JP2601670B2 - Heat treatment method for steel rails with excellent falling load resistance - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for heat-treating steel rails related to the improvement of falling-weight characteristics required for heavy-load railway steel rails in cold regions.

(Prior art) In recent years, the speeding up of trains and the heavy loading of freight wagons have been widely promoted for the purpose of improving the efficiency of rail transport. It is about to thrive. Under such a severe rail operating environment, high strength has been achieved by a fine pearlite structure in order to maintain and improve the rail life.

(Problems to be solved by the invention) However, most of the research on the rail material so far has focused on only the rail head, and some of them have cooling for the purpose of controlling bending from the rail bottom. Although there was
It is not intended to achieve stable high strength, nor does it limit the cooling temperature range or cooling rate.Therefore, there is no study on the rail bottom, which affects the falling load characteristics required for heavy load railways in cold regions. Not even.

(Means for Solving the Problems) In view of such circumstances, an object of the present invention is to provide a rail having excellent falling load resistance characteristics. 800 to 450 ° C while accelerating and cooling the rail head from the austenitic area
Rail bottom pearlite average hardness, characterized by accelerated cooling at a cooling rate of 1 to 5 ° C. / sec is in the heat treatment method for steel excellent rail drop resistance bearing properties is H _B 320 or between.

In the present invention, the component composition of the steel rail is not particularly limited, but among others, a component system capable of stably obtaining a pearlite structure, that is, C: 0.55 to 0.85%, Si: 0.20 to 1.
20%, Mn: 0.50 to 1.50%, or Cr: 0.10 to 0.80
%, And preferably a component system containing one or more elements such as Nb, V, and Ti as necessary and the balance being iron.

In addition, in the heat treatment of the rail bottom, the bending of the rail can be controlled by combining it with accelerated cooling heat treatment for strengthening the head, and harmful tensile residual stress can be compressed by accelerated cooling of the rail bottom. It can be changed to residual stress.

(Structure and Example of the Invention) Hereinafter, the present invention will be described in detail.

According to the present invention, the steel bottom is cooled to 800 to 450 ° C. while performing accelerated cooling at a speed exceeding air cooling in accordance with various characteristics such as strength, hardness and the like that require the rail head from the austenite region after rolling the steel rail. The space is accelerated and cooled at a cooling rate of 1 to 5 ° C./sec using a gas or a gas-liquid cooling medium. Here 80
The setting of 0 to 450 ° C. is for defining the cooling rate, not merely for defining the range of cooling. 80
0 ° C. indicates a sufficient austenite region, and 450 ° C. is a temperature at which the pearlite transformation is sufficiently completed, and the temperature range during this period includes a discontinuous cooling portion due to pearlite transformation heat generation.

In limiting the accelerated cooling rate at the bottom of the rail,
Detailed experimental research was performed.

First, before performing the drop weight test, the entire rail length of 1.3 m was -60
The sample was cooled to and maintained at a temperature of ℃, and a falling weight was dropped from a height of 5 m onto a rail head having a distance between fulcrums of 1 m, and the presence or absence of breakage was examined using a falling weight of 1000 kgf as a test condition.

As the test rail, steel A of the current ordinary carbon steel rail component shown in Table 1 and steel B of the low alloy steel rail were used. As a basic test, the effect of the bottom of the rail on the weight drop characteristics was investigated by changing the pearlite grain size (block size) and hardness by reheating / accelerated cooling heat treatment. Thereafter, the rail bottom after rolling was accelerated and cooled, and a drop weight test was performed.
The test results are shown in FIG. In each of the test conditions, the presence or absence of breakage of the three rails after the drop load test is shown in the figure.

As a result, the rail breaks from the bottom surface,
New findings have been obtained that the finer the austenite grains at the bottom of the rail are, the less likely it is to break, and the higher the strength of the bottom is, the less likely it is to break. In general, it has been considered that the higher the strength, the more the embrittlement occurs and the falling weight property deteriorates. Therefore,
In order to improve the drop weight characteristics of the rolling cool left rail is effective strengthening of the rail bottom, drop average hardness having a pearlite structure at low temperatures by the high strength of the rail bottom of the above H _B 320 It became clear that the heavy characteristics could be greatly improved. The pearlite block size depends on the austenite grain size, but also has a correlation with the strength, and the higher the strength, the finer the grains. The average pearlite block grain size at the bottom of the rail obtained by cooling after the end of rail rolling is about ASTM No.6
However, the ASTM No.
Graining into 7, and increasing the strength is also effective in improving the falling weight characteristics through the graining.

Here was a lower cooling rate 1 ° C. / sec, the rail bottom average hardness is less than this is because not a H _B 320 or more, the drop weight characteristics of the left rolling cooling rail is not improved. Further, the upper limit of the cooling rate is set to 5 ° C./sec, because if the cooling rate exceeds this, a bainite structure or a martensite structure is generated on the surface of the rail bottom, and the dropping characteristics are significantly deteriorated. For the above reasons, the cooling rate between 800 and 450 ° C was set to 1 to 5 ° C / sec.

 Hereinafter, specific examples of the present invention will be described.

Table 2 shows the breaking ratio of the rails of the steel A and the steel B after the dropping test under the heat treatment conditions of the heat treatment of the present invention and the comparative heat treatment, in which accelerated cooling was performed from the austenite region after the end of the rail rolling.

As shown in Table 2 and Figure 1 (Effect of the Invention), the cooling from the austenite region after rail rolling completion to obtain a rail bottom average hardness than H _B 320, between eight hundred to four hundred fifty ° C. 1 to 5 ° C
By accelerated cooling at a cooling rate of / sec, a steel rail having excellent falling weight characteristics can be manufactured.

[Brief description of the drawings]

FIG. 1 shows the relationship between the average hardness at the bottom and the pearlite block size in a rail drop test. The dashed line in the figure is an experimental line indicating the falling load rupture limit obtained by the reheating heat treatment of the rail bottom. △ is a comparative steel rail, and ○ is a steel rail of the present invention.

 ──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Yoshiaki Makino 1-1-1 Edamitsu, Yawatahigashi-ku, Kitakyushu-city, Fukuoka Prefecture Inside Nippon Steel Corporation Yawata Works (56) References JP-A-62-227041 ( JP, A)

Claims (1)

(57) [Claims] 1. A rail bottom part, wherein the rail bottom is accelerated and cooled at a cooling rate of 1 to 5 ° C./sec from 800 to 450 ° C. while accelerating and cooling the rail head from the austenite region after the rail rolling. heat treatment method of steel rail pearlite average hardness and excellent drop resistance bearing properties is H _B 320 or more.