EP0187904B1 - Method of heat treating pearlitic rail steels - Google Patents

Abstract

The present invention relates to a method for thermal treatment of pearlitic rail steel. For increasing strength and wear resistance steels with the claimed composition are produced with a fine lamellar pearlite structure by heat treatment. During the first cycle the rail head portion is heated in a sufficient depth of up to 50 mm by means of a burner or an inductive system to an austenitization temperature of about 950 DEG to 1050 DEG C. Thereafter the heated head portion is cooled by means of compressed air in such a way that in a first step by blowing a large amount of air the temperature of the rail head portion is cooled within 10 to 20 s to 650 DEG to 600 DEG C. before the area of the pearlitic transformation. In a second step with throttled blowing compared to the first step in the area of the pearlitic transformation the rail head portion is cooled within 2 to 4 minutes to about 400 DEG C. until finishing the pearlitic transformation. Then the rail head portion is again heated for 4 to 6 minutes to a temperature of about 600 DEG to 650 DEG C. and then rapidly cooled by means of water or other appropriate quenching media to a temperature of less than 100 DEG C.

Description

The invention relates to a method for the heat treatment of pearlitic rail steels.

Processes for the heat treatment of rail steels, in which the entire rail or only the rail head are heated to the austenitizing temperature and then accelerated in such a way that a fine-lamellar pearlitic structure results from conversion in the lower pearlite stage, are steel and iron from the publication DE-Z ", 1970, No. 17, page 926 ff., Known. The aim of such heat treatment processes is, in particular for rails with directional analyzes according to quality 90 A according to UIC leaflet 860-V or according to the standard quality according to AREA (0.60 to 0.75% carbon, 0.80 to 1.30% manganese , max. 0.50% silicon) on the running surface of the rail to a depth of at least 10 mm to set a fine pearlitic structure, whereby this fine pearlitic structure results in increased resistance to wear and a four to six times longer service life compared to non-heat-treated rails .

The usual austenitizing temperature for these processes is 850 to 900 ° C. The heating takes place in an oven, inductively or by burner. The accelerated cooling is achieved by quenching in oil or blowing with water vapor, a water spray or compressed air.

The hardness values achieved with these processes are between 320 and 380 HV on the running surface. Depending on the heat treatment process, the hardness towards the middle of the rail head drops gradually or with a steep transition to 280 to 300 HV.

Such heat-treated rails are used in routes with high traffic volumes, tight curve radii and / or with axle loads over 200 kN as well as in switches.

However, the strength and wear properties of the above are sufficient for particularly extreme loads. heat-treated splints. An increase in strength while maintaining the desired fine pearlitic structure, which is favorable for the wear properties, by adding strength-increasing alloy elements such as chromium, manganese, nickel and molybdenum is not possible, since in the heat treatment described, when these alloy elements are added instead of a transformation in the lower pearlite stage, the transformation partly. in the bainite and martensite stage, and thus structures are formed which have an unfavorable effect on the wear properties and the break resistance.

High-strength pearlitic rail steels are known from the literature reference DE-Z Technische Mitteilungen Krupp, Werkberichte, Vol. 37 (1979), H. 3, pages 79-87 and 89-94, which have a fine-grained structure with a small lamella spacing and a small thickness Cementite and ferrite lamellae after adding max. 1.4% chromium and max. 2% nickel, based on a steel with approx. 0.75% carbon and approx. 1% manganese tensile strengths up to 1350 N / mm ² in the naturally hard air-cooled state. However, an increase in this strength through heat treatment, for example accelerated cooling into the region of the lower pearlite stage, leads to the above-mentioned undesirable proportions of bainite and martensite in the structure.

• 0,2 bis 0,85 Gew.-% Kohlenstoff, 0 bis 1,5 Gew.- % Silizium, 0,5 bis 2,5 Gew.-% Mangan, max. 0,06 Gew.-% Phosphor, max. 0,06 Gew.-% Schwefel, 0 bis 1,5 Gew.-% Chrom, 0 bis 1,0 Gew.-% Nickel, 0 bis 0,6 Gew.-% Molybdän, sowie mindestens einem der folgenden Elemente 0,05 bis 0,2 Gew.- % Vanadium, 0,01 bis 0,1 Gew.-% Niob, 0,015 bis 0,1 Gew.-% Aluminium, 0,15 bis 0,3 Gew.-% Zirkon in Verbindung mit 0,003 bis 0,030 Gew.-% Stickstoff.

Rail steels are known from DE-A 2 113 418 with an analysis of:

• 0.2 to 0.85% by weight carbon, 0 to 1.5% by weight silicon, 0.5 to 2.5% by weight manganese, max. 0.06% by weight phosphorus, max. 0.06% by weight of sulfur, 0 to 1.5% by weight of chromium, 0 to 1.0% by weight of nickel, 0 to 0.6% by weight of molybdenum, and at least one of the following elements 0, 05 to 0.2% by weight of vanadium, 0.01 to 0.1% by weight of niobium, 0.015 to 0.1% by weight of aluminum, 0.15 to 0.3% by weight of zirconium in combination with 0.003 to 0.030 wt% nitrogen.

These steels have a ferritic-pearlitic structure with a grain size smaller than 8 according to the A.S.T.M. norm.

These steels achieve their strength values either after normalizing or after a controlled rolling process, the highest strength value shown in the examples being 81 kg / mm ^2, corresponding to approx. 795 N / mm ² or approx. 220 HV. A heat treatment downstream of the rail rolling to increase the strength values at least in the rail head is not provided. Because of the relatively low tensile strength values, the wear resistance of these steels is not guaranteed under extreme loads.

DE-C-2541978 discloses a process for the heat treatment of switch parts made of steels with a composition of 0.6 to 0.8% carbon, 0.05 to 0.5% silicon and 0.8 to 1.3% manganese , in which the running surface of the switch part is heated to austenitizing temperature by means of a burner or inductor and then blown with compressed air in order to quench it in the range of pearlite conversion from 650 to 450 ° C. and subsequently in one or more stages with blowing being throttled compared to the first stage to be deterred so that the temperature range set in the tread before the pearlite transformation is not exceeded and is maintained in the region of the fastest pearlite transformation until the completion of the transformation, after which the water is quenched to below 100.degree.

With this process, tensile strength values up to a maximum of 1200 N / mm ² (approx. 350 HV) can be achieved in a heat-treated condition up to a depth of 15 mm from the driving surface.

It is therefore an object of the present invention to further develop methods for the heat treatment of rail steels of the type mentioned at the outset such that hardness values of> 380 HV are achieved while maintaining the fine-lamellar pearlitic structure on the rail surface, the hardness also being at a depth of 15 mm below the driving surface should be above 360 HV.

This problem is solved by the method according to the main claim. Preferred embodiments can be found in the dependent claims 2-5.

The addition of vanadium of the order of 0.05 to 0.20% in conjunction with the envisaged nitrogen content of 0.010 to 0.025%, in particular, ensures that after the heat treatment in the ferrite lamellae of the finely streaked perlite finely dispersed precipitates of vanadium nitrides or Vanadium carbonitrides are present. These finely dispersed precipitates cause an increase in strength (precipitation hardening) which overlaps the phase boundary hardening by setting a small lamella spacing. The hardness of the rails heat-treated according to the invention is thereby increased compared to a vanadium-free steel.

The decisive factor for the success according to the invention in the process steps is, in particular, the combination of the austenitizing temperature of 950 to 1050 ° C., which is higher than in the prior art, in conjunction with the renewed heating of the rail head to 600 to 650 ° C. after completion of the pearlite transformation. The reheating of the rail head leads to a complete separation of the finest vanadium carbonitride particles from the supersaturated solution, in which vanadium, nitrogen and carbon are at 950 to 1050 ° C after austenitizing. With the accelerated cooling to temperatures of approximately 400 ° C., the elimination of the vanadium carbonitride particles can only take place incompletely; therefore the desired increase in strength will only partially occur.

If the vanadium carbides or vanadium carbonitrides are finely dispersed, the rail head is then quickly cooled to a temperature below 100 ° C using water or other suitable quenching media.

The reheating of the rail head to 600 to 650 ° C thus causes a complete hardening by excretion of the finest vanadium carbides or vanadium carbonitrides. The previous cooling of the rail head to approx. 400 ° C ensures that the conversion in the pearlite stage is complete and that the subsequent hardening can proceed with a high germ density.

The addition of niobium of the order of 0.02 to 0.10% provided according to the invention hardly brings about any additional precipitation hardening. It is designed to prevent grain growth in combination with the intended aluminum additives from 0.010 to 0.070% during austenitizing heating.

The features of the subclaims represent advantageous embodiments of the invention.

So it is advisable to use a limited steel analysis in the individual elements according to claim 2. For accelerated cooling, further liquid media, such as water or steam, can be added to the compressed air, in particular in the first cooling stage.

The features of claim 5 represent an economical process variant, according to which the rails are treated from the hot forming heat at final roll temperatures of 950 to 1000 ° C. In this variant of the method, heating of the cooled rail to the austenitizing temperature is eliminated, which saves energy costs. In this modification of the process, the rail head is first brought to a fine-lamellar pearlitic structure by blowing with compressed air, and after the pelite conversion has ended at 400 ° C, the rail head is heated again to 600 to 650 ° C and then the entire rail (head, bridge and foot) is mixed with water or other suitable quenching media is rapidly cooled to a temperature below 100 ° C.

The method according to the invention is explained in more detail below by means of two graphic representations. ;

• Fig. 1 den Temperatur-Zeit-Ablauf des Wärmebehandlungsverfahrens für einen Stahl mit definierter Analyse,
• Fig. 2 den Härteverlauf erfindungsgemäß wärmebehandelter Schienen im Schienenkopf in bestimmten Abständen von der Lauffläche.

Show it

• 1 shows the temperature-time sequence of the heat treatment process for a steel with a defined analysis,
• Fig. 2 shows the hardness profile according to the invention heat-treated rails in the rail head at certain distances from the tread.

• 0,75% C, 0,46% Si, 1,05% Mn, 0,10% V, 0,04% Nb, 0,020% Al, 0,015% N, Rest Eisen und übliche Verunreinigungen aufgetragen.

1 shows the temperatures to be set when the method according to the invention is carried out in ° C. during certain method steps A to G as a function of the time in minutes for a rail steel with a chemical composition of

• 0.75% C, 0.46% Si, 1.05% Mn, 0.10% V, 0.04% Nb, 0.020% Al, 0.015% N, balance iron and usual impurities.

• A - Erwärmen auf Austenitisierungstemperatur
• B - Halten bei Austenitisierungstemperatur
• C - Schnelle Abkühlung bis zum Beginn der Perlitumwandlung
• D - Gedrosselte Abkühlung auf rd. 400°C
• E - Wiedererwärmen auf rd. 600°C
• F - Halten bei etwa 600°C
• G - Abkühlung auf etwa 100°C.

It means procedural steps

• A - heating to austenitizing temperature
• B - Hold at austenitizing temperature
• C - Rapid cooling until pearlite transformation begins
• D - Reduced cooling to approx. 400 ° C
• E - reheating to approx. 600 ° C
• F - Hold at around 600 ° C
• G - cooling to about 100 ° C.

2, the hardness HV is plotted in the rail head as a function of the distance in mm from the rail running surface, in the form of a scattering band for rails heat-treated according to the invention with the following directional analysis:
0.73 to 0.80% C, 0.40 to 0.50% Si, 0.90 to 1.20% Mn, 0.09 to 0.12% V, 0.03 to 0.05% Nb, 0.015 to 0.040% Al, 0.012 to 0.018% N, balance iron.

The lower area of the scatter band applies to
0.73 to 0.75% C, 0.40 to 0.43% Si, 0.90 to 0.95% Mn, 0.09 to 0.10% V, 0.03 to 0.05% Nb, 0.015 to 0.040% Al, 0.012 to 0.014% N.

The upper area of the scatter band applies to

0.78 to 0.80% C, 0.47 to 0.50% Si, 1.15 to 1.20% Mn, 0.11 to 0.12% V, 0.03 to 0.05% Nb, 0.015 to 0.040% Al, 0.016 to 0.018% N.

The tensile strength scale drawn next to the hardness scale shows the conversion of the hardness HV (Vickers hardness values) into strength values N / mm ² (N = Newton).

Within the above-mentioned directional analysis, when the heat treatment method according to the invention is carried out, hardness values of 400 to 445 HV corresponding to a strength of 1350 to 1500 N / mm ^{2 are} achieved on the rail running surface.

At a depth of 15 mm below the rail running surface (= running surface), the hardness values in a range from 380 to 425 HV are far above the required values of 360 HV.

The hardness profile of the splint heat-treated according to FIG. 1 with the chemical composition specified there roughly meets the mean values of the scatter band according to FIG. 2.

Claims (5)

1. A process for the thermal treatment of rails, to increase strength and resistance to wear with the formation of a fine lamellar pearlitic structure, of steels having

0.55 to 0.82% carbon

0.25 to 0.50% silicon

0.80 to 1.30% manganese

≤ 0.035% phosphorus

≤ 0.040% sulphur

≤ 0.30% chromium

≤ 0.10% nickel

≤ 0.05% molybdenum

0.05 to 0.20% vanadium

0.02 to 0.10% niobium

0.010 to 0.025% nitrogen

0.01 to 0.070% aluminium

residue iron and the usual impurities due to smelting, wherein for increased strength and resistance to wear the rail head is heated to an adequate depth of up to 50 mm as it passes through the furnace by means of burners or inductively to an austenitization temperature of 950 to 1050°C and then cooled with compressed air, a large quantity of which is blown on to the rail head in a first stage within 10 to 20 seconds to reduce its temperature from 650 to 600°C before the range of pearlite transformation, a smaller quantity of compressed air being applied in a second stage within 2 to 4 minutes in the range of pearlite transformation to reduce the temperature of the rail head to about 400°C until the completion of the pearlite transformation, with the formation of a fine lamellar pearlitic structure, whereafter the rail head is again heated to 600 to 650°C for a period of 4 to 6 minutes and then rapidly cooled by water or other suitable quenching media to a temperature below 100°C.

2. A process according to claim 1, characterized in that the steels subjected to the thermal treatment have an analysis of

0.70 to 0.80% carbon

0.40 to 0.50% silicon

0.90 to 1.20% manganese

≤ 0.035% phosphorus

≤ 0.040% sulphur

≤ 0.30% chromium

≤ 0.10% nickel

≤ 0.02% molybdenum

0.08 to 0.12% vanadium

0.02 to 0.05% niobium

0.012 to 0.018% nitrogen

0.010 to 0.050% aluminium

residue iron and the usual impurities.

3. A process according to one of claims 1 and 2, characterized in that liquid media, such as water or steam, are admixed to the compressed air, more particularly in the first cooling stage.

4. A process according to one of claims 1 to 3, characterized in that the austenitization temperature is 950 to 1000°C.

5. A process according to one of claims 1 to 4, characterized in that coming from the hot deformation heat the rails are treated at final rolling temperatures of 950 to 1000°C, compressed air first being blown on to the rail head to confer a final lamellar pearlitic structure thereon, while after completion of pearlite transformation at 400°C the rail head is again heated to 600 to 650°C, whereafter the whole rail (head, web and base) is rapidly cooled by water or other suitable quenching media to a temperature below 100°C.

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