JP2016053191A - Pearlitic high carbon steel rail excellent in ductility and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a pearlitic high carbon steel rail having enhanced ductility and a manufacturing method therefor.SOLUTION: There is provided a pearlitic high carbon steel rail containing C:0.70 to 1.40%, Si:0.10 to 2.00%, Mn:0.10 to 2.00%, P:0.025% or less, S:0.025% or less and, if needed, Cr, Mo, Co, B, Cu, Ni, Mg, Ca, REM, Zr, N, or Al, with two kinds of Ti, Nb and V each in a range of 0.0005 to 0.0060% and the remaining one in the range of 0 to 0.0080%, and the balance Fe with inevitable impurities, where a metallic structure of a rail top is a pearlite, and in a pearlite in any cross section of the rail top, the number of two or more kinds of a Ti-based deposit, a Nb-based deposit, a V-based deposit and a composite deposit having a diameter of 10 nm to 100 nm inclusive is 50,000 to 1,000,000 per area to be checked of 1 mm.SELECTED DRAWING: None

Description

  The present invention relates to a pearlitic high carbon steel rail and a manufacturing method thereof. In particular, the present invention relates to a pearlite high carbon steel rail having high ductility and a manufacturing method thereof.

  Steel having a pearlite structure has excellent wear resistance. For this reason, steel having a pearlite structure is used as a material suitable for railroad rails. In recent years, in order to further improve transportation efficiency in heavy-duty railways that transport coal and iron ore, the increase of cargo loading has been promoted. Along with this, since the weight per train increases, the load applied to the rail has increased. As a result, the wear of the rail head in contact with the wheel is promoted, and the life of the rail is reduced compared to the conventional case. For this reason, measures for shortening the rail replacement cycle accompanying a decrease in rail life are also an issue.

  Against this background, the development of rail steel with improved wear resistance is in progress. There are two ways. One method is to increase the hardness of the rail head. The other is a method of increasing the cementite density in the lamella in the pearlite structure by setting the carbon (C) content of the rail steel represented by Patent Document 1 as a hypereutectoid composition exceeding 0.85 mass%.

  However, in any method, there is a drawback that ductility and toughness are lowered as compared with general rail steel (Non-Patent Document 1).

  In order to improve the ductility and toughness of the pearlite structure, it is generally effective to refine the structural unit called “pearlite block” in which the crystal orientation of ferrite is the same in the pearlite structure (Non-patent Document 2).

  The following three methods are considered to be effective for miniaturizing the pearlite block. The first is the refinement of austenite grains before pearlite transformation. Secondly, the degree of supercooling from austenite. Thirdly, the number of pearlite nucleation is increased by utilizing heterogeneous nucleation sites such as inclusions. Among these methods, the simplest method is to refine austenite grains.

  The following three methods are mentioned as a method for refining austenite grains. First, lowering the reheating temperature at the time of reheating the steel pieces for rail rolling. Secondly, lowering the final rolling temperature during hot rolling. Thirdly, the increase in the cross-sectional area reduction rate (increase in the amount of strain) during hot rolling. By these methods, the austenite grains immediately after rolling can be refined. However, in an actual production line, there is conveyance from the finish rolling process to the accelerated cooling process, and it takes time to start accelerated cooling. In the meantime, even if the above three methods are applied and the austenite grains are refined under the final pressure of the finish rolling process, the austenite grains become coarse due to grain growth, so that the austenite grains are not refined eventually.

  Further, in the first and second methods among the methods for refining austenite grains, in hypereutectoid steel, a brittle structure called pro-eutectoid cementite that lowers ductility and toughness may be generated at the austenite grain boundaries before pearlite transformation. Lowering the reheating temperature or lowering the final rolling temperature during hot rolling means lowering the accelerated cooling start temperature. When the accelerated cooling heating temperature decreases, proeutectoid cementite is likely to be generated. Therefore, a rail with excellent ductility cannot be obtained.

  In response to such problems, pearlite rails that have been refined by improving the ductility of pearlite blocks by using pinning by precipitates and inhibiting austenite grain growth have been developed (patents). Reference 2).

  However, in the case of the pearlite rail of Patent Document 2 described above and the manufacturing method thereof, there are problems such as ensuring the roll formability and reducing ductility due to the generation of proeutectoid cementite inside the head. This is because in Patent Document 2, it is necessary to reheat at a low temperature in order to finely disperse AlN.

  In addition, a technique using Ti in addition to Al (Patent Document 3) has been developed. Ti has the following properties. Since Ti is a strong deoxidizing element in the molten steel in the steelmaking process, it is easy to crystallize a Ti-based oxide. Ti tends to crystallize coarser carbides in the molten steel at the end of solidification accompanied by solidification segregation. When N in the molten steel is high, Ti tends to crystallize nitrides and carbonitrides. These products become particles of about several μm to several tens of μm due to aggregation or coarsening in the molten steel. The pinning force of austenite grains increases as the particle diameter decreases and the number of particles increases. If coarse particles are generated in the molten steel, it may not be possible to secure a sufficient number of particles having a small particle size effective for pinning. For this reason, in order to increase pinning particle | grains, a large amount of precipitate production | generation elements will be added, efficiency is bad, and since addition amount cost increases, it is economically bad. Furthermore, since coarse particles serve as a starting point of fracture, there is a problem that ductility and impact characteristics (toughness) are lowered.

Japanese Patent No. 3078461 JP 2002-302737 A Japanese Patent No. 5053187

Japanese Industrial Standards JIS E 1101 "Normal rails and special rails for turnouts" Takahashi, Nagumo, Asano, Journal of the Japan Institute of Metals, Volume 42, 1978, P708-715. Revision 4 Metal Data Book, Maruzen Co., Ltd.

  This invention is made | formed in view of the said situation, and makes it a subject to provide the pearlite type | system | group high carbon steel rail provided with high ductility, and its manufacturing method.

  When the present inventors diligently studied, they found the following. Ti-based precipitates (for example, TiC, TiN, Ti (C, N)), Nb-based precipitates (for example, NbC, NbN, Nb (C, N)), V-based precipitation in austenite during hot rolling (E.g., VC, VN, V (C, N)) or Ti, Nb or V composite precipitates (e.g., Ti-Nb-based precipitates, Ti-V-based precipitates, Nb-V-based precipitates, Ti- Nb-V-based precipitates) are finely precipitated. Next, austenite grain growth is suppressed between rolling passes and from final rolling to accelerated cooling. Then, the pearlite block size can be miniaturized. By this method, a pearlite rail having high ductility can be obtained.

Hereinafter, various aspects of the present invention will be described.
(1) The pearlitic high carbon steel rail according to one embodiment of the present invention has a chemical composition of mass%, C: 0.70 to 1.40%, Si: 0.10 to 2.00%, Mn: 0.10 to 2.00%, P: 0.025% or less, S: 0.025% or less, Cr: 0 to 2.00%, Mo: 0 to 0.50%, Co: 0 to 2.00 %, B: 0 to 0.0050%, Cu: 0 to 1.00%, Ni: 0 to 1.00%, Mg: 0 to 0.0200%, Ca: 0 to 0.0200%, REM: 0 -0.0500%, Zr: 0-0.0200%, N: 0-0.0200%, Al: 0-1.00%, and two of Ti, Nb, and V are each 0.0005 -0.0060% of the range, the remaining one is in the range of 0-0.008%, the remainder consists of Fe and impurities, Is pearlite, and in the pearlite in an arbitrary cross section of the rail head, the total of two or more of Ti-based precipitates, Nb-based precipitates, V-based precipitates or composite precipitates having a diameter of 10 nm to 100 nm is There are 50,000 to 1,000,000 pieces per 1 mm ^{2 of the} test area.
(2) The pearlitic high carbon steel rail described in (1) has a chemical composition of mass%, Cr: 0.05 to 2.00%, Mo: 0.01 to 0.50%, Co : 0.10 to 2.00%, B: 0.0005 to 0.0050%, Cu: 0.05 to 1.00%, Ni: 0.01 to 1.00%, Mg: 0.0005 to 0 0.0200%, Ca: 0.0005-0.0200%, REM: 0.0005-0.0500%, Zr: 0.0001-0.0200%, N: 0.0020-0.0200%, and Al : You may contain 1 type (s) or 2 or more types of 0.0020-1.00%.
(3) The manufacturing method of the pearlite type | system | group high-carbon steel rail which concerns on 1 aspect of this invention is a chemical component mass%, C: 0.70-1.40%, Si: 0.10-2.00% , Mn: 0.10 to 2.00%, Ti, Nb, and V are in the range of 0.0005 to 0.0060% each, and the remaining one is 0 to 0.008% When forming the steel strip for rolling a rail into a rail by hot rolling, the reheating temperature of the steel strip for rolling is set to 1200 ° C or higher, and the final rolling of the hot rolling is performed in a range of 850 to 1050 ° C. Accelerated cooling after hot rolling is performed at an average cooling rate of 2 to 30 ° C./sec from an austenite temperature range where the rail head surface temperature is 700 ° C. or higher to a temperature range of 550 ° C. to 650 ° C., to at least 400 ° C. Allow to cool.

  According to one embodiment of the present invention, the structure of rail steel and the size and number of precipitates can be controlled. Since the austenite grain growth after the final rolling in the hot rolling process can be inhibited by fine precipitates, a fine pearlite block can be obtained. Since a fine pearlite block can be obtained, the rail can have high ductility. As a result, the service life of the rail can be greatly improved.

Graph showing the relationship between Ti content and total elongation Cross-sectional schematic diagram showing the head of the rail A graph showing the relationship between the number of precipitates having a diameter of 10 nm or more and 100 nm or less and the total elongation SEM photograph showing an example of precipitates in the metal structure of the rail Tensile specimen collection position Graph showing the relationship between C amount and total elongation

  In general, when using a precipitate to inhibit (pinning) the crystal growth of the parent phase (austenite in the present application), the precipitate preferably has the following two characteristics. One is “the size of individual precipitates is small”, and the other is “the number density (volume fraction) of small precipitates is large”. The more these are satisfied, the stronger the ability to inhibit crystal grain growth.

  Inventors examined refinement | miniaturization of the austenite grain before a pearlite transformation in order to refine | miniaturize a pearlite structure | tissue in rail steel (to obtain a fine pearlite block). At this time, attention was paid to utilization of precipitates (carbides, nitrides, carbonitrides) by addition of Ti, Nb, and V. The effect of pinning was observed when the amount of each element added was large. However, coarse precipitates that are thought to be formed at high temperatures in austenite and coarse crystallized substances that are thought to be formed in molten steel during solidification are mixed. It sometimes went down. That is, coarse precipitates and crystallized substances are factors that impair ductility. On the other hand, when the addition amount of each element of Ti, Nb, and V is small, generation of coarse precipitates and crystallized substances that are harmful to ductility is not observed, and fine precipitates having a size effective for pinning are generated. . However, since the number density of precipitates is small, the effect of inhibiting the growth of austenite grains is low, and as a result, pearlite cannot be refined. That is, it is impossible to achieve both “small size of individual precipitates” and “high number density (volume fraction) of precipitates”.

  Precipitates composed of Ti, Nb, and V elements are treated as having the same effect. Moreover, since it may form a composite precipitate, it is easily recognized as a similar element. For this reason, if the total addition amount of each element of Ti, Nb, and V is large, it behaves in the same manner as the addition amount is large, and coarse composite precipitates are generated, and “the size of each precipitate is small”. It is easy to be misunderstood. However, these are separate elements. If the inventors satisfy the “small size of individual precipitates” and add another additive element that satisfies the “small size of individual precipitates”, then “the size of the individual precipitates” I thought that it would be possible to achieve both “small” and “large number density (volume fraction) of precipitates”. Therefore, by adding two or more kinds of Ti, Nb, and V in small amounts, it is possible to avoid generation of coarse precipitates and crystallized substances that are harmful to ductility, and to provide fine precipitates that are effective in inhibiting austenite grain growth. It was found that the austenite grains were refined by securing the amount, and the ductility was improved by refinement of the pearlite structure. Hereinafter, embodiments of the present invention will be described in detail.

(1) Reasons for limiting chemical components and metal structures (1-a) Reasons for limiting chemical components First, the reasons for limiting the chemical components of rail steel to the claims will be described in detail. Hereinafter, the mass% in the composition is simply described as%.

  C is an effective element that promotes pearlite transformation and ensures wear resistance. If the C content is lower than 0.70%, a pearlite structure cannot be obtained stably, and proeutectoid ferrite harmful to wear resistance is likely to be produced. That is, the present invention has a high carbon pearlite structure and has an issue of improving the ductility of a highly wear-resistant rail, so the lower limit of the C content is 0.70%. When the amount of C is equal to or higher than the eutectoid concentration, the volume ratio of cementite in the pearlite structure increases as the amount of C increases, and the wear resistance improves. When the amount of C exceeds 1.40%, generation of pro-eutectoid cementite which is harmful to ductility becomes remarkable. For this reason, the amount of C is limited to 0.70 to 1.40%. More preferably, it is 0.80-1.30%, More preferably, it is 0.90-1.20%.

  Si is a component utilized as a deoxidizing material. Further, it is an element that improves the wear resistance by improving the hardness (strength) of the pearlite structure by solid solution strengthening of ferrite in the pearlite structure. However, when the Si amount is less than 0.10%, deoxidation is insufficient, and a coarse oxide is mixed in the pearlite structure, and the oxide becomes a starting point of destruction, so that the ductility is likely to be lowered. If the Si content exceeds 2.00%, the ferrite becomes brittle and the ductility of the rail decreases. For this reason, the amount of Si is desirably 0.10 to 2.00%. More preferably, it is 0.20-1.50%, More preferably, it is 0.30-1.40%.

  Mn has the effect of delaying the pearlite transformation. When Mn is added, when accelerated cooling is performed at the same cooling rate, the pearlite transformation temperature is lowered and the lamella spacing of the pearlite structure is refined. This increases the hardness (strength) of the rail head and improves wear resistance. However, when the amount of Mn is less than 0.10%, these effects are small. In addition, if the Mn content is less than 0.10%, not only a pearlite structure with low hardness and low wear resistance is generated, but also the degree of supercooling at the time of pearlite transformation with insufficient hardenability cannot be secured sufficiently, and the coarse pearlite structure Thus, the ductility is lowered. When the Mn content exceeds 2.00%, the hardenability is remarkably increased, and a bainite structure that deteriorates the wear resistance and a martensite structure that is harmful to ductility are easily generated on the rail head surface. Therefore, the Mn content is desirably 0.10 to 2.00%. More preferably, it is 0.20-1.20%, More preferably, it is 0.30-1.00%.

  P is an impurity element in steel. It is possible to control the amount of P by performing refining in a converter. If the amount of P exceeds 0.025%, the ductility of the rail steel decreases. Therefore, the P content is preferably 0.025% or less. Preferably it is 0.020% or less.

  S is an impurity element in steel. It is possible to control the amount of S by performing desulfurization with a hot metal ladle. When the amount of S exceeds 0.025%, coarse sulfides such as MnS as inclusions are likely to be generated. In this case, stress concentration around the inclusions causes early rail breakage, and ductility decreases. For this reason, the amount of S is desirably 0.025% or less. Preferably it is 0.020% or less.

  Ti is added in a small amount to steel, so that it precipitates as fine Ti-based precipitates (eg, TiC, TiN, Ti (C, N)) in austenite in the hot rolling process, or other elements (V, Nb). Etc.). The precipitation of Ti-based precipitates or composite precipitation with Ti, V, Nb, etc. occurs in austenite, thereby suppressing the growth of austenite grains after recrystallization, miniaturization of austenite grains, and ductility of rail steel To improve. However, if the amount of Ti is less than 0.0005%, the number of fine precipitates is insufficient, the effect of refining austenite grains cannot be sufficiently expected, and no improvement in ductility is observed. If the Ti content exceeds 0.0060%, coarse Ti-based precipitates are likely to be generated, and the effect of suppressing the grain growth of austenite becomes small, so the ductility of the rail steel is not improved by Ti. Furthermore, when it exceeds 0.0080%, the ductility of rail steel will be reduced. This is thought to be because, when solidified from molten steel to a steel strip for rolling a rail, a coarse crystallized product is likely to be generated in the solidified segregation portion, and it becomes a starting point of fracture due to the use characteristics of the rail. For this reason, the amount of Ti is desirably 0.0005 to 0.0060%. If the Nb amount and the V amount are 0.0005 to 0.0060%, respectively, the improvement in ductility due to Nb and V is not impaired, so the Ti amount is 0 to less than 0.0005% or more than 0.0060 to 0.0080. % Or less is also acceptable.

  Nb is added in a small amount to steel, so that it precipitates as fine Nb-based precipitates (for example, NbC, NbN, Nb (C, N)) in austenite in the hot rolling process, or other elements (Ti, V Etc.). The precipitation of Nb-based precipitates or composite precipitation with Nb, Ti, V, etc. occurs in austenite, thereby suppressing the growth of austenite grains after recrystallization, miniaturization of austenite grains, and ductility of rail steel To improve. However, if the Nb content is less than 0.0005%, the number of fine precipitates is insufficient, and the effect of refining austenite grains cannot be expected sufficiently, and no improvement in ductility is observed. On the other hand, if the Nb content exceeds 0.0060%, coarse Nb-based precipitates are likely to be generated, and the effect of suppressing the grain growth of austenite becomes small, so the ductility of the rail steel is not improved by Nb. Furthermore, if the Nb amount exceeds 0.0080%, the ductility of the rail steel is lowered. When solidifying from molten steel to a steel strip for rolling a rail, a coarse crystallized product is likely to be generated in the solidified segregation part, and there is a concern that it may become a starting point of fracture due to the use characteristics of the rail. Therefore, the Nb amount is preferably 0.0005 to 0.0060%. If the amount of Ti and the amount of V are 0.0005 to 0.0060%, respectively, the improvement in ductility by Ti and V is not impaired, so the amount of Nb is 0 to less than 0.0005% or more than 0.0060 to 0.0080. % Or less is also acceptable.

  V is precipitated as fine V-based precipitates (for example, VC, VN, V (C, N)) in austenite in the hot rolling process, or other elements (Ti, Nb) by adding a small amount to steel. Etc.). The precipitation of V-based precipitates or composite precipitation with V, Nb, Ti, etc. occurs in austenite, thereby suppressing the growth of austenite grains after recrystallization, miniaturization of austenite grains, and ductility of rail steel To improve. However, if the amount of V is less than 0.0005%, the number of fine precipitates is insufficient, the effect of refining austenite grains cannot be sufficiently expected, and no improvement in ductility is observed. Further, if the V content exceeds 0.0060%, single or other Ti or Nb and coarse composite precipitates are formed, and the effect of suppressing the grain growth of austenite becomes small, so V improves the ductility of the rail steel. do not do. Furthermore, if the amount of V exceeds 0.0080%, the ductility of the rail steel is lowered. This is thought to be because, when solidified from molten steel to a steel strip for rolling a rail, a coarse crystallized product is likely to be generated in the solidified segregation portion, and it becomes a starting point of fracture due to the use characteristics of the rail. For this reason, the V amount is preferably 0.0005 to 0.0060%. If the Ti content and the Nb content are 0.0005 to 0.0060%, respectively, the ductility improvement by Ti and Nb is not impaired, so the V content is 0 to less than 0.0005% or more than 0.0060 to 0.0080. % Or less is also acceptable.

Although Ti, Nb, and V have been described above, precipitates of the same size and number are generated within the above range.
If all of Ti, Nb, and V are within the range of 0.0005 to 0.0060%, the pinned pinning of the austenite grain growth of the fine precipitate results in accelerated cooling compared to the case where the precipitate is not applied. The previous austenite becomes finer, the pearlite structure after accelerated cooling becomes finer, and the total elongation improves. However, as described above, among the three types of Ti, Nb, and V, two types are within the above range, and the remaining type is 0 to 0.0080% even if the other type is outside the range of 0.0005 to 0.0060%. If it is within the range, the effect of miniaturization is slightly inferior, but the effect of improving the total elongation is obtained. This effect will be described in detail.

  In the laboratory, the inventors measured C content: 1.0%, Si content: 0.7%, Mn content 0.7%, P content 0.01%, S content: 0.008%, precipitates. A steel in which the amount of Ti was changed on the basis of the amount of the generated element V: 0.0055% and the amount of Nb: 0.004% was melted and cast. These steel ingots were made into hot rolled sheets under the same heating, rolling and cooling conditions, and then the total elongation was evaluated by a tensile test.

  FIG. 1 shows the relationship between Ti content and total elongation. The total elongation was highest when the Ti content was between 0.0005 and 0.0060%, that is, when all the precipitate-generating elements were within the limited range. When it was added more than 0.0060% to 0.0080%, the total elongation slightly decreased, and when the amount added exceeded 0.0080%, the total elongation further decreased.

As a result of observing the structure in detail, when all the precipitate-generating elements are within the above-mentioned limited range, coarse carbides, nitrides, and carbonitrides that reduce the toughness were not confirmed. When the amount of Ti exceeds 0.0060% to 0.0080%, trace amounts of coarse carbides, nitrides, and carbonitrides due to Ti have been confirmed, but suppression of austenite grain growth of fine precipitates due to Nb and V is suppressed. As a result, the effect of improving the ductility due to the refinement of the pearlite structure was sufficiently exhibited. When the Ti amount exceeds 0.0080%, the number of coarse carbides, nitrides, and carbonitrides due to Ti increases, and the total elongation improvement effect due to the austenite grain refinement effect due to the fine precipitates due to Nb and V It is thought that it was damaged.
When the amount of Ti is less than 0.0005%, the austenite refinement effect due to the fine precipitates due to Ti cannot be obtained, but the coarse precipitates of Ti do not impair the toughness, and the fine precipitates due to Nb and V are sufficient. An austenite grain refinement effect can be obtained.
This behavior was also observed when the remaining type was Nb or V instead of Ti.
From the above, in order to finely disperse precipitates, pin the austenite grain growth during hot rolling, refine the pearlite structure, and improve the ductility, two of Ti, Nb, and V are each The range is 0.0005 to 0.0060%, and the remaining one type is desirably 0.008% or less.

  In addition, the rail manufactured with the above component composition is Cr, Mo, for the purpose of improving wear resistance and ductility by increasing the hardness (strength) of the pearlite structure and preventing softening of the heat affected zone. One or more elements of Co, B, Cu, Ni, Mg, Ca, REM, Zr, N, and Al may be contained as necessary.

  Cr raises the equilibrium transformation point of pearlite. When Cr is added at the same cooling rate, the difference between the equilibrium transformation temperature and the pearlite transformation temperature, that is, the degree of supercooling is increased, and the lamella spacing of the pearlite structure is made fine. Thus, Cr is an element contributing to high hardness (strength). At the same time, Cr is an element that improves the wear resistance by strengthening the cementite phase and improving the hardness (strength) of the pearlite structure. However, the effect is small when the Cr content is less than 0.05%. Moreover, when excessive Cr addition exceeding 2.00% is performed, hardenability will increase remarkably, and the bainite structure which deteriorates abrasion resistance and the martensitic structure which reduces ductility will form on the rail head surface. It becomes easy. Therefore, the Cr content may be limited to 0.05 to 2.00%.

  Mo, like Cr, is an element that increases the equilibrium transformation point of pearlite and, as a result, refines the pearlite structure, thereby contributing to higher hardness (strength) and improving 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. In addition, when Mo is added in excess of 0.50%, hardenability is remarkably increased, and a bainite structure that deteriorates wear resistance and a martensite structure that decreases ductility are formed on the rail head surface. It becomes easy. For this reason, you may limit Mo content to 0.01 to 0.50%.

  Co is an element that dissolves in the ferrite in the pearlite structure and improves the hardness (strength) of the pearlite structure by solid solution strengthening. Further, Co is an element that improves ductility by increasing the transformation energy of pearlite and making the pearlite structure fine. If the Co content is less than 0.10%, these effects cannot be expected. Moreover, when Co is added exceeding 2.00%, the ferrite in a pearlite structure will become remarkably embrittled, and the ductility of rail steel will fall remarkably. For this reason, the Co content may be limited to 0.10 to 2.00%.

  B is the effect of inhibiting the growth of austenite grains by forming iron boride at the austenite grain boundary, the effect of refining the formation of proeutectoid cementite structure in hypereutectoid steel, and the cooling rate of the pearlite transformation temperature. It is an element that prevents the deterioration of the ductility of the rail and extends the life by the effect of reducing the dependency and making the hardness distribution of the head uniform. If B is less than 0.0005%, the effect is not sufficient, and no improvement is observed in the formation of pro-eutectoid cementite and the hardness distribution of the rail head. Moreover, when B is added exceeding 0.0050%, a coarse iron-bore boride will produce | generate in an austenite grain boundary, and the ductility of rail steel will fall large. Therefore, the B content may be limited to 0.0005 to 0.0050%.

  Cu is an element that dissolves in the ferrite in the pearlite structure and improves the hardness (strength) of the pearlite structure by solid solution strengthening. If the amount of Cu is less than 0.05%, the effect cannot be expected. Moreover, when Cu is added exceeding 1.00%, a bainite structure that is harmful to the wear resistance of the rail head and a martensite structure that is harmful to ductility are likely to be generated due to remarkable improvement in hardenability. Furthermore, the ferrite in the pearlite structure becomes extremely brittle due to excessive solid solution strengthening, and the ductility of the rail steel is lowered. In addition, it may cause embrittlement during hot rolling and reduce the ductility of the rail. Therefore, the Cu content may be limited to 0.05 to 1.00%.

  Ni is an element that prevents embrittlement during hot rolling due to the addition of Cu, and at the same time, increases the hardness (strength) of pearlite steel by solid solution strengthening in ferrite. However, if the amount of Ni is less than 0.01%, the effect is remarkably small. Moreover, when Ni is added exceeding 1.00%, the ductility of the ferrite phase in a pearlite structure will fall remarkably, and the ductility of rail steel will fall. For this reason, Ni content may be limited to 0.01 to 1.00%.

  When Mg is added, it combines with O, S, Al or the like to form fine oxides or sulfides in the molten steel or austenite. These oxides and sulfides suppress the growth of austenite grains in the reheating process of hot rolling, and aim to refine the austenite grains. Thus, Mg is an element effective for improving the ductility of the pearlite structure. Furthermore, MgO, MgS finely disperses MnS, forms a thin Mn band around MnS, contributes to the formation of pearlite transformation, and as a result, by reducing the pearlite block size, the ductility of the pearlite structure To improve. However, if the amount of Mg is less than 0.0005%, the effect is weak. Moreover, when Mg is added exceeding 0.0200%, a coarse oxide of Mg is generated in the molten steel, and the ductility of the rail steel is lowered. Therefore, the Mg content may be limited to 0.0005 to 0.0200%.

  Ca has a strong binding force with S, forms a sulfide as CaS, CaS finely disperses MnS, forms a Mn dilute band around MnS, and contributes to the generation of pearlite transformation. As a result, it is an effective element for improving the ductility of the pearlite structure by reducing the pearlite block size. 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 the ductility of the rail steel is lowered, so the Ca content is 0.0005 to 0.00. It may be limited to 0200%.

REM is a deoxidation / desulfurization element, and when it is contained, REM generates oxysulfide (REM ₂ O ₂ S) and becomes a nucleus of Mn sulfide inclusions. Since the oxysulfide that is the nucleus has a high melting point, it suppresses the stretching of the Mn sulfide inclusions after rolling. As a result, REM finely disperses MnS, which is an inclusion, relieves stress concentration around the inclusion, and prevents a decrease in ductility. 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. On the other hand, when the amount of REM exceeds 0.0500%, hard REM oxysulfide is generated, and due to stress concentration, early rupture tends to occur and ductility decreases. For this reason, you may limit REM content to 0.0005 to 0.0500%.

  Note that REM is a rare earth element such as Ce, La, Pr, or Nd. The above content limits the total content of all these REMs. If the sum of the contents is within the above range, the same effect can be obtained even if each element is in a single form or a form in which each element is contained in a composite form (a form in which two or more elements are contained).

Zr generates ZrO ₂ in molten steel. ZrO ₂ has good lattice matching with austenite, so it becomes a solidification nucleus in the rail steel whose solidification primary crystal is austenite, increases the equiaxed crystallization rate of the solidification structure, and suppresses the formation of a segregation zone at the center of the slab. . Therefore, in hypereutectoid steel rails, Zr is an element that suppresses the formation of pro-eutectoid cementite structure formed in the segregated portion. However, when the amount of Zr is less than 0.0005%, the number of ZrO ₂ -based inclusions is small, and a sufficient effect as a solidification nucleus is not exhibited. As a result, a pro-eutectoid cementite structure is generated in the segregation part, and the ductility of the rail steel is reduced. On the other hand, if the amount of Zr exceeds 0.0200%, a large amount of coarse Zr-based inclusions are generated, which becomes a starting point of fracture and the ductility of the rail steel is lowered. Therefore, the Zr content may be limited to 0.0005 to 0.2000%.

  N is an element that combines with Ti, Nb, and V in molten steel or austenite to generate nitrides and carbonitrides. However, when the N content is less than 0.0020%, the effect is weak and does not contribute to the formation of fine precipitates. If the N content exceeds 0.0200%, most of Ti and Nb crystallize coarsely as nitrides and carbonitrides in the molten steel. The crystallized nitride and carbonitride are not dissolved in austenite in the reheating stage during hot rolling, and suppress grain growth of austenite during hot rolling and immediately after hot rolling. For this reason, fine Ti and Nb nitrides and carbonitrides cannot be produced. In addition, coarse nitrides and carbonitrides are the starting points for fracture, and therefore reduce ductility. Therefore, the N amount may be limited to 0.0020% to 0.0200%.

  Al is a component useful as a deoxidizer. In addition, it is an element that moves the eutectoid transformation temperature to the higher temperature side and the amount of eutectoid carbon to the higher carbon side, and is an effective element for increasing the strength of the pearlite structure and suppressing the formation of the proeutectoid cementite structure. However, if the Al content is less than 0.0020%, the effect is weak. Moreover, when Al is added exceeding 1.00%, coarse alumina inclusions are generated in the molten steel, and the ductility of the rail steel is lowered. In addition, coarse inclusions become the starting point of fatigue damage when using rails, and oxides are generated during welding, which significantly deteriorates the mechanical properties of the weld. For this reason, you may limit Al content to 0.0020 to 1.00%.

  The rail according to one aspect of the present embodiment contains the above components, and the balance contains iron and impurities. Examples of impurities include those contained in raw materials such as ores and scraps, and those mixed in the manufacturing process.

  Rail steel composed of the above composition is melted by a conventional method such as a converter or an electric furnace, and this molten steel is cast by hot-rolling by the ingot / bundling method or continuous casting method. Casting. The steel strip for hot rolling is formed into a rail by hot rolling. After hot rolling, accelerated cooling is performed by a cooling device from the austenite region. Alternatively, after hot rolling, the steel slab cooled to near room temperature by being allowed to cool is reheated to the austenite region, and then accelerated cooling is performed.

(1-b) Reason for limitation of metal structure The metal structure of the head of the rail of the present invention will be described. The metal structure of the head is preferably a pearlite structure with excellent wear resistance, but depending on the component system and the accelerated cooling conditions, a small amount of pro-eutectoid ferrite structure, pro-eutectoid cementite structure, bainite structure and martensite structure Site organization may be mixed. Even if these structures are mixed in a trace amount, the rail characteristics are not adversely affected. Therefore, in the cross-section of the rail head (see Fig. 2), the area ratio is 5% in the pro-eutectoid ferrite structure, pro-eutectoid cementite structure, A bainite structure or a martensite structure may be included. In other words, the area ratio of the pearlite structure of the rail head is set to 95% or more. Therefore, the upper limit is 100%. The pearlite structure in the present application is a state where the area ratio of the pearlite structure is 95% or more.

(1-c) Reasons for limiting the size and number of precipitates generated on the rail head Further, Ti-based precipitates, Nb-based precipitates, V-based precipitates, composite precipitates (for example, Ti) in steel in the rail of the present invention The reasons for limiting the size and number of the -Nb-based precipitates, Ti-V-based precipitates, Nb-V-based precipitates, and one or more Ti-Nb-V-based precipitates) will be described.

  The size of the precipitate is measured by the following method. It is determined by measuring the average particle size of the precipitates observed by the method described later. When the precipitate is close to a perfect circle on the photograph (two-dimensionally), the diameter of the area equal to the precipitate is taken as the average particle diameter. The average particle diameter of the ellipsoidal and rectangular parallelepiped precipitates is not the true spherical shape, but the average value of the major axis (long side) and the minor axis (short side).

  Precipitates having an average particle diameter of less than 10 nm have a pinning effect on austenite grain growth, but are difficult to measure the number during observation, and are therefore excluded from the size limitation. On the other hand, when the average particle diameter of the precipitate exceeds 100 nm, the pinning force is insufficient, the austenite grain growth cannot be sufficiently suppressed, and the pearlite structure cannot be refined. For this reason, the size of the precipitate to be measured is limited to 10 to 100 nm.

The authors conducted a hot rolling experiment simulating rail rolling in order to verify how much the precipitates of 10 to 100 nm are dispersed, which is effective in improving ductility. In order to change the number of precipitates based on steel containing C = 1.10%, Si = 0.60%, Mn = 0.80%, P = 0.012%, S = 0.010%, Ti, Nb, and V were added as appropriate. The ingot adjusted for dissolution was placed in a heating furnace and held at a furnace temperature of 1250 ° C. for 60 minutes. The ingot extracted from the heating furnace was finally rolled at a temperature of 950 ° C. after rough shaping. After the hot rolling, the rolled plate surface was subjected to accelerated cooling from 800 ° C. Accelerated cooling was applied until the surface reached 600 ° C. The cooling rate during that time was 6 ° C./sec.
The result of the tensile test of the hot rolled sheet produced in the laboratory is shown in FIG. As shown in FIG. 3, an improvement in total elongation (ductility) is recognized in the range where 50,000 to 1,000,000 precipitates of 10 to 100 nm exist per 1 mm ² . The reason why the total elongation is not improved outside this range will be described below.
Even if precipitates having an average particle size of 10 to 100 nm are produced, if the number of produced particles is less than 50,000 per 1 mm ² , the austenite grain growth suppressing effect is weak and ductility is not improved. On the other hand, when the number exceeds 1,000,000 per 1 mm ² , the deformation of the pearlite structure is constrained, and the ductility is reduced. Therefore, precipitates in the steel is limited to the range 1 mm 50,000 to 1,000,000 pieces per ^2.

Here, a method for measuring the density and size of the precipitate will be described. The corroded surface at an arbitrary position in the head section is observed using a scanning electron microscope (SEM), or an extracted replica sample and a thin film sample are prepared and observed using a transmission electron microscope (TEM). As an example, FIG. 4 shows an example in which precipitates are observed using a scanning electron microscope. The number of precipitates of 10 nm or more and 100 nm or less is measured for an area of at least 1000 μm ² or more. This measurement result is converted into the number per unit area. For example, when one visual field is observed as 100 mm × 80 mm at a magnification of 20,000 times, the observation area per visual field is 20 μm ² , so at least 50 visual fields are randomly observed. In this case, if the number of precipitates of 10 to 100 nm or less is 100 in 50 fields (1000 μm ² ), the particle density can be converted to 100,000 per 1 mm ² .

(2) Manufacturing conditions Next, manufacturing conditions for manufacturing the rail of the present invention will be described.
The rail of the present invention is manufactured through a step of hot rolling a steel slab to form the rail and a step of performing accelerated cooling.

(2-a) Heating temperature In the reheating step of the steel strip for rail rolling during hot rolling, if the reheating temperature is less than 1200 ° C, coarse Ti-based precipitates generated during cooling after casting ( For example, TiC, TiN, Ti (C, N)), Nb-based precipitates (for example, NbC, NbN, Nb (C, N)), V-based precipitates (for example, VC, VN, V (C, N)), or Composite precipitates (for example, Ti-Nb-based precipitates, Ti-V-based precipitates, Nb-V-based precipitates, Ti-Nb-V-based precipitates) are partially undissolved and finely precipitated in austenite during rolling. The number of precipitates to be reduced is reduced, the effect of inhibiting the growth of austenite grains cannot be sufficiently exhibited, and the effect of improving the ductility of the rail is lowered. For this reason, the heating temperature is preferably 1200 ° C. or higher. Accordingly, coarse Ti-based precipitates (for example, TiC, TiN, Ti (C, N)), Nb-based precipitates (for example, NbC, NbN, Nb (C, N)), V-based precipitates (for example, VC, VN, V (C, N)) or composite precipitates (for example, Ti—Nb-based precipitates, Ti—V-based precipitates, Nb—V-based precipitates, Ti—Nb—V-based precipitates) This is possible if the internal temperature is 1200 ° C. or higher and the holding time is 30 minutes or longer.

(2-b) Final rolling temperature of hot rolling Ti, Nb, V dissolved in the steel in the reheating process of the steel slab for rail rolling is the final finishing rolling in the finishing rolling process during hot rolling. The dislocations introduced into the austenite at the time of rolling can be precipitated as fine and large amounts as nucleation sites. However, when the final finish rolling temperature exceeds 1050 ° C. at the top, not only the dislocations that are nucleation sites disappear quickly due to recovery, but also some of the precipitates formed on the dislocations become coarse, and austenite Since the grain growth inhibiting effect is reduced, the pearlite structure cannot be sufficiently refined. When the final rolling temperature is lower than 850 ° C., the recrystallization of austenite is partially suppressed (there is an unrecrystallized portion), and coarse austenite grains are mixed, so that a uniform fine pearlite structure cannot be obtained as a result. The effect of improving ductility cannot be fully exhibited. For this reason, it is preferable to perform the final finish rolling in the case of hot rolling between 850-1050 degreeC.

(2-c) Reason for limitation of acceleration cooling start temperature Next, the reason for limitation of the cooling start temperature performed after hot rolling will be described. If the rail temperature, especially the rail head temperature, falls below 700 ° C before accelerated cooling, pearlite with low hardness, which is harmful to wear resistance, is generated on the rail head surface (top, corner, head side). End up. In hypereutectoid steel rails, a large amount of pro-eutectoid cementite that is harmful to ductility is generated at the austenite grain boundaries. For this reason, the cooling start temperature is set to 700 ° C. or higher. In the hypereutectoid steel rail, the higher the cooling start temperature, the more the generation of proeutectoid cementite can be suppressed, so the more preferable cooling start temperature is 740 ° C. or higher.

(2-d) Reason for limiting cooling stop temperature The cooling stop temperature for accelerated cooling will be described. When the cooling stop temperature is below 550 ° C at the top of the rail, bainite that is harmful to wear resistance and martensite that reduces ductility are generated on the rail head surface (top, corner, head side). It becomes easy. In addition, when cooling is stopped in a temperature range exceeding 650 ° C., the temperature rises further due to double heat, and a pearlite structure with low hardness is generated on the surface and inside, and the wear resistance necessary for the rail is reduced. . Moreover, in the hypereutectoid steel rail, pro-eutectoid cementite which is harmful to ductility is generated. For this reason, the stop temperature of acceleration cooling shall be 550-650 degreeC.

(2-e) Reason for limiting cooling rate Next, the range of the accelerated cooling rate will be described. The cooling rate is a value obtained by dividing the temperature drop from the start of accelerated cooling to the cooling stop by the time required for cooling. If the accelerated cooling rate on the surface of the rail head is less than 2 ° C./sec, the high hardness of the rail head cannot be achieved under the rail manufacturing conditions, and it is difficult to ensure the wear resistance of the rail head. Furthermore, depending on the carbon content and alloy composition of the steel, a pro-eutectoid cementite structure is formed, and the ductility of the head of the rail is lowered. On the other hand, when the accelerated cooling rate exceeds 30 ° C./sec, it becomes difficult to control the cooling stop temperature, and a bainite structure harmful to wear resistance and a martensite structure harmful to ductility are generated. For this reason, the range of the accelerated cooling rate of the rail head surface is set to 2 to 30 ° C./sec.

(2-f) Measures after Stopping Accelerated Cooling After accelerated cooling from 700 ° C. or higher austenite to 550 to 650 ° C. at 2 to 30 ° C./sec, the mixture is allowed to cool to 400 ° C. If the rail head surface reaches 400 ° C., it may be cooled at an arbitrary cooling rate thereafter.

Next, examples of the present invention will be described.
<Example 1>
Table 1 shows C, Si, Mn, Ti, Nb, V and other components of the steel strip for rail rolling used in the examples. The structure of the steel strip for rail rolling is as follows.

(1) Steel slab for rail rolling whose chemical component is within the limited range of the present invention (20 steel types of the present steel component, steel codes: A to T)
(2) Steel slab for rail rolling whose chemical composition is outside the limited range of the present invention (comparative steel composition 14 steel types, steel code: bo)

  The components of these steels were adjusted by converter and secondary refining, and cast into rail rolling steel pieces by continuous casting method.

* Underlined parts are outside the scope of the present invention

  Table 2 shows the result of observing the microstructure of the head of the rail manufactured by hot rolling using the steel rolling steel slab of the steel code shown in Table 1, and the result of measuring precipitates of 10 to 100 nm The result of the total elongation obtained by the tensile test is shown. The configuration of each rail is as follows. In addition, about all the rails, the reheating temperature of the steel slab for rolling shall be 1200 degreeC or more, the final rolling of hot rolling shall be the range of 850-1050 degreeC, and accelerated cooling after hot rolling is carried out on the rail head surface. The production conditions were such that the temperature was cooled from an austenite temperature range of 700 ° C or higher to a temperature range of 550 ° C to 650 ° C at an average cooling rate of 2 to 30 ° C / sec and allowed to cool to at least 400 ° C. However, the final rolling temperature of hot rolling has deviated from some rails. The rail whose final rolling temperature was off was handled as a comparative rail.

(1) Microstructure, rails having a number of precipitates of 10 to 100 nm within the limited range of the present invention (rails of the present invention, rail codes: A-1, B-1, C-1, D-1, E-1, F-1, G-1, H-1, I-1, J-1, K-1, L-1, M-1, N-1, O-1, P-1, Q-1, R- 1, S-1, T-1)

(2) Rail whose chemical composition is outside the limited range of the present invention (comparison rail, rail code: b-1, c-1, d-1, e-1, f-1, g-1, h-1, i) -1, j-1, k-1, l-1, m-1, n-1, o-1)

(3) Although the steel component is within the limited range of the present invention, since the hot rolling temperature deviates from the above limit, the number of precipitates of 10 to 100 nm is out of the limited range of the present invention (comparison rail, rail). Code: H-2, K-2)

The tensile test conditions are as follows.
`` Head tensile test ''
Tester: Universal small tensile tester Test piece shape: JIS Z2201 No. 4 Similar Test piece sampling position: Collected 5mm below the head surface as the center of the test piece (see Fig. 5)
Parallel part length: 40 mm, parallel part diameter: 6 mm, distance between stretch measurement scores: 21 mm
Tensile speed: 10 mm / min, test temperature: normal temperature (20 ° C.)

The microstructure observation method is as follows.
"Method for observing the structure of pearlite, bainite, martensite"
Observation sample: Rail head cut out perpendicular to the longitudinal direction of the rail Corrosion method: 10 sec immersion in Nital (see Non-Patent Document 3)
Observation method: Optical microscope, 200 times

"Structural observation method of proeutectoid cementite"
Observation sample: Rail head section cut perpendicular to the longitudinal direction of the rail Corrosion method: Immersion in boiling sodium picrate (see Non-Patent Document 3)
Observation method: Optical microscope, 200 times

* Underlined parts are outside the scope of the present invention

  As shown in Table 2, the present invention rail has a C amount, Si amount, Mn amount, Ti amount, Nb amount, and V amount within the ranges limited to the above, and 10 to 100 nm compared to the comparative rail. By keeping the number of precipitates within the above-mentioned limited range, pro-eutectoid ferrite, bainite structure, which is harmful to the wear resistance of the steel rail, pro-eutectoid cementite, martensite structure, coarse precipitates, etc. that adversely affect ductility A pearlite rail with excellent ductility could be produced without being formed.

  On the other hand, since the amount of C of the code | symbol b-1 was higher than the range limited above, ductility fell because the pro-eutectoid cementite which is harmful to ductility produced | generated in large quantities.

  Reference c-1 has a pearlite structure because the Si amount was lower than the range defined above, but a low-hardness pearlite structure with low wear resistance and a coarse oxide formed due to insufficient deoxidation. Therefore, the oxide became the starting point of destruction, and the total elongation was not improved.

  Reference d-1 has a pearlite structure due to the addition of Si exceeding the above-defined range, but the ferrite was remarkably strengthened (embrittled), so that the total elongation was lowered.

  The symbol e-1 has a pearlite structure because the amount of Mn was lower than the range defined above, but since the hardenability could not be improved, a low-hardness pearlite structure with low wear resistance was generated, And since the degree of supercooling was small, nucleation of pearlite was small, resulting in a coarse pearlite structure, and the total elongation was not improved.

  With reference to the symbol f-1, a large amount of bainite structure harmful to wear resistance was generated on the surface of the rail head due to the addition of Mn exceeding the range defined above.

  In the sign g-1, coarse precipitates are mixed in the pearlite structure due to the addition of V exceeding the limited range described above, and the amount of Nb is less than the limited range, so the number of 10-100 nm precipitates is small. Thus, the grain growth of austenite was not inhibited and the total elongation was not improved.

  Similarly to g-1, since the amount of V was larger than the above range, the sign h-1 was mixed with a coarse precipitate in the pearlite structure, and the amount of Ti was less than the above limited range. The number of precipitates decreased, the austenite grain growth was not inhibited, and the total elongation was not improved.

  Since the sign i-1 had a larger amount of Nb than the above range, coarse precipitates were mixed in the pearlite structure, and since the V amount was smaller than the limited range, the number of precipitates of 10 to 100 nm was reduced. The austenite grain growth was not inhibited, and the total elongation was not improved.

  Since the amount of Nb and the amount of V of J-1 were larger than the above-mentioned limited ranges, coarse precipitates were generated, the austenite grain growth could not be suppressed, and the total elongation was not improved.

  Since the amount of Ti and the amount of V of the sign k-1 were larger than the above-mentioned limited ranges, coarse precipitates were generated, austenite grain growth could not be suppressed, and total elongation was not improved.

  Since the sign 1-1 was larger in the Nb content and V content than the above-mentioned limited ranges, coarse precipitates were generated, the austenite grain growth could not be suppressed, and the total elongation was not improved.

  Since the code | symbol m-1 had more P amounts than the said limited range, ductility fell.

In code n-1, since the amount of S was larger than the limited range, coarse MnS was generated, and ductility was lowered due to early fracture due to stress concentration.
The symbol o-1 is Ti, Nb, and V of Ti and V are within the above range, but the added amount of Nb exceeds 0.080%, so that coarse carbides, nitrides, carbonitriding harmful to ductility The amount of product produced was increased, and the total elongation was not improved even with the effects of the present invention.

  In addition, the code H-2 has a chemical component in the above-mentioned limited range and a pearlite structure was obtained. However, since the final rolling temperature of hot rolling exceeded 1050 ° C., a precipitate of 10 to 100 nm was obtained as a result. The total elongation was not improved because the number of was less than the limited range.

  The sign K-2 had a chemical composition within the above-mentioned limited range and a pearlite structure was obtained. However, since the final rolling temperature of hot rolling was lower than 850 ° C., the formation of precipitates became remarkable, resulting in 10 to 10. The total elongation was not improved due to the restraint of deformation by the precipitates, in which the number of precipitates of 100 nm was larger than the above-mentioned limited range.

  FIG. 6 shows the relationship between the C content and total elongation of each steel. In the present invention, the total elongation can be improved as compared with the same C content by keeping the number of chemical components and the number of precipitates of 10 to 100 nm within the limited range. The amount of C is directly related to the wear resistance of the rail, and the total elongation is the resistance to bending as a structural material of the rail (breakage resistance). If the rail of the present invention has the same wear resistance (C content), it is more difficult to break. .

<Example 2>
Rails with hot rolling conditions (reheating temperature, final rolling temperature) and accelerated cooling conditions (cold start temperature, stop temperature, cooling rate) after hot rolling are changed using the steel of symbol Q shown in Table 1. Manufactured. After accelerated cooling, it was allowed to cool to 400 ° C. Table 3 shows the total elongation values obtained as a result of the microstructure observation result, the precipitate observation result, and the tensile test of the head of the manufactured rail. The structure of the manufactured rail is as follows.

(1) Rail whose hot rolling conditions and accelerated cooling conditions are within the limited range of the present invention:
(Invention rail manufacturing method, 8 pieces, code K-31 to 38)

(2) Rail whose hot rolling conditions and accelerated cooling conditions are outside the limited range of the present invention:
(Comparison rail manufacturing method, 8 pieces, code K-41 to 48)

* Underlined parts are outside the scope of the present invention

  As shown in Table 3, the rails (reference numerals K-31 to 38) created by the rail manufacturing method of the present invention are compared with the comparative rail steels (K-41 to 48) in hot rolling conditions and accelerated cooling conditions. Therefore, a large amount of fine precipitates were generated and dispersed, and the total elongation was improved.

  On the other hand, since the reheating temperature of the code K-41 was lower than the limited range, Ti-based, Nb-based, and V-based precipitates were partially in an insoluble state. As a result, the number of precipitates finely precipitated in austenite during hot rolling was reduced, the austenite grain growth inhibition effect could not be sufficiently exhibited, and the total elongation was not improved.

  Code K-42, the final rolling temperature of hot rolling exceeded 1050 ° C., so the dislocation of nucleation sites introduced into austenite at the time of hot rolling is accelerated, and the number of precipitates generated is small. In addition, since the generated precipitate was coarsened, there was no inhibitory effect on austenite grain growth, and the total elongation was not improved.

  No. K-43 has a final rolling temperature lower than 850 ° C., making it difficult for the austenite to recrystallize. In addition, a large amount of fine precipitates was generated, so that recrystallization was suppressed and partially unrecrystallized. Coarse austenite grains were mixed, a uniform fine pearlite structure was not obtained, and the total elongation was not improved.

  No. K-44, the hot rolling conditions were within the limited range, but the start temperature of accelerated cooling was lower than the limited temperature, and a low hardness pearlite with low wear resistance was formed on a part of the head surface. Since the pro-eutectoid cementite produced | generated and harmful to ductility produced | generated, the effect of this invention was not acquired.

  With the sign K-45, the cooling stop temperature of accelerated cooling was higher than the above limited temperature, so low hardness pearlite with low wear resistance was generated on part of the head surface, and proeutectoid cementite harmful to ductility was generated. Therefore, the effect of the present invention was not obtained.

  In the sign K-46, the cooling stop temperature of the accelerated cooling was below the limited range, and martensite harmful to ductility was generated on the rail head surface, so that the total elongation was lowered.

  In the case of K-47, since the accelerated cooling rate was lower than the above-mentioned limited range, a large amount of proeutectoid cementite was generated in addition to the generation of low-hardness pearlite harmful to wear resistance, and the total elongation was lowered.

  In code K-48, since the accelerated cooling rate exceeded the limited range, martensite harmful to ductility was generated on the head surface, and thus the total elongation decreased.

Claims (3)

Chemical composition is mass%,
C: 0.70 to 1.40%
Si: 0.10 to 2.00%,
Mn: 0.10 to 2.00%,
P: 0.025% or less,
S: 0.025% or less,
Cr: 0 to 2.00%
Mo: 0 to 0.50%,
Co: 0 to 2.00%
B: 0 to 0.0050%,
Cu: 0 to 1.00%,
Ni: 0 to 1.00%,
Mg: 0 to 0.0200%,
Ca: 0 to 0.0200%,
REM: 0-0.0500%,
Zr: 0 to 0.0200%,
N: 0 to 0.0200%
Al: 0 to 1.00%
2 out of Ti, Nb, and V are each in the range of 0.0005 to 0.0060%, the remaining one is in the range of 0 to 0.0080%, and the balance is Fe and impurities Ti-based precipitates, Nb-based precipitates, V-based precipitates or composites having a diameter of 10 nm or more and 100 nm or less in the pearlite in the rail head at any cross section of the rail head. A pearlitic high carbon steel rail excellent in ductility, characterized in that there are 50,000 to 1,000,000 precipitates per 1 mm ^{2 of} test area. The chemical composition is mass%,
Cr: 0.05 to 2.00%,
Mo: 0.01 to 0.50%,
Co: 0.10 to 2.00%,
B: 0.0005 to 0.0050%,
Cu: 0.05 to 1.00%,
Ni: 0.01-1.00%,
Mg: 0.0005 to 0.0200%,
Ca: 0.0005 to 0.0200%,
REM: 0.0005 to 0.0500%
Zr: 0.0005 to 0.0200%,
N: 0.0020 to 0.0200%, and Al: 0.0020 to 1.00%,
The pearlitic high carbon steel rail excellent in ductility according to claim 1, comprising one or more of the following. % By mass
C: 0.70 to 1.40%
Si: 0.10 to 2.00%,
Mn: 0.10 to 2.00%
Containing
At least two of Ti, Nb, and V are in the range of 0.0005 to 0.0060%, and the remaining one is in the range of 0 to 0.0080% by hot rolling. When modeling on the rail, the reheating temperature of the steel slab for rolling is 1200 ° C or higher, the final rolling of the hot rolling is in the range of 850 to 1050 ° C,
Accelerated cooling after hot rolling is cooled at an average cooling rate of 2 to 30 ° C / sec from an austenite temperature range where the rail head surface temperature is 700 ° C or higher to a temperature range of 550 ° C to 650 ° C,
A method for producing a pearlitic high carbon steel rail excellent in ductility characterized by cooling to at least 400 ° C.