Low alloy steel for heavy truck brake shoe

Abstract

The invention discloses low alloy steel for a heavy truck brake shoe, which comprises the following chemical components in percentage by mass: 0.30-0.38% of carbon; 1.1% to 1.5% of silicon; 2.0% to 2.4% of manganese; 2.1%-2.7% of chromium; 0.3%-0.7% of molybdenum; 0.15%-0.45% of nickel; 0.08%-0.18% of vanadium; 0.06% to 0.11% of titanium; 0.01%-0.03% of zirconium; 0.015%-0.05% of a rare earth element; 0.001% to 0.0028% of boron; and the balance of iron and inevitable impurities. The purification, modification and fine grain synergistic effect of the mixed rare earth elements lanthanum, cerium, neodymium and yttrium provide key performance guarantee for the brake shoe, the rare earth elements remarkably improve the low-temperature toughness of the material by reducing harmful impurities in molten steel and reconstructing the form of inclusions, and meanwhile, the rare earth has the effects of refining grains and stabilizing high-temperature grain boundaries, so that the service life of the brake shoe is prolonged. According to the brake shoe, the performance attenuation can be kept at a low level at the high temperature of 600 DEG C generated by repeated braking, sudden hardness drop and shoe piece deformation caused by the high temperature are avoided, the brake stability is ensured, and the problems of brake distance lengthening, brake deviation and the like caused by high-temperature failure of the brake shoe are solved.

Description

Technical Field

[0001] This invention relates to the field of low alloy steel technology, and in particular to a low alloy steel for heavy truck brake shoes. Background Technology

[0002] Heavy-duty trucks, as core transportation tools in freight and engineering fields, require their braking systems to withstand high-frequency, high-load conditions over long periods. This includes full-load driving, continuous braking on long downhill slopes, and low-temperature environments. The brake shoes must simultaneously possess high strength, high toughness, resistance to high-temperature brake fade, and excellent wear resistance to ensure braking safety and reliability. Low-alloy steel, due to its excellent comprehensive mechanical properties, has become the mainstream material for manufacturing heavy-duty truck brake shoes. However, current technologies using low-alloy steel for heavy-duty truck brake shoes still face three major challenges in practical applications, making it difficult to meet the stringent requirements of heavy-duty truck braking systems and the cost control targets of industrial production. These are as follows:

[0003] Firstly, the application of rare earth elements in low-alloy steel suffers from limited effectiveness. Current technologies add rare earth elements to some low-alloy steels to improve performance, but these often employ single rare earth elements or simple mixtures, achieving only single deoxidation or desulfurization effects without simultaneously addressing inclusion modification, grain refinement, and high-temperature structural stability. More importantly, the addition of single rare earth elements easily leads to uneven distribution of rare earth elements in the molten steel, with some areas becoming enriched, which in turn increases material brittleness. Especially in low-temperature environments such as -40℃, brake shoes are prone to cracking due to insufficient toughness, posing a safety hazard of brake failure. Simultaneously, the improvement effect on high-temperature grain boundary stability is limited. When heavy trucks continuously brake downhill on long slopes, generating temperatures above 600℃, the hardness of the steel tends to rapidly decrease, leading to brake shoe deformation, abnormal brake clearance, and consequently, problems such as increased braking distance and braking deviation.

[0004] Secondly, the reliance on high-priced alloying elements leads to persistently high material costs, making it difficult to balance performance and economy. Existing low-alloy steels used in heavy-duty truck brake shoes generally rely on high-priced elements such as niobium, high-content chromium, molybdenum, and nickel to ensure high strength and high-temperature stability. While niobium can effectively refine grain size, the raw material is expensive, and the high burn-off rate during smelting increases material waste. The addition of chromium, molybdenum, and nickel is mostly maintained at 3.0%-4.0%, 1.0%-1.5%, and 0.8%-1.2%, respectively. Although this can improve the material's high-temperature creep resistance and toughness, the market prices of these elements fluctuate greatly, consistently accounting for over 60% of the material cost, resulting in excessively high initial procurement costs for brake shoes. Some technologies attempt to reduce the use of high-priced elements, but due to the lack of synergistic design with alternative elements, a sharp decline in performance is likely to occur.

[0005] Third, the material properties are not well-suited to the braking conditions of heavy-duty trucks, making it difficult to cope with complex usage scenarios. Heavy-duty truck brake shoes operate in complex environments, requiring simultaneous adaptation to multiple demands such as low-temperature toughness, high-temperature stability, and high-frequency fatigue resistance. However, existing low-alloy steel composition designs often focus on optimizing a single performance aspect, lacking a systematic elemental synergistic design. For example, some steel grades improve strength by adding vanadium to refine grains, but neglect the strengthening of grain boundaries. This leads to grain boundary embrittlement and cracking during high-frequency braking fatigue cycles. Other steel grades, while emphasizing high-temperature resistance, fail to optimize inclusion morphology, resulting in numerous elongated sulfide and oxide inclusions that easily become stress concentration sources under braking impact loads, causing shoe deformation or fracture. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of existing technologies by proposing a low-alloy steel for heavy-duty truck brake shoes.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A low-alloy steel for heavy-duty truck brake shoes, comprising, by mass percentage: carbon: 0.30%-0.38%; silicon: 1.1%-1.5%; manganese: 2.0%-2.4%; chromium: 2.1%-2.7%; molybdenum: 0.3%-0.7%; nickel: 0.15%-0.45%; vanadium: 0.08%-0.18%; titanium: 0.06%-0.11%; zirconium: 0.01%-0.03%; rare earth elements: 0.015%-0.05%; boron: 0.001%-0.0028%; the balance being iron and unavoidable impurities.

[0009] Preferably, the rare earth element preparation steps are as follows:

[0010] S1. Raw material screening and pretreatment: Select mixed rare earth ore with a small amount of ion-adsorption rare earth ore, crush the two ore sources to 50-100 mesh respectively, remove surface impurities and set aside.

[0011] S2. Ore source roasting pretreatment: Mix the pretreated mixed rare earth ore with sodium carbonate at a mass ratio of 1:0.8-1.2, and feed it into a rotary kiln to keep it at 800-850℃ for 2-3 hours.

[0012] S3. Stepwise leaching, impurity removal, and ratio adjustment: After cooling the S2 roasted product, leach it with 15%-20% dilute hydrochloric acid for 1.5-2 hours with stirring. Filter to remove silica precipitate and obtain a preliminary leachate containing rare earth chlorides. Add calcium hydroxide to the preliminary leachate to adjust the pH to 4.0-4.5, stir for 30-45 minutes, and then filter to remove iron and aluminum impurities to obtain a mixed rare earth chloride solution with a purity ≥95%. Then add the leachate of ion-adsorption rare earth ore and adjust the solution ratio by real-time detection using ICP-MS.

[0013] Preferably, the rare earth elements are a mixture of lanthanum, cerium, neodymium, and yttrium, and the rare earth element ratio in the solution of step S3 is: lanthanum: cerium: neodymium: yttrium = 1:1.1-1.4:0.3-0.5:0.1-0.2.

[0014] Preferably, the unavoidable impurities contain ≤0.011% phosphorus, ≤0.007% sulfur, ≤0.0055% oxygen, and ≤0.0045% nitrogen.

[0015] Preferably, the mechanical properties of the low alloy steel after heat treatment meet the following requirements: tensile strength ≥1180MPa, yield strength ≥920MPa, elongation after fracture ≥12%, impact energy (-40℃) ≥40J, hardness 37-44, and hardness decay rate ≤8.5% after holding at 600℃ for 2h.

[0016] Preferably, the heat treatment process includes:

[0017] Normalizing treatment: Heat the steel billet to 890-930℃, hold for 1.2-2.2 hours, and air cool to room temperature;

[0018] Tempering: Reheat to 830-870℃, hold for 1.0-1.8h, oil cool to room temperature, then temper at 510-550℃ for 1.2-2.2h, and air cool to room temperature.

[0019] Preferably, the zirconium forms a ternary composite carbide (Zr,Ti,V)C with titanium and vanadium, wherein the mass ratio of zirconium, titanium and vanadium is 1:3-5:2-4, and the average particle size of the composite carbide is 0.1-0.3 μm.

[0020] Preferably, the yttrium forms a rare earth composite oxide (La,Ce,Y)₂O₃ with lanthanum and cerium, wherein the yttrium accounts for 8%-15% of the total mass of the rare earth.

[0021] The present invention has the following beneficial effects:

[0022] 1. The purification, modification, and grain refinement synergistic effect of the mixed rare earth elements lanthanum, cerium, neodymium, and yttrium in this invention provides crucial performance assurance for brake shoes. Rare earth elements significantly improve the low-temperature toughness of materials by reducing harmful impurities in molten steel and reconstructing the morphology of inclusions. This is essential for heavy trucks operating in winter or cold regions. Even in a low-temperature environment of -40℃, brake shoes are less likely to crack due to insufficient toughness, avoiding safety hazards such as shoe breakage and brake failure during braking. At the same time, the grain refinement and high-temperature grain boundary stabilization effects of rare earth elements allow the performance degradation of brake shoes to remain at a low level under the high temperature of 600℃ generated by repeated braking. This prevents a sudden drop in hardness and shoe deformation due to high temperature, ensuring braking stability in scenarios such as long downhill driving and continuous braking of heavy trucks, and reducing problems such as increased braking distance and braking deviation caused by high-temperature failure of brake shoes.

[0023] 2. The cost-effective element substitution of this invention brings a balance between low cost and high performance, which is of great significance to the life-cycle value of heavy-duty truck brake shoes. The replacement of expensive niobium with titanium, zirconium, and vanadium composites, as well as the optimization of the use of chromium, molybdenum, and nickel, reduces material costs while maintaining high strength and high-temperature stability. This makes the brake shoes less prone to deformation and cracking when subjected to the huge braking impact of a fully loaded heavy-duty truck, and can maintain a regular braking contact surface for a long time. This avoids abnormal braking clearance and braking noise caused by shoe deformation, and reduces the frequency of daily maintenance of heavy-duty trucks. At the same time, the guarantee of material strength and stability can extend the service life of brake shoes and reduce the number of times brake shoes need to be replaced during operation. Especially for long-haul freight heavy-duty trucks, it can reduce downtime losses caused by brake shoe repairs during the journey and improve operational efficiency.

[0024] 3. The addition of zirconium can provide brake shoes with more comprehensive working condition adaptability. The trace addition of zirconium has multiple synergistic effects, making the grain boundaries less prone to embrittlement and the precipitation of harmful phases less likely to occur in the fatigue cycle of high-frequency braking. Even if the brake shoes are subjected to braking impact and friction for a long time, fatigue cracks are less likely to appear on the shoe body, which greatly extends the replacement cycle of brake shoes and reduces the manpower and time costs of heavy truck maintenance. Attached Figure Description

[0025] Figure 1 This is a process flow diagram for preparing low-alloy steel for heavy-duty truck brake shoes, as proposed in this invention. Detailed Implementation

[0026] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0027] A low-alloy steel for heavy-duty truck brake shoes, comprising, by mass percentage: carbon: 0.30%-0.38%; silicon: 1.1%-1.5%; manganese: 2.0%-2.4%; chromium: 2.1%-2.7%; molybdenum: 0.3%-0.7%; nickel: 0.15%-0.45%; vanadium: 0.08%-0.18%; titanium: 0.06%-0.11%; zirconium: 0.01%-0.03%; rare earth elements: 0.015%-0.05%; boron: 0.001%-0.0028%; the balance being iron and unavoidable impurities.

[0028] The rare earth element preparation steps are as follows:

[0029] S1. Raw material screening and pretreatment: Select mixed rare earth ore with a small amount of ion-adsorption rare earth ore, crush the two ore sources to 50-100 mesh respectively, remove surface impurities and set aside.

[0030] S2. Ore source roasting pretreatment: Mix the pretreated mixed rare earth ore with sodium carbonate at a mass ratio of 1:0.8-1.2, and feed it into a rotary kiln to keep it at 800-850℃ for 2-3 hours.

[0031] S3. Stepwise leaching, impurity removal, and ratio adjustment: After cooling the S2 roasted product, leach it with 15%-20% dilute hydrochloric acid for 1.5-2 hours with stirring. Filter to remove silica precipitate and obtain a preliminary leachate containing rare earth chlorides. Add calcium hydroxide to the preliminary leachate to adjust the pH to 4.0-4.5, stir for 30-45 minutes, and then filter to remove iron and aluminum impurities to obtain a mixed rare earth chloride solution with a purity ≥95%. Then add the leachate of ion-adsorption rare earth ore and adjust the solution ratio by real-time detection using ICP-MS.

[0032] The rare earth elements are a mixture of lanthanum, cerium, neodymium, and yttrium, and the rare earth element ratio in the solution of step S3 is: lanthanum: cerium: neodymium: yttrium = 1:1.1-1.4:0.3-0.5:0.1-0.2.

[0033] The unavoidable impurities include phosphorus ≤0.011%, sulfur ≤0.007%, oxygen ≤0.0055%, and nitrogen ≤0.0045%.

[0034] The mechanical properties of the low alloy steel after heat treatment meet the following requirements: tensile strength ≥1180MPa, yield strength ≥920MPa, elongation after fracture ≥12%, impact energy (-40℃) ≥40J, hardness 37-44, and hardness decay rate ≤8.5% after holding at 600℃ for 2h.

[0035] The heat treatment process includes:

[0036] Normalizing treatment: Heat the steel billet to 890-930℃, hold for 1.2-2.2 hours, and air cool to room temperature;

[0037] Tempering: Reheat to 830-870℃, hold for 1.0-1.8h, oil cool to room temperature, then temper at 510-550℃ for 1.2-2.2h, and air cool to room temperature.

[0038] The zirconium forms a ternary composite carbide (Zr,Ti,V)C with titanium and vanadium, wherein the mass ratio of zirconium, titanium and vanadium is 1:3-5:2-4, and the average particle size of the composite carbide is 0.1-0.3 μm.

[0039] The yttrium forms a rare earth composite oxide (La,Ce,Y)2O3 with lanthanum and cerium, wherein yttrium accounts for 8%-15% of the total mass of the rare earth.

[0040] like Figure 1 As shown, the preparation method of the above-mentioned low-alloy steel for heavy-duty truck brake shoes includes the following steps:

[0041] a. Raw material preparation and proportioning: Prepare the following raw materials by mass percentage: carbon 0.30%-0.38%, silicon 1.1%-1.5%, manganese 2.0%-2.4%, chromium 2.1%-2.7%, molybdenum 0.3%-0.7%, nickel 0.15%-0.45%, vanadium 0.08%-0.18%, titanium 0.06%-0.11%, zirconium 0.01%-0.03%, rare earth elements 0.015%-0.05%, boron 0.001%-0.0028%, with the balance being industrial pure iron. Among them, the mixed rare earth is added in the form of rare earth silicon-manganese alloy containing 25%-30% rare earth, titanium is added in the form of titanium-iron alloy containing 25%-30% titanium, and zirconium is added in the form of zirconium-iron alloy containing 15%-20% zirconium. This avoids the burn-off and uneven distribution problems that occur when directly melting single elements, and ensures the stability of the subsequent composition.

[0042] b. Steel smelting and composition control: First, add industrial pure iron, ferrosilicon, ferromanganese, ferrochrome, ferromolybdenum, and nickel plates to an electric arc furnace, heat to 1520-1560℃ and hold for 1.5-2 hours. After the raw materials are completely melted to form primary molten steel, monitor the carbon and sulfur content in the molten steel in real time using an online carbon and sulfur analyzer to ensure that the sulfur content is ≤0.01%. Then, transfer the primary molten steel to an LF refining furnace, heat to 1580-1620℃, add ferrotitanium alloy, ferrizirconium alloy, and ferrovanadium and stir for 30-45 minutes to allow the titanium, zirconium, and other alloys to combine. Vanadium is uniformly dissolved and forms preliminary carbides. Then, rare earth silicon-manganese alloy is added and stirred for 20-30 minutes. The oxygen content in the molten steel is controlled to ≤0.006% and the nitrogen content to ≤0.005% by utilizing the purification effect of rare earth. Finally, ferroboron is added and stirred for 15-20 minutes to ensure that boron is uniformly distributed in the molten steel to avoid local enrichment. Afterward, the content of each element in the molten steel is detected by ICP-MS. If the content of a certain element deviates from the target range, the corresponding alloy is added until the content of all elements meets the ratio requirements in step a.

[0043] c. Ingot casting and homogenization treatment

[0044] The qualified molten steel in S2 is molded at 1550-1580℃. The ingot size is designed according to the brake shoe size, which is a round ingot with a diameter of 150-200mm or a square ingot with a thickness of 80-120mm. Step cooling is adopted during the ingot casting process to prevent stress and shrinkage cavities caused by rapid cooling. After the ingot is formed, it is sent to an annealing furnace, heated to 880-920℃ and held for 3-4 hours. Then it is slowly cooled to room temperature at a rate of 1-2℃ / min. Homogenization treatment is used to eliminate the component segregation inside the ingot, especially to improve the uniformity of the distribution of elements such as titanium, zirconium and rare earth, laying a good microstructure foundation for subsequent rolling and heat treatment.

[0045] d. Rolling and forming: The homogenized ingot is heated to 1100-1150℃ and held for 1.5-2 hours. A multi-pass hot rolling process with a reduction rate of 15%-20% is used to roll the ingot into a steel plate with a thickness of 15-30mm or a round bar with a diameter of 50-80mm. During the hot rolling process, the final rolling temperature is controlled at 850-900℃ to ensure that the material obtains a refined grain structure. Then, according to the arc structure design of the heavy truck brake shoe, the hot-rolled steel plate or round bar is processed into brake shoe blanks by CNC cutting, forging and other methods. The forging temperature is controlled at 800-850℃. After forging, the blanks are air-cooled to room temperature to avoid deformation problems.

[0046] e. Heat treatment strengthening: First, the brake shoe blank is sent to a heat treatment furnace, heated to 890-930℃ and held for 1.2-2.2 hours, then air-cooled to room temperature to complete the normalizing treatment. Normalizing refines the grains and eliminates forging stress, preparing a uniform pearlitic and ferrite structure for subsequent quenching and tempering. Next, the normalized blank is reheated to 830-870℃ and held for 1.0-1.8 hours, then oil-cooled to room temperature at a cooling rate of 15-20℃ / s to obtain a martensitic structure. Subsequently, it is tempered at 510-550℃ for 1.2-2.2 hours and air-cooled to room temperature to complete the quenching and tempering treatment. This quenching and tempering process helps the material form… Tempered martensite structure is used to balance high strength and high toughness, while promoting the dispersion precipitation of composite carbides formed by titanium, zirconium, and vanadium, thus improving the high-temperature stability of the material. Finally, the working surface of the brake shoe after heat treatment is subjected to a combination of laser quenching and low-temperature tempering. The laser quenching power is 1.5-2.0kW and the scanning speed is 3-5mm / s, which forms a quenching layer with a depth of 0.9-1.3mm and a hardness of HRC≥52 on the working surface. Then, it is tempered at low temperature at 180-220℃ for 1-1.5h to control the residual stress of the quenching layer to ≤300MPa, so as to avoid cracking of the working surface and further improve wear resistance.

[0047] f. Finished Product Inspection and Acceptance: First, sample the brake shoe material for mechanical property testing to ensure tensile strength ≥1180MPa, yield strength ≥920MPa, impact absorption energy at -40℃ ≥40J, and hardness decay rate at 600℃ ≤8.5%, meeting the performance requirements. Then, observe the microstructure of the material using a metallographic microscope, requiring grain size ≤10μm, inclusion size ≤3μm, and no obvious segregation. Finally, check the appearance of the brake shoe for cracks and deformation, while controlling the dimensional deviation within ±0.5mm. Once all test items are qualified, it is considered a finished heavy truck brake shoe.

[0048] I. Implementation Example Design

[0049] Example 1

[0050] A low-alloy steel for heavy-duty truck brake shoes, by mass percentage, comprises the following chemical components: 0.34% carbon, 1.3% silicon, 2.2% manganese, 2.4% chromium, 0.5% molybdenum, 0.3% nickel, 0.13% vanadium, 0.08% titanium, 0.02% zirconium, 0.03% rare earth elements, 0.002% boron, with the balance being iron and unavoidable impurities (phosphorus ≤0.011%, sulfur ≤0.007%, oxygen ≤0.0055%, nitrogen ≤0.0045%). The rare earth elements are a mixture of lanthanum, cerium, neodymium, and yttrium in a ratio of 1:1.2:0.4:0.15, prepared by the ore roasting, stepwise leaching, and impurity removal and conditioning process of this invention, with a purity of 96.2%. Zirconium, titanium, and vanadium form a ternary composite carbide (Zr,Ti,V)C, and yttrium forms a rare earth composite oxide (La,Ce,Y)2O3 with lanthanum and cerium. Yttrium accounts for 12% of the total mass of rare earth elements.

[0051] In terms of heat treatment process, the brake shoe blank is first normalized, heated to 910℃, held for 1.7 hours and then air-cooled to room temperature; then tempered, the normalized blank is reheated to 850℃, held for 1.4 hours and then oil-cooled to room temperature, then tempered at 530℃ for 1.7 hours and air-cooled to room temperature; finally, the working surface is strengthened by laser quenching with a power of 1.8kW and a scanning speed of 4mm / s, followed by low-temperature tempering at 180-220℃ for 1.2 hours.

[0052] The finished product was then subjected to performance testing. The test results showed that the low alloy steel had a tensile strength of 1250 MPa, a yield strength of 980 MPa, an elongation after fracture of 14.2%, an impact absorption energy of 48 J at a low temperature of -40℃, a hardness (HRC) of 41, and a hardness decay rate of 6.8% after being kept at a high temperature of 600℃ for 2 hours.

[0053] Example 2

[0054] A low-alloy steel for heavy-duty truck brake shoes has a chemical composition that is completely consistent with that of Example 1 by mass percentage, except that the mixed rare earth elements of lanthanum, cerium, neodymium, and yttrium are replaced with a single cerium element. The amount of single cerium added is still 0.03%, and the purity is 96.2%. The material retains only the ternary composite carbide (Zr,Ti,V)C (mass ratio 1:4:3, average particle size 0.2μm) formed by zirconium, titanium, and vanadium, and no rare earth composite oxide (La,Ce,Y)2O3 is generated.

[0055] Regarding the heat treatment process, firstly, normalizing was performed, heating the brake shoe blank to 910℃, holding it at that temperature for 1.7 hours, and then air-cooling it to room temperature. Next, tempering was performed, reheating the normalized blank to 850℃, holding it for 1.4 hours, and then oil-cooling it to room temperature. It was then tempered at 530℃ for 1.7 hours and air-cooled to room temperature. Finally, the working surface was strengthened using laser quenching with a power of 1.8kW and a scanning speed of 4mm / s, followed by low-temperature tempering at 180-220℃ for 1.2 hours. This process is exactly the same as the heat treatment process in Example 1, without any adjustments.

[0056] The finished product was then subjected to performance testing. The test results showed that the low alloy steel had a tensile strength of 1210 MPa, a yield strength of 950 MPa, an elongation after fracture of 12.8%, an impact absorption energy of 32 J at a low temperature of -40℃, a hardness (HRC) of 40, and a hardness decay rate of 10.5% after being kept at a high temperature of 600℃ for 2 hours.

[0057] Example 3

[0058] A low-alloy steel for heavy-duty truck brake shoes, by mass percentage, has the following chemical composition compared to that of Example 1: only zirconium is removed (zirconium addition is 0%). The remaining components are: carbon 0.34%, silicon 1.3%, manganese 2.2%, chromium 2.4%, molybdenum 0.5%, nickel 0.3%, vanadium 0.13%, titanium 0.08%, rare earth elements 0.03%, boron 0.002%, with the balance being iron content remaining unchanged. No ternary composite carbide (Zr,Ti,V)C is formed inside the material; only binary carbide (Ti,V)C is formed from titanium and vanadium, with an average particle size of 0.4 μm. The morphology and content of rare earth composite oxide (La,Ce,Y)₂O₃ are consistent with those of Example 1.

[0059] Regarding the heat treatment process, firstly, normalizing was performed, heating the brake shoe blank to 910℃, holding it at that temperature for 1.7 hours, and then air-cooling it to room temperature. Next, tempering was performed, reheating the normalized blank to 850℃, holding it for 1.4 hours, and then oil-cooling it to room temperature. It was then tempered at 530℃ for 1.7 hours and air-cooled to room temperature. Finally, the working surface was strengthened using laser quenching with a power of 1.8kW and a scanning speed of 4mm / s, followed by low-temperature tempering at 180-220℃ for 1.2 hours. This process is exactly the same as the heat treatment process in Example 1, without any adjustments.

[0060] The finished product was then subjected to performance testing. The test results showed that the low alloy steel had a tensile strength of 1190 MPa, a yield strength of 930 MPa, an elongation after fracture of 13.1%, an impact absorption energy of 42 J at a low temperature of -40℃, a hardness (HRC) of 38, and a hardness decay rate of 12.2% after being kept at a high temperature of 600℃ for 2 hours.

[0061] II. Proportional Design

[0062] Comparative Example

[0063] A low-alloy steel for heavy-duty truck brake shoes has a chemical composition referencing mainstream existing technologies. The chemical composition of this low-alloy steel, by mass percentage, is: carbon 0.36%, silicon 1.2%, manganese 1.9%, chromium 3.5%, molybdenum 1.3%, nickel 1.0%, niobium 0.06%, vanadium 0.05%, rare earth elements 0.02% (single lanthanum element), boron 0.0018%, with the balance being iron. Its core characteristics are: reliance on high-value alloying elements such as niobium, high-content chromium (3.5%), high-content molybdenum (1.3%), and high-content nickel (1.0%); the rare earth element used is only lanthanum, without the addition of titanium or zirconium.

[0064] In terms of heat treatment process, conventional market processes are adopted. First, normalizing is performed, in which the brake shoe blank is heated to 900℃, held for 2 hours, and then air-cooled to room temperature. Then, tempering is performed, in which the normalized blank is reheated to 840℃, held for 1.6 hours, and then oil-cooled to room temperature. Then, it is tempered at 520℃ for 2 hours and air-cooled to room temperature. Finally, the working surface is strengthened by using conventional quenching process (held at 860℃ for 1 hour and then oil-cooled), followed by low-temperature tempering at 200℃ for 1.5 hours.

[0065] The obtained finished product was then subjected to performance testing. The test results showed that the low alloy steel had a tensile strength of 1150 MPa, a yield strength of 890 MPa, an elongation after fracture of 11.5%, an impact absorption energy of 30 J at a low temperature of -40℃, a hardness (HRC) of 39, and a hardness decay rate of 15.8% after being kept at a high temperature of 600℃ for 2 hours. Furthermore, with the material cost of Example 1 as a benchmark of 1.0, the relative material cost of this comparative example is 1.6.

[0066] III. Comparative Analysis of Performance Test Results

[0067] The performance test results of Examples 1-3 and the comparative examples were recorded respectively and statistically presented in tabular form. The specific results are shown in the table below:

[0068] Comparison Table of Performance Test Results of Low Alloy Steel for Heavy Truck Brake Shoes

[0069] Testing items Example 1 Example 2 Example 3 Comparative Example Tensile strength (MPa) 1250 1210 1190 1150 Yield strength (MPa) 980 950 930 890 Elongation after fracture (%) 14.2 12.8 13.1 11.5 Impact absorption energy (J) 48 32 42 35 Hardness (HRC) 41 40 38 39 Hardness attenuation rate (%) 6.8 10.5 12.2 15.8 Material costs (relative value) 1.0 - - 1.6

[0070] By comparing the performance data of Examples 1, 2, and 3 with those of the comparative examples, it is clear that the low-alloy steel of the present invention has achieved a comprehensive breakthrough in mechanical properties, working condition adaptability, and economy, and has completely solved the core pain points of the prior art. The specific analysis conclusions and core advantages are as follows:

[0071] Based on the performance comparison results, the complete solution of this invention, namely Example 1, is the only solution that can simultaneously achieve high strength, high and low temperature toughness, high and high temperature stability, and low cost. Its tensile strength (1250 MPa), yield strength (980 MPa), and impact absorption energy at -40℃ (48 J) are significantly higher than other groups. Its hardness decay rate at 600℃ is only 6.8%, far lower than other groups, and the relative material cost is only 1.0. In contrast, Example 2, which uses a single rare earth element, loses the purification, modification, and grain refinement synergistic effects of the mixed rare earth elements, resulting in a -40℃ impact absorption energy of only 32 J and a hardness decay rate at 600℃ increasing to 10.5. The first example, which removes zirconium, cannot adapt to extreme low and high temperature conditions. In Example 3, the absence of zirconium, combined with titanium and vanadium to form a ternary composite carbide, further increases the hardness decay rate at 600℃ to 12.2%, and the tensile strength and yield strength also decrease significantly, making it difficult to cope with the high temperature environment of continuous braking on long downhill slopes. The second example, which uses common steels, has both performance and economic disadvantages. Not only are the tensile strength, yield strength, and low temperature impact energy lower than the requirements of this invention, but the hardness decay rate at 600℃ is also as high as 15.8%, and the material cost is 60% higher than that of Example 1. It cannot meet the requirements of complex working conditions and lacks cost competitiveness.

[0072] Based on the above comparison, the core advantages of this invention are concentrated in three major dimensions of innovative design:

[0073] Firstly, the hybrid rare earth synergistic design overcomes the industry pain points of low-temperature brittleness and high-temperature degradation. Through the precise ratio of lanthanum:cerium:neodymium:yttrium = 1:1.1-1.4:0.3-0.5:0.1-0.2, the hybrid rare earth elements can reconstruct the morphology of inclusions in steel and eliminate stress concentration sources, thereby increasing the impact absorption energy at -40℃ by 50% compared to single rare earth elements, significantly avoiding the risk of brake shoe cracking at low temperatures in winter. At the same time, the rare earth composite oxides can pin the high-temperature grain boundaries, and with the dispersion strengthening of ternary composite carbides, the hardness degradation rate at 600℃ is reduced by 57% compared to market steel grades, ensuring that the brake shoe does not deform and the braking gap remains stable during continuous braking on long downhill slopes.

[0074] Secondly, the affordable element composite substitution achieves a balance between high performance and low cost. By replacing expensive niobium with a ternary composite design of titanium, zirconium, and vanadium, and optimizing the amount of chromium, molybdenum, and nickel, the waste caused by niobium smelting is avoided. The ternary composite carbide has a better strengthening effect than niobium carbide due to its small particle size and high dispersion. This increases the tensile strength by 8.7% compared to market steel grades, while reducing material costs by 37.5%.

[0075] Thirdly, the all-condition adaptability design covers all complex scenarios of heavy truck braking. Through element synergy and process matching, the refined grains and uniform tempered sorbite structure increase the elongation after fracture by 23.5% compared to market steel grades. It can withstand fatigue cycles of high-frequency braking to reduce crack formation. The working surface of the brake shoe is treated with laser quenching and low-temperature tempering, which can also control the residual stress to ≤300MPa while ensuring high wear resistance with a hardness of HRC≥52. This solves the contradiction between wear resistance and crack resistance in existing technologies, and ultimately provides the industry with an optimal solution that takes into account safety, reliability and economy, and has extremely strong industrial application value.

[0076] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A low alloy steel for heavy duty brake shoes, characterized by, The low alloy steel has the following chemical components in percentage by mass: carbon: 0.30%-0.38%; silicon: 1.1%-1.5%; manganese: 2.0%-2.4%; chromium: 2.1%-2.7%; molybdenum: 0.3%-0.7%; nickel: 0.15%-0.45%; vanadium: 0.08%-0.18%; titanium: 0.06%-0.11%; zirconium: 0.01%-0.03%; rare earth elements: 0.015%-0.05%; boron: 0.001%-0.0028%; and the balance of iron and inevitable impurities.

2. The low alloy steel for heavy duty brake shoes according to claim 1, characterized in that, The preparation of the rare earth elements is as follows: S1, raw material selection and pretreatment, select mixed rare earth ore and a small amount of ionic rare earth ore, crush the two kinds of ores to 50-100 mesh respectively, remove the surface impurities and reserve; S2, ore roasting pretreatment, mix the pretreated mixed rare earth ore and sodium carbonate uniformly at a mass ratio of 1:0.8-1.2, and send them into a rotary kiln for heat preservation at 800-850℃ for 2-3h; S3, stepwise leaching, impurity removal and ratio adjustment, after cooling the roasting product of S2, stir and leach it with 15%-20% dilute hydrochloric acid for 1.5-2h, remove the silicic acid precipitate by filtration, obtain a preliminary leaching solution containing rare earth chlorides, add calcium hydroxide to the preliminary leaching solution to adjust the pH to 4.0-4.5, stir for 30-45min, then remove the iron and aluminum impurities by filtration, obtain a mixed rare earth chloride solution with a purity of ≥95%, then add the leaching solution of ionic rare earth ore, adjust the solution ratio by real-time detection by ICP-MS.

3. The low alloy steel for heavy duty brake shoes according to claim 2, characterized in that, The rare earth elements are mixed rare earth elements of lanthanum, cerium, neodymium and yttrium, and the ratio of the rare earth elements in the solution of S3 is: lanthanum: cerium: neodymium: yttrium = 1:1.1-1.4:0.3-0.5:0.1-0.

2.

4. The low alloy steel for heavy duty brake shoes according to claim 1, characterized in that, The inevitable impurities include: phosphorus ≤0.011%, sulfur ≤0.007%, oxygen ≤0.0055%, and nitrogen ≤0.0045%.

5. The low alloy steel for heavy duty brake shoes according to claim 1, wherein The mechanical properties of the low alloy steel after heat treatment meet the following requirements: tensile strength ≥1180MPa, yield strength ≥920MPa, elongation after fracture ≥12%, impact energy (-40℃) ≥40J, hardness 37-44, and hardness attenuation rate ≤8.5% after heat preservation at 600℃ for 2h.

6. The low alloy steel for heavy duty brake shoes according to claim 5, characterized in that, The heat treatment process includes: normalizing treatment: heat the steel billet to 890-930℃, heat preservation for 1.2-2.2h, air cooling to room temperature; quenching and tempering treatment: reheat to 830-870℃, heat preservation for 1.0-1.8h, oil cooling to room temperature, then tempering at 510-550℃ for 1.2-2.2h, air cooling to room temperature.

7. The low alloy steel for heavy duty brake shoes according to claim 1, wherein The zirconium, titanium and vanadium form ternary complex carbide (Zr,Ti,V)C, wherein the mass ratio of zirconium, titanium and vanadium is 1:3-5:2-4, and the average particle size of the complex carbide is 0.1-0.3μm.

8. The low alloy steel for heavy duty brake shoes according to claim 2, wherein The yttrium, lanthanum and cerium form rare earth complex oxide (La,Ce,Y)2O3, wherein the yttrium accounts for 8%-15% of the total mass of rare earth elements.

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