A heat treatment process for eliminating mixed crystals of a low-carbon gear steel after warm forging or hot forging

Abstract

The present application relates to a kind of low carbon gear steel material warm forging or hot forging after eliminating the heat treatment process of forging mixed crystal, the heat treatment process is to the mixed crystal of forging blank and is annealed, keeps warm 3-3.5 hours, with furnace cooling to 500 DEG C or less, out of furnace air cooling, annealing temperature is determined according to the A1 temperature of steel grade and Al / N ratio, specific annealing temperature is determined according to empirical formula t=A1-K·Al / N, wherein K indicates constant, applicable to Al grain refinement steel, usually 25~35 is taken.The forging blank with mixed crystal structure can be eliminated by reheat treatment process, to ensure that the structure and grain size after subsequent carburizing heat treatment meet the standard requirements.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of special steel smelting, and particularly relates to a heat treatment process for eliminating mixed crystals of low-carbon gear steel after warm forging or hot forging. BACKGROUND

[0002] Gear parts are generally formed by hot forging, warm forging or cold forging of rolled materials. When the forging temperature or cold deformation is unreasonable, the gear forging blanks after forging may have uneven organization, thereby producing mixed crystals. The grain size and uniformity of the forging blanks with mixed crystals after carburizing heat treatment are also uneven. Therefore, taking heat treatment measures to save the steel parts with mixed crystals after forging is an important means to ensure that the parts have fine and uniform grains after carburizing heat treatment.

[0003] The disclosure No. CN111982956B provides a method for determining the elimination of mixed crystal organization of ultra-low carbon steel based on a thermal simulation testing machine. The method includes the following test conditions: heating the sample to a first target temperature Ac3+30-Ac3+50℃, then cooling to a second target temperature Ar3±20℃, and deforming at a deformation ratio of 83%-89% at the second target temperature. Multiple test samples are obtained by performing multiple tests according to the test conditions. If a part of the samples has mixed crystal organization, and another part of the samples has mixed crystal organization elimination, the test conditions corresponding to the samples with mixed crystal organization are compared with the test conditions corresponding to the samples with mixed crystal organization elimination; the second target temperature and the selection set of deformation amount of the mixed crystal organization elimination of the ultra-low carbon steel are determined. This method can obtain the organization state of the simulated rolled sample, and has guiding significance for the production process of steel.

[0004] The disclosure No. CN114226618B provides a reverse control process for the final forging forming of large shaft forgings based on mixed crystal regulation. In view of the mixed crystal problem of large shaft parts, a multi-pass non-continuous hot forging experiment and an isothermal heat treatment experiment are combined to explore the mixed crystal degree evolution and effect on the forging-heat treatment process interface, determine the process tolerance of uneven forging grains, establish the best forging process window for the final forging forming of large shaft forgings, and explore and propose a reverse control forging process analysis and design method for the final forging forming of large shaft forgings, so that the final forging forming process of large shaft forgings can finally realize the fine and uniform control of the grain organization of large forgings.

[0005] Publication No. CN 115058566B discloses a method for improving the grain uniformity of Cr-Mo-V heat-resistant alloy steel pipe. The method comprises sequentially subjecting the steel pipe to normalizing heat treatment and tempering heat treatment; the normalizing heat treatment comprises heating, natural cooling and controlled cooling steps in sequence; wherein the natural cooling comprises: naturally cooling the heated steel pipe in air, and the time of the natural cooling is 1.6 seconds / mm to 5 seconds / mm calculated according to the wall thickness of the steel pipe; the controlled cooling is cooled to below 400℃ at a cooling rate of ≥28℃ / min. The method can improve the phenomenon of abnormal grain growth or mixed crystal caused by heat treatment in the prior art, thereby improving the uniformity of grain size, the morphology and proportion of ferrite, pearlite and bainite, and comprehensively improving the strength and toughness of the steel pipe and the grain size homogeneity of thick-walled steel pipe.

[0006] Publication No. CN113862433B discloses a bevel gear fine-grain control method, comprising the following steps: S1. Material selection: using 22CrMoH steel alloyed with Al and N as raw material; S2. Forging treatment: the material of step S1 is forged, the heating temperature is ≥1180℃, the final forging temperature is 900-1100℃, and the forged part is quenched to below 200℃ after forging; S3. Normalizing treatment: after the quenched forged part is heated and pre-treated, it is air-cooled and then put into a normalizing furnace for isothermal heating and holding, and then air-cooled after taking out; S4. The material after normalizing treatment is carburized and quenched and low-temperature tempered to obtain a gear product; by adjusting the alloy composition and the quenching process after forging, the mixed crystal phenomenon in the normalizing structure is effectively eliminated, and the deformation after heat treatment is small. The product qualification rate of heat treatment is improved, and the quality stability is increased.

[0007] A medium carbon microalloy steel and a method for eliminating its mixed crystal structure are disclosed in CN113088825B. The medium carbon microalloy steel comprises the following chemical components by weight percentage: carbon 0.30-0.40%, silicon 0.10-0.50%, manganese 0.90-1.20%, chromium 0.10-0.30%, aluminum 0.010-0.050%, nickel 0.10-0.30%, copper 0.00-0.20%, molybdenum 0.00-0.050%, phosphorus 0.000-0.020%, sulfur 0.000-0.020%, vanadium 0.020-0.060%, nitrogen 0.0030-0.0100%, and the balance is iron. The method for eliminating the mixed crystal structure is also disclosed. The uneven carbonitride can be fully solid-solved into austenite by high-temperature solid-solution treatment, i.e. the material containing mixed crystal structure is kept at 1000-1250℃ for 1-2h. The carbonitride can be fully and uniformly precipitated after low-temperature aging treatment, i.e. the material after high-temperature solid-solution treatment is rapidly cooled to 700-850℃, and then kept for 1-1.5h. The normalizing heat treatment mode eliminates the mixed crystal structure of the forged piece and refines the grains, so that the treated medium carbon microalloy steel has high strength and good toughness, and meets the requirements of medium carbon microalloy steel for rail transit axles.

[0008] CN112824545B discloses a process method for solving the serious mixed crystal of high-temperature bearing steel W9Cr4V2Mo. The heat treatment process method for eliminating mixed crystal structure is summarized based on the mixed crystal structure and the distribution state of coarse and fine grains of the forged piece through multiple process tests and inspections. The specific steps are as follows: a, material preheating: preheat the W9Cr4V2Mo steel bearing ring forged piece with serious mixed crystal to 750±10℃ and keep for 2 hours; b, high-temperature heating: heat the forged piece of step a to 940±10℃ at a speed of 150℃ per hour and keep for 2 hours; c, isothermal spheroidizing heating: furnace cool the forged piece of step b to 740±10℃ at a speed of 50℃ per hour and keep for 6-7 hours; d, spheroidizing treatment: heat the forged piece of step c to 860±10℃ and keep for 6-7 hours, then cool to 740±10℃ at a speed of 50℃ per hour, keep for 6-7 hours, then cool to 680℃ at a speed of 20℃ per hour, and furnace cool to 550-500℃ and air cool. The average grain size of the product after heat treatment reaches 8.5 grade (standard 7 grade or finer), and the mixed crystal phenomenon is eliminated. The W9Cr4V2Mo steel bearing ring forged piece completely meets the requirements in terms of technical quality.

[0009] A method for eliminating mixed crystals in S31035 high-alloy austenitic heat-resistant steel, the conventional S31035 high-alloy austenitic heat-resistant steel pipe preparation process: hot extrusion -> cold rolling -> high-temperature solid solution treatment, removes the cold rolling link on the basis of the conventional S31035 high-alloy austenitic heat-resistant steel pipe preparation process, and introduces a hot rolling process. 23 C6 dissolution temperature 30℃-100℃ In the hot rolling process, the continuous grain boundary phase M 23 C6 is precipitated to pin the grain boundary and inhibit recrystallized grain growth. After hot rolling, high-temperature solid solution treatment is further conducted at 1200℃-1220℃, the grain boundary precipitated phase M 23 C6 is completely dissolved, the structure is completely recrystallized, the mixed crystals are finally eliminated, the grains are refined, and the structure is uniform. The method can effectively eliminate mixed crystals in S31035 high-alloy austenitic heat-resistant steel pipes, improve the internal structure, and improve the qualified rate of products.

[0010] Although the method of the above-mentioned application can effectively improve the mixed crystal condition of the steel to some extent by process improvement, etc., unlike the present application, the process control process of these applications is relatively complex, some are not easy to operate, and some have high cost. The heat treatment process of the mixed crystal part is simple and easy to operate, and has strong operability. SUMMARY

[0011] The technical problem to be solved by the present application is to provide a heat treatment control method for eliminating mixed crystals of low-carbon gear rolling stock after hot forging and warm forging caused by improper forging temperature or deformation. The simple and easy-to-operate heat treatment method makes the grain size of the forged part small and uniform, greatly improving the fatigue strength and life of the carburized gear steel.

[0012] The production process of gear steel is generally: electric furnace or converter-external refining-VD or RH vacuum degassing-continuous casting or mold injection-rolling into steel- cutting or sawing-finishing-(surface and internal flaw detection)-packaging.

[0013] The production process of gear blanks is generally: rolling stock blanking-heating (the temperature is generally controlled at 750-1250℃, below 900℃ for warm forging)-forging-piling in the material basket for cold stacking-isothermal annealing-rough machining-detection of hardness, structure, grain size, etc.

[0014] The composition control technology principle for refining the grain size of gear steel: the carburizing temperature of carburized gear steel is relatively high (generally ≥930℃), and the time is long, the grains in the steel are easy to grow under long-time high temperature, causing mixed crystals or coarse grains, which seriously affects the fatigue life of the gear. Therefore, in the production process of gear steel, a certain amount of Al and N is added to the steel, and the Al / N ratio is specified to be reasonable, so that a large number of AlN particles that prevent grain growth can exist in the steel.

[0015] The principle of grain refinement control technology in rolling and forging process: ensure the Al, N composition from the metallurgical aspect, so that there are enough second phase AlN particles in the steel, which is the basis of ensuring the refined grain, whether the actual product grain can be refined also depends on whether the forging, rolling and heat treatment process after steelmaking is reasonable. AlN particles play a key role in austenite grain size It mainly precipitates at the grain boundary, pins the austenite grain boundary, and only when there are enough AlN particles in the steel to precipitate at the grain boundary can the austenite grain be uniform and fine. The crystal defects formed during deformation in the austenite high temperature zone are beneficial to the diffusion of Al, N atoms and the precipitation of AlN phase, effectively inhibiting the migration of grain boundary, and easy to obtain uniform and fine austenite grain size. When deforming in the austenite low temperature zone, the dynamic recovery speed is slow, and more lattice defects are retained, which increases the nucleation rate of austenite, refines the initial austenite grain, and the smaller the initial grain size, the greater the interface energy, and the greater the grain growth tendency, accompanied by the decrease of deformation temperature, mixed crystal organization appears.

[0016] The technical principle of eliminating the mixed crystal of the forged blank is similar to the control principle of refining the grain size of the rolled or forged material, and the purpose is to create favorable conditions to make more uniform and fine AlN particles precipitate in the steel. When the rolled material is re-forged into a forged part (such as a gear blank), if the forging temperature, deformation and other factors are not reasonably controlled, the number of AlN particles precipitated is small or the size is large, and the pinning force of the particles to prevent the growth of austenite grains is not enough or does not work, which will cause the grain of the forged blank to be uneven. The number of AlN particles precipitated is closely related to the temperature control. According to the principle of thermodynamics and dynamics of AlN particle precipitation in steel, due to the difference in Al and N solid solubility product, there is a peak value of AlN particle precipitation in a certain temperature range, which corresponds to the precipitation of more fine and uniform AlN particles. Therefore, the mixed crystal organization of the forged blank can be eliminated by reheat treatment process to ensure that the organization and grain size after carburizing heat treatment meet the standard requirements.

[0017] The technical scheme adopted by the present application to solve the above problems is: a heat treatment process for eliminating mixed crystal of forged parts after warm forging or hot forging of low-carbon gear steel, annealing treatment is carried out on the mixed crystal forged blank, the annealing temperature is determined according to the Al temperature and Al / N ratio of the steel grade, A1 is the phase transition point when the eutectoid steel passes through the PSK line during heating or cooling: 727 ℃, at this time, the transformation between pearlite and austenite occurs, in the process of "equilibrium transformation", the austenite transformation point or eutectoid transformation point of the steel. The specific annealing temperature is determined according to the empirical formula t=A1-K·Al / N (K: constant, applicable to Al grain refined steel, usually take 25~35; not applicable to Ti grain refined steel), the temperature is kept for 3-3.5 hours, and the furnace is cooled to below 500 ℃, and then the furnace is discharged and air cooled.

[0018] Further, the forging blank is heated to 500 DEG C after being put into the furnace, so that the blank is not prone to stress and cracks, and the heating speed can not be controlled in this stage, so that the cost is saved and the blank is rapidly heated;

[0019] Further, the forging blank is heated to 500 DEG C after being put into the furnace, so that the blank is not prone to stress and cracks, and the heating speed can not be controlled in this stage, so that the cost is saved and the blank is rapidly heated;

[0020] Further, the forging blank is heated to 500 DEG C after being put into the furnace, so that the blank is not prone to stress and cracks, and the heating speed can not be controlled in this stage, so that the cost is saved and the blank is rapidly heated;

[0021] Further, the forging blank is heated to 500 DEG C after being put into the furnace, so that the blank is not prone to stress and cracks, and the heating speed can not be controlled in this stage, so that the cost is saved and the blank is rapidly heated;

[0022] The method is suitable for low-carbon gear steels such as 20CrMo, 20MnCr5, SAE8620H and 20CrH.

[0023] Compared with the prior art, the method has the advantages that:

[0024] The forging blank with mixed crystals is annealed, the annealing temperature is determined according to the A1 temperature and Al / N ratio of the steel, the number of AlN particles precipitated is closely related to the temperature control, according to the principle of thermodynamics and dynamics of AlN particles precipitated in the steel, there is a peak value of AlN particle precipitation in a certain temperature range due to the difference in Al and N solid solubility product, and a large number of fine and uniform AlN particles are precipitated, so that the forging blank with mixed crystal structure can be eliminated by reheat treatment process, so as to ensure that the structure and grain size after subsequent carburizing heat treatment meet the standard requirements. The forging grain size becomes fine and uniform through the simple and operable heat treatment method, and the fatigue strength and service life of the carburizing gear steel are greatly improved. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 It is a heat treatment process diagram of the application.

[0026] Figure 2 It is a detection result diagram of grain size before and after improvement of example 1 of the application.

[0027] Figure 3 It is a detection result diagram of grain size before and after improvement of example 2 of the application.

[0028] Figure 4 It is a detection result diagram of grain size before and after improvement of example 3 of the application.

[0029] Figure 5The detection result chart of the grain size before and after the improvement of the embodiment 4 of the present application. DETAILED DESCRIPTION

[0030] The technical solutions of the present application are described in more detail in combination with the preferred embodiments of the present application. However, these embodiments are only descriptions of the preferred embodiments of the present application, and cannot impose any limitation on the scope of the present application. EMBODIMENT

[0031] (1) The steel grade of this embodiment is SCr420H. The chemical composition of the steel is as follows: C: 0.22%, Si: 0.24%, Mn: 0.79%, Cr: 1.21%, Ni: 0.02%, S: 0.005%, P: 0.018%, Al: 0.038, Cu: 0.02%, N: 0.016, and the balance is Fe and inevitable impurities.

[0032] (2) The forging process of the steel product is as follows: blanking → electric furnace heating → forging temperature 1180℃ → air cooling after forging. The grain size sample of the forging is taken and detected at 930℃×4 hours, water quenching, and the mixed grain size is 8.5-3 grade, and the grain below 5 grade accounts for 15%.

[0033] (3) The forging heat treatment: the annealing process is performed according to the requirements in the heat treatment process chart, and the annealing temperature is set to 640-650℃.

[0034] (4) The forging after heat treatment is cut to take samples for grain size detection: 930℃×4 hours, water quenching, and the grain size is detected to be 8.0-6.0 grade. EMBODIMENT

[0035] (1) The steel grade of this embodiment is 20MnCrS5. The actual chemical composition of the steel is as follows: C: 0.21%, Si: 0.11%, Mn: 1.35%, Cr: 1.21%, Mo: 0.02%, Ni: 0.12%, S: 0.026%, P: 0.021%, Al: 0.035%, Cu: 0.02%, N: 0.013%, and the balance is Fe and inevitable impurities.

[0036] (2) The forging process of the steel product is as follows: blanking → electric furnace heating → forging temperature 850℃ → air cooling after forging. The grain size sample of the forging is taken and detected at 930℃×4 hours, water quenching, and the mixed grain size is 8.5-3 grade, and the grain below 5 grade accounts for 20%.

[0037] (3) The forging heat treatment: the annealing process is performed according to the requirements in the heat treatment process chart, and the annealing temperature is set to 625-635℃.

[0038] (4) The forged piece after heat treatment is cut for grain size detection: 930℃x4 hours, water quenching, and the grain size is detected to be 8.0-6.0 grade. Example 1

[0039] (1) The steel grade of this example is SAE8620H. The chemical composition of the steel is as follows: C: 0.18%, Si: 0.24%, Mn: 0.82%, Cr: 0.55%, Mo: 0.21%, Ni: 0.46%, S: 0.003%, P: 0.015%, Al: 0.026, Cu: 0.02%, N: 0.0091, and the balance is Fe and inevitable impurities.

[0040] (2) The forging process of the steel product is: blanking→electric furnace heating→forging temperature 870℃→air cooling after forging. The grain size sample of the forged piece is detected at 930℃x4 hours, water quenching, and the grain size is detected to be mixed crystal 7.5-2 grade, and the grain size below 5 grade accounts for 25%.

[0041] (3) The forged piece is heat treated: the annealing process is performed according to the requirements in the heat treatment process chart, and the annealing temperature is set to be 630-640℃.

[0042] (4) The forged piece after heat treatment is cut for grain size detection: 930℃x4 hours, water quenching, and the grain size is detected to be 8.0-6.0 grade. Example 2

[0043] (1) The steel grade of this example is 20CrMoH. The actual chemical composition of the steel is as follows: C: 0.19%, Si: 0.21%, Mn: 0.78%, Cr: 1.12%, Mo: 0.16%, Ni: 0.04%, S: 0.005%, P: 0.019%, Al: 0.031%, Cu: 0.04%, N: 0.0087%, and the balance is Fe and inevitable impurities.

[0044] (2) The forging process of the steel product is: blanking→electric furnace heating→forging temperature 1150℃→air cooling after forging. The grain size sample of the forged piece is detected at 930℃x4 hours, water quenching, and the grain size is detected to be mixed crystal 8-2 grade, and the grain size below 5 grade accounts for 30%.

[0045] (3) The forged piece is heat treated: the annealing process is performed according to the requirements in the heat treatment process chart, and the annealing temperature is set to be 605-615℃.

[0046] (4) The forged piece after heat treatment is cut for grain size detection: 930℃x4 hours, water quenching, and the grain size is detected to be 8.5-6.0 grade.

[0047] While the preferred embodiments of the application have been described above in detail, it is to be understood that various modifications and alterations to the preferred embodiments will occur to persons skilled in the art. Any such modifications or alterations are intended to fall within the scope of the application.

Claims

1. A heat treatment process for eliminating mixed grains in low-carbon gear steel after warm or hot forging, characterized in that... The heat treatment process involves annealing the mixed-grain forging blank, holding it at a temperature of 3-3.5 hours, furnace cooling to below 500°C, and then air cooling after removal from the furnace. The annealing temperature is determined based on the Al temperature and Al / N ratio of the steel grade. The specific annealing temperature is determined according to the empirical formula T=A1-K·Al / N, where K represents a constant, applicable to Al-refined grain steel, and is taken as 25~35. The steel grades are 20CrMo, 20MnCr5, SAE8620H, and 20CrH steel.

2. The heat treatment process for eliminating mixed crystals in low-carbon gear steel after warm or hot forging according to claim 1, characterized in that: The forging blank is first loaded into the furnace and then heated to 500°C to ensure that the blank is not prone to stress and cracks. The heating rate is not controlled during this stage.

3. The heat treatment process for eliminating mixed grains in low-carbon gear steel after warm or hot forging according to claim 1, characterized in that: The heating rate of the forging blank between 500℃ and the annealing and holding section is 4℃~7℃ / min.

4. The heat treatment process for eliminating mixed grains in low-carbon gear steel after warm or hot forging according to claim 1, characterized in that: After the forging blank has been annealed and held at a certain temperature, it is slowly cooled to 500°C in the furnace before being taken out of the furnace.

5. The heat treatment process for eliminating mixed grains in low-carbon gear steel after warm or hot forging according to claim 1, characterized in that: The annealing and holding temperature of the forging is controlled at 600-650℃.

6. The heat treatment process for eliminating mixed grains in low-carbon gear steel after warm or hot forging according to claim 1, characterized in that: The heat treatment process is suitable for gear steels with low carbon and refined Al grains.

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