JP7522985B1 - Rail and manufacturing method thereof - Google Patents

Abstract

Provided is a rail with excellent fatigue crack propagation resistance in the sole of the rail, which has a chemical composition containing 0.70-1.20 mass% C, 0.10-1.20 mass% Si, 0.10-1.50 mass% Mn, 0.035 mass% or less P, 0.020 mass% or less S, and 0.05-2.00 mass% Cr, with the balance being Fe and unavoidable impurities, the area ratio of pearlite structures at a depth of 1 mm from the surface in the center of the sole of the rail is 95% or more, and the value of HC, expressed by the following formula (1), satisfies the following formula (2).
Record
HC={(12×[%C])+([%Si]/10)+([%Mn]/20)+([%Cr]/18)} ^2・・・ (1)
HC≦(120×GS)/(1.1×BS)・・・(2)

Description

The present invention relates to a rail and a method for manufacturing the same.

In high-axle-load railways, which are primarily used for transporting ore, the load on the axles of freight cars is much higher than that of passenger cars, and the environment in which the rails are used is also harsh. For this reason, steel with a pearlite structure has traditionally been used for the rails, with an emphasis on wear resistance.

In recent years, efforts have been made to further increase the load capacity of freight cars in order to improve the efficiency of rail transport. In addition, the number of wheels passing over the rails is also increasing due to increased transport capacity.

When wheels pass over the rails, they are subjected to repeated tensile and compressive stresses. As the load weight and the number of wheels passing over the rails increases, breakage occurs at the sole of the rail, and the frequency of rail replacement tends to increase year by year. For this reason, there is a growing demand for rail steel with improved fatigue crack propagation resistance at the sole of the rail.

Under the above-mentioned circumstances, various studies have been conducted with the aim of further improving the breakage resistance of the rail bottom.For example, Patent Document 1 proposes a rail steel with a pearlite structure containing 0.65-1.40% C, in which at least a portion of the range from the bottom surface to a depth of 10 mm contains 200 or more pearlite blocks with grain sizes of 1-15 μm per 0.2 ^mm2 of the test area.

Patent Document 2 proposes a rail containing, by mass, 0.65-1.20% C, 0.05-2.00% Si, 0.05-2.00% Mn, with the remainder being Fe and unavoidable impurities, with 97% or more of the head and bottom surfaces being pearlite structures, with the surface hardness of the pearlite structure being in the range of Hv320-500, the maximum surface roughness being 180 μm or less, and the ratio of the surface hardness to the maximum surface roughness being 3.5 or more.

Patent Document 3 discloses a rail in which the rail head is accelerated cooled from the austenite region after rolling, while the rail bottom is accelerated cooled between 800 and 450°C at a cooling rate of 1 to 5°C/sec, resulting in an average pearlite hardness at the rail bottom of HB320 or more.

JP 2006-57127 A International Publication No. 2011/021582 Japanese Patent Application Laid-Open No. 1-139724

However, the above conventional technologies have the following problems that need to be solved. When the C content is set to 0.65-1.40% as in the technologies described in Patent Documents 1 and 2, depending on the heat treatment conditions, a hard and brittle pro-eutectoid cementite structure is formed at the prior austenite grain boundaries, so no improvement in fatigue damage resistance can be expected. Furthermore, with the technology described in Patent Document 3, depending on the combination of ingredients and manufacturing conditions, a pro-eutectoid cementite structure may also be formed, resulting in an increase in the fatigue crack propagation rate, so it is difficult to say that material control is sufficient.

The present invention has been made to advantageously solve the above-mentioned problems, and aims to provide a rail having excellent fatigue crack propagation resistance at the bottom of the rail, together with a manufacturing method thereof.

In order to solve the above problems, the inventors have produced rails with different contents of C, Si, Mn and Cr, and have thoroughly investigated the structure and fatigue crack propagation resistance of the rail base. As a result, they have found a component parameter (HC parameter expressed by formula (1) described later) defined by the C content, Si content, Mn content and Cr content, and have found that the value of this parameter is related to the amount of pro-eutectoid cementite. Furthermore, they have found that by controlling the value of the HC parameter to a value equal to or smaller than the value of the parameter consisting of the prior austenite grain size and pearlite block size of the rail base, excellent fatigue crack propagation resistance of the rail base can be obtained even if a large amount of pro-eutectoid cementite is present. More details are as follows.

The present inventors have found that the reason why the presence of pro-eutectoid cementite increases the fatigue crack propagation rate is that, as shown in the schematic diagram of FIG. 1, in the pro-eutectoid cementite part present in the plastic zone at the tip of the fatigue crack, the {100} plane of ferrite in pearlite adjacent to the pro-eutectoid cementite part undergoes brittle fracture first. Furthermore, the present inventors have found that by controlling the ratio of the prior austenite grain size, which is the formation site of the structure, to the pearlite block size, which corresponds to the structural unit of the brittle fracture, according to the amount of pro-eutectoid cementite produced, the frequency of encounter between the plastic zone formed at the tip of the fatigue crack and the {100} plane of ferrite in pearlite is reduced, and brittle crack propagation can be suppressed. In other words, even when a large amount of pro-eutectoid cementite is present, it has been found that the aforementioned effect of suppressing the fatigue crack propagation rate can be stably obtained by coarsening the prior austenite grain size or by finely refining the pearlite block size.

The present invention is based on the above findings, and has the following gist and configuration.
[1] C: 0.70 to 1.20% by mass,
Si: 0.10 to 1.20 mass%,
Mn: 0.10 to 1.50 mass%,
P: 0.035% by mass or less,
S: 0.020% by mass or less, and
Cr: 0.05 to 2.00 mass%;
The balance is Fe and unavoidable impurities.
The area ratio of pearlite structure at a depth of 1 mm from the surface of the center of the sole of the rail is 95% or more,
A rail whose HC value expressed by the following formula (1) satisfies the following formula (2).
Record
HC={(12×[%C])+([%Si]/10)+([%Mn]/20)+([%Cr]/18)} ^2・・・ (1)
HC≦(120×GS)/(1.1×BS)・・・(2)
here,
[%C], [%Si], [%Mn] and [%Cr] are the contents (mass%) of C, Si, Mn and Cr, respectively.
GS is the prior austenite grain size (μm) at the center of the rail sole,
BS is the pearlite block size (μm) at the center of the rail sole.
[2] The composition further comprises:
V: 0.30% by mass or less,
Cu: 1.0 mass% or less,
Ni: 1.0 mass% or less,
Nb: 0.05% by mass or less,
Mo: 2.0% by mass or less,
Al: 0.07% by mass or less,
W: 1.0% by mass or less,
Co: 1.0 mass% or less,
B: 0.005% by mass or less,
Ti: 0.05% by mass or less,
Sb: 0.05% by mass or less,
Mg: 0.01% by mass or less,
Ca: 0.02% by mass or less, and
Sn: 0.05% by mass or less,
The rail of [1], which contains at least one selected from the group consisting of:
[3] A method for manufacturing a rail according to [1] or [2],
A steel material having the chemical composition of [1] or [2] is heated at a heating temperature of 1350°C or less, hot rolled under conditions in which the rolling finish temperature at the center of the rail foot sole is 850°C or more, then cooled at an average cooling rate _CR1 (°C/sec) until the temperature at the center of the rail foot sole decreases from 850°C to 750°C, and then cooled at an average cooling rate _CR2 (°C/sec) until the temperature at the center of the rail foot sole 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,
A manufacturing method for a rail, wherein an average cooling rate CR ₁ (°C/sec) and an average cooling rate CR ₂ (°C/sec) satisfy the following formulas (3) and (4), respectively.
Record
HC/300≦CR ₁ ≦3.0...(3)
HC/120≦ _CR2 ≦6.0...(4)

According to the present invention, it is possible to provide a rail with excellent fatigue crack propagation resistance at the sole of the rail, together with a manufacturing method thereof. The rail of the present invention contributes to extending the service life of rails for high axle load railways and preventing railway accidents, and is industrially useful. Furthermore, the rail manufacturing method of the present invention is industrially useful, as it can stably improve the fatigue crack propagation resistance at the sole of the rail by optimizing the heat treatment conditions after hot rolling.

FIG. 2 is a schematic diagram showing the influence of pro-eutectoid cementite, prior austenite grain size, and pearlite block size on fatigue crack propagation rate. FIG. 2 is a cross-sectional view of a rail showing each portion of the rail and positions for taking test specimens for observing prior austenite grain size and pearlite block size. FIG. 4 is a diagram showing the cross-sectional area and thickness of a rail foot portion. FIG. 2 is a diagram showing the positions at which test pieces for a fatigue crack propagation test are taken. 5A, 5B, and 5C are diagrams illustrating the shape of a test piece for a fatigue crack propagation test, in which Fig. 5A shows a front view, Fig. 5B shows a side view, and Fig. 5C shows an enlarged front view of a notch portion.

<Rail parts>
First, the designations of each part of the rail in the present invention will be explained with reference to the rail cross-sectional view of Figure 2. In the rail 1 shown in Figure 2, 11 is the rail head, 12 is the rail web, and 13 is the rail foot, and the underside of the rail foot 13 is called the rail sole 14. The central part of the rail sole is the part located near the center in the width direction of the rail sole, and for example, if the width dimension of the rail sole 14 is W, then the central part of the rail sole is the area within a width range of ±0.075 x W from the width center of the rail sole 14.
In the following, the rail head, rail web, rail foot, rail sole, and rail sole center will also be referred to as the head, web, foot, sole, and sole center, respectively.

<Rail composition>
Next, the chemical composition of the steel for the rail of the present invention will be described. In the following description, "%" refers to "mass %" unless otherwise specified.

C: 0.70-1.20%
C is an essential element for ensuring the strength of the pearlite structure, i.e., fatigue damage resistance. If the C content is less than 0.70%, it is difficult to obtain excellent fatigue crack propagation resistance properties of the sole. Also, if 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. Note that proeutectoid cementite may exist even when the C content is 1.20% or less, but the influence can be avoided by controlling the prior austenite grain size and block size in the center of the sole to satisfy the above formula (2). From these points of view, the C content is 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%.

Si: 0.10-1.20%
In addition to its effect as a deoxidizer, Si is an element that contributes to lowering the fatigue crack propagation rate by increasing the pearlite equilibrium transformation temperature and reducing the lamellar spacing. From this point of view, the Si content must be 0.10% or more, but if it exceeds 1.20%, the weldability will deteriorate due to the high bonding strength of Si with oxygen. Furthermore, since Si has the effect of shifting the eutectoid point to the low C side, excessive addition of Si promotes the formation of pro-eutectoid cementite, leading to an increase in the fatigue crack propagation rate. From these points of view, the Si content is set to the range of 0.10 to 1.20%, preferably 0.15 to 1.10%, and more preferably 0.20 to 1.00%.

Mn: 0.10-1.50%
Mn is an element that contributes to the reduction of the fatigue crack propagation rate by lowering the pearlite transformation temperature and reducing the lamellar spacing. If the Mn content is less than 0.10%, sufficient effect cannot be obtained. On the other hand, if the Mn content exceeds 1.50%, martensite structure is likely to be generated, and hardening and embrittlement occur during heat treatment and welding of the rail, and the material is likely to deteriorate. Furthermore, since Mn has the effect of shifting the eutectoid point to the low C side, excessive addition promotes the formation of pro-eutectoid cementite and leads to an increase in the fatigue crack propagation rate. From these points, the Mn content is in the range of 0.10 to 1.50%, preferably in the range of 0.20 to 1.40%, and more preferably in the range of 0.30 to 1.30%.

P: 0.035% or less
If P is contained in an amount exceeding 0.035%, it deteriorates ductility. Therefore, the P content is set to 0.035% or less, and preferably 0.020% or less. The lower limit of the P content is not particularly limited and may be 0%, but in industry, it is usually more than 0%, and excessively reducing the P content leads to an increase in refining costs. From the viewpoint of economic efficiency, it is preferable that the P content is set to 0.001% or more.

S: 0.020% or less
S is an element that exists in steel mainly in the form of A-type inclusions, but if the S content exceeds 0.020%, the amount of these inclusions increases significantly and coarse inclusions are generated, which leads to a deterioration in the cleanliness of the steel. Therefore, the S content is set to 0.020% or less, preferably 0.015% or less, and more preferably 0.010% or less. The lower limit of the S content is not particularly limited and may be 0%, but it is usually more than 0% industrially, and excessively reducing the S content leads to an increase in refining costs. From the viewpoint of economic efficiency, it is preferable to set the S content to 0.0005% or more.

Cr: 0.05-2.00%
Cr is an element that contributes to lowering the fatigue crack propagation rate by increasing the pearlite equilibrium transformation temperature and reducing the lamellar spacing. If the Cr content is less than 0.05%, fatigue crack propagation cannot be sufficiently suppressed. On the other hand, if the Cr content exceeds 2.00%, the hardenability of the steel increases and martensite is more likely to form. In addition, if the steel is manufactured under conditions in which martensite does not form, proeutectoid cementite forms at the prior austenite grain boundaries, increasing the fatigue crack propagation rate. From these points of view, the Cr content is set to the range of 0.05 to 2.00%, preferably 0.10 to 1.60%, and more preferably 0.15 to 1.40%.

Furthermore, in the composition of the present invention, it is not sufficient for each element to satisfy the above range; it is important to control the value of the component parameter HC, which corresponds to the amount of pro-eutectoid cementite, to a value equal to or smaller than the value of a specified parameter consisting of the prior austenite grain size GS and the pearlite block size BS in the center of the sole of the rail.

The value of the component parameter HC corresponding to the amount of pro-eutectoid cementite is calculated by the following formula (1).
HC={(12×[%C])+([%Si]/10)+([%Mn]/20)+([%Cr]/18)} ² ... (1)
Here, [%C], [%Si], [%Mn] and [%Cr] are the respective contents (mass%) of C, Si, Mn and Cr.
The HC value is preferably 75-200, more preferably 90-150, from the viewpoint of achieving both high strength of pearlite structure and suppression of pro-eutectoid cementite, that is, ensuring fatigue damage resistance.

In the present invention, a predetermined parameter consisting of the prior austenite grain size GS and the pearlite block size BS is expressed as (120×GS)/(1.1×BS). The value of the parameter and the value of HC satisfy the relationship of the following formula (2).
HC≦(120×GS)/(1.1×BS)・・・(2)
here,
GS is the prior austenite grain size (μm) at the center of the rail sole,
BS is the pearlite block size (μm) at the center of the rail sole.

The component composition of the rail used in the present invention may contain, in addition to the above essential components, at least one selected from the following:
V: 0.30 mass% or less, Cu: 1.0 mass% or less, Ni: 1.0 mass% or less, Nb: 0.05 mass% or less, Mo: 2.0 mass% or less, Al: 0.07 mass% or less, W: 1.0 mass% or less, Co: 1.0 mass% or less, B: 0.005 mass% or less, Ti: 0.05 mass% or less, Sb: 0.05 mass% or less, Mg: 0.01 mass% or less, Ca: 0.02% by mass or less, and Sn: 0.05% by mass or less.
The optional elements mentioned above will now be described.

V: 0.30% or less
V is an element that forms carbonitrides in steel and disperses and precipitates in the matrix, improving the wear resistance of the steel. If the V content exceeds 0.30%, workability deteriorates and the alloy cost, i.e., the rail manufacturing cost, increases. From these points of view, when the composition contains V, it is preferable that the V content be set at an upper limit of 0.30%. From the viewpoint of exerting the effect of improving wear resistance, the V content is preferably 0.001% or more. The V content is more preferably in the range of 0.001 to 0.15%.

Cu: 1.0% or less
Like Cr, Cu is an element that can further increase the strength of steel through solid solution strengthening. If the Cu content exceeds 1.0%, Cu cracking is likely to occur, so if the composition contains Cu, the Cu content is preferably 1.0% or less. From the viewpoint of increasing strength, the Cu content is preferably 0.001% or more. The more preferable range of the Cu content is 0.001 to 0.5%.

Ni: 1.0% or less
Ni is an element that can increase the strength of steel without deteriorating ductility. In addition, when added in combination with Cu, it is possible to suppress Cu cracking, so it is desirable to also contain Ni when the composition contains Cu. However, if the Ni content exceeds 1.0%, the hardenability of the steel increases, the amount of martensite and bainite produced increases, and the wear resistance and fatigue damage resistance of the rail head tend to decrease. From these points of view, when the composition contains Ni, the Ni content is preferably 1.0% or less. From the viewpoint of increasing strength, the Ni content is preferably 0.001% or more. A more preferable range of the Ni content is 0.001 to 0.5%.

Nb: 0.05% or less
Nb is an element that combines with C in the steel and precipitates as carbides during and after hot rolling to form a rail, effectively reducing the size of pearlite colonies, thereby greatly improving wear resistance, fatigue damage resistance, and ductility, and greatly contributing to the extension of the rail's life. However, even if the Nb content exceeds 0.05%, the effect of improving wear resistance and fatigue damage resistance saturates, and no effect commensurate with the increase in content can be obtained. From these points of view, when the composition contains Nb, the upper limit of the Nb content is preferably 0.05%. From the viewpoint of obtaining a sufficient effect on the extension of the rail's life, the Nb content is preferably 0.001% or more. A more preferable range of the Nb content is 0.001 to 0.03%.

Mo: 2.0% or less
Mo is an element that can further increase the strength of steel through solid solution strengthening. Mo also has the effect of shifting the eutectoid point to the high C side, and therefore also has the effect of suppressing the formation of pro-eutectoid cementite. However, if the Mo content exceeds 2.0%, the amount of bainite formed in the steel increases, and the wear resistance of the rail head decreases. From these points of view, 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. The more preferable range of the Mo content is 0.001 to 1.0%.

Al: 0.07% or less
Al is an element that can be added as a deoxidizer. If the Al content exceeds 0.07%, a large amount of oxide-based inclusions are generated in the steel due to the high bonding strength of Al with oxygen, resulting in a decrease in the ductility of the steel. Therefore, when the composition contains Al, the Al content is preferably 0.07% or less. There is no particular lower limit for the Al content, but it is preferably 0.001% or more for deoxidization. The more preferable range of the Al content is 0.001 to 0.03%.

W: 1.0% or less
W is an element that precipitates as carbides during and after hot rolling to form the rail shape, improving the strength and ductility of the rail through precipitation strengthening. If the W content exceeds 1.0%, martensite is formed in the steel, resulting in a decrease in ductility. From these points of view, when the composition contains W, the W content is preferably 1.0% or less. There is no particular lower limit for the W content, but in order to achieve the above-mentioned effect 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%.

Cobalt: 1.0% or less
Co is an element that can increase the pearlite equilibrium transformation temperature and refine the lamellar spacing, thereby further increasing the strength of the steel. Co also has the effect of suppressing the precipitation of proeutectoid cementite. If the Co content exceeds 1.0%, martensite is formed in the steel, resulting in a decrease in ductility. From these points of view, when the composition contains Co, the Co content is preferably 1.0% or less. There is no particular limit to the lower limit of the Co content, but it is preferably 0.001% or more in order to increase the strength. A more preferable range of the Co content is 0.001 to 0.5%.

B: 0.005% or less
B is an element that precipitates as nitrides in steel during and after hot rolling to form the rail shape, improving the strength and ductility of the steel through precipitation strengthening. If the B content exceeds 0.005%, martensite is formed, resulting in a decrease in the ductility of the steel. From these points of view, when the composition contains B, the B content is preferably 0.005% or less. There is no particular lower limit for the B content, but in order to achieve the effect of improving the strength and ductility described above, it is preferably 0.001% or more. A more preferred range for the B content is 0.001 to 0.003%.

Ti: 0.05% or less
Ti is an element that precipitates in steel as carbides, nitrides, or carbonitrides during and after hot rolling to form a rail shape, improving the strength and ductility of the steel through precipitation strengthening. If 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 points of view, when the composition contains Ti, the Ti content is preferably 0.05% or less. There is no particular lower limit for the Ti content, but in order to achieve the above-mentioned effect 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%.

Sb: 0.05% or less
Sb is an element that has a remarkable effect of preventing decarburization of steel during reheating of rail steel material in a heating furnace before hot rolling. If the Sb content exceeds 0.05%, it has a negative effect on the ductility and toughness of steel, so if the composition contains Sb, the Sb content is preferably 0.05% or less. There is no particular lower limit for the Sb content, but in order to exert the effect of reducing the decarburized layer, it is preferably 0.001% or more. A more preferable range of the Sb content is 0.001 to 0.03%.

Mg: 0.01% or less
Mg is an element that combines with oxygen to precipitate MgO, thereby further increasing strength. If the Mg content exceeds 0.01%, the increase in MgO adversely affects the ductility and toughness of the steel, so if the composition contains Mg, the Mg content is preferably 0.01% or less. There is no particular lower limit for the Mg content, but in order to achieve the above-mentioned strength-improving effect, it is preferably 0.001% or more. A more preferred range for the Mg content is 0.001 to 0.005%.

Ca: 0.02% or less
Ca is an element that combines with oxygen to precipitate CaO, thereby further increasing strength. If the Ca content exceeds 0.02%, the increase in CaO adversely affects the ductility and toughness of the steel, so if the composition contains Ca, the Ca content is preferably 0.02% or less. There is no particular lower limit for the Ca content, but in order to achieve the above-mentioned strength-improving effect, it is preferably 0.001% or more. A more preferred range for the Ca content is 0.001 to 0.01%.

Sn: 0.05% or less
Sn is an element that has a remarkable effect of preventing decarburization of steel during reheating of rail steel material in a heating furnace before hot rolling. If the Sn content exceeds 0.05%, it has a negative effect on the ductility and toughness of the steel, so if the composition contains Sn, the Sn content is preferably 0.05% or less. There is no particular lower limit for the Sn content, but in order to exert the effect of reducing the decarburized layer, it is preferably 0.001% or more. A more preferable range of the Sn content is 0.001 to 0.01%.

In the composition of the steel for the rail of the present invention, the balance of the above essential and optional components consists of Fe and unavoidable impurities. Here, examples of unavoidable impurities include N and O, with N being permitted up to 0.008% and O being permitted up to 0.004%. Note that, depending on the conditions of raw materials, materials, manufacturing equipment, etc., impurities other than N and O may be inevitably mixed into the steel. Examples of raw materials include iron ore, reduced iron, and scrap. The above impurities are permitted as long as they do not impede the objectives 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, and REM.

<Rail microstructure>
The rail of the present invention is a pearlitic rail. From the viewpoint of fatigue crack propagation resistance, the microstructure at a depth of 1 mm from the surface of the central part of the sole of the rail has pearlite at an area ratio of 95.0% or more. The area ratio of pearlite may be 100%. The remaining structure other than pearlite at a depth of 1 mm from the surface of the central part of the sole is acceptable if its area ratio is 5.0% or less, and may be 0%. Among them, pro-eutectoid cementite is acceptable if its area ratio is 3.0% or less because it does not significantly affect fatigue crack propagation resistance. Examples of the remaining structure other than pro-eutectoid cementite include ferrite, bainite, and martensite.
The microstructure and area ratio of the central part of the sole can be measured by the measuring method described in the examples below.

(GS grain size of former austenite in the center of the sole)
In the rail of the present invention, the prior austenite grain size GS in the central sole portion is preferably 30 to 140 μm, and from the viewpoint of preventing an excessive decrease in ductility and toughness, it is more preferably 30 to 80 μm.
The prior austenite grain size GS in the center of the sole can be measured by the measuring method described in the examples below.

(Perlite block size BS)
In the rail of the present invention, the pearlite block size BS in the center of the sole is preferably 15 to 45 μm, and more preferably 15 to 30 μm, because by reducing the pearlite block size, which corresponds to the structural unit of brittle fracture, it is possible to further shorten the length of brittle crack propagation.
The perlite block size BS in the center of the sole can be measured by the measuring method described in the Examples below.

<Rail shape>
The shape of the rail in the present invention is not particularly limited, and may be any shape described in JIS E 1101:2001, BS EN13674-1:2011, American Railway Engineering and Maintenance-of-Way Association (AREMA), or the like.

<Rail manufacturing method>
The method for manufacturing a rail of the present invention will now be described. The rail of the present invention can be manufactured by sequentially carrying out the following treatments (1) to (3) on a steel material having the above-mentioned chemical composition.
(1) Hot rolling
(2) Primary cooling
(3) Secondary cooling

The steel material used as the rail material has the above-mentioned rail composition and can be manufactured by any method. In general, it is preferable to manufacture the steel material by casting, particularly continuous casting.

(1) Hot rolling heating temperature: 1350°C or less Prior to hot rolling, the steel material is heated to a heating temperature of 1350°C or less. If the heating temperature exceeds 1350°C, the steel material may partially melt due to excessive heating, which may cause defects inside the rail. There is no particular lower limit for the heating temperature, but it is preferably 1150°C or more in order to reduce the deformation resistance during rolling.

Rolling finish temperature: 850℃ or higher. The heated steel material is hot-rolled into the shape of a rail. Here, finish rolling refers to one pass of rolling using the final rolling groove of one final rolling mill.
The rolling finish temperature in hot rolling is 850°C or higher. If the rolling finish temperature is lower than 850°C, the rolling is performed in the low austenite temperature range, and not only is processing strain introduced into the austenite grains, but the elongation of the austenite grains becomes significant. The increase in the austenite grain boundary area increases the number of nucleation sites of pro-eutectoid cementite, and as a result, the fatigue crack propagation resistance decreases. Therefore, the rolling finish temperature is 850°C or higher, and preferably 900°C or higher. There is no particular upper limit to the rolling finish temperature, but if the prior austenite grain size becomes extremely coarse, the ductility and toughness decrease, so it is preferably 1050°C or lower. Here, the rolling finish temperature is the surface temperature of the center of the rail sole at the entry side of the rolling mill in the final rolling pass, and can be measured with a radiation thermometer.

The reduction ratio (thickness reduction ratio) in the thickness direction of the rail base in the finish rolling, which is the final rolling pass, is preferably set to a value larger than the area reduction ratio of the corresponding pass. For example, the area reduction ratio may be set to 11%, and the thickness reduction ratio at the center of the rail base may be set to 15% or more. This allows a large strain to be introduced at the center of the rail base, which further promotes the refinement of the structure after pearlite transformation and effectively increases the fatigue strength of the base. By designing the groove shape so that the width dimension of the base is expanded in this finish rolling, it is possible to stably perform rolling with a relatively large thickness reduction ratio compared to the area reduction ratio.
Here, the area reduction of the rail base is calculated based on the area of the shaded portion marked with S in the cross-sectional view of a rail in Fig. 3, using the cross-sectional area of the base before finish rolling as _S0 and the cross-sectional area of the base after finish rolling as _S1 , as follows: Area reduction (%) = {( _S0 - _S1 )/ _S0 } x 100.
The thickness reduction rate of the rail base is calculated based on the height marked with H in the figure, where the thickness of the base before finish rolling is _H0 and the cross-sectional area of the base after finish rolling is _H1 , as follows: Thickness reduction rate (%) = {( _{H0 -H1} )/ _H0 } x 100.
Other hot rolling conditions are not particularly limited.

(2) Primary cooling
Average cooling rate from 850℃ to 750℃ (CR ₁ [℃/sec]): HC/300≦CR ₁ ≦3.0
Next, accelerated cooling is performed. In this case, cooling from 850°C to 750°C is defined as primary cooling. The temperature range from 850°C to 750°C corresponds to the temperature range in which pro-eutectoid cementite is formed, and if the average cooling rate _CR1 in this temperature range is less than HC/300 [°C/sec], the amount of pro-eutectoid cementite increases. As a result, cracks are likely to occur at the interface of the pro-eutectoid cementite structure, and the fatigue damage resistance of the rail may decrease. Therefore, the average cooling rate _CR1 in primary cooling is set to HC/300 [°C/sec] or more, and preferably HC/200 [°C/sec] or more.

On the other hand, if the average cooling rate _CR1 of the primary cooling exceeds 3.0°C/sec, martensite structure may be generated, and ductility and fatigue damage resistance may decrease. Therefore, the average cooling rate _CR1 of the primary cooling is set to 3.0°C/sec or less, and preferably 2.0°C/sec or less.

(3) Secondary cooling
Average cooling rate from 750°C to the cooling stop temperature T (CR ₂ [°C/sec]): HC/120≦CR ₂ ≦6.0
Secondary cooling is performed following the primary cooling. The secondary cooling is performed from 750 ° C to a cooling stop temperature T, which is a temperature in the range of 400 to 650 ° C. If the average cooling rate from the secondary cooling start temperature of 750 ° C to the cooling stop temperature T of the secondary cooling in the range of 400 to 650 ° C is less than HC / 120 [ ° C / sec ], the pearlite block size becomes coarse and the lamellar spacing also becomes coarse. That is, the pearlite block size corresponding to the structure unit of brittle fracture becomes large, and as a result, brittle crack propagation occurs longer, and the fatigue damage resistance of the rail may decrease. In addition, the cooling time in the low temperature range increases, so productivity decreases and the manufacturing cost of the rail may increase. Therefore, the average cooling rate CR ₂ of the secondary cooling is set to HC / 120 [ ° C / sec ] or more, preferably HC / 100 [ ° C / sec ] or more.

On the other hand, if the average cooling rate _CR2 of the secondary cooling exceeds 6.0°C/sec, martensite structure may be generated, and ductility and fatigue damage resistance may decrease. Therefore, the average cooling rate _CR2 of the secondary cooling is set to 6.0°C/sec or less, and preferably 4.0°C/sec or less.

The value of HC in relation to the average cooling rates CR ₁ and CR ₂ can be calculated by the above formula (1), and is preferably 75-200, more preferably 90-150.

In both primary and secondary cooling, the temperature used to determine the average cooling rate is the surface temperature at the center of the sole of the rail, which can be measured with a radiation thermometer. The cooling stop temperature T during secondary cooling is the surface temperature at the center of the sole of the rail after accelerated cooling has stopped (before reheating), measured with a radiation thermometer.

The accelerated cooling method is not particularly limited, and can be performed, for example, by cooling using online heat treatment equipment. The cooling medium is not particularly limited, and one or more selected from air, spray water, mist, etc. can be used, but it is preferable to use air. For example, the average cooling speed of the rail sole can be controlled by providing multiple air injection devices at the sole and adjusting the injection time and pressure of the air ejected from each device.

In the manufacturing method of the present invention, it is important to cool the surface temperature of the center of the rail foot sole after hot rolling so as to satisfy a predetermined average cooling rate, and as long as this condition is satisfied, the cooling method for other parts of the rail (e.g., the rail head, etc.) is not particularly limited. For example, accelerated cooling may be performed on the rail foot, and other parts of the rail (e.g., the rail head, etc.) may be allowed to cool, or accelerated cooling may be performed in the same way as for the base.

(4) Others After cooling, the rail material may be subjected to known treatments, for example, cold roller straightening.

The present invention will be explained in more detail below with reference to examples. However, the present invention is not limited to the examples, and appropriate modifications can be made within the scope of the spirit of the present invention, all of which are included in the technical scope of the present invention.

For steel materials having the composition shown in Table 1, the steel materials were heated and hot-rolled under the conditions shown in Table 2, and accelerated cooling was performed after hot rolling to produce 60 kg rail materials conforming to JIS E1101. After cooling was stopped, the steel materials were allowed to cool. The rolling finish temperature in Table 2 is the surface temperature of the center of the rail sole at the final rolling mill entry side measured with a radiation thermometer. The cooling stop temperature T in Table 2 is the surface temperature of the center of the rail sole at the time of stopping cooling in the secondary cooling measured with a radiation thermometer. The average cooling rate was calculated by converting the temperature change from the start of cooling to the stop of cooling for the primary cooling (from 850°C to 750°C) and the secondary cooling (from 750°C to the cooling stop temperature T) per unit time (seconds) to obtain the cooling rate (°C/sec).
The finish rolling in the hot rolling process was performed under the condition of a thickness reduction rate of 15% at the center of the base part. Under these conditions, the thickness reduction rate is larger than the area reduction rate of 11% in the finish rolling process.
The accelerated cooling was performed by air injection using an air injection device.

The obtained rails are pearlitic rails. For each rail, the prior austenite grain size GS, pearlite block size BS, and fatigue crack propagation resistance were evaluated. The test results are shown in Table 2. Each evaluation item is explained in detail below.

<Rail microstructure>
The method for measuring the microstructure and area ratio of the central sole is as follows.
Test pieces for microstructure observation were taken from a position 1 mm deep from the surface of the center of the rail sole shown in Figure 2, embedded in resin, and mirror-polished, and then a surface perpendicular to the rolling direction at a position 1 mm deep from the surface was observed at 200x magnification using an optical microscope in 10 fields of view (each field was 300 μm × 400 μm). The area ratio of the constituent phases was determined for each field, and the average value was taken as the area ratio of the microstructure.

<Previous austenite grain size GS>
The method for measuring the prior austenite grain size GS in the center of the sole is as follows.
A test piece for microstructural observation was taken from a position 1 mm deep from the surface of the center of the rail sole shown in Figure 2, embedded in resin, mirror-polished, and then niter-etched to reveal the pro-eutectoid cementite precipitated at the prior austenite grain boundaries. A surface perpendicular to the rolling direction at a position 1 mm deep from the surface was observed at 200x magnification using a scanning electron microscope. The grain size of the area surrounded by the pro-eutectoid cementite was measured by tracing using image analysis software. More than 400 areas were measured, and the average value was taken as the prior austenite grain size GS.

<Perlite block size BS>
The method for measuring the perlite block size BS in the center of the sole is as follows.
A test piece for microstructure observation was taken from a position 1 mm deep from the surface of the center of the rail sole shown in Figure 2. After embedding in resin and mirror polishing, orientation analysis was performed using EBSD (Electron Backscatter Diffraction Pattern). Grain boundaries where the misorientation of adjacent crystal orientations is 15° or more were defined as pearlite block boundaries, and the average value of the grain size measured in a circle equivalent was defined as the pearlite block size BS. The measurement area was 300 μm square, the measurement step was 0.3 μm intervals, and measurement points with a Confidex index of 0.1 or less, which indicates the reliability of the measurement orientation, were excluded from the measurement. Furthermore, crystal grains on the edge of the measurement area were also excluded from the measurement.

<Fatigue crack propagation resistance>
A fatigue crack propagation test specimen was taken from the center of the rail sole, shown in Figure 4, at a depth of 1 mm from the surface, and a fatigue crack propagation test was conducted.
FIG. 5 is a schematic diagram showing an example of a test piece, where FIG. 5(a) is a front view, FIG. 5(b) is a side view, and FIG. 5(c) is an enlarged front view of the notch. The test piece is a plate-like piece with a width W of 20 mm, a height H of 100 mm, and a thickness B of 5 mm in FIG. 5, and a notch was formed at one end of the width at the center H/2 of the height H. The notch was formed on the center side of the sole of the rail. The notch had a length L of 2 mm and a width C of 0.2 mm, and the end of the notch had a curvature R of 0.1 mm. The stress ratio (R ratio = minimum stress/maximum stress) was set to 0.2, and the fatigue crack propagation rate da/dN (m/cycle) was measured in the stress intensity factor range ΔK = 20 MPa m ^1/2 to evaluate the fatigue crack propagation resistance. If the value of da/dN was 7.0×10 ^-8 or less, it was evaluated that the fatigue crack propagation inhibition performance was present.

As shown in Table 2, the fatigue crack propagation rates of the rail materials of the examples (Test Nos. 1 to 40 in Table 2) all satisfied the condition of 7.0×10 ⁻⁸ or less, and it was confirmed that the rail materials had fatigue crack propagation suppression performance. On the other hand, the comparative examples (Test Nos. 41 to 48 and 50 to 54 in Table 2) in which the component composition of the rail material did not satisfy the conditions of the present invention, the pearlite area ratio did not satisfy the conditions of the present invention, or the condition of the above formula (2) did not satisfy the condition, had fatigue crack propagation rates da/dN (m/cycle) exceeding 7.0×10 ⁻⁸ . In Test No. 49, the heating temperature was too high, so part of the steel material melted during heating. For this reason, it was not possible to use it for rolling due to concerns about fracture during rolling, and the characteristics were not evaluated.

According to the present invention, it is possible to provide a rail with excellent fatigue crack propagation resistance at the sole of the rail, together with a manufacturing method thereof. The rail of the present invention contributes to extending the service life of rails for high axle load railways and preventing railway accidents, and is industrially useful. Furthermore, the rail manufacturing method of the present invention is industrially useful, as it can stably improve the fatigue crack propagation resistance at the sole of the rail by optimizing the heat treatment conditions after hot rolling.

1 Rail 11 Rail head (head)
12 Rail bottom (bottom)
13 Rail foot (foot)
14 Rail sole (sole)

Claims (3)

C: 0.70 to 1.20 mass%
Si: 0.10 to 1.20 mass%,
Mn: 0.10 to 1.50 mass%,
P: 0.035% by mass or less,
S: 0.020% by mass or less, and
Cr: 0.05 to 2.00 mass%;
The balance is Fe and unavoidable impurities.
The area ratio of pearlite structure at a depth of 1 mm from the surface of the center of the sole of the rail is 95.0% or more,
A rail whose HC value expressed by the following formula (1) satisfies the following formula (2).
Record
HC={(12×[%C])+([%Si]/10)+([%Mn]/20)+([%Cr]/18)} ^2・・・ (1)
HC≦(120×GS)/(1.1×BS)・・・(2)
here,
[%C], [%Si], [%Mn] and [%Cr] are the contents (mass%) of C, Si, Mn and Cr, respectively.
GS is the prior austenite grain size (μm) at the center of the rail sole,
BS is the pearlite block size (μm) at the center of the rail sole. The composition further comprises:
V: 0.30% by mass or less,
Cu: 1.0 mass% or less,
Ni: 1.0 mass% or less,
Nb: 0.05% by mass or less,
Mo: 2.0% by mass or less,
Al: 0.07% by mass or less,
W: 1.0% by mass or less,
Co: 1.0 mass% or less,
B: 0.005% by mass or less,
Ti: 0.05% by mass or less,
Sb: 0.05% by mass or less,
Mg: 0.01% by mass or less,
Ca: 0.02% by mass or less, and
Sn: 0.05% by mass or less,
The rail according to claim 1, comprising at least one selected from the group consisting of: A method for manufacturing a rail according to claim 1 or 2,
A steel material having the chemical composition as set forth in claim 1 or 2 is heated at a heating temperature of 1350°C or less, hot rolled under conditions in which the rolling finish temperature at the center of the rail foot sole is 850°C or more, cooled at an average cooling rate _CR1 (°C/sec) until the temperature at the center of the rail foot sole decreases from 850°C to 750°C, and then cooled at an average cooling rate _CR2 (°C/sec) until the temperature at the center of the rail foot sole decreases from 750°C to a cooling stop temperature T,
here,
The cooling stop temperature T is a temperature in the range of 400 to 650°C,
A manufacturing method for a rail, wherein an average cooling rate CR ₁ (°C/sec) and an average cooling rate CR ₂ (°C/sec) satisfy the following formulas (3) and (4), respectively.
Record
HC/300≦CR ₁ ≦3.0...(3)
HC/120≦ _CR2 ≦6.0...(4)

Images