Hafnium microalloyed gear steel with low heat treatment deformation and preparation method thereof

Abstract

The invention discloses low-heat-treatment-deformation hafnium microalloyed gear steel and a preparation method thereof, and belongs to the technical field of high-temperature carburized gear steel. The gear steel comprises the following components in percentage by mass: 0.17%-0.23% of C, 0.15%-0.35% of Si, 0.80%-1.10% of Mn, 1.00%-1.30% of Cr, 0.01%-0.10% of Ti, 0.02%-0.10% of Hf, less than or equal to 0.020% of P, less than or equal to 0.015% of S and the balance of Fe and impurities, and the ratio of Hf to Ti is controlled to be 0.4-1.5. The preparation method comprises the steps of smelting, continuous casting, rolling, controlled cooling and low-deformation heat treatment. Through Hf microalloying, the high-temperature stability of HfN / C and the thermal expansion property matched with a matrix are utilized, after carburization is conducted at the temperature larger than or equal to 930 DEG C, ASTM 8-grade or above superfine grains can still be kept, the quenching stress is remarkably reduced, the tooth direction deviation of the gear is smaller than or equal to 0.05 mm, unification of high-temperature efficient carburization and extremely low deformation is achieved, and the high-precision gear is suitable for manufacturing of new energy automobiles and other high-precision gears.

Description

Technical Field

[0001] This invention belongs to the field of advanced steel materials, specifically a hafnium microalloyed gear steel with low heat treatment deformation and its preparation method. It particularly relates to the technology of reducing post-heat deformation by replacing titanium (Ti) with hafnium (Hf), which is suitable for the precision manufacturing of high-speed, high-torque transmission gears in new energy vehicles. Background Technology

[0002] With the trend of electric new energy vehicle transmission systems developing towards higher speeds (>15,000 rpm) and higher torques (>300 Nm), the requirements for gear load-bearing capacity and fatigue performance are becoming increasingly stringent. Carburizing and quenching are key processes for improving the surface hardness and wear resistance of gears; however, deformation control during heat treatment has become a core bottleneck restricting the accuracy and reliability of gear transmissions. Traditional gear steels widely employ titanium (Ti) microalloying technology, which refines the initial grains by forming TiN particles that pin austenite grain boundaries. However, this technology reveals significant defects when dealing with higher carburizing temperatures (≥930℃) to increase the carburizing rate and layer depth: TiN particles have insufficient high-temperature stability and are prone to coarsening (average size >1 μm) or even partial dissolution during long-term high-temperature carburizing. According to Zener's pinning theory, the pinning force (P) Z ) and particle diameter (P D Inversely proportional to the strength and toughness of the material, coarsening of particles directly leads to a decrease in pinning force and a sharp drop in austenite grain boundary migration resistance, resulting in severe grain growth. In practice, after carburizing Ti-microalloyed steel at temperatures above 930℃, the austenite grain size often deteriorates from the initial grade 8 to below grade 6 (ASTM E112, average grain diameter ≥45 μm). Coarse grain structure not only impairs the material's strength and toughness but also leads to uneven distribution of quenching transformation stress, causing significant increases in gear tooth deviation and out-of-tolerance roundness of the inner hole, often necessitating the addition of a straightening process to correct deformation. This process not only significantly increases production costs but may also introduce harmful residual stress, increasing the risk of early gear cracking.

[0003] To suppress grain coarsening in Ti microalloyed steel, existing techniques typically limit the carburizing temperature to 930°C or below. However, according to the formula for the diffusion coefficient of carbon in austenite (D... C∝exp(-Q / RT)), a decrease in temperature exponentially slows down the carbon atom diffusion rate. For example, at 930℃ compared to 950℃, the carbon diffusion activation energy Q increases by approximately 18%, leading to a significant decrease in carburizing efficiency. To achieve the deep, effective hardened layer (typically >1.5 mm) required for high-torque gears (>300 Nm) in new energy vehicles, the carburizing time needs to be significantly extended, resulting in increased energy consumption and production costs. More seriously, to compensate for the slow carbon diffusion at low temperatures, the carbon potential in the furnace is often forced to be increased in actual production. This easily leads to excessively high carbon concentration on the gear surface (up to 1.6%C or more), while insufficient carbon penetration in the core or deeper layers, resulting in a larger carbon concentration difference (ΔC) between the surface and core, and a steeper carbon concentration gradient (dC / dx) (up by 30%-50%). This steep carbon gradient exacerbates the structural stress during quenching, not only increasing deformation uncertainty but also easily inducing surface microcracks, severely damaging the contact fatigue and bending fatigue life of the gear.

[0004] To address these issues, the industry has made various attempts. For example, Chinese patent CN120384248A discloses a gear steel using Nb, Ti, and B composite microalloying, aiming to achieve high-temperature carburizing at 980℃ by forming various carbonitrides to pin grain boundaries. However, its core technology still partially relies on TiN particles, and the coarsening and dissolution trend of this precipitate at long-term high temperatures has not been fundamentally resolved, making it difficult to stably maintain ultrafine grains. Furthermore, its manufacturing process lacks sufficient consideration for proactive control of heat treatment deformation. Another example is Chinese patent CN115094309B, which discloses a Cr-Ni-Mo carburizing steel containing Nb, improving the microstructure and properties during high-temperature carburizing by adding Nb and Al and employing a complex heat treatment process. However, the AlN precipitate it relies on also faces the risk of dissolution at temperatures above 950℃, the grain refinement effect is unstable at extreme temperatures, and its deformation control relies excessively on cumbersome process parameter adjustments, failing to fundamentally solve the problems of carbon concentration gradient and stress concentration caused by high carbon potential.

[0005] Therefore, developing a new type of gear steel material and its supporting preparation method that can maintain ultra-fine and stable austenitic grains under high-temperature (≥930℃) carburizing conditions, while effectively reducing quenching stress and controlling gear heat treatment deformation, is of urgent practical significance and important industrial value for meeting the precision manufacturing needs of high-end gears for new energy vehicles. Summary of the Invention

[0006] In view of this, the purpose of this invention is to provide a hafnium microalloyed gear steel with low heat treatment deformation and its preparation method. By utilizing the high-temperature stability and thermal expansion coordination of hafnium, the problem of grain coarsening and large deformation during high-temperature carburizing of traditional gear steel is solved, thereby achieving high-temperature and efficient carburizing and significantly reducing heat treatment deformation, thus meeting the requirements of high-precision gear manufacturing.

[0007] To achieve the above objectives, the present invention provides the following technical solution: This invention first proposes a hafnium microalloyed gear steel with low heat treatment deformation. Its chemical composition by mass percentage is as follows: C: 0.17%~0.23%, Si: 0.15%~0.35%, Mn: 0.80%~1.10%, Cr: 1.00%~1.30%, Ti: 0.01%~0.10%, Hf: 0.02%~0.10%, P≤0.020%, S≤0.015%, with the balance being Fe and unavoidable impurities. The mass percentage ratio of Hf to Ti, Hf / Ti, is controlled between 0.4 and 1.5.

[0008] Furthermore, its chemical composition by mass percentage is as follows: C: 0.19%–0.22%, Si: 0.18%–0.32%, Mn: 0.85%–1.08%, Cr: 1.08%–1.28%, Ti: 0.03%–0.08%, Hf: 0.04%–0.07%.

[0009] Furthermore, the mass percentage ratio of Hf to Ti, Hf / Ti, is preferably 0.5 to 2.33.

[0010] Furthermore, its oxygen content [O] is ≤15ppm and its nitrogen content [N] is 80~120ppm.

[0011] Furthermore, after heat treatment at a carburizing temperature of ≥930℃, the austenite grain size is maintained at ASTM grade 8 or above, the tooth profile deviation Fβ of the finished gear is ≤0.05mm, and the end face runout is ≤0.03mm.

[0012] This invention also proposes a method for preparing hafnium microalloyed gear steel with low heat treatment deformation as described above, comprising the following steps: (1) Alloy element ratio design: According to the chemical composition range described in claim 1, high-purity metal raw materials are selected for formulation; (2) Primary refining: Primary refining is carried out using an electric arc furnace or converter, and the refining temperature reaches 1580-1650℃; (3) LF-VD secondary refining: Refining is carried out under vacuum conditions ≤1.0Pa and argon gas stirring is used. The refining time is not less than 45 minutes. (4) Continuous casting: Electromagnetic stirring process is adopted, the superheat is controlled at 20-35℃, and the casting speed is 0.6-0.8 m / min to obtain continuous casting billet; (5) Heating and rolling: After heating the billet to 1150~1200℃ and holding it at that temperature, rolling is carried out. The roughing temperature is 1050~1100℃, the finishing temperature is controlled at 900~950℃, and the total reduction rate is not less than 70%. (6) Controlled cooling: The steel is cooled to 600-650°C at a cooling rate of 30-50°C / s using an ultra-fast cooling process, and then air-cooled to room temperature; (7) Low deformation heat treatment: The steel obtained in step (6) or the gear workpiece made therefrom is subjected to low deformation heat treatment, which includes pretreatment, carburizing and quenching and tempering.

[0013] Furthermore, the low-deformation heat treatment in step (7) specifically includes: Pretreatment: Under a protective atmosphere, heat to 450-550℃ at a rate of ≤100℃ / h and hold for 0.1-3 hours, then cool with the furnace to 300-350℃ at a rate of ≤80℃ / h and then air cool. Carburizing and quenching: includes carburizing treatment and quenching, wherein the carburizing treatment includes a strong carburizing stage at 880-940℃ and a diffusion stage at 840-880℃; the quenching adopts a two-stage oil quenching method, in which the workpiece is first cooled in quenching oil at 110-130℃, and then transferred to quenching oil at 70-90℃ for further cooling. Tempering: Heat to 160-190℃ at a rate of ≤100℃ / h, hold for 0.1-5 hours, and then air cool.

[0014] Furthermore, in the carburizing and quenching step, the carbon potential in the strong carburizing stage is controlled at 0.85-1.25%, and the carbon potential in the diffusion stage is controlled at 0.75-0.85%, and the time of the diffusion stage is 1 / 3 to 1 / 2 of the time of the strong carburizing stage.

[0015] Furthermore, in step (7), before carburizing and quenching, there is also a loading step. The loading step uses a vertical clamping method combining a fractal topological support structure and a spherical crown microplane array to fix the gear workpiece. The fractal topological support structure includes three sets of ribs evenly distributed in a 120° circumferential direction. The contact part between the ribs and the end face of the gear workpiece is the spherical crown microplane array.

[0016] Furthermore, in the spherical microplane array, the base diameter of a single spherical cap is 1-2 mm, the radius of curvature is 0.5-1.0 mm, and the center-to-center distance between adjacent spherical caps is 5 ± 0.5 mm; the surface of the spherical microplane is provided with a composite coating, which includes a CrAlN underlayer with a thickness of 3-5 μm and a MoS2 surface layer with a thickness of 1-2 μm.

[0017] The technical principle of this invention is as follows: Replacing titanium (Ti) with hafnium (Hf) in gear steel effectively reduces gear deformation. The key lies in the superior high-temperature stability and grain boundary pinning ability of HfN compounds. HfN has a much higher melting point than traditional TiN, stronger binding energy, and greater stability at high temperatures. This allows it to maintain an ultrafine particle state for a long time in the carburizing temperature environment, significantly inhibiting grain coarsening. Through a dual pinning mechanism—the physical barrier of nanoparticles and the segregation and dragging effect of elements at grain boundaries—Hf effectively locks austenite grain boundaries, ensuring that the material maintains a fine-grained structure after high-temperature carburizing, fundamentally improving the dimensional stability of the gear. Simultaneously, the introduction of Hf significantly optimizes the carburizing process window. It allows for higher carburizing temperatures, thereby accelerating carbon atom diffusion and significantly shortening the time required for deep carburizing. More importantly, this high-temperature carburizing process reduces the surface carbon concentration, making the carbon content gradient change more gradual and directly alleviating the structural stress during quenching. Notably, the thermal expansion coefficient matching between HfC and the steel matrix is ​​significantly better than that of TiC. This coordinated thermal expansion behavior significantly reduces interfacial stress generated during quenching and cooling, effectively suppressing microcrack initiation and drastically reducing the overall gear deformation rate. The energy savings from high-temperature carburizing, coupled with production optimization that eliminates the straightening process, keep the increase in material costs within a controllable range. This technological approach provides crucial support for the efficient manufacturing of high-precision gears for new energy vehicles.

[0018] The key breakthrough of this invention lies in its innovative use of hafnium to replace traditional titanium microalloying, completely solving the problem of grain coarsening during high-temperature carburizing of gear steel. Through the unique high-temperature stability and thermal expansion compatibility of hafnium compounds, it achieves, for the first time, a significant increase in carburizing temperature while maintaining an ultra-fine grain structure, greatly suppressing quenching deformation. Simultaneously, it optimizes carbon distribution uniformity, reduces the risk of stress concentration, and eliminates the need for straightening processes, providing a groundbreaking material solution for the manufacturing of high-precision, high-reliability gears for new energy vehicles.

[0019] Hafnium (Hf) is the key element in this invention, and deformation control is achieved through multiple mechanisms. First, the HfC precipitates exhibit extremely high thermal stability, with a dissolution temperature more than 250°C higher than that of conventional TiC, and maintain a size distribution of ≤50nm even at a carburizing temperature of 950°C. Through the Zener pinning mechanism, these nanoprecipitates can provide pinning pressure of ≥0.5MPa, effectively suppressing austenite grain boundary migration. Second, Hf forms a stable HfS compound with sulfur, reducing the sulfur content at grain boundaries to below 30ppm and significantly improving grain boundary bonding energy. Third, Hf promotes uniform distribution of precipitates within the grain by altering the austenite / precipitate interface energy, avoiding localized stress concentration. This multi-scale microstructure control stabilizes the austenite grain size at ASTM 8-9, providing a microstructure basis for low-deformation quenching. Considering the above factors, the Hf content is 0.02~0.10%.

[0020] Precise control of the carbon content is based on phase change thermodynamics. According to Andrews' empirical formula, controlling the carbon content between 0.17% and 0.23% keeps the Ms point within a high range. A higher Ms point means a reduced phase change driving force and a decrease in phase change stress of approximately 25%. Simultaneously, this carbon content range ensures a carburized layer hardness of HRC58-62 and a core strength ≥1200 MPa, achieving an optimal balance between strength and deformation characteristics. By controlling the carbon activity within the range of 0.35-0.45, carburizing kinetics can be optimized to obtain a gentler carbon concentration gradient.

[0021] Manganese (Mn) is a double-edged sword; its hardenability can be used to prevent its segregation. According to the Grossmann equation, for every 0.1% increase in Mn content, the critical diameter D... c An increase of approximately 1.5 mm is achieved. Controlling Mn at 0.80%-1.10% ensures that the core of gears with diameters below 50 mm achieves over 90% martensite. Simultaneously, through the grain boundary fixation effect coupled with Hf, the Mn segregation index is controlled within the range of 1.15-1.25 (compared to 1.6-1.8 in traditional processes), effectively suppressing banded structures. This compositional uniformity reduces the difference in phase transformation expansion coefficients between different regions to 2.1 × 10⁻⁶. -6 Below / K, deformation consistency is significantly improved.

[0022] Chromium (Cr) exerts its effects through a dual mechanism of solid solution strengthening and carbide formation. For every 0.1% increase in Cr content, the hardenability factor increases by 4-6 points. Maintaining Cr content between 1.00% and 1.30% optimizes hardenability without promoting δ-ferrite formation. Simultaneously, Cr forms 6-complex carbides with Hf. These carbides maintain dimensional stability during austenitization through the Ostwald ripening inhibition mechanism, resulting in a lower coarsening rate constant compared to conventional methods.

[0023] The unique role of silicon (Si) is manifested in solid solution strengthening and control of the carburizing process. Si reduces the diffusion coefficient of carbon in austenite through the dragging effect, resulting in a smoother carbon concentration distribution in the carburized layer, with a concentration gradient reduced by approximately 30%. Simultaneously, Si increases the residual compressive stress in the carburized layer to above -450 MPa, enhancing its resistance to deformation. However, Si must be controlled within a certain range to avoid promoting the formation of an internal oxide layer.

[0024] Titanium (Ti) is used as an auxiliary element for hydrogen nitride (Hf). Traditional gear steel uses Ti to refine grain size, but its stability at high temperatures is inferior to that of Hf. Retaining a small amount of Ti (0.01%-0.10%) allows it to form a composite (Ti,Hf)(C,N) precipitate with Hf. This precipitate is more dispersed, helping Hf to more reliably control the grain size. However, the amount must be strictly controlled; if too much Ti is added, it will form coarse nitrides, which can become the starting point for fatigue cracks.

[0025] P is controlled at ≤0.020% to avoid temper embrittlement caused by grain boundary segregation. According to grain boundary segregation kinetics, this content level can reduce the tendency for temper embrittlement at 500℃. S is controlled at ≤0.015% to limit the amount of MnS inclusions, while reserving sufficient S element for HfS formation. O and N are controlled at ≤15ppm and 80-120ppm, respectively, to optimize the distribution of precipitated phases without compromising toughness.

[0026] The beneficial effects of this invention are as follows: This invention replaces titanium (Ti) with hafnium (Hf) in gear steel. Through the high-temperature stability of HfN or HfC (melting point higher than TiN or TiC) and the dual pinning mechanism (increased Zener pinning force, high grain boundary segregation energy dragging grain boundary migration), grain coarsening during high-temperature carburizing (≥930℃) is suppressed, maintaining a stable grain size of ASTM grade 8 (average diameter ≤22 μm), while Ti-microalloyed steel only reaches grade 6 (≥45 μm). Regarding carburizing process optimization, Hf microalloying allows carburizing temperatures ≥930℃ (Ti-microalloyed steel ≤930℃). With increased temperature, the carbon diffusion coefficient increases by 40%, and the carburizing time for the 1.5 mm hardened layer is significantly shortened; simultaneously, the surface carbon concentration decreases, significantly reducing the carbon concentration gradient and decreasing quenching stress and microcracks. Furthermore, HfC has a lower coefficient of thermal expansion than TiC, closer to that of the steel matrix, reducing thermal mismatch stress at the quenching interface during quenching, thus reducing gear deformation. In addition, although the addition of Hf increases the cost increment, this is offset by energy saving in the process and the elimination of the straightening process, so the overall cost increase can be controlled at a level comparable to that of Ti microalloyed steel.

[0027] Specifically, this invention achieves the following technical effects through hafnium (Hf) microalloying design: (1) Excellent high-temperature grain stability: Utilizing the high melting point and strong pinning force of HfN / C, the grain size is stably maintained at ASTM grade 8 or above (average diameter ≤22μm) after carburizing at ≥930℃. Compared with traditional Ti microalloyed steel (grain coarsening to grade 6, ≥45μm), it fundamentally inhibits the performance degradation caused by grain growth. (2) Efficient carburizing and gentle carbon gradient: The upper limit of carburizing temperature is increased, the carbon diffusion coefficient is increased by about 40%, the deep carburizing time (such as 1.5mm) is shortened, and a lower carbon potential is allowed, which reduces the carbon concentration on the surface and the carbon concentration gradient distribution is gentle, effectively reducing the stress of quenching structure and the risk of microcracks. (3) Extremely low quenching deformation: HfC is more compatible with the thermal expansion coefficient of the steel matrix, which significantly reduces the stress at the quenching interface; combined with the optimized heat treatment process, the gear tooth deviation is ≤0.05mm and the end face runout is ≤0.03mm, realizing straightening-free production; (4) Balance between overall performance and cost: While ensuring high surface hardness (HRC58-62) and core strength (≥1200MPa), the addition of Hf increases the cost of raw materials, but the increase in overall cost is controllable through energy-saving carburizing and eliminating the straightening process. Attached Figure Description

[0028] To make the objectives, technical solutions, and beneficial effects of this invention clearer, the following figures are provided for illustration: Figure 1 The image shows the core microstructure (500x magnification) of the gear steel obtained in Example 1 of this invention after carburizing, quenching and tempering. The microstructure is uniform and fine tempered martensite with high consistency, providing a matrix with high strength, high toughness and uniform stress distribution for the gear, which is the microscopic basis for achieving low heat treatment. Figure 2 This is a schematic diagram of the tooth deviation (Fβ) detection report of the gear after processing in Embodiment 2 of the present invention. The maximum tooth deviation value is controlled at 4.5μm, which is far better than the target requirement (≤0.05mm), directly proving that the present invention has excellent effect in controlling gear heat treatment deformation. Figure 3 The image shows a comparison of the austenite grain size of traditional Ti microalloyed steel and the gear steel of Example 3 of this invention after the same heat treatment. The grain size of the steel of this invention is significantly smaller (ASTM grade 8), which proves that the Hf element imparts ultra-high high-temperature stability to the grains and effectively inhibits grain coarsening. Figure 4 The image shows the micro Vickers hardness gradient distribution curve from the surface to the core of the gear steel after carburizing and quenching in Example 4 of this invention. The effective hardened layer depth reaches 0.88 mm, and the hardness transition is smooth without steep drops, indicating that the carbon concentration distribution is uniform and the gradient stress is small. This is key evidence for achieving high load-bearing capacity and low deformation risk. Detailed Implementation

[0029] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, so that those skilled in the art can better understand and implement the present invention. However, the embodiments described are not intended to limit the present invention.

[0030] This invention provides a hafnium microalloyed gear steel with low heat treatment deformation and its preparation method. The core of the hafnium microalloyed gear steel lies in its unique chemical composition design, which, by mass percentage, contains: C: 0.17%–0.23%, Si: 0.15%–0.35%, Mn: 0.80%–1.10%, Cr: 1.00%–1.30%, Ti: 0.01%–0.10%, Hf: 0.02%–0.10%, P≤0.020%, S≤0.015%, with the balance being Fe and unavoidable impurities. Furthermore, the mass percentage ratio of Hf to Ti (Hf / Ti) is controlled between 0.4 and 1.5.

[0031] Preferably, the C content can be 0.19%–0.22%, Si 0.18%–0.32%, Mn 0.85%–1.08%, Cr 1.08%–1.28%, Ti 0.03%–0.08%, Hf 0.04%–0.07%, and the Hf / Ti ratio can be 0.5–2.33. Furthermore, through refining control, the oxygen content [O] is ≤15 ppm, and the nitrogen content [N] is 80–120 ppm.

[0032] After heat treatment at a carburizing temperature ≥930℃, the austenite grain size in the hafnium microalloyed gear steel of this embodiment is maintained at ASTM grade 8 or above, the tooth direction deviation Fβ of the finished gear is ≤0.05mm, and the end face runout is ≤0.03mm.

[0033] The method for preparing hafnium microalloyed gear steel with low heat treatment deformation in this embodiment includes the following steps: (1) Alloy element ratio design: According to the chemical composition range described in claim 1, high-purity metal raw materials are selected for formulation. In this embodiment, the high-purity metal materials include: hafnium metal, titanium metal, chromium metal, manganese metal, iron silicon, high-carbon iron, and the rest are pure iron and unavoidable impurities. All metal materials are high-purity raw materials and do not contain industrial waste metals.

[0034] (2) Primary refining: Primary refining is carried out using an electric arc furnace or converter. After melting and clearing, the refining period begins. The refining temperature reaches 1580-1650℃ and the refining time is no less than 40 minutes.

[0035] (3) LF-VD secondary refining: Refining is carried out under vacuum conditions ≤1.0Pa, with argon stirring throughout the process. The charging sequence is as follows: pure iron and high-carbon iron are added with the furnace; metallic hafnium and metallic titanium are added from the high-level silo; metallic chromium, metallic manganese, and iron-silicon are added from the alloy silo. After the materials added with the furnace are melted and cleared, the metal from the high-level silo is added. After complete melting, deoxidation and alloying are carried out, and finally, the metal from the alloy silo is added. The refining time is not less than 45 minutes, the white slag holding time is not less than 25 minutes, and the soft argon blowing time is not less than 15 minutes; the smelting composition is sampled and analyzed at the furnace front, and then the composition is fine-tuned according to the design requirements; after adjusting to the target composition, the casting is carried out at a temperature of 1520~1550℃.

[0036] (4) Continuous casting: Electromagnetic stirring process is adopted, the superheat is controlled at 20-35℃, and the casting speed is 0.6-0.8 m / min to obtain continuous casting billet.

[0037] (5) Heating and rolling: After heating the billet to 1150-1200℃ and holding it for 2-4 hours, it is taken out of the furnace for rolling. The rough rolling temperature is 1050-1100℃, the finishing rolling temperature is controlled at 900-950℃, and the total reduction rate is not less than 70%.

[0038] (6) Controlled cooling: The steel is cooled to 600-650°C at a cooling rate of 30-50°C / s using an ultra-fast cooling process, and then air-cooled to room temperature.

[0039] (7) Low deformation heat treatment: The steel obtained in step (6) or the gear workpiece made therefrom is subjected to low deformation heat treatment, which includes: loading, pre-cleaning, pretreatment, carburizing and quenching, post-cleaning and tempering.

[0040] The loading process is as follows: To minimize the supporting stress during heating and quenching, this embodiment employs a vertical clamping method combining a fractal topological support structure and a spherical cap-shaped microplane array to fix the gear workpiece, which is a gear or gear shaft. The fractal topological support structure includes three sets of ribs evenly distributed circumferentially at 120°. The contact portion between the ribs and the end face of the gear workpiece is the spherical cap-shaped microplane array. Specifically, in the spherical cap-shaped microplane array, the base diameter of a single spherical cap is 1-2 mm, the radius of curvature is 0.5-1.0 mm, and the center-to-center distance between adjacent spherical caps is 5 ± 0.5 mm. In this embodiment, the surface of the spherical cap-shaped microplane array is coated with a composite coating, which includes a 3-5 μm thick CrAlN underlayer and a 1-2 μm thick MoS2 surface layer.

[0041] The pretreatment process is as follows: stepped preheating treatment, under a protective atmosphere (such as nitrogen), heating to 450-550℃ at a rate of ≤100℃ / h, holding at that temperature for 0.1-3 hours, and then cooling in the furnace to 300-350℃ at a rate of ≤80℃ / h followed by air cooling. This process can homogenize machining stress.

[0042] The carburizing and quenching process includes carburizing and quenching. Carburizing is performed at 840-940℃ using a dual-carburizing process of high-temperature intensive carburizing and cooling diffusion. After carburizing, quenching is then performed. Specifically, the carburizing process includes a high-temperature intensive carburizing stage and a cooling diffusion stage: the intensive carburizing stage temperature is 880-940℃, and the carbon potential is controlled at 0.85-1.25%C; the diffusion stage temperature is 840-880℃, and the carbon potential is controlled at 0.75-0.85%C. The diffusion stage lasts for 1 / 3 to 1 / 2 of the time of the intensive carburizing stage.

[0043] Specifically, the quenching process is carried out in a dual-temperature zone quenching tank. After carburizing, the workpiece is transferred to the quenching tank. The quenching employs a two-stage cooling method: first, the workpiece is immersed in quenching oil at 110–130°C for rapid cooling, and then transferred to quenching oil at 70–90°C for further cooling before being removed from the furnace. Specifically, the quenching tank is separated and maintained by a guide tube and a circulation system, maintaining an upper high-temperature zone (110±10°C) and a lower low-temperature zone (80±10°C). The workpiece is transferred between the two temperature zones using a robotic arm.

[0044] Tempering: Heat to 160-190℃ at a rate of ≤100℃ / h, hold for 0.1-5 hours and then air cool to eliminate residual stress and stabilize the microstructure.

[0045] The gear steel prepared by the method described in this embodiment maintains an austenitic grain size of ASTM grade 8 or higher (average grain diameter ≤22μm) after high-temperature carburizing at ≥930℃. The manufactured gears exhibit minimal deformation, with tooth deviation (Fβ) controlled to ≤0.05mm and end face runout ≤0.03mm, achieving high-precision, straightening-free production. The gear surface hardness reaches HRC58-62, with uniform effective hardened layer depth and a core tensile strength ≥1200MPa.

[0046] The specific embodiments of the present invention will be further described below with reference to specific examples, but the present invention is not limited to the following examples.

[0047] Example 1 The hafnium microalloyed gear steel with low heat treatment deformation in this embodiment has the following specific chemical composition (mass percentage): C: 0.22%, Si: 0.18%, Mn: 0.85%, Cr: 1.08%, Ti: 0.08%, Hf: 0.04%, P: 0.012%, S: 0.006%, with the balance being Fe and unavoidable impurities. The Hf / Ti mass ratio is 0.5.

[0048] Its preparation method includes: electric arc furnace smelting, LF+VD refining (final [O]=10ppm, [S]=0.005%), continuous casting (superheat 28℃, casting speed 0.85m / min), rolling (heating temperature 1190℃, final rolling 880℃), controlled cooling (rapid cooling to 610℃ followed by air cooling).

[0049] The blank was used to manufacture gears. After the gears were made, the following heat treatment was performed: pretreatment at 520℃ for 2 hours → furnace cooling to 330℃; carburizing: strong carburizing at 910℃ (1.18%C) for 5 hours → diffusion (0.83%C) for 2 hours; quenching (845℃ for 60 minutes → double-stage oil quenching); tempering at 180℃ for 3.5 hours.

[0050] like Figure 1 The image shows the microstructure of the gear core after treatment in this embodiment. The microstructure is uniform and fine tempered martensite with good consistency, providing excellent matrix conditions for low deformation. The gear tooth deviation is 3 μm. The end face runout is 0.02 mm (≤0.03 mm). The effective hardened layer depth is 0.83 mm, the surface hardness is 750 HV, and the gradient is gentle. This embodiment demonstrates that when Hf / Ti = 0.5, the material possesses fine grains, low deformation, and high hardened layer consistency.

[0051] Example 2 The hafnium microalloyed gear steel with low heat treatment deformation in this embodiment has the following specific chemical composition (mass percentage): C: 0.19%, Si: 0.32%, Mn: 1.08%, Cr: 1.28%, Ti: 0.02%, Hf: 0.02%, P: 0.019%, S: 0.009%, with the balance being Fe and unavoidable impurities. The Hf / Ti mass ratio is 1.0.

[0052] Its preparation method includes: converter smelting, LF+VD refining (final [O]=13ppm, [S]=0.009%), continuous casting (superheat 22℃, casting speed 0.75m / min), rolling (heating temperature 1210℃, final rolling 860℃), controlled cooling (rapid cooling to 640℃ followed by air cooling).

[0053] The blank is used to manufacture gear parts. After the gear parts are made, they are subjected to low deformation heat treatment: pretreatment: 480℃×3h→slow cooling in furnace to 310℃; carburizing: strong carburizing at 925℃ with 1.22%C×3.5h→diffusion with 0.84%C×1.5h; quenching: 848℃×55min→double-stage oil quenching; tempering: 165℃×4h.

[0054] like Figure 2 The diagram shows a schematic of the tooth profile deviation detection report for the gear after processing in this embodiment. The difference in maximum tooth profile deviation before and after heating is only 0.015 mm, demonstrating the excellent deformation control capability of this invention. The grain size is effectively controlled at level 8, with no obvious mixed grains appearing. The effective hardened layer depth is 0.85 mm, and the core hardness is 430 HV. This embodiment shows excellent deformation control when Hf / Ti=1.5, and the ratio is within the scope of the claims, reinforcing the scope limitation significance.

[0055] Example 3 The hafnium microalloyed gear steel with low heat treatment deformation in this embodiment has the following specific chemical composition (mass percentage): C: 0.17%, Si: 0.35%, Mn: 0.80%, Cr: 1.30%, Ti: 0.10%, Hf: 0.04%, P: 0.020%, S: 0.015%, with the balance being Fe and unavoidable impurities. The Hf / Ti mass ratio is 0.4.

[0056] Its preparation method includes: electric arc furnace smelting, LF+VD refining (final [O]=15ppm, [S]=0.012%), continuous casting (superheat 35℃, casting speed 0.70m / min), rolling (heating temperature 1180℃, final rolling 890℃), controlled cooling (rapid cooling to 600℃ followed by air cooling).

[0057] Gears were manufactured using this blank and then subjected to low-deformation heat treatment: Pretreatment: 550℃×2h → slow cooling in the furnace to 350℃; Carburizing: 930℃ strong carburizing 1.25%C×3h → diffusion 0.85%C×1.2h; Quenching: 850℃×50min → double-stage oil quenching; Tempering: 190℃×2h.

[0058] like Figure 3The image shows a grain size comparison. The left-hand comparative example (traditional Ti microalloyed steel, with only Hf replaced by an equal amount of Ti) exhibits severe grain coarsening to ASTM grade 5.5 (approximately 45 μm) after carburizing at 930℃. The right-hand example of the hafnium microalloyed gear steel, under the same stringent process, maintains a grain size of ASTM grade 8 (approximately 22 μm), fully demonstrating the superior ability of Hf microalloying to suppress grain growth at high temperatures. The maximum tooth deviation difference before and after heating is only 0.013 mm. The carbon concentration gradient is gentle, with no microcracks. This example verifies the high-temperature grain stability at Hf / Ti = 0.4.

[0059] Example 4 The hafnium microalloyed gear steel with low heat treatment deformation in this embodiment has the following specific chemical composition (mass percentage): C: 0.23%, Si: 0.15%, Mn: 1.10%, Cr: 1.00%, Ti: 0.067%, Hf: 0.10%, P: 0.010%, S: 0.010%, with the balance being Fe and unavoidable impurities. The Hf / Ti mass ratio is approximately 1.5.

[0060] Its preparation method includes: electric arc furnace smelting, LF+VD refining (final [O]=9ppm, [S]=0.008%), continuous casting (superheat 20℃, casting speed 0.90m / min), rolling (heating temperature 1220℃, final rolling 850℃), controlled cooling (rapid cooling to 650℃ followed by air cooling).

[0061] Gears were manufactured using this blank and then subjected to low-deformation heat treatment: Pretreatment: 450℃×2.5h → slow cooling in the furnace to 300℃; Carburizing: 890℃ strong carburizing 1.15%C×6h → diffusion 0.80%C×2.5h; Quenching: 830℃×70min → double-stage oil quenching; Tempering: 160℃×3h.

[0062] like Figure 4 The figure shows the hardness gradient curve of this embodiment. The curve shows that the effective hardened layer depth reaches 0.8 mm, and the transition from the surface to the high-hardness core is extremely smooth, proving that carbon diffusion is sufficient, the concentration gradient is gentle, and the internal stress is small. The surface hardness of this gear reaches 760 HV. The austenite grain size is tested to meet ASTM 8.5 grade. The maximum tooth deviation difference before and after heat treatment is only 0.013 mm. This embodiment shows that even if the Hf content is used up to 0.10% of the upper limit of the claim and an optimized Hf / Ti ratio (about 1.5) is formed with Ti, the present invention can still obtain excellent hardened layer performance and extremely low heat treatment deformation, fully verifying the effectiveness and rationality of the composition range defined in the claims.

[0063] Comparative Example 1 The comparative steel composition was similar to that of Example 1, but the Ti content was adjusted to 0.12%, the Hf content to 0.03%, and the Hf / Ti ratio to 0.25 (exceeding the lower limit of the claims). It was treated with the same process (carburizing at 930°C).

[0064] Testing revealed that the austenite grain size reached ASTM grade 6.5 (significantly coarsened), with mixed grains appearing. The maximum tooth deviation difference before and after heating was only 0.033 mm. Insufficient Hf content and excessive Ti forming coarse TiN resulted in insufficient pinning force, leading to increased grain boundary migration and deformation.

[0065] Comparative Example 2 The comparative steel composition was similar to that of Example 1, but the Hf content was adjusted to 0.15%, the Ti content to 0.02%, and the Hf / Ti ratio was 7.5 (exceeding the upper limit of the claims). It was processed using the same method.

[0066] Testing revealed that the austenite grain size met ASTM grade 7 (partial coarsening), exhibiting mixed grain structure. The maximum tooth deviation difference before and after heating was only 0.028 mm. Excessive Hf led to localized aggregation of precipitates, exacerbated thermal expansion mismatch, stress concentration, and failure of deformation control.

[0067] As can be seen from the above embodiments and comparative examples, strictly controlling the Hf / Ti ratio within the range of 0.4 to 1.5 (as in Examples 1-4) can ensure the dispersed distribution of precipitates, grain refinement, and minimal deformation. Exceeding the limit (Comparative Examples 1-2) leads to grain coarsening or stress imbalance, which demonstrates the critical importance of the scope of the claims and verifies the high-temperature carburizing stability and low deformation advantages of the present invention.

[0068] The above-described embodiments are merely preferred embodiments provided to fully illustrate the present invention, and the scope of protection of the present invention is not limited thereto. Equivalent substitutions or modifications made by those skilled in the art based on the present invention are all within the scope of protection of the present invention. The scope of protection of the present invention is defined by the claims.

Claims

1. A hafnium microalloyed gear steel with low heat treatment deformation, characterized in that: The chemical composition by mass percentage is as follows: C: 0.17%–0.23%, Si: 0.15%–0.35%, Mn: 0.80%–1.10%, Cr: 1.00%–1.30%, Ti: 0.01%–0.10%, Hf: 0.02%–0.10%, P≤0.020%, S≤0.015%, with the balance being Fe and unavoidable impurities. The mass percentage ratio of Hf to Ti, Hf / Ti, is controlled between 0.4 and 1.

5.

2. The hafnium microalloyed gear steel with low heat treatment deformation according to claim 1, characterized in that: Its chemical composition by mass percentage is as follows: C: 0.19%–0.22%, Si: 0.18%–0.32%, Mn: 0.85%–1.08%, Cr: 1.08%–1.28%, Ti: 0.03%–0.08%, Hf: 0.04%–0.07%.

3. The hafnium microalloyed gear steel with low heat treatment deformation according to claim 1 or 2, characterized in that: The mass percentage ratio of Hf to Ti, Hf / Ti, is further preferably 0.5 to 2.

33.

4. The hafnium microalloyed gear steel with low heat treatment deformation according to claim 1, characterized in that: Its oxygen content [O] is ≤15ppm and its nitrogen content [N] is 80~120ppm.

5. The hafnium microalloyed gear steel with low heat treatment deformation according to claim 1, characterized in that: After heat treatment at a carburizing temperature ≥930℃, the austenite grain size is maintained at ASTM grade 8 or above, the tooth deviation Fβ of the finished gear is ≤0.05mm, and the end face runout is ≤0.03mm.

6. A method for preparing hafnium microalloyed gear steel with low heat treatment deformation as described in any one of claims 1-5, characterized in that: Includes the following steps: (1) Alloy element ratio design: According to the chemical composition range described in claim 1, high-purity metal raw materials are selected for formulation; (2) Primary refining: Primary refining is carried out using an electric arc furnace or converter, and the refining temperature reaches 1580-1650℃; (3) LF-VD secondary refining: Refining is carried out under vacuum conditions ≤1.0Pa and argon gas stirring is used. The refining time is not less than 45 minutes. (4) Continuous casting: Electromagnetic stirring process is adopted, the superheat is controlled at 20-35℃, and the casting speed is 0.6-0.8 m / min to obtain continuous casting billet; (5) Heating and rolling: After heating the billet to 1150~1200℃ and holding it at that temperature, rolling is carried out. The roughing temperature is 1050~1100℃, the finishing temperature is controlled at 900~950℃, and the total reduction rate is not less than 70%. (6) Controlled cooling: The steel is cooled to 600-650°C at a cooling rate of 30-50°C / s using an ultra-fast cooling process, and then air-cooled to room temperature; (7) Low deformation heat treatment: The steel obtained in step (6) or the gear workpiece made therefrom is subjected to low deformation heat treatment, which includes pretreatment, carburizing and quenching and tempering.

7. The method for preparing hafnium microalloyed gear steel with low heat treatment deformation according to claim 6, characterized in that: The low-deformation heat treatment in step (7) specifically includes: Pretreatment: Under a protective atmosphere, heat to 450-550℃ at a rate of ≤100℃ / h and hold for 0.1-3 hours, then cool with the furnace to 300-350℃ at a rate of ≤80℃ / h and then air cool. Carburizing and quenching: includes carburizing treatment and quenching, wherein the carburizing treatment includes a strong carburizing stage at 880-940℃ and a diffusion stage at 840-880℃; the quenching adopts a two-stage oil quenching method, in which the workpiece is first cooled in quenching oil at 110-130℃, and then transferred to quenching oil at 70-90℃ for further cooling. Tempering: Heat to 160-190℃ at a rate of ≤100℃ / h, hold for 0.1-5 hours, and then air cool.

8. The method for preparing hafnium microalloyed gear steel with low heat treatment deformation according to claim 7, characterized in that: In the carburizing and quenching process, the carbon potential in the strong carburizing stage is controlled at 0.85-1.25%, and the carbon potential in the diffusion stage is controlled at 0.75-0.85%, with the diffusion stage lasting for 1 / 3 to 1 / 2 of the strong carburizing stage.

9. The method for preparing hafnium microalloyed gear steel with low heat treatment deformation according to claim 6, characterized in that: In step (7), before carburizing and quenching, there is also a loading step. The loading step uses a vertical clamping method combining a fractal topological support structure and a spherical crown microplane array to fix the gear workpiece. The fractal topological support structure includes three sets of ribs evenly distributed in a 120° circumferential direction. The contact part between the ribs and the end face of the gear workpiece is the spherical crown microplane array.

10. The method for preparing hafnium microalloyed gear steel with low heat treatment deformation according to claim 9, characterized in that: In the spherical microplane array, the base diameter of a single spherical cap is 1-2 mm, the radius of curvature is 0.5-1.0 mm, and the center-to-center distance between adjacent spherical caps is 5 ± 0.5 mm; the surface of the spherical microplane is provided with a composite coating, which includes a CrAlN underlayer with a thickness of 3-5 μm and a MoS2 surface layer with a thickness of 1-2 μm.

Images