Preheat treatment process of high-toughness bainite steel forge piece

Abstract

The invention provides a preheat treatment process of a high-toughness bainite steel forge piece, which comprises the following steps: step 1, temperature equalization of the forge piece: placing the forge piece in a heating furnace, heating the forge piece to 300-350 DEG C at a heating rate of less than or equal to 80 DEG C / h, and preserving heat for 4-5 hours; step 2, temperature equalization of the forge piece: heating to 900-950 DEG C at a heating speed of less than or equal to 100 DEG C / h, preserving heat for 2-4 hours, then cooling to 700 DEG C along with the furnace, and preserving heat for 2 hours; and 3, hydrogen diffusion annealing is conducted, specifically, the furnace is cooled to 650-680 DEG C, heat preservation is conducted for 24-36 h, and then discharging and air cooling are conducted to the room temperature. The technical problems of deformation, cracking, stripping and chipping, low percent of pass and short service life of the forge piece treated by a traditional preheat treatment process are solved; the structure uniformity of the bainite steel forge piece is effectively improved, forging stress is eliminated, grains are refined, and the mechanical property of the bainite steel forge piece is comprehensively improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of chemical metallurgy and metal heat treatment, and particularly relates to a preliminary heat treatment process for high-strength and tough bainitic steel forgings. Background Art

[0002] Bainitic steel is widely used in the field of railway frog due to its excellent comprehensive mechanical properties, such as high strength, good toughness and wear resistance. However, during the forging process of bainitic steel forgings, problems such as stress generated by forging deformation and tissue inhomogeneity will have an adverse impact on their final properties. Preliminary heat treatment is a heat treatment that must be carried out before the machining and final heat treatment of forgings. Through preliminary heat treatment, the hardness of the forgings can be reduced, residual stress can be eliminated, tissue heredity can be eliminated, and tissue preparation can be made for the final heat treatment. As a key process for improving the structure and properties of forgings, preliminary heat treatment is crucial for improving the quality of bainitic steel forgings. However, traditional preliminary heat treatment processes often have difficulty in precisely controlling the tissue morphology and properties of bainitic steel forgings, resulting in defects such as deformation, cracking, spalling and chipping in the forgings during subsequent heat treatment, machining or use, which are manifested as spalling defects on the frog, reducing the qualification rate and service life of the frog products. In view of this, the following improved technical solutions are proposed. Summary of the Invention

[0003] The technical problem solved by the present invention: to provide a preliminary heat treatment process for high-strength and tough bainitic steel forgings, to solve the technical problems of deformation, cracking, spalling and chipping, low qualification rate and short service life of the forgings processed by the traditional preliminary heat treatment process; to effectively improve the tissue uniformity of bainitic steel forgings, eliminate forging stress, refine grains, and comprehensively improve the mechanical properties of bainitic steel forgings.

[0004] The technical solution adopted by the present invention: a preliminary heat treatment process for high-strength and tough bainitic steel forgings, comprising the following steps:

[0005] Step 1, equalizing the temperature of the forging: placing the forging in a heating furnace, heating it to 300 - 350°C at a heating rate of ≤80°C / h, and holding for 4 - 5 hours.

[0006] Step 2, equalizing the temperature of the forging: then heating it to 900 - 950°C at a heating rate of ≤100°C / h, holding for 2 - 4 h, and then cooling it in the furnace to 700°C and holding for 2 h.

[0007] Step 3, hydrogen-expansion annealing: cooling it in the furnace to 650 - 680°C and holding for 24 - 36 h, and then taking it out of the furnace and air-cooling it to room temperature.

[0008] In the above technical solution, further: it also includes subsequent austempering steps and low-temperature tempering steps.

[0009] In the above technical solution, preferably: the forging comprises the following chemical components by weight percentage: C 0.20 - 0.30 wt%, Si 1.5 - 2 wt%, Mn 1.5 - 2 wt%, Cr 1 - 1.5 wt%, Mo 0.1 - 0.5 wt%, Ni 0.2 - 0.3 wt%, V 0.1 - 0.2 wt%, N ≤ 100 ppm, O ≤ 20 ppm, H ≤ 1 ppm, and the balance is Fe and unavoidable impurities.

[0010] In the above technical solution, preferably: the forging has a grain size of grade 10, a tensile strength ≥ 1400 MPa, a yield strength ≥ 1100 MPa, an impact energy at room temperature + 20 °C ≥ 80 J, and an impact energy at - 40 °C ≥ 60 J.

[0011] The present invention also claims the application of a forging manufactured by any of the above-mentioned preliminary heat treatment processes, and the forging is used for the manufacture of railway switches.

[0012] Advantages of the present invention compared with the prior art: The forging of the present invention has a grain size of grade 10, a tensile strength ≥ 1400 MPa, a yield strength ≥ 1100 MPa, an impact energy at room temperature + 20 °C ≥ 80 J, and an impact energy at - 40 °C ≥ 60 J, effectively improving the microstructure uniformity of bainitic steel forgings, eliminating forging stress, refining grains, and comprehensively improving the mechanical properties of bainitic steel forgings. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1 It is a process curve graph of the preliminary heat treatment process for the high-strength and tough bainitic steel forging of the present invention;

[0014] Figure 2 It is a microstructural diagram of the high-strength and tough bainitic steel forging of the present invention;

[0015] Figure 3 It is a grain size diagram of the high-strength and tough bainitic steel forging of the present invention;

[0016] Figure 4 It is a process flow chart of the preliminary heat treatment process of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0017] The following will combine the accompanying drawings in the embodiments of the present invention Figures 1-4 , and clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts belong to the scope of protection of the present invention.

[0018] A preliminary heat treatment process for a high-strength and tough bainitic steel forging includes the following steps:

[0019] Example 1:

[0020] Step 1, Forging soaking: Place the forging in a heating furnace and heat it to 300°C at a heating rate of ≤80°C / h, and hold for 4 hours.

[0021] Step 2, Forging soaking: Then heat it to 900°C at a heating rate of ≤100°C / h, hold for 2h, and then cool it in the furnace to 700°C and hold for 2h.

[0022] Step 3, Hydrogen-expansion annealing: Cool it in the furnace to 650°C and hold for 24h, and then take it out of the furnace and air-cool to room temperature.

[0023] In the above embodiment, further: It also includes subsequent isothermal quenching step and low-temperature tempering step.

[0024] In the above embodiment, preferably: The forging includes the following chemical components by weight percentage: C 0.20wt%, Si 1.5wt%, Mn 1.5wt%, Cr 1wt%, Mo 0.1wt%, Ni 0.2wt%, V 0.1wt%, N 70ppm, O 15ppm, H 0.45ppm, and the balance is Fe and inevitable impurities.

[0025] The tensile strength of the forging in Example 1 reaches 1449MPa, the yield strength reaches 1114MPa, the impact energy Ku2 at room temperature +20°C is 88J, the impact energy Ku2 at -40°C is 70J, the grain size is 10 grades, the hydrogen content is 0.58ppm. Compared with the forging of the same material without being treated by the process of the present invention, the grain microstructure is granular bainite, the grains are significantly refined, the performance is significantly improved, the tissue uniformity is significantly improved, and no obvious defects are found.

[0026] Example 2:

[0027] Step 1, Forging soaking: Place the forging in a heating furnace and heat it to 320°C at a heating rate of ≤80°C / h, and hold for 4.5 hours.

[0028] Step 2, Forging soaking: Then heat it to 930°C at a heating rate of ≤100°C / h, hold for 3h, and then cool it in the furnace to 700°C and hold for 2h.

[0029] Step 3, Hydrogen-expansion annealing: Cool it in the furnace to 665°C and hold for 30h, and then take it out of the furnace and air-cool to room temperature.

[0030] In the above embodiment, further: It also includes subsequent isothermal quenching step and low-temperature tempering step.

[0031] In the above embodiments, preferably, the forging includes the following chemical components by weight percentage: C 0.28wt%, Si 1.57wt%, Mn 1.74wt%, Cr 1.23wt%, Mo 0.35wt%, Ni 0.27wt%, V 0.13wt%, N 78ppm, O 17ppm, H 0.9ppm, and the balance is Fe and unavoidable impurities.

[0032] Example 3:

[0033] Step 1, soaking of the forging: Place the forging in a heating furnace and heat it to 340°C at a heating rate of ≤80°C / h, and hold for 4.5 hours.

[0034] Step 2, soaking of the forging: Then heat it to 930°C at a heating rate of ≤100°C / h, hold for 3h, and then cool it in the furnace to 700°C and hold for 2h.

[0035] Step 3, hydrogen diffusion annealing: Cool it in the furnace to 660°C and hold for 32h, and then take it out of the furnace and air-cool it to room temperature.

[0036] In the above embodiments, further: It also includes subsequent isothermal quenching step and low-temperature tempering step.

[0037] In the above embodiments, preferably, the forging includes the following chemical components by weight percentage: C 0.29wt%, Si 1.66wt%, Mn 1.67wt%, Cr 1.22wt%, Mo 0.38wt%, Ni 0.24wt%, V 0.13wt%, N 78ppm, O 15ppm, H 0.48ppm, and the balance is Fe and unavoidable impurities.

[0038] Example 4:

[0039] Step 1, soaking of the forging: Place the forging in a heating furnace and heat it to ~350°C at a heating rate of ≤80°C / h, and hold for 5 hours.

[0040] Step 2, soaking of the forging: Then heat it to 950°C at a heating rate of ≤100°C / h, hold for 4h, and then cool it in the furnace to 700°C and hold for 2h.

[0041] Step 3, hydrogen diffusion annealing: Cool it in the furnace to 680°C and hold for 36h, and then take it out of the furnace and air-cool it to room temperature.

[0042] In the above embodiments, further: It also includes subsequent isothermal quenching step and low-temperature tempering step.

[0043] In the above embodiments, preferably: the forging comprises the following chemical components by weight percentage: C 0.30 wt%, Si 2 wt%, Mn 2 wt%, Cr 1.5 wt%, Mo 0.5 wt%, Ni 0.3 wt%, V 0.2 wt%, N 100 ppm, O 20 ppm, H 1 ppm, and the balance is Fe and unavoidable impurities.

[0044] The tensile strength of the forging in Example 4 is 1448 MPa, the yield strength reaches 1119 MPa, the impact energy Ku2 at room temperature + 20 °C is 84 J, the impact energy Ku2 at -40 °C is 64 J, the grain size is grade 10, and the hydrogen content is 0.13 ppm. It also shows better performance than that processed by traditional processes, with uniform and fine microstructure, and the microstructure is granular bainite, meeting the performance requirements of high-quality bainite steel forgings.

[0045] The present invention also claims the application of a forging manufactured by any of the above-mentioned preliminary heat treatment processes, and the forging is used for the manufacture of railway switches.

[0046] The mechanism of action of the present invention is as follows: The isothermal operation in Step 1 aims to preliminarily homogenize the internal structure of the bainite steel, reduce the thermal stress during the subsequent annealing process, and prevent the forging blank from cracking during the subsequent heating stage due to a large temperature difference between the inside and the outside. The heating rate of ≤ 80 °C / h is relatively slow, which helps the internal temperature of the forging to rise evenly and avoid non-uniform structure caused by local overheating. The tissue uniformity is the key to obtaining high-performance steel because it ensures that the performance of each part of the steel is consistent when stressed. Insulating for 4 - 5 hours within the temperature range of 300 - 350 °C can further promote the homogenization of the internal structure of the forging. This step helps to eliminate the internal stress that may be generated during the processing of the forging and lay a good foundation for the subsequent quenching and tempering treatments. Appropriate preheating and heat preservation treatments can promote the refinement of the internal grains of the forging. Fine grains help to improve the strength and toughness of the steel because grain boundaries can hinder the movement of dislocations, thereby increasing the strength of the material. At the same time, fine grains can also provide more crack propagation paths and improve the toughness of the material. By optimizing the preheating and heat preservation conditions, it can be ensured that the forging has more stable mechanical properties during subsequent processing and use, which helps to reduce the risk of failure caused by fluctuations in material properties. Adopting a fixed heating rate and heat preservation time can more precisely control the heat treatment process of the forging, which helps to maintain the consistency of the forging performance in mass production, improve production efficiency and quality stability.

[0047] Step 2: Heat at a heating rate of ≤100 °C / h to 900 - 950 °C and hold for 2 - 4 h. The purpose is to fully austenitize the structure of the forging. Then cool in the furnace to 700 °C and hold for 2 h. The purpose is to refine the structure and reduce the hardness of the forging blank. Holding at a high temperature of 900 - 950 °C can make the austenite structure inside the forging more uniform. Austenite is the main phase of steel at high temperatures, and its uniformity has an important influence on the formation and distribution of bainite during the subsequent cooling process. Uniformly distributed austenite helps to form a uniform and fine bainite structure during cooling, thereby improving the strength and toughness of the steel. By controlling the heating rate and holding time, the bainite structure can be refined to a certain extent. Fine bainite structure helps to improve the wear resistance and fatigue resistance of the steel. Cooling in the furnace to 700 °C and holding for 2 hours helps to stabilize the transformation process from austenite to bainite and reduce the formation of unfavorable structures, such as the formation of coarse Widmanstätten structure, etc. The heating rate of ≤100 °C / h and the holding time of 2 - 4 h provide an accurate range for process control, which helps to maintain the consistency of the forging performance during mass production. Slow heating and holding treatment help to reduce defects such as cracks and deformation that may occur during the heat treatment process. Compared with rapid heating at high temperatures, using a lower heating rate and appropriate holding time can reduce energy consumption, which helps to reduce production costs and conforms to the current environmental protection and sustainable development concepts. The treatment in Step 2 not only optimizes the organizational structure of the forging but also provides a good foundation for its subsequent tempering, surface treatment, etc., which helps to comprehensively improve the mechanical properties and service life of the steel.

[0048] For the hydrogen-expansion annealing operation in Step 3, furnace cool to 650 - 680 °C and hold for 24 - 36 h. The purpose is to promote the dissipation of hydrogen, and then take the workpiece out of the furnace and air cool to room temperature. During forging, hydrogen is easily absorbed and dispersed into the material, thus reducing the strength and toughness of the material. Through the hydrogen-expansion annealing operation, hydrogen molecules can diffuse out of the material at high temperature and be discharged with the atmosphere during furnace cooling, thus effectively removing hydrogen from the forging. The heat preservation treatment during the hydrogen-expansion annealing process helps to optimize the microstructure of the forging. Holding for 24 - 36 hours within the temperature range of 650 - 680 °C can promote the homogenization and refinement of the internal structure of the forging, reduce structural defects, and improve the overall performance of the material. For the forging after hydrogen removal, its strength and toughness will be significantly improved. At the same time, the microstructure optimization during the hydrogen-expansion annealing process also helps to improve the mechanical properties such as wear resistance and fatigue resistance of the forging, which makes the forging have better reliability and durability during subsequent processing and use. The hydrogen-expansion annealing operation has the characteristics of high process stability and controllability. By precisely controlling the heat preservation temperature and time, it can ensure that the forging obtains consistent performance during the heat treatment process, which helps to maintain the consistency of forging performance during mass production, improve production efficiency and quality stability. The hydrogen-expansion annealing operation is relatively simple and easy to control, without additional energy consumption. At the same time, no harmful substances are generated during this process, which is harmless to the environment and operators, and this conforms to the current environmental protection and sustainable development concepts, helping to reduce industrial pollution and waste of resources.

[0049] The subsequent isothermal quenching and low-temperature tempering operations are used to enable bainitic steel forgings to obtain uniform and fine grains, and the residual stress is significantly reduced. Among them, isothermal quenching is one of the key steps in the heat treatment of bainitic steel. In this step, the forging is rapidly cooled to the bainite transformation temperature range and maintained at this temperature for a period of time to promote the formation of bainite structure. Low-temperature tempering is carried out after isothermal quenching. Its main purpose is to eliminate the internal stress generated by quenching, stabilize the structure, and prevent grain growth. During the isothermal quenching process, the forging is kept in the bainite transformation temperature range for a period of time, which helps to homogenize the internal stress distribution and reduce the residual stress caused by quenching. Low-temperature tempering can further release the residual stress inside the forging and make the material more stable, which helps to reduce problems such as cracks and deformation caused by stress concentration. Through the combined process of isothermal quenching and low-temperature tempering, a good balance between strength and toughness can be obtained. This makes bainitic steel forgings have better resistance to deformation and fracture when subjected to high loads and impacts. Small and uniform grains and low residual stress help improve the wear resistance and fatigue resistance of forgings, which enables forgings to maintain stable performance during long-term use. Austempering and low-temperature tempering processes are highly stable and controllable. By precisely controlling process parameters (such as temperature, time, etc.), it can ensure that forgings obtain consistent performance during heat treatment, which helps to maintain the consistency of forging performance in mass production and improve production efficiency and quality stability.

[0050] From the above description, it can be found that: after testing, the preparatory heat treatment process of the present invention, compared with the traditional process, can produce forgings with a grain size of 10, a hydrogen content of ≤1ppm, a tensile strength of ≥1400MPa, a yield strength of ≥1100MPa, an impact energy of ≥80J at room temperature +20℃, and an impact energy of ≥60J at -40℃, which effectively improves the strength and toughness of high-strength and high-toughness bainitic steel forgings and meets the use requirements of high-performance and high-strength and high-toughness bainitic steel forgings in the fields of railway turnouts.

[0051] In summary, the present invention effectively solves the technical problems of deformation, cracking, flaking, low pass rate and short life of forgings treated by traditional preliminary heat treatment process; improves the uniformity of the structure of bainitic steel forgings, eliminates forging stress, refines the grains, and comprehensively improves the mechanical properties of bainitic steel forgings.

[0052] The above preferred embodiments are not intended to limit the scope of implementation of the present invention, so all equivalent changes made to the contents described in the claims of the present invention should be included in the scope of the claims of the present invention.

Claims

1. A preliminary heat treatment process for high-strength and toughness bainitic steel forgings, characterized in that: The steps include: Step 1, forging temperature: place the forging in a heating furnace, heat to 300-350°C at a heating rate of ≤80°C / h, and keep warm for 4-5 hours; Step 2, forging temperature: heat to 900-950℃ at a heating rate of ≤100℃ / h, keep warm for 2-4h, then cool to 700℃ and keep warm for 2h; Step 3, hydrogen expansion annealing: cool the furnace to 650-680℃ and keep it for 24-36h, then take it out of the furnace and air cool it to room temperature.

2. The preliminary heat treatment process according to claim 1, characterized in that: A subsequent isothermal quenching step and a low temperature tempering step are also included.

3. The preliminary heat treatment process according to claim 1, characterized in that: The forging comprises the following chemical components by weight percentage: C 0.20-0.30wt%, Si 1.5-2wt%, Mn 1.5-2wt%, Cr 1-1.5wt%, Mo 0.1-0.5wt%, Ni0.2-0.3wt%, V 0.1-0.2wt%, N≤100ppm, O≤20ppm, H≤1ppm, and the balance is Fe and unavoidable impurities.

4. The preliminary heat treatment process according to claim 1 or 3, characterized in that: The forging has a grain size of grade 10, a tensile strength of ≥1400MPa, a yield strength of ≥1100MPa, an impact energy of ≥80J at room temperature, and an impact energy of ≥60J at -40°C.

5. An application of a forging manufactured by the preliminary heat treatment process according to any one of claims 1 to 4, characterized in that: The forging is used for manufacturing railway frogs.

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