RU2487178C1 - Method for thermal treatment of rails - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: accelerated cooling is carried out upon completion of hot rolling with air or air with admixture of water along a head and a foot of a rail in two stages, first in the temperature range from ≥900°C to ≤870°C along the head with the cooling speed of 2-15°C/s, and along the foot with the speed of 1-10°C/s, and then in the temperature range from 870°C to 650°C and less along the head with the cooling speed of 2-20°C/s, and along the foot with the cooling speed of 1-15°C/s, at the same time in each specific case the cooling speed along the head differs from the cooling speed along the foot, and the total time of accelerated cooling makes 90-145 sec. Steel, if required, additionally contains one or more elements selected from the group including wt %: chrome from 0.04 to 1.10, vanadium from 0.03 to 0.09, nitrogen from 0.007 to 0.020, niobium from 0.001 to 0.06.

EFFECT: provision of balanced complex of mechanical properties of rails from carbon and low-alloyed steel, reduced curvature of rails in the vertical plane, reduction of forces during cold straightening to the minimum and provision of minimum residual stresses of a rail with length of 100-200 m.

2 cl, 4 tbl

Description

The invention relates to the field of ferrous metallurgy, in particular to methods for heat treatment of railway rails.

A known method of heat treatment of rails of high carbon steel containing 0.90-1.20% carbon, providing accelerated cooling at a speed of 5-20 ° C / s of the edge sections of the sole from temperatures ≥650 ° C, then the head, neck and central part of the sole subjected to cooling at a speed of 1-10 ° C / s (JP No. 4267334, C21D 9/04).

A significant drawback of this heat treatment method is that the rail head and sole are cooled at the same speed, as a result of which the rails are bent and there is a need for cold straightening of the rails, which adversely affects the value of residual stresses.

There are also known methods of heat treatment of rails made of carbon or low alloy steel, providing for accelerated cooling of the rail from the temperature of the austenitic region in the range of 750-650 ° C (JP No. 4267267, C21D 9/04), accelerated cooling at a speed of 5-15 ° C / s to temperature 650-500 ° С (RU No. 2113511, C21D 9/04), accelerated cooling at a speed of 10-30 ° С / s to a temperature of 750-600 ° С (RU No. 97121881, C21D 9/04, С22С 38/04) , accelerated cooling at a speed of 5-15 ° C / s to a temperature of 650-500 ° C (RU No. 96123715, C21D 9/04), accelerated cooling of the surface layer of the rail head from Ar1 at a speed of 1-10 ° C / s and 2-20 ° C / s at a depth of ≥20 mm (JP No. 3731934, C21D 9/04).

A significant drawback of these methods of heat treatment of rails is the lack of regulated cooling of the bottom of the rails, which adversely affects their curvature.

The closest technical solution is the method of heat treatment of rails, in which a rail 100-200 m long made of steel containing 0.90-1.20% carbon, is cooled to ≤600 ° C at a speed of 1 after 200 seconds after the end of hot rolling -10 ° C / s from the temperature range of the austenitic structure of the head, neck and sole of the rail (JP No. 4272410, C21D 9/04).

Significant disadvantages of this cooling method are as follows.

1. The inability to use for heat treatment of rails made of low alloy steel, as well as carbon steel with a lower carbon content.

2. The lack of straightness of the rails in the vertical plane due to the lack of differentiated (different speed) cooling of the head and sole of the rail.

The objective of the invention is to provide a balanced set of mechanical properties on rails made of carbon and low alloy steel, reducing the curvature of the rails in the vertical plane, minimizing forces during cold dressing and ensuring minimal residual stresses.

The problem is achieved in that in the known method of heat treatment of steel rails with a length of 100-200 m, including accelerated cooling after hot rolling, according to the invention, accelerated cooling with air or air with admixture of water along the head and bottom of the rail in two stages, first in the temperature range from ≥900 ° С to ≤870 ° С along the head with a cooling rate of 2-15 ° С / s, and on the sole with a speed of 1-10 ° С / s, and then in the temperature range from 870 ° С to 650 ° C or less along the head with a cooling rate of 2-20 ° C / s, and on the sole with a cooling rate of 1-15 ° C / s, and in each case, the head cooling rate differs from the sole cooling rate, and the total accelerated cooling time is 90-145 s. Moreover, the steel, if necessary, additionally contains one or more elements selected from the group comprising from 0.04 to 1.10% wt. chromium, from 0.03 to 0.09% wt. vanadium, from 0.007 to 0.020% wt. nitrogen and from 0.001 to 0.06% wt. niobium.

The claimed limits are selected experimentally, based on the requirements for straightness, microstructure and residual stresses of rails made of carbon and low alloy steels.

The choice of the declared cooling rates in the temperature ranges from no less than 900 ° C to no more than 870 ° C and from no more than 870 ° C to 650 ° C and less is due to the need to obtain a pearlite structure, as well as to achieve a minimum temperature gradient over the rail section to achieve its required straightness and residual stresses.

When cooling the rail head and sole in the temperature range from not less than 900 ° C to not more than 870 ° C with a cooling rate of less than 2 ° C / s and 1 ° C / s, respectively, the required performance of the cooling device is not provided. At a cooling rate of the head and sole more than 15 ° C / s and 10 ° C / s, respectively, the likelihood of formation of a bainitic and martensitic structure from the surface of the head and sole increases.

The cooling of the rail head and sole in the temperature range from no more than 870 ° C to 650 ° C or less with a cooling rate of respectively less than 2 ° C / s and 1 ° C / s does not provide the required level of mechanical properties of differentially hardened rails. At a cooling rate of the head and sole more than 20 ° C / s and 15 ° C / s, respectively, the likelihood of formation of a bainitic and martensitic structure from the surface of the head and sole increases.

The claimed cooling rates are provided by the design of the cooling device, which includes sections 2400 mm long. Each section consists of 4 perforated boxes, three of which are placed above the head and one along the middle of the rail sole. Each box is connected to the pipeline. By adjusting the air pressure with the help of automated throttle control pneumatic valves, the necessary cooling rates are maintained, providing the necessary set of mechanical properties and the minimum temperature gradient between the head and the sole of the rail.

The inventive chemical composition of the steel is selected based on the following premises.

Chrome increases the hardenability of rail steel and increases the strength of perlite due to the formation of alloyed cementite. With a chromium content of less than 0.04%, its effect is negligible, an increase in its content of more than 1.10% leads to the formation of martensite.

To improve ductility and toughness of rail steel, one or more of the following elements may be added: vanadium, niobium and nitrogen. The introduction of nitrogen in combination with vanadium and niobium allows to obtain crushed austenite grain, which provides an increase in resistance to brittle fracture. The presence of vanadium and niobium in this case allows us to achieve the necessary solubility of nitrogen in the compounds. When the nitrogen content is less than 0.007%, its effect is insignificant and it is impossible to provide grain grinding, and more than 0.020% there may be cases of spotted segregation and "nitrogen" boiling (bubbles in steel). The selected content of vanadium and niobium provides the desired toughness due to carbonitride hardening. When the concentration of vanadium is less than 0.03% and niobium is less than 0.001%, their effect is negligible. When vanadium is introduced in steel more than 0.09% and niobium more than 0.06%, the amount of carbonitrides increases, which lead to a decrease in the toughness of steel.

Rail steel (table 1) was smelted in a 100-ton electric arc furnace DSP-100 I7 and cast in a continuous casting machine. The resulting billets were heated and rolled onto P65 rails 25-100 m long, which were subjected to differentiated hardening (i.e., accelerated cooling of the profile elements at different speeds) in a cooling device. After the hot rolling, the rails at a temperature of 950-1080 ° C were positioned in the "on the sole" position and one at a time set into the cooling device. The rail was fixed in the cooling device using a clamping device. Rail cooling was carried out in two stages. At the first stage, cooling of the rail head and sole in the temperature range from 900-1030 ° C to 730-870 ° C was carried out respectively at a speed of 2-15 ° C / s and 1-10 ° C / s. At the second stage, cooling of the rail head and sole in the temperature range from 730-870 ° C to 540-640 ° C was carried out respectively at a speed of 2-20 ° C / s and 1.5-15 ° C / s. After cooling, the rail with a temperature of 540-640 ° C was dispensed from the cooling device and transferred to the refrigerator for cooling to a temperature of 60 ° C and below.

Table 1 The chemical composition of rail steel Rail number Mass fraction of elements,% FROM Mn Si V Nb Cr Ni Cu S P N ₂ one 0.74 0.85 0.62 0.06 - 0.30 0.06 0,07 0.008 0.009 0,020 2 0.87 0.95 0.39 - 0.05 0.40 0.10 0.09 0.009 0.012 0.008 3 0.83 0.95 0.40 0.10 - 0.15 0.10 0.11 0.006 0.017 0.017 four 0.84 0.80 0.60 0.08 - 0.25 0.15 0.15 0.012 0.015 0.015 5 0.79 0.90 0.43 0,07 0.01 0.58 0.08 0.15 0.005 0.011 0.009 6 0.85 0.90 0.35 0.10 - 0.05 0.09 0.10 0.007 0.013 0.018 7 0.79 0.78 0.55 0.08 0.001 0.80 0.06 0.05 0.013 0.010 0.012

After cooling, the microstructure of the metal was investigated, and the deflection arrow and residual stresses were determined.

The technological parameters of the cooling of the rails are shown in table 2.

table 2 Rail number Rail cooling mode I Stage Stage II Total cooling time, s Rail temperature after cooling, ° С Cooling time, s Cooling start temperature, ° C The cooling rate, ° C / s Cooling time, s Cooling start temperature, ° C The cooling rate, ° C / s head sole head sole head sole head sole head sole one 110 960 900 2 one 10 740 790 twenty fifteen 120 540 640 2 thirty 960 900 3 2 60 870 840 5.5 3,5 90 540 630 3 40 1030 960 four 2,5 65 870 860 5 four 105 545 600 four 70 995 925 2,5 1,5 75 820 820 3.6 2.6 145 550 625 5 70 995 940 3,5 3 thirty 750 730 7 5 one hundred 540 580 6 fifteen 960 910 7 5 115 855 835 2.6 2 130 556 605 7 fifteen 980 920 fifteen 10 one hundred 755 770 2 1,5 115 555 620

The results of measurements of the arrow of the deflection and residual stresses, as well as studies of the microstructure are presented in table 3, the results of mechanical tests are presented in table 4.

Table 3 Rail number Rail deflection arrow, mm Groove divergence, mm Rail microstructure one 10 1.4 Perlite 2 10 1,2 Perlite 3 5 1,1 Perlite four 6 1,0 Perlite 5 10 1,2 Perlite 6 5 1,0 Perlite 7 fifteen 1,2 Perlite

Table 4 Mechanical test results Rail number σ σw δ5 ψ Hardness KCU at a temperature of + 20 ° C N / mm ² % HB10 HB22 Nvsh НВпод HBPC J / cm ² one 900 1320 eleven 34 388 373 343 352 395 29th thirty 2 930 1300 12 36 388 373 352 363 388 27 31 3 980 1333 10.5 33 395 388 352 352 398 35 33 four 980 1320 13 38 388 375 363 363 389 thirty 28 5 930 1380 fifteen 37 395 385 363 345 398 32 31 6 890 1312 11.5 34 388 375 345 363 390 34 31 7 910 1360 12.5 33 383 373 331 345 388 32 32

Note: NVPCG - hardness on the rolling surface of the rail head; HB10, HB22 - hardness at a distance of 10 and 22 mm, respectively; НВш - hardness in the neck; НВпод - hardness in the sole.

The proposed method of heat treatment of rails made it possible to obtain a balanced set of mechanical properties with a satisfactory pearlite structure, as well as to improve the straightness of the rails and provide low residual stresses.

Claims (2)

1. The method of heat treatment of rails made of steel with a length of 100-200 m, comprising accelerated cooling after the end of hot rolling, characterized in that accelerated cooling is carried out with air or air with an admixture of water along the head and bottom of the rail in two stages, first in the temperature range from ≥900 ° C to <870 ° C on the head with a cooling rate of 2-15 ° C / s, and on the sole at a speed of 1-10 ° C / s, and then in the temperature range from 870 ° C to 650 ° C or less on the head with a cooling rate of 2-20 ° C / s, and on the sole with a cooling rate of 1-15 ° C / s, while in In each specific case, the head cooling rate differs from the sole cooling rate, and the total accelerated cooling time is 90-145 s. 2. The method according to claim 1, characterized in that the steel, if necessary, additionally contains one or more elements selected from the group comprising, wt.%: Chromium from 0.04 to 1.10, vanadium from 0.03 to 0.09, nitrogen from 0.007 to 0.020, niobium from 0.001 to 0.06.