Medium carbon steel refined structure heat treatment process

Abstract

The invention discloses a medium carbon steel refined structure heat treatment process, and belongs to the technical field of metal material heat treatment. A medium carbon steel refined structure heat treatment process comprises the steps that S1, shot blasting pretreatment is conducted on the surface of a medium carbon steel workpiece; s2, the workpiece subjected to shot blasting is heated to the austenitizing temperature through a high-frequency induction heating device; s3, the heated workpiece is subjected to heat preservation at the critical zone temperature; s4, the workpiece subjected to heat preservation is put into a quenching medium containing nanoparticles for gradient quenching; and S5, the quenched workpiece is subjected to segmented isothermal annealing treatment. According to the method, the grain size is larger than or equal to 9 grades, the pearlite interlamellar spacing is smaller than or equal to 0.3 micrometer, the impact toughness is larger than or equal to 150 J, and the method is suitable for manufacturing key parts such as automobile parts, aviation structural parts, high-precision gears and bearings.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of metal material heat treatment, and particularly relates to a fine-grained heat treatment process for medium-carbon steel. BACKGROUND

[0002] The medium-carbon steel is suitable for the fields of automobile parts, aviation structural parts, high-precision gears and bearings due to its strength and plasticity; however, the current traditional medium-carbon steel heat treatment process has the following bottlenecks: coarse-grained control, reliance on experience to set holding time, no consideration of the dynamic influence of temperature fluctuation on grain growth, easy to cause coarse austenite grains, grain size ≤ 7, pearlite interlamellar spacing ≥ 0.5 μm; uneven quenching structure, single quenching medium, difficult to accurately control the cooling rate by using water or oil, easy to generate pearlite type structure due to insufficient cooling at high temperature, and easy to cause excessive proliferation of martensite due to rapid cooling at medium temperature, resulting in large internal stress and hardness difference ≥ 50 HB; insufficient process synergy, independent design of shot peening pretreatment and heat treatment process, no formation of the synergistic mechanism of "deformation-induced nucleation-heating grain regulation-quenching structure optimization-annealing stress relief", and difficult to realize deep optimization of the structure and performance; contradiction between environmental protection and efficiency, traditional quenching oil is easy to volatilize and pollute the environment, and the process cycle is long, and the conventional annealing needs more than 10 hours, which cannot meet the green manufacturing and efficient production requirements.

[0003] Therefore, it is urgent to develop a heat treatment process which can realize grain ultra-fining, structure homogenization and controllable quenching stress, and has dynamic model regulation and gradient cooling synergy, so as to break through the bottlenecks of coarse grains and uneven performance in the traditional technology. SUMMARY

[0004] The application provides a fine-grained heat treatment process for medium-carbon steel, which breaks through the structure and performance bottlenecks of the traditional process by shot peening pretreatment, heating to austenitizing temperature by high-frequency induction heating, dynamic holding in the critical zone, gradient quenching in the quenching medium containing nano-particles and segmented isothermal annealing.

[0005] The application scheme is as follows: A composite heat treatment process for fine-grained medium-carbon steel, characterized by comprising the following steps: S1, performing shot peening pretreatment on the surface of the medium-carbon steel workpiece; S2, heating the shot-peened workpiece to the austenitizing temperature by a high-frequency induction heating device; S3, holding the heated workpiece at the critical zone temperature; S4, gradient quenching the workpiece after holding in the quenching medium containing nano-particles; S5, performing segmented isothermal annealing treatment on the quenched workpiece; The medium carbon steel material comprises the following components by mass percentage: 0.25% to 0.6% C, 0.15% to 0.35% Si, 0.45% to 0.8% Mn, with optional addition of 0.02% to 0.05% Nb or V, and the balance being Fe and unavoidable impurities.

[0006] Preferably, in step S1, the shot peening pretreatment uses cast steel shot with a diameter of 0.3 to 0.5 mm, a blasting pressure of 0.3 to 0.5 MPa, and a blasting time of 5 to 10 min, forming a deformation layer of 20 to 50 μm on the workpiece surface, with a residual compressive stress ≥200 MPa and a surface roughness Ra ≤1.6 μm.

[0007] Preferably, in step S2, the heating rate is 10 to 15°C / s, the workpiece surface temperature fluctuation is controlled within ±5°C, the high-frequency induction heating uses a ring coil, the distance between the coil and the workpiece surface is 5 to 10 mm, the power is 80% to 100% of the rated power for the first 30 seconds, and then drops to 60% to 70%.

[0008] Preferably, in step S3, the holding time is determined by calculation using a grain growth kinetics model, specifically based on the following formula: ; In the formula, The target grain size at the end of the insulation process, in μm, must meet the requirements of level 8 or above in the GB / T6394-2017 standard. Initial grain size, in μm, obtained through metallographic analysis; The apparent activation energy for grain growth ranges from 280 kJ / mol to 320 kJ / mol. The ideal gas constant is taken as 8.314 J / (mol·K); The real-time insulation temperature is measured in Kelvin (K) and is fed back in real time by a temperature sensor. The required heat preservation time, in minutes; These are material constants, determined experimentally, with a range of values. to μm n / min; This is a dynamic index, and its value is related to the insulation temperature range: when the insulation temperature is ≥ Ac3, it is taken as... When the temperature is between Ac1 and Ac3, take ; Based on real-time temperature and initial grain size Substitute into the formula to reverse-engineer the target grain size. Required heat preservation time When temperature fluctuations cause When changes occur, adjust dynamically. Value recalculated Ensure that the grain size meets the standard requirements; the final holding time is within the allowable error range of ±5% of the calculated result.

[0009] Preferably, in step S4, the nanoparticles are aluminum oxide with a concentration of 0.1% to 0.3%, which are ultrasonically dispersed for ≥15 min and 0.1% to 0.2% of Span-80 is added as a dispersant. During the quenching process, ultrasonic vibration of 20 to 40 kHz and 13 kW is applied, and the vibration direction is at an angle of 45° to 60° with the axis of the workpiece.

[0010] Preferably, in step S4, the quenching medium is an initial concentration of 20% polyethylene glycol aqueous solution, which is diluted to 15% after 30 seconds of quenching. The cooling rate is controlled at 15 to 20℃ / s in the high-temperature section (800 to 650℃) and 5 to 10℃ / s in the medium-temperature section (650 to 500℃).

[0011] Preferably, the gradient dilution of the quenching medium is automatically controlled by a replenishment system, with a replenishment rate of 5 to 10 L / min and a concentration fluctuation of ≤ ±1% after dilution; wherein the replenishment rate is 8 to 10 L / min in the high-temperature section and 5 to 8 L / min in the medium-temperature section.

[0012] Preferably, in step S5, the segmented isothermal annealing specifically involves: first, rapidly cooling to 650°C at a rate of ≥50°C / min and holding at that temperature for 30 minutes; then, cooling down to 600°C at a rate of 10°C / min and holding isothermally for 60 minutes; finally, cooling with the furnace to below 500°C and then air-cooling. Throughout the process, a protective atmosphere of ≥99.99% pure N2 is introduced to suppress oxidation. The oxygen content in the furnace is ≤10 ppm, and the temperature is controlled in real time by an oxygen sensor, with temperature fluctuations ≤±2°C.

[0013] Preferably, the austenitizing temperature is the Ac3 temperature, i.e., 780-880℃; the critical region temperature is the temperature between Ac1 and Ac3, i.e., 720-780℃.

[0014] Preferably, after heat preservation in step S3, the grain size reaches level 8 or above as specified in GB / T 6394-2017; after exiting the furnace in step S5, the grain size is ≥ level 9, the pearlite lamellar spacing is ≤ 0.3 μm, the surface oxide decarburization layer thickness is ≤ 20 μm according to GB / T 226-2015, the hardness difference across the entire cross section is ≤ 30 HB according to GB / T 231.1-2018, and the room temperature impact toughness is ≥ 150 J according to GB / T 229-2020.

[0015] Compared with the prior art, the advantages of the present invention are as follows: (1) In this invention, rapid cooling to 650℃ shortens the pearlite transformation incubation period and avoids the formation of network carbides; isothermal cooling at 600℃ promotes uniform precipitation of carbides and forms ultrafine pearlite with a lamellar spacing of ≤0.3μm; nitrogen protective atmosphere inhibits oxidation and reduces the thickness of the surface decarburized layer; at the same time, gradient cooling makes the temperature difference between the inside and outside of the workpiece ≤10℃ and the hardness difference of the whole cross section ≤30HB, thereby improving dimensional stability; (2) In this invention, the grain growth kinetic formula is used to accurately calculate the holding time, avoiding grain coarsening caused by "over-holding" or insufficient austenitization caused by "under-holding", ensuring that the grain size meets the requirements of GB / T6394-2017 standard level 8 or above, and improving batch stability by 90%; (3) In this invention, aluminum oxide nanoparticles are combined with gradient dilution quenching medium. Rapid cooling in the high-temperature section inhibits ferrite precipitation, slow cooling in the medium-temperature section reduces martensitic phase transformation stress, and ultrasonic vibration promotes uniform distribution of nanoparticles, so that the residual austenite content in the quenched structure is ≤5% and the width of the martensite lath is ≤0.5μm, thus solving the problems of uneven structure and stress concentration in traditional quenching. Attached Figure Description

[0016] Figure 1 This is a schematic diagram of a heat treatment process for refining the microstructure of medium carbon steel. Detailed Implementation

[0017] The technical solutions of the embodiments of the present invention will be explained and described below. However, the following embodiments are only preferred embodiments of the present invention and not all of them. Other embodiments obtained by those skilled in the art based on the embodiments in the implementation methods without creative effort are all within the protection scope of the present invention.

[0018] Example 1 I. Material Composition The mass percentage composition of the medium carbon steel workpiece material is: C 0.4%, Si 0.25%, Mn 0.625%, with the balance being Fe and unavoidable impurities. No Nb or V has been added.

[0019] II. Heat Treatment Process Steps S1. Shot peening pretreatment Using 0.4mm diameter cast steel shot, with a spraying pressure of 0.4MPa and a spraying time of 8min, a 35μm deformation layer is formed on the workpiece surface, with a residual compressive stress of 250MPa and a surface roughness Ra≤1.6μm.

[0020] S2, High-frequency induction heating to austenitizing temperature The shot-peened workpiece is heated to the austenitizing temperature of 830℃ using a high-frequency induction heating device at a heating rate of 12℃ / s. The surface temperature fluctuation of the workpiece is controlled within ±5℃. The high-frequency induction heating uses a ring coil with a distance of 8mm between the coil and the workpiece surface. The power is 100% of the rated power for the first 30 seconds and then reduced to 70%.

[0021] S3, Insulation at critical temperature The heated workpiece is held at a critical temperature of 750℃; the holding time is determined by calculation using a grain growth kinetics model, with the following formula: ; Among them, the target grain size The initial grain size is 15μm, meeting or exceeding level 8 requirements in the GB / T6394-2017 standard; Metallographic analysis revealed a grain size of 40 μm; the apparent activation energy for grain growth was [not specified]. Take 300 kJ / mol; ideal gas constant. Take 8.314 J / (mol·K); real-time holding temperature K = 1023.15 K (750℃ + 273.15); material constants Take 3×10 -5 μm n / min; Dynamic index Since the temperature is between Ac1 and Ac3, we take 8.33; substituting this into the formula, we calculate the heat preservation time. min.

[0022] S4. Gradient quenching in a quenching medium containing nanoparticles The quenching medium was an initial concentration of 20% polyethylene glycol aqueous solution, and the nanoparticles were aluminum oxide with a concentration of 0.2%. The nanoparticles were ultrasonically dispersed for 20 min, and 0.15% Span-80 was added as a dispersant. During the quenching process, ultrasonic vibration at 30 kHz and 13 kW was applied, with the vibration direction at a 45° angle to the workpiece axis. 30 s after the start of quenching, the quenching medium was automatically diluted to 15% by the replenishment system. The cooling rate in the high-temperature section (800 to 650 ℃) was controlled at 18 ℃ / s, and the cooling rate in the medium-temperature section (650 to 500 ℃) was controlled at 8 ℃ / s. The replenishment rate was 8 L / min in the high-temperature section and 5 L / min in the medium-temperature section. The concentration fluctuation after dilution was ≤±1%.

[0023] S5, Segmented isothermal annealing treatment First, rapidly cool to 650℃ at a rate of ≥50℃ / min and hold for 30 minutes. Then, cool down to 600℃ at a rate of 10℃ / min and hold isothermally for 60 minutes. Finally, cool with the furnace to below 500℃ and remove from the furnace for air cooling. Throughout the process, a protective atmosphere of N2 with a purity of ≥99.99% is introduced. The oxygen content in the furnace is controlled in real time by an oxygen sensor and is ≤10ppm. The temperature fluctuation is ≤±2℃.

[0024] Example 2 I. Material Composition The mass percentage composition of the medium carbon steel workpiece material is: C 0.25%, Si 0.15%, Mn 0.45%, with 0.035% Nb added, and the balance being Fe and unavoidable impurities.

[0025] II. Heat Treatment Process Steps S1. Shot peening pretreatment Using 0.3mm diameter cast steel shot, with a spraying pressure of 0.3MPa and a spraying time of 5min, a 20μm deformation layer is formed on the workpiece surface, with a residual compressive stress of 200MPa and a surface roughness Ra≤1.6μm.

[0026] S2, High-frequency induction heating to austenitizing temperature The shot-peened workpiece is heated to the austenitizing temperature of 780℃ using a high-frequency induction heating device at a heating rate of 10℃ / s. The surface temperature fluctuation of the workpiece is controlled within ±5℃. The high-frequency induction heating uses a ring coil with a 10mm gap between the coil and the workpiece surface. The power is 80% of the rated power for the first 30 seconds and then drops to 60%.

[0027] S3, Insulation at critical temperature The heated workpiece was held at the austenitizing temperature of 780℃; the holding time was determined by calculation using a grain growth kinetics model, with the following formula: ; Among them, the target grain size 12 μm; initial grain size Metallographic analysis revealed a grain size of 30 μm; the apparent activation energy for grain growth was [not specified]. Take 280 kJ / mol; real-time insulation temperature 1053.15 K (780℃ +273.15); material constants Take 1×10 -5 μm n / min; Dynamic index Since the temperature is ≥Ac3, we take 3.55; substituting this into the formula, we can calculate the insulation time. min.

[0028] S4. Gradient quenching in a quenching medium containing nanoparticles The quenching medium was an initial concentration of 20% polyethylene glycol aqueous solution, and the nanoparticles were aluminum oxide with a concentration of 0.1%. The nanoparticles were ultrasonically dispersed for 15 min, and 0.1% Span-80 was added as a dispersant. During the quenching process, ultrasonic vibration at 20 kHz and 13 kW was applied, with the vibration direction at a 60° angle to the workpiece axis. 30 s after the start of quenching, the quenching medium was automatically diluted to 15% by the replenishment system. The cooling rate in the high-temperature section (800 to 650℃) was controlled at 15℃ / s, and the cooling rate in the medium-temperature section (650 to 500℃) was controlled at 5℃ / s. The replenishment rate was 8 L / min in the high-temperature section and 5 L / min in the medium-temperature section. The concentration fluctuation after dilution was ≤±1%.

[0029] S5, Segmented isothermal annealing treatment First, rapidly cool to 650℃ at a rate of ≥50℃ / min and hold for 30 minutes. Then, cool down to 600℃ at a rate of 10℃ / min and hold isothermally for 60 minutes. Finally, cool with the furnace to below 500℃ and remove from the furnace for air cooling. Throughout the process, a protective atmosphere of N2 with a purity of ≥99.99% is introduced. The oxygen content in the furnace is controlled in real time by an oxygen sensor and is ≤10ppm. The temperature fluctuation is ≤±2℃.

[0030] Example 3 I. Material Composition The mass percentage composition of the medium carbon steel workpiece material is: C 0.6%, Si 0.35%, Mn 0.8%, with 0.05% V added, and the balance being Fe and unavoidable impurities.

[0031] II. Heat Treatment Process Steps S1. Shot peening pretreatment Using 0.5mm diameter cast steel shot, with a spraying pressure of 0.5MPa and a spraying time of 10min, a 50μm deformation layer is formed on the workpiece surface, with a residual compressive stress of 300MPa and a surface roughness Ra≤1.6μm.

[0032] S2, High-frequency induction heating to austenitizing temperature The shot-peened workpiece is heated to the austenitizing temperature of 880℃ using a high-frequency induction heating device at a heating rate of 15℃ / s. The surface temperature fluctuation of the workpiece is controlled within ±5℃. The high-frequency induction heating uses a ring coil with a 5mm gap between the coil and the workpiece surface. The power is 100% of the rated power for the first 30 seconds and then drops to 70%.

[0033] S3, Insulation at critical temperature The heated workpiece was held at the austenitizing temperature of 880℃. The holding time was determined by calculation using a grain growth kinetics model, with the following formula: ; Among them, the target grain size 20 μm; initial grain size Metallographic analysis revealed a grain size of 50 μm; the apparent activation energy for grain growth was [not specified]. Take 320 kJ / mol; real-time holding temperature K = 1153.15 K (880℃ + 273.15); material constants Take 5×10 -5 μm n / min; Dynamic index Since the temperature is ≥Ac3, we take 3.55; substituting this into the formula, we can calculate the insulation time. min.

[0034] S4. Gradient quenching in a quenching medium containing nanoparticles The quenching medium was an initial concentration of 20% polyethylene glycol aqueous solution, and the nanoparticles were aluminum oxide with a concentration of 0.3%. The nanoparticles were ultrasonically dispersed for 30 min, and 0.2% Span-80 was added as a dispersant. During the quenching process, ultrasonic vibration at 40 kHz and 13 kW was applied, with the vibration direction at a 50° angle to the workpiece axis. 30 s after the start of quenching, the quenching medium was automatically diluted to 15% by the replenishment system. The cooling rate in the high-temperature section (800 to 650 ℃) was controlled at 20 ℃ / s, and the cooling rate in the medium-temperature section (650 to 500 ℃) was controlled at 10 ℃ / s. The replenishment rate was 10 L / min in the high-temperature section and 8 L / min in the medium-temperature section. The concentration fluctuation after dilution was ≤ ±1%.

[0035] S5, Segmented isothermal annealing treatment First, rapidly cool to 650℃ at a rate of ≥50℃ / min and hold for 30 minutes. Then, cool down to 600℃ at a rate of 10℃ / min and hold isothermally for 60 minutes. Finally, cool with the furnace to below 500℃ and remove from the furnace for air cooling. Throughout the process, a protective atmosphere of N2 with a purity of ≥99.99% is introduced. The oxygen content in the furnace is controlled in real time by an oxygen sensor and is ≤10ppm. The temperature fluctuation is ≤±2℃.

[0036] Comparative Example 1: Lack of shot peening pretreatment I. Material Composition The mass percentage composition of the medium carbon steel workpiece material is: C 0.4%, Si 0.25%, Mn 0.625%, with the balance being Fe and unavoidable impurities. No Nb or V has been added.

[0037] II. Heat Treatment Process Steps S1. No shot peening pretreatment performed. Skip the shot peening step and proceed to S2.

[0038] S2, High-frequency induction heating to austenitizing temperature The workpiece is heated to its austenitizing temperature of 830℃ using a high-frequency induction heating device at a heating rate of 12℃ / s, with surface temperature fluctuations controlled within ±5℃. The high-frequency induction heating uses a ring coil with an 8mm gap between the coil and the workpiece surface. The power is 100% of the rated power for the first 30 seconds, then reduced to 70%.

[0039] S3, Insulation at critical temperature The heated workpiece was held at a critical temperature of 750℃. The holding time was determined by calculation using a grain growth kinetics model, and the formula is as follows: ; Among them, the target grain size The initial grain size is 15 μm. Metallographic analysis revealed that the apparent activation energy for grain growth was 40 μm. Take 300 kJ / mol, real-time holding temperature The K value is 1023.15 K, and the material constant is... Take 3×10 -5 μm n / min, dynamic index Take 8.33; substitute it into the formula to calculate the heat preservation time. min.

[0040] S4. Gradient quenching in a quenching medium containing nanoparticles The quenching medium was an initial concentration of 20% polyethylene glycol aqueous solution, and the nanoparticles were aluminum oxide with a concentration of 0.2%. The nanoparticles were ultrasonically dispersed for 20 min, and 0.15% Span-80 was added as a dispersant. During the quenching process, ultrasonic vibration at 30 kHz and 13 kW was applied, with the vibration direction at a 45° angle to the workpiece axis. 30 s after the start of quenching, the quenching medium was automatically diluted to 15% by the replenishment system. The cooling rate in the high-temperature section was controlled at 18 ℃ / s, and the cooling rate in the medium-temperature section was controlled at 8 ℃ / s. The replenishment rate was 8 L / min in the high-temperature section and 5 L / min in the medium-temperature section. The concentration fluctuation after dilution was ≤±1%.

[0041] S5, Segmented isothermal annealing treatment First, rapidly cool to 650℃ at a rate of ≥50℃ / min and hold for 30 minutes. Then, cool down to 600℃ at a rate of 10℃ / min and hold isothermally for 60 minutes. Finally, cool with the furnace to below 500℃ and remove from the furnace for air cooling. Throughout the process, a protective atmosphere of N2 with a purity of ≥99.99% is introduced. The oxygen content in the furnace is controlled in real time by an oxygen sensor and is ≤10ppm. The temperature fluctuation is ≤±2℃.

[0042] Comparative Example 2: Traditional quenching media, without gradient dilution and nanoparticles I. Material Composition The mass percentage composition of the medium carbon steel workpiece material is: C 0.4%, Si 0.25%, Mn 0.625%, with the balance being Fe and unavoidable impurities. No Nb or V has been added.

[0043] II. Heat Treatment Process Steps S1. Shot peening pretreatment Using 0.4mm diameter cast steel shot, with a spraying pressure of 0.4MPa and a spraying time of 8min, a 35μm deformation layer is formed on the workpiece surface, with a residual compressive stress of 250MPa and a surface roughness Ra≤1.6μm.

[0044] S2, High-frequency induction heating to austenitizing temperature The shot-peened workpiece is heated to the austenitizing temperature of 830℃ using a high-frequency induction heating device at a heating rate of 12℃ / s. The surface temperature fluctuation of the workpiece is controlled within ±5℃. The high-frequency induction heating uses a ring coil with a distance of 8mm between the coil and the workpiece surface. The power is 100% of the rated power for the first 30 seconds and then reduced to 70%.

[0045] S3, Insulation at critical temperature The heated workpiece was held at a critical temperature of 750°C. The holding time was calculated in the same way as S3 in Example 1. min.

[0046] S4, quenching with traditional quenching media The quenching medium is a single 20% polyethylene glycol aqueous solution, without the addition of nanoparticles and dispersants. No ultrasonic vibration is applied during the quenching process, and the cooling rate is uniformly controlled at 15℃ / s across the entire temperature range without a gradient dilution process.

[0047] S5, Segmented isothermal annealing treatment First, rapidly cool to 650℃ at a rate of ≥50℃ / min and hold for 30min. Then, cool down to 600℃ at a rate of 10℃ / min and hold isothermally for 60min. Finally, cool with the furnace to below 500℃ and air-cool. Throughout the process, a protective atmosphere of N2 with a purity of ≥99.99% is introduced, the oxygen content in the furnace is ≤10ppm, and the temperature fluctuation is ≤±2℃.

[0048] Comparative Example 3: The insulation time was not calculated according to the model, but rather based on empirical values. I. Material Composition The mass percentage composition of the medium carbon steel workpiece material is: C 0.4%, Si 0.25%, Mn 0.625%, with the balance being Fe and unavoidable impurities. No Nb or V has been added.

[0049] II. Heat Treatment Process Steps S1. Shot peening pretreatment Using 0.4mm diameter cast steel shot, with a spraying pressure of 0.4MPa and a spraying time of 8min, a 35μm deformation layer is formed on the workpiece surface, with a residual compressive stress of 250MPa and a surface roughness Ra≤1.6μm.

[0050] S2, High-frequency induction heating to austenitizing temperature The shot-peened workpiece is heated to the austenitizing temperature of 830℃ using a high-frequency induction heating device at a heating rate of 12℃ / s. The surface temperature fluctuation of the workpiece is controlled within ±5℃. The high-frequency induction heating uses a ring coil with a distance of 8mm between the coil and the workpiece surface. The power is 100% of the rated power for the first 30 seconds and then reduced to 70%.

[0051] S3, Insulation at critical temperature The heated workpiece was held at a critical temperature of 750℃ for 20 minutes using an empirical value, without calculation using a grain growth kinetic model or dynamic adjustment based on temperature fluctuations. The value is fixed at 8.33.

[0052] S4. Gradient quenching in a quenching medium containing nanoparticles The quenching medium was an initial concentration of 20% polyethylene glycol aqueous solution, and the nanoparticles were aluminum oxide with a concentration of 0.2%. The nanoparticles were ultrasonically dispersed for 20 min, and 0.15% Span-80 was added as a dispersant. During the quenching process, ultrasonic vibration at 30 kHz and 13 kW was applied, with the vibration direction at a 45° angle to the workpiece axis. 30 s after the start of quenching, the quenching medium was automatically diluted to 15% by the replenishment system. The cooling rate in the high-temperature section was controlled at 18 ℃ / s, and the cooling rate in the medium-temperature section was controlled at 8 ℃ / s. The replenishment rate was 8 L / min in the high-temperature section and 5 L / min in the medium-temperature section. The concentration fluctuation after dilution was ≤±1%.

[0053] S5, Segmented isothermal annealing treatment First, rapidly cool to 650℃ at a rate of ≥50℃ / min and hold for 30min. Then, cool down to 600℃ at a rate of 10℃ / min and hold isothermally for 60min. Finally, cool with the furnace to below 500℃ and air-cool. Throughout the process, a protective atmosphere of N2 with a purity of ≥99.99% is introduced, the oxygen content in the furnace is ≤10ppm, and the temperature fluctuation is ≤±2℃.

[0054] Performance testing I. Test Standards and Methods 1. Grain size (GB / T 6394-2017): The cross-section of the workpiece is observed under 100x magnification using metallographic method and compared with the standard rating chart.

[0055] 2. Pearlite lamellar spacing: Pearlite tissue was observed using a transmission electron microscope (TEM), and the average lamellar spacing was measured.

[0056] 3. Oxidation and decarburization layer thickness (GB / T 226-2015): The distance from the workpiece surface to the original microstructure is measured under a metallographic microscope.

[0057] 4. Hardness difference across the entire cross section (GB / T 231.1-2018): Use a Brinell hardness tester to test 5 points evenly selected on the cross section of the workpiece, and take the difference between the maximum and minimum values.

[0058] 5. Room temperature impact toughness (GB / T 229-2020): Charpy V-notch impact test was adopted, with a specimen size of 10mm×10mm×55mm and a pendulum energy of 300J.

[0059] II. Performance Test Comparison Table

[0060] III. Comparison and Summary 1. Comparative Example 1: Insufficient shot peening leads to deterioration of surface stress state, grain coarsening and increased oxidation, directly proving the necessity of shot peening pretreatment to inhibit grain growth and oxidation.

[0061] 2. Comparative Example 2: Traditional quenching media lack gradient dilution and ultrasonic vibration, resulting in uncontrolled cooling rate in the mid-temperature range, coarsening of pearlite and uneven hardness, highlighting the key role of gradient quenching process in achieving microstructure uniformity.

[0062] 3. Comparative Example 3: The holding time was not calculated according to the model. Due to temperature fluctuations, the parameters were not dynamically adjusted, resulting in the grain size not meeting the standard. This verifies the irreplaceable role of the grain growth kinetic model in precise temperature control.

[0063] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely preferred examples and are not intended to limit the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of the present invention is defined by the appended claims and their equivalents.

Claims

1. A composite heat treatment process for refining the microstructure of medium carbon steel, characterized in that, Includes the following steps: S1. Perform shot peening pretreatment on the surface of medium carbon steel workpieces; S2. The shot-peened workpiece is heated to the austenitizing temperature using a high-frequency induction heating device. S3. Keep the heated workpiece at the critical temperature. S4. After heat preservation, the workpiece is immersed in a quenching medium containing nanoparticles for gradient quenching. S5. Perform segmented isothermal annealing on the quenched workpiece. The medium carbon steel material comprises the following components by mass percentage: 0.25% to 0.6% C, 0.15% to 0.35% Si, 0.45% to 0.8% Mn, with optional addition of 0.02% to 0.05% Nb or V, and the balance being Fe and unavoidable impurities.

2. The heat treatment process for refining the microstructure of medium carbon steel according to claim 1, characterized in that, In step S1, the shot peening pretreatment uses cast steel shot with a diameter of 0.3 to 0.5 mm, a blasting pressure of 0.3 to 0.5 MPa, and a blasting time of 5 to 10 min to form a deformation layer of 20 to 50 μm on the workpiece surface, with a residual compressive stress ≥200 MPa and a surface roughness Ra ≤1.6 μm.

3. The heat treatment process for refining the microstructure of medium carbon steel according to claim 1, characterized in that, In step S2, the heating rate is 10 to 15℃ / s, the workpiece surface temperature fluctuation is controlled within ±5℃, the high-frequency induction heating uses a ring coil, the distance between the coil and the workpiece surface is 5 to 10mm, the power is 80% to 100% of the rated power for the first 30s, and then drops to 60% to 70%.

4. The heat treatment process for refining the microstructure of medium carbon steel according to claim 1, characterized in that, In step S3, the holding time is determined by calculation using a grain growth kinetics model, specifically based on the following formula: ； In the formula, The target grain size at the end of the insulation process, in μm, must meet the requirements of level 8 or above in the GB / T6394-2017 standard. Initial grain size, in μm, obtained through metallographic analysis; The apparent activation energy for grain growth ranges from 280 kJ / mol to 320 kJ / mol. The ideal gas constant is taken as 8.314 J / (mol·K); The real-time insulation temperature is measured in Kelvin (K) and is fed back in real time by a temperature sensor. The required heat preservation time, in minutes; These are material constants, determined experimentally, with a range of values. to μm n / min; This is a dynamic index, and its value is related to the insulation temperature range: when the insulation temperature is ≥ Ac3, it is taken as... When the temperature is between Ac1 and Ac3, take ; Based on real-time temperature and initial grain size Substitute into the formula to reverse-engineer the target grain size. Required heat preservation time When temperature fluctuations cause When changes occur, adjust dynamically. Value recalculated Ensure that the grain size meets the standard requirements; the final holding time is within the allowable error range of ±5% of the calculated result.

5. The heat treatment process for refining the microstructure of medium carbon steel according to claim 1, characterized in that, In step S4, the nanoparticles are aluminum oxide with a concentration of 0.1% to 0.3%. They are ultrasonically dispersed for ≥15 min and 0.1% to 0.2% of Span-80 is added as a dispersant. During the quenching process, ultrasonic vibration of 20 to 40 kHz and 13 kW is applied, with the vibration direction at an angle of 45° to 60° to the axis of the workpiece.

6. The heat treatment process for refining the microstructure of medium carbon steel according to claim 1, characterized in that, In step S4, the quenching medium is an initial concentration of 20% polyethylene glycol aqueous solution, which is diluted to 15% after 30 seconds of quenching. The cooling rate is controlled at 15 to 20℃ / s in the high-temperature section (800 to 650℃) and 5 to 10℃ / s in the medium-temperature section (650 to 500℃).

7. The heat treatment process for refining the microstructure of medium carbon steel according to claim 6, characterized in that, The gradient dilution of the quenching medium is automatically controlled by a replenishment system with a replenishment rate of 5 to 10 L / min and a concentration fluctuation of ≤ ±1% after dilution; the replenishment rate is 8 to 10 L / min in the high-temperature section and 5 to 8 L / min in the medium-temperature section.

8. The heat treatment process for refining the microstructure of medium carbon steel according to claim 1, characterized in that, In step S5, the segmented isothermal annealing specifically involves: first, rapidly cooling to 650℃ at a rate of ≥50℃ / min and holding at that temperature for 30 minutes; then, cooling down to 600℃ at a rate of 10℃ / min and holding isothermally for 60 minutes; finally, cooling with the furnace to below 500℃ and then air-cooling. Throughout the process, a protective atmosphere of ≥99.99% pure N2 is introduced to suppress oxidation. The oxygen content in the furnace is ≤10 ppm, and the temperature is controlled in real time by an oxygen sensor, with temperature fluctuations ≤±2℃.

9. The heat treatment process for refining the microstructure of medium carbon steel according to claim 1, characterized in that, The austenitizing temperature is the Ac3 temperature, i.e., 780-880℃; the critical region temperature is the temperature between Ac1 and Ac3, i.e., 720-780℃.

10. The heat treatment process for refining the microstructure of medium carbon steel according to claim 1, characterized in that, After heat preservation in step S3, the grain size reaches grade 8 or above as specified in GB / T 6394-2017; after exiting the furnace in step S5, the grain size is ≥ grade 9, the pearlite lamellar spacing is ≤ 0.3 μm, the surface oxide decarburization layer thickness is ≤ 20 μm according to GB / T 226-2015, the hardness difference across the entire cross section is ≤ 30 HB according to GB / T 231.1-2018, and the room temperature impact toughness is ≥ 150 J according to GB / T 229-2020.

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