JPH1177160A - Method and device for straightening rail - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce a tensile residual stress generated at the head and the sole of a rail and to improve belly brittleness resistance/crack propagation property by adjusting the rolling reduction of the rail head/sole, imparting a downward warp and then, straightening the rail with a roller straightening machine arranged in a zigzag shape. SOLUTION: A rail cambered by cooling after hot rolling is light-reduction- rolled by upper/lower rolls 2, 3 of a light reducing rolling mill. At this time, at the same time when a compression residual stress is imparted to a head/sole, the most of a camber is eliminated. Immediately thereafter, the rail 1 is passed through a roller straightening machine and is slightly bent by each of the rollers 4-10. At this time, since the most of the camber is already eliminated, the rail passes the straightening machine without greatly changing a residual stress from the light reduced state. By this method, since the light reduced rolling is applied from the beginning to the end of the rail 1, the most of edge bending without being straightened by the straightening is straightened.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and an apparatus for suppressing or controlling the generation of residual stress in straightening rail manufacturing.

[0002]

2. Description of the Related Art In order to correct the shape of rails in the vertical direction, vertical rollers are corrected using horizontal rolls. In this correction process, tensile stress in the longitudinal direction is generated at the head and the bottom and compressive stress is generated at the column as residual stress. These residual stresses cause the head to bend upward in the opposite direction and the foot to bend downward, so that a brittle crack in the abdomen may significantly promote crack propagation. In the past, in the United States, there was actually a rail breakage accident due to the propagation of abdominal brittle cracks, which became a major problem, and this residual stress distribution was considered as one of the causes.

[0003] Therefore, a method of metallurgically or mechanically relaxing such residual stress has been proposed.

As a metallurgical residual stress control method, there is known a method disclosed in Japanese Patent Application Laid-Open No. Hei 2-282426, "a method utilizing pearlite transformation and aimed at preventing brittle cracks from coming out to the top of the head". However, the abdominal brittle crack itself is not so short.

As a mechanical residual stress control method, Japanese Patent Application Laid-Open No. Hei 6-112216 discloses a "method of reducing residual stress by reducing plastic deformation in roller correction" disclosed in JP-A-7-185660. "Method of lightly rolling with a small-diameter roll at the stage after roller straightening" is known.

[0006] Among them, the latter technique applies a compressive stress to the surface layer of the head and sole of the rail, so that a residual stress distribution that suppresses the propagation of brittle cracks in the abdomen is obtained, and is relatively simple as a means. Nevertheless, it is very effective in that the residual stress distribution is drastically improved. In particular, the reason why the small-diameter roll is used is that plastic deformation is limited to the top of the head and sole of the foot, so that residual stress can be applied without substantially changing the overall shape.

Further, it has been found that this technique can provide a rail having a long service life of 400 million tons or more as shown in the embodiment without reducing productivity.

However, in the method disclosed in Japanese Patent Application Laid-Open No. Hei 6-320216, a shape defect such as a warpage may occur especially after the correction. This warpage reaches a maximum of 60 mm per 12 m (300 m in radius of curvature, 1.67 × 10 ⁻³ m ^{−1 in} terms of second-order derivative).
It far exceeds the warpage amount of 19 mm per 12 m, which is the REA standard. In such a case, it is necessary to take measures such as performing press straightening on the finish or applying bending straightening again.
In particular, it is not preferable to perform the bending correction again because the effect of controlling the residual stress is lost.

As disclosed in Japanese Patent Application Laid-Open No. Hei 7-185660, there is a technique for reducing the strength of correction to control residual stress. However, it cannot always cope with a large warp after cooling. However, it has been clarified that when the load is lower than a certain level, the residual stress also decreases.

[0010]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a roller straightener with reduced tensile residual stress generated on the crown and sole, or to reduce the burden of roller straightening to reduce the tension between the crown and sole. An object of the present invention is to manufacture a rail having good anti-abdominal brittle crack propagation characteristics by roller straightening while reducing residual stress.

[0011]

At first, the present inventors attempted a number of light reduction rolling experiments in which a rail straightened by a roller straightening machine was pressed down in the vertical direction and rolling analysis related thereto, and performed warpage after light reduction. We have studied a method for controlling residual stress that does not cause cracks.

In this study, it was possible to suppress the upward warpage by making the lower roll larger in diameter than the upper roll or by increasing the widthwise curvature of the roll.

On the other hand, the rails warp so that their heads are on the inside (upward warpage) due to the difference in the amount of heat released between the upper and lower parts due to cooling after rolling. The primary purpose of roller straightening is to eliminate this warpage.

If the load of the roller straightening is high, a high tensile residual stress is generated at the top of the head and the sole of the foot. However, it has been found that when the load is lower than a certain level, the residual stress also decreases. Therefore, a method has been conceived in which light rolling is performed so as to cause a downward warp to reduce the warp, and then a straight rail is obtained by straightening the rollers.

The present invention has been completed on the basis of these findings.
It is characterized in that cold rolling with different reduction amounts of the top and soles is performed before straightening the horizontal roller. The specific summary is as follows.

(1) A rail straightening method characterized in that a downward warpage is given to a rail by adjusting a rolling amount of a rail head and a sole, and a straightening is performed by a roller straightening machine arranged in a staggered manner.

(2) A rail straightening apparatus characterized in that a cold rolling mill for imparting warpage to a rail is arranged on the entry side of a roller straightening machine arranged in a staggered manner with respect to the rail.

(3) The rail straightening device according to the above (2), wherein cold rolling mills having different rolling reductions between the rail head and the sole are arranged so as to give a warp to the rail.

(4) The rail straightening apparatus according to (2) or (3), wherein cold rolling mills having different upper and lower roll diameters are arranged.

(5) The rail straightening apparatus according to the above (2) or (3), wherein cold rolling mills having upper and lower rolls having different curvatures in the width direction are arranged.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail.

FIG. 1 shows a schematic view of the apparatus of the present invention.

As shown in FIG. 1, the rail 1 warped by cooling after hot rolling is lightly rolled by upper and lower rolls 2 and 3 for light rolling by a cold rolling mill. At this time, most of the warpage is eliminated at the same time as compressive residual stress is applied to the top of the head and the sole. Immediately after this, the rail 1 passes through roller straightening and is slightly bent by each of the rollers 4-10. At this time, since most of the warpage has already been eliminated, the residual stress passes through the straightening without largely changing from the state where the pressure is lightly reduced.

In this method, since the light rolling by the cold rolling mill is performed from the beginning to the end of the rail, most of the curving that cannot be corrected by the roller correction is also corrected.

The rolling methods having different upper and lower rolling reductions include:
The lower roll may have a larger diameter than the upper roll, or the curvature in the roll width direction may be increased. Changing the roll diameter is simpler in changing the roll diameter than changing the widthwise curvature.

When the diameter of the lower roll is made larger than that of the upper roll, the lower roll surface pressure is dispersed, so that the upper roll is extended, and downward warpage occurs. This tendency is greater as the difference between the upper and lower diameter ratios increases.

On the other hand, when the load increases, the compressive residual stress increases, but the downward warpage also increases accordingly.

On the other hand, if the roll diameter and the curvature in the roll width direction are reversed, upward warpage will occur.

Therefore, there is a correlation between the upper and lower roll diameter ratio, the light reduction load, and the target residual stress, respectively.
It is desirable to adjust these appropriate values in conjunction with the correction amount.

[0030]

EXAMPLES The present inventors performed light reduction rolling on rails before straightening, and performed roller straightening.

The light rolling mill at this time has an upper roll diameter of 25.
0 mm, the lower roll diameter was 350 mm, and the roll diameter ratio was 1.4. In this rolling, only the lower roll is driven in order to minimize the difference between the upper and lower peripheral speeds.

The load of light reduction rolling performed as an experiment was 40
There are three types: 0 kN, 600 kN, and 800 kN.

Immediately after the light pressure reduction, the roller diameter is 800 mm,
Roller straightening is performed by a straightening machine in which rollers are arranged in a zigzag pattern with a pitch of 1500 mm (between upper and lower sides). The correction condition at this time is that the maximum contact surface pressure is 1200 in elasticity calculation.
MPa, which is a lighter condition than usual.

The rail thus obtained was straight with little bending.

FIG. 2 is a diagram showing the residual stress distribution of the present invention and the conventional method. In the figure, in the present invention, the roll diameter ratio is 1.4,
FIG. 3 shows the residual stress distribution of a steel sheet which was corrected at a maximum contact surface pressure with a strength of 1200 MPa after light pressure reduction. In the conventional method as a comparison, the contact surface pressure is 180 ° without light reduction.
It shows the residual stress of a rail that has been corrected to a strength of 0 MPa (normal strength). In FIG. 2, reference numeral 11 denotes a residual stress distribution when normal strength is corrected without light reduction, 12 denotes a residual stress distribution in the present invention (light load 800 kN), and 13 denotes a residual stress distribution in the present invention (light reduction). Load 600 kN) and 14 show the residual stress distribution (light load 400 kN) in the present invention.

In the case of normal strength correction, the rail is straight, but very high residual stress is generated at the top of the head and the sole of the foot. In addition, it was not possible to adjust the end bending.

On the other hand, the rails that have been corrected after light pressure reduction are:
Similarly, it is straight and the bend is slight. In addition, no strong tensile residual stress was generated at the crown and soles.

Table 1 shows a method for evaluating the abdominal part brittle crack propagation characteristics of these rails by placing the rail horizontally at 450 mm.
The notch of length is shown, and the opening amount is shown. In addition, the same table shows the number of times until a fracture is performed by performing a fatigue test in which the foot side is pulled and oscillated at Δσ = 200 MPa.

[0039]

[Table 1] The opening amount is negative, the cut is in the closing direction, and the residual stress is controlled so that the crack is hardly propagated. In addition, since the tensile residual stress on the foot side is reduced, it can be seen that the fatigue life is longer with the residual stress controlled.

[0040]

According to the present invention, while maintaining the shape accuracy of the roller straightening,
Rails with excellent abdominal brittle crack propagation characteristics and high fatigue strength can be obtained. In addition, edge bending has been reduced.

[Brief description of the drawings]

FIG. 1 is a diagram showing an apparatus embodying the present invention.

FIG. 2 is a diagram showing an example of a difference in residual stress distribution between the present invention and a conventional condition.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Warped rail before straightening 2 Upper roll of light rolling mill 3 Lower roll of light rolling mill 4 Roller of straightening machine 5 Roller of straightening machine 6 Roller of straightening machine 7 Roller of straightening machine 8 Roller of straightening machine 9 Roller for straightening machine 10 Roller for straightening machine 11 Residual stress distribution when normal strength straightening is performed without light reduction 12 Residual stress distribution in the present invention (light reduction load 800k
N) 13 Residual stress distribution in the present invention (600k under light rolling load)
N) 14 Residual stress distribution in the present invention (400k under light rolling load)
N)

Claims (5)

[Claims] 1. A rail straightening method comprising: adjusting a rolling amount of a rail head and a sole to give a warp to a rail; and performing a straightening by a roller straightening machine arranged in a staggered manner. 2. A rail straightening apparatus comprising: a cold rolling mill for imparting a warp to a rail; and a roller straightening machine arranged in a staggered manner with respect to the rail. 3. The rail straightening apparatus according to claim 2, wherein cold rolling mills having different rolling reductions between the rail head and the sole are arranged so as to give a warp to the rail. 4. The rail straightening apparatus according to claim 2, wherein cold rolling mills having different upper and lower roll diameters are arranged. 5. The rail straightening apparatus according to claim 2, wherein cold rolling mills in which upper and lower rolls have different curvatures in the width direction are arranged.