A track shoe for a large earthmover and a method of making the same

Abstract

The present invention relates to a crawler shoe for a large bulldozer and a preparation method thereof. The weight percentages of its chemical components are as follows: C: 0.25 - 0.31%, Si: 0.36 - 0.80%, Mn: 1.15 - 1.30%, P ≤ 0.015%, S ≤ 0.010%, Cr: 0.70 - 0.90%, B: 0.0015 - 0.0030%, Ti: 0.025 - 0.055%, Al: 0.02 - 0.05%, N: 0.003 - 0.006%, H ≤ 0.0002%, O ≤ 0.0025%, As ≤ 0.0080%, and the rest is iron and trace impurities. The C range in the same cross-section is ≤ 0.03%, the Mn range is ≤ 0.04%, and the Cr range is ≤ 0.05%. Its hardenability value is 40HRC < J25 ≤ 45HRC, 36HRC < J30 ≤ 42HRC. The present invention can achieve that all the metallographic structures obtained by natural cooling after hot rolling of the crawler shoe in winter with a relatively low ambient temperature (below 0°C) are ferrite + pearlite, without the bainite structure with poor toughness, and have good shearing and punching properties. At the same time, the hardenability, wear resistance, and fatigue resistance of the crawler shoe are further improved, and surface defects are effectively controlled.

Description

Technical Field

[0001] This invention belongs to the field of materials metallurgy technology. Specifically, this invention relates to a track plate for a large bulldozer and its preparation method. Background Technology

[0002] Track shoes come in single-tooth, double-tooth, and triple-tooth types to meet the needs of different machine models and operating conditions. Bulldozers commonly use single-tooth track shoes. Track shoes are manufactured using various processes such as casting, stamping, machining, and rolling. Compared to other methods, rolling offers high efficiency, low cost, and superior overall performance, making it the mainstream process for track shoe manufacturing. However, for the ultra-large single-tooth track plates used in large bulldozers, not only is the rolling process difficult, but the hardenability requirements of the material are also higher than those of ordinary track plates. To solve this problem, CN202210783391.3 discloses a high hardenability steel for ultra-large engineering machinery tracks and its production method. The track steel, by weight percentage, is composed of the following chemical composition: C 0.23~0.30%, Si 0.15~0.35%, Mn 1.05~1.35%, P≤0.025%, S≤0.015%, Cr 0.60~0.90%, B 0.0010~0.0030%, Ti 0.015~0.055%, Al 0.01~0.06%, with the remainder being iron and unavoidable impurities. Hardenability and wear resistance are improved by increasing the Cr content from 0.3% to 0.60-0.90%. However, because Cr significantly alters the cooling transformation behavior of steel, increasing the Cr content shifts the continuous cooling transformation curve of steel to the right and reduces the critical cooling rate for bainite transformation. This leads to changes in the microstructure of the track plate under natural cooling conditions after rolling in winter at low ambient temperatures (below 0°C), resulting in the formation of bainite with poor plasticity. This causes cracking problems during shearing and punching, resulting in insufficient environmental adaptability of this patented technology. On the other hand, in order to solve the problem that brittle phases such as Widmanstätten and bainite are prone to appear in the metallographic structure after the Cr content is increased, the patent adopts a lower final rolling temperature (not greater than 930℃). This not only requires waiting for the temperature before finishing rolling, which affects the rolling rhythm and reduces production efficiency, but also, for high hardenability steel, at such a low final rolling temperature, the head of the rolled piece with a faster temperature drop is very likely to form martensite under the action of cooling water, making the hardness significantly higher than the roll hardness. This causes the head of the rolled piece to bite into the roll surface and form a pit on the rolled piece during subsequent rolling. Summary of the Invention

[0003] To address the deficiencies of the existing technologies, the present invention provides a crawler shoe for large bulldozers and its preparation method. It can achieve that all the metallographic structures obtained by natural cooling after hot rolling of the crawler shoes in winter with relatively low ambient temperatures (below 0°C) are ferrite + pearlite, without the bainite structure with poor toughness. It has good shearing and punching properties. At the same time, the hardenability, wear resistance, and fatigue resistance of the crawler shoes are further improved, and surface defects are effectively controlled.

[0004] To achieve the above objectives, the present invention adopts the following solutions:

[0005] A crawler shoe for large bulldozers, the weight percentages of its chemical components are: C: 0.25 - 0.31%, Si: 0.36 - 0.80%, Mn: 1.15 - 1.30%, P ≤ 0.015%, S ≤ 0.010%, Cr: 0.70 - 0.90%, B: 0.0015 - 0.0030%, Ti: 0.025 - 0.055%, Al: 0.02 - 0.05%, N: 0.003 - 0.006%, H ≤ 0.0002%, O ≤ 0.0025%, As ≤ 0.0080%, and the rest is iron and trace impurities. The C range in the same cross-section ≤ 0.03%, the Mn range ≤ 0.04%, and the Cr range ≤ 0.05%.

[0006] For the crawler shoe involved in the present invention, its hardenability value is 40HRC < J25 ≤ 45HRC, 36HRC < J30 ≤ 42HRC, enabling the crawler shoe to have a thicker hardened layer depth and better wear resistance.

[0007] The crawler shoe for large bulldozers provided by the present invention has a plate width of 300 - 400 mm, an equivalent thickness of the tooth root of 40 - 60 mm, a tooth height of 85 - 120 mm, and a web thickness of 20 - 40 mm.

[0008] As a preferred embodiment, the weight percentages of the chemical components of the crawler shoe for large bulldozers are: C: 0.26 - 0.29%, Si: 0.40 - 0.70%, Mn: 1.15 - 1.25%, P ≤ 0.013%, S ≤ 0.008%, Cr: 0.75 - 0.85%, B: 0.0015 - 0.0025%, Ti: 0.03 - 0.05%, Al: 0.02 - 0.04%, N: 0.003 - 0.005%, H ≤ 0.0002%, O ≤ 0.0020%, As ≤ 0.0080%, and the rest is iron and trace impurities. The C range in the same cross-section ≤ 0.02%, the Mn range ≤ 0.03%, and the Cr range ≤ 0.04%.

[0009] Carbon (C) is the most economical element for increasing the strength of steel, but as the C content increases, the toughness of the steel deteriorates. C is also the main element for increasing the surface hardness after quenching, which can significantly improve the surface wear resistance and extend the service life of the material. Taking all factors into consideration, the C content is determined to be controlled within the range of 0.25% to 0.31%.

[0010] Si: Si is a deoxidizer that must be added during the steelmaking process. After deoxidation, a small amount remains in the molten steel. Upon cooling, it can dissolve in ferrite to increase strength and improve wear resistance. Experimental studies have shown that when the Si content is low (below 1%), increasing the Si content shifts the continuous cooling transformation curve of the steel to the left, increasing the critical cooling rate for bainite transformation, which is the opposite of the effect of Cr on the continuous cooling transformation curve. Therefore, this invention offsets the problem of bainite formation at low temperatures caused by the increase of Cr by increasing the Si content to 0.36-0.80%. At the same time, the increase in Si content can also improve the hardenability of the steel, thereby increasing the J30 value and extending the service life of the track plates.

[0011] Mn has a solid solution strengthening effect, which can improve strength. Mn can also combine with S to form MnS, which improves the plasticity of steel at high temperature and prevents hot cracks from occurring during continuous casting. However, Mn is also an element that is prone to segregation. Adding too much will cause serious segregation in the core of the billet, resulting in performance deterioration. Therefore, this invention controls the Mn content in the range of 1.15 to 1.30%.

[0012] P: An unavoidable impurity element in the ore. For this invention, P is a harmful element that will reduce the toughness of the material and cause brittleness. Its content should be reduced as much as possible during the smelting process. However, removing P will increase the cost. Under the premise of not affecting the toughness, P ≤ 0.015% is sufficient.

[0013] S: An unavoidable impurity element introduced from the ore. For this invention, S is a harmful element, leading to reduced high-temperature toughness and causing hot brittleness. Furthermore, S is one of the main sources of inclusions in steel. Its content should be minimized during smelting, or more Mn should be added to eliminate its hot brittleness. Without affecting its use, S ≤ 0.010% is acceptable.

[0014] Cr: A common alloying element in steel, Cr can improve the strength, hardenability, and wear resistance of steel. It can also significantly alter the cooling transformation behavior of steel materials. With increasing Cr content, the continuous cooling transformation curve of steel shifts to the right, making it easier for bainite to form and leading to machining cracking. This invention increases the Si content based on existing technology to offset the effect of Cr on bainite transformation. Therefore, this invention controls the Cr content within the range of 0.70% to 0.90%.

[0015] B: Residual elements that are difficult to avoid in metallurgical auxiliary materials have a significant impact on the toughness of steel. B element tends to agglomerate at grain boundaries, reducing the low-temperature impact toughness of steel. However, B is also a highly efficient element for improving hardenability. Adding a trace amount can significantly improve the hardenability of steel. Taking all factors into consideration, the B element content is controlled within the range of 0.0015% to 0.0030%.

[0016] Ti: Ti is more reactive than B and combines more easily with N, playing a role in fixing nitrogen and thus increasing the content of acid-soluble B. Under the same B content, adding Ti can further improve hardenability. However, excessive Ti content will deteriorate toughness and cause track plates to crack. Controlling the Ti content within the range of 0.025% to 0.055% can effectively improve hardenability without reducing toughness.

[0017] Al: Al is a commonly used element for deoxidation of molten steel. It can combine with oxygen in steel to form Al2O3, which floats to the surface and is removed from the molten steel, thus preventing boron from losing its ability to improve hardenability due to oxidation. In addition, Al has an adverse effect on the castability of molten steel, and adding too much can easily lead to nozzle blockage. Taking all factors into consideration, the Al content is controlled within the range of 0.02% to 0.05%.

[0018] Nitrogen (N): Nitrogen promotes the precipitation of microalloying elements such as Nb, V, and Ti, thereby increasing strength. It can also combine with Al to form AlN at austenite grain boundaries, inhibiting grain growth and improving toughness. However, excessive N can lead to strain aging, causing brittle fracture of the steel after processing. Through experimental research and comparison, controlling the N content within the range of 0.003–0.006% achieves the optimal balance between strength and toughness in track plates.

[0019] H: H is a harmful element in steel, causing defects such as internal cracks and white spots, and severely reducing the toughness of steel. Therefore, H must be controlled at an extremely low level. Controlling the H content to ≤0.0002% can meet the requirements for the use of track plates.

[0020] O: O is a harmful element in steel, often existing in the form of inclusions, which disrupts the continuity of the steel matrix and leads to toughness and fatigue failure. However, in order to control the C content in steel, oxygen must be blown during steelmaking, resulting in a certain amount of O in the molten steel, which is removed by refining later. In order to meet the requirements of toughness and fatigue, and to meet economic production, this invention controls the O content within the range of ≤0.0025%.

[0021] As: An unavoidable harmful element that significantly reduces the toughness of steel. As is a common associated element in iron ore, entering the molten steel from the molten iron during the smelting process. To meet the requirements of track plates, the content should be controlled at ≤0.0080%.

[0022] Range is a parameter characterizing the degree of segregation of steel (or billet) composition. A smaller range indicates that the steel (or billet) composition is more uniform and there are no local areas where the content of elements such as C, Mn, and Cr is higher than that of the matrix. In order to control the generation of abnormal structures (bainite, martensite, etc.) during the cooling process, and considering the guarantee capability of the preparation method of this invention, the range of C is specified to be ≤0.03%, the range of Mn is ≤0.04%, and the range of Cr is ≤0.05%.

[0023] The present invention also provides a method for preparing the above-mentioned track plates for large bulldozers, comprising the following steps:

[0024] Step S1: Use low-As molten iron and scrap steel for converter smelting and LF+ vacuum refining. The As content in the molten iron is ≤80ppm, and the proportion of molten iron in the raw materials for converter steelmaking is not less than 80%.

[0025] Step S2: Cast steel billets using a continuous casting machine. The continuous casting machine is equipped with an end electromagnetic stirrer with a current of 450A to 600A. The straightening rollers of the continuous casting machine are equipped with a pressing function with a pressing amount of 5-15mm.

[0026] Step S3: Use a slow cooling pit to slowly cool the steel billet;

[0027] Step S4: The steel billet is heated, rolled, and cooled to form track plates.

[0028] Furthermore, in the above preparation method, in step S3, the slow cooling pit is first baked until the pit wall temperature reaches 750°C. Then, the high-temperature steel billet from the continuous casting line is placed into the slow cooling pit for slow cooling to reduce the temperature gradient between the pit wall and the steel billet. After the slow cooling time is greater than 24h to 48h, the billet is removed from the slow cooling pit to homogenize the steel billet composition, reduce core segregation, and prevent abnormal structures such as bainite and martensite from appearing in the core due to the higher content of C, Mn, and Cr elements compared to the surrounding areas.

[0029] Furthermore, in the above preparation method, in step S4, the billet is rough rolled in 9 to 13 passes during the rolling process, including 1 to 2 descaling passes, which loosen and peel off the iron oxide scale on the surface of the rolled piece. After the descaling passes, a turning operation (≥90°) is added to remove the loosened and peeled iron oxide scale from the rolled piece. This compensates for the fact that the iron oxide scale is tightly adhered and difficult to remove with high-pressure water descaling, resulting in pits formed in the finished product due to the iron oxide scale being pressed in, thus improving the surface quality of the track plate.

[0030] Furthermore, in the above preparation method, in step S4, the final rolling temperature is 940–1000°C. A full ferrite + pearlite microstructure can be obtained without low-temperature rolling, achieving efficient production and high surface quality of track plates.

[0031] This invention addresses the issue that increased chromium (Cr) leads to bainite formation in track plates at low temperatures, worsening machinability. By increasing the silicon (Si) content, the adverse effects of Cr are mitigated, resulting in track plates with a microstructure entirely composed of ferrite and pearlite. The increased Si content further improves the hardenability of the steel, increases the depth of the hardened layer, and enhances the wear resistance of the track plates. Using low-As molten iron significantly reduces the risk of track plate fracture. Continuous casting employs end-stage electromagnetic stirring with current controlled between 450A and 600A, reducing elemental segregation in the core, increasing the proportion of equiaxed crystals, and improving toughness. Slow cooling of the billet further eliminates elemental segregation in the billet core, reduces the C, Mn, and Cr elemental range, prevents the formation of abnormal structures such as bainite, and better ensures that the hot-rolled track plate microstructure is entirely composed of ferrite and pearlite. Using a straightening machine to reduce the height of the continuously cast billet by 5-15mm utilizes the positive temperature gradient of the billet to achieve high-penetration deformation, eliminating casting defects in the central region of the billet and improving the fatigue life of the track plates. The descaling and steel-turning process during rolling can further reduce the residual iron oxide scale after high-pressure water descaling, thus improving the surface quality of the track plates. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the cross-sectional shape of a single-tooth track plate;

[0033] 1—Single tooth track plate;

[0034] 11—tooth cusp;

[0035] 12 – Tooth root;

[0036] D—Equivalent thickness of tooth root;

[0037] Figure 2 The microstructure of the track plate prepared using existing technology (containing a large amount of bainite);

[0038] Figure 3 The microstructure of the track plate (without bainite) was prepared using the technique of this invention;

[0039] Figure 4 The metallographic structure (without bainite) of the central part of the track plate was prepared using the technology of this invention. Detailed Implementation

[0040] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0041] A type of track pad for a large bulldozer (see attached) Figure 1 The chemical composition of the track plate and its preparation method are compared with that of existing large-size track plates in Table 1.

[0042] Table 1. Comparison of chemical composition (wt%) of existing technology and track plates of the present invention

[0043]

[0044] Compared with existing technologies, the chemical composition of this invention also requires that the C range of the steel in the same cross section be ≤0.03%, the Mn range be ≤0.04%, and the Cr range be ≤0.05%.

[0045] Examples 1-3

[0046] The preparation method of track plates for large bulldozers, as shown in Table 2 for Examples 1-3, includes the following steps:

[0047] Step S1: Use low-As molten iron and scrap steel for converter smelting and LF+ vacuum refining. The As content in the molten iron is ≤80ppm, and the mass ratio of molten iron in the raw materials for converter steelmaking is not less than 80%.

[0048] Step S2: Cast steel billets using a continuous casting machine. The continuous casting machine is equipped with an end electromagnetic stirrer with a current of 450A to 600A. The straightening rollers of the continuous casting machine are equipped with a pressing function with a pressing amount of 5-15mm.

[0049] Step S3: Use a slow cooling pit to slowly cool the steel billet;

[0050] Step S4: After heating, rolling, and cooling, the steel billet is made into track plates, the shape of which is as follows. Figure 1 As shown.

[0051] In step S3, the slow cooling pit is first baked until the pit wall temperature reaches 750°C. Then, the high-temperature steel billet from the continuous casting line is placed into the slow cooling pit for slow cooling to reduce the temperature gradient between the pit wall and the steel billet. After the slow cooling time is greater than 24h to 48h, the billet is removed from the slow cooling pit to homogenize the steel billet composition, reduce core segregation, and prevent abnormal structures such as bainite and martensite from appearing in the core due to the higher content of C, Mn, and Cr elements compared to the surrounding areas.

[0052] In step S4, the billet is rough rolled in 9 to 13 passes during the rolling process, including 1 to 2 descaling passes, which loosen and peel off the iron oxide scale on the surface of the rolled piece. After the descaling passes, a turning operation (≥90°) is added to remove the loosened and peeled iron oxide scale from the rolled piece. This compensates for the fact that the iron oxide scale is tightly adhered and difficult to remove with high-pressure water descaling, which leads to pits formed in the finished product due to the iron oxide scale being pressed in, thus improving the surface quality of the track plate.

[0053] In step S4, the final rolling temperature is 940–1000℃. A fully ferrite + pearlite microstructure can be obtained without low-temperature rolling, achieving efficient production and high surface quality of track plates.

[0054] Table 2. Chemical composition (wt%) of the track plates for large bulldozers prepared in Examples 1-3.

[0055]

[0056] Table 3 shows the compositional range (wt%) of the track plates for large bulldozers prepared in Examples 1-3.

[0057] chemical composition C Mn Cr Example 1 0.02 0.03 0.04 Example 2 0.02 0.02 0.03 Example 3 0.01 0.03 0.03

[0058] Samples were taken from the track steel sections prepared in Examples 1-3 and processed. The end hardenability was tested according to GB / T 225, with a normalizing temperature of 900±10℃, a normalizing holding time of 30-35 min, and a quenching temperature of 870±5℃. The end hardenability J25 and J30 indices of the track plates of this invention and the prior art were tested respectively. The end hardenability data of the track plates of this invention and the prior art are shown in Table 4.

[0059] Table 4. Hardenability data of track plate ends in Examples 1-3

[0060]

[0061] As shown in Table 4, compared with the track plates prepared by the present invention, the hardenability of the track steel ends after heat treatment is improved, especially the J30 value, which is significantly improved, confirming the improvement effect of the present invention on the hardenability of the track plates. The teeth of the single-tooth track plates used in large bulldozers are thick and high (the maximum thickness can reach 60mm, and the maximum can reach 120mm). Due to the limitation of the hardenability of the steel in the existing technology, the hardness of the teeth drops rapidly from the surface to the core after quenching, resulting in insufficient wear resistance and affecting the service life of the track plates. The present invention improves the J25 and J30 values, so that the largest specification track plates achieve a higher hardness from the surface to the core after heat treatment, thereby improving the wear resistance of the track plates.

[0062] In winter, when the ambient temperature is low (below 0°C), the microstructure of track plates prepared using existing technologies and the present invention was analyzed separately, see [the table below]. Figure 2 , Figure 3 and Figure 4 .from Figure 2 It is evident that the metallographic structure of track plates produced in winter using existing technology consists of a ferrite + pearlite matrix with a large amount of bainite distributed on top. As a brittle phase, the presence of bainite can cause cracking of the track plates during punching, shearing, and perforation. Figure 3 It is evident that the microstructure of the track plates produced in winter using the technology of this invention consists entirely of ferrite and pearlite, with no bainite. Due to the implementation of end-stage electromagnetic stirring and slow cooling of the cast billet, the range of C, Mn, and Cr is reduced, and the degree of segregation is significantly decreased. Figure 4 It is evident that even the central part of the track plate is entirely composed of ferrite and pearlite, without bainite, which meets the requirements for punching, shearing, and drilling.

[0063] The upper and lower limits of the process parameters (such as temperature, time, etc.) and the range values ​​of the present invention can all achieve the method, and examples are not listed here.

[0064] All aspects not described in detail in this invention can be covered using conventional technical knowledge in the field.

[0065] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A track pad for a large bulldozer, characterized in that, The chemical composition of the track plates for the large bulldozer, by weight percentage, is as follows: C: 0.25–0.31%, Si: 0.36–0.80%, Mn: 1.15–1.30%, P≤0.015%, S≤0.010%, Cr: 0.70–0.90%, B: 0.0015–0.0030%, Ti: 0.025–0.055%, Al: 0.02–0.05%, N: 0.003–0.006%, H≤0.0002%, O≤0.0025%, As≤0.0080%, with the remainder being iron and trace impurities. The C range for the same cross-section is ≤0.03%, the Mn range is ≤0.04%, and the Cr range is ≤0.05%. A method for manufacturing track pads for large bulldozers includes the following steps: Step S1: Use low-As molten iron and scrap steel for converter smelting and LF+ vacuum refining. The As content in the molten iron is ≤80ppm, and the proportion of molten iron in the raw materials for converter steelmaking is not less than 80%. Step S2: Cast steel billets using a continuous casting machine. The continuous casting machine is equipped with an end electromagnetic stirrer with a current of 450A to 600A. The straightening rollers of the continuous casting machine are equipped with a pressing function with a pressing amount of 5-15mm. Step S3: Use a slow cooling pit to slowly cool the steel billet; Step S4: The steel billet is heated, rolled, and cooled to form track plates; In step S4, there are 1 to 2 descaling passes in the roughing rolling passes to loosen and peel off the iron oxide scale on the surface of the rolled piece. After the descaling passes, a ≥90° steel turning operation is added to remove the loosened and peeled iron oxide scale from the rolled piece. The final rolling temperature is 940 to 1000°.

2. The track pads for large bulldozers as described in claim 1, characterized in that, The chemical composition by weight percentage of the track plates used in the large bulldozer is as follows: C: 0.26–0.29%, Si: 0.40–0.70%, Mn: 1.15–1.25%, P≤0.013%, S≤0.008%, Cr: 0.75–0.85%, B: 0.0015–0.0025%, Ti: 0.03–0.05%, Al: 0.02–0.04%, N: 0.003–0.005%, H≤0.0002%, O≤0.0020%, As≤0.0080%, with the remainder being iron and trace impurities. The C range on the same cross-section is ≤0.02%, the Mn range is ≤0.03%, and the Cr range is ≤0.04%.

3. The track pads for large bulldozers as described in claim 1, characterized in that, The track plate has a hardenability value of 40 HRC. <J25≤45HRC,36HRC<J30≤42HRC。 4. The track pads for large bulldozers as described in claim 1, characterized in that, The track plates have a width of 300-400mm, an equivalent thickness of 40-60mm for the tooth heel, a tooth height of 85-120mm, and a web thickness of 20-40mm.

5. The track pads for large bulldozers as described in claim 1, characterized in that, In step S3, the slow cooling pit is first baked. After the pit wall temperature reaches 750°C, the high-temperature steel billet from the continuous casting line is placed into the slow cooling pit for slow cooling. The slow cooling time is greater than 24 hours before the billet is removed from the slow cooling pit.

6. The track pads for large bulldozers as described in claim 1, characterized in that, In step S4, the billet is rough rolled in 9 to 13 passes during the rolling process.

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