A control method for reducing banded structure of high strength beam steel

Abstract

The application discloses a control method for reducing banded structure of high-strength girder steel, and belongs to the technical field of metallurgical rolling. The method is characterized by low segregation smelting, dynamic soft reduction continuous casting, high-temperature homogenization heating, two-stage controlled rolling, gradient ultrafast cooling and stack slow cooling full-process collaborative control, so that segregation is inhibited from the source, austenite is refined, uniform nucleation is promoted, and banded structure inheritance and growth are blocked. The obtained girder steel has stable banded structure of not more than 1.5 levels, yield strength of not less than 550 MPa, tensile strength of not less than 650 MPa, elongation of not less than 22%, and no cracks in 180-degree bending. The method cancels offline heat treatment, reduces cost and increases efficiency, is suitable for conventional hot rolling production lines, can be mass-produced in an industrialized manner, and significantly improves the formability and fatigue life of the girder steel.

Description

Technical Field

[0001] This invention belongs to the field of metallurgical rolling technology, specifically relating to a method for controlling the reduction of banded structure in high-strength beam steel, which is particularly applicable to the production of hot-rolled high-strength steel for automobile beams. Background Technology

[0002] Automotive frame steel is a core material for commercial vehicle chassis, requiring high strength, high plasticity, high fatigue performance, and low anisotropy. Banded structure is a typical microstructural defect in hot-rolled high-strength steel, characterized by alternating bands of ferrite and pearlite along the rolling direction. This leads to anisotropy in mechanical properties, bending cracking, and reduced fatigue life, severely restricting the service safety of the frame steel.

[0003] In existing technologies, mitigating banded structures often relies on offline heat treatments such as diffusion annealing and normalizing, which have problems such as long processes, high energy consumption, increased costs, and low production efficiency. Conventional controlled rolling and cooling have limited effect on improving the hereditary banded structure caused by dendrite segregation, and it is difficult to stably control the banding level to ≤1.5.

[0004] Therefore, developing a control method for reducing the banded structure of high-strength beam steel that requires no offline heat treatment, involves full-process collaboration, is low-cost, and industrially scalable has become a pressing technical challenge in this field.

[0005] A search of the following three patent documents, which are in the same technical field as this invention, reveals that this invention possesses outstanding novelty and inventiveness:

[0006] Prior art 1: CN112517638B provides a method for controlling the strip structure of hot-rolled high-strength steel for cold bending. This prior art uses conventional controlled rolling and a single laminar flow cooling method to improve the strip structure, without employing dynamic light-pressure continuous casting and high-temperature long-term homogenization heating processes. The cooling regime is only a single cooling mode, without setting up a three-stage gradient cooling path of ultra-fast cooling + air cooling transition + laminar flow cooling, and still relies on offline heat treatment assistance. It cannot stably control the strip structure to level 1.5 or below without adding processes. The process economy and the accuracy of microstructure control are both weaker than the present invention.

[0007] Existing technology 2: CN114574773A provides a method for producing 610MPa grade low-cost hot-rolled high-strength beam strip. This existing technology does not employ continuous casting dynamic light reduction technology, resulting in insufficient source control of center segregation and hereditary banded structure. The rolling and cooling processes still use the traditional TMCP process, with a low cooling rate and no gradient control, which cannot effectively suppress the directional precipitation of ferrite and pearlite. The final banded structure level is mostly between 2.0 and 3.0, and the anisotropy control effect is poor, which is significantly different from the whole-process homogenization control approach of this invention.

[0008] Existing technology 3: CN110819773A provides a heat treatment process for controlling the banded structure of low-alloy high-strength weather-resistant steel. This existing technology uses offline multi-stage normalizing and isothermal heat treatment as its core means, which significantly increases the production process and energy consumption. It does not achieve online integrated control, and the process route is complex and costly. It cannot be adapted to efficient continuous hot rolling production lines, and is fundamentally different from the technical solution of this invention, which does not require offline heat treatment and achieves precise online control throughout the entire process. Summary of the Invention

[0009] This invention provides a method for controlling the banded structure of high-strength beam steel. Through the coordinated control of the entire process, including low-segregation smelting, homogenized continuous casting, high-temperature homogenization heating, two-stage controlled rolling, gradient ultra-fast cooling, and stacking slow cooling, the method achieves stable control of the banded structure to ≤1.5 level, while ensuring high strength, high toughness, and good formability. It also eliminates offline heat treatment and reduces production costs.

[0010] Specifically, this invention proposes for the first time a comprehensive, coordinated control method for the entire process: low-segregation smelting—dynamic light-pressure continuous casting—high-temperature homogenization heating—two-stage controlled rolling—gradient ultra-fast cooling—stacking slow cooling. This method suppresses segregation at its source through light-pressure continuous casting, achieves compositional homogenization through high-temperature homogenization, and achieves phase transformation homogenization through three-stage gradient cooling. Without any offline heat treatment, the banded microstructure of high-strength beam steel can be stably controlled at grade 1.5 or below, while simultaneously ensuring high strength, high toughness, and excellent formability. The overall technical solution and key process combinations described above are not disclosed in the aforementioned prior art.

[0011] To achieve the above objectives, the present invention mainly adopts the following technical solutions:

[0012] This invention provides a method for controlling the banded structure of high-strength beam steel, comprising the following steps:

[0013] (1) Smelting and refining: The converter smelting + LF + RH vacuum refining is adopted. The chemical composition is controlled by weight percentage as follows: C 0.06~0.08%, Si 0.05~0.15%, Mn 1.20~1.80%, P≤0.015%, S≤0.005%, Nb 0.03~0.06%, Ti 0.01~0.03%, Alt 0.020~0.050%, with the balance being Fe and unavoidable impurities; the pure circulation time after vacuum treatment is ≥8min, and the TO in the molten steel is ≤15ppm and N≤40ppm;

[0014] (2) Continuous casting: constant speed dynamic light reduction continuous casting is adopted, superheat 15~30℃, casting speed 0.90~1.20m / min, total reduction at the end of solidification 3.0~6.0mm, billet surface temperature ≥850℃ hot delivery and hot charging;

[0015] (3) Heating: The temperature of the billet soaking zone is 1200~1250℃, the holding time is 60~120min, and the heating rate of the heating zone is ≤8℃ / min to prevent overheating and decarburization, and to achieve full diffusion and homogenization of carbon and manganese.

[0016] (4) Controlled rolling:

[0017] - Rough rolling: Rolling in the recrystallization zone, with an initial rolling temperature of 1050~1100℃, a single-pass deformation of 15~25%, and a cumulative deformation of ≥65%, to achieve full recrystallization and coarsening of austenite;

[0018] - Finish rolling: Rolling in the non-recrystallization zone, with an initial rolling temperature of 880~920℃ and a final rolling temperature of 820~860℃; the cumulative deformation of the last 3 passes is ≥55%, forming a high-density deformation zone and an intragranular ferrite nucleation core;

[0019] The initial rolling temperature in the recrystallization zone of rough rolling is 50~1100℃, and the cumulative deformation is ≥65%; the initial rolling temperature in the non-recrystallization zone of finish rolling is 880~920℃, the final rolling temperature is 820~860℃, and the cumulative deformation after the three passes of finish rolling is ≥55%.

[0020] (5) Three-stage controlled cooling:

[0021] It adopts a three-stage cooling system: gradient ultra-fast cooling + air cooling transition + laminar flow cooling.

[0022] - Ultra-fast cooling section: Started within 0.5s after final rolling, cooling rate 35~50℃ / s, cooling to 700~730℃;

[0023] - Air cooling section: Air cooling for 2~5 seconds to promote dispersion and homogenization of the tissue;

[0024] - Laminar flow cooling section: cooling rate 10~20℃ / s, winding temperature 580~620℃;

[0025] (6) Slow cooling after coiling and stacking: The steel coils are coiled in close rows and stacked for slow cooling for ≥24 hours to eliminate internal stress and stabilize the structure.

[0026] In some implementations, in step (2), the continuous casting adopts a water distribution system of weak corner cooling + strong surface cooling.

[0027] In some implementations, in step (5), the ultra-fast cooling is initiated within 0.5 s after final rolling.

[0028] In some implementations, the resulting high-strength beam steel has a banded structure grade ≤ 1.5.

[0029] In some embodiments, the resulting high-strength beam steel has a yield strength ≥550MPa, tensile strength ≥650MPa, elongation ≥22%, and no cracks when bent at 180°.

[0030] The present invention mainly adopts the following technical principles:

[0031] 1. Low phosphorus and sulfur content, and high purity reduce inclusions and segregation factors;

[0032] 2. Dynamic light pressure inhibits central segregation and blocks the source of hereditary banded lesions;

[0033] 3. High-temperature homogenization promotes the diffusion of alloying elements and reduces microsegregation;

[0034] 4. Two-stage controlled rolling refines austenite and introduces deformation bands, increasing the number of uniform ferrite nucleation sites;

[0035] 5. Gradient ultrafast cooling suppresses the growth of grain boundary ferrite bands, forces phase transformation homogenization, and significantly reduces banded structure.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] (1) Controllable banded tissue: banded grade is stable ≤1.5, and anisotropy is significantly reduced;

[0038] (2) Excellent performance: yield strength ≥550MPa, tensile strength ≥650MPa, elongation ≥22%, no cracks when bent at 180°;

[0039] (3) Low cost: The elimination of offline diffusion annealing / normalizing reduces the cost per ton of steel by 80-120 yuan;

[0040] (4) Industrialization: It is compatible with conventional hot rolling production lines, with stable processes and strong batch replication. Attached Figure Description

[0041] Figure 1 This is a microstructure diagram (banded structure grade 1.0) of the high-strength beam steel obtained in Example 1 of the present invention.

[0042] Figure 2 This is a microstructure diagram (banded structure grade 1.5) of the high-strength beam steel obtained in Example 2 of the present invention.

[0043] Figure 3 Microstructure of high-strength beam steel obtained using conventional processes (banded structure, grade 3.0–4.0) is shown for comparison. Detailed Implementation

[0044] The present invention will be further described in detail below with reference to specific embodiments, but the scope of protection of the present invention is not limited by the embodiments.

[0045] Example 1

[0046] Production specifications: 6.0mm × 1500mm 610L high-strength beam steel

[0047] 1. Chemical composition (wt%):

[0048] C 0.08%, Si 0.12%, Mn 1.50%, P 0.012%, S 0.003%, Nb 0.045%, Ti 0.020%, Als 0.035%, balance Fe and unavoidable impurities.

[0049] 2. Continuous casting: superheat 22℃, casting speed 1.05m / min, light reduction 4.5mm, hot charging temperature 890℃.

[0050] 3. Heating: Heat to 1230℃, hold for 90 minutes, heating rate ≤8℃ / min.

[0051] 4. Rolling: Rough rolling starts at 1080℃, with a cumulative deformation of 70%; finish rolling starts at 900℃ and finishes at 840℃, with a deformation of 58% in the last 3 passes.

[0052] 5. Cooling: 0.4s after final rolling, ultra-fast cooling is started, with a cooling rate of 40℃ / s to 715℃, followed by air cooling for 3s, laminar flow cooling at a cooling rate of 15℃ / s, and coiling at 600℃.

[0053] 6. After winding, stack and cool slowly for 24 hours.

[0054] Test results: Grade 1.0 band-like tissue, microscopic examination revealed... Figure 1 As shown; yield strength 590MPa, tensile strength 695MPa, elongation 24.5%; no cracks after 180° bending, cold bending qualified.

[0055] Example 2

[0056] Production specifications: 8.0mm × 1800mm 700L high-strength beam steel

[0057] 1. Chemical composition (wt%):

[0058] C 0.07%, Si 0.10%, Mn 1.65%, P 0.013%, S 0.004%, Nb 0.050%, Ti 0.025%, Als 0.040%, balance Fe and unavoidable impurities.

[0059] 2. Continuous casting: superheat 25℃, casting speed 1.00m / min, light reduction 5.0mm, hot charging temperature 870℃.

[0060] 3. Heating: Heat to 1240℃, hold for 100 min, heating rate ≤8℃ / min.

[0061] 4. Rolling: Rough rolling starts at 1090℃, with a cumulative deformation of 68%; finish rolling starts at 910℃ and finishes at 850℃, with a deformation of 60% in the last 3 passes.

[0062] 5. Cooling: 0.4s after final rolling, ultra-fast cooling is started, with a cooling rate of 45℃ / s to 720℃, followed by air cooling for 4s, laminar flow cooling at a cooling rate of 18℃ / s, and coiling at 610℃.

[0063] 6. After winding, stack and cool slowly for 24 hours.

[0064] Test results: Grade 1.5 band-like tissue, microscopic examination revealed... Figure 2 As shown; yield strength 680MPa, tensile strength 780MPa, elongation 23.0%; no cracks after 180° bending, cold bending qualified.

[0065] Comparative Example

[0066] The conventional process is adopted: no dynamic light pressing, final rolling temperature of 780-800℃, cooling method is single laminar flow cooling (cooling rate of 15-20℃ / s), and coiling temperature of 650℃.

[0067] Test results: Band-like tissue, grade 3.0-4.0, microstructure visible. Figure 3 As shown, the elongation is ≤18%, it is prone to cracking when cold-bent, and the anisotropy is obvious.

[0068] In summary, this invention achieves a significant reduction in the banded structure of high-strength beam steel through precise control of the entire process, suppressing segregation at the source, homogenizing the structure during the process, and forcing a uniform phase transformation at the end. This results in outstanding performance and cost advantages, making it suitable for large-scale industrial production.

[0069] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for controlling the banded structure of high-strength beam steel, characterized in that, Includes the following steps: (1) Smelting and refining: The converter smelting + LF + RH vacuum refining is adopted. The chemical composition is controlled by weight percentage as follows: C 0.06~0.08%, Si 0.05~0.15%, Mn 1.20~1.80%, P≤0.015%, S≤0.005%, Nb 0.03~0.06%, Ti 0.01~0.03%, Alt 0.020~0.050%, with the balance being Fe and unavoidable impurities; the pure circulation time after vacuum treatment is ≥8min, and the TO in the molten steel is ≤15ppm and N≤40ppm; (2) Continuous casting: constant speed dynamic light reduction continuous casting is adopted, superheat 15~30℃, casting speed 0.90~1.20m / min, total reduction at the end of solidification 3.0~6.0mm, billet surface temperature ≥850℃ hot delivery and hot charging; (3) Heating: The temperature of the billet soaking zone is 1200~1250℃, the holding time is 60~120min, and the heating rate of the heating zone is ≤8℃ / min; (4) Controlled rolling: The initial rolling temperature in the recrystallization zone of rough rolling is 50~1100℃, and the cumulative deformation is ≥65%; the initial rolling temperature in the non-recrystallization zone of finish rolling is 880~920℃, and the final rolling temperature is 820~860℃, with a cumulative deformation of ≥55% in the three passes after finish rolling; (5) Three-stage controlled cooling: Ultra-fast cooling section: cooling rate 35~50℃ / s, cooling to 700~730℃; Air cooling section: air cooling for 2 to 5 seconds; Laminar flow cooling section: cooling rate 10~20℃ / s, winding temperature 580~620℃; (6) Slow cooling after winding and stacking: slow cooling time ≥ 24h.

2. The method according to claim 1, characterized in that, In step (2), the continuous casting adopts a water distribution system of weak corner cooling + strong surface cooling.

3. The method according to claim 1, characterized in that, In step (5), the ultra-fast cooling is initiated within 0.5s after the final rolling.

4. The method according to claim 1, characterized in that, The resulting high-strength beam steel has a banded structure grade ≤ 1.

5.

5. The method according to any one of claims 1-4, characterized in that, The resulting high-strength beam steel has a yield strength ≥550MPa, tensile strength ≥650MPa, elongation ≥22%, and no cracks when bent at 180°.

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