Steel for range-extender electric vehicle reducer gear and manufacturing method thereof

Abstract

This invention relates to a steel for gears in a range-extended electric vehicle reducer and its manufacturing method, belonging to the field of iron and steel metallurgy. The chemical composition of the steel, by weight percentage, is: C: 0.19–0.22%, Si: 0.20–0.30%, Mn: 0.40–0.55%, Cr: 0.60–0.80%, P: ≤0.018%, S: ≤0.003%, Mo: 0.23–0.28%, Nb: 0.047–0.074%, N: 0.0070–0.0110%, B: 0.0019–0.0023%, Nb / N ≥ 6.7, with the balance being Fe and unavoidable impurities. The steel exhibits fine grain size at high temperatures; after holding at 970℃ for 5 hours, the grain size of the water-quenched steel is grade 7.0–9.0. After oil quenching at 870℃ and tempering at 200℃, the hardness of the steel at a position 11mm from the quenched end face is 32HRC-35HRC, and the microstructure is martensite + bainite, with a martensite content of 50%-60%. The tensile strength of the steel after oil quenching at 870℃ and tempering at 200℃ can be stably controlled between 1150MPa and 1300MPa. The gear steel of this application is a high-temperature, high-strength, high-hardness, fine-grained gear steel product that meets the stringent requirements for gear steel used in range-extended electric vehicle reducers.

Description

Technical Field

[0001] This invention belongs to the field of iron and steel metallurgy, and specifically relates to gear steel and its manufacturing method. Background Technology

[0002] Range-extended electric vehicles (EREVs) utilize a battery pack to power the vehicle when the battery is fully charged, which in turn powers the electric motor. When the battery is low, an EREV can use a range extender (internal combustion engine) to directly store energy in the battery pack, thus increasing its driving range. Pure electric vehicles, on the other hand, rely on external power sources for energy storage, making EREVs significantly more flexible. EREVs alleviate range anxiety while retaining the driving characteristics of electric vehicles. The reducer in an EREV is an independent component consisting of gear drives, worm gear drives, or gear-worm gear drives enclosed in a rigid housing. It matches the speed and transmits torque between the motor and the driven machine, ensuring the motor operates within its highest efficiency range. EREV reducers are characterized by high speeds and longer operating times compared to ordinary electric vehicles, thus placing high demands on the strength, hardness, grain size, and fatigue life of the gear steel used. Summary of the Invention

[0003] To meet the requirements for gear steel in range-extended electric vehicle reducers, this invention develops a type of gear steel for range-extended electric vehicle reducers and its manufacturing method. The high-temperature grain size of the steel (water quenching after holding at 970℃ for 5 hours) reaches grade 7.0-9.0. After quenching and tempering treatment (oil quenching at 870℃ and tempering at 200℃), the hardness J11 of the steel at a position 11mm from the quenched end face is 32HRC-35HRC, and the martensite content at this position is 50%-60%. At the same time, the tensile strength of the steel in this heat-treated state is 1150MPa-1300MPa.

[0004] The technical solution of the present invention is: a steel for gears of a range-extended electric vehicle reducer. The chemical composition of the steel of the present invention is as follows (by weight percentage): C: 0.19-0.22%, Si: 0.20-0.30%, Mn: 0.40-0.55%, Cr: 0.60-0.80%, P: ≤0.018%, S: ≤0.003%, Mo: 0.23-0.28%, Nb: 0.047-0.074%, N: 0.0070-0.0110%, B: 0.0019-0.0023%, Nb / N ≥ 6.7, with the balance being Fe and unavoidable impurities.

[0005] The main functions and design basis of each chemical element in the steel of this invention are as follows:

[0006] C: Carbon is the most important element affecting the tensile strength and hardness of steel. The addition of an appropriate amount of carbon to the steel in this invention ensures stable tensile strength. Simultaneously, to ensure a hardness of 32HRC-35HRC at a position 11mm from the quenched end face, where the martensite content is 50%-60%, the selected carbon content must be stable and not fluctuate significantly. Therefore, the carbon content is selected within the range of 0.19-0.22%.

[0007] Si: Silicon is added in this invention to provide deoxidation and reduction. The silicon content is selected in the range of 0.20% to 0.30%.

[0008] Mn: Adding a certain amount of manganese to steel can improve its hardenability. However, excessive manganese will increase the tendency for steel grain coarsening and reduce its temper brittleness. Therefore, the manganese content should be selected within the range of 0.40% to 0.55%.

[0009] Cr: Chromium is added to the steel of this invention to improve its hardenability and to form chromium carbides during the high-temperature carburizing process (this steel is a high-temperature carburizing steel, and the carburizing heat treatment process involves holding at 970℃ for 5 hours), thereby improving the wear resistance and fatigue strength of the gear surface. The chromium content is selected in the range of 0.60% to 0.80%.

[0010] P: Phosphorus is a harmful element in steel, increasing its cold brittleness and reducing its plasticity. The P content in this invention is ≤0.018%.

[0011] Sulfur readily forms elongated sulfides in steel. The tensile strength of steel decreases significantly in areas where these elongated sulfides accumulate. The presence of sulfur in the steel of this invention is detrimental to the overall stability of the steel's tensile strength. Therefore, the sulfur content in the steel of this invention should be strictly controlled, with S ≤ 0.003%.

[0012] Molybdenum (Mo) in the carburized layer after high-temperature carburizing can reduce the formation of a continuous network of carbides at grain boundaries and decrease the amount of residual austenite in the carburized layer, thereby increasing the wear resistance and fatigue strength of the gear carburized layer and improving the fatigue life of the gear. Therefore, the molybdenum content in this invention is selected in the range of 0.23-0.28%.

[0013] Niobium (Nb) forms niobium nitride, which has a grain-refining effect. Niobium nitride has a high dissolution temperature in steel, especially in low-carbon gear steel, where the dissolution temperature exceeds 1000℃. Therefore, niobium nitride retains excellent grain boundary pinning ability even at high temperatures (970℃). Trace amounts of niobium can improve the strength of steel without affecting its plasticity or toughness. The niobium content selected in this invention ranges from 0.047% to 0.074%.

[0014] Nitrogen in steel preferentially combines with niobium to form niobium nitride, which can precipitate at grain boundaries. Niobium nitride can improve the strength of grain boundaries at high temperatures and increase the fatigue life of steel. However, excessive nitrogen will combine with boron added later in the process of making the boron lose its ability to improve the hardenability of the steel. Moreover, nitrogen has very low solubility in ferrite at room temperature. Supersaturated nitrogen in steel will precipitate as Fe4N during room temperature storage, making the steel brittle and reducing its fatigue life. Therefore, the design range of nitrogen content in the steel of this invention is N: 0.0070~0.0110%, and it needs to meet the requirement of Nb / N≥6.7.

[0015] B: The main purpose of adding trace amounts of boron to the steel of this invention is to significantly increase the hardenability of the steel, thereby saving manganese, chromium, and molybdenum alloys. Because boron has a strong affinity for nitrogen, to ensure that all added boron exists in elemental form, it needs to be added after vacuum degassing in an RH furnace. After a long smelting process, the nitrogen has completely combined with niobium to form stable niobium nitride. At this point, the added boron will exist entirely in elemental form, and this elemental boron fully plays its role in improving the hardenability of the steel, thus stabilizing its hardenability. Since boron has a significant impact on the hardenability of steel, controlling the boron content within a narrow range can effectively reduce fluctuations in the hardenability and tensile strength of the steel. This provides a strong guarantee for the stable control of the hardness at a position 11mm from the quenching end face, the stable control of the microstructure content at this position, and the stable control of the tensile strength of the steel under this heat treatment state (870℃ oil quenching + 200℃ tempering). Therefore, the boron element in the steel of this invention is selected within a narrow range B: 0.0019-0.0023%.

[0016] The manufacturing method for the steel used in the reducer gear of the above-mentioned range-extended electric vehicle includes the following process: KR mechanical bidirectional stirring → converter smelting → refining → vacuum degassing treatment → continuous casting of square billets → rolling into round bars. Specifically, it includes the following process steps:

[0017] (1) First, the KR mechanical bidirectional stirring method is used for efficient desulfurization of molten iron. Two coaxial, concentric, cross-shaped stirring heads made of refractory material are inserted to a certain depth below the surface of the molten iron, and the two cross-shaped stirring heads are rotated at high speed in opposite directions. This efficient bidirectional mechanical stirring action makes the desulfurizing agent particles more uniform, dispersed and refined in the molten iron, thereby achieving rapid, efficient and deep desulfurization. The sulfur content of the desulfurized molten iron can be stably controlled at ≤0.002%, and the desulfurization time is 10-15 minutes. After desulfurization, the desulfurization slag floating on the surface of the molten iron must be completely removed.

[0018] (2) The converter uses desulfurized molten iron, molybdenum-containing scrap steel, and calcium molybdate (CaMoO4) for smelting. The molybdenum content of the smelted steel is controlled at 0.23-0.28%. Traditional smelting processes mainly use ferromolybdenum as an additive for smelting molybdenum alloy steel. However, ferromolybdenum is very expensive and its production process causes environmental pollution. A small amount of inexpensive primary raw material molybdenum oxide (MoO3) is also used as an additive for molybdenum alloy steel. However, molybdenum oxide (MoO3) has a low boiling point of only 1155℃. During the converter smelting process, molybdenum oxide is added to the high-temperature molten steel (around 1600℃) and volatilizes quickly, resulting in a low molybdenum yield and unnecessary waste. This invention adds calcium molybdate (CaMoO4), which is relatively stable at high temperatures (around 1600℃), and stable molybdenum-containing scrap steel during the converter smelting process. This avoids the volatilization of molybdenum, improves the molybdenum yield, and reduces the production cost of molybdenum-containing steel.

[0019] (3) In the early stage of refining, MnN wire is first fed in to adjust the N content in the molten steel to 0.0070-0.0110%. Then, according to the nitrogen content in the molten steel, ferroniobium is added to the molten steel. After refining, the Nb / N ratio in the molten steel is ≥6.7, ensuring that all nitrogen elements in the molten steel combine with niobium elements to form niobium nitride. The refined molten steel is then degassed in an RH furnace under vacuum. After vacuum degassing, FeB is added to the molten steel to adjust the boron content in the molten steel to 0.0019-0.0023%. Since the nitrogen elements in the steel have completely combined with niobium elements during the smelting process to form niobium nitride, the boron elements added after the RH vacuum degassing treatment exist entirely in elemental form, which plays a role in improving the hardenability and tensile strength of the steel.

[0020] (4) After vacuum degassing, the molten steel is transferred to the continuous casting process. The continuous casting process adopts full-process protective casting treatment, and nitrogen-free refractory materials are preferred in the continuous casting production process.

[0021] Preferably, the surface of the refined molten steel is immediately covered with a nitrogen-free covering agent; inert argon gas is used as the lifting gas during the vacuum degassing process. These measures prevent nitrogen addition to the molten steel during the refining to continuous casting process, ensuring that the Nb / N ratio in the molten steel is ≥6.7, and that elemental boron in the molten steel is not nitrided.

[0022] This invention relates to a steel for gears in a range-extended electric vehicle reducer. The steel composition is low in carbon and sulfur, with added molybdenum and trace amounts of niobium and nitrogen to ensure an Nb / N ratio ≥ 6.7. The invention employs KR mechanical bidirectional stirring to achieve rapid, efficient, and deep desulfurization; the addition of molybdenum-containing scrap steel and calcium molybdate to the converter increases molybdenum yield and reduces production costs; MnN wire is fed during the initial refining stage to adjust the nitrogen content, followed by the addition of ferroniobium to ensure an Nb / N ratio ≥ 6.7; and FeB is added after RH vacuum degassing to adjust the boron content in the molten steel.

[0023] The surface of the refined molten steel is immediately covered with a nitrogen-free covering agent. In the RH vacuum degassing process, inert argon gas is used as the lifting gas. The continuous casting process adopts full-process protective casting treatment. In addition, nitrogen-free refractory materials are used in the continuous casting production process. The above measures prevent nitrogen addition in the molten steel from refining to continuous casting, ensuring that the Nb / N ratio in the molten steel is ≥6.7 and that the elemental boron in the molten steel is not nitrided. The final product is a steel for gears in a range-extended electric vehicle reducer. The high-temperature grain size of the steel (after holding at 970℃ for 5 hours and then water quenching) is controlled at grade 7.0-9.0. After oil quenching at 870℃ and tempering at 200℃, the hardness of the steel at a position 11mm away from the quenched end face is 32HRC-35HRC. The martensite content at this position is 50%-60%, and the remaining structure is bainite. At the same time, under this heat treatment state (oil quenching at 870℃ + tempering at 200℃), the tensile strength of the steel is 1150MPa-1300MPa.

[0024] Compared with the prior art, the advantages of the present invention are as follows:

[0025] (1) The composition uses low carbon and low sulfur, and adds a certain amount of molybdenum and trace amounts of niobium and nitrogen to ensure Nb / N≥6.7. The narrow chemical composition design ensures a narrow range of steel grain size grades, and the hardness, microstructure and tensile strength of the steel are more stable after quenching.

[0026] (2) First, the KR mechanical bidirectional stirring method is used for efficient desulfurization of molten iron. Two coaxial, concentric, cross-shaped stirring heads made of refractory material are inserted to a certain depth below the surface of the molten iron ladle, and these two cross-shaped stirring heads are rotated at high speed in opposite directions. This efficient bidirectional mechanical stirring action makes the desulfurizing agent particles more uniform, dispersed, and refined in the molten iron, thereby achieving rapid, efficient, and deep desulfurization. The sulfur content of the desulfurized molten iron can be stably controlled at ≤0.002%, and the desulfurization time is 10-15 minutes. Compared with the existing KR mechanical unidirectional stirring method, the KR mechanical bidirectional stirring method has a shorter desulfurization time, requires less desulfurizing agent, and has a better desulfurization effect.

[0027] (3) The converter uses desulfurized molten iron, molybdenum-containing scrap steel, and calcium molybdate (CaMoO4) for smelting. The molybdenum content of the smelted steel is controlled at 0.23-0.28%. Traditional smelting processes mainly use ferromolybdenum as an additive for smelting molybdenum alloy steel. However, ferromolybdenum is very expensive and its production process causes environmental pollution. A small amount of inexpensive primary raw material molybdenum oxide (MoO3) is also used as an additive for molybdenum alloy steel. However, molybdenum oxide (MoO3) has a low boiling point of only 1155℃. During the converter smelting process, molybdenum oxide is added to the high-temperature molten steel (around 1600℃) and volatilizes quickly, resulting in a low molybdenum yield and unnecessary waste. This invention adds calcium molybdate (CaMoO4), which is relatively stable at high temperatures (around 1600℃), and stable molybdenum-containing scrap steel during the converter smelting process. This avoids the volatilization of molybdenum, improves the molybdenum yield, and reduces the production cost of molybdenum-containing steel.

[0028] (4) In the early stage of refining, MnN wire is fed in first to adjust the N content in the molten steel to 0.0070-0.0110%. Then, ferroniobium is added to the molten steel. After refining, the Nb / N ratio in the molten steel is ≥6.7, ensuring that all nitrogen elements in the molten steel combine with niobium elements to form niobium nitride. The refined molten steel is then degassed in an RH furnace under vacuum. After vacuum degassing, FeB is added to the molten steel to adjust the boron content in the molten steel to 0.0019-0.0023%. Since the nitrogen elements in the steel have completely combined with niobium elements in the early stage to form niobium nitride, the boron elements added after the RH vacuum degassing treatment exist entirely in elemental form, which plays a role in improving the hardenability and tensile strength of the steel.

[0029] (4) The surface of the refined molten steel is immediately covered with a nitrogen-free covering agent; argon gas is used as the lifting gas in the RH vacuum degassing process; the continuous casting process adopts full-process protective casting treatment, and nitrogen-free refractory materials are used in the continuous casting production process. The above measures prevent nitrogen addition in the molten steel after refining and during the continuous casting process, ensuring that the Nb / N ratio in the molten steel is ≥6.7, which can completely prevent the nitriding of elemental boron in the molten steel.

[0030] (5) A range-extended electric vehicle reducer gear steel produced according to this invention has the characteristics of fine and stable high-temperature grain size; hardness of 32HRC-35HRC at 11mm from the quenched end face after quenching and low-temperature tempering; microstructure of martensite + bainite with martensite content of 50%-60%; and tensile strength of steel after oil quenching at 870℃ and tempering at 200℃ can be stably controlled at 1150MPa-1300MPa. Attached Figure Description

[0031] Figure 1 The microstructure of the gear steel of Embodiment 1 of the present invention after oil quenching at 870℃ and tempering at 200℃ is shown at 11mm from the quenched end face, magnified 500 times.

[0032] Figure 2 The microstructure of the gear steel in Embodiment 2 of the present invention after oil quenching at 870℃ and tempering at 200℃ is shown at 11mm from the quenched end face, magnified 500 times. Detailed Implementation

[0033] The present invention will be further described in detail below with reference to the accompanying drawings. The embodiments described are exemplary and intended to explain the present invention, and should not be construed as limiting the present invention. Embodiments 1 and 2:

[0034] The two embodiments involve a method for manufacturing steel for gears in a range-extended electric vehicle reducer: KR mechanical bidirectional stirring → 100t converter → 100t refining → RH vacuum degassing treatment → continuous casting of square billets (240mm*240mm) → rolling into round bars. Two batches of range-extended electric vehicle reducer gear steel were manufactured.

[0035] First, efficient desulfurization of molten iron was achieved using the KR mechanical bidirectional stirring method. The sulfur content of the desulfurized molten iron was 0.0010% (Example 1) and 0.0008% (Example 2); the desulfurization time was 11 min (Example 1) and 15 min (Example 2). Desulfurized molten iron, molybdenum-containing scrap steel, and calcium molybdate (CaMoO4) were added to the converter for smelting. MnN wire was fed in during the early refining stage to adjust the nitrogen content of the molten steel to 0.0073% (Example 1) and 0.0108% (Example 2). Then, ferroniobium was added. After refining, the niobium content was 0.053% (Example 1) and 0.073% (Example 2), and the Nb / N ratio was 7.26 (Example 1) and 6.76 (Example 1). The surface of the refined molten steel was immediately covered with a nitrogen-free covering agent, and then the molten steel was subjected to vacuum degassing in an RH furnace. Inert argon was used as the lifting gas during the RH furnace vacuum degassing process. After vacuum degassing, FeB was added to adjust the boron content of the molten steel to 0.0019% (Example 1) and 0.0023% (Example 2). The continuous casting process employed full-process protective casting treatment, and nitrogen-free refractory materials were used in the continuous casting production process. The nitrogen content of the steel remained unchanged after casting; the nitrogen content in Example 1 was 0.0073%, and in Example 2 it was 0.0108%. This ensured that the Nb / N ratio in the molten steel was ≥6.7. Elemental boron in the steel was not nitrided. The cast billets (240mm*240mm) were rolled into round bars; in Example 1, they were rolled into Φ45mm, and in Example 2, into Φ60mm.

[0036] The smelting composition of the steels obtained in Examples 1 and 2 is shown in Table 1.

[0037] Table 1 Smelting composition (wt%)

[0038]

[0039] The high-temperature grain size of the steels obtained in Examples 1 and 2 is shown in Table 2.

[0040] Table 2

[0041] High-temperature grain size (water quenching after holding at 970℃ for 5 hours) Example 1 Level 7.5 Example 2 Level 9.0

[0042] The steels obtained in Examples 1 and 2, after oil quenching at 870°C and tempering at 200°C, showed a hardness of 32.3 HRC (Example 1) and 34.9 HRC (Example 2) at a position 11 mm from the quenched end face. The martensite content at this position was 50% (Example 1) and 60% (Example 2), with the remainder being bainite. A photograph of the microstructure at this position is shown below. Figure 1 (Example 1) and Figure 2 (Example 2)

[0043] The steels obtained in Examples 1 and 2, after being oil quenched at 870°C and tempered at 200°C, had tensile strengths of 1159 MPa (Example 1) and 1292 MPa (Example 2), respectively.

[0044] This invention relates to gear steel for range-extended electric vehicle reducers. The steel composition is low in carbon and sulfur, with added molybdenum and trace amounts of niobium and nitrogen to ensure an Nb / N ratio ≥ 6.7. This narrower chemical composition design ensures a narrower range of grain size grades, resulting in more stable hardness, microstructure, and tensile strength after quenching. This invention employs KR mechanical bidirectional stirring to achieve rapid, efficient, and deep desulfurization; the addition of molybdenum-containing scrap steel and calcium molybdate to the converter increases the molybdenum yield and reduces production costs; MnN wire is fed in the early stage of refining to adjust the nitrogen content, followed by the addition of ferroniobium to ensure Nb / N ≥ 6.7; FeB is added after RH vacuum degassing to adjust the boron content in the molten steel; the surface of the refined molten steel is immediately covered with a nitrogen-free covering agent; argon gas is used as the lifting gas in the RH vacuum degassing process; full-process protective casting treatment is adopted in the continuous casting process; and nitrogen-free refractory materials are used in the continuous casting production process. These measures prevent nitrogen addition in the molten steel from refining to continuous casting, ensuring that the Nb / N ratio in the molten steel is ≥ 6.7 and that elemental boron in the molten steel is not nitrided.

Claims

1. A type of steel for gears in a range-extended electric vehicle reducer, characterized in that: The chemical composition of the steel, by weight percentage, is: C: 0.19–0.22%, Si: 0.20–0.30%, Mn: 0.40–0.55%, Cr: 0.60–0.80%, P: ≤0.018%, S: ≤0.003%, Mo: 0.23–0.28%, Nb: 0.047–0.074%, N: 0.0070–0.0110%, B: 0.0019–0.003%. 0.0023%, Nb / N≥6.7, balance is Fe and unavoidable impurities; after oil quenching at 870℃ and tempering at 200℃, the hardness at a position 11mm from the quenched end face is 32HRC-35HRC, the microstructure is martensite + bainite, and the martensite content is 50%-60%; after oil quenching at 870℃ and tempering at 200℃, the tensile strength of the steel can be stably controlled at 1150MPa-1300MPa.

2. The steel for the gears of the range-extended electric vehicle reducer according to claim 1, characterized in that: The high-temperature grain size of steel is fine; after holding at 970℃ for 5 hours, the grain size of water-quenched steel is grade 7.0-9.

0.

3. A method for manufacturing steel for the gears of the range-extended electric vehicle reducer as described in claim 1, characterized in that: Production process: KR mechanical bidirectional stirring molten iron pretreatment → converter smelting → refining → vacuum degassing treatment → continuous casting of square billets → rolling into round bars. Specific control methods include:

1. The KR mechanical bidirectional stirring method is used for desulfurization of molten iron. Two coaxial, concentric, cross-shaped stirring heads of different sizes, made of refractory material, are inserted below the surface of the molten iron ladle. The two cross-shaped stirring heads are rotated at high speed in opposite directions, coaxially and concentrically. The bidirectional mechanical stirring makes the desulfurizing agent particles more uniform, dispersed and refined in the molten iron, achieving deep desulfurization. The sulfur content of the molten iron after desulfurization is stably controlled at ≤0.002%, and the desulfurization time is 10-15 minutes. After the desulfurization treatment, the desulfurization slag floating on the surface of the molten iron is completely removed.

2. In the converter smelting stage, the desulfurized molten iron from step one, molybdenum-containing scrap steel, and calcium molybdate are used as smelting raw materials. The molybdenum content of the molten steel after converter smelting is controlled at 0.23-0.28%.

3. In the early stage of refining, MnN wire is first fed in to adjust the N content in the molten steel to 0.0070-0.0110%. Then, according to the nitrogen content in the molten steel, ferroniobium is added to the molten steel so that the Nb / N ratio in the molten steel is ≥6.7 after refining. This ensures that all the nitrogen elements in the molten steel combine with the niobium elements to form niobium nitride. The refined molten steel is then vacuum degassed. After vacuum degassed, FeB is added to the molten steel to adjust the boron content in the molten steel to 0.0019-0.0023%. Because the nitrogen elements in the steel have completely combined with the niobium elements during the smelting process to form niobium nitride, the boron elements added after the vacuum degassed treatment are all in elemental form. IV. After vacuum degassing, the molten steel is transferred to the continuous casting process. The continuous casting process adopts full-process protective casting treatment, and nitrogen-free refractory materials are used in the continuous casting production process.

4. The method for manufacturing steel for the reducer gear of a range-extended electric vehicle according to claim 3, characterized in that: The surface of the refined molten steel is immediately covered with a nitrogen-free covering agent.

5. The method for manufacturing steel for the reducer gear of a range-extended electric vehicle according to claim 3, characterized in that: In step three, the vacuum degassing process uses inert argon gas as the booster gas.

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