JP3117916B2 - Manufacturing method of pearlitic rail with excellent wear resistance - Google Patents

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 pearlitic rail which has greatly improved abrasion resistance required for a rail of a heavy load railway.

[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, with the recent improvement in the heat treatment technology for increasing the strength of rails, the following high-strength (high-hardness) rails exhibiting a fine pearlite structure using eutectoid carbon steel have been developed, and have been developed for heavy load railways. Rail life in curved sections has been dramatically improved. Heat treatment rail for ultra-high load with sorbite head or fine pearlite head (Japanese Patent Publication No. 54-2549)
No. 0). A method for producing a low-alloy heat-treated rail in which an alloy such as Cr and Nb is added to improve not only wear resistance but also a decrease in hardness of a welded portion (see Japanese Patent Publication No. 59-19173). The features of these rails are high strength (high hardness) rails exhibiting a fine pearlite structure by eutectoid carbon-containing steel,
The aim was to improve wear resistance.

[0004] However, in recent years, heavy-load railways overseas have been strongly increasing the load of cargo in order to further increase the efficiency of rail transportation, and especially the rails with the sharp curves use the developed rails. However, abrasion resistance cannot be secured, and a reduction in rail life due to wear has become a problem. From such a background, development of a rail having wear resistance higher than that of the current eutectoid carbon-containing high-strength rail has been required.

[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. As another solution, it is conceivable to use a material exhibiting a metal structure with higher wear resistance than the pearlite structure as rail steel.However, in a rolling wear environment such as rails and wheels, a fine pearlite structure is used. At present, no material that is inexpensive and has excellent wear resistance has not been found.

[0006] 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 plate-like hard cementite structure.
The present inventors have analyzed the wear mechanism of the pearlite structure, first of all, a soft ferrite phase was squeezed out by passing through the wheel, and then only a hard cementite phase was laminated immediately below the rolling surface, and the wear resistance was secured. I confirmed that. Therefore, the present inventors have improved the hardness of the pearlite structure in order to improve the wear resistance, and at the same time, have increased the amount of carbon, the plate-like hard cementite phase in the pearlite structure ensuring the wear resistance. It has been found through experiments that the wear resistance is dramatically improved by increasing the ratio and increasing the cementite phase density immediately below the rolling surface.

However, when the amount of carbon is increased in order to improve the wear resistance, proeutectoid cementite is apt to be formed on the rail head, and there is a problem that the toughness and ductility of the rail are greatly reduced. To solve this problem, Japanese Patent Application No. Hei 7-4
A method of accelerating and cooling the rail head has been developed as shown in the specification of No. 6753. However, when the concentration of carbon or the like in the slab central segregation portion is extremely severe, the rail column portion where the accelerated cooling is not performed is used. There was a problem that proeutectoid cementite was easily formed and toughness of the column portion was lowered. However, despite these problems, rails that prevent the formation of pro-eutectoid cementite in the column and have a high cementite ratio in the head and a stable pearlite structure with high hardness The production method has not been sufficiently studied.

Accordingly, the present inventors have studied experimentally a cooling method for suppressing the formation of pro-eutectoid cementite in the column portion and for stably forming a pearlite structure having a high cementite ratio and a high hardness at the head. did. As a result, the rail head immediately after hot rolling or reheated was heated to 1 to 10 ° C / s.
By accelerated cooling with ec, the cementite ratio is high, the pearlite structure with high hardness is generated stably, and at the same time, in the rail column portion, the high temperature range just before the start of pearlite transformation where pro-eutectoid cementite is easily formed is 1 to 1. 10 ° C / sec
It was confirmed that the generation of proeutectoid cementite could be suppressed by accelerated cooling with.

Based on the above results, the present inventors first increased the carbon content of the rail steel, and at the same time, increased the heat resistance of the rail steel in order to improve the wear resistance of the rail head and to prevent the toughness of the rail column from decreasing. Accelerated cooling of the rail head and pillars immediately after or during re-rolling ensures the hardness of the rail head and the toughness of the rail pillars, enabling the manufacture of high-strength rails with excellent wear resistance. confirmed. That is, an object of the present invention is to provide, at low cost, a rail in which abrasion resistance required for a rail of a heavy load railway is greatly improved.

[0010]

The present invention attains the above object, and its gist is that the weight percent
And C: more than 0.85 to 1.20%, Si: 0.10
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 to 0.050%,
Co: 0.10 to 2.00%, B: 0.0005
Hot rolled steel rails containing one or more than 0.0050% and the balance consisting of iron and unavoidable impurities, or heated to high temperatures for the purpose of heat treatment At the head and column of the steel rail,
The head is accelerated and cooled at a rate of 1 to 10 ° C./sec from an austenitic zone temperature to a temperature of 700 to 500 ° C., and a column is heated at a temperature of 1 to 10 ° C. to a temperature of 750 to 600 ° C. This is a method for producing a pearlitic rail having excellent wear resistance, characterized by accelerated cooling at a rate of ° C./sec.

Hereinafter, the present invention will be described in detail. First, the reason why the chemical components of the rail are limited as described above in the present invention will be described. C is an effective element that promotes pearlite transformation and secures abrasion resistance. As a normal rail steel, 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 the wear resistance cannot be ensured, and furthermore, proeutectoid ferrite, which is a starting point of fatigue damage inside the rail head. Is easily generated. On the other hand, if the C content exceeds 1.20%, a large amount of proeutectoid cementite is formed on the rail head and the column after the heat treatment, and the ductility and toughness are reduced. Therefore, the C content is more than 0.85 to 1.20. %.

Si is an element which improves strength by solid solution hardening into a ferrite phase in a 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 enhances the hardenability, thereby contributing to an increase in strength and further suppressing the formation of proeutectoid cementite.
If the content is less than 0.10%, the effect is small, the hardness of the rail head after the heat treatment is reduced, and proeutectoid 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. Although this small amount of martensite structure does not significantly affect the toughness and wear resistance of the rail, in order to stably generate the pearlite structure in the heat treatment, the Mn content must be 0.10 to 0.10.
It is desirable to be in the range of 1.00%.

Further, one or more of the following elements may be added to a rail manufactured with the above-mentioned component composition as required for the purpose of improving strength, ductility and toughness. Cr: 0.05 to 1.00%, Mo: 0.01 to
0.20%, V: 0.02 to 0.30%, Nb:
0.002 to 0.050%, Co: 0.10 to 2.00
%, B: 0.0005 to 0.005%

Next, the reason why these chemical components are determined as described above will be described. Cr is an element that raises the equilibrium transformation point of pearlite and consequently refines the pearlite structure to contribute to high strength, and at the same time, enhances the wear resistance by strengthening the cementite phase in the pearlite structure. If the content is less than 0.05%, the effect is small, and excessive addition exceeding 1.00% generates a large amount of martensite structure and embrittles the steel. Therefore, the Cr content 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. Although this small amount of martensite structure does not significantly affect the toughness and wear resistance of the rail, the amount of Cr should be in the range of 0.05 to 0.60% in order to stably generate the pearlite structure during heat treatment. It is desirable.

Mo, like Cr, raises the equilibrium transformation point of pearlite and consequently refines the pearlite structure, thereby contributing to higher strength and improving wear resistance. The effect is small, 0.2
Excessive addition exceeding 0% lowers the pearlite transformation rate and tends to generate a martensite structure harmful to toughness. Therefore, the amount of Mo added was limited to 0.01 to 0.20%.

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

Nb is an effective element for forming Nb charcoal / nitride and refining austenite grains like V, and its effect of suppressing austenite grain growth is higher than V in a temperature range (around 1200 ° C.). Acts to improve ductility and toughness of rails. The effect cannot be expected if it is less than 0.002%, and no further effect can 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 that increases the transformation energy of pearlite to improve the strength by making the pearlite structure finer, but its effect cannot be expected if it is less than 0.10%, and 2.00%. The effect reaches the saturation range even if an excessive addition exceeding 0.1 is performed, so that the Co content is set to 0.1.
Limited to 10-2.00%.

B has an effect of suppressing proeutectoid cementite generated from the prior austenite grain boundary and is an effective element for stably generating a pearlite structure. However, if less than 0.0005%, the effect is weak, and 0.005%.
If it is added in excess of 50%, coarse carbon borides of B are formed and the ductility and toughness of the rail are deteriorated.
005 to 0.0050%.

The rail steel having the above-mentioned composition is melted in a commonly used melting furnace such as a converter or an electric furnace, and the molten steel is subjected to an ingot-bulking method or a continuous casting method.
Further, it is manufactured as a rail through hot rolling. next,
This hot-rolled rail holding high-temperature heat, or accelerated cooling of the head and pillars of the rail heated to a high temperature for the purpose of heat treatment, to improve the hardness of the pearlite structure of the rail head, Prevents formation of columnar proeutectoid cementite structure. Here, FIG. 1 shows the names of the wear-resistant pearlite-based rails of the present invention in a cross section. The rail has a head: 1, a column: 2, and a bottom: 3, and the column: 2 is a portion sandwiched between the head: 1 and the bottom: 3.

Next, the reason why the respective cooling stop temperature ranges and the accelerated cooling rates are determined as described above will be described in detail. First, the reason why the rail head is accelerated and cooled at a rate of 1 to 10 ° C./sec from an austenite region temperature to an accelerated cooling stop temperature of 700 to 500 ° C. will be described. When accelerated cooling is stopped at a temperature exceeding 700 ° C., immediately after accelerated cooling,
In addition, a large amount of pearlite structure having low hardness is generated, and the hardness required for the wear resistance of the rail head cannot be secured.
The temperature was limited to 0 ° C. or lower. Further, when accelerated cooling to less than 500 ° C., sufficient spontaneous reheating from the inside of the rail cannot be expected after accelerated cooling, and a martensite structure harmful to the toughness and wear resistance of the rail is likely to be generated. C. or higher.

The reason why the accelerated cooling rate is limited to 1 to 10 ° C./sec is that if the accelerated cooling rate is less than 1 ° C./sec, the pearlite is coarse and has low hardness in a high temperature range during accelerated cooling. The rate was limited to 1 ° C./sec or more because a large amount of microstructure was generated, the hardness required for the wear resistance of the rail head could not be secured, and a large amount of proeutectoid cementite harmful to the toughness and ductility of the rail was generated. When accelerated cooling is performed at a cooling rate of 10 ° C./sec or more, pearlite transformation does not end during accelerated cooling, and martensite harmful to rail toughness and wear resistance during accelerated cooling and in the natural cooling region thereafter. Since a different structure such as carbon or bainite is generated, the rate is limited to 1 to 10 ° C./sec.

Next, the rail column portion is moved from the austenite region temperature to the accelerated cooling stop temperature of 750 to 600 ° C. by 1 to 1.
The reason for accelerated cooling at 10 ° C./sec will be described. 75
When accelerated cooling is stopped at a temperature exceeding 0 ° C, proeutectoid cementite is generated along the center segregation line of the column in the subsequent natural cooling region, and the toughness of the rail column is greatly reduced,
It was limited to 750 ° C or lower. Further, when accelerated cooling to less than 600 ° C., the pearlite transformation does not end in the subsequent natural cooling region, a martensite structure is easily generated along the central segregation line, and the toughness of the rail column portion is greatly reduced.
The temperature was limited to 600 ° C. or higher.

The reason why the accelerated cooling rate is limited to 1 to 10 ° C./sec is that when the accelerated cooling rate is less than 1 ° C./sec, pro-eutectoid cementite is formed along a center segregation line in a high temperature range during accelerated cooling. However, since the toughness of the rail column greatly decreases,
C / sec or more. Further, when accelerated cooling is performed at a cooling rate exceeding 10 ° C./sec, the pearlite transformation does not end in the subsequent natural cooling region, a martensite structure is generated along the segregation line, and the toughness of the rail column portion decreases. Therefore, 1
It was limited to 10 ° C / sec.

Therefore, in order to manufacture a rail having a pearlite-based structure and excellent wear resistance, the formation of a coarse and low-hardness pearlite structure at the rail head is prevented, and ductility, toughness and toughness are reduced. In order to prevent the formation of pro-eutectoid cementite, bainite and martensitic structures which are detrimental to abrasion resistance, a temperature of from 700 to 500 from the austenite region temperature.
Cooling to a temperature between 1 ° C and 1-10 ° C / sec is accelerated to stably generate a pearlite structure with high hardness. Further, in the rail column portion, a pro-eutectoid cementite structure harmful to toughness is not formed. 750 from the austenite temperature range
It is necessary to accelerate and cool down to a temperature between ６００600 ° C. at 1１０10 ° C./sec to stably generate a pearlite structure.

The metal structure of the rail is preferably a pearlite structure. However, depending on the component system, cooling rate, and segregation state of the material, a trace amount of proeutectoid cementite is present in the pearlite structure of the rail head and column. May be generated. However, the formation of a small amount of proeutectoid cementite in the pearlite structure does not significantly affect the ductility, toughness, wear resistance and strength of the rail. Includes mixed organizations.

It is desirable to use a gas-liquid mixture such as air or mist as a cooling medium at the time of accelerated cooling. In addition, the cooling speed of the rail head and the pillar portion at the time of accelerated cooling can be secured by independently controlling the amount and speed of the cooling medium injected at the time of cooling. In addition, regarding the hardness of the rail head after accelerated cooling, H
It is desirable to be v320 or more. Note that this Hv3
In the area of hardness of 20 or more, in order to secure the rail life,
As shown in FIG. 2, it is desirable that the depth of the rail top part a and the rail head corner part b be at least 20 mm from the head surface (including the entire head side) as a starting point.

[0029]

Next, an embodiment of the present invention will be described. Table 1-1 and Table 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 evaluation results of the wear characteristics of the material of the rail head using the Nishihara-type abrasion tester shown in FIG. The results of evaluation of the impact characteristics by the impact tester are also shown. In the figure, 4 is a rail test piece, 5
Is a partner material, and 6 is a cooling nozzle. Table 3 shows the chemical composition and cooling conditions of the comparative rail steel. Table 4
Shows the hardness and structure of the rail head after cooling, the results of the evaluation of the wear characteristics of the rail head material using the Nishihara type abrasion tester shown in FIG. 3, and the structure of the center segregation part of the rail column, using the Charpy impact tester. Impact characteristic evaluation results are also shown.

The configuration of the rail is as follows. -Rail steel of the present invention (16 pieces) Symbols: A to P: Heat-treated rails in which the rail head and the column in the above-mentioned component range are subjected to accelerated cooling within the above-mentioned limited range. -Comparative rail steel (7 pieces) Code: Q to W: Comparative rail (Q to S) made of eutectoid carbon-containing steel and heat-treated rail (T ~ W).

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

The impact test conditions were as follows. -Test piece: JIS No. 3 2mm U notch Charpy impact test piece-Test temperature: Normal temperature (+ 20 ° C)

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

[Table 3]

[0036]

[Table 4]

[0037]

[Table 5]

[0038]

The relationship between the head hardness (Hv) of the present invention and the comparative rail steel and the wear amount (g / 700,000 times) is shown in FIG. FIG. 5 shows the relationship between the column impact value (J / cm ² ) and the head wear amount. As shown in FIG. 4, the rail steel of the present invention has a higher carbon content than the eutectoid carbon-containing comparative rail steel (symbols: Q to S) and, at the same time, is subjected to heat treatment, so that it has the same hardness as the comparative rail steel. The wear amount is small and the wear resistance is greatly improved. In addition, as shown in FIG. 5, the rail steel of the present invention is subjected to an appropriate heat treatment on the rail column to generate columnar proeutectoid cementite even in a high carbon component as compared with the comparative rail steel (code: T to W). Can be prevented, and the wear resistance of the rail head and the toughness of the column can be sufficiently ensured. As described above, according to the present invention, it is possible to provide a rail having excellent wear resistance in a heavy-load railway.

[Brief description of the drawings]

FIG. 1 is a diagram showing names in a cross section of a rail.

FIG. 2 is a diagram showing names of respective parts in a cross section of a rail head axis.

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

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

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

[Explanation of symbols]

 1: head 2: pillar 3: bottom a: rail top b: rail head corner 4: rail test piece 5: mating material 6: cooling nozzle

Continuation of the front page (56) References JP-A-6-279929 (JP, A) JP-A-63-28824 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 9 / 00-9/44 C21D 9/50 C22C 38/00-38/60

Claims (2)

(57) [Claims] 1. The composition contains, by weight, C: more than 0.85 to 1.20%, Si: 0.10 to 1.00%, and Mn: 0.10 to 1.50%, with the balance being iron. In addition, in a steel rail having high temperature heat obtained by hot-rolling steel consisting of unavoidable impurities or a steel rail heated to a high temperature for the purpose of heat treatment, the head is raised from austenite temperature to 700 ° C. Accelerated cooling at a temperature of ~ 500 ° C at 1-10 ° C / sec, and further, accelerated cooling of the column at a temperature of 1-10 ° C / sec from austenite zone temperature to a temperature of 750-600 ° C A method for producing a pearlitic rail with excellent wear resistance. 2. In% by weight, C: more than 0.85 to 1.20%, Si: 0.10 to 1.00%, Mn: 0.10 to 1.50%, Cr: 0.05-1.00%, Mo: 0.01-0.20%, V: 0.02-0.30%, Nb: 0.002-0.05%, Co: 0.10-2. B: one or more kinds of B: 0.0005 to 0.005%, and a high temperature heat-retaining rail obtained by hot-rolling a steel containing iron and unavoidable impurities with the balance being iron, or At the head and column of the steel rail heated to a high temperature for the purpose of heat treatment, the head is shifted from the austenitic temperature to 7%.
The temperature is accelerated and cooled at a rate of 1 to 10 ° C./sec to a temperature between 00 and 500 ° C., and the column is further cooled from the austenite region temperature by 75 ° C.
A method for producing a pearlitic rail having excellent wear resistance, characterized by accelerated cooling at a temperature of 0 to 600C at a rate of 1 to 10C / sec.