JP4846476B2 - Method for producing pearlitic rails with excellent wear resistance and ductility - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the wear-resistance and the ductility of a rail with which the structure of the head part of the rail is made to fine and the hardness is held in a prescribed range by controlling the composition and the finish-rolling condition of a steel retaining still recrystallizing austenite structure and controlling a heat-treatment condition thereafter. <P>SOLUTION: To a steel slab for rolling to the rail composed of 0.85-1.40% C, 0.05-2.00% Si, 0.05-2.00% Mn and the balance Fe with inevitable impurities, at least a rough-rolling and a finish-rolling are applied to the manufacture of the rail. In the finish-rolling, in the temperature range of the lower than Arcm point - &ge;700&deg;C to the surface temperature of the rail head part, the rolling is applied at &ge;20% accumulated reduction surface ratio of the bend part, and on the head part surface of the rail just after completing the rolling, the still recrystallizing austenite structure is generated at &ge;50% and thereafter, the head part surface of the rail after finish-rolling is acceleratively cooled at least to 550&deg;C with 5-50&deg;C/sec cooling speed within 200 sec after completing the finish-rolling. <P>COPYRIGHT: (C)2008,JPO&amp;INPIT

Description

The present invention provides a rail for use in heavy load railways, a manufacturing method of pearlitic rails for the purpose of improving the wear resistance and ductility of the head at the same time.

High carbon pearlitic steels have been used as a superior wear resistance or al railway rail material. However, since the carbon content is very high, there is a problem that ductility and toughness are low.
For example, in an ordinary carbon steel rail having a carbon content of 0.6 to 0.7 mass% shown in Non-Patent Document 1, an impact value at room temperature in a JIS No. 3 U-notch Charpy impact test is about 12 to 18 J / cm ² . When such a rail is used in a low temperature region such as a cold region, there is a problem that a brittle fracture is caused by a minute initial defect or a fatigue crack.
In recent years, rail steels have been further increased in carbon to improve wear resistance, and as a result, there has been a problem that ductility and toughness are further lowered.

  In general, to improve the ductility and toughness of pearlite steel, it is effective to refine the pearlite structure (pearlite block size), specifically, to refine the austenite structure before pearlite transformation and refine the pearlite structure. It is said. Methods for achieving austenite refinement include a reduction in rolling temperature during hot rolling, an increase in rolling reduction, and a heat treatment by low-temperature reheating after rail rolling. In addition, as a method for reducing the pearlite structure, there is promotion of pearlite transformation from austenite grains using transformation nuclei.

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

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

  Therefore, it has been demanded to develop a method for producing a high carbon steel rail that can secure the formability during rolling and can refine the pearlite structure after rolling without performing low-temperature reheating. In order to solve this problem, a method for producing a high carbon steel rail as shown in Patent Documents 1 to 3 below has been developed. The main feature of these rails is the continuous rolling under small reduction by utilizing the fact that the austenite grains of high carbon steel are relatively low temperature and are easy to recrystallize even with a small reduction amount in order to refine the pearlite structure. Thus, finely sized grains are obtained and the ductility and toughness of pearlite steel are improved.

In the disclosed technique of Patent Document 1, in the finish rolling of a steel rail containing high carbon steel, a high ductility rail can be provided by rolling for three or more consecutive passes in a predetermined time between passes.
Further, in the disclosed technique of Patent Document 2, in the finish rolling of a steel rail containing high carbon steel, rolling is performed continuously for two passes or more at a predetermined time between passes, and further, after the continuous rolling, accelerated cooling is performed after the rolling. By performing the above, a high wear resistance and high toughness rail can be provided.
Furthermore, in the disclosed technique of Patent Document 3, in the finish rolling of a steel rail containing high carbon steel, cooling is performed between passes, and further, continuous rolling is performed, and then accelerated cooling is performed after rolling. High toughness rails can be provided.

  However, in the disclosed technologies of Patent Documents 1 to 3, depending on the combination of the carbon content of steel, the temperature during continuous hot rolling, the number of rolling passes and the time between passes, the austenite structure cannot be refined and the pearlite structure is coarse. There is a problem that ductility and toughness are not improved.

  In particular, in steel with a high carbon content, the grain growth rate immediately after rolling is large, so depending on the selection of the time between passes during continuous rolling, the growth of austenite grains between passes becomes significant, and the continuous indicated above. Even if the rolling method or cooling between passes is performed, there is a problem that the austenite structure cannot be refined, the pearlite structure becomes coarse, and the ductility and toughness are not improved.

  Therefore, a method for producing a high carbon steel rail as shown in Patent Document 4 below has been developed as a method for suppressing grain growth immediately after rolling in a steel having a high carbon content and making the austenite structure finer. The main feature of this rail manufacturing method is that the time between passes during continuous rolling is controlled by the amount of carbon in the steel and the number of rollings in order to suppress the growth of austenite grains, and the addition amount of V, Nb, and N To obtain fine grains and improve the ductility and toughness of pearlite steel. Thus, in the finish rolling of steel rails containing high carbon steel, the time between passes during continuous rolling is controlled by the amount of carbon in the steel and the number of rollings, the amount of addition of V, Nb, and N is adjusted. Accelerated cooling can provide a rail with excellent wear resistance and ductility.

  However, in the disclosed technique of Patent Document 4, depending on the combination of the addition amount of V, Nb, etc. and the cooling rate of accelerated cooling performed immediately after rolling, grain growth of the austenite structure cannot be suppressed, and the pearlite structure that is the final structure is coarse. The ductility and toughness are not improved, the grain growth is excessively suppressed, the austenite structure becomes very fine, and even after heat treatment, the hardness of the pearlite structure does not increase due to the decrease in hardenability. There is a problem that the wear resistance of the rail cannot be ensured.

Japanese Unexamined Patent Publication No. 7-173530 JP 2001-234238 A JP 2002-226915 A JP 2005-290544 A JISE1101-1990

  Against this background, it has become desirable to provide a pearlite-based rail with excellent wear resistance that achieves a stable refinement of the pearlite structure and has improved ductility in steel with a high carbon content. The present invention has been devised in view of the above-described problems, and the object thereof is to stably and simultaneously improve the wear resistance and ductility required for heavy-duty railroad rails. It is for the purpose.

  The method for manufacturing a pearlite rail according to the present invention controls the rolling temperature of the head surface, the cumulative reduction ratio of the head, and then, by applying an appropriate heat treatment, the duct head and the wear resistance of the rail head are improved. The gist is to improve it stably. Specifically, in a pearlitic rail containing high carbon, in order to stably improve the ductility of the rail head, the residual amount of unrecrystallized austenite structure on the head surface immediately after rolling and the formation of a fine cementite structure are generated. Control and achieve pearlite structure refinement, and also perform accelerated cooling to ensure wear resistance. The configuration of the present invention is as follows.

(A) In mass%, C: 0.85 to 1.40%, Si: 0.05 to 2.00%, Mn: 0.05 to 2.00%, the balance being Fe and inevitable A method for producing a pearlite-based rail excellent in wear resistance and ductility by performing at least rough rolling and finish rolling on a rail rolling steel slab comprising impurities,
In the finish rolling, in a temperature range where the rail head surface temperature is less than the Arcm point to 700 ° C. or higher, rolling is performed so that the cumulative reduction in area of the head is 20% or more. A recrystallized austenite structure is generated at an area ratio of 50% or more, and a cementite structure is generated at the austenite grain boundary in the non-recrystallized austenite structure on the rail head surface immediately after the finish rolling,
Thereafter, the rail head surface after finish rolling is acceleratedly cooled to at least 550 ° C. at a cooling rate of 5 to 50 ° C./sec within 200 sec after completion of finish rolling.
(B) Furthermore, in mass%,
Cr: 0.05 to 2.00%, Mo: 0.01 to 0.50%, V: 0.005 to 0.500%, Nb: 0.002 to 0.050, B: 0.0001 to 0 .0050%, Co: 0.003-2.00%, Cu: 0.01-1.00%, Ni: 0.01-1.00%, Ti: 0.0050-0.0500%, Mg: 0.0005 to 0.0200%, Ca: 0.0005 to 0.0150, Al: 0.010 to 1.00%, Zr: 0.0001 to 0.2000%, N: 0.0060 to 0.0200 1 or 2 or more types in%, The manufacturing method of the pearlite type rail as described in (A) characterized by the above-mentioned .

  ADVANTAGE OF THE INVENTION According to this invention, the wear resistance and ductility of a head requested | required with the rail of a heavy load railway can be improved stably simultaneously in a pearlite system rail.

  As a mode for carrying out the present invention, a method for producing a pearlite rail excellent in wear resistance and ductility will be described in detail below. Hereinafter, the mass in the composition is simply described as%.

  First, the present inventors performed hot rolling simulating rail rolling using a high carbon steel (0.85 to 1.40%) with a changed carbon content, and a conventional austenite single layer. In addition to the region, the relationship between the austenite grain behavior and the temperature and area reduction ratio during hot rolling was investigated up to the range of the two-phase region of the austenite phase and the cementite phase. As a result, in high carbon steel, by rolling in the two-phase region of the austenite phase and the cementite phase below the Arcm transformation point, which was difficult to apply due to the problem of formation of cementite structure harmful to ductility and toughness, A large amount of unrecrystallized austenite grains (flat coarse grains) in which the initial austenite grains remain without being recrystallized, and the cementite structure formed at the austenite grain boundaries becomes fine around the unrecrystallized austenite grains. Found to generate.

  Next, the inventors confirmed the behavior of the fine cementite structure and non-recrystallized austenite grains after the rolling by experiments. As a result, if the reduction in area during rolling exceeds a certain value, the amount of unrecrystallized austenite structure is secured, and the unrecrystallized austenite structure is recrystallized during natural cooling after rolling, resulting in fine austenite. It was confirmed that recrystallization into grains and further refinement of the cementite structure formed at the unrecrystallized austenite grain boundaries.

  Furthermore, the present inventors investigated the behavior of fine austenite grains and fine cementite structure during natural cooling after rolling. As a result, it was found that the fine cementite structure existing in the austenite grain boundary suppresses the grain growth of the austenite grain by the pinning action, and the fine grain is maintained for a long time.

  Based on these results, the present inventors examined a method for stably improving the ductility by utilizing this fine cementite structure and non-recrystallized austenite structure. Laboratory rolling and heat treatment experiments were conducted and ductility was evaluated by a tensile test. As a result, in order to refine the pearlite structure and improve the ductility stably, it is necessary to secure the amount of unrecrystallized austenite structure generated immediately after rolling, and the unrecrystallized austenite structure is fine. In order to recrystallize to austenite grains, in addition to a certain range of natural cooling time from the end of rolling to the start of heat treatment, there is a coarse fine cementite structure that was acting on pinning of fine austenite grains In order not to adversely affect the ductility, it has been found that there is a certain range in the natural cooling time from the end of rolling to the start of heat treatment.

  In addition to these findings, the present inventors searched for a method of directly using this non-recrystallized austenite structure in order to further improve the ductility. As a result of lab rolling and heat treatment experiments, the time of natural cooling after the end of rolling was shortened, and in the state where the non-recrystallized austenite structure did not recrystallize, by performing accelerated cooling, from inside the non-recrystallized austenite structure It was confirmed that a large amount of fine pearlite structure was formed and the ductility was further improved.

  From the above-mentioned knowledge, the present inventors finely controlled the rail rolling temperature and the cumulative area reduction rate within a certain range when manufacturing a high-carbon steel slab by hot rolling as a rail. Ductile and wear resistance of the rail head by making a certain amount of cementite structure and a predetermined amount of unrecrystallized austenite structure remain, and then heat-treating within a certain period of natural cooling to refine the pearlite structure. It was found to secure at the same time.

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

(1) Reasons for limiting chemical components of rail rolling steel slab C is an element effective in promoting pearlite transformation and ensuring wear resistance. If the C content is less than 0.85%, the volume ratio of the cementite phase in the pearlite structure cannot be secured, and the wear resistance cannot be maintained in heavy-duty railways. Further, the fine cementite structure that has been acting on the pinning of the fine austenite grains is not generated, and the ductility is not improved. On the other hand, when the C content exceeds 1.40%, in this production method, the cementite structure that has been acting on the pinning becomes coarse, and no improvement in ductility is observed. In addition, a large amount of coarse pro-eutectoid cementite structure is formed after heat treatment, and wear resistance and ductility are reduced. For this reason, the amount of C was limited to 0.85 to 1.40%.

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

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

  In addition, in this invention, about the chemical component of the steel strip for rail rolling, components other than C, Si, and Mn are not particularly limited, but if necessary, Cr: 0.05 to 2.00%, Mo: 0.01 to 0.50%, V: 0.005 to 0.500%, Nb: 0.002 to 0.050, B: 0.0001 to 0.0050%, Co: 0.003 to 2 0.00%, Cu: 0.01-1.00%, Ni: 0.01-1.00%, Ti: 0.0050-0.0500%, Mg: 0.0005-0.0200%, Ca: One or more of 0.0005 to 0.0150, Al: 0.010 to 1.00%, Zr: 0.0001 to 0.2000%, N: 0.0060 to 0.0200% are contained. It is desirable. The reason why such a component range is desirable is as follows.

  Cr: 0.05 to 2.00%: Cr is an element that makes the pearlite structure fine and contributes to high hardness (strength) and improves wear resistance. However, when the Cr content is less than 0.05%, the effect is small. On the other hand, if the Cr content exceeds 2.00%, a large amount of martensite structure harmful to wear resistance and ductility is generated, so the Cr addition amount is preferably 0.05 to 2.00%.

  Mo: 0.01 to 0.50%: Mo is an element that contributes to increasing the hardness (strength) by making the pearlite structure fine, 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 if the amount of Mo exceeds 0.50%, a martensite structure harmful to ductility is generated. 0.50% is desirable.

  V: 0.005 to 0.500%: V is an element effective for forming nitrides and carbonitrides, improving ductility, and simultaneously improving hardness (strength). However, if the amount of V is less than 0.005%, the effect cannot be sufficiently expected, and if the amount of V exceeds 0.500%, coarse precipitates that start fatigue damage are generated. The addition amount is preferably 0.005 to 0.500%.

  Nb: 0.002 to 0.050: Nb is an element effective for forming nitrides and carbonitrides, improving ductility, and at the same time improving hardness (strength). Further, it is an element that raises the temperature range of non-recrystallized austenite and stabilizes the non-recrystallized austenite structure. However, if the Nb content is less than 0.002%, it cannot be expected, and if the Nb content exceeds 0.050%, coarse precipitates that start fatigue damage are generated. ~ 0.050% is desirable.

  B: 0.0001 to 0.0050%: B is an element that prevents the deterioration of the ductility of the rail and increases the life by miniaturizing the formation of pro-eutectoid cementite structure and uniforming the hardness distribution of the head. is there. However, if the amount of B is less than 0.0001%, the effect is not sufficient, and if the amount of B exceeds 0.0050%, coarse precipitates are generated. .0050% is desirable.

  Co: 0.003 to 2.00%: Co is an element that improves the hardness (strength) of the pearlite structure, and further, a fine ferrite structure formed by contact with the wheel on the wear surface of the rail head. It is an element that further refines and improves wear resistance. However, if the Co content is less than 0.003%, the effect cannot be expected. Further, if the Co content exceeds 2.00%, spalling damage occurs on the rolling surface, so the Co addition amount is preferably 0.003 to 2.00%.

  Cu: 0.01 to 1.00%: Cu is an element that improves the hardness (strength) of the pearlite structure. However, if the amount of Cu is less than 0.01%, the effect cannot be expected. Further, if the amount of Cu exceeds 1.00%, a martensite structure harmful to wear resistance is generated, so the amount of Cu added is preferably 0.01 to 1.00%.

  Ni: 0.01 to 1.00%: Ni is an element for increasing the hardness (strength) of pearlite steel. However, when the amount of Ni is less than 0.01%, the effect is remarkably small. If the Ni content exceeds 1.00%, spalling damage occurs on the rolling surface. For this reason, the Ni addition amount is desirably 0.01 to 1.00%.

  Ti: 0.0050 to 0.0500%: Ti is a component effective for forming nitrides and carbonitrides, improving ductility, and at the same time improving hardness (strength). Further, it is an element that raises the temperature range of non-recrystallized austenite and stabilizes the non-recrystallized austenite structure. However, when the amount of Ti is less than 0.0050%, the effect is small. On the other hand, if the Ti content exceeds 0.0500%, coarse precipitates are generated and the ductility of the rail is greatly reduced. Therefore, the Ti addition amount is preferably 0.0050 to 0.0500%.

  Mg: 0.0005 to 0.0200%: Mg is an element effective in reducing the austenite grains and the pearlite structure and improving the ductility of the pearlite structure. However, if the amount of Mg is less than 0.0005%, the effect is weak. Further, if the Mg content exceeds 0.0200%, a coarse oxide of Mg is generated and the ductility of the rail is lowered. Therefore, the Mg addition amount is desirably 0.0005 to 0.0200%.

  Ca: 0.0005 to 0.0150%: Ca is an element that contributes to the generation of the pearlite transformation and, as a result, is effective in improving the ductility of the pearlite structure. However, when the Ca content is less than 0.0005%, the effect is weak. Further, if the Ca content exceeds 0.0150%, a coarse oxide of Ca is generated and the ductility of the rail is lowered. Therefore, the Ca addition amount is preferably 0.0005 to 0.0150%.

  Al: 0.010 to 1.00%: Al is an element effective for increasing the strength of the pearlite structure and suppressing the formation of a pro-eutectoid cementite structure. However, when the Al content is 0.010% or less, the effect is weak. Further, if the Al content exceeds 1.00%, coarse alumina inclusions are generated and the ductility of the rail is lowered. Therefore, the Al addition amount is preferably 0.010 to 1.00%.

  Zr: 0.0001 to 0.2000%: Zr is an element that suppresses the formation of a pro-eutectoid cementite structure generated in the segregation part. However, if the amount of Zr is 0.0001% or less, a pro-eutectoid cementite structure is formed and the ductility of the rail is lowered. On the other hand, if the amount of Zr exceeds 0.2000%, a large amount of coarse Zr-based inclusions are generated and the ductility of the rail is lowered. Therefore, the amount of Zr added is preferably 0.0001 to 0.2000%.

  N: 0.0060 to 0.0200%: N is an element effective for enhancing the ductility of the pearlite structure and at the same time improving the hardness (strength). However, if the N content is less than 0.0060%, the effect is weak. On the other hand, if the N content exceeds 0.0200%, it becomes difficult to make a solid solution in the steel, and bubbles are generated as a starting point of fatigue damage. Therefore, the N addition amount is 0.0060 to 0.0200%. desirable. In the rail steel, N is contained as a maximum of about 0.0050% as an impurity. Therefore, in order to make N amount into the above range, it is necessary to intentionally add N.

  In the present invention, the steel strip for rail rolling having the above-described composition is melted in a commonly used melting furnace such as a converter or an electric furnace, and this molten steel is ingot / slabed or continuously. Casted.

(2) Reason for limiting rolling temperature range Next, the reason why the rolling temperature range of the rail head surface of finish rolling is limited to the above-mentioned claims will be described in detail. In addition, before finish rolling is performed, rough rolling and intermediate rolling are performed on the steel pieces for rail rolling.

  When rolling in a state where the rail head surface temperature is equal to or higher than the Arcm transformation point, a fine cementite structure is not formed, a pinning action of austenite grains is not obtained, and austenite grains become coarse, resulting in pearlite after rolling and heat treatment. The structure is not refined and ductility is not improved. Moreover, when rolling in a temperature range below 700 ° C., the cementite structure generated during rolling becomes coarse, and the wear resistance and ductility of the rail are greatly reduced. Furthermore, the rolling formability is greatly reduced, and rail rolling becomes difficult. For this reason, the range of the rolling temperature of the rail head surface is set to a range of less than Arcm transformation point to 700 ° C. or more, preferably less than 850 ° C. to 700 ° C. or more.

The Arcm transformation point varies depending on the carbon content of the steel and the alloy composition. Experimental verification is required to accurately determine the Arcm transformation point. In order to obtain these values simply, the equilibrium diagram of the Fe-F e ₃ C system published in metallurgical textbooks (for example, steel materials, edited by the Japan Institute of Metals) based on the carbon content alone is used. It is desirable to read from. The Arcm transformation point in the component system of the present invention is a value 20-30 ° C. lower than the Acm line in the equilibrium diagram.

(3) Reason for limiting the cumulative area reduction rate of the head Next, the reason why the cumulative area reduction rate of the rail head of finish rolling is limited to the above claims will be described in detail.

  When the cumulative reduction in area of the rail head is less than 20%, the residual amount of the predetermined non-recrystallized austenite structure cannot be obtained, the amount of strain in the non-recrystallized austenite structure decreases, and the rolling temperature of the present invention In the range, the austenite structure after recrystallization is not refined and the austenite structure is coarsened. Further, the cementite structure generated at the non-recrystallized austenite grain boundaries is coarsened, and the pinning action of the austenite grains cannot be obtained. Further, in the subsequent heat treatment, a pearlite structure is not generated from the deformation band of the processed non-recrystallized austenite structure, and as a result, the pearlite structure becomes coarse and the ductility of the rail does not improve. For this reason, the cumulative surface reduction rate of the rail head is limited to 20% or more.

  Here, the cumulative surface reduction rate of the rail head will be described. The cumulative area reduction ratio is a reduction ratio of the area of the head section after the final rolling pass to the area of the head section before the first rolling pass in finish rolling. Therefore, no matter what rolling pass is present during finish rolling, if the head cross-sectional shapes of the first rolling pass and the final rolling pass are the same, the cumulative area reduction ratio is the same.

In addition, although there is no particular limitation on the upper limit value of the cumulative reduction in area of the rail head of the finish rolling, about 50% is practical in order to ensure the formability of the rail head and ensure the dimensional system. Is the upper limit.
In the present invention, the number of rolling passes at the time of finish rolling and the time between rolling passes are not particularly limited, but the recovery of strain in unrecrystallized austenite grains during the rolling is suppressed, and the cementite structure is further refined. In order to obtain a fine pearlite structure after natural cooling and heat treatment, the number of rolling passes is preferably 4 or less, and the maximum time between passes is preferably 6 sec or less.

(4) Reason for limiting the residual amount of the non-recrystallized austenite structure on the head surface Next, the reason for limiting the residual amount of the non-recrystallized austenite structure on the head surface to the above claims will be described in detail.

If the residual amount of the non-recrystallized austenite structure on the rail head surface is less than 50% by area ratio, a fine austenite structure cannot be obtained sufficiently in the recrystallization that occurs during the subsequent cooling, and a coarse pearlite structure is not obtained. The production amount increases and ductility does not improve. Further, in the subsequent heat treatment, the fine pearlite structure generated from the inside of the non-recrystallized austenite structure is not sufficiently generated, the generation amount of coarse pearlite structure is increased, and the ductility of the rail is not improved. For this reason, the residual amount of the non-recrystallized austenite structure on the head surface is limited to 50% or more by area ratio.

The upper limit value of the area ratio in the residual amount of the non-recrystallized austenite structure is not particularly limited, but about 80% is substantially the upper limit in the temperature range of the present invention.

  Moreover, the production amount of the non-recrystallized austenite structure immediately after rolling can be confirmed by cutting the short rail from the long rail and quenching immediately after the rail rolling. A sample is cut out from the hardened rail head, and after polishing, etched with a mixed solution of sulfonic acid and picric acid, the austenite structure can be confirmed. Note that the non-recrystallized austenite structure is flatter in the rolling direction and coarser than the recrystallized austenite structure, and thus can be classified with an optical microscope. The area ratio can be calculated by approximating the recrystallized austenite structure to an ellipse, obtaining the area, and calculating the ratio from the ratio to the visual field area. The details of the measurement method are not particularly limited, but the field magnification is preferably 100 times and the number of fields is 5 or more.

  It should be noted that quenching of the short rail for investigating the amount of the above-mentioned non-recrystallized austenite structure greatly reduces productivity. Therefore, in order to easily confirm the amount of unrecrystallized austenite structure formed, the relationship between the reaction force value of the hot rolling mill and the amount of unrecrystallized austenite structure formed immediately after rolling was investigated.

  FIG. 1 is a summary of the results of rolling experiments using steel with a carbon content of 0.85 to 1.40%. Relationship between the reaction force value of the rolling mill divided by the reaction force value at the rolling temperature of 950 ° C. with the same cumulative area reduction ratio (hereinafter abbreviated as “reaction force ratio”) and the amount of unrecrystallized austenite structure formed immediately after rolling There was a linear correlation, and when the reaction force ratio exceeded 1.40, it was confirmed that the amount of unrecrystallized austenite structure immediately after rolling exceeded 50%. Therefore, in actual operation, it becomes possible to control the amount of unrecrystallized austenite structure produced using this reaction force ratio as an index.

(5) Reason for limitation of heat treatment condition after finish rolling Next, the reason for limitation of the heat treatment condition of the rail head surface after finish rolling will be described in detail.

  First, the reason why the heat treatment start conditions performed for stably improving the ductility using the fine austenite grains obtained from this non-recrystallized austenite structure is limited as described above.

  When accelerated cooling is started over 200 sec after the end of rolling, the fine cementite structure generated in the recrystallized austenite grain boundaries becomes coarse, the pinning action of the austenite grains decreases, the grain growth becomes remarkable, and the unrecrystallized austenite structure The recrystallized austenite structure becomes coarse, and the ductility of the pearlite structure after heat treatment decreases. In addition, ductility decreases due to coarsening of the fine cementite structure. For this reason, the heat treatment start time was limited to within 200 sec after the end of rolling.

  Although there is no particular limitation on the lower limit of the accelerated cooling start time, in order to sufficiently generate a fine pearlite structure from the inside of the non-recrystallized austenite structure, immediately after rolling so as not to recover the strain in rolling. It is desirable to perform accelerated cooling. Therefore, about 0 to 10 sec after the end of rolling is substantially the lower limit.

  Although the cooling method from the end of finish rolling to the start of accelerated cooling is not limited, natural cooling or slow cooling is desirable. This is because when natural cooling or slow cooling is performed after rolling, the non-recrystallized austenite structure immediately after rolling is recrystallized and the refinement of austenite grains is promoted. In addition, natural cooling after rolling is cooling naturally in air | atmosphere, without performing any heating and cooling processes after rolling. Further, the slow cooling is a range of a cooling rate of 2 ° C./sec or less.

  Next, the range of the accelerated cooling rate on the rail head surface will be described. When the accelerated cooling rate of the rail head is less than 5 ° C./sec, under this manufacturing condition, the cementite structure becomes coarse during accelerated cooling, and the wear resistance and ductility of the rail head are reduced. Also, the high hardness of the rail head cannot be achieved, and it becomes difficult to ensure the wear resistance of the rail head. Furthermore, depending on the steel composition, a pro-eutectoid cementite structure and a pro-eutectoid ferrite structure are formed, and the wear resistance and ductility of the rail head are reduced. When the accelerated cooling rate exceeds 50 ° C./sec, a martensitic structure is generated under the present manufacturing conditions, and the duct head and toughness of the rail head are greatly reduced. For this reason, the range of the accelerated cooling rate of the rail head is limited to the range of 5 to 50 ° C./sec.

  Finally, the range of the accelerated cooling temperature on the rail head surface will be described. When the accelerated cooling of the rail head is stopped at a temperature exceeding 550 ° C., excessive recuperation is generated from the inside of the rail after the accelerated cooling is completed. As a result, the pearlite transformation temperature rises due to the temperature rise, the pearlite structure cannot have a high hardness, and the wear resistance cannot be ensured. Further, the pearlite structure becomes coarse, and the ductility of the rail head also decreases. For this reason, it was limited to perform accelerated cooling to at least 550 ° C.

The temperature at which accelerated cooling of the rail head surface starts is not particularly limited, but 680 ° C. higher than the pearlite transformation temperature is required to stably increase the hardness (strength) of the pearlite structure by heat treatment. is there.
In addition, the lower limit of the temperature at which accelerated cooling of the rail head is terminated is not particularly limited, but the hardness of the rail head is ensured, and the generation of a martensite structure that is likely to be generated in the segregated portion inside the head is generated. In order to prevent this, the lower limit is substantially 400 ° C.

  Here, the part of the rail will be described. FIG. 2 shows the names of the rail parts. In the present invention, as shown in FIG. 2, the rail head portion is a portion located above a horizontal line passing through a point A that intersects each other when the lower surface of the head side portion 3 is extended. It is a part including the part 2 and the head side part 3. The area reduction rate at the time of hot rolling can be calculated from the reduction rate of the cross-sectional area of the portion indicated by hatching. Further, the temperature of the rail head surface during rolling can control the austenite grains during rolling by controlling the temperature of the head surface of the top 1 and the head corner 2, thereby improving the ductility of the rail. be able to.

  In addition, the area ratio of the non-recrystallized austenite structure on the head surface immediately after the end of rolling can be obtained by measuring the position of the rail head at a depth of 6 mm from the head surface of the top (code: 1) shown in FIG. The entire surface can be represented.

  Furthermore, the accelerated cooling rate and the accelerated cooling stop temperature in the heat treatment after rolling described above are the head surface of the top part (reference numeral: 1) and the head corner part (reference numeral: 2) shown in FIG. If the temperature is measured within a depth of 3mm from the surface of the part, the entire rail head can be represented. By controlling the temperature and cooling rate of this part, fine pearlite with excellent wear resistance and ductility. You can get an organization.

  In the present manufacturing method, the refrigerant in the accelerated cooling is not particularly limited, but in order to ensure a predetermined cooling rate and to reliably control the cooling conditions at each part of the rail, air, mist, and a mixture of air and mist are used. It is desirable to perform predetermined cooling on the outer surface of each part of the rail using a refrigerant.

In the present manufacturing method, the hardness of the rail head is not particularly limited, but it is desirable to ensure a hardness of Hv 350 or higher in order to ensure wear resistance in heavy-duty railways.
Moreover, the metal structure of the head of the steel rail manufactured by this manufacturing method is preferably a pearlite structure including a fine cementite structure. However, depending on the selection of the component system and accelerated cooling conditions, In some cases, a very small amount of pro-eutectoid ferrite structure, pro-eutectoid cementite structure and bainite structure which are coarse carbides may be formed. However, even if a small amount of these structures are formed in a pearlite structure containing a fine cementite structure, it does not significantly affect the wear resistance and ductility of the rail. The structure of the part includes a mixture of some pro-eutectoid ferrite structure, pro-eutectoid cementite structure and bainite structure.

  The amount and size of the fine cementite structure in the pearlite structure are not particularly mentioned, but it is desirable that the austenite grains can be pinned without affecting the wear resistance and ductility.

Next, examples of the present invention will be described.
Table 1 shows the chemical composition of the test rail steel. Table 2 shows the finish rolling conditions, the area ratio of the non-recrystallized austenite structure, the heat treatment conditions of the rails manufactured by the rail manufacturing method of the present invention using the test rail steels (steel: A to J) shown in Table 1. Furthermore, the microstructure of the 2 mm position below the rail head surface, the hardness, the total elongation value of the tensile test conducted by collecting the test piece from the position shown in FIG. 3, and the test piece taken from the position shown in FIG. The results of the wear test conducted by the method shown in Fig. 5 are also shown.

表３は、表１に示す供試レール鋼（鋼：Ａ〜Ｎ）を用いて、比較レール製造方法で製造したレールの、仕上げ圧延条件、未再結晶オーステナイト組織の面積比率、熱処理条件、さらには、レール頭表面下２ｍｍ位置のミクロ組織、硬さ、図３に示す位置から試験片を採取して行った引張試験の全伸び値、図４に示す位置から試験片を採取し、図５に示す方法で行った摩耗試験の結果も併記した。

Table 3 shows the finish rolling conditions, the area ratio of the non-recrystallized austenite structure, the heat treatment conditions of the rails manufactured by the comparative rail manufacturing method using the test rail steels (steel: A to N) shown in Table 1. Shows the microstructure and hardness at a position 2 mm below the rail head surface, the total elongation value of the tensile test conducted by collecting the test piece from the position shown in FIG. 3, and the test piece taken from the position shown in FIG. The results of the wear test conducted by the method shown in Fig. 6 are also shown.

(1) Invention rail manufacturing method (19) Reference numerals 1 to 19
A pearlite rail produced within the above-mentioned limited component range and finish rolling and heat treatment conditions within the above-mentioned limited range.
(2) Comparative heat treatment rails (15) Reference numerals 20 to 34
Steel: 20 to 23: Rail manufactured under the heat treatment conditions immediately after hot rolling within the above limited range outside the above limited component range.
Steel: 24 to 29: Rail produced from rail steel within the above limited component range under finish rolling conditions outside the above limited range.
Steel: 30 to 34: Rail manufactured from rail steel within the above limited component range under heat treatment conditions outside the above limited range.

  FIG. 6 shows the relationship between the amount of carbon and the total elongation of the results of the head tensile test of the rail manufactured by the rail manufacturing method of the present invention shown in Table 2 and the rail manufactured by the comparative rail manufacturing method shown in Table 3. It is. FIG. 7 shows the relationship between the amount of carbon and the amount of wear of the head wear test results of the rail manufactured by the rail manufacturing method of the present invention shown in Table 2 and the rail manufactured by the comparative rail manufacturing method shown in Table 3. is there.

Various test conditions are as follows.
1. Head tensile tester: Universal small tensile tester Test piece shape: JIS No. 4 similar parallel part length: 30 mm, parallel part diameter: 6 mm, distance between elongation measurement scores: 25 mm
Test piece sampling position: 6mm below the rail head surface (see Fig. 5)
Tensile speed: 10 mm / min, test temperature: normal temperature (20 ° C.)
2. Abrasion tester: Nishihara type wear tester (see Fig. 7)
Test piece shape: disk-shaped test piece (outer diameter: 30 mm, thickness: 8 mm)
Test piece sampling position: 2mm below the rail head surface (see Fig. 6)
Test load: 686 N (contact surface pressure 640 MPa)
Slip rate: 20%
Opposite material: Pearlite steel (Hv380)
Atmosphere: Air cooling: Forced cooling with compressed air (flow rate: 100 Nl / min)
Repeat count: 700,000 times

  As shown in Tables 2 and 3, the rail steel of the present invention (steel: 1 to 19) is within a certain range in which the addition amount of C, Si and Mn is larger than that of the comparative rail steel (steel: 20 to 23). Therefore, it does not generate pro-eutectoid ferrite, pro-eutectoid cementite structure, martensite structure, etc. that adversely affect the wear resistance and ductility of the rail, and a pearlite structure with excellent wear resistance and ductility is generated. .

  Moreover, as shown in Table 2, Table 3, and FIG. 6, this invention rail steel (steel: 1-19) is a fixed range with finish rolling conditions compared with comparative rail steel (steel: 24-29). Therefore, the duct head is improved in ductility when the fine pearlite structure is stably generated and the carbon content of the steel is the same. In addition, the rail steel of the present invention (steel: 1 to 19) has a heat treatment condition within a certain range as compared with the comparative rail steel (steel: 30 to 34), so that a fine pearlite structure can be stably formed. When the carbon content of the steel is the same, the duct head has further improved ductility.

  Furthermore, as shown in Table 2, Table 3, and FIG. 7, the rail steel of the present invention (steel: 1 to 19) has a certain range of finish rolling conditions as compared with the comparative rail steel (steel: 24, 25). Therefore, a fine pearlite structure is stably generated and wear resistance is ensured. Moreover, since the rail steel of the present invention (steel: 1 to 19) is within a certain range of heat treatment conditions as compared with the comparative rail steel (steel: 30 to 33), it is a detrimental effect on wear resistance. Generation of cementite structure and martensite structure is suppressed, and wear resistance is ensured.

  As described above, according to the present invention, in rail production, the structure of the rail head used in heavy-duty railways is controlled by controlling the steel composition, finish rolling conditions, and further heat treatment conditions, and the hardness It is possible to improve the wear resistance and ductility of the rail.

Results of rolling experiments using steel with carbon content of 0.85 to 1.40% were obtained by dividing the reaction force ratio (reaction force value of the rolling mill by the reaction force value at the rolling temperature of 950 ° C. with the same cumulative area reduction rate). (Value) and the figure shown by the relationship between the production amount of the non-recrystallized austenite structure immediately after rolling. The figure which showed the name in the head cross-section surface position of the rail manufactured with the rail manufacturing method of this invention. The figure which showed the test piece collection position in the tension test shown in Table 2 and Table 3. FIG. The figure which showed the test piece collection position in the abrasion test shown in Table 2 and Table 3. FIG. The figure which showed the outline | summary of the abrasion test. The figure which showed the result of the head tensile test of the rail manufactured with the rail manufacturing method of this invention shown in Table 2, and the rail manufactured with the comparative rail manufacturing method shown in Table 3 by the relationship between carbon amount and total elongation value. The figure which showed the result of the head abrasion test of the rail manufactured with the rail manufacturing method of this invention shown in Table 2, and the rail manufactured with the comparative rail manufacturing method shown in Table 3 by the relationship between carbon amount and the amount of wear.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Top part 2 ... Head corner part 3 ... Head side part 4 ... Rail test piece 5 ... Counterpart material 6 ... Cooling nozzle

Claims (2)

In mass%, C: 0.85 to 1.40%, Si: 0.05 to 2.00%, Mn: 0.05 to 2.00%, with the balance being Fe and inevitable impurities A method of manufacturing a pearlite rail excellent in wear resistance and ductility by performing at least rough rolling and finish rolling on a steel strip for rail rolling,
In the finish rolling, in a temperature range where the rail head surface temperature is less than the Arcm point to 700 ° C. or higher, rolling is performed so that the cumulative reduction in area of the head is 20% or more. A recrystallized austenite structure is generated at an area ratio of 50% or more, and a cementite structure is generated at the austenite grain boundary in the non-recrystallized austenite structure on the rail head surface immediately after the finish rolling,
Thereafter, the rail head surface after finish rolling is acceleratedly cooled to at least 550 ° C. at a cooling rate of 5 to 50 ° C./sec within 200 sec after completion of finish rolling. Furthermore, in mass%,
Cr: 0.05 to 2.00%, Mo: 0.01 to 0.50%, V: 0.005 to 0.500%, Nb: 0.002 to 0.050, B: 0.0001 to 0 .0050%, Co: 0.003-2.00%, Cu: 0.01-1.00%, Ni: 0.01-1.00%, Ti: 0.0050-0.0500%, Mg: 0.0005 to 0.0200%, Ca: 0.0005 to 0.0150, Al: 0.010 to 1.00%, Zr: 0.0001 to 0.2000%, N: 0.0060 to 0.0200 The method for producing a pearlite rail according to claim 1, wherein the pearlite rail contains 1 type or 2 types or more.

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