JPH06316727A - Production of bainitic steel rail excellent in surface damage resistance - Google Patents

Abstract

PURPOSE:To prevent the occurrence of rolling fatigue damage at the surface of a railhead and to prolong the service life of a rail for rapid transit railway. CONSTITUTION:A steel, which has a composition containing 0.15-0.45% C, 0.15-1.00% Si, 0.20-2.00% Mn, and 0.20-2.00% Cr and containing, besides the above, one or >=2 elements among Mo, Cu, Ni, Ti, V, Nb, and B, if necessary, is used. The bainitic steel rail excellent in surface damage resistance can be obtained by subjecting the railhead, after the completion of hot rolling or after heating up to a high temp. for the purpose of heat treatment, to accelerated cooling through the region between the austenite region temp. and the ordinary temp. at a rate of (1 to 10) deg.C/sec. The resulting rail has superior surface damage resistance as compared with the conventional rails, and the service life of the rail can be remarkably improved.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a rail having a bainite structure which is effective in improving rolling fatigue damage resistance of a rail head surface required in a high speed operation section of a passenger railway.

[0002]

2. Description of the Related Art In recent years, as a means for improving the efficiency of rail transportation, an increase in on-vehicle load and an increase in train operating speed have been attempted. Such efficient rail transportation means that the rail usage environment becomes severe, and further improvement of rail materials has been demanded. Specifically, the wear of the rail laid in the sharp curve section sharply increases, and fatigue damage frequently occurs from inside the rail gauge corner (GC) which is the main contact position between the rail and the wheel. As a countermeasure against this, the following method has been conventionally adopted. An as-rolled alloy steel rail containing a large amount of alloy elements such as Cr and Mo (Japanese Patent Laid-Open No. 50-140316). Heat-treated rails manufactured by accelerating cooling of the rail head or the whole without adding an alloy (Japanese Patent Laid-Open Publication No. 5 (1999) -58242).
5-23885). A low alloy heat treatment rail (Japanese Patent Publication No. 59-19173) in which not only wear resistance and damage resistance but also a decrease in hardness of a welded portion is improved by adding an alloy having a relatively low content. The feature of these rails is that they are high-strength rails that exhibit a fine pearlite structure due to the high carbon content steel, and their purpose is to improve wear resistance and internal fatigue damage resistance. .

On the other hand, in rails in straight and gentle curve sections where wear or internal fatigue damage does not pose a problem, rolling fatigue damage occurs on the rail head surface due to repeated contact between the wheel and the rail, and peeling or fatigue generated from the rail head surface occurs. There are some cases where cracks branch inside the rail head and cause transverse crack damage. This typical damage is called "crown surface shelling" or "dark spot damage" that occurs mainly in straight sections of high-speed railways such as the Shinkansen. However, in such a section, despite the occurrence of the damage as described above, the conventional as-rolled rail having a pearlite structure is used.

Rolling fatigue damage originating from the rail head surface is generated mainly on a rail in a straight line or a relaxation curve section of a passenger railway after a certain specific period (train passing tonnage). As a result of investigating the cause of the above-mentioned damage, the present inventors have confirmed that the cause is that a fatigue damage layer caused by repeated contact between the wheel and the rail accumulates on the rail head surface portion. As a measure against this,
There is a method of periodically grinding the rail head surface with a grinder or the like, but there are problems that the grinding vehicle and its working cost are expensive, and that the grinding time cannot be sufficient due to the train operation interval. Another possible solution is to increase the wear rate of the rail head surface and remove this fatigue layer by wear before fatigue damage accumulates. Generally, the wear characteristics of rail steels are dominated by the hardness of the pearlite structure, and the hardness of the rail steels may be reduced to promote wear. However, if the hardness is simply reduced, plastic deformation is generated on the surface of the rail head, and the rail head surface damage such as cracks and peeling frequently occurs, so that the above-mentioned damage occurs in rail steel with a pearlite structure. Was difficult to prevent.

[0005]

The pearlite structure that has been used as a rail steel has been a layered structure of a low hardness ferrite structure and a plate-like hard cementite structure, and is soft on the raceway surface through which the wheel passes. The ferrite structure is squeezed out, and only hard cementite is piled up,
Work hardening is added to ensure wear resistance. However, at the same time, there is a problem that a laminar structure flow (metal flow) is always generated in the rail inner direction on the track surface, and crack damage occurs along the metal flow depending on the lubrication and stress conditions.

On the other hand, with a ferrite structure and a ferrite / pearlite mixed structure having a larger amount of wear than the pearlite structure, it is difficult to secure strength as a rail material, and at the same time, it is difficult to maintain wear resistance at the head. there were. In addition, since the bainite structure, which has a larger amount of wear than the pearlite structure and is relatively easy to secure strength, is a structure in which granular fine cementite is dispersed in a soft ferrite structure ground, so the cementite also wears along with the ferrite ground when the wheel is running. It is possible to remove the fatigue damage layer at the rail head surface portion by easing it easily and accelerating the wear. However, in order to exhibit a bainite structure as rolled and to secure the strength as a rail material, it is necessary to add a large amount of alloying elements such as Cr, which causes a problem such as an increase in rail component cost. It was

Therefore, in order to solve this problem, the inventors of the present invention set the head of the rail hot-rolled to retain high temperature heat or the head heated to a high temperature for the purpose of heat treatment from the austenite region temperature. It was confirmed by experiments that this problem can be solved by a rail with a low alloy and bainite structure by accelerated cooling up to room temperature. That is, an object of the present invention is to provide a rail excellent in surface damage resistance at low cost by using the above manufacturing process.

[0008]

The present invention achieves the above objects, and the gist of the invention is that C: 0.15 to 0.45% Si: 0.15% by weight.
~ 1.00% Mn: 0.20-2.00% Cr: 0.20
.About.2.00% and further Mo: 0.01-0.60% Cu: 0.05
~ 0.50% Ni: 0.05-2.00% Ti: 0.01
-0.05% V: 0.03-0.30% Nb: 0.01
~ 0.05% B: 0.0005 to 0.0050% of one or more kinds of rails, the balance of which is hot rolled steel having the balance iron and unavoidable impurities, or a rail that retains high temperature heat, or Manufacture of bainite steel rail with excellent surface damage resistance, which is characterized by accelerating cooling the head of a rail heated to a high temperature for the purpose of heat treatment at a temperature of austenite to room temperature at 1 to 10 ° C / sec. Is the law.

The present invention will be described in detail below. First, the reason why the chemical composition of the rail is determined as described above will be described. C is an essential element for ensuring a certain hardness, and if it is less than 0.15%, it becomes difficult to secure the wear resistance as rail steel, and if it exceeds 0.45%, it is harmful to the occurrence of surface damage. Pearlite structure is generated, and the bainite transformation rate is greatly reduced, and a martensite structure, which does not completely complete the bainite transformation and is harmful to the toughness of the rail, is generated. Limited

Si is an element that improves the strength by forming a solid solution in the ferrite matrix in the bainite structure, but if the content is 0.15% or less, the improvement in the strength cannot be expected. Further, if it exceeds 1.00%, surface flaws are likely to occur during rail rolling, and further, island martensite structure is generated in the bainite structure, degrading the toughness of the rail, and therefore 0.15 to 1.00. Limited to%.

Similar to C, Mn has the effect of enhancing the hardenability of steel and making the ferrite grains finer and simultaneously improving the strength and toughness, but if it is less than 0.20%, that effect is small, and if it is 2.00%. If it exceeds, a pearlite structure, which is harmful to the occurrence of surface damage, is generated in the bainite structure.
It was limited to 0 to 2.00%.

[0012] Cr is an important element for finely dispersing cementite in the bainite structure to secure the strength, but if it is less than 0.20%, different structures such as a ferrite structure and a ferrite-pearlite structure are present in the bainite structure. Generated, the wear resistance at the head is greatly reduced. Also,
If it exceeds 2.00%, the bainite transformation rate is significantly reduced, and the martensite structure harmful to the toughness of the rail is generated without completely completing the bainite transformation.
Limited to 2.00%.

In addition, one or more kinds of the following elements are added to the rail manufactured with the above-mentioned component composition, if necessary, for the purpose of preventing toughness, ductility, strength, and material deterioration during welding. Mo: 0.01-0.60% Cu: 0.05
~ 0.50% Ni: 0.05-2.00% Ti: 0.01
-0.05% V: 0.03-0.30% Nb: 0.01
~ 0.05% B: 0.0005-0.0050%

Next, the reason why these chemical components are defined as described above will be explained. Mo is an element that is indispensable for strengthening / stabilizing the bainite structure and preventing temper embrittlement during welding, like Cr, but if its content is less than 0.01%, its effect is not sufficient and exceeds 0.60%. Since the bainite transformation rate is greatly reduced and the bainite transformation is not completely completed and a martensitic structure harmful to the toughness of the rail is generated, the content is limited to 0.01 to 0.60%. Cu
Is an element that improves the strength of steel without impairing its toughness. The effect is greatest in the range of 0.05 to 0.50%, and if it exceeds 0.50%, red heat embrittlement occurs, so the range was limited to 0.05 to 0.50%. Ni is an element that stabilizes austenite grains, and has the effect of lowering the bainite transformation temperature, refining the bainite structure, and improving the toughness, but if it is less than 0.05%, the effect is extremely small, and 2.00 %, The bainite transformation rate is significantly reduced, and a martensitic structure harmful to the toughness of the rail is easily generated.
The range was limited to 0%.

By utilizing the fact that precipitated Ti (C, N) does not dissolve even at high temperatures, Ti contributes to the refinement of austenite crystal grains during rolling and heating of the rail. But 0.0
If less than 1%, the effect is small, and if more than 0.05%, T
The iN is coarsened and becomes a nucleus of fatigue damage inside the rail, which is harmful and is limited to 0.01 to 0.05%. V is V
The precipitation of (C, N) can strengthen the bainite structure, but if 0.03% or less, the effect is not sufficient, and if V is 0.30% or more, addition of V causes coarsening of V (C, N). To cause embrittlement rather than 0.03 to
It was limited to 0.30%. Nb is an austenite grain refinement element and can improve the toughness and ductility of the rail steel, but if it is 0.01% or less, its effect is not sufficient, and if it is 0.05% or more, the intermetallic compound of Nb is present. Since it is generated and causes embrittlement, it is limited to 0.01 to 0.05%. B has an effect of suppressing the generation of ferrite generated from austenite grain boundaries, and is an effective element for stably generating a bainite structure. However, 0.
The effect is weak at 0005% or less, and if 0.0050% or more is added, a coarse compound of B is formed and the rail material is deteriorated, so the content is limited to 0.0005 to 0.0050%.

The rail steel composed of the above-described composition is melted in a commonly used melting furnace such as a converter or an electric furnace, and the molten steel is ingot-segmented or continuously cast,
Further, it is manufactured as a rail through hot rolling. next,
The reason for setting the cooling conditions as described above will be described. Rails composed of the above component composition are melted in a commonly used melting furnace such as a converter or an electric furnace.
In the ingot / segmentation or continuous casting method, the rail for hot rolling the material for rail rolling has high temperature heat, or the head of the rail heated to high temperature for the purpose of heat treatment is changed from austenite temperature to normal temperature. 1-10 ℃ / sec
The rail manufactured by accelerated cooling with a low alloy can stably generate a bainite structure by accelerated cooling.

The reason for setting the cooling rate as described above will be described in detail. First, the reason why accelerated cooling is performed in the range of 1 to 10 ° C./sec from the temperature of austenite to room temperature will be described. The reason for limiting the accelerated cooling rate to the range of 1 to 10 ° C / sec is that when the above component system is cooled at 1 ° C / sec or less, different structures such as a ferrite structure and a pearlite structure are generated in the bainite structure, and It was difficult to secure strength and wear resistance, and further, it was limited to 1 ° C / sec or more in order to induce surface damage. Also, 10
Cooling at ℃ / sec or more causes martensitic transformation without completely ending the bainite transformation, hard martensitic structure is generated in the bainite structure, and adversely affects the toughness of the rail. Limited
That is, in the present invention, by performing accelerated cooling at a temperature of 1 to 10 ° C./sec from the austenite region temperature to room temperature, a low alloy and stable bainite structure can be obtained. A gas-liquid mixture such as air or mist is used as a cooling medium during accelerated cooling. The rail having a bainite structure produced by the method of the present invention as described above has the surface damage resistance required as a rail for a high-speed passenger railway.

[0018]

EXAMPLES Next, examples of the present invention will be described. Table 1 shows the chemical composition and cooling conditions of the present invention steel and the comparative steel. Table 2 shows the hardness of the steels of the present invention and comparative steels, the results of measuring the amount of wear after repeated 500,000 times under dry conditions in the Nishihara-type wear test, and disk test pieces obtained by scaling the shapes of rails and wheels to 1/4. Figure 3 shows the surface damage occurrence life of water-lubricated rolling fatigue test.

The structure of the rail constituent material is as follows. -Invention rail (8) Code A, code B, code C, code D, code E, code F, code G, code H: Rail exhibiting bainite structure manufactured by accelerated cooling of the rail head. Comparative rails (three) Reference code I: Rail exhibiting bainite structure naturally cooled after rolling Reference code J: Hot piece state immediately after completion of rolling or pearlite structure in which only rail head is heat-treated by reheating after completion of rolling Showing the reference sign K: a rail showing a pearlite structure that is naturally cooled after rolling

The test conditions were as follows. Abrasion test conditions (common to all test rails) -Testing machine: Nishihara-type abrasion tester-Test piece shape: Disc-shaped test piece (outer diameter: 30 mm, inner diameter: 1
6mm, thickness: 8mm) ・ Test load: 490N ・ Slip ratio: 9% ・ Mating material: Tempered martensitic steel (Hv35
0) ・ Atmosphere: In air ・ Number of repetitions: 500,000 times

Rolling Fatigue Test ・ Testing Machine: Rolling Fatigue Testing Machine ・ Shape of test piece: Disc-shaped test piece (outer diameter: 200 mm, rail material cross-sectional shape: 1/4 model of 60K rail) ・ Test load: 1.0 ton (Radial load) -Atmosphere: Drying + water lubrication (60cc / min) -Rotational speed: Drying; 100 rpm, water lubrication; 300 rpm-Number of repetitions: 0 to 5000 times in a dry state, and then until water damage causes damage

[0022]

[Table 1]

[0023]

[Table 2]

As is clear from Table 2, the rails A of the present invention,
B, C, D, E, F, G, and H have a large amount of wear and greatly improve the rolling fatigue damage occurrence life, as compared with the rails J and K having the conventional pearlite structure. Further, the rolling fatigue damage occurrence life is greatly improved as compared with the rail I having the as-rolled bainite structure.

[0025]

As described above, according to the present invention, it is possible to provide a rail having a bainite structure excellent in rolling fatigue damage resistance of the rail surface required in a high-speed operation section, and its industrial effect is extremely large. .

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location C22C 38/54

Claims (2)

[Claims] 1. By weight%, C: 0.15 to 0.45% Si: 0.15 to 1.00% Mn: 0.20 to 2.00% Cr: 0.20 to 2.00% However, the rail that holds high temperature heat obtained by hot rolling steel with the balance being iron and inevitable impurities, or the head of the rail that is heated to high temperature for the purpose of heat treatment is A method for manufacturing a bainite steel rail with excellent surface damage resistance, which is characterized by accelerated cooling at 10 ° C / sec. 2. C: 0.15 to 0.45% Si: 0.15 to 1.00% Mn: 0.20 to 2.00% Cr: 0.20 to 2.00% by weight Further, Mo: 0.01 to 0.60% Cu: 0.05 to 0.50% Ni: 0.05 to 2.00% Ti: 0.01 to 0.05% V: 0.03 to 0.30% Nb: 0.01 to 0.05% B: 0.0005 to 0.0050% One or two or more types, with the balance being iron and unavoidable impurities, hot-rolled high temperature Resistance to surface damage, characterized by accelerating cooling of the rail holding the heat of the rail or the head of the rail heated to a high temperature for the purpose of heat treatment at a temperature of 1 to 10 ° C / sec from the austenite temperature to room temperature Excellent bainite steel rail manufacturing method.