A production method for improving the height of the hardness low point distribution of a heat-treated rail cross section

Abstract

The application discloses a production method for improving the distribution height of the hardness low point of a heat-treated rail cross section, which comprises the following steps: conventional smelting, pouring into a blank after refining, stack slow cooling, heating the cast blank, rolling, cooling, and naturally cooling to room temperature. The application can improve the distribution height of the hardness low point of the rail cross section from about 3-9 mm to not more than 0.5 mm, eliminate the hardness low point in the pre-polishing stage after the rail is laid, and improve the service performance of the rail under the premise that the mechanical properties of the rail are ensured, the tensile strength is 1080-1380 MPa, the elongation after fracture is 10-16%, the Brinell total hardness of the rail head tread is 320-420 HB, the Rockwell hardness of the first point of the rail head cross section is 34-44 HRC, the Rockwell hardness of the last point of the rail head cross section is greater than or equal to 34 HRC, and the metallographic structure of the whole cross section of the rail is fine lamellar pearlite and a small amount of ferrite.

Description

Technical Field

[0001] This invention relates to a method for producing steel rails, specifically a method for increasing the distribution height of low-hardness points on the cross-section of heat-treated steel rails. This invention is applicable to all steel grades and compositions of existing steel rails. Background Technology

[0002] With the increasing weight of train axles and the continuous advancement and increase in train speed and operating density, wear and fatigue damage to rails during service have significantly increased, severely impacting their service life. Domestic and international rail manufacturers generally employ online heat treatment processes to accelerate the cooling of hot-rolled rails, increasing their strength and hardness, and improving their wear resistance and fatigue resistance to meet the demands of small-radius curve railways and heavy-haul railways.

[0003] To ensure the performance and service life of heat-treated rails, the railway industry standard TB / T 2344.1-2020 specifies the performance of heat-treated rails in detail, especially the distribution of hardness in the cross-section of the rail. It requires that the Rockwell hardness at each point on the cross-section of the heat-treated rail gradually and uniformly decreases from the surface to the interior, and that abrupt changes in hardness are not allowed.

[0004] Currently, the main online heat treatment process for rails adopts a traveling-type method. Several sets of cooling units are arranged after the rolling mill. The hot-rolled rails pass through the cooling units at a certain speed, and a certain cooling intensity is applied to the rail head by spraying cooling media (air, water mist, etc.). This accelerated cooling transforms the austenite structure into fine lamellar pearlite, thereby improving strength and hardness. Heat-treated rails produced using this traveling-type heat treatment process have low-hardness points 3-9mm below the rail head surface, causing the hardness distribution of the rail head cross-section to show a trend of initially low, then high, and then decreasing again. This is more pronounced at the gauge angle of the rail head. To ensure that the low-hardness points meet the standard requirements, the overall hardened layer hardness is usually increased. However, this method fails to adequately meet the requirement of the railway industry standard TB / T2344.1-2020, which stipulates that the hardness value should gradually and uniformly decrease from the surface to the interior, without any abrupt changes. In addition, during the heat treatment of rails, the phase transformation of the rail head gradually occurs from the surface to the inside. As the cooling is accelerated, the remaining heat of the rail head gradually decreases. If the applied cooling intensity is not reasonable, the cooling rate will be greater towards the inside of the rail head, causing the internal hardness to gradually increase, and even harmful structures such as martensite that are not allowed to exist.

[0005] Search results:

[0006] Chinese Patent Publication No. CN1083013C discloses a "Heat Treatment Method and Apparatus for Producing High-Strength Steel Rails Using Residual Heat from Rolling". The method involves continuously feeding the hot-rolled steel rails, which are kept at a high temperature in the austenitic region, into a unit equipped with a heat treatment device. Cooling medium is sprayed onto the steel rails through nozzles, so that the steel rails are uniformly and acceleratedly cooled, resulting in a fine pearlitic structure with gradually decreasing hardness. The heat-treated steel rails produced can meet the development requirements of railway transportation towards high speed, heavy load, and large capacity.

[0007] Chinese Patent Publication No. CN 1793403 A discloses a "Pearlite Heat-Treated Steel Rail and its Production Method", which includes smelting, rolling and heat treatment. The steel rail is cooled from 650-880℃ to 400-500℃ at a cooling rate of 1-10℃ / s, or the steel rail at room temperature is first heated to 850-1100℃, then naturally cooled to 650-880℃ and then cooled to 400-500℃ at a cooling rate of 1-10℃ / s, and then naturally cooled to room temperature. The produced steel rail has good wear resistance.

[0008] Chinese Patent Publication No. CN 102220545 A discloses a "High-Carbon High-Strength Heat-Treated Rail with Excellent Wear Resistance and Plasticity and its Manufacturing Method". The rail is heated to 680-900℃ after rolling, cooled to 400-500℃ at a cooling rate of 1.5-10℃ / s, and then naturally cooled to room temperature. The rail has a tensile strength ≥1330MPa, elongation ≥9%, rail head hardness ≥380HB, hardened layer depth ≥25mm, and fine pearlite microstructure. It has excellent wear resistance and plasticity and can meet the requirements of heavy-haul railways.

[0009] Chinese Patent Publication No. CN 85109735 discloses a "Method and Equipment for Heat Treatment of Rails". The method involves preparing rails at a temperature not lower than the austenitic region, arranging a nozzle device around the rail head so that the nozzles can spray a gaseous cooling medium onto the rail head; determining the distance between the nozzle and the rail head based on the required hardness grade of the rail head and the carbon equivalent in the rail; moving the nozzle device to achieve the required distance between the nozzle device and the rail head; and spraying the gaseous cooling medium onto the rail head at a predetermined flow rate for a certain time to cool it, thereby achieving the required hardness grade at the rail head.

[0010] Analysis of the above literature reveals that it only addresses methods to improve the strength, tread hardness, and rail head hardening layer depth of heat-treated rails. It fails to solve the problem of raising the low-hardness points in the rail cross-section to near the upper surface, ensuring that the finished product does not have these low-hardness points. Therefore, the rails described cannot meet the needs of trains operating at increasingly higher speeds and densities. Addressing the insufficient distribution of low-hardness points in the heat-treated rail cross-section is a pressing issue that needs to be resolved to meet market demands. Summary of the Invention

[0011] This invention aims to overcome the shortcomings of existing technologies and provide a production method for improving the distribution height of low hardness points in the cross-section of heat-treated rails. This method ensures that the rail's mechanical properties are maintained at a tensile strength of 1080–1380 MPa, elongation after fracture of 10–16%, Brinell hardness of the rail head tread of 320–420 HB, Rockwell hardness of the first point on the rail head cross-section of 34–44 HRC, Rockwell hardness of the last point on the rail head cross-section of ≥32 HRC, and a metallographic structure of fine lamellar pearlite and a small amount of ferrite throughout the rail cross-section. The low hardness points can be eliminated during the pre-grinding stage after the rail is installed on the track, thus improving the rail's performance.

[0012] Measures to achieve the above objectives:

[0013] A production method for increasing the distribution height of low hardness points on the cross-section of heat-treated steel rails, comprising the following steps:

[0014] 1) After conventional smelting and refining, the billet is cast into a billet. During casting, the superheat of the tundish temperature is controlled at 25-30℃, the casting speed is 0.5-1.2m / min, and conventional protective gas is used for protection throughout the casting process.

[0015] 2) Stack the billets for slow cooling, with a cooling time of not less than 24 hours;

[0016] 3) Heat the billet, controlling the heating temperature at 1250-1350℃ and the heating time at 150-300 minutes;

[0017] 4) Rolling is carried out, and the roughing rolling temperature is controlled at 1050-1150℃; the finishing rolling temperature is controlled at 840-900℃.

[0018] 5) Cool the rolled rails, controlling the initial cooling temperature at 690–790℃; cool the rail heads according to different cross-sectional specifications of the rails, and control the total cooling time at 50–100 seconds.

[0019] The rail head is cooled in the following manner:

[0020] A. During the ≤6s period after the rail head begins to cool, control the cooling rate to be between 0.8 and 1.55℃ / s.

[0021] B. During the cooling period of >6s to ≤40s for the rail head, control the cooling rate to be between 2.5 and 8℃ / s.

[0022] C. During the cooling period of >40s to 100s for the rail head, the cooling rate is set at 10 to 20 seconds per second.

[0023] The speed is reduced by 20-40% compared to the previous time, and cooling is stopped when the temperature of the rail head drops to 480-560℃.

[0024] The rail head is cooled simultaneously with the rail base, and the cooling intensity in the middle of the rail base is 80-96% of the cooling intensity in the rail head.

[0025] D. Allow to cool naturally to room temperature.

[0026] Preferably, the cooling rate is controlled at 1.0 to 1.45℃ / s within ≤6s of the rail head beginning to cool.

[0027] Preferably, the cooling rate is controlled at 3 to 6.5℃ / s during the cooling time of the rail head from >6s to ≤40s.

[0028] Preferably, during the >40s to 100s cooling period of the rail head, the cooling rate is reduced by 25% to 36% every 10 to 20 seconds compared to the previous cooling rate.

[0029] The key point is that, during the cooling of rail heads with different cross-sectional specifications, the total cooling time of 50 to 100 seconds is positively correlated with the size of the cross-sectional specifications.

[0030] The key point is that this method is applicable to the chemical composition of all rail steels in the prior art.

[0031] The role and mechanism of each raw material and main process in this invention

[0032] The reason why the heating temperature is controlled at 1250-1350℃ and the heating time is 150-300min in this invention is to ensure that the billet is heated evenly and completely austenitized, while ensuring that the temperature of the initial rolling, final rolling and heat treatment processes meets the requirements.

[0033] The reason why this invention controls the roughing rolling temperature at 1050-1150℃ and the finishing rolling temperature at 840-900℃ is that it is necessary to control the growth of austenite grains in the rolled workpiece during the rolling process, while ensuring that the cooling temperature during the heat treatment process meets the design requirements, so that the performance of the rail after heat treatment meets the requirements.

[0034] The reason this invention controls the initial cooling temperature at 690–790°C and the total cooling time at 50–100 seconds is that online accelerated cooling of the austenitic rail after hot rolling utilizes the residual heat of the rail itself. Accelerated cooling increases the phase transformation kinetics of austenite to pearlite, resulting in a pearlitic microstructure with finer interlamellar spacing, thus improving the rail's strength and hardness. If the initial accelerated cooling temperature is too high, the time required to cool to the phase transformation temperature is longer, resulting in higher consumption of cooling medium without increasing the phase transformation undercooling. Furthermore, the billet heating temperature, initial rolling temperature, and final rolling temperature must be correspondingly increased, leading to higher energy consumption and coarser original austenite grains. Conversely, if the initial cooling temperature is too low, insufficient phase transformation undercooling occurs under effective cooling rates, resulting in insufficiently fine interlamellar spacing in the pearlitic microstructure of the rail head.

[0035] The reason this invention accelerates cooling after rail rolling and applies different cooling rates to the rail head at different time points during accelerated cooling is as follows:

[0036] During the initial 6 seconds of accelerated cooling of the rail head, a low cooling rate is adopted, controlled at 0.8-1.55℃ / s; preferably, the cooling rate is controlled at 1.0-1.45℃ / s. This is because at the beginning of accelerated cooling, the metal on the rail head surface rapidly transforms from austenite to pearlite due to convective heat transfer, forming a thin pearlite layer. An interface between pearlite and austenite forms at the inward position. The inner austenitic metal transfers heat to the surface pearlite through thermal conduction, and the heat is ultimately carried away by the cooling medium through convection, causing the inner austenitic metal to transform back into pearlite. Because austenite has poor thermal conductivity, heat is not easily conducted from austenite to pearlite, resulting in a low cooling rate at the initially formed pearlite-austenite interface, leading to low hardness points. The higher the cooling rate applied at the beginning of accelerated cooling of the rail head, the thicker the pearlite layer formed. Consequently, the location of low hardness points will be further inward along the thickness direction. By applying a low cooling rate at the beginning of accelerated cooling of the rail head, the location of low hardness points can be controlled within 0.5mm inward from the surface. After the rail is installed on the track, the low hardness points can be eliminated by pre-grinding, thereby improving the performance of the rail.

[0037] During the cooling period of >6s to ≤40s for the rail head, the cooling rate is controlled at 2.5 to 8℃ / s; preferably, the cooling rate is controlled at 3 to 6.5℃ / s. This is because the surface of the rail head forms a pearlite and austenite interface, and at this time, it is necessary to quickly remove the surface heat through convection to improve the internal heat conduction rate, thereby ensuring the performance after heat treatment.

[0038] During the >40s to 100s cooling period of the rail head, the cooling rate is reduced by 20% to 40% every 10 to 20 seconds, preferably by 25% to 36%. This is because as the pearlite phase transformation proceeds inward, the area of ​​the high-temperature region inside the rail head continuously decreases, and the cooling rate generated by heat conduction gradually increases. Therefore, the cooling rate applied to the rail head needs to be gradually reduced. In addition, the further in the rail head you go, the more severe the component segregation becomes, and the more likely it is to produce undesirable structures such as martensite. Therefore, the cooling rate is reduced by 20% to 40% every 10 to 20 seconds until the rail head temperature drops to 480 to 560°C.

[0039] During the accelerated cooling process of the rail head, the temperature of the rail head drops rapidly, which inevitably causes it to bend towards the rail head due to cooling contraction. Therefore, while the rail head is being accelerated cooled, the rail base is also being accelerated cooled. The intensity of the cooling medium applied is 75-95% of that of the rail head cooling medium to balance part of the rail head contraction, ensure that the heat treatment process does not cause accidents such as steel jamming, and ensure smooth production.

[0040] Compared with the prior art, this invention, while ensuring the mechanical properties of the rail are tensile strength of 1080-1380 MPa, elongation after fracture of 10-16%, total Brinell hardness of rail head tread of 320-420 HB, Rockwell hardness of the first point of rail head cross section of 34-44 HRC, Rockwell hardness of the last point of rail head cross section ≥34 HRC, and the metallographic structure of the entire rail cross section is fine lamellar pearlite and a small amount of ferrite, increases the depth of the low hardness point of the rail cross section from about 3-9 mm to no more than 0.5 mm. The low hardness point can be eliminated during the pre-grinding stage after the rail is installed on the track, thereby improving the performance of the rail. Attached Figure Description

[0041] Figure 1 This is a metallographic diagram of the rail obtained by the present invention. Detailed Implementation

[0042] The present invention will now be described in detail:

[0043] Table 1 is a list of chemical components for each embodiment of the present invention;

[0044] Table 2 is a list of the main process parameters for each embodiment and comparative example of the present invention;

[0045] Table 3 is a list of rail head cooling process parameters for various embodiments and comparative examples of the present invention;

[0046] Table 4 is a list of test results of the mechanical properties of rails in various embodiments and comparative examples of the present invention.

[0047] Table 5 is a list of Rockwell hardness of rail cross sections in various embodiments and comparative examples of the present invention.

[0048] Table 6 is a list of Vickers hardness values ​​of rail cross-sections at different distances from the surface along line B in various embodiments and comparative examples of the present invention.

[0049] Table 7 is a list of Vickers hardness values ​​of the C-line of the rail cross section at different distances from the surface in various embodiments and comparative examples of the present invention.

[0050] The various embodiments of the present invention are produced according to the following steps.

[0051] 1) After conventional smelting and refining, the billet is cast into a billet. During casting, the superheat of the tundish temperature is controlled at 25-30℃, the casting speed is 0.5-1.2m / min, and conventional protective gas is used for protection throughout the casting process.

[0052] 2) Stack the billets for slow cooling, with a cooling time of not less than 24 hours;

[0053] 3) Heat the billet, controlling the heating temperature at 1250-1350℃ and the heating time at 150-300 minutes;

[0054] 4) Rolling is carried out, and the roughing rolling temperature is controlled at 1050-1150℃; the finishing rolling temperature is controlled at 840-900℃.

[0055] 5) Cool the rolled rails, controlling the initial cooling temperature at 690–790℃; cool the rail heads according to different cross-sectional specifications of the rails, and control the total cooling time at 50–100 seconds.

[0056] The rail head is cooled in the following manner:

[0057] A. During the ≤6s period after the rail head begins to cool, control the cooling rate to be between 0.8 and 1.55℃ / s.

[0058] B. During the cooling period of >6s to ≤40s for the rail head, control the cooling rate to be between 2.5 and 8℃ / s.

[0059] C. During the cooling period of >40s to 100s for the rail head, the cooling rate is set at 10 to 20 seconds per second.

[0060] The speed is reduced by 20-40% compared to the previous time, and cooling is stopped when the temperature of the rail head drops to 480-560℃.

[0061] The rail head is cooled simultaneously with the rail base, and the cooling intensity in the middle of the rail base is 80-96% of the cooling intensity in the rail head.

[0062] D. Allow to cool naturally to room temperature.

[0063] Table 1. List of chemical components (wt%) of various embodiments and comparative examples of the present invention.

[0064]

[0065] Table 2. List of main process parameters for each embodiment and comparative example of the present invention.

[0066]

[0067] Table 3. List of rail head cooling process parameters for various embodiments and comparative examples of the present invention.

[0068]

[0069] Continued from Table 3

[0070]

[0071] The tensile strength, elongation after fracture, tread hardness, and metallographic structure of the rails obtained in the examples and comparative examples were determined according to the methods specified in the TB / T 2344.1-2020 rail standard, and the results are shown in Table 4. The Rockwell hardness test results along lines A, B, C, D, and E of the rail cross-section were performed according to the requirements of TB / T 2344.1-2020 standard for cross-sectional hardness testing of heat-treated rails, and are shown in Table 5. Vickers hardness tests were performed at different distances from the surface along lines B and C of the rail cross-section, and the results are shown in Tables 6 and 7.

[0072] Table 4. List of mechanical property test results for each embodiment and comparative example of the present invention.

[0073]

[0074] Table 5. List of Rockwell hardness of rail cross sections in various embodiments and comparative examples of the present invention.

[0075]

[0076] Continued from Table 5

[0077]

[0078] Table 6. Vickers hardness HV0.2 at different distances from the surface of the B line in the cross-section of the rail in various embodiments and comparative examples of the present invention.

[0079]

[0080] Table 7. Vickers hardness / HV0.2 at different distances from the C-line of the rail cross-section to the surface in various embodiments and comparative examples of the present invention.

[0081]

[0082] As shown in Tables 4 to 7, the rails in Examples 1-8 exhibit good mechanical properties and metallographic structure, meeting all relevant standard requirements. Along lines A, B, C, D, and E of the rail cross-section, the Rockwell hardness shows a uniform decreasing trend from the surface to the interior. In Comparative Examples 1 and 2, the Rockwell hardness of the rail cross-section shows a soft spot at the first point (5 mm below the surface). However, in the heat-treated rail produced using this method, the soft spot appears 0.2-0.4 mm inward from the surface. In comparison, the method of this invention effectively improves the distribution of low-hardness points while ensuring the microstructure and mechanical properties of the heat-treated rail.

[0083] This specific embodiment is merely a best example and is not intended to limit the implementation of the technical solution of the present invention.

Claims

1. A production method for increasing the height of low-point distribution of hardness in the cross-section of heat-treated steel rails, comprising the following steps: 1) After smelting and refining, the billet is cast into a billet. During casting, the superheat of the tundish temperature is controlled at 25-30℃, the casting speed is 0.5-1.2m / min, and protective gas is used throughout the casting process. 2) Stack the billets for slow cooling, with a cooling time of not less than 24 hours; 3) Heat the billet, controlling the heating temperature at 1250-1350℃ and the heating time at 150-300 minutes; 4) Rolling is carried out, and the roughing rolling temperature is controlled at 1050-1150℃; the finishing rolling temperature is controlled at 840-900℃. 5) Cool the rolled rails, controlling the initial cooling temperature at 690–790℃; cool the rail heads according to different cross-sectional specifications of the rails, and control the total cooling time at 50–100 seconds. The rail head is cooled in the following manner: A. During the ≤6s period after the rail head begins to cool, control the cooling rate to be between 0.8 and 1.55℃ / s. B. During the cooling period of >6s to ≤40s for the rail head, control the cooling rate to be between 2.5 and 8℃ / s. C. During the cooling period of >40s to 100s for the rail head, the cooling rate is set at 10 to 20 seconds per second. The speed is reduced by 20-40% compared to the previous time, and cooling is stopped when the temperature of the rail head drops to 480-560℃. The rail head is cooled simultaneously with the rail base, and the cooling intensity in the middle of the rail base is 80-96% of the cooling intensity in the rail head. D. Allow to cool naturally to room temperature.

2. The production method for increasing the height of low-point distribution of hardness in the cross-section of heat-treated steel rails as described in claim 1, characterized in that: Within ≤6 seconds of the rail head beginning to cool, the cooling rate should be controlled at 1.0~1.45℃ / s.

3. The production method for increasing the height of low-point distribution of hardness in the cross-section of heat-treated steel rails as described in claim 1, characterized in that: During the cooling period of >6s to ≤40s for the rail head, the cooling rate should be controlled at 3 to 6.5℃ / s.

4. The production method for increasing the height of low-point distribution of hardness in the cross-section of heat-treated steel rails as described in claim 1, characterized in that: During the >40s to 100s cooling period of the rail head, the cooling rate is set at 10 to 20 seconds per second. The speed decreased by 25-36% compared to the previous time.

5. The production method for increasing the distribution height of low points in the cross-sectional hardness of heat-treated steel rails as described in claim 1, characterized in that: In the cooling of rail heads with different cross-sectional specifications, the total cooling time of 50 to 100 seconds is positively correlated with the size of the cross-sectional specifications.

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