Guide rail and manufacturing method thereof

Abstract

Provided is a guide rail having excellent fatigue crack propagation resistance in the sole portion thereof. A guide rail having a component composition containing 0.70 to 1.20 mass% of C, 0.10 to 1.20 mass% of Si, 0.10 to 1.50 mass% of Mn, 0.035 mass% or less of P, 0.020 mass% or less of S, and 0.05 to 2.00 mass% of Cr, with the remainder being Fe and unavoidable impurities; the area ratio of the pearlite structure at a depth of 1 mm from the surface of the sole center portion of the guide rail is 95% or more, and the value of HC represented by formula (1) satisfies formula (2). HC = {(12 * [% C]) + ([% Si] / 10) + ([% Mn] / 20) + ([% Cr] / 18)} 2... (1) HC < = (120 * GS) / (1.1 * BS)... (2)

Description

Technical Field

[0001] This invention relates to guide rails and their manufacturing methods. Background Technology

[0002] For heavy-duty railways primarily used for transporting ore, the load on freight car axles is far greater than that on passenger cars, and the operating environment of the guide rails is also more demanding. Therefore, from the perspective of prioritizing wear resistance, guide rails have always primarily used steel with a pearlitic microstructure.

[0003] In recent years, to improve the efficiency of rail transport, the load capacity of freight cars has been increasing. Furthermore, due to the increased transport capacity, the number of wheels traveling on the rails has also increased.

[0004] The passage of wheels applies repeated tensile and compressive stresses to the guide rails. With increasing load weight and the number of wheels passing over the guide rails, wear and tear occurs at the rail feet, leading to a tendency for guide rail replacements to increase annually. Therefore, guide rail steel with improved fatigue crack propagation resistance at the rail feet is needed.

[0005] Against this backdrop, various studies have been conducted to further improve the breakage resistance of the bottom of the guide rail. For example, Patent Document 1 proposes a guide rail in which, in a guide rail steel having a pearlitic microstructure containing 0.65–1.40% C, at least a portion extending from the bottom surface to a depth of 10 mm, every 0.2 mm... 2 The tested area contained more than 200 pearlite particles with a diameter of 1–15 μm.

[0006] Patent document 2 proposes a guide rail containing, by mass percent, 0.65-1.20% C, 0.05-2.00% Si and 0.05-2.00% Mn, with the remainder containing Fe and unavoidable impurities. More than 97% of the head surface and bottom surface are pearlite, the surface hardness of the pearlite portion is in the range of Hv320-500, the maximum surface roughness is less than 180 μm, and the ratio of surface hardness to maximum surface roughness is more than 3.5.

[0007] Patent document 3 discloses a guide rail as follows: while the head of the guide rail is accelerated to cool from the austenitic region after the guide rail is rolled, the bottom surface of the guide rail is accelerated to cool at a cooling rate of 1 to 5°C / second between 800 and 450°C, and the average hardness of the pearlite at the bottom of the guide rail is HB320 or higher.

[0008] Existing technical documents

[0009] Patent documents

[0010] Patent Document 1: Japanese Patent Application Publication No. 2006-57127

[0011] Patent Document 2: International Publication No. 2011 / 021582

[0012] Patent Document 3: Japanese Patent Application Publication No. 1-139724 Summary of the Invention

[0013] However, the aforementioned prior art still has the following unresolved problems. As with the technologies described in Patent Documents 1 and 2, if the C content is 0.65–1.40%, a hard and brittle proeutectoid cementite structure is formed at the original austenite grain boundaries depending on the heat treatment conditions; therefore, an improvement in fatigue damage resistance cannot be expected. Furthermore, with the technology described in Patent Document 3, depending on the combination of composition and manufacturing conditions, a proeutectoid cementite structure is sometimes also formed, resulting in an increased fatigue crack propagation rate; therefore, it is difficult to say that the material control is sufficient.

[0014] The present invention was made to advantageously solve the above-mentioned problems, and its purpose is to provide a guide rail with excellent fatigue crack propagation resistance characteristics at the foot of the guide rail and a method for manufacturing the same.

[0015] To address the aforementioned problems, the inventors manufactured guide rails with varying contents of C, Si, Mn, and Cr, and conducted in-depth investigations into the microstructure and fatigue crack propagation characteristics of the guide rail feet. The results showed that the compositional parameter (HC parameter represented by equation (1) described later), defined by the C, Si, Mn, and Cr contents, is correlated with the amount of proeutectoid cementite. Furthermore, it was discovered that by controlling the HC parameter to be the same as or smaller than the value of the parameter composed of the original austenite grain size and pearlite block size of the guide rail feet, excellent fatigue crack propagation characteristics of the guide rail feet can be obtained even in the presence of a large amount of proeutectoid cementite. More details are as follows.

[0016] The inventors have discovered that the fatigue crack propagation rate increases due to the presence of proeutectoid cementite for the following reasons: Figure 1 As shown in the schematic diagram, in the proeutectoid cementite portion present in the plastic zone at the tip of a fatigue crack, the {100} facet of the pearlite ferrite adjacent to the proeutectoid cementite portion undergoes brittle fracture first. Furthermore, the inventors have discovered that by controlling the ratio of the pre-austenite grain size (which forms the site of formation) to the pearlite block size (corresponding to the brittle fracture microstructure) based on the amount of proeutectoid cementite formed, the frequency of encounters between the plastic zone at the tip of the fatigue crack and the {100} facet of the pearlite ferrite is reduced, thereby suppressing brittle crack propagation. That is, it is known that even in the presence of a large amount of proeutectoid cementite, by coarsening the pre-austenite grain size or refining the pearlite block size, the aforementioned fatigue crack propagation rate suppression effect can be stably obtained.

[0017] Based on the above insights, the main structure of this invention is as follows.

[0018] [1] A guide rail having the following composition: containing C: 0.70 to 1.20 wt%, Si: 0.10 to 1.20 wt%, Mn: 0.10 to 1.50 wt%, P: less than 0.035 wt%, S: less than 0.020 wt%, and Cr: 0.05 to 2.00 wt%, with the remainder consisting of Fe and unavoidable impurities;

[0019] The area ratio of pearlite tissue at a depth of 1 mm from the center of the guide rail foot is over 95%.

[0020] The value of HC represented by the following equation (1) satisfies the following equation (2).

[0021] remember

[0022] HC={(12×[%C])+([%Si] / 10)+([%Mn] / 20)+([%Cr] / 18)} 2 ···(1)

[0023] HC≤(120×GS) / (1.1×BS)···(2)

[0024] here,

[0025] [%C], [%Si], [%Mn], and [%Cr] represent the contents (mass%) of C, Si, Mn, and Cr, respectively.

[0026] GS represents the original austenite grain size (μm) in the center of the guide rail's foot.

[0027] BS is the size (μm) of the pearlite block in the center of the guide rail's foot.

[0028] [2] According to the guide rail of [1], wherein the above composition further contains at least one of the following: V: less than 0.30 wt%, Cu: less than 1.0 wt%, Ni: less than 1.0 wt%, Nb: less than 0.05 wt%, Mo: less than 2.0 wt%, Al: less than 0.07 wt%, W: less than 1.0 wt%, Co: less than 1.0 wt%, B: less than 0.005 wt%, Ti: less than 0.05 wt%, Sb: less than 0.05 wt%, Mg: less than 0.01 wt%, Ca: less than 0.02 wt%, and Sn: less than 0.05 wt%.

[0029] [3] A method for manufacturing a guide rail, which is the method for manufacturing a guide rail as described in [1] or [2].

[0030] A steel billet with composition [1] or [2] is heated to a temperature below 1350°C and hot-rolled at a rolling end temperature of 850°C or higher at the center of the guide rail foot. Then, it is cooled at an average cooling rate CR1 (°C / second) until the temperature at the center of the guide rail foot changes from 850°C to 750°C. Next, it is cooled at an average cooling rate CR2 (°C / second) until the temperature at the center of the guide rail foot changes from 750°C to the cooling stop temperature T.

[0031] here,

[0032] The aforementioned cooling stop temperature T is a temperature range of 400–650°C.

[0033] The average cooling rate CR1 (°C / sec) and the average cooling rate CR2 (°C / sec) satisfy the following equations (3) and (4), respectively.

[0034] remember

[0035] HC / 300≤CR1≤3.0···(3)

[0036] HC / 120≤CR2≤6.0···(4)

[0037] According to the present invention, a guide rail with excellent fatigue crack propagation resistance at the guide rail foot portion and a method thereof are provided. The guide rail of the present invention contributes to extending the service life of guide rails for heavy axle load railways and preventing railway accidents, thus being industrially beneficial. Furthermore, the manufacturing method of the guide rail of the present invention, by optimizing the heat treatment conditions after hot rolling, can stably improve the fatigue crack propagation resistance at the guide rail foot portion, which is also industrially beneficial. Attached Figure Description

[0038] Figure 1 This is a schematic diagram illustrating the influence of proeutectoid cementite, proto-austenite grain size, and pearlite block size on fatigue crack propagation velocity.

[0039] Figure 2 It is a cross-sectional view of the guide rail showing the various parts of the guide rail and the location of the test piece used for observation of the original austenite grain size and pearlite block size.

[0040] Figure 3 It is a diagram showing the cross-sectional area and thickness of the guide rail feet.

[0041] Figure 4 This is a diagram showing the location of the test piece used in fatigue crack propagation testing.

[0042] Figure 5 This is an explanatory diagram illustrating the shape of the test piece used in fatigue crack propagation tests. Figure 5 (a) represents the front view. Figure 5 (b) represents the side view. Figure 5(c) indicates an enlarged front view of the cut section. Detailed Implementation

[0043] <Part of the guide rail>

[0044] First, refer to Figure 2 The cross-sectional view of the guide rail is provided to illustrate the names of the various parts of the guide rail of this invention. Figure 2 In the guide rail 1 shown, 11 represents the guide rail head, 12 represents the guide rail belly, and 13 represents the guide rail foot. The back side of the guide rail foot 13 is referred to as the guide rail foot 14. The center of the guide rail foot is the portion located near the center in the width direction of the guide rail foot. For example, if the width dimension of the guide rail foot 14 is set to W, the center of the guide rail foot is the area within the range of ±0.075 × W from the center of the width of the guide rail foot 14.

[0045] The guide rail head, guide rail belly, guide rail foot, guide rail foot ball, and guide rail foot center are sometimes referred to as head, belly, foot, foot ball, and foot center, respectively.

[0046] <Composition of the guide rail>

[0047] Next, the composition of the steel used in the guide rail of the present invention will be described. Unless otherwise specified, "%" in the following description means "mass %".

[0048] C: 0.70~1.20%

[0049] C is an essential element for ensuring the strength of pearlitic microstructure, i.e., its resistance to fatigue damage. When the C content is less than 0.70%, it is difficult to obtain excellent fatigue crack propagation characteristics in the foot region. In addition, when the C content exceeds 1.20%, a large amount of proeutectoid cementite is generated at the austenite grain boundaries during cooling after hot rolling, leading to an increase in the fatigue crack propagation rate. It should be noted that proeutectoid cementite can also exist when the C content is below 1.20%, but its influence can be avoided by controlling the original austenite grain size and block size in the central part of the foot to satisfy the above formula (2). From these points of view, the C content is preferably in the range of 0.70 to 1.20%, preferably in the range of 0.70 to 0.89%, and more preferably in the range of 0.70 to 0.85%.

[0050] Si: 0.10–1.20%

[0051] In addition to its effect as a deoxidizer, silicon (Si) also contributes to reducing fatigue crack propagation speed by increasing the equilibrium phase transformation temperature of pearlite and refining the lamellar spacing. From this perspective, the Si content should be 0.10% or higher, but exceeding 1.20% leads to deterioration of weldability due to Si's high affinity for oxygen. Furthermore, Si shifts the eutectoid point towards the lower carbon side; therefore, excessive addition promotes the formation of proeutectoid cementite, resulting in an increase in fatigue crack propagation speed. From these perspectives, a Si content in the range of 0.10–1.20% is preferred, in the range of 0.15–1.10%, and more preferably in the range of 0.20–1.00%.

[0052] Mn: 0.10~1.50%

[0053] Mn is an element that helps reduce fatigue crack propagation speed by lowering the pearlite phase transformation temperature and refining the lamellar spacing. When the Mn content is less than 0.10%, the effect is insufficient. On the other hand, when the Mn content exceeds 1.50%, martensitic structures are easily formed, which solidify and become brittle during heat treatment and welding of the guide rails, leading to material degradation. Furthermore, Mn has the effect of shifting the eutectoid point towards the lower carbon side; therefore, excessive addition promotes the formation of proeutectoid cementite, resulting in an increase in fatigue crack propagation speed. From these perspectives, an Mn content in the range of 0.10–1.50% is preferred, in the range of 0.20–1.40%, and more preferably in the range of 0.30–1.30%.

[0054] P: below 0.035%

[0055] When phosphorus (P) content exceeds 0.035%, ductility deteriorates. Therefore, the P content is 0.035% or less, preferably 0.020% or less. There is no particular limitation on the lower limit of the P content; it can be 0%, but in industry, exceeding 0% is common practice, as excessively reducing the P content leads to increased refining costs. From an economic point of view, it is preferable to have a P content of 0.001% or more.

[0056] S: below 0.020%

[0057] Sulfur (S) is an element that mainly exists in steel as Al-series inclusions. However, when the S content exceeds 0.020%, the amount of these inclusions increases significantly, and coarse inclusions are formed, thus leading to a deterioration in the cleanliness of the steel. Therefore, the S content is preferably 0.020% or less, more preferably 0.015% or less, and even more preferably 0.010% or less. There is no particular limitation on the lower limit of the S content; it can be 0%, but in industry, exceeding 0% is common practice, as excessively reducing the S content leads to an increase in refining costs. From an economic point of view, it is preferable to have an S content of 0.0005% or more.

[0058] Cr: 0.05–2.00%

[0059] Cr is an element that helps reduce the fatigue crack propagation rate by increasing the equilibrium phase transformation temperature of pearlite and narrowing the lamellar spacers. When the Cr content is less than 0.05%, fatigue crack propagation cannot be sufficiently suppressed. On the other hand, when the Cr content exceeds 2.00%, the hardenability of the steel increases, and martensite is more easily formed. Furthermore, when manufacturing is carried out under conditions where martensite does not form, proeutectoid cementite forms at the original austenite grain boundaries, increasing the fatigue crack propagation rate. From these perspectives, a Cr content in the range of 0.05–2.00% is preferred, in the range of 0.10–1.60%, and more preferably in the range of 0.15–1.40%.

[0060] Furthermore, it is not sufficient for the individual elements of the present invention to merely satisfy the above-mentioned ranges. It is important to control the value of the composition parameter HC corresponding to the amount of proeutectoid carburized body to be the same as or smaller than the value of the specified parameter consisting of the original austenite grain size GS and the pearlite block size BS in the center of the guide rail foot.

[0061] The value of the composition parameter HC corresponding to the amount of preeutectoid carburized material is obtained by the following formula (1).

[0062] HC={(12×[%C])+([%Si] / 10)+([%Mn] / 20)+([%Cr] / 18)} 2 ···(1)

[0063] Here, [%C], [%Si], [%Mn] and [%Cr] represent the contents (mass%) of C, Si, Mn and Cr, respectively.

[0064] From the perspective of balancing the high strength of the pearlite structure and the suppression of proeutectoid cementite, i.e., ensuring fatigue damage resistance, the value of HC is preferably 75 to 200, and more preferably 90 to 150.

[0065] In this invention, the specified parameter, which is composed of the original austenite grain size GS and the pearlite block size BS, is represented by (120×GS) / (1.1×BS). The value of this parameter and the value of HC satisfy the following equation (2).

[0066] HC≤(120×GS) / (1.1×BS)···(2)

[0067] here,

[0068] GS represents the original austenite grain size (μm) in the center of the guide rail's foot.

[0069] BS is the size (μm) of the pearlite block in the center of the guide rail's foot.

[0070] In addition to the essential components mentioned above, the guide rail used in this invention may also contain at least one of the following components.

[0071] V: less than 0.30% by mass, Cu: less than 1.0% by mass, Ni: less than 1.0% by mass, Nb: less than 0.05% by mass, Mo: less than 2.0% by mass, Al: less than 0.07% by mass, W: less than 1.0% by mass, Co: less than 1.0% by mass, B: less than 0.005% by mass, Ti: less than 0.05% by mass, Sb: less than 0.05% by mass, Mg: less than 0.01% by mass, Ca: less than 0.02% by mass, and Sn: less than 0.05% by mass.

[0072] The following is an explanation of any of the above elements.

[0073] V: Below 0.30%

[0074] Vitamin V (V) is an element that forms carbonitrides in steel and disperses into the matrix, thereby improving the wear resistance of steel. When the V content exceeds 0.30%, workability deteriorates, and alloy costs, i.e., the manufacturing cost of guide rails, also increase. From these perspectives, when the composition contains V, the V content is preferably capped at 0.30%. From the viewpoint of exhibiting an effect of improving wear resistance, the V content is preferably 0.001% or more. A more preferred range for V content is 0.001% to 0.15%.

[0075] Cu: below 1.0%

[0076] Like Cr, Cu is an element that can further increase the strength of steel through solid solution strengthening. When the Cu content exceeds 1.0%, Cu cracks are prone to occur; therefore, when Cu is present in the composition, the Cu content is preferably 1.0% or less. From the viewpoint of increasing strength, the Cu content is preferably 0.001% or more. A more preferred range for Cu content is 0.001% to 0.5%.

[0077] Ni: below 1.0%

[0078] Ni is an element that can achieve high strength in steel without deteriorating its ductility. Furthermore, it can suppress Cu cracking by being added in combination with Cu; therefore, when the composition contains Cu, Ni is also preferred. However, when the Ni content exceeds 1.0%, the hardenability of the steel further increases, and the formation of martensite and bainite increases, tending to reduce the wear resistance and fatigue damage resistance of the guide rail head. From these perspectives, when the composition contains Ni, the Ni content is preferably 1.0% or less. From the viewpoint of achieving high strength, the Ni content is preferably 0.001% or more. A more preferred range for Ni content is 0.001% to 0.5%.

[0079] Nb: below 0.05%

[0080] Nitrogen (Nb) precipitates as carbides after hot rolling and bonding with carbon (C) in steel during the forming of guide rails. It effectively contributes to the refinement of pearlite pellet size, resulting in significantly improved wear resistance, fatigue resistance, and ductility, thus significantly extending the lifespan of the guide rails. However, even when the Nb content exceeds 0.05%, the improvement in wear resistance and fatigue resistance becomes saturated, and the effect does not match the increased content. From these perspectives, when the composition contains Nb, the upper limit of the Nb content is preferably 0.05%. From the viewpoint of obtaining a sufficiently long lifespan for the guide rails, the Nb content is preferably 0.001% or more. A more preferred range for the Nb content is 0.001% to 0.03%.

[0081] Mo: 2.0% or less

[0082] Mo is an element that can further increase the strength of steel through solid solution strengthening. In addition, Mo has the effect of shifting the eutectoid point towards the high-carbon side, thus also inhibiting the formation of proeutectoid cementite. However, when the Mo content exceeds 2.0%, the amount of bainite formed in the steel increases, reducing the wear resistance of the guide rail head. From these perspectives, when the composition contains Mo, the Mo content is preferably 2.0% or less. From the viewpoint of increasing strength, the Mo content is preferably 0.001% or more. A more preferred range for Mo content is 0.001% to 1.0%.

[0083] Al: below 0.07%

[0084] Al is an element that can be added as a deoxidizer. When the Al content exceeds 0.07%, due to Al's high affinity for oxygen, a large number of oxide inclusions are formed in the steel, resulting in a decrease in the steel's ductility. Therefore, when the composition contains Al, the Al content is preferably 0.07% or less. There is no particular limitation on the lower limit of the Al content, but for deoxidation purposes, it is preferably 0.001% or more. A more preferred range for the Al content is 0.001% to 0.03%.

[0085] W: Below 1.0%

[0086] W precipitates as carbides during and after hot rolling to form the guide rail shape, and its precipitation strengthening enhances the strength and ductility of the guide rail. When the W content exceeds 1.0%, martensite is formed in the steel, resulting in reduced ductility. From these points of view, when the composition contains W, the W content is preferably 1.0% or less. There is no particular limitation on the lower limit of the W content, but to exhibit the aforementioned effects of improving strength and ductility, it is preferably 0.001% or more. A more preferred range for the W content is 0.001% to 0.5%.

[0087] Co: less than 1.0%

[0088] Co is an element that can further increase the strength of steel by raising the equilibrium phase transformation temperature of pearlite and refining the lamellar spacing. In addition, Co also inhibits the precipitation of proeutectoid cementite. When the Co content exceeds 1.0%, martensite is formed in the steel, resulting in reduced ductility. From these perspectives, when the composition contains Co, the Co content is preferably 1.0% or less. There is no particular limitation on the lower limit of the Co content, but for higher strength, it is preferably 0.001% or more. A more preferred range for the Co content is 0.001% to 0.5%.

[0089] B: Below 0.005%

[0090] Boron (B) precipitates in steel as nitrides during and after hot rolling to form a guide rail shape, and its precipitation strengthening enhances the strength and ductility of the steel. When the B content exceeds 0.005%, martensite is formed, resulting in a decrease in the ductility of the steel. From these perspectives, when the composition contains B, the B content is preferably 0.005% or less. While there is no particular limitation on the lower limit of the B content, it is preferably 0.001% or more to exhibit the aforementioned effects of improving strength and ductility. A more preferred range for the B content is 0.001% to 0.003%.

[0091] Ti: below 0.05%

[0092] Ti is an element that precipitates in steel as carbides, nitrides, or carbonitrides during and after hot rolling to form a guide rail shape, thereby improving the strength and ductility of the steel through precipitation strengthening. When the Ti content exceeds 0.05%, coarse carbides, nitrides, or carbonitrides are formed, resulting in a decrease in the ductility of the steel. From these viewpoints, when the composition contains Ti, the Ti content is preferably 0.05% or less. There is no particular limitation on the lower limit of the Ti content, but in order to exhibit the aforementioned effects of improving strength and ductility, it is preferably 0.001% or more. A more preferred range for the Ti content is 0.001% to 0.03%.

[0093] Sb: below 0.05%

[0094] Sb is an element that significantly prevents decarburization of guide rail billets during reheating in a furnace before hot rolling. When the Sb content exceeds 0.05%, it adversely affects the ductility and toughness of the steel; therefore, when the composition contains Sb, the Sb content is preferably 0.05% or less. While there is no particular limitation on the lower limit of the Sb content, it is preferably 0.001% or more to exhibit the effect of reducing the decarburized layer. A more preferred range for the Sb content is 0.001% to 0.03%.

[0095] Mg: less than 0.01%

[0096] Mg is an element used to combine with oxygen to precipitate MgO, thereby achieving further high strength. When the Mg content exceeds 0.01%, the increase in MgO adversely affects the ductility and toughness of the steel. Therefore, when the composition contains Mg, the Mg content is preferably 0.01% or less. There is no particular limitation on the lower limit of the Mg content, but to exhibit the aforementioned strength-enhancing effect, it is preferably 0.001% or more. A more preferred range for the Mg content is 0.001% to 0.005%.

[0097] Ca: below 0.02%

[0098] Ca is an element used to combine with oxygen to precipitate CaO, thereby achieving further high strength. When the Ca content exceeds 0.02%, the increase in CaO adversely affects the ductility and toughness of the steel. Therefore, when the composition contains Ca, the Ca content is preferably 0.02% or less. There is no particular limitation on the lower limit of the Ca content, but to exhibit the aforementioned strength-enhancing effect, it is preferably 0.001% or more. A more preferred range for the Ca content is 0.001% to 0.01%.

[0099] Sn: less than 0.05%

[0100] Sn is an element that significantly prevents decarburization of the steel during the reheating of guide rail billets in a reheating furnace before hot rolling. When the Sn content exceeds 0.05%, it adversely affects the ductility and toughness of the steel; therefore, when the composition contains Sn, the Sn content is preferably 0.05% or less. There is no particular limitation on the lower limit of the Sn content, but to exhibit the effect of reducing the decarburized layer, it is preferably 0.001% or more. A more preferred range for the Sn content is 0.001% to 0.01%.

[0101] In the steel composition of the guide rail of the present invention, the remaining portion of the aforementioned essential components and optional components consists of Fe and unavoidable impurities. Examples of unavoidable impurities include N and O, with N permitted up to 0.008% and O up to 0.004%. It should be noted that, depending on the raw materials, materials, manufacturing equipment, etc., impurities other than N and O may sometimes unavoidably mix into the steel. Examples of raw materials include iron ore, reduced iron, and scrap. The mixing of the aforementioned impurities is permitted as long as it does not impede the purpose of the present invention. Examples of impurities other than N and O include Pb, Zr, Bi, Zn, Se, As, Te, Tl, Cd, Hf, Ag, Hg, Ga, Ge, REM, etc.

[0102] <Microstructure of guide rails>

[0103] The guide rail of this invention is a pearlitic guide rail. From the viewpoint of fatigue crack propagation resistance, the microstructure at a depth of 1 mm from the center of the guide rail's surface has a pearlite content of 95.0% or more in terms of area. The area ratio of pearlite can be 100%. The remaining microstructure at a depth of 1 mm from the center of the guide rail's surface, excluding pearlite, is permissible if its area ratio is 5.0% or less, or can be 0%. Specifically, proeutectoid cementite, if its area ratio is 3.0% or less, will not significantly affect the fatigue crack propagation resistance and is therefore permissible. Examples of the remaining microstructure besides proeutectoid cementite include ferrite, bainite, and martensite.

[0104] The microstructure and area ratio of the central part of the sole can be determined by the measurement method described in the embodiments below.

[0105] (Original austenite grain size GS in the center of the sole)

[0106] The original austenite grain size GS in the central part of the foot of the guide rail of the present invention is preferably 30 to 140 μm, and more preferably 30 to 80 μm from the viewpoint of preventing excessive reduction in ductility and toughness.

[0107] The original austenite grain size GS in the center of the sole can be determined by the measurement method described in the embodiments below.

[0108] (Pearlite block size BS)

[0109] The pearlite block size BS in the center of the foot of the guide rail of the present invention is preferably 15 to 45 μm. By reducing the pearlite block size, which corresponds to the tissue unit of brittle fracture, the length of brittle crack propagation can be further shortened, and therefore it is more preferably 15 to 30 μm.

[0110] The pearlite block size BS in the center of the sole can be determined by the measurement method described in the embodiments below.

[0111] <The shape of the guide rail>

[0112] The shape of the guide rail of the present invention is not particularly limited, and can be the shape of the guide rail described in JIS E 1101:2001, BS EN 13674-1:2011, American Railway Engineering and Maintenance-of-Way Association (AREMA), etc.

[0113] <Methods for Manufacturing Guide Rails>

[0114] The manufacturing method of the guide rail of the present invention will be described. The guide rail of the present invention can be manufactured by sequentially performing the following processes (1) to (3) on a steel billet having the above-described composition.

[0115] (1) Hot rolling

[0116] (2) Primary cooling

[0117] (3) Secondary cooling

[0118] The steel billet used as the guide rail blank has the composition of the guide rail described above and can be manufactured by any method. Generally, casting, especially continuous casting, is preferred for manufacturing the steel billet.

[0119] (1) Hot rolling

[0120] Heating temperature: below 1350℃

[0121] Before hot rolling, the steel billet is heated to a temperature below 1350°C. If the heating temperature exceeds 1350°C, the billet partially melts due to excessive heating, posing a risk of defects inside the guide rails. There is no particular lower limit to the heating temperature, but to reduce deformation resistance during rolling, a temperature above 1150°C is preferred.

[0122] Rolling end temperature: above 850℃

[0123] The heated steel billet is hot-rolled to form the shape of the guide rail. Here, finish rolling refers to the final rolling pass of a single final rolling mill.

[0124] The rolling end temperature in hot rolling is 850°C or higher. When the rolling end temperature is below 850°C, rolling is performed in the low-temperature region of austenite, which not only introduces processing strain into the austenite grains but also significantly increases the elongation of the austenite grains. Due to the increase in the austenite grain boundary area, the number of nucleation sites for proeutectoid cementite increases, resulting in a decrease in fatigue crack propagation resistance. Therefore, the rolling end temperature is 850°C or higher, preferably 900°C or higher. There is no particular upper limit to the rolling end temperature, but if the original austenite grain size becomes extremely coarse, the ductility and toughness decrease, so it is preferable to be below 1050°C. Here, the rolling end temperature is the surface temperature of the center of the guide rail foot on the mill entrance side of the final rolling pass, which can be measured using a radiation thermometer.

[0125] The reduction rate (thickness reduction rate) in the thickness direction of the guide foot in the finishing mill, which becomes the final rolling pass, is preferably a value greater than the section reduction rate of that pass. For example, a section reduction rate of 11% can be cited, while the thickness reduction rate at the center of the guide foot is 15% or more. This allows for the introduction of large strain at the center of the guide foot, further promoting the refinement of the microstructure after the pearlite phase transformation, and effectively improving the fatigue strength of the foot. It should be noted that by designing a pass with a wider foot width in this finishing mill, rolling with a relatively large thickness reduction rate relative to the section reduction rate can be stably performed.

[0126] Here, the reduction rate of the cross-section of the guide rail foot is... Figure 3 Based on the area of ​​the oblique line portion represented by S in the cross-sectional diagram of the guide rail, the cross-sectional area of ​​the bottom before finishing rolling is set as S0, and the cross-sectional area of ​​the bottom after finishing rolling is set as S1. The value is calculated by the cross-sectional reduction rate (%) = {(S0-S1) / S0}×100.

[0127] The thickness reduction rate of the guide rail foot is a value calculated based on the height represented by H in this figure, with the thickness of the bottom before finishing rolling set as H0 and the cross-sectional area of ​​the bottom after finishing rolling set as H1, by the thickness reduction rate (%) = {(H0-H1) / H0}×100.

[0128] There are no special restrictions on other hot-rolled conditions.

[0129] (2) Primary cooling

[0130] Average cooling rate from 850℃ to 750℃ (CR1 [℃ / sec]): HC / 300 ≤ CR1 ≤ 3.0

[0131] Next, accelerated cooling is performed. At this stage, cooling from 850°C to 750°C is considered a primary cooling step. The temperature range of 850°C to 750°C corresponds to the formation temperature range of proeutectoid cementite. If the average cooling rate CR1 in this temperature range is less than HC / 300 [°C / sec], the amount of proeutectoid cementite increases. Therefore, cracks are easily generated at the interface of the proeutectoid cementite structure, posing a risk of reduced fatigue resistance. Therefore, the average cooling rate CR1 of the primary cooling step is HC / 300 [°C / sec] or higher, preferably HC / 200 [°C / sec] or higher.

[0132] On the other hand, when the average cooling rate CR1 of a single cooling cycle exceeds 3.0 °C / second, there is a risk of martensitic formation, which reduces ductility and fatigue resistance. Therefore, the average cooling rate CR1 of a single cooling cycle is 3.0 °C / second or less, preferably 2.0 °C / second or less.

[0133] (3) Secondary cooling

[0134] Average cooling rate (CR2 [°C / sec]) from 750°C to cooling stop temperature T: HC / 120 ≤ CR2 ≤ 6.0

[0135] A secondary cooling process is performed after the initial cooling. The secondary cooling is from 750°C to the cooling stop temperature T, which is a temperature range of 400–650°C. When the average cooling rate from the secondary cooling start temperature (750°C) to the cooling stop temperature T within the range of 400–650°C is less than HC / 120 [°C / sec], in addition to the pearlite bulk size becoming larger, the lamellar spacing also becomes coarser. That is, the pearlite bulk size, which corresponds to brittle fracture microstructure, becomes larger, resulting in longer brittle crack propagation, and consequently, a risk of reduced fatigue resistance of the guide rail. Furthermore, the increased cooling time in the low-temperature region risks reduced productivity and increased manufacturing costs for the guide rail. Therefore, the average cooling rate CR2 of the secondary cooling is HC / 120 [°C / sec] or higher, preferably HC / 100 [°C / sec] or higher.

[0136] On the other hand, when the average cooling rate CR2 of the secondary cooling exceeds 6.0 °C / s, there is a risk of martensitic structure formation, resulting in reduced ductility and fatigue resistance. Therefore, the average cooling rate CR2 of the secondary cooling is below 6.0 °C / s, preferably below 4.0 °C / s.

[0137] The HC value related to the average cooling rate CR1 and the average cooling rate CR2 can be obtained by the above formula (1), preferably 75 to 200, more preferably 90 to 150.

[0138] In both primary and secondary cooling, the average cooling rate is determined by the surface temperature of the center of the guide rail feet, which can be measured using a radiation thermometer. The cooling stop temperature T during secondary cooling is obtained by measuring the surface temperature of the center of the guide rail feet after accelerated cooling has stopped (before reheating) using a radiation thermometer.

[0139] There are no particular limitations on the method of accelerating cooling; for example, it can be achieved by using cooling equipment with online heat treatment. There are no particular limitations on the cooling medium; one or more selected from air, water spray, mist, etc., can be used, but air is preferred. For example, by installing multiple air jet devices on the sole of the foot and adjusting the jetting time and pressure of the air from each device, the average cooling rate of the guide rail sole can be controlled.

[0140] In the manufacturing method of the present invention, it is important that after hot rolling, the guide rail foot is cooled at a predetermined average cooling rate to meet the surface temperature of the central portion of the guide rail foot. If this condition is met, the cooling method for other parts of the guide rail (e.g., the guide rail head) is not particularly limited. For example, the guide rail foot can be accelerated and the other parts of the guide rail (e.g., the guide rail head) can be cooled naturally, or the same accelerated cooling method can be applied as to the foot.

[0141] (4) Other

[0142] The cooled guide rail material can be subjected to known treatments, such as cold roller straightening.

[0143] Example

[0144] The present invention will be described in more detail below with reference to embodiments, but the present invention is not limited to the embodiments and may be appropriately modified within the scope suitable for the spirit of the present invention, and all such modifications are included in the technical scope of the present invention.

[0145] For steel billets with the composition shown in Table 1, the billets were heated and hot-rolled under the conditions shown in Table 2, followed by accelerated cooling to produce 60kg guide rail material according to JIS E1101. After cooling was stopped, the billets were allowed to cool naturally. The rolling end temperature in Table 2 is the value obtained by measuring the surface temperature of the center of the guide rail foot on the final mill inlet side using a radiation thermometer. The cooling stop temperature T in Table 2 is the value obtained by measuring the surface temperature of the center of the guide rail foot at the end of the secondary cooling using a radiation thermometer. The average cooling rate (°C / sec) is calculated by converting the temperature change from the start of cooling to the end of cooling into an average unit time (seconds) for both primary cooling (from 850°C to 750°C) and secondary cooling (from 750°C to the cooling stop temperature T).

[0146] The finishing rolling in hot rolling is carried out under conditions where the thickness reduction rate at the center of the foot is 15%. Under these conditions, the thickness reduction rate is relatively large compared to the 11% reduction rate in cross-section during finishing rolling.

[0147] Accelerated cooling is achieved by using air jets from an air jet device.

[0148]

[0149]

[0150] The obtained guide rails are pearlite system guide rails. The original austenite grain size (GS), pearlite block size (BS), and fatigue crack propagation resistance were evaluated for each guide rail. Table 2 shows the experimental results. The following provides a detailed explanation of each evaluation item.

[0151] <Microstructure of guide rails>

[0152] The method for measuring the microstructure and area ratio of the central part of the sole is as follows.

[0153] by Figure 2 As shown, a test slide for tissue observation was taken from a depth of 1 mm from the center of the guide rail foot surface, embedded in resin, and mirror-polished. The surface perpendicular to the long side of the rolling process at a depth of 1 mm was observed using an optical microscope at 200x magnification in 10 fields of view (each field of view being 300 μm × 400 μm). The area fraction of the constituent phase was determined for each field of view, and its average value was taken as the area fraction of the microstructure.

[0154] <Original Austenite Grain Size GS>

[0155] The method for determining the original austenite grain size (GS) in the central part of the sole is as follows.

[0156] by Figure 2 As shown, a test piece for microstructure observation was taken from a depth of 1 mm from the center of the guide rail foot surface. It was embedded in resin, mirror-polished, and then etched with nitric acid alcohol to reveal the proeutectoid cementite precipitated at the proto-austenite grain boundaries. A scanning electron microscope was used at 200x magnification to observe the surface perpendicular to the long rolling edge at a depth of 1 mm from the surface. The grain size of the region surrounded by proeutectoid cementite was determined using image analysis software. Measurements were taken from over 400 regions, and the average value was taken as the proto-austenite grain size (GS).

[0157] <Pearlite block size BS>

[0158] The method for measuring the pearlite block size BS in the center of the sole is as follows.

[0159] by Figure 2The specimens used for tissue observation were taken from a depth of 1 mm from the center of the guide rail foot surface, as shown. After being embedded in resin and mirror-polished, orientation analysis was performed using EBSD (Electron Backscatter Diffraction Pattern). Grain boundaries with an orientation difference of 15° or more between adjacent crystals were defined as pearlite bulk boundaries. The grain size was measured using circular equivalents and averaged, and the resulting value was taken as the pearlite bulk size BS. The measurement area was 300 μm square, with a measurement step size of 0.3 μm. Measurement points with a confidence index of 0.1 or less, indicating the reliability of the measurement orientation, were excluded from the measurement. Furthermore, grains at the edges of the measurement area were also removed from the measurement.

[0160] <Fatigue Crack Propagation Characteristics>

[0161] by Figure 4 As shown, a fatigue crack propagation test was conducted using a fatigue crack propagation test piece taken from a depth of 1 mm from the center of the guide rail foot surface.

[0162] Figure 5 This is a schematic diagram representing an example of a test piece. Figure 5 (a) represents the front view. Figure 5 (b) represents the side view. Figure 5 (c) shows an enlarged front view of the cut area. The test piece is... Figure 5 A plate with a width W = 20 mm, a height H = 100 mm, and a thickness B = 5 mm has a notch formed at one end of the width portion H / 2 of the height H. The notch is located on the center side of the guide rail's foot. The notch has a length L = 2 mm, a width C = 0.2 mm, and an end curvature R = 0.1 mm. The stress ratio (R ratio = minimum stress / maximum stress) is 0.2, and the measured stress intensity factor range is ΔK = 20 MPa·m. 1 / 2 The fatigue crack propagation velocity da / dN (m / cycle) is used to evaluate fatigue crack propagation resistance. If the value of da / dN is 7.0 × 10⁻⁶, the fatigue crack propagation resistance is evaluated. －8 The following evaluation indicates that it has fatigue crack propagation suppression performance.

[0163] As shown in Table 2, the fatigue crack propagation velocity of the guide rail material of the invention example (tests No. 1 to 40 in Table 2) all meet the requirement of 7.0 × 10⁻⁶. －8 The following exhibits fatigue crack propagation suppression performance. On the other hand, comparative examples (tests No. 41-48, 50-54 in Table 2) whose guide rail material composition does not meet the conditions of this invention, or whose pearlite area ratio does not meet the conditions of this invention, or whose conditions of formula (2) above are not met, have fatigue crack propagation da / dN (m / cycle) exceeding 7.0 × 10⁻⁶.－8 In Experiment No. 49, due to excessively high heating temperature, part of the steel billet melted during heating. Therefore, due to concerns about breakage during rolling, it could not be supplied for rolling, and thus, property evaluation could not be performed.

[0164] Industrial availability

[0165] According to the present invention, a guide rail with excellent fatigue crack propagation resistance at the guide rail foot portion and a method thereof are provided. The guide rail of the present invention contributes to extending the service life of guide rails for heavy axle load railways and preventing railway accidents, thus being industrially beneficial. Furthermore, the manufacturing method of the guide rail of the present invention, by optimizing the heat treatment conditions after hot rolling, can stably improve the fatigue crack propagation resistance at the guide rail foot portion, which is also industrially beneficial.

[0166] Symbol Explanation

[0167] 1 guide rail

[0168] 11. Guide rail head (head)

[0169] 12-rail abdominal section (abdomen)

[0170] 13. Guide rail feet (feet)

[0171] 14-rail footrest (foot area)

Claims

1. A guide rail having the following composition: containing C: 0.70-1.20% by mass, Si: 0.10-1.20% by mass, Mn: 0.10-1.50% by mass, P: less than 0.035% by mass, S: less than 0.020% by mass and Cr: 0.05-2.00% by mass, with the remainder consisting of Fe and unavoidable impurities; The area fraction of pearlite tissue at a depth of 1 mm from the center of the guide rail foot is over 95.0%. The value of HC represented by the following equation (1) satisfies the following equation (2). HC={(12×[%C])+([%Si] / 10)+([%Mn] / 20)+([%Cr] / 18)} 2 ···(1) HC≤(120×GS) / (1.1×BS)···(2) here, [%C], [%Si], [%Mn], and [%Cr] represent the contents of C, Si, Mn, and Cr, respectively, in % by mass. GS represents the original austenite grain size in the center of the guide rail's foot, in μm. BS is the size of the pearlite block in the center of the guide rail's foot, in μm.

2. The guide rail according to claim 1, wherein, The composition further comprises at least one of the following: V: less than 0.30% by mass, Cu: less than 1.0% by mass, Ni: less than 1.0% by mass, Nb: less than 0.05% by mass, Mo: less than 2.0% by mass, Al: less than 0.07% by mass, W: less than 1.0% by mass, Co: less than 1.0% by mass, B: less than 0.005% by mass, Ti: less than 0.05% by mass, Sb: less than 0.05% by mass, Mg: less than 0.01% by mass, Ca: less than 0.02% by mass, and Sn: less than 0.05% by mass.

3. A method for manufacturing a guide rail, which is the method for manufacturing a guide rail according to claim 1 or 2. A steel billet having the composition described in claim 1 or 2 is heated to a temperature below 1350°C and hot-rolled at a rolling end temperature of 850°C or higher at the center of the guide rail feet. It is then cooled at an average cooling rate CR1 until the temperature at the center of the guide rail feet decreases from 850°C to 750°C, and then cooled at an average cooling rate CR2 until the temperature at the center of the guide rail feet decreases from 750°C to the cooling stop temperature T. here, The cooling stop temperature T is a temperature in the range of 400 to 650°C. The average cooling rates CR1 and CR2 satisfy the following equations (3) and (4), respectively, where, The units for CR1 and CR2 are ℃ / second. HC / 300≤CR1≤3.0···(3) HC / 120≤CR2≤6.0···(4).

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