Method for grain refinement of low-cost high-strength steel
By combining pre-tempering, gradient inverse austenitization, and staged quenching, the problems of coarse grains and insufficient strength and toughness in Nb-modified Mo steel have been solved, achieving grain refinement and performance improvement of low-cost high-strength steel, which is suitable for industrial mass production.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- GUANGXI UNIVERSITY OF TECHNOLOGY
- Filing Date
- 2026-05-15
- Publication Date
- 2026-07-21
AI Technical Summary
Existing Nb-to-Mo technology and its associated heat treatment processes cannot simultaneously achieve both low cost and high strength and toughness, resulting in coarse grains and uneven microstructure, which cannot meet the requirements of engineering applications.
A combined process of pre-tempering, gradient reverse austenitization, and staged quenching is adopted, including chemical composition design and controlled rolling. Through gradient heating reverse austenitization and staged salt bath quenching, a uniform and refined martensitic structure is formed.
This process achieves ultra-fine grain size, improves the low-temperature toughness and strength of steel, reduces production costs, and ensures process stability and synergistic performance improvement.
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Figure CN122428087A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of heat treatment and microalloying crystallization technology for metallic materials, and in particular to a low-cost method for refining the grain size of high-strength steel. Background Technology
[0002] High-strength low-alloy steel, with its high strength, good ductility and toughness, and high cost-effectiveness, is widely used in key fields such as engineering machinery, rail transportation, and energy equipment. The performance improvement of this type of steel mainly relies on the synergistic effect of microalloying design and heat treatment processes. Microalloying elements such as Mo, Nb, and Ti can combine with C and N in the steel to form nanoscale carbonitrides, achieving grain refinement and strength enhancement through precipitation strengthening and grain boundary pinning.
[0003] In existing microalloying technologies, Mo can effectively refine second-phase particles and reduce the coarsening rate of precipitates, significantly improving the strength and toughness of steel. However, Mo is expensive, and adding large amounts would increase steel production costs, limiting its large-scale application. Therefore, partially replacing expensive Mo with low-cost Nb has become the mainstream research direction for cost reduction and efficiency improvement in high-strength low-alloy steel.
[0004] However, existing Nb-to-Mo technology and its associated heat treatment processes have many drawbacks, making it difficult to simultaneously achieve both low cost and high strength and toughness: (1) After Nb partially replaces Mo, the grain boundary pinning ability of the precipitated phase (Ti,Nb)C in hot-rolled steel is insufficient, which easily leads to abnormally large austenite grains and uneven microstructure distribution, resulting in a simultaneous decrease in the strength, plasticity and low-temperature toughness of the steel, which cannot meet the requirements of engineering applications. (2) Conventional heat treatment process adopts a two-step process of pre-tempering and constant temperature reheating quenching. The reheating process has a fast heating rate and a single heat preservation method, which can easily lead to local coarsening of austenite grains, aggregation and growth of precipitates, and loss of grain boundary pinning fine grain effect. (3) Traditional quenching uses a single cooling rate, which easily leads to uneven width of martensite laths, low proportion of large-angle grain boundaries, and limited improvement in the low-temperature toughness of steel. (4) Existing conventional processes cannot achieve the coordinated control of three aspects: dispersed precipitation of precipitates, uniform austenite grains, and refined martensite structure. It is difficult to achieve ultra-fine grains and stable performance improvement while reducing the amount of Mo.
[0005] Currently, none of the publicly available technical documents and patents propose a combined process of pre-tempering, gradient heating reverse austenitization, and staged quenching. This process cannot effectively solve the technical problems of coarse grains, decreased strength and toughness, and poor process stability after Nb-substituted Mo, thus restricting the industrial application of low-cost, high-strength Nb-substituted Mo steel. Summary of the Invention
[0006] The purpose of this invention is to provide a low-cost method for refining the grains of high-strength steel, which can solve the problems of coarse grains, low strength and toughness, and poor process stability in the existing Nb-Mo substitution method for preparing high-strength steel.
[0007] To solve the above-mentioned technical problems, the technical solution adopted by the present invention includes the following steps: S1. Prepare Nb-substituted Mo hot-rolled high-strength steel with the following chemical composition by mass percentage: C: 0.25-0.28%, Si: 0.25-0.30%, Mn: 1.0-1.3%, Ti: 0.1-0.2%, Cr: 0.3-0.4%, Mo: 0.05-0.10%, Nb: 0.05-0.09%, P≤0.01%, S≤0.005%, with the balance being Fe; S2. Pre-tempering treatment: The hot-rolled high-strength steel obtained in step S1 is held at 530–570℃ for 2.5–3.5 hours. S3. Gradient reverse austenitization treatment: The hot-rolled high-strength steel after pre-tempering in step S2 is subjected to gradient heating reverse austenitization treatment. First, it is heated to 820–840℃ and held for 3–5 min. Then, it is heated to 870–890℃ at a rate of 3–6℃ / min and held for 4–6 min. S4. Graded quenching treatment: After the austenitizing treatment in step S3 is completed, the hot-rolled high-strength steel is first immersed in a mixed salt bath at 130–150℃ for pre-cooling for 3–5 seconds, and then water-quenched to room temperature to obtain a fine-grained structure; the mixed salt bath is 55wt% KNO3 + 45wt% NaNO2.
[0008] A more specific technical solution than the above-mentioned technical solution is that the hot-rolled high-strength steel described in step S1 is prepared sequentially through vacuum melting, forging, controlled rolling, and laminar flow cooling processes.
[0009] In some possible implementations, the cumulative roughing reduction rate is 0.6, the cumulative finishing reduction rate is 0.8, and the final rolling temperature is 870°C during the controlled rolling process.
[0010] In some possible implementations, the laminar cooling rate is 15°C / s, and the cooling termination temperature is 615°C.
[0011] In some possible implementations, after the pre-tempering treatment in step S2, the furnace is slowly cooled to room temperature.
[0012] In some possible implementations, the heating rate to 820-840°C in step S3 is 8-12°C / min.
[0013] In some possible implementations, the resulting high-strength steel, as tested by EBSD, has an average effective grain size of 2.5-3.0 μm and large-angle grain boundaries with an orientation difference >15°, accounting for 70% or more of the total number of grain boundaries.
[0014] In some possible implementations, a (Ti,Nb)C and (Ti,Mo)C composite nano-precipitates are formed inside the high-strength steel, with the precipitate size ranging from 5 to 40 nm, and are dispersedly distributed at grain boundaries and within grains.
[0015] By adopting the above technical solution, the present invention has the following beneficial effects compared with the prior art: 1. Significant grain refinement effect: The average effective grain size of the high-strength steel obtained is controlled at 2.5-3.0μm, which is more than 60% finer than the conventional hot-rolled grain. The structure is uniform with no mixed grains or coarse grain regions, laying the foundation for improved strength and toughness. 2. High proportion of large-angle grain boundaries: The proportion of large-angle grain boundaries is ≥70%, which effectively hinders crack propagation, significantly improves the low-temperature toughness of steel, and has an impact energy of ≥50J at -20℃, meeting the requirements of low-temperature engineering applications. 3. Stable and dispersed precipitates: (Ti,Nb)C and (Ti,Mo)C composite nano-precipitates are formed in the steel, with a size of 5-40nm. They are uniformly dispersed in the grain boundaries and within the grains, and the grain boundary pinning effect is long-lasting, which can effectively inhibit the growth of austenite grains at high temperatures. 4. Low cost and high performance synergy: The amount of Mo used is reduced by 40%-60% compared with all-Mo high-strength steel, which significantly reduces the raw material production cost. At the same time, the steel yield strength is ≥1100MPa, tensile strength is ≥1350MPa, and elongation is ≥15%, achieving the performance level of all-Mo high-strength steel. 5. Stable and controllable process: The gradient heating and staged quenching design reduces the impact of temperature fluctuations on the process effect. The process is simple and easy to operate, making it suitable for industrial mass production. Attached Figure Description
[0016] Figure 1 This is a SEM grain morphology image of the high-strength steel prepared in Example 1 of the present invention.
[0017] Figure 2 This is a TEM composite nano-precipitated phase morphology image of the high-strength steel prepared in Example 1 of the present invention.
[0018] Figure 3 This is an EBSD large-angle grain boundary distribution map of the high-strength steel prepared in Example 1 of the present invention. Detailed Implementation
[0019] The following are specific embodiments of the present invention. It should be noted that the present invention is not limited to the following specific embodiments. All equivalent modifications made based on the technical solutions of this application fall within the protection scope of the present invention.
[0020] Example 1: The grain refinement method in this embodiment includes the following steps: S1. Prepare Nb-substituted Mo hot-rolled high-strength steel with the following chemical composition by mass percentage: C: 0.26%, Si: 0.28%, Mn: 1.15%, Ti: 0.15%, Cr: 0.35%, Mo: 0.07%, Nb: 0.07%, P: 0.01%, S: 0.003%, with the balance being Fe. The raw material with the above composition is subjected to vacuum induction melting and forging, followed by controlled rolling treatment. The roughing reduction rate is 0.6, the finishing reduction rate is 0.8, the final rolling temperature is 870℃, and then it is laminar cooled to 615℃ at a rate of 15℃ / s, and then air-cooled to room temperature to obtain an 8mm thick high-strength steel plate. S2. Pre-tempering treatment: The high-strength steel plate obtained in step S1 is placed in a heat treatment furnace and held at 550℃ for 3 hours. After the holding period, it is slowly cooled to room temperature with the furnace. The purpose of this step is to promote the precipitation of nanoscale carbides in the steel matrix. These carbides are mainly (Ti,Nb)C and (Ti,Mo)C, which lay the foundation for grain boundary pinning in the subsequent reverse austenitization process. S3. Gradient reverse austenitizing treatment: The pre-tempered high-strength steel plate is placed back into the heat treatment furnace for gradient heating reverse austenitizing treatment, which is divided into two stages: (1) Low temperature preheating section: Heat to 820℃ at a rate of 10℃ / min and hold for 4min to achieve local austenite nucleation and avoid rapid grain growth in the early stage of nucleation; (2) Gradient heating section: The temperature is slowly increased to 880℃ at a rate of 5℃ / min and held for 5min to complete the uniform reverse austenitization. At the same time, the nano carbides precipitated in the early stage are used to pin the grain boundaries, inhibit the growth of austenite grains, and achieve uniform refinement of austenite grains. S4. Graded quenching treatment: After austenitization, the high-strength steel plate is immediately subjected to graded salt bath quenching treatment, which is divided into two stages: (1) First-stage salt bath precooling: The high-strength steel plate is quickly immersed in a 140℃ mixed salt bath for 4 seconds for precooling; wherein, the mixed salt bath is 55wt% KNO3 + 45wt% NaNO2; The purpose of this first-stage salt bath precooling is to precisely control the degree of undercooling, stabilize the austenite state, inhibit abnormal growth of martensite laths, reduce quenching internal stress, avoid cracking and deformation, and ensure uniform and refined microstructure. (2) Secondary water quenching: The high-strength plate, which has been pre-cooled in a salt bath, is quickly transferred to room temperature water quenching to obtain a uniform and refined tempered martensite structure, and finally achieves ultra-fine grains.
[0021] The high-strength steel plate obtained by the grain refinement method in this embodiment has the following properties: average effective grain size 1.76μm, large-angle grain boundary ratio 70.5%, yield strength 1120MPa, tensile strength 1360MPa, impact energy at -20℃ 62J, and elongation 15.5%.
[0022] like Figure 1 As shown, the high-strength steel exhibits a fine grain size, with an average effective grain size of 2.61 μm; Figure 2 As shown, the composite nano-precipitated phase is distributed at grain boundaries and within grains; as Figure 3 As shown, Figure 3 The black lines in the diagram represent large-angle grain boundaries with an orientation difference greater than 15°, which account for more than 70% of all grain boundaries.
[0023] Example 2: The grain refinement method in this embodiment includes the following steps: S1. Prepare Nb-substituted Mo hot-rolled high-strength steel with the following chemical composition by mass percentage: C: 0.25%, Si: 0.25%, Mn: 1.0%, Ti: 0.1%, Cr: 0.3%, Mo: 0.10%, Nb: 0.05%, P: 0.008%, S: 0.005%, with the balance being Fe. The raw material with the above composition is subjected to vacuum induction melting and forging, followed by controlled rolling. The roughing reduction rate is 0.6, the finishing reduction rate is 0.8, the final rolling temperature is 870℃, and then it is laminar cooled to 615℃ at a rate of 15℃ / s, and then air-cooled to room temperature to obtain an 8mm thick high-strength steel plate. S2. Pre-tempering treatment: The high-strength steel plate obtained in step S1 is placed in a heat treatment furnace and held at 540℃ for 3.5 hours. After the holding period, it is cooled to room temperature with the furnace. S3. Gradient reverse austenitizing treatment: The pre-tempered high-strength steel plate is placed back into the heat treatment furnace for gradient heating reverse austenitizing treatment, which is divided into two stages: (1) Low temperature preheating section: Heat to 825℃ at a rate of 9℃ / min and hold for 5min; (2) Gradient heating section: Heat to 875℃ at a slow rate of 4℃ / min and hold for 6min; S4. Graded quenching treatment: (1) First-stage salt bath precooling: The high-strength steel plate is quickly immersed in a 130℃ mixed salt bath for 5 seconds for precooling; wherein, the mixed salt bath is 55wt% KNO3 + 45wt% NaNO2; (2) Secondary water quenching: The high-strength plate, which has been pre-cooled in a salt bath, is quickly transferred to room temperature water quenching to obtain a uniform and refined tempered martensite structure, and finally achieves ultra-fine grains.
[0024] The high-strength steel sheet obtained by the grain refinement method in this embodiment has the following performance tests: average effective grain size 2.88μm, large-angle grain boundary ratio 70.8%, yield strength 1105MPa, tensile strength 1345MPa, impact energy at -20℃ 59J, and elongation 15.2%.
[0025] Example 3: The grain refinement method in this embodiment includes the following steps: S1. Prepare Nb-substituted Mo hot-rolled high-strength steel with the following chemical composition by mass percentage: C: 0.28%, Si: 0.30%, Mn: 1.3%, Ti: 0.2%, Cr: 0.4%, Mo: 0.05%, Nb: 0.09%, P: 0.008%, S: 0.003%, balance Fe. The raw material with the above composition is subjected to vacuum induction melting and forging, followed by controlled rolling treatment. The roughing reduction rate is 0.6, the finishing reduction rate is 0.8, the final rolling temperature is 870℃, and then it is laminar cooled to 615℃ at a rate of 15℃ / s, and then air-cooled to room temperature to obtain an 8mm thick high-strength steel plate. S2. Pre-tempering treatment: The high-strength steel plate obtained in step S1 is placed in a heat treatment furnace and held at 570℃ for 2.5 hours. After the holding period, it is slowly cooled to room temperature with the furnace. S3, Gradient inverse austenitization treatment: (1) Low temperature preheating section: Heat to 840℃ at a rate of 8℃ / min and hold for 3min; (2) Gradient heating section: slowly heat up to 890℃ at a rate of 6℃ / min and hold for 4min; S4. Graded quenching treatment: After austenitization, the high-strength steel plate is immediately subjected to graded salt bath quenching treatment, which is divided into two stages: (1) First-stage salt bath precooling: The high-strength steel plate is quickly immersed in a 150℃ mixed salt bath for 3 seconds for precooling; wherein, the mixed salt bath is 55wt% KNO3 + 45wt% NaNO2; (2) Secondary water quenching: The high-strength plate, which has been pre-cooled in a salt bath, is quickly transferred to room temperature water quenching to obtain a uniform and refined tempered martensite structure, and finally achieves ultra-fine grains.
[0026] The high-strength steel sheet obtained by the grain refinement method in this embodiment has the following performance test results: average effective grain size 2.61μm, large-angle grain boundary ratio 71.2%, yield strength 1145MPa, tensile strength 1380MPa, impact energy at -20℃ 65J, and elongation 15.8%.
[0027] Example 4: The grain refinement method in this embodiment includes the following steps: S1. Preparation of Nb-substituted Mo hot-rolled high-strength steel with the following chemical composition by mass percentage: C: 0.27%, Si: 0.27%, Mn: 1.2%, Ti: 0.18%, Cr: 0.38%, Mo: 0.10%, Nb: 0.08%, P: 0.008%, S: 0.003%, balance Fe; The raw material with the above composition is subjected to vacuum induction melting and forging, followed by controlled rolling treatment, wherein the rough rolling reduction rate is 0.6, the finish rolling reduction rate is 0.8, the final rolling temperature is 870℃, and then laminar cooling is carried out at a rate of 15℃ / s to 615℃, and then air cooling is carried out to room temperature to obtain 8mm thick high-strength steel plate; S2. Pre-tempering treatment: The high-strength steel plate obtained in step S1 is placed in a heat treatment furnace and held at 560℃ for 3 hours. After the holding is completed, it is cooled to room temperature with the furnace. S3, Gradient inverse austenitization treatment: (1) Low temperature preheating section: Heat to 835℃ at a rate of 11℃ / min and hold for 4min; (2) Gradient heating section: slowly heat to 885℃ at a rate of 5℃ / min and hold for 5min; S4. Graded quenching treatment: (1) First-stage salt bath precooling: The high-strength steel plate is rapidly immersed in a 130℃ mixed salt bath for 4 seconds for precooling; wherein, the mixed salt bath is 55wt% KNO3 + 45wt% NaNO2; (2) Secondary water quenching: The high-strength plate, which has been pre-cooled in a salt bath, is quickly transferred to room temperature water quenching to obtain a uniform and refined tempered martensite structure, and finally achieves ultra-fine grains.
[0028] The high-strength steel sheet obtained by the grain refinement method in this embodiment has the following performance test results: average effective grain size 2.69μm, large-angle grain boundary ratio 70.9%, yield strength 1130MPa, tensile strength 1370MPa, impact energy at -20℃ 64J, and elongation 15.6%.
[0029] Comparative Example 1: The chemical composition and preparation process of the steel in this comparative example are exactly the same as those in Example 1; The heat treatment process is as follows: the prepared steel billet is pre-tempered, held at 550℃ for 3 hours, then heated to 880℃ and held at that temperature for 5 minutes, and then quenched in water to room temperature in one go. Performance test results: average effective grain size 3.85μm, large-angle grain boundary ratio 57.3%, yield strength 1080MPa, tensile strength 1320MPa, impact energy at -20℃ 42J, elongation 12.8%.
[0030] Comparative Example 2: The chemical composition and preparation process of the steel in this comparative example are exactly the same as those in Example 1; Without pre-tempering, the prepared steel billet is directly subjected to gradient reverse austenitization treatment. First, it is heated to 830℃ at 10℃ / min and held for 4min; then, it is heated to 880℃ at 5℃ / min and held for 5min.
[0031] Graded quenching: 140℃ salt bath, mixed salt bath of 55wt% KNO3 + 45wt% NaNO2 pre-cooled for 4s, then transferred to room temperature water quenching to room temperature.
[0032] Performance test results: average effective grain size 3.52μm, large-angle grain boundary ratio 61.5%, yield strength 1095MPa, tensile strength 1330MPa, impact energy at -20℃ 48J, elongation 13.6%.
[0033] Comparative Example 3: The chemical composition and preparation process of the steel in this comparative example are exactly the same as those in Example 1; The heat treatment process is as follows: the prepared steel billet is pre-tempered, held at 550℃ for 3 hours, and then heated to 880℃ and held at that temperature for 5 minutes. Graded quenching: 140℃ salt bath, mixed salt bath of 55wt% KNO3 + 45wt% NaNO2 pre-cooled for 4s, then transferred to room temperature water quenching to room temperature.
[0034] Performance testing: average effective grain size 3.31μm, large-angle grain boundary ratio 63.8%, yield strength 1100MPa, tensile strength 1335MPa, impact energy at -20℃ 51J, elongation 14.1%.
[0035] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.
Claims
1. A method for refining the grain size of low-cost, high-strength steel, characterized in that, Includes the following steps: S1. Prepare Nb-substituted Mo hot-rolled high-strength steel with the following chemical composition by mass percentage: C: 0.25-0.28%, Si: 0.25-0.30%, Mn: 1.0-1.3%, Ti: 0.1-0.2%, Cr: 0.3-0.4%, Mo: 0.05-0.10%, Nb: 0.05-0.09%, P≤0.01%, S≤0.005%, with the balance being Fe; S2. Pre-tempering treatment: The steel billet obtained in step S1 is held at 530–570℃ for 2.5–3.5 hours. S3. Gradient reverse austenitizing treatment: The hot-rolled high-strength steel after pre-tempering in step S2 is subjected to gradient heating reverse austenitizing treatment. First, it is heated to 820-840℃ and held for 3-5 minutes. Then, it is heated to 870-890℃ at a rate of 3-6℃ / min and held for 4-6 minutes. S4. Graded quenching treatment: After the austenitizing treatment in step S3 is completed, the hot-rolled high-strength steel is first immersed in a mixed salt bath at 130-150℃ for pre-cooling for 3-5 seconds, and then water-quenched to room temperature to obtain a fine-grained structure; the mixed salt bath is 55wt% KNO3 + 45wt% NaNO2.
2. The method for refining the grain size of low-cost high-strength steel according to claim 1, characterized in that: The hot-rolled high-strength steel described in step S1 is prepared sequentially through vacuum melting, forging, controlled rolling, and laminar flow cooling processes.
3. The method for refining the grain size of low-cost high-strength steel according to claim 2, characterized in that: In the controlled rolling process, the cumulative roughing reduction rate is 0.6, the cumulative finishing reduction rate is 0.8, and the final rolling temperature is 870℃.
4. The method for refining the grain size of low-cost high-strength steel according to claim 2, characterized in that: The laminar flow cooling rate is 15℃ / s, and the cooling termination temperature is 615℃.
5. The method for refining the grains of low-cost high-strength steel according to claim 1, characterized in that: After the pre-tempering treatment in step S2, the furnace is slowly cooled to room temperature.
6. The method for refining the grain size of low-cost high-strength steel according to claim 1, characterized in that: In step S3, the heating rate to 820-840℃ is 8-12℃ / min.
7. The method for refining the grain size of low-cost high-strength steel according to claim 1, characterized in that: The high-strength steel obtained was tested by EBSD and found that the average effective grain size was 2.5-3.0 μm, and large-angle grain boundaries with an orientation difference of >15° accounted for 70% or more of the total number of grain boundaries.
8. The method for refining the grains of low-cost high-strength steel according to claim 1, characterized in that: The high-strength steel forms (Ti,Nb)C and (Ti,Mo)C composite nano-precipitates inside, with the precipitate size ranging from 5 to 40 nm, and is dispersed in the grain boundaries and within the grains.