JPS63143220A - Heat treatment of rail - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention is directed to cooling a high-temperature rail that has heat above the austenite range temperature after hot rolling or for the purpose of heat treatment. The present invention relates to a heat treatment method for manufacturing strength rails.

(Conventional technology) In recent years, rail transportation has become more oriented toward higher axle loads and faster speeds, which has resulted in severe wear and fatigue of rail heads, and the characteristics required of rails have become even more stringent. Curved rails require high-strength rails with excellent wear resistance and damage resistance.

A rail that can satisfy these requirements is a steel rail with a fine pearlite structure, and it is known from past research that such a rail exhibits excellent properties such as wear resistance and damage resistance. ing.

As a manufacturing method for such a steel rail, for example, carbon steel as disclosed in JP-A-50-140316 is coated with S l , Mn , Nl , Cr , Mo , 'r
As-rolled alloy steel rails having a composition system obtained by adding elements such as In addition to cooling the rail that retains rolling heat after hot rolling from a predetermined temperature range,
There is a heat-treated rail that is obtained by controlling cooling in a certain temperature range. There are various methods for manufacturing geothermal treated rails, but among them, the heat treatment method that allows a fine pearlite structure to be obtained relatively stably is the method disclosed in JP-A No. 59-133322, in which the geothermal treatment rail is dissolved and stirred as a cooling medium. There is a method of heat-treating the rolled rail by immersing it in a flowing molten salt bath at a temperature above the austenite range.

(Problems to be Solved by the Invention) However, all of the conventional rail manufacturing methods described above have the following problems. In other words, as-rolled rails with added alloying elements and controlled composition,
It is necessary to surround a large amount of the alloying element, and since the alloying element is expensive, there is a problem of high cost.

Next, the rail manufacturing method in which water or other liquid is injected as a cooling medium into the head of the rail that has been heated to a high temperature to forcibly cool it is particularly suitable for direct heat treatment using rolling heat, which prevents shape changes during cooling. It requires a strong restraint device and a very long cooling zone considering the rolling interval, which is disadvantageous in terms of equipment and cost.

In addition, in the case of immersion heat treatment in a molten salt bath that is dissolved at a predetermined temperature and is stirred and fluidized by blowing out gas from the lower part of the bath, in the case of direct heat treatment using rolling heat, cooling time is limited due to rolling intervals, etc. In particular, the untransformed austenite in the segregated area is transferred to the subsequent molten salt cleaning process without completely completing the pearlite transformation, and there is a risk that it will be rapidly cooled in the cleaning process, resulting in the formation of a micromartensitic structure.

The present invention has been made to solve these conventional problems, and provides a heat treatment method that can produce a high-strength rail that stably has a fine pearlite structure over the entire cross section of the rail.

(Means and effects for solving the problems) In order to achieve the above-mentioned object, the present invention aims, in particular, to ensure a stable fine pearlite structure without generating a micromartensite structure due to segregation etc. This is a heat treatment method for rails.

In the present invention, a high-temperature rail having heat above the austenite region temperature heated for the purpose of finishing hot rolling or for heat treatment is first heated by about 6 points above the Ms point by an operation as shown in FIG.
Molten salt bath No. 1 that heats and melts in the temperature range of 00℃ and stirs and flows.
Cool by immersing in a cooling tank for a predetermined period of time. In the figure, 1 indicates a thermometer.

Next, the temperature was set at a temperature at which the untransformed austenite including the segregated part could complete the pearlite transformation in the shortest time, that is, the temperature range from the temperature corresponding to the nose of the TTT curve to 150°C or less of that temperature. This is a heat treatment method for a rail, which is characterized in that the rail is immersed in a second constant temperature bath in a molten salt bath that is stirred and fluidized, and pearlite transformation including the segregated portion is promptly completed. According to this method, not only the rail head but also the whole rail cross section can completely eliminate micromartensite, which is particularly likely to be generated due to segregation, and it is possible to stably obtain a no-I-lite structure.

The present invention will be explained in more detail below.

Steel composition system that can stably obtain a pearlite structure, that is, C: 0.55 to 0.85%, SI: 0.20 to 1.20
%.

Mn: 0150-1.50%, or Cr
:Basically, the steel rail has a composition with 0.10~0.80% added, and if necessary, one or more of Nb, V, Ti, Co, AL, etc., and the balance Fs. ,
After hot rolling, the material is sawn to a predetermined size (length) or heated for the purpose of heat treatment and is in a temperature range above the austenite region, and is then heat treated by the above method.First, the material shown in FIG. 1 cooling tank for a predetermined time, preferably 30 seconds to 120 seconds, and forced cooling.The required immersion time Tc is based on the rail standard, cooling start temperature θie, first cooling tank molten salt temperature θS1, carbon equivalent Ceq, and rail strength production target. Level (Hv: position -10 degrees below the center surface of the top of the rail)
is determined by equation (1).

Tc=a+θ1c(b-)-c@θIc)+θ1(d-
) −osθfi1) −(1) However, θs1 =
al+Hv"(bl+cl・Hv)+Ceq・(dl+
el*ceq)Tc; Rail immersion time in the first cooling tank (S
) θ1c; Cooling start temperature of the top of the rail (°C) θ1; Temperature of the first cooling tank molten salt bath (C) Hv; Target manufacturing strength at the -10 position below the central surface of the top of the rail C@q: Carbon equivalent of the rail (C+Si/24+M
n/6+Cr/6) & r b r e Hd H
@ T 11 + b I He 1 * d 1 +
@1: Coefficient related to rail shape This corresponds to the time required for the vicinity of the center of the head of the Ted heat-treated rail to reach the temperature (kr3) at which pearlite transformation begins. Further, the molten salt bath is preferably stirred by, for example, jetting gas (air, nitrogen gas, etc.) from a pipe provided at the bottom of the molten salt bath, and fluidly stirring the entire bath by making full use of the buoyancy of the gas. As the gas flow rate increases, the flow in the molten salt bath becomes more intense and the heat transfer also improves, but according to the present method, the flow rate may be as low as 0.5 Nm'/min per unit area In''.

The rail, which has been cooled for a predetermined time Tc, continues to have a temperature θ near the nose of the TTT curve. . , @ to θ. . ,.

It is immersed in a second constant-temperature bath molten salt bath maintained at a temperature range of up to -150°C to complete the pearlite transformation. The temperature of the molten salt in the second constant temperature bath varies depending on the shape of the rail, but is θ□. The temperature up to -100°C is the most desirable temperature for completing the pearlite transformation in the center of the rail head in a short time, and the immersion time usually only needs to be about 60 seconds.

Once the rail has undergone transformation, the molten salt adhering to it is first blown away with a gas such as air, and then the rail is cleaned by water spray or hot water immersion, and then straightened and refined.

As described above, according to the method of the present invention, it is possible to reliably prevent the formation of harmful micromartensitic structures, and to manufacture high-strength rails with stable fine and auritic structures.

EXAMPLES Steel compositions shown in Table 1 were hot rolled into 132 pound/yard rails, sawn, and heat treated using the method of the present invention.

Table 1 (vt%) The heat treatment conditions were Hv > 360 at the rail head target strength level (lower center surface of the top of the head - iow position), cooling start temperature θie#'i6o℃, and first cooling tank molten salt bath temperature. θs1
#400℃, molten salt bath stirring nitrogen gas flow rate QN2 #
It was set at 0.5 Nm"/mln-m".

In this case, the first cooling tank immersion time Tc is 65 from equation (1).
It will be over in seconds. In addition, the temperature of the second constant temperature bath molten salt bath is set to θ12#470°C, which is 100°C or less from the TTT curve nose.
The immersion time in the constant temperature bath was about 60 seconds, and the stirring gas was the same as in the first bath.

The heat treatment results are the rail head hardness distribution shown in Figure 2,
The mechanical properties shown in the table were obtained, and microscopic observation showed that the entire cross section was a fine 9-lite structure with no harmful micromartensite structure observed in the segregated areas.

Table 2 (Effects of the Invention) As described above, according to the heat treatment method of the present invention, the high-temperature rail heated for the purpose of rolling or heat treatment can be treated without applying particularly severe treatment conditions. Payinite and martensitic structures are harmful to the cross section of the rail head.

In particular, it is possible to reliably prevent the formation of a micromartensite structure in the segregated portion, and to stably produce a high-strength rail with a fine pearlite structure.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing one embodiment of the present invention, and FIG. 2 (
4) is the second diagram showing the heat treatment results in Examples of the present invention.
Figure ω) is a partial sectional view of the rail showing the measurement part. l: Thermometer.

Claims (1)

[Claims] A high-temperature rail that is heated to a temperature higher than the austenite range temperature after hot rolling or for the purpose of heat treatment is melted by heating to a temperature in the range of 600°C above the Ms point of the rail and placed in a molten salt bath that is stirred and fluidized. immersion for 30 seconds or more and 120 seconds, followed by heating and melting in a temperature range from the temperature corresponding to the TTT curve nose to 150 ° C. or less,
A method for heat treatment of a rail, which comprises immersing the rail in a molten salt bath that is stirred and fluidized to transform untransformed austenite into pearlite.