A variable-hardness single-tooth track steel and its preparation method

Abstract

This invention discloses a variable-hardness single-tooth track steel, the composition and weight percentage of which are: C 0.32-0.36%, Si 0.63-0.73%, Mn 1.3-1.7%, P≤0.006%, S≤0.005%, Nb 0.017-0.026%, Als 0.062-0.075%, Cr 0.82-0.92%, Co 0.73-0.85%, Ti 0.037-0.046%, H≤0.00006%, O≤0.0008%; the remainder is Fe and impurity elements. This invention mainly employs quenching and tempering processes tailored to the shape characteristics of the track steel, imparting different hardness and toughness to different parts of the track steel. This effectively improves the wear resistance and toughness of the track teeth, increases the service life of the track steel, and reduces maintenance and replacement costs, making it suitable for widespread application.

Description

Technical Field

[0001] This invention belongs to the field of steel preparation technology, specifically relating to a variable hardness single-tooth track steel and its preparation method. Background Technology

[0002] Large-pitch single-tooth track steel is mainly used to manufacture tracks for the chassis of large mining machinery, and currently it is mainly imported. Since mining machinery (such as bulldozers and excavators) is mainly used in coal mines, gold mines, iron mines and other mining areas, track steel is required to have good grip and wear resistance. The track steel adopts a single-foot type, and grip is achieved by pressing the track teeth into the road surface. The track teeth are connected to the web plate to achieve gripping and movement.

[0003] The mining area is characterized by irregularly shaped ore deposits, leading to severe wear on the track teeth during movement. The track teeth typically have a lifespan of only 6-7 months. Track steel consists of track plates and track teeth. The track plates primarily bear the weight of the entire mining machinery, approximately 70-80 tons, while the track teeth experience significant wear due to contact with the ground (sand, ore). Track steel failure usually manifests as track plate breakage due to load, and track teeth typically become unusable after about 7 months of wear. Therefore, track plates require good toughness, and track teeth require good hardness to improve wear resistance. Track steel is an asymmetrical cross-section steel, with the track plates and track teeth having different thicknesses and heights. The thickness of the track teeth gradually decreases from the root to the tip, making performance control extremely difficult. To ensure a proper match between the wear resistance of the track teeth and the toughness of the track plates, and to extend the service life of the track steel, it is necessary to adjust the hardness of different parts of the track steel. This ensures that the track teeth have high hardness and high wear resistance, while the track plates have low hardness and high toughness, thus achieving a match between the wear resistance and toughness of different parts of the track plates. Summary of the Invention

[0004] The main problem of this invention is to address the problems and shortcomings of the existing technology by providing a variable hardness large pitch track steel and its manufacturing method. By imparting different hardness and toughness to different parts of the track teeth and track plates, and by quenching the track steel according to the cooling method of the track steel shape characteristics, the wear resistance of the track teeth is improved, while ensuring that the track plates have good toughness and do not break, thereby improving the service life of the track steel and effectively reducing the maintenance and replacement costs of the track steel.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] A variable-hardness single-tooth track steel has the following composition and weight percentage content: C 0.32-0.36%, Si 0.63-0.73%, Mn 1.3-1.7%, P≤0.006%, S≤0.005%, Nb 0.017-0.026%, Als 0.062-0.075%, Cr 0.82-0.92%, Co 0.73-0.85%, Ti 0.037-0.046%, H≤0.00006%, O≤0.0008%; the remainder is Fe and impurity elements.

[0007] Furthermore, the variable hardness single-tooth track steel includes a track tooth, a track web, and a junction between the track web and the track tooth. The metallographic structure of the track tooth is tempered martensite, and the metallographic structure of the track web is lower bainite. The metallographic structure of the junction between the track web and the track tooth is ferrite and pearlite, wherein the ferrite content is 70-68% and the pearlite content is 30-32%.

[0008] Furthermore, the junction of the track web and the track teeth includes an arc-shaped connection portion of the track teeth and the track web, as well as a portion of the track teeth and track web connected thereto. The length of the track teeth connected to the arc-shaped connection portion accounts for 5 to 10% of the total length of the outer contour line of the track teeth (the oblique line from the intersection point corresponding to the end extension line), and the length of the track web connected to the arc-shaped connection portion accounts for 15 to 20% of the total length of the track web (from the end face of the web).

[0009] Furthermore, the junction between the track web and the track teeth also includes a portion (arc-shaped connection portion) that connects to other special structures in the track steel.

[0010] Furthermore, in the variable hardness large pitch single-tooth track steel, the hardness of the track teeth is 520-530 HBW, and the tensile strength is 1960-2010 MPa; the hardness of the track web is 410-420 HBW, and the tensile strength is 1460-1500 MPa; the hardness of the joint between the track web and the track teeth is 330-350 HBW, and the tensile strength is 960-1000 MPa.

[0011] The above-mentioned method for preparing a variable hardness, large pitch, single-tooth track steel includes the following steps:

[0012] 1) Deep desulfurization of molten iron is carried out using a converter smelting process, and the tapping temperature is controlled at 1650-1660℃;

[0013] 2) Add quicklime, lightly calcined dolomite, fluorite, and ore to make white slag for deep deoxidation and desulfurization. Then add quartz sand and control the slag alkalinity to 0.8-1.2. The LF furnace treatment time should not be less than 42 minutes.

[0014] 3) RH vacuum dehydrogenation and oxygen treatment;

[0015] 4) Continuous casting of molten steel is carried out using a large ladle with a long nozzle and a crystallizer immersion nozzle for protection during casting; the billet casting speed is controlled at 0.6 to 0.7 m / min.

[0016] 5) Cover the billet with an insulation cover for slow cooling at a rate of 0.2–0.4 °C / s to promote the escape of hydrogen from the billet and reduce the hydrogen content in the billet;

[0017] 6) Heat the steel billet, controlling the temperature of the soaking zone to be 1220~1240℃, and the heating time to be 180~210min;

[0018] 7) Perform rough rolling and control the initial rolling temperature to be no lower than 1150℃;

[0019] 8) Perform finishing rolling, controlling the final rolling temperature of the web plate to 820-840℃, the final rolling temperature of the web plate and track tooth joint to 840-870℃, and the final rolling temperature of the track teeth to 680-700℃.

[0020] 9) The quenching process is carried out using a dual-liquid (water and polymer quenching fluid) + air cooling method. The specific steps include: first, heating and holding the precision-rolled track steel at a certain temperature; then, quenching the track teeth in water, cooling the track web in polymer quenching fluid, and cooling the joint between the track steel web and the track teeth by air spraying; and the cooling rates of water quenching, polymer quenching fluid quenching, and air spraying decrease sequentially.

[0021] 10) To ensure the hardness and toughness of different parts of the track steel after double liquid quenching, different heating temperatures and tempering times are used for tempering treatment of the track teeth, track plates, track webs and track tooth joints.

[0022] In the above scheme, the deep desulfurization step of molten iron removes sulfur from the molten iron to below 0.005%.

[0023] In the above scheme, the RH vacuum dehydrogenation and oxygen treatment steps adopt a low vacuum degree, pressure ≤40Pa, and treatment time of more than 22min.

[0024] In the above scheme, during the continuous casting step of molten steel, the immersion depth is 75-85mm; the temperature of the molten steel in the tundish is controlled to be 10-25℃ above the liquidus temperature line of the corresponding steel grade.

[0025] Furthermore, a universal rolling mill unit is used for rough rolling.

[0026] In the above scheme, the roughing mill adopts a double stand. The first stand reciprocates for 7 to 9 passes to roll the square billet into an intermediate billet. The total compression ratio is 1.8 to 2.0, and the rolling speed is 2.0 to 2.1 m / s. The second stand reciprocates for 3 to 5 passes, adopts a large compression ratio, the total compression ratio is 3.1 to 3.2, and the rolling speed is 3.2 to 3.4 m / s.

[0027] In the above scheme, the finishing rolling adopts a double-stand reversible continuous casting method, with a total compression ratio of 1.4 to 1.5 and a rolling speed of 4.4 to 4.5 m / s. Cooling water is used to control the final rolling temperature of different parts of the track steel. Specifically, the final rolling temperature of the track web is 820 to 840°C, the final rolling temperature of the joint between the track web and the track teeth is 840 to 870°C, and the final rolling temperature of the track teeth is 680 to 700°C.

[0028] In the above scheme, the heating and heat preservation step in step 9) uses a temperature of 890-895℃ and a heat preservation time of 100-110min.

[0029] In the above scheme, the cooling rate of water quenching is 40-45℃ / s; the cooling rate in polymer quenching liquid is 15-20℃ / s; and the air jet cooling rate is 3-5℃ / s.

[0030] In the above scheme, the tempering process includes: heating the track teeth to 180-190°C in a heating furnace, holding for 50-60 minutes, and air-cooling to room temperature; heating the track web and track tooth joint to 210-220°C in a heating furnace, holding for 50-60 minutes, and air-cooling to room temperature; heating the track web to 240-250°C in a heating furnace, holding for 50-60 minutes, and air-cooling to room temperature; selecting different heating temperatures for different parts ensures that the stress obtained after tempering is consistent; if a simultaneous tempering process (at the same temperature) is used for all parts, the tempering temperature is the same, and carbon atoms will segregate during the tempering process of each part to form ξ carbides. Due to the different thicknesses and inconsistent amounts of carbides in each part, stress cracking and other problems are easily caused.

[0031] The modification principle of this invention includes:

[0032] 1) Adding 1.3-1.7% Mn and 0.82-0.92% Cr to rail steel can increase the thickness of the hardened layer after quenching; adding 0.73-0.85% Co can reduce the tempering time.

[0033] 2) The track steel after precision rolling is quenched using a double-liquid + air-cooling method. Firstly, heat treatment helps to achieve different hardness and toughness in different parts of the track steel. The track steel is heated to 890–895℃ to austenitize the entire cross-section, held for 100–110 minutes to ensure bending austenitization and maintain the austenite grain size at level 8–9, obtaining fine austenite grains to prepare for quenching. The track teeth are then quenched in water to obtain a fully martensitic structure with a tensile strength of 1960–2010 MPa and a hardness of 520–530 HB, exhibiting good wear resistance. Properties: The track web is cooled in polymer quenching liquid at a cooling rate of 15-20℃ / s to obtain a lower bainitic structure, which has a good balance of hardness and toughness, with a tensile strength of 1460-1500MPa and a hardness of 410-420HB. The joint between the track steel web and the track teeth is cooled by air spray at a cooling rate of 3-5℃ / s. Air spray cooling is used to ensure that the ferrite grains in this area are refined to reduce pearlite deviation, with a tensile strength of 960-1000MPa and a hardness of 330-350HB, connecting the track teeth and track plates with excellent strength and toughness.

[0034] 3) Tempering the track steel after double liquid quenching and air cooling effectively eliminates the uneven stress caused by different microstructures in the track web, track teeth, and connecting parts, thereby ensuring uniform stress across the entire cross-section. In particular, since the thickness of different parts of the track steel is different, heating the track teeth, track web, and other parts to different temperatures and then holding them at these temperatures can promote the consistency of stress after tempering under different thickness conditions, effectively avoiding problems such as cracks or deformation caused by stress.

[0035] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0036] 1) This invention imparts different hardness and toughness to different parts of the track teeth and track plates, and optimizes the quenching and tempering process of the track steel according to the cooling method of the track steel shape characteristics. This can effectively improve the wear resistance of the track teeth, while ensuring that the track plates have good toughness and do not break, significantly improving the service life of the track steel by more than 3 times.

[0037] 2) The production method involved in this invention is relatively simple, low in cost, and easy to promote and apply. It can solve the problem of frequent replacement of tracks in engineering machinery and reduce the cost of track steel use and maintenance. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of the quenching treatment of track steel using a dual-liquid + air cooling method according to an embodiment of the present invention; in the figure, 1 is the track tooth; 2 is the air spray system; 3 is the quenching pool containing polymer coolant; 4 is the track web; 5 is the joint between the track web and the track tooth; 6 is the air spray system; 7 is the quenching box containing cooling water. Detailed Implementation

[0039] To further understand the present invention, preferred embodiments of the present invention are described below in conjunction with examples. These descriptions are only for further illustrating the features and advantages of the present invention, and are not intended to limit the scope of the claims of the present invention.

[0040] The track steel after precision rolling is quenched using a double-liquid + air-cooling method. (See below for details.) Figure 1 . Figure 1 In the track section, 1 represents the track teeth, which are in direct contact with the ground and require high hardness; 4 represents the track web, which supports the track teeth and requires relatively high hardness; and 5 represents the junction between the track web and the track teeth, connecting the track teeth and the track plate, and requires good toughness. After the track steel is rolled into shape, it is reheated to 890–895℃ and held for 100–110 minutes before the track teeth (…) are then… Figure 1 1) Place it in a quenching box containing cooling water. Figure 1 Rapid cooling (40-45℃ / s) is applied to the track web to obtain a high-hardness martensitic structure; Figure 1 4) Place the sample into a quenching tank containing polymer quenching fluid (PETROFER polymer quenching fluid). Figure 1 3) Cool to room temperature at a relatively slow rate (15-20℃ / s) to obtain a lower bainitic structure with appropriate strength and toughness; for the joint between the track web and the track teeth ( Figure 1 5) The air jet system is adopted. Figure 1 Middle 2 and Figure 1 6) Slowly cool it (3-5℃ / s) to obtain a pearlitic and ferrite structure with good toughness; then heat the track teeth in a heating furnace to 180-190℃, hold for 50-60 min, and air cool to room temperature; heat the track web and track tooth joint in a heating furnace to 210-220℃, hold for 50-60 min, and air cool to room temperature; heat the track web to 240-250℃ in a heating furnace, hold for 50-60 min, and air cool to room temperature.

[0041] Example 1

[0042] A type of variable-hardness, large-pitch, single-tooth track steel has the following chemical composition and weight percentage content: C 0.32%, Si 0.64%, Mn 1.45%, P 0.002%, S 0.003%, Nb 0.019%, Als 0.066%, Cr 0.85%, Co 0.74%, Ti 0.038%, H 0.00004%, O 0.0005%, with the remainder being Fe and impurity elements. The specific preparation steps are as follows:

[0043] 1) The sulfur content of molten iron is reduced to below 0.005% by spraying magnesium powder, and the steel is smelted in a converter with the tapping temperature controlled at 1658℃;

[0044] 2) Add quicklime, lightly calcined dolomite, fluorite, and ore to make white slag. Use silicon deoxidation. Then add 46 kg of quartz sand per ton of molten steel. Control the slag basicity at 0.8. LF furnace treatment time is 45 min.

[0045] 3) RH vacuum dehydrogenation and oxygen treatment: Low vacuum degree is adopted, vacuum pressure ≤40Pa, and the molten steel is treated under vacuum conditions for 22min;

[0046] 4) Continuous casting of molten steel is carried out using a large ladle with a long nozzle and a crystallizer immersion nozzle for protection during casting, with an immersion depth of 75mm; the temperature of the molten steel in the tundish is set at 12℃ above the liquidus temperature line of its steel grade; the billet casting speed is controlled at 0.67m / min.

[0047] 5) Cover the billet with an insulation cover for slow cooling at a rate of 0.28℃ / s to promote the escape of hydrogen from the billet and reduce the hydrogen content in the billet;

[0048] 6) Heat the steel billet, controlling the temperature of the soaking zone to 1225℃ and the heating time to 185min;

[0049] 7) A universal rolling mill is used for roughing, and its initial rolling temperature is controlled at 1180℃. The roughing process uses a two-stand mill. The first stand reciprocates for 7 passes to roll the square billet into an intermediate billet. The total compression ratio is 1.85, and the rolling speed is 2.05 m / s. The second stand reciprocates for 3 passes with a large compression ratio of 3.12 and a rolling speed of 3.25 m / s.

[0050] 8) Finish rolling is carried out using a double stand with a total compression ratio of 1.42 and a rolling speed of 4.42 m / s. Cooling water is used to control the final rolling temperature of different parts of the track steel. The final rolling temperature of the web is 820℃, the final rolling temperature of the web and track tooth joint is 845℃, and the final rolling temperature of the track teeth is 680℃.

[0051] 9) The track steel after precision rolling is quenched using a double-liquid + air-cooling method. The specific steps include: heating to 890℃, holding for 105 minutes, and then quenching using a double-liquid method; the track teeth are quenched in water at a cooling rate of 40℃ / s; the track web is cooled in polymer quenching liquid at a cooling rate of 16℃ / s; the joint between the track steel web and the track teeth is cooled by air jet cooling at a cooling rate of 3.2℃ / s.

[0052] 10) Tempering treatment of track steel after double liquid quenching, the specific steps include: heating the track teeth to 180°C in a heating furnace, holding for 50 minutes, and air cooling to room temperature; heating the track web and track tooth joint to 210°C in a heating furnace, holding for 50 minutes, and air cooling to room temperature; heating the track web to 240°C in a heating furnace, holding for 50 minutes, and air cooling to room temperature.

[0053] Example 2

[0054] A type of variable-hardness, large-pitch, single-tooth track steel has the following composition and weight percentage content: C 0.34%, Si 0.66%, Mn 1.55%, P 0.002%, S 0.003%, Nb 0.017%, Als 0.068%, Cr 0.87%, Co 0.79%, Ti 0.037%, H 0.00003%, O 0.0006%, with the remainder being Fe and impurity elements. The specific preparation steps are as follows:

[0055] 1) The sulfur content of molten iron is reduced to below 0.004% by spraying magnesium powder, and the steel is smelted in a converter with the tapping temperature controlled at 1655℃;

[0056] 2) Add active lime, lightly calcined dolomite, fluorite, and ore to make white slag. Use silicon deoxidation, then add 47 kg of quartz sand per ton of molten steel. Control the slag basicity at 0.9 and the LF furnace treatment time is 44 min.

[0057] 3) RH vacuum dehydrogenation and oxygen treatment: Low vacuum degree is adopted, vacuum pressure ≤40Pa, and the molten steel is treated under vacuum conditions for 21 minutes;

[0058] 4) Continuous casting of molten steel is carried out using a large ladle with a long nozzle and a crystallizer immersion nozzle for protection during casting, with an immersion depth of 75mm; the temperature of the molten steel in the tundish is set at 15℃ above the liquidus temperature line of its steel grade; the billet casting speed is controlled at 0.65m / min.

[0059] 5) Cover the billet with an insulation cover for slow cooling at a rate of 0.25℃ / s to promote the escape of hydrogen from the billet and reduce the hydrogen content in the billet;

[0060] 6) Heat the steel billet, controlling the temperature of the soaking zone to 1228℃ and the heating time to 192min;

[0061] 7) A universal rolling mill is used for roughing, and its initial rolling temperature is controlled at 1160℃. The roughing process uses a two-stand mill. The first stand reciprocates for 7 passes to roll the square billet into an intermediate billet with a total compression ratio of 1.87 and a rolling speed of 2.06 m / s. The second stand reciprocates for 3 passes with a large compression ratio of 3.14 and a rolling speed of 3.29 m / s.

[0062] 8) Finish rolling is carried out using a double stand with a total compression ratio of 1.43 and a rolling speed of 4.46 m / s. Cooling water is used to control the final rolling temperature of different parts of the track steel. The final rolling temperature of the web is 830℃, the final rolling temperature of the web and track tooth joint is 849℃, and the final rolling temperature of the track teeth is 685℃.

[0063] 9) The track steel after precision rolling is quenched using a double-liquid + air-cooling method. The specific steps include: heating to 892℃, holding for 102 minutes, and then quenching using a double-liquid method; the track teeth are quenched in water at a cooling rate of 42℃ / s; the track web is cooled in a polymer quenching liquid at a cooling rate of 17℃ / s; and the joint between the track steel web and the track teeth is cooled by air jet cooling at a cooling rate of 3.5℃ / s.

[0064] 10) Tempering treatment of track steel after double liquid quenching, the specific steps include: heating the track teeth to 185°C in a heating furnace, holding for 55 minutes, and air cooling to room temperature; heating the track web and track tooth joint to 215°C in a heating furnace, holding for 55 minutes, and air cooling to room temperature; heating the track web to 245°C in a heating furnace, holding for 55 minutes, and air cooling to room temperature.

[0065] Example 3

[0066] A type of variable-hardness, large-pitch, single-tooth track steel has the following chemical composition and weight percentage content: C 0.36%, Si 0.71%, Mn 1.68%, P 0.002%, S 0.002%, Nb 0.025%, Als 0.074%, Cr 0.91%, Co 0.84%, Ti 0.045%, H 0.00003%, O 0.0004%, with the remainder being Fe and impurity elements. The specific preparation steps are as follows:

[0067] 1) The sulfur content of molten iron is reduced to below 0.003% by spraying magnesium powder, and the steel is smelted in a converter with the tapping temperature controlled at 1658℃;

[0068] 2) Add active lime, lightly calcined dolomite, fluorite, and ore to make white slag. Use silicon deoxidation. Then add 48 kg of quartz sand per ton of molten steel. Control the slag basicity at 1.0. The LF furnace treatment time is 43 min.

[0069] 3) RH vacuum dehydrogenation and oxygen treatment: Low vacuum degree is adopted, vacuum pressure ≤40Pa, and the molten steel is treated under vacuum conditions for 24min;

[0070] 4) Continuous casting of molten steel is carried out using a large ladle with a long nozzle and a crystallizer immersion nozzle for protection during casting, with an immersion depth of 84 mm; the temperature of the molten steel in the tundish is set at 18°C ​​above the liquidus temperature line of the steel grade; the billet casting speed is controlled at 0.68 m / min.

[0071] 5) Cover the billet with an insulation cover for slow cooling at a rate of 0.35℃ / s to promote the escape of hydrogen from the billet and reduce the hydrogen content in the billet;

[0072] 6) Heat the steel billet, controlling the temperature of the soaking zone to 1237℃ and the heating time to 195min;

[0073] 7) A universal rolling mill is used for roughing, and its initial rolling temperature is controlled at 1175℃. The roughing process uses a two-stand mill. The first stand reciprocates for 7 passes to roll the billet into an intermediate billet with a total compression ratio of 2.0 and a rolling speed of 2.1 m / s. The second stand reciprocates for 3 passes with a large compression ratio of 3.2 and a rolling speed of 3.39 m / s.

[0074] 8) Finish rolling is carried out using a double stand with a total compression ratio of 1.49 and a rolling speed of 4.48 m / s. Cooling water is used to control the final rolling temperature of different parts of the track steel. The final rolling temperature of the web is 835℃, the final rolling temperature of the web and track tooth joint is 863℃, and the final rolling temperature of the track teeth is 696℃.

[0075] 9) The track steel after precision rolling is quenched using a double-liquid + air-cooling method. The specific steps include: heating to 895℃, holding for 106 minutes, and then quenching using a double-liquid method; the track teeth are quenched in water at a cooling rate of 45℃ / s; the track web is cooled in polymer quenching liquid at a cooling rate of 18℃ / s; and the joint between the track steel web and the track teeth is cooled by air jet cooling at a cooling rate of 4.7℃ / s.

[0076] 10) Tempering treatment of track steel after double liquid quenching, the specific steps include: heating the track teeth to 190°C in a heating furnace, holding for 58 minutes, and air cooling to room temperature; heating the track web and track tooth joint to 220°C in a heating furnace, holding for 58 minutes, and air cooling to room temperature; heating the track web to 250°C in a heating furnace, holding for 58 minutes, and air cooling to room temperature.

[0077] The heat treatment of single-tooth track steel using the method described in this invention requires precise control of the quenching and tempering temperatures, and the hardness and microstructure of each part must be controlled within the range required by this invention. During the quenching process, cooling of three parts needs to be initiated simultaneously. In the experiment, the track teeth were cooled first with water, and after 10 seconds, cooling of the track web, track steel web, and the track tooth junction was initiated. This resulted in excessively rapid cooling of the track teeth, generating tensile stress and causing cracking of the track web, track steel web, and track tooth junction. Based on the experimental results, simultaneously initiating cooling of the track teeth, track plates, track web, and track tooth junction until cooling to room temperature (20°C) effectively improves the performance of the resulting track steel.

[0078] The variable hardness, large pitch single-tooth track steel obtained in Examples 1-3 (pitch 280mm, total width 322mm, tooth height 93mm) was compared with the commonly used track steel 25MnB to compare the strength and hardness of different parts of the two different track steels. The main chemical composition and their mass percentage in the 25MnB track steel were: C 0.25%, Mn 0.13%, Si 0.26%, P 0.018%, S 0.011%, Cr 0.4%, Ti 0.03%, B 0.002%, with the remainder being Fe and impurity elements. The heat treatment process involved heating the entire cross-section to 890℃, holding for 260min, water quenching the entire cross-section, then heating to 230℃, holding for 150min, and air cooling to room temperature. The tensile strength, hardness, and microstructure of different parts of the above four track steel compositions were compared, and the results are shown in Table 1.

[0079] Table 1. Performance test results of Examples 1-3 and commercially available single-tooth track steel.

[0080]

[0081]

[0082] As can be seen from the table, the variable hardness large pitch track steel obtained by the present invention has a service life that is at least 3.4 times longer than that of comparative example 1.

[0083] This invention is not limited to the embodiments described above. Those skilled in the art can make various improvements and modifications without departing from the principles of this invention, and these improvements and modifications are also considered within the scope of protection of this invention. Contents not described in detail in this specification are prior art known to those skilled in the art.

Claims

1. A variable-hardness single-tooth track steel, characterized in that, Its composition and weight percentage content are as follows: C 0.32~0.36%, Si 0.63~0.73%, Mn 1.3~1.7%, P≤0.006%, S≤0.005%, Nb 0.017~0.026%, Als 0.062~0.075%, Cr 0.82~0.92%, Co 0.73~0.85%, Ti 0.037~0.046%, H≤0.00006%, O≤0.0008%; the remainder is Fe and impurity elements. The variable hardness single-tooth track steel includes track teeth, track web, and the joint between the track web and track teeth. The metallographic structure of the track teeth is tempered martensite, and the metallographic structure of the track web is lower bainite. The metallographic structure of the joint between the track web and track teeth is ferrite and pearlite, wherein the ferrite content is 70-68% and the pearlite content is 30-32%.

2. The variable hardness single-tooth track steel according to claim 1, characterized in that, In the variable hardness single-tooth track steel, the hardness of the track teeth is 520~530HBW and the tensile strength is 1960~2010MPa; the hardness of the track web is 410~420HBW and the tensile strength is 1460~1500MPa; the hardness of the joint between the track web and the track teeth is 330~350HBW and the tensile strength is 960~1000MPa.

3. The method for preparing the variable hardness single-tooth track steel according to any one of claims 1 to 2, characterized in that, Includes the following steps: 1) Deep desulfurization of molten iron is carried out using a converter smelting process, and the tapping temperature is controlled at 1650~1660℃; 2) Add quicklime, lightly calcined dolomite, fluorite, and ore to make white slag for deep deoxidation and desulfurization. Then add quartz sand and control the slag alkalinity to 0.8~1.

2. The LF furnace treatment time should not be less than 42 minutes. 3) RH vacuum dehydrogenation and oxygen treatment; 4) Continuous casting of molten steel is carried out using a large ladle with a long nozzle and a crystallizer immersion nozzle for protection during casting, and the billet casting speed is controlled at 0.6~0.7m / min; 5) Cover the billet with an insulation cover and cool it slowly at a rate of 0.2~0.4℃ / s to promote the escape of hydrogen from the billet and reduce the hydrogen content in the billet; 6) Heat the steel billet, controlling the temperature of the soaking zone to be 1220~1240℃, and the heating time to be 180~210min; 7) Perform rough rolling and control the initial rolling temperature to be no lower than 1150℃; 8) Perform finishing rolling, controlling the final rolling temperature of the web plate to 820~840℃ and the final rolling temperature of the track teeth to 680~700℃; 9) The quenching process is carried out by a dual-liquid + air cooling method. The specific steps include: first, heating and holding the precision-rolled track steel at a certain temperature; then, quenching the track teeth in water, cooling the track web in a polymer quenching liquid, and cooling the joint between the track steel web and the track teeth by air spraying; and the cooling rates of water quenching, polymer quenching liquid quenching and air spraying cooling decrease sequentially. 10) To ensure the hardness and toughness of different parts of the track steel after double liquid quenching, different heating temperatures and tempering times are used for tempering treatment of the track teeth, track plates, track webs and track tooth joints.

4. The preparation method according to claim 3, characterized in that, The RH vacuum dehydrogenation and oxygen treatment steps employ a low vacuum degree, with a pressure ≤40Pa and a treatment time of 22 minutes or more.

5. The preparation method according to claim 3, characterized in that, The roughing mill adopts a two-stand mill. The first stand reciprocates and rolls 7 to 9 times to roll the square billet into an intermediate billet. The total compression ratio of the first stand is 1.8 to 2.0, and the rolling speed is 2.0 to 2.1 m / s. The second stand reciprocates for 3 to 5 passes. The second stand adopts a large compression ratio, with a total compression ratio of 3.1 to 3.2 and a rolling speed of 3.2 to 3.4 m / s.

6. The preparation method according to claim 3, characterized in that, The finishing rolling adopts a two-stand reversible continuous rolling method with a total compression ratio of 1.4~1.5 and a rolling speed of 4.4~4.5m / s. Cooling water is used to control the final rolling temperature of different parts of the track steel. Specifically, the final rolling temperature of the track web is 820~840℃, the final rolling temperature of the joint between the track web and the track teeth is 840~870℃, and the final rolling temperature of the track teeth is 680~700℃.

7. The preparation method according to claim 3, characterized in that, The heating and heat preservation step described in step 9) uses a temperature of 890~895℃ and a heat preservation time of 100~110min.

8. The preparation method according to claim 3, characterized in that, The cooling rate for water quenching is 40~45℃ / s; the cooling rate for polymer quenching liquid is 15~20℃ / s; and the cooling rate for air spray cooling is 3~5℃ / s.

9. The preparation method according to claim 3, characterized in that, The tempering process includes: heating the track teeth to 180-190°C in a heating furnace, holding the temperature for 50-60 minutes, and then air-cooling to room temperature; heating the track web and the joint between the track teeth in a heating furnace to 210-220°C, holding the temperature for 50-60 minutes, and then air-cooling to room temperature; and heating the track web to 240-250°C in a heating furnace, holding the temperature for 50-60 minutes, and then air-cooling to room temperature.

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