BR112016022007B1 - rail and manufacturing method - Google Patents

Abstract

RAIL AND MANUFACTURING METHOD The present invention provides a rail that exhibits excellent wear resistance and reduced hardness variation along the length of a rail. The rail has a chemical composition that contains 0.60% to 1.0% C, 0.1% to 1.5% Si, 0.01% to 1.5% Mn, 0.035% or less P , 0.030% or less of S and 0.1% to 2.0% of Cr, with the remainder being Fe and any impurities. The surface hardness of the rail exhibits variation equal to or less than (More or less) 15 HB points along the length of the rail.

Description

FIELD OF TECHNIQUE

[001] The present invention relates to a rail, especially a rail having high hardness and small hardness variation, and also to a method of manufacturing the rail.

FUNDAMENTALS OF THE INVENTION

[002] In freight cars, used on railways to transport cargo and ore, the weight of the cargo tends to be higher than in passenger cars, which results in heavy loads acting on the axles of the cars and an environment of intense contact between the freight car wheels and the rails. Under these conditions, the rails are expected to exhibit wear resistance and these are conventionally made of perlite frame steel.

[003] In recent years, the trend has been even higher load and mineral weights, and so on, in order to improve the efficiency of rail transport, which further increased wear and shortened the life of the rails. . In this way, it seeks to improve the wear resistance of the rails in order to prolong their useful life, having been proposed numerous rails of high hardness in which the hardness of the rail is reinforced.

[004] For example, PTL 1, PTL 2, PTL 3 and PTL 4 each reveal a hypereutectoid rail with increased cementite content and a method of fabrication of that rail. In addition, PTL 5, PTL 6 and PTL 7 each describe a technique in which hardness is increased by refining the lamellar spacing of a perlite structure in steel with carbon content at the level of the eutetoid composition.

[005] In relation to a rail manufacturing method, PTL 8 proposes a method for manufacturing a high hardness rail having fatigue resistance in the internal region of the upper part of the billet (head). When rolling a steel rail plate by this method, the finish rolling is carried out at a temperature on the billet surface between 850 °C and 1050 °C to finish the finish, and after a time interval between passes of at least 3 seconds and at most 1 minute, one pass or a plurality of steps is carried out for lamination of the final finish at a temperature on the billet surface between 800 °C and 950 °C and with a reduction in the lamination of 10% or less per step. After that, accelerated cooling begins, at a cooling rate of 2 °C/s to 4 °C/s for 0.1 second to 10 seconds, in order to cool the temperature in less than 5 mm from the surface from the billet and corner of the rail to the transformation temperature Ar1 or lower, and cooling proceeds at a maximum surface cooling rate of at least 4 °C/s and at maximum 30 °C/s.

[006] PTL 9 describes a method for fabricating a rail with high tenacity that exhibits a perlite metallic structure. In this method, after rough rolling of a plate of low alloy steel or carbon steel, containing 0.60% to 1.00% C, for rail form, the continuous finishing roll is performed by three or more steps. of lamination at a rail surface temperature of 850 °C to 1000 °C with rolling reduction of a cross-sectional area of 5% to 30% per step and 10 seconds or less between rolling steps and then the rail is allowed to cool or is cooled from 700 °C or more to a temperature in a range of 500 °C to 700 °C at a rate of 2 °C/s to 15 °C/s.

[007] In addition, PTL 10 describes a method for fabricating a pearlite rail having superior wear resistance and ductility, in which at least rough lamination and finish lamination are performed on a steel plate, for rail lamination , which contains, in % by mass, 0.65% to 1.20% of C, 0.05% to 2.00% of Si and 0.05% to 2.00% of Mn, the remainder being Fe and any impurities. In finish lamination, lamination is carried out at a surface temperature in the rail billet of at most 900 °C and at least the Ar3 transformation point or the Arcm transformation point, an accumulated reduction rate in equal billet area or greater than 20% and with a reaction force ratio equal to or greater than 1.25, which is a value obtained by dividing a roller reaction force value by a reaction force value for the same accumulated reduction rate in rolling area and temperature of 950 °C. After finish lamination, the billet surface is cooled to 550 °C or less at a cooling rate of 2 °C/sec to 30 °C/sec by accelerated cooling or natural cooling.

[008] It is expected that the rails used in high axle load railways, whose main examples are rail systems for the transport of cargo and ore, are expected to have superior wear resistance in order to improve the durability of the rails and, in response, several rail proposals have been put forward with a focus on increasing hardness.

List of quotes patent literature

[009] PTL 1: JP 4272385 B PTL 2: JP 3078461 B PTL 3: JP 3081116 B PTL 4: JP 3513427 B PTL 5: JP 4390004 B PTL 6: JP 2009-108396 A PTL 7: JP 2009-235515 A PTL 8: JP 3423811 B PTL 9: JP 3113137 B PTL 10: JP 2008-50687 A

SUMMARY OF THE INVENTION Technique Problem

[010] A rail is manufactured by hot rolling a steel raw material so that its length is equal to or greater than 100 m, and the hardness of the rail, in the sense of the rail length, varies depending on the manufacturing method. Consequently, the rail, when placed, may suffer uneven wear and thus be unable to sufficiently demonstrate its effects. Although it is extremely important, therefore, to reduce the hardness variation in the longitudinal direction of the rolling, document PTL 1-10 makes no mention of this hardness variation.

[011] In consideration of the foregoing, the present invention aims to provide a rail that exhibits excellent wear resistance and reduced variation in hardness along the length of the rail, and also a method of manufacturing the rail.

Technique Solution

[012] The inventors prepared sample bodies as samples of steel materials with pearlite structures corresponding to rails of different hardness, and conducted a wear test on the specimens in order to investigate a relationship between hardness and wear. The investigation results are shown in Figure 1.

[013] The wear test is a comparative study in which the actual contact conditions between a pearlitic steel rail and a wheel were simulated using a Nishihara-type wear test device that makes it possible to assess the wear resistance in a short period of time. The test was conducted as illustrated in Figure 2, rotating a Nishihara-type wear specimen 1, 30 mm in outer diameter, taken as a sample of a billet, in contact with a tire specimen 2 . The arrows in Figure 2 indicate the rotation directions of Nishihara type specimen 1 and tire specimen 2, respectively. The tire specimen was obtained by sampling on a round bar 32 mm in diameter from a normal billet stipulated by standard JIS E1101, subjecting the round bar to heat treatment so that it has a tempered martensite structure and Brinell hardness (Brinell load 29.4 kN) of 370 HB and subsequently processing the round bar to assume the shape illustrated in Figure 2. Nishihara-type wear specimens 1 were taken from two locations in a billet 3 as illustrated in Figure 3. A specimen taken from a surface layer of billet 3 is denoted as Nishihara wear specimen 1a, and a specimen taken from an inner part of billet 3 is denoted as Nishihara type wear specimen 1b. The center, in the longitudinal direction, of the Nishihara-type wear specimen 1b taken from the inside of the billet 3, is located at a depth of 24 mm to 26 mm (average value: 25 mm) from an upper surface of the billet 3 The test was conducted under dry environment conditions, and the wear was measured after 1.8 x 105 rotations under conditions of a contact pressure of 1.2 GPa, slip ratio of -10% and rotation speed of 750 rpm (tire test piece: 750 rpm). Wear was calculated from the difference in the mass of the specimen, measured before and after the test.

[014]As illustrated in Figure 1, wear resistance increases as hardness increases. For example, the wear resistance of a rail with a hardness equal to or greater than 400 HB can be improved by 15% when compared to a rail subjected to common heat treatment (370 HB). However, if the hardness exhibits a large amount of variation along the length of the rail, a difference in wear behavior for hard and soft parts arises. For example, in a situation where the hardness in spots is 415 HB and exhibits a variation equal to or less than ±15 (that is, the hardness varies within a range from a minimum of 400 HB to a maximum of 430 HB), wear changes from 0.37 g to 0.3 g and thus exhibits a variation equal to or less than 20%. On the other hand, in a situation where the hardness in spots is 415 HB and exhibits a variation equal to ±30 (ie, the hardness varies in a range from a minimum of 385 HB to a maximum of 445 HB), the wear changes from 0.40 g to 0.27 g and thus exhibits a variation of 33%. Taking the foregoing into consideration, reducing the hardness variation in the longitudinal direction of an accompanying rail while increasing the hardness of the rail allows the rail to wear evenly and contributes to improving its service life. It is preferable for wear to be as even as possible along the length as wear continues due to contact between the rail and wheels during use. Taking into account the test results described above, the variation in hardness along the length of the track is preferably such that the variation in wear is equal to or less than 20%. The inventors have found that a variation in surface hardness of ±15 HB or less ensures superior wear resistance along the length direction and contributes to improved rail life. This discovery led to the present invention.

[015] Specifically, the primary attributes of the present invention are as follows:

[016] (1) A rail comprising a chemical composition that contains (consists), in % by mass: 0.60% to 1.0% of C; 0.1% to 1.5% Si; 0.01% to 1.5% Mn; 0.035% or less P; 0.030% or less of S; and 0.1% to 2.0% Cr, with the remainder being Fe and eventual impurities, where the surface hardness of a rail billet exhibits ±15 HB variation between points or less in one direction of the rail length.

[017] In this descriptive report, the variation in surface hardness in the direction of the length of the rail refers to the difference between an average value of Brinell hardness of the upper region of the billet, calculated from measurements made at intervals of 5 m in one direction the length of the roll over the entire length of the rail (eg 25 m to 100 m), and the Brinell hardness value measured at each of the measuring points. In other words, the variation in surface hardness is equal to or less than ±15 HB between points along the length of the rail means that when an average value for Brinell hardness is calculated from all hardness values measured in 5 m intervals (ie measured values at 6 points in the case of a total length of 25 m, 11 points in the case of a total length of 50 m and 21 points in the case of a total length of 100 m), the maximum difference in Brinell hardness between the mean value and the values for the measurement points is equal to or less than ±15 points. Note that Brinell hardness is measured after removing 0.5 mm or more of a decarburized layer using a grinder or similar.

[018] (2) The trail described in (1), in which the chemical composition still contains, in % by mass, one or more of: 1.0% or less of Cu; 0.5% or less Ni; 0.5% or less Mo; and 0.15% or less of V.

[019] (3) The rail described in (1) or (2), where the surface hardness of the rail billet is equal to or greater than 400 HB.

[020] (4) The rail described in any one of (1) to (3), where the variation in surface hardness is equal to or less than ±10 HB between points.

[021] (5) A rail manufacturing method comprising: heating to 1200 °C or more a steel raw material having a chemical composition containing (consists), in % by mass: 0.60% a 1.0% C, 0.1% to 1.5% Si, 0.01% to 1.5% Mn, 0.035% or less P, 0.030% or less S and 0.1% to 2.0% Cr, with the remainder being Fe and any impurities; hot rolling the raw steel material after heating, the hot rolling being carried out in such a way that the rolling, along the length of a rail, in a region whose temperature is not higher than 1000 °C, is performed over a plurality of steps with a time interval between steps showing a variation equal to or less than 15 seconds along the length of the rail, an accumulated area reduction rate equal to or greater than 40% for a part that forms a billet and a final finishing temperature equal to or greater than 900 °C; and cool the rail billet after hot rolling, from a cooling start temperature of 800°C or more to a cooling end temperature of 600°C or less at a cooling rate of 1°C/ at 10 °C/s.

[022](6) The rail manufacturing method described in (5), in which the chemical composition also contains, in % by mass, one or more between: 1.0% or less of Cu; 0.5% or less Ni; 0.5% or less Mo; and 0.15% or less of V.

[023] (7) The rail fabrication method described in (5) or (6), where the rate of cooling in the cooling exhibits variation equal to or less than ±1 °C/s in the length direction of the rail.

Advantageous Effect

[024] The present invention allows to minimize the variation of hardness along the length of a rail, and effectively improves the durability of the rail (prolongs the useful life of the rail), especially in the case of a rail that is placed in an environment of high axle load like a heavy-duty railroad or a mining railroad and thus demonstrates a significant effect on industrial use.

BRIEF DESCRIPTION OF THE DRAWINGS

[025] In the attached drawings:

[026] Figure 1 is a graph illustrating a relationship between the hardness and wear of the rail material;

[027] Figure 2 illustrates a Nishihara-type wear specimen whose wear resistance is evaluated, in which (a) is a plan view and (b) is a side view; and

[028] Figure 3 is a cross-sectional view of a billet illustrating sampled positions of Nishihara wear specimens.

DETAILED DESCRIPTION OF THE INVENTION

[029] First, the reasons for the limitations on each component in the chemical composition of a rail will be explained. When components are expressed as “%”, this refers to “% by mass” unless otherwise specified.

[030]C: 0.60% to 1.0%

[031]C is an important element in a pearlitic rail to form cementi-ite, increase hardness and strength, and improve wear resistance. However, these effects are small when the C content is less than 0.60% and therefore the lower limit for the C content is 0.60%. On the other hand, although an increase in C content, and thus an increase in cementite content, is expected to increase hardness and strength, an increase in C content also decreases ductility. Furthermore, an increase in the C content widens the temperature range Y + θ and promotes the softening of a zone affected by heat. Taking these influences into account, the upper limit for the C content is 1.0%. The C content is preferably in a range between 0.73% and 0.85%.

[032]Si: 0.1% to 1.5%

[033]Si is added to the rail material as a deoxidizing material and in order to raise the equilibrium transformation temperature (TE) and strengthen the perlite structure (increases the hardness by refining the lamellar structure). However, these effects are small when the Si content is less than 0.1%. On the other hand, an increase in Si content promotes decarbonization and the formation of defects on the rail surface. Therefore, the upper limit for the Si content is 1.5%. The Si content is preferably in a range between 0.5% and 1.3%.

[034]Mn: 0.01% to 1.5%

[035]Mn has the effect of reducing the actual temperature of transformation into perlite and narrowing the lamellar spacing of the perlite, and is an effective element to achieve high hardness. However, these effects are small when the Mn content is less than 0.01%. On the other hand, adding more than 1.5% Mn to improve hardenability facilitates transformation to bainite or martensite. Therefore, the upper limit for the Mn content is 1.5%. The Mn content is preferably in a range between 0.3% and 1.2%.

[036] P: 0.035% or less

[037]P content greater than 0.035% decreases the toughness and ductility. Therefore, the upper limit for the P content is 0.035%. A preferable range for P content has an upper limit of 0.0025%. On the other hand, taking into account the higher cost of manufacturing steel when special refining or similar procedures are carried out, the lower limit for the P content is preferably 0.001%.

[038]S: 0.030% or less

[039]S forms MnS with coarse grain extending towards the lamination and decreases ductility and toughness. Therefore, the upper limit for the S content is 0.030%. On the other hand, restricting the S content to less than 0.0005% requires a significant increase in the cost of steelmaking, which is due, for example, to a large increase in the time of the steelmaking process. Therefore, the lower limit for the S content is preferably 0.0005%. The S content is preferably between 0.001% and 0.015%.

[040]Cr: 0.1% to 2.0%

[041]Cr raises the equilibrium transformation temperature (TE), contributes to the refinement of pearlite lamellar spacing and increases hardness and strength. For such effects to be obtained, it is necessary to add 0.2% or more of Cr. On the other hand, adding more than 2.0% Cr increases the occurrence of defects in the weld while also increasing the hardenability and promoting the formation of martensite. Therefore, the upper limit for the Cr content is 2.0%. The Cr content is more preferably in a range between 0.26% and 1.00%.

[042] In addition to the chemical components described above, one or more of the following may be added: 1.0% or less of Cu, 0.5% or less of Ni, 0.5% or less of Mo and 0 .15% or less of V.

[043]Cu: 1.0% or less

[044]Cu is an element that can provide even higher hardness by strengthening the solid solution. Cu also has the effect of suppressing decarbonization. For these effects to be obtained, preferably, 0.01% or more of Cu is added. On the other hand, adding more than 1.0% Cu makes it more likely for surface cracks to occur during casting or continuous rolling. Therefore, the upper limit for the Cu content is preferably 1.0%. Furthermore, the Cu content is more preferably in a range between 0.05% and 0.6%.

[045] Ni: 0.5% or less

[046] Ni is an effective element to improve toughness and ductility. Ni is also an effective element to inhibit Cu cracking by addition combined with Cu. Therefore, in a situation where Cu is added, preferably Ni is also added. However, these effects are not visible when the Ni content is less than 0.01%. Therefore, in a situation where Ni is added, the lower limit for the Ni content is preferably equal to or greater than 0.01%. On the other hand, adding more than 0.5% Ni increases the hardening capacity and promotes the formation of martensite. Therefore, the upper limit for the Ni content is preferably 0.5%. The Ni content is more preferably in a range between 0.05% and 0.50%.

[047] Mo: 0.5% or less

[048] Mo is an effective element to increase strength, but this effect is small when the Mo content is less than 0.01%. Therefore, the lower limit for the Mo content is preferably 0.01%. On the other hand, adding more than 0.5% Mo causes the formation of martensite as a result of the increased hardening capacity and radically decreases toughness and ductility. Therefore, the upper limit for the Mo content is preferably 0.5%. The Mo content is more preferably in a range between 0.05% and 0.30%.

[049]V: 0.15% or less

[050]V forms VC, VN, or the like, as a fine precipitate in ferrite and is an element that contributes to achieving high hardness through precipitation strengthening of ferrite. The solvation temperature of VC or VN is sufficiently lower than that of Ti or Nb so that it has little influence on the recrystallization behavior of austenite during rolling and therefore has little influence on the variation of properties in the direction of the length of the rail. In addition, V also acts as a hydrogen entrapment site, and could be predicted to exhibit a late fracture-inhibiting effect. Therefore, 0.001% or more of V is preferably added. On the other hand, when more than 0.15% V is added, the effects described above reach saturation, and the cost of alloying increases dramatically. Therefore, the upper limit for the V content is preferably 0.15%. The content of V content is more preferably in a range between 0.005% and 0.12%.

[051]The balance excluding the components mentioned above is formed by Fe and any impurities.

[052] For example, one can allow up to 0.006% N and 0.003% O as eventual impurities. Furthermore, although Al is effective as a deoxidizing material, Al forms AlN in agglomerate form, which significantly decreases the fatigue characteristics of laminates. Therefore, the Al content is preferably equal to or less than 0.003%. Nb and Ti are also contained as possible impurities as described below.

[053]Nb: 0.003% or less

[054]Ti: 0.003% or less

[055]Nb and Ti are effective elements to improve hardness and wear resistance by forming carbides or carbonitrides that strengthen the matrix. However, Nb and Ti are detrimental elements that promote variation in the rail hardness in the longitudinal direction and therefore are not generally added, although 0.003% of accidentally mixed Nb and Ti are permissible. Specifically, the addition of Nb or Ti causes changes in hardness to a large extent according to the heating, rolling, or cooling conditions of the material, and thus causes changes in hardness along the length of the rolling to be associated. more sensitively to variation in these conditions. In metallurgical terms, the inhomogeneity of heated austenite particles is promoted and, at the same time, the inhibition of austenite recrystallization during rolling and a change in the temperature of transformation into perlite associated with them are greatly increased compared to steel in which Nb and Ti are not added, and this can promote hardness variation.

[056] In addition to the chemical composition described above, it is essential that the surface hardness exhibits variation in points equal to or less than ± 15 HB in the direction of the length of the rail. The reason for this is that the change in rail wear reaches 20% or more if the variation in point hardness is greater than ± 15 HB. Furthermore, it is more preferred that the surface hardness exhibits variation in points equal to or less than ± 10 HB along the length of the rail, as the variation in hardness in points equal to or less than ± 10 HB allows to restrict the change in wear of the rail to less than 15%.

[057] The following is a specific description of the manufacturing conditions of the rail.

[058] First, the steel raw material used is preferably continuous cast steel, obtained through the continuous casting of molten steel that has been adjusted to the chemical composition described above by means of steel fabrication processes, such as a process in blast furnace, molten iron pretreatment, a process in a converter and RH degassing.

[059]The raw steel material is hot rolled to define a rail shape by rolling common in gauge or universal. The reasons for the limitations imposed on conditions during heating and rolling described above are explained below, as well as on conditions during subsequent cooling.

[060]Heating temperature before hot rolling: 1200 °C or higher

[061]The heating of raw material produced from steel requires 1200 °C or more. This is done with the main objective of sufficiently reducing the deformation resistance to enable the use of a lighter load for lamination and also with the objective of homogenization. In order to sufficiently obtain these effects, the heating temperature must be equal to or greater than 1200 °C. Although it is not necessary to define a specific upper limit, the heating temperature is preferably equal to or less than 1300 °C from the point of view of suppressing a loss in scale and decarbonization.

[062]Lamination in one direction of the length of the rail, in a region whose temperature is not greater than 1000 °C, performed over a plurality of steps with a time interval between steps exhibiting an equal or lower variation at 15 seconds along the length of the rail

[063]Heated steel raw material, as described above, is defined in rail form by hot rolling. In hot rolling, it is important that a plurality of rolling steps at temperatures not exceeding 1000 °C be carried out by rolling repeatedly in a single direction in order to minimize the variation in a time interval between steps. Note that the time interval between steps refers to the interval between a time when a given longitudinal part (rolling direction) of a laminated material such as a rail is bitten by a cylinder and a time when the given part is then bitten by the cylinder. The time interval between steps differs most at the top (front end) of the laminated material as a rail and the bottom (ex-rear end) of the laminated material as a rail.

[064] In conventional reverse lamination, during an interval between biting an upper laminated part (front end) by the cylinder in a given step and the beginning of the bite in a next step, the next step is performed in order, feeding first a lower laminated portion (ex-rear end) to the cylinder, which lengthens the time interval between steps for the upper laminated portion. On the other hand, after the lower laminated part (rear end) has passed through a certain step, the lower part is first bitten by the cylinder in the next step, which shortens the time interval between steps. The difference in the time interval between the steps for the leading edge and the trailing edge described above, which is a characteristic of reverse rolling, influences the state of the austenite structure and also influences the variation in hardness after transformation to pearlite. In contrast, when continuous lamination is carried out in one direction, the difference in the time interval between steps for a leading edge of a rolled material is fundamentally small. Therefore, the inhomogeneity of the austenite structure arising from the above-described difference in the time interval between steps can be resolved. It is therefore necessary that the difference mentioned above in the time interval between steps is equal to or less than 15 seconds. In other words, a difference in time interval between steps equal to or less than 15 seconds can suppress the hardness variation along the length of the rail. The difference in time interval between steps is preferably equal to or less than 12 seconds.

[065]The above stipulations are conditions to be applied to rolling performed at 1000 °C or less in hot rolling. Reverse lamination can be used for lamination carried out in a region whose temperature does not exceed 1000 °C, a representative example of which is rough lamination. In other words, as long as rolling at 1000 °C or less is carried out continuously in one direction, a preceding stage of rolling in a region whose temperature is not higher than 1000 °C can be carried out freely. In hot rolling, two to seven rolling steps are preferably carried out at 1000°C or less. The reason for this is that single-step lamination requires a large lamination load and makes molding difficult, while more than seven passes tends to cause a very inhomogeneous austenite state and increase hardness variation.

[066] Accumulated reduction rate in an area equal to or greater than 40% for a part that forms a billet

[067] The required rate for cumulative reduction in area of the lamination carried out at 1000 °C or less is equal to or greater than 40%. The reason for this is that it is necessary to process 40% or more area reduction at 1000 °C or less in order to further refine the austenite recrystallization. If the area reduction rate for rolling at 1000 °C or less is less than 40%, the refinement of the austenite recrystallization is insufficient and coarse austenite may partially remain, which results in increased hardness variation in the rail length direction (rolling direction).

[068] Final finishing temperature equal to or greater than 900 °C

[069]When performing continuous lamination in a single direction in order to reduce the variation in the time interval between passes along the entire length of the laminated material, a final finishing temperature of 900 °C or greater is preferred. The reason for this is that if the final finishing temperature is less than 900 °C, the global hardness decreases and its variation increases for reasons such as a decrease in the start temperature of the in-line heat treatment cooling, carried out consecutively after the lamination , and the promotion of transformation to perlite (transformation at higher temperature). Therefore, the final finishing temperature is preferably equal to or greater than 900°C in order to avoid a decrease in hardness as described above.

[070]Cooling is carried out consecutively after hot rolling under the following conditions.

[071] Billet cooling, from a cooling start temperature of 800 °C or greater to a cooling end temperature of 600 °C or less at a cooling rate of 1 °C/s to 10 °C/ s

[072] Firstly, the start temperature of cooling is preferably equal to or greater than 800 °C. Specifically, a cooling start temperature below 800 °C may not allow sufficient supercooling or allow sufficient surface hardness to be achieved. The temperature required for the end of cooling is equal to or less than 600 °C. Sufficient hardness cannot be obtained if the cool-down end temperature is above 600 °C. Although no specific lower limit is provided, saturation is reached in terms of hardness when cooling is performed to 400°C or less, and productivity is adversely affected by the increased cooling time. Therefore, the cooling is preferably stopped at 400 °C or more.

[073]The cooling rate is in a range of 1 °C/s to 10 °C/s. A cooling rate greater than 10 °C/s does not allow enough time for the transformation into pearlite, causes the formation of bainite and martensite, and thus reduces toughness, ductility and fatigue strength. On the other hand, a cooling rate of less than 1 °C/s does not allow sufficient hardness to be obtained. The cooling rate is preferably in a range between 2 °C/s and 8 °C/s.

[074] In addition, the cooling rate preferably exhibits variation equal to or less than ±1°C/s in the longitudinal direction of the rolling mill. Restricting the variation in the cooling rate to ±1 °C/s or less reduces the variation in the lamellar spacing of the pearlite, makes it possible to achieve a variation in hardness equal to or less than ± 10 HB, and reduces the variation in wear resistance and variation in fatigue strength in the longitudinal direction of the rail.

[075]Cooling carried out consecutively after hot rolling is preferably carried out by cooling with forced air or mist. Forced air cooling is an accelerated cooling in which air is forcibly blown against the billet. Mist cooling involves mixing air and water and blowing a mist of water against the billet.

[076] In order to control and minimize the variation in the cooling rate in the longitudinal direction of the rolling mill, in the case of forced air cooling, for example, it is necessary to control the air pressure at intervals of 5 m or less (preferably 3 m or less), adjust the in-line air pressure according to the variation in temperature of the rail in the longitudinal direction, measured before cooling, and carry out the control such that the cooling rate remains constant along the length. In the case of mist cooling, cooling is preferably carried out by controlling the amount of water and pressure in the longitudinal direction in the same way as described above.

[077] Through the chemical composition described above and the execution of the rolling and cooling described above, it is possible to obtain a pearlitic steel rail that has a surface hardness preferably equal to or greater than 400 HB 400 and that exhibits a variation on surface hardness at points equal to or less than ± 15 HB along the length of the rail In other words, a homogeneous, high-hardness pearlitic steel rail can be obtained that exhibits little variation in hardness along the length of the rolling mill.

EXAMPLES

*Os teores de Nb, Ti, sol Al, N e O são como eventuais impurezas.[078] Steels were produced with the chemical compositions shown in Table 1, and cast steels obtained by continuous casting of these were subjected to heating, hot rolling and cooling to make a 136-pound rail or a 141-pound rail for each steel. The manufacturing conditions are shown together with the results of the investigation of surface hardness and its variation in Table 2. Table 1

*The contents of Nb, Ti, sol Al, N and O are possible impurities.

[079] In this descriptive report, the variation in the time interval between steps in the rolling conditions indicates the difference between the time elapsed from a front end of a rolled material being rolled to the front end being rolled next and the time elapsed from a trailing edge of the rolled material being rolled to the trailing edge being rolled next. As explained in more detail above, when the lamination is carried out by conventional reverse lamination, the time interval between steps is extended for an upper laminated part and shortened for a lower laminated part. Thus, the difference in the time interval between steps to the front end (top) and rear end (bottom) of the laminated material is especially evident in reverse lamination. In contrast, the difference in the time interval between steps associated with a leading edge and a trailing edge of a laminated material is smaller in one-way continuous lamination and therefore the inhomogeneity of a produced structure can be remedied. as shown in Table 2.

[080] Note that the cool-down start temperature and the cool-down end temperature are results for the surface temperature of a corner of the rail measured by a thermal imager. The rail's cooling rate is an average value of cooling rates measured from start and end coolant temperatures and cooldown times measured at intervals of 5 m lengthwise. Regarding the variation of the cooling rate in the length direction, it was determined whether the difference between a higher and a lower value in the variation of the cooling rates was greater than ±1 °C/s or was less than or equal to ±1 °C/s.

[081] In addition, the surface hardness of the billet and the microstructure of each of the manufactured rails were evaluated. The surface hardness of the billet was assessed by removing 0.5 mm or more of a decarburized layer using a shredder and measuring the Brinell hardness of points at 5 m intervals along the length of the rail. Likewise, microscopic samples were cut and their structures observed.

Controle do resfriamento no sentido do comprimento: Não ^ Variação > 1 °C/s Controle do resfriamento no sentido do comprimento: Sim ^ Variação < 1 °C/s Tabela 2 (continuação)

Controle do resfriamento no sentido do comprimento: Não ^ Variação > 1 °C/s Controle do resfriamento no sentido do comprimento: Sim ^ Variação < 1 °C/s Tabela 2 (continuação)

Controle do resfriamento no sentido do comprimento: Não ^ Variação > 1 °C/s Controle do resfriamento no sentido do comprimento: Sim ^ Variação < 1 °C/s[082]The evaluation results are shown in Table 2. Table 2

Cooling control lengthwise: No ^ Variation > 1 °C/s Cooling control lengthwise: Yes ^ Variation < 1 °C/s Table 2 (continued)

Cooling control lengthwise: No ^ Variation > 1 °C/s Cooling control lengthwise: Yes ^ Variation < 1 °C/s Table 2 (continued)

Cooling control lengthwise: No ^ Variation > 1 °C/s Cooling control lengthwise: Yes ^ Variation < 1 °C/s

[083] The hardness of rails according to the present invention exhibited an extremely small variation equal to or less than ± 15 HB in the direction of the rail length, while the hardness of rails that departed from the scope of the present invention in terms either or both, the chemical composition and the conditions of the lamination, exhibited a variation greater than ± 15 HB.

Claims (2)

1. Rail, CHARACTERIZED by the fact that it comprises a chemical composition that consists, in % by mass: 0.60% to 1.0% of C; 0.1% to 1.5% Si; 0.01% to 1.5% Mn; 0.035% or less P; 0.030% or less of S; and 0.1% to 2.0% Cr, and optionally one or more of: 0.006% or less N; 0.003% or less of O; 0.003% or less of Al; 0.003% or less of Nb; 0.003% or less of Ti; 1.0% or less of Cu; 0.5% or less Ni; 0.5% or less Mo; and 0.15% or less of V; the remainder being Fe and any impurities, where the surface hardness of a rail billet is 400 HB or greater and the surface hardness of the rail billet exhibits a variation equal to or less than ± 10 HB points or less in the direction of the length of the rail . 2. Rail manufacturing method, CHARACTERIZED by the fact that the method comprises: heating, from 1200 °C to 1300 °C, a steel raw material having a chemical composition consisting, in % by mass: 0 .60% to 1.0% C, 0.1% to 1.5% Si, 0.01% to 1.5% Mn, 0.035% or less P, 0.030% or less S, and 0.1% to 2.0% Cr, and optionally one or more of: 0.006% or less N; 0.003% or less of O; 0.003% or less of Al; 0.003% or less of Nb; 0.003% or less of Ti; 1.0% or less of Cu; 0.5% or less Ni; 0.5% or less Mo; and 0.15% or less of V; the remainder being Fe and any impurities; hot rolling the steel raw material after heating, the hot rolling being carried out continuously in a single direction such that the rolling, along the length of a rail, in a region whose temperature is not greater than 1000 °C, is performed over 2 to 7 steps with a difference in time between steps of 15 seconds or less along the length of the rail, an accumulated area reduction rate equal to or greater than 40% for a part that forms a billet and a final finishing temperature equal to or greater than 900 °C; and cool the rail billet after hot rolling from a start-cooling temperature of 800 °C or greater to an end-cooling temperature of 400 °C to 600 °C at a cooling rate of 1 °C /s at 10 °C/s, where the rate of cooling on cooling varies by ± 1 °C/s or less along the length of the rail.

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