JP6852761B2 - Rails and their manufacturing methods - Google Patents

Description

The present invention relates to a rail, and more particularly to a rail having excellent wear resistance, fatigue damage resistance, and delayed fracture resistance. The present invention also relates to a method for manufacturing the rail.

In high-axle heavy railways used for transporting ore, the load applied to the axles of freight cars is much higher than that of passenger cars, and the rail usage environment is harsh. As a material for rails used in such an environment, steel having a pearlite structure has been mainly used from the viewpoint of emphasizing wear resistance.

However, in recent years, the weight loaded on freight cars has been further increased in order to improve the efficiency of transportation by rail, and further improvement in wear resistance, fatigue damage resistance, and delay fracture resistance is required. .. The high-axle heavy railway is a railway having a large load weight per train or freight car (for example, a load weight of about 150 tons or more).

For example, Patent Documents 1 and 2 propose to increase the amount of C in the rail material to more than 0.85% by mass and 1.20% by mass or less. Further, Patent Documents 3 and 4 propose that the amount of C is more than 0.85% by mass and 1.20% by mass or less, and that the rail head is heat-treated. As described above, the wear resistance is improved by increasing the amount of C and increasing the cementite fraction.

On the other hand, the rails in the curved section of the high-axis heavy railway are subject to rolling stress due to the wheels and slipping force due to centrifugal force, so that the rails are more severely worn and fatigue damage due to slipping occurs. If the amount of C is simply increased as in the techniques proposed in Patent Documents 1 to 4, proeutectoid cementite is produced depending on the heat treatment conditions, and the amount of cementite in the brittle pearlite layered structure is increased. Therefore, the above technique cannot be expected to improve fatigue damage resistance.

Therefore, Patent Document 5 proposes a technique of suppressing the formation of proeutectoid cementite by adding Al and Si to improve fatigue damage resistance.

Further, Patent Document 6 proposes a technique for improving wear resistance and internal fatigue damage resistance by setting the Vickers hardness of the surface layer of the rail head to Hv370 or higher.

Patent Document 7 proposes that the Vickers hardness is set to Hv300 to 500 and the service life of the rail is improved by controlling the pearlite block at the rail head.

Further, as disclosed in Patent Documents 8 to 11, there is also known a technique for preventing delayed fracture of rails by controlling the form and amount of A-based inclusions.

Japanese Unexamined Patent Publication No. 8-109439 Japanese Unexamined Patent Publication No. 8-144016 Japanese Unexamined Patent Publication No. 8-246100 Japanese Unexamined Patent Publication No. 8-246101 Japanese Unexamined Patent Publication No. 2002-69585 Japanese Unexamined Patent Publication No. 10-195601 Japanese Unexamined Patent Publication No. 2003-293086 Japanese Unexamined Patent Publication No. 2000-328190 Japanese Unexamined Patent Publication No. 6-279928 Japanese Unexamined Patent Publication No. 6-279850 Japanese Unexamined Patent Publication No. 6-279929

However, if the strength of the rail is increased, the risk of delayed fracture increases, so that the conventional technique as proposed in Patent Documents 1 to 7 cannot sufficiently prevent delayed fracture. Further, in the technique proposed in Patent Document 5, there is a problem that an oxide that is a starting point of fatigue damage is generated by adding Al. Therefore, it has been found that it is difficult to achieve both wear resistance and fatigue damage resistance in a steel rail having a pearlite structure (hereinafter referred to as "pearlite rail").

Further, in the conventional technique as proposed in Patent Documents 8 to 11, the toughness and ductility of the rail are improved by controlling the A-based inclusions, but good delayed fracture resistance can always be obtained. I couldn't say that.

In this way, the rail has excellent wear resistance, fatigue damage resistance, and delayed fracture resistance, which has a sufficient life even in the harsh usage environment of recent years only by controlling the pearlite structure and inclusion morphology. Was not realized.

An object of the present invention is to solve the above-mentioned various problems, and to provide a rail having excellent wear resistance, fatigue damage resistance, and delayed fracture resistance as compared with conventional pearlite rails. With the goal.

In order to solve the above problems, the inventors have produced rails having different V and N contents, and have diligently investigated the structure, fatigue damage resistance and delayed fracture resistance. As a result, by optimizing the area ratio of bainite, which is a structure with excellent fatigue damage resistance, and controlling the ratio of V content to N content within a specific range, excellent wear resistance and fatigue damage resistance It has been found that a rail having both properties and delayed fracture resistance can be obtained.

The present invention is based on the above findings, and its gist structure is as follows.

1. 1. By mass%
C: 0.70 to 1.00%,
Si: 0.25 to 2.00%,
Mn: 0.25 to 1.50%,
P: 0.035% or less,
S: 0.0005 to 0.010%,
Cr: 0.25 to 1.50%,
V: 0.005 to 0.120%,
N: Contains 0.0015 to 0.0150%,
The balance consists of Fe and unavoidable impurities, and
It has a component composition in which the [% V] / [% N] value defined as the ratio of the V content [% V] to the N content [% N] is 6.0 to 30.0.
The total area ratio of pearlite and bainite in the region from the surface of the head to a depth of 0.5 mm to 25 mm is 95% or more, and the area ratio of bainite in the region is 20% or more.
A rail having a Vickers hardness of Hv360 or higher in the region.

2. The component composition is further increased by mass%.
Cu: 1.0% or less,
Ni: 1.0% or less,
The rail according to 1 above, which contains 1 or 2 or more selected from the group consisting of Nb: 0.05% or less and Mo: 1.0% or less.

3. 3. The component composition is further increased by mass%.
Al: 0.07% or less,
W: 1.0% or less,
B: 0.005% or less,
The rail according to 1 or 2 above, which contains 1 or 2 or more selected from the group consisting of Ti: 0.05% or less and Sb: 0.05% or less.

4. A steel material having the component composition according to any one of 1 to 3 above is hot-rolled under the condition of rolling finish temperature: 850 to 950 ° C. to form a rail shape.
Accelerated cooling start temperature: 700 ° C or higher, accelerated cooling stop temperature: 400 to 600 ° C, average cooling rate: 3.0 to 20.0 ° C / s.
A method for manufacturing a rail, which comprises secondary cooling at an average cooling rate from the accelerated cooling stop temperature to 350 ° C.: 0.2 to less than 2.0 ° C./s.

The rail of the present invention has far superior wear resistance, fatigue damage resistance, and delayed fracture resistance as compared with the conventional pearlite rail. Therefore, the rail of the present invention can be extremely suitably used as a rail for high-axis heavy railroads, and contributes to extending the life of the rail and preventing railroad accidents.

It is a schematic diagram which shows the measurement position of Vickers hardness. It is a schematic diagram which shows the test method of the wear resistance by the Nishihara type wear tester. It is a schematic diagram which shows the sampling position of a rail test piece 2. It is a schematic diagram which shows the test method of the fatigue damage resistance by a Nishihara type wear tester. It is a schematic diagram which shows the sampling position of the SSRT test piece 4. It is a schematic diagram which shows the shape and size of the SSRT test piece 4.

Hereinafter, embodiments of the present invention will be described. The following description shows a preferred embodiment of the present invention, and the present invention is not limited to the following description.

[Ingredient composition]
The rail of the present invention has the above-mentioned component composition. In other words, the rail of the present invention is made of steel having the above-mentioned composition. Hereinafter, the reason for limiting the composition of steel to the above range will be described. Unless otherwise specified, "%" representing the content of each component means "mass%".

C: 0.70 to 1.00%
C is an element necessary for ensuring the strength of the pearlite and bainite structures, that is, the wear resistance, and the wear resistance improves as the C content increases. If the C content is less than 0.70%, it is difficult to obtain excellent wear resistance as compared with the conventional pearlite rail. Therefore, the C content is 0.70% or more, preferably 0.75% or more. On the other hand, when the C content exceeds 1.00%, proeutectoid cementite is formed at the austenite grain boundaries during the pearlite transformation after hot rolling, and the fatigue damage resistance is remarkably lowered. Therefore, the C content is 1.00% or less, preferably 0.85% or less.

Si: 0.25 to 2.00%
Si is an element that acts as a deoxidizer and also serves as a reinforcing element for pearlite and bainite structures. In order to obtain the effect of adding Si, the Si content is 0.20% or more, preferably 0.50% or more. On the other hand, when the Si content exceeds 2.00%, the weldability deteriorates due to the high bonding force of Si with oxygen. Further, due to the high hardenability of Si, a martensite structure is formed from the surface layer to the inside of the rail, and the fatigue damage resistance is lowered. Therefore, the Si content is 2.00% or less, preferably 1.20% or less.

Mn: 0.25 to 1.50%
Mn is an element necessary for forming a bainite structure and improving its strength. In order to obtain the effect of adding Mn, the Mn content is 0.20% or more, preferably 0.40% or more. On the other hand, if the Mn content exceeds 1.50%, a martensite structure is likely to be formed, and the material deteriorates due to hardening or embrittlement when the rail is heat-treated or welded. Further, since excessive addition of Mn lowers the equilibrium transformation temperature of pearlite, it becomes difficult to obtain a desired hardness due to the coarsening of the lamellar interval. Therefore, the Mn content is 1.50% or less, preferably 0.80% or less.

P: 0.035% or less When the P content exceeds 0.035%, ductility decreases. Therefore, the P content is 0.035% or less, preferably 0.020% or less. On the other hand, since the smaller the amount of P, the more preferable it is, the lower limit of the P content is not particularly limited and may be 0%. May be greater than 0%. In addition, since excessively low P causes an increase in refining time and an increase in cost, the P content is preferably 0.001% or more.

S: 0.0005 to 0.010%
S is mainly present in the steel as an A-based inclusion. When the S content exceeds 0.010%, the amount of inclusions increases remarkably, the inclusions become coarse, and the cleanliness of the steel material deteriorates. Therefore, the S content is 0.010% or less, preferably 0.008% or less. On the other hand, if the S content is less than 0.0005%, the steelmaking cost will increase. Therefore, the S content is set to 0.0005% or more.

Cr: 0.25 to 1.50% or less Cr is an element necessary for forming a bainite structure and improving its strength. In order to obtain the effect of adding Cr, the Cr content is 0.20% or more, preferably 0.60% or more. On the other hand, if the Cr content exceeds 1.50%, a martensite structure is likely to be formed, and the material deteriorates due to hardening or embrittlement when the rail is heat-treated or welded. Therefore, the Cr content is 1.50% or less, preferably 1.20% or less.

V: 0.005 to 0.120%
V is an element having the effect of forming a carbonitride, dispersing and precipitating in the matrix, and improving wear resistance and delayed fracture resistance. In order to obtain the above effect, the V content is 0.005% or more, preferably 0.010% or more. On the other hand, if the V content exceeds 0.120%, the workability deteriorates and the alloy cost also increases, so that the rail manufacturing cost increases. Therefore, the V content is 0.120% or less, preferably 0.100% or less.

N: 0.0015 to 0.0150%
N is an element having the effect of forming a nitride, dispersing and precipitating in the matrix, and improving wear resistance and delayed fracture resistance. In order to obtain the above effect, the N content is 0.0015% or more, preferably 0.0030% or more. On the other hand, when the N content exceeds 0.0150%, coarse nitrides are formed in the rail, and fatigue damage resistance and delayed fracture resistance are lowered. Therefore, the N content is 0.0150% or less, preferably 0.0100% or less.

[% V] / [% N] value: 6.0 to 30.0
If the [% V] / [% N] value defined as the ratio of the V content [% V] (mass%) to the N content [% N] (mass%) is less than 6.0, the delay tolerance is reduced. The amount of deposited V-nitride that contributes to the improvement of fracture property is insufficient, and the delayed fracture resistance is lowered. On the other hand, when the [% V] / [% N] value exceeds 30.0, coarse V-nitrides are precipitated, and the delayed fracture resistance is lowered. Therefore, the [% V] / [% N] values are set to 6.0 to 30.0.

The rail in one embodiment of the present invention may have a component composition consisting of the above elements, the balance Fe and unavoidable impurities. Further, the case where other trace component elements are contained in place of a part of the Fe within a range that does not substantially affect the action and effect of the present invention also belongs to the present invention. Examples of the unavoidable impurities include O (oxygen). The content of O as an unavoidable impurity is not particularly limited, but is preferably 0.004% or less.

Further, in another embodiment of the present invention, the above-mentioned component composition can further optionally contain 1 or 2 or more selected from the following group A.
(Group A)
Cu: 1.0% or less Ni: 1.0% or less Nb: 0.05% or less Mo: 1.0% or less

Cu: 1.0% or less Cu is an element having an action of further improving the strength of steel by solid solution strengthening like Cr. However, if the Cu content exceeds 1.0%, Cu cracking is likely to occur. Therefore, when Cu is added, the Cu content is 1.0% or less, preferably 0.5% or less. On the other hand, the lower limit of the Cu content is not particularly limited, but is preferably 0.005% or more.

Ni: 1.0% or less Ni is an element that can improve the strength of steel without reducing ductility. However, when the Ni content exceeds 1.0%, the hardenability of the steel is further increased, and as a result, martensite is generated, and the wear resistance and the fatigue damage resistance are lowered. Therefore, when Ni is added, the Ni content is set to 1.0% or less, preferably 0.5% or less. On the other hand, the lower limit of the Ni content is not particularly limited, but is preferably 0.005% or more. By adding Ni together with Cu, Cu cracking can be suppressed. Therefore, especially when Cu is added, it is preferable to add Ni as well.

Nb: 0.05% or less Nb is combined with C in steel during hot rolling and after hot rolling and is precipitated as carbide. Since the carbide is effective for refining the particle size of the old austenite, the addition of Nb can further improve wear resistance, fatigue damage resistance, and ductility. However, when the Nb amount exceeds 0.05%, the effect of improving the wear resistance and the fatigue damage resistance is saturated, and the effect commensurate with the increase in the Nb amount cannot be obtained. Therefore, when Nb is added, the Nb content is set to 0.05% or less, preferably 0.03% or less. On the other hand, the lower limit of the Nb content is not particularly limited, but from the viewpoint of sufficiently obtaining the effect of adding Nb, the Nb content is preferably 0.001% or more.

Mo: 1.0% or less Mo is an element that is effective in producing bainite and has an effect of further improving the strength of steel by solid solution strengthening. However, when the Mo content exceeds 1.0%, martensite is formed in the steel, and the wear resistance and fatigue damage resistance are lowered. Therefore, when Mo is added, the Mo content is 1.0% or less, preferably 0.5% or less. On the other hand, the lower limit of the Mo content is not particularly limited, but from the viewpoint of sufficiently obtaining the effect of adding Mo, the Mo content is preferably 0.005% or more.

Further, in another embodiment of the present invention, the above-mentioned component composition can further optionally contain 1 or 2 or more selected from the following group B.
(Group B)
Al: 0.07% or less W: 1.0% or less B: 0.005% or less Ti: 0.05% or less Sb: 0.05% or less

Al: 0.07% or less Al is an element that can be added as an antacid. However, when the Al content exceeds 0.07%, a large amount of oxide-based inclusions are formed in the steel due to the high binding force of Al with oxygen, and as a result, the ductility of the steel is lowered. Therefore, when Al is added, the Al content is 0.07% or less, preferably 0.03% or less. On the other hand, the lower limit of the Al content is not particularly limited, but is preferably 0.001% or more for deoxidation.

W: 1.0% or less W is precipitated as carbide during and after hot rolling, and the strength and ductility of the rail are further improved by precipitation strengthening. However, if the W content exceeds 1.0%, martensite is formed in the steel, resulting in a decrease in ductility. Therefore, when W is added, the W content is set to 1.0% or less, preferably 0.5% or less. On the other hand, the lower limit of the W content is not particularly limited, but in order to obtain a sufficient effect of adding W, the W content is preferably 0.001% or more, more preferably 0.005% or more. ..

B: 0.005% or less B has the effect of further improving hardenability by segregating into the old austenite grain boundaries, and is an element effective for the formation of bainite structure. However, if the B content exceeds 0.005%, martensite is formed, and as a result, wear resistance and fatigue damage resistance are lowered. Therefore, when B is added, the B content is set to 0.005% or less, preferably 0.003% or less. On the other hand, the lower limit of the B content is not particularly limited, but in order to sufficiently obtain the effect of adding B, the B content is preferably 0.001% or more.

Ti: 0.05% or less Ti precipitates as carbides, nitrides, or carbonitrides during and after hot rolling, and the strength and ductility of steel are further improved by precipitation strengthening. However, if the Ti content exceeds 0.05%, coarse carbides, nitrides, or carbonitrides are formed, resulting in reduced ductility of the steel. Therefore, when Ti is added, the Ti content is set to 0.05% or less, preferably 0.03% or less. On the other hand, the lower limit of the Ti content is not particularly limited, but in order to obtain a sufficient effect of adding Ti, the Ti content is preferably 0.001% or more, and more preferably 0.005% or more. preferable.

Sb: 0.05% or less Sb is an element having an effect of preventing decarburization of steel when the steel material is heated before hot rolling. However, if the Sb content exceeds 0.05%, it adversely affects the ductility and toughness of the steel. Therefore, when Sb is added, the Sb content is set to 0.05% or less, preferably 0.03% or less. On the other hand, the lower limit of the Sb content is not particularly limited, but in order to fully exert the effect of adding Sb, the Sb content is preferably 0.001% or more, preferably 0.005% or more. More preferred.

In addition, only one of the group A element and the group B element may be added, or both may be added. In other words, in another embodiment of the present invention, the component composition is one or two or more selected from the group A and one or two or more selected from the group B, or both. Further, it can be arbitrarily contained.

[Microstructure on the surface of the head]
Pearlite + bainite: 95% or more The wear resistance and fatigue damage resistance of steel vary greatly depending on the microstructure. Pearlite and bainite have excellent wear resistance and fatigue damage resistance as compared with martensite structure of the same hardness. Therefore, excellent wear resistance and fatigue damage resistance can be obtained by setting the total area ratio of pearlite and bainite on the surface layer of the rail head to 95% or more. The total area ratio is preferably 98% or more. On the other hand, the upper limit of the total area ratio is not particularly limited and may be 100%.

Bainite: 20% or more In addition, since bainite is more easily worn than pearlite, it has the effect of improving familiarity in contact between the wheel and rail in the initial stage of use. By improving the familiarity between the wheel and the rail, the contact area between the wheel and the rail is increased, the load surface pressure on the rail by the wheel is reduced, and the fatigue damage resistance of the rail is improved. In order to obtain the above effect, the area ratio of bainite on the surface layer of the rail head is set to 20% or more. On the other hand, the upper limit of the bainite area ratio is not particularly limited and may be 100%, but from the viewpoint of ensuring a better balance between wear resistance and fatigue damage resistance, it is preferably 50% or less. ..

The structure other than pearlite and bainite in the microstructure of the rail of the present invention is not particularly limited. If the total area ratio is 5% or less, the existence of other tissues is allowed. Examples of the other structure include one or both of ferrite and martensite.

Here, the area ratio of each of the above tissues refers to the area ratio in the region from the surface of the rail head to a depth of 0.5 mm to a depth of 25 mm. The area ratio can be measured by the method described in Examples.

[Vickers hardness of the surface layer of the head]
If the Vickers hardness of the surface layer of the rail head is less than Hv360, the wear resistance of the steel is lowered and the service life of the rail is shortened. Therefore, the Vickers hardness of the surface layer of the head is set to Hv360 or more, preferably 400 or more. When the above-mentioned steel structure is satisfied with the above-mentioned composition, the Vickers hardness of the obtained head surface layer is less than Hv500. When the Hv is 500 or more, martensite is generated, so that the fatigue damage resistance of the steel is lowered. Therefore, if the upper limit of the Vickers hardness of the surface layer of the head is intentionally stated, it is less than Hv500. From the viewpoint of ensuring wear resistance and fatigue damage resistance, it is preferably less than Hv480.

[Production method]
Next, a method for manufacturing a rail according to an embodiment of the present invention will be described. The rail of the present invention can be produced by sequentially applying the following treatments (1) to (3) to a steel material having the above-mentioned composition.
(1) Hot rolling (2) Accelerated cooling (primary cooling)
(3) Secondary cooling

The steel material used as a material can be produced by any method, but in general, it is preferable to produce the steel material by casting, particularly continuous casting. The steel material may be a piece of steel.

(1) Hot rolling First, the steel material is hot-rolled to form a rail shape. In the present invention, the method of hot rolling is particularly limited because the microstructure of the rail finally obtained can be controlled by controlling the rolling finish temperature in the hot rolling and the cooling conditions after the hot rolling. It can be done by any method. The heating temperature in the heating prior to hot rolling is preferably 1150 to 1350 ° C. By setting the heating temperature to 1150 ° C. or higher, the deformation resistance during rolling can be further reduced. Further, by setting the heating temperature to 1350 ° C. or lower, it is possible to prevent the steel material from being partially melted due to an excessive temperature rise and causing defects inside the rail.

Rolling finish temperature: 850 to 950 ° C
When the rolling finish temperature in the hot rolling is lower than 850 ° C., the rolling is performed in the austenite low temperature region, and not only the processing strain is introduced into the austenite crystal grains but also the elongation of the austenite crystal grains is remarkable. Become. The introduction of dislocations and the increase in austenite grain boundary area increase pearlite nucleation sites, resulting in a significant decrease in wear resistance. This is because the pearlite colony size becomes finer, but the pearlite nucleation site increases, the pearlite transformation start temperature rises, and the lamellar spacing of the pearlite layer becomes coarser. Further, since the pearlite transformation is promoted, it becomes difficult to obtain bainite having an area ratio of 20% or more, and the fatigue damage resistance is also lowered. Therefore, the rolling finish temperature is set to 850 ° C. or higher. On the other hand, when the rolling finish temperature exceeds 950 ° C., the austenite crystal grains become coarse, so that martensite is easily generated and the fatigue damage resistance is lowered. Therefore, the rolling finish temperature is set to 950 ° C. or lower. The rolling finish temperature referred to here is the temperature of the side surface of the rail head on the entry side of the final rolling mill, and can be measured by a radiation thermometer.

(2) Accelerated cooling Accelerated cooling start temperature: 700 ° C or higher Next, accelerated cooling is performed. At that time, if the accelerated cooling start temperature is lower than 700 ° C., the area ratio of the bainite structure is lowered, and the fatigue damage resistance of the rail is lowered. In addition, since the lamellar spacing of the pearlite structure becomes coarse, the internal hardness of the rail head also decreases. Therefore, the acceleration cooling start temperature is set to 700 ° C. or higher. On the other hand, the upper limit of the accelerated cooling start temperature is not particularly limited, but the accelerated cooling stop temperature is inevitably equal to or lower than the rolling finish temperature.

Accelerated cooling stop temperature: 400-600 ° C
When the accelerated cooling stop temperature in the accelerated cooling is higher than 600 ° C., the area ratio of the bainite structure is lowered, and the fatigue damage resistance of the rail is lowered. In addition, since the lamellar spacing of the pearlite structure becomes coarse, the internal hardness of the rail head also decreases. Therefore, the accelerated cooling stop temperature is set to 600 ° C. or lower, preferably 550 ° C. or lower. On the other hand, when the accelerated cooling stop temperature is lower than 400 ° C., martensite is generated, and the wear resistance and fatigue damage resistance are lowered. Therefore, the accelerated cooling stop temperature is set to 400 ° C. or higher.

Average cooling rate: 3.0-20.0 ° C / s
If the average cooling rate in the accelerated cooling is less than 3.0 ° C./s, the area ratio of the bainite structure is lowered and the fatigue damage resistance of the rail is lowered. In addition, since the lamellar spacing of the pearlite structure becomes coarse, the internal hardness of the rail head also decreases. Therefore, the average cooling rate is 3.0 ° C./s or higher, preferably 5.0 ° C./s or higher. On the other hand, when the average cooling rate exceeds 20.0 ° C./s, a martensite structure is formed, and wear resistance and fatigue damage resistance are lowered. Therefore, the average cooling rate is 20.0 ° C./s or less, preferably 15.0 ° C./s or less.

(3) Secondary cooling After the above accelerated cooling is stopped, secondary cooling is performed. In the secondary cooling, the average cooling rate in the temperature range from the accelerated cooling stop temperature to 350 ° C. is 0.2 ° C./s or more and less than 2.0 ° C./s. When the average cooling rate is 2.0 ° C./s or higher, the bainite transformation is not completed, and a martensite structure is formed from the partially untransformed austenite phase. As a result, the wear resistance and fatigue damage resistance of the rail are reduced. Therefore, the average cooling rate is less than 2.0 ° C./s, preferably 1.5 ° C./s or less. On the other hand, when the average cooling rate is less than 0.2 ° C./s, the area ratio of the bainite structure is lowered, the fatigue damage resistance is lowered, and the cooling time in a low temperature region is increased, so that the productivity is increased. Will decrease and the rail manufacturing cost will increase. Therefore, the average cooling rate is 0.2 ° C./s or higher, preferably 0.5 ° C./s or higher.

In the above-mentioned accelerated cooling and secondary cooling, the cooling start temperature, the cooling stop temperature, and the temperature for determining the cooling rate are all the surface temperature of the side surface of the rail head and can be measured by a radiation thermometer. ..

Next, the present invention will be described in more detail based on Examples. However, the present invention is not limited by the following examples, and can be appropriately modified within a range that can be adapted to the gist of the present invention, all of which are included in the technical scope of the present invention. Is done.

Rails were manufactured by the following procedure and their characteristics were evaluated.

First, the steel material (steel piece) having the composition shown in Table 1 was hot-rolled under the conditions shown in Table 2 to form a rail shape. Then, accelerated cooling and secondary cooling were further performed under the conditions shown in Table 2, and then allowed to cool to obtain a rail. In addition, Comparative Example No. For 40, after the acceleration cooling was stopped, the temperature was maintained at 630 ° C., which is the acceleration cooling stop temperature, for 250 seconds, and then the secondary cooling was performed. The accelerated cooling start temperature was 700 ° C. or higher in each example.

The rolling finish temperature shown in Table 2 is a value obtained by measuring the temperature of the side surface of the rail head on the entry side of the final rolling mill with a radiation thermometer. The accelerated cooling stop temperature is a value obtained by measuring the temperature of the surface layer on the side surface of the rail head at the time of cooling stop with a radiation thermometer.

A rail having a pearlite structure was prepared in order to be used as a reference in the evaluation of wear resistance, fatigue damage resistance, and delayed fracture resistance, which will be described later (No. 1). Hereinafter, this pearlite rail is referred to as a "reference material".

For each of the obtained rails, the microstructure of the head surface layer, the Vickers hardness of the head surface layer, wear resistance, fatigue damage resistance, and delayed fracture resistance were evaluated. The evaluation method will be described below.

(Microstructure on the surface of the head)
A sample for microstructure observation was taken from the head of the obtained rail. The sample was taken from 6 places per rail so that the observation positions were 0.5 mm, 5 mm, 10 mm, 15 mm, 20 mm, and 25 mm in depth from the surface of the rail head. The collected test pieces were polished on the surface prior to observation and then corroded with nital. The type of tissue was identified by 400x cross-sectional observation using an optical microscope, and the area ratio of each tissue of pearlite structure and bainite structure was determined by image analysis. The average of the area ratio at each depth was taken as the area ratio on the surface layer of the head.

(Vickers hardness on the surface of the head)
As shown in FIG. 1, the Vickers hardness in the region from the surface of the rail head 1 to a depth of 0.5 mm to a depth of 25 mm was measured at a load of 98 N and a pitch of 0.5 mm. The maximum and minimum values of the measured Vickers hardness were determined.

(Abrasion resistance)
It is desirable to evaluate the wear resistance by actually laying the rail, but it takes a long time for the test. Therefore, in this embodiment, the wear resistance is determined by a test simulating the actual contact conditions between the internal high-hardness rail and the wheel using a Nishihara-type wear tester that can evaluate the wear resistance in a short time. evaluated.

2A and 2B are schematic views showing a test method by a Nishihara type wear tester, where FIG. 2A is a side view and FIG. 2B is a front view. This test is carried out by rotating a rail test piece 2 (Nishihara type wear test piece) having an outer diameter of 30 mm in contact with the tire test piece 3 as shown in FIG.

The rail test piece 2 was collected from two locations on the rail head 1 as shown in FIG. The one collected from the surface layer of the rail head 1 was designated as the surface layer rail test piece 2a, and the one collected from the inner side was designated as the internal rail test piece 2b.

On the other hand, the tire test piece 3 was produced by the following procedure. First, a round bar having a diameter of 32 mm was collected from the head of an ordinary rail specified in JIS E1101. Next, the round bar was heat-treated so that the Vickers hardness (load 98N) was Hv390 and the structure was a tempered martensite structure. The round bar after the heat treatment was processed into the shape shown in FIG. 2 to obtain a tire test piece 2.

In the test, the rail test piece 2 and the tire test piece 3 were rotated in the directions indicated by arrows in FIG. 2, and the amount of wear (weight reduction) of the rail test piece 2 after 100,000 rotations was measured. The test was carried out in a dry state, and the test conditions were: contact pressure: 1.2 GPa, slip ratio: -10%, rotation speed of rail test piece 1: 675 times / min (rotation speed of tire test piece was 750 times / min). ). Tables 3 and 4 show the amount of wear (g) between the surface layer and the inside measured by the test using the surface rail test piece 2a and the test using the internal rail test piece 2b, respectively.

Tables 3 and 4 also show the amount of improvement in wear resistance (%) based on the amount of wear of the heat-treated pearlite rail (No. 1) as the above-mentioned "reference material". The amount of improvement in the wear resistance of the rail test piece is calculated by {(wear amount of reference material (g) -wear amount of rail test piece (g)) / (wear amount of reference material (g))} × 100. did. Here, as the wear amount of the rail test piece, the average value of the wear amount of the surface layer rail test piece 2a and the wear amount of the internal rail test piece 2b was used. The amount of improvement is preferably 5% or more.

(Fatigue damage resistance)
The evaluation of fatigue resistance and damage resistance was also carried out using the Nishihara type wear tester in the same manner as the evaluation of wear resistance. However, as the rail test piece 2, as shown in FIG. 4, a test piece having a diameter of 30 mm, in which the contact surface with the tire test piece 3 is a curved surface having a radius of curvature of 15 mm, was used. As the rail test piece 2, the surface layer rail test piece 2a and the internal rail test piece 2b collected from two places of the rail head 1 were used as shown in FIG. 3, as in the evaluation of the abrasion resistance described above. Further, the tire test piece 3 was also prepared by the same method as the above-mentioned evaluation of wear resistance.

In the test, the rail test piece 2 and the tire test piece 3 were rotated in the directions indicated by the arrows in FIG. 4, respectively. The test environment was oil-lubricated, and the test was conducted under the conditions of contact pressure: 2.2 GPa, slip ratio: -20%, and rail test piece rotation speed: 600 rpm (tire test piece rotation speed is 750 rpm).

The surface of the rail test piece 2 was observed every 25,000 times to confirm whether or not a crack of 0.5 mm or more was generated. The rotation speeds at the time when a crack of 0.5 mm or more occurs are shown in Tables 3 and 4 as the rotation speeds until the occurrence of fatigue damage. The rotation speed can be regarded as an index of fatigue damage resistance (fatigue damage life).

Tables 3 and 4 also show the amount of improvement in fatigue damage resistance (%) based on the number of revolutions until the occurrence of fatigue damage in the heat-treated pearlite rail (No. 1) as the above-mentioned "reference material". did. The amount of improvement in the fatigue damage resistance of the rail test piece is {(rotation speed until fatigue damage occurs in the rail test piece-rotation speed until fatigue damage occurs in the reference material) / (until fatigue damage occurs in the reference material). Rotation speed)} × 100 was calculated. Here, as the rotation speed of the rail test piece until the occurrence of fatigue damage, the average value of the measured values of the surface layer rail test piece 2a and the measured values of the internal rail test piece 2b was used. The amount of improvement is preferably 20% or more.

(Delay resistance)
The SSRT (Slow Strain Rate Technique) test was conducted under the following two conditions, and the delayed fracture susceptibility was evaluated from the obtained elongation values.

First, as shown in FIG. 5, the SSRT test piece 4 was sampled centering on a position 25 mm deep from the upper surface of the rail head 1. The shape and dimensions of the SSRT test piece 4 are as shown in FIG. Except for the screw part and R part, ▽▽▽ finish was applied, and the parallel part was emery-polished to # 600.

Two of the same SSRT test pieces 4 were prepared, one for SSRT test in air and the other for SSRT test in aqueous ammonium thiocyanate solution. The SSRT test in the atmosphere was carried out by mounting the SSRT test piece 4 on the test device and performing the SSRT test piece in the air at a strain rate of 3.3 × 10 ^{-6 / s at 25 ° C.} E0 was measured. The SSRT test in an aqueous solution of ammonium thiocyanate was carried out in a 20 mass% aqueous solution of ammonium thiocyanate at a strain rate of 3.3 × 10 ^-6 / sec at 25 ° C., and the SSRT test piece in the aqueous solution of ammonium thiocyanate was subjected to the SSRT test. Elongation E1 was measured.

Using the obtained E0 and E1, the delayed fracture (DF) susceptibility, which is an index of the delayed fracture resistance, was calculated. Here, the delayed fracture susceptibility (%) is defined as 100 × (1-E1 / E0). The values of delayed fracture susceptibility obtained are shown in Tables 3 and 4.

Tables 3 and 4 also show the amount of improvement in delayed fracture resistance (%) based on the delayed fracture sensitivity of the heat-treated pearlite rail (No. 1) as the above-mentioned "reference material". The amount of improvement in delayed fracture susceptibility was calculated by {(delayed fracture susceptibility of the rail test piece-delayed fracture susceptibility of reference material) / (delayed fracture susceptibility of reference material)} x 100. The amount of improvement is preferably 10% or more.

As can be seen from the evaluation results shown in Tables 3 and 4, the rail satisfying the conditions of the present invention has a wear resistance of 10% or more, a fatigue damage resistance of 20% or more, and a fatigue damage resistance of 20% or more as compared with the pearlite rail as a reference material. The delayed fracture property was improved by 10% or more. On the other hand, the rail of the comparative example that does not satisfy the conditions of the present invention is inferior to the rail of the present invention in at least one of abrasion resistance, abrasion resistance, and fatigue damage resistance.

1 Rail head 2 Rail test piece 2a Surface rail test piece 2b Internal rail test piece 3 Tire test piece 4 SSRT test piece

Claims (4)

By mass%
C: 0.70 to 1.00%,
Si: 0.25 to 2.00%,
Mn: 0.25 to 1.50%,
P: 0.035% or less,
S: 0.0005 to 0.010%,
Cr: 0.25 to 1.50%,
V: 0.005 to 0.120%,
N: Contains 0.0015 to 0.0150%,
The balance consists of Fe and unavoidable impurities, and
It has a component composition in which the [% V] / [% N] value defined as the ratio of the V content [% V] to the N content [% N] is 6.0 to 30.0.
The total area ratio of pearlite and bainite in the region from the surface of the head to a depth of 0.5 mm to 25 mm is 95% or more, and the area ratio of bainite in the region is 20% or more.
A rail having a Vickers hardness of Hv360 or higher in the region. The component composition is further increased by mass%.
Cu: 1.0% or less,
Ni: 1.0% or less,
The rail according to claim 1, which contains 1 or 2 or more selected from the group consisting of Nb: 0.05% or less and Mo: 1.0% or less. The component composition is further increased by mass%.
Al: 0.07% or less,
W: 1.0% or less,
B: 0.005% or less,
The rail according to claim 1 or 2, which contains 1 or 2 or more selected from the group consisting of Ti: 0.05% or less and Sb: 0.05% or less. A steel material having the component composition according to any one of claims 1 to 3 is hot-rolled under the condition of rolling finish temperature: 850 to 950 ° C. to form a rail shape.
Accelerated cooling start temperature: 700 ° C or higher, accelerated cooling stop temperature: 400 to 600 ° C, average cooling rate: 3.0 to 20.0 ° C / s.
An average cooling rate from the accelerated cooling stop temperature to 350 ° C.: A rail manufacturing method for secondary cooling at 0.2 ° C./s or more and less than 2.0 ° C./s.
The total area ratio of pearlite and bainite in the region from the surface of the head of the rail to a depth of 0.5 mm to 25 mm is 95% or more, and the area ratio of bainite in the region is 20% or more. A method for manufacturing a rail, wherein the Vickers hardness in the above is Hv360 or more .

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