A method for producing an anti-grain-coarsening cold extrusion carburizing gear steel

Abstract

This invention discloses a method for producing cold-extruded carburized gear steel with anti-grain coarsening properties, belonging to the field of alloy steel manufacturing and heat treatment technology. The gear steel has the following chemical composition by mass percentage: C 0.17%~0.23%, Si 0.15%~0.40%, Mn 0.8%~1.50%, P ≤0.025%, S ≤0.035%, Cr 0.80%~1.50%, Nb 0.030%~0.080%, Al 0.020%~0.060%, Ti ≤0.005%, B ≤0.0010%, N 0.010%~0.025%, with the balance being Fe and unavoidable impurities. The production process includes converter smelting, LF refining, RH vacuum treatment, continuous casting, billet heating, and rolling. This method optimizes alloy composition and process control, making it less likely for the austenite grains of steel to coarsen after high-temperature carburizing. After carburizing, the steel can be directly quenched without normalizing treatment, resulting in a fine-grained martensite structure. This effectively simplifies the gear production process, reduces energy consumption, and minimizes deformation and oxidation decarburization, while ensuring that the gears have high surface hardness, high wear resistance, and excellent core toughness.

Description

Technical Field

[0001] This invention belongs to the field of alloy steel manufacturing and heat treatment technology, and relates to a production method of cold extrusion carburizing gear steel with anti-grain coarsening properties. 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 cold-extruded gear manufacturing process typically includes blanking → spheroidizing annealing → surface treatment → cold extrusion forming → carburizing → normalizing → quenching → tempering, using 20CrMnTiH material. Because spheroidizing annealing and cold extrusion can cause abnormal grain growth during carburizing, normalizing is necessary after carburizing to eliminate the coarse grains formed during the process. However, this normalizing step increases production time, energy consumption, and manufacturing costs, and may also cause part deformation and oxidation decarburization.

[0003] Therefore, developing a new type of cold-extruded gear steel that does not coarsen after high-temperature carburizing, does not require normalizing, and can be directly quenched to obtain excellent comprehensive mechanical properties would be of great significance for simplifying the process, reducing production costs, and improving product competitiveness. Summary of the Invention

[0004] 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.

[0005] The technical solution of the present invention: A method for producing cold-extruded carburized gear steel with anti-grain coarsening properties, the process flow includes converter smelting → LF furnace refining → VD / RH vacuum treatment → continuous casting → billet heating → rolling. The chemical composition of the steel is C=0.17%~0.23%, Si=0.15%~40%, Mn=0.8%~1.50%, P≤0.025%, S≤0.035%, Cr=0.80%~1.50%, Nb=0.030%~0.080%, Al=0.020%~0.060%, Ti≤0.005%, B≤0.0010%, N=0.010%~0.025%, with the remainder being Fe and unavoidable impurity elements; the key process steps include: 1) Converter smelting: The final C value of the converter is 0.05%~0.10%, the tapping temperature is 1600~1640℃, 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 1570~1600℃, and the refining time is 40~70 minutes; nitrogen enrichment is adopted using a nitrogen line; 3) Continuous casting: Full-process protective casting, using a special protective slag for low-C steel; tundish superheat ≤40℃. Crystallizer electric stirring current 150A, frequency 3Hz; end-stage electric stirring current 200A, frequency 2.5Hz; billet cross-sectional area not less than 30000 mm². 2 ; 4) Heating and rolling: The temperature of the high-temperature section is 1210~1250℃, and the heating time is not less than t minutes, where t = billet thickness mm × 0.7 min / mm; the billet exit temperature is ≥1150℃, the final rolling temperature is ≥920℃, the temperature on the cooling bed is ≥750℃, and it is slowly cooled on the cooling bed with a cooling rate ≤1℃ / s.

[0006] The obtained samples were held at 950±10℃ for 5 hours, and the austenite grain size was tested by direct hardening method to be no less than grade 7; the banded structure grade of round steel was ≤2.0; the end hardenability test samples were heat treated with a normalizing temperature of 900±20℃ and a quenching temperature of 930±15℃, J9=30.0~42.0J, J15=25.0~35.0J, and the hardness test values ​​at the same distance from the end were not greater than 4HRC.

[0007] The round steel was processed into cold-extruded gears, which were then carburized at 900℃~930℃. The original austenite grain size after carburizing was ≥8. After carburizing, the workpiece was directly and slowly cooled to 830℃~870℃ for quenching. After quenching, it was subjected to low-temperature tempering at 160℃~200℃. The resulting gears met the performance requirements.

[0008] The beneficial effects of this invention are as follows: Through rational composition and process design, the nanoscale precipitates of AlN and Nb (C, N) are fully utilized to pin the austenite grain boundaries, improving the austenite's resistance to coarsening during carburizing. This ensures that the cold-extruded gears do not coarsen after carburizing, eliminating the need for additional fine-grain heat treatment, thus improving production efficiency, reducing energy consumption, and minimizing 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 gear production process is simplified, energy consumption is reduced, and significant economic and environmental benefits are achieved. Attached Figure Description

[0009] Figure 1 Photograph of the grain size of the round steel produced in Example 1.

[0010] Figure 2The metallographic structure of the carburized layer of the gear produced in Example 1.

[0011] Figure 3 The grain size of the carburized layer of the gear produced in Example 1. Detailed Implementation Example 1

[0012] A method for producing cold-extruded carburized gear steel with anti-grain coarsening properties, the process flow includes converter smelting → LF furnace refining → RH vacuum treatment → continuous casting → billet heating → rolling, the chemical composition by mass percentage is C=0.19%, Si=0.25%, Mn=1.00%, P=0.011%, S=0.015%, Cr=1.12%, Nb=0.046%, Al=0.035%, Ti=0.003%, B=0.0004%, N=0.012%, 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 0.09%, P is 0.010%, the tapping temperature is 1620℃, top slag is added during the tapping process, and argon is blown throughout the furnace process; no elements such as Ti, B, Ni, Mo, etc. that increase hardenability are added during the converter and refining smelting process. 2) LF Refining: The refining outlet temperature is 1594℃, and the refining time is 51 minutes; 3) Continuous casting: Full-process protective casting, no slag removal in the ladle, using special protective slag for low-C steel; tundish superheat 26~28℃. Crystallizer electric agitation current 150A, frequency 3Hz; end-point electric agitation current 200A, frequency 2.5Hz. Billet cross-sectional area 57600mm². 2 ; 4) Heating and rolling: The high temperature section temperature is 1210~1220℃, and the heating time is 210 minutes; the billet exit temperature is 1165℃, the final rolling temperature is 943-956℃, the upper cooling bed temperature is 761~775℃, and it is slowly cooled on the cooling bed at a cooling rate of 0.5℃ / s.

[0013] The obtained samples were held at 950±10℃ for 6 hours. The austenite grain size was grade 8 by direct hardening method; the banded structure of the round steel was grade 1.5; the end hardenability sample was normalized at 900±20℃ and quenched at 930±15℃, with J9=34.2J and J15=27.6J. The hardness test values ​​at the same distance from the end were not greater than 2HRC.

[0014] The round steel was processed into cold-extruded gears, which were then carburized at 910℃, resulting in a pre-austenite grain size of grade 9. After carburizing, the workpiece was directly and slowly cooled to 860℃ for quenching. After quenching, it was subjected to low-temperature tempering at 180℃, and the resulting gears met the performance requirements.

[0015] Example 2 A method for producing cold-extruded carburized gear steel with anti-grain coarsening properties, the process flow includes converter smelting → LF furnace refining → RH vacuum treatment → continuous casting → billet heating → rolling, the chemical composition by mass percentage is C=0.20%, Si=0.20%, Mn=1.08%, P=0.012%, S=0.012%, Cr=1.19%, Nb=0.055%, Al=0.030%, Ti=0.002%, B=0.0004%, N=0.013%, 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 0.085%, P is 0.011%, the tapping temperature is 1621℃, top slag is added during the tapping process, and argon is blown throughout the furnace process; no elements such as Ti, B, Ni, Mo, etc. that increase hardenability are added during the converter and refining smelting process. 2) LF Refining: The refining outlet temperature is 1589℃, and the refining time is 46 minutes; 3) Continuous casting: Full-process protective casting, no slag removal from the ladle, using special protective slag for low-C steel; tundish superheat 30-34℃. Crystallizer electric agitation current 150A, frequency 3Hz; end-point electric agitation current 200A, frequency 2.5Hz. Billet cross-sectional area 78400mm². 2 ; 4) Heating and rolling: The high temperature section temperature is 1220~1235℃, and the heating time is 230 minutes; the billet exit temperature is 1203℃, the final rolling temperature is 977~995℃, the upper cooling bed temperature is 795~801℃, and it is slowly cooled on the cooling bed at a cooling rate of 0.3℃ / s.

[0016] The obtained samples were held at 950±10℃ for 6 hours. The austenite grain size was grade 8 by direct hardening method; the banded structure of the round steel was grade 1.5; the end hardenability sample was normalized at 900±20℃ and quenched at 930±15℃, with J9=35.1J and J15=28.6J. The hardness test values ​​at the same distance from the end were not greater than 2HRC.

[0017] The round steel was processed into cold-extruded gears, which were then carburized at 910℃, resulting in a pre-austenite grain size of grade 9. After carburizing, the workpiece was directly and slowly cooled to 860℃ for quenching. After quenching, it was subjected to low-temperature tempering at 180℃, and the resulting gears met the performance requirements.

[0018] Comparative Example 1: The existing production method for cold-extruded carburized gear steel includes a process flow of converter smelting → LF furnace refining → RH vacuum treatment → continuous casting → billet heating → rolling. The chemical composition (mass percentage) is C=0.17%-0.23%, Si=0.17%-0.37%, Mn=0.85%-1.20%, P≤0.030%, S≤0.035%, Cr=1.00%~1.45%, Nb≤0.005%, Al=0.010%~0.050%, Ti=0.04%-0.10%, B≤0.0008%, N=0.010%~0.025%, with the remainder being Fe and unavoidable impurities. The desired round steel is obtained through converter smelting, refining, continuous casting, and rolling. The obtained samples were kept at 950±10℃ for 6 hours, and the austenite grain size was tested by direct hardening method to be ≥5 grade; the banded structure of round steel was ≤3 grade; the end hardenability of the sample was tested by heat treatment at normalizing temperature of 900±20℃ and quenching temperature of 930±15℃, J9=30-42J, J15=22-35J.

[0019] The round steel was processed into cold-extruded gears, which were then carburized at 910℃. After carburizing, grade 0 austenite grains were present. After carburizing, the gears were normalized, then heated to 860℃ for quenching, and finally tempered at 180℃ to achieve the required performance. Furthermore, the carburized gears exhibited significant deformation and required extensive grinding.

Claims

1. A production method of an anti-grain-coarsening cold extrusion carburizing gear steel, the process flow comprising converter smelting → LF furnace refining → VD / RH vacuum treatment → continuous casting → billet heating → rolling, characterized in that: The chemical composition (mass percentage) is C = 0.17%~0.23%, Si = 0.15%~40%, Mn = 0.8%~1.50%, P ≤ 0.025%, S ≤ 0.035%, Cr = 0.80%~1.50%, Nb = 0.030%~0.080%, Al = 0.020%~0.060%, Ti ≤ 0.005%, B ≤ 0.0010%, N = 0.010%~0.025%, with the remainder being Fe and unavoidable impurity elements; the key process steps are as follows: 1) Converter smelting: The final C value of the converter is 0.05%~0.10%, the tapping temperature is 1600~1640℃, 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 1570~1600℃, and the refining time is 40~70 minutes; nitrogen enrichment is adopted using a nitrogen line; 3) continuous casting: full protection pouring, using low C steel special protection slag; tundish liquid superheat ≤ 40℃. crystallizer electric stirring current 150 A, frequency 3 Hz, end electric stirring current 200 A, frequency 2.5 Hz; casting blank cross section area not less than 30000 mm 2 ; 4) Heating and rolling: The temperature of the high-temperature section is 1210~1250℃, and the heating time is not less than t minutes, where t = billet thickness mm × 0.7 min / mm; the billet exit temperature is ≥1150℃, the final rolling temperature is ≥920℃, the temperature on the cooling bed is ≥750℃, and it is slowly cooled on the cooling bed with a cooling rate ≤1℃ / s.

2. The method for producing cold-extruded carburized gear steel with anti-grain coarsening according to claim 1, characterized in that: The obtained samples were held at 950±10℃ for 5 hours, and the austenite grain size was tested by direct hardening method to be no less than grade 7; the banded structure grade of round steel was ≤2.0; the end hardenability test samples were heat treated with a normalizing temperature of 900±20℃ and a quenching temperature of 930±15℃, J9=30.0~42.0J, J15=25.0~35.0J, and the hardness test values ​​at the same distance from the end were not greater than 4HRC.

3. The method for producing cold-extruded carburized gear steel with anti-grain coarsening according to claim 1, characterized in that: The round steel is processed into cold-extruded gears and then carburized at 900~930℃. The original austenite grain size after carburizing is ≥8. After carburizing, the workpiece is directly and slowly cooled to 830~870℃ for quenching. After quenching, the gears are subjected to low-temperature tempering at 160~200℃, and the resulting gears meet the performance requirements.

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