Low-cost high-toughness bucket tooth steel and method of making same

Abstract

This invention belongs to the field of metallurgy, and specifically relates to a low-cost, high-toughness steel for bucket teeth and its preparation method. The chemical composition of the steel, by weight percentage, is: C: 0.22%–0.28%, Si: 1.14%–1.26%, Mn: 1.0%–1.8%, Cr: 1.3%–1.5%, S: ≤0.002%, P: ≤0.005%, Alt: 0.03%–0.06%, with the balance being Fe and unavoidable impurities. This invention utilizes a medium-carbon, low-manganese, and low-chromium alloy system, without adding precious alloying elements such as Ni and Mo. It employs a 390-510mm cross-section continuous casting billet, combined with the rolling process for bucket tooth steel, and a quenching and tempering heat treatment process, thereby improving both the yield strength and excellent toughness of the steel.

Description

Technical Field

[0001] This invention belongs to the field of metallurgy, and specifically relates to a low-cost, high-toughness steel for bucket teeth and its preparation method. Background Technology

[0002] The application of bucket tooth steel essentially stems from the contradiction between the engineering field's development needs for 'high efficiency, low cost, and high reliability' and the insufficient performance of traditional materials. By precisely matching the "impact-wear" composite working conditions of the bucket teeth, it solves the long-standing industry pain points of "vulnerability, high-frequency replacement, and high cost." This not only improves the operational efficiency of individual machines but also supports large-scale, continuous production in core sectors of the national economy such as mining and infrastructure. It is a crucial material foundation for the "domesticization and high-end development" of modern engineering equipment. For example, the NM450-NM550 series bucket tooth steel developed by domestic companies like Baosteel and Ansteel has gradually replaced imported steel, reducing the industry's dependence on foreign materials. For excavators, whether electric or hydraulic, the bucket teeth are crucial components during operation, and the performance of the bucket tooth steel directly affects their working efficiency and economic benefits. During excavator operation, the bucket teeth endure intense composite impact loads such as compression, shearing, and bending, as well as severe wear. The heat treatment process directly affects the microstructure, performance, and quality of the bucket teeth; therefore, in-depth research on excavator bucket tooth materials is fundamental to ensuring the proper service life of these components.

[0003] Therefore, due to the limitations of working conditions, bucket tooth steel not only needs high strength but also high impact toughness to achieve a "balance between wear resistance and toughness". From the perspective of engineering application safety, in order to meet the impact toughness requirements of bucket tooth steel, the purpose of this invention is to provide a bucket tooth steel with ultra-high toughness and low cost. This steel has higher impact toughness requirements than traditional bucket tooth steel, reasonable combination of chemical components, and is weldable, meeting the usage requirements of engineering machinery equipment.

[0004] In the prior art, Chinese patent document CN102400050A discloses a bucket tooth and its preparation method. This method involves adding alloying elements such as B, Cr, and Mn to ensure the comprehensive mechanical properties of the steel, including strength and toughness. The steel is then produced by die casting and forging into a square billet, followed by rolling and tempering heat treatment to obtain a bucket tooth steel with excellent toughness and a microstructure of tempered bainite + tempered sorbite. However, although Examples 1-4 meet the toughness requirements for bucket tooth steel, die casting has been largely phased out in actual production due to its cumbersome process and low yield. Continuous casting has replaced die casting in actual production. Furthermore, the quality of die-cast billets cannot be guaranteed compared to continuous casting. Chinese patent document CN112647013A discloses a JN28Cr2 steel for large bucket teeth and its preparation method, which adds multiple alloying elements such as Mo, V, Mn, Ni, and Ti. The thickness of the continuously cast billet is 220-260 mm. The preparation methods include converter smelting, refining, heating, and rolling. However, although the product can ensure high impact toughness due to the addition of multiple alloying elements, the production cost is high, making it uncompetitive in the market and difficult to actually supply, thus limiting its overall application in industrial production. Summary of the Invention

[0005] The purpose of this invention is to provide a low-cost, high-toughness steel for bucket teeth and its preparation method. By using a medium-carbon, low-manganese, and low-chromium alloy system without adding precious alloying elements such as Ni and Mo, and employing a 390-510mm cross-section continuous casting billet, combined with the rolling process for bucket teeth steel and a quenching and tempering heat treatment process, the yield strength of the steel is improved, and excellent strength and toughness are obtained.

[0006] To achieve the above objectives, the present invention employs the following technical solution: A low-cost, high-toughness steel for bucket teeth, the chemical composition of which, by weight percentage, is: C: 0.22%–0.28%, Si: 1.14%–1.26%, Mn: 1.0%–1.8%, Cr: 1.3%–1.5%, S: ≤0.002%, P: ≤0.005%, Alt: 0.03%–0.06%, with the balance being Fe and unavoidable impurity elements.

[0007] The steel used for bucket teeth has a yield strength ≥1400MPa, tensile strength ≥1850MPa, elongation after fracture ≥11%, reduction of area ≥40%, and U-shaped impact energy (KU2) ≥80J.

[0008] The diameter of the finished steel used for bucket teeth is 40-230mm.

[0009] The microstructure of the finished steel is 81%–89% tempered sorbite + 11%–19% retained austenite.

[0010] The reasons for limiting the composition of the steel used for bucket teeth are as follows: (1) Carbon: Carbon forms cementite (Fe3C) with iron, which significantly improves the hardness and strength of steel through solid solution strengthening and precipitation strengthening. Carbon is a key element in the martensitic transformation, and the martensitic structure formed after quenching gives the surface of the bucket teeth high wear resistance. Too high a carbon content (e.g., >0.28%) will reduce impact toughness and increase the risk of welding cracks; too low a carbon content will result in insufficient hardness. Therefore, the carbon content should be controlled between 0.22% and 0.28%.

[0011] (2) Silicon: Achieves solid solution strengthening effect: Silicon atomic radius is 11% larger than that of iron, causing lattice distortion and increasing yield strength by about 50%. Improves hardenability: Silicon inhibits the decomposition of supercooled austenite, delays the transformation of pearlite and bainite, and allows martensite to form in the core of the bucket teeth, thus improving overall strength. Antioxidant properties: Silicon forms a dense SiO2 film on the steel surface, improving high-temperature oxidation resistance and making it suitable for high-temperature wear environments.

[0012] Excessive silicon content (>1.26%) may lead to decreased toughness and needs to be used in combination with elements such as manganese. Excessive Si content is detrimental to the weldability of steel. Therefore, this invention controls its content at 1.14~1.26%.

[0013] (3) Manganese: Mn is a strong austenite stabilizing element and an effective element for improving the hardenability of steel. Manganese expands the austenite phase region and reduces the critical cooling rate, allowing the bucket teeth to obtain a martensitic structure during air cooling or oil cooling. However, when too much Mn is added to the steel, it will coarsen the grains, increase the carbon equivalent, and thus affect the weldability of the steel, and cause temper brittleness. Therefore, this invention selects an Mn content of 1.0%-1.8%.

[0014] (4) Sulfur: S easily forms FeS and MnS inclusions in steel, causing hot brittleness and significantly reducing the toughness of steel. Therefore, the sulfur content in steel should be reduced as much as possible.

[0015] (5) Phosphorus: P often accumulates at grain boundaries in steel, which disrupts the continuity of the matrix, significantly reduces the toughness of steel, worsens weldability, and easily causes cold brittleness. Therefore, the P content in steel should be reduced as much as possible.

[0016] (6) Chromium: Cr is a strong carbide-forming element. It forms stable carbides with C in steel, which play a role in strength at room temperature and high temperature. In order for this steel to have sufficient strength, the Cr content range is set to 1.3~1.5% in this invention.

[0017] (7) Aluminum: Aluminum mainly plays a role in nitrogen fixation and deoxidation. The AlN formed by the combination of Al and N can effectively refine the grains, but if the content is too high, it will impair the toughness of the steel, and the billet and steel plate will be prone to cracking. Therefore, the present invention controls its content (Alt) to be between 0.03 and 0.06%.

[0018] In this invention, the steel for bucket teeth is smelted using a converter + LF + RH refining process, which ensures precise control over the steel composition and gas content. The continuous casting process is protected throughout the pouring and billet slow cooling process, which ensures the internal quality of the billet. Controlled rolling ensures the refinement of the initial microstructure. The post-rolling heat treatment uses a quenching + tempering process, which can fully guarantee the strength and toughness of the steel plate.

[0019] A method for preparing low-cost, high-toughness bucket tooth steel includes converter + LF + RH refining, continuous casting, furnace heating, rolling, quenching heat treatment, and tempering heat treatment; specifically including: 1) The converter + LF + RH refining process includes: desulfurization of the reducing slag in the LF furnace to reduce inclusions and adjust the composition. Then, the molten steel is degassed in the RH vacuum furnace, ensuring that the total degassed time in the RH furnace is 15-20 minutes. The H and O contents are measured to ensure that [H] ≤ 2ppm.

[0020] 2) The continuous casting includes: full-process protective casting, casting speed control of 0.2 to 0.4 m / min, reducing secondary oxidation during the continuous casting process, reducing the content of inclusions in the steel, and improving the purity of the steel, so as to ensure that the center segregation of the billet is not higher than C1.0 grade.

[0021] Slow cooling and shot blasting process for billets: After the continuous casting billets with a cross-sectional size of 390-510mm are removed from the production line, they are stacked in a slow cooling pit for slow cooling, so that the gas in the billet can be fully diffused and discharged, minimizing the gas content of the billet. The slow cooling time is ≥48 hours, and the surface of the billet is cleaned by shot blasting.

[0022] 3) The heating process in the furnace includes: feeding the billet into a walking beam furnace with an average heating rate of 9-10℃ / min; heating to 1150-1250℃ to ensure high-temperature rolling; and holding the billet at the core temperature until it reaches the surface temperature for at least 1 hour to ensure that the alloying elements in the steel are fully dissolved and to ensure the uniformity of the composition and properties of the final product.

[0023] The rolling process includes: removing iron oxide scale with high-pressure water before rolling, with an initial rolling temperature of 1050-1150℃ and a final rolling temperature of 900-1000℃.

[0024] The quenching heat treatment includes: offline quenching of the rolled steel plate. Alloying elements such as Cr, Mn, and Si in the steel can increase the hardenability of large-diameter round steel. The steel of this invention has been determined to have a ferrite-austenite equilibrium phase transformation end temperature AC3 of 796℃, a quenching temperature of AC3+ (50~100℃), preferably 855~885℃, and a holding time of 2~5min / mm. The purpose is to achieve uniform austenitization of the microstructure while ensuring uniform austenite grain size.

[0025] The tempering heat treatment includes: for quenched steel, in order to release the internal stress of quenched steel, the preferred tempering temperature is 530-570℃, the holding time is 4-7 min / mm, and the holding time is sufficient to ensure that the alloying elements in the steel are effectively precipitated in the form of alloy cementite (Fe3Cr) to form equiaxed ferrite, thereby increasing the ferrite content in the tempered sorbite structure. After tempering, the steel is air-cooled. The purpose of tempering is to obtain a structure dominated by tempered sorbite.

[0026] Compared with the prior art, the beneficial effects of the present invention are: 1) This invention uses the combined action of Mn, Cr and C, along with trace alloying elements, to control the morphology and quantity of the second phase precipitates, thereby improving its strength and toughness.

[0027] 2) The diameter of the finished steel for bucket teeth manufactured using this technical solution is 40-230mm, and its yield strength is ≥1400MPa, tensile strength is ≥1850MPa, elongation after fracture is ≥11%, reduction of area is ≥40%, and U-shaped impact energy is ≥80J.

[0028] 3) This invention employs a series of methods, including shot blasting of the upper and lower surfaces of the billet, controlled heating during the heating process, high-pressure water descaling, and high-temperature rolling, to ensure that the steel has good surface quality.

[0029] 4) This invention ensures that alloying elements in steel are effectively precipitated in the form of alloy cementite (Fe3Cr) by performing high-temperature tempering treatment at low temperature, forming equiaxed ferrite, increasing the ferrite content in the tempered sorbite structure, and improving the toughness of the steel.

[0030] The steel for bucket teeth of this invention combines strength and toughness, as well as good weldability and efficient and economical production process. It is suitable for the expanding market for bucket tooth steel and the ever-increasing application requirements, and belongs to the high-end alloy structural steel with broad prospects. Attached Figure Description

[0031] Figure 1 This is a metallographic microstructure diagram of Example 1 of the present invention. Detailed Implementation

[0032] The present invention will be further described in detail below with reference to the embodiments, but the scope of protection of the present invention is not limited thereto. Experimental methods for which specific conditions are not specified in the embodiments are generally determined according to national / industry standards; if there is no corresponding national / industry standard, then they are performed according to general international standards, conventional conditions, or conditions recommended by the manufacturer.

[0033] The following embodiments are merely some preferred implementations of the present invention and do not limit the scope and technical means of the invention. The production process of the steel for bucket teeth of the present invention is as follows: molten iron pretreatment - converter smelting - ladle refining (LF+RH) - continuous casting - slow cooling by stacking - billet surface treatment - heating - rolling - heat treatment - packaging and warehousing. Table 1 shows the composition involved in each embodiment, Table 2 shows the billet heating, rolling, and cooling processes of the embodiments, Table 3 shows the heat treatment process of the embodiments, Table 4 shows the comprehensive mechanical properties of the embodiments, and Table 5 shows the microstructure and the proportion of each component. The metallographic structure of Example 1 is as follows: Figure 1 As shown.

[0034] Table 1 Chemical composition (wt%) of embodiments of the present invention: Table 2. Smelting, slab heating, rolling, and cooling processes in the examples: Table 3. Heat treatment process of the examples: Table 4. Comprehensive mechanical properties of the final state of the steel plate: Table 5. Microstructures and proportions of each component in the examples: This invention employs a new composition design for the steel used in bucket teeth with diameters ranging from 40 to 230 mm. The steelmaking process utilizes continuous casting to produce continuously cast billets with cross-sections of 390-510 mm. These billets are then subjected to high-temperature hot rolling, followed by quenching and high-temperature tempering heat treatment to obtain a well-matched strength and toughness profile, resulting in high-strength and high-toughness bucket teeth steel.

[0035] Obviously, the above embodiments are merely illustrative examples and not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A low-cost, high-toughness steel for bucket teeth, characterized in that, The chemical composition of the steel, by weight percentage, is as follows: C: 0.22%~0.28%, Si: 1.14%~1.26%, Mn: 1.0%~1.8%, Cr: 1.3%~1.5%, S≤0.002%, P≤0.005%, Alt: 0.03%~0.06%, with the balance being Fe and unavoidable impurity elements.

2. The low-cost, high-toughness steel for bucket teeth according to claim 1, characterized in that, The steel used for bucket teeth has a yield strength ≥1400MPa, tensile strength ≥1850MPa, elongation after fracture ≥11%, reduction of area ≥40%, and U-shaped impact energy ≥80J.

3. The low-cost, high-toughness steel for bucket teeth according to claim 1, characterized in that, The diameter of the finished steel used for bucket teeth is 40-230mm.

4. The low-cost, high-toughness steel for bucket teeth according to claim 1, characterized in that, The microstructure of the finished steel is 81%–89% tempered sorbite + 11%–19% retained austenite.

5. A method for preparing low-cost, high-toughness bucket tooth steel as described in any one of claims 1-4, comprising converter + LF + RH refining, continuous casting, heating, rolling, quenching heat treatment, and tempering heat treatment; characterized in that, The quenching heat treatment includes: quenching temperature of 855~885℃, and holding time of 2~5min / mm; The tempering heat treatment includes: tempering temperature of 530-570℃, holding time of 4-7 min / mm, and air cooling after tempering.

6. The method for preparing low-cost, high-toughness steel for bucket teeth according to claim 5, characterized in that, The converter + LF + RH refining process includes: desulfurization of reducing slag in the LF furnace, and degassing of molten steel in the RH vacuum furnace. The total degassing time is 15-20 minutes, ensuring that [H] ≤ 2ppm.

7. The method for preparing low-cost, high-toughness steel for bucket teeth according to claim 5, characterized in that, The continuous casting includes: casting speed control of 0.2 to 0.4 m / min, and slow cooling time of ≥48 hours.

8. The method for preparing low-cost, high-toughness steel for bucket teeth according to claim 5, characterized in that, The heating includes heating to 1150-1250°C and holding the temperature for at least 1 hour.

9. The method for preparing low-cost, high-toughness bucket tooth steel according to claim 5, characterized in that, The rolling process includes: an initial rolling temperature of 1050–1150°C and a final rolling temperature of 900–1000°C.

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