Medium carbon structural steel for cold forging hub bearing and continuous spheroidizing annealing process of medium carbon structural steel

Abstract

The invention relates to the technical field of metal heat treatment, in particular to medium-carbon structural steel for cold forging hub bearings and a continuous spheroidizing annealing process thereof.The medium-carbon structural steel for the cold forging hub bearings in a hot rolling state is slowly heated to the temperature below Ac1 and then heated to the temperature within the Ac1-Ac3 two-phase region temperature range for heat preservation, the medium-carbon structural steel is cooled to Ar3 according to the set cooling speed for heat preservation, and then the medium-carbon structural steel for the cold forging hub bearings is obtained; and cooling to the temperature below Ar1 according to the set cooling speed, preserving heat, entering a non-heat-supply section, slowly cooling, and discharging from the furnace. According to the method, control over the heating rate below Ac1, heating near Ac3 and grading isothermal treatment near Ar3 and below Ar1 are introduced into a traditional spheroidizing annealing system, optimization regulation and control over the carbide form and distribution state are achieved, the morphology stability of spheroidizing carbide is improved, abnormal structure generation is inhibited, the spheroidizing rate is larger than or equal to 80%, finally, the hardness of round steel is stably controlled within the interval of 160-170 HBW, and the spheroidizing quality is improved. And the work hardening sensitivity and the cracking tendency in the material cold forming process are effectively reduced, and the plasticity and the surface quality of the material are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of metal heat treatment, in particular to a medium-carbon structural steel for cold-forged hub bearing and a continuous spheroidizing annealing process thereof. BACKGROUND

[0002] Hub bearing is a core component in the automobile suspension system, which bears the important functions of vehicle weight support and rotation driving, and is long-term in high load, alternating stress and harsh working environment, so the material used must have excellent strength, toughness and fatigue performance. With the improvement of vehicle safety and service life requirements, hub bearing manufacturing is gradually shifting from machining to high-precision and high-efficiency cold forging process. Cold forging is a near-net forming process, in which the blank is accurately formed by a die in a plastic state, and has excellent dimensional repeatability. Thanks to the high stiffness of the die and the closed mold control, the cold forging product has small tolerance and concentrated size distribution, which can greatly reduce the subsequent machining allowance, even realize the "less cutting" or "no cutting" manufacturing of the parts, greatly improve the consistency and operation reliability of the parts assembly, and the material utilization rate can be more than 85%, and even more than 95% after optimization design; at the same time, due to the full material flow and high forming efficiency, multi-station cold header can be used for high-speed production, and the unit production cost is lower than that of machining or hot forging and finishing process. In addition, the cold forging process can improve the surface density and organizational strength of the product, which is beneficial to prolong the service life and reduce the quality cost.

[0003] To realize stable forming under the conditions of high precision and large deformation, the steel used must have good cold plasticity, low yield strength and uniformity of organization, and these properties are highly dependent on spheroidizing annealing before cold forging. Hub bearing and its connecting parts (such as hub stud, shaft sleeve, etc.) need to bear multi-directional composite load in use, including radial load, axial impact, torque transmission, etc., and also have good fatigue resistance and structural stiffness. Compared with high-carbon steel or low-carbon steel, medium-carbon structural steel containing 0.45% to 0.60% carbon can obtain higher strength and hardness after induction quenching, and also maintain good toughness inside, which exactly meets the comprehensive requirements of such parts for strength and toughness.

[0004] In medium-carbon structural steel, the initial microstructure after hot rolling or normalizing is typically composed of ferrite and pearlite clumps. These coarse pearlite clumps are often distributed in blocky aggregates, with the cementite inside arranged in dense lamellar layers. This results in low interfacial energy, insufficient driving force for the carbide "decomposition-spheroidization-reprecipitation" process, and a significant lag in the spheroidization process, requiring a long isothermal time to achieve spheroidization. Slight deviations in temperature control can also lead to coarsening, and the surrounding fine pearlite or dispersed ferrite regions spheroidize faster, resulting in a bimodal structure after annealing, with some regions fully spheroidized and some remaining lamellar pearlite. The lamellar cementite that is not fully spheroidized still retains a sharp-angled interface structure, which can easily induce microcracks under multi-directional stress conditions and become a potential crack source during cold forging. Failure is particularly prone to occur in strain concentration areas, seriously affecting the uniformity of material properties and cold forging stability. Summary of the Invention

[0005] The present invention addresses the above-mentioned technical issues and overcomes the shortcomings of the prior art, providing a medium-carbon structural steel for cold-forged wheel hub bearings and a continuous spheroidizing annealing process therefor. By optimizing the heat treatment path, precisely controlling the heating and isothermal processes, and rationally designing the cooling curve, the present invention achieves full spheroidization and refinement of the lamellar cementite in the microstructure, resulting in a uniformly distributed spheroidal carbide structure with an appropriate particle size, thereby significantly improving the material's cold working performance, stability, and surface quality. The process provided by the present invention can achieve a spheroidization rate of 80% or higher and a hardness of 180 HBW or lower, significantly improving the processing consistency and yield rate of cold-forged wheel hub bearing steel, meeting the needs of modern high-efficiency manufacturing and low-cost production.

[0006] In a first aspect, the present invention provides a continuous spheroidizing annealing process for medium carbon structural steel for cold forged wheel hub bearings, comprising the following steps:

[0007] Step 1: providing hot-rolled medium-carbon structural round steel for cold-forged wheel hub bearings as raw material;

[0008] Step 2: Slow heating stage: medium carbon structural round steel is laid out one by one on the feeding platform and slowly heated to a temperature below Ac1 in a continuous heat treatment furnace;

[0009] Step 3, two-phase zone heat preservation stage: heating the above-mentioned medium carbon structural round steel to the temperature range of Ac1 to Ac3 two-phase zone and keeping it warm;

[0010] Step 4, first stage isothermal cooling: cooling the medium carbon structural steel kept warm in the two-phase region to Ar3 at a set cooling rate for keeping warm;

[0011] Step 5, second stage isothermal cooling: cool the medium carbon round structural steel after the first stage isothermal cooling treatment at a set cooling rate until the furnace is cooled to below Ar1 for insulation, then enter the non-heating section for slow cooling before being taken out of the furnace.

[0012] Further, the chemical composition of the medium carbon structural round steel includes, in mass percentage: C: 0.49-0.57%, Si: 0.16-0.39%, Mn: 0.55-1.05%, Cr: 0.20-0.50%, Al: 0.030-0.055%, P: ≤0.012%, S: ≤0.006%, N: ≥0.008%, and the rest is base Fe and inevitable impurities.

[0013] Further, the microstructure of the hot-rolled cold-forged hub bearing medium carbon structural round steel is ferrite + pearlite group, the hardness is 230-255 HB, and the diameter range is 25-70 mm.

[0014] Further, in the step two, heating to a temperature below Ac1 at a speed of ≤120℃ / h, and the Ac1 temperature is 700-710℃; the medium carbon structural steel has a high carbon content and coarse original pearlite group, and if the heating is too fast, the coarse pearlite group will be austenitized first. This will lead to the "escape" of the spheroidizing process, that is, the coarse region cannot form spherical carbides through the decomposition mechanism after the formation of austenite, but plate-like cementite is precipitated during cooling, which is not conducive to spheroidizing; for Cr-containing alloy steel, Cr element tends to enter the carbide phase to form (Fe, Cr)3C. This type of carbide has high thermal stability and slow dissolution rate, and traditional rapid heating will make it unstable. Slow heating improves the nucleation quality and stability of (Fe, Cr)3C, so that it can exist stably in the subsequent austenite region and be more easily spheroidized rather than dissolved and re-precipitated.

[0015] Further, in the step three, heating to a high temperature section of 755-765℃ in the two-phase region temperature range of Ac1-Ac3 at a speed of >120℃ / h for 40-60 minutes of short holding time; this is because high temperature accelerates the diffusion rate of carbon atoms, rapidly promotes the disintegration of the original pearlite layer, and makes the locally supersaturated carbon easy to form fine spherical carbides on the grain boundaries or residual carbides. The holding time is usually 40-60 minutes, and short-time treatment can quickly scatter the clusters to achieve the redistribution of carbon concentration in austenite, avoid carbon enrichment or carbide coarsening caused by long-time heating, and is beneficial to the formation of new carbide particles with smaller size and dispersed distribution.

[0016] Further, in the step four, the medium-carbon structural steel after the two-phase zone holding is cooled to Ar3 temperature (the initial temperature of austenite transforming into ferrite) at a rate of ≤40℃ / h, and is held for 126-185 minutes, and Ar3 temperature is 705-715℃; at this time, the round steel is in a transition state of "austenite coexisting with a small amount of newly generated ferrite", the diffusion ability of carbon in austenite is still strong, and the generation of ferrite will gradually push the carbon in the remaining austenite, so that the carbon concentration is locally increased, which promotes the precipitation of carbides along the grain boundaries or the old cementite residual sites, and is more likely to spheroidize. If direct rapid cooling, the original carbides may be rapidly precipitated in an unstable state to form coarse rod-shaped or chain-shaped structures, and holding near Ar3 can delay the nucleation rate of ferrite and inhibit the coarsening of carbides, which is beneficial to obtain uniformly sized and dispersed spherical carbides, and improve the deformation consistency and plasticity of subsequent cold forging processing.

[0017] Further, in the step five, the medium-carbon structural steel after the first stage cooling isothermally treated is cooled to below Ar1 at a rate of ≤40℃ / h, and then is subjected to a second stage isothermal treatment for 244-312 minutes, and Ar1 temperature is 650-670℃; at this time, the austenite has been basically completely transformed, and there is no power to initiate new austenite decomposition or microstructure evolution, but the low-temperature diffusion is still ongoing, and carbon atoms slowly gather to the existing carbides, which promotes the slow growth, shape rounding and size equalization of the carbides formed in the previous spheroidizing process, further reduces the residual stress and interface energy in the microstructure, thereby improving the cold working performance and stability of the material. Although this stage is not the main stage of spheroidizing annealing, it plays an important auxiliary role in microstructure optimization and performance regulation. After isothermal treatment, the round steel is taken out of the furnace after slow cooling for 5-6 hours without heating, and is air-cooled.

[0018] In the second aspect, the application further provides a medium-carbon structural steel for cold-forged hub bearings, which is prepared by using the continuous spheroidizing annealing process of any one of the first aspect.

[0019] Further, the spheroidizing rate of the medium-carbon structural steel for cold-forged hub bearings is ≥80%.

[0020] Further, the hardness of the medium-carbon structural steel for cold-forged hub bearings is between 160-170HBW.

[0021] The application has the following beneficial effects:

[0022] (1) The application provides a continuous spheroidizing annealing process for medium-carbon structural steel for cold-forged hub bearing, which realizes the optimization and control of the morphology and distribution state of carbides by introducing the heating rate control below Ac1 + heating near Ac3 + staged isothermal treatment near Ar3 and below Ar1 in the traditional spheroidizing annealing system, improves the morphology stability of spheroidized carbides, and inhibits abnormal structure generation, with a spheroidizing rate of greater than or equal to 80%, and the hardness of the final round steel is stably controlled in the range of 160-170 HBW, effectively reducing the work hardening sensitivity and crack tendency in the cold forming process of the material, and improving the plasticity and surface quality of the material;

[0023] (2) The application aims to solve the key problems of pearlite group, spheroidizing difficulty, uneven structure and narrow temperature control window in the spheroidizing annealing process before cold forging of medium-carbon structural steel, and realizes the full spheroidization and refinement of lamellar cementite in the structure, and obtains a uniform distribution and appropriate particle size of spherical carbide structure, so that the cold working performance, stability and surface quality of the material are significantly improved. Through the process provided by the application, the spheroidizing rate of the material can be greater than or equal to 80%, and the hardness is less than or equal to 180 HBW, which greatly improves the processing consistency and yield of the cold-forged hub bearing steel, and meets the needs of modern efficient manufacturing and low-cost production. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 The spheroidized structure diagram of the medium-carbon structural steel for cold-forged hub bearing obtained in Example 1 of the application. DETAILED DESCRIPTION

[0025] For forging manufacturing enterprises, high-quality spheroidized structure is the key to ensure the stability of cold forging process and the mechanical properties of parts. The process can realize efficient spheroidizing treatment of medium-carbon steel on a continuous production line, which is more than 20% shorter in time and 15-25% lower in energy consumption than the traditional box annealing process, and the spheroidizing rate is more stable, which significantly improves the consistency and cold forming performance of the product. The forging market represented by hub bearings and other automobile key components is developing towards precision and light weight, and the popularization of this process can help enterprises to stably produce in batches, reduce the repair rate and improve the product grade, which has direct cost reduction and benefit increasing benefits.

[0026] For steel enterprises, the traditional spheroidizing annealing process has problems such as long cycle, high cost and difficult to control the structure, which has been difficult to meet the supply demand of high-end cold forging steel. The application relies on continuous annealing route and structure controllable mechanism, and can be widely applied to medium-carbon structural steel varieties, realizing the system optimization from material design, heat treatment process to terminal performance. This technology improves the proportion of high-end products, reduces customer returns and technical disputes caused by organizational deviation, and effectively promotes the transformation and upgrading of enterprises towards “customization, high quality and integration”.

[0027] The continuous spheroidizing annealing process for the medium-carbon structural steel for cold-forged hub bearing provided by the application, combined with heat treatment path optimization and microstructure evolution control, significantly improves the spheroidizing efficiency and microstructure uniformity of the material, and has good engineering adaptability and industrialization promotion prospects. From the perspective of upstream and downstream cooperation, the process not only improves the forming quality and processing efficiency of the forging manufacturing enterprise, but also provides a high-stability and high-value-added heat treatment solution for medium-carbon structural steel for steel enterprises. For specific examples, see Examples 1-3.

[0028] Example 1

[0029] The mass percentage of the round steel is: C: 0.52%, Si: 0.30%, Mn: 0.78%, Cr: 0.37%, Al: 0.035%, P: 0.007%, S: 0.001%, N: 0.0095%, and the balance is iron. The incoming material is a hot-rolled round steel with a diameter of 25 mm, and the microstructure is mainly ferrite + pearlite, with a hardness of 230-247 HBW.

[0030] This example includes the following steps:

[0031] (1) Furnace entry: The hot-rolled round steel is laid flat on the feeding rack, with a stacking height of 100 mm, not more than 4 layers. After the control console confirms the material, the roller is started to run, and the round steel is slowly sent into the continuous heat treatment furnace;

[0032] (2) Preheating: heating at a rate of 120℃ / h to 700℃ below Ac1;

[0033] (3) Two-phase zone heating and holding: heating to 755℃ in the two-phase zone for 20 minutes, and holding for 40 minutes;

[0034] (4) First stage cooling isothermal: the alloy structural steel in the two-phase zone is cooled at a rate of 30℃ / h to 715℃ near Ar3, and held for 126 minutes;

[0035] (5) Second stage cooling isothermal: the round steel after the first stage isothermal is cooled at a rate of 30℃ / h to 670℃ below Ar1, and held for 244 minutes; then slowly cooled for 5 hours in the non-heating section and discharged.

[0036] The main properties of the medium-carbon structural round steel spheroidized according to Example 1 in the continuous heat treatment furnace are shown in Table 1. The spheroidizing rate of Φ25mm round steel is >80%, and the hardness of the round steel cross section at different positions is <180HBW. Figure 1The spheroidized structure of the continuous furnace spheroidizing annealing process of Example 1. The present application precisely selects and controls the isothermal annealing temperature, balances the transformation driving force and stability, adopts two-step annealing or multi-stage slow cooling, gradually disperses the pearlite group structure, promotes the spheroidization of cementite, and controls the cooling rate and holding time, which prevents the coarsening of carbides and ensures sufficient spheroidization.

[0037] Table 1 Main properties of round steel spheroidizing annealed according to the process flow of Example 1

[0038]

[0039] Example 2

[0040] The mass percentage of round steel is: C: 0.56%, Si: 0.29%, Mn: 0.83%, Cr: 0.40%, Al: 0.041%, P: 0.006%, S: 0.002%, N: 0.0112%, and the balance is iron. The incoming material is hot-rolled round steel with a diameter of 50 mm, and the structure is mainly ferrite + pearlite, with a hardness of 241-253 HBW.

[0041] This example includes the following steps:

[0042] (1) Furnace entry: The hot-rolled round steel is laid flat on the feeding rack, with a stacking height of 150 mm, not more than 3 layers, and the roller is started after the console confirms the material and slowly sent into the continuous heat treatment furnace;

[0043] (2) Preheating: heated to 710℃ below Ac1 at a heating rate of 115℃ / h;

[0044] (3) Two-phase zone heating and holding: heated to 760℃ in the two-phase zone for 30 minutes, and held for 50 minutes;

[0045] (4) First stage cooling and isothermal: the alloy structural steel in the two-phase zone is cooled to 710℃ near Ar3 at a cooling rate of 35℃ / h, and held for 165 minutes;

[0046] (5) Second stage cooling and isothermal: the round steel after the first stage isothermal is cooled to 660℃ below Ar1 at a cooling rate of 35℃ / h, and held for 287 minutes; then slowly cooled for 6 hours through the non-heating section and discharged.

[0047] The main properties of the medium carbon structural round steel spheroidizing annealed according to Example 2 are shown in Table 2. The spheroidization rate of Φ50mm round steel is >80%, and the hardness of the round steel cross section at different positions is <180HBW.

[0048] Table 2 Main properties of round steel spheroidizing annealed according to the process flow of Example 2

[0049]

[0050] Example 3

[0051] The round steel has the following mass percentage: C: 0.57%, Si: 0.38%, Mn: 1.03%, Cr: 0.49%, Al: 0.044%, P: 0.008%, S: 0.001%, N: 0.0120%, and the balance is iron, and the round steel is in a hot-rolled state, with a diameter of 70 mm, and the structure is mainly ferrite + pearlite, and the hardness is 240-255 HBW.

[0052] The embodiment includes the following steps:

[0053] (1) entering the furnace: the hot-rolled round steel is laid flat on the feeding rack, with a stacking height of 140 mm, and the height does not exceed 2 layers, and the console confirms the material and then starts the roller operation, and slowly sends the continuous heat treatment furnace;

[0054] (2) preheating: heating at a heating rate of 110℃ / h to 710℃ below Ac1;

[0055] (3) heating and holding in the two-phase region: heating to 765℃ in the two-phase region for 27 minutes, and holding for 60 minutes;

[0056] (4) first stage cooling and isothermal: the alloy structural steel in the two-phase region is cooled at a cooling rate of 40℃ / h to 705℃ near Ar3, and held for 185 minutes;

[0057] (5) second stage cooling and isothermal: the round steel after the first stage isothermal is cooled at a cooling rate of 40℃ / h to 650℃ below Ar1, and held for 312 minutes; and then slowly cooled for 6 hours through the non-heating section and discharged.

[0058] The main properties of the medium-carbon structural round steel spheroidized according to the process flow of Example 3 in the continuous heat treatment furnace are shown in Table 3. The spheroidization rate of the Φ70mm round steel is >80%, and the hardness of the round steel at different positions of the cross section is <180HBW.

[0059] Table 3 Main properties of round steel spheroidized according to the process flow of Example 3

[0060]

[0061] The present application optimizes the heat treatment path, accurately controls the heating and isothermal process, and reasonably designs the cooling curve, so as to realize the full spheroidization and refinement of the lamellar cementite in the structure, obtain a uniform distribution and appropriate particle size of the spherical carbide structure, and significantly improve the cold working performance, stability and surface quality of the material. Through the process provided by the present application, the spheroidization rate of the material is ≥80%, and the hardness is ≤180HBW, which greatly improves the processing consistency and yield of the cold-forged hub bearing steel.

[0062] In addition to the embodiments described above, the present application can have other implementation manners. Any technical solutions formed by equivalent replacement or equivalent transformation fall within the protection scope of the present application.

Claims

1. A continuous spheroidizing annealing process for medium carbon structural steel for cold forged wheel hub bearings, characterized in that: The following steps are involved: Step 1: providing hot-rolled medium-carbon structural round steel for cold-forged wheel hub bearings as raw material; Step 2, slow heating stage: slowly heat the aforementioned medium carbon structural round steel to a temperature below Ac1; Step 3, two-phase zone heat preservation stage: heating the above-mentioned medium carbon structural round steel to the temperature range of Ac1 to Ac3 two-phase zone and keeping it warm; Step 4, first stage isothermal cooling: cooling the medium carbon structural steel kept warm in the two-phase region to Ar3 at a set cooling rate for keeping warm; Step 5, second stage isothermal cooling: cool the medium carbon round structural steel after the first stage isothermal cooling treatment at a set cooling rate until the furnace is cooled to below Ar1 for insulation, then enter the non-heating section for slow cooling before being taken out of the furnace.

2. The continuous spheroidizing annealing process according to claim 1, characterized in that: The chemical composition of the medium-carbon structural round steel includes, by mass percentage, C: 0.49-0.57%, Si: 0.16-0.39%, Mn: 0.55-1.05%, Cr: 0.20-0.50%, Al: 0.030-0.055%, P: ≤0.012%, S: ≤0.006%, N ≥0.008%, and the rest is matrix Fe and unavoidable impurities.

3. The continuous spheroidizing annealing process according to claim 1, characterized in that: The hot-rolled medium-carbon structural round steel for cold-forged wheel hub bearings has a structure of ferrite+pearlite clusters, a hardness of 230-255 HB, and a diameter range of 25-70 mm.

4. The continuous spheroidizing annealing process according to claim 1, characterized in that: In the step 2, heating is performed at a rate of ≤120°C / h to a temperature lower than Ac1, where Ac1 is 700-710°C.

5. The continuous spheroidizing annealing process according to claim 1, characterized in that: In the step 3, the temperature is heated to 755-765° C. in the high temperature section of the Ac1-Ac3 two-phase temperature range at a rate of >120° C. / h, and short-term heat preservation is performed for 40-60 minutes.

6. The continuous spheroidizing annealing process according to claim 1, characterized in that: In the step 4, the medium carbon structural steel after the two-phase region insulation is cooled to the Ar3 temperature at a rate of ≤40°C / h and kept for 126-185 minutes, and the Ar3 temperature is 705-715°C.

7. The continuous spheroidizing annealing process according to claim 1, characterized in that: In the step 5, the medium carbon structural steel that has undergone the first stage of cooling isothermal treatment is cooled to below Ar1 at a rate of ≤40°C / h and then subjected to the second stage of isothermal treatment for 244-312 minutes, with the Ar1 temperature being 650-670°C.

8. A medium carbon structural steel for cold forged wheel hub bearings, characterized in that: The continuous spheroidizing annealing process according to any one of claims 1 to 7 is used for preparation.

9. The medium carbon structural steel for cold forged hub bearings according to claim 8, characterized in that: The spheroidization rate of the medium carbon structural steel used for the cold forged wheel hub bearing is ≥80%.

10. The medium carbon structural steel for cold forged hub bearings according to claim 8, characterized in that: The hardness of the medium carbon structural steel used in the cold forged wheel hub bearing is between 160-170 HBW.

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