JPH09111352A - Production of pearlitic rail excellent in wear resistance - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the wear resistance of a hyper-eutectoid rail steel by a rapid accelerated cooling heat treating method. SOLUTION: A steel rail contg. >0.85 to 1.4% C, 0.10 to 1.00% Si and 0.10 to 1.50% Mn and furthermore contg., at need, one or >=two kinds among 0.05 to 0.80% Cr, 0.10 to 0.50% Mo, 0.02 to 0.30% V and 0.002 to 0.050% Nb is subjected to accelerated cooling from the temp. in an austenitic region at 10 to 30 deg.C/sec, and the accelerated cooling is stopped at the temp. in which pearlitic transformation progresses by at least 70%. Thus, by the rapid accelerated cooling covering deterioration in the hardenability of the hyper-eutectoid steel, the high strength wear resistant rail can be produced.

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 steel rail which greatly improves the wear resistance required for a rail in a curved section of a heavy load railway.

[0002]

2. Description of the Related Art In heavy-duty railways such as mining railways, the weight of trains loaded is increasing with the aim of improving the efficiency of transportation. The wear significantly increases, and further improvement in wear resistance is required for high-strength rails called premium rails.

As a conventional high-strength rail and its manufacturing method, a technique in which the pearlite structure of the eutectoid steel component is refined by heat treatment occupies the mainstream, and the following methods are representative ones. Heat-treating rails for super-heavy loads whose head has a sorbite structure or a fine pearlite structure (Japanese Patent Publication No.
490). A method for producing a high-strength rail of 130 kgf / mm ² or more (Patent Registration No. 1567914) in which accelerated rolling is performed at a rate of 1 to 4 ° C / sec between 850 and 500 ° C after completion of rolling or from a reheated austenite temperature range. The alloy of Cr, Nb, etc. is added to improve the wear resistance as well as the hardness decrease of the welded joint, and the low alloy steel is 850
To 550 ° C. in 50 to 400 seconds for accelerated cooling, a method for manufacturing a heat-treated rail (Japanese Patent Publication No. 59-1917).
(See Japanese Patent Publication No. 3).

A characteristic of these rails is that eutectoid carbon-containing steel is used as a basic component, and this is heat-treated to make the pearlite structure finer and have higher strength, and to improve wear resistance. However, in recent years, heavy-duty mining railways have been attempting to increase the cargo loading for the purpose of further improving the efficiency of railway transportation. It is required to improve wear resistance for extending the life.

[0005]

In order to improve the wear resistance of the above-described eutectoid pearlite steel rail, it is necessary to further strengthen the rail steel. The method is to add an alloy and quench it. It is conceivable to improve the property or further improve the accelerated cooling rate. However, as a method for increasing the strength by heat-treating the existing eutectoid pearlite steel, both the alloy addition and the cooling rate improvement measures have already reached their limits, and a uniform pearlite structure is generated without the formation of martensite or bainite structure. Stable rail head surface hardness with Hv400
It was impossible to achieve the above.

The present invention provides a method for manufacturing a pearlite rail having excellent wear resistance while solving the above-mentioned conventional problems.

[0007]

DISCLOSURE OF THE INVENTION The inventors of the present invention have invented a heat treatment method for stably obtaining a hypereutectoid pearlite structure, paying attention to an increase in carbon content which directly affects improvement of wear resistance. Fig. 1 shows the results of a laboratory comparison of the wear resistance of eutectoid and hypereutectoid steels. With the increase in carbon content, hypereutectoid steels show a dramatic increase in hardness even if they have the same hardness (strength). It was found that the wear resistance was improved. As the point of interest of the heat treatment method, as shown in Fig. 2 comparing the continuous cooling transformation diagrams of eutectoid steel and hypereutectoid steel, when the carbon content was increased, the pearlite nose was shorter than that of the eutectoid steel component material. It was found that the pearlite transformation was likely to occur due to the movement to the time side. That is, it was found that in order to obtain high strength during heat treatment of the hyper-eutectoid steel rail, it is necessary to further increase the accelerated cooling rate as compared with the conventional eutectoid component steel. In addition, to prevent the formation of pro-eutectoid cementite that causes embrittlement, which is another problem of hyper-eutectoid steel, improving the accelerated cooling rate is effective and prevents the formation of pro-eutectoid cementite at austenite grain boundaries. As a result, it has been found that the wear resistance can be expected to be improved by further increasing the carbon content.

The present invention is based on such knowledge, and the gist thereof is% by weight, C: 0.8.
More than 5% to 1.4%, Si; 0.10% to 1.00
%, Mn; 0.10% to 1.50%, and optionally Cr; 0.05% to 0.80%, M
o; 0.01% to 0.50%, V; 0.02% to 0.
Steel rail containing 30%, Nb; 0.002% to 0.050% of one kind or two or more kinds, and the balance being steel and iron and unavoidable impurities hot-rolled to retain high temperature heat. Or, the head of a steel rail heated to a high temperature for the purpose of heat treatment is heated from the austenite region temperature to over 10 to 3
This is a method for producing a pearlite steel rail having excellent wear resistance, which is characterized by accelerating cooling at 0 ° C./sec, stopping accelerating cooling at a temperature at which pearlite transformation has progressed by 70% or more, and then allowing to cool.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. First, the reason why the chemical composition of the rail is determined as described above in the present invention will be described. C is one of the important constituent elements of the present invention, and is an element indispensable for improving wear resistance. In conventional eutectoid rail steel, C
Although the amount of C added was 0.60% to 0.85%, if the amount of C was 0.85% or less, it was insufficient for further improvement in wear resistance. Therefore, the lower limit was set to more than 0.85%. Further, when the amount of C added exceeds 1.4%, pro-eutectoid cementite that causes embrittlement at the γ grain boundary of the rail head after heat treatment is generated, so the upper limit value was limited to 1.4%.

Si is one of the main elements for deoxidizing rail steel and is an element which forms a solid solution with the ferrite structure in the pearlite structure and contributes to the strength improvement.
%, The effect is not fully exhibited, and 1.00%
If the content exceeds the range, the rail becomes brittle and the weld bondability is impaired, so the addition amount was limited to 0.10% to 1.00%.

Like Mn, Mn is also effective as a deoxidizing element, and is an element effective in increasing strength by lowering the pearlite transformation temperature and improving hardenability.
It is also effective in suppressing the formation of pro-eutectoid cementite.
However, if it is less than 0.10%, its effect is small, and if it exceeds 1.50%, the addition amount of 0.10 is added in order to easily generate a martensite structure that causes embrittlement in the segregated portion.
% To 1.50%.

Further, the rail manufactured with the above-mentioned component composition, if necessary, one or more kinds of the following elements are added for the purpose of improving strength, ductility and toughness. Cr;
0.05% to 0.80%, Mo; 0.01% to 0.
50%, V; 0.02% to 0.30%, Nb;
0.002% to 0.050%

Next, the reason why these chemical components are limited as described above will be explained. Cr is an element effective for refining and increasing the strength of the pearlite structure, and by strengthening the cementite phase in the pearlite structure, contributes to further improvement in wear resistance. But 0.05
If it is less than 0.8%, the effect is small, and if it exceeds 0.80%, a martensitic structure is generated in the segregated portion, so 0.8
It was limited to 0% or less.

Mo, like Cr, is an element that makes the pearlite structure finer, but if it is less than 0.01%, its effect is not recognized, and if more than 0.50% of Mo is added, martensite is contained in the segregated portion more than Cr. Since it is easy to generate
It was limited to 1% to 0.50%.

V is an element that forms V carbon / nitride in the cooling process after hot working, and improves the plastic deformation resistance by precipitation hardening of V carbon / nitride, and during the heat treatment for reheating to the austenite region, V Contributes to the improvement of ductility and toughness through the action of suppressing γ grain growth. However, 0.0
If it is added in an amount of 2% or less, the effect cannot be expected.
Even if added over 30%, no further effect can be expected, so the content was limited to 0.02% to 0.30%.

Nb, like V, forms Nb carbon / nitride, but in the case of Nb, it precipitates at a low temperature during rolling and suppresses recrystallization of γ grains, and pearlite from inside the processed γ grains is generated. It promotes transformation and contributes to improvement of ductility and toughness. Also, in the reheating heat treatment, as in V, the charcoal deposited during heating
Nitride contributes to the improvement of ductility and toughness through the effect of suppressing γ grain growth. However, if less than 0.002%, the effect cannot be expected, and if more than 0.050% is added, no further effect can be expected, so 0.002% to 0.05
Limited to 0%.

Next, the reason why the accelerated cooling rate and the accelerated cooling stop timing are set as described above will be explained. First,
As shown in the continuous cooling transformation diagram of Fig. 2, the reason for limiting the austenite temperature to more than 10 to 30 ° C / sec is that the hyper-eutectoid rail steel has a pearlite nose on a shorter time side than the eutectoid rail steel. The position where the nose exists corresponds to more than 10 to 30 ° C./sec in the range of the components of the present invention. In continuous cooling, the pearlite transformation heat is forcibly suppressed, and if it is cooled at a constant rate as it is, the martensite structure will be mixed into the pearlite structure, but in the actual heat treatment of the rail, due to the volume of the rail head, Once the pearlite metamorphosis nose is reached, the pearlite metamorphosis is sufficiently promoted by the mass of the rail. However, 1
When the temperature is 0 ° C./sec or less, the pearlite transformation heat generation is large, and the pearlite transformation occurs at a high temperature, so that sufficient strengthening cannot be achieved. Also, if cooled at a cooling rate of over 30 ° C / sec, most of the pearlite nose does not reach the pearlite nose, or even if it reaches the yoshinba pearlite nose, 70% or more of the pearlite transformation cannot be expected, Martensite structure is mixed in after cooling with insufficient pearlite transformation.

The purpose of stopping the cooling at 70% or more of the pearlite transformation is that when the accelerated cooling of more than 10 to 30 ° C./sec is continued to a low temperature, the inside of the rail head where microscopic segregation exists is not transformed. As it is cooled as it is, it scatters island-shaped martensite structure, which is harmful for rail use. Therefore, accelerated cooling is stopped in the pearlite nose and when 70% or more pearlite transformation is generated, and the rail head is stopped. It is necessary to sufficiently promote the pearlite transformation in the segregated portion by the amount of heat of the portion. Here, as a guideline for determining the pearlite transformation amount of 70% or more, when measuring the cooling rate with a thermocouple mounted on the rail head surface, pearlite transformation heat is generated and about 70% immediately before the temperature rise is stopped. It corresponds to the pearlite transformation amount of.

From the above idea of the accelerated cooling rate and the accelerated cooling stop time, the range of the accelerated cooling rate exceeds 10 to 30 ° C.
/ sec, and the accelerated cooling stop time was limited to 70% or more of the pearlite transformation. As a means for obtaining the cooling rate of 10 to 30 ° C./sec, a predetermined cooling rate is obtained by air injection cooling, mist cooling, water / air mixed injection cooling, or a combination thereof.

After the accelerated cooling is stopped, cooling is performed. The cooling rate in standing cooling is usually 1 ° C./sec or less, and virtually no martensitic transformation occurs even at low temperatures.

[0021]

Next, an embodiment of the present invention will be described. Table 1 shows the chemical composition of the rail steels of the present invention and the comparative steels, the accelerated cooling rate during the heat treatment of the rails, and the pearlite structure fraction at the time when the accelerated cooling is stopped. Further, Table 2 shows the hardness (Hv) of the head surface after the heat treatment of the rail, the wear amount after the Nishihara-type wear test, and the wear test result of the rail-head material by the Nishihara-type wear tester shown in FIG. . In the figure, 1 is a rail test piece,
2 is a mating member and 3 is a cooling nozzle.

The wear test conditions were as follows.・ Testing machine; Nishihara abrasion tester ・ Test piece shape: Disc-shaped test piece (outer diameter 30 mm, thickness 8 mm) ・ Test load: 686 N ・ Slip rate: 20% ・ Mating material: Perlite steel (Hv390) ・ Atmosphere; Atmosphere (Forced cooling by compressed air) -Number of repetitions: 700,000 times

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

EFFECTS OF THE INVENTION The hyper-eutectoid perlite rail of the present invention is superior in wear resistance to the conventional eutectoid perlite steel even with the same hardness, and significantly improves the wear resistance of the rail outside the curved section. Also, since there is no formation of proeutectoid ferrite that is the starting point of internal fatigue cracks generated inside the gauge corners of outer rails laid in sharp curve sections, it has excellent internal fatigue damage resistance and rapid acceleration. The combination of cooling and cooling stop also dramatically improved rail heat treatment productivity.

[Brief description of the drawings]

FIG. 1 shows a comparison of the Nishihara-type wear test characteristics of a conventional eutectoid perlite rail and a hypereutectoid perlite rail steel of the present invention.

FIG. 2 shows continuous cooling transformation diagrams of eutectoid rail steel and hyper-eutectoid rail steel after heating at 1000 ° C.

FIG. 3 shows a schematic diagram of a Nishihara-type wear test.

FIG. 4 shows the relationship between the amount of wear and the hardness of the test piece performed by the Nishihara-type wear tester, and shows the wear characteristics of the steel of the present invention and the comparative steel in comparison.

[Explanation of symbols]

 A Austenite structure P Pearlite structure M Martensite structure 1 Rail test piece 2 Counterpart material 3 Cooling nozzle ○ Comparative steel ● Steel of the present invention

Claims (2)

[Claims] 1. By weight%, C: more than 0.85% to 1.4%, Si: 0.10% to 1.00%, Mn; 0.10% to 1.50%, and the balance. Of steel consisting of iron and unavoidable impurities is hot-rolled to a steel rail that retains high-temperature heat, or the head of a steel rail that has been heated to a high temperature for the purpose of heat treatment is heated from austenite region temperature to over 10 to 30 A method for producing a pearlite steel rail having excellent wear resistance, which comprises accelerating cooling at ℃ / sec, stopping accelerating cooling at a temperature at which pearlite transformation has progressed by 70% or more, and then allowing to cool. 2. By weight%, C: more than 0.85% to 1.4%, Si: 0.10% to 1.00%, Mn; 0.10% to 1.50%, Further, Cr: 0.05% to 0.80%, Mo: 0.01% to 0.50%, V: 0.02% to 0.30%, Nb: 0.002% to 0.050%, 1 Steel containing at least one kind or two or more kinds and the balance being iron and unavoidable impurities, hot-rolled steel rail holding high temperature heat, or head of steel rail heated to high temperature for the purpose of heat treatment From austenite temperature to 10
Accelerated cooling at super ~ 30 ℃ / sec, pearlite transformation is 70
% The method of manufacturing a pearlite steel rail with excellent wear resistance, characterized in that accelerated cooling is stopped at a temperature that has progressed by not less than%, and then it is allowed to cool.