JPH09137227A - Production of high wear resistant pearlite rail - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the wear resistance required of a high-load railroad rail. SOLUTION: A steel rail, which has a composition containing >0.85-1.40% C, 0.10-1.00% Si, and 0.10-1.50% Mn and further containing, if necessary, one or >=2 elements among Cr, Mo, V, Nb, Co, and B, and has a high-temp. heat after hot rolling, or a steel rail, heated to high temp. for the purpose of heat treatment, is used. The railhead of this steel rail is subjected to accelerated cooling through the temp. region from austenitic region temp. to 750-600 deg.C at a rate of (>10 to 30) deg.C/sec and successively subjected to controlled cooling through the temp. region from 750-600 deg.C to 550-450 deg.C at a rate of (1 to <10) deg.C/sec. In this rail steel, wear resistance is remarkably improved without deteriorating toughness, as compared with the reference rail steel, by increasing carbon content and simultaneously applying proper heat treatment to form a pearlitic structure.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a pearlite rail having a significantly improved wear resistance required for a heavy rail rail.

[0002]

2. Description of the Related Art As means for increasing the efficiency of rail transportation, train speed has been increased and train load weight has been increased. Such an increase in the efficiency of rail transportation implies a severer use environment for rails, and further improvements in rail materials have been required. Specifically, rails laid on curved sections of heavy-duty railways abroad have rapidly increased in wear, and have become problematic in terms of rail wear life.

However, due to the recent improvement in the heat treatment technology for increasing the strength of rails, the following high-strength (high-hardness) rails having a fine pearlite structure using eutectoid carbon steel have been developed, and have been developed for heavy-duty railways. We have dramatically improved the rail life in curved sections. Heat-treating rail for super heavy load with sorbite structure or fine pearlite structure on the head (Japanese Patent Publication No. 54-2549).
No. 0). A method for manufacturing a low alloy heat treatment rail, in which alloys such as Cr and Nb are added to improve not only wear resistance but also decrease in hardness of welded portions (see Japanese Patent Publication No. 59-19173). The characteristics of these rails are high-strength (high hardness) rails that exhibit a fine pearlite structure made of steel containing eutectoid carbon.
The purpose was to improve wear resistance.

However, in recent years, in overseas heavy-duty railways, in order to further improve the efficiency of railway transportation, the loading of cargo has been strongly promoted. Especially, in the case of a steep curve rail, the rail developed above is used. However, wear resistance cannot be ensured, and the reduction of rail life due to wear has become a problem. Against this background, there has been a demand for the development of a rail having wear resistance higher than that of the current high-strength rail containing eutectoid carbon.

[0005]

In a conventional rail steel exhibiting a pearlite structure of an eutectoid carbon component, in order to improve abrasion resistance, a method of reducing the lamellar spacing in the pearlite structure and improving hardness. Is used. However, in the case of a rail steel exhibiting a pearlite structure of a eutectoid carbon component, the current hardness is the upper limit (Hv420). In addition, there was a problem that a bainite or martensite structure was generated, and the wear resistance and toughness of the rail were reduced. Another possible solution is to use a material with a metal structure that has higher wear resistance than the pearlite structure as the rail steel, but in rolling wear environments such as rails and wheels, it is better than the fine pearlite structure. At present, however, no material that is inexpensive and has excellent wear resistance has been found.

The pearlite structure of the eutectoid carbon component conventionally used as a rail steel has a layered structure of a ferrite structure having a low hardness and a hard cementite structure having a plate shape.
As a result of analyzing the wear mechanism of the pearlite structure, the present inventors first squeeze out a soft ferrite phase by passing through a wheel, and then only a hard cementite phase is laminated immediately below the rolling surface to ensure wear resistance. I confirmed that. Therefore, the present inventors have improved the hardness of the pearlite structure in order to improve the wear resistance, at the same time, increase the carbon content, to secure the wear resistance of the plate-like hard cementite phase in the pearlite structure. It was found by experiments that the wear resistance was dramatically improved by increasing the ratio and increasing the cementite phase density just below the rolling surface.

However, if the amount of carbon is increased to improve wear resistance, proeutectoid cementite is likely to be formed on the rail head, and there is a problem that ductility and toughness of the rail are greatly reduced.

It is known that the pro-eutectoid cementite formed when the amount of carbon is increased is formed in a high temperature region where the cooling rate is slow and before the pearlite transformation starts. Therefore, as a method of preventing the formation of this pro-eutectoid cementite, there is also a method of subjecting a rail immediately after hot rolling or a reheated rail to accelerated cooling. However, depending on the composition of the components and the segregation state of the slab, a small amount of pro-eutectoid cementite is generated in the old austenite grain boundaries even if accelerated cooling is performed, and the toughness and ductility at the rail head, which is important as the basic performance of the rail, is reduced. There was a problem of making it happen. However, in spite of such problems, a rail manufacturing method for preventing the formation of a small amount of pro-eutectoid cementite and for stably forming a pearlite structure having a high cementite ratio and a high hardness is It has not been fully examined.

Therefore, the present inventors have conducted experiments to examine a cooling method for suppressing the formation of pro-eutectoid cementite. As a result, by accelerating and cooling the high temperature region immediately after hot rolling or just before the start of pearlite transformation in which proeutectoid cementite is formed, the reheated rail head is accelerated at a high cooling rate of more than 10 to 30 ° C./sec. It was confirmed that the formation of precipitated cementite can be suppressed. Furthermore, the present inventors examined a method of increasing the hardness of the pearlite structure after suppressing the formation of pro-eutectoid cementite. As a result, by performing controlled cooling at a cooling rate of 1 to 10 ° C / sec immediately after accelerated cooling until the start of pearlite transformation, it was confirmed that a pearlite structure with high hardness and excellent wear resistance was stably generated. Confirmed by.

From the above results, in order to improve the wear resistance of the rail and to secure the toughness and ductility, the present inventors first increase the carbon content of the rail steel and, at the same time, immediately after hot rolling or reheating. By accelerating cooling the rail head and then performing controlled cooling, the formation of proeutectoid cementite, which is harmful to the toughness and ductility of the rail, is prevented, and the pearlite structure has a high cementite ratio and excellent wear resistance. It was confirmed that a high-strength rail exhibiting the above can be manufactured. That is, it is an object of the present invention to provide a rail having a significantly improved wear resistance required for a heavy-duty railway rail at low cost.

[0011]

The present invention achieves the above-mentioned object, and the gist of the invention is% by weight.
And C: more than 0.85 to 1.40%, Si: 0.10.
.About.1.00%, Mn: 0.10 to 1.50%, and if necessary, Cr: 0.05 to 1.00.
%, Mo: 0.01 to 0.20%, V: 0.02
~ 0.30%, Nb: 0.002-0.05%, C
o: 0.10 to 2.00%, B: 0.0005 to
Steel rails containing 0.005% of 1 or 2 or more types, the balance of which is iron and unavoidable impurities, hot-rolled, steel rails that retain high temperature heat, or heated to high temperature for the purpose of heat treatment. The steel rail head is heated from austenite temperature to a temperature between 750 and 600 ° C in excess of 10 to 3
Accelerated cooling at 0 ° C / sec, followed by 1-1 from 750 to 600 ° C to 550 to 450 ° C
It is a method for producing a high wear-resistant pearlite rail characterized by controlled cooling at less than 0 ° C / sec.

Hereinafter, the present invention will be described in detail. First, the reason why the chemical composition of the rail is limited as described above in the present invention will be described. C is an effective element that promotes pearlite transformation and secures wear resistance. As a normal rail steel, the C content is 0.60 to 0.85%.
However, if the C content is 0.85% or less, the cementite density in the pearlite structure for improving wear resistance cannot be ensured, and the proeutectoid ferrite that becomes the starting point of rail head internal fatigue damage is added. Are easily generated. Further, when the C content exceeds 1.40%, proeutectoid cementite is generated at the rail head even if the above heat treatment is performed, and the ductility and toughness decrease, so the C content is set to more than 0.85 to 1.40%. Limited

Si is an element which improves strength by solid solution hardening into a ferrite phase in the pearlite structure.
If it is less than 0.10%, the effect cannot be expected sufficiently, and if it exceeds 1.00%, the rail becomes brittle and the weldability decreases, so the Si content is limited to 0.10 to 1.00%. did.

Mn is an element that lowers the pearlite transformation temperature and contributes to the strengthening by increasing the hardenability and further suppresses the formation of pro-eutectoid cementite.
If the content is less than 0.10%, the effect is small, the hardness of the rail head after heat treatment is lowered, and pro-eutectoid cementite is easily generated. Further, when the content exceeds 1.50%, a martensite structure harmful to the toughness of the rail is easily generated, so the Mn content is limited to 0.10 to 1.50%. In addition,
When Mn exceeds 1.00%, in the high-carbon component system,
Depending on the combination of the added elements and the cooling conditions, a minute amount of martensite structure may be generated in the segregated portion. This small amount of martensite structure does not significantly affect the toughness and wear resistance of the rail, but in order to stably generate the pearlite structure during heat treatment, the Mn content is 0.10 to 0.10.
It is desirable to set it in the range of 1.00%.

In addition, one or more of the following elements are added to the rail manufactured with the above-mentioned component composition, if necessary, for the purpose of improving strength, ductility and toughness. Cr: 0.05-1.00%, Mo: 0.01-
0.20%, V: 0.02-0.30%, Nb:
0.002-0.05%, Co: 0.10-2.00
%, B: 0.0005 to 0.005%

Next, the reason why these chemical components are defined as described above will be explained. Cr is an element that raises the equilibrium transformation point of pearlite and, as a result, makes the pearlite structure finer and contributes to higher strength, and at the same time enhances the wear resistance by strengthening the cementite phase in the pearlite structure. If less than 0.05%, its effect is small, and excessive addition exceeding 1.00% produces a large amount of martensite structure and embrittles the steel, so the Cr addition amount is limited to 0.05 to 1.00%. did. In addition, when Cr is 0.
If it exceeds 60%, in the high-carbon component system, a small amount of martensite structure may be generated in the segregated portion depending on the combination of the added elements and the cooling conditions. This small amount of martensite structure does not significantly affect the toughness and wear resistance of the rail, but in order to stably generate the pearlite structure during heat treatment, the Cr content is set to 0.05 to 0.60%. Is desirable.

Mo is an element which, like Cr, raises the equilibrium transformation point of pearlite and, as a result, contributes to higher strength by making the pearlite structure finer and improves wear resistance, but if it is less than 0.01%. The effect is small, 0.2
Excessive addition of more than 0% lowers the pearlite transformation rate and facilitates the formation of a martensitic structure detrimental to toughness, so the Mo addition amount was limited to 0.01 to 0.20%.

V increases the strength by precipitation hardening due to V carbon / nitride produced in the cooling process during hot rolling, and further suppresses the growth of crystal grains when heat treatment is performed at a high temperature. It is an effective component for refining austenite grains and improving the strength and toughness required for rails, but if it is less than 0.02%, the effect cannot be expected,
Since no further effect can be expected even if it exceeds 30%, the V content is limited to 0.02 to 0.30%.

Similar to V, Nb is an effective element that forms Nb carbon / nitride to refine austenite grains, and its austenite grain growth suppressing effect is up to a temperature range higher than V (near 1200 ° C.). Acts to improve the ductility and toughness of the rail. The effect cannot be expected if it is less than 0.002%, and further effect cannot be expected even if an excessive addition exceeding 0.050% is performed. Therefore, the Nb amount is set to 0.1.
002 to 0.050%.

Co is an element which increases the transformation energy of pearlite and makes the pearlite structure finer to improve the strength, but if it is less than 0.10%, its effect cannot be expected, and 2.00%. The effect reaches the saturation region even if an excessive amount of Co is added, so that the Co content is set to 0.
Limited to 10-2.00%.

[0021] B has an effect of suppressing the pro-eutectoid cementite formed from the former austenite grain boundaries, and is an element effective for stably forming the pearlite structure. However, if less than 0.0005%, the effect is weak, and 0.005%.
If added in excess of 5%, coarse carbon boride of B will be formed, and the ductility and toughness of the rail will be deteriorated.
It was limited to 005 to 0.005%.

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,
This hot-rolled rail that retains high-temperature heat, or the head of the rail that has been heated to a high temperature for the purpose of heat treatment, is accelerated and cooled, and then controlled cooling is performed to harden the pearlite structure of the rail head. Improve the quality.

Next, the reason why each cooling stop temperature range and the accelerated cooling rate are set as described above will be described in detail. First, from the austenite region temperature to the accelerated cooling stop temperature 750
The reason why accelerated cooling is performed at a temperature of up to 600 ° C at 10 to 30 ° C / sec will be described.

If accelerated cooling is stopped at a temperature higher than 750 ° C., proeutectoid cementite is formed in the high temperature region during the subsequent controlled cooling, and the toughness and ductility of the rail are greatly reduced, so the temperature was limited to 750 ° C. or lower. Further, if accelerated cooling is performed to less than 600 ° C., the pearlite transformation does not end during the subsequent controlled cooling, and a different structure such as martensite or bainite, which is harmful to the toughness and wear resistance of the rail, is easily generated. Limited to above ℃.

The reason for limiting the accelerated cooling rate to more than 10 to 30 ° C./sec is that when the temperature is 10 ° C./sec or less, a small amount of proeutectoid cementite is generated in the high temperature region during accelerated cooling depending on the component system, and Since the toughness and ductility at the head are reduced,
Limited to over 10 ° C / sec. In addition, when accelerated cooling is performed at a cooling rate of over 30 ° C / sec, pearlite transformation does not end during subsequent controlled cooling, and different structures such as martensite and bainite that are harmful to the toughness and wear resistance of rails are generated. Since it becomes easy, it was limited to 1 to 10 ° C / sec.

Next, the temperature from 750 to 600 ° C. to the controlled cooling stop temperature 550 to 450 ° C.
The reason for controlled cooling at less than 10 ° C / sec will be described.
If the controlled cooling is stopped at a temperature higher than 550 ° C, a large amount of coarse and low hardness pearlite structure is generated immediately after the controlled cooling, and the hardness required for the wear resistance of the rail head cannot be secured. Limited to: Further, if controlled cooling is performed to less than 450 ° C., sufficient natural recuperation from the inside of the rail cannot be expected after accelerated cooling, and a martensite structure harmful to the toughness of the rail is generated in the segregation portion, etc.
It was limited to 0 ° C or higher.

The controlled cooling rate is limited to 1 to less than 10 ° C./sec, that is, when the controlled cooling rate is less than 1 ° C./sec, pearlite is coarse and has a low hardness in a high temperature region during controlled cooling. Since a large amount of structure is generated and the hardness required for the wear resistance of the rail head cannot be secured, the hardness is limited to 1 ° C / sec or more. Further, if controlled cooling is performed at a cooling rate of 10 ° C / sec or more, pearlite transformation does not end during controlled cooling, and martens which is harmful to rail toughness and wear resistance during controlled cooling and in the natural recuperation region thereafter. Since different structures such as sites and bainite are generated, it is limited to less than 1 to 10 ° C / sec.

Therefore, in order to manufacture a rail having a pearlite structure and excellent in wear resistance, as shown in FIG. 1, the austenite temperature is controlled so that proeutectoid cementite harmful to the toughness and ductility of the rail is not formed. 750-600
Accelerated cooling up to a temperature between 10 ℃ and 10 ℃ ～ 30 ℃ / sec,
Further, in order not to form a coarse and low hardness pearlite structure and martensite or bainite structure, which is harmful to toughness and wear resistance, a temperature between 750 to 600 ° C. and 550 to 450 ° C. Up to 1-10 ℃ / sec
It is necessary to perform controlled cooling at a temperature lower than the above and to stably generate a pearlite structure having high hardness in a low temperature range.

Depending on the selection of the component system and the cooling rate, the pearlite transformation starts during accelerated cooling from the austenite region temperature, and the transformation is completed in the subsequent controlled cooling region, as well as during and after controlled cooling. The pearlite transformation may start in the thermal region and the transformation may be completed. However, since the pearlite structure generated in the accelerated cooling stop temperature range and the controlled cooling stop range has a high hardness and does not significantly affect the wear resistance of the rail, the pearlite structure of the present invention is austenite. Both the pearlite structure generated in the accelerated cooling region from the zone temperature and the pearlite structure generated in the controlled cooling region and the natural recuperation region after the accelerated cooling are included.

It is desirable to use a gas-liquid mixture such as air or mist as the cooling medium at the time of accelerated cooling, and to ensure wear resistance with respect to the hardness of the rail head after accelerated cooling and controlled cooling. It is desirable that the Hv is 320 or more. In addition, the region of hardness of Hv320 or more,
In order to secure the rail life, as shown in FIG. 2, at least a depth of 2 from the head surface (including the entire head side part) as a starting point in the rail crown a and the rail head corner b.
It is desirable that the range is 0 mm.

[0031]

Next, an embodiment of the present invention will be described. Tables 1-1 and 1-2 show the chemical composition and cooling conditions of the rail steel of the present invention. Further, Table 2 shows the hardness and structure of the rail head after cooling, the wear characteristic evaluation result of the rail head material by the Nishihara abrasion tester shown in FIG. 3, and the rail head impact characteristic by the Charpy impact tester. The evaluation results are also shown.
In the figure, 1 is a rail test piece, 2 is a mating member, and 3 is a cooling nozzle. In addition, Table 3-1 shows the chemical composition and cooling conditions of the comparative rail steel. Further, Table 3-2 shows the hardness and structure of the rail head after cooling, and the wear characteristic evaluation result of the rail head material by the Nishihara wear tester shown in FIG.
The results of rail head impact characteristics evaluation using a Charpy impact tester are also shown.

The structure of the rail is as follows. -Invention rail steel (16 pieces) Codes: A to P: Heat-treated rails in which the rail head is subjected to accelerated cooling within the above-mentioned limited range and subsequently controlled cooling within the above-mentioned component range. -Comparative rail steel (7 pieces) Codes: Q to W: Comparative rails (Q to S) made of eutectoid carbon-containing steel and the rail head within the above-mentioned component range were subjected to accelerated cooling or controlled cooling outside the above-mentioned limited range. Heat treatment rail (T-W).

The wear test conditions were as follows.・ Testing machine: Nishihara abrasion tester (see Fig. 3) ・ Shape of test piece: Disc-shaped test piece (outer diameter: 30 mm, thickness: 8 mm) ・ Test load: 686N ・ Slip ratio: 20% ・ Mating material: perlite Steel (Hv390) -Atmosphere: In the air-Cooling: Forced cooling with compressed air (flow rate: 100 Nl / min) -Number of repetitions: 700,000 times

The impact test conditions were as follows.・ Test piece: JIS No. 2 mm U-notch Charpy impact test piece ・ Test temperature: Room temperature (+ 20 ° C)

[0035]

[Table 1]

[0036]

[Table 2]

[0037]

[Table 3]

[0038]

[Table 4]

[0039]

[Table 5]

[0040]

The relationship between head hardness (Hv) and wear amount (g / 700,000 cycles) of the present invention and comparative rail steel is shown in FIG. 4, and the relationship between impact value and wear resistance amount is shown in FIG. It was shown to. As shown in FIG. 4, the rail steel of the present invention has a carbon content higher than that of the comparative rail steel containing eutectoid carbon (symbols: Q to S), and at the same time, heat treatment is performed to obtain the same hardness as the comparative rail steel. The amount of wear is small and the wear resistance is greatly improved. Further, as shown in FIG. 5, the rail steel of the present invention is subjected to an appropriate heat treatment on the head to obtain a comparative rail steel (codes: T to W).
Compared with the above, a pearlite structure can be stably obtained even in a high carbon component, and it becomes possible to secure wear resistance and sufficient toughness. As described above, according to the present invention, it is possible to provide a rail having excellent wear resistance in a heavy-duty railway.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a rail heat treatment manufacturing method.

FIG. 2 is a view showing the names of various parts in a rail head axial cross section.

FIG. 3 is a schematic diagram of a Nishihara-type abrasion tester.

FIG. 4 is a diagram comparing the wear test results of the rail steel of the present invention shown in Table 1 and the comparative rail steel shown in Table 2 (symbols: Q to S) in terms of the hardness of the head and the wear amount.

FIG. 5 is a diagram showing the relationship between the impact value and the wear amount of the head of the present invention rail steel shown in Table 1 and the comparative rail steel (reference numeral: T to W) shown in Table 2.

[Explanation of symbols]

 a: Rail top part b: Rail head corner part 1: Rail test piece 2: Mating material 3: Cooling nozzle

Claims (2)

[Claims] 1. By weight%, C: more than 0.85 to 1.40%, Si: 0.10 to 1.00%, Mn: 0.10 to 1.50%, and the balance iron and The head of a steel rail that retains high temperature heat, or the head of a steel rail that is heated to a high temperature for the purpose of heat treatment, is obtained by hot rolling steel consisting of unavoidable impurities. Temperature up to more than 10-30 ℃ / s
Accelerated cooling with ec, and then 1-10 ℃ / s from 750-600 ℃ to 550-450 ℃
A method for manufacturing a high wear resistant perlite rail, which is characterized by controlled cooling at a temperature less than ec. 2. By weight, C: more than 0.85 to 1.40%, Si: 0.10 to 1.00%, Mn: 0.10 to 1.50%, and Cr: 0. .05 to 1.00%, Mo: 0.01 to 0.20%, V: 0.02 to 0.30%, Nb: 0.002 to 0.05%, Co: 0.10 to 2.00 %, B: 0.0005 to 0.005% of one or more kinds of steel rails, hot-rolled steel containing the balance of iron and unavoidable impurities, the steel rail having high temperature heat, or The head of a steel rail heated to a high temperature for the purpose of heat treatment is accelerated cooled from 10 to 30 ° C / sec from the austenite temperature to a temperature between 750 and 600 ° C, and then continuously between 750 and 600 ° C. From the temperature of 550 to 450 ℃
A method for producing a high wear-resistant pearlite rail, which comprises controlled cooling up to a temperature between 1 and 10 ° C./sec.