Heat treatment process for eliminating forge piece mixed crystals after low-carbon gear steel warm forging or hot forging

Abstract

The invention relates to a heat treatment process for eliminating forge piece mixed crystals after warm forging or hot forging of low-carbon gear steel, which comprises the following steps: annealing a forge piece blank with mixed crystals, preserving heat for 3-3.5 hours, cooling to below 500 DEG C along with a furnace, discharging and cooling in air, determining the annealing temperature according to the A1 temperature and the Al / N ratio of the steel grade, and determining the specific annealing temperature according to an empirical formula t = A1-K.Al / N, wherein K represents a constant, is suitable for Al refined grain steel, and is usually 25-35. The forging blank with the mixed crystal structure can be eliminated through a re-heat treatment process, so that the structure and the grain size after subsequent carburizing heat treatment meet the standard requirements.

Description

Technical Field

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

[0002] The production of gear parts is generally hot forging, warm forging or cold forging of rolled materials. If the forging temperature or cold forging deformation is unreasonable, the gear forging blank may produce uneven structure after forging, thus producing mixed crystals. The grain size and uniformity of mixed crystal forgings after carburizing heat treatment are also uneven. Therefore, taking heat treatment rescue measures for mixed crystal steel parts after forging is an important means to ensure that the grains of parts after carburizing heat treatment are small and uniform.

[0003] Publication No. CN111982956B provides a method for determining the elimination of mixed crystal structure of ultra-low carbon steel based on a thermal simulation test machine. The method includes test conditions: heating the sample to a first target temperature in the range of Ac3+30~Ac3+50℃, then cooling it to a second target temperature Ar3±20℃, and deforming it at a deformation ratio of 83%~89% at the second target temperature. Perform multiple tests according to the test conditions to obtain multiple tested samples. If the organizational state of a part of the samples is a mixed crystal structure and the organizational state of another part of the samples is the elimination of the mixed crystal structure, the test conditions corresponding to the samples that obtain the mixed crystal structure are compared with the test conditions corresponding to the samples that obtain the elimination of the mixed crystal structure; determine the second target temperature for the elimination of the mixed crystal structure of the ultra-low carbon steel and the selected set of deformation amounts. This method can obtain the organizational state of the sample after simulated rolling, which is of guiding significance to the production process of steel.

[0004] Publication No. CN 114226618B is a reverse control process for the final forging of large shaft forgings based on mixed crystal regulation. Aiming at the mixed crystal problem of large shaft forgings "due to their size", this paper combines multi-pass discontinuous hot forging experiments with isothermal heat treatment experiments to explore the mixed crystal problem. By analyzing the evolution and effect of the mixed crystal degree on the forging-heat treatment process interface, the process tolerance of forging grain non-uniformity is determined, and the optimal forging process window for the final forging of large shaft forgings is established. On this basis, the reverse control forging process analysis and design method for the final forging of large shaft forgings is explored and proposed, so as to innovate the final forging process of large shaft forgings and ultimately achieve the fine and homogenous control of the grain structure of large forgings.

[0005] Publication No. CN 115058566B proposes a method for improving the grain uniformity of Cr-Mo-V heat-resistant alloy steel pipe. The method comprises sequentially performing the following treatments on the steel pipe: normalizing heat treatment and tempering heat treatment; the normalizing heat treatment comprises sequentially heating, natural cooling and controlled cooling steps; wherein the natural cooling comprises: naturally cooling the heated steel pipe in air, and the natural cooling time is 1.6 seconds / mm to 5 seconds / mm according to the wall thickness of the steel pipe; the controlled cooling is cooling to below 400°C at a cooling rate of ≥28°C / min. The method of the present invention 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, as well as the metallographic structure morphology and proportion of ferrite, pearlite and bainite, and comprehensively improving the strength and toughness of the steel pipe and the homogeneity of grain size of the thick-walled steel pipe.

[0006] Publication No. CN113862433B A spiral bevel gear fine grain control method, comprising the following steps: The spiral bevel gear fine grain control method, comprising the following steps: S1. Material selection: Al, N micro-alloyed 22CrMoH steel is used 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 forging is quenched to below 200℃; S3. Normalizing treatment: After the quenched forging is pre-treated by heating and heat preservation, it is placed in a normalizing furnace for isothermal heating and heat preservation after air cooling and then taken out of the furnace for air cooling; S4. The material after normalizing treatment is carburized, quenched and tempered at low temperature to obtain a gear product; By adjusting the alloy composition and coordinating the quenching process after forging, the mixed crystal phenomenon in the normalizing structure is effectively eliminated, and the deformation amount after heat treatment is small. Improve the qualified rate of heat-treated products and increase quality stability.

[0007] Publication No. CN113088825B discloses a medium carbon microalloy steel and a method for eliminating mixed crystal structures thereof. The invention discloses a medium carbon microalloy steel, comprising the following chemical components in 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 invention also discloses a method for eliminating the mixed crystal structure, wherein the material containing the mixed crystal structure is kept in an environment of 1000-1250°C for 1-2 hours through high temperature solution treatment, so that the non-uniform carbonitrides can be fully dissolved in austenite. The low temperature aging treatment is to rapidly cool the material after the high temperature solution treatment to 700-850°C, and then carry out a heat treatment, preferably the heat treatment time is 1-1.5 hours, so that the carbonitrides can be fully and evenly precipitated, and then the heat treatment method of normalizing is carried out, so that the mixed crystal structure of the forging is eliminated, and refined grains are obtained, so that the treated medium carbon microalloyed steel has high strength and good toughness, and meets various requirements of medium carbon microalloyed steel for rail transit axles.

[0008] Publication No. CN112824545B discloses a process for solving the serious mixed crystal problem of high-temperature bearing steel W9Cr4V2Mo. Aiming at the mixed crystal structure and the distribution of coarse and fine grains of forgings, a heat treatment process for eliminating the mixed crystal structure is summarized and proposed through a number of process tests and inspections. The specific steps are as follows: a. Material preheating: preheat the W9Cr4V2Mo steel bearing ring forging with serious mixed crystals to 750±10℃ and keep it warm for 2 hours; b. High temperature heating: heat the forging of step a to 940±10℃ at a rate of 150℃ per hour and keep it warm for 2 hours; c. Isothermal spheroidizing heating: furnace cool the forging of step b to 740±10℃ at a rate of 50℃ per hour and keep it warm for 6-7 hours; d. Spheroidizing treatment: heat the forging of step c to 860±10℃ and keep it warm for 6-7 hours, then cool it to 740±10℃ at a rate of 50℃ per hour, keep it warm for 6-7 hours, then cool it to 680℃ at a rate of 20℃ per hour, furnace cool it to 550-500℃ and air cool it out of the furnace. The average grain size of the heat-treated product after quenching reaches 8.5 grade (standard grade 7 or finer), and the mixed crystal phenomenon is eliminated. It fully meets the technical quality requirements of W9Cr4V2Mo steel bearing ring forgings.

[0009] Publication No. CN113584263B A method for eliminating mixed crystals in S31035 high alloy austenitic heat resistant steel. Conventional S31035 high alloy austenitic heat resistant steel pipe preparation process: hot extrusion → cold rolling → high temperature solution treatment, remove the cold rolling step, and introduce the hot rolling process. The hot rolling temperature range is lower than the M in the sample. 23 C6 dissolution temperature is 30℃~100℃. During hot rolling, the continuous grain boundary phase M is induced by strain. 23 C6 precipitation pins the grain boundary and inhibits the growth of recrystallized grains. After hot rolling, it is subjected to high temperature solution treatment at 1200℃~1220℃, and the grain boundary precipitation phase M 23 C6 is completely dissolved back, the structure is completely recrystallized, and finally the mixed crystals are eliminated, the grains are refined, and the structure is uniform. The method of the present invention can effectively eliminate the mixed crystals in the S31035 high alloy austenitic heat-resistant steel pipe, improve the internal structure, and increase the qualified rate of the product.

[0010] Although the above-mentioned inventions have effectively improved the mixed crystal condition of steel to a certain extent through process improvement, etc., unlike the present invention, the process control process of these inventions is relatively complicated, some are not easy to operate, and some are more expensive. The present invention has the characteristics of simple and easy heat treatment process for mixed crystal parts and strong operability. Summary of the invention

[0011] The technical problem to be solved by the present invention is to provide a heat treatment control method for eliminating mixed crystals in forging blanks caused by improper forging temperature or deformation after hot forging and warm forging of low-carbon gear rolled materials in accordance with the above-mentioned prior art. Through a simple and highly operable heat treatment method, the grain size of the forging becomes fine and uniform, thereby greatly improving the fatigue strength and life of carburized gear steel.

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

[0013] The production process of gear blanks is generally as follows: unrolling - heating (temperature is generally controlled at 750-1250°C, of ​​which 900°C or below is warm forging) - forging - stacking in a collection basket for cooling - isothermal annealing - rough machining - testing of hardness, structure, grain size, etc.

[0014] Principle of composition control technology for refining gear steel grains: The carburizing temperature of carburized gear steel is high (generally ≥930℃) and the time is long. The grains in the steel are easy to grow under long-term high temperature, resulting in mixed crystals or coarse crystals, which have a serious impact on the fatigue life of the gears. Therefore, in the production process of gear steel, in order to ensure uniform and fine grains, a certain amount of Al and N are often added to the steel, and the Al / N ratio is specified to reach a reasonable value. The purpose is to ensure that there are a large number of AlN particles in the steel to prevent grain growth.

[0015] The principle of grain refinement control technology in rolling and forging process: ensuring the given composition of Al and N from the metallurgical aspect, so that there are enough second phase AlN particles in the steel, is only the basis for ensuring grain refinement. Whether the grain of the actual product can be refined depends on whether the forging, rolling and heat treatment process after steelmaking is reasonable. AlN particles play a key role in the austenite grain size , which mainly precipitates at the grain boundaries and pins the austenite grain boundaries. Only when there are enough AlN particles in the steel that precipitate at the grain boundaries can the austenite grains be uniform and fine. The crystal defects formed during the deformation process in the austenite high temperature zone are conducive to the diffusion of Al and N atoms and the precipitation of AlN phases, effectively inhibiting the migration of grain boundaries, and easily obtaining 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 and refines the initial austenite grains. The finer the initial grains, the greater the interfacial energy, the greater the tendency of grain growth, and the mixed crystal structure appears with the decrease of deformation temperature.

[0016] The technical principle of eliminating mixed crystals in forging blanks is similar to the control principle of refining the grain size of rolled or forged materials mentioned above, with the purpose of creating favorable conditions for the precipitation of more fine and uniformly distributed AlN particles in the steel. When the rolled material is re-forged into a forging (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 insufficient or ineffective, which will cause uneven grains in the forging blank. The number of AlN particles precipitated is closely related to temperature control. According to the analysis of the thermodynamic and kinetic process principle of AlN particles precipitation in steel, due to the different solubility products of Al and N, there is a peak value of AlN particles precipitation within a certain temperature range, corresponding to the precipitation of more fine and uniform AlN particles. Therefore, the forging blank with mixed crystal structure can be eliminated by re-heat treatment process to ensure that the structure and grain size after subsequent carburizing heat treatment meet the standard requirements.

[0017] The technical solution adopted by the present invention to solve the above problems is: A heat treatment process for eliminating mixed crystals of forgings after warm forging or hot forging of low-carbon gear steel, annealing treatment of mixed crystal forging blanks, annealing temperature is determined according to the A1 temperature and Al / N ratio of the steel grade, A1 refers to the phase transformation point of eutectoid steel when passing through the PSK line during heating or cooling: 727°C, at which time the transformation between pearlite and austenite occurs, in the process of "equilibrium transformation", the austenite transformation point or eutectoid transformation point of steel. The specific annealing temperature is determined according to the empirical formula t=A1-K·Al / N (K: constant, applicable to Al refined grain steel, usually 25~35; not applicable to Ti refined grain steel), keep warm for 3-3.5 hours, cool to below 500°C with the furnace, and air cool out of the furnace.

[0018] Furthermore, the forging blank of the present invention is heated to 500° C. after being loaded into the furnace, so as to ensure that the blank is not prone to stress and cracks. The heating speed may not be controlled at this stage, so that the temperature can be increased quickly while saving costs; Furthermore, the present invention requires that the heating rate of the forging blank from 500°C to the annealing and holding stage be 4°C to 7°C / min. In order to prevent the generation of large structural stress, this temperature range is close to the phase transition temperature, so it is necessary to control it by a suitable heating rate. Furthermore, the present invention requires that after the forging blank is annealed and heat-insulated, it must be slowly cooled to 500° C. before being taken out of the furnace; Furthermore, the annealing and holding temperature of the forging of the present invention is generally controlled at 600-650°C.

[0019] This method is suitable for low carbon gear steel grades such as 20CrMo, 20MnCr5, SAE8620H, 20CrH, etc.

[0020] Compared with the prior art, the advantages of the present invention are: The present invention performs annealing treatment on the mixed crystal forging blank, and the annealing temperature is determined according to the Al temperature and Al / N ratio of the steel type. The number of AlN particles precipitated is closely related to the temperature control. According to the analysis of the thermodynamic and kinetic process principle of AlN particles precipitating in steel, due to the difference in the solid solubility product of Al and N, there is a peak value of AlN particles precipitation within a certain temperature range, corresponding to the precipitation of more fine and uniform AlN particles. Therefore, the forging blank with mixed crystal structure can be eliminated by re-heat treatment process to ensure that the structure and grain size after subsequent carburizing heat treatment meet the standard requirements. The present invention makes the grain size of the forging small and uniform through a simple and highly operable heat treatment method, which greatly improves the fatigue strength and life of the carburized gear steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 It is a heat treatment process diagram of the present invention.

[0022] Figure 2 This is a graph showing the detection results of the grain size before and after improvement of Example 1 of the present invention.

[0023] Figure 3 This is a graph showing the detection results of the grain size before and after improvement of Example 2 of the present invention.

[0024] Figure 4 This is a graph showing the detection results of the grain size before and after improvement of Example 3 of the present invention.

[0025] Figure 5 This is a graph showing the detection results of the grain size before and after improvement of Example 4 of the present invention. DETAILED DESCRIPTION

[0026] The technical solution of the present invention is described in more detail in conjunction with the preferred embodiments of the present invention. However, these embodiments are only descriptions of the preferred implementation methods of the present invention and cannot impose any limitation on the scope of the present invention. Example

[0027] (1) The steel type in this embodiment is SCr420H. The chemical composition of the steel is 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 unavoidable impurities.

[0028] (2) The forging process of the steel product is: blanking → electric furnace heating → forging temperature 1180℃ → air cooling after forging. The grain size sample of the forging was subjected to 930℃×4 hours, water quenching, and the grain size was tested. The mixed crystal was 8.5-3 level, and the proportion of grains below level 5 was 15%.

[0029] (3) Heat treatment of forgings: The annealing process is carried out according to the requirements of the heat treatment process diagram, and the annealing temperature is set at 640~650℃.

[0030] (4) After heat treatment, samples are cut from forgings for grain size testing: 930℃×4 hours, water quenching, and the grain size is tested to be 8.0-6.0. Example

[0031] (1) The steel type in this embodiment is 20MnCrS5. The actual chemical composition of the steel is 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 unavoidable impurities.

[0032] (2) The forging process of the steel product is: blanking → electric furnace heating → forging temperature 850℃ → air cooling after forging. The grain size sample of the forging is subjected to 930℃×4 hours, water quenching, and the grain size is tested. The mixed crystal is 8.5-3 level, and the grains below level 5 account for 20%.

[0033] (3) Heat treatment of forgings: The annealing process is carried out according to the requirements of the heat treatment process diagram, and the annealing temperature is set at 625~635℃.

[0034] (4) After heat treatment, samples are cut from forgings for grain size testing: 930℃×4 hours, water quenching, and the grain size is tested to be 8.0-6.0. Example

[0035] (1) The steel grade of this embodiment is SAE8620H. The chemical composition of the steel is 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 unavoidable impurities.

[0036] (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 forging is subjected to 930℃×4 hours, water quenching, and the grain size is tested. The mixed crystal is 7.5-2 level, and the grains below level 5 account for 25%.

[0037] (3) Heat treatment of forgings: The annealing process is carried out according to the requirements of the heat treatment process diagram, and the annealing temperature is set at 630~640℃.

[0038] (4) After heat treatment, samples are cut from forgings for grain size testing: 930℃×4 hours, water quenching, and the grain size is tested to be 8.0-6.0. Example

[0039] (1) The steel type in this embodiment is 20CrMoH. The actual chemical composition of the steel is 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 unavoidable impurities.

[0040] (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 forging was subjected to 930℃×4 hours, water quenching, and the grain size was tested. The mixed crystal was level 8-2, and the grains below level 5 accounted for 30%.

[0041] (3) Heat treatment of forgings: The annealing process is carried out according to the requirements of the heat treatment process diagram, and the annealing temperature is set at 605~615℃.

[0042] (4) After heat treatment, samples are cut from forgings for grain size testing: 930℃×4 hours, water quenching, and the grain size is tested to be 8.5-6.0.

[0043] Although the preferred embodiments of the present invention are described in detail above, it should be clearly understood that various modifications and variations are possible for those skilled in the art. Any modification, equivalent substitution, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A heat treatment process for eliminating mixed crystals in forgings after warm forging or hot forging of low carbon gear steel, characterized in that The heat treatment process is to anneal the mixed crystal forging blank, keep it warm for 3-3.5 hours, cool it to below 500°C with the furnace, and air cool it out of the furnace. The annealing temperature is determined according to the A1 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, which is suitable for Al refined grain steel and is usually 25-35.

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

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

4. The heat treatment process for eliminating mixed crystals in forgings after warm or hot forging of low carbon gear steel according to claim 1, characterized in that: After the forging blank is annealed and heat-insulated, it is slowly cooled to 500° C. in the furnace and then taken out of the furnace.

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

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

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