JPS62227041A - Rail having superior resistance to impact fracture and its manufacture - Google Patents

Abstract

PURPOSE:To manufacture a rail having superior resistance to impact fracture by treating a hot rolled rail under specified conditions so as to provide a pearlite structure to the head and a tempered martensite structure to the midsection and the bottom. CONSTITUTION:The head of a hot rolled rail kept at a temp. between a temp. above the A1 transformation point and 1,300 deg.C by its own potential heat is cooled at a rate at which a pearlite structure or a fine pearlite structure is formed. At the same time, the midsection and/or the bottom is hardened by rapid cooling to cause martensitic transformation. The hardened part is then tempered by heating to a temp. between 400 deg.C and a temp. below the A1 transformation point and immediately it is rapidly cooled. Thus, the resistance of the rail to impact fracture is improved.

Description

[Detailed Description of the Invention] (Industrial Application Field) The present invention provides impact fracture resistance that prevents immediate brittle fracture from damaged parts at the head and bottom of the rail due to impact caused by passing trains. This article relates to an excellent rail and a method for manufacturing the same.

(Prior Art and Problems to be Solved by the Invention) Recently, railways have become faster and have higher axle loads at the expense of improving transportation efficiency. The most concerning problem is brittle fracture of rails due to impact loads, which is caused by fatigue cracks from shearing flaws at the top of the rails and corrosion pits at the bottom. This is a problem that leads to complex fractures in which cracks and rails split into multiple pieces. Since these have a high possibility of directly leading to serious accidents, the demand for rails to withstand impact loads and breakage is increasing year by year, as a result of a derailment accident in the United States where a rail was broken over a length of approximately 100 m or more.

However, conventionally, since the rails are made of high carbon steel, their fracture toughness is extremely low, as is typical of brittle steel, and the fracture toughness value has been improved by adding fA of the alloying elements and heat-treating the pearlite. Even if you try, KIC
The value can be improved to at most 150 KIIf/min -3'', and in any case, it is only about 50 to 100 steps, and it has not yet reached the point where the initiation and propagation of brittle cracks can be stopped.

Therefore, the present inventors conducted extensive investigation and research in order to improve the fracture toughness value of the rail in order to address the above-mentioned problems. As a result, it was found that the fracture toughness value can be significantly improved by using a tempered martensitic structure, and brittle fracture does not occur up to about -40C. However, it is known that the wear resistance and damage resistance of the tempered martensitic structure of the rail are inferior to the pearlite structure and the micro-a-pearlite structure. Therefore, in order to prevent the occurrence and propagation of brittle cracks due to impact loads while imparting wear resistance and damage resistance to only the rail head, it is necessary to
Only the rail head is made of pearlite structure or fine/slightly light structure, but the rail abdomen and/or bottom, where brittle cracks occur and propagate, are made of tempered martensite to prevent dangerous fractures. I know what to do, friend.

(The present invention has been made based on the above-mentioned findings, and its main purpose is to provide a rail that exceeds the A1 transformation point after hot rolling and retains heat at an O temperature of 1300°C or less. While the head is cooled at a rate at which it exhibits a 4-light structure or a fine arm-light structure, the abdomen and/or the bottom of the rail are rapidly cooled to cause martensitic transformation, and then the abdomen of the rail is further cooled. Alternatively, there is provided a rail having excellent impact fracture resistance, and a method for manufacturing the same, characterized in that the bottom portion or both of these portions are heated to a temperature of 400° C. or higher but not exceeding the A1 transformation point, and then rapidly cooled immediately.

The present invention will be explained in detail below.

First, for convenience of explanation, the names of each part of the rail will be described with reference to FIG. 1 is the head of the rail, 2 is the abdomen, and 3 is the bottom.

The present invention has completed hot rolling and exceeded the Al transformation point at 1300°C.
While the head of the rail, which retains heat at the following temperature, is cooled at a speed that produces a pearlite m-cell or fine/f-light structure, the rail abdomen or bottom, or both, are first rapidly cooled and completely quenched. Perform cooling to cause multi/site metamorphosis. Subsequently, the hardened portion of the abdomen or bottom of the rail, or both thereof, is heated to a temperature of 400° C. or higher but not exceeding the A1 transformation point to perform tempering, and then immediately rapidly cooled.

First, the reason why the cooling start temperature range during quenching is set above the Al transformation point and below 1300°C will be described. The reason why the minimum cooling start temperature must exceed the A1 transformation point is that after the heat-treated portion is austenitized, it is rapidly cooled to cause martensitic transformation.

The reason why the minimum cooling start temperature is set to 1300° C. or lower is that rapid cooling from a high temperature exceeding 1300° C. tends to cause quench cracking, and the crystal grains become coarse, resulting in a decrease in ductility.

The reason for rapid cooling is to cause martensitic transformation, whereas slow cooling with compressed air, etc. will cause martensitic transformation.
This is because the condition will not occur and perlite will deteriorate.

As the refrigerant, water or solipul liquid can be used.

Next, we set the heating temperature range when tempering the Akatsuki part to 400°C.
The reason why the temperature is set at a temperature not exceeding .degree. C. or higher and not exceeding the AI transformation point will be described. The reason why the tempering temperature is set to 4001: or more is to obtain a ductile tempered martensitic structure.If it is lower than 400°C, the martensite structure will not be sufficiently tempered and will become a heterogeneous tempered martensitic structure. This is because a suitable fracture toughness value cannot be obtained. The reason why the temperature is set so as not to exceed the Al transformation point is that if the temperature exceeds the A1 transformation point, the material will become austenitic and the martensitic structure obtained by the quenching process will disappear.

The reason for rapid cooling immediately after tempering is to obtain as large a pressure and compressive residual stress as possible. In this case, it may be rapidly cooled to room temperature, or it may be rapidly cooled to about 200°C and then left to cool.

Since residual stress acts as an average stress, it is known that it also affects brittle fracture characteristics.

That is, tensile residual stress has an adverse effect, while compressive residual stress effectively affects brittle fracture properties. In the present invention,
It is also intended to utilize the effect of this compressive residual stress during the Sung Dynasty.

As described above, the rail manufactured by the method of the present invention prevents the occurrence and propagation of brittle cracks by creating a tempered martensitic structure in the areas where brittle cracks occur and propagates, and at the same time prevents the occurrence and propagation of brittle cracks from the structure. Residual stress is also added to further enhance the impact fracture resistance of the rail itself.

(Example) Next, one embodiment of the present invention will be described.

AREAI 3 at a temperature of about 850°C after hot rolling
After the 6 tb rail V was attached, Soripul liquid was sprinkled all over the abdomen and bottom of the rail, and the rail was rapidly cooled to room temperature.

Thereafter, the entire abdomen and bottom were heated to 550° C. using a high-frequency tied induction heating device, and then rapidly cooled with water. As a result, the rail head is a normal 9-light group 2, but
The entire rail abdomen and bottom part had a tempered martensitic structure with a good fracture toughness of 1 straight, as shown in the photograph of the 8 profiled metal structure shown in Figure 2. As shown in Figure 3, for the rail of the present invention, in order to investigate the fracture resistance of the rail, a semicircular notch with a radius of 5 layers and a depth of 5 mm was made over the entire width of the bottom cross section of the rail. A lightning strike test was carried out at room temperature by dropping a 1 tonf weight from a height of 4.2 m to determine whether or not it would break. As a result, as is clear from Table 1, although both the conventional plain carbon steel rail and the head heat-treated rail were of the same weight, the rail of the present invention did not break, and had a higher resistance to fracture. It can be seen that it is very damaged.

[Brief explanation of drawings]

Figure 1 is an explanatory diagram explaining the names of each part of the rail, Figure 2 is a reproduction of a photograph showing the microscope metal assembly at the bottom of the rail abdomen used in the example, and Figure 3 (a) is the example. FIG. 3 is a diagram showing the basic mold test method in FIG. 1: Rail head 2: Rail abdomen 3: 'IIIL bottom = 1 J -°\Honda Kodaira,] -i ~t,ri
, = i, :: 5j Figure 2 2 Me μ Procedural amendment form U), 22 Showa g/June 2016 Telephone 0 1 Director-General Uga, if 1'! B. 1. Indication of the case B. 7 Showa, </2018 Patent Application No. 2, No. 4°: 2°3°, Relationship with 19 cases. Address To the east (2-6-2 Marunouchi, Daita-ku, Tokyo Shigesu Building 330 Name (3667) 11-5 to Teruo Taniyama, amendment order FJ (Xr
7 Showa/noon (11 near day)
4. 31p, :V. 1. , ij →1i=-→-pupil i→-+9-4H], 7. Subject of amendment 8, Contents of amendment Amended as shown in the attached document Original application"9J, +lit document and drawings, ode to the following article amended In Note 1, page 10, lines 3 and 4, the phrase "a copy of a photograph showing a microscopic metallographic structure," should be corrected to read "a photograph showing a microscopic metallographic structure." ``Figure 2'' in the 2 drawings is corrected as shown in the attached sheet.

Claims (1)

[Scope of Claims] 1. A rail with excellent impact fracture resistance, characterized in that the head part is a pearlite structure or a fine pearlite structure, and the abdomen and/or bottom part is a tempered martensitic structure. 2. After hot rolling, the head of the rail, which has exceeded the A_1 transformation point and retains heat at a temperature of 1300°C or less, is cooled at a rate at which it exhibits a pearlite structure or a fine pearlite structure, while cooling the rail abdomen or bottom, or both. Impact fracture resistance characterized by rapid cooling to cause martensitic transformation, then heating the abdomen or bottom of the rail, or both, to a temperature of 400°C or higher, but not exceeding the A_1 transformation point, and then immediately cooling rapidly. A method for manufacturing rails with excellent properties.