of the rail is reduced. For this reason, the Si content was limited to 0.10 to 2.00%. In order to stabilize the formation of the pearlite structure and improve the wear resistance of the rail, the Si content is desirably 0.20 to 1.50%.

  Mn is an element that enhances the hardenability of the rail and stabilizes the pearlite transformation, and at the same time, refines the lamella spacing of the pearlite structure, secures the hardness of the pearlite structure, and further improves the wear resistance of the rail. However, if the Mn content is less than 0.10%, the effect is small, a soft pro-eutectoid ferrite structure is generated, and the wear resistance of the rail is reduced. On the other hand, when the Mn content exceeds 2.00%, the hardenability of the rail is significantly increased, a bainite structure or a martensite structure is generated at the head of the rail, and the wear resistance of the rail is reduced. For this reason, the Mn content was limited to 0.10 to 2.00%. In order to stabilize the formation of the pearlite structure and improve the wear resistance of the rail, the Mn content is desirably 0.20 to 1.50%.

  P is an impurity element contained in steel. By performing appropriate refining in the converter, it is possible to control the P content. If the P content exceeds 0.0250%, the pearlite structure is embrittled, the ductility of the plastic deformation region of the rail is reduced, peeling is generated on the worn surface, and the wear resistance of the rail is reduced. When the P content exceeds 0.0250%, the toughness of the pearlite structure causes the rail toughness to decrease. For this reason, the P content is limited to 0.0250% or less. Although the lower limit of the P content is not limited, the lower limit of the P content may be set to about 0.0020% in consideration of the dephosphorization ability in the refining step.

  S is an impurity element contained in steel. It is possible to control the S content by performing desulfurization in a hot metal pot. If the S content exceeds 0.0250%, coarse MnS-based sulfide inclusions are easily formed, and coarse voids due to stress concentration in the plastic deformation region of the rail head due to contact between the rail and the wheel. It forms and peels off on the worn surface, resulting in reduced wear resistance. On the other hand, if the S content exceeds 0.0250%, stress concentration is generated around coarse MnS-based sulfide inclusions, and rail toughness is reduced. For this reason, the S content was limited to 0.0250% or less. Although the lower limit of the S content is not limited, the lower limit of the S content may be set to about 0.0020% in consideration of the desulfurization ability in the refining process.

  Pb is an impurity element contained in steel. As described above, usually in the manufacture of rails using blast furnace molten iron as a main raw material, the use of scrap is restricted or the amount of scrap used is restricted, and the quality of the scrap is controlled. About 0.0020% of Pb is not mixed. According to the investigation by the present inventors, the amount of Pb is usually limited to about 0 to 3 ppm (0.00000 to 0.0003%) in rails for producing molten steel by the blast furnace method. However, the present inventors can suppress a significant decrease in rail toughness even if the Pb content is increased to 0.0020%, as long as the metal structure and hardness of the rail head surface are appropriately controlled as described later. It was found that the toughness required for the rail could be secured. That is, even if the Pb content is implicitly limited as an undesired impurity according to the common technical knowledge, the rail toughness according to the present embodiment does not significantly deteriorate. For this reason, the upper limit of the Pb content is limited to 0.0020% or less.

When the upper limit of Pb is set to 0.0020%, it is possible to reduce the manufacturing cost of the rail by increasing the amount of scrap used or decreasing the grade of scrap. On the other hand, when the amount of scrap used is increased and the quality is reduced, mixing of about 0.0005% of Pb is inevitable. Therefore, the lower limit of Pb is set to 0.0005%.
In order to further maintain the toughness of the rail by controlling the Pb content, it is desirable to control the Pb content to 0.0015% or less. In order to keep the toughness of the rail even higher, it is desirable to control the Pb content to 0.0010% or less. On the other hand, in order to further reduce the manufacturing cost of the rail, the lower limit of the Pb content may be set to 0.006%, 0.007%, or 0.008%.

  Furthermore, the rail manufactured with the above-mentioned composition has improved wear resistance and internal fatigue damage resistance due to an increase in the hardness of the pearlite structure, improved toughness, prevention of softening of the weld heat affected zone, and improvement of the inside of the head. One or two kinds of Cr and Mo in group a, one kind or two kinds of Co in group b, B in group c, one or two kinds of Cu and Ni, and group e One or more of V, Nb, and Ti, one or more of f, Mg, Ca, and REM, g of Zr, h of Al, and i of N If necessary, one or more groups may be contained. However, even if these elements are not contained, the rail according to the present embodiment can exhibit its effect, so the lower limit of the content of these elements is 0%.

  Cr and Mo in the a group reduce the lamella spacing of the pearlite structure by increasing the equilibrium transformation point, and improve the hardness of the rail. Co of group b dissolves in the ferrite phase having a pearlite structure to refine the lamellar structure immediately below the rolling surface of the rail, thereby increasing the hardness of the worn surface. B in group c reduces the cooling rate dependency of the pearlite transformation temperature and makes the hardness distribution of the rail head uniform. Cu and Ni in the d group form a solid solution in the ferrite in the pearlite structure, increase the hardness of the rail, and improve the toughness of the pearlite structure. V, Nb, and Ti in group e improve the fatigue strength of the pearlite structure by precipitation hardening of carbides and nitrides generated during the hot rolling of the rail and the subsequent cooling of the rail. In addition, V, Nb, and Ti in the e group stably generate carbides and nitrides when the rail is reheated, and prevent the heat-affected zone of the weld joint from softening. Mg, Ca, and REM in the f group finely disperse the MnS-based sulfide and reduce internal fatigue damage generated from inclusions. By increasing the equiaxed crystallization ratio of the solidified structure, Zr in the g group suppresses the formation of a segregation zone at the center of the slab and suppresses the formation of a proeutectoid cementite structure and a martensite structure. Al in the h group shifts the eutectoid transformation temperature to a higher temperature side, suppresses the formation of a proeutectoid cementite structure, and improves the wear resistance of the pearlite structure. N in group i promotes pearlite transformation by segregating at austenite grain boundaries and improves the toughness of the rail. Further, N in group i promotes the precipitation of carbides and nitrides of V in the subsequent cooling process after hot rolling, and improves the fatigue resistance of the pearlite structure.

<Group a>

  Cr is an element that raises the equilibrium transformation temperature and increases the degree of undercooling, thereby making the lamella spacing of the pearlite structure finer, improving the hardness of the pearlite structure, and improving the wear resistance of the rail. In order to obtain the above-described effects, the Cr content is preferably set to 0.10% or more. On the other hand, if the Cr content exceeds 2.00%, the hardenability of the rail is significantly increased, a bainite structure or a martensite structure is generated at the rail head, and the wear resistance of the rail may be reduced. Therefore, the Cr content is preferably set to 0.10 to 2.00%. In order to stabilize the formation of the pearlite structure and improve the wear resistance of the rail, the Cr content is desirably 0.20 to 1.00%.

  Mo is an element that raises the equilibrium transformation temperature and increases the degree of undercooling, thereby reducing the lamella spacing of the pearlite structure, improving the hardness of the pearlite structure, and improving the wear resistance of the rail. In order to obtain the above-described effects, the Mo content is preferably set to 0.01% or more. On the other hand, when the Mo amount exceeds 0.50%, the transformation speed is remarkably reduced, a martensite structure is formed on the rail head, and the wear resistance of the rail may be reduced. Therefore, it is desirable that the Mo content be 0.01 to 0.50%.

<Group b>

  Co is an element that forms a solid solution with the ferrite phase having a pearlite structure, refines the lamellar structure of the pearlite structure immediately below the rolling surface, improves the hardness of the rolling surface, and improves the wear resistance of the rail. In order to obtain the above-described effects, the Co content is preferably set to 0.01% or more. On the other hand, if the Co content exceeds 1.00%, the above effect is saturated, and the lamellar structure cannot be refined according to the Co content. On the other hand, if the Co content exceeds 1.00%, the economic efficiency is reduced due to an increase in alloy cost. For this reason, the Co content is desirably set to 0.01 to 1.00%.

<Group c>

B forms iron carbide boride (Fe ₂₃ (CB) ₆ ) at the austenite grain boundary, reduces the cooling rate dependency of the pearlite transformation temperature by the effect of promoting pearlite transformation, and reduces the temperature from the head surface to the inside of the rail. It is an element that makes the hardness distribution uniform and prolongs the life of the rail. In order to obtain the above-described effects, the B content is preferably set to 0.0001% or more. On the other hand, if the B content exceeds 0.0050%, coarse iron carbide borides are generated, promoting brittle fracture, and the toughness of the rail may be reduced. Therefore, it is desirable that the B content be 0.0001 to 0.0050%.

<D group>

  Cu is an element that forms a solid solution in the ferrite phase having a pearlite structure, improves the hardness of the rail by solid solution strengthening, and improves the wear resistance of the rail. In order to obtain the above-described effects, the Cu content is preferably set to 0.01% or more. On the other hand, when the Cu content exceeds 1.00%, martensite structure is generated at the head of the rail due to remarkable hardenability of the rail, and the wear resistance of the rail may be reduced. Therefore, the Cu content is preferably set to 0.01 to 1.00%. In order to secure the hardness of the rail head and to suppress the formation of a martensite structure that is easily formed in a segregated portion or the like, it is desirable to control the Cu content to 0.20% or less.

Ni is an element that improves the toughness of the pearlite structure, and at the same time, improves the hardness of the rail by solid solution strengthening and improves the wear resistance of the rail. Furthermore, in the weld heat affected zone, Ni is an element that combines with Ti and precipitates as a fine intermetallic compound of Ni ₃ Ti, thereby suppressing the softening of the rail by precipitation strengthening. When Cu is contained in the rail, Ni suppresses embrittlement of the grain boundary. In order to obtain the above-described effects, the Ni content is preferably set to 0.01% or more. If the amount of Ni exceeds 1.00%, a martensite structure is generated at the rail head due to remarkable improvement in hardenability of the rail, and the wear resistance of the rail may be reduced. Therefore, it is desirable that the Ni content be 0.01 to 1.00%.

<Group e>

  V is an element that increases the hardness (strength) of the pearlite structure by precipitation hardening by carbon / nitride generated in the cooling process after hot rolling, and improves the fatigue damage resistance inside the rail head. is there. In order to obtain the above-described effects, the V content is preferably set to 0.01% or more. On the other hand, if the V content exceeds 0.50%, the number of fine carbon / nitride particles becomes excessive, the pearlite structure becomes brittle, and the fatigue damage resistance of the rail may decrease. For this reason, the V content is desirably set to 0.01 to 0.50%.

  Nb is an element that increases the hardness of the pearlite structure and improves the fatigue damage resistance inside the rail head by precipitation hardening by Nb carbide and Nb nitride generated in the cooling process after hot rolling. Also, in the heat-affected zone reheated to a temperature range below the Ac1 point, Nb stably generates Nb carbide and Nb nitride from a low temperature range to a high temperature range, and prevents softening of the heat-affected zone of a welded joint. It is an effective element to do. In order to obtain the above-described effects, the Nb content is preferably set to 0.0010% or more. On the other hand, if the Nb content exceeds 0.0500%, the precipitation hardening of carbides and nitrides of Nb becomes excessive, the pearlite structure itself becomes brittle, and the fatigue damage resistance of the rail may decrease. Therefore, it is desirable that the Nb content be 0.0010 to 0.0500%.

  Ti is an element that increases the hardness of the pearlite structure and improves the fatigue resistance inside the rail head by precipitation hardening by Ti carbide and Ti nitride generated in the cooling process after hot rolling. In addition, by utilizing the fact that Ti carbide and Ti nitride precipitated during reheating during welding do not dissolve in the matrix, Ti refines the structure of the heat-affected zone heated to the austenite region, It is an effective component for preventing embrittlement of the part. In order to obtain the above effects, the amount of Ti is preferably set to 0.0030% or more. On the other hand, if the Ti content exceeds 0.0500%, coarse carbides of Ti and nitrides of Ti are generated, and stress cracks are apt to be generated around these carbides, so that fatigue cracks are easily formed, and the fatigue damage resistance of the rail is increased. May decrease. Therefore, it is desirable that the Ti content be 0.0030 to 0.0500%.

<F group>

  Mg combines with S to form fine sulfide (MgS), which is an element that finely disperses MnS, reduces stress concentration around MnS, and improves fatigue damage resistance of rails. is there. In order to obtain the above-described effects, the Mg content is preferably set to 0.0005% or more. On the other hand, when the amount of Mg exceeds 0.0200%, a coarse oxide of Mg is generated, and a stress crack is easily generated around the coarse oxide, so that a fatigue crack is easily generated and the fatigue damage resistance of the rail decreases. May be. Therefore, it is desirable that the amount of Mg is 0.0005 to 0.0200%.

  Ca forms a sulfide (CaS) because it has a strong bonding force with S, and this CaS disperses MnS finely, reduces stress concentration around MnS, and improves the fatigue damage resistance of the rail. It is. In order to obtain the above-described effects, the Ca content is preferably set to 0.0005% or more. On the other hand, if the Ca content exceeds 0.0200%, a coarse oxide of Ca is generated, and the stress concentration around the coarse oxide facilitates the generation of fatigue cracks, thereby reducing the fatigue damage resistance of the rail. May be. For this reason, it is desirable that the amount of Ca be 0.0005 to 0.0200%.

REM is a deoxidizing / desulfurizing element, which generates REM oxysulfide (REM ₂ O ₂ S) and serves as a nucleus for generating Mn sulfide-based inclusions. Oxysulfide (REM ₂ O ₂ S) has a high melting point, and thus suppresses the stretching of Mn sulfide-based inclusions after rolling. As a result, REM finely disperses MnS, reduces stress concentration around MnS, and improves the fatigue damage resistance of the rail. In order to obtain the above effects, it is preferable that the amount of REM is 0.0005% or more. On the other hand, if the REM amount exceeds 0.0500%, coarse and hard REM oxysulfide (REM ₂ O ₂ S) is generated, and fatigue cracks are easily generated due to stress concentration around the oxysulfide. The fatigue resistance of the rail may be reduced. For this reason, it is desirable that the REM content be 0.0005 to 0.0500%.

  REM is a rare earth metal such as Ce, La, Pr or Nd. The “REM content” means the sum of the contents of all these REM elements. If the total content is within the above range, the same effect can be obtained regardless of whether the type of REM element is one or two or more.

<G group>

Since Zr generates ZrO ₂ inclusions having good lattice matching with γ-Fe, γ-Fe becomes a solidification nucleus of a high-carbon rail steel that is a primary solidification crystal, and increases the equiaxed crystallization rate of the solidification structure. This is an element that suppresses the formation of a segregation zone at the center of the slab and suppresses the formation of a martensite structure generated in the rail segregation portion. In order to obtain the above effects, the Zr content is preferably set to 0.0001% or more. On the other hand, if the Zr content exceeds 0.0200%, a large amount of coarse Zr-based inclusions are generated, and the stress concentration around the coarse inclusions tends to cause fatigue cracks, resulting in fatigue resistance of the rail. Performance may be reduced. Therefore, it is desirable that the Zr content be 0.0001 to 0.0200%.

<Group h>

  Al is a component that functions as a deoxidizing material. Al is an element that raises the eutectoid transformation temperature, suppresses the formation of the proeutectoid cementite structure, and improves the wear resistance of the pearlite structure. In order to obtain the above-described effects, the Al content is preferably set to 0.0100% or more. On the other hand, if the Al content exceeds 1.00%, it becomes difficult to form a solid solution of Al in the steel, and coarse alumina-based inclusions are formed, and fatigue cracks are easily generated from the coarse inclusions. The fatigue resistance of the rail may be reduced. Further, when the Al content exceeds 1.00%, an oxide is generated during rail welding, and the rail weldability may be significantly reduced. Therefore, it is desirable that the Al content be 0.0100 to 1.00%.

<Group i>

  N is an element effective to promote pearlite transformation from austenite grain boundaries by segregating to austenite grain boundaries, and to improve rail toughness mainly by reducing the size of pearlite blocks. Further, when N and V are simultaneously contained, precipitation of carbonitrides of V is promoted in the cooling process after hot rolling of the rail, the hardness of the pearlite structure is increased, and the fatigue resistance of the rail is improved. In order to obtain the above-described effects, the N content is preferably set to 0.0050% or more. On the other hand, if the N amount exceeds 0.0200%, it becomes difficult to form a solid solution of N in the steel, and air bubbles serving as starting points of fatigue damage may be easily generated. Therefore, it is desirable that the N content be 0.0050 to 0.0200%.

  Among the above-mentioned elements, Cu, Ni, Cr, and Mo may be mixed in from raw material scrap. Even in this case, the characteristics of the rail according to the present embodiment are not impaired as long as the components of Cu, Ni, Cr, and Mo are within the above ranges.

  The balance of the chemical composition of the rail according to the present embodiment contains Fe and impurities. Here, the impurity means what is mixed in from the raw material scrap or the manufacturing environment when the steel material is industrially manufactured. The impurities may be contained within a range that does not adversely affect the characteristics of the rail according to the present embodiment.

(2) Reasons for Restricting Necessary Ranges of Metallic Structure and Pearlite Structure Next, in the present embodiment, 95% (area ratio) or more of a range (head surface) from the outer surface of the head to a depth of 20 mm is used. The reason for this will be described in detail.

  First, the reason why the amount of the pearlite structure is set to 95% by area or more will be described.

  It is most important to ensure wear resistance at the rail head that comes into contact with the wheels. As a result of investigating the relationship between the metal structure and the wear resistance, it was confirmed that the pearlite structure most improved the wear resistance of the rail. The pearlite structure has sufficient hardness even with a small amount of alloying elements. Therefore, in order to improve the abrasion resistance of the rail, the structure is limited to the pearlite structure.

  Next, the reason why the required range of the metal structure (the structure including the pearlite structure) in which the pearlite structure is included at a rate of 95% or more in area ratio is limited to a range of at least 20 mm from the outer surface of the head.

  If the area containing 95% by area or more of the pearlite structure is less than 20 mm from the outer surface of the head, the area is too small to ensure the required wear resistance of the rail head, and thus a sufficient rail. It becomes difficult to secure the service life. Further, in order to further improve the wear resistance of the rail, it is desirable that a structure including a pearlite structure of 95 area% or more is provided from the outer surface of the head to a depth of about 30 mm.

  As described above, the range from the outer surface of the head, which is the combined area of the head corner 2 and the crown 1 to the depth of 20 mm, is referred to as the front surface part (3a, hatched part). If a structure containing 95% by area or more of a pearlite structure having a predetermined hardness is arranged on the front surface portion 3a from the surface of the head corner portion 2 and the top portion 1 to a depth of 20 mm, the wear resistance of the rail is secured. it can.

  Therefore, it is desirable to arrange the structure including the pearlite structure of 95% by area or more in the front part 3a where the wheel is mainly in contact and where wear resistance is required, and other structures where wear resistance is not required. The portion may have a pearlite structure of less than 95 area%.

  Further, it is desirable that the metal structure of the front surface portion 3a of the rail of the present embodiment is a structure including a pearlite structure of 95% by area or more as described above. However, depending on the component system of the rail and the heat treatment manufacturing method, a small amount of a proeutectoid ferrite phase, a proeutectoid cementite phase, a bainite cementite phase, a bainite structure or a martensite structure with an area ratio of less than 5% is mixed into the structure in addition to the pearlite structure. Sometimes. However, the inclusion of these structures does not have a great adverse effect on the wear resistance of the head surface. Phase, a proeutectoid cementite phase, a bainite structure, and a martensite structure. In other words, the metal structure of the top of the rail according to the present embodiment only needs to have a pearlite structure in an area ratio of 95% or more, and in order to sufficiently secure the wear resistance of the rail, it is necessary to use the area ratio of the top of the rail. It is desirable that 98% or more of the metal structure is a pearlite structure.

  The measurement of the area ratio of the pearlite structure of the head surface portion is performed by observing a metal structure of a cross section ranging from the outer surface of the head to at least a depth of 20 mm with an optical microscope of 200 times or more for at least 10 visual fields. This can be done by calculating the average of the rates.

(3) Reasons for Limiting the Hardness of the Tissue Containing the Pearlite Structure Next, the reason for limiting the hardness of the tissue including the pearlite structure in the frontal region to the range of Hv300 to 500 in the present embodiment will be described. If the hardness of the structure including the pearlite structure in the head portion is less than Hv300, the wear of the rail proceeds, and it becomes difficult to secure the wear resistance required for the rail. Further, if the hardness of the pearlite structure in the head portion exceeds Hv500, minute cracks are likely to be generated on the outer surface of the head in contact with the wheels due to embrittlement of the structure including the pearlite structure, and the rail surface resistance damage It is difficult to secure the performance. For this reason, the hardness of the structure including the pearlite structure in the head surface is limited to the range of Hv300 to 500. In addition, since the rails do not contact and wear resistance is not required at locations other than the head surface, the hardness of the tissue at locations other than the head surface is not particularly limited.

  For example, a rail in which the hardness of the rail at a position at a depth of 2 mm and a position at a depth of 20 mm from the outer surface of the head is within the above-mentioned range is determined to be a rail satisfying the above-mentioned requirements regarding the hardness of the head surface. You. In addition, it is desirable to measure the hardness of the rail at each measurement position by measuring 10 or more points at the measurement position and calculating the average value thereof. In the top surface of the rail according to this embodiment, the pearlite structure accounts for 95% or more in area ratio, but other structures (proeutectoid cementite phase, proeutectoid ferrite phase, martensitic structure, bainite structure) in a range of less than 5 area%. This is because the hardness of the structure including the pearlite structure may not be able to be represented by one point measurement due to the presence of the structure.

(4) Reasons for Limiting the Number Density of Pb Oxide Inclusions Next, in the rail according to the present embodiment, the cross-section at a depth of 2 to 20 mm from the outer surface of the head has a particle size of 1.0 to 5 mm. The reason why it is preferable to set the number density of Pb oxide-based inclusions of 0.0 μm to 100 or less per 1000 μm ^{2 of the} area to be detected will be described. The Pb oxide usually exists in steel as an inclusion consisting of only the Pb oxide, or exists as an inclusion consisting of the Pb oxide and another alloy element attached around the Pb oxide. In the present embodiment, these inclusions are referred to as “Pb oxide-based inclusions”.

  First, the present inventors studied the control of Pb oxide-based inclusions generated in the pearlite structure in order to further stably improve the toughness of the rail steel. In the converter refining process, oxygen is blown into molten steel, lime is introduced, impurities such as P are removed, then alloying elements are added, and the amount of oxygen is controlled by introducing FeSi (ferrosilicon). The pearlite steel was manufactured by changing the size and number of the oxide-based inclusions, and the toughness of the pearlite steel was evaluated by an impact test. As a result, it became clear that, for the same Pb content, there were a pearlite steel having an improved impact value and a pearlite steel having an adversely reduced impact value.

  Then, the present inventors investigated in detail the fracture starting point portion of the pearlite steel having a reduced impact value in order to effectively improve the impact value. As a result, it was found that a large amount of Pb oxide-based inclusions having a particle size in the range of 1.0 to 5.0 μm were present at the fracture starting point. The present inventors have found that the breakdown is promoted by stress concentration on the Pb oxide-based inclusions.

Furthermore, the present inventors lab-melted steel having a changed Pb content in order to clarify the effect of the amount of Pb oxide-based inclusions, and thereafter controlled the oxygen amount, and An experiment simulating stirring was performed, and the pearlite steel in which the molten steel stirring time, the molten steel standing time after the stirring treatment was controlled, and the number of Pb oxide-based inclusions per unit area was changed, and the toughness was evaluated. .
[Alloy composition of rail steel]
● Base steel component 1.00% C-0.60% Si-0.60% Mn-0.0150% P-0.0120% S
● Pb content: changed within the range of 0.0000 to 0.0020% [Rail steel manufacturing conditions]
● Molten steel treatment after converter refining Molten steel stirring time with inert gas: 0.5 to 4 minutes Molten steel standing time after stirring treatment: 10 to 40 minutes ● Hot rolling conditions Reheating temperature: 1250 ° C ⇒ Final rolling temperature : 1000 ° C
● Heat treatment conditions Cooling rate: 6 ° C / sec, cooling start temperature: 800 ° C, stop temperature: 580 ° C
[Basic properties of rail steel]
● Metal structure of the rail head: Perlite (95% or more area ratio)
● Hardness of the rail head: Hv400-420
● Number of Pb oxides per unit area: 0 to 200/1000 μm ²
(Particle size range: 1.0 to 5.0 μm)
[Method for measuring particle size and number of Pb oxide inclusions]
● Pretreatment: Diamond polishing of cross section ● Equipment: Scanning electron microscope Magnification: 1000-2000
● Pre-analysis: Conduct component analysis using an electron beam microanalyzer to identify complex inclusions of Pb and O.
● Measurement position: Observed in a visual field centered on a depth of 5 mm starting from the outer surface of the head ● Particle size range of Pb oxide inclusions to be evaluated: 1.0 to 5.0 μm
● Measurement of particle size: Only Pb oxide inclusions are selected, the area is determined, and the particle size is calculated by the diameter of a circle corresponding to the area.
● Calculation of the number: Observation was performed in 20 visual fields, the number of Pb oxide-based inclusions was counted, converted to the number per 1000 μm ² , and the average value was calculated.
[Toughness evaluation test]
● Test method: Impact test ● Specimen: JIS No. 3 2mm U notch ● Specimen sampling position: 2mm below outer surface of rail head (see Fig. 6, 4mm below notch)
● Test temperature: room temperature (+ 20 ℃)
FIG. 3 shows the results of the impact test. The horizontal axis is the number of Pb oxide-based inclusions (particle size range: 1.0 to 5.0 μm) per unit area (pieces / 1000 μm ² ), and the vertical axis is the impact value. As a result, it was confirmed that the impact value significantly changed depending on the number of Pb oxide-based inclusions per unit area.

Specifically, it was found that when the number of Pb oxide-based inclusions per unit area was 100 or less per 1000 μm ^{2 of the} test area, stress concentration was relaxed, and the impact value was further improved. Therefore, the number of Pb oxide-based inclusions having a particle size of 1.0 to 5.0 μm per unit area is limited to 100 or less per 1000 μm ^{2 of the} test area.

Note that Pb oxide-based inclusions tend to accumulate at the center of the slab during casting. When the Pb content is small, the Pb oxide-based inclusion may not be present in the vicinity of the surface of the head shell for confirming the presence. Therefore, the lower limit of the number density of Pb oxide-based inclusions having a particle size of 1.0 to 5.0 μm is not defined and may be 0 in some cases. On the other hand, when this oxide is present as an object to be counted, 1/1000 μm ² or more may be set as the control range.

  The control target position of the Pb oxide-based inclusion is a cross section in a range of 2 to 20 mm in depth from the outer surface of the head. When the depth is less than 2 mm from the outer surface of the head, the presence of Pb oxide-based inclusions is small, and it is difficult to evaluate the inclusions. Since the toughness required for the rail itself is not a required area, a range less than 2 mm in depth from the outer surface of the head is not controlled.

  Further, the reason why the particle size of the target Pb oxide-based inclusions was limited to the range of 1.0 to 5.0 μm is that, as described above, the starting point of the fracture of the rail steel was observed. This is because Pb oxide-based inclusions having a particle size of 1.0 to 5.0 μm were present. This is because it has been confirmed that Pb oxide-based inclusions in this particle size range cause stress concentration and cause a decrease in impact value.

  Note that Pb oxide-based inclusions having a particle size of less than 1.0 μm and Pb oxide-based inclusions having a particle size of more than 5.0 μm may be present in the steel. However, it was found that Pb oxide inclusions having a particle size of less than 1.0 μm did not significantly affect the impact value because stress concentration around the Pb oxide inclusions was small. The Pb oxide-based inclusions having a particle diameter of more than 5.0 μm have a large stress concentration around the Pb oxide inclusions, but the number thereof is very small. It turned out to be meaningless. Therefore, the number density of the Pb oxide-based inclusions having a particle size of less than 1.0 μm and the number density of the Pb oxide-based inclusions having a particle size of more than 5.0 μm are not specified.

(5) Method of Manufacturing Rail of Present Embodiment In a normal method of manufacturing a rail, first, pig iron manufactured in a blast furnace is refined in a converter refining process. In the converter step, oxygen is blown into molten steel, lime is charged, and impurities such as P are removed. Then, an alloy element is added to the molten steel in a ladle, and the amount of oxygen in the molten steel is controlled by introducing FeSi or the like. Further, if necessary, the amount of hydrogen, the amount of nitrogen, the amount of oxygen, and the like of the molten steel are controlled by a vacuum degassing device. This molten steel is cast by an ingot-bulking method or a continuous casting method to produce a bloom, and then the bloom is hot-rolled into a rail shape. Further, the rail is subjected to heat treatment for the purpose of controlling the metal structure and hardness of the rail head as required.

  In the rail manufacturing method of the present embodiment, scrap is introduced into molten steel as an auxiliary material during refining in a converter. After the converter refining, the alloy is added to the ladle, and the amount of oxygen is controlled by introducing FeSi or the like. Furthermore, in the rail manufacturing method of the present embodiment, the alloy, the molten steel of the ladle with controlled oxygen content is subjected to stirring with an inert gas, the molten steel stirring time, and the molten steel standing time after the stirring process is controlled, The number density of Pb oxide-based inclusions is controlled. Next, molten steel is cast into a bloom, and the bloom is hot-rolled to form a rail shape. If necessary, heat treatment is performed on the rail after hot rolling for the purpose of controlling the metal structure of the rail head and controlling the hardness.

(5-1) Refining conditions (control of Pb content)
First, the method for controlling the Pb content will be described. In the method for manufacturing a rail according to the present embodiment in which scrap is used as an auxiliary material, in order to control the Pb content, it is desirable to control the oxygen amount in a ladle after smelting molten steel in a converter refining process. . The amount of oxygen in the molten steel is measured in advance, and the amount of oxygen is preferably controlled by introducing FeSi or the like in an amount corresponding to the amount of oxygen in the molten steel. Further, the residual oxygen amount is controlled by a vacuum degassing device as needed. Utilizing this remaining oxygen, Pb is generated as an oxide of Pb. If the Pb content is excessive, it is necessary to discharge this oxide from the steel and control the Pb content in the molten steel.

(5-2) Conditions for stirring and standing of molten steel (control of the number of Pb oxide-based inclusions)
Next, a method for controlling the number of Pb oxide-based inclusions will be described. In the rail according to the present embodiment, in a cross section at a position of 2 to 20 mm deep from the outer surface of the head, 100 Pb oxide-based inclusions having a particle size of 1.0 to 5.0 μm per 1000 μm ^{2 of the} test area. In order to control the Pb oxide in this manner, in the rail manufacturing method according to the present embodiment, in the ladle after the converter refining, the molten steel is stirred with an inert gas, It is necessary to control the molten steel stirring time and the molten steel standing time after the stirring treatment to control the number of Pb oxide-based inclusions.

  Specifically, when molten steel is stirred with an inert gas in a ladle after converter refining, the reaction between the remaining oxygen and Pb is promoted, and Pb oxide-based inclusions are generated. Further, stirring of the molten steel has an effect of promoting agglomeration of Pb oxide-based inclusions in the molten steel and promoting floating removal of the Pb oxide-based inclusions. Therefore, in order to control the number of Pb oxide-based inclusions, it is desirable that the stirring time of the molten steel be 2 minutes or more. However, long-time stirring increases the viscosity of the molten steel due to a decrease in the temperature of the molten steel, and lowers the levitation of Pb oxide-based inclusions. Therefore, when controlling the number of Pb oxide-based inclusions, it is desirable that the stirring time be in the range of 2.0 to 4.0 minutes.

  Further, it is necessary to stir the molten steel in the ladle after the converter refining after sufficiently stirring the molten steel with an inert gas in order to float and separate the Pb oxide-based inclusions generated in the molten steel. . In order to promote the floating separation of Pb oxide inclusions, it is desirable that the molten steel after stirring using an inert gas is allowed to stand for 30 minutes or more. However, even if the standing time is extremely increased, the temperature of the molten steel decreases, the viscosity of the molten steel increases, and the floating property of the Pb oxide-based inclusions decreases, so that the floating separation effect is saturated. Therefore, when controlling the number of Pb oxide-based inclusions, it is desirable that the molten steel standing time be in the range of 30.0 to 40.0 minutes.

  Therefore, in order to keep the number density of the Pb oxide-based inclusions (particle size range: 1.0 to 5.0 μm) at a predetermined value, the stirring time of the molten steel is set to 2 to 4 minutes, and the standing time of the molten steel is set to It needs to be 30 to 40 minutes.

(5-3) Hot Rolling Conditions The hot rolling conditions are set in the following condition range in order to maintain the pearlite structure and control the hardness of the rail head.

  First, the hot rolling conditions will be described.

  First, the bloom reheating temperature will be described. If the bloom reheating temperature is less than 1000 ° C., hot formability can be ensured in rail rolling, and rolling flaws occur, making rail production difficult. In addition, since the melting point is lowered depending on the amount of carbon or alloy of steel, if the reheating temperature exceeds 1400 ° C., the steel may be melted, and it may be difficult to manufacture rails. Therefore, the bloom reheating temperature is in the range of 1000-1400 ° C.

  Next, the final rolling temperature will be described.

  If the final rolling temperature is lower than 750 ° C., the pearlite transformation starts immediately after the rolling, so that the rail cannot be hardened in the heat treatment after the rolling is completed, and the wear resistance cannot be secured. On the other hand, when the final rolling temperature exceeds 1100 ° C., austenite grains after rolling are coarsened, hardenability is greatly increased, and a bainite structure harmful to wear resistance is formed on the rail head. In this case, the wear resistance of the rail is reduced, and the minimum ductility required for the rail cannot be secured. Therefore, the final rolling temperature is in the range of 750 to 1100 ° C.

  Other hot rolling conditions are not particularly limited. In order to secure the hardness of the rail head, ordinary groove rolling of the rail may be performed while controlling the bloom reheating temperature and the final rolling temperature as described above. For example, after rough rolling of a billet, intermediate rolling by a reverse rolling mill is performed over a plurality of passes, followed by finish rolling by a continuous rolling mill for two or more passes. What is necessary is just to control in the said temperature range.

(5-4) Heat treatment conditions The heat treatment conditions after the hot rolling are performed in the following condition range in order to maintain the pearlite structure and control the hardness of the rail head.

  First, the cooling rate will be described. If the cooling rate is less than 1 ° C./sec, the pearlite transformation temperature increases, the rail cannot be hardened, and the abrasion resistance of the rail cannot be ensured. On the other hand, if the cooling rate exceeds 20 ° C./sec, in the present component system, a bainite structure or a martensite structure is generated at the rail head, and the wear resistance of the rail is reduced. Therefore, the cooling rate is in the range of 1 to 20 ° C./sec.

  Next, the cooling start temperature will be described. If the cooling start temperature is less than 700 ° C., in the present component system, a pearlite structure is generated in a high temperature range before accelerated cooling, so that the rail cannot be hardened and the wear resistance of the rail cannot be secured. If the cooling start temperature is lower than 700 ° C., a pro-eutectoid cementite structure may be formed, and the wear resistance of the rail may be reduced. On the other hand, if the starting temperature exceeds 900 ° C., the hardenability of the rail is greatly increased, and a bainite structure harmful to the wear resistance is generated at the rail head, and the wear resistance is reduced. Therefore, the cooling start temperature is set in the range of 700 to 900 ° C.

  Next, the cooling stop temperature will be described. If the cooling stop temperature exceeds 650 ° C., in the present component system, pearlite transformation starts in a high temperature range immediately after cooling, so that many pearlite structures with low hardness are generated. As a result, the hardness of the head cannot be secured, and it becomes difficult to secure the required wear resistance of the rail. When accelerated cooling is performed to less than 500 ° C., in the present component system, a large amount of bainite structure harmful to wear resistance is generated immediately after cooling is stopped. As a result, it becomes difficult to secure the required wear resistance of the rail. Therefore, the cooling stop temperature is in the range of 500 to 650 ° C.

  The type of the heat treatment refrigerant for the rail is not particularly limited. In order to impart the wear resistance to the rail, the hardness of the rail is controlled by air cooling, mist cooling, mixed cooling of water and air, or a combination thereof, thereby cooling the rail at the time of heat treatment. Is controlled as described above. If the cooling rate of the rail is within the above range, the rail after hot rolling may be naturally cooled to room temperature.In this case, the above-described cooling start temperature and cooling stop temperature are defined as follows. You can ignore it.

  Next, examples of the present invention will be described. In Example 1, the effect of the chemical composition and the manufacturing method on the configuration and characteristics of the rail was investigated, and in Example 2, the effect of manufacturing the rail of the present invention using scrap marketed as an auxiliary material was investigated. did.

Tables 1-1 to 1-4 show the chemical components and various characteristics of the rail of the present invention, and Tables 2-1 and 2-2 show the chemical components and various characteristics of the comparative rail. Table 1-1 and Table 2-2 show the chemical component values, the microstructure of the head, the hardness of the head, and the number density (Pb oxidation) of Pb oxide-based inclusions having a particle size of 1.0 to 5.0 μm. Object number density). Further, the results of the wear test and the impact test performed by the method shown in FIG. 4 are also shown in the table. In Tables 1-1 to 3-2, the samples whose structures are described as "P" indicate that the pearlite (P) amount was 95 area% or more, and were described as "P + F". The samples having a pearlite content of less than 95 area% and the remainder were proeutectoid ferrite (F) showed that the sample described as “P + B + M” had a pearlite content of less than 95 area%. Indicates that the balance was bainite (B) and martensite (M). In the sample described as “P + C”, the pearlite amount was less than 95 area%, and the balance was proeutectoid cementite ( C). In Tables 1-1 to 3-2, the samples in which the number (number / 1000 μm ² ) of Pb oxide-based inclusions having a particle size of 1.0 to 5.0 μm is described as “excess” are the above-mentioned particles. It shows that the number of Pb oxide-based inclusions in the diameter range was more than 100/1000 μm ² .

The outline of the production steps and production conditions of the present invention examples and comparative examples shown in Tables 1-1 to 2-2 are as follows.
● Overall process Refining process: Injecting scrap into pig iron as auxiliary material in a converter, blowing oxygen into molten steel, introducing lime, removing impurities such as P → Component adjustment (ladle): adding alloy, adding FeSi, etc. Control of oxygen amount by injection → Casting (bloom) → Reheating → Hot rolling → Heat treatment The auxiliary materials of the present invention and comparative examples are produced in the manufacturing industry, such as cut sheets, punched chips, cutting chips, and chips. Scrap generated at the stage (referred to as factory-generated scrap or high-grade scrap) and scrap made of Pb-plated steel sheet were used. This is because in order to investigate the effect of Pb on the characteristics of the rail, it is necessary to control the content of the alloy element with high accuracy. High-grade scrap and Pb-plated steel sheets are suitable for controlling the amount of alloying elements in the rails because the amount of alloying elements in the rails is small.

The production conditions of the present invention examples and comparative examples shown in Tables 1-1 to 2-2 are as follows.
● Hot rolling conditions Reheating temperature: 1000-1400 ° C, final rolling temperature: 750-1100 ° C
● Heat treatment (cooling) conditions after hot rolling Cooling rate: 1 to 20 ° C / sec, cooling start temperature: 700 to 900 ° C, cooling stop temperature: 500 to 650 ° C
Further, in order to control the number density of the Pb oxide-based inclusions, the components of the molten steel were adjusted under the following conditions.
● Molten steel treatment after converter refining Molten steel stirring time by inert gas: 0.5 to 4 minutes Molten steel standing time after stirring treatment: 10 to 40 minutes Note that Table 3-1 and Table 3-2 show Some detailed manufacturing conditions of the inventive rail are described. In addition, C1 to C5 were manufactured using the chemical components of the steel of the examples described in “Steel Component No.” in Table 3-1. By further optimizing the manufacturing conditions, the number density of Pb oxide-based inclusions can be controlled, and the toughness can be further improved.

  The details of the rail of the present invention example and the rail of the comparative example shown in the table are as follows.

(1) Rail of the present invention (51)
Invention Examples A1 to A51: Rails in which the chemical component values, the microstructure of the head and the hardness of the head are within the scope of the present invention.

Invention Examples A1 to A10, A12, A14, A16, A17, A19, A21, A23, A25, A27 to A32, A34, A36, A38, A40, A42, A44, A46, A48, A50: Chemical component values, head table The microstructure of the part and the hardness of the head surface are within the scope of the present invention.
Invention Examples A11, A13, A15, A18, A20, A22, A24, A26, A33, A35, A37, A39, A41, A43, A45, A47, A49, A51: Chemical component value, head surface microstructure, head A rail having a hardness of a surface portion and a number density of Pb oxide-based inclusions having a particle size of 1.0 to 5.0 μm within the range of the present invention.

(2) Comparative rails (14)
Comparative Examples B1 to B14 (14 pieces): Rails in which one or more of the chemical component value, the microstructure of the head and the hardness of the head are out of the range of the present invention.
Comparative Examples B1 to B8 (eight): Rails in which the contents of C, Si, Mn, P, and S are out of the range of the present invention.
Comparative Examples B9 to B14 (six): Rails whose Pb content is out of the range of the present invention.

[Measurement method of hardness of rail head surface]
● Measuring device: Vickers hardness tester (load 98N)
● Sampling of test specimen for measurement: Cutting out a sample from the cross section of the rail head surface section ● Pretreatment: Polishing the cross section with diamond abrasive grains having a particle size of 1 μm ● Measurement method: Calculation head for measuring hardness according to JIS Z 2244 2 mm below outer surface: hardness measurement at 20 points at a depth of 2 mm from the outer surface of the head of the cross section of the head surface shown in FIG. 2, and the average value of the hardness obtained at this position is calculated. Hardness.
20 mm below the outer surface of the head: The hardness of 20 points is measured at a position at a depth of 20 mm from the outer surface of the head in the cross section of the head surface shown in FIG. 2, and the average value of the hardness obtained by the measurement is calculated at the position. And hardness.
Pass / fail judgment of head surface hardness: When both the hardness at the position of 2 mm below the surface of the head outer surface and the hardness at the position of 20 mm below the surface of the head outer surface are within the scope of the present invention, the head surface hardness is passed. Judged.
[Method for measuring particle size and number of Pb oxide inclusions]
● Pretreatment: Diamond polishing of cross section ● Equipment: Scanning electron microscope Magnification: 1000-2000
● Pre-analysis: Conduct component analysis using an electron beam microanalyzer to identify complex inclusions of Pb and O.
● Measurement position: Observed in a visual field centered on a depth of 5 mm starting from the outer surface of the head ● Particle size range of Pb oxide inclusions to be evaluated: 1.0 to 5.0 μm
● Measurement of particle size: Only Pb oxide inclusions are selected, the area is determined, and the particle size is calculated by the diameter of a circle corresponding to the area.
● Calculation of the number: Observation was performed in 20 visual fields, the number of Pb oxide-based inclusions was counted, converted to the number per 1000 μm ² , and the average value was calculated.

[Wear test]
● Testing machine: Nishihara type abrasion testing machine (see Fig. 4)
● Test piece shape (rail material 4): Disc-shaped test piece (outer diameter: 30 mm, thickness: 8 mm)
● Specimen sampling position: Position equivalent to 2 mm below the head outer surface (head surface, see Fig. 5)
● Test load: 686N (contact surface pressure 640MPa)
● Slip ratio: 20%
● Material (wheel material 5): Pearlite steel (Hv380)
● Atmosphere: In the air ● Cooling: Forced cooling by compressed air injection from cooling air nozzle 6 (flow rate: 100 Nl / min)
● Number of repetitions: 700,000

[Impact test]
● Testing machine: Impact testing machine ● Test piece shape: JIS No.2 2mm U notch ● Test piece sampling position: Position corresponding to 2mm below the outer surface of the rail head outer surface (see Fig. 6, notch position 4mm below)
● Test temperature: room temperature (20 ℃)

  As shown in Tables 1-1 to 2-2, the rails (A1 to A51) of the present invention contained the contents of C, Si, Mn, P, and S within a limited range, and showed a proeutectoid ferrite phase. By suppressing the formation of a proeutectoid cementite phase, a bainite structure, and a martensite structure to make the head surface part a pearlite structure of 95 area% or more, the abrasion resistance and the abrasion resistance are higher than those of the comparative example rails (B1 to B8). Improved toughness.

  Furthermore, the rails (A1 to A51) of the present invention can have higher toughness than the rails (B9 to B14) of the comparative example by keeping the Pb content within a limited range.

  FIG. 7 shows the relationship between the Pb content of the rails (A27 to A32) of the present invention and the impact value. As shown in FIG. 7, in the rails (A27 to A32) of the example of the present invention, the Pb content was further controlled to 0.0015% or less, or 0.0010% or less, whereby the toughness was stably improved. .

FIG. 8 shows the relationship between the carbon content and the impact value of the rails (A10 to A51) of the example of the present invention. In the rails (A10 to A51) of the present invention, the toughness is further improved by controlling the number density of Pb oxide-based inclusions having a particle size of 1.0 to 5.0 μm to 100 or less per 1000 μm ^{2 of the} test area. Could be improved.

  The wear test is an evaluation test that reproduces an actual track. As a result of elucidating the correlation between the wear test and the actual orbit of the pearlite steel having a carbon content of 0.75 to 1.20%, when the wear amount in this test is 1.20 gf or less, the carbon content is 0.75 to 1. It has been confirmed that 20% of the rails have improved wear resistance on actual tracks. Here, the presence or absence of improvement in wear resistance was determined based on the evaluation criteria of a wear amount of 1.20 gf or less.

This impact test is an evaluation test for confirming the required toughness of the rail steel. As a result of analyzing the impact value of pearlite steel (0.75 to 1.20% C) so far, if the impact value of this test is 10.0 J / cm ² or more, it is confirmed that the toughness of the rail is secured on the actual track. Have been. Here, it was determined whether toughness was ensured based on an impact value of 10.0 J / cm ² or more as an evaluation criterion.

  In addition, as shown in Tables 3-1 and 3-2, hot rolling and heat treatment of the head were performed under certain conditions to suppress the formation of a proeutectoid cementite phase, a bainite structure, and a martensite structure, and By ensuring the hardness of the tissue, wear resistance and toughness can be improved. Furthermore, in the ladle after the converter refining process, by controlling the stirring time and the standing time of the molten steel after the stirring treatment, the number density of the carbide of Pb can be controlled, and the toughness can be further improved.

  Examples of the present invention were prepared under the same manufacturing conditions as in Example 1 using general scraps available on the market as sub-materials, and the structure and characteristics were evaluated in the same manner as in Example 1. The results are shown in Table 4-1 and Table 4-2.

  As shown in Table 4-2, a rail having excellent characteristics was obtained by controlling the Pb content within a predetermined range even when scrap distributed in the market was an auxiliary material.

1: Top of head 2: Head corner 3: Rail head 3a: Head surface (range from head corner and top surface to a depth of 20 mm, shaded area)
4: Rail material 5: Wheel material 6: Air nozzle for cooling

Claims (3)

In unit mass%,
C: 0.75 to 1.20%,
Si: 0.10 to 2.00%,
Mn: 0.10-2.00%,
P ≦ 0.0250%,
S ≦ 0.0250%,
Pb: 0.0005 to 0.0020%,
The balance consists of Fe and impurities,
95% or more of the area from the outer surface of the head to a depth of 20 mm is a pearlite structure, and
A rail having excellent wear resistance and toughness, wherein the hardness from the outer surface of the head to the depth of 20 mm is in the range of Hv 300 to 500. The number density of Pb oxide-based inclusions having a particle size of 1.0 to 5.0 μm in a cross section having a depth of 2 to 20 mm from the outer surface of the head is not more than 100 per 1000 μm ^{2 of the} test area. The rail with excellent wear resistance and toughness according to claim 1, characterized in that: Furthermore, in unit mass%,
a group: one or two of Cr: 2.00% or less and Mo: 0.50% or less;
Group b: Co: 1.00% or less,
Group c: B: 0.0050% or less,
d group: one or two of Cu: 1.00% or less, and Ni: 1.00% or less;
Group e: one or more of V: 0.50% or less, Nb: 0.0500% or less, and Ti: 0.0500% or less;
Group f: one or more of Mg: 0.0200% or less, Ca: 0.0200% or less, and REM: 0.0500% or less;
g group: Zr: 0.0200% or less,
h group: Al: 1.00% or less,
3. The rail having excellent wear resistance and toughness according to claim 1, wherein one or more groups of i group: N: 0.0200% or less are contained. 4.

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