EP0049004A1 - Process for making rails, and rails so produced - Google Patents

Abstract

Process for making rails consisting, at the exit of the hot-rolling mill, in leaving the rail in the open air for at least 45 seconds, then in introducing it into a tank containing an aqueous bath heated to a temperature above 75 DEG C, and preferably substantially to boiling point, the said rail being introduced in a substantially horizontal position but with the flange at the bottom.

Description

The present invention relates to an improved process for manufacturing rails and in particular high resistance rails.

• - résistance à la rupture élevée : au moins 1080 MPa au bourrelet pour les aciers à haute résistance,
• - allongement : au moins égal à 10 %,
• - déformation : minimale après traitement.

Its purpose is to obtain, in the hot rolling, and without the addition of alloying elements, a rail having, after cooling, the following mechanical characteristics:

• - high breaking strength: at least 1080 MPa at the bead for high strength steels,
• - elongation: at least equal to 10%,
• - deformation: minimal after treatment.

The process is especially applicable to steels containing 0.4 to 0.85% of C and from 0.4 to 1% of Mn, 0.1 to 0.4% of Si, and preferably from 0.6 to 0 , 85% C, and 0.6-0.8% Mn.

It is not outside the scope of the invention to apply the process to steels containing a certain amount of alloying elements, for example up to 1% chromium or up to 0.3% molybdenum, or up to 0.15% vanadium.

It is well known, at the exit of the hot rolling mill, to subject the laminates to a cooling operation, more or less accelerated by immersing them in a tank containing an aqueous bath which may be at its boiling temperature. In the case of wire rod, for example, higher mechanical properties are obtained which are more homogeneous than those obtained with air cooling.

Such treatment with boiling water is also already known for rails, in particular by Belgian patent number 754.416, but it leads to very high thermal gradients between the bead and the pad, during treatment, and to permanent deformation. important rail.

In order to remedy these drawbacks, it has already been proposed to carry out differential cooling on the bead and the shoe. According to this process described for example in Belgian patent n ° 854.834, the bead is subjected to accelerated cooling by quenching in boiling water mechanically stirred, while the shoe is cooled in air or in still water to 100 ° C.

• - accélérer le refroidissement du bourrelet et donc obtenir une résistance élevée,
• - uniformiser les refroidissements du patin et du bourrelet et dès lors, minimiser les déformations permanentes.

These treatments have a double objective, namely

• - accelerate the cooling of the bead and therefore obtain a high resistance,
• - standardize the cooling of the pad and the bead and therefore minimize permanent deformation.

However, the effective implementation of this process on an industrial scale presents great technological difficulties.

The object of the present invention is precisely a particularly simple process which makes it possible to eliminate the drawbacks mentioned above.

The process which is the subject of the present invention is essentially characterized in that, at the outlet from the hot rolling mill, the rail is left in the open air for at least 45 seconds, and preferably until the center of the bead reaches a temperature between 780 ° C and 680 ° C, then the rail is introduced into a tank containing an aqueous bath brought to a temperature above 75 ° C, and preferably substantially at boiling temperature, the said rail y being introduced in a substantially horizontal position, but with the shoe down (so-called "standing" position). Advantageously, the duration of stay in the open air is less than 400 seconds; this duration varies according to the profile of the rail and its temperature at the end of rolling. Furthermore, the duration of the immersion of the rail in the aqueous bath is at least equal to that necessary to obtain at least 80% of the allotropic transformation of the steel in the bead, and preferably between 50 seconds and 90 seconds .

The position of the rail unexpectedly plays an essential role in the kinetics of cooling in water. In fact, when the rail is immersed in the "shoe down" position, the heat exchanges are particularly intense on the upper face of the bead and greatly slowed down. on the underside of the skate. The bead cools further. quickly and the pad slower, whereas the reverse occurs when the rail is submerged in the "pad up" position (so-called "inverted" position).

• - atteindre une résistance supérieure à 1080 MPa dans le bourrelet avec un acier moins chargé en manganèse (C : 0,6 à 0,85 %, Mn : 0,6 à 0,8 %); la dureté et donc la résistance sont sensiblement inférieures dans le même rail immergé dans les mêmes conditions, mais en position "patin en haut" (figure 1);
• - limiter la teneur en manganèse de l'acier, ce qui permet à la fois de réaliser une économie et de minimiser le risque d'apparition de martensite aux extrémités du patin;
• - limiter, le cas échéant, les teneurs en éléments de micro- alliages (Cr, Mo, V, Si,...),
• - réaliser une meilleure homogénéité de température entre le bourrelet et le patin au cours du refroidissement et dès lors, une meilleure homogénéité des caractéristiques mécaniques. On ramène les déformations permanentes du rail entre des limites acceptables.

The acceleration of the cooling of the bead and the slowing down of the cooling of the pad of a high resistance rail immersed in the "pad down" position have the following advantages:

• - reach a resistance greater than 1080 MPa in the bead with a steel less loaded with manganese (C: 0.6 to 0.85%, Mn: 0.6 to 0.8%); the hardness and therefore the resistance are significantly lower in the same submerged rail under the same conditions, but in the "pad up" position (Figure 1);
• - limit the manganese content of the steel, which both saves money and minimizes the risk of the appearance of martensite at the ends of the skate;
• - limit, where appropriate, the contents of micro-alloy elements (Cr, Mo, V, Si, ...),
• - Achieve better temperature uniformity between the bead and the pad during cooling and therefore, better homogeneity of the mechanical characteristics. The permanent deformations of the rail are brought back between acceptable limits.

As regards the lower resistance steel rails, one can also cite as an advantage, an increase in weldability and a reduction (for example of the order of 0.5%) of their manganese content, as well as, where appropriate, the contents of micro-alloy elements.

The field of the invention also extends to the different kinds of rails obtained by means of the methods described above.

The appended figure, given by way of nonlimiting example, makes it possible to realize the differences in mechanical properties that can be obtained on a given rail, depending on whether its introduction into the water bath is carried out in standing position (that is to say with the pad down, in accordance with the present invention), or in an inverted position (pad up).

Position a of the rail represents the "standing" position, that is to say with the shoe at the bottom, position b of the rail represents the inverted position, that is to say with the shoe at the top.

The rail steel contains 0.72% C and 0.63% Mn. The rail was cooled in the open air for 45 seconds, then immersed in an aqueous bath at boiling temperature.

The zone marked 1 corresponds to a breaking strength less than or equal to 1080 MPa (≤ 321 HB); the zone marked 2 corresponds to a breaking strength of 1080 to 1180 MPa; the zone marked 3 corresponds to a breaking strength of between 1180 and 1226 MPa (350 to 380 HB).

Claims (8)

1. A method of manufacturing rails, characterized in that at the outlet of the hot rolling mill, the rail is left in the open air for at least 45 seconds, then it is introduced into a tank containing an aqueous bath brought to a temperature higher than 75 ° C., and preferably substantially at boiling point, the said rail being introduced therein in a substantially horizontal position, but with the shoe at the bottom. 2. Method according to claim 1, characterized in that the duration of the cooling of the rail in the open air is less than 400 seconds. 3. Method according to either of Claims 1 and 2, characterized in that the cooling in the open air lasts until the temperature at the center of the bead of the rail is between 780 ° C and 680 ° C. 4. Method according to either of claims 1 to 3, characterized in that the duration of the immersion of the rail in the aqueous bath is at least equal to that necessary to obtain at least 80% of the allotropic transformation steel in the rail bead. 5. Method according to claim 4, characterized in that the duration of the immersion of the rail in the aqueous bath is between 50 seconds and 90 seconds. 6. Method according to claims 1 to 5, characterized in that the composition of the steel verifies the following relationships: 0.4% ≤ C ≤ 0.85%, and preferably 0.6% ≤ C ≤ 0.85% 0.4% 4 Mn ≤ 1%, and preferably 0.6% ≤ Mn ≤ 0.8% 0.1 to 0.4% Si. 7. Method according to either of claims 1 to 5, characterized in that it is applied to steels containing, individually or together, up to 1% of chromium, up to 0.3% of molybdenum and up to 0.15% vanadium. 8. Rails as described and claimed above.

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