A high-strength, high-hardenability 20CrMnTiH gear steel and its preparation method

Abstract

This invention belongs to the field of special steel materials technology, and relates to a high-strength, high-hardenability 20CrMnTiH gear steel and its preparation method. The composition (%) is: C: 0.19–0.22, Si: 0.18–0.25, Mn: 1.05–1.15, Cr: 1.10–1.20, Ti: 0.045–0.055, V: 0.08–0.12, Al: 0.020–0.035, P≤0.010, S≤0.005, TO≤8ppm, N≤45ppm, H≤1ppm; the balance is Fe and unavoidable impurities. This invention solves the technical bottlenecks of insufficient hardenability, low strength, and large performance fluctuations in traditional 20CrMnTiH gear steel through composition design, composite microalloying control, full-process purity control, and integrated innovation of heat treatment processes. It provides a gear steel that can be industrially produced and has excellent comprehensive performance.

Description

Technical Field

[0001] This invention belongs to the field of special steel materials technology, specifically relating to a high-strength, high-hardenability 20CrMnTiH gear steel and its preparation method, which is suitable for gear components that bear heavy loads, high speeds, and impact loads in high-end automobiles, engineering machinery, wind power equipment and other fields. Background Technology

[0002] 20CrMnTiH gear steel is one of the most widely used carburizing gear steels in the industrial field due to its moderate cost and good machinability. However, its traditional products generally have defects such as insufficient hardenability, low strength, and difficulty in achieving both toughness and fatigue performance. Specifically, it is characterized by insufficient core hardness and uneven hardenability layer depth. Under heavy load, low temperature or high frequency impact conditions, it is prone to failure phenomena such as tooth surface spalling and tooth root fracture, which cannot meet the stringent requirements of high-end equipment for gear materials of "high strength, high hardenability and high reliability".

[0003] In the existing technology, there are two main ways to improve the hardenability and strength of 20CrMnTiH gear steel: one is to increase the content of alloying elements (Cr, Mn), but this easily leads to severe segregation of the cast billet, which increases the difficulty of subsequent processing and significantly reduces toughness; the other is to optimize the heat treatment process, but due to the inherent purity and composition uniformity, the performance improvement is limited and it is difficult to achieve a simultaneous breakthrough in strength and hardenability.

[0004] Furthermore, in the traditional 20CrMnTiH gear steel smelting process, deoxidation and desulfurization are incomplete, resulting in a high oxygen content (typically 15-20 ppm) and large-sized sulfide and oxide inclusions. This further exacerbates fluctuations in hardenability and instability in mechanical properties, leading to significant deformation during gear heat treatment and a shortened service life. Therefore, developing a 20CrMnTiH gear steel that balances high strength, high hardenability, and good toughness and machinability, along with its industrial preparation method, has become a pressing technical challenge for the industry.

[0005] Based on this, the present invention solves the technical bottlenecks of insufficient hardenability, low strength, and large performance fluctuation of traditional 20CrMnTiH gear steel by integrating and innovating precise narrow-range composition design, composite micro-alloying control, full-process purity control, and suitable heat treatment process. It provides a high-strength, high-hardenability 20CrMnTiH gear steel that can be industrially produced and has excellent comprehensive performance. Summary of the Invention

[0006] To overcome the shortcomings of existing 20CrMnTiH gear steel, such as insufficient hardenability, low strength, and difficulty in balancing toughness and machinability, a high-strength, high-hardenability 20CrMnTiH gear steel is provided. This steel has a hardenability J9 ≥ 40 HRC, a hardenability bandwidth ≤ 3 HRC, and a tensile strength ≥ 1250 MPa, while also possessing good low-temperature toughness and machinability. Furthermore, a stable, cost-controllable preparation method suitable for large-scale industrial production is provided.

[0007] To achieve the technical objectives, the present invention adopts the following technical solution; First, a chemical composition of high-strength, high-hardenability 20CrMnTiH gear steel is provided, consisting of the following components by mass percentage: C 0.19~0.22%, Si 0.18~0.25%, Mn 1.05~1.15%, Cr 1.10~1.20%, Ti 0.045~0.055%, V 0.08~0.12%, Al 0.020~0.035%, P≤0.010%, S≤0.005%, TO≤8ppm, N≤45ppm, H≤1ppm, with the balance being Fe and unavoidable impurities; Key considerations for ingredient design: The carbon content is controlled within a narrow range of 0.19% to 0.22%, which ensures the hardness and wear resistance of the tooth surface after carburizing, while avoiding excessive carbon content that would reduce the toughness of the core and increase the difficulty of processing. At the same time, it lays the foundation for improving hardenability.

[0008] Mn and Cr composite regulation: The Mn content is increased to 1.05-1.15%, and the Cr content is controlled at 1.10-1.20%. The synergistic effect of the two significantly improves the hardenability of the steel, promotes the martensitic transformation, and inhibits the growth of austenite grains during the carburizing process, thus balancing strength and toughness.

[0009] Ti and V microalloying synergy: The Ti content is controlled at 0.045-0.055%, which combines with N and C in the steel to form fine and dispersed TiN and TiC precipitates, refining the grains; the V content is added at 0.08-0.12%, which further refines the austenite grains, improves the hardenability, strength and toughness of the steel, and solves the problems of insufficient hardenability and low core strength of traditional 20CrMnTiH gear steel.

[0010] Alt is controlled at 0.020-0.035% to achieve deep deoxidation, reduce oxide inclusions in steel, refine grains, improve steel purity, and enhance fatigue performance; the content of harmful elements such as P, S, TO, N, and H is strictly controlled to avoid performance defects caused by harmful inclusions and gases.

[0011] Secondly, a method for preparing high-strength, high-hardenability 20CrMnTiH gear steel is provided, including: converter, LF refining, RH vacuum degassing, continuous casting, and rolling; 1. BOF converter smelting: Control the tapping temperature to 1600–1670℃; Specifically, a scrap steel to molten iron ratio of 8:2 is adopted, and low-phosphorus, low-sulfur molten iron (P≤0.010%, S≤0.005%) is selected; the control endpoints are C≥0.08% and P≤0.008%. The steel tapping process employs slag-free tapping technology, with the simultaneous addition of silicon-manganese alloy and ferrotitanium for pre-deoxidation, preventing secondary oxidation of the molten steel and laying a foundation for purity in subsequent refining.

[0012] 2. LF Refining: High-alkalinity slag refining, combined deoxidation and deep desulfurization, with core components controlled within a narrow range of ±0.015%; Specifically, the process involves composite deoxidation, deep desulfurization, and precise composition control: slag ratio (wt%): CaO 55-62%, Al2O3 23-29%, MgO 6-9%, CaF2 4-7%, with alkalinity controlled at 6.0-7.0; deoxidation employs a staged deoxidation method using a composite deoxidation of "aluminum particles + silicon carbide," first precipitation deoxidation followed by diffusion deoxidation, controlling the (FeO+MnO) content in the slag to ≤0.6%, achieving deep deoxidation; desulfurization utilizes strong argon stirring to ensure S ≤0.003%; composition fine-tuning controls the core elements C, Mn, Cr, Ti, and V within a narrow range of ±0.015%, ensuring compositional uniformity.

[0013] 3. RH vacuum degassing: Vacuum degree ≤67Pa, maintain for ≥25min, degas and feed with aluminum wire and calcium treatment; Specifically, a high-vacuum argon gas circulation is used (other flow rates are 50-150 m³ / h). 3 ( / h) efficiently removes H and O gases from steel, ensuring H≤0.5ppm and TO≤8ppm; after vacuum feeding, the aluminum content is finely adjusted to ensure Al is stable at 0.025~0.035%; calcium treatment is performed before ladle hoisting, with calcium wire feeding amount of 100~140 m / furnace.

[0014] 4. Full-process protection for continuous casting: Argon protection throughout the process, superheat ≤25℃, pulling speed ≤1.1m / min, combined with electromagnetic stirring and end light pressing, wherein the end light pressing amount is ≤10mm; Specifically, argon gas protection is used throughout the process from ladle to tundish to crystallizer. An alkaline carbon-free covering agent is used in the tundish to prevent secondary oxidation of the molten steel. The superheat of continuous casting is controlled at 20-25℃, and the casting speed is 0.9-1.1 m / min. A slow casting process with low superheat is adopted to reduce dendrite segregation and inclusion aggregation in the billet. Electromagnetic stirring in the crystallizer and at the end, combined with light reduction at the end (5-10 mm), improves the density and compositional uniformity of the billet and avoids internal defects in the billet.

[0015] 5. Heated rolling process: High-temperature diffusion of the cast billet, with a diffusion temperature of ≤1220℃ and a time of ≤5h; controlled rolling and controlled cooling, with an initial rolling temperature of ≥1100℃ and a final rolling temperature of ≤900℃; slow cooling after rolling for ≥24h.

[0016] Specifically, the billet is first heated and subjected to high-temperature diffusion (1180~1220℃, 3~5h) to eliminate dendritic segregation and refine the as-cast structure; the initial rolling temperature is 1100~1150℃ and the final rolling temperature is 860~890℃. A controlled rolling and controlled cooling process is adopted, and after rolling, it is subjected to windproof slow cooling (slow cooling time ≥24h) to avoid uneven structure after rolling and ensure uniform hardness of the rolled material.

[0017] The performance indicators of 20CrMnTiH gear steel prepared based on the above composition and process are as follows: Hardenability: J9 = 40~44HRC, hardenability bandwidth ≤3HRC, CPK ≥1.33; Mechanical properties: tensile strength ≥1250MPa, yield strength ≥1000MPa, elongation ≥15%, reduction of area ≥58%, low temperature impact toughness AKU2 ≥50J; Purity: TO≤8ppm, S≤0.005%, H≤0.5ppm, N≤45ppm; Inclusions: Class A ≤ 1.0, Class B ≤ 0.5, Class C 0, Class D ≤ 0.5, Class DS ≤ 1.0, no large inclusions > 15μm; Machining performance: Brinell hardness HBW220~250, good machinability, and after carburizing, the tooth surface hardness can reach HRC60~64, which meets the requirements of gear machining and use.

[0018] Beneficial effects: (1) The 20CrMnTiH gear steel prepared by this invention has significantly improved hardenability and extremely narrow bandwidth; hardenability J9≥40HRC, bandwidth≤3HRC (optimal 2.6HRC), which is far superior to traditional 20CrMnTiH. The core hardness of large cross-section gears is uniform, deformation is controllable, and heat treatment consistency is extremely strong. Moreover, the tensile strength is ≥1250MPa (maximum 1300MPa), which is significantly improved compared to traditional 20CrMnTiH; the low-temperature impact toughness AKU2≥50J, which combines high strength and high toughness, meeting the requirements of gear processing and use.

[0019] (2) TO≤8ppm, H≤1ppm, maximum inclusions≤13μm, purity and fatigue life are significantly improved, harmful inclusions and gases are controlled to the level of high-end gear steel; contact fatigue life reaches 1.18×10 7 ~1.32×10 7 This is 2.7 to 3 times that of traditional 20CrMnTiH products.

[0020] (3) The present invention utilizes Ti+V composite microalloying to form a dispersed precipitate phase, which significantly refines the austenite grains, prevents carburization growth, and results in a high proportion of full martensite in the core structure, resulting in small performance fluctuations, high reliability, and stable structure. Detailed Implementation

[0021] To further verify the effectiveness of the present invention, a detailed description is provided below with reference to specific embodiments and comparative examples. Embodiments 1-3 are the technical solutions of the present invention, and comparative examples are provided. The specific parameters and performance test results are as follows.

[0022] I. Testing Standards and Methods for Each Indicator 1. Hardenability J9 (HRC) (1) Standard: GB / T 225-2006 "End-quenching test method for hardenability of steel (Jominy test)" (domestic arbitration standard); (2) Method: Prepare a normalized pretreated sample (Φ30mm×100mm), and after normalized pretreatment, process a standard end-quenched sample (Φ25mm×100mm) and quench it at the specified process temperature; spray water to cool the end of the sample, and test the Rockwell hardness (HRC) at the 9mm (J9) position along the sample axis from the end; directly obtain the hardness value of the J9 point, which represents the hardness level of the steel at a 9mm hardened layer depth, and is the core indicator for judging the hardenability of gear steel.

[0023] 2. Hardenability Bandwidth (HRC) (1) Standard: Same hardenability: GB / T 225-2006; at the same time, it must meet the tolerance requirements for hardenability bandwidth in GB / T 5216-2014 "Structural Steel with Guaranteed Hardenability"; (2) Method: For multiple samples of the same batch and specifications, the full hardenability curve was tested according to the Jominy end-quenching method; At the target hardening location, the difference between the maximum and minimum hardness values ​​of all samples is the hardenability bandwidth.

[0024] 3. Tensile strength (MPa) (1) Standard: GB / T 228.1-2021 Metallic materials, tensile testing - Part 1: Room temperature test method (domestic room temperature tensile arbitration standard); (2) Method: Prepare round bar tensile specimens (Φ15mm gauge length specimens, gauge length L0=5d) according to the standard; perform longitudinal tensile testing at room temperature using a universal testing machine, and record the maximum tensile force before the specimen breaks; calculate the tensile strength Rm = maximum tensile force Fm / original cross-sectional area S0 of the specimen according to the formula, with the unit being MPa.

[0025] 4. Yield strength (MPa) (1) Standard: Same as tensile strength GB / T 228.1-2021; (2) Method: The tensile strength was measured simultaneously in the same tensile test as the tensile strength; for steel with obvious yielding phenomenon, the lower yield strength ReL of the yield plateau was measured; for steel without obvious yielding, the specified plastic extension strength Rp0.2 (i.e. the stress when the plastic deformation is 0.2%) was measured; the unit is MPa, which represents the ability of steel to resist plastic deformation.

[0026] 5. Elongation A (%) (1) Standard: Same as tensile strength GB / T 228.1-2021; (2) Method: After the tensile test is broken, align the fracture surfaces of the specimen and measure the gauge length Lu after the fracture. Calculate the elongation A according to the formula: Elongation A = (Lu - L0) / L0 × 100%, where L0 is the original gauge length.

[0027] 6. Impact energy AKU2 (J) (1) Standard: GB / T 229-2020 "Charpy Impact Test Method for Metallic Materials" (Domestic Arbitration Standard for Impact Performance); (2) Method: Prepare standard U-notch impact specimens (10mm×10mm×55mm, U-notch depth 2mm); The impact energy absorbed when the specimen breaks at room temperature is recorded using a pendulum impact tester. This value is called AKU2, and the unit is joules (J).

[0028] 7. Maximum inclusion size (μm) (1) Standard: GB / T 10561-2023 "Standard rating chart microscopic test method for determination of non-metallic inclusion content in steel"; (2) Method: Samples were taken at 1 / 2 radius of the round steel to prepare standard metallographic specimens (without scratches or deformation); under a metallographic microscope, the maximum length of all non-metallic inclusions in the field of view was counted according to the standard specified field of view, in μm. 8. Exposure fatigue life (times) (1) Standard: GB / T 14229-2021 Gear Contact Fatigue Test Method; (2) Method: Gear bench test: Prepare standard test gears and run them under specified load, speed and lubrication conditions, and record the number of cycles in which pitting and peeling occur on the tooth surface.

[0029] (a) Comparison of chemical composition (%, ppm) in Table 1

[0030] As shown in Table 1, compared with Comparative Example 1 (traditional 20CrMnTiH), this application has higher Mn, Cr, and Ti contents, and adds 0.08~0.12% V, reducing the content of harmful elements and gases to 1 / 3~1 / 5, thus ensuring hardenability and purity from the source. Compared with Comparative Examples 2 and 3 (V-free microalloying), under similar matrix compositions, the addition of V is a key variable for improving hardenability, strength, and toughness, and its dosage control is crucial.

[0031] (II) Preparation process

[0032] 1. The processes in Embodiments 1-3 of the present invention are as follows:

[0033] (1) BOF converter smelting: The scrap steel to molten iron ratio is 8:2 (by mass), and low-phosphorus, low-sulfur molten iron is selected (P≤0.010%, S≤0.005%); the tapping temperature is controlled at 1600~1670℃, and the final concentration is C≥0.08%, P≤0.008%; The steel tapping process employs slag-free tapping technology, with the simultaneous addition of silicon-manganese alloy and ferrotitanium for pre-deoxidation, preventing secondary oxidation of the molten steel and laying a foundation for purity in subsequent refining.

[0034] (2) LF refining: composite deoxidation + deep desulfurization + precise composition control: slag ratio (wt%): CaO 55~62%, Al2O3 23~29%, MgO 6~9%, CaF 24~7%, alkalinity controlled at 6.0~7.0; deoxidation adopts the "aluminum particles + silicon carbide" composite deoxidation method in stages, first precipitation deoxidation and then diffusion deoxidation, controlling the total amount of FeO and MnO in the slag, that is, (FeO+MnO)≤0.6%, to achieve deep deoxidation; desulfurization adopts strong argon gas stirring to make S≤0.003%; composition fine adjustment is controlled in a narrow range of ±0.015% for core elements such as C, Mn, Cr, Ti, and V to ensure composition uniformity.

[0035] (3) RH vacuum degassing: vacuum degree ≤67Pa, holding time ≥25min, high vacuum argon gas circulation (other flow rate 50~150m3 / h) is used to efficiently remove H and O gases from steel, so that H≤0.5ppm and TO≤8ppm; after vacuum feeding, the aluminum content is finely adjusted to ensure that Al is stable at 0.025~0.035%, and calcium treatment is carried out before hoisting.

[0036] (4) Full-process protection continuous casting: Argon gas protection is used throughout the process from ladle to tundish to crystallizer. Alkaline carbon-free covering agent is used in the tundish to prevent secondary oxidation of molten steel. The superheat of continuous casting is controlled at 20-25℃, the casting speed is 0.9-1.1m / min, and a slow casting process with low superheat is adopted to reduce dendrite segregation and inclusion aggregation in the billet. The crystallizer and end electromagnetic stirring + end light pressing (reduction amount 5-10mm) improve the density and composition uniformity of the billet and avoid internal defects in the billet.

[0037] (2) Heating and rolling process: The billet is first heated and high temperature diffusion (1180~1220℃×3~5h) to eliminate dendritic segregation and refine the as-cast structure; the rolling start temperature is 1100~1150℃ and the final rolling temperature is 860~890℃. The controlled rolling and controlled cooling process is adopted. After rolling, the material is cooled slowly in the sheltered place (slow cooling time ≥24h) to avoid uneven structure after rolling and ensure uniform hardness of the rolled material.

[0038] 2. Specific group and parameter settings are shown in Table 2. Table 2. Process differences between the examples and comparative examples

[0039] The example adopts BOF+LF+RH double refining combined with high vacuum degassing, with vacuum degree ≤67Pa and holding for ≥25min, combined with continuous casting under full argon protection, superheat 20~25℃, casting speed 0.9~1.1m / min, crystallizer, end electromagnetic stirring and other processes.

[0040] Comparative Example 1 uses only LF, no RH, high superheat, and fast pulling speed. This process results in poor deoxidation and degassing, heavy segregation, and large inclusions. Comparative Examples 2 and 3 use RH, but no V is added, which limits the refinement of grains and the improvement of hardenability.

[0041] (III) Comparison of performance test results in Table 3

[0042] The test results in Table 3 show that J9 in the examples is 41~43 HRC with a bandwidth of 2.6~2.8 HRC; Comparative Example 1 is only 36 HRC with a bandwidth of 5.1 HRC; Comparative Examples 2~3 are 38~39 HRC with a bandwidth of 3.7~3.9 HRC. This indicates that the composition designed in this application effectively improves the basic hardenability through Mn-Cr composite, and further improves and narrows the bandwidth by combining Ti-V synergy.

[0043] Furthermore, the tensile strength of the embodiments is 1270~1300MPa, and AKU2=51~56J; Comparative Example 1 is only 1040MPa and 31J; Comparative Examples 2~3 are 1120~1160MPa and 40~43J; This shows that the present invention significantly improves strength and toughness through V microalloying, solving the problem of low strength and poor toughness of traditional steel cores.

[0044] Meanwhile, considering the data on purity and fatigue life, the maximum inclusion size in the example was 11~13μm, and the contact fatigue life was 1.18×10⁻⁶. 7 ~1.32×10 7 Comparative Example 1 had an inclusion size of 31 μm and a lifetime of only 4.4 × 10⁻⁶. 6 This invention employs high-purity control and micro-alloying throughout the entire process, resulting in a significant improvement in fatigue life and enhanced reliability under heavy loads.

[0045] In summary, this invention, based on dual innovations in composition and process, specifically achieves high hardenability, high strength, high toughness, and high fatigue resistance through narrow-range hardenability, Ti-V microalloying, and high-purity smelting. The prepared 20CrMnTiH gear steel exhibits outstanding performance stability, with a hardenability bandwidth ≤3HRC and good batch-to-batch consistency, making it suitable for mass production of high-end automotive / engineering machinery gears. Simultaneously, it boasts low cost, stable process, and large-scale production capability, making it a viable alternative to imported and traditional 20CrMnTiH steels, with broad market application prospects.

[0046] Note: The above embodiments are only used to illustrate the present invention and are not intended to limit the technical solutions described in the present invention. Therefore, although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the present invention. All technical solutions and improvements that do not depart from the spirit and scope of the present invention should be covered within the scope of the claims of the present invention.

Claims

1. A high-strength, high-hardenability 20CrMnTiH gear steel, characterized in that, It is composed of the following components by mass percentage: C 0.19–0.22%, Si 0.18–0.25%, Mn 1.05–1.15%, Cr 1.10–1.20%, Ti 0.045–0.055%, V 0.08–0.12%, Al 0.020–0.035%, P≤0.010%, S≤0.005%, TO≤8ppm, N≤45ppm, H≤1ppm, with the balance being Fe and unavoidable impurities; The gear steel has a hardenability J9 ≥ 40 HRC, a hardenability bandwidth ≤ 3 HRC, and a tensile strength ≥ 1250 MPa.

2. The high-strength, high-hardenability 20CrMnTiH gear steel according to claim 1, characterized in that, S≤0.003%, TO≤7.6ppm, H≤0.5ppm.

3. The high-strength, high-hardenability 20CrMnTiH gear steel according to claim 1, characterized in that, Low-temperature impact energy AKU2≥50J, maximum inclusion size≤13μm, contact fatigue life≥1.18×10 7 Second-rate.

4. A method for preparing the high-strength, high-hardenability 20CrMnTiH gear steel according to any one of claims 1 to 3, characterized in that, The preparation methods include, in sequence: converter smelting, LF refining, RH vacuum degassing, full-process protected continuous casting, and heated rolling; (1) Converter smelting: control the tapping temperature to 1600~1670℃; (2) LF refining: high alkalinity slag refining, compound deoxidation and deep desulfurization, core components are controlled within a narrow range of ±0.015%; (3) RH vacuum degassing: vacuum degree ≤67Pa, maintain for ≥25min, feed aluminum wire after degassing and treat with calcium; (4) Full-process protection continuous casting: Argon protection throughout the process, superheat ≤25℃, casting speed ≤1.1m / min, combined with electromagnetic stirring and end light pressing, wherein the end light pressing amount is ≤10mm; (5) Heating and rolling: High-temperature diffusion of the billet, diffusion temperature ≤1220℃, time ≤5h; controlled rolling and controlled cooling, initial rolling temperature ≥1100℃, final rolling temperature ≤900℃; slow cooling after rolling ≥24h.

5. The method for preparing high-strength, high-hardenability 20CrMnTiH gear steel according to claim 4, characterized in that, The LF refining slag system consists of CaO 55%–62%, Al2O3 23%–29%, MgO 6%–9%, ​​CaF 24%–7%, and basicity 6.0–7.

0. It adopts composite deoxidation of aluminum particles and silicon carbide, with (FeO+MnO) ≤0.6% in the slag and S ≤0.003% after desulfurization.

6. The method for preparing high-strength, high-hardenability 20CrMnTiH gear steel according to claim 4, characterized in that, After RH vacuum degassing, the aluminum wire is fed with Al controlled at 0.025-0.035%, and the calcium wire is fed with 100-140 / furnace after calcium treatment.

7. The method for preparing high-strength, high-hardenability 20CrMnTiH gear steel according to claim 4, characterized in that, The continuous casting superheat is 20-25℃, the casting speed is 0.9-1.1 m / min, and the end reduction is 5-10 mm.

8. The method for preparing high-strength, high-hardenability 20CrMnTiH gear steel according to claim 4, characterized in that, The billet heating and diffusion temperature is 1180~1220℃, and the time is 3~5 h; the initial rolling temperature is 1100~1150℃, and the final rolling temperature is 860~890℃.

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