Hf microalloyed narrow-hardenability band 20CrMnTi gear steel and preparation method thereof

Abstract

The invention discloses Hf microalloyed narrow-hardenability gear steel with 20CrMnTi and a preparation method of the Hf microalloyed narrow-hardenability gear steel, and belongs to the technical field of advanced steel and iron materials. The gear steel is prepared from the following components in percentage by mass: 0.17 to 0.22 percent of C, 0.20 to 0.35 percent of Si, 0.80 to 1.00 percent of Mn, 1.00 to 1.20 percent of Cr, 0.04 to 0.06 percent of Ti, 0.02 to 0.04 percent of Hf and 0.15 to 0.30 percent of Mo, wherein the ratio of the Hf to the Ti is 0.4 to 0.8. The preparation method comprises the processes of vacuum induction melting, LF-VD duplex refining, electromagnetic stirring continuous casting, hot rolling and controlled rolling and controlled cooling. Nano HfC is preferentially formed through Hf microalloying, TiC coarsening and pinning grain boundaries are effectively inhibited, and a phase change path is reconstructed, so that the hardenability bandwidth (J9-J15) is stably controlled to be smaller than or equal to 4 HRC, the problems of uneven tooth surface hardness, heat treatment distortion, high tooth breakage risk and the like caused by large hardenability fluctuation of traditional gear steel are solved, and the service life of the gear steel is prolonged. The method is particularly suitable for manufacturing high-precision gears of new energy automobiles and the like.

Description

Technical Field

[0001] This invention belongs to the field of advanced steel materials technology, specifically a 20CrMnTi gear steel with narrow hardenability band and Hf microalloying and its preparation method. By controlling the distribution of precipitated phases and grain boundary migration behavior through Hf, the hardenability bandwidth (J9 / J15) is stably controlled within the range of ≤5HRC, solving the problem of excessive hardenability bandwidth (≥8HRC) of traditional 20CrMnTiH due to TiC coarsening and compositional fluctuations, and meeting the stringent requirements of high-precision gears for hardness uniformity. Background Technology

[0002] Gears are core components of mechanical transmission systems, and their performance directly affects the reliability, efficiency, and lifespan of the equipment. 20CrMnTiH gear steel, a classic Cr-Mn-Ti carburizing gear steel, has long been widely used in automotive transmissions and other fields due to its excellent hardenability, strength, and processing properties. Its core mechanism lies in the microalloying effect of titanium (Ti), which forms fine Ti(C,N) precipitates that pin austenite grain boundaries, inhibiting grain growth and thus fundamentally controlling hardenability.

[0003] However, with the rapid development of the new energy vehicle industry, drive motors are evolving towards higher speeds (>20,000 rpm) and higher torques (>400 Nm), placing unprecedentedly stringent demands on the load-bearing capacity, fatigue life, and transmission accuracy of gears. Traditional 20CrMnTiH gear steel exhibits increasingly prominent inherent defects when dealing with such extreme conditions, with the core issue being an excessively large hardenability bandwidth (the hardness difference between points J9 and J15 in the Jominy curve). Actual production shows that the hardenability bandwidth of traditional materials is typically as high as 8-10 HRC. An excessively wide hardenability band directly leads to uneven hardness distribution on the gear tooth surface and core (fluctuations can exceed 3 HRC), which in turn triggers a series of engineering failure risks: First, uneven hardness causes stress concentration, which can easily induce early fatigue cracks under high torque alternating loads, increasing the risk of tooth breakage; Second, hardenability fluctuations affect the uniformity of carbon diffusion during carburizing, causing the designed carburized layer depth to fluctuate beyond the standard, affecting gear meshing accuracy and increasing operating noise; Third, a wide hardenability band is also one of the main reasons for excessive distortion (such as tooth distortion) in gears after carburizing and quenching. To meet accuracy requirements, subsequent finishing processes such as gear grinding are often required, significantly increasing manufacturing costs.

[0004] The root causes of excessive hardenability bandwidth lie primarily in two aspects: First, the difficulty in controlling TiC precipitates. Fluctuations in Ti content (typically between 0.04-0.10%) easily lead to a discrete size distribution of TiC precipitates, forming a large number of coarse TiC particles (up to 1-20 μm). These coarse TiC particles act as a microscopic "thermal barrier" during quenching, severely disrupting the local cooling rate and becoming the core source of uneven quenching. Second, macroscopic segregation of alloying elements. During continuous casting solidification, elements that improve hardenability, such as Mn and Cr, tend to accumulate between dendrites, leading to differences in austenite stability in micro-regions and further exacerbating the dispersion of hardness distribution.

[0005] To compress the hardenability bandwidth, the industry has attempted various technical improvement paths, but all have significant limitations: while electromagnetic stirring or light reduction processes can alleviate macroscopic segregation to some extent, their effect on suppressing the coarsening of precipitates such as TiC is limited, and they increase production costs, ultimately making it difficult to stably reduce the hardenability bandwidth below 7 HRC; using rare earth elements for inclusion modification treatment can refine inclusions, but it easily leads to the production problem of nodule formation in continuous casting nozzles, resulting in poor practical application effects and stability; using ultra-low oxygen smelting ([O]≤10 ppm) to improve the purity of molten steel cannot solve the problem of uncontrolled precipitate formation caused by Ti solution fluctuations, and the hardenability bandwidth still fluctuates significantly. None of these solutions have fundamentally solved the hardenability fluctuation problem dominated by TiC coarsening.

[0006] The published patent technologies also reflect the technical challenges in this field. For example, Chinese patent CN116254470B proposes a gear steel with Nb-Ti composite microalloying, attempting to stabilize austenite grains through the synergistic precipitation of Ti and Nb carbonitrides. However, data from actual embodiments show that the hardenability bandwidth (J9-J15) is still close to or exceeds 5 HRC, failing to achieve narrower bandwidth control. The fundamental reason is that it still relies on traditional Ti microalloying, failing to avoid the problems of TiC size dispersion and coarsening; at the same time, the added NbC coarsens during high-temperature carburizing above 930℃, resulting in a decrease in pinning force and grain control instability at high temperatures. Another patent application, CN117344209A, focuses on achieving high-temperature (1000℃) carburizing through niobium microalloying. In order to pursue high-temperature grain refinement and low heat treatment deformation, it deliberately reduces the hardenability of the steel (e.g., J9≤32 HRC). This approach, which sacrifices core strength and martensite transformation, may lead to risks such as soft spots on the tooth surface and reduced load-bearing capacity. Furthermore, its process is complex, costly, and lacks sufficient empirical evidence on the uniformity of precipitated phases. Summary of the Invention

[0007] In summary, the manufacturing of high-end gears for new energy vehicles urgently requires a new type of gear steel material capable of stably controlling the hardenability band within a narrower range (e.g., ≤4 HRC). This necessitates new technical solutions that can effectively suppress the coarsening of precipitates such as TiC, ensuring microstructure uniformity, while simultaneously maintaining good strength, toughness, machinability, and cost control. Recent theoretical studies have shown that hafnium (Hf), as a strong carbide-forming element, produces carbides (HfC) with extremely high thermal stability, maintaining submicron or even nanometer-scale sizes at high temperatures, and providing grain boundary pinning forces far exceeding those of TiC. This offers a new potential approach to solving the TiC coarsening problem. However, excessive Hf forms coarse HfC, affecting hardenability and thus worsening the hardenability band. How to combine Hf microalloying technology with the existing 20CrMnTi gear steel composition system and industrial production process, clarify the synergistic ratio of Hf and Ti, and achieve the effective precipitation and distribution of nano-sized HfC through reasonable preparation process, so as to achieve a leapfrog compression of hardenability bandwidth while ensuring cost advantage, is still a technical gap that needs to be solved.

[0008] In view of this, the purpose of the present invention is to provide a 20CrMnTi gear steel with narrow hardenability band by Hf micro-alloying and its preparation method. By pinning grain boundaries with HfC and compressing the phase transformation range of the C curve, the hardenability band is ≤4 HRC, thus achieving the goal of eliminating grinding of tooth distortion and high reliability service.

[0009] To achieve the above objectives, the present invention provides the following technical solution: This invention first proposes a narrow hardenability band 20CrMnTi gear steel with Hf microalloying. Its chemical composition, by mass percentage, is: C: 0.17%–0.22%, Si: 0.20%–0.35%, Mn: 0.80%–1.00%, Cr: 1.00%–1.20%, Ti: 0.04%–0.06%, Hf: 0.02%–0.04%, Mo: 0.15%–0.30%, with the balance being Fe and unavoidable impurities. Furthermore, the mass percentage ratio of Hf to Ti, Hf / Ti, is 0.4–0.8.

[0010] Furthermore, among the unavoidable impurities, P ≤ 0.020% and S ≤ 0.020%.

[0011] Furthermore, the hardenability bandwidth of the gear steel, i.e. the hardness difference between positions J9 and J15 in the end quenching test, is not greater than 4 HRC.

[0012] Furthermore, after carburizing heat treatment, the austenitic grain size of the gear steel is not lower than ASTM grade 8.

[0013] Furthermore, the gear steel contains nanoscale HfC precipitates.

[0014] This invention also proposes a method for preparing 20CrMnTi gear steel with narrow hardenability bands and Hf microalloying as described above, comprising the following steps: (1) Alloy element ratio: Weigh the raw materials according to the composition range described in claim 1 and mix them; (2) Vacuum induction melting: The melting is carried out in a vacuum induction furnace under the condition that the vacuum degree is not higher than 1.0 Pa, the melting temperature is 1580~1650℃, the melting time is not less than 40 min, and the tapping temperature is 1520~1550℃. (3) LF-VD double refining: LF furnace refining is carried out for a refining time of not less than 45 min and white slag holding time of not less than 25 min; then it is transferred to VD furnace and held under vacuum of not more than 67 Pa for not less than 15 min; finally, soft blowing argon treatment is carried out for a time of not less than 15 min; the oxygen content of the final molten steel is not more than 15 ppm and the hydrogen content is not more than 1.5 ppm. (4) Continuous casting: Electromagnetic stirring is used to control the superheat of molten steel to 20-35℃ and the casting speed to 0.6-0.8 m / min, so as to obtain a billet with an equiaxed crystal ratio of not less than 40%; (5) Hot rolling: The billet is heated to 1150-1200℃ and held for 2-4 hours, and then rolled. The rough rolling temperature is 1050-1100℃, the finish rolling temperature is 900-950℃, and the total reduction rate is not less than 70%. (6) Controlled cooling: The rolled material is rapidly cooled to 600-650°C at an ultra-fast cooling rate of 30-50°C / s, and then air-cooled to room temperature to obtain steel.

[0015] Furthermore, in step (3), the LF furnace refining process adopts the white slag process and includes the step of feeding calcium wire to perform inclusion modification treatment.

[0016] Furthermore, in step (4), the continuous casting is a fully protected casting process, and the continuous casting process adopts end electromagnetic stirring with a stirring current intensity of 300-500 A and a frequency of 3-5 Hz.

[0017] Furthermore, in step (5), the single-pass reduction rate in the finishing rolling stage is not less than 20%.

[0018] Furthermore, in step (6), after the rolled material is rapidly cooled to 600-650°C, it is held at this temperature for 10-30 minutes for relaxation treatment, and then air-cooled to room temperature.

[0019] The principle of this invention is as follows: The core innovative value of Hf lies in reconstructing the thermodynamic / kinetic process of martensitic phase transformation: Under optimized addition, Hf promotes core martensite transformation through a triple mechanism: First, Hf's nanophase pins the austenite grain boundaries, increasing the grain size of 960℃ carburizing to ASTM grade 8-9 (compared to grade 5-6 in traditional processes), increasing the total grain boundary area, providing dense sites for martensite nucleation, and improving the martensite core transformation. Second, Hf's high affinity for C causes nano-HfC to precipitate preferentially, significantly consuming free N atoms in the steel, resulting in subsequent TiC growth forming small-sized TiC due to C shortage. The small TiC, because its size is below the thermal diffusion characteristic length, eliminates the problem of uneven local cooling rates caused by coarse TiC acting as a thermal barrier—its thermal conductivity mismatch with the matrix no longer forms a macroscopic low-temperature region, allowing Jominy sample 15... The cooling rate fluctuation at the mm position is reduced; the improved cooling rate uniformity leads to an increase in the martensite transformation amount in this region, resulting in a narrower final hardness fluctuation range; thirdly, the HfC semi-coherent interface significantly weakens the proeutectoid ferrite nucleation driving force by adsorbing and consuming Mn / Cr elements and enriching carbon, forcing austenite to skip the ferrite precipitation temperature range under slow cooling conditions and directly enter the intermediate temperature transformation region; at this time, the strong strain field of HfC (ε≥0.015) hinders bainite shearing nucleation and prolongs its transformation incubation period; at the same time, the local enrichment of carbon causes the pearlite nucleation temperature to shift upward, while the lattice softening effect caused by solute redistribution in austenite raises the martensite initiation temperature. The synergistic effect of these three phase transformation path reconstructions ultimately ensures that the core achieves complete martensite transformation at a low cooling rate (<30℃ / s), reducing the hardenability bandwidth. The breakthrough of this invention lies in the hafnium-titanium synergistic mechanism: that is, HfC inhibits TiC coarsening, provides excellent grain boundary pinning force, and maintains the fine grain structure of high-temperature austenite.

[0020] The key breakthrough of this invention lies in the synergistic design of partially replacing titanium with hafnium, overcoming the bottleneck of hardenability technology while maintaining cost advantages. Although reducing the titanium content weakens the traditional grain boundary pinning effect, innovative component ratios and process coupling achieve precise suppression of TiC coarsening by nanoscale HfC particles and reconstruct the phase transformation path—simultaneously blocking non-martensitic phase transformation, delaying intermediate-temperature transformation, and optimizing martensite formation conditions. Combined with elemental segregation control technology in the continuous casting process, the hardenability bandwidth is stably compressed to an industry-leading level, providing an industrial solution for new energy gears that combines grinding-free quenching characteristics with long-term service reliability.

[0021] Compositional design basis: Hf is the core microalloying element of this invention. Hf has an extremely high affinity for C, forming nanoscale HfC precipitates. These nanophases effectively pin austenite grain boundaries, inhibit high-temperature grain growth, and provide nucleation sites for martensitic transformation, thereby significantly refining the microstructure. Simultaneously, the formation of HfC consumes C atoms in the steel, limiting the formation of coarse TiC and eliminating the main factor contributing to hardenability fluctuations at the source. However, excessive Hf content will form coarse HfC carbides, impairing toughness and increasing costs. Considering all factors, the mass percentage of Hf should be controlled between 0.02% and 0.4%.

[0022] Carbon (C) is a fundamental element for ensuring hardenability and strength. Its content directly affects the hardenability and final strength of steel, and is crucial for obtaining high-hardness martensite. Higher C content helps improve tooth surface hardness and contact fatigue strength. However, excessively high C content significantly reduces toughness, increases the risk of brittle fracture, and promotes the formation and aggregation of harmful carbides. Considering both strength and toughness, the C mass percentage should be precisely controlled between 0.17% and 0.22%.

[0023] Manganese (Mn) is an important solid solution strengthening and hardenability-enhancing element. Mn stabilizes austenite, significantly improving the hardenability of steel and ensuring that even the core of large-section gears can achieve a martensitic structure. It can also improve the matrix strength through solid solution strengthening. However, Mn is a highly segregating element; excessive content can exacerbate the formation of banded structures, leading to microstructure inhomogeneity and anisotropy, and deteriorating fatigue performance. To balance hardenability and homogeneity, the mass percentage of Mn should be controlled between 0.80% and 1.00%.

[0024] Si is an effective deoxidizer and solid solution strengthening element. Si improves hardenability and significantly enhances the strength of steel through solid solution strengthening. It also inhibits the formation of certain brittle carbides during carburizing, which is beneficial to toughness. However, excessive Si content can severely impair the plasticity and toughness of steel and promote the formation of a decarburized layer. To balance strength and plasticity, the mass percentage of Si should be strictly controlled between 0.20% and 0.35%.

[0025] Ti is a traditional microalloying element that needs to be controlled synergistically with Hf. Ti forms fine TiN / C precipitates, which pin grain boundaries and control austenite grain size. In this invention, it works synergistically with Hf to optimize the precipitate system. However, excessive Ti content can lead to the formation of coarse, liquid-precipitated TiN, which becomes a fatigue crack initiation point. Furthermore, excessive Ti can fix C and N in the steel, reducing hardenability. To achieve the best synergistic effect with Hf, the mass percentage of Ti needs to be precisely controlled between 0.04% and 0.06%.

[0026] The beneficial effects of this invention are as follows: This invention achieves revolutionary control over the hardenability bandwidth of 20CrMnTi gear steel by introducing trace amounts of hafnium (Hf) and forming a synergistic ratio with titanium (Ti) (Hf / Ti=0.4-0.8). The core technical effect of this invention is that it stably compresses the hardenability bandwidth (J9-J15) of traditional materials from ≥8 HRC to ≤4 HRC.

[0027] The technical effects of this invention stem from the multiple synergistic mechanisms of Hf microalloying: First, the preferentially formed nanoscale HfC effectively pins austenite grain boundaries, significantly refining the carburized grain size to ASTM 8-9 grade, laying the foundation for uniform martensite nucleation; Second, HfC consumes carbon atoms, strongly suppressing the formation of coarse TiC, fundamentally eliminating the main source of fluctuations leading to uneven cooling; Third, the HfC interface characteristics optimize local solute distribution, reconstruct phase transformation paths, suppress the formation of proeutectoid ferrite and bainite, and promote complete martensitic transformation in the core even at low cooling rates.

[0028] The narrow and stable hardenability band directly translates into superior engineering performance: the gear tooth surface hardness is highly uniform after carburizing and quenching, significantly reducing stress concentration and the risk of early tooth breakage; heat treatment distortion is significantly reduced, potentially eliminating the need for subsequent gear grinding processes and lowering costs; simultaneously, the material maintains a good balance of strength and toughness while achieving high hardness. Based on existing mainstream production processes (LF-VD-continuous casting-controlled rolling and controlled cooling), this invention achieves a leapfrog improvement in performance, providing a highly reliable and low-cost gear material solution for demanding applications such as new energy vehicles. Attached Figure Description

[0029] To make the objectives, technical solutions, and beneficial effects of this invention clearer, the following figures are provided for illustration: Figure 1 (a) is a TEM image of the steel obtained in Example 1; (b) is a TEM image of the steel obtained in Comparative Example 1. By comparison, it is found that the nanoscale HfC precipitate formed by Hf microalloying is the structural basis for achieving a narrow hardenability band. Figure 2 The metallographic structure at J15 of the end-quenched sample in Example 1 shows a full lath martensite structure, proving that the addition of Hf optimizes the phase transformation path and suppresses non-martensite transformation even under slow cooling conditions. Figure 3 The stress-strain curve of the steel in Example 2 shows that the material has excellent strength-plasticity matching, indicating that Hf microalloying maintains good toughness while improving hardness. Figure 4 The microstructure at J15 of the end-quenched sample in Comparative Example 1 shows a distinct mixed structure of proeutectoid ferrite and bainite, which corresponds to the wide hardenability band of traditional Ti microalloyed steel. Detailed Implementation

[0030] 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.

[0031] The core of this invention lies in providing a 20CrMnTi gear steel with narrow hardenability bands and Hf microalloying, and its preparation method. The typical chemical composition of the gear steel, by mass percentage, is: C: 0.17–0.22%, Si: 0.20–0.35%, Mn: 0.80–1.00%, Cr: 1.00–1.20%, Ti: 0.04–0.06%, Hf: 0.02–0.04%, Mo: 0.15–0.30%, P≤0.020%, S≤0.020%, with the balance being Fe and unavoidable impurities. The mass percentage ratio of Hf to Ti (Hf / Ti) is controlled between 0.4 and 0.8, which is crucial for achieving preferential precipitation of nanoscale HfC and effectively suppressing TiC coarsening, thereby obtaining a narrow hardenability band.

[0032] Specifically, the hardenability bandwidth of the gear steel, i.e., the hardness difference between positions J9 and J15 in the end-quench test, is no greater than 4 HRC. After carburizing heat treatment, the austenite grain size of the gear steel is no less than ASTM grade 8. The gear steel contains nanoscale HfC precipitates.

[0033] The preparation method of Hf microalloyed narrow hardenability band 20CrMnTi gear steel in this embodiment includes the following steps.

[0034] (1) Alloy element ratio: Weigh the raw materials according to the composition range mentioned above and mix them.

[0035] (2) Vacuum induction melting: The melting is carried out in a vacuum induction furnace under the condition that the vacuum degree is not higher than 1.0 Pa, the melting temperature is 1580~1650℃, the melting time is not less than 40 min, and the tapping temperature is 1520~1550℃.

[0036] (3) LF-VD Dual Refining: LF furnace refining is performed for no less than 45 min, with white slag holding time no less than 25 min; then transferred to VD furnace, and held under vacuum no higher than 67 Pa for no less than 15 min; finally, soft argon blowing is performed for no less than 15 min; the final molten steel oxygen content is no higher than 15 ppm, and hydrogen content is no higher than 1.5 ppm. Specifically, the LF furnace refining adopts the white slag process and includes the step of feeding calcium wire for inclusion modification treatment.

[0037] (4) Continuous casting: Electromagnetic stirring is used to control the superheat of molten steel to 20-35℃, the casting speed to 0.6-0.8 m / min, and end electromagnetic stirring is applied with a current of 300-500 A and a frequency of 3-5 Hz to obtain a billet with an equiaxed crystal ratio of not less than 40%.

[0038] (5) Hot rolling: The billet is heated to 1150-1200℃ and held for 2-4 hours, and then rolled. The rough rolling temperature is 1050-1100℃, the finishing rolling temperature is 900-950℃, the total reduction rate is not less than 70%, and the single-pass reduction rate in the finishing rolling stage is not less than 20%.

[0039] (6) Controlled cooling: The rolled material is rapidly cooled to 600-650°C at an ultra-fast cooling rate of 30-50°C / s, held at this temperature for 10-30 min for relaxation treatment, and then air-cooled to room temperature to obtain steel.

[0040] The following is an explanation through specific examples.

[0041] Example 1 This embodiment aims to illustrate the preparation and properties of gear steel under the preferred composition and process parameters of the present invention.

[0042] 1. Gear steel The Hf-microalloyed narrow-hardenability 20CrMnTi gear steel of this embodiment has the following chemical composition by mass percentage: C: 0.20%, Si: 0.28%, Mn: 0.90%, Cr: 1.10%, Ti: 0.06%, Hf: 0.03%, Mo: 0.22%, P: 0.015%, S: 0.012%, with the balance being Fe. The calculated Hf / Ti ratio is 0.5, which is within the optimal range of 0.4-0.8 recommended by this invention.

[0043] In this embodiment, all raw materials are high-purity metals, including chromium (99.8%), manganese (99.7%), molybdenum (99.9%), hafnium (99.5%), iron-titanium alloy (Ti:70%), iron-silicon alloy (Si:75%), and pure iron (99.9%). No industrial waste metals are used, thus controlling the introduction of impurity elements from the source.

[0044] 2. Preparation method The preparation method of Hf microalloyed narrow hardenability band 20CrMnTi gear steel in this embodiment includes the following steps.

[0045] (1) Alloy element ratio: The alloy is formulated according to the mass percentages mentioned above in this embodiment, specifically: C: 0.20%, Si: 0.28%, Mn: 0.90%, Cr: 1.10%, Ti: 0.06%, Hf: 0.03%, Mo: 0.22%, P: 0.015%, S: 0.012%, with the balance being Fe.

[0046] (2) Vacuum induction melting: Pure iron, metallic manganese, and metallic molybdenum are added to a 10kg vacuum induction furnace along with the furnace charge. The furnace is evacuated to 0.8Pa before melting begins. After the furnace charge is completely melted, metallic chromium and iron-titanium alloy are added from the high-level hopper. After complete melting, a pre-deoxidation operation is performed. Finally, metallic hafnium and iron-silicon alloy are added from the alloy hopper. The refining temperature during the melting period is controlled at 1600℃, and the refining time is 50min. Samples are taken in front of the furnace for spectral analysis. After fine-tuning the composition based on the results, the steel is tapped at 1535℃.

[0047] (3) LF-VD Dual Refining: Molten steel was transferred to the LF furnace and refined using a white slag process for 50 minutes, with a white slag holding time of 30 minutes. During this period, calcium wire was added via a wire feeder to modify inclusions. Then, it was transferred to the VD furnace, evacuated to 65 Pa and held at that pressure for 18 minutes for deep degassing and inclusion removal. Finally, soft argon blowing was performed for 18 minutes to promote the flotation of fine inclusions. Testing showed that the final oxygen content was 12 ppm and the hydrogen content was 1.3 ppm, meeting the high purity requirements.

[0048] (4) Continuous casting: The electromagnetic stirring continuous casting process with full protection was adopted. The superheat of the molten steel was controlled at 28℃, the casting speed was 0.7m / min, and the electromagnetic stirring parameters at the end were set to a current intensity of 400A and a frequency of 4Hz. The equiaxed crystal ratio of the obtained continuous casting billet reached 45%, which significantly reduced the compositional segregation.

[0049] (4) Hot rolling: The continuously cast billet is heated to 1180℃ and held for 3 hours to homogenize the temperature. Then rough rolling and finish rolling are carried out. The rough rolling temperature is 1080℃ and the finish rolling temperature is 920℃. The total reduction rate reaches 75%. The single-pass reduction rate in the finish rolling stage is controlled between 20-25%, which effectively promotes the refinement of recrystallized grains.

[0050] (5) Controlled cooling: The steel is rapidly cooled to 620°C at a cooling rate of 40°C / s using an ultra-fast cooling process. It is then held at this temperature for 20 minutes for relaxation treatment, and finally air-cooled to room temperature to obtain the final steel product.

[0051] 3. Performance and tissue characterization: 3.1 Microstructure The steel prepared in this embodiment was observed by scanning electron microscopy for deep corrosion, such as... Figure 1 As shown, the results indicate that the precipitated phase is small in size and uniformly distributed. Figure 1 The display shows nanoscale HfC precipitates formed by Hf microalloying, which contrasts sharply with the coarse TiC particles in the comparative example, intuitively demonstrating the core mechanism of this invention that preferentially forms nanoscale HfC to limit TiC coarsening.

[0052] 3.2 Hardenability The steel prepared in this embodiment was subjected to an end-quenching test (Jominy Test), and the results are shown in Table 1: Table 1. End-quench hardness test results (HRC) As shown in Table 1, the calculated hardenability bandwidth (hardness difference between J9 and J15) of the invention is as follows: the hardness of J9 is 40 HRC, the hardness of J15 is 38.5 HRC, and the hardenability bandwidth (J9-J15) is only 1.5 HRC, which meets the requirement of ≤4 HRC and is far lower than the 7.5 HRC of the comparative example.

[0053] 3.3 Metallographic Structure The metallographic structure at J15 of the end-quenched sample is as follows Figure 2 As shown, its microstructure is almost entirely lath martensite, containing only a very small amount of retained austenite. This indicates that even at a relatively slow cooling rate, the steel of this invention can still suppress the formation of proeutectoid ferrite and bainite, achieving a highly uniform martensitic transformation, which is direct microstructural evidence for its extremely narrow hardenability bandwidth.

[0054] Example 2 The Hf-microalloyed narrow-hardenability 20CrMnTi gear steel of this embodiment has the following chemical composition by mass percentage: C: 0.22%, Si: 0.35%, Mn: 0.80%, Cr: 1.20%, Ti: 0.04%, Hf: 0.02%, Mo: 0.30%, P: 0.020%, S: 0.008%, Fe balance. The Hf / Ti ratio is 0.5, which is within the optimal range of 0.4-0.8 recommended by this invention.

[0055] In this embodiment, the process parameters for preparing Hf microalloyed narrow hardenability band 20CrMnTi gear steel are similar to those in Example 1, but have been optimized and adjusted for compositional characteristics: the finishing rolling temperature is set to 930℃, the ultra-fast cooling rate is 35℃ / s, and the relaxation time is 25min.

[0056] The obtained steel has a uniform microstructure, and the distribution of precipitates is similar to that of Example 1 but slightly sparser. End-quench tests showed a bandwidth of 2.0 HRC.

[0057] Mechanical property tests show that the material has an excellent strength-ductility balance, and its typical stress-strain curve is as follows: Figure 3 As shown.

[0058] Example 3 The Hf-microalloyed narrow-hardenability 20CrMnTi gear steel of this embodiment has the following chemical composition by mass percentage: C: 0.19%, Si: 0.28%, Mn: 0.90%, Cr: 1.10%, Ti: 0.05%, Hf: 0.02%, Mo: 0.22%, P: 0.015%, S: 0.020%, with the balance being Fe. The calculated Hf / Ti ratio is 0.4, which is at the lower limit of the recommended range of 0.4-0.8 in this invention, and is used to verify the performance at the lower limit of the ratio.

[0059] The preparation method of gear steel in this embodiment includes the following steps, and the process parameters are optimized and adjusted based on Example 1 to match the composition characteristics: Alloy element ratio: Weigh out the high-purity raw materials according to the above composition, including metallic chromium (99.8%), metallic manganese (99.7%), metallic molybdenum (99.9%), metallic hafnium (99.5%), iron-titanium alloy (Ti:70%), iron-silicon alloy (Si:75%) and pure iron (99.9%).

[0060] Vacuum induction melting: Melting for 45 minutes at a vacuum of 0.9 Pa and a melting temperature of 1590℃, with a tapping temperature of 1530℃.

[0061] LF-VD duplex refining: LF refining time 48 min, white residue held for 28 min, calcium feed line for denaturation treatment; VD vacuum degree 65 Pa, pressure held for 16 min; soft argon blowing time 16 min. Final oxygen content 13 ppm, hydrogen content 1.4 ppm.

[0062] Continuous casting: molten steel superheat 25℃, casting speed 0.65m / min, end electromagnetic stirring current 350A, frequency 3.5Hz, equiaxed crystal ratio 42%.

[0063] Hot rolling: The billet is heated to 1170℃ and held for 3.5 hours. The rough rolling temperature is 1070℃, the finishing rolling temperature is 910℃, the total reduction rate is 72%, and the single-pass reduction rate of finishing rolling is 22%.

[0064] Controlled cooling: Ultra-fast cooling to 630℃ at 35℃ / s, followed by relaxation treatment for 15 minutes and then air cooling.

[0065] Metallographic analysis revealed that the microstructure at J15 was entirely lath martensite, containing a small amount of bainite (<5%), which is consistent with... Figure 2 The comparison showed that the martensitic transformation was more complete, but the uniformity of the cooling rate was slightly inferior to that of Example 1. TEM observation revealed a similar precipitate distribution to Example 1, but the HfC size was slightly larger (due to the lower Hf content), still predominantly nanoscale. Reference Figure 1(a) HfC precipitate characteristics. In this example, HfC effectively suppressed TiC coarsening, but the pinning density was lower than that in Example 1. The end-quench test results are shown in Table 2. The hardness of J9 was 39.5 HRC, the hardness of J15 was 37.0 HRC, and the hardenability bandwidth (J9-J15) was 2.5 HRC, which met the requirement of ≤4 HRC. This proves that narrow bandwidth control can still be achieved when Hf / Ti=0.4, but the bandwidth is slightly higher than that in Example 1 (1.5 HRC), indicating that the reduction of Hf content has a slight impact on uniformity.

[0066] This embodiment shows that the core mechanism of the invention is still effective when Hf / Ti=0.4, but optimizing the Hf content to the upper limit can further improve the performance.

[0067] Example 4 The chemical composition of the gear steel in this embodiment is: C: 0.17%, Si: 0.20%, Mn: 1.00%, Cr: 1.00%, Ti: 0.05%, Hf: 0.04%, Mo: 0.25%, P: 0.016%, S: 0.010%, with the balance being Fe. The Hf / Ti ratio is 0.8, which is at the upper limit of the recommended range and is used to verify the effect of high Hf content.

[0068] The preparation process is basically the same as in Example 1, except that the parameters are adjusted for high Hf content: Vacuum induction melting: melting temperature 1620℃, time 42 minutes, tapping temperature 1540℃ to ensure Hf is fully dissolved.

[0069] LF-VD refining: LF refining time 50 minutes, white residue maintained for 30 minutes; VD holding pressure for 17 minutes; final oxygen content 11 ppm, hydrogen content 1.2 ppm.

[0070] Continuous casting: superheat 30℃, casting speed 0.75m / min, electromagnetic stirring current 450A, frequency 4.5Hz, equiaxed crystal ratio 48%.

[0071] Hot rolling: Finishing temperature 940℃, total reduction rate 78%, single pass reduction rate 24% to promote recrystallization.

[0072] Controlled cooling: Cooling rate 45℃ / s to 640℃, relaxation treatment for 25 minutes.

[0073] Metallographic observation revealed that J15 was entirely martensite with no proeutectoid ferrite. (Reference) Figure 2 This demonstrates that the high Hf content further optimizes the phase transition kinetics. TEM shows that the nano-HfC precipitated phase has a high density and more uniform size (compared to...). Figure 1(a) Similar to, but with a slight tendency for HfC aggregation due to higher Hf content; TiC showed significant inhibition, with no coarse particles. End-quench tests showed J9 hardness at 40.5 HRC and J15 hardness at 38.0 HRC, with a bandwidth of 2.5 HRC (same as Example 3), but the core hardness was higher, indicating that the high Hf content strengthened the phase transformation path. Stress-strain curves and Figure 3 Similarly, the tensile strength reaches 1250MPa and the elongation is 12%, showing an excellent balance between strength and plasticity.

[0074] This embodiment verifies that bandwidth control is stable and core strength is improved when Hf / Ti=0.8, but it should be noted that excessive Hf content may increase costs.

[0075] Comparative Example 1 The traditional 20CrMnTi composition is used: C: 0.20%, Si: 0.28%, Mn: 0.90%, Cr: 1.10%, Ti: 0.08%, Mo: 0.22%, P: 0.015%, S: 0.012%, Fe balance. No Hf is added.

[0076] The preparation process is the same as in Example 1.

[0077] The results showed that there were a large number of coarse TiC precipitates of 1-5 μm in the microstructure, and the metallographic structure at J15 of the end-quenched sample was as follows. Figure 4 As shown, it has a mixed ferrite and bainite microstructure with a hardenability bandwidth of 7.5 HRC.

[0078] Summary of the results of this embodiment: Through the synergistic effect of Hf microalloying (0.02%-0.04%) and Ti, combined with optimized preparation process, nanoscale HfC precipitates are formed in gear steel, effectively suppressing TiC coarsening, refining grains, and optimizing the phase transformation path, thereby stabilizing the hardenability bandwidth within the range of ≤4 HRC. This embodiment shows that after adding Hf, the hardenability bandwidth decreased from 7.5 HRC in the comparative example to 1.5-2.0 HRC, achieving precise control of the narrow hardenability band, and providing a material basis for the manufacturing of high-end gears for new energy vehicles.

[0079] Comparative Example 2 The chemical composition of this comparative gear steel is: C: 0.20%, Si: 0.28%, Mn: 0.90%, Cr: 1.10%, Ti: 0.06%, Hf: 0.018%, Mo: 0.22%, P: 0.015%, S: 0.012%, with the balance being Fe. The Hf / Ti ratio is 0.3, lower than the lower limit of 0.4 required by the invention, and is used to compare the effect of ineffective proportions.

[0080] The process parameters were exactly the same as in Example 1 to isolate component variables.

[0081] Microstructure: TEM observation showed that the HfC precipitate was sparse and uneven in size (some >100 nm), while the TiC was significantly coarsened (size 1-10 μm), compared with Comparative Example 1. Figure 1 (b) Similar to coarse TiC, this demonstrates that insufficient Hf content cannot effectively suppress TiC coarsening. The end-quench test showed a hardness of 38.0 HRC for J9 and 32.0 HRC for J15, with a bandwidth of 6.0 HRC, far exceeding the 4 HRC limit. The large hardness fluctuation at J15 highlights the problem of uneven cooling rate. The microstructure at J15 is a mixture of ferrite (approximately 20%) and bainite, similar to... Figure 4 The microstructure is similar to that of Comparative Example 1, indicating that the proeutectoid phase transition was not suppressed. When Hf / Ti < 0.4, the pinning force of HfC is insufficient, TiC coarsening dominates the cooling fluctuations, the phase transition path reverts to the traditional mode, and bandwidth control fails.

[0082] This comparative example demonstrates the necessity of Hf / Ti ≥ 0.4; below this value, the inventive advantage is lost.

[0083] Comparative Example 3 The chemical composition of this comparative gear steel is as follows: C: 0.18%, Si: 0.30%, Mn: 0.85%, Cr: 1.05%, Ti: 0.04%, Hf: 0.04%, Mo: 0.20%, P: 0.017%, S: 0.011%, with the balance being Fe. The Hf / Ti ratio is 1, which is higher than the upper limit of 0.8 required by the invention, and is used to verify the negative impact of excessive Hf.

[0084] The process is the same as in Example 1, but the melting temperature is increased to 1640°C to handle high Hf content.

[0085] TEM showed excessive HfC precipitation, with some particles coarsened (up to 50-100 nm), and stress concentration at the interface with the matrix. (Reference) Figure 1 (a) The ideal nanophase, in this comparative example, HfC aggregation weakens pinning uniformity. The end-quench test showed J9 hardness of 41.0 HRC and J15 hardness of 36.0 HRC, with a bandwidth of 5.0 HRC. Although lower than the 7.5 HRC of Comparative Example 1, these are still >4 HRC, and the core hardness decreases, indicating that the phase transformation path is partially optimized but not optimal. At J15, martensite and bainite are mixed (bainite accounts for 15%), similar to... Figure 4 However, the effect was relatively mild, indicating that while excessive Hf inhibited ferrite formation, the bainite transformation delay was insufficient. Reaching the upper limit of Hf content increased raw material costs without a significant performance improvement, resulting in low cost-effectiveness. This comparative example shows that when Hf / Ti > 0.8, the bandwidth control effect diminishes and may introduce toughness risks, emphasizing the importance of the upper limit of the ratio.

[0086] 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 narrow hardenability band 20CrMnTi gear steel with Hf microalloying, characterized in that: The chemical composition by mass percentage is as follows: C: 0.17%–0.22%, Si: 0.20%–0.35%, Mn: 0.80%–1.00%, Cr: 1.00%–1.20%, Ti: 0.04%–0.06%, Hf: 0.02%–0.04%, Mo: 0.15%–0.30%, with the balance being Fe and unavoidable impurities; and the mass percentage ratio of Hf to Ti, Hf / Ti, is 0.4–0.

8.

2. The Hf microalloyed narrow hardenability band 20CrMnTi gear steel according to claim 1, characterized in that: Of the unavoidable impurities, P ≤ 0.020% and S ≤ 0.020%.

3. The Hf microalloyed narrow hardenability band 20CrMnTi gear steel according to claim 1 or 2, characterized in that: The hardenability bandwidth of the gear steel, i.e. the hardness difference between positions J9 and J15 in the end quenching test, is not greater than 4 HRC.

4. The Hf microalloyed narrow hardenability band 20CrMnTi gear steel according to claim 1 or 2, characterized in that: After carburizing heat treatment, the austenitic grain size of the gear steel is not lower than ASTM grade 8.

5. The Hf microalloyed narrow hardenability band 20CrMnTi gear steel according to claim 1 or 2, characterized in that: The gear steel contains nanoscale HfC precipitates.

6. A method for preparing Hf microalloyed narrow hardenability band 20CrMnTi gear steel as described in any one of claims 1-5, characterized in that: Includes the following steps: (1) Alloy element ratio: Weigh the raw materials according to the composition range described in claim 1 and mix them; (2) Vacuum induction melting: The melting is carried out in a vacuum induction furnace under the condition that the vacuum degree is not higher than 1.0 Pa, the melting temperature is 1580~1650℃, the melting time is not less than 40 min, and the tapping temperature is 1520~1550℃. (3) LF-VD dual refining: LF furnace refining is carried out for a refining time of not less than 45 min and white slag holding time of not less than 25 min; then it is transferred to VD furnace and held under vacuum of not more than 67 Pa for not less than 15 min; finally, soft blowing argon treatment is carried out for a time of not less than 15 min; the oxygen content of the final molten steel is not more than 15 ppm and the hydrogen content is not more than 1.5 ppm. (4) Continuous casting: Electromagnetic stirring is used to control the superheat of molten steel to 20-35℃ and the casting speed to 0.6-0.8 m / min, so as to obtain a billet with an equiaxed crystal ratio of not less than 40%; (5) Hot rolling: The billet is heated to 1150-1200℃ and held for 2-4 hours, and then rolled. The rough rolling temperature is 1050-1100℃, the finish rolling temperature is 900-950℃, and the total reduction rate is not less than 70%. (6) Controlled cooling: The rolled material is rapidly cooled to 600-650°C at an ultra-fast cooling rate of 30-50°C / s, and then air-cooled to room temperature to obtain steel.

7. The method for preparing Hf microalloyed narrow hardenability band 20CrMnTi gear steel according to claim 6, characterized in that: In step (3), the LF furnace refining process adopts the white slag process and includes the step of feeding calcium wire to perform inclusion modification treatment.

8. The method for preparing Hf microalloyed narrow hardenability band 20CrMnTi gear steel according to claim 6, characterized in that: In step (4), the continuous casting is a fully protected casting process, and the continuous casting process adopts end electromagnetic stirring with a stirring current intensity of 300-500 A and a frequency of 3-5 Hz.

9. The method for preparing Hf microalloyed narrow hardenability band 20CrMnTi gear steel according to claim 6, characterized in that: In step (5), the single-pass reduction rate in the finishing rolling stage shall not be less than 20%.

10. The method for preparing Hf microalloyed narrow hardenability band 20CrMnTi gear steel according to claim 6, characterized in that: In step (6), after the rolled material is rapidly cooled to 600-650°C, it is held at this temperature for 10-30 minutes for relaxation treatment, and then air-cooled to room temperature.

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