Gear steel for controlling banded structure through hafnium microalloying and preparation method thereof

Abstract

The invention discloses gear steel using hafnium microalloying to control a banded structure and a preparation method thereof, the gear steel comprises the following chemical components in percentage by mass: 0.12%-0.21% of C, 0.15%-0.35% of Si, 0.80%-1.20% of Mn, 1.00%-1.30% of Cr, 0.03%-0.08% of Ti, 0.08%-0.12% of Hf, less than or equal to 0.015% of P and less than or equal to 0.005% of S, and the ratio of Hf to Ti is 1.5-3.0. The preparation method comprises the steps of vacuum induction melting, LF-VD duplex refining, electromagnetic stirring continuous casting and controlled rolling and controlled cooling. According to the method, the HfO2 / HfS nano precipitated phase is formed in the molten steel, MnS nucleation sites are preempted from a solidification source, dendritic segregation of the Mn element is effectively inhibited, and therefore the banded structure level of the final steel is stably controlled to be 1.5 level or below. The obtained gear steel is uniform in structure height, has the advantages of small heat treatment deformation, uniform carburized layer and long fatigue life, replaces high-cost rare earth with relatively cheap hafnium element, is stable and controllable in process, and is particularly suitable for manufacturing high-precision and high-reliability gears of new energy automobiles and the like.

Description

Technical Field

[0001] This invention relates to the field of advanced steel materials technology, specifically to a gear steel with banded structure controlled by hafnium microalloying and its preparation method. By controlling the segregation of Mn element through hafnium (Hf) microalloying, the banded structure is significantly suppressed or even eliminated. Background Technology

[0002] Gears are the core components of mechanical transmission systems, and their performance directly determines transmission efficiency, reliability, and service life. Gear steel, as a key material for gear manufacturing, has a unique microstructure that is one of the decisive factors affecting the final dimensional accuracy, mechanical properties, and service stability of gears.

[0003] Cr-Mn-Ti carburized gear steels, represented by 20CrMnTi, have long dominated the automotive transmission and construction machinery industries due to their excellent hardenability, strength-toughness balance, and heat treatment process stability. However, with the rapid development of the new energy vehicle industry, the requirements for high-speed (e.g., above 20,000 rpm), high instantaneous torque (e.g., above 400 Nm), long service life (million-kilometer level), and high noise and vibration (NVH) performance of drive motors have posed unprecedented challenges to gear steels. The banded structure (composed of alternating ferrite and pearlite bands) defect commonly found in traditional gear steels exposes a series of serious problems under these extreme conditions, becoming a key technical bottleneck restricting its application in high-end new energy vehicle transmission systems.

[0004] Banded microstructure is a microstructure formed in steel during solidification and hot working due to insufficient homogenization of dendritic segregation of alloying elements (especially Mn). This results in alternating bands of ferrite and pearlite parallel to the rolling direction during subsequent cooling. This microstructure inhomogeneity causes multiple problems: First, it leads to severe deformation and loss of precision during heat treatment. There is a significant difference in the coefficients of thermal expansion between ferrite and pearlite (e.g., ferrite bands have a coefficient of thermal expansion of approximately 14.5 × 10⁻⁶). -6 / K, the pearlite band is approximately 12.8 × 10 -6 / K). During carburizing and quenching, this difference generates uneven phase transformation stress, leading to unpredictable torsional deformation of gear parts, such as tooth deviation and excessive end face runout. For 20CrMnTi gears with a banded microstructure reaching level 3, the deformation after quenching often fails to meet the stringent requirements of millisecond-level meshing accuracy and low noise for new energy gears.

[0005] Secondly, it significantly degrades fatigue performance and service life. The abrupt change in mechanical properties at the interface between the soft phase (ferrite) and the hard phase (pearlite) in banded microstructure easily becomes a stress concentration point and a microcrack initiation site. Studies show that up to 80% of contact fatigue cracks tend to propagate along ferrite bands. Compared to homogeneous microstructure, the contact fatigue life of grade 3 banded gear steel can decrease by up to two orders of magnitude. Furthermore, ferrite bands have a higher hydrogen diffusion coefficient, which, under complex conditions such as wet slippage and high pressure, exacerbates hydrogen accumulation in the soft bands, significantly reducing the material's hydrogen embrittlement critical stress intensity factor and inducing early brittle fracture risk, posing a threat to the safety of high-speed transmission systems.

[0006] Furthermore, it causes uneven carburized layer depth and makes process control difficult. The diffusion rate of carbon atoms in ferrite is much higher than that in pearlite. This difference in diffusion rate leads to inconsistent penetration depth of carbon in different regions of the banded structure during carburizing, causing the designed carburized layer depth (e.g., 1.5 mm) to fluctuate too much in the actual product, increasing the difficulty of heat treatment quality control and the scrap rate of parts.

[0007] To compensate for the defects caused by the banded structure, existing technologies often have to rely on cumbersome remedial processes such as hot straightening, fine grinding after carburizing, and graded quenching. This not only significantly increases manufacturing costs and time, but also makes it difficult to fundamentally guarantee batch stability and product reliability.

[0008] To address the issue of banded microstructure control, the industry has explored various approaches. For example, Chinese patent application CN117512301A proposes rare-earth titanium microalloying, but this primarily targets ordinary automotive structural steel. While the low-carbon, low-manganese design mitigates segregation to some extent, it doesn't fundamentally solve the problem of Mn dendritic segregation and relies on expensive and volatile rare-earth elements (Ce), making the process complex. Another example is Chinese patent CN120485637A, which proposes Nb / Ti / RE composite microalloying. However, this still involves high-cost rare-earth elements (Y), has a narrow process window, presents significant challenges to stability in large-scale production, and fails to completely eliminate the potential damage to fatigue performance caused by coarse TiN inclusions, while also not achieving thorough control over banded microstructure.

[0009] In summary, the upgrading of the new energy vehicle industry has placed higher demands on the microstructure uniformity of gear steel materials. Developing a novel gear steel and its stable preparation method that can effectively suppress elemental segregation from the solidification source, thoroughly improve the banded microstructure, and simultaneously possess excellent hardenability, low deformation characteristics during heat treatment, and good economic efficiency has become a key technical problem urgently needing breakthroughs in this field. Summary of the Invention

[0010] In view of this, the purpose of this invention is to provide a gear steel with controlled banded structure using hafnium microalloying and its preparation method. By precisely controlling the Hf content and Hf / Ti ratio, Mn element segregation is suppressed from the solidification source, and the banded structure is stably controlled below level 1.5, thereby significantly improving the uniformity of the steel structure and reducing heat treatment deformation to meet the requirements of high precision and high reliability for gears in new energy vehicles.

[0011] To achieve the above objectives, the present invention provides the following technical solution: This invention first proposes a gear steel with a banded structure controlled by hafnium microalloying. Its chemical composition, by mass percentage, includes: C: 0.12%~0.21%, Si: 0.15%~0.35%, Mn: 0.80%~1.20%, Cr: 1.00%~1.30%, Ti: 0.03%~0.08%, Hf: 0.08%~0.12%, P≤0.015%, S≤0.005%, with the balance being Fe and unavoidable impurities; and the mass percentage ratio of Hf to Ti, Hf / Ti, is 1.5~3.0.

[0012] Furthermore, the banded structure grade of the gear steel is no higher than grade 1.5.

[0013] This invention also proposes a method for preparing gear steel using hafnium microalloying to control the banded structure as described above, comprising the following steps: (1) Batching: Alloy elements are proportioned according to the stated mass percentage and Hf / Ti ratio; (2) Vacuum induction melting: Melting is carried out under a vacuum of less than 1.0 Pa, with a melting temperature of 1580~1650℃ and a time of not less than 50 minutes; (3) Refining and casting: Refining is carried out using the LF-VD dual process, and the soft blowing argon time is controlled; the casting temperature is 1520~1550℃; (4) Continuous casting: Electromagnetic stirring continuous casting process is adopted, the superheat is controlled at 15~30℃, and the casting speed is 0.7~0.9 m / min; (5) Hot rolling: The continuous casting billet is heated at 1180~1220℃ and held at that temperature, followed by rough rolling and finish rolling. The total reduction rate is ≥75%, and the final rolling temperature of finish rolling is 880~930℃. (6) Controlled cooling: The ultra-fast cooling process is adopted to cool to 550-600℃ at a cooling rate of 40-60℃ / s, and then air-cooled to room temperature.

[0014] Furthermore, in step (2), the specific feeding sequence of the vacuum induction melting is as follows: first, add pure iron and metallic nickel and melt them, then add metallic chromium and metallic titanium, and after complete melting, perform deoxidation and alloying, and finally add metallic hafnium.

[0015] Furthermore, in step (3), the white slag holding time in the LF-VD refining process is ≥30 min and the soft blowing argon time is ≥20 min.

[0016] Furthermore, in step (3), the refining temperature is not lower than 1580℃.

[0017] Furthermore, in step (4), the current intensity of the electromagnetic stirring is 200~400 A and the frequency is 2~5 Hz.

[0018] Furthermore, in step (5), the continuously cast billet is heated at 1180~1220℃ and held for 2~3 hours before rough rolling and finish rolling.

[0019] Furthermore, in step (5), the initial rolling temperature of the rough rolling is 1080~1130℃.

[0020] The technical principle of this invention is as follows: The banded structure is essentially a two-phase stratification caused by the dendritic segregation of Mn / C elements. During the solidification of traditional gear steel (such as 20CrMnTi), elements like Mn and Si accumulate between dendrites, forming a periodic distribution of Mn-depleted regions (ferrite bands) and Mn-rich regions (pearlite bands). This invention achieves segregation blocking through the control of hafnium (Hf) nanophase precipitation. HfO2 / HfS nanopinning: The affinity of Hf for oxygen / sulfur (the enthalpy of mixing of HfO2 is -1080 kJ / mol) is much higher than that of Mn (MnO is -362 kJ / mol), forming nano-sized HfO2 / HfS in the early stages of steel solidification. These nanoprecipitates preempt MnS nucleation sites, reducing the amount of MnS precipitation and cutting off the nucleation of the banded structure at its source. Simultaneously, these Hf nanophases act as α-Fe solidification nuclei, refining the dendrite arm spacing, promoting the formation of equiaxed crystals, significantly reducing the diffusion distance of solute elements, and suppressing segregation. Combined with electromagnetic stirring, this reduces the segregation tendency of Mn, disrupting the compositional basis for banded structures. Furthermore, this invention also achieves microstructure homogenization through high-temperature pinning of the Hf nanophases and phase deformation nucleus regulation, suppressing the amplification of banded defects caused by microstructure inheritance effects during hot rolling. During the hot-rolling austenitization stage, the Hf nanophases maintain nanoscale dispersion due to their ultra-high melting point. Their Zener pinning force is more than twice that of traditional TiN / C, limiting the original austenite grain size to ≤30μm (ASTM grade 9). During cooling, the refined austenite grain boundaries increase the density of γ→α phase deformation nuclei, causing discontinuous ferrite precipitation at the grain boundaries, blocking the formation of traditional 50μm wide continuous ferrite bands. Meanwhile, the Hf nanophase pins at the phase transition interface, inhibiting the growth of proeutectoid ferrite, and ultimately obtaining a uniform microstructure with a smaller bandwidth.

[0021] The key breakthrough of this invention lies in replacing high-cost rare earth elements (such as La / Ce) with relatively inexpensive hafnium (Hf), significantly reducing alloy costs while improving process stability. Although the low-rare-earth design weakens the modification capability of traditional MnS inclusions, innovative compositional control (0.08-0.12% Hf) and process synergy enable targeted suppression of Mn segregation by nano-HfO2 / HfS, significantly reducing the banded structure and refining its morphology; the high-temperature pinning of the Hf nanophase refines the original austenite grains, blocking the continuous growth of ferrite bands. This provides a material basis for new energy gears that combines high uniformity, low deformation risk, and high mass production stability.

[0022] The composition design of this invention focuses on hafnium (Hf) microalloying. By precisely controlling the content of each element, Mn segregation is suppressed from the solidification source, significantly reducing the banded structure and refining its morphology, while ensuring that the material possesses excellent heat treatment stability, hardenability, and corrosion resistance. Specifically, the role and control range of each element are as follows.

[0023] Hf is the key alloying element for achieving banded microstructure control in this invention. Hf has an extremely strong affinity for oxygen and sulfur, preferentially forming nanoscale HfO2 / HfS precipitates over Mn. These nanophases effectively occupy MnS nucleation sites, significantly reducing the number and size of MnS inclusions, thereby cutting off the nucleation pathways for Mn segregation and banded microstructure at the solidification source. Simultaneously, the Hf nanophases can act as heterogeneous nucleation nuclei for α-Fe, refining dendrite arm spacing, promoting equiaxed crystal formation, shortening solute diffusion distance, and further suppressing segregation. During hot working, the ultra-high melting point Hf nanophases provide strong Zener pinning force, inhibiting austenite grain growth and blocking the continuous formation of proeutectoid ferrite bands. However, if the Hf content is too low, the above effects are insufficient; if it is too high, coarse inclusions may form, which can impair toughness. Considering all factors, the mass percentage of Hf is strictly controlled between 0.08% and 0.12%.

[0024] Carbon (C) is a key element for ensuring the core strength and hardenability of gear steel. It ensures that the material possesses sufficient basic mechanical properties and is compatible with existing heat treatment processes. However, C exacerbates Mn segregation and widens the performance difference between pearlite and ferrite in the final microstructure, and is a significant factor in the formation and solidification of banded structures. Taking all factors into consideration, the C mass percentage is strictly controlled between 0.12% and 0.21%.

[0025] Mn is the core segregating element that needs to be controlled in this invention. Mn is a strong austenite stabilizing element, and its dendritic segregation is the direct cause of the formation of ferrite / pearlite banded structures. This invention does not completely eliminate Mn, but rather suppresses its segregation behavior by adding Hf. Simultaneously, the presence of Mn is crucial for ensuring the hardenability of the steel. Therefore, its content needs to be controlled at a low level while ensuring basic hardenability to reduce the driving force of segregation. Taking all factors into consideration, the mass percentage of Mn should be controlled between 0.8% and 1.2%.

[0026] Si primarily exists as a deoxidizer and also possesses some solid solution strengthening properties. However, Si is a ferrite-forming element; excessive content will promote the precipitation of proeutectoid ferrite, thereby exacerbating the banded structure and severely impairing the material's plasticity and toughness. Therefore, its content needs to be controlled at a low level, only meeting basic deoxidation requirements. Considering all factors, the mass percentage of Si should be controlled between 0.15% and 0.35%.

[0027] Ti, as a strong carbonitride forming element, complements Hf in this invention. Ti can form stable TiN, effectively pinning grain boundaries at high temperatures and inhibiting austenite grain growth, providing a finer initial microstructure for Hf to exert its nano-pinning effect. However, excessive Ti content can lead to the formation of coarse Ti(C,N) inclusions, which become crack initiation sites and negatively impact fatigue and impact properties. Considering all factors, the mass percentage of Ti should be controlled between 0.03% and 0.08%.

[0028] P and S are extremely harmful impurity elements and must be controlled with extreme rigor. S is a prerequisite for the formation of MnS inclusions, and its content directly determines the amount of MnS, thus contributing to the banded structure. P readily segregates at grain boundaries, leading to grain boundary embrittlement and significantly reducing the material's impact toughness and resistance to hydrogen embrittlement, which contradicts the high reliability goal of this invention. Therefore, the content of both must be reduced to a minimum. Taking all factors into consideration, the mass percentage of P should be controlled below 0.015%, and the mass percentage of S should be controlled below 0.005%.

[0029] The beneficial effects of this invention are as follows: This invention utilizes hafnium microalloying to control the banded structure of gear steel. Through hafnium (Hf) microalloying and related processes, the performance of gear steel is comprehensively improved. The core technological effects are: fundamentally suppressing the banded structure by utilizing the HfO2 / HfS nanophase to preempt MnS nucleation sites, blocking Mn dendrite segregation, and stably controlling the banded structure to ≤1.5 level (compared to 3 level in traditional processes), significantly improving the uniformity of the microstructure. This results in three core advantages: (1) The heat treatment deformation is minimal. Because the thermal expansion difference of the ferrite / pearlite band is eliminated, the tooth profile and tooth direction accuracy are high after carburizing and quenching, which meets the requirements of new energy gear production without straightening. (2) Fatigue life and reliability are greatly improved. The uniform structure avoids stress concentration at the interface of soft and hard materials and hydrogen embrittlement sensitive path. The contact fatigue life is expected to be improved by two orders of magnitude. (3) Excellent carburization uniformity, consistent carbon diffusion rate, small fluctuation in carburization layer depth, and stable product quality.

[0030] In addition, replacing expensive rare earth elements with Hf reduces raw material costs while ensuring performance, and the entire process (vacuum melting, electromagnetic stirring continuous casting, controlled rolling and controlled cooling) is highly controllable, with excellent mass production stability and economy. Attached Figure Description

[0031] 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 metallographic structure of the gear steel obtained in Example 1; as shown. Figure 1 As shown, the microstructure of this steel is uniform and fine, with no obvious banded structure characteristics. Ferrite (light-colored phase) and pearlite (dark-colored area) are diffusely and uniformly distributed. According to the standard evaluation, its banded structure level is 1.0. Figure 2 The image shows a comparison of the metallographic structure of the steel in Example 2 (left) and the conventional 20CrMnTi steel in the comparative example (right). The steel on the left is the steel of the present invention, which has a uniform structure. The steel in the comparative example on the right has obvious parallel distribution of white ferrite bands and dark pearlite bands, exhibiting a typical banded structure morphology, and is rated as 3.0. Figure 3 This is a diagram showing the tooth profile accuracy inspection of the gear steel after carburizing and quenching treatment in Example 3; as shown. Figure 3 As shown, gears made from the steel of this invention maintain good tooth profile, high precision, and small deformation after heat treatment, meeting the requirements of high precision and low deformation for gears in new energy vehicles. Detailed Implementation

[0032] 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.

[0033] This invention provides a gear steel with controlled banded microstructure using hafnium microalloying and its preparation method. The gear steel, through specific compositional design, particularly controlling the hafnium (Hf) content and its ratio to titanium (Ti), combined with optimized smelting, continuous casting, and rolling processes, suppresses manganese (Mn) dendritic segregation from the solidification source, thereby obtaining a highly uniform microstructure with a banded microstructure level not exceeding 1.5.

[0034] Specifically, the gear steel of this invention utilizes hafnium microalloying to control the banded structure. Its chemical composition, by mass percentage, includes: C: 0.12%~0.21%, Si: 0.15%~0.35%, Mn: 0.80%~1.20%, Cr: 1.00%~1.30%, Ti: 0.03%~0.08%, Hf: 0.08%~0.12%, P≤0.015%, S≤0.005%, with the balance being Fe and unavoidable impurities. Furthermore, the mass percentage ratio of Hf to Ti (Hf / Ti) is 1.5~3.0. The banded structure grade of the gear steel is no higher than 1.5.

[0035] The present invention relates to gear steel and its preparation method, comprising the following steps.

[0036] (1) Batching: Alloy elements are proportioned according to the mass percentage and Hf / Ti ratio.

[0037] (2) Vacuum induction melting: Melting is carried out under a vacuum of less than 1.0 Pa at a temperature of 1580~1650℃ for a time of not less than 50 minutes, followed by casting at 1520~1550℃. Specifically, the specific order of adding materials in vacuum induction melting is as follows: first add pure iron and metallic nickel and melt them thoroughly, then add metallic chromium and metallic titanium, and after complete melting, deoxidize and alloy them, and finally add metallic hafnium.

[0038] (3) Refining and casting: The LF-VD dual process is used for refining, and the soft blowing argon time is controlled; the casting temperature is 1520~1550℃. Specifically, in the LF-VD refining process, the white slag holding time is ≥30 min, the soft blowing argon time is ≥20 min, and the refining temperature is not lower than 1580℃.

[0039] (4) Continuous casting: Electromagnetic stirring continuous casting process is adopted, with superheat controlled at 15~30℃ and casting speed at 0.7~0.9 m / min. Specifically, the current intensity of electromagnetic stirring is 200~400 A and the frequency is 2~5 Hz.

[0040] (5) Hot rolling: After heating the continuous casting billet at 1180~1220℃ and holding it for 2~3h, rough rolling and finish rolling are carried out. The total reduction rate is ≥75%. The starting rolling temperature of rough rolling is 1080~1130℃, and the finishing rolling temperature is 880~930℃.

[0041] (6) Controlled cooling: The ultra-fast cooling process is adopted to cool to 550-600℃ at a cooling rate of 40-60℃ / s, and then air-cooled to room temperature.

[0042] The specific embodiments of the present invention will be further described below through specific examples.

[0043] Example 1 The gear steel is formulated with the following chemical composition by mass percentage: C: 0.18%, Si: 0.25%, Mn: 1.05%, Cr: 1.15%, Ti: 0.05%, Hf: 0.10%, P: 0.012%, S: 0.003%, with the balance being Fe and unavoidable impurities. The Hf / Ti mass ratio is 2.0.

[0044] The preparation method of gear steel includes the following steps: (1) Ingredients: Alloy elements are proportioned according to the mass percentage and Hf / Ti ratio mentioned above in this embodiment.

[0045] (2) Smelting: Smelting was carried out in a vacuum induction furnace. First, pure iron and metallic nickel were added, and the vacuum was drawn to 0.8 Pa before heating and smelting began. After the furnace charge was completely melted, metallic chromium and metallic titanium were added through the high-level hopper. After all the furnace charge was completely melted and reached 1600°C, deoxidation and alloying were carried out. Finally, metallic hafnium was added from the alloy hopper. The smelting temperature was maintained at 1600°C, and the smelting time was 60 minutes.

[0046] (3) Refining and casting: The molten steel is transferred to the LF-VD duplex refining unit for refining at a temperature of 1600℃ for 60 minutes. The white slag is maintained for 35 minutes, followed by soft argon blowing for 25 minutes. After refining, casting is prepared at a casting temperature of 1540℃.

[0047] (4) Continuous casting: The electromagnetic stirring continuous casting process is adopted for casting. The superheat of the molten steel is controlled at 20℃ and the billet pulling speed is 0.8 m / min. The electromagnetic stirrer is turned on, the current intensity is set to 300A and the frequency is 3Hz to obtain a continuous casting billet with uniform composition.

[0048] (5) Hot rolling: The continuously cast billet is fed into a heating furnace and held at 1200℃ for 2.5 hours. Then it is rolled, with the rough rolling starting temperature at 1100℃ and the finishing rolling finishing temperature at 900℃, and the total reduction rate is 78%.

[0049] (6) Controlled cooling: The rolled steel immediately enters the ultra-fast cooling system and is rapidly cooled to 580°C at a cooling rate of 50°C / s. Then it is removed from the cooling device and cooled to room temperature in the air to obtain gear steel.

[0050] Metallographic examination was performed on the gear steel obtained in this embodiment, and its microstructure is as follows: Figure 1 As shown, there is no obvious continuous banded structure in the microstructure, and ferrite and pearlite are evenly distributed. According to GB / T 13299 standard, its banded microstructure grade is 1.0. Specifically, as... Figure 1 As shown, the microstructure of this gear steel is uniform and fine, without obvious banded structure characteristics. Ferrite (light-colored phase) and pearlite (dark-colored area) are dispersed and uniformly distributed. According to the standard evaluation, its banded structure level is 1.0.

[0051] Example 2 The gear steel is formulated with the following chemical composition by mass percentage: C: 0.15%, Si: 0.20%, Mn: 0.90%, Cr: 1.25%, Ti: 0.05%, Hf: 0.08%, P: 0.010%, S: 0.004%, with the balance being Fe and unavoidable impurities. The Hf / Ti mass ratio is 1.6.

[0052] The preparation method of the gear steel is the same as in Example 1, except that the continuous casting superheat is 25℃, the hot rolling finishing temperature is 920℃, and the ultra-rapid cooling rate is 45℃ / s. The resulting steel has a banded microstructure grade of 1.5, with fine and uniform grains. A metallographic comparison was made between the steel from this example and the conventional 20CrMnTi steel of the comparative example, and the results are as follows... Figure 2 As shown.

[0053] Specifically, Figure 2 The images show a comparison of the metallographic structures of the gear steel obtained in this embodiment (left) and the conventional 20CrMnTi steel (right) as a comparative example. The gear steel on the left has a uniform microstructure; the steel on the right, as a comparative example, exhibits clearly parallel white ferrite bands and dark pearlite bands, displaying a typical banded microstructure morphology, and is rated as grade 3.0.

[0054] Example 3 The gear steel is formulated with the following chemical composition by mass percentage: C: 0.20%, Si: 0.30%, Mn: 1.15%, Cr: 1.08%, Ti: 0.06%, Hf: 0.11%, P: 0.014%, S: 0.002%, with the balance being Fe and unavoidable impurities. The Hf / Ti mass ratio is 1.83.

[0055] The preparation method and steps of gear steel are the same as in Example 1, wherein the continuous casting electromagnetic stirring current is 350A, the hot-rolled billet heating temperature is 1210℃, and the ultra-fast cooling termination temperature is 600℃.

[0056] The resulting steel exhibited a banded microstructure grade of 1.5. To further verify its performance in practical applications, the steel from this embodiment was machined into gear samples and subjected to 930°C carburizing and quenching treatment, followed by testing of its tooth profile accuracy. Figure 3 As shown, the steel of this invention has high tooth profile accuracy and small deformation after heat treatment.

[0057] Specifically, Figure 3 This is a diagram showing the tooth profile accuracy inspection of the gear steel obtained in this embodiment after carburizing and quenching treatment. Figure 3 As shown, the gears manufactured using the gear steel of this embodiment maintain good tooth profile, high precision, and small deformation after heat treatment, meeting the requirements of high precision and low deformation for gears in new energy vehicles.

[0058] Example 4 To verify the effectiveness of this invention at the lower limits of composition and Hf / Ti ratio, this embodiment prepared a gear steel with the following chemical composition (by mass percentage): C: 0.12%, Si: 0.15%, Mn: 0.80%, Cr: 1.00%, Ti: 0.08%, Hf: 0.12%, P: 0.015%, S: 0.005%, with the balance being Fe and unavoidable impurities. The Hf / Ti mass ratio is 1.5, precisely within the lower limit of the range required by this invention.

[0059] The preparation method is the same as in Example 1, and the key process parameters are strictly controlled within the range described in this invention: vacuum induction melting temperature 1580℃, refining temperature 1580℃, continuous casting superheat 15℃, finishing rolling temperature 880℃, and ultra-fast cooling rate 40℃ / s.

[0060] Metallographic examination of the steel obtained in this embodiment showed that its banded structure level was 1.5. This indicates that even when the contents of each major alloying element and the Hf / Ti ratio are at the lower limits defined in the claims, the present invention can still effectively suppress Mn segregation and control the banded structure to an excellent level below 1.5, proving that the technical solution of the present invention has a broad and effective process window.

[0061] Example 5 To verify the effectiveness of this invention at the upper limits of the composition and the Hf / Ti ratio, this embodiment prepared a gear steel with the following chemical composition (by mass percentage): C: 0.21%, Si: 0.35%, Mn: 1.20%, Cr: 1.30%, Ti: 0.03%, Hf: 0.09%, P: 0.010%, S: 0.003%, with the balance being Fe and unavoidable impurities. The Hf / Ti mass ratio is 3.0, precisely within the upper limit of the range required by this invention.

[0062] The preparation method is the same as in Example 1. The key process parameters are controlled as follows: vacuum induction melting temperature 1650℃, refining temperature 1650℃, continuous casting superheat 30℃, finishing rolling temperature 930℃, and ultra-fast cooling.

[0063] Metallographic analysis results show that the banded structure of the steel in this embodiment is grade 1.5. This fully demonstrates that, under the upper limit composition conditions of the claims of this invention, the mechanism of suppressing segregation through Hf nanoprecipitates remains highly efficient, ensuring a uniform microstructure and further consolidating the rationality and feasibility of the claims of this invention.

[0064] Comparative Example 1 In contrast, a conventional 20CrMnTi gear steel without Hf was smelted, with the following composition: C: 0.20%, Si: 0.25%, Mn: 1.10%, Cr: 1.20%, Ti: 0.06%, P: 0.015%, S: 0.010%, and the balance being Fe. Its preparation process was exactly the same as in Example 1.

[0065] Specifically, the microstructure of this traditional 20CrMnTi gear steel is shown in [the image]. Figure 2 As shown on the right, distinctly parallel white ferrite bands and dark pearlite bands are visible, with a banded structure rating of 3.0, indicating significant microstructural inhomogeneity. After carburizing and quenching using the same method, the gear tooth profile deforms significantly, increasing the difficulty of precision control.

[0066] Comparative Example 2 To illustrate the adverse effects of an Hf / Ti ratio below the range of this invention (1.5~3.0), this comparative example modifies the composition of Example 1: keeping the Hf content unchanged at 0.10%, the Ti content is significantly increased to 0.20%, resulting in an Hf / Ti ratio of approximately 0.5, far below the lower limit required by this invention. The remaining components are essentially the same as in Example 1. The preparation process is also completely identical to that of Example 1.

[0067] Metallographic analysis revealed a distinct banded structure in the comparative steel, achieving a rating of 2.5. This is because excessive Ti content easily forms coarse Ti(C,N) inclusions, which can not only become fatigue crack initiation sites but also consume C and N elements in the steel, affecting the effectiveness of Hf in forming nano-precipitates. Simultaneously, the relatively insufficient Hf (relative to Ti) prevents it from fully occupying MnS nucleation sites and effectively refining the solidification structure, resulting in a significant weakening of the synergistic inhibition of Mn segregation and ultimately making it difficult to control the formation of banded structures.

[0068] Comparative Example 3 To illustrate the problems arising when the Hf / Ti ratio exceeds the range of this invention, this comparative example modifies the composition of Example 1: the Ti content is reduced to 0.02%, while the Hf content is maintained at 0.12%, resulting in an Hf / Ti ratio of 6.0, which is far higher than the upper limit required by this invention. The remaining components are similar to those of Example 1. The preparation process is exactly the same as in Example 1.

[0069] The evaluation revealed that the banding control effect of the comparative steel was unsatisfactory, receiving a rating of 2.0. The mechanism is as follows: the Ti content was too low, resulting in an insufficient number of TiN particles capable of effectively pinning austenite grain boundaries at high temperatures. This failed to provide a refined initial austenite microstructure basis for the Hf nanophase. Although the Hf content was sufficient, the lack of fine initial grain boundaries as nucleation sites weakened the pinning and nucleation regulation effects of the Hf nanophase during the phase transformation process, ultimately affecting the uniformity of the microstructure and leading to an increased banding level.

[0070] Examples 4 and 5 demonstrate that the boundaries of the component range and Hf / Ti ratio range (1.5~3.0) required by the present invention are effective and necessary, and excellent results with banded tissue grade ≤1.5 can be stably obtained within this range.

[0071] Comparative Examples 2 and 3 clearly demonstrate, through negative examples, that once the Hf / Ti ratio deviates from the optimized range set by this invention (whether too low or too high), it is difficult to achieve the desired banded structure control effect, even using the same preparation process. This underscores the crucial importance of precisely controlling the Hf to Ti ratio for fully leveraging their synergistic effect and achieving the objectives of this invention.

[0072] In summary, this invention, by strictly controlling the Hf content to 0.08%~0.12% and synergistically controlling the Ti content to maintain the Hf / Ti ratio within the range of 1.5~3.0, coupled with an optimized process centered on high-vacuum melting, electromagnetic stirring continuous casting, and controlled rolling and cooling, effectively suppresses Mn dendritic segregation from the solidification source, successfully and stably controlling the banded microstructure level below 1.5. The gear steel provided by this invention has outstanding advantages such as highly uniform microstructure, small quenching deformation, good carburizing uniformity, and long fatigue life. Its preparation method has a wide process window, strong controllability, and relatively low cost, making it particularly suitable for manufacturing gears for new energy vehicles with extremely high precision and reliability requirements.

[0073] 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 gear steel with a banded microstructure controlled by hafnium microalloying, characterized in that: The chemical composition, by mass percentage, includes: C: 0.12%~0.21%, Si: 0.15%~0.35%, Mn: 0.80%~1.20%, Cr: 1.00%~1.30%, Ti: 0.03%~0.08%, Hf: 0.08%~0.12%, P≤0.015%, S≤0.005%, with the balance being Fe and unavoidable impurities; and the mass percentage ratio of Hf to Ti, Hf / Ti, is 1.5~3.

0.

2. The gear steel with hafnium microalloying for controlling banded microstructure according to claim 1, characterized in that: The banded structure grade of the gear steel is no higher than 1.

5.

3. A method for preparing gear steel using hafnium microalloying to control banded microstructure as described in claim 1 or 2, characterized in that: Includes the following steps: (1) Batching: Alloy elements are proportioned according to the stated mass percentage and Hf / Ti ratio; (2) Vacuum induction melting: Melting is carried out under a vacuum of less than 1.0 Pa, with a melting temperature of 1580~1650℃ and a time of not less than 50 minutes; (3) Refining and casting: Refining is carried out using the LF-VD dual process, and the soft blowing argon time is controlled; the casting temperature is 1520~1550℃; (4) Continuous casting: Electromagnetic stirring continuous casting process is adopted, the superheat is controlled at 15~30℃, and the casting speed is 0.7~0.9 m / min; (5) Hot rolling: The continuous casting billet is heated at 1180~1220℃ and held at that temperature, followed by rough rolling and finish rolling. The total reduction rate is ≥75%, and the final rolling temperature of finish rolling is 880~930℃. (6) Controlled cooling: The ultra-fast cooling process is adopted to cool to 550-600℃ at a cooling rate of 40-60℃ / s, and then air-cooled to room temperature.

4. The method for preparing gear steel according to claim 3, characterized in that: In step (2), the specific feeding sequence of the vacuum induction melting is as follows: first, add pure iron and metallic nickel and melt them, then add metallic chromium and metallic titanium, and after complete melting, perform deoxidation and alloying, and finally add metallic hafnium.

5. The method for preparing gear steel according to claim 3, characterized in that: In step (3), the white slag holding time is ≥30 min and the soft argon blowing time is ≥20 min in the LF-VD refining process.

6. The method for preparing gear steel according to claim 3, characterized in that: In step (3), the refining temperature shall not be lower than 1580℃.

7. The method for preparing gear steel according to claim 3, characterized in that: In step (4), the current intensity of the electromagnetic stirring is 200~400 A and the frequency is 2~5 Hz.

8. The method for preparing gear steel according to claim 3, characterized in that: In step (5), the continuously cast billet is heated at 1180~1220℃ and held for 2~3 hours before rough rolling and finish rolling.

9. The method for preparing gear steel according to claim 3, characterized in that: In step (5), the initial rolling temperature of the rough rolling is 1080~1130℃.

Images