Structure refining heat treatment process for overheated bainite non-quenched and tempered steel front axle
By using annealing and normalizing processes, the microstructure of the overheated bainitic non-quenched and tempered steel front shaft is controlled to transform into ferrite and pearlite, solving the problem of coarse grains caused by overheating, achieving grain refinement and performance improvement, reducing scrap rate, and promoting the industrial application of bainitic non-quenched and tempered steel front shafts.
Patent Information
- Application Number
- CN202511096934.4
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-08-06
- Publication Date
- 2025-11-07
AI Technical Summary
Bainitic non-quenched and tempered steel front shafts exhibit coarse grains or mixed-grain structures when overheated, leading to a decrease in strength, toughness, and fatigue performance. Conventional normalizing treatment is difficult to effectively repair this, resulting in a high scrap rate and limiting its industrial application.
By measuring the CCT curve and using annealing and normalizing processes, the microstructure of the front shaft of the overheated bainitic non-quenched and tempered steel was controlled to transform into ferrite and pearlite. The ferrite/pearlite grain boundaries were used as austenite nucleation sites to achieve grain refinement. Furthermore, the formation of martensite was avoided by controlling the cooling rate, thus achieving microstructure and property restoration.
The grain size of the overheated bainitic non-quenched and tempered steel front shaft was significantly refined, improving mechanical properties such as yield strength, tensile strength and impact energy, reducing scrap rate and increasing product qualification rate.
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Abstract
Description
TECHNICAL FIELD
[0001] The application belongs to the technical field of automobile parts manufacturing and heat treatment, and particularly relates to a microstructure refining heat treatment process for a superheated bainite non-quenched and tempered steel front axle. BACKGROUND
[0002] Under the background of low-carbon economy, the application of bainite non-quenched and tempered steel is more and more extensive. The bainite non-quenched and tempered steel workpiece usually obtains bainite organization by air cooling or natural cooling using the residual heat after hot forming. The thermal stress and phase change stress of the workpiece during the cooling process are small, and the deformation of the workpiece can be effectively controlled. Meanwhile, the bainite can have the plasticity and toughness of high-temperature transformation products and the strength of low-temperature transformation products, and good comprehensive mechanical properties can be obtained. Therefore, the bainite non-quenched and tempered steel has been paid attention to by domestic and foreign scholars in recent years, and is a kind of non-quenched and tempered steel material which develops rapidly in engineering application. However, when the bainite non-quenched and tempered steel is used to manufacture a large-specification heavy truck front axle, the following problems may exist. In the actual production process, the bainite non-quenched and tempered steel front axle is usually formed by forging using a round bar. The round bar is inductively heated to about 1200 DEG C and then hot formed. However, the inductive heating often has temperature fluctuations. When the heating temperature exceeds 1250 DEG C, the workpiece is overheated, which causes the grain of the front axle product to be coarse or mixed crystal organization to appear, seriously damages the strength and toughness and fatigue performance of the front axle, and causes the performance of the workpiece to be unable to meet the use requirements. In engineering, the organization of the overheated workpiece is usually repaired by normalizing treatment and then reheat treatment. However, the non-quenched and tempered steel has a significant organization inheritance phenomenon, and it is difficult to refine and repair the organization by the conventional normalizing treatment, which causes the waste product rate of the bainite non-quenched and tempered steel front axle to be too high in the actual production process, and seriously restricts the industrialization and popularization and application thereof.
[0003] Therefore, it is the key to promote the large-scale engineering application of the heavy truck bainite non-quenched and tempered steel front axle to realize the organization repair and mechanical property improvement of the overheated non-quenched and tempered steel front axle by using a heat treatment process. SUMMARY
[0004] To solve the above technical problems, the application provides a microstructure refining heat treatment process for an overheated bainite non-quenched and tempered steel front axle, and the main technical idea is as follows: (1) the CCT curve of the overheated bainite non-quenched and tempered steel is accurately measured, and a suitable annealing process is formulated to make the overheated bainite non-quenched and tempered steel front axle re-austenitized into ferrite and pearlite type organization, avoid forming bainite and martensite type organization, and eliminate the influence of alloy organization inheritance; (2) on this basis, recrystallization is caused by normalizing treatment to refine the grain, and the organization transformation in the cooling process is controlled by a suitable cooling process, so that the overheated bainite non-quenched and tempered steel organization and mechanical properties are repaired.
[0005] The technical problem solved by the present application is solved by the following technical scheme: a microstructure refining heat treatment process of a superheated bainite non-quenched and tempered steel front axle, the superheated bainite non-quenched and tempered steel front axle has the following chemical components in percentage by weight: C 0.20-0.28%, Mn 1.7-2.6%, Si 0.3-0.6%, Cr 0.4-0.8%, B 0.001-0.005%, Ti 0.02-0.03%, Nb 0.02-0.05%, N 0.008-0.012%, S 0.030-0.065%, P≤0.025%, O≤0.0015%, and the rest is Fe and inevitable impurity elements; By measuring the CCT curve of the superheated bainite non-quenched and tempered steel front axle, the annealing process is used to completely austenitize the superheated bainite non-quenched and tempered steel front axle, and the re-austenitized superheated bainite non-quenched and tempered steel front axle is converted into ferrite and pearlite type microstructure; the CCT curve measurement method is a prior art, and will not be described here; Then, the annealed superheated bainite non-quenched and tempered steel front axle is recrystallized by using the normalizing process, and bainite transformation is carried out.
[0006] Preferably, the annealing process comprises: Step 1) austenitizing heat treatment: the superheated bainite front axle is heated to 850-900 DEG C and held for 1-3 h to ensure that the superheated bainite non-quenched and tempered steel front axle is completely austenitized; Step 2) step cooling treatment: the superheated bainite non-quenched and tempered steel front axle is cooled at a cooling rate of 80-120 DEG C / h to 750-700 DEG C and held for 1-3 h to make the superheated bainite non-quenched and tempered steel front axle have sufficient ferrite microstructure transformation; Then, the superheated bainite non-quenched and tempered steel front axle is cooled at a cooling rate of 30-60 DEG C / h to 680-650 DEG C and held for 3-7 h to make the superheated bainite non-quenched and tempered steel front axle have sufficient pearlite microstructure transformation; Finally, the furnace is cooled to below 400 DEG C and discharged to ensure that all austenite is converted into ferrite / pearlite type microstructure.
[0007] Preferably, the normalizing process comprises: Step S1) austenitizing heat treatment: the annealed superheated bainite non-quenched and tempered steel front axle is reheated to 850-950 DEG C and held for 1-3 h to ensure that the superheated bainite non-quenched and tempered steel front axle is completely austenitized; Step S2) cooling control: The superheated bainite non-quenched and tempered steel front axle after austenitizing heat treatment is discharged and air-cooled or air-cooled, and by controlling the cooling rate, the superheated bainite non-quenched and tempered steel front axle has sufficient bainite transformation and realizes tempering treatment.
[0008] Preferably, the cooling speed of the bainite transformation process is specifically: the cooling speed is greater than or equal to 5 ℃ / min at a temperature greater than or equal to 550 ℃, so as to avoid the generation of ferrite and pearlite in the structure; The cooling speed is controlled to be 2-10 ℃ / min in the temperature range of 550-300 ℃, so as to perform sufficient bainite transformation and avoid the generation of martensite in the structure.
[0009] Preferably, the cooling speed of the tempering treatment is specifically: the cooling speed is less than or equal to 2 ℃ / min at a temperature less than or equal to 300 ℃, and the tempering is performed by using the residual temperature.
[0010] Preferably, the grain size of the over-heated bainite non-quenched and tempered steel front axle after the heat treatment is greater than or equal to 8. The mechanical properties of the structure after the repair are: the yield strength Rp0.2 is greater than or equal to 760 MPa, the tensile strength Rm is greater than or equal to 960 MPa, the elongation A after the fracture is greater than or equal to 16%, the reduction of area Z is greater than or equal to 45%, and the impact energy AKU2 at room temperature is greater than or equal to 65 J.
[0011] Compared with the prior art, the present application has the following beneficial effects: Based on the chemical composition of the over-heated bainite non-quenched and tempered steel front axle, the CCT curve of the over-heated bainite non-quenched and tempered steel front axle is determined, the over-heated bainite non-quenched and tempered steel front axle is completely austenitized by using the annealing treatment process, the over-heated bainite non-quenched and tempered steel front axle after the re-austenitization is transformed into ferrite and pearlite type structure, the recrystallization of the over-heated bainite non-quenched and tempered steel front axle after the annealing treatment is performed by using the normalizing treatment process, and the bainite transformation is performed. By transforming the over-heated bainite non-quenched and tempered steel front axle after the re-austenitization into the ferrite and pearlite type structure, the over-heated non-quenched and tempered steel front axle is used as the nucleation site of the austenite in the austenitization heating process of the normalizing treatment, the multi-point nucleation of the austenite realizes the re-division and refinement of the austenite grains of the over-heated non-quenched and tempered steel front axle, the purpose of the structure refinement is achieved, the bainite structure obtained by the recrystallization is refined, the structure and the mechanical properties of the over-heated bainite non-quenched and tempered steel front axle are significantly repaired, and the qualified rate of the non-quenched and tempered steel front axle product in the actual production process is improved. BRIEF DESCRIPTION OF DRAWINGS
[0012] Figure 1 The grain size morphology of the over-heated bainite non-quenched and tempered steel front axle before the heat treatment is shown in the figure; Figure 2 The metallographic structure of the over-heated bainite steel front axle after the annealing treatment of the embodiment 1 is shown in the figure; Figure 3 The metallographic structure of the over-heated bainite steel front axle after the annealing treatment of the embodiment 2 is shown in the figure; Figure 4 The metallographic structure of the over-heated bainite steel front axle after the annealing treatment of the embodiment 3 is shown in the figure; Figure 5 Microstructure of the over-heated bainite steel front axle after annealing treatment of example 4; Figure 6 Microstructure of the over-heated bainite steel front axle after microstructure repair of example 1; Figure 7 Microstructure of the over-heated bainite steel front axle after microstructure repair of example 2; Figure 8 Microstructure of the over-heated bainite steel front axle after microstructure repair of example 3; Figure 9 Microstructure of the over-heated bainite steel front axle after microstructure repair of example 4. DETAILED DESCRIPTION
[0013] The technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the drawings.
[0014] The technical solution adopted by the present application is a microstructure refining heat treatment process for an over-heated bainite non-quenched and tempered steel front axle of a heavy truck, and the chemical composition of the over-heated bainite non-quenched and tempered steel front axle in percentage by weight is as follows: C 0.20-0.28%, Mn 1.7-2.6%, Si 0.3-0.6%, Cr 0.4-0.8%, B 0.001-0.005%, Ti 0.02-0.03%, Nb 0.02-0.05%, N 0.008-0.012%, S 0.030-0.065%, P≤0.025%, O≤0.0015%, and the rest is Fe and inevitable impurity elements.
[0015] The non-quenched and tempered steel has good hardenability, and the microstructure of the over-heated bainite non-quenched and tempered steel front axle is usually bainite / martensite, but the microstructure has significant genetic characteristics, and it is difficult to refine and repair the microstructure through conventional normalizing treatment. Therefore, the present application focuses on solving the microstructure and mechanical property repair problem of the over-heated bainite non-quenched and tempered steel front axle with such alloy composition.
[0016] The microstructure refining heat treatment process for the over-heated bainite non-quenched and tempered steel front axle comprises the following steps: Step 1: annealing treatment The annealing process provided by the present application comprises the following steps: Step 1) austenitizing heat treatment The over-heated bainite front axle is heated to 850-900℃ and held for 1-3h, and the specific heating rate and holding time are determined according to the actual charging amount of the workpiece to ensure complete austenitization of the front axle.
[0017] Step 2) segmented cooling treatment After the above austenitizing heating, the pre-axle is cooled at a cooling rate of 80-120℃ / h to 750-700℃ and then kept for 1-3h to make the ferrite structure fully transformed; then cooled at a cooling rate of 30-60℃ / h to 680-650℃ and kept for 3-7h to make the pearlite structure fully transformed; and then the workpiece is cooled to below 400℃ out of the furnace to ensure that all the austenite is transformed into ferrite / pearlite type structure.
[0018] Step 2: normalizing treatment The normalizing process provided by the present application comprises the following steps: Step 1) austenitizing treatment The non-quenched and tempered steel pre-axle after the above annealing treatment is reheated to 850-950℃ and kept for 1-3h, and the specific heating rate and holding time are determined according to the actual loading amount of the workpiece to ensure that the pre-axle is completely austenitized, thereby preparing for the subsequent normalizing treatment. During the austenitizing heating process, the grain boundaries and phase interfaces of ferrite / pearlite can be used as austenite nucleation sites, and the multi-point nucleation of austenite can realize the re-division and refinement of the austenite grains of the overheated non-quenched and tempered steel pre-axle.
[0019] Step 2) cooling control The non-quenched and tempered steel pre-axle after the re-austenitizing treatment is air-cooled or air-cooled after being taken out of the furnace, and the cooling rate of the workpiece is ≥5℃ / min when the temperature is above 550℃ to avoid the formation of ferrite and pearlite in the structure; the cooling rate is controlled at 2-10℃ / min in the temperature range of 550-300℃ to make the bainite fully transformed and avoid the formation of martensite in the structure, thereby reducing the toughness of the material; and the cooling rate is ≤2℃ / min when the temperature is below 300℃ to make the workpiece tempered by using the residual temperature, thereby reducing the stress of the pre-axle and improving the plasticity and toughness thereof.
[0020] The grain size of the overheated bainite non-quenched and tempered steel pre-axle after the microstructure refinement heat treatment process is refined to 8 or more, and the mechanical properties are significantly improved, with the yield strength Rp0.2≥760MPa, the tensile strength Rm≥960MPa, the elongation A≥16%, the reduction of area Z≥45%, and the room temperature impact energy AKU2≥65J.
[0021] Examples 1-4 Examples 1-4 are bainite non-quenched and tempered steel pre-axles for heavy trucks, and the percentage of the chemical components is shown in Table 1. The bainite steel pre-axles of Examples 1-4 are overheated due to the excessively high temperature in the forging heating process, the pre-axle structure appears coarse grains and mixed crystals, the impact energy (AKU2) is less than 20J, the elongation is less than 13%, and the mechanical properties cannot meet the use requirements. The microstructure and mechanical properties are repaired by using the microstructure refinement heat treatment method provided by the present application, and the specific heat treatment process is shown in Table 2. The microstructure characteristics and mechanical property indexes before and after the microstructure refinement heat treatment are listed in Table 3.
[0022] Table 1 Chemical composition of bainite non-quenched and tempered steel (wt. %)
[0023] Table 2 Process parameters of microstructure refining heat treatment of each example
[0024] Table 3 Comparison of microstructure and mechanical properties of each example before and after repair
[0025] Figure 1 It is a prior austenite grain size morphology diagram of superheated bainite, and the grain size is about 4 levels. Figures 2-5 They are metallographic structures of superheated bainite non-quenched and tempered steel prior shafts after annealing treatment of examples 1-4, and ferrite + pearlite structures are obtained; Figures 6-9 They are metallographic structures of examples 1-4 after prior shaft microstructure repair, and bainite structures are obtained, and the microstructure is refined.
[0026] The application can effectively solve the microstructure and mechanical property repair problem of superheated bainite non-quenched and tempered steel prior shaft, improve the qualified rate of non-quenched and tempered steel prior shaft products in the actual production process, and break through the technical bottleneck restricting the popularization and application of bainite non-quenched and tempered steel prior shaft.
Claims
1. A process for microstructural refinement heat treatment of a superheated bainitic non-quenched and tempered steel front axle, characterized by, The chemical composition of the superheated bainite non-quenched and tempered steel front axle by weight percentage: C 0.20-0.28%, Mn 1.7-2.6%, Si 0.3-0.6%, Cr 0.4-0.8%, B 0.001-0.005%, Ti 0.02-0.03%, Nb 0.02-0.05%, N 0.008-0.012%, S 0.030-0.065%, P ≤0.025%, O ≤0.0015%, and the rest is Fe and inevitable impurity elements; The superheated bainite non-quenched and tempered steel front axle is completely austenitized by annealing treatment process, and the re-austenitized superheated bainite non-quenched and tempered steel front axle is transformed into ferrite and pearlite type structure by measuring the CCT curve of the superheated bainite non-quenched and tempered steel front axle. Then, the recrystallization of the annealed superheated bainite non-quenched and tempered steel front axle occurs by normalizing treatment process, and the bainite transformation occurs.
2. The process for microstructural refinement heat treatment of over- hardened bainitic non-quenched and tempered steel front axle as claimed in claim 1 wherein, The annealing treatment process comprises: Step 1) austenitizing heating treatment: the superheated bainite front axle is heated to 850-900℃ and held for 1-3h to ensure that the superheated bainite non-quenched and tempered steel front axle is completely austenitized; Step 2) staged cooling treatment: the superheated bainite non-quenched and tempered steel front axle is cooled to 750-700℃ at a cooling rate of 80-120℃ / h and held for 1-3h to make the superheated bainite non-quenched and tempered steel front axle fully transform into ferrite structure; Then, the superheated bainite non-quenched and tempered steel front axle is cooled to 680-650℃ at a cooling rate of 30-60℃ / h and held for 3-7h to make the superheated bainite non-quenched and tempered steel front axle fully transform into pearlite structure; Finally, the superheated bainite non-quenched and tempered steel front axle is furnace-cooled to below 400℃ and discharged to ensure that all austenite is transformed into ferrite / pearlite type structure.
3. The microstructural refinement heat treatment process for a superheated bainitic non-quenched and tempered steel front axle as claimed in claim 1 or 2, wherein, The normalizing treatment process comprises: Step S1) austenitizing heating treatment: the annealed superheated bainite non-quenched and tempered steel front axle is reheated to 850-950℃ and held for 1-3h to ensure that the superheated bainite non-quenched and tempered steel front axle is completely austenitized; Step S2) cooling control: The austenitized superheated bainite non-quenched and tempered steel front axle is discharged and air-cooled or air-cooled, and the cooling rate is controlled to make the superheated bainite non-quenched and tempered steel front axle fully transform into bainite and realize tempering treatment.
4. The process for microstructural refinement heat treatment of over- hardened bainitic non-quenched and tempered steel front axle as claimed in claim 3 wherein, The cooling rate during the bainite transformation process is specifically: the cooling rate is ≥5℃ / min at a temperature above 550℃ to avoid the generation of ferrite and pearlite in the structure; The cooling rate is controlled at 2-10℃ / min at a temperature of 550-300℃ to fully transform into bainite and avoid the generation of martensite in the structure.
5. The process for microstructural refinement heat treatment of over- hardened bainitic non-quenched and tempered steel front axle as claimed in claim 3 wherein, The cooling rate during the tempering treatment is specifically: the cooling rate is ≤2℃ / min at a temperature below 300℃ to utilize the residual temperature for tempering.
6. The process for microstructural refinement heat treatment of over- hardened bainitic non-quenched and tempered steel front axle as claimed in claim 1 wherein: The grain size of the superheated bainite non-quenched and tempered steel front axle after heat treatment is above 8 levels; The mechanical properties after the structure repair are: yield strength Rp0.2 ≥760MPa, tensile strength Rm ≥960MPa, elongation A ≥16%, reduction of area Z ≥45%, and room temperature impact energy AKU2 ≥65J.