Medium-carbon high-chromium non-quenched and tempered bainite molded steel automobile steering knuckle and preparation process thereof

Abstract

The invention discloses a medium-carbon high-chromium non-quenched and tempered bainite type steel automobile steering knuckle and a preparation process thereof.The preparation process comprises the following steps that S1, a medium-carbon high-chromium bainite type steel blank is heated and subjected to complete austenitizing treatment, and a heated blank is obtained; s2, the heated blank is subjected to hot die forging forming treatment, and a forge piece is obtained; and S3, the obtained forged piece starts to be cooled within a preset time interval, and the medium-carbon high-chromium non-quenched and tempered bainite molded steel automobile steering knuckle is obtained. Through raw material component design and process optimization, traditional quenching and high-temperature tempering hardening and tempering procedures are omitted, microscopic structures and mechanical properties meeting use performance requirements are directly obtained, it is ensured that under the condition that parts are not subjected to hardening and tempering treatment, needed comprehensive properties such as high strength and high toughness can be achieved, the production process is remarkably simplified, and the production cost is reduced. The manufacturing period is shortened, and the energy consumption and the cost are reduced.

Description

Technical Field

[0001] The invention relates to the technical field of hot processing and forming of metal materials, in particular to a medium-carbon high-chromium non-quenched and tempered bainite steel automobile steering knuckle and a preparation process thereof. Background Art

[0002] As a key component in the automotive steering system, the steering knuckle supports the front wheel and drives it around the kingpin. It also must withstand various impact loads from the road. Therefore, the knuckle must possess comprehensive mechanical properties, including high strength, high toughness, high fatigue strength, and good wear resistance, to ensure vehicle stability and safety. Currently, automotive steering knuckles are primarily manufactured from medium-carbon alloy steels, such as quenched and tempered steels like 40Cr, 42CrMo, and 35CrMo. The traditional production process typically includes blanking, heating, multi-fire die forging, trimming and punching, quenching and high-temperature tempering (i.e., quenching and tempering), and final machining. While this process is mature and reliable, it also has some significant drawbacks. First, the quenching and tempering heat treatment not only increases the production process and manufacturing cycle time, but also consumes significant energy and increases costs. Second, the quenching process can generate significant internal stresses and deformation, which not only complicates subsequent correction and machining but also can cause quenching cracks, reducing product yield. Furthermore, the quenching media used in the quenching and tempering process (such as oil or water-based solutions) can be environmentally polluting.

[0003] To overcome the shortcomings of these traditional processes, researchers have begun exploring the application of non-quenched and tempered steels, particularly controlled rolling and controlled cooling (CRCC) steels. These steels achieve the desired microstructure and mechanical properties directly through precise control of the thermomechanical processing and cooling processes, thus eliminating the need for traditional heat treatment. Bainitic non-quenched and tempered steels have attracted widespread attention due to their excellent strength-toughness balance. Bainite, particularly lower bainite or granular bainite, can, under certain conditions, offer higher strength and better toughness than tempered troostite or troostite after quenching and tempering, particularly in terms of impact toughness and fatigue performance. However, achieving a uniform and ideal bainite structure and ensuring stable performance during the controlled cooling process after forging for automotive steering knuckle forgings, which have complex shapes and widely varying cross-sectional dimensions, remains a significant challenge. Firstly, the complex geometry of the steering knuckle results in significant differences in cooling rates across various parts, making uniform cooling and microstructural transformation difficult. Secondly, the narrow bainite transformation zone in the continuous cooling transformation curve (CCT) of conventional medium-carbon or low-alloy steels is susceptible to pearlite or martensite transformations, necessitating extremely precise control of the cooling process. In recent years, researchers have attempted to broaden the bainite transformation zone, improve the hardenability of steel, and optimize performance by increasing the content of alloying elements. Chromium can significantly improve the hardenability of steel, shifting the CCT curve to the lower right, thereby broadening the temperature and time window for bainite transformation, reducing sensitivity to cooling rate, and providing the possibility of controlled cooling for bainitization of complex forgings. However, designing a complete forging and controlled cooling process for specific medium-carbon, high-chromium bainitic steels suitable for complex forgings such as automotive steering knuckles, ensuring that uniform microstructure and excellent comprehensive mechanical properties that meet the requirements of use are obtained without quenching and tempering treatment, remains a technical challenge that needs to be solved urgently. Existing non-quenched and tempered steel forging technologies often have difficulty ensuring cooling uniformity when facing complex-shaped parts, resulting in uneven microstructure and performance; or fail to fully tap the potential of medium-carbon, high-chromium bainitic steels; or the process control window is too narrow and the stability is poor, making it difficult to achieve stable industrial production.

[0004] Therefore, it is necessary to provide an improved medium-carbon high-chromium non-quenched and tempered bainitic steel automobile steering knuckle forging forming process method to achieve efficient, low-cost, high-performance automobile steering knuckle manufacturing. Summary of the Invention

[0005] In view of this, the present application provides a medium-carbon high-chromium non-quenched and tempered bainitic steel automobile steering knuckle and a preparation process thereof, which are used to solve the problem of how to achieve efficient, low-cost, and high-performance automobile steering knuckle manufacturing.

[0006] In order to achieve the above technical objectives, this application adopts the following technical solutions: In a first aspect, the present application provides a process for preparing a medium-carbon, high-chromium, non-quenched and tempered bainitic steel automobile steering knuckle, comprising the following steps: S1. The medium carbon high chromium bainite steel billet is heated and completely austenitized to obtain a heated billet; S2. The heated billet is subjected to hot die forging to obtain a forging; S3. The obtained forging begins cooling treatment within a preset time interval to obtain a medium-carbon, high-chromium, non-quenched and tempered bainitic steel automobile steering knuckle.

[0007] Preferably, in step S1, the medium-carbon high-chromium bainitic steel blank includes the following components, by mass percentage: carbon (C): 0.30% to 0.50%, chromium (Cr): 1.0% to 2.5%, manganese (Mn): 0.8% to 1.8%, silicon (Si): 0.20% to 0.80%, molybdenum (Mo): 0.10% to 0.50%, vanadium (V): 0.05% to 0.20%, boron (B): 0.0005% to 0.0030%, sulfur (S) ≤ 0.025%, phosphorus (P) ≤ 0.025%, and the balance is iron (Fe) and impurities.

[0008] Preferably, in step S1, the temperature of the complete austenitizing treatment is 1180-1250° C., and the treatment atmosphere is a polyaromatic atmosphere.

[0009] Preferably, in step S2, the hot die forging process is a multi-step closed die forging process; the hot die forging process includes a pre-forging process and a final forging process.

[0010] Preferably, in step S2, the temperature of the pre-forging treatment is 1150-1230°C, and the temperature of the final forging treatment is higher than the Ac3 point of the medium-carbon high-chromium bainite steel blank.

[0011] Preferably, in step S3, the cooling treatment method includes one or more of air cooling, atomization cooling, spray cooling, isothermal cooling, continuous cooling-isothermal cooling, and residual heat insulation slow cooling.

[0012] Preferably, in step S3, the preset time interval is ≤15s.

[0013] Preferably, in step S3, the cooling process is as follows: cooling at 550-800°C at a rate of ≥10°C / s, and then cooling at 350-550°C at a rate of 0.5-5°C / s.

[0014] Preferably, the method further comprises the steps of: performing correction and / or shot peening after the cooling treatment.

[0015] In a second aspect, the present application provides a medium-carbon, high-chromium, non-quenched and tempered bainitic steel automobile steering knuckle, which includes lower bainite and / or granular bainite accounting for ≥90% by mass.

[0016] The beneficial effects of this application are as follows: 1. Significantly simplifies the production process and shortens the manufacturing cycle: This invention achieves a bainite structure that meets performance requirements by precisely controlling the forging and post-forging cooling processes. This completely eliminates the necessary quenching and tempering heat treatment (quenching + high-temperature tempering) and its associated auxiliary processes (such as normalizing or annealing before heat treatment) required in traditional processes. This significantly shortens the entire production process, improves production efficiency, and can reduce the manufacturing cycle by over 30%.

[0017] 2. Significantly Reduced Energy Consumption and Production Costs: Eliminating the energy-intensive quenching and tempering heat treatment process directly saves significant amounts of electricity and fuel. This also reduces heat treatment equipment investment, maintenance costs, and associated labor costs. Overall, this significantly reduces the unit production cost of steering knuckles and enhances product competitiveness.

[0018] 3. Improved Product Performance and Reliability: Through optimized steel composition and precisely controlled process parameters, this invention achieves a uniform microstructure dominated by fine bainite. Compared to the tempered troostite / bainite structure of conventional quenched and tempered steel, this bainite structure typically exhibits higher strength (especially yield strength), better toughness (especially low-temperature impact toughness), and superior fatigue performance. Furthermore, by avoiding the quenching process, quenching stress and deformation are reduced, lowering the risk of quenching cracks. This improves the product's dimensional stability and internal quality, thereby enhancing the safety and reliability of the steering knuckle.

[0019] 4. Improved part quality uniformity: This invention pays special attention to the problem of controlling cooling uniformity for the complex shape of the steering knuckle. Through the designed cooling strategy, it strives to ensure that different parts of the forging (such as flanges, arms, journals and other areas with different thicknesses) can obtain a relatively consistent cooling process and final structure, thereby ensuring the uniformity and stability of the mechanical properties of the entire part, which is crucial for key safety parts.

[0020] 5. Environmentally friendly: Omitting the quenching and tempering heat treatment reduces exhaust emissions from fossil fuel combustion and indirect emissions from electricity consumption. At the same time, it avoids the use of media that may pollute the environment, such as quenching oil, and is more in line with the development trend of green manufacturing.

[0021] 6. Process adaptability and stability: The medium-carbon, high-chromium bainitic steel used has a wider bainite transformation window, which reduces the extreme sensitivity to cooling rate. This makes the controlled cooling process of the present invention have better process stability and feasibility for industrial application compared to ordinary non-quenched and tempered steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is the microstructure diagram of the bainite steel used in this application; Figure 2 This is a hardness diagram detected after forging by the process method of the present invention; Figure 3 This is the stress-strain diagram detected after forging using the process method of the present invention. DETAILED DESCRIPTION

[0023] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention.

[0024] The present application provides a process for preparing a medium-carbon, high-chromium, non-quenched and tempered bainitic steel automobile steering knuckle, comprising the following steps: S1. The medium carbon high chromium bainite steel billet is heated and completely austenitized to obtain a heated billet; S2. The heated billet is subjected to hot die forging to obtain a forging; S3. The obtained forging begins cooling treatment within a preset time interval to obtain a medium-carbon, high-chromium, non-quenched and tempered bainitic steel automobile steering knuckle.

[0025] This application uses medium-carbon, high-chromium bainitic steel within the specified range. The billet is heated to a fully austenitized state, forged within an appropriate temperature range, and targeted controlled cooling is implemented immediately after final forging. The cooling path is designed to avoid the pearlite and proeutectoid ferrite transformation zones on the steel's continuous cooling transformation curve (CCT curve) and promote the maximum transformation of supercooled austenite to a microstructure dominated by bainite. This eliminates the traditional quenching and high-temperature tempering process, directly achieving microstructure and mechanical properties that meet performance requirements. By precisely controlling the forging and cooling processes, the parts achieve the required high strength, high toughness, and other comprehensive properties without tempering, significantly simplifying the production process, shortening the manufacturing cycle, and reducing energy consumption and costs. By closely integrating the forging process with a specific controlled cooling strategy after forging, the phase transformation is actively guided toward bainite by controlling the austenite state after final forging and a precisely designed cooling path, suppressing other undesirable phase transformations.

[0026] The present application performs cooling treatment immediately after hot die forging, which is the core step to achieve the non-tempering goal and directly determines the final microstructure and mechanical properties. After the hot die forging treatment is completed (final forging is completed), the forging must be immediately removed from the forging die and quickly transferred to a specially designed controlled cooling system for cooling. The controlled cooling strategy aims to enable the cooling curve of the forging as a whole (especially the key stress-bearing parts) to successfully avoid the pearlite and ferrite transformation zones on the CCT curve and fall into the bainite transformation zone as completely as possible, ultimately obtaining a microstructure dominated by fine needle-shaped lower bainite, granular bainite or a mixture of the two, while avoiding the generation of a large amount of coarse upper bainite, blocky martensite or untransformed residual austenite. Then, a metallographic microscope is used to observe and compare the differences in the organizational characteristics of different slices, such as the morphology, size and distribution of bainite, to determine the uniformity of the organization and performance of different parts of the entire forging.

[0027] In some embodiments, in step S1, the medium-carbon high-chromium bainitic steel blank includes the following components, calculated by mass percentage: carbon (C): 0.30% to 0.50%, chromium (Cr): 1.0% to 2.5%, manganese (Mn): 0.8% to 1.8%, silicon (Si): 0.20% to 0.80%, molybdenum (Mo): 0.10% to 0.50%, vanadium (V): 0.05% to 0.20%, boron (B): 0.0005% to 0.0030%, sulfur (S) ≤ 0.025%, phosphorus (P) ≤ 0.025%, and the balance is iron (Fe) and impurities.

[0028] In this example, the steel's compositional design improves hardenability and broadens the bainite transformation zone, laying the material foundation for achieving controlled cooling of complex parts without quenching and tempering. The presence of chromium, in particular, helps inhibit pearlite transformation, making it easier to avoid the pearlite zone during cooling, thereby promoting the bainite phase transformation and improving the overall mechanical properties of the final product.

[0029] Specifically, the effects of selecting this component design are as follows: Carbon (C): A carbon content in the medium range (0.30% to 0.50%) is essential for achieving high strength in steel. Too low a content results in insufficient strength, while too high a content significantly reduces the steel's toughness and ductility, increasing its tendency to quench cracking. This range helps achieve sufficient strength while maintaining good overall performance.

[0030] Chromium (Cr): Containing a relatively high content (1.0%–2.5%), Cr is a key element in this invention. Cr significantly improves the hardenability of steel and strongly inhibits pearlite transformation, pushing the pearlite transformation "nose" on the CCT curve further to the right, thereby significantly widening the cooling rate window for bainite transformation. This allows steering knuckles with complex shapes and uneven wall thicknesses to effectively avoid the pearlite region during controlled cooling after forging, even with some cooling rate fluctuations. This is crucial for achieving non-quenching and tempering treatment. Furthermore, as an alloying element, Cr raises the steel's phase transformation temperature and refines grain size, thereby helping to control phase transformation and facilitating the transformation of fine austenite grains to bainite during cooling. Furthermore, Cr, dissolved as a solid solution in the iron matrix, causes lattice distortion, hinders dislocation slip, and contributes to improved tempering stability (although high-temperature tempering is not performed in this invention).

[0031] Manganese (Mn): Contents range from 0.8% to 1.8%. Mn is also an effective element for improving hardenability and lowering the critical transformation temperature. It works synergistically with Cr to optimize the steel's phase diagram, altering the stability of austenite and refining the austenite grains, lowering the transformation temperature, further enhancing hardenability, and expanding the bainite transformation zone. Mn also forms MnS inclusions with S, reducing internal friction, facilitating plastic deformation at high temperatures, and improving the steel's hot workability. However, excessive Mn content can increase the steel's sensitivity to overheating and temper brittleness.

[0032] Silicon (Si): With a content of 0.20% to 0.80%, Si in steel can inhibit the precipitation of cementite during the bainite transformation process, helping to form carbide-free bainite or bainite containing a small amount of dispersed carbides. Because this structure is mainly composed of fine ferrite and a small amount of carbides, it generally has better plasticity and toughness than structures containing a large amount of cementite. In addition, it can also refine the grain size, effectively hinder the movement of dislocations, and improve the strength of the material. The solid solution strengthening effect of Si can form Fe-Si alloys, causing lattice distortion, hindering dislocation movement, and increasing the elastic limit; it can directly increase its yield strength. Considering that the presence of carbide-free bainite increases its tensile strength, the yield strength ratio is also improved.

[0033] Molybdenum (Mo): Mo content ranges from 0.10% to 0.50%. Mo strongly enhances hardenability, effectively inhibiting pearlite and ferrite transformations and further broadening the isothermal or continuous cooling transformation range of bainite. Mo also improves the steel's high-temperature strength and creep resistance, refines grain size, and enhances tempering stability. In this invention, Mo is primarily utilized to enhance hardenability, ensuring bainite can be obtained even in thick sections.

[0034] Vanadium (V): Added in trace amounts (0.05%-0.20%), V is a strong carbonitride-forming element. During the austenitization process, some undissolved V (C, N) particles effectively prevent austenite grain growth, ensuring fine original austenite grains after forging. According to the Hall-Petch relationship, the smaller the grain size, the higher the yield strength and tensile strength of the material. Fine grains provide more grain boundaries, which effectively hinder the movement of dislocations, increase the strength of the material, and contribute to the toughness and strength of the final bainite structure. During the cooling process, fine V (C, N) precipitation may also occur, producing a secondary strengthening effect.

[0035] Boron (B): Added in trace amounts (0.0005%–0.0030%), Boron is an extremely effective hardenability-enhancing element. Even minimal amounts can significantly inhibit the nucleation of proeutectoid ferrite. The addition of Boron delays the temperature at which austenite transforms to ferrite or cementite, allowing more austenite to remain at elevated temperatures. This, in turn, promotes martensite formation during quenching, significantly improving hardenability. This effect is particularly pronounced in medium-carbon steels. The presence of Boron ensures effective entry into the bainite transformation zone even at relatively slow cooling rates. However, Boron additions must be carefully controlled; excessive amounts can reduce hardenability and lead to hot brittleness.

[0036] Sulfur (S) and phosphorus (P): As harmful impurities, their content must be strictly controlled at a low level (≤0.025%) to reduce the adverse effects on the toughness, plasticity and fatigue properties of steel.

[0037] Adding a small amount of nickel (0.2%-0.5%) further improves toughness.

[0038] Steel bars or billets meeting the aforementioned composition requirements are selected and accurately cut to the weight and dimensions required for the steering knuckle forging. The billets should undergo surface inspection before forging to remove defects such as cracks and folds. Through the synergistic effect of the aforementioned components, this steel grade possesses excellent hot working plasticity, superior hardenability, and a wide bainite transformation range, laying the material foundation for subsequent non-quenched and tempered forging processes.

[0039] In some embodiments, in step S1 , the temperature of the complete austenitizing treatment is 1180-1250° C., and the treatment atmosphere is a polyaromatic atmosphere.

[0040] In this embodiment, the temperature range is limited to ensure that the alloying elements are fully dissolved, eliminate structural heterogeneity within the billet, prevent oxidation and decarburization, maintain good thermoplasticity, and facilitate subsequent forging. An appropriate protective atmosphere can reduce the risk of oxidation and decarburization, ensuring the quality of the steel. The specific operation and mechanism of action are as follows: the billet is placed in a heating furnace (preferably a protective atmosphere heating furnace, such as a nitrogen-based atmosphere or an exothermic atmosphere furnace to prevent severe oxidation and decarburization) and heated, with the heating temperature controlled between 1180°C and 1250°C. This temperature range is selected to ensure that the steel is completely austenitized, allowing the alloying elements to fully dissolve, while eliminating structural heterogeneity within the billet and achieving good thermoplasticity, facilitating subsequent forging. The heating time is determined according to the billet size. After the core of the billet reaches the target temperature, it is then kept warm for a period of time (for example, 30-60 minutes per 25mm of diameter or thickness) to ensure temperature uniformity.

[0041] In some embodiments, in step S2, the hot die forging process is a multi-step closed die forging process; the hot die forging process includes a pre-forging process and a final forging process.

[0042] In this embodiment, the hot die forging process can effectively distribute the deformation, ensure smooth metal flow, fill the mold cavity, and form a clear outline. In the manufacture of complex parts such as steering knuckles, reasonable deformation control is crucial to ensure metal flow and filling in various parts of the part, which helps to improve the dimensional accuracy and mechanical properties of the final product. The hot die forging process uses induction heating instead of furnace heating, which has the advantages of fast heating speed, less oxidation and decarburization, and easy automation control. In addition to hot die forging presses, screw presses or electric screw presses can also be used as forging equipment.

[0043] It's important to note that during the pre-forging and final forging stages, deformation must be appropriately distributed to ensure smooth metal flow, filling the die cavity, and achieving a clear outline. The total deformation should be large enough to break up the original coarse grains and refine the austenite grains through dynamic recrystallization or work hardening. As those skilled in the art will appreciate, for complex parts like steering knuckles, particular attention must be paid to metal flow and filling in areas such as the arm, flange, and kingpin hole.

[0044] In some embodiments, in step S2, the temperature of the pre-forging treatment is 1150-1230°C, and the temperature of the final forging treatment is higher than the Ac3 point of the medium-carbon high-chromium bainite steel blank.

[0045] In this embodiment, at the temperature of the pre-forging treatment, the steel has good plasticity and low deformation resistance, which is conducive to metal filling the mold cavity. The final forging temperature needs to be higher than the Ac3 point of the steel, which is above 800°C for the steel type of this application, and is preferably controlled in the range of 900°C ~ 1000°C. Ensure that when the final forging is completed, the steel is still in a stable single-phase austenite state and the austenite grains are refined by deformation. The final forging temperature should not be too high to prevent the merging of grains during the recrystallization process, thereby avoiding excessive grain growth; the final forging temperature should not be too low to avoid entering the two-phase region, resulting in uneven structure or excessive work hardening, and to ensure that there is sufficient time and driving force for subsequent controlled cooling and bainite transformation.

[0046] In some embodiments, in step S3, the cooling treatment method includes one or more of air cooling, atomization cooling, spray cooling, isothermal cooling, continuous cooling-isothermal cooling, and residual heat insulation slow cooling.

[0047] In this embodiment, these methods can be flexibly selected according to the specific shape, size and actual conditions of the steering knuckle and the actual conditions of the production line, aiming to solve the problem of uneven cooling of complex forgings, ensure that the entire part obtains uniform bainite structure and stable mechanical properties, and adapt to the needs of different application scenarios.

[0048] Specifically, there are several types of cooling treatment: Controlled air cooling (forced air cooling): Forgings are placed on an air cooling device (such as an air cooling conveyor or air cooling stand). The average cooling rate over the forging surface is controlled by adjusting fan power, air outlet distribution, forging spacing, and placement. The goal is to rapidly cool the forging in the high-temperature zone (e.g., 800°C to 550°C), passing the pearlite transformation "nose." Cooling is then continued in the intermediate-temperature bainite transformation zone (550°C to 300°C, depending on the steel grade's CCT curve) at a relatively slow rate sufficient to complete the bainite transformation. For forgings with complex shapes and significant thickness variations, achieving precise control and uniformity of cooling rates across all parts requires optimizing air cooling parameters through thermocouple temperature measurement, thermal simulation calculations, and extensive process testing.

[0049] Mist cooling or spray cooling: During the high-temperature rapid cooling phase, water mist or a specific water-based coolant is used for short, intermittent, or zone-controlled spraying to more quickly advance through the pearlite transformation zone. Forced air cooling or slow cooling is then used to complete the bainite transformation. This solution offers high cooling capacity and control flexibility, but it requires high design and control precision of the spray system, increasing equipment complexity and cost.

[0050] Isothermal cooling or continuous cooling-isothermal cooling (a combination of continuous and isothermal cooling): The forging is rapidly cooled from the final forging temperature (e.g., by forced air cooling or a short spraying process) to the bainite transformation temperature range (350°C–500°C). The forging is then quickly transferred to a holding furnace or salt bath for isothermal treatment for a sufficient time (e.g., 30 minutes to 2 hours, as determined by the isothermal transformation kinetics curve (TTT)) to allow for the complete transformation of austenite to bainite. The forging is then removed from the furnace and air-cooled to room temperature. Alternatively, a slow cooling or isothermal stage can be introduced within the bainite transformation temperature range during the continuous cooling process. This approach allows for the most precise control of the phase transformation process and yields the optimal bainite structure.

[0051] Residual heat insulation and slow cooling (leveraging the residual heat of the forging die and subsequent stacking / covering for insulation and slow cooling): For forgings with thicker walls or relatively simple shapes, the residual heat of the forging die and the heat capacity of the forging itself can be utilized. After final forging, the forgings can be stacked in a specific manner and covered (such as with insulation felt, asbestos blankets, or buried in slow-cooling ash / sand) to achieve a slower cooling process, keeping the cooling curve precisely within the bainite transformation zone. This method is simple and low-cost, but has poor control accuracy and is significantly affected by environmental factors, making it suitable only under specific conditions.

[0052] Preferably, the cooling treatment method includes one or more of air cooling, continuous cooling, and isothermal cooling. The key is to ensure that the cooling curves of the steering knuckle forging during the cooling process effectively promote bainite phase transformation at both the core and the surface through precise design and strict control of the cooling path.

[0053] In some embodiments, in step S3, the preset time interval is ≤15s.

[0054] In this embodiment, the product is quickly removed from the final forging die and quickly transferred to a controlled cooling system for cooling, ensuring that the supercooled austenite is transformed into bainite to the maximum extent possible. Timely cooling treatment helps to avoid unnecessary phase transformations, such as the formation of proeutectoid ferrite or martensite, thereby ensuring the organizational uniformity and performance stability of the final product.

[0055] In some embodiments, trimming and punching can be performed while hot after forging, or after cooling. If hot trimming and punching are performed, they need to be completed quickly to avoid excessive temperature drop that affects subsequent cooling.

[0056] In some embodiments, in step S3, the cooling process is as follows: cooling at 550-800°C at a rate of ≥10°C / s, then cooling at 350-550°C at a rate of 0.5-5°C / s; and finally cooling to room temperature.

[0057] In this embodiment, cooling is performed at a rate of ≥10°C / s from 550°C to 800°C to quickly pass through the pearlite and ferrite transformation zones; then a slower, controlled average cooling rate (0.5-5°C / s) is adopted in the bainite phase transformation temperature range (350-550°C) to ensure sufficient bainite transformation, thereby obtaining excellent strength, toughness and fatigue properties.

[0058] In some embodiments, the method further comprises the step of performing straightening and / or shot peening after the cooling process.

[0059] In this embodiment, the correction may still produce certain residual stress and slight deformation due to the controlled cooling process. If necessary, cold correction or warm correction (below the bainite transformation temperature) can be performed; shot peening can remove surface oxide scale and introduce surface compressive stress to improve fatigue strength.

[0060] In some embodiments, after the cooling treatment, the following subsequent processing steps are also included: machining, performing necessary machining on the steering knuckle, such as processing the kingpin hole, bearing mounting hole, connecting flange surface, etc., to achieve the final product size and precision requirements; inspection, conducting a comprehensive inspection of the finished product in terms of size, appearance, hardness, mechanical properties (tensile, impact, fatigue) and internal quality (such as non-destructive testing), taking samples for tensile testing, and recording their stress-strain relationship curves to determine their yield strength and tensile strength; taking samples for impact testing on an impact testing machine, and recording the impact energy absorption value to evaluate the toughness of the material; taking samples for fatigue testing on a fatigue testing machine according to relevant standards to determine their fatigue limit; taking samples into groups and taking out samples for testing and comparison after a specified time period to evaluate their stability to ensure that the product meets the design and use requirements.

[0061] The present application provides a medium-carbon, high-chromium, non-quenched and tempered bainitic steel automobile steering knuckle, which includes lower bainite and / or granular bainite with a mass proportion of ≥90%; wherein the total amount of pearlite, proeutectoid ferrite and martensite is less than 10%.

[0062] The bainite-based microstructure of this application gives the steering knuckle excellent comprehensive mechanical properties, including high yield strength, good impact toughness and excellent fatigue life, making it a key safety component with high performance and reliability.

[0063] The present invention is further described below through specific examples.

[0064] Example 1 A process for preparing a medium-carbon, high-chromium, non-quenched and tempered bainitic steel automobile steering knuckle comprises the following steps: S1. Select the round bar blank with the chemical composition (mass percentage) as follows: C 0.38%, Cr 1.65%, Mn 1.30%, Si 0.45%, Mo 0.25%, V 0.12%, B 0.0015%, S 0.018%, P 0.020%, and the balance Fe ( Figure 1 The forging blank is a medium-carbon high-chromium bainitic steel blank having a diameter of Φ80 mm, a length of L=150 mm, and a weight of approximately 6.0 kg. The forging blank is heated in a continuous nitrogen-based atmosphere protection heating furnace. The heating temperature is set to 1220±10°C. The total heating time of the blank in the furnace is 150 minutes, of which the holding time at 1220°C is 60 minutes to ensure that the core of the blank is completely austenitized and the temperature is uniform, thereby obtaining a heated blank. S2. A 1600-ton electric screw press performs three-step die forging on the heated blank. For pre-forging, the heated blank is removed from the furnace and quickly (<10 seconds) transferred to the press's first die cavity for initial forming. At this point, the blank temperature is approximately 1180°C. The pre-forged piece is then transferred to the second die cavity for further forming. The primary deformation is distributed, ensuring the metal flows roughly toward the various arms and flanges of the steering knuckle. The operating temperature is approximately 1100°C. Finally, the forging is transferred to the final forging die for final filling and shaping. The final forging temperature is strictly controlled at 960±20°C. After final forging, the forging is essentially formed, with a smooth surface and no defects such as folds or underfill. S3. After the final forging is completed, a manipulator is used to remove the hot steering knuckle forging from the final forging die within 10 seconds and quickly place it on a forced air cooling line. The air cooling line is equipped with multiple sets of axial flow fans with adjustable angles and wind speeds, as well as tooling for supporting and flipping the forgings. The fan start and stop and air volume are controlled by a pre-set program to implement differentiated cooling for different parts of the steering knuckle. The cooling target parameters are set as follows: from 960℃ to 550℃, the average cooling rate is controlled at 15±3℃ / s, and the pearlite transformation zone is quickly passed. From 550℃ to 350℃, the air volume is reduced to control the average cooling rate to 1.5±0.5℃ / s, ensuring that the bainite is cooled. There is enough time for a full bainite phase transformation in the transformation temperature range. After the temperature drops below 350°C, the forced air cooling is stopped and the forging is allowed to cool naturally to room temperature in the air. The temperature changes of key parts are monitored in real time by an online infrared thermometer during the entire controlled cooling process, and feedback adjustment can be made as needed. After the forging is cooled to room temperature, trimming and punching operations are performed, and then the steering knuckle surface is shot peened with steel shots with a diameter of 0.8mm to remove oxide scale, improve surface finish, and form a residual compressive stress layer about 0.2mm deep on the surface. The steering knuckle is then precision machined for the kingpin hole, bearing hole, flange surface, etc. to obtain a medium-carbon, high-chromium non-quenched and tempered bainitic steel automobile steering knuckle.

[0065] Testing and Evaluation The performance test of the medium carbon high chromium non-quenched and tempered bainitic steel automobile steering knuckle of Example 1 was carried out, and the process and results are as follows: Hardness test: Samples were taken from different parts of the steering knuckle (arm, flange, journal) to test the Brinell hardness. The average hardness was HBW 300±20, and the hardness distribution was uniform. The results are as follows: Figure 2 As shown; Metallographic structure observation: Sampling was carried out for metallographic analysis, and the results showed that the microstructure was mainly composed of fine lower bainite and a small amount of granular bainite, with no obvious pearlite and proeutectoid ferrite, and the martensite content was extremely low (<5%). The grain size rating was 7-8.

[0066] Mechanical properties test: Samples were taken from designated locations of the forgings and processed into standard specimens for testing. The typical mechanical properties obtained are as follows: tensile strength Rm ≥ 1050 MPa, yield strength Rp0.2 ≥ 850 MPa, elongation A ≥ 12%, cross-sectional shrinkage Z ≥ 45%, and impact energy Akv ≥ 45 J at -40°C. Figure 3 This is the stress-strain diagram detected after forging using the process method of the present invention.

[0067] Fatigue test: The finished steering knuckle is subjected to bench fatigue life test, and the result meets the fatigue life cycle number required by the design.

[0068] Using conventional 42CrMo steel as a comparative example, a steering knuckle of the same model was produced after quenching and tempering (860°C oil quenching followed by 580°C tempering). A series of comparative examples with gradient variations in Cr and Mn content were developed for this steel. For example, four comparative examples were set for Cr content at 1%, 2%, 2.5%, and 3%, respectively; and three comparative examples were set for Mn content at 0.5%, 1.5%, and 2%, respectively. Samples of the final forged parts were taken and subjected to hardness testing, metallographic structure observation, mechanical property testing, and fatigue testing. The test results were recorded and compared.

[0069] In terms of process, for the comparative examples of the heating process in the examples, forging blanks with the same chemical composition and size are not calcined, but are directly heated to the forging temperature before forging. The remaining processes remain the same. The final formed parts are sampled and tested, and the test results are recorded and compared in sequence. For the comparative examples of the heating process temperature changes in the examples, a series of comparative examples with heating temperature changes can be prepared, with the temperatures being 800°C, 1000°C, and 1400°C, respectively, and the total heating time being 150 minutes. The remaining processes remain the same, and the final formed parts are sampled and tested, and the mechanical property data of the examples and comparative examples at different calcination temperatures are recorded.

[0070] Compared with the steering knuckle in the comparative example, the steering knuckle produced in this embodiment has the following characteristics: the mechanical properties are comparable or even slightly superior in terms of yield strength ratio and impact toughness; the production cycle is shortened by 40%; the comprehensive manufacturing cost is reduced by about 25%; the product dimensional accuracy is more stable; the subsequent machining allowance can be appropriately reduced; and the scrap rate is significantly reduced.

[0071] Compared with the existing technology, the present application optimizes the composition design of medium carbon high chromium bainitic steel, so that it has good thermoplasticity to adapt to the forging of complex shapes of steering knuckles, and can more easily and stably obtain bainite structure in the subsequent controlled cooling; by precisely controlling the key parameters such as heating temperature, deformation degree, and final forging temperature during the forging process, a favorable austenite state (such as suitable grain size and uniformity) is created for the subsequent bainite transformation; an effective post-forging controlled cooling process for the complex geometric shapes of steering knuckles is designed and implemented, which can achieve a reasonable cooling rate range. Within the specified range, the precipitation of pearlite and proeutectoid ferrite is suppressed, and the formation of martensite is avoided or reduced as much as possible, so as to promote the maximum transformation of supercooled austenite into bainite structure with excellent performance, and ensure the uniformity of structure and performance of different parts of the entire forging; through the above process integration, the traditional quenching and tempering heat treatment process is finally omitted, and medium-carbon high-chromium bainitic steel automobile steering knuckle with high strength, high toughness, high fatigue life and stable and reliable performance is directly produced, thereby significantly shortening the production cycle, reducing energy consumption and manufacturing costs, and reducing potential defects and environmental problems related to heat treatment.

[0072] The above are only preferred specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or replacements that can be easily thought of by any technician familiar with this technical field within the technical scope disclosed by the present invention should be covered by the scope of protection of the present invention.

Claims

1. A process for preparing a medium carbon high chromium non-quenched and tempered bainite steel automobile steering knuckle, characterized in that: The following steps are involved: S1. The medium carbon high chromium bainite steel billet is heated and completely austenitized to obtain a heated billet; S2. The heated billet is subjected to hot die forging treatment to obtain a forging; S3. The obtained forging begins cooling treatment within a preset time interval to obtain the medium-carbon high-chromium non-quenched and tempered bainitic steel automobile steering knuckle.

2. The process for preparing the medium carbon high chromium non-quenched and tempered bainite steel automobile steering knuckle according to claim 1, characterized in that: In step S1, the medium-carbon high-chromium bainitic steel blank includes the following components, by mass percentage: carbon (C): 0.30% to 0.50%, chromium (Cr): 1.0% to 2.5%, manganese (Mn): 0.8% to 1.8%, silicon (Si): 0.20% to 0.80%, molybdenum (Mo): 0.10% to 0.50%, vanadium (V): 0.05% to 0.20%, boron (B): 0.0005% to 0.0030%, sulfur (S) ≤ 0.025%, phosphorus (P) ≤ 0.025%, and the balance is iron (Fe) and impurities.

3. The process for preparing the medium carbon high chromium non-quenched and tempered bainite steel automobile steering knuckle according to claim 1, characterized in that: In step S1, the temperature of the complete austenitizing treatment is 1180-1250° C., and the treatment atmosphere is a polyaromatic atmosphere.

4. The process for preparing the medium carbon high chromium non-quenched and tempered bainite steel automobile steering knuckle according to claim 1, characterized in that: In step S2, the hot die forging process is a multi-step closed die forging process; the hot die forging process includes a pre-forging process and a final forging process.

5. The process for preparing the medium carbon high chromium non-quenched and tempered bainite steel automobile steering knuckle according to claim 4, characterized in that: In step S2, the temperature of the pre-forging treatment is 1150-1230°C, and the temperature of the final forging treatment is higher than the Ac3 point of the medium-carbon high-chromium bainite steel billet.

6. The process for preparing the medium carbon high chromium non-quenched and tempered bainite steel automobile steering knuckle according to claim 1, characterized in that: In step S3, the cooling treatment method includes one or more of air cooling, atomization cooling, spray cooling, isothermal cooling, continuous cooling-isothermal cooling, and residual heat insulation slow cooling.

7. The process for preparing the medium carbon high chromium non-quenched and tempered bainite steel automobile steering knuckle according to claim 1, characterized in that: In step S3, the preset time interval is ≤15s.

8. The process for preparing the medium carbon high chromium non-quenched and tempered bainite steel automobile steering knuckle according to claim 1, characterized in that: In step S3, the cooling process is as follows: cooling at 550-800°C at a rate of ≥10°C / s, and then cooling at 350-550°C at a rate of 0.5-5°C / s.

9. The process for preparing the medium carbon high chromium non-quenched and tempered bainite steel automobile steering knuckle according to claim 1, characterized in that: The method further comprises the steps of: performing straightening and / or shot peening after the cooling treatment.

10. A medium carbon high chromium non-quenched and tempered bainitic steel automobile steering knuckle obtained by the preparation process according to any one of claims 1 to 9, characterized in that: It includes lower bainite and / or granular bainite accounting for ≥90% by mass.

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