A high-strength steel for reducer gears of new energy vehicles and a manufacturing method thereof

Abstract

The present invention relates to a high-strength steel for a reducer gear of a new energy vehicle and a manufacturing method thereof. The chemical composition mass percentage of the gear steel is C: 0.13-0.18%, Si: ≤0.15%, Mn: 1.10-1.35%, Cr: 1.20-1.45%, P: ≤0.012%, S: ≤0.003%, Ni: 0.15-0.35%, Mo: 0.05-0.20%, Al: 0.015-0.050%, Nb: 0.015-0.050%, N: 0.0085-0.0225%, B: 0.0015-0.0035%, and the balance is Fe and unavoidable impurities. After the process steps of primary refining, refining, vacuum degassing, continuous casting, hot conveying, rolling and isothermal heat treatment, a high-strength steel for reducer gears of new energy vehicles is manufactured. The banded structure of the steel is ≤1.5, the hardness of the steel is ≤160HBW, the austenite grain size of the steel is ≥8, there is no crack defect on the surface of the steel, and the tensile strength of the steel after 870℃ oil quenching + 200℃ tempering is ≥1180MPa.

Description

Technical Field

[0001] The present invention belongs to the technical field of special steel smelting, and specifically relates to a high-strength steel for reducer gears of new energy vehicles and a manufacturing method thereof. Background Art

[0002] The speed of the gears of the new energy vehicle reducer is very high during the transmission process. Some high-quality new energy vehicle reducer gears can even reach more than 10,000r / min. The gears are constantly subjected to large external forces during high-speed operation. The gears of the new energy vehicle reducer must have high strength, good organizational uniformity, and good assembly accuracy to ensure that the new energy vehicle reducer has a long fatigue life. The corresponding steel must have excellent banded structure, low hardness, no crack defects on the surface to facilitate cold forging of steel, fine austenite grains and high strength.

[0003] Patent No. CN112981271A discloses a method for manufacturing steel for electric vehicle reducer gears, which belongs to the field of iron-based alloy technology, and in particular to a gear steel. The element composition design involved is: by mass percentage, C: 0.20-0.35%, Si: 0.05-0.35%, Mn: 1.20-1.60%, Cr: 1.10-1.50%, Mo: 0.20-0.50%, S: ≤0.035%, P: ≤0.015%, Cu: ≤0.20%, Ni: ≤0.20%, Al: 0.010-0.080%, N: 0.0060-0.0260%, Al / N: 1.5-3.9, and the balance is Fe and unavoidable impurities. Smelting process: pre-treated molten iron and selected scrap steel → primary smelting in electric furnace → refining in LF furnace → vacuum degassing in VD furnace → continuous casting billet → continuous casting billet shot blasting → heating in walking beam furnace → high-pressure water descaling → continuous casting billet rolling → hot-rolled round steel surface flaw detection + internal ultrasonic flaw detection - quality inspection - storage. The element composition combined with the smelting process meets the requirements of high strength and high fatigue life for electric vehicle reducer steel, and at the same time has gained significant advantages in hardenability and grain uniformity. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a high-strength steel for reducer gears of new energy vehicles and a manufacturing method thereof in view of the above-mentioned prior art, so as to ensure that the steel has fine austenite grains and high tensile strength.

[0005] The technical solution adopted by the present invention to solve the above-mentioned problem is: a high-strength steel for reducer gears of new energy vehicles, wherein the chemical composition of the gear steel is C: 0.13-0.18%, Si: ≤0.15%, Mn: 1.10-1.35%, Cr: 1.20-1.45%, P: ≤0.012%, S: ≤0.003%, Ni: 0.15-0.35%, Mo: 0.05-0.20%, Al: 0.015-0.050%, Nb: 0.015-0.050%, N: 0.0085-0.0225%, B: 0.0015-0.0035%, and the balance is Fe and unavoidable impurities.

[0006] The banded structure of the steel is ≤ grade 1.5, the hardness of the steel is ≤ 160 HBW, the austenite grain size of the steel is ≥ grade 8, there are no crack defects on the surface of the steel, and the tensile strength of the steel after 870°C oil quenching + 200°C tempering is ≥ 1180 MPa.

[0007] The main functions and design basis of each chemical element of the steel of the present invention are:

[0008] C: Carbon is the most important element that affects the strength of steel. The higher the carbon content, the higher the tensile strength of the steel; the hardness of the steel will also increase accordingly. In order to ensure that the steel of the present invention has a lower hardness, the present invention adopts a low-carbon design, and the carbon content selection range is 0.13-0.18%.

[0009] Si: The steel of the present invention is used for cold forging. To prevent the cold work hardening phenomenon during the cold forging process, the silicon content in the steel needs to be strictly controlled. The selection range of the silicon content of the present invention is Si≤0.15%.

[0010] Mn: Manganese is a carbide-forming element. It enters cementite to replace a portion of iron atoms. Manganese in steel will reduce the critical transformation temperature, refine pearlite, and indirectly improve the strength of pearlite. The Mn content in the present invention is selected in the range of 1.10-1.35%.

[0011] Cr: Chromium can increase the hardenability and strength of steel, but the hardness of the steel will also increase. The Cr content in the present invention is selected in the range of 1.20-1.45%.

[0012] P: The present invention belongs to cold forging steel. Phosphorus can significantly reduce the tensile strength of steel, especially at low temperatures. The selection range of phosphorus content in the present invention is P≤0.012%.

[0013] S: Sulfur exists in the steel of the present invention in the form of iron sulfide (FeS). FeS and Fe form a low melting point (985°C) compound. The premature melting of FeS compounds during the continuous casting and straightening process and the rolling process will cause cracks on the steel surface. The selection range of the S content of the present invention is S≤0.003%.

[0014] Ni: Nickel can improve the tensile strength of steel. However, nickel is a precious metal. The content of Ni in the present invention is selected within a range of 0.15 to 0.35%.

[0015] Mo: Molybdenum can effectively improve the tensile strength of steel. However, molybdenum is a precious metal. The Mo content of the present invention is selected within a range of 0.05-0.20%.

[0016] Al, N, Nb: Aluminum combines with oxygen in steel to deoxidize, AlN formed by combining with nitrogen can refine the grain size of steel, and NbN formed by combining with nitrogen also refines the grain size of steel. The content of Al in the present invention is selected in the range of 0.015-0.050%, the content of N in the range of 0.0085-0.0225%, and the content of Nb in the range of 0.015-0.050%.

[0017] B: Boron in the present invention mainly serves to increase the hardenability of steel and improve the tensile strength of steel, thereby saving other more expensive metals. The content of B in the present invention is selected within the range of B: 0.0015-0.0035%.

[0018] The above-mentioned high-strength steel for reducer gears of new energy vehicles and the manufacturing method thereof include the following process steps:

[0019] The molten steel smelted by the primary refining-refining-vacuum degassing process is hoisted to the continuous casting platform for casting. The continuous casting crystallizer is cooled by strong cooling to form a high-strength chill layer with a thickness of more than 10mm on the surface of the ingot. The chill layer is composed of fine equiaxed crystal bands with different orientations. The high-strength fine equiaxed crystal bands ensure that the continuous casting ingot will not produce surface crack defects during the subsequent straightening process. In order to prevent excessive stress on the surface of the continuous casting ingot during the cooling process, the continuous casting ingot adopts a hot delivery process with a hot delivery temperature of ≥600℃.

[0020] The continuous casting billet with no crack defects on the surface and low stress is hot sent to the heating furnace and heated to 1180-1280℃. The heated continuous casting billet needs to undergo three stages of rolling deformation, namely rough rolling, intermediate rolling and finishing rolling. The temperature of the rolling deformation process in these three stages should be carried out at a relatively high plastic temperature. The method for determining the high plastic temperature is: the continuous casting billet is heated to different temperatures (the heating temperature range is 750-1100℃), and then the continuous casting billet is forged with the same force. The high plastic temperature is the one that can achieve large deformation after forging. The low stress on the surface of the rolled material ensures that the surface of the billet will not crack during the heating process of the billet; the rolling deformation at a high plastic temperature ensures that no crack defects will occur on the surface of the material during the rolling process.

[0021] The rolled steel is sent to an isothermal furnace for isothermal heat treatment. The isothermal heat treatment is divided into three stages. The first stage is isothermal normalizing treatment. The purpose of this stage is to fully precipitate all the NbC and AlN particles that have not yet been completely precipitated in the rolled material in the shortest time. The method for determining the optimal normalizing holding temperature is to use a scanning electron microscope to observe the precipitation rate of NbC and AlN particles in the high-temperature rolled material at different temperatures in the range of 850-950℃. When NbC and AlN particles have the highest precipitation rate, this temperature is The second stage is rapid cooling, the temperature drop rate of the steel is ≥10℃ / s, and the steel is rapidly cooled to 20℃ below the Ar1 temperature (the Ar1 temperature of the present invention is 705℃). The purpose of this stage is to reduce the precipitation of banded pro-eutectoid ferrite before the eutectoid phase transformation of the steel; the third stage is isothermal annealing treatment, the purpose of this stage is to keep the temperature below the Ar1 temperature for a period of time at 20℃, so that the austenite structure is completely transformed into fine and evenly distributed ferrite and pearlite structure, thereby reducing the banding of the steel. Finally, air cooling is performed, and the hardness of the steel is reduced to below 160HBW.

[0022] The present invention is aimed at high-strength reducer gear steel for new energy vehicles. The composition is designed to be low-carbon, and appropriate amounts of nickel and molybdenum elements are added, and trace amounts of aluminum, nitrogen, and niobium elements are added. Through the subsequent heat treatment process, the AlN and NbN masses are fully precipitated to ensure that the steel has fine austenite grains and high tensile strength. In the continuous casting process of the present invention, the billet is subjected to a strong cooling process in the crystallizer to form a high-strength quenching layer of more than 10 mm on the surface of the billet; the continuous casting billet adopts a hot delivery process to reduce the stress on the surface of the billet; the continuous casting billet is subjected to rough rolling, intermediate rolling, and finish rolling at a high plastic temperature to ensure that the surface of the produced steel is free of crack defects. After rolling, the steel is subjected to three-stage isothermal heat treatment, which has the following functions: ① NbC and AlN particles are fully precipitated at the fastest rate; ② the precipitation of banded pro-eutectoid ferrite before the eutectoid phase transformation of the steel is reduced; ③ the austenite structure is completely transformed into fine and evenly distributed ferrite and pearlite structure, thereby reducing the banding of the steel and reducing the hardness of the steel to below 160HBW. The high-strength reducer gear steel for new energy vehicles finally produced is characterized by: the banded structure of the steel is ≤1.5, the hardness of the steel is ≤160HBW, the austenite grain size of the steel is ≥8, there is no crack defect on the surface of the steel, and the tensile strength of the steel after 870℃ oil quenching + 200℃ tempering is ≥1180MPa.

[0023] Compared with the prior art, the advantages of the present invention are:

[0024] The present invention is aimed at the steel for high-strength reducer gears of new energy vehicles. The composition adopts a low-carbon design, adds appropriate amounts of nickel and molybdenum elements, and adds trace amounts of aluminum, nitrogen, and niobium elements. Through the subsequent heat treatment process, AlN and NbN are fully precipitated to ensure that the steel has fine austenite grains and high tensile strength. In the continuous casting process, a strong cooling process is immediately adopted after the high-temperature ingot is discharged from the crystallizer, so that a high-strength chilling layer of more than 10 mm is formed on the surface of the ingot; the continuous casting ingot adopts a hot delivery process to reduce the stress on the surface of the ingot; the continuous casting ingot is subjected to rough rolling, intermediate rolling, and finish rolling at a high plastic temperature to ensure that the surface of the produced steel is free of crack defects. After rolling, the steel is subjected to three-stage isothermal heat treatment, and the functions of each stage are as follows: ① NbC and AlN particles are fully precipitated at the fastest rate ② a fine and evenly distributed ferrite structure is formed to reduce the ferrite banded structure. ③ The hardness of the steel is reduced to below 160HBW. Through reasonable steelmaking, rolling and isothermal heat treatment processes, a high-strength reducer gear steel for new energy vehicles was successfully manufactured, filling the domestic gap. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 This is the longitudinal banded structure of Example 1 of the present invention (X100 times), and the banded structure grade is 0.5.

[0026] Figure 2 This is the longitudinal banded structure of Example 2 of the present invention (X100 times), and the banded structure grade is 0.5. DETAILED DESCRIPTION

[0027] The technical solution of the present invention is described in more detail in conjunction with the preferred embodiments of the present invention. However, these embodiments are only descriptions of the preferred implementation methods of the present invention and cannot impose any limitation on the scope of the present invention.

[0028] Example 1 and Example 2:

[0029] A method for manufacturing high-strength reducer gear steel for new energy vehicles involved in two embodiments: 100t electric furnace primary refining → 100t refining furnace refining → VD furnace vacuum degassing → continuous casting square billet (300mmX340mm) → continuous casting billet hot delivery → continuous casting billet heating → rough rolling → intermediate rolling → finishing rolling → isothermal furnace isothermal heat treatment. Two batches of high-strength reducer gear steel for new energy vehicles are manufactured.

[0030] The molten steel smelted by the electric furnace-refining-vacuum degassing process is hoisted to the continuous casting platform for casting. After the ingot comes out of the crystallizer, it is strongly cooled. The thickness of the surface chilled layer is 15 mm (Example 1) and 17 mm (Example 2); the hot delivery temperature of the continuous casting ingot is 633°C (Example 1) and 626°C (Example 2).

[0031] The continuous casting billet is hot sent to a heating furnace and heated to 1180-1280° C. The heated continuous casting billet needs to be rolled and deformed into Φ52mm (Example 1) and Φ95mm (Example 2) through three stages of rough rolling, intermediate rolling and finish rolling, with the rough rolling temperature being 1050° C., the intermediate rolling temperature being 1000° C. and the finish rolling temperature being 975° C.

[0032] The rolled steel is fed into an isothermal furnace for isothermal heat treatment. The first stage is isothermal normalizing treatment, the isothermal temperature is 925°C, and the isothermal time is 52min (Example 1) and 95min (Example 2). In the second stage, the steel is cooled by strong wind, the temperature drop rate is 12°C / s (Example 1) and 11°C / s (Example 2), rapidly cooled to 685°C, kept warm for 208min (Example 1) and 380min (Example 2), and finally air-cooled out of the furnace.

[0033] The chemical composition of the steels obtained in Examples 1 and 2 is shown in Table 1;

[0034] Table 1 (wt%)

[0035]

[0036] The steels prepared in Examples 1 and 2 have no crack defects on their surfaces, and the banded structure grade, hardness, and austenite grain size grade of the steels are specifically shown in Table 2;

[0037] Table 2

[0038] Specification Banded tissue Steel hardness Austenite grain size (grade) Example 1 Φ52mm 0.5 147HBW 8.5 Example 2 Φ95mm 0.5 158HBW 9.0

[0039] The steels prepared in Examples 1 and 2 were oil quenched at 870°C and tempered at 200°C. The tensile strength of the steels is shown in Table 3.

[0040] Table 3

[0041] Specification tensile strength Example 1 Φ52mm 1198MPa Example 2 Φ95mm 1287MPa

[0042] Although the preferred embodiments of the present invention are described in detail above, it should be clearly understood that various modifications and variations are possible for those skilled in the art. Any modification, equivalent substitution, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for manufacturing high-strength reducer gear steel for new energy vehicles, characterized in that The chemical composition of the steel is C: 0.13-0.18%, Si: ≤0.15%, Mn: 1.10-1.35%, Cr: 1.20-1.45%, P: ≤0.012%, S: ≤0.003%, Ni: 0.15-0.35%, Mo: 0.05-0.20%, Al: 0.015-0.050%, Nb: 0.015-0.050%, N: 0.0085-0.0225%, B: 0.0015-0.0035%, and the balance is Fe and unavoidable impurities. The main steps of the method are: 1) The molten steel smelted by the primary refining-refining-vacuum degassing process is hoisted to the continuous casting platform for casting; 2) The continuous casting billet with no surface crack defects and low stress is hot fed into the heating furnace and heated to 1180-1280℃. The heated continuous casting billet needs to undergo three stages of rolling deformation: rough rolling, intermediate rolling and finish rolling. 3) The rolled steel is sent to an isothermal furnace for isothermal heat treatment. The isothermal heat treatment is divided into three stages. The first stage is isothermal normalizing treatment. The purpose of this stage is to fully precipitate all the NbC and AlN particles that have not yet been completely precipitated in the rolled material in the shortest time; the second stage is rapid cooling. The purpose of this stage is to reduce the precipitation of strip-shaped pro-eutectoid ferrite before the eutectoid phase transformation of the steel; the third stage is isothermal annealing treatment. The purpose of this stage is to keep the temperature below Ar1 temperature for a period of time at 20°C to allow the austenite structure to be completely transformed into fine and evenly distributed ferrite and pearlite structure, thereby reducing the banding of the steel. Finally, air cooling is performed to reduce the hardness of the steel to below 160HBW; The method for determining the best normalizing holding temperature during isothermal normalizing treatment is to use a scanning electron microscope to observe the precipitation rate of NbC and AlN particles inside the high-temperature rolled material at different temperatures in the range of 850-950℃. When the NbC and AlN particles have the highest precipitation rate, this temperature is the best normalizing temperature. The temperature drop rate of the steel in the rapid cooling stage is ≥10℃ / s, and the steel is rapidly cooled to 20℃ below the Ar1 temperature, and the Ar1 temperature is 705℃.

2. The method for manufacturing high-strength reducer gear steel for new energy vehicles according to claim 1, characterized in that The banded structure of the gear steel is ≤ grade 1.5, the hardness of the steel is ≤160HBW, the austenite grain size of the steel is ≥grade 8, there is no crack defect on the surface of the steel, and the tensile strength of the steel after 870°C oil quenching + 200°C tempering is ≥1180MPa.

3. The method for manufacturing high-strength reducer gear steel for new energy vehicles according to claim 1, characterized in that The continuous casting crystallizer is cooled by strong cooling, so that a high-strength chill layer with a thickness of more than 10 mm is formed on the surface of the ingot. The chill layer is composed of fine equiaxed crystal bands with different orientations. The high-strength fine equiaxed crystal bands ensure that the continuous casting ingot will not produce surface crack defects during the subsequent straightening process.

4. According to claim 3, a method for manufacturing high-strength reducer gear steel for new energy vehicles, characterized in that In order to prevent excessive stress on the surface of the continuous casting billet during the cooling process, the continuous casting billet adopts a hot delivery process with a hot delivery temperature ≥ 600℃.

5. The method for manufacturing high-strength reducer gear steel for new energy vehicles according to claim 1, characterized in that The temperatures of the three-stage rolling deformation process are all carried out at a relatively high plastic temperature. The method for determining the high plastic temperature is: the continuous casting billet is heated to different temperatures, and then the continuous casting billet is forged with the same force. The high plastic temperature is when the continuous casting billet can achieve large deformation after forging.

6. The method for manufacturing high-strength reducer gear steel for new energy vehicles according to claim 5, characterized in that The continuous casting slab is heated to a temperature of 750-1100°C.

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