Heat treatment process for improving fracture toughness of large vanadium-containing railway wheel

Abstract

The invention discloses a heat treatment process for improving fracture toughness of a large-sized vanadium-containing railway wheel, which comprises the following steps: S1, a rolled blank wheel is loaded into a progressive annular heating furnace, the wheel sequentially passes through five large sections, namely a preheating section, a heating section I, a heating section II, a heating section III and a soaking section in the furnace, and the total heating time is 3.0-4.5 hours; the soaking section is equally divided into five small areas, the five small areas are numbered as a soaking 1 # area to a soaking 5 # area in sequence, and the temperature of the soaking 1 # area to the soaking 2 # area is controlled to be (T + T) + / -5 DEG C; the temperature of the soaking area 3 to the soaking area 5 is controlled to be T + / -5 DEG C; wherein T is the target soaking temperature, 850 < = T < = 900 DEG C, and 25 < = T < = 40 DEG C; s2, the wheel subjected to heating and heat preservation is taken out for tread quenching; s3, tempering is conducted; by means of the process, the fracture toughness level and uniformity of the large-specification vanadium-containing railway wheel can be effectively improved, and good obdurability matching is achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of railway wheel manufacturing, and particularly relates to a heat treatment process for improving the fracture toughness of large-sized vanadium-containing railway wheels. Background Art

[0002] Ferritic-pearlitic railway wheels have excellent wear resistance, thermal stability and machinability, and have been widely used in countries and regions around the world. As a key running component of trains, wheels are subjected to complex thermal-mechanical loads during service. Improving the strength-ductility matching relationship of wheels plays an important role in ensuring their safe and reliable service. Among them, fracture toughness is an important index to measure the ability to resist the unstable propagation of cracks, and has an important impact on potential failure risks such as rim cracks and braking hot cracks during the service of wheels.

[0003] With the accelerating evolution of the high-speed and heavy-haul processes of railways, higher requirements are put forward for the fracture toughness and strength-ductility matching of wheels, which has led to the trend of microalloying treatment in the development of high-quality wheels. By adding microalloying elements, the strength of wheels can be significantly improved. However, due to the competitive relationship between strength and toughness, it is very difficult to further improve the fracture toughness while obtaining high strength, and the factors affecting fracture toughness are numerous and complex. Therefore, in a sense, the ability to control fracture toughness can better reflect the production technology level of wheels. In addition, for large-sized wheels with large wheel diameters and large rim thicknesses, the "size effect" will inevitably increase the difficulty of controlling the microstructure and property uniformity of the wheels.

[0004] Therefore, how to significantly improve the fracture toughness of large-sized vanadium-containing railway wheels without affecting the conventional mechanical properties such as the strength of the wheels has become a technical problem.

[0005] After retrieval, for example, in Chinese Patent, application publication number: CN 103741047 A, application publication date: April 23, 2014, a medium-carbon steel wheel steel for railway locomotives and a wheel preparation method for improving fracture toughness are disclosed. Based on the standard EN 13262 "Railway applications - Wheelsets and bogies - Wheels - Product requirements", the contents of Si, Mn, Ni, Cr, and Als in ER7 wheel steel are adjusted and designed, and a high-temperature normalizing process is added before the conventional heat treatment process, that is, the whole wheel is heated to a temperature higher than the conventional quenching heating temperature and then taken out of the furnace and air-cooled to room temperature to improve the fracture toughness by improving the microstructure uniformity of the wheels.

[0006] For another example, in a Chinese patent, the application publication number is: CN 103741021 A, and the application publication date is: April 23, 2014. It discloses a high-toughness wheel steel for railway wagons and a wheel preparation method. The Si, Mn, Ni, and Als in the CL65 wheel steel are adjusted and designed, and a low-temperature normalizing process is added before the conventional heat treatment process, that is, the whole wheel is heated to a temperature lower than the conventional quenching heating temperature and then taken out of the furnace and air-cooled to room temperature, so as to improve the fracture toughness by refining the grain structure of the wheel.

[0007] However, the technical measures disclosed in the above two patents will inevitably lead to a significant increase in the heat treatment process time due to the two heat treatments, disrupting the production rhythm and affecting the actual production efficiency, and are not very suitable for mass production with a fast rhythm and large output. Summary of the Invention

[0008] To solve the problems of low fracture toughness and poor uniformity of large-sized vanadium-containing wheels, the present invention provides a heat treatment process for improving the fracture toughness of large-sized vanadium-containing railway wheels. This process can effectively improve the fracture toughness level and uniformity of large-sized vanadium-containing railway wheels and obtain good strength and toughness matching.

[0009] The technical solution adopted by the present invention is as follows:

[0010] A heat treatment process for improving the fracture toughness of large-sized vanadium-containing railway wheels, the heat treatment process comprising the following steps:

[0011] S1: Load the rolled and formed blank wheel into a progressive ring heating furnace. The wheel successively experiences five major sections in the furnace: a preheating section, a heating section I, a heating section II, a heating section III, and a soaking section. The total heating time is 3.0 - 4.5 h;

[0012] S2: Take out the wheel after heating and insulation and perform tread quenching so that a pearlite structure is completely formed in the area between the tread surface of the wheel and the transition between the rim and the web;

[0013] S3: Tempering, the purpose is to decompose the martensite structure formed by the strong cooling in the S2 stage on the tread surface layer, facilitate subsequent machining, and at the same time release and improve the heat treatment residual compressive stress generated in the S2 stage;

[0014] In step S1, the temperature controls of the four major sections of the preheating section, the heating section I, the heating section II, and the heating section III are respectively (T - 90) ± 30 °C, (T - 50) ± 20 °C, (T - 15) ± 15 °C, and T ± 10 °C; where T is the target soaking temperature, and 850 ≤ T ≤ 900 °C is set.

[0015] In step S1, the residence times of the four major sections of the preheating section, heating section I, heating section II, and heating section III account for 18 - 20%, 8 - 10%, 8 - 10%, and 18 - 20% of the total heating time respectively, and the remaining time is allocated to the soaking section, but the holding time of the soaking section is not less than 1.25 h.

[0016] In step S1, the soaking section is equally divided into 5 small zones, numbered soaking zone 1# to soaking zone 5# in sequence. The temperature of soaking zone 1# to soaking zone 2# is controlled at (T + △T) ± 5°C; the temperature of soaking zone 3# to soaking zone 5# is controlled at T ± 5°C; where 25 ≤ △T ≤ 40°C.

[0017] In step S2, the water cooling time of the wheel on the quenching table is 300 - 500 s.

[0018] In step S2, the total water flow rate of the quenching table is 110 - 130 tons per hour, and there are 6 box - type spray guns evenly distributed circumferentially. The water flow rate of each spray gun is equal, and the flow velocity is 7 - 10 m / s.

[0019] In step S3, the tempering average temperature is 470 - 520°C, the tempering time ≥ 4 h, and after tempering, it is taken out of the furnace and air - cooled.

[0020] The blank outer diameter of the large - size vanadium - containing railway wheel is 900 - 1300 mm, and the rim thickness is 80 - 120 mm.

[0021] The weight percentage of V contained in the large - size vanadium - containing railway wheel is 0.04 - 0.20%.

[0022] Furthermore, the chemical composition of the large - size vanadium - containing railway wheel in terms of weight percentage is: C 0.50 - 0.60%; Si 0.20 - 0.80%; Mn 0.70 - 1.00%; P ≤ 0.015%; S ≤ 0.015%; V 0.04 - 0.20%; Cr 0.15 - 0.30%; Al ≤ 0.015%; Ti ≤ 0.005%; N (60 - 90) × 10 -4 %; the balance is Fe and unavoidable impurity elements.

[0023] The single - value of the fracture toughness of the large - size vanadium - containing railway wheel ≥ 75 MPa·m 1 / 2 and the average value ≥ 80 MPa·m 1 / 2 , and the fracture toughness range ≤ 12 MPa·m 1 / 2 .

[0024] Vanadium, as a strong carbide and nitride forming element, the solubility of vanadium in steel is affected by the heating temperature. The higher the heating temperature, the higher the solubility of vanadium. In the subsequent cooling and phase transformation stage, the precipitation amount of carbide and nitride is more, and the precipitation strengthening effect is stronger. While obtaining higher strength, it also has a challenging impact on the strength-ductility matching relationship. After the wheel of the present invention is heated section by section until the end of heating section III, a certain degree of austenitization process has been carried out at this time, and the target set soaking temperature is basically reached. A certain proportion of vanadium has been dissolved into the austenite matrix during this process. Subsequently, through the soaking zone 1# to soaking zone 2# at a higher temperature, the solid solution ratio of vanadium is further increased at this time, achieving full dissolution or complete dissolution. Then, through the soaking process in soaking zone 3# to soaking zone 5# with the same temperature as the target temperature, due to the decrease in solubility, the vanadium dissolved in the matrix will inevitably "precipitate", pre-precipitate from the matrix and exist in the form of a vanadium-containing second phase. Since the vanadium-containing second phase pre-precipitated at high temperature has a semi-coherent interface with the austenite matrix, compared with the coherent interface, the misfit degree is larger, and the bonding strength of the two-phase interface is weaker. Therefore, the vanadium-containing second phase pre-precipitated in the matrix has limited contribution to the strength of the final material, which provides favorable conditions for toughness reserve.

[0025] Since vanadium atoms are not easily segregated at grain boundaries, once vanadium precipitates are formed at grain boundaries, a vanadium-depleted microzone will be generated around the precipitates. If precipitation continues, the vanadium atoms dissolved in the grains need to be transported to the grain boundaries in the form of volume diffusion. The potential barrier that this diffusion needs to cross is very large. Therefore, once a vanadium-containing second phase is formed at the grain boundaries, it will no longer have the ability to nucleate continuously. Thereafter, the nucleation mainly occurs in the grains. In other words, the vanadium-containing second phase pre-precipitated from the austenite matrix in the present invention mainly precipitates in the grains.

[0026] In the subsequent cooling and phase transformation stage, the supercooled austenite transforms into ferrite-pearlite. Ferrite not only undergoes pro-eutectoid transformation at grain boundaries, but also because the planar lattice misfit degree between the ferrite and the vanadium-containing second phase pre-precipitated in the grains is much lower than its misfit degree with austenite, the vanadium-containing second phase pre-precipitated in the grains becomes a secondary site (nucleation catalyst) for ferrite nucleation, effectively inducing heterogeneous nucleation of ferrite in the grains, thereby increasing the number of ferrite nuclei. Here, this type of ferrite is named "intragranular ferrite".

[0027] As a ferritic plastic soft phase, its interface is relatively "weak" and plays an important role in the process of crack initiation and propagation. The appearance of intragranular ferrite increases the phase interface in the microstructure, making the crack propagation path tend to bypass the hard pearlite clusters and travel near the softer ferrite. The stress peak at the crack tip entering the ferrite interface is relaxed, resulting in plastic passivation. The propagation path bends and bifurcates multiple times, and the total resistance encountered during crack propagation increases. Multiple complex deflections occur and consume more energy, leading to an increase in the fracture absorption work, and thus macroscopically showing higher fracture toughness.

[0028] The heat treatment process for improving the fracture toughness of large-sized vanadium-containing railway wheels provided by the present invention is based on the reasonable distribution of quenching heating, and develops a temperature control technology of "high-temperature heating and low-temperature soaking" in the soaking section, realizing the pre-precipitation of vanadium-containing second phases to induce intragranular ferrite. By changing the weight contribution of vanadium precipitation strengthening and synergistically optimizing the ferrite morphology distribution, the fracture toughness level and uniformity are significantly improved.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] (1) Through reasonable heating time distribution and the temperature control measures of "high-temperature heating and low-temperature soaking" in the soaking section, the vanadium dissolved in the matrix undergoes "precipitation", pre-precipitates from the matrix and exists in the form of vanadium-containing second phases with a semi-coherent relationship with the matrix, and serves as a nucleation catalyst for inducing intragranular ferrite in the subsequent cooling phase transformation stage.

[0031] (2) The existence of intragranular ferrite increases the phase interface in the microstructure, making the crack propagation path tend to bypass the hard pearlite clusters and travel near the softer ferrite. The total resistance encountered during crack propagation increases. Multiple complex deflections occur and consume more energy, leading to an increase in the fracture absorption work, and thus macroscopically showing higher fracture toughness.

[0032] (3) Compared with the conventional process, using the heat treatment process provided by the present invention can significantly improve the fracture toughness level and uniformity without reducing the strength and hardness of the wheel rim, and obtain a single fracture toughness value ≥ 75 MPa·m 1 / 2 , an increase rate ≥ 24%, an average value ≥ 80 MPa·m 1 / 2 , an increase rate ≥ 13%, controlling the fracture toughness difference ≤ 12 MPa·m 1 / 2 , a decrease rate ≥ 44%, and the strength and toughness are well matched.

[0033] (4) The process is simple and easy to implement, does not require equipment modification, and is convenient for industrial implementation and application. Description of the Drawings

[0034] Figure 1It is a schematic diagram of the cross-section of a wheel rim. Point A is the transition between the rim and the spoke. After quenching treatment, the area between the tread surface of the wheel and the worn limit point and point A is completely formed into pearlite structure;

[0035] Figure 2 It is the microstructure of the wheel in Example 1. Among them, in addition to pearlite and proeutectoid ferrite in a discontinuous network along the grain boundaries, intragranular ferrite in the shape of ellipsoids or particles is also precipitated on the pearlite matrix, as indicated by the arrows in the figure;

[0036] Figure 3 It is the SEM morphology of intragranular ferrite in the microstructure of the wheel in Example 1;

[0037] Figure 4 It is the microstructure of the wheel in Comparative Example 1, and no intragranular ferrite is seen;

[0038] Figure 5 It is the second vanadium-containing phase pre-precipitated from the matrix of the wheel in Example 1, and it has a semi-coherent interface with the austenite matrix;

[0039] Figure 6 It is a schematic diagram of the sampling positions of the tensile specimens and impact specimens of the wheel rims of Examples 1-3 and Comparative Examples 1-3;

[0040] Figure 7 It is a schematic diagram of the sampling position of the fracture toughness of the wheel rims of Examples 1-3 and Comparative Examples 1-3. Specific Embodiments

[0041] The present invention will be described in detail below in conjunction with the embodiments.

[0042] Example 1

[0043] The chemical composition of the wheel in Example 1 is shown in Table 1, and the balance is Fe and inevitable impurity elements. The outer diameter of the wheel is 920 mm, and the rim thickness is 83 mm. To improve the fracture toughness, heat treatment is carried out according to the following steps:

[0044] S1: Load the rolled blank wheel into a progressive ring heating furnace. The wheel goes through five major sections in the furnace in turn: the preheating section, the heating section I, the heating section II, the heating section III, and the soaking section. The total heating time is 3.0 h. The residence time in the first four major sections accounts for 20%, 8%, 8%, and 20% of the total heating time in turn, and the remaining time is allocated to the soaking section. Control the temperatures of the first four major sections to be 790±30 °C, 830±20 °C, 865±15 °C, and 880±10 °C respectively. The soaking section is equally divided into 5 small zones, numbered soaking zone 1# to soaking zone 5# in turn. Control the temperatures of soaking zone 1# to soaking zone 2# to be 910±5 °C, and control the temperatures of soaking zone 3# to soaking zone 5# to be 880±5 °C.

[0045] S2: Take out the wheel after heating and heat preservation, move it to the horizontal quenching table, and cool it by spraying cooling water onto the tread surface of the wheel. The water spraying cooling time is 320 s, the total water flow rate of the quenching table is 110 tons per hour, and there are 6 box-type spray guns evenly distributed circumferentially. The water flow rate of each spray gun is equal, and the flow velocity is 7 m / s, ensuring that the area between the surface of the wheel tread to the worn limit and point A is completely formed with pearlite structure, as Figure 1 shown.

[0046] S3: Take out the wheel after water spraying cooling, and transfer it to the tempering furnace through the roller conveyor chain bed for tempering treatment in 5 - 10 min. The tempering uniform temperature is 490 °C, and the tempering time is 4.5 h. The purpose is to decompose the martensite structure formed by the strong cooling in the S2 stage on the tread surface, which is convenient for subsequent machining, and at the same time release and improve the heat treatment residual compressive stress generated in the S2 stage. After tempering, it is taken out of the furnace and air-cooled, and then machined to obtain the finished wheel.

[0047] Comparative Example 1

[0048] The chemical composition of the wheel in Comparative Example 1 is shown in Table 1, and the balance is Fe and inevitable impurity elements. The outer diameter of the wheel is 922.5 mm, and the rim thickness is 81 mm. Its composition and specification dimensions are similar to those of the wheel in Example 1.

[0049] The heating system of the first four major sections of the wheel in Comparative Example 1 in the ring heating furnace is exactly the same as that of the wheel in Example 1. The difference is that the 5 small positions in the soaking section are all controlled at 880 ± 10 °C, and no small-scale heating-up and then cooling-down treatment is carried out. The water spraying cooling and tempering treatment systems of the wheel in Comparative Example 1 are exactly the same as those of the wheel in Example 1.

[0050] As Figure 2 shown, the microstructure of the wheel in Example 1 is pearlite and proeutectoid ferrite in a discontinuous network along the grain boundaries. At the same time, ellipsoidal or granular ferrite will precipitate on the pearlite matrix, as Figure 2 indicated by the arrow in, that is, intragranular ferrite. Its SEM morphology is as Figure 3 shown. The interface between the intragranular ferrite and the pearlite matrix is clearly layered and the interface is flat and smooth. Looking at the wheel in Comparative Example 1, as Figure 4 shown, no intragranular ferrite appears in its microstructure.

[0051] The formation of intragranular ferrite in the wheel of Example 1 is due to the wheel being first heated at a high temperature and then soaked at a low temperature in the soaking section of the heating furnace. During this process, due to the decrease in solubility, the vanadium dissolved in the matrix must "precipitate", pre-precipitate from the matrix and exist in the form of a vanadium-containing second phase, as Figure 5As shown, since the pre-precipitated vanadium-containing second phase at high temperature has a semi-coherent interface with the austenite matrix, compared with the coherent interface, the misfit degree is larger and the bonding strength of the two-phase interface is weaker. Therefore, the pre-precipitated vanadium-containing second phase in the matrix has a limited contribution to the strength of the final material, which provides favorable conditions for toughness reserve. In the subsequent cooling phase transformation stage, the supercooled austenite transforms into ferrite-pearlite. Ferrite not only undergoes pro-eutectoid precipitation at the grain boundaries, but also because the planar lattice misfit degree between the ferrite and the pre-precipitated vanadium-containing second phase in the grains is much lower than that between the ferrite and the austenite, the pre-precipitated vanadium-containing second phase in the grains becomes the nucleation catalyst for ferrite, effectively inducing heterogeneous nucleation of ferrite in the grains.

[0052] As Figure 6 shown, tensile specimens are taken 15 mm below the middle of the tread surface of the finished wheel (i.e., corresponding to the position marked 1# in Figure 6 ), and the tensile mechanical properties of the rims of the wheels in Example 1 and Comparative Example 1 are tested according to GB / T 228.1—2010 "Metallic materials - Tensile testing - Part 1: Method of test at room temperature". As Figure 6 shown, Charpy impact specimens are taken at three corresponding positions on the rim cross-section (i.e., corresponding to the positions marked 1#, 2#, and 3# in Figure 6 ), and the -20°C impact performance of the wheels in Example 1 and Comparative Example 1 is tested according to GB / T 229—2007 "Metallic materials - Charpy pendulum impact test method". As Figure 1 shown, three points are taken at 10, 20, 30, and 40 mm below the tread profile along the rim cross-section (near the rim, middle of the tread, near the outer side), and the Brinell hardness of the wheels in Example 1 and Comparative Example 1 is tested according to GB / T 231.1—2002 "Metallic materials - Brinell hardness test - Part 1: Test method". As Figure 7 shown, six compact tensile specimens are taken evenly distributed circumferentially in the rim part, and the linear elastic plane strain fracture toughness of the wheels in Example 1 and Comparative Example 1 is tested according to GB / T 4161—2007 "Metallic materials - Plane-strain fracture toughness K IC test method". The test results of the cross-section hardness show that the average hardness values of the wheel in Example 1 at 10, 20, 30, and 40 mm from the tread are 285, 279, 275, and 270 HB respectively, and the average hardness values of the wheel in Comparative Example 1 at 10, 20, 30, and 40 mm from the tread are 287, 278, 274, and 272 HB respectively. The other corresponding test results are shown in Table 2.

[0053] It can be seen that compared with the conventional process, using the present invention can significantly improve the fracture toughness and the level and uniformity of low-temperature impact toughness while not reducing the strength and hardness of the rim, and obtain a better strength-toughness matching.

[0054] Table 1 Main chemical components (wt%) of the wheels in the examples and comparative examples

[0055]

[0056] Table 2 Test Results of Rim Tensile Properties, Impact Energy and Fracture Toughness

[0057]

[0058]

[0059] Example 2

[0060] The chemical composition of the wheel in Example 2 is shown in Table 1, and the balance is Fe and inevitable impurity elements. The outer diameter of the wheel is 1098 mm, and the rim thickness is 102 mm. To improve the fracture toughness, heat treatment is carried out according to the following steps:

[0061] S1: Load the rolled and formed blank wheel into a progressive ring heating furnace. The wheel goes through five major sections in the furnace: the preheating section, heating section I, heating section II, heating section III, and soaking section. The total heating time is 3.5 h. The residence times in the first four major sections account for 18%, 8%, 9%, and 19% of the total heating time respectively, and the remaining time is allocated to the soaking section. Control the temperatures of the first four major sections to be 800±30°C, 840±20°C, 875±15°C, and 890±10°C respectively. The soaking section is evenly divided into 5 small zones, numbered soaking zone 1# to soaking zone 5# in sequence. Control the temperatures of soaking zone 1# to soaking zone 2# to be 930±5°C, and control the temperatures of soaking zone 3# to soaking zone 5# to be 890±5°C.

[0062] S2: Take out the wheel after heating and holding, move it to a horizontal quenching table, and cool it by spraying cooling water towards the tread of the wheel. The spraying cooling time is 400 s. The total water flow rate of the quenching table is 120 tons per hour. There are 6 box-type spray guns evenly distributed circumferentially, and the water flow rate of each spray gun is equal, with a flow velocity of 9 m / s, ensuring that the area between the tread surface of the wheel to the worn limit and point A is completely formed with pearlite structure, as Figure 1 shown

[0063] S3: Take out the wheel after spraying cooling, transfer it to a tempering furnace through a roller conveyor chain bed for 5 - 10 minutes for tempering treatment. The tempering average temperature is 520°C, and the tempering time is 5 h. The purpose is to decompose the martensite structure formed on the tread surface due to strong cooling in the S2 stage, facilitate subsequent machining, and at the same time release and improve the heat treatment residual compressive stress generated in the S2 stage. After tempering, take it out of the furnace and air cool it, and then carry out machining to obtain the finished wheel.

[0064] Comparative Example 2

[0065] For Comparative Example 2, the chemical composition of the wheel is shown in Table 1, and the balance is Fe and inevitable impurity elements. The outer diameter of the wheel is 1092 mm, and the rim thickness is 100 mm. Its composition and specification dimensions are similar to those of the wheel in Example 2.

[0066] For the heating regime of the wheel in Comparative Example 2 in the first four major sections of the annular heating furnace, it is exactly the same as that of the wheel in Example 2. The difference is that the temperature of the 5 small zones in the soaking section is controlled at 890 ± 10 °C, and no small-scale temperature increase and then decrease treatment is carried out. The water spray cooling and tempering treatment regime of the wheel in Comparative Example 2 is exactly the same as that of the wheel in Example 2.

[0067] Referring to Example 1 and Comparative Example 1, the rim tensile mechanical properties, -20 °C impact properties, fracture toughness and cross-sectional hardness of the wheels in Example 2 and Comparative Example 2 were tested. The cross-sectional hardness test results show that the average hardness values at 10, 20, 30, and 40 mm from the tread of the wheel in Example 2 are 281, 275, 270, and 266 HB respectively, and the average hardness values at 10, 20, 30, and 40 mm from the tread of the wheel in Comparative Example 2 are 284, 276, 270, and 265 HB respectively. The other corresponding test results are shown in Table 2.

[0068] It can be seen that compared with the conventional process, adopting the present invention can significantly improve the fracture toughness and the level and uniformity of low-temperature impact toughness while not reducing the strength and hardness of the rim, and a better strength and toughness matching is obtained.

[0069] Example 3

[0070] The chemical composition of the wheel in Example 3 is shown in Table 1, and the balance is Fe and inevitable impurity elements. The outer diameter of the wheel is 1280 mm, and the rim thickness is 120 mm. To improve the fracture toughness, the heat treatment is carried out according to the following steps:

[0071] S1: Load the rolled and formed blank wheel into a progressive annular heating furnace. The wheel successively experiences five major sections in the furnace: the preheating section, the heating section I, the heating section II, the heating section III, and the soaking section. The total heating time is 4.5 h. The residence times in the first four major sections account for 19%, 8%, 8%, and 20% of the total heating time respectively, and the remaining time is allocated to the soaking section. Control the temperatures of the first four major sections to be 770 ± 30 °C, 810 ± 20 °C, 845 ± 15 °C, and 860 ± 10 °C respectively. The soaking section is equally divided into 5 small zones, numbered successively as soaking zone 1# to soaking zone 5#. Control the temperatures of soaking zone 1# to soaking zone 2# to be 885 ± 5 °C, and control the temperatures of soaking zone 3# to soaking zone 5# to be 860 ± 5 °C.

[0072] S2: Take out the wheel after heating and heat preservation, move it to the horizontal quenching table, and cool it by spraying cooling water towards the tread of the wheel. The spraying cooling time is 480 s, the total water flow rate of the quenching table is 130 tons per hour, there are 6 box-type spray guns evenly distributed circumferentially, the water flow rate of each spray gun is equal, and the flow velocity is 10 m / s, ensuring that the area between the surface of the wheel tread to the worn limit and point A is completely formed into pearlite structure, as Figure 1 shown.

[0073] S3: Take out the wheel after spraying cooling, and transfer it to the tempering furnace through the roller chain plate bed for tempering treatment in 5 - 10 min. The tempering uniform temperature is 470 °C, and the tempering time is 6 h. The purpose is to decompose the martensite structure formed by strong cooling in the S2 stage on the tread surface, facilitate subsequent machining, and at the same time release and improve the heat treatment residual compressive stress generated in the S2 stage. After tempering, take it out of the furnace and air cool it, and then carry out machining to obtain the finished wheel.

[0074] Comparative Example 3

[0075] The chemical composition of the wheel in Comparative Example 3 is shown in Table 1, and the balance is Fe and inevitable impurity elements. The outer diameter of the wheel is 1290 mm, and the rim thickness is 119 mm. Its composition and specification dimensions are similar to those of the wheel in Example 3.

[0076] The heating system of the wheel in Comparative Example 3 in the first four major sections of the ring heating furnace is exactly the same as that of the wheel in Example 3. The difference is that the 5 small positions in the soaking section are all controlled at 860 ± 10 °C, and no small-scale heating and then cooling treatment is carried out. The spraying cooling and tempering treatment systems of the wheel in Comparative Example 3 are exactly the same as those of the wheel in Example 3.

[0077] Referring to Example 1 and Comparative Example 1, the rim tensile mechanical properties, -20 °C impact properties, fracture toughness and cross-section hardness of the wheels in Example 3 and Comparative Example 3 are tested. The cross-section hardness test results show that the average hardness values at 10, 20, 30, 40 mm from the tread of the wheel in Example 3 are 279, 274, 268, 265 HB respectively, and the average hardness values at 10, 20, 30, 40 mm from the tread of the wheel in Comparative Example 3 are 281, 275, 269, 262 HB respectively. The other corresponding test results are shown in Table 2.

[0078] It can be seen that compared with the conventional process, adopting the present invention can significantly improve the fracture toughness and low-temperature impact toughness level and uniformity without reducing the strength and hardness of the rim, and obtain better strength and toughness matching.

[0079] The above detailed description of a heat treatment process for improving the fracture toughness of large-sized vanadium-containing railway wheels with reference to the embodiments is illustrative rather than restrictive. Several embodiments can be enumerated within the defined scope. Therefore, changes and modifications without departing from the general concept of the present invention shall fall within the protection scope of the present invention.

Claims

1. A heat treatment process for improving the fracture toughness of large-sized vanadium-containing railway wheels, characterized in that, The heat treatment process includes the following steps: S1: Load the rolled and formed blank wheel into a progressive ring heating furnace. The wheel successively goes through five major sections in the furnace, namely the preheating section, heating section I, heating section II, heating section III, and soaking section. The total heating time is 3.0 - 4.5 h; S2: Take out the wheel after heating and insulation and perform tread quenching; S3: Temper; In step S1, the soaking section is equally divided into 5 small zones, numbered soaking zone 1# to soaking zone 5# in sequence. Control the temperature of soaking zone 1# to soaking zone 2# to be (T + △T) ± 5 °C; control the temperature of soaking zone 3# to soaking zone 5# to be T ± 5 °C; where T is the target soaking temperature, 850 ≤ T ≤ 900 °C, 25 ≤ △T ≤ 40 °C.

2. The heat treatment method according to claim 1, wherein, The outer diameter of the blank of the large - sized vanadium - containing railway wheel is 900 - 1300 mm, and the rim thickness is 80 - 120 mm.

3. The heat treatment process according to claim 1 or 2, characterized in that, In step S1, the temperature control of the four major sections of the preheating section, heating section I, heating section II, and heating section III are respectively (T - 90) ± 30 °C, (T - 50) ± 20 °C, (T - 15) ± 15 °C, and T ± 10 °C.

4. The heat treatment method according to claim 1 or 2, characterized in that In step S1, the residence time ratios of the four major sections of the preheating section, heating section I, heating section II, and heating section III in the total heating time are 18 - 20%, 8 - 10%, 8 - 10%, and 18 - 20% in sequence. The remaining time is allocated to the soaking section, but the holding time of the soaking section is not less than 1.25 h.

5. The heat treatment method according to claim 1 or 2, characterized in that, In step S2, the water - spraying cooling time of the wheel on the quenching table is 300 - 500 s.

6. The heat treatment method according to claim 1 or 2, characterized in that, In step S2, the total water flow rate of the quenching table is 110 - 130 tons per hour. There are 6 box - type spray guns evenly distributed circumferentially. The water flow rate of each spray gun is equal, and the flow velocity is 7 - 10 m / s.

7. The heat treatment method according to claim 1 or 2, characterized in that, In step S3, the tempering soaking temperature is 470 - 520 °C, the tempering time ≥ 4 h, and after tempering, it is taken out of the furnace and air - cooled.

8. The heat treatment method according to claim 1 or 2, characterized in that, The weight percentage of V contained in the large - sized vanadium - containing railway wheel is 0.04 - 0.20%.

9. The heat treatment method according to claim 1 or 2, characterized in that The chemical composition of the large - sized vanadium - containing railway wheel is calculated by weight percentage as follows: C 0.50 - 0.60%; Si 0.20 - 0.80%; Mn 0.70 - 1.00%; P ≤ 0.015%; S ≤ 0.015%; V 0.04 - 0.20%; Cr 0.15 - 0.30%; Al ≤ 0.015%; Ti ≤ 0.005%; N (60 - 90) × 10 -4 %; the balance is Fe and inevitable impurity elements.

10. The heat treatment method according to claim 1 or 2, characterized in that, The fracture toughness single value of the large-size vanadium-containing railway wheel is ≥75MPa·m 1 / 2 , average value ≥80MPa·m 1 / 2 , fracture toughness is poor ≤12MPa·m 1 / 2 .

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