JP2002249825A - Method for producing high strength bainitic rail - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a high strength bainitic rail which has wear resistance and flaking damage resistance as compared with those of the conventional ones and has stability in wear resistance higher than that heretofore. SOLUTION: A steel containing, by mass, 0.3 to 0.4% C, <=1% Si, 0.4 to 2.5% Mn, 0.05 to 0.5% Nb, <=0.035% P and <=0.035% S is hot-rolled to form a rail stock. In a cooling stage after the hot rolling, the heat of the rail in the rail stock is cooled at the average cooling rate of 1 to 7 deg.C/s within the temperature range from an austenitic region to the completion of bainitic transformation.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a bainite type rail having many sharply curved sections such as an overseas mining railway and suitable for use under high axial load conditions, and particularly to wear resistance and anti-flaking damage. The present invention relates to a method for manufacturing a high-strength bainite type rail having excellent properties.

[0002]

2. Description of the Related Art In a railway mainly transporting heavy goods such as ores, the load applied to the wheel axle of a vehicle is much larger than that of a general railway passenger car. Conventionally, pearlite steel has been used as rail steel used in railways from the viewpoint of emphasizing wear resistance. In recent years, the C content of pearlite steel has been increased to increase the cementite fraction,
Attempts have been made to further improve wear resistance. JP-A-8-109439 and JP-A-8-144016 disclose highly wear-resistant pearlite rails having a C content of 0.85% to 1.20%, JP-A-8-246100, JP-A-8-246101. In the official gazette, the C content is 0.85% to 1.
There is disclosed a pearlite-based rail technology which is 20% and has excellent wear resistance in which a heat treatment is applied to the rail head.

[0003] On the other hand, a rail in a curved section of a high axle heavy railway is subjected to rolling stress due to wheels and sliding force due to centrifugal force, so that the rail becomes more severe and flaking damage due to sliding occurs. Flaking damage is a type of rolling fatigue damage.In the curved section, the rail top surface and the rolling surface of the wheel slide in directions other than the rolling direction, and plastic deformation occurs immediately below the rail top surface. Is a phenomenon that occurs. As measures for preventing flaking damage, it is known to increase the yield point or proof stress as a material of the rail steel, and to reduce carbides such as brittle cementite, but as described above, pearlite is aimed at improving wear resistance. When the C content of the steel is increased, the yield strength cannot be expected because the yield strength of the pearlite structure is almost determined by the layer spacing of the pearlite layered structure, and the amount of the brittle cementite (Fe ₃ C) phase increases. Due to the increase, the resistance to flaking damage decreases. Therefore, it has been difficult to satisfy both the wear resistance and the resistance to flaking damage in a rail steel having a pearlite structure.

As a high-strength rail replacing the pearlite system, a rail using a bainite structure is disclosed in Japanese Patent Laid-Open No. 11-1.
No. 52520 discloses this. In this technology, for a low-alloy bainite microstructure rail with low strength, the rail head is accelerated and cooled from the austenite region temperature, and subsequently maintained at 400 to 300 ° C. for a certain period of time,
An object of the present invention is to provide a rail exhibiting a high-strength bainite structure. However, Japanese Patent Application Laid-Open No. H11-152520 discloses a technique for promoting abrasion while maintaining hardness, and a facility for stably maintaining a long rail in a uniform state at a high temperature for a certain time over the entire length. There is a problem in that the use of is extremely expensive industrially. Also,
No consideration is given to flaking damage resistance.

As a method for solving the above-mentioned problem, a method for manufacturing a rail having a bainite structure excellent in wear resistance and flaking damage resistance is disclosed in Japanese Patent Application Laid-Open No. 2000-32813.
No. 8 discloses this. JP-A-2000-328
Although it is possible to manufacture bainite-type rails having very excellent characteristics by using the method of JP-A-138, the stability of wear resistance is not considered. The state in which the stability of wear resistance is high is a state in which the difference between the internal hardness and the surface hardness of the rail is small and the hardness of the rail is stable. If a rail with low stability of wear resistance is used, if the surface layer of the rail head is removed for any reason and the wear progresses in the long term use, the wear inside the rail where the hardness is low is reduced. Is promoted, and the life may be shortened.
Therefore, in order to further improve the life of the rail, the hardness does not decrease not only on the surface of the rail head but also inside the rail, the difference between the internal hardness of the rail and the surface hardness is small, The development of rails with high stability is desired.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has been found to provide abrasion resistance and flaking damage resistance as compared with conventional hypoeutectoid, eutectoid and hypereutectoid pearlite rail steels. An object of the present invention is to provide a method for manufacturing a high-strength bainite-type rail which is excellent in both of these characteristics and has a higher abrasion resistance stability than ever before.

[0007]

Means for Solving the Problems In order to solve the above problems, the present inventors have studied the optimum alloy components and manufacturing conditions on the premise of a bainite structure having excellent flaking resistance. As a result, by finely dispersing the Nb carbide in the steel, it is possible to improve both the flaking resistance and the wear resistance, and the fine dispersion of the Nb carbide in the steel mitigates the decrease in hardness inside the rail. ,
It was found that the stability of wear resistance was also improved, and the present invention was completed.

[0008] The present invention has been made based on the above-mentioned findings, and the first invention of the present invention has a C: 0.3 to 0.3% by mass.
0.4%, Si: 1% or less, Mn: 0.4-2.5%,
Nb: 0.05 to 0.5%, P: 0.035% or less,
S: Steel containing 0.035% or less is hot-rolled to form a rail material, and in a cooling process after the hot rolling, bainite transformation of the rail head of the rail material from the austenite region is completed. A high-strength bainite-type rail, characterized in that the temperature range is cooled at an average cooling rate of 1 ° C./s or more and 7 ° C./s or less. In the second invention, the steel component is, in addition to the steel component of the first invention, in mass%, Cr: 1.5% or less, Cu: 1% or less,
Ni: 1% or less, Mo: 2% or less, V: 0.2% or less. Case 3
The invention according to the first or second invention, wherein the heating temperature in the hot rolling is 1100 ° C. or higher and 135
0 ° C or less, the rolling end temperature is 750 ° C or more,
In the cooling process after hot rolling, cooling from a temperature range of at least 450 ° C. to a temperature range of 400 ° C. to 150 ° C. is performed at an average cooling rate of 1 ° C./s to 7 ° C./s. This is a method for manufacturing a high-strength bainite type rail.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION According to the present invention, the C content is reduced to 0.3 to 0.4%, which is remarkably reduced as compared with the conventional pearlite type rail, and the microstructure is made uniform by adding 0.05 to 0.5% of Nb. In order to improve wear resistance by finely dispersing Nb carbide in steel with higher hardness than Fe ₃ C, a rail with a component composition that aimed to improve flaking resistance as
The main point is to cool the temperature range until the bainite transformation is completed at a specific cooling rate. Due to the fine dispersion of Nb carbide in the steel, a decrease in hardness inside the rail is reduced, and the stability of wear resistance is improved. Hereinafter, the reasons for limiting the chemical components and the production conditions in the present invention will be described in detail.

First, the reasons for limiting the chemical components of the steel used in the present invention will be described.

C: 0.3% to 0.4%. C is an essential element for ensuring the required strength and wear resistance of the rail.
If it is less than 0.3%, it is difficult to sufficiently secure Nb carbide necessary for wear resistance. On the other hand, when the content exceeds 0.4%, it is difficult to stabilize the hardness of the bainite structure, and the flaking resistance is significantly reduced. Therefore, the amount of C is 0.3-0.4%
Limited to.

Nb: 0.05-0.5%. Nb not only increases the hardenability and turns the microstructure into bainite, but also binds with C in the steel and precipitates finely as carbide after rolling. Increase hardness. As a result, the abrasion resistance and the flaking resistance are greatly improved, the stability of the abrasion resistance is also improved, and the rail life is greatly contributed. However, if it is less than 0.05%, a sufficient effect cannot be obtained. 0.5%
If added in excess of, the weldability will deteriorate. In addition, since the effect of improving wear resistance and anti-flaking properties by adding Nb is saturated at 0.2%, from an economic viewpoint, the amount of added Nb is 0.05 to 0.
It is preferably set to 2%. Therefore, the Nb amount is 0.05 to 0.5%,
Preferably, it is limited to 0.05 to 0.2%.

Si: 1% or less. Si is effective as a deoxidizing agent, but if it exceeds 1%, the weldability is degraded due to the high bonding force of Si with oxygen. Therefore, the amount of Si is limited to 1% or less.

Mn: 0.4 to 2.5%. Mn increases the hardenability and promotes the formation of bainite,
It is an element that contributes to high strength and high ductility of rails.
However, if the content is less than 0.4%, a sufficient effect cannot be obtained, and if it exceeds 2.5%, a martensitic structure is easily generated due to micro-segregation of steel, and hardening or embrittlement occurs during heat treatment and welding, resulting in deterioration of the material. . Therefore, the amount of Mn is limited to 0.4 to 2.5%.

P: 0.035% or less. P exceeding 0.035% deteriorates toughness. Therefore, the P content is limited to 0.035% or less.

S: 0.035% or less. S is mainly present in the steel in the form of inclusions, but if it exceeds 0.035%, the amount of these inclusions increases significantly and causes embrittlement of the material.
Limited to 35% or less.

In the present invention, one or more of the following steel components are added in addition to the above steel components as required.

Cr: 1.5% or less. Cr is an element for increasing the hardenability and promoting the formation of bainite in the structure. However, if it exceeds 1.5%, it may be a factor that impairs weldability due to the high bonding force of Cr with oxygen. Therefore, when added, the Cr content is limited to 1.5% or less.

Cu: 1% or less. Cu is an element for achieving higher strength by solid solution strengthening. However, if it exceeds 1%, Cu cracks occur. Therefore, when adding
Limit the amount to 1% or less.

Ni: 1% or less. Ni is an element for further improving toughness and increasing strength by solid solution strengthening. In addition, in order to suppress Cu cracking by adding Cu in combination, it is desirable to add Ni when Cu is added. However, if it exceeds 1%, the effect of improving strength and toughness saturates, which is economically undesirable. Therefore, when adding, the amount of Ni is limited to 1% or less.

Mo: 2% or less. Mo is an element for promoting the formation of bainite in the structure. However, when the content exceeds 2%, the hardenability becomes too high, so that a martensite structure is easily generated, and the flaking resistance is lowered. Therefore, when adding, the amount of Mo is limited to 2% or less.

V: 0.2% or less. Since V precipitates after rolling in association with C in steel, the hardness of the rail head is increased by precipitation strengthening to the inside of the rail head, improving wear resistance, improving the stability of wear resistance, and improving rail life. It is effective to further extend. However, even if added in excess of 0.2%, the effect is saturated, which is not economically preferable. Therefore, when adding, the amount of V is limited to 0.2% or less.

The other components are substantially Fe. Substantially Fe means that the substance containing other trace elements including unavoidable impurities can be included in the scope of the present invention, as long as the effects of the present invention are not lost.

Next, the reasons for limiting the manufacturing conditions will be described. In the present invention, a steel having the above-described composition is heated and hot-rolled to form a rail, and the head of the rail is controlled and cooled in a cooling process after the hot rolling to manufacture a bainite-based rail.

The heating temperature before rolling is preferably from 1100 ° C. to 1350 ° C. In order to accurately roll the rail shape,
High-temperature heating that reduces deformation resistance is effective, and specifically,
This is because 1100 ° C. or higher is preferable. Further, if the heating is performed at a high temperature exceeding 1350 ° C., there is a possibility that scratches may occur during heating or rolling, so the upper limit of the heating temperature is preferably 1350 ° C. or less.

The rolling end temperature is preferably 750 ° C. or higher. If the rolling end temperature is too low, the deformation resistance will increase,
This is because it may be difficult to satisfy the dimensional accuracy of the rail shape. Therefore, the rolling end temperature is preferably set to 750 ° C. or higher.

The rail head is cooled at an average cooling rate of 1 ° C./s or more and 7 ° C./s or less at an average cooling rate, with the start of controlled cooling from the austenite region and the end of controlled cooling until completion of bainite transformation. The reason is that if the controlled cooling start temperature is in the two-phase region of austenite / ferrite, the resulting microstructure becomes a mixed structure of ferrite and bainite or martensite and bainite, and the wear resistance and flaking resistance are reduced. is there. Unless the temperature range until the bainite transformation is completed is not controlled and cooled, the microstructure and hardness distribution are uneven at the rail head, and the stability of wear resistance and the stability of flaking are obtained. Because there is no. In order to obtain a microstructure with excellent wear and flaking resistance, the average cooling rate during cooling must be 1
It is necessary to secure at least ° C / s. The cooling rate is 7 ° C
If it exceeds / s, martensite is easily generated,
Stable abrasion resistance and flaking resistance cannot be obtained. Therefore, the average cooling rate is set to 7 ° C./s or less.

It is necessary to start the controlled cooling from the austenite region.
The temperature may be higher than or equal to ° C. Unless controlled cooling is started from a temperature higher than the temperature at which bainite transformation or ferrite transformation starts, a high-strength bainite structure excellent in wear resistance and flaking resistance may not be obtained, but the case of the component range of the present invention If the cooling rate is about 450 ° C. or higher, the austenite region is not present and transformation does not start at 450 ° C. or higher. Therefore, if the controlled cooling start temperature is 450 ° C. or higher, a stable bainite structure can be obtained stably. Therefore, the controlled cooling start temperature is preferably set to 450 ° C. or higher.

It is necessary to perform the controlled cooling until the bainite transformation is completed.
The temperature may be 400 ° C or lower and 150 ° C or higher. In most cases, the bainite transformation is completed at 400 ° C or lower and 200 ° C or higher within the range of the components proposed in the present invention. However, when controlled cooling to below 150 ° C, distortion may occur. By performing controlled cooling until the bainite transformation is completed, the inside of the rail head stably exhibits high-strength bainite. Therefore, the controlled cooling stop temperature is preferably set to 150 ° C. or more and 400 ° C. or less.

[0030]

Embodiments of the present invention will be described below.

Example 1 A steel (steel type: A-1 to A-27) having the components shown in Table 1 was heated to 1250 ° C., rolled at 920 ° C., formed into a rail material, and then subjected to controlled cooling. Starting temperature 850
℃, cooling rate 1.5 ~ 3 ℃ / sec, control cooling stop temperature 300 ℃, the test piece was taken from the rail manufactured by cooling the head under the conditions of wear, flaking occurrence life, test load 98N performing measurement of the Vickers hardness (H _V) was evaluated wear resistance and flaking resistance.

With respect to wear resistance, it is most desirable to evaluate the amount of wear by actually laying the rails, but in this embodiment, a comparative test in which actual contact conditions were simulated using a Nishihara type wear tester. Was done. A 30mm diameter Nishihara type abrasion test specimen was taken from the rail head and contact pressure: 1.4GP
a, The amount of wear after 100,000 rotations was measured under a slip condition of -10% and drying conditions. The pearlite-type heat-treated rail with 0.71% (0.71% C) of C currently used as the rail is used as the standard for wear, and it is 3% less than the pearlite-type heat-treated rail (0.80 g / h). When a value with a small amount of wear was obtained, it was determined that the wear resistance was sufficiently improved.

Regarding the flaking resistance, it is most preferable to actually lay the rail in a curved section to evaluate the occurrence of flaking. In the present embodiment, similarly to the abrasion resistance, the Nishihara-type abrasion test is performed. A comparative test was performed using a machine to simulate the contact conditions where actual flaking occurs. A Nishihara type abrasion test piece with a diameter of 30 mm and a radius of curvature of 15 mm applied to the contact surface was sampled from the rail head, and contact pressure: 2.1 GPa, slip rate: -20%, 25,000 rotations under oil lubrication conditions The contact surface of the test piece was observed each time, and the time at which cracks or peeling occurred was defined as the flaking occurrence life. The 0.71% C pearlite heat-treated rail currently used as a rail is used as the standard for the flaking generation life. The flaking generation life is 1.5 times longer than the flaking generation life (3.5h) of this pearlite heat-treated rail. If it was, it was judged that the flaking resistance was sufficiently improved.

Table 2 shows the measurement results of the amount of wear, the life of occurrence of flaking, and the hardness. Steel type A-27 is a pearlitic heat treated rail of 0.71% C, and No.1-27 is a reference material. No. 1-1, 2, 3, 2 using steel types A-1, 2, 3, 21, 22, 23, 24 with low C content
In Examples 1, 22, 23, and 24, the wear amount was not reduced by 3% or more compared to Reference Material No. 1-27, and it could not be said that the wear resistance was improved. In addition, C types higher than the present invention steel types A-8, 9, 1
No. 1-8, 9, 10, 12, 1 using 0, 12, 14, 18, 19, 26
For 4, 18, 19, and 26, the microstructure exhibited a coarse pearlite or a mixed structure of martensite and bainite, and the flaking generation life was less than 1.5 times the reference material. No.1-13 for steel type A-13 due to low Mn content
Had poor abrasion resistance. In addition, steel type A-25 has a large amount of Mo,
No. 1-25 had a high hardness due to a mixed structure of martensite and bainite, but was inferior in flaking resistance. On the other hand, steel types A-4, 5, 6, 7, 11, 15, 16, 17, and 20 whose components satisfy the claims were used.
No. 1-4, 5, 6, 7, 11, 15, 16, 17, 20 are abrasion resistant,
Excellent properties were shown in all of the flaking resistance.

[0035]

[Table 1]

[0036]

[Table 2]

FIG. 1 is based on the data of Nos. 1-1 to 1-10,
2 is a diagram illustrating the effect of the amount of C on abrasion resistance and flaking resistance. C content within the scope of the present invention: 0.3 to 0.4%
It can be seen that the bainite steel obtained excellent characteristics in both wear resistance and flaking resistance.

Example 2 A steel having the components shown in Table 3 was heated to 1280 ° C., hot rolling was completed at 900 ° C., a controlled cooling start temperature of 800 ° C., a cooling rate of 0.5 to 3 ° C./sec, Using the rail cooled under the condition of the controlled cooling stop temperature of 350 ° C., the wear amount, the flaking generation life, and the hardness were measured in the same manner as in Example 1. Steel types B-1 to B-11 mainly change the content of Nb, and steel type B-12 is a 0.71% C reference material currently used as a pearlite-type heat treatment rail. Table 4 shows the measurement results of the amount of wear, the life of flaking, and the hardness. FIG. 2 shows the relationship between the amount of Nb, the amount of wear, and the life of the occurrence of flaking from the data of Nos. 2-1 to 2-11, and illustrates the effect of the amount of Nb on the wear resistance and the resistance to flaking. It is.

[0039]

[Table 3]

[0040]

[Table 4]

As is clear from Table 4 and FIG. 2, No. 2-1 and No. 2 using steel types B-1 and B2 having a low Nb content have a large amount of wear and are compared with No. 2-12 of the reference material. And a reduction in wear of more than 3%
No more than twice the life of flaking occurred. Therefore, neither the wear resistance nor the flaking resistance was improved.
In Nos. 2-3 to 2-11 containing 0.05% or more of Nb, excellent abrasion resistance and flaking resistance were obtained. Since the weldability decreases when Nb is added at 0.5% or more, it was found that good wear resistance and flaking resistance can be obtained by adding 0.05% to 0.5% Nb. However, the effect of improving wear resistance by adding Nb was saturated at about 0.08%, and the effect of improving flaking resistance was saturated at about 0.2%. Therefore, from an economic viewpoint, it is preferable to set the Nb addition amount: 0.05 to 0.2%.

(Example 3) As shown in Table 5, steel type C-1 whose composition is within the scope of the present invention and 0.71% C steel type C-type currently used as a pearlite-type heat-treated rail as a reference material.
The steel of No. 2 was manufactured by changing the control cooling start temperature, the control cooling stop temperature, and the average cooling rate of the control cooling as shown in Table 6, and the rails were manufactured. A measurement was made. Regarding the hardness of the rail, a test piece was also taken from the inside of the rail, and the hardness of the rail surface (surface at the top of the rail) and the inside (12.5 mm below the top of the rail) was measured. Some were evaluated as having high stability of wear resistance. Table 6 shows the manufacturing conditions and the amount of abrasion, the life of flaking, the difference between internal hardness and surface hardness.
(Nos. 3-1 to 3-11).

[0043]

[Table 5]

[0044]

[Table 6]

As is clear from Table 6, Nos. 3-1 and 2 having a low cooling start temperature had a mixed structure of ferrite and bainite, and were not only inferior in wear resistance but also no improvement in flaking resistance. Was. Nos. 3-3 and 4 with slow cooling rates also had a mixed structure of ferrite and bainite, and were inferior in wear resistance. No.3- Cooling rate is remarkably fast exceeding 7 ℃ / s
5, No. 3-6 with extremely low cooling stop temperature has a structure containing martensite, poor abrasion resistance and flaking resistance, a large difference between internal hardness and surface hardness, abrasion resistance The stability was also low. No. 3-7, which has a high cooling stop temperature, had poor wear resistance and low stability of wear resistance. Cooling stop temperature is slightly higher than the range of the present invention No. 3-9, abrasion resistance, although flaking resistance is excellent,
The stability of wear resistance was low. On the other hand, No. 3-8 and No. 10 whose production conditions all satisfy the range of the present invention have good abrasion resistance, flaking resistance, and stability of abrasion resistance.

[0046]

According to the present invention, it is possible to manufacture a high-strength bainite type rail excellent in abrasion resistance and flaking resistance suitable for high axle heavy railways and the like. In addition, it is possible to manufacture a high-strength bainite type rail having a small difference in hardness between the surface and the inside of the rail and having higher wear resistance and stability than conventional bainite type rails, so that the rail life is further improved. .

[Brief description of the drawings]

FIG. 1 is a view showing the influence of the amount of C on abrasion resistance and flaking resistance.

FIG. 2 is a graph showing the effect of the amount of Nb on wear resistance and flaking resistance.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat ゛ (Reference) C22C 38/12 C22C 38/12 38/58 38/58 F-term (Reference) 4E002 AC06 BC07 BC10 CB01 4K032 AA05 AA11 AA12 AA14 AA16 AA17 AA19 AA20 AA22 AA23 AA27 AA29 AA31 AA36 BA00 CA02 CA03 CC03 CC04 CD02 4K042 AA04 BA01 BA03 CA05 CA06 CA08 CA09 CA13 DA06 DE01 DE06

Claims (3)

[Claims] C .: 0.3 to 0.4% by mass, S
i: 1% or less, Mn: 0.4 to 2.5%, Nb: 0.0
5 to 0.5%, P: 0.035% or less, S: 0.035
% Or less, and a rail material is formed by hot rolling, and in a cooling process after the hot rolling, the temperature range of the rail head of the rail material from the austenite region to the completion of bainite transformation is reduced. A method for manufacturing a high-strength bainite-type rail, wherein the rail is cooled at an average cooling rate of 1 ° C / s or more and 7 ° C / s or less. 2. C: 0.3 to 0.4% by mass, S:
i: 1% or less, Mn: 0.4 to 2.5%, Nb: 0.0
5 to 0.5%, P: 0.035% or less, S: 0.035
%: Cr: 1.5% or less, Cu: 1%
%, Ni: 1% or less, Mo: 2% or less, V: 0.2
% Or less of steel containing at least one of the following types of steel, hot-rolled to form a rail material, and then the head of the rail material is heated to a temperature range from the austenite region to completion of bainite transformation. A method for manufacturing a high-strength bainite-type rail, wherein the rail is cooled at an average cooling rate of 1 ° C / s or more and 7 ° C / s or less. 3. The heating temperature in hot rolling is 1100 ° C.
The temperature is not less than 1350 ° C., the rolling end temperature is 750 ° C. or more, and in the cooling process after hot rolling, the average cooling rate is from at least 450 ° C. to 400 ° C. to 150 ° C. The method for producing a high-strength bainite-type rail according to claim 1 or 2, wherein the heating is performed at 1 ° C / s or more and 7 ° C / s or less.