JP4757957B2 - Perlite rail with excellent wear resistance and toughness - Google Patents

Description

The present invention relates to a pearlite rail intended to simultaneously improve wear resistance and toughness of a head in a rail used in overseas freight railroads.
This application claims priority on October 31, 2008 based on Japanese Patent Application No. 2008-281847 for which it applied to Japan, and uses the content here.

  Along with economic development, new development of natural resources such as coal is underway. Specifically, mining is being carried out in areas that have been undeveloped until now and have severe natural environments. Along with this, the track environment has become extremely severe in overseas freight railroads that transport resources. For rails, in addition to wear resistance more than ever, toughness in cold regions has been demanded. Against this background, there has been a demand for the development of a rail having wear resistance and high toughness that is higher than that of current high-strength rails.

In general, it is said that to improve the toughness of pearlite steel, it is effective to refine the pearlite structure, specifically, to refine the austenite structure before pearlite transformation and to refine the pearlite block size. 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, for pearlite transformation from austenite grains using transformation nuclei, there are problems such as difficulty in controlling the amount of transformation nuclei and instability of pearlite transformation from within grains. Could not be achieved.

  In order to drastically improve the toughness of pearlite structure rails due to these problems, a method is used in which pearlite transformation is performed by accelerated cooling and then the pearlite structure is refined by performing low-temperature reheating after rail rolling. I came. However, in recent years, the carbon of rails has been increased to improve wear resistance, and coarse carbides remain undissolved in the austenite grains during the low-temperature reheating heat treatment described above, reducing the ductility and toughness of the pearlite structure after accelerated cooling. There is a problem such as. 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 ensures 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 that the austenite grains of high-carbon steel are utilized at a relatively low temperature and easily recrystallized even with a small reduction amount in order to refine the pearlite structure. As a result, finely sized grains are obtained by continuous rolling under small pressure, and the ductility and toughness of pearlite steel are improved (see, for example, Patent Documents 1, 2, and 3).

Patent Document 1 discloses that, in the finish rolling of a steel rail containing a high carbon content, a high ductility rail can be provided by rolling for three or more consecutive passes in a predetermined time between rolling passes.
Further, in Patent Document 2, in finish rolling of a steel rail containing high carbon, by performing continuous rolling for two or more passes at a predetermined time between passes, and further performing accelerated cooling after rolling after performing continuous rolling. It discloses that a high wear resistance and high toughness rail can be provided.
Furthermore, Patent Document 3 provides a high wear resistance and high toughness rail by performing accelerated rolling after rolling after cooling between passes in finish rolling of steel rails containing high carbon steel. We disclose what we can do.

  However, in the disclosed technologies of Patent Documents 1 to 3, a certain level of austenite structure can be refined by a combination of the temperature during continuous hot rolling, the number of rolling passes and the time between passes, and a slight improvement in toughness is recognized. However, there is a problem that the effect is not recognized for the fracture starting from the inclusions present in the steel, and the toughness is not drastically improved.

  In addition, austenite grain growth is rapid in high carbon steel. For this reason, the austenite structure refined | miniaturized by rolling grows a grain after rolling, and there exists a problem that toughness does not improve in the rail after heat processing even if accelerated cooling is performed.

Therefore, in order to suppress the formation of typical inclusions in the rail, that is, MnS and Al ₂ O ₃ , the addition of Ca, the reduction of oxygen, and the reduction of Al were studied. The characteristics of these production methods are that in the hot metal pretreatment, the addition of Ca makes MnS harmless as CaS, and furthermore, the addition of deoxidizing elements and vacuum treatment are applied to reduce oxygen as much as possible. These technologies have been studied (see, for example, Patent Documents 4, 5, and 6).

  In the technique of Patent Document 4, a method for producing a high carbon silicon killed high-clean molten steel in which MnS-based elongation inclusions are reduced by means of fixing the amount of Ca as appropriate and fixing S as CaS is proposed. In this technology, segregated and concentrated S in the solidification process reacts with the segregated and concentrated Ca and calcium silicate generated in molten steel, and is sequentially fixed as CaS, thereby suppressing the formation of MnS elongation inclusions. It is to be done.

  In the technique of Patent Document 5, a method for producing a high carbon high-cleanness molten steel is proposed in which MnO inclusions are reduced and MnS elongation inclusions precipitated from MnO are reduced. In this technique, after melting in an air refining furnace, after steel is discharged in an undeoxidized or weakly deoxidized state, the dissolved oxygen is reduced to 30 ppm or less by vacuum treatment at a vacuum degree of 1 Torr or less. Next, Al and Si are added, and then Mn is added. As described above, the number of secondary deoxidation products serving as crystallization nuclei of MnS crystallized in the final solidified portion is decreased, and the MnO concentration in the oxide is decreased. Thereby, crystallization of MnS is suppressed.

  In the technique of patent document 6, the manufacturing method of the high carbon highly clean molten steel which reduced the amount of oxygen in steel, and the amount of Al is proposed. This technique can produce a rail with excellent damage resistance by limiting the total oxygen amount based on the relationship between the total oxygen value of oxide inclusions and damage. Furthermore, the damage resistance of the rail is further improved by limiting the amount of solute Al or the composition of inclusions to a preferable range.

  The disclosed techniques of Patent Documents 4 to 6 control the form and amount of MnS and Al-based inclusions generated at the steel slab stage. However, in rail rolling, the form of inclusions changes during hot rolling. In particular, Mn sulfide inclusions that have been stretched in the longitudinal direction by rolling serve as the failure start point of the rail, so that it is possible to stably improve the damage resistance and toughness of the rail only by controlling the inclusions in the steel slab stage There is a problem that can not be.

Moreover, in order to suppress the grain growth of the austenite structure after rolling, application of precipitates was studied. The characteristics of these manufacturing methods are to suppress grain growth by adding alloys, precipitating carbonitrides, and pinning the austenite structure, resulting in a refined structure after heat treatment and improved toughness. (For example, Patent Document 7).
In the technique of Patent Document 7, V and Nb are added to precipitate V and Nb carbonitrides. Furthermore, accelerated cooling according to the addition amounts of V and Nb is performed, grain growth of the austenite structure after rolling is controlled, the pearlite structure is refined, and the toughness of the rail is improved.

The disclosed technique of Patent Document 7 suppresses grain growth by adding an alloy, precipitating carbonitride, and pinning an austenite structure. However, the amount of alloy carbonitride produced varies greatly depending on the rolling temperature and the amount of reduction. As a result, large variations occur in the suppression of grain growth, resulting in partial grain coarsening, and the alloy carbonitride alone cannot stably improve the damage resistance and toughness of the rail. There's a problem.
Moreover, the disclosed technique of Patent Document 7 is only to achieve the miniaturization of the austenite structure. There is no effect on damage from Mn sulfide inclusions stretched in the longitudinal direction by rolling, and there is a problem that the damage resistance and toughness of the rail cannot be improved stably.

  Furthermore, in the disclosed technologies of Patent Documents 4 to 7, the structure is embrittled due to fluctuations in the steel components, particularly the fluctuations in the components mixed as impurities, and inclusion control and precipitates due to alloy addition and oxygen reduction. There is a problem that the damage resistance and toughness of the rail cannot be improved stably only by refining the austenite structure by the application of.

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

Japanese Unexamined Patent Publication No. 7-173530 JP 2001-234238 A JP 2002-226915 A JP-A-5-171247 Japanese Patent Laid-Open No. 5-263121 Japanese Patent Laid-Open No. 2001-220651 JP 2007-291413 A

  The present invention was created in view of the above-mentioned problems, and has an object to provide a pearlite rail that is improved in the wear resistance and toughness of the head at the same time, which is particularly required for rails of overseas freight railroads. And

The gist of the present invention is as follows.
The pearlite rail of the present invention is in mass%, C: 0.65 to 1.20%, Si: 0.05 to 2.00%, Mn: 0.05 to 2.00%, P ≦ 0.0150. %, S ≦ 0.0100%, Ca: 0.0005 to 0.0200%, and the balance is made of steel containing Fe and inevitable impurities. In the head of the rail, the head surface portion having a depth of up to 10 mm starting from the surface of the head corner portion and the top of the head is a pearlite structure, and the hardness of the pearlite structure is Hv 320 to 500. In an arbitrary cross section in the longitudinal direction in the pearlite structure (cross section parallel to the length direction of the rail), the amount of Mn sulfide inclusions having a long side of 10 to 100 μm per unit area is 10 to 200 / mm ² . Exists.
Here, Hv refers to the Vickers hardness defined in JIS B7774.
In the pearlite-based rail of the present invention, the steel contains, by mass%, one or two of Mg: 0.0005 to 0.0200% and Zr: 0.0005 to 0.0100%, 500 to 50000 Mg-based oxides, Zr oxides, and Mn sulfide-based inclusions having a particle diameter of 5 to 100 nm per unit area in an arbitrary transverse section (cross section parallel to the rail width direction) in the pearlite structure May be present in an amount of / mm ² .
The steel may further contain one or more of steel components described in the following (1) to (9) in mass%.
(1) Co: 0.01 to 1.00%
(2) Cr: 0.01-2.00%, Mo: One or two of 0.01-0.50% (3) V: 0.005-0.50%, Nb: 0.002- 1 type or 2 types of 0.050% (4) B: 0.0001 to 0.0050%
(5) Cu: 0.01 to 1.00%
(6) Ni: 0.01 to 1.00%
(7) Ti: 0.0050 to 0.0500%
(8) Al: more than 0.0100 to 1.00%
(9) N: 0.0060 to 0.0200%

  According to the present invention, the composition, structure, and hardness of rail steel are controlled, and in addition to this, P and S are reduced, Ca is added, and the number of Mn sulfide inclusions is controlled. As a result, the wear resistance and toughness of the pearlite structure can be improved, and in particular, the service life of overseas railroad rails can be improved. Furthermore, by adding Mg and Zr and controlling the number of fine Mn sulfide inclusions and Mg and Zr oxides, the toughness of the pearlite structure can be further improved, and the service life can be further improved. it can.

It is a figure which shows the name in the cross section (cross section perpendicular | vertical with respect to a longitudinal direction) of this invention rail steel. Using steel with a carbon content of 1.00% with a P content of 0.0150% or less, varying the S content, and further melting steel with Ca, Mg, and Zr added to simulate rolling conditions equivalent to rails It is the result of having carried out the laboratory melting rolling experiment which was done, and the result of having done the impact test, and is a figure which shows the influence of Ca addition, Mg, Zr addition in the relationship between S amount and an impact value. It is a figure which shows the observation position of the Mn sulfide type inclusion of rail steel in Claim 1. It is a figure which shows the observation position of the Mn sulfide type inclusion, Mg type oxide, and Zr oxide of rail steel in Claim 2. It is a figure which shows the sampling position of the test piece in an abrasion test. It is a figure which shows the outline | summary of an abrasion test. It is a figure which shows extraction | collection of the test piece in an impact test. It is a figure which shows the relationship between the amount of carbon in the abrasion test result of this invention rail steel, and comparison rail steel (steel No. 48, 50, 51, 52, 53, 64, 66, 67) and the amount of wear. It is a figure which shows the relationship between the carbon content and the impact value in the impact test result of this invention rail steel and comparative rail steel (steel No. 49, 51, 53, 65, 66, 68). It is a figure which shows the relationship between the carbon amount and the impact value in the impact test result of this invention rail steel and comparative rail steel (Steel No. 54-63, the addition amount of P, S, and Ca is outside the limited range of this invention). is there. The relationship between the carbon content and the impact value in the impact test results of the rail steels of the present invention (steel Nos. 11 to 13, 18 to 20, 24 to 26, 29 to 31, 33 to 35, 36 to 38, 45 to 47) is shown. FIG.

Hereinafter, a pearlite rail excellent in wear resistance and toughness will be described in detail as an embodiment of the present invention. In addition, the containing unit in an alloy element is the mass%, and is only described as% below.
FIG. 1 shows a cross section perpendicular to the longitudinal direction of a pearlitic rail excellent in wear resistance and toughness of the present invention. The rail head portion 3 includes a top portion 1 and head corner portions 2 located at both ends of the top portion 1. One of the head corner portions 2 is a gauge corner (GC) portion that mainly contacts the wheel.
A range from the surface of the head corner portion 2 and the top of the head 1 to a depth of 10 mm is referred to as a head surface portion 3a (solid hatched portion). Further, a range up to a depth of 20 mm starting from the surfaces of the head corner portion 2 and the top portion 1 is indicated by reference numeral 3b (dotted line hatched portion).

First, the present inventors examined a steel component system that adversely affects the toughness of the rail. Using a steel with a changed carbon content and a steel with a changed content of P and S, we conducted test melting and hot rolling experiments simulating hot rolling conditions equivalent to rails, and prototyped rails Was made. And the impact value of the prototype was measured by the impact test, and the influence of the P and S contents on the impact value was examined.
As a result, it was confirmed that the impact value of the steel having a pearlite structure of Hv 320 to 500 level is improved when the P and S contents are both reduced to a certain level or less.
Furthermore, as a result of examining the optimum content by the combination of P and S, it was confirmed that the impact value was greatly improved by reducing the amount of addition of any element to a certain level at the same time.

Next, in order to further improve the impact value of the rail, the inventors proceeded to elucidate the factors governing the impact value. As a result, in the rail having a low impact value, it was confirmed that a large number of Mn sulfide inclusions extending in the longitudinal direction by hot rolling existed and became the starting point of fracture.
Therefore, the present inventors have elucidated the generation mechanism of Mn sulfide inclusions extending in the longitudinal direction. In rail manufacture, the steel slab is once reheated to 1200 to 1300 ° C. and hot rolled. The relationship between the hot rolling conditions and the form of MnS was investigated. As a result, it was confirmed that the soft Mn sulfide inclusions easily cause plastic deformation and easily extend in the rail longitudinal direction when the rolling temperature is high and the rolling amount during rolling is large.

Therefore, the present inventors examined a method for suppressing the stretching of the Mn sulfide-based inclusion itself. As a result of test melting and hot rolling experiments, it was confirmed that Mn sulfide inclusions were produced with various oxides as nuclei. Furthermore, as a result of investigating the hardness of the oxide and the morphology of the Mn sulfide inclusions, it was confirmed that this stretching can be suppressed by hardening the inclusions that are the core of the Mn sulfide inclusions.
Furthermore, the present inventors examined a hard inclusion which becomes a nucleus of the Mn sulfide-based inclusion. As a result of laboratory experiments using oxides with a high melting point, it was discovered that Ca, which has a relatively high melting point, forms sulfides in addition to oxides to form an aggregate of CaO—CaS. In addition, since CaS is highly compatible with Mn sulfide inclusions, it is confirmed that Mn sulfide inclusions are efficiently generated in the Ca oxide and sulfide aggregate (CaO-CaS). I found it.
Note that the term “consistency” refers to a difference in lattice constant (distance between atoms) on a crystal plane in the crystal structure of two metals. It is considered that the smaller this difference is, the higher the consistency is, that is, the two metals are easily bonded.

Next, in order to perform the above verification, the present inventors conducted test melting and hot rolling experiments using steel added with Ca. As a result, the Mn sulfide inclusions produced by using the Ca oxide and sulfide aggregate (CaO-CaS) as the core hardly stretched after hot rolling, and consequently stretched in the longitudinal direction. It was confirmed that Mn sulfide inclusions were reduced.
Furthermore, as a result of performing an impact test using this steel, in the steel in which Ca is added and there are few stretched Mn sulfide inclusions, the fracture from the stretched Mn sulfide inclusions is reduced, and the impact value is reduced. It was confirmed to improve.

  In addition, the present inventors also tested the relationship between the addition amount of Ca and the addition amount of S in which oxide and sulfide form an aggregate in order to further suppress the stretching of Mn sulfide-based inclusions, and test melting and hot rolling It examined by experiment. As a result, by controlling the ratio of the addition amount of S and Ca, Ca sulfide is moderately generated and finely dispersed, thereby further extending the elongation of Mn sulfide inclusions after rolling. It was confirmed that it could be suppressed.

Furthermore, in addition to the suppression of stretched Mn sulfide inclusions that adversely affect toughness, the present inventors utilize Mn sulfide inclusions and oxides to increase the grain growth of the austenite structure after hot rolling. The suppression method was examined. As a result of test melting and hot rolling experiments, in order to stably suppress the grain growth of the austenite structure, a nano-sized oxide or Mn sulfide system can be used as a pinning element instead of the conventional alloys. It was found that it was necessary to finely disperse inclusions.
Therefore, the present inventors examined a method for finely dispersing oxides and Mn sulfide inclusions. As a result, it was confirmed that the oxides of Mg and Zr do not aggregate and are finely and uniformly dispersed. Furthermore, both Mg-based oxides and Zr oxides have good compatibility with Mn sulfide inclusions, and Mn sulfide inclusions are also finely dispersed with this fine oxide as a core. Was confirmed.

  Next, the inventors conducted a hot rolling experiment using steel to which Mg and Zr were added. As a result, it was confirmed that nano-sized oxides and Mn sulfide inclusions were finely dispersed, and the grain growth of the austenite structure after rolling could be suppressed. Furthermore, as a result of an impact test using this steel, it was confirmed that the impact value was improved by refinement of the pearlite structure in the steel to which Mg and Zr were added.

  The present inventors changed the amount of S by using a steel having a carbon content of 1.00% with a P content of 0.0150% or less, and further added Ca, Mg, and Zr to prepare an experimental steel. Test dissolution. Next, a laboratory rolling experiment simulating the hot rolling conditions equivalent to the rail was performed, and a prototype of the rail was produced. Then, the impact value of the prototype was measured by an impact test, and the influence of the amount of S and the addition of Ca, Mg, and Zr on the impact value was investigated. The hardness of the material was adjusted to the Hv400 level by controlling heat treatment conditions.

  FIG. 2 shows the relationship between the amount of S (ppm) and the impact value. It was confirmed that the impact value is improved when the S content is reduced to 0.0100% or less when the P content is 0.0150% or less in the steel (● mark) with the C content of 1.00%. Moreover, it was confirmed from the result of the steel (■ mark) to which Ca was added that by adding Ca, the stretched Mn sulfide inclusions were controlled and the impact value was improved. Furthermore, from the result of steel added with Ca, Mg, Zr (Δ mark), by adding Mg and Zr in addition to Ca, nano-sized oxides and Mn sulfide inclusions are finely dispersed. As a result, it was confirmed that the impact value was remarkably improved.

Based on the above research results, the present invention having the above-described constituent elements has been completed. The components of the present invention will be described below.
(1) Reasons for limiting chemical components of steel In the pearlite rail of the present invention, the reasons for limiting the chemical components of steel to the above-described numerical ranges will be described in detail.
C is an effective element that promotes pearlite transformation and ensures wear resistance. When the amount of C is less than 0.65%, this component system cannot maintain the minimum strength and wear resistance required for the rail. On the other hand, when the C content exceeds 1.20%, a large amount of coarse pro-eutectoid cementite structure is generated, and wear resistance and toughness are lowered. For this reason, C addition amount is limited to 0.65-1.20%. In order to secure sufficient wear resistance, the C content is preferably 0.90% or more.

  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 toughness. However, when the Si content is less than 0.05%, these effects cannot be expected sufficiently. On the other hand, if the amount of Si exceeds 2.00%, a lot of surface defects are generated during hot rolling, and weldability deteriorates due to generation of oxides. Furthermore, the hardenability is remarkably increased, and a martensite structure that is harmful to the wear resistance and toughness of the rail is generated. For this reason, Si addition amount is limited to 0.05 to 2.00%. In order to secure hardenability and to suppress the formation of martensite structure that is harmful to wear resistance and toughness, the Si content is preferably 0.20 to 1.30%.

  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. On the other hand, if the amount of Mn exceeds 2.00%, the hardenability is remarkably increased, and a martensite structure that is harmful to wear resistance and toughness is easily generated. For this reason, Mn addition amount is limited to 0.05 to 2.00%. In order to secure hardenability and suppress the formation of martensite structure that is harmful to wear resistance and toughness, the Mn content is desirably 0.40 to 1.30%.

P is an element inevitably contained in steel. There is a correlation between the amount of P and toughness. When the amount of P increases, the pearlite structure becomes brittle due to embrittlement of the ferrite phase, and brittle fracture, that is, rail damage is likely to occur. For this reason, in order to improve toughness, it is desirable that the amount of P is low. As a result of confirming the correlation between the impact value and the P amount in the laboratory, when the P amount is reduced to 0.0150% or less, the segregation of P is remarkably reduced, and the embrittlement of the pearlite structure which is the starting point of fracture is suppressed. It was confirmed that the impact value was greatly improved. From this result, the amount of P is limited to 0.0150% or less. The lower limit of the amount of P is not limited, but considering the dephosphorization ability in the refining process, it is considered that the amount of P is about 0.0020% which is the limit in actual production.
Note that the process of reducing P (reducing the amount of P) not only increases the refining cost but also deteriorates productivity. Therefore, in view of economic efficiency, it is desirable that the P content be 0.0030 to 0.0100% in order to stably improve the impact value.

S is an element inevitably contained in steel. There is a correlation between the amount of S and toughness, and when the amount of S increases, stress concentration occurs due to coarsening of MnS and increase in density, and brittle fracture, that is, rail damage is likely to occur. For this reason, in order to improve toughness, it is desirable that the amount of S is low. As a result of confirming the correlation between the impact value and the amount of S in the laboratory, when the amount of S is reduced to 0.0100% or less, the amount of Mn sulfide inclusions that are the starting point of fracture is reduced. It was confirmed that the embrittlement of the pearlite structure is suppressed and the impact value is greatly improved by suppressing the stretching and refinement of the Mn sulfide inclusions by adding Zr and Mg. From this result, the amount of S is limited to 0.0100% or less. The lower limit of the amount of S is not limited, but considering the desulfurization capability in the refining process, the amount of S of about 0.0010% is considered to be the limit for actual production.
Note that the process of reducing S (reducing the amount of S) not only increases the refining cost but also deteriorates productivity. Therefore, in view of economic efficiency, in order to suppress the generation of stretched Mn sulfide inclusions and to stably improve the impact value, it is desirable that the S amount is 0.0060 or less.
In order to further improve the impact value, in order to stably generate fine Mn sulfide inclusions that pin the austenite structure and to suppress the formation of stretched Mn sulfide inclusions, the amount of S is reduced to 0. 0020-0.0035% is desirable.

Ca is a deoxidation / desulfurization element. When Ca is added, an oxide and sulfide of Ca generate an aggregate (CaO—CaS). This aggregate serves as a production nucleus of Mn sulfide inclusions, and suppresses stretching of the Mn sulfide inclusions after rolling. Furthermore, nano-sized Mn sulfide inclusions are formed using this as a nucleus. Ca is an element having such an effect. If the amount of Ca is less than 0.0005%, the effect is small, and it is insufficient as a production nucleus of Mn sulfide inclusions. On the other hand, if the Ca content exceeds 0.0200%, depending on the oxygen content in the steel, the number of single hard CaO that does not become the core of Mn sulfide inclusions increases. As a result, the toughness of the rail steel is greatly reduced. For this reason, Ca addition amount is limited to 0.0005 to 0.0200%.
In order to improve the impact value by reliably suppressing the formation of stretched Mn sulfide inclusions and suppressing the formation of hard CaO that is harmful to toughness and not the core of Mn sulfide inclusions. The Ca addition amount is desirably in the range of 0.0015 to 0.0150%. Further, in order to further improve the impact value, it is necessary to stably generate fine Mn sulfide inclusions that pin the austenite structure, and to suppress the coarsening of the Mn sulfide inclusions. It is desirable to set it as 0.0020 to 0.0080%.

As described above, S and Ca generate an aggregate of oxide and sulfide (CaO—CaS). This aggregate serves as a nucleus of Mn sulfide-based inclusions and greatly affects the stretching of Mn sulfide-based inclusions. Therefore, control of the S addition amount and the Ca addition amount is important. Therefore, hot rolling experiments were carried out by melting the test steel with varying amounts of S and Ca. As a result, when the ratio of the Ca addition amount to the S addition amount (S / Ca) is within a specific range, Ca oxides and sulfides are appropriately generated and finely dispersed. Further, it was found that stretching of Mn sulfide inclusions after rolling can be further suppressed.
Specifically, when the value of S / Ca is less than 0.45, the number of single hard CaOs that do not become nuclei of Mn sulfide inclusions slightly increases. As a result, the toughness of the rail steel may be reduced. Moreover, when the value of S / Ca exceeds 3.00, the number of sulfide aggregates (CaO—CaS) serving as nuclei of the Mn sulfide inclusions is reduced, and the Mn sulfide inclusions are stretched. Be encouraged. As a result, the toughness of the rail steel may be reduced. For this reason, it is more desirable to make the value of S / Ca into the range of 0.45-3.00.

In this invention, it is preferable to contain 1 type or 2 types of Mg and Zr.
Mg is a deoxidizing element and is an element that mainly combines with O to form a complex with a fine nano-sized oxide (MgO) or sulfide (MgS). Using this composite as a nucleus, nano-sized Mn sulfide inclusions are formed. As a result, the grain growth of the austenite structure after rolling is suppressed, the structure of the rail steel is refined, and the toughness of the pearlite structure can be improved. However, if the amount of Mg is less than 0.0005%, the amount of fine oxide (MgO) and sulfide (MgS) composites produced is small, and the effect of suppressing the grain growth of the austenite structure after rolling cannot be sufficiently obtained. . When the amount of Mg exceeds 0.0200%, a coarse oxide of Mg is generated, and the toughness of the rail is lowered. At the same time, fatigue damage occurs from the coarse oxide. For this reason, Mg addition amount is limited to 0.0005 to 0.0200%.
A sufficient amount of fine oxides (MgO) for pinning the austenite structure and oxides (MgO) and sulfides (MgS) forming nano-sized Mn sulfide inclusions are secured. In order to sufficiently suppress the generation of coarse oxides that are harmful to fatigue damage and improve the impact value, it is desirable that the Mg addition amount be in the range of 0.0010 to 0.0050%.

Zr is a deoxidizing element, and is an element that mainly combines with O to form a fine nano-sized oxide (ZrO ₂ ). This oxide is finely and uniformly dispersed, and further nano-sized Mn sulfide inclusions are formed using this oxide as a nucleus. As a result, the grain growth of the austenite structure after rolling is suppressed, the structure of the rail steel is refined, and the toughness of the pearlite structure can be improved. However, if the amount of Zr is less than 0.0005%, the amount of fine oxide (ZrO ₂ ) produced is small, and the effect of suppressing the grain growth of the austenite structure after rolling cannot be sufficiently obtained. If the amount of Zr exceeds 0.0100%, a coarse oxide of Zr is generated, and the toughness of the rail is lowered, and at the same time, fatigue damage occurs from the coarse precipitate. For this reason, the amount of Zr added is limited to 0.0005 to 0.0100%.
Incidentally, fine oxides pinning austenitic structure (ZrO _2), oxide forming the Mn sulfide-based inclusions nanosize (ZrO ₂₎ securing a sufficient amount of generated and harmful coarse to fatigue damage In order to sufficiently suppress the formation of an oxide and improve the impact value, it is desirable that the amount of Mg added is in the range of 0.0010 to 0.0050%.

  In addition, the rail manufactured with the above composition has improved hardness (strengthening) of pearlite structure and pro-eutectoid ferrite structure, improved toughness, prevention of softening of weld heat affected zone, and cross-sectional hardness distribution inside rail head. One or more elements selected from Co, Cr, Mo, V, Nb, B, Cu, Ni, Ti, Al, and N are added as necessary for the purpose of controlling Is preferred.

The main addition purpose and action effect of the above elements are shown below.
Co refines the lamellar structure and ferrite grain size of the wear surface and improves the wear resistance of the pearlite structure. Cr and Mo increase the pearlite equilibrium transformation point, and ensure the hardness of the pearlite structure mainly by reducing the pearlite lamella spacing. V and Nb generate carbides and nitrides during hot rolling and the subsequent cooling process, thereby suppressing the growth of austenite grains. Furthermore, the toughness and hardness of the pearlite structure are improved by precipitation hardening in the ferrite structure and pearlite structure. 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 dependency 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 weld heat affected zone and prevents embrittlement of the weld joint. Al moves the eutectoid transformation temperature to the high temperature side and increases the hardness of the pearlite structure. N promotes pearlite transformation by segregating at austenite grain boundaries. And toughness is improved by refining the pearlite block size.

The reasons for limiting these components will be described in detail below.
Co is dissolved in the ferrite phase in the pearlite structure. As a result, the fine ferrite structure formed by contact with the wheels on the wear surface of the rail head is further refined to improve wear resistance. If the Co content is less than 0.01%, the ferrite structure cannot be refined, and the effect of improving the wear resistance cannot be expected. Even if the Co content exceeds 1.00%, the above effect is saturated, and the ferrite structure cannot be refined according to the addition amount. In addition, the economic efficiency decreases due to the increase in the alloy addition cost. For this reason, Co addition amount is limited to 0.01 to 1.00%.

  Cr increases the equilibrium transformation temperature and, as a result, refines the ferrite structure and pearlite structure and contributes to higher hardness (strength). At the same time, the cementite phase is strengthened to improve the hardness (strength) of the pearlite structure. However, if the Cr content 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 an excessive amount of Cr exceeding 2.00% is added, the hardenability is increased and a martensite structure is generated. As a result, a sprig damage starting from the martensite structure occurs at the head corner and the top, and the surface damage resistance is reduced. For this reason, Cr addition amount is limited to 0.01 to 2.00%.

  Mo, like Cr, increases the equilibrium transformation temperature, and as a result, contributes to higher hardness (strength) by making the ferrite structure and pearlite structure finer. Thus, Mo is an element that improves the hardness (strength), but if the amount of Mo 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. Moreover, if the Mo amount exceeds 0.50% and excessive Mo is added, the transformation rate is significantly reduced. As a result, the sprig damage starting from the martensite structure occurs at the head corner and the top, and the surface damage resistance is reduced. For this reason, Mo addition amount is limited to 0.01 to 0.50%.

  V refines austenite grains due to the pinning effect of V carbide and V nitride when heat treatment is performed at a high temperature. Furthermore, precipitation hardening by V carbide and V nitride generated in the cooling process after hot rolling increases the hardness (strength) of the ferrite structure and pearlite structure and at the same time improves the toughness. V is an element effective for obtaining such an effect. In the heat-affected zone reheated to a temperature range below the Ac1 point, V is 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. Element. However, if the amount of V is less than 0.005%, the effect cannot be sufficiently expected, and no improvement in the hardness or toughness of the ferrite structure or pearlite structure is observed. On the other hand, if the V content exceeds 0.50%, precipitation hardening of V carbides and nitrides becomes excessive, and the toughness of the ferrite structure and pearlite structure decreases. As a result, the sprig damage occurs at the head corner and the top, and the surface damage resistance is reduced. For this reason, V addition amount is limited to 0.005 to 0.50%.

  Nb, like V, refines austenite grains by the pinning effect of Nb carbide or Nb nitride when heat treatment is performed at a high temperature. Furthermore, the hardness (strength) of the ferrite structure and the pearlite structure is increased and the toughness is improved by precipitation hardening with Nb carbide and Nb nitride generated in the cooling process after hot rolling. Nb is an effective element for obtaining such an effect. 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 the weld joint heat-affected zone It is an effective element for preventing softening. However, if the amount of Nb is less than 0.002%, the effect cannot be expected, and no improvement in the hardness or toughness of the ferrite structure or pearlite structure is observed. On the other hand, if the Nb content exceeds 0.050%, precipitation hardening of Nb carbides and nitrides becomes excessive, and the toughness of the ferrite structure and pearlite structure decreases. As a result, the sprig damage occurs at the head corner and the top, and the surface damage resistance is reduced. For this reason, the Nb addition amount is limited to 0.002 to 0.050%.

B forms iron boride (Fe ₂₃ (CB) ₆ ) at the austenite grain boundaries and promotes pearlite transformation. Due to this pearlite transformation promoting effect, the cooling rate dependency of the pearlite transformation temperature is reduced, and a more uniform hardness distribution can be obtained from the head surface to the inside of the rail. For this reason, the life of the rail can be extended. 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 generated, resulting in a decrease in toughness. For this reason, B addition amount is limited to 0.0001 to 0.0050%.

  Cu is an element that improves the hardness (strength) of the pearlite structure by forming a solid solution in the ferrite phase in the ferrite structure or the pearlite structure and strengthening the solid solution. If the amount of Cu is less than 0.01%, the effect cannot be expected. On the other hand, if the Cu content exceeds 1.00%, a martensitic structure that is harmful to toughness is generated due to a significant improvement in hardenability. As a result, the sprig damage occurs at the head corner and the top, and the surface damage resistance is reduced. For this reason, the amount of Cu is 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 Ni ₃ Ti that is a composite compound with Ti is finely precipitated, and softening is suppressed by precipitation strengthening. When the amount of Ni is less than 0.01%, the effect is remarkably small. When the amount of Ni exceeds 1.00%, the toughness of the ferrite structure and pearlite structure is significantly reduced. As a result, the sprig damage occurs at the head corner and the top, and the surface damage resistance is reduced. For this reason, Ni addition amount is 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, when the Ti content is less than 0.0050%, the effect is small, and when the Ti content exceeds 0.0500%, coarse Ti carbides or Ti nitrides are generated, and the toughness of the rail is lowered. At the same time, fatigue damage occurs from coarse precipitates. For this reason, Ti addition amount is limited to 0.0050 to 0.050%.

  Al is an essential component as a deoxidizing material. Moreover, it is an element that moves the eutectoid transformation temperature to the high temperature side, and is an element that contributes to increasing the hardness (strength) of the pearlite structure. If the Al content is 0.0100% or less, the effect is weak. On the other hand, if the Al content exceeds 1.00%, it is difficult to make a solid solution in the steel, and coarse alumina inclusions are generated. As a result, the toughness of the rail is reduced, and at the same time, fatigue damage is generated from coarse precipitates. Furthermore, since oxides are generated during welding and weldability is significantly reduced, the amount of Al added is limited to more than 0.0100 to 1.00%.

  N promotes ferrite and pearlite transformation from the austenite grain boundary by segregating to the austenite grain boundary. Thereby, toughness can be improved 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 is limited to 0.0060-0.0200%.

(2) Reason for limiting the pearlite structure region and hardness of the rail head surface portion 3a Next, the reason why the rail head surface portion 3a is a pearlite structure and its hardness is limited to a range of Hv320 to 500. explain.
First, the reason for limiting the hardness of the pearlite structure to the range of Hv320 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 head surface portion 3a of the rail, 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 surface portion 3a is greatly reduced. Further, when the hardness of the pearlite structure exceeds Hv500, the toughness of the pearlite structure is remarkably lowered, and the damage resistance of the rail head surface portion 3a is lowered. For this reason, the hardness of a pearlite structure | tissue is 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 the head surface portion 3a of the rail steel will be described.
Here, the head surface portion 3a of the rail indicates a range (solid hatched portion) up to a depth of 10 mm starting from the surfaces of the head corner portion 2 and the top portion 1 as shown in FIG. If the pearlite structure of the above component range is arranged at this site, wear due to contact with the wheel can be suppressed, and the wear resistance of the rail can be improved.

Moreover, the pearlite structure | tissue of hardness Hv320-500 is arrange | positioned in the range 3b to the depth of 20 mm from the surface of the head corner part 2 and the head top part 1, ie, at least in the shaded part of the dotted line in FIG. It is preferable that the wear resistance in the case of further wear to the inside of the rail head due to contact with the wheel is further ensured, 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 surface of the rail head 3 where the wheel and the rail mainly contact each other, and the other part may be a metal structure other than the pearlite structure.
In addition, as a method for obtaining a pearlite structure having a hardness of Hv 320 to 500 in the vicinity of the surface of the rail head 3, as described later, a high temperature rail head 3 having an austenite region after rolling or after reheating is used. It is desirable to perform accelerated cooling.

Of the rail head portion 3 in the present invention, the above-described head surface portion 3a or the metal structure in the range 3b up to a depth of 20 mm including the head surface portion 3a is preferably composed only of the 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 with an area ratio of 5% or less may be mixed in the pearlite structure. However, even if these structures are mixed in a content of 5% or less, the wear resistance and toughness of the rail head 5 are not greatly adversely affected. Also included are those in which a structure, a pro-eutectoid cementite structure, a bainite structure, a martensite structure and the like are mixed at a content of 5% or less.
In other words, in the rail head portion 5 in the present invention, the above-described head surface portion 3a or the metal structure in the range 3b including the head surface portion 3a up to a depth of 20 mm may be 95% or more if it is a pearlite structure. In order to sufficiently secure wear resistance and toughness, it is desirable that 98% or more of the head metal structure is a pearlite structure.
In the column of the microstructure in Table 1 and Table 2 to be described later, a content of 5% or less means a content of 5% or less, and a content other than a pearlite structure does not describe a trace amount of more than 5%. Means (outside of the present invention).

(3) Reason for limiting the number per unit area of Mn sulfide inclusions having a long side of 10 to 100 μm In the present invention, the length of the long side of the Mn sulfide inclusion having an arbitrary cross-section in the longitudinal direction as an evaluation target The reason why the thickness is limited to the range of 10 to 100 μm will be described in detail.
In this component system, as a result of investigating the length of the long side of Mn sulfide inclusions and the actual track damage (damage when using the actual rail), the failure of the rail is caused by stress concentration. It was confirmed that it was generated from the end of the sulfide inclusion. Therefore, as a result of test melting of steel with varying lengths of Mn sulfide inclusions and conducting hot rolling experiments, the number of Mn sulfide inclusions having a long side length of 10 to 100 μm and rail damage resistance Was confirmed to have a good correlation. Therefore, the evaluation object of the number of Mn sulfide inclusions is limited to the long side length of 10 to 100 μm.

  It should be noted that the effect of Mn sulfide inclusions with a long long side with a significant stress concentration on the damage resistance is larger, and the effect of Mn sulfide inclusions with a short long side is shorter. The impact is small. However, in the steel of the present invention, there are few Mn sulfide inclusions exceeding 100 μm in length, which is inappropriate for grasping the characteristics of the steel, and that Mn sulfide inclusions less than 10 μm in length are Since there is little influence on the damage property, Mn sulfide inclusions having the indicated size were evaluated.

Next, in the present invention, in an arbitrary cross section in the longitudinal direction (cross section parallel to the length direction of the rail), the number of Mn sulfide inclusions having a long side of 10 to 100 μm per unit area is 10 to 200 / mm. ^The reason for limiting to ² will be described in detail.
If the total number of Mn sulfide inclusions with a long side of 10 to 100 μm exceeds 200 / mm ² per unit area, the number of Mn sulfide inclusions will be excessive in this component system, and stress concentration will occur around the inclusions. This increases the possibility of rail damage. The impact value cannot be improved even in the mechanical properties of steel. In addition, when the total number of Mn sulfide inclusions having a long side of 10 to 100 μm is less than 10 / mm ² per unit area, trap sites that adsorb inevitable hydrogen remaining in steel in this component system. Is significantly reduced. This increases the possibility of inducing hydrogen defects (hydrogen embrittlement), which may impair the damage resistance of the rail. Therefore, the total number of Mn sulfide inclusions having a long side length of 10 to 100 μm is limited to 10 to 200 / mm ² per unit area.
In addition, in this limitation, Mn sulfide inclusions are Mn sulfide inclusions produced by using an aggregate of Ca oxide and sulfide (CaO-CaS) as a nucleus, and other Mn sulfide inclusions. Both interventions are evaluated.

As for the number of Mn sulfide inclusions, a sample is cut out from the longitudinal section of the rail head 3 where rail damage is apparent as shown in FIG. 3, and the sulfide inclusions are measured. The rail longitudinal section of each sample cut out is mirror-polished, and Mn sulfide inclusions are examined with an optical microscope in an arbitrary section. Then, the number of inclusions of the limited size is counted, and this is calculated as the number per unit section. The representative value of each rail steel was the average value of the number per unit cross section of these 20 visual fields. Although the measurement site | part of Mn sulfide type inclusion is not specifically limited, It is desirable to measure the range of 3-10 mm in depth from the surface of the rail head 5 used as the starting point of damage.
Further, in order to further reduce the influence of Mn sulfide inclusions that are the starting point of fracture, to suppress hydrogen defects, and to stably improve the breakage resistance of the rail, the long side is 10 to 100 μm. It is desirable to control the total number of Mn sulfide inclusions in a range of ^{20 to} 180 / mm ² per unit area.

(4) Reasons for limiting the number per unit area of Mg-based oxides, Zr oxides, and Mn sulfide-based inclusions having a particle size of 5 to 100 nm In the present invention, in any cross section, the particle size is 5 to 100 nm. The Mg-based oxide, Zr oxide, and Mn sulfide-based inclusion are preferably present in an amount of 500 to 50000 pieces / mm ² per unit area.
The reason why the particle size of the Mg-based oxide, Zr oxide and Mn sulfide-based inclusions to be evaluated is limited to the range of 5 to 100 nm will be described in detail.
If the particle size of the Mg-based oxide, Zr oxide, and Mn sulfide-based inclusion is in the range of 5 to 100 nm, a sufficient pinning effect is exhibited at the grain boundary when formed in the austenite structure. As a result, it was confirmed that the pearlite structure was refined and the toughness was improved reliably without adversely affecting the damage resistance of the rail. Therefore, the evaluation targets of Mg-based oxides, Zr oxides, and Mn sulfide-based inclusions are limited to the range of 5 to 100 nm in particle size.
The pinning effect is more effective as many inclusions having a fine particle size exist. However, for Mg-based oxides, Zr oxides, and Mn sulfide-based inclusions having a particle size of less than 5 nm, Measurement is very difficult. In addition, as described above, the pinning effect cannot be obtained for Mg-based oxides, Zr oxides, and Mn sulfide-based inclusions having a particle size exceeding 100 nm. As described above, Mg-based oxides, Zr oxides, and Mn sulfide-based inclusions with the above sizes are evaluated.

Next, in a preferred embodiment, the amount (number) per unit mm ² of Mg-based oxide, Zr oxide and Mn sulfide-based inclusions having a particle size of 5 to 100 nm in an arbitrary cross section in the longitudinal direction is set to 500 to 50,000. The reason for limiting will be described in detail.
When the total number of Mg-based oxides, Zr oxides, and Mn sulfide-based inclusions having a particle diameter of 5 to 100 nm is less than 500 / mm ² per unit area, the pinning effect in the austenite structure after rolling is sufficient. It does not appear. For this reason, a pearlite structure coarsens and the toughness of a rail does not improve. In addition, when the total number of Mg-based oxides, Zr oxides, and Mn sulfide-based inclusions having a particle size of 5 to 100 nm exceeds 50000 / mm ² per unit area, precipitation becomes excessive and the pearlite structure itself is brittle. And the toughness of the rail decreases. Therefore, the total number of Mg-based oxides, Zr oxides, and Mn sulfide-based inclusions having a particle size of 5 to 100 nm is limited to 500 to 50000 pieces / mm ² per unit area.

In this limitation, Mg-based oxides and Zr oxides partially include composite oxides such as Mn sulfides, and Mn sulfide-based inclusions include Mg oxides and Zr oxides. A fine oxide such as an oxide or Ca oxide is generated as a nucleus.
The Mg-based oxide, Zr oxide, and Mn sulfide-based inclusion are observed as follows, and the particle size and number are measured. First, a thin film is collected from an arbitrary cross section shown in FIG. 4 and observed at a magnification of 50000 to 500000 using a transmission electron microscope. The particle size of the precipitate is obtained as a diameter of a circle corresponding to the area of each precipitate by observation.
The precipitates are observed in 20 fields of view, the number of precipitates corresponding to a predetermined diameter of 5 to 100 nm is counted, and this is converted into the number per unit area. The representative value of rail steel is the average value of these 20 fields of view. In addition, although the measurement site | part of Mg type oxide, Zr oxide, and Mn sulfide type inclusion is not specifically limited, the range of 3-10 mm in depth from the surface of the rail head surface part 3a in which toughness is required is measured. It is desirable to do.

(5) Manufacturing method of rail steel of this invention Although the rail steel of this invention which has said component composition and microstructure is not specifically limited, Usually, it manufactures with the following method. First, smelting is performed in a commonly used melting furnace such as a converter or an electric furnace to obtain molten steel. And using this molten steel, the steel slab for rolling (steel ingot) is manufactured by the ingot-making / splitting method or the continuous casting method. Further, after reheating the steel slab to 1200 ° C. or higher, several passes of hot rolling are performed to form a rail. Then, a rail is manufactured by performing heat processing (reheating, cooling).
In particular, in the hot metal stage, normal de-S and de-P are performed (de-P, de-S treatment), and sufficient de-S and de-P are performed in a commonly used melting furnace such as a converter and an electric furnace ( (De-P, De-S treatment). Next, the Mn sulfide inclusions are controlled by adding Ca. Furthermore, nano-sized oxides and Mn sulfide inclusions are finely dispersed by adding Mg and Zr as necessary.
Details of the manufacturing conditions are shown below.

In the hot metal stage, it is desirable to carefully carry out normal de-P treatment and de-S treatment in order to achieve low P and low S.
About de-S, it is desirable to spend time in the hot metal ladle (the pre-stage of converter refining), add CaO sufficiently, and discharge CaS as slag.
In addition, the addition of CaO here is a method performed when reducing S from hot metal with a very high amount of S. Unlike the addition of a CaO—Si alloy that is added to generate an aggregate of Ca oxide and sulfide (CaO—CaS), which will be described later, there is no influence.
As for de-P, in order to prevent remelting of P from slag containing P (such as P ₂ O ₅ ) separated by de-P during refining in a converter, it is desirable to discharge slag during refining.

Next, the Mn sulfide inclusions are controlled by adding Ca.
It is desirable to add Ca in the refining process before casting. As a method of adding Ca, it is desirable to add a Ca alloy (Ca—Si alloy or the like) wire or a Ca alloy lump with a ladle or to blow Ca alloy powder.
As the Ca alloy, a Ca—Si alloy (such as 50Ca-50Si), a Fe—Si—Ca alloy (such as Fe-30Si-30Ca), and a Ni—Ca alloy (such as 90Ni-10Ca) are used. Since Ca has a high vapor pressure, when pure Ca is added, splash of molten steel is generated, or slag on the molten metal surface is involved and the molten steel cleanliness deteriorates. Also, the yield is low. Then, adding Ca alloy, for example, Ca-Si alloy, is widely performed. By alloying, the activity of Ca is lower than in the case of Ca alone, so evaporation at the time of addition becomes relatively gentle and the yield is also improved.
A lower Ca concentration in the alloy is preferable in that the yield is improved and the occurrence of splash during addition is suppressed. However, it is necessary to select a Ca alloy composition in consideration that elements other than Ca (such as Si) are added simultaneously.

In order to prevent aggregation and segregation of Ca oxide and sulfide aggregates (CaO-CaS), after addition, the molten steel is stirred by Ar bubbling in the ladle, etc. It is desirable to raise inclusions. When the amount of molten steel is 200 t or more, it is desirable to perform stirring for about 5 to 10 minutes. Excessive stirring is not preferable because the inclusions are coarsened by agglomeration and coalescence of the inclusions.
From the viewpoint of securing the yield of Ca, it is advantageous to add at the end of the refining process. Ca may be added into the tundish in the casting process, not in the refining process. It is necessary to adjust the addition rate of the Ca alloy according to the throughput during casting (the amount of casting per hour). In this case, since the molten steel stirring after the addition of Ca is limited to the inside of the tundish and the mold, the uniformity of the Ca concentration is slightly inferior to that of the addition of the ladle. Therefore, in order to prevent aggregation and segregation of Ca oxide and sulfide aggregates (CaO—CaS) in the casting stage, it is desirable to stir the molten steel being solidified by electromagnetic force or the like. It is also desirable to optimize the shape of the casting nozzle in order to control the flow of molten steel during casting.
Further, in order to efficiently generate CaS having high consistency with Mn sulfide inclusions, it is desirable to adjust the oxygen amount of the molten steel so as to suppress the generation of excessive CaO. In order to adjust the oxygen amount in advance, it is desirable to deoxidize in advance with Al, Si or the like.

In order to finely disperse fine nano-sized oxides and Mn sulfide inclusions, pure metal Mg, Mg alloy (Fe- Si-Mg, Fe-Mn-Mg, Fe-Si-Mn-Mg, Si-Mg), or Zr alloy (Fe-Si-Zr, Fe-Mn-Mg-Zr, Fe-Si-Mn-Mg-Zr) ) Is desirable. Furthermore, in order to prevent agglomeration and segregation at the casting stage, it is desirable to stir the molten steel being solidified by electromagnetic force or the like. It is also desirable to optimize the shape of the casting nozzle in order to control the flow of molten steel during casting.
The order in which Ca, Mg and Zr are added is not specified, but high-carbon steel with a small amount of oxygen has a relatively weak oxidizing power in order to efficiently generate Ca, Mg and Zr oxides. It is desirable to add Ca first, and then add Mg or Zr having strong oxidizing power.

In hot rolling, the temperature at which the final forming is performed is preferably in the range of 900 to 1000 ° C. from the viewpoint of securing the shape and material.
In addition, with respect to the heat treatment after hot rolling, in order to obtain a pearlite structure having a hardness of Hv 320 to 500 in the rail head 3, the high temperature rail head 3 having an austenite region after rolling or reheating is used. It is desirable to perform accelerated cooling. As an accelerated cooling method, heat treatment (and cooling) is performed by a method as described in Patent Document 8 (JP-A-8-246100), Patent Document 9 (JP-A-9-111352), and the like. Thus, a predetermined structure and hardness can be obtained.
In addition, in order to heat-process by reheating after rail rolling, it is desirable to heat a rail head part or the whole rail with a flame or a high frequency.

Next, examples of the present invention will be described.
Tables 1 to 6 show chemical components of the test rail steel. The balance consists of Fe and inevitable impurities. Rail steel having the composition shown in Tables 1 to 6 was produced by the following method.
Demetalization and desulfurization were performed at the hot metal stage, and further, sufficient dephosphorization and desulfurization were performed in a commonly used melting furnace such as a converter and an electric furnace to obtain molten steel. By adding Ca to the molten steel, Mn sulfide inclusions were controlled, or by adding Mg and Zr, nano-sized oxides and Mn sulfide inclusions were finely dispersed. And the steel ingot was manufactured by the continuous casting method, and also hot rolling was performed with respect to the steel ingot. Then, it was set as the rail by heat-processing.

(A) Measurement of the number of Mn sulfide inclusions FIG. 3 shows the observation position of the Mn sulfide inclusions in the rail steel defined in claim 1.
As shown in FIG. 3, a sample was cut out from a region having a depth of 3 to 10 mm from the rail head surface including the head surface portion 3 a in the longitudinal section of the obtained rail steel. And the number (piece / mm < ² >) per unit area of the Mn sulfide type inclusion whose long side is 10-100 micrometers was calculated | required by the method mentioned above.

(B) Measurement of the number of Mn sulfide inclusions, Mg oxides, and Zr oxides FIG. 4 shows the Mn sulfide inclusions, Mg oxides, and Zr oxidations of the rail steel defined in claim 2. Indicates the observation position of the object.
As shown in FIG. 4, a sample was cut out from a region having a depth of 3 to 10 mm from the rail head surface including the head surface portion 3 a in the cross section of the obtained rail steel. And the number per unit area (pieces / mm ² ) of Mg-based oxides, Zr oxides, and Mn sulfide-based inclusions having a particle diameter of 5 to 100 nm was determined by the method described above.

(C) Observation of microstructure of head surface portion 3a and measurement of hardness A sample was cut from the surface of the rail head portion 3 at a depth of 4 mm. The observation surface was etched with a nital etchant after polishing. Based on JIS G 0551, the microstructure of the observation surface was observed with an optical microscope.
Moreover, according to JIS B7774, the Vickers hardness Hv of the cut-out sample was measured. The Vickers hardness was measured by loading a diamond indenter on a sample with a load of 98 N (10 kgf). The table indicated (Hv, 98N).
The obtained results are shown in Tables 7-12. In addition, in the table | surface, * 1 head material shows that it is a material of the site | part of depth 4mm from the surface of the rail head 5. FIG.

(D) Abrasion test FIG. 5 illustrates the sampling position of the test piece in the abrasion test, and the numbers in the figure indicate dimensions (mm).
As shown in FIG. 5, a disk-shaped test piece was cut out from a region including the head surface portion 3a in the rail steel. Then, as shown in FIG. 6, a disk-shaped test piece (rail test piece 4) is arranged on one of the two rotating shafts facing each other, and a mating member 5 is placed on the other rotating shaft. Arranged. In a state where a predetermined load is applied to the rail test piece 4, the rail test piece 4 and the mating member 5 are brought into contact with each other. In this state, the two rotating shafts were rotated at a predetermined rotation speed while cooling by supplying compressed air from the cooling nozzle 6. And after rotating 700,000 times, the reduction | decrease amount (abrasion amount) of the weight of the rail test piece 4 was measured.
The conditions of the wear test are shown below.
Testing machine: Nishihara type abrasion testing machine (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: In the air Cooling: Forced cooling with compressed air (flow rate: 100 Nl / min)
Repeat count: 700,000 times

(E) Head Impact Test FIG. 7 illustrates the sampling position of the test piece in the impact test.
As shown in FIG. 7, a test piece was cut out from the rail width (cross section) direction so that the region including the head surface portion 3 a in the cross section of the rail steel became the notch bottom. Then, an impact test was performed on the obtained test piece under the following conditions, and an impact value (J / cm ² ) was measured.
Tester: Impact tester Test piece shape: JIS3 2mm U-notch Test piece sampling position: 2mm below the rail head surface (see Fig. 7)
Test temperature: Normal temperature (20 ° C)

The obtained results are shown in Tables 13-15. In the table, the wear test result of * 2 is the result of the above-described wear test, and the reduction amount (g) of the weight of the rail test piece 13 is shown as the wear amount. The impact test result of * 3 is the result of the head impact test described above, and indicates the impact value (J / cm ² ). In addition, it means that toughness is excellent, so that an impact value (J / cm < ² >) is large.
In this evaluation, when the amount of wear after 700,000 times was 1.5 g or less, it was evaluated that the wear resistance was excellent. Since the impact value at 20 ° C varies greatly depending on the carbon content of the steel, the standard value indicating the superiority or inferiority of the characteristics has not been set. did.

(1) Invention rails (47), steel codes 1 to 47
Steel No. 3, 4, 7, 8, 11-14, 17-19, 21-25, 29, 30, 32-34, 36, 37, 43, 45, 46: The chemical components are within the limited range of the present invention. Long side: A pearlite rail excellent in wear resistance and toughness in which the number of Mn sulfide inclusions of 10 to 100 μm, the microstructure of the rail head, and the hardness are within the limited ranges of the present invention.
Steel No. 1, 2, 5, 6, 9, 10, 15, 16, 20, 26-28, 31, 35, 38-42, 44, 47: The chemical component is within the limited range of the present invention, and the long side: 10 The number of Mn sulfide inclusions of ˜100 μm, the particle size: the number of Mg-based oxides, Zr oxides and Mn sulfide-based inclusions of 5 to 100 nm, the microstructure of the rail head, and the hardness are the limited range of the present invention. It is a pearlitic rail with excellent wear resistance and toughness.

(2) Comparison rail (21 pieces), steel sign 48-68
Steel No. 48-53: A rail in which components of C, Si, and Mn are outside the scope of the present invention.
Steel No. 54-55: Rails with components P and S outside the scope of the present invention.
Steel No. 56-57: Rail whose Ca component is outside the scope of the present invention.
Steel No. 58-63: The rail of which the component of P, S, and Ca is outside the range of this invention.
Steel No. 64-66: A rail whose chemical composition is within the scope of the present invention but whose head microstructure is outside the limited range of the present invention.
Steel No. 67-68: A rail whose chemical composition is within the scope of the present invention, but whose head hardness is outside the above-mentioned limited range of the present invention.

As shown in Tables 1 to 15, in the present rail steel (steel Nos. 1 to 47), compared with the comparative rail steel (steel Nos. 48 to 53), the chemical components of steel C, Si, and Mn are present. Within the scope of the invention. Therefore, it is possible to stably obtain a pearlite structure having a hardness within the limited range of the present invention without generating a pro-eutectoid ferrite structure, a pro-eutectoid cementite structure and a martensite structure that adversely affect wear resistance and toughness. It becomes.
In this invention rail steel (steel No. 1-47), compared with comparative rail steel (steel No. 64-68), the microstructure of a head contains a pearlite structure, and hardness is within the limits of the present invention. is there. For this reason, the wear resistance and toughness of the rail can be improved.

FIG. 8 shows the results of wear tests of the rail steel of the present invention (steel Nos. 1-47) and comparative rail steel (steel Nos. 48, 50, 51, 52, 53, 64, 66, 67).
The chemical components of steel C, Si, and Mn are within the limited range of the present invention, thereby preventing the formation of proeutectoid ferrite structure and martensite structure that adversely affect wear resistance, and the hardness is limited by the present invention. Within. As described above, the wear resistance can be greatly improved in any carbon amount.

FIG. 9 shows the results of an impact test of the rail steel of the present invention (steel Nos. 1-47) and comparative rail steel (steel Nos. 49, 51, 53, 65, 66, 68).
The chemical components of steel C, Si, and Mn are within the limited range of the present invention, thereby preventing the formation of proeutectoid cementite structure and martensite structure that adversely affect toughness, and the hardness is within the limited range of the present invention. And As described above, the toughness can be greatly improved at any carbon content.

As shown in FIG. 10, the present invention rail steel (steel Nos. 1 to 47) has a limited range of addition of P, S, and Ca compared to the comparative rail steel (steel Nos. 54 to 63). Is within. For this reason, the toughness of the rail of a pearlite structure | tissue can be improved greatly in any carbon amount.
Furthermore, as shown in FIG. 11, Ca is added to the rail steel of the present invention (steel Nos. 11 to 13, 18 to 20, 24 to 26, 29 to 31, 33 to 35, 36 to 38, and 45 to 47). Furthermore, the Ca addition amount is optimized. Thereby, Mn sulfide inclusions are controlled, and the number thereof is within the limited range of the present invention. For this reason, the toughness of the rail of a pearlite structure | tissue can be improved. In addition to the above, when Mg and Zr are added, oxides and Mn sulfide inclusions are finely dispersed, and the number of Mg oxide, Zr oxide, and Mn sulfide inclusions is 500 to 50,000. / Mm ² . Thereby, the toughness of the rail of a pearlite structure | tissue can further be improved.

  The pearlite rail of the present invention has excellent wear resistance and toughness that are higher than those of current high-strength rails. For this reason, this invention can be applied suitably as a rail used in a remarkably severe track environment like a rail for a freight railroad that transports natural resources mined in a region where the natural environment is severe.

1: head portion 2: head corner portion 3: rail head portion 3a: head surface portion 3b: range from the head corner portion and the top surface to a depth of 20 mm 4: rail test piece 5: mating material 6 : Cooling nozzle

Claims (11)

% By mass
C: 0.65 to 1.20%,
Si: 0.05 to 2.00%,
Mn: 0.05-2.00%
P ≦ 0.0150%,
S ≦ 0.0100%,
Ca: 0.0005 to 0.0200% is contained,
The balance consists of steel containing Fe and inevitable impurities,
In the head of the rail, the head surface part consisting of a range up to a depth of 10 mm starting from the surface of the head corner part and the top of the head is the pearlite structure,
And the hardness of the said pearlite structure | tissue is Hv320-500,
A pearlite rail characterized in that, in an arbitrary cross section in the longitudinal direction in the pearlite structure, Mn sulfide inclusions having a long side of 10 to 100 μm are present in an amount of 10 to 200 / mm ² per unit area. . The steel contains, in mass%, one or two of Mg: 0.0005 to 0.0200%, Zr: 0.0005 to 0.0100%,
In an arbitrary cross section in the pearlite structure, Mg-based oxides, Zr oxides, and Mn sulfide-based inclusions having a particle size of 5 to 100 nm are present in an amount of 500 to 50000 pieces / mm ² per unit area. The pearlite rail according to claim 1.   The pearlite rail according to claim 1 or 2, wherein the steel further contains Co: 0.01 to 1.00% by mass.   The said steel is the mass%, and also contains 1 type or 2 types of Cr: 0.01-2.00%, Mo: 0.01-0.50%, The 1-3 characterized by the above-mentioned. The pearlite rail according to any one of the above.   The said steel is the mass%, and also contains 1 type or 2 types of V: 0.005-0.50%, Nb: 0.002-0.050%, The 1-4 characterized by the above-mentioned. The pearlite rail according to any one of the above.   The pearlite rail according to any one of claims 1 to 5, wherein the steel further contains B: 0.0001 to 0.0050% in mass%.   The pearlite rail according to any one of claims 1 to 6, wherein the steel further contains Cu: 0.01 to 1.00% by mass.   The pearlite rail according to any one of claims 1 to 7, wherein the steel further contains Ni: 0.01 to 1.00% by mass%.   The pearlite rail according to any one of claims 1 to 8, wherein the steel further contains, by mass%, Ti: 0.0050 to 0.0500%.   The pearlite rail according to any one of claims 1 to 9, wherein the steel further contains Al: more than 0.0100 to 1.00% by mass%.   The pearlite rail according to any one of claims 1 to 10, wherein the steel contains mass% and further contains N: 0.0060 to 0.0200%.

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