DEEPLY-HARDENED-SURFACE TURNOUT RAIL WITH HIGH DEGREE OF UNDERCOOLING AND THE PREPARATION METHOD THEREOF

Abstract

The invention relates to a turnout rail production technology, in particular to a deeply-hardened-surface turnout rail with high degree of undercooling and the preparation method thereof. The invention aims to solve the technical problem by providing a deeply-hardened-surface turnout rail with high degree of undercooling featured in even hardness distribution and a deeply hardened surface layer and the preparation method thereof. The method is described as follows: feeding molten iron for converter smelting.fwdarw.furnace rear argon blowing station.fwdarw.LF refining.fwdarw.RH vacuumization.fwdarw.casting steel blanks.fwdarw.slow cooling in the slow cooling pit.fwdarw.austenitic homogenization.fwdarw.rail rolling.fwdarw.heat treatment; in the converter smelting process, adding 0.2-0.3% Cr, 0.04-0.06 V and 0.75-0.80% C; the heat treatment process is divided into two cooling stages. The turnout rail prepared with the method described in the invention has a deeper deeply-hardened surface layer; the hardness is distributed more evenly, the anti-contact fatigue performance is higher and the resistance to wearing is ideal.

Claims

1. A method for preparing a deeply-hardened-surface turnout rail with a high degree of undercooling, said method comprising the following sequential steps: feeding molten iron for converter smelting; blowing argon into the molten iron in a furnace rear argon blowing station; LF refining; RH vacuumization; casting steel blanks; slow cooling in a slow cooling pit; austenitic homogenization; rail rolling; and heat treatment, wherein 0.2-0.3% Cr, 0.04-0.06 V and 0.75-0.80% C are added to the molten iron during the converter smelting and the heat treatment step is divided into two cooling stages.

2. The method according to claim 1, wherein the austenitic homogenization is conducted at a temperature of 1,000° C.-1,300° C. and for a duration of 200-500 minutes.

3. The method according to claim 1, wherein a total deformation during rolling is 85-95%.

4. The method according to claim 1, wherein the heat treatment step comprises the step of treating a rolled rail in a heat treatment unit with residual heat; the temperature when feeding the rolled rail into the heat treatment unit is 800-850° C.

5. The method according to claim 1, wherein the heat treatment step lasts for 110 seconds; wherein, for the first 80 seconds after a rolled rail is fed into a heat treatment unit, the rolled rail is cooled at a speed of 3-5° C./s; for the last 30 seconds, the rolled rail is cooled at a speed of 0.5-2° C./s.

6. The method according to claim 1, wherein, after heat treatment, the rail is naturally cooled down to a temperature below 100° C. and then straightened by vertical and horizontal straightening machines.

7. A deeply-hardened-surface turnout rail with a high degree of undercooling prepared by the method of claim 1.

8. The deeply-hardened-surface turnout rail with the high degree of undercooling according to claim 7, wherein chemical components of the rail by weight percentage are as follows: C0.75-0.80%, Si0.1-0.6%, Mn0.6-1.3%, P≤0.020%, S≤0.020%, Cr0.2-0.3%, V0.04-0.06%; the rest including Fe and unavoidable impurities.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The invention will be described in conjunction with the following drawings, wherein:

[0022] FIG. 1 shows the locations for hardness inspection of turnout rail section in embodiments and the comparative examples.

[0023] FIG. 2 shows the marks for the locations for hardness inspection of turnout rail section in embodiments and comparative examples.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0024] In details, the invention provides a method for preparing a deeply-hardened-surface turnout rail with high degree of undercooling. The method comprises the following steps:

[0025] Feeding molten iron for converter smelting.fwdarw.furnace rear argon blowing station.fwdarw.LF refining.fwdarw.RH vacuumization.fwdarw.casting steel blanks.fwdarw.slow cooling in the slow cooling pit.fwdarw.austenitic homogenization.fwdarw.rail rolling.fwdarw.heat treatment; in the converter smelting process, adding 0.2-0.3% Cr, 0.04-0.06 V and 0.75-0.80% C; the heat treatment process is divided into two cooling stages.

[0026] In the present invention, 0.75-0.80% C, 0.2-0.3% Cr and 0.04-0.06% V are added in the smelting process. Wherein, C and Cr are added to move the C curve rightwards and thus improve hardenability of the turnout rail. V is mainly for precipitation hardening so that the hardness is distributed more evenly at the rail head, the anti-contact fatigue performance is better and the resistance to wearing is ideal.

[0027] In the invention, the temperature for austenitic homogenization is 1,000° C.-1,300° C. and the duration is 200-500 minutes. The purpose is to allow large and uniform original austenitic grain size, promote homogenization of components and guarantee evenness and controllability of the pearlite structure after rail rolling and heat treatment.

[0028] In the invention, the heat treatment process includes two-stage cooling: the entire heat treatment process takes 110 seconds.

[0029] Stage 1 (pre-phase change): due to a unit weight greater than 60 kg/m, the rail web of a turnout rail is about twice that of an ordinary symmetric rail. As a result, the rolled turnout rail has a high heat capacity, with the rail surface temperature as high as 900-1,000° C. High finishing rolling temperature results in that the degree of undercooling cannot be further increased and the heat at the center of rail head cannot be dissipated in the follow-up heat treatment process.

[0030] Therefore, in stage 1, forced cooling is conducted on the rolled turnout rail. That is, for the first 80 seconds after the rolled rail is fed into the heat treatment unit, cooling is performed at a speed of 3 -5° C./s, with the purpose of increase the degree of undercooling, reduce heat capacity at the center of the rail, increase the phase change drive force at the center and improve center hardness. When cooling in stage 1 is too slow, the ideal cooling effect cannot be achieved; when cooling is too fast, the rail surface is cooled too fast while the center cannot be cooled fast enough due to the high heat capacity, there will be significant transition in hardness gradient of the rail, and the expected even transition of hardness gradient cannot be achieved.

[0031] In stage 2, i.e. the last 30 seconds, cooling is performed at a speed of 0.5-2° C./s, both the surface and the center of the turnout rail are beyond the phase change point, in which case the cooling speed can be reduced accordingly for further dissipation of heat at the center.

[0032] The invention not only increases the degree of under cooling of turnout rails, but also significantly improves the deeply hardened surface layer. The prepared turnout rail shows significant improvement in wearing performance and anti-contact fatigue performance.

[0033] The following embodiments are provided to further illustrate the invention.

TABLE-US-00001 TABLE 1 Chemical components (%) of the turnout rails in embodiments and comparative examples Chemical elements (%) Item C Si Mn P S Cr V Embodiment 1 0.75 0.10 0.62 0.010 0.010 0.21 0.04 Embodiment 2 0.76 0.15 0.68 0.011 0.006 0.22 0.04 Embodiment 3 0.76 0.20 0.76 0.013 0.005 0.22 0.04 Embodiment 4 0.77 0.27 0.84 0.014 0.007 0.23 0.04 Embodiment 5 0.79 0.32 0.92 0.015 0.008 0.23 0.05 Embodiment 6 0.78 0.37 1.01 0.015 0.011 0.23 0.05 Embodiment 7 0.79 0.42 1.10 0.013 0.013 0.24 0.06 Embodiment 8 0.80 0.53 1.20 0.012 0.015 0.24 0.06 Embodiment 9 0.80 0.59 1.29 0.011 0.011 0.25 0.06 Comparative 0.70 0.65 0.55 0.010 0.010 0.05 0.03 example 1 Comparative 0.77 0.34 1.01 0.015 0.009 0.23 0.03 example 2 Comparative 0.78 0.33 1.02 0.016 0.008 0.24 0.07 example 3 Comparative 0.79 0.35 1.03 0.014 0.007 0.25 0.07 example 4

TABLE-US-00002 TABLE 2 Treatment processes and structures in embodiments and comparative examples Cooling speed Cooling speed in stage 1 in stage 2 Item (° C./s) (° C./s) Structure Embodiment 1 3 0.5 P Embodiment 2 3 0.5 P Embodiment 3 3 0.5 P Embodiment 4 4 1 P Embodiment 5 4 1 P Embodiment 6 4 1 P Embodiment 7 5 2 P Embodiment 8 5 2 P Embodiment 9 5 2 P Comparative 0 0 P example 1 Comparative 2 0.3 P example 2 Comparative 2.5 0.3 P example 3 Comparative 6 3 M example 4

[0034] The rest process parameters are the same for embodiments and comparative examples.

[0035] Samples are taken from rail sections for hardness testing as shown in the drawings. See table 3 for details.

TABLE-US-00003 TABLE 3 Hardness inspection in embodiments and comparative examples Section hardness (HBW 2.5/187.5) HBW HBW HBW Formula Item A1 A3 B1 B2 C1 C2 D1 E1 1 2 3 Range Result Embodiment 321 316 318 315 319 320 319 319 319.3 317.5 316.0 6 0.17 1 Embodiment 322 317 319 319 319 320 320 320 320.0 319.5 317.0 5 1.30 2 Embodiment 325 320 322 321 322 322 323 323 323.0 321.5 320.0 5 0.30 3 Embodiment 351 353 351 354 350 353 348 350 350.7 353.5 353.0 4 1.43 4 Embodiment 353 355 355 356 352 355 351 353 353.3 355.5 355.0 4 1.17 5 Embodiment 355 356 356 356 353 356 352 355 354.7 356.0 356.0 3 0.53 6 Embodiment 362 363 363 364 363 363 360 368 362.7 363.5 363.0 2 0.63 7 Embodiment 365 365 365 364 365 367 362 360 365.0 365.5 365.0 3 0.50 8 Embodiment 367 368 364 366 364 368 364 363 365.0 367.0 368.0 4 0.20 9 Comparative 315 316 314 313 312 313 310 310 313.7 313.0 316.0 6 −2.07 example 1 Comparative 325 314 322 314 326 303 322 323 324.3 308.5 314.0 23 −9.63 example 2 Comparative 336 326 333 315 334 334 333 334 334.3 324.5 326.0 21 −4.83 example 3 Comparative 374 357 374 343 375 375 374 374 374.3 359.0 357.0 32 −4.93 example 4

[0036] Table 3 shows that all embodiments meet HBW2-0.6*HBW3-0.4*HBW1>0, indicating that the hardness of the rail prepared with the method in the invention decreases uniformly from the surface to the center, and the hardness is greater at the depth.

[0037] Samples are respectively taken from the rail heads for wearing testing in embodiments and comparative examples. The results are given in table 4.

TABLE-US-00004 TABLE 4 Rail head wearing in embodiments and comparative examples in the invention Test parameters Number of rotation Wearing Item Load (N) (ten-thousand times) loss (g) Embodiment 1 980 10 0.27 Embodiment 2 980 10 0.29 Embodiment 3 980 10 0.28 Embodiment 4 980 10 0.25 Embodiment 5 980 10 0.23 Embodiment 6 980 10 0.22 Embodiment 7 980 10 0.21 Embodiment 8 980 10 0.20 Embodiment 9 980 10 0.19 Comparative 980 10 0.42 example 1 Comparative 980 10 0.38 example 2 Comparative 980 10 0.32 example 3 Comparative 980 10 0.22 example 4

[0038] Samples are respectively taken from the rail heads for contact fatigue testing in embodiments and comparative examples. The results are given in table 5.

TABLE-US-00005 TABLE 5 Contact fatigue of the rails in embodiments and comparative examples in the invention Rotation Contact fatigue/ Contact Slip speed ten-thousand Item stress/MPa frequency/% rpm times Embodiment 1 1,350 5 1,000 25 Embodiment 2 1,350 5 1,000 26 Embodiment 3 1,350 5 1,000 27 Embodiment 4 1,350 5 1,000 42 Embodiment 5 1,350 5 1,000 43 Embodiment 6 1,350 5 1,000 44 Embodiment 7 1,350 5 1,000 45 Embodiment 8 1,350 5 1,000 46 Embodiment 9 1,350 5 1,000 47 Comparative 1,350 5 1,000 20 example 1 Comparative 1,350 5 1,000 21 example 2 Comparative 1,350 5 1,000 22 example 3 Comparative 1,350 5 1,000 23 example 4

[0039] According to above results, the method described in the invention can effectively increase the hardness of the deeply hardened surface layer and significantly improve the wearing performance and anti-contact fatigue performance of the rail. The turnout rail prepared with the method in the invention applies to heavy-loaded railways and high-speed railways with heavy axle loads and high density.