Rolling process for eliminating heat treatment mixed crystals of 20CrMnTi gear steel

Abstract

The invention discloses a rolling process for eliminating heat treatment mixed crystals of 20CrMnTi gear steel, which comprises the following steps of: S1, heating a 20CrMnTi continuous casting blank, and controlling the temperature of a soaking zone at 900-1030 DEG C; s2, the heated blank is rolled, the rolling temperature is controlled to range from 900 DEG C to 1000 DEG C, and the single-pass rolling deformation is controlled to range from 20% to 35%; and S3, the rolled bar is subjected to a slow cooling process, and the cooling speed is controlled to be 0.5-1 DEG C / s. According to the method, the mixed crystal and heredity problems caused by casting blank dendritic segregation and out-of-control second-phase particle precipitation behavior are solved, the heredity path of austenite mixed crystal is blocked from the source, the microstructure uniformity of the 20CrMnTi gear steel is remarkably improved, the austenite mixed crystal phenomenon in the high-temperature carburizing process can be eliminated to the maximum extent, and the service life of the 20CrMnTi gear steel is prolonged. The comprehensive performance of the gear after heat treatment is improved, and the fatigue life and the size precision of subsequent gear products are effectively guaranteed.

Description

Technical Field

[0001] This invention relates to a rolling process, and more particularly to a rolling process for eliminating mixed crystals in 20CrMnTi gear steel after heat treatment. Background Technology

[0002] As automobiles and construction machinery upgrade towards higher load capacity, longer lifespan, and higher precision, the requirements for material properties in the core gears of their transmission systems have significantly increased. Gears, in particular, that withstand alternating loads require sufficient surface hardness after carburizing heat treatment, and uniform austenite grain size; otherwise, uneven microstructure can easily lead to early fatigue fracture. 20CrMnTi gear steel, due to its excellent hardenability and toughness, has become the dominant raw material for this type of high-end gear. The high-temperature carburizing heat treatment temperature for 20CrMnTi gear steel is typically 950–1000℃, with a holding time of 4–8 hours. However, the high-temperature carburizing heat treatment process easily induces abnormal austenite grain growth, forming a mixed-grain structure; this defect severely reduces the fatigue life and dimensional accuracy of the gears, becoming a key bottleneck restricting the quality of high-end transmission components.

[0003] The mixed-grain phenomenon during high-temperature carburizing of gear steel is closely related to the inherited characteristics of the microstructure of 20CrMnTi steel during bar rolling. This steel grade is prone to dendritic segregation during continuous casting solidification, leading to a non-uniform distribution of alloying elements such as C, Mn, and Cr, as well as second-phase particles such as Al and N. In the subsequent high-temperature carburizing heat treatment stage, the differences in the segregation of alloying elements and the dissolution and elution behavior of second-phase particles induce uneven growth of austenite grains in local areas, ultimately forming a mixed-grain structure with significant differences in grain size. Studies have shown that the process parameters during bar rolling, especially the heating regime, deformation temperature, and deformation amount, are the core parameters for controlling the distribution of second-phase particles and grain uniformity. If the process control at this stage is unreasonable, it will directly lead to mixed-grain defects in the later carburizing heat treatment of gear steel.

[0004] The existing controlled rolling and cooling process for 20CrMnTi gear steel bars focuses on refining grains, but lacks specific control over the uniformity of austenite during the rolling process, making it difficult to avoid the generation of hereditary microstructure in gear steel bars.

[0005] Chinese patent application CN103192012A discloses a rolling production process for 20CrMnTi gear steel, which uses high-temperature homogenization at 1150-1250℃ and conventional rolling temperature control. Although this can ensure smooth rolling, it cannot suppress the induction of mixed crystals by element segregation, and mixed crystal defects are still prone to occur after carburizing.

[0006] In their paper "Control of Banded Structure in 20CrMnTi Hot-Rolled Strip Steel" (Rolling Steel, 2021, No. 6, pp. 91-94, 123), Ai Yuzhong et al. reduced the influence of elemental segregation on the microstructure and significantly improved the uniformity of the microstructure by optimizing slab heating, finishing rolling parameters, and post-rolling cooling processes. However, high temperature and long-term heating will increase energy consumption.

[0007] In his paper "Research on Controlled Rolling and Cooling Process of 20CrMnTiH Round Steel" (Proceedings of the 11th China Iron and Steel Annual Conference - S03. Rolling and Heat Treatment, 2017), Wang Chao proposed using a heating temperature above 1200℃ and an initial rolling temperature of 1170℃ to 1190℃ to meet the requirements for low-grade banded structure and fine grains in gear steel. Grain refinement was achieved by controlling the final rolling temperature and post-rolling cooling process. However, the multi-wave cooling method used after rolling places stringent requirements on the nozzle distribution and water volume control, resulting in a long parameter adjustment cycle when switching to different specifications of round steel.

[0008] In his article "The Influence of Cold Deformation on Mixed Grain Size in Carburized 20CrMnTi and Process Improvement" (Special Steel Technology, 2022, Vol. 1, pp. 24-26), Kong Xiangwei found that annealing or normalizing pretreatment before carburizing can effectively improve the mixed grain size problem during carburizing, with normalizing showing better results. However, the additional pretreatment process prolongs the production cycle and increases energy consumption costs.

[0009] Patent document CN119351680A discloses a heat treatment process for eliminating mixed grains in low-carbon gear steel after warm or hot forging. This process, through annealing the mixed-grain forging blank, promotes the uniform precipitation of AlN particles and pins grain boundaries, ensuring that the microstructure and grain size meet standards after subsequent carburizing. However, prolonged heat preservation and slow cooling reduce production efficiency, and the need for precise testing of the Al / N ratio for each batch of steel to match the annealing temperature increases testing costs and process complexity.

[0010] Patent application CN121065445A discloses a heat treatment method for optimizing the microstructure and refining the grains of gear steel, applicable to cold-forged gear parts. It involves adding a pretreatment after cold forging and before carburizing to suppress austenite grain growth and avoid mixed grains. However, this pretreatment can easily cause slight deformation in some complex-shaped parts, and it is only suitable for SCR420H steel with specific Al and N contents, lacking adaptability to gear steels with other compositions.

[0011] In summary, existing bar rolling technologies generally suffer from problems such as complex processes, high costs, demanding equipment requirements, or poor results in improving the uniformity of austenitic microstructure and suppressing / eliminating mixed grains during carburizing heat treatment in 20CrMnTi steel. There is a lack of efficient, low-cost, and easily industrially applicable solutions. This has resulted in the problem of mixed grains during carburizing heat treatment remaining a long-standing quality bottleneck for 20CrMnTi gear steel bar rolling products.

[0012] Therefore, developing an integrated rolling process that optimizes key process parameters of billet heating and rolling to suppress microsegregation inheritance in billets, improve the uniformity of rolled austenite structure, and thus eliminate mixed crystals in subsequent carburizing heat treatment, has significant industrial application value and practical significance for breaking through the quality bottleneck of high-end gear materials and reducing enterprise production costs. Summary of the Invention

[0013] The technical problem to be solved by the present invention is to provide a rolling process for eliminating mixed crystals in 20CrMnTi gear steel during heat treatment that can suppress the inheritance of microsegregation in the cast billet.

[0014] To solve the above-mentioned technical problems, the technical solution adopted by the present invention includes the following steps: S1, placing the 20CrMnTi continuous casting billet in a heating furnace for heating, and controlling the temperature of the soaking zone at 900-1030℃; S2. The heated billet is rolled at a temperature of 900–1000℃, and the deformation per pass is controlled at 20–35%. S3. The rolled bar undergoes a slow cooling process, with the cooling rate controlled at 0.5~1℃ / s.

[0015] Furthermore, in step S1, the temperature of the preheating section is controlled at 500-800℃ and the temperature of the heating section is controlled at 800-950℃.

[0016] Furthermore, in step S1, the heat equalization time is 30–60 min.

[0017] Furthermore, step S2 employs multi-pass continuous rolling.

[0018] The principle of this invention: The essence of mixed-crystal structure in carburized 20CrMnTi steel lies in the difference in grain boundary migration induced by the uneven pinning force of the second-phase particles. Due to dendritic segregation, the continuously cast billet of this steel grade forms solute-rich and solute-poor regions, and the dissolution behavior of the second-phase particles in different regions shows significant differences. According to classical materials science theory, the growth of austenite grains during carburizing heat treatment is mainly controlled by the grain boundary migration rate, and the pinning effect of the second-phase particles is the key factor in inhibiting grain boundary migration. When the heating temperature is in the range of 1030℃ to 1080℃, the concentration of Al and N elements in the solute-rich region is high and the solubility is low. Undissolved AlN particles generate strong pinning force through the Zener pinning mechanism, effectively hindering grain boundary migration and keeping the austenite grains in this region fine. In contrast, the concentration of Al and N elements in the solute-poor region is low, the dissolution of AlN particles and the sharp decrease in pinning force lead to rapid grain boundary migration. The driving force of grain boundary migration is greater than the resistance, causing abnormal growth of individual grains, ultimately forming a mixed-crystal structure of "fine grains + coarse grains".

[0019] Based on the above mechanism, in order to suppress the abnormal growth of austenite grains during the carburizing heat treatment process, this invention is based on the idea of ​​"thermodynamic regulation (controlled dissolution) - kinetic regulation (controlled recrystallization) - phase transformation regulation (controlled inheritance)". Through the synergistic regulation of heating, rolling and cooling processes, the inheritance of mixed crystals is cut off. The specific principle is as follows: This invention first strictly controls the homogenization temperature of the pre-rolled billet in the heating furnace within the range of 950–1030°C. The scientific basis for selecting this temperature range is as follows: On the one hand, under this temperature condition, AlN particles in both the solute-poor and solute-rich regions do not dissolve, which helps to suppress the growth of austenite grains during heating and mitigate the adverse effects of elemental microsegregation on the inhomogeneity of the original austenite structure. On the other hand, avoiding the AlN dissolution region above 1030°C prevents the uncontrolled growth of austenite grains caused by the complete dissolution of AlN particles in the solute-poor region, which would result in the loss of pinning for grain boundary migration. Meanwhile, AlN particles in the solute-rich region do not dissolve under this temperature condition, and their coarsening is also suppressed, thus effectively inhibiting grain growth. Within this temperature range, the dissolution and coarsening rates of the second-phase particles in both the solute-poor and solute-rich regions are effectively controlled, thereby establishing a uniform grain boundary pinning force field throughout the material and avoiding austenite grain size inhomogeneity during the heating stage. This lays the foundation for obtaining a uniform, non-mixed-grain original austenite structure.

[0020] In terms of rolling technology, a low-temperature rolling process of 900–1000℃ combined with a deformation amount of 20–35% is adopted. This process parameter setting conforms to the basic laws of metal plastic deformation and recrystallization. According to dislocation theory, plastic deformation at lower temperatures can generate higher dislocation density and deformation energy storage, which provides sufficient driving force for the subsequent static recrystallization process. When the deformation amount reaches more than 20%, the stored deformation energy is sufficient to trigger static recrystallization in both solute-poor and solute-rich regions. According to the principle of recrystallization kinetics, new strain-free grains can completely transform the pre-deformation austenite grains into a fine, uniform equiaxed grain structure through nucleation and growth mechanisms, ensuring the reliability of microstructure refinement and homogenization in solute-poor and solute-rich regions, and eliminating the differences in austenite recrystallization behavior caused by microsegregation in the cast billet.

[0021] In terms of cooling control, a ferrite + pearlite microstructure is obtained through the aforementioned slow cooling process. This design is based on the fundamental principles of solid-state phase transformation. According to the Fe-C phase diagram and TTT curve characteristics, controlling the cooling rate can prevent the formation of non-equilibrium structures (such as bainite and martensite). Non-equilibrium structures are prone to microstructural inheritance during subsequent reheating due to their specific crystallographic orientation relationship (KS relationship) with the original austenite. In contrast, ferrite and pearlite in the equilibrium microstructure provide more nucleation sites during reaustitization. According to the nucleation rate formula, the increase in nucleation sites directly leads to an increase in the austenite nucleation rate, thereby obtaining fine and uniform initial grains. This process conforms to the fundamental laws of phase transformation thermodynamics and kinetics, solves the problem of non-equilibrium structure-induced inheritance, and fundamentally blocks the microstructural inheritance path of mixed crystals.

[0022] In summary, based on the principles of second-phase pinning and recrystallization in materials science, by controlling the heating temperature to 950–1030℃ to avoid the partial dissolution zone of AlN, utilizing low-temperature rolling at 900–1000℃ and a suitable deformation amount of 20–35% to promote the static recrystallization behavior of the solute-poor and solute-rich zones, and then obtaining a balanced microstructure through slow cooling, the triple regulation synergistic effect eliminates the mixed crystal inheritance of 20CrMnTi gear steel from the source, and achieves uniform and refined microstructure after carburizing heat treatment.

[0023] The beneficial effects of the above technical solution are as follows: This invention, through precise and coordinated control of heating temperature, rolling temperature, and deformation per pass, coupled with a "low-temperature heating + low-temperature rolling" process, solves the problem of mixed crystals and their inheritance caused by uncontrolled dendrite segregation and second-phase particle precipitation in the cast billet. It blocks the inheritance path of austenite mixed crystals at the source, significantly improving the microstructure uniformity of 20CrMnTi gear steel. This minimizes the austenite mixed crystal phenomenon during high-temperature carburizing, enhances the overall performance of the gear after heat treatment, and yields a ferrite + pearlite microstructure, effectively ensuring the fatigue life and dimensional accuracy of subsequent gear products. This invention features a simple process, controllable cost, and strong industrial applicability. It is suitable for the industrial production of 20CrMnTi gear steel bars with diameters ranging from Φ15 to 100 mm, especially for the pre-manufacturing processes of high-end gears, providing gear steel bars with uniform microstructure for subsequent carburizing heat treatment. Attached Figure Description

[0024] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0025] Figure 1 This is a metallographic diagram of the rolled sample obtained in Example 1 after heat treatment; Figure 2 This is a metallographic diagram of the rolled sample obtained in Example 2 after heat treatment. Detailed Implementation

[0026] Example 1: The rolling process for eliminating mixed crystals in the heat treatment of 20CrMnTi gear steel is described in detail below.

[0027] S1. Billet heating: Place the 20CrMnTi continuous casting billet in a continuous heating furnace, control the preheating section temperature to 700℃, the heating section temperature to 900℃, and the soaking section temperature to 1030℃, and the soaking time to 60 minutes, to ensure that the internal temperature of the billet is uniform and that the AlN second phase particles do not dissolve.

[0028] S2. Low-temperature rolling: The heated billet is continuously rolled in multiple passes at a rolling temperature of 1000℃, with a single-pass deformation of 20%.

[0029] S3. Post-rolling cooling: After rolling, the material is cooled at a low rate of 0.7℃ / s to room temperature to obtain a balanced ferrite + pearlite structure.

[0030] The rolled sample obtained in this embodiment was subjected to pseudo-carburizing heat treatment at 980℃ for 6 hours, followed by metallographic examination; the results are as follows: Figure 1 As shown, the austenite grain size rating reaches ASTM level 8, the microstructure is uniform and fine, and no mixed crystal phenomenon was observed.

[0031] Example 2: The rolling process for eliminating mixed crystals in the heat treatment of 20CrMnTi gear steel is described in detail below.

[0032] S1. Billet heating: Place the 20CrMnTi continuous casting billet in a continuous heating furnace, control the preheating section temperature to 600℃, the heating section temperature to 850℃, and the soaking section temperature to 980℃, and the soaking time to 60 minutes, to ensure that the internal temperature of the billet is uniform and that the AlN second phase particles do not dissolve.

[0033] S2. Low-temperature rolling: The heated billet is continuously rolled in multiple passes at a rolling temperature of 950℃, with a single-pass deformation of 33%.

[0034] S3. Post-rolling cooling: After rolling, the material is cooled at a low rate of 1.0℃ / s to room temperature to obtain a balanced ferrite + pearlite structure.

[0035] The rolled sample obtained in this embodiment was subjected to pseudo-carburizing heat treatment at 980℃ for 6 hours, followed by metallographic examination; the results are as follows: Figure 2 As shown, the austenite grain size rating reaches ASTM level 8, the microstructure is uniform and fine, and no mixed crystal phenomenon was observed.

[0036] Example 3: The rolling process for eliminating mixed crystals in the heat treatment of 20CrMnTi gear steel is described in detail below.

[0037] S1. Billet heating: Place the 20CrMnTi continuous casting billet in a continuous heating furnace, control the preheating section temperature to 500℃, the heating section temperature to 800℃, and the soaking section temperature to 900℃, and the soaking time to 50 minutes, to ensure that the internal temperature of the billet is uniform and that the AlN second phase particles do not dissolve.

[0038] S2. Low-temperature rolling: The heated billet is continuously rolled in multiple passes at a rolling temperature of 900℃, with a single-pass deformation of 35%.

[0039] S3. Post-rolling cooling: After rolling, the material is cooled at a low rate of 0.8℃ / s to room temperature to obtain a balanced ferrite + pearlite structure.

[0040] The rolled sample obtained in this embodiment was subjected to pseudo-carburizing heat treatment at 980℃ for 6 hours. After metallographic examination, the austenite grain size rating reached ASTM level 8, the microstructure was uniform and fine, and no mixed grain phenomenon was observed.

[0041] Example 4: The rolling process for eliminating mixed crystals in the heat treatment of 20CrMnTi gear steel is described in detail below.

[0042] S1. Billet heating: Place the 20CrMnTi continuous casting billet in a continuous heating furnace, control the preheating section temperature to 800℃, the heating section temperature to 950℃, and the soaking section temperature to 1000℃, and the soaking time to 30 minutes, to ensure that the internal temperature of the billet is uniform and that the AlN second phase particles do not dissolve.

[0043] S2. Low-temperature rolling: The heated billet is continuously rolled in multiple passes at a rolling temperature of 970℃, with a single-pass deformation of 25%.

[0044] S3. Post-rolling cooling: After rolling, the material is cooled at a low rate of 0.5℃ / s to room temperature to obtain a balanced ferrite + pearlite structure.

[0045] The rolled sample obtained in this embodiment was subjected to pseudo-carburizing heat treatment at 980℃ for 6 hours. After metallographic examination, the austenite grain size rating reached ASTM level 8, the microstructure was uniform and fine, and no mixed grain phenomenon was observed.

Claims

1. A rolling process for eliminating mixed grains in 20CrMnTi gear steel after heat treatment, characterized in that, The process includes the following steps: S1, placing the 20CrMnTi continuous casting billet in a heating furnace for heating, with the temperature of the soaking zone controlled at 900-1030℃; S2. The heated billet is rolled at a temperature of 900–1000℃, and the deformation per pass is controlled at 20–35%. S3. The rolled bar undergoes a slow cooling process, with the cooling rate controlled at 0.5~1℃ / s.

2. The rolling process for eliminating mixed grains in 20CrMnTi gear steel after heat treatment according to claim 1, characterized in that: In step S1, the temperature of the preheating section is controlled at 500-800℃ and the temperature of the heating section is controlled at 800-950℃.

3. The rolling process for eliminating mixed grains in 20CrMnTi gear steel after heat treatment according to claim 1, characterized in that: In step S1, the heating time is 30 to 60 minutes.

4. A rolling process for eliminating mixed grains in 20CrMnTi gear steel during heat treatment, as described in claim 1, 2, or 3, characterized in that: Step S2 employs multi-pass continuous rolling.

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