JP5867263B2 - Rail with excellent delayed fracture resistance - Google Patents

Description

  The present invention relates to a rail intended to improve toughness and delayed fracture resistance in a high-strength rail used in overseas freight railways.

  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, higher wear resistance has been demanded. Against this background, there has been a demand for the development of a rail having wear resistance higher than that of current high-strength rails.

  In order to improve the wear resistance of rail steel, the following rails have been developed. The main features of these rails are to increase the carbon content of the steel, to increase the volume ratio of the cemetite phase in the pearlite lamella, and to increase the strength at the same time to improve wear resistance (for example, Patent Document 1). 2).

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

  In the published technology of Patent Document 2, hypereutectoid steel (C: more than 0.85 to 1.20%) is used to increase the cementite volume ratio in the lamellae in the pearlite structure and at the same time control the hardness. A rail having excellent wear resistance can be provided.

  However, in the disclosed techniques of Patent Documents 1 and 2, the wear resistance can be improved by increasing the volume ratio of the cemetite phase in the pearlite structure and simultaneously increasing the strength. However, when the strength is increased, the risk of delayed fracture due to residual hydrogen in the steel increases, and there is a problem that the rail is liable to break.

  In general, to suppress the occurrence of delayed fracture due to residual hydrogen, it is said that it is effective to increase the number of hydrogen trap sites in the steel, disperse the hydrogen accumulation sites, and reduce the amount of hydrogen in the steel. Yes.

  Therefore, development of a high-strength rail in which inclusions, which are hydrogen accumulation sites, are dispersed has been demanded. In order to solve this problem, the following high-strength rails have been developed. The main features of these rails are to suppress delayed fracture by increasing the number of hydrogen trap sites in the steel and dispersing the hydrogen accumulation sites (see, for example, Patent Documents 3 and 4).

In the disclosed technologies of Patent Documents 3 and 4, the inclusion of A-based (MnS) and C-based (SiO ₂ , CaO) inclusions, which are hydrogen trap sites, is dispersed in the pearlite structure, and the amount of hydrogen in the steel is controlled. By doing so, a rail excellent in delayed fracture resistance can be provided.

  However, in the disclosed technologies of Patent Documents 3 and 4, depending on the type of inclusions, the failure may be a starting point, and even if the amount of inclusions is controlled, delayed fracture cannot be suppressed stably, and rail breakage occurs. There was a problem that made it easier.

Japanese Patent Application Laid-Open No. 08-144016 Japanese Patent Laid-Open No. 08-246100 JP 2007-277716 A JP 2008-050684 A Japanese Patent Laid-Open No. 09-111352

  One aspect of the present invention has been devised in view of the above-described problems, and in particular, provides a rail aimed at improving delayed fracture resistance required for rails of overseas freight railroads. For the purpose.

(1) By mass%, C: 0.88 to 1.20%, Si: 0.10 to 2.00%, Mn: 0.10 to 2.00%, P ≦ 0.0100%, S: 0 In a steel rail containing 0.0250 to 0.0350%, the balance being Fe and inevitable impurities, the U value shown in Formula 1 in mass% is in the range of 50 to 200, and the head of the steel rail A rail characterized in that 95% or more of the head surface part in a range up to a depth of 20 mm starting from the surface of the corner part and the top of the head part is a pearlite structure.
U = P × S × 10 ⁶ Formula 1

(2) The rail according to (1), characterized in that, in mass%, the U value shown in Formula 1 is in the range of 50 to 150.
(3) The rail as set forth in (2), characterized in that, in mass%, H ≦ 2.0 ppm or less.

(4) In addition, the rail in any one of the above (1) to (3) may selectively contain one or more of the following components (a) to (o) in mass%. Can do.
(A) Mg: 0.0005 to 0.0200%,
(B) Ca: 0.0005 to 0.0200%,
(C) REM: 0.0005 to 0.0500%,
(D) Cr: 0.01 to 2.00%
(E) Mo: 0.01 to 0.50%
(F) Co: 0.01 to 1.00%,
(G) B: 0.0001 to 0.0050%,
(H) Cu: 0.01 to 1.00%,
(I) Ni: 0.01-1.00%
(J) V: 0.005 to 0.50%,
(K) Nb: 0.001 to 0.050%,
(L) Ti: 0.0030 to 0.0500%,
(M) Zr: 0.0001 to 0.0200%,
(N) N: 0.0060 to 0.0200%
(O) N: 0.0060 to 0.0200%

  According to one aspect of the present invention, by controlling the composition and structure of rail steel, the amount of MnS sulfide in the steel, that is, the content of S and the content of P which is an impurity element, It is possible to improve the delayed fracture resistance of rails used in overseas freight railways and greatly improve the service life.

The figure which showed the relationship between S addition amount and a limit stress value. The figure which showed the relationship between the amount of P addition in S addition amount 0.0200%, 0.0250%, 0.0350%, and the critical stress value of delayed fracture. S amount 0.0200%, 0.0250%, shows the relationship between the S amount and P addition amount of the product (P × S × 10 ⁶⁾ and delayed threshold stress fracture in 0.0350%. The figure in the head cross-sectional surface position of the rail excellent in the delayed fracture resistance of this invention, and the figure which showed the area | region where a pearlite structure | tissue is required. The relationship between the S addition amount and the critical stress value of delayed fracture of the present rail steel (reference symbols A1 to A51) and comparative rail steel (reference symbols B11 to B20) shown in Tables 1 and 2 is the product of the S addition amount and the P addition amount ( The figure arranged in the range of the value of P × S × 10 ⁶ ). The present invention rail steels shown in Table 1 ((code A 12 ~A13, A16~A17, A 21 ~A22, A 24 ~A25, A 27 ~A28, A 32 ~A33, A 36 ~A37, A 40 ~A43, The relationship between the S addition amount of A 46 to A47 and A 50 to A51) and the critical stress value of delayed fracture is the range of the product of the S addition amount and the P addition amount (P × S × 10 ⁶ ), that is, S addition The figure shown by the relationship between the control of the amount and P addition amount and the hydrogen amount control. The schematic diagram which showed the delayed fracture test method.

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. However, the present invention is not limited to the following description, and it will be easily understood by those skilled in the art that modes and details can be variously changed without departing from the spirit and scope of the present invention. Therefore, the present invention should not be construed as being limited to the description of the embodiments below.
As this embodiment, a rail excellent in delayed fracture resistance will be described in detail. Hereinafter, the mass% in the composition is simply described as%.

  First, in order to improve the delayed fracture resistance of the rail, the present inventors first studied to improve the characteristics of inclusions that are hydrogen trap sites. As a result of investigating the cheapest inclusions that have little influence on the properties of the rail, the soft MnS sulfide produced from S contained as an iron impurity and Mn generally added as a strengthening element is the starting point of fracture. However, it was found to be cheap and promising as a hydrogen trap site.

  Next, the inventors examined the amount of MnS-based sulfide, that is, the optimum range of the amount of S added through experiments. Carbon amount 1.0% (0.25% Si-1.0% Mn-0.0010-0.0200% P) as a base component, hydrogen amount 2.5ppm, S addition amount 0.0050-0. The steel changed to 0400% is melted, rail rolling and heat treatment are performed, and the head surface (the range from the outer surface of the head up to a depth of 20 mm) is pulled to the head using a rail with a pearlite structure. Delayed fracture characteristics were evaluated by a three-point bending test under stress. The delayed fracture characteristics were performed by a three-point bending (span length: 1.5 m) method so that a tensile stress acts on the head. The stress condition was 200 to 400 MPa, the stress load time was 500 hours, and the maximum value of the stress when not broken when loaded for 500 hours was defined as the critical stress value.

  FIG. 1 shows the relationship between the amount of MnS sulfide, that is, the amount of S added and the critical stress value. When the S content is 0.0200% or more, the MnS sulfide effectively acts as a hydrogen trap site, the critical stress value of delayed fracture increases to 250 MPa or more, and the critical stress value with the increase of the S content. Showed a tendency to increase. In addition, when the S content exceeds 0.0350%, MnS-based sulfides become excessive, and MnS-based sulfides become coarse and increase in the generation density, thereby promoting stress concentration and embrittlement of the structure. The critical stress value for fracture decreased to 250 MPa or less. From these results, there is an optimum range for the amount of added S, that is, the amount of MnS-based sulfide, and in order to improve delayed fracture resistance, the amount of added S is in the range of 0.0200 to 0.0350%. Confirmed that you need to control.

  Furthermore, the present inventors examined a method for further improving the delayed fracture characteristics. In the range of 0.0200 to 0.0350%, which is the optimum S addition amount, the delayed fracture resistance was analyzed in detail. As a result, as shown in FIG. 1, it was found that the critical stress value of delayed fracture greatly fluctuated in the range of S addition amount of 0.0200 to 0.0350%, and variation in delayed fracture characteristics occurred. It was.

  Therefore, the present inventors investigated the cause of this variation. As a result, the hydrogen embrittlement fracture surface peculiar to the delayed fracture and the normal brittle fracture surface where the fracture occurred instantaneously existed at the starting point where the delayed fracture occurred. When the fracture surface morphology was examined in detail, it was found that the area of the hydrogen embrittlement fracture surface was remarkably small in the rails with variations in delayed fracture characteristics.

  Furthermore, the present inventors investigated the cause of the remarkably reduced area of the hydrogen embrittlement fracture surface. As a result of the component analysis of the rail having a remarkably small fracture surface area of hydrogen embrittlement, it was confirmed that the content of impurities, particularly P, in the steel was remarkably high.

  From these results, the delayed fracture resistance greatly fluctuates, and the cause of the variation in delayed fracture characteristics is that the increase in the P content in the steel promotes the embrittlement of the steel and the formation of hydrogen embrittlement fracture surfaces. It was presumed that the normal brittle fracture generated later was promoted.

  In order to verify this result, the present inventors examined the optimal range of P addition amount by experiment. Carbon amount 1.0% (0.25% Si-1.0% Mn-0.005-0.0200% P) as a base component, hydrogen amount 2.5ppm, S addition amount 0.0200%, 0 0.02% and 0.0350% steel is melted, rail rolling and heat treatment are performed, and a rail with a pearlite structure in the head surface (the range from the outer surface of the head to a depth of 20 mm) is used. The delayed fracture characteristics were evaluated by a three-point bending test in which tensile stress was applied to the head. The delayed fracture characteristics were performed by a three-point bending (span length: 1.5 m) method so that a tensile stress acts on the head. The stress condition was 200 to 400 MPa, the stress load time was 500 hours, and the maximum value of the stress when not broken when loaded for 500 hours was defined as the critical stress value.

  FIG. 2 shows the relationship between the P addition amount and the critical stress value. When the P content was controlled within the range of 0.0100% or less, the delayed fracture resistance was greatly improved and the critical stress value increased to 300 MPa or more in any steel with S content. Further, in any steel with S content, when the P content is reduced to a certain range, the limit stress value reaches a peak, and in order to obtain the effect of reducing the P content, there is a lower limit value for the P content. I understood.

  As a result of investigating this result in more detail, when the P content is 0.0100% or less, the range in which the critical stress value is greatly improved has a strong correlation with the S content of steel, and effectively improves the delayed fracture resistance. In order to make it, it discovered that the control which considered S content and P content of steel mutually was required.

Therefore, the present inventors have studied the optimum range in consideration of the S content and P content of steel. As a result of various numerical analyses, the U value of Formula 1 shown below consisting of the product of the S addition amount (mass%) and the P addition amount (mass%) falls within the range of 50 to 200. It was found that the critical stress value can be effectively improved by considering the S content and the P content.
U = P × S × 10 ⁶ Formula 1

  FIG. 3 shows the results of the delayed fracture test in FIG. 2 organized by the relationship between the product of the S addition amount and the P addition amount. When the U value of Equation 1 is 200 or less, the critical stress value increases and the delayed fracture resistance is greatly improved in any steel with any S content. Further, in any steel with S content, when the U value of Formula 1 reaches around 50, the critical stress value is saturated in steel with any S content, and the delayed fracture resistance is further improved. I can't hope.

  Furthermore, the present inventors have examined in detail the relationship between the U value of Equation 1 and the critical stress value. As a result, it was found that by controlling the U value of Formula 1 to 150 or less, the delayed fracture resistance is greatly improved in any S-content steel.

  In addition to controlling these MnS-based sulfides, the present inventors have studied a method for further improving delayed fracture characteristics. By applying secondary refining (degassing) strengthening of molten steel and dehydrogenation treatment with steel slabs, the hydrogen content in the steel is controlled to 2.0 ppm or less, thereby delaying fracture in any steel with any S content. It was confirmed that the critical stress value of the steel was further improved and the delayed fracture resistance was further improved.

  That is, the present invention controls the composition and structure of rail steel, controls the amount of MnS sulfide in the steel, that is, controls the content of S and the content of P which is an impurity element, The present invention relates to a rail intended to improve the delayed fracture resistance of rails used in overseas freight railways and greatly improve the service life by reducing the content of.

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

(1) Reasons for limiting chemical components of steel In the 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. If the C content is 0.85% or less, the present component system cannot maintain the minimum strength and wear resistance required for the rail. Further, a pro-eutectoid ferrite structure that is soft and easily accumulates strain is formed, and delayed fracture is likely to occur. On the other hand, when the C content exceeds 1.20%, a large amount of pro-eutectoid cementite structure with low toughness is generated, and delayed fracture is likely to occur. For this reason, C addition amount was limited to 0.88 to 1.20%. In addition, in order to stabilize the formation of the pearlite structure and improve the delayed fracture resistance, it is desirable that the amount of C added is more than 0.85 to 1.10%.

  Si is an element that dissolves in the ferrite phase of the pearlite structure, increases the hardness (strength) of the rail head, and improves the wear resistance. Furthermore, it is an element that suppresses the formation of a pro-eutectoid cementite structure with low toughness and suppresses the occurrence of delayed fracture. However, if the Si amount is less than 0.10%, these effects cannot be expected sufficiently. Moreover, when the amount of Si exceeds 2.00%, many surface defects are generated during hot rolling. Further, the hardenability is remarkably increased, and a martensitic structure with low toughness is generated in the head surface portion, so that delayed fracture is likely to occur. For this reason, Si addition amount was limited to 0.10 to 2.00%. In addition, in order to stabilize the production | generation of a pearlite structure | tissue and to improve a delayed fracture resistance, it is desirable to make Si addition amount 0.20 to 1.50%.

  P is an element inevitably contained in steel. When refining in a converter, the P content is reduced to 0.0010 to 0.0250%. There is a good correlation between the P content and the critical stress value of delayed fracture. If the P content exceeds 0.0100%, improvement of the critical stress value cannot be expected, and stable improvement of delayed fracture resistance is difficult. Become. For this reason, the P content is limited to 0.0100% or less. The lower limit of the P content is not limited, but considering the dephosphorization ability in the refining process, it is considered that the P content is about 0.0020% which is the limit in actual production.

S is an element inevitably contained in steel. When desulfurization is performed in the hot metal ladle, the S content is reduced to 0.0030 to 0.0500%. There is a correlation between the S content and the amount of MnS sulfide produced, and as the S content increases, MnS sulfide increases. If the S content is less than 0.0200%, an increase in the amount of MnS sulfide produced cannot be expected, and it is difficult to ensure delayed fracture resistance. On the other hand, if the S content exceeds 0.0350%, the coarsening of MnS-based sulfides and the increase in the generation density cause stress concentration and embrittlement of the structure, and delayed fracture tends to occur. For this reason, S content was limited to 0.0200-0.0350%.
Moreover, in order to increase the production amount of MnS-based sulfides and to reliably improve the delayed fracture resistance, the S content is limited to 0.0250 to 0.0350% as limited to one embodiment of the present invention. .

  Mn is an element that enhances hardenability 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 improves the wear resistance. However, if the amount of Mn is less than 0.10%, the effect is small, the formation of a pro-eutectoid ferrite structure that is soft and easily accumulates strain is induced, and it becomes difficult to ensure wear resistance and delayed fracture resistance. 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 toughness is generated in the head surface portion, which easily causes delayed fracture. For this reason, Mn addition amount was limited to 0.10 to 2.00%. In order to stabilize the formation of the pearlite structure and improve the delayed fracture resistance, the Mn addition amount is desirably 0.20 to 1.50%.

  H is an element that causes delayed fracture. When the H content of the steel slab (bloom) before rail rolling exceeds 2.0 ppm, the amount of H accumulated at the interface of the MnS-based sulfide increases, and the occurrence of delayed fracture is promoted. Therefore, in one embodiment of the present invention, the H content is limited to 2.0 ppm or less. The lower limit of the H content is not limited, but considering the secondary refining (degassing) capacity in the refining process and the dehydrogenation capacity of the steel slab, the H content of about 1.0 ppm is actually produced. It is thought that it will be the limit at.

  In addition, rails manufactured with the above component composition have improved delayed fracture resistance due to fine dispersion of MnS-based sulfides, improved wear resistance due to increased hardness (strength) of pearlite structure, improved toughness, welding Mg, Ca, REM, Cr, Mo, Co, B, Cu, Ni, V, Nb, Ti, Zr, N for the purpose of preventing softening of the heat affected zone and controlling the cross-sectional hardness distribution inside the rail head One or more elements are added as required.

  Here, Mg, Ca, and REM finely disperse MnS-based sulfides. Cr and Mo raise the equilibrium transformation point, refine the lamella spacing of the pearlite structure, and improve the hardness. Co refines the base ferrite structure of the wear surface and increases the hardness of the wear surface. 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 pearlite structure and increases the hardness. Ni improves the toughness and hardness of the pearlite structure, and at the same time prevents softening of the heat-affected zone of the weld joint. V, Nb, and Ti suppress the growth of austenite grains by carbides and nitrides generated by hot rolling and the subsequent cooling process, and further improve the toughness and hardness of the pearlite structure by precipitation hardening. In addition, carbides and nitrides are stably generated during reheating, and softening of the weld joint heat-affected zone is prevented. Zr increases the equiaxed crystallization rate of the solidified structure, thereby suppressing the formation of a segregation zone at the center of the slab and suppressing the formation of proeutectoid cementite structure and martensite structure. N is mainly added for the purpose of promoting pearlite transformation by segregating at austenite grain boundaries and making the pearlite structure fine.

  Mg is an element that combines with S to form fine sulfides, and MgS finely disperses MnS, increases the top sites of hydrogen, and improves delayed fracture resistance. However, if it is less than 0.0005%, the effect is weak, and if added over 0.0200%, a coarse oxide of Mg is generated, and rail breakage is likely to occur due to stress concentration. For this reason, the amount of Mg was limited to 0.0005 to 0.0200%.

  Ca is an element that has a strong bonding force with S and forms a sulfide as CaS. Further, CaS finely disperses MnS, increases the top sites of hydrogen, and improves delayed fracture resistance. However, if it is less than 0.0005%, the effect is weak, and if added over 0.0200%, a coarse oxide of Ca is generated, and rail breakage is likely to occur due to stress concentration. For this reason, the amount of Ca was limited to 0.0005 to 0.0200%.

REM is a deoxidation / desulfurization element. When added, REM generates REM oxysulfide (REM ₂ O ₂ S), which serves as a nucleus of Mn sulfide inclusions. In addition, since the melting point of oxysulfide (REM ₂ O ₂ S) as the nucleus is high, stretching of Mn sulfide inclusions after rolling is suppressed. As a result, it is an element that finely disperses MnS, increases the top site of hydrogen, and improves delayed fracture resistance. However, if the amount of REM is less than 0.0005%, the effect is small, and it is insufficient as a production nucleus of MnS-based sulfide. When the amount of REM exceeds 0.0500%, hard REM oxysulfide (REM ₂ O ₂ S) is generated, and rail breakage easily occurs due to stress concentration. For this reason, the amount of REM added is limited to 0.0005 to 0.0500%.

  Note that REM is a rare earth metal such as Ce, La, Pr, or Nd. The above addition amount is limited to the addition amount of all these REMs. As long as the total sum of the total addition amounts is within the above range, the same effect can be obtained regardless of whether the form is single or composite (two or more types).

  Cr is an element that raises the equilibrium transformation temperature, refines the lamella spacing of the pearlite structure by increasing the degree of supercooling, and improves the hardness (strength) of the pearlite structure and bainite 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. Further, if excessive addition exceeding the Cr amount of 2.00% is performed, the hardenability is remarkably increased, and a martensite structure harmful to toughness is generated on the rail head surface portion and the like, and delayed fracture is likely to occur. For this reason, Cr addition amount was limited to 0.01 to 2.00%.

  Mo, like Cr, is an element that raises the equilibrium transformation temperature, refines the lamella spacing of the pearlite structure by increasing the degree of supercooling, and improves the hardness (strength) of the pearlite structure. However, 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. Further, if the Mo amount exceeds 0.50%, the transformation rate is remarkably reduced, and a martensite structure harmful to toughness is generated in the rail head surface portion and the like, and delayed fracture is likely to occur. For this reason, Mo addition amount was limited to 0.01 to 0.50%.

  Co is dissolved in the ferrite phase of the pearlite structure, and on the wear surface of the rail head surface, the fine ferrite structure formed by contact with the wheel is further refined and the wear resistance is improved by increasing the hardness. It is an element to be made. However, if the Co content is less than 0.01%, refinement of the ferrite structure is not promoted, and an effect of improving wear resistance cannot be expected. On the other hand, when the Co content exceeds 1.00%, the above effects are saturated, and the ferrite structure cannot be refined according to the added amount. In addition, the economic efficiency decreases due to the increase in the alloy addition cost. For this reason, Co addition amount was limited to 0.01 to 1.00%.

B forms an iron boride (Fe ₂₃ (CB) ₆ ) at the austenite grain boundary, reduces the cooling rate dependence of the pearlite transformation temperature due to the effect of pearlite transformation, and is more uniform from the head surface to the inside. It is an element that imparts a hardness distribution to the rail and extends the life of the rail. However, if the amount of B is less than 0.0001%, the effect is not sufficient, and no improvement is observed in the hardness distribution of the rail head. On the other hand, if the amount of B exceeds 0.0050%, coarse ferrocarbon borides are generated, and rail damage is likely to occur due to stress concentration. For this reason, B addition amount was limited to 0.0001 to 0.0050%.

  Cu is an element that dissolves in the ferrite phase of the pearlite structure, improves hardness (strength) by solid solution strengthening, and improves wear resistance. However, if it is less than 0.01%, the effect cannot be expected. On the other hand, when the Cu content exceeds 1.00%, a martensitic structure that is harmful to toughness is generated in the rail head surface portion and the like due to remarkable hardenability improvement, and delayed fracture is likely to occur. For this reason, Cu addition amount was limited to 0.01 to 1.00%.

Ni is an element that improves the toughness of the pearlite structure and at the same time improves the hardness (strength) by solid solution strengthening and improves the wear resistance. Further, in the weld heat affected zone, Ni ₃ Ti intermetallic compound is finely precipitated in combination with Ti, and is an element that suppresses softening by precipitation strengthening. Moreover, it is an element which suppresses the embrittlement of a grain boundary in Cu addition steel. However, when the Ni content is less than 0.01%, these effects are remarkably small. When the Ni content exceeds 1.00%, martensite is harmful to the toughness of the rail head surface due to a significant improvement in hardenability. Tissue is generated, making delayed fracture easier to occur. For this reason, Ni addition amount was limited to 0.01 to 1.00%.

  V is effective for improving the toughness of the pearlite structure by refining the austenite grains by the pinning effect by precipitation of V carbides and V nitrides when normal hot rolling or heat treatment at a high temperature is performed. Element. Furthermore, it is an element that increases the hardness (strength) of the pearlite structure and improves the wear resistance by precipitation hardening with V carbides and V nitrides generated in the cooling process after hot rolling. Moreover, in the heat affected zone reheated to a temperature range below the Ac1 point, it is an element effective for generating V carbide and V nitride in a relatively high temperature range and preventing softening of the heat affected zone of the weld joint. is there. However, if the V content is less than 0.005%, these effects cannot be expected sufficiently, and no improvement in toughness or hardness (strength) is observed. On the other hand, if the V content exceeds 0.50%, precipitation hardening of V carbide and nitride becomes excessive, the pearlite structure becomes brittle, and the toughness of the rail is lowered. For this reason, V addition amount was limited to 0.005-0.50%.

  Nb, like V, refines austenite grains by the pinning effect of Nb carbide or Nb nitride and improves the toughness of the pearlite structure when normal hot rolling or heat treatment heated to a high temperature is performed. Is an effective element. Furthermore, it is an element that increases the hardness (strength) of the pearlite structure and improves the wear resistance by precipitation hardening with Nb carbide and Nb nitride generated in the cooling process after hot rolling. Further, in the heat-affected zone reheated to a temperature range below the Ac1 point, Nb carbide and Nb nitride are stably generated from a low temperature range to a high temperature range, and softening of the weld joint heat-affected zone is prevented. It is an effective element. However, if the Nb content is less than 0.0010%, these effects cannot be expected, and an improvement in the toughness and hardness (strength) of the pearlite structure is not recognized. On the other hand, if the amount of Nb exceeds 0.050%, precipitation hardening of Nb carbides and nitrides becomes excessive, the pearlite structure becomes brittle, and the toughness of the rail decreases. For this reason, Nb addition amount was limited to 0.0010 to 0.050%.

  Ti is effective for improving the toughness of pearlite structure by refining austenite grains due to the pinning effect when Ti carbide or Ti nitride precipitates during normal hot rolling or heat treatment at high temperatures. Element. Furthermore, it is an element that increases the hardness (strength) of the pearlite structure and improves the wear resistance by precipitation hardening with 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, the structure of the heat affected zone heated to the austenite region is refined and the weld joint becomes brittle. It is an effective ingredient to prevent However, when the Ti content is less than 0.0030%, these effects are small. If the Ti content exceeds 0.0500%, coarse Ti carbides and Ti nitrides are generated, and rail breakage is likely to occur due to stress concentration. For this reason, Ti addition amount was limited to 0.0030-0.0500%.

  Zr has a good lattice matching with γ-Fe because the ZrO2 inclusion is a solidified nucleus of high-carbon rail steel that is a solidified primary crystal, and by increasing the equiaxed crystallization rate of the solidified structure, It is an element that suppresses the formation of a segregation zone at the center of the slab and suppresses the formation of martensite and proeutectoid cementite structure generated in the rail segregation part. However, if the amount of Zr is less than 0.0001%, the number of ZrO2 inclusions is small and does not exhibit a sufficient effect as a solidification nucleus. As a result, martensite and a pro-eutectoid cementite structure are generated in the segregated portion, and the toughness of the rail is lowered. On the other hand, if the amount of Zr exceeds 0.0200%, a large amount of coarse Zr-based inclusions are generated, and rail breakage is likely to occur due to stress concentration. For this reason, the Zr addition amount is limited to 0.0001 to 0.0200%.

  N is an element effective for improving the toughness by promoting the pearlite transformation from the austenite grain boundary by segregating at the austenite grain boundary and mainly by refining the structure. Also, by adding simultaneously with V and Al, the precipitation of VN and AlN is promoted, and when a normal hot rolling or heat treatment is performed at a high temperature, the austenite grains are made fine by the pinning effect of VN or AlN. It is an element effective for improving the toughness of pearlite structure and bainite structure. However, when the N content is less than 0.0060%, these effects are weak. On the other hand, when the N content exceeds 0.0200%, it is difficult to make a solid solution in the steel, and bubbles that become the starting point of fatigue damage are generated, and rail breakage is likely to occur. For this reason, N addition amount was limited to 0.0060-0.0200%.

  Al is an essential component as a deoxidizing material. Further, it is an element that moves the eutectoid transformation temperature to the high temperature side, contributes to increasing the hardness (strength) of the pearlite structure, and improves the wear resistance of the pearlite structure. However, when the Al content is less than 0.0100%, the effect is weak. Further, if the Al content exceeds 1.00%, it becomes difficult to make a solid solution in the steel, and coarse alumina inclusions are generated, fatigue damage is generated from the coarse precipitates, and brittle fracture is redundant. As a result, the toughness of the rail decreases. Furthermore, oxides are generated during welding, and weldability is significantly reduced. For this reason, Al addition amount was limited to 0.0100 to 1.00%.

  Rail steel composed of the above component composition is melted in a commonly used melting furnace such as a converter, electric furnace, etc., and this molten steel is ingot / bundled or continuous casting, It is manufactured as a rail through hot rolling. Furthermore, heat treatment is performed for the purpose of controlling the metal structure of the rail head as necessary.

(2) Reason for limiting metal structure In the rail of the present invention, the reason why at least a part of the head surface of the rail is limited to the pearlite structure will be described in detail.

First, the reason for limiting to the pearlite structure will be described.
It is most important to ensure wear resistance at the rail head surface 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 was the best. Furthermore, it was confirmed by experiment that the delayed fracture resistance does not decrease by using a pearlite structure. Therefore, it was limited to a pearlite structure for the purpose of ensuring wear resistance and delayed fracture resistance.

  Here, FIG. 4 shows the designation of the rail head section surface position excellent in delayed fracture resistance of the present invention, and the area where pearlite structure is required. 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 20 mm is referred to as a head surface portion (3a, hatched portion). As shown in FIG. 4, if the pearlite structure is arranged on the head surface part up to a depth of 20 mm starting from the surface of the head corner part 2 and the top of the head part 1, the wear resistance is ensured in the rail head surface part. In addition, the delayed fracture resistance can be improved.

  Therefore, it is desirable to place the pearlite structure on the head surface where the wheels and rails are mainly in contact with each other, and wear resistance and delayed fracture resistance are required. The other parts where these characteristics are not required are pearlite structures. Other metal structures may be used.

  The hardness of these metal structures is not particularly limited. It is desirable to adjust the hardness according to the track condition to be laid. In order to ensure sufficient wear resistance, it is desirable to control the hardness to about Hv 300 to 500. As a method for obtaining a pearlite structure having a hardness of Hv 300 to 500, it is desirable to select an appropriate alloy and perform accelerated cooling on a high-temperature rail head having an austenite region after rolling or reheating. As a method of accelerated cooling, a predetermined structure and hardness can be obtained by performing heat treatment by a method as described in Cited Document 2, Cited Document 5, and the like.

  The metal structure of the head surface portion of the rail of the present invention is desirably a pearlite structure as described above. However, depending on the component system of the rail and the heat treatment production method, a very small 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 these structures. However, even if these structures are mixed, there is no significant adverse effect on the delayed fracture resistance of the rail and the wear resistance of the head surface. Also included is a small amount of pro-eutectoid ferrite structure, pro-eutectoid cementite structure, bainite structure, and martensite structure. In other words, the metal structure of the head surface portion of the rail of the present invention may be 95% or more if it is a pearlite structure. To ensure delayed fracture resistance and sufficiently improve the wear resistance, the head surface metal structure is sufficient. It is desirable to make 98% or more of the pearlite structure. In Tables 1 and 2, all the structures other than the pearlite structure described in the microstructure column mean an area ratio exceeding 5%.

(3) Reason for limiting number of U values shown in Formula 1 in mass% The reason why the U value shown in Formula 1 in mass% in one embodiment of the present invention is limited to a range of 50 to 200 will be described in detail.

  Considering the S content and P content of steel, the optimum range for improving delayed fracture characteristics was studied. As a result of various numerical analyses, the S content and P content of the steel are mutually considered by controlling the U value of Equation 1 shown below consisting of the product of the S addition amount and the P addition amount. It was found that the critical stress value of delayed fracture can be improved effectively.

  Furthermore, as a result of examining the optimum range, when the U value shown in Formula 1 exceeds 200, the P content in the steel becomes excessive, and improvement in the critical stress value of delayed fracture is not recognized. Moreover, when the U value shown in Formula 1 is less than 50, the effect of reducing the P content in the steel is saturated, the limit stress value of excess delayed fracture reaches a peak, and improvement of the effect is not recognized. For this reason, U value of Formula 1 shown below which consists of the product of S addition amount shown in Formula 1 and P addition amount was limited to the range of 50-200.

  Next, in one embodiment of the present invention, in addition to controlling S: 0.0250 to 0.0350%, the reason why the U value shown in Formula 1 in mass% is limited to the range of 50 to 150 is described below. This will be described in detail.

When the value shown in Formula 1 is controlled to 150 or less, it becomes possible to secure the amount of MnS sulfide, which is a hydrogen trap site, and to reduce the content of P, and ultimately control hydrogen embrittlement and destroy hydrogen embrittlement. A delay occurs in brittle fracture, and the delayed fracture resistance is greatly improved. For this reason, the U value of Formula 1 shown below consisting of the product of the S addition amount and the P addition amount shown in Formula 1 is limited to a range of 50 to 150.
U = P × S × 10 ⁶ Formula 1

(4) P amount control method A desirable manufacturing method for controlling the P amount will be described.
In the hot metal stage, P is contained in a large amount as an impurity. In general, the amount of P is controlled in a converter. In the converter, CaO is added and P is discharged into the slag as CaO · P ₂ O ₅ . When refining in a general converter, the amount of P is reduced to 0.0020 to 0.0300%. It is desirable to control the P content to 0.0100 or less by controlling the time of dephosphorization treatment in this converter and the amount of CaO added. As a result, the critical stress value is improved, and the delayed fracture resistance is stably improved.

(5) S amount control method A desirable manufacturing method for controlling the S amount will be described.
In the hot metal stage, S is contained in a large amount as an impurity. In general, the amount of S is controlled in a hot metal ladle before refining in a converter. In the hot metal ladle, CaO is added and S is discharged into the slag as CaS. When desulfurization is performed in a general hot metal ladle, the amount of S is reduced to 0.0030 to 0.0500%. It is desirable to control the amount of S to 0.0200 to 0.0350% or 0.0250 to 0.0350% by adjusting the desulfurization time and the amount of CaO added. As a result, the amount of MnS-based sulfide generated is increased and the delayed fracture resistance is improved.

(6) Control method of product of S addition amount and P addition amount (U = P × S × 10 ⁶ )
A method for controlling the product of S addition amount and P addition amount (U = P × S × 10 ⁶ ) will be described.
In general, hot metal refining is performed in the order of desulfurization in a hot metal ladle before refining the converter and dephosphorization in the converter. After desulfurization in the hot metal ladle, the molten steel is analyzed, and the dephosphorization amount in the converter is determined based on the amount of S. By controlling the dephosphorization time and the amount of CaO added, the S addition amount and P Control U value of product of addition amount. As a result, it is possible to secure the amount of MnS-based sulfide generated and reduce the P content, suppress hydrogen embrittlement, delay from hydrogen embrittlement to final brittle fracture, and have delayed fracture resistance. Greatly improved.

(7) Control method of H amount A desirable manufacturing method for controlling the H amount that further improves the delayed fracture characteristics will be described.
In the hot metal stage, H is contained as an impurity. Control of the amount of H is generally performed by secondary refining (degassing) after the converter. In secondary refining, the ladle is evacuated and H in the steel is discharged. It is desirable to control the amount of H to 2.0 ppm or less by controlling the treatment time in this secondary refining and to further improve the delayed fracture resistance.

  In addition, hydrogen may enter from the atmosphere after the refining and increase the amount of hydrogen in the steel slab after casting. In such a case, it is desirable to apply a method of diffusing hydrogen inside the steel slab by cooling the steel slab or reheating the steel slab.

Next, examples of the present invention will be described.
Table 1 shows the chemical composition and various characteristics of the rail of the present invention. Table 1 shows the chemical component value, the product of the S addition amount and the P addition amount (U = P × S × 10 ⁶ ), the microstructure of the head surface portion, and the hardness of the head surface portion. Furthermore, the result (limit stress value) of the delayed fracture test performed by the method shown in FIG. 7 is also shown. Note that the microstructure of the head surface part includes a mixture of a very small amount of pro-eutectoid ferrite structure, pro-eutectoid cementite structure, bainite structure and martensite structure of 5% or less in area ratio.

Table 2 shows the chemical composition and various properties of the comparative rail. Table 1 shows the chemical component value, the product of the S addition amount and the P addition amount (U = P × S × 10 ⁶ ), the microstructure of the head surface portion, and the hardness of the head surface portion. Furthermore, the result (limit stress value) of the delayed fracture test performed by the method shown in FIG. 7 is also shown. Note that the microstructure of the head surface part includes a mixture of a very small amount of pro-eutectoid ferrite structure, pro-eutectoid cementite structure, bainite structure and martensite structure of 5% or less in area ratio.

  In addition, the outline of the manufacturing process and manufacturing conditions of the rails of the present invention and the comparative rail shown in Tables 1 and 2 are as shown below.

  Molten steel ⇒ component adjustment (hot metal ladle: desulfurization, converter: dephosphorization, secondary refining: dehydrogenation) ⇒ casting (bloom) ⇒ reheating (1250 ° C) ⇒ hot rolling (finishing temperature 950 ° C) ⇒ heat treatment (starting temperature) 800 ℃, accelerated cooling) ⇒ Cooling

<Method of hydrogen content analysis>
The method of analyzing the hydrogen amount of the rails of the present invention and the comparative rail shown in Tables 1 and 2 is as follows.
(1) Analytical process: Sample molten steel from the mold at the time of steel piece casting (2) Sample holding method: Rapid cooling after sampling ⇒ Immersion in liquid nitrogen (3) Analytical method Thermal conductivity method
Sample size: A cylinder with a diameter of 6 mm and a thickness of 1 mm Heating temperature: 1900 ° C. (Induction heating of a sample on a graphite crucible)
Atmosphere: Inert gas (Ar)
Carrier gas: N2
Analyzer: Thermal conductivity detector

<Measurement method of hardness>
The microstructure of the head surface part of the present invention rail and the comparative rail shown in Tables 1 and 2 and the hardness of the head surface part were performed at a position 3 mm deep from the surface of the rail head surface part. The hardness was measured with a Vickers hardness meter. The measuring method is as shown below.
(1) Pretreatment: Rail cutting ⇒ Cross section polishing.
(2) Measuring method: Measured according to JIS Z 2244.
(3) Measuring machine: Vickers hardness meter (load 98N).
(4) Measurement location: a position 3 mm deep from the rail head surface.
(5) Number of measurements: It is desirable to measure 5 or more points and use the average value as the representative value of the steel rail.

<Conditions for delayed fracture test>
The conditions of the delayed fracture test of the rails of the present invention and the comparative rail shown in Tables 1 and 2 are as shown below.
(1) Rail shape: 136 pound rail (67 kg / m)
(2) Delayed fracture test
Test method: 3-point bending (span length: 1.5 m, see FIG. 6)
Test posture: Load applied to the bottom of the rail (tensile stress acting on the head).
Stress condition: 200 to 500 MPa (rail head surface)
Stress load time: 500 hours (3) Limit stress value: Maximum value of stress when unstressed when loaded for 500 hours at a predetermined stress

Details of the rails of the present invention and comparative rails shown in Tables 1 and 2 are as shown below.
(1) Invention rail (49)
Symbols A1 to A51: Rails in which the chemical component value, the product of the addition amount of S and the addition amount of P (P × S × 10 ⁶ ), and the microstructure of the head surface portion are within the scope of the present invention.
(2) Comparison rail (20)
Symbols B1 to B10 (10): A rail in which the addition amount of C, Si, Mn, P, and S and the microstructure of the head surface part are outside the scope of the present invention.
Symbols B11 to B20: rails whose product of the S addition amount and the P addition amount (P × S × 10 ⁶ ) is outside the scope of the present invention.

  As shown in Tables 1 and 2, the present invention rails (reference symbols A1 to A51) have a limited range of addition amounts of steel C, Si, Mn, P, and S compared to the comparative rails (reference symbols B1 to B10). By accommodating the inner surface, the formation of pro-eutectoid ferrite structure, pro-eutectoid cementite structure, bainite structure, and martensite structure can be suppressed, and by controlling the head surface part to pearlite structure, the delayed fracture characteristics can be improved.

In addition, as shown in Tables 1 and 2 and FIG. 5, the rail steel of the present invention (reference symbols A1 to A51) is C, Si, Mn, P, compared to the comparative rail steel (reference symbols B11 to B20). In addition to the addition amount of S, the product of the addition amount of S and the addition amount of P (U = P × S × 10 ⁶ ) is kept within a limited range, thereby improving delayed fracture characteristics at the same addition amount of S. be able to.

Also, Table 1, Table 2, as shown in FIG. 6, the present invention rail steels (code A 12 ~A13, A16~A17, A 21 ~A22, A 24 ~A25, A 27 ~A28, A 32 ~A33, a 36 ~A37, a 40 ~A43, a 46 ~A47, a 50 ~A51) further controls the value of the S amount and P addition amount of the product (U = P × S × 10 6), in which In addition, the delayed fracture characteristics can be further improved by controlling the H addition amount.

1: head portion 2: head corner portion 3: rail head portion 3a: head surface portion (range up to a depth of 20 mm starting from the head corner portion and the surface of the head portion, hatched portion)

Claims (4)

In mass%, C: 0.88 to 1.20%, Si: 0.10 to 2.00%, Mn: 0.10 to 2.00%, P ≦ 0.0100%, S: 0.0250 to In a steel rail containing 0.0350% and the balance being Fe and inevitable impurities, the U value shown in Formula 1 in mass% is in the range of 50 to 200, and the head corner portion of the steel rail and A rail characterized in that 95% or more of the head surface portion, which has a depth of 20 mm starting from the surface of the top of the head, is a pearlite structure.
U = P × S × 10 ⁶ Formula 1 The rail according to claim 1, wherein the U value shown in Formula 1 is in the range of 50 to 150 in terms of mass%.   The rail according to claim 2, further comprising, by mass%, H: 2.0 ppm or less. In mass%,
Mg: 0.0005 to 0.0200%,
Ca: 0.0005 to 0.0200%,
REM: 0.0005 to 0.0500%,
Cr: 0.01 to 2.00%
Mo: 0.01 to 0.50%,
Co: 0.01-1.00%,
B: 0.0001 to 0.0050%,
Cu: 0.01 to 1.00%,
Ni: 0.01-1.00%,
V: 0.005 to 0.50%,
Nb: 0.0010 to 0.050%,
Ti: 0.0030 to 0.0500%,
Zr: 0.0001 to 0.0200%,
N: 0.0060-0.0200%,
Al: 0.0100 to 1.00%
The rail according to any one of claims 1 to 3, comprising one or more of the following.

Images