Production method of high-strength and high-toughness gear steel without normalizing treatment

Abstract

The application discloses a production method of high-strength and high-toughness gear steel without normalizing treatment. The gear steel adopts a Cr-Mn based hardenability system and is micro-alloyed by V, Nb and Al. The production process comprises smelting, continuous casting, controlled rolling and controlled cooling, and a key heat treatment process of directly slow cooling to quenching temperature after high-temperature carburizing. The V, Nb and Al carbonitrides pin the grain boundaries at high temperature, and inhibit the growth of austenite grains during the carburizing process. The application enables the gear steel to be directly quenched without normalizing treatment after high-temperature carburizing, and fine crystal structure and high comprehensive performance are obtained, so that the production process is greatly simplified, and the cost and energy consumption are reduced.

Description

Technical Field

[0001] This invention belongs to the field of alloy steel manufacturing and heat treatment technology, and relates to a production method for high-strength and high-toughness gear steel that can be directly quenched without normalizing after high-temperature carburizing. Background Technology

[0002] Carburized gear steel is a key material for manufacturing high-load gears in automobiles, wind power equipment, and construction machinery. The traditional gear manufacturing process typically includes blanking → forging → machining → carburizing → normalizing → quenching → tempering. The purpose of normalizing after carburizing is twofold: first, to eliminate coarse grains and network carbides formed during carburizing; and second, to homogenize and refine the austenite, preparing the microstructure for subsequent quenching. However, this normalizing process increases production time, energy consumption, and manufacturing costs, and may also cause part deformation and oxidation decarburization.

[0003] With the application of high-temperature carburizing technology in the temperature range of 980~1050℃, the carburizing efficiency has been significantly improved. However, the increase in carburizing temperature has intensified the growth of austenite grains, making normalizing treatment after carburizing almost necessary. Otherwise, a uniform and fine martensite structure cannot be obtained after quenching, which seriously affects the fatigue strength and toughness of the gears.

[0004] Therefore, developing a new type of gear steel that can be directly quenched after high-temperature carburizing without normalizing and obtain excellent comprehensive mechanical properties is of great significance for simplifying the process, reducing production costs, and improving product competitiveness. Summary of the Invention

[0005] The purpose of this invention is to provide a production method for high-strength, high-toughness gear steel that does not require normalizing after high-temperature carburizing. This gear steel, through optimized alloy composition design, allows for the formation of a fine-grained martensitic structure through direct quenching after high-temperature carburizing, even without normalizing. This avoids the precipitation of network carbides, thereby simplifying the heat treatment process, reducing energy consumption, and ensuring that the gear products possess high surface hardness, high wear resistance, and excellent core toughness.

[0006] The technical solution of the invention: A method for producing high-strength, high-toughness gear steel without normalizing treatment, comprising the following process flow: converter smelting → LF furnace refining → RH vacuum treatment → continuous casting → billet heating → rolling, characterized in that: the chemical composition by mass percentage is C=0.15%~0.20%, Si=0.10~25%, Mn=1.30%~1.70%, P≤0.025%, S=0.010%~0.035%, Cr=1.30%~1.70%, Nb=0.010%~0.050%, Al=0.020%~0.050%, Ti≤0.005%, B≤0.0010%, V=0.01~0.05%, N=0.015%~0.025%, with the remainder being Fe and unavoidable impurity elements; including the following key process steps: 1) Converter smelting: The final C of the converter is 0.05%~0.10%, P≤0.012%, the tapping temperature is 1610~1640℃, top slag is added during the tapping process, and argon is blown throughout the furnace. Avoid adding elements that increase hardenability such as Ti, B, Ni, and Mo during the converter and refining smelting process.

[0007] 2) LF refining: The refining outlet temperature is 1640~1650℃, and the refining time is 50~70 minutes.

[0008] 3) Continuous casting: Full-process protective casting, using special protective slag for low-C steel; ladle steel retention ≥ 2 tons; tundish steel superheat 25~35℃. Crystallizer electric stirring current 180A, frequency 5Hz; end electric stirring current 200A, frequency 5Hz; billet cross-sectional area not less than 72000mm². 2。

[0009] 4) Heating and rolling: The temperature of the high-temperature section is 1230~1260℃, and the heating time is not less than h minutes, where h = billet thickness mm × 1min / mm; the billet exit temperature is ≥1180℃, the final rolling temperature is 950~1000℃, the upper cooling bed temperature is 700~750℃, the steel is laid with teeth on the cooling bed, and after leaving the cooling bed, it is placed in the heat preservation pit for more than 12 hours.

[0010] The samples obtained by the above method were kept at 1000±10℃ for 5 hours. The austenite grain size was tested by direct hardening method and was not less than grade 7, with the maximum grain size not exceeding 40um. The banded structure grade of the round steel was ≤2.0. The end hardenability test sample was normalized at 950±20℃ and quenched at 930±15℃. J9=38.0~44.0J, J15=32.0~38.0J, and the hardness test value range at the same distance from the end was not greater than 4HRC.

[0011] The machined gear workpiece is gas carburized at 980℃~1020℃. After carburizing, the workpiece is directly and slowly cooled to a quenching temperature of 830℃~870℃ (no need for furnace normalizing), held at this temperature for 0.5~1.5 hours, and then oil quenched or high-pressure gas quenched. After quenching, it is tempered at 160℃~200℃ for 2~4 hours to relieve stress and stabilize the microstructure.

[0012] The key points of this invention are as follows: Firstly, the multi-element microalloying design: Cr and Mn improve hardenability and ensure core strength. V, Nb, and Al form stable V(C,N), Nb(C,N), and AlN precipitates, which can strongly pin grain boundaries during high-temperature carburizing, effectively inhibiting austenite grain growth. This is the core of achieving carburizing without normalizing, and even at high carburizing temperatures, the grains can still remain fine (ASTM grade 7 and above). Secondly, the process synergy: by controlling rolling and cooling, a good original microstructure is prepared for subsequent processing. More importantly, by precisely controlling the cooling path after carburizing, the material is directly and slowly cooled to the optimal quenching temperature range and held at that temperature, avoiding the precipitation of harmful carbides and further grain growth, thus homogenizing the austenite composition and making perfect preparations for direct quenching, thereby eliminating the need for a normalizing process.

[0013] The beneficial effects of this invention are: eliminating the need for reheating and normalizing after carburizing, shortening the production cycle by more than 30%, reducing energy consumption by more than 25%, and reducing part deformation and oxidation. The composition design ensures the structural stability of the steel under high-temperature carburizing conditions and broadens the process window. The simplified process, reduced energy consumption, and significant economic and environmental benefits are all evident. Attached Figure Description

[0014] Figure 1 Photograph of the grain size of the round steel produced in Example 1.

[0015] Figure 2 The image shows a SEM image of the round steel produced in Example 1.

[0016] Figure 3 Photograph of the grain size of the carburized layer of the gear made from round steel produced in Example 1.

[0017] Figure 4 Photograph of the core grain size of a gear made from round steel produced in Example 1 after carburizing. Detailed Implementation Example 1

[0018] A method for producing high-strength, high-toughness gear steel without normalizing treatment, comprising the following process flow: converter smelting → LF furnace refining → RH vacuum treatment → continuous casting → billet heating → rolling, characterized in that: the chemical composition by mass percentage is C=0.18%, Si=0.18%, Mn=1.51%, P=0.008%, S=0.020%, Cr=1.49%, Nb=0.041%, Al=0.031%, Ti=0.003%, B=0.0006%, V=0.03%, N=0.0181%, with the remainder being Fe and unavoidable impurity elements; including the following key process steps: 1) Converter smelting: The final C of the converter is 0.08%, P is 0.009%, the tapping temperature is 1620℃, top slag is added during the tapping process, and argon is blown throughout the furnace. Elements that increase hardenability, such as Ti, B, Ni, and Mo, are avoided during the converter and refining processes.

[0019] 2) LF refining: The refining outlet temperature is 1642℃, and the refining time is 58 minutes.

[0020] 3) Continuous casting: Full-process protective casting is required; slag dumping from the ladle is strictly prohibited; special protective slag for low-C steel is used; 2 tons of steel are retained in the ladle; the superheat of molten steel in the tundish is 26-28℃. The crystallizer electric agitation current is 180A, frequency 5Hz; the end-stage electric agitation current is 200A, frequency 5Hz. The cross-sectional area of ​​the cast billet is 78400mm². 2 .

[0021] 4) Heating and rolling: The high temperature section temperature is 1220~1240℃, and the heating time is 302 minutes; the billet exit temperature is 1189℃, the final rolling temperature is 982~990℃, the upper cooling bed temperature is 721~735℃, the steel is placed on the cooling bed with toothed cloth, and after leaving the cooling bed, it is placed in the heat preservation pit for 16 hours.

[0022] The samples obtained by the above method were kept at 1000±10℃ for 5 hours. The austenite grain size was tested by direct hardening method and was grade 8, with the maximum grain size not exceeding 34 μm. The banded structure of the round steel was grade 1.5. The end hardenability test sample was normalized at 950±20℃ and quenched at 930±15℃. J9=41.0J, J15=35.0J, and the hardness test value range at the same distance from the end was 3HRC.

[0023] The machined gear workpiece was gas carburized at 980℃. After carburizing, the workpiece was directly and slowly cooled to a quenching temperature of 870℃, held at that temperature for 0.5 hours, and then oil quenched. After quenching, it was tempered at 180℃ for 2 hours to relieve stress and stabilize the microstructure. Example 2

[0024] A method for producing high-strength, high-toughness gear steel without normalizing treatment, comprising the following process flow: converter smelting → LF furnace refining → RH vacuum treatment → continuous casting → billet heating → rolling, characterized by the following chemical composition by mass percentage: C=0.18%, Si=0.21%, Mn=1.49%, P=0.009%, S=0.025%, Cr=1.52%, Nb=0.043%, Al=0.029%, Ti=0.002%, B=0.0007%, V=0.03%, N=0.0192%, with the remainder being Fe and unavoidable impurity elements; including the following key process steps: 1) Converter smelting: The final C of the converter is 0.08%, P is 0.010%, the tapping temperature is 1616℃, top slag is added during the tapping process, and argon is blown throughout the furnace process; avoid adding elements that increase hardenability such as Ti, B, Ni, and Mo during the converter and refining smelting process.

[0025] 2) LF refining: The refining outlet temperature is 1648℃, and the refining time is 59 minutes.

[0026] 3) Continuous casting: Full-process protective casting is required; slag dumping from the ladle is strictly prohibited; special protective slag for low-C steel is used; ≥2 tons of steel are retained in the ladle; the superheat of molten steel in the tundish is 32℃. The crystallizer electric agitation current is 180A, frequency 5Hz; the end-stage electric agitation current is 200A, frequency 5Hz. The cross-sectional area of ​​the cast billet is 78400mm². 2 .

[0027] 4) Heating and rolling: The high temperature section temperature is 1225~1232℃, and the heating time is 310 minutes; the billet exit temperature is 1199℃, the final rolling temperature is 971~985℃, the upper cooling bed temperature is 735~741℃, the steel is placed on the cooling bed with toothed cloth, and after leaving the cooling bed, it is placed in the heat preservation pit for 18 hours.

[0028] The samples obtained by the above method were kept at 1000±10℃ for 5 hours. The austenite grain size was tested by direct hardening method and found to be grade 8.5, with the maximum grain size not exceeding 36μm. The banded structure of the round steel was grade 1.0. The end hardenability test sample was heat treated at a normalizing temperature of 950±20℃ and a quenching temperature of 930±15℃. J9=39.0J, J15=36.0J, and the hardness test values ​​at the same distance from the end were not greater than 3HRC.

[0029] The machined gear workpiece was gas carburized at 980℃. After carburizing, the workpiece was directly and slowly cooled to a quenching temperature of 870℃ (no need for furnace normalizing), held at that temperature for 0.5 hours, and then oil quenched. After quenching, it was tempered at 180℃ for 2 hours to relieve stress and stabilize the microstructure.

Claims

1. A method for producing high-strength, high-toughness gear steel without normalizing treatment, the process flow including converter smelting → LF furnace refining → RH vacuum treatment → continuous casting → billet heating → rolling, characterized in that: The steel's chemical composition (mass percentage) is: C = 0.15%~0.20%, Si = 0.15~35%, Mn = 1.10%~1.70%, P ≤ 0.025%, S = 0.010%~0.035%, Cr = 1.10%~1.70%, Nb = 0.010%~0.050%, Al = 0.020%~0.050%, Ti ≤ 0.005%, B ≤ 0.0010%, V = 0.01~0.05%, N = 0.015%~0.025%, with the remainder being Fe and unavoidable impurity elements; it includes the following key process steps: 1) Converter smelting: The final C of the converter is 0.05%~0.10%, P≤0.012%, the tapping temperature is 1610~1640℃, top slag is added during the tapping process, and argon is blown throughout the furnace process; avoid adding elements that increase hardenability such as Ti, B, Ni, and Mo during the converter and refining smelting process; 2) LF refining: The refining outlet temperature is 1640~1650℃, and the refining time is 50~70 minutes; 3) Continuous casting: Full-process protective casting, using special protective slag for low-C steel; ladle steel retention ≥ 2 tons; tundish steel superheat 25~35℃. Crystallizer electric stirring current 180A, frequency 5Hz; end electric stirring current 200A, frequency 5Hz; billet cross-sectional area not less than 72000mm². 2 ; 4) Heating and rolling: The temperature of the high-temperature section is 1230~1260℃, and the heating time is not less than h minutes, where h = billet thickness mm × 1min / mm; the billet exit temperature is ≥1180℃, the final rolling temperature is 950~1000℃, the steel is laid on the cooling bed with toothed cloth, and after leaving the cooling bed, it is placed in the heat preservation pit for more than 12 hours.

2. The production method of high-strength, high-toughness gear steel without normalizing treatment according to claim 1, characterized in that: The obtained samples were held at 1000±10℃ for 5 hours. The austenite grain size was tested using the direct hardening method and was not less than grade 7, with the maximum grain size not exceeding 40μm. The banded structure grade of the round steel was ≤2.

0. The end-hardenability samples were heat-treated with a normalizing temperature of 950±20℃ and a quenching temperature of 930±15℃. The J9 was 38.0~44.0J, and the J15 was 32.0~38.0J. The hardness test values ​​at the same distance from the end were not greater than 4HRC.

3. The production method of high-strength, high-toughness gear steel without normalizing treatment according to claim 1, characterized in that: After machining, the gear workpiece is gas carburized at 1000~1020℃. After carburizing, the workpiece is directly and slowly cooled to a quenching temperature of 830~870℃ and held at that temperature for 0.5~1.5 hours. Then, it is oil quenched or high-pressure gas quenched. After quenching, it is tempered at 160~200℃ for 2~4 hours to eliminate stress and stabilize the structure.

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